CA2257058A1 - Suppression of polarization hole burning with an acousto-optic modulator - Google Patents
Suppression of polarization hole burning with an acousto-optic modulator Download PDFInfo
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Abstract
An apparatus and method for reducing polarization hole buming in a rare-earth-doped fiber amplifier within an optical communication system by converting an optical carrier having a characteristic wavelength into a polarization-rotating optical carrier is disclosed. The apparatus includes a polarization-fixing device optically coupled in the optical transmission system to transform the optical carrier to a polarized optical carrier, and an acousto-optic modulator positioned to receive a first portion of the polarized optical carrier and to orthogonally convert the polarization of the polarized optical carrier and to shift the polarized optical carrier by a modulation frequency. The apparatus and method further includes a polarization beam combiner optically coupled to receive the orthogonally polarization converted and frequency shifted polarized signal and a portion of the original polarized optical carrier. The polarization beam combiner produces a polarization-rotating carrier signal that is transmitted downstream in the optical communication system to a rare-earth-doped fiber amplifier.
Description
CA 022~70~8 1998-12-30 - "SUPPRESSION OF POLARIZATION HOLE BURNING WITH AN ACOUSTO-OPTIC MODULATOR"
BACKGROUND OF THE INVENTION
The present invention relates generally to methods and systems for su~.pressi, lg pola,i~dlion hole burning in rare-earth doped fiber amplifiers. More particularly, the present invention relates to methods and systems for su,uplessing pola,i~dlion hole burning using acousto-optic modulation to vary a state of polari~dlion of an input signal.
Long dislance optical communication systems have been known to suffer from various polarization dependent effects that may cause a signal-to-noise ratio of the system to lessen. Polarization hole burning (PHB) is one of the polari~alion dependent phenomena that can severely impair the pel ru",lance of erbium-doped fiber amplifiers (EDFAs) located in optical fiber communication systems. PHB occurs when a strong, polarized optical signal is launched into an EDFA and causes anisotropic saturation of the amplifier. This effect, which is related to the pop~ tion inversion dynamics of the EDFA, depresses the gain of the EDFA for light with the same polari~dlion as the saturating signal. Thus, PHB causes a signal having a state of polari~dlion (SOP) orthogonal to the saturating signal to have a gain greater than that of the saturating signal.
In a chain of saturated EDFAs, amplified spontaneous el"ission (ASE) noise can accumulate faster in the polarization o, II,ogonal to a saturating il ~rur~lalion signal than along the polarization parallel to the signal. ASE ol lhogonal to a saturating signal will accumulate at each amplifier stage of the l.anslnission line. The build-up of orthogonal ~-ASE reduces the signal-to-noise ratio (SNR) of the optical l,ansr"ission system, thus causing possible errors in the received data stream. Accordingly, it is des' ~le to reduce the effects of PHB in amplified systems in order to maintain a system with good SNR
chara~;tel i~lics.
O~.erdli"g EDFAs in gain co",pressiol1 helps to cause the u"desired PHB effect.
The degree of gain compression Cp indicates the difrerence of gain of the amplifier in its operative condition of prop~gation of a slgnal with 1~Y~ optical power (i.e., a non-saturating signal ex,ue, ienc;ng maximum gain, called "Go") with' respect to the value experienced by the optical signal in the power level condi;ion at which-it is operdting (G). An amplifier's operaling gain in decibels can be measured with a saturating signal of input power Si as CA 022~70~8 1998-12-30 - the following:
G=So-Si (1) where So is the saturated output power. Accordingly, the amount of gain compression equals the following:
Cp = Go - G. (2) The gain in the orthogonal polari~dlion on the other hand can be measured using a probe signal with an input polarization orthogonal to the saturating signal as the following:
Po-Pi=G+~G (3) Pi and Po being the input and output power of the probe signal. In equation (3) ~G
10 corresponds to the PHB value.
Moreover the amount of PHB increases as the amplifier goes deeper into gain co",pression. Figure 1 is a graph of experimental measurements showing the relalionshi~.
between the amount of gain compression and the amount of PHB in an EDFA. As shown in this graph the amount of PHB is only about 0.08 dB for a single EDFA that operates 15 with 3 dB of gain compression. However, as the gain co",pr~ssion increases so does the PHB. When the EDFA operates in a saturated condition with Cp equal to about 9-10 dB
the PHB is more sig"ificdnt and quantiri~le at around 0.2 dB per EDFA.
Furthermore the amount of PHB in an EDFA depends on the degree of polarization (DOP) of the saturating signal passing through the amplifier. Figure 2 is a 20 graph of experimental results on an EDFA operating at 10 dB of gain compression. As can be seen from this graph of Figure 2 as the degree of polarization of the saturating signal di~"i.,ishes from 100% the vaiidlion of gain induced by PHB also diminishes. This fact illustrates that the delete, ious effects from PHB may be lessened by varying the state of polaii,dlion. PHB can be reduced by scrambling the SOP of the l,ans",itled optical 25 signal at a rate that is much higher than 1/t~, where t, is the ani.,ol,~p-c saturation time.
P~ec~use an EDFA takes about 0.5 msec to reach a gain stable con.lition after variation of a signal's SOP the signal's SOP should be scrambled at about 10 kHz or more in order to overcome the PHB phenomenon.
The literature has proposed several a" dnge-nents for mitigating PHB effects in 30 optical communication systems. EP 615,356 and ~ ;. Patent No. 5,491 576 d; vlose a techr,. ue for reducing nonlinear signal dey, dddlion by simultaneously launching two optical signals of different v:_~/elens~tl,s cor"pardble power levels, and suL,-~;ldntially olll,ogonal relative polar,~dtions into the same l,dnsr".ssion path. The resulting overall CA 022~70~8 1998-12-30 - transmitted signal is therefore essentially u"polari~ed, and the impact of detrimental pola, i~dlion dependent effects within the l,ans",ission system are r~po~ ledly minimized.
The combined signal is modu'ated by a polari~dlion independent optical modulator so that both wavelengtl, COI"ponenls of the combined signal carry the same data, or each5 wavelength path is separ~tely modulated prior to their combination. Similar disclosure of a system that launches two signals of dirrerenl wavelengths can be found in Bergano et al., "Polari~dlion Hole-Buming in Erbium-Doped Fiber-Amplifier T,dnsl"ission Systems,"
ECOC '94, pp. 621-628.
U.S. Patent No. 5,107,358 describes a method and apparatus for lldnsr"itling 10 i"ror",alion and dete~ti.,g it after propa3alion through a waveguide by means of a coherent optical detector. In particular, Fig. 3 shows a transmitter comprising an optical source generating a single carrier signal which is fed to a modulator. An optical splitter generates two ver~ions of the modulated signal. The first version is fed to a first polarization cor,l,."er, while the second version is fed via a frequency shifting circuit to a 15 second polari~dlion conl-~l'er. The pola-i~alion of this signal is adjusted by the second conl"JI' er to be orthogonal to the polari~alion of the signal from the first controller. The orthogonally pola,i~ed signals are then co",b-..,ed by a polari~dlion selective coupler for transmission.
It should be understood that in all the exarr,r'es described in the '358 patent, the 20 two optical carrier frequencies will typically be separated by two to three times the bit rate in Herk. Applicants have observed that by superposing an optical signal with a version of the same having orthogonal pola,i~dlion and being shifted in frequency by two to three times the bit rate, an optical signal with a bandY.idll, of the same magnitude (two to three times the bit rate) is obtained. The bandwidth of the filters to be used at the receiver must 25 be equal to or greater than the signal bandwidth. Due to this large filter bandwidth, the noise at the receivcr, in the case of a long ~i~.t-ance amplified optical telecommu".~alion system, would be too high to allow a good signal reception, particularly for a bit rate greater than 1 GbiVs.
It is also known from, for example, U.S. Patent No. 5,327,511 and l lei~,n,ann et al., 30 ~EIectro-optic ~ola,i~dlion scramblers for optically ~ar~plified long-haul l,dnsl"ission systems,~ ECOC '94, pp. 629-632, to generate a cafrrier signal having a single wavelength, modul7tç the carrier signal with data, and then send the mod~ ted carrier signal through a pola,i~dtion mod~ator or scr~m~'er to help ~ "~viale the effects of pola,i~dlion hole .
CA 022~70~8 1998-12-30 - buming. These documents ~ close the use of a lithium niobate-based electro-optic modl ~tor with a single path for passi"g the carrier wavelength and modulating its polari~-dlion at, for examr'e, modu'~tion frequencies of 40 kHz and 10.66 GHz. These polarization modulators or s~, dn,b'er~ create highly ~ando~ ed polari~dlion states for the 5 signal. Such devices affect the output polarization accord;"g to a control signal and use relatively high levels of power.
From Elect,on-.~s Letters, Vol. 30, No.18, p.1500-1501, September 1,1994 an acousto optical Ti:LiNbO3 device is known whose transducer is placed at 1/3 of the interaction length, which forms a polarization-i~,dependent optical depolarizer consisling of 10 two or more sections of a wavelength tunable TE-TM converter, s~ 'e to suppress pola, i~dlion hole-buming in EDFAs. The authors present a double stage depolarizer with a < 0.03 residual degree of polari~dLion.
As well, acousto-optical waveguide devices are known that provide a polarizationrotation to an input optical signal and modulate the signal with an acoustic wave from a modulation source. Relevant publications include, for example, EP 737,880, EP 757,276 and M. Rehage et al., "Wavelength-Selective rolarisdlion Analyser with Integrated Ti:LiNbO3 Acousto-Optical TE-TM Converter," Electronics Letters, vol. 30, no.14, July 7, 1994.
Applicants have found that the known techniques for minimizing pola, i~alion hole burning using electro-optic modulators to rotate the polari~dlion of a carrier signal require undesirably high levels of power. As well, Applicants have discovered that the known techr, ques for providing a pola,i~dlion-rotating signal for an erbium~oped fiber amplifier require a much wider band width than is pnd~ticdlly acceptable for a receiver in an optical t,ansr"ission system. F~"ll,er",or~, systems employing two sources at dirrerenl wavelen~tl,s are difficult to implement, due to the p~b!ems in selecting the sources and in stabilizing their wavelengtl ,s. WDM l, dnsr"ission by this system would be verycomplicated and expensive.
SUMMARY OF THE INVENTION
In a~~ordance with the present invention, all optical t, ansr"ission system has been developed to help reduce pola,i~dtion hole buming in a rare-earth-doped fiber amplifier by converting an optical carrier signal having a chdldctt:lialic wavelengtl, into a pola,i~dlion-rotdli"g optical carrier. The system el"~'~ys an ~cousto-optic modul~tor that modul~tes a CA 022~70~8 1998-12-30 - portion of the optical carrier. The acousto-optic mod~ ~?tor causes an orthogonal rotation of the pola, i~dlion of the portion of the optical carrier. A polari~dlion beam combiner then combines the mod~ 'ed and orthogonal signal from the acousto-optic modu~tor with the remainder of the original optical carrier signal to produce a polari~dlion-,utdLi,)g optical 5 carrier. The pola, i~dlion-rotating optical carrier is i"se~led into the optical communication system for eventual use within a rare-earth-doped fiber amplifier.
To obtain the adva"lages and in accor~lance with the purpose of the invention, as embodied and broadly described herein, an appardtus for reducing polari~dlion hole buming in a rare-earth-doped fiber amplifier within an optical communication system by 10 converting an optical carrier having a ~;hardcte,i~lic wavelength and an initial state of polarization into a pola,i~dlion-rotating optical carrier, includes an acousto-optic modulator and a pola,i~alion beam co",b-..,er. The ~cousto-optic modulator has a carrier input optically coupled to receive a first portion of the polarized optical carrier, a modulation input electrically coupled to receive an RF modul~tion frequency, and a modulator output.
15 The acousto-optic modulator includes circuitry for orthogonally converting pola, i~alion of the polarized optical carrier and shifting the polarized optical carrier frequency by the modulation frequency. The pola, i~dlion beam combiner has a first input optically coupled to receive the orthogonally SOP (State of Polari~dlion) -converted and frequency-shifted polarized signal, a second input optically coupled to receive a second portion of the 20 polal i~ed optical carrier, and an output optically coupled to the rare-earth-doped fiber amplifier downstream in the optical communication system.
In anotl,er aspect, the invention includes an optical l,dnsruill0r for reducing polari~dtion hole buming in a rare-earth-doped fiber amplifier within an opticalcommunication system having an optical source for l,dnsn,itling an optical carrier having 25 an initial state of polali~dlion, a splitter, a modu~ation source for providing a modu'-'ion signal, an acousto-optic modul~tor, an attenuator, and a polaii~lion beam col"bil,er. The splitter is positioned do~h"~t,eai" from the optical source, has an input, a first output, and a second output, and divides the optical carriem~c~iv0d at the input between the first output and the second output. The acousto-optic modl ~-'or has a carrier input optically coupled 30 to the first output of the splitter, a modu'~,on input ~el,ectrically coupled to the RF
modl ~-'ion source, and a mod~ ~atcr output. The acoi ~sto-optic modu'-'or includes circuitry for oi ll ,ogonally converting pola, i alion of the optical carrier and frequency shifting the optical carrier by the frequency of the modu'ation signal. The polari~dlion beam . .
CA 022~70~8 1998-12-30 combiner has a first input optically coupled to receive the orthogonally polari~dlion converted and frequency-shifted optical signal, a second input optically coupled to the attenuator, and an output optically coupled to the rare-earth-doped fiber amplifier dow"~l,ear" in the optical communication system.
In anotl,er aspect, the pr~sent invention includes a method of su~pressing polari~alion hole buming in a rare-earth-doped fiber amplifier within an opticalcommunication system including the steps of splitting an optical carrier signal into a first sub-carrier signal and a second sub-carrier signal, and rotali"9 orthogonally the polari~alion of the first sub-carrier signal and modulating the first sub~arrier signal with a 10 RF modu~tion frequency to create an o, ll ,ogonal-modulated sub-carrier signal. The method further includes the steps of combining the orthogonal-modulated sub-carrier signal and the second sub-carrier signal to produce a pola~ i~dlion-rotating carrier signal, and passi"g the polarization-,utaling carrier signal downstream in the optical communication system to the rare-earth-doped fiber amplifier.
- In a further aspect, the present invention includes an acousto-optic mod~ or for rotating the polari~alion of an optical carrier signal, col"prisi"g: a substrate of a birefringent and photo-elastic material; a first port on the substrate for receiving the optical carrier signal from an optical waveguide; a splitter having an input coupled to the first port, a first output, and a second output; a first optical waveguide branch coupled at one end to 20 the first output of the splitter; a second optical waveguide branch coupled at one end to the second output of the splitter; an acoustic waveguide on the substrate including at least a portion of the first optical w~veguide branch; an acoustic wave generdtor positioned on the substrate over at least a portion of the ~coustic waveguide; and a polal i~alion splitter having a first input coupled to another end of the first optical waveguide branch, a second 25 input coupled to another end of the second waveguide branch, and an output.
It is to be ~" ,der~lood that both the fort:gc ,9 general description and the following detailed desc~i~Jtion are ex ~mplaly and ex~,lanatory only and are not re~l,i.;ti~Je of the invention as claimed. The f~llo.~,;"g descri~ tion, as well as the practioe of the invention, set forth and suggest additional advantages and purposes of this invention.
~ !
BRIEF DESCRIPTION OF T~E DRAWINGS
The acco"~panying d~ ;. ,gs, which are incorporated in and constitute a part of this spe-,ifi~lion, illustrate embodiments of the invention, and together with the des." i~Jtion, CA 022~70~8 1998-12-30 explain the advantages and principles of the invention.
Fig. 1 is a graph illu~lldling a ~elationship bet~r,ecn PHB and gain compression for a double stage erbium-doped fiber amplifier;
Fig. 2 is a graph illu~tldtillg a rel.~lionship between PHB and the degree of 5 pola,i~dlion of an optical infor",dtion signal for a double stage EDFA with Cp=10 dB;
Fig. 3 is a block diayldrll showing an optical communication system using a polari~dlion modulator according to one embodiment of the present invention;
Fig. 4 is a top view of an embodiment of a pola~ i~dlion mod~ tor for use in theoptical communication system d~p-. ~ed in Fig. 3;
Fig. 5 is a block ,liagrai" of an experimental setup for the optical commu"icalion system clep cted in Fig. 3; and Fig. 6 is a graph showing experimental results using the test setup of Fig. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made to various embodiments according to this invention examples of which are shown in the accompanying drawings and will be obvious from the description of the invention. In the drawings the same rerer~:nce numbers represent the same or similar elements in the different drawings whenever possible.
As generally referenced at 300 in Fig. 3 an optical communication system consi~lenl with the present invention includes a polari~dlion modu~-tor that reduces polari~dlion hole burning in a rare-earth-doped fiber amplifier. The optical communication system shown gener~lly at 300 cG",p,ises an optical source (OS) 305 for l,anslY,itli"g an optical carrier, a polari~ation-fixing device (PC) 310 and a pola,i~dlion modu~?tor 312.
Polali~dtiol) mod~~tor 312 includes a splitter 315, an RF modu~~fion source 325 for providing a modu'~fion signal, an ~cousto-optic modu'~'or 320, an attenuator 330, and a pGlari~dtioo beam combiner 335.
As ~fert:nced at 305 in Fig. 3 the optical source for transmitting an optical carrier CGIllpl ises a laser diode or similar cor"ponent for producing an optical signal having a relatively fixed v:avelengtl,. Optical source 305 generales the relatively fixed v:avelengtl, as a carrier signal that may be modulat~ by vario~us, techn-.q_es within the optical communication system 300 as des~ iL,ed in more détail below. For exan~le, optical source 305 is an AT&T DFB se",iconductor laser having Model No. 246AH and operating at a nominal wavelengll, in vacuum of 1556.7 nm, having a line bandwidth of less than 100 CA 022~70~8 1998-12-30 MHz.
Downstream from optical souroe 305, optical communication system 300 may include an electro-optic modu'?tor (EOM) 304 for mod~ ting an information signal onto the carrier signal produoed by optical souroe 305. As is readily known to one of ordinary 5 skill in the art, electro-optic or data modu~ator 304 may be a Mach-Zehnder i, lt~, rerometer or equivalent device for providing an amplitude modu'?tion on the optical carrier according to an electromagnetic signal introduoed by source (RF) 345. The electromagnetic signal may be, for example, an RF signal containing data to be transmitted across optical communication system 300. The use of data modulator 304 is optional for the practioe of 10 the pr~serlt invention but provides the feature of inserting infcrmation onto the carrier signal. As an alter"alive to data modu'~tor 304, optical souroe 305 can be directly modulated. Multiple souroes 305 at difrar~ril emission wavelengths or a multiplewavelength source may be used in case of wavelength-division-multiplexing (WDM) l, ans,nission.
Downstream from optical source 305, and possibly also data mod~ tor 304, polari~ation-fixing device 310 is optically coupled to l,ansr~r"~ the optical carrier from optical source 305 into an optical carrier having a fixed SOP corresponding to a preferred input SOP of polari~dtion modulator 312. Naturally, if data modulator 340 is used within the optical communication system 300 descril.ed herein, polarization-fixing device 310 will 20 convert the optical carrier that has been modulated with data by the data modu'~tor 340 into a constant ~iOP optical carrier. Polarization-fixing devioe 310 is preferably a pola, i~dtion conl,~'ler that cG",prises a series of loops of an optical fiber that have an angular adjustment to provide a sele-oled and fixed polali~dlion for a signal output from the polali~dlion cont,."er. This type of pola,i~dlion conl~l'er, which is readily known in the 25 field, may be obtained in the marketrl~ce or manufactured as desired by one of ordinary skill in the art. Altemative devioes for the polali~alion-fixing device 310 include a polal i~dlion-maintaining fiber, a polari~dlion-maintaining splitter, or a polari~dlion stabilizer. Other structures not explicitly listed may altematively be chosen for pGlal i~dlion-fixing devioe 310 such that the output of devioe 310 provides an optical signal having a 30 fixed pGlali~alion. - ~' The optical communication system 300 for re~ducing pola, i~dlion hole buming further includes a polali~dlion modulator shown generally as 312 in Fig. 3. Polari,dlion CA 022~70~8 1998-12-30 modulator 312 includes splitter 315 positioned dc~ eam from the polari~dlion-fixing device 310. Splitter 315 has an input 317, a first output 318, and a second output 319, for example into first and second sub-carrier signals. r,eferdbly, splitter 315 is a 3 dB coupler of the fused fiber variety that divides the pola, i~ed optical carrier received at input 317 from polari~dlion-fixing device 310 tetwecn the outputs of 318 and 319.
In addition, the polari~dtion modu~tor 312 consisle"l with the present inventionfurther includes an ~cousto-optic modu~or (AOM) 320 positioned downstream from splitter 315. Acousto-optic mod~ator 320 has a carrier input 321 optically coupled to the first output 318 of splitter 315. In this way, a portion of the polali~ed optical carrier passed 10 by polarization-fixing device 310 is received by acousto-optic mod~ ~ator 320 via input port 321. Acousto-optic modubtor 320 also includes a mod~ tion input 322, a mod~ tor output 323, and additional output 324. ModlJ'?tion input 322 is optically coupled to a modulation source (RF) 325 that provides a relatively fixed elec1,ul,,dy, ,elic frequency to acousto-optic modu~ator 320. AOM 320 is prererdbly a waveguide device made on a 15 LiNbO3 substrate, e.g., as desu,ibed in a paper by S. Schmid et al., Post Deadline Paper ThP1, pp. 21-24, Proceedings of the 7th European Conrar~nce on Integrated Optics, Delft, The Netherlands, April 3-6, 1995. For a waveguide AOM made on a LiNbO3 substrate, the frequency v of the RF signal is, for example, about 172.6 MHz for an optical signal at a wavelength ~ = 1556.7 mm. The change in RF frequency ~v required to tune the AOM20 after a change ~ of optical signal wavelengll, (tuning slope) is in the above example ~v/~ 120 kHz/nm. If a plurality of optical signals at different wavol~ngll,s are input to AOM 320, modul~tion source 325 will advant~geously provide a cor,esponding number of modulation signals, each tuned to one opticdl signal.
As explained more fully below, acousto-optic mod~ tor 320 modulates the 25 pGla, i~ed carrier by the mod~ ~'?tion signal received at the modulation input 322, thereby orthogonally converting the pola, i~dtion of the pola, i~ed carrier. That is, ~cousto-optic modu~tor 320 will provide a TE->TM or TM-~TE conversion of the received pGla- i~ed carrier signal. If polali~dlion-fixing device 310 sets the polali~dlion of the carrier signal at the TE (transverse electric) mode, acousto-optic modu~-~or 320 will ol ll ,ogonally rotate the 30 TE mode to the TM (transverse ",ag"eti-.~ mode, or~ vfce versa. Also, ~coust~optic modulator will shift the optical frequency of the pola,if~ed carrier signal at the frequency of the RF modulating signal.
Coupled to the second output 319 of splitter 315 is an attenuator 330. Attenuator CA 022~70~8 l998-l2-30 330 may comprise an adjustable attenuator or a fixed attenuator depending on thepreferred design implementdlion. Attenuator 330 serves to adjust the magnitude of the portion of the polarized optical carrier receivod from the second output 319 of splitter 315 so that this second portion has a magnitude subslanlially equal to the magnitude of the orthogonal-modulated signal exiting from ~cousto-optic mod~ ~?tor 320 via output 323. As a result polari~dlion beam combiner (PBC) 335 of Fig. 3 receives an o~ lhogonally-shifted and modu~'ed polarized signal from acousto-optic modu'~tor 320 and a pottion of the original polarized optical carrier from attenuator 330 where the two received signals by polari~dlion beam combiner 335 have substa"lially the same magnUude. As mentioned 10 attenuator 330 may be used to equalize the magnitudes of the two signals received by pola, i~dlion beam combiner 335. Altematively, splitter 315 may be an u"balanced splitter or coupler specifically designed with a ratio between the first output 318 and the second output 319 so that the two signals eventually received by polarization beam combiner 335 have substantially the same magnitudes.
As mentioned polarization beam combiner 335 is positioned dow"~l,ea", from both the acousto-optic modulator 320 and the optional attenuator 330. rolari~dlion beam combiner 335 has a first input 336 optically coupled to receive the orthogonally-shifted and modulated polal i~ed signal from output 323 of acousto-optic modulator 320. As well polarization beam combiner 335 has a second input 337 optically coupled to receive a portion of the polarized optical carrier from splitter 315 which may be passed via attenuator 330. In a known fashion polarization beam Gombiner 335 will combine the orthogonally-polarization converted and frequency-shifted polari~ad signal received from ~cousto-optic modu'~tsr 320 with the portion of the original polarized optical signal received from splitter 315 to produce a polal i~dlion-rotating carrier signal. This pola, i~dlion-totali"g carrier signal will have sul,sld-,lially the same wavelerigtl, as the original carrier signal generdled by optical source 305, but will have a state of polari~dlion that will vary at a rate proportional to the modulation frequency generdled by mod~ fion source 325. In the preferred embodiment, this modulation frequency is about 172.6 MHz.
As a result, the overall polai i~dtion modulator 312 of the present invention, as defined by splitter 315, ~cousto-optic modul ~tcr 32~; attenuat~os- 330 and pola"~dtion beam comb.. ,er 335 changes the state of pola, i~dtion of the origina~ carrier signal at a very high rate. This rate of change of the state of pola- i~dtiGn eYceeds the respol1se time of an erbium-doped fiber amplifier, which is defined by 1/t, where t, is the ar,isot~p-~ saturation time.
CA 022~70~8 1998-12-30 - Typically, t" 2 0.5 ~s for erbium-doped fiber amplifiers.
rolari~dlion beam co,nb-.. ,er 335 is, for exar"~'e, Model PB100-1 L-1S-FP by JDS-FITEL. Polari~dlion beam combiner 335 also has an output 338 optically coupled to at least one rare-earth-doped fiber amplifier 340 positioned dow"a~,edm in the optical 5 communication system 300. The rare-earth-doped fiber amplifier is preferably an erbium-doped fiber amplifier. Single-stage, two-stage or multiple-stage amplifiers can be used. It is possible to use a plurality of amplifiers separdled from each other by links of long distance t,dnsmission fiber (not shown). In a test setup, a pola,i~dtion filter (Glen-Thomson prism) was positioned doJ~ L ,~l~ea", from polari~dlion beam combiner 335 for 1G detecting rotation of the signal polari~dlion. A polari~dlion filter, hoNever, is normally not co,np,iaed in an apparat.Js for reducing pola,i~dtion hole buming as herein described.
As in conventional optical communication systems such as 300, a receiver system 350 is located at the end of the communication system 300 to receive and detect i"ror,.,dlion l,dnsr,litted along the optical path. Receiver 350 may include demultiplexing 15 circuitry for a wavelength division multiplexer ar p' ~ation and may serve to detect and demodulate the optical carrier signal containing data modu~ted by data modulator 304 upstream in the optical communication system 300.
Fig. 4 illustrates a prefer,ed embodiment for pGlal i~dlion modu~tor 312.
Integrated acousto-optical devices, such as that shown as 312 in Fig. 4, are known whose 20 operation is based on the interactions b,etv:een light signals, prop~g~li. ,g in waveguides obtained on a substrate of a b..~r,ingenl and photo-elastic material, and acoustic waves propag~ ,g at the surface of the substrate, generdted through suitable transducers. The interaction between a polal i~ed optical signal and an acoustic wave produces a pola, i~dlion conversion of the signal, that is, a rotation of the polal i~dlion of the optical 25 signal's TE and TM components.
rolari~dtion mod~ 'or 312 in Fig. 4 generally co,np,ises a substrate 410,- an optical coupler 315 formed with optical waveguides within substrate 410, an acoustic waveguide 420 on substrate 410, an electro-acoustic transducer 430, first optical waveguide branch 440, second optical waveguide branch 450, ~coustic cladding 460, and 30 polal i~dlion beam combiner 335.
The substrate 410 p~erdbly is a crystal of li~hium niobate (LiNbO3) cut perpendicularly to the x-axis with optical waveguide brdnches 440 and 450 oriented along the crystal's y-axis. Altematively, another b.rer,i.)gènt, photo ela~.lic and piezoelect,ic ... . ~ . . ...
CA 022~70~8 1998-12-30 material may be used such as LiTaO3 TeO2 or CaMoO4.
Coupler 315 is formed of an optical waveguide within substrate 410 and having aninput 317 ccp~!e of being connected to an optical fiber (not shown) from upstream components in the optical communication system 300 such as pola,i~dlion-fixing device 5 310. The output polaii~alion of pola,i~dlion-fixing device 310 is preferably selected so as to match the TE or TM prop~g~tion mode of optical wavegu ~es 440 450 of polarization modu~?tor 312. Coupler 315 splits its optical path into first optical branch 440 at a first output 318 and a second optical branch 450 at a second output 319. Coupler 315 is subsldntially pola, i~dlion independen~.
~ 10 First optical branch 440 passes through ~coustic waveguide 420 to form an ~cousto-optic converter. The second optical waveguide branch 450 bypasses the acousto-optic converter and rejoins with the first optical waveguide branch 440 within polarization beam co".~i.,er 335.
Electro-acoustic transducer 430 is placed in acoustic waveguide 420 15 communicating with the first optical waveguide branch 440 so as to form the acoustic converter. The electro-acoustic transducer 430 is formed by interdigital electrodes cap~ e of generdli"g a radio-frequency (RF) surface acoustic wave. Optical signals received at input port 317 of coupler 315 and propag~ ,g along the first optical waveguide branch 440 interact with an acoustic wave prop~g~ting through acoustic waveguide 420.
20 The acoustic wave within ~coustic waveguide 420 is made so that the intensity profile of the surface acoustic wave has a peak in the central portion of the acoustic waveguide 420 and two troughs at the ends of the same waveguide. Optical signals propagating along the first optical waveguide branch 440 interact with the acoustic wave having an intensity increasi"g up to halfway along the path and de~asi"g in the other half in an area having 25 a preselec~ed i"terd~lion length. The acousticwaveguide 420 is circu",sc,ibed by ~coustic cladding 460 ui,erei., the speed of the ~coustic waves generaled by electro-~coustic transducer 430 is higher than in the acoustic waveguide 420.
The polari~dtion beam combiner 335 is pr~ferdbly formed by an evanescent wave pola, i~ttion splitter or dir~;tional coupler comprising a central optical waveguide with pairs of input wavegu;des 336 and 337. The operd~l7n of pola,i~dtion splitter/combiner 335 is desc,iLed in EP 737 880 at col.12-14 which~document is incor~ordted by r~fer~,-ce. Outputs 338 and 339 provide orthogonally-separdted signals for exiting the modulator 312 for conne,;tion to an output optical fiber for l~ansr"ission downstream in the CA 022~70~8 l998-l2-30 - optical communication system 300.
The operdtion of polari~dlion modu~ator 312 of Fig.4 according to the present invention is as follows. When an app,upriale modu~tion signal, e.g., an RF signal of 172.6 MHz, is applied from modlJl~fion source 325 to electro-acoustic transducer 430, 5 transducer 430 generates a respective RF surface acoustic wave having a driving acoustic frequency conesponding to the optical resonance wavelenyll" e.g., ~ = 1556.7 nm. At this fesonance wavelength, the polarization conversion TE-~TM or TM-~TE takes place. Optical signals enter polarization modulator 312 from polari~dlion-fixing device 310 with a fixed pola, i~dlion of either TE or TM. As the received optical signals prop~gate 10 through the first optical waveguide branch 440, they are l,ansrur,,,ed into their cor,-asponding orthogonal con,ponerils. That is, if the ll:ceived signals have a polari~dlion of TE, their pola,i~dlion is rotated to its orthogonal component TM, or vice versa. Also, they receive freguency shifts with an ~bs~lute value equal to the RF signal frequency.
~olari~dtion splitter, or combiner, 335 combines the modulated optical signal from 15 first optical waveguide branch 440 with the unmodulated optical signal from second optical waveguide branch 450. Outputs 338 and 339 provide orthogonally-separated signals.
Combining the polarkdtion modulated and frequency shifted optical signal with the unmodulated signal results in the optical signal at output 338 with a rotating state of polarization. As described, this rotating state of polarization will occur at a rate dictated in part by RF source 325 and will prererdbly take place at a rate in excess of 1/t5, where tS is the anisotropic saturation time of a fiber amplifier, such as amplifier 340 shown in Fig. 3 downstream from pola,i~dlion modulator 312. Consequently, polari~dtion modulator 312 illustrated in Fig. 3 provides a concise and erficie, It implementation of an acousto-optic mod~ ~~tor for generdting a pGlari~ation-rotating optical signal that helps to suppress pola,i~dliGn hole buming in an EDFA.
The f~ i, .9 describes the experimental tests and results for the present invention as previously set forth. Fig. 5 illustrates a test setup generally refer~:nced at 600 for ex~,e,i,nentally analyzing the optical communication system and transmitter of Fig. 3. As shown in Fig. 5, an optical source 610 in the fomm of a laser diode (AT&T Model 246AH) operatil,g at a nominal v avelength of 15g6.7 nm ir~ vacuum provided an optical carrier signal to a pold,i~ation co, lt, ~'ler for 620. roldl icdtion conl~l'er 620 was a series of optical fiber loops arranged to have an angular fl;,p'~cel"ent for controlling the polal i~alion of the optical carrier r ecei\rcd from laser source 610. An output from the CA 022~70~8 1998-12-30 pola,i~alion co,lt,."er 620 was optically coupled to a pola,i~dlion modulator 630, which comprised the components of pola, i~dlion modulator 312.
At the output of the polari~dlion mod~ator 630, the test setup dep, ~ed generally as 600 in Fig. 5 included a coupler 640 that split the output signal from pola,i~dlion modulator 630 in a 90:10 ratio. Ten percent (10%) of the polali~dlion modulator output was split by coupler 640 and fed through pola, i~ing filter 6$0 and photodiode 660 so that it could be detected and analyzed within oscilloscope 670. Coupler 640 directed 90% of the output from polarization mod~ ~ator 630 to an attenuator 675. Another coupler 680 having a splitting ratio of 90:10 was positioned dol~. ,sl~eam from attenuator 675 and split 10% of the signal from attenuator 675 to a power meter 685 for detection. Coupler 680 divided 90% of the output signal from attenuator 675 and passed it to an erbium-doped fiber amplifier 690. Amplifier 690 was a double stage EDFA pumped with 1480 nm laser diodes. For an input power of -15dBm, the amplifier had an output power of 9 dBm, a gain G = 24 dB, Gc= 12 dB, N,= 6.5 dB, ~ = 0.92, and P~, = 0.94 IlW. After amplifying the received polarization-rotating carrier signal from coupler 680, EDFA 690 passed the signal through polarization controller 697 to an optical signal analyzer 698 for detection and - analysis. Polarization conl, ~"er 697 was used to set the polarization of the amplified signal received from EDFA 690 to a state accept-~'e by the optical signal analyzer 698.
Fig. 6 shows the 9, dph-c ~' test results obtained by optical signal analyzer 698 under various test conditions and referenced generally as 700. Signal trace 710 depicts the spectra leceived at the output of EDFA 690 when no modulation or rotation of the signal pola, i~dlion was imp!e.,lenled. In other words, signal 710 in Fig. 6 illustrates the output from EDFA 690 when the input signal to that amplifier had a degree of polal i~dlion equal to 100%. Signal trace 720 depicts the output from EDFA 690 when the degree of pola, i~dlion of the input signal was 36%. Signal trace 730 shows the output signal from EDFA 690 with an input signal that had a degree of pola~ i~dlion of 6%, as received dol.~":il,ean, from pola,i~dlion modulator 630.
As shown in Fig. 6, the amplified spontaneous emission (ASE) noise was the highest in signal 710 with a degree of pola,i~dtion of 100%, and least in signal trace 730 with an input signal having a degree of-tola,i~dlior~! e,qual to 6%. C~r,lparing the two results in signal traces 710 and 730, it can be seen4hat the ASE noise drops by about 0.24 dB when the polali~dtion modulator 630 is used.~ This~drop in ASE noise cor,esponds approxi,ndlely to the quantity of gain ~aridlion caused by pola,i~dlion hole . .
CA 022~70~8 1998-12-30 burning present in an EDFA with a polari~ed saturating signal. In particular, Fig. 2 shows that for 10 dB of gain co",pression, the pGlari~dlion hole buming in de~ els corresponds to about 0.2 dB for a degree of polari~dlion equal to 100%. Thus, polari~alion modulator 630 provides a siyl ,i~icant decrease in signal fading caused by pola~ i~dlion hole burning Applicants have also determined that the above~escril,ed invention is effective in reducing pola,i~dlion hole buming in wavelenglh-division-multiplexing (WDM) optical transmission systems. As is readily known to one of ordinary skill in the art, in a WDM
system, a plurality of optical sources generdtes carrier frequencies for the channels in the l,ansl"ission system. One or more of the channels is modu~-ted with information, and the 10 channels are multiplexed and then transmitted down a common optical fiber. Repeaters or optical amplifiers along the l,ansmission path may boost the channel levels for passage across a long distance. At a receiver end, a demultiplexer separates the channels to respe.ti~/e paths, and a receiver obtains the mod~ ~ated i~ If o",~alion from a particular channel. For such a WDM system, polai i~dtion hole buming can be reduced by using a 15 single polari~dlion modulator as descril,ed above that is positioned downstream from the multiplexer. In this fashion, the polari~dlion of all the channels in the WDM system can be rotated. Altematively, a plurality of pola, i~dtion-rotating modulators can be used prior to multiplexing all the channels so that a group of adjacenl or interleaved channels may have their polarization individually rotated. Furthermore, ApF'i~anls believe that polarization 20 hole burning can be reduced in a WDM system by rotating the polarization of less than all of the channels in the WDM system, and even only one cl,annel in the WDM system.It will be appareril to those skilled in the art that various modifications and vai idlions can be made to the system and method of the present invention without depa, li"g from the spirit or scope of the invention. The presenl invention covers the 25 modifications and varidliGns of this invention provided they come within the scope of the appended claims and their equivalents.
BACKGROUND OF THE INVENTION
The present invention relates generally to methods and systems for su~.pressi, lg pola,i~dlion hole burning in rare-earth doped fiber amplifiers. More particularly, the present invention relates to methods and systems for su,uplessing pola,i~dlion hole burning using acousto-optic modulation to vary a state of polari~dlion of an input signal.
Long dislance optical communication systems have been known to suffer from various polarization dependent effects that may cause a signal-to-noise ratio of the system to lessen. Polarization hole burning (PHB) is one of the polari~alion dependent phenomena that can severely impair the pel ru",lance of erbium-doped fiber amplifiers (EDFAs) located in optical fiber communication systems. PHB occurs when a strong, polarized optical signal is launched into an EDFA and causes anisotropic saturation of the amplifier. This effect, which is related to the pop~ tion inversion dynamics of the EDFA, depresses the gain of the EDFA for light with the same polari~dlion as the saturating signal. Thus, PHB causes a signal having a state of polari~dlion (SOP) orthogonal to the saturating signal to have a gain greater than that of the saturating signal.
In a chain of saturated EDFAs, amplified spontaneous el"ission (ASE) noise can accumulate faster in the polarization o, II,ogonal to a saturating il ~rur~lalion signal than along the polarization parallel to the signal. ASE ol lhogonal to a saturating signal will accumulate at each amplifier stage of the l.anslnission line. The build-up of orthogonal ~-ASE reduces the signal-to-noise ratio (SNR) of the optical l,ansr"ission system, thus causing possible errors in the received data stream. Accordingly, it is des' ~le to reduce the effects of PHB in amplified systems in order to maintain a system with good SNR
chara~;tel i~lics.
O~.erdli"g EDFAs in gain co",pressiol1 helps to cause the u"desired PHB effect.
The degree of gain compression Cp indicates the difrerence of gain of the amplifier in its operative condition of prop~gation of a slgnal with 1~Y~ optical power (i.e., a non-saturating signal ex,ue, ienc;ng maximum gain, called "Go") with' respect to the value experienced by the optical signal in the power level condi;ion at which-it is operdting (G). An amplifier's operaling gain in decibels can be measured with a saturating signal of input power Si as CA 022~70~8 1998-12-30 - the following:
G=So-Si (1) where So is the saturated output power. Accordingly, the amount of gain compression equals the following:
Cp = Go - G. (2) The gain in the orthogonal polari~dlion on the other hand can be measured using a probe signal with an input polarization orthogonal to the saturating signal as the following:
Po-Pi=G+~G (3) Pi and Po being the input and output power of the probe signal. In equation (3) ~G
10 corresponds to the PHB value.
Moreover the amount of PHB increases as the amplifier goes deeper into gain co",pression. Figure 1 is a graph of experimental measurements showing the relalionshi~.
between the amount of gain compression and the amount of PHB in an EDFA. As shown in this graph the amount of PHB is only about 0.08 dB for a single EDFA that operates 15 with 3 dB of gain compression. However, as the gain co",pr~ssion increases so does the PHB. When the EDFA operates in a saturated condition with Cp equal to about 9-10 dB
the PHB is more sig"ificdnt and quantiri~le at around 0.2 dB per EDFA.
Furthermore the amount of PHB in an EDFA depends on the degree of polarization (DOP) of the saturating signal passing through the amplifier. Figure 2 is a 20 graph of experimental results on an EDFA operating at 10 dB of gain compression. As can be seen from this graph of Figure 2 as the degree of polarization of the saturating signal di~"i.,ishes from 100% the vaiidlion of gain induced by PHB also diminishes. This fact illustrates that the delete, ious effects from PHB may be lessened by varying the state of polaii,dlion. PHB can be reduced by scrambling the SOP of the l,ans",itled optical 25 signal at a rate that is much higher than 1/t~, where t, is the ani.,ol,~p-c saturation time.
P~ec~use an EDFA takes about 0.5 msec to reach a gain stable con.lition after variation of a signal's SOP the signal's SOP should be scrambled at about 10 kHz or more in order to overcome the PHB phenomenon.
The literature has proposed several a" dnge-nents for mitigating PHB effects in 30 optical communication systems. EP 615,356 and ~ ;. Patent No. 5,491 576 d; vlose a techr,. ue for reducing nonlinear signal dey, dddlion by simultaneously launching two optical signals of different v:_~/elens~tl,s cor"pardble power levels, and suL,-~;ldntially olll,ogonal relative polar,~dtions into the same l,dnsr".ssion path. The resulting overall CA 022~70~8 1998-12-30 - transmitted signal is therefore essentially u"polari~ed, and the impact of detrimental pola, i~dlion dependent effects within the l,ans",ission system are r~po~ ledly minimized.
The combined signal is modu'ated by a polari~dlion independent optical modulator so that both wavelengtl, COI"ponenls of the combined signal carry the same data, or each5 wavelength path is separ~tely modulated prior to their combination. Similar disclosure of a system that launches two signals of dirrerenl wavelengths can be found in Bergano et al., "Polari~dlion Hole-Buming in Erbium-Doped Fiber-Amplifier T,dnsl"ission Systems,"
ECOC '94, pp. 621-628.
U.S. Patent No. 5,107,358 describes a method and apparatus for lldnsr"itling 10 i"ror",alion and dete~ti.,g it after propa3alion through a waveguide by means of a coherent optical detector. In particular, Fig. 3 shows a transmitter comprising an optical source generating a single carrier signal which is fed to a modulator. An optical splitter generates two ver~ions of the modulated signal. The first version is fed to a first polarization cor,l,."er, while the second version is fed via a frequency shifting circuit to a 15 second polari~dlion conl-~l'er. The pola-i~alion of this signal is adjusted by the second conl"JI' er to be orthogonal to the polari~alion of the signal from the first controller. The orthogonally pola,i~ed signals are then co",b-..,ed by a polari~dlion selective coupler for transmission.
It should be understood that in all the exarr,r'es described in the '358 patent, the 20 two optical carrier frequencies will typically be separated by two to three times the bit rate in Herk. Applicants have observed that by superposing an optical signal with a version of the same having orthogonal pola,i~dlion and being shifted in frequency by two to three times the bit rate, an optical signal with a bandY.idll, of the same magnitude (two to three times the bit rate) is obtained. The bandwidth of the filters to be used at the receiver must 25 be equal to or greater than the signal bandwidth. Due to this large filter bandwidth, the noise at the receivcr, in the case of a long ~i~.t-ance amplified optical telecommu".~alion system, would be too high to allow a good signal reception, particularly for a bit rate greater than 1 GbiVs.
It is also known from, for example, U.S. Patent No. 5,327,511 and l lei~,n,ann et al., 30 ~EIectro-optic ~ola,i~dlion scramblers for optically ~ar~plified long-haul l,dnsl"ission systems,~ ECOC '94, pp. 629-632, to generate a cafrrier signal having a single wavelength, modul7tç the carrier signal with data, and then send the mod~ ted carrier signal through a pola,i~dtion mod~ator or scr~m~'er to help ~ "~viale the effects of pola,i~dlion hole .
CA 022~70~8 1998-12-30 - buming. These documents ~ close the use of a lithium niobate-based electro-optic modl ~tor with a single path for passi"g the carrier wavelength and modulating its polari~-dlion at, for examr'e, modu'~tion frequencies of 40 kHz and 10.66 GHz. These polarization modulators or s~, dn,b'er~ create highly ~ando~ ed polari~dlion states for the 5 signal. Such devices affect the output polarization accord;"g to a control signal and use relatively high levels of power.
From Elect,on-.~s Letters, Vol. 30, No.18, p.1500-1501, September 1,1994 an acousto optical Ti:LiNbO3 device is known whose transducer is placed at 1/3 of the interaction length, which forms a polarization-i~,dependent optical depolarizer consisling of 10 two or more sections of a wavelength tunable TE-TM converter, s~ 'e to suppress pola, i~dlion hole-buming in EDFAs. The authors present a double stage depolarizer with a < 0.03 residual degree of polari~dLion.
As well, acousto-optical waveguide devices are known that provide a polarizationrotation to an input optical signal and modulate the signal with an acoustic wave from a modulation source. Relevant publications include, for example, EP 737,880, EP 757,276 and M. Rehage et al., "Wavelength-Selective rolarisdlion Analyser with Integrated Ti:LiNbO3 Acousto-Optical TE-TM Converter," Electronics Letters, vol. 30, no.14, July 7, 1994.
Applicants have found that the known techniques for minimizing pola, i~alion hole burning using electro-optic modulators to rotate the polari~dlion of a carrier signal require undesirably high levels of power. As well, Applicants have discovered that the known techr, ques for providing a pola,i~dlion-rotating signal for an erbium~oped fiber amplifier require a much wider band width than is pnd~ticdlly acceptable for a receiver in an optical t,ansr"ission system. F~"ll,er",or~, systems employing two sources at dirrerenl wavelen~tl,s are difficult to implement, due to the p~b!ems in selecting the sources and in stabilizing their wavelengtl ,s. WDM l, dnsr"ission by this system would be verycomplicated and expensive.
SUMMARY OF THE INVENTION
In a~~ordance with the present invention, all optical t, ansr"ission system has been developed to help reduce pola,i~dtion hole buming in a rare-earth-doped fiber amplifier by converting an optical carrier signal having a chdldctt:lialic wavelengtl, into a pola,i~dlion-rotdli"g optical carrier. The system el"~'~ys an ~cousto-optic modul~tor that modul~tes a CA 022~70~8 1998-12-30 - portion of the optical carrier. The acousto-optic mod~ ~?tor causes an orthogonal rotation of the pola, i~dlion of the portion of the optical carrier. A polari~dlion beam combiner then combines the mod~ 'ed and orthogonal signal from the acousto-optic modu~tor with the remainder of the original optical carrier signal to produce a polari~dlion-,utdLi,)g optical 5 carrier. The pola, i~dlion-rotating optical carrier is i"se~led into the optical communication system for eventual use within a rare-earth-doped fiber amplifier.
To obtain the adva"lages and in accor~lance with the purpose of the invention, as embodied and broadly described herein, an appardtus for reducing polari~dlion hole buming in a rare-earth-doped fiber amplifier within an optical communication system by 10 converting an optical carrier having a ~;hardcte,i~lic wavelength and an initial state of polarization into a pola,i~dlion-rotating optical carrier, includes an acousto-optic modulator and a pola,i~alion beam co",b-..,er. The ~cousto-optic modulator has a carrier input optically coupled to receive a first portion of the polarized optical carrier, a modulation input electrically coupled to receive an RF modul~tion frequency, and a modulator output.
15 The acousto-optic modulator includes circuitry for orthogonally converting pola, i~alion of the polarized optical carrier and shifting the polarized optical carrier frequency by the modulation frequency. The pola, i~dlion beam combiner has a first input optically coupled to receive the orthogonally SOP (State of Polari~dlion) -converted and frequency-shifted polarized signal, a second input optically coupled to receive a second portion of the 20 polal i~ed optical carrier, and an output optically coupled to the rare-earth-doped fiber amplifier downstream in the optical communication system.
In anotl,er aspect, the invention includes an optical l,dnsruill0r for reducing polari~dtion hole buming in a rare-earth-doped fiber amplifier within an opticalcommunication system having an optical source for l,dnsn,itling an optical carrier having 25 an initial state of polali~dlion, a splitter, a modu~ation source for providing a modu'-'ion signal, an acousto-optic modul~tor, an attenuator, and a polaii~lion beam col"bil,er. The splitter is positioned do~h"~t,eai" from the optical source, has an input, a first output, and a second output, and divides the optical carriem~c~iv0d at the input between the first output and the second output. The acousto-optic modl ~-'or has a carrier input optically coupled 30 to the first output of the splitter, a modu'~,on input ~el,ectrically coupled to the RF
modl ~-'ion source, and a mod~ ~atcr output. The acoi ~sto-optic modu'-'or includes circuitry for oi ll ,ogonally converting pola, i alion of the optical carrier and frequency shifting the optical carrier by the frequency of the modu'ation signal. The polari~dlion beam . .
CA 022~70~8 1998-12-30 combiner has a first input optically coupled to receive the orthogonally polari~dlion converted and frequency-shifted optical signal, a second input optically coupled to the attenuator, and an output optically coupled to the rare-earth-doped fiber amplifier dow"~l,ear" in the optical communication system.
In anotl,er aspect, the pr~sent invention includes a method of su~pressing polari~alion hole buming in a rare-earth-doped fiber amplifier within an opticalcommunication system including the steps of splitting an optical carrier signal into a first sub-carrier signal and a second sub-carrier signal, and rotali"9 orthogonally the polari~alion of the first sub-carrier signal and modulating the first sub~arrier signal with a 10 RF modu~tion frequency to create an o, ll ,ogonal-modulated sub-carrier signal. The method further includes the steps of combining the orthogonal-modulated sub-carrier signal and the second sub-carrier signal to produce a pola~ i~dlion-rotating carrier signal, and passi"g the polarization-,utaling carrier signal downstream in the optical communication system to the rare-earth-doped fiber amplifier.
- In a further aspect, the present invention includes an acousto-optic mod~ or for rotating the polari~alion of an optical carrier signal, col"prisi"g: a substrate of a birefringent and photo-elastic material; a first port on the substrate for receiving the optical carrier signal from an optical waveguide; a splitter having an input coupled to the first port, a first output, and a second output; a first optical waveguide branch coupled at one end to 20 the first output of the splitter; a second optical waveguide branch coupled at one end to the second output of the splitter; an acoustic waveguide on the substrate including at least a portion of the first optical w~veguide branch; an acoustic wave generdtor positioned on the substrate over at least a portion of the ~coustic waveguide; and a polal i~alion splitter having a first input coupled to another end of the first optical waveguide branch, a second 25 input coupled to another end of the second waveguide branch, and an output.
It is to be ~" ,der~lood that both the fort:gc ,9 general description and the following detailed desc~i~Jtion are ex ~mplaly and ex~,lanatory only and are not re~l,i.;ti~Je of the invention as claimed. The f~llo.~,;"g descri~ tion, as well as the practioe of the invention, set forth and suggest additional advantages and purposes of this invention.
~ !
BRIEF DESCRIPTION OF T~E DRAWINGS
The acco"~panying d~ ;. ,gs, which are incorporated in and constitute a part of this spe-,ifi~lion, illustrate embodiments of the invention, and together with the des." i~Jtion, CA 022~70~8 1998-12-30 explain the advantages and principles of the invention.
Fig. 1 is a graph illu~lldling a ~elationship bet~r,ecn PHB and gain compression for a double stage erbium-doped fiber amplifier;
Fig. 2 is a graph illu~tldtillg a rel.~lionship between PHB and the degree of 5 pola,i~dlion of an optical infor",dtion signal for a double stage EDFA with Cp=10 dB;
Fig. 3 is a block diayldrll showing an optical communication system using a polari~dlion modulator according to one embodiment of the present invention;
Fig. 4 is a top view of an embodiment of a pola~ i~dlion mod~ tor for use in theoptical communication system d~p-. ~ed in Fig. 3;
Fig. 5 is a block ,liagrai" of an experimental setup for the optical commu"icalion system clep cted in Fig. 3; and Fig. 6 is a graph showing experimental results using the test setup of Fig. 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Reference will now be made to various embodiments according to this invention examples of which are shown in the accompanying drawings and will be obvious from the description of the invention. In the drawings the same rerer~:nce numbers represent the same or similar elements in the different drawings whenever possible.
As generally referenced at 300 in Fig. 3 an optical communication system consi~lenl with the present invention includes a polari~dlion modu~-tor that reduces polari~dlion hole burning in a rare-earth-doped fiber amplifier. The optical communication system shown gener~lly at 300 cG",p,ises an optical source (OS) 305 for l,anslY,itli"g an optical carrier, a polari~ation-fixing device (PC) 310 and a pola,i~dlion modu~?tor 312.
Polali~dtiol) mod~~tor 312 includes a splitter 315, an RF modu~~fion source 325 for providing a modu'~fion signal, an ~cousto-optic modu'~'or 320, an attenuator 330, and a pGlari~dtioo beam combiner 335.
As ~fert:nced at 305 in Fig. 3 the optical source for transmitting an optical carrier CGIllpl ises a laser diode or similar cor"ponent for producing an optical signal having a relatively fixed v:avelengtl,. Optical source 305 generales the relatively fixed v:avelengtl, as a carrier signal that may be modulat~ by vario~us, techn-.q_es within the optical communication system 300 as des~ iL,ed in more détail below. For exan~le, optical source 305 is an AT&T DFB se",iconductor laser having Model No. 246AH and operating at a nominal wavelengll, in vacuum of 1556.7 nm, having a line bandwidth of less than 100 CA 022~70~8 1998-12-30 MHz.
Downstream from optical souroe 305, optical communication system 300 may include an electro-optic modu'?tor (EOM) 304 for mod~ ting an information signal onto the carrier signal produoed by optical souroe 305. As is readily known to one of ordinary 5 skill in the art, electro-optic or data modu~ator 304 may be a Mach-Zehnder i, lt~, rerometer or equivalent device for providing an amplitude modu'?tion on the optical carrier according to an electromagnetic signal introduoed by source (RF) 345. The electromagnetic signal may be, for example, an RF signal containing data to be transmitted across optical communication system 300. The use of data modulator 304 is optional for the practioe of 10 the pr~serlt invention but provides the feature of inserting infcrmation onto the carrier signal. As an alter"alive to data modu'~tor 304, optical souroe 305 can be directly modulated. Multiple souroes 305 at difrar~ril emission wavelengths or a multiplewavelength source may be used in case of wavelength-division-multiplexing (WDM) l, ans,nission.
Downstream from optical source 305, and possibly also data mod~ tor 304, polari~ation-fixing device 310 is optically coupled to l,ansr~r"~ the optical carrier from optical source 305 into an optical carrier having a fixed SOP corresponding to a preferred input SOP of polari~dtion modulator 312. Naturally, if data modulator 340 is used within the optical communication system 300 descril.ed herein, polarization-fixing device 310 will 20 convert the optical carrier that has been modulated with data by the data modu'~tor 340 into a constant ~iOP optical carrier. Polarization-fixing devioe 310 is preferably a pola, i~dtion conl,~'ler that cG",prises a series of loops of an optical fiber that have an angular adjustment to provide a sele-oled and fixed polali~dlion for a signal output from the polali~dlion cont,."er. This type of pola,i~dlion conl~l'er, which is readily known in the 25 field, may be obtained in the marketrl~ce or manufactured as desired by one of ordinary skill in the art. Altemative devioes for the polali~alion-fixing device 310 include a polal i~dlion-maintaining fiber, a polari~dlion-maintaining splitter, or a polari~dlion stabilizer. Other structures not explicitly listed may altematively be chosen for pGlal i~dlion-fixing devioe 310 such that the output of devioe 310 provides an optical signal having a 30 fixed pGlali~alion. - ~' The optical communication system 300 for re~ducing pola, i~dlion hole buming further includes a polali~dlion modulator shown generally as 312 in Fig. 3. Polari,dlion CA 022~70~8 1998-12-30 modulator 312 includes splitter 315 positioned dc~ eam from the polari~dlion-fixing device 310. Splitter 315 has an input 317, a first output 318, and a second output 319, for example into first and second sub-carrier signals. r,eferdbly, splitter 315 is a 3 dB coupler of the fused fiber variety that divides the pola, i~ed optical carrier received at input 317 from polari~dlion-fixing device 310 tetwecn the outputs of 318 and 319.
In addition, the polari~dtion modu~tor 312 consisle"l with the present inventionfurther includes an ~cousto-optic modu~or (AOM) 320 positioned downstream from splitter 315. Acousto-optic mod~ator 320 has a carrier input 321 optically coupled to the first output 318 of splitter 315. In this way, a portion of the polali~ed optical carrier passed 10 by polarization-fixing device 310 is received by acousto-optic mod~ ~ator 320 via input port 321. Acousto-optic modubtor 320 also includes a mod~ tion input 322, a mod~ tor output 323, and additional output 324. ModlJ'?tion input 322 is optically coupled to a modulation source (RF) 325 that provides a relatively fixed elec1,ul,,dy, ,elic frequency to acousto-optic modu~ator 320. AOM 320 is prererdbly a waveguide device made on a 15 LiNbO3 substrate, e.g., as desu,ibed in a paper by S. Schmid et al., Post Deadline Paper ThP1, pp. 21-24, Proceedings of the 7th European Conrar~nce on Integrated Optics, Delft, The Netherlands, April 3-6, 1995. For a waveguide AOM made on a LiNbO3 substrate, the frequency v of the RF signal is, for example, about 172.6 MHz for an optical signal at a wavelength ~ = 1556.7 mm. The change in RF frequency ~v required to tune the AOM20 after a change ~ of optical signal wavelengll, (tuning slope) is in the above example ~v/~ 120 kHz/nm. If a plurality of optical signals at different wavol~ngll,s are input to AOM 320, modul~tion source 325 will advant~geously provide a cor,esponding number of modulation signals, each tuned to one opticdl signal.
As explained more fully below, acousto-optic mod~ tor 320 modulates the 25 pGla, i~ed carrier by the mod~ ~'?tion signal received at the modulation input 322, thereby orthogonally converting the pola, i~dtion of the pola, i~ed carrier. That is, ~cousto-optic modu~tor 320 will provide a TE->TM or TM-~TE conversion of the received pGla- i~ed carrier signal. If polali~dlion-fixing device 310 sets the polali~dlion of the carrier signal at the TE (transverse electric) mode, acousto-optic modu~-~or 320 will ol ll ,ogonally rotate the 30 TE mode to the TM (transverse ",ag"eti-.~ mode, or~ vfce versa. Also, ~coust~optic modulator will shift the optical frequency of the pola,if~ed carrier signal at the frequency of the RF modulating signal.
Coupled to the second output 319 of splitter 315 is an attenuator 330. Attenuator CA 022~70~8 l998-l2-30 330 may comprise an adjustable attenuator or a fixed attenuator depending on thepreferred design implementdlion. Attenuator 330 serves to adjust the magnitude of the portion of the polarized optical carrier receivod from the second output 319 of splitter 315 so that this second portion has a magnitude subslanlially equal to the magnitude of the orthogonal-modulated signal exiting from ~cousto-optic mod~ ~?tor 320 via output 323. As a result polari~dlion beam combiner (PBC) 335 of Fig. 3 receives an o~ lhogonally-shifted and modu~'ed polarized signal from acousto-optic modu'~tor 320 and a pottion of the original polarized optical carrier from attenuator 330 where the two received signals by polari~dlion beam combiner 335 have substa"lially the same magnUude. As mentioned 10 attenuator 330 may be used to equalize the magnitudes of the two signals received by pola, i~dlion beam combiner 335. Altematively, splitter 315 may be an u"balanced splitter or coupler specifically designed with a ratio between the first output 318 and the second output 319 so that the two signals eventually received by polarization beam combiner 335 have substantially the same magnitudes.
As mentioned polarization beam combiner 335 is positioned dow"~l,ea", from both the acousto-optic modulator 320 and the optional attenuator 330. rolari~dlion beam combiner 335 has a first input 336 optically coupled to receive the orthogonally-shifted and modulated polal i~ed signal from output 323 of acousto-optic modulator 320. As well polarization beam combiner 335 has a second input 337 optically coupled to receive a portion of the polarized optical carrier from splitter 315 which may be passed via attenuator 330. In a known fashion polarization beam Gombiner 335 will combine the orthogonally-polarization converted and frequency-shifted polari~ad signal received from ~cousto-optic modu'~tsr 320 with the portion of the original polarized optical signal received from splitter 315 to produce a polal i~dlion-rotating carrier signal. This pola, i~dlion-totali"g carrier signal will have sul,sld-,lially the same wavelerigtl, as the original carrier signal generdled by optical source 305, but will have a state of polari~dlion that will vary at a rate proportional to the modulation frequency generdled by mod~ fion source 325. In the preferred embodiment, this modulation frequency is about 172.6 MHz.
As a result, the overall polai i~dtion modulator 312 of the present invention, as defined by splitter 315, ~cousto-optic modul ~tcr 32~; attenuat~os- 330 and pola"~dtion beam comb.. ,er 335 changes the state of pola, i~dtion of the origina~ carrier signal at a very high rate. This rate of change of the state of pola- i~dtiGn eYceeds the respol1se time of an erbium-doped fiber amplifier, which is defined by 1/t, where t, is the ar,isot~p-~ saturation time.
CA 022~70~8 1998-12-30 - Typically, t" 2 0.5 ~s for erbium-doped fiber amplifiers.
rolari~dlion beam co,nb-.. ,er 335 is, for exar"~'e, Model PB100-1 L-1S-FP by JDS-FITEL. Polari~dlion beam combiner 335 also has an output 338 optically coupled to at least one rare-earth-doped fiber amplifier 340 positioned dow"a~,edm in the optical 5 communication system 300. The rare-earth-doped fiber amplifier is preferably an erbium-doped fiber amplifier. Single-stage, two-stage or multiple-stage amplifiers can be used. It is possible to use a plurality of amplifiers separdled from each other by links of long distance t,dnsmission fiber (not shown). In a test setup, a pola,i~dtion filter (Glen-Thomson prism) was positioned doJ~ L ,~l~ea", from polari~dlion beam combiner 335 for 1G detecting rotation of the signal polari~dlion. A polari~dlion filter, hoNever, is normally not co,np,iaed in an apparat.Js for reducing pola,i~dtion hole buming as herein described.
As in conventional optical communication systems such as 300, a receiver system 350 is located at the end of the communication system 300 to receive and detect i"ror,.,dlion l,dnsr,litted along the optical path. Receiver 350 may include demultiplexing 15 circuitry for a wavelength division multiplexer ar p' ~ation and may serve to detect and demodulate the optical carrier signal containing data modu~ted by data modulator 304 upstream in the optical communication system 300.
Fig. 4 illustrates a prefer,ed embodiment for pGlal i~dlion modu~tor 312.
Integrated acousto-optical devices, such as that shown as 312 in Fig. 4, are known whose 20 operation is based on the interactions b,etv:een light signals, prop~g~li. ,g in waveguides obtained on a substrate of a b..~r,ingenl and photo-elastic material, and acoustic waves propag~ ,g at the surface of the substrate, generdted through suitable transducers. The interaction between a polal i~ed optical signal and an acoustic wave produces a pola, i~dlion conversion of the signal, that is, a rotation of the polal i~dlion of the optical 25 signal's TE and TM components.
rolari~dtion mod~ 'or 312 in Fig. 4 generally co,np,ises a substrate 410,- an optical coupler 315 formed with optical waveguides within substrate 410, an acoustic waveguide 420 on substrate 410, an electro-acoustic transducer 430, first optical waveguide branch 440, second optical waveguide branch 450, ~coustic cladding 460, and 30 polal i~dlion beam combiner 335.
The substrate 410 p~erdbly is a crystal of li~hium niobate (LiNbO3) cut perpendicularly to the x-axis with optical waveguide brdnches 440 and 450 oriented along the crystal's y-axis. Altematively, another b.rer,i.)gènt, photo ela~.lic and piezoelect,ic ... . ~ . . ...
CA 022~70~8 1998-12-30 material may be used such as LiTaO3 TeO2 or CaMoO4.
Coupler 315 is formed of an optical waveguide within substrate 410 and having aninput 317 ccp~!e of being connected to an optical fiber (not shown) from upstream components in the optical communication system 300 such as pola,i~dlion-fixing device 5 310. The output polaii~alion of pola,i~dlion-fixing device 310 is preferably selected so as to match the TE or TM prop~g~tion mode of optical wavegu ~es 440 450 of polarization modu~?tor 312. Coupler 315 splits its optical path into first optical branch 440 at a first output 318 and a second optical branch 450 at a second output 319. Coupler 315 is subsldntially pola, i~dlion independen~.
~ 10 First optical branch 440 passes through ~coustic waveguide 420 to form an ~cousto-optic converter. The second optical waveguide branch 450 bypasses the acousto-optic converter and rejoins with the first optical waveguide branch 440 within polarization beam co".~i.,er 335.
Electro-acoustic transducer 430 is placed in acoustic waveguide 420 15 communicating with the first optical waveguide branch 440 so as to form the acoustic converter. The electro-acoustic transducer 430 is formed by interdigital electrodes cap~ e of generdli"g a radio-frequency (RF) surface acoustic wave. Optical signals received at input port 317 of coupler 315 and propag~ ,g along the first optical waveguide branch 440 interact with an acoustic wave prop~g~ting through acoustic waveguide 420.
20 The acoustic wave within ~coustic waveguide 420 is made so that the intensity profile of the surface acoustic wave has a peak in the central portion of the acoustic waveguide 420 and two troughs at the ends of the same waveguide. Optical signals propagating along the first optical waveguide branch 440 interact with the acoustic wave having an intensity increasi"g up to halfway along the path and de~asi"g in the other half in an area having 25 a preselec~ed i"terd~lion length. The acousticwaveguide 420 is circu",sc,ibed by ~coustic cladding 460 ui,erei., the speed of the ~coustic waves generaled by electro-~coustic transducer 430 is higher than in the acoustic waveguide 420.
The polari~dtion beam combiner 335 is pr~ferdbly formed by an evanescent wave pola, i~ttion splitter or dir~;tional coupler comprising a central optical waveguide with pairs of input wavegu;des 336 and 337. The operd~l7n of pola,i~dtion splitter/combiner 335 is desc,iLed in EP 737 880 at col.12-14 which~document is incor~ordted by r~fer~,-ce. Outputs 338 and 339 provide orthogonally-separdted signals for exiting the modulator 312 for conne,;tion to an output optical fiber for l~ansr"ission downstream in the CA 022~70~8 l998-l2-30 - optical communication system 300.
The operdtion of polari~dlion modu~ator 312 of Fig.4 according to the present invention is as follows. When an app,upriale modu~tion signal, e.g., an RF signal of 172.6 MHz, is applied from modlJl~fion source 325 to electro-acoustic transducer 430, 5 transducer 430 generates a respective RF surface acoustic wave having a driving acoustic frequency conesponding to the optical resonance wavelenyll" e.g., ~ = 1556.7 nm. At this fesonance wavelength, the polarization conversion TE-~TM or TM-~TE takes place. Optical signals enter polarization modulator 312 from polari~dlion-fixing device 310 with a fixed pola, i~dlion of either TE or TM. As the received optical signals prop~gate 10 through the first optical waveguide branch 440, they are l,ansrur,,,ed into their cor,-asponding orthogonal con,ponerils. That is, if the ll:ceived signals have a polari~dlion of TE, their pola,i~dlion is rotated to its orthogonal component TM, or vice versa. Also, they receive freguency shifts with an ~bs~lute value equal to the RF signal frequency.
~olari~dtion splitter, or combiner, 335 combines the modulated optical signal from 15 first optical waveguide branch 440 with the unmodulated optical signal from second optical waveguide branch 450. Outputs 338 and 339 provide orthogonally-separated signals.
Combining the polarkdtion modulated and frequency shifted optical signal with the unmodulated signal results in the optical signal at output 338 with a rotating state of polarization. As described, this rotating state of polarization will occur at a rate dictated in part by RF source 325 and will prererdbly take place at a rate in excess of 1/t5, where tS is the anisotropic saturation time of a fiber amplifier, such as amplifier 340 shown in Fig. 3 downstream from pola,i~dlion modulator 312. Consequently, polari~dtion modulator 312 illustrated in Fig. 3 provides a concise and erficie, It implementation of an acousto-optic mod~ ~~tor for generdting a pGlari~ation-rotating optical signal that helps to suppress pola,i~dliGn hole buming in an EDFA.
The f~ i, .9 describes the experimental tests and results for the present invention as previously set forth. Fig. 5 illustrates a test setup generally refer~:nced at 600 for ex~,e,i,nentally analyzing the optical communication system and transmitter of Fig. 3. As shown in Fig. 5, an optical source 610 in the fomm of a laser diode (AT&T Model 246AH) operatil,g at a nominal v avelength of 15g6.7 nm ir~ vacuum provided an optical carrier signal to a pold,i~ation co, lt, ~'ler for 620. roldl icdtion conl~l'er 620 was a series of optical fiber loops arranged to have an angular fl;,p'~cel"ent for controlling the polal i~alion of the optical carrier r ecei\rcd from laser source 610. An output from the CA 022~70~8 1998-12-30 pola,i~alion co,lt,."er 620 was optically coupled to a pola,i~dlion modulator 630, which comprised the components of pola, i~dlion modulator 312.
At the output of the polari~dlion mod~ator 630, the test setup dep, ~ed generally as 600 in Fig. 5 included a coupler 640 that split the output signal from pola,i~dlion modulator 630 in a 90:10 ratio. Ten percent (10%) of the polali~dlion modulator output was split by coupler 640 and fed through pola, i~ing filter 6$0 and photodiode 660 so that it could be detected and analyzed within oscilloscope 670. Coupler 640 directed 90% of the output from polarization mod~ ~ator 630 to an attenuator 675. Another coupler 680 having a splitting ratio of 90:10 was positioned dol~. ,sl~eam from attenuator 675 and split 10% of the signal from attenuator 675 to a power meter 685 for detection. Coupler 680 divided 90% of the output signal from attenuator 675 and passed it to an erbium-doped fiber amplifier 690. Amplifier 690 was a double stage EDFA pumped with 1480 nm laser diodes. For an input power of -15dBm, the amplifier had an output power of 9 dBm, a gain G = 24 dB, Gc= 12 dB, N,= 6.5 dB, ~ = 0.92, and P~, = 0.94 IlW. After amplifying the received polarization-rotating carrier signal from coupler 680, EDFA 690 passed the signal through polarization controller 697 to an optical signal analyzer 698 for detection and - analysis. Polarization conl, ~"er 697 was used to set the polarization of the amplified signal received from EDFA 690 to a state accept-~'e by the optical signal analyzer 698.
Fig. 6 shows the 9, dph-c ~' test results obtained by optical signal analyzer 698 under various test conditions and referenced generally as 700. Signal trace 710 depicts the spectra leceived at the output of EDFA 690 when no modulation or rotation of the signal pola, i~dlion was imp!e.,lenled. In other words, signal 710 in Fig. 6 illustrates the output from EDFA 690 when the input signal to that amplifier had a degree of polal i~dlion equal to 100%. Signal trace 720 depicts the output from EDFA 690 when the degree of pola, i~dlion of the input signal was 36%. Signal trace 730 shows the output signal from EDFA 690 with an input signal that had a degree of pola~ i~dlion of 6%, as received dol.~":il,ean, from pola,i~dlion modulator 630.
As shown in Fig. 6, the amplified spontaneous emission (ASE) noise was the highest in signal 710 with a degree of pola,i~dtion of 100%, and least in signal trace 730 with an input signal having a degree of-tola,i~dlior~! e,qual to 6%. C~r,lparing the two results in signal traces 710 and 730, it can be seen4hat the ASE noise drops by about 0.24 dB when the polali~dtion modulator 630 is used.~ This~drop in ASE noise cor,esponds approxi,ndlely to the quantity of gain ~aridlion caused by pola,i~dlion hole . .
CA 022~70~8 1998-12-30 burning present in an EDFA with a polari~ed saturating signal. In particular, Fig. 2 shows that for 10 dB of gain co",pression, the pGlari~dlion hole buming in de~ els corresponds to about 0.2 dB for a degree of polari~dlion equal to 100%. Thus, polari~alion modulator 630 provides a siyl ,i~icant decrease in signal fading caused by pola~ i~dlion hole burning Applicants have also determined that the above~escril,ed invention is effective in reducing pola,i~dlion hole buming in wavelenglh-division-multiplexing (WDM) optical transmission systems. As is readily known to one of ordinary skill in the art, in a WDM
system, a plurality of optical sources generdtes carrier frequencies for the channels in the l,ansl"ission system. One or more of the channels is modu~-ted with information, and the 10 channels are multiplexed and then transmitted down a common optical fiber. Repeaters or optical amplifiers along the l,ansmission path may boost the channel levels for passage across a long distance. At a receiver end, a demultiplexer separates the channels to respe.ti~/e paths, and a receiver obtains the mod~ ~ated i~ If o",~alion from a particular channel. For such a WDM system, polai i~dtion hole buming can be reduced by using a 15 single polari~dlion modulator as descril,ed above that is positioned downstream from the multiplexer. In this fashion, the polari~dlion of all the channels in the WDM system can be rotated. Altematively, a plurality of pola, i~dtion-rotating modulators can be used prior to multiplexing all the channels so that a group of adjacenl or interleaved channels may have their polarization individually rotated. Furthermore, ApF'i~anls believe that polarization 20 hole burning can be reduced in a WDM system by rotating the polarization of less than all of the channels in the WDM system, and even only one cl,annel in the WDM system.It will be appareril to those skilled in the art that various modifications and vai idlions can be made to the system and method of the present invention without depa, li"g from the spirit or scope of the invention. The presenl invention covers the 25 modifications and varidliGns of this invention provided they come within the scope of the appended claims and their equivalents.
Claims (16)
1. An apparatus for reducing polarization hole burning in a rare-earth-doped fiber amplifier within an optical communication system by converting an optical carrier having a characteristic wavelength and an initial state of polarization into a polarization-rotating optical carrier, comprising:
an acousto-optic modulator having a carrier input optically coupled to receive a first portion of the optical carrier, a modulation input electrically coupled to receive a modulation frequency, and a modulator output, the acousto-optic modulator including circuitry for orthogonally converting the polarization of the polarized optical carrier and shifting the polarized optical carrier frequency by the modulation frequency;
and a polarization beam combiner having a first input optically coupled to receive the orthogonally-polarization converted and frequency-shifted polarized signal, a second input optically coupled to receive a second portion of the optical carrier, and an output optically coupled to the rare-earth-doped fiber amplifier downstream in the optical communication system.
an acousto-optic modulator having a carrier input optically coupled to receive a first portion of the optical carrier, a modulation input electrically coupled to receive a modulation frequency, and a modulator output, the acousto-optic modulator including circuitry for orthogonally converting the polarization of the polarized optical carrier and shifting the polarized optical carrier frequency by the modulation frequency;
and a polarization beam combiner having a first input optically coupled to receive the orthogonally-polarization converted and frequency-shifted polarized signal, a second input optically coupled to receive a second portion of the optical carrier, and an output optically coupled to the rare-earth-doped fiber amplifier downstream in the optical communication system.
2. The apparatus according to claim 1, further comprising:
a polarization-fixing device optically coupled to the carrier input to transformthe initial state of polarization of the optical carrier to a predetermined state of polarization.
a polarization-fixing device optically coupled to the carrier input to transformthe initial state of polarization of the optical carrier to a predetermined state of polarization.
3. The apparatus according to claim 2, wherein the polarization-fixing device is one of a polarization controller, polarization-maintaining fiber, and polarization stabilizer.
4. The apparatus according to claim 1, further comprising:
an attenuator coupled to the second input of the polarization beam combiner for attenuating the second portion of the optical carrier to a magnitude substantially equal to a magnitude of the orthogonally-polarization converted and frequency-shifted polarized signal.
an attenuator coupled to the second input of the polarization beam combiner for attenuating the second portion of the optical carrier to a magnitude substantially equal to a magnitude of the orthogonally-polarization converted and frequency-shifted polarized signal.
5. The apparatus according to claim 1, further comprising:
a splitter coupled to the carrier input of the acousto-optic modulator and the second input of the polarization beam combiner, the splitter providing a higher magnitude of the optical carrier to the carrier input than to the second input.
a splitter coupled to the carrier input of the acousto-optic modulator and the second input of the polarization beam combiner, the splitter providing a higher magnitude of the optical carrier to the carrier input than to the second input.
6. The apparatus according to claim 1, wherein the modulation frequency is about 172 MHz.
7. An optical transmitter for reducing polarization hole burning in a rare-earth-doped fiber amplifier within an optical communication system, comprising:
an optical source for transmitting an optical carrier having an initial state ofpolarization;
a splitter, positioned downstream from the optical source, having an input, a first output, and a second output, and dividing the optical carrier received at the input between the first output and the second output;
a modulation source for providing a modulation signal;
an acousto-optic modulator having a carrier input optically coupled to the first output of the splitter, a modulation input electrically coupled to the modulation source, and a modulator output, the acousto-optic modulator including circuitry for orthogonally converting polarization of the optical carrier and frequency shifting the optical carrier by the frequency of the modulation signal;
an attenuator optically coupled to the second output of the splitter; and a polarization beam combiner having a first input optically coupled to receive the orthogonally polarization converted and frequency-shifted optical signal, a second input optically coupled to the attenuator, and an output optically coupled to the rare-earth-doped fiber amplifier downstream in the optical communication system.
an optical source for transmitting an optical carrier having an initial state ofpolarization;
a splitter, positioned downstream from the optical source, having an input, a first output, and a second output, and dividing the optical carrier received at the input between the first output and the second output;
a modulation source for providing a modulation signal;
an acousto-optic modulator having a carrier input optically coupled to the first output of the splitter, a modulation input electrically coupled to the modulation source, and a modulator output, the acousto-optic modulator including circuitry for orthogonally converting polarization of the optical carrier and frequency shifting the optical carrier by the frequency of the modulation signal;
an attenuator optically coupled to the second output of the splitter; and a polarization beam combiner having a first input optically coupled to receive the orthogonally polarization converted and frequency-shifted optical signal, a second input optically coupled to the attenuator, and an output optically coupled to the rare-earth-doped fiber amplifier downstream in the optical communication system.
8. The apparatus according to claim 7, further comprising:
a polarization-fixing device optically coupled to the input of the splitter to transform the initial state of polarization of the optical carrier to a predetermined state of polarization.
a polarization-fixing device optically coupled to the input of the splitter to transform the initial state of polarization of the optical carrier to a predetermined state of polarization.
9. The apparatus according to claim 8, wherein the polarization-fixing device is one of a polarization controller, polarization-maintaining fiber, and polarization stabilizer.
10. The apparatus according to claim 7, wherein frequency of the modulation signal is about 172 MHz.
11. A method of suppressing polarization hole burning in a rare-earth-doped fiber amplifier within an optical communication system, comprising the steps of:splitting an optical carrier signal into a first sub-carrier signal and a secondsub-carrier signal;
rotating orthogonally polarization of the first sub-carrier signal and modulating the first sub-carrier signal with a modulation frequency to create anorthogonal-modulated sub-carrier signal;
combining the orthogonal-modulated sub-carrier signal and the second sub-carrier signal in a polarization beam combiner to produce a polarization-rotating carrier signal; and passing the polarization-rotating carrier signal downstream in the optical communication system to the rare-earth-doped fiber amplifier.
rotating orthogonally polarization of the first sub-carrier signal and modulating the first sub-carrier signal with a modulation frequency to create anorthogonal-modulated sub-carrier signal;
combining the orthogonal-modulated sub-carrier signal and the second sub-carrier signal in a polarization beam combiner to produce a polarization-rotating carrier signal; and passing the polarization-rotating carrier signal downstream in the optical communication system to the rare-earth-doped fiber amplifier.
12. The method according to claim 11, further comprising the step of:
fixing a state of polarization of the optical carrier signal prior to splitting the optical carrier signal.
fixing a state of polarization of the optical carrier signal prior to splitting the optical carrier signal.
13. The method according to claim 11 further comprising the step of:
attenuating the second sub-carrier signal prior to combining the second sub-carrier signal with the orthogonal-modulated sub-carrier signal.
attenuating the second sub-carrier signal prior to combining the second sub-carrier signal with the orthogonal-modulated sub-carrier signal.
14. The method according to claim 13, wherein the attenuating step includes the substep of:
attenuating the second sub-carrier signal to a magnitude substantially equal to a magnitude of the orthogonal-modulated sub-carrier signal.
attenuating the second sub-carrier signal to a magnitude substantially equal to a magnitude of the orthogonal-modulated sub-carrier signal.
15. The method according to claim 11, wherein in the splitting step includes thesubstep of:
splitting the optical carrier signal into the first sub-carrier signal with a magnitude greater than a magnitude of the second sub-carrier signal.
splitting the optical carrier signal into the first sub-carrier signal with a magnitude greater than a magnitude of the second sub-carrier signal.
16. An acousto-optic modulator for rotating the polarization of an optical carrier signal, comprising:
a substrate of a birefringent and photo-elastic material;
a first port on the substrate for receiving the optical carrier signal from an optical waveguide;
a splitter having an input coupled to the first port, a first output, and a second output;
a first optical waveguide branch coupled at one end to the first output of the splitter;
a second optical waveguide branch coupled at one end to the second output of the splitter;
an acoustic waveguide on the substrate including at least a portion of the first optical waveguide branch;
an acoustic wave generator positioned on the substrate over at least a portion of the acoustic waveguide; and a polarization splitter having a first input coupled to another end of the firstoptical waveguide branch, a second input coupled to another end of the second waveguide branch, and an output.
a substrate of a birefringent and photo-elastic material;
a first port on the substrate for receiving the optical carrier signal from an optical waveguide;
a splitter having an input coupled to the first port, a first output, and a second output;
a first optical waveguide branch coupled at one end to the first output of the splitter;
a second optical waveguide branch coupled at one end to the second output of the splitter;
an acoustic waveguide on the substrate including at least a portion of the first optical waveguide branch;
an acoustic wave generator positioned on the substrate over at least a portion of the acoustic waveguide; and a polarization splitter having a first input coupled to another end of the firstoptical waveguide branch, a second input coupled to another end of the second waveguide branch, and an output.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| EP97123015 | 1997-12-31 | ||
| EP97123015.6 | 1997-12-31 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2257058A1 true CA2257058A1 (en) | 1999-06-30 |
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| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2257058 Abandoned CA2257058A1 (en) | 1997-12-31 | 1998-12-30 | Suppression of polarization hole burning with an acousto-optic modulator |
Country Status (6)
| Country | Link |
|---|---|
| JP (1) | JPH11262093A (en) |
| AR (1) | AR014217A1 (en) |
| AU (1) | AU9817198A (en) |
| BR (1) | BR9805741A (en) |
| CA (1) | CA2257058A1 (en) |
| NZ (1) | NZ333527A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113053272A (en) * | 2021-03-12 | 2021-06-29 | 青岛理工大学 | Multifunctional cinema step convertible indicating spotlight and using method |
-
1998
- 1998-12-22 NZ NZ33352798A patent/NZ333527A/en unknown
- 1998-12-23 AU AU98171/98A patent/AU9817198A/en not_active Abandoned
- 1998-12-29 BR BR9805741-3A patent/BR9805741A/en not_active Application Discontinuation
- 1998-12-30 AR ARP980106742 patent/AR014217A1/en unknown
- 1998-12-30 CA CA 2257058 patent/CA2257058A1/en not_active Abandoned
-
1999
- 1999-01-04 JP JP16599A patent/JPH11262093A/en active Pending
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113053272A (en) * | 2021-03-12 | 2021-06-29 | 青岛理工大学 | Multifunctional cinema step convertible indicating spotlight and using method |
Also Published As
| Publication number | Publication date |
|---|---|
| AU9817198A (en) | 1999-07-22 |
| NZ333527A (en) | 2000-08-25 |
| JPH11262093A (en) | 1999-09-24 |
| AR014217A1 (en) | 2001-02-07 |
| BR9805741A (en) | 1999-12-28 |
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| Date | Code | Title | Description |
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| FZDE | Dead |