WO2001071951A2

WO2001071951A2 - Method and system for non-soliton transmission of short pulse signals via an optical fiber

Info

Publication number: WO2001071951A2
Application number: PCT/US2001/007414
Authority: WO
Inventors: Ilya M. Fishman; Yu Sheng Bai
Original assignee: Optimight Communications, Inc.
Priority date: 2000-03-16
Filing date: 2001-03-08
Publication date: 2001-09-27
Also published as: WO2001071951A3

Abstract

Short return-to-zero format pulse signals are propagated via an optical fiber network in a non-soliton transmission mode. For suppression of non-linear effects in a transmission line, the short optical pulses are stretched by a dispersion stretching device to duration equal to or shorter than 1/2 bit period of the digital signal, and propagated via the transmission line. The propagated optical signals are compressed by a dispersion compressing device to duration of the initial short optical pulses, and detected for obtaining the transmitted-digital signal in RZ mode.

Description

METHOD AND SYSTEM FOR NON-SOLITON TRANSMISSION OF SHORT PULSE SIGNALS VIA AN OPTICAL FIBER

FIELD OF THE INVENTION The present invention relates to optical multi-channel transmission for a multichannel optical fiber communication network, and more particularly to a Wavelength-Division Multiplexing (WDM) optical fiber transmission system and method for non-soliton transmission of short pulse signals via an optical fiber.

BACKGROUND OF THE INVENTION

The transmission format of signals propagated through optical fibers may be either nonreturn-to-zero (NRZ) or return-to-zero (RZ). In NRZ format, the duration of light pulses used for transmission of digital "l"s is equal to bit duration of the transmitted signal. In RZ format, the duration of "l"s does not exceed one half of bit duration. The majority of fiber transmission lines deployed in commercial networks employ the

SONET transmission standard with NRZ modulation format. The NRZ format is widely utilized because it requires cheaper equipment for both transmission and reception systems. Conventionally, transmission systems comprise internally or externally modulated continuous wave (CW) lasers, and reception systems comprise detection and clock recovery electronics. However, the NRZ transmission format has limitations that are detrimental to development of multichannel WDM systems with reach over 2,000 km and optical add-drop and switching capability. In conventional terrestrial WDM systems, after optical signals propagate approximately 500-600 km, they have to be regenerated by optical-to-electronic conversion, clock recovery and retiming. The optoelectronic conversion becomes necessary because after 500-600 km, the signal-to-noise ratio decreases below the level of reliable signal detection. The maximum transmission distance could be increased by reducing optical amplifier spacing or by increasing transmitted power per channel. Reduction of optical amplifier spacing that would increase the signal level at the amplifier input and result in dramatic increase of signal to noise ratio, is not acceptable for terrestrial applications. It is also impossible to increase transmitted power per channel in the existing NRZ format because of nonlinear interaction of coherent electromagnetic fields propagating in WDM channels.

The advanced WDM networks contemplate wide utilization of add/drop and optical cross-connect functionality. NRZ format causes substantial limitations for these applications due to mutual coherence of fields in adjacent WDM channels causing coherent cross-talk in optical Add-Drop and Optical Cross-Connect devices. CW semiconductor lasers are characterized by a high degree of temporal and spatial coherence. Substitution of single- line coherent light sources with transmission sources having multiple spectral lines may substantially reduce major coherent cross-talk problems in multichannel networks. These transmission sources are disclosed in the US patent application "Method and System of Transmitting Optical Signals Generated by Multi-Line Sources via WDM Optical Network." filed by the assignee of the present patent application on March 16, 2000. The most efficient way to generate the desired multi-line spectrum is to periodically generate short optical pulses having duration substantially shorter than one bit, and code the signal in RZ format. Introduction of multi-line sources or equivalently, short pulse sources complicates the transmission technology to a certain extent, but increases the number of degrees of freedom of the transmission system and allows for improved transmission quality and overall performance of WDM transmission system.

It is known [L. Boivin et al, Receiver Sensitivity Improvement by Impulse Coding, IEEE Phot. Tech. Lett. v. 9, p. 684, 1997], that because of the high peak intensity, RZ signal improves the receiver sensitivity and signal-to-noise-ratio (SNR) over NRZ signal for given average signal and noise power. In linear signal transmission, the only major difference between RZ and NRZ is that RZ requires twice as large transmission bandwidth. Thus if the transmission line has sufficient redundant bandwidth, as in the case of optical fiber, it is always preferable to transmit digital signal in RZ format. However, it is also known that nonlinear effects do exist in optical fiber transmission. The conventional NRZ transmission systems usually operate at the power level just below the onset of nonlinear effects. Thus the high peak power in RZ signal could induce large nonlinear effects and degrade system performance. In fact, it was argued by some authors [F. Forghieri et al, RZ Versus NRZ in Nonlinear WDM Systems, IEEE Phot. Tech. Lett, v. 9, p. 1035, 1997] that in a generic WDM system, nonlinear effect would wipe out all the gains in receiver sensitivity form RZ, and NRZ-based systems generally would outperform RZ based

systems.

Generation and propagation of short optical pulses as optical solitons is a well-known technique [L.F. Mollenauer, J.P. Gordon, and P. V. Mamyshev "Solitons in High Bit-Rate, Long-Distance Transmission", Optical Fiber Communication Systems_* ^Vo1- MA_* Chapter 12, Ed. I. P. Kaminow and T. L. Koch, Academic Press, San Diego, 1997]. According to soliton technology, light pulses of about 30 ps duration and peak power of several dBm were generated and propagated along fiber spans of several million kilometers without significant shape degradation. The trend of the pulse to expand due to linear dispersion was compensated by contraction due to self-phase modulation; the balance between linear and nonlinear contributions could be maintained only within a certain range of pulse power. To support solition transmission, optical amplifiers should be closely spaced to maintain the power range within requred limits. After many stages of optical amplification, the accumulation of noise results in soliton timing jitter (known as Gordon-Haus effect) that is the major source of transmission errors. This timing jitter practically eliminates advantages of RZ data transmission. To reduce the Gordon-Haus effect and allow for extra long propagation of the transmission signal, special filtering schemes were developed. However, these schemes are generally too complex for deployment in the real transmission systems. Another major disadvantage of soliton transmission is that the optical amplifiers have to be spaced at distances much shorter than customary for commercial long-haul networks. As a result, the soliton transmission systems, although studied for almost 20 years have not found commercial applications. Yet another approach to transmit short pulses over optical fibers is to reduce the signal power launching to the transmission line, so that the peak power of the pulses is comparable to that of NRZ signals. This approach would also require short optical amplifier span and limit its application in practical systems.

SUMMARY OF THE INVENTION

A method and a system thereof for non-soliton transmission of short pulses allows for propagation and detection of short return-to-zero type pulses to increase substantially a network reach and optical add/drop capability within contemporary fiber networks. Short laser pulses of the required spectrum are generated by a multi-line light source. Duration of these short pulses has to be substantially shorter than a bit period of the transmission signal. Short pulses are modulated by a transmission signal and are stretched by a stretcher device. The duration of stretched modulated short pulses is comparable to the bit period of the transmission line. The peak power of the stretched pulses is ~2 times the average signal power, similar to that in NRZ format. The stretched modulated short pulses are propagated via a transmission line. The stretched pulses may form short pulses again due to local dispersion in the propagation. However, this occurs only after a certain distance so that the signals have experienced appreciable attenuation, and nonlinear interaction becomes negligible. Thus the propagation of the stretched modulated short pulses has a linear characteristic. Propagated short pulses are compressed to obtain duration comparable to the duration of generated short laser pulses. To detect the transmission signal the propagated short pulses are modulated as retum-to-zero optical pulses.

Short pulses may be obtained by external or internal modulation of the laser source. The duration of short pulses is in a range of about 20ps to 30ps.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other features and advantages of the present invention will be described by way of example with references to the accompanying drawings in which:

Figure 1 shows a schematic diagram of a non-soliton transmission of short pulse signals via optical fiber according to the present invention.

Figure 2 shows a span of dispersion compensated optical fiber transmission line.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The general idea of the present invention is to fully utilize the optical spectrum within each WDM window by generating short optical pulses. The optical power associated with the short optical pulse is substantially higher than the CW power spread over the bit period duration. To diminish the nonlinear effects, the short pulses are, before propagation through the link, stretched into quasi-CW mode. The optical link is dispersion compensated so that the total chromatic dispersion of the link is about zero. Due to local dispersion in the link, the stretched pulses may form short pulses again in the propagation; however, this occurs only after certain distance, typically >20 km, so that the signals have seen appreciable attenuation and nonlinear interaction becomes negligible. After propagating quasi-CW power through the link, the pulses are compressed back to initial duration, and detected as RZ signals. Referring to Figure 1 , the system of the present invention comprises a continuous wave (CW) laser 10. The CW laser 10 is coupled to an intensity modulator 20 for generating short pulses. The duration of the pulses is equal to or shorter than one half of the bit period of the transmission signal. Duration of the short pulses should not be less than _α/β, where β is a bandwidth of the optical channel of the

transmission line and _α is a form factor related to the shape of the pulse. In conventional WDM

optical fiber systems, the bandwidth value is in a range between 50 GHz to 100 GHz, and it is known that for a Gaussian type pulse _α = 0.45. For bit rate in a range of 2.5 to 10 GHz

corresponding to bit period in a range of 400 ps to 100 ps, the pulse duration can be in a range from 20 ps to 200ps. Intensity modulator 30 converts the electrical signal into an optical pulse sequence, and then modulates the optical pulse train.

According to different embodiments of the present invention the intensity modulator 30 or intensity modulator 20 may be connected directly to the CW laser 10. When the CW laser 10 is connected to the intensity modulator 20, the combination of this two elements can be replaced by a pulsed laser, for example a mode-locked laser, a direct- modulated semiconductor laser, or and integrated semiconductor laser-modulated package. When the sequence of the intensity modulators are inverted, the combination of CW laser 10 coupled to the intensity modulator 30 can be replaced by a direct modulated semiconductor laser or and integrated semiconductor laser-modulator package.

A dispersion stretching device 40 stretches the short laser pulses, which are modulated with a transmission signal. The duration of stretched laser pulses is equal to or greater than Vz bit period of the transmission signal. The stretched pulses are propagated via a transmission line A-B. The integral dispersion of the transmission line A-B should be zero. A dispersion compensation device 50 is disposed within an output of the transmission line A-B for compressing the propagated laser pulses to their initial width, e.g. equal to or less than Vi of the bit period of the transmission signal. Over the predetermined wavelength range, the dispersion device 40 and dispersion compensating device 50 have the same value but opposite sign of the group-velocity dispersion and dispersion slope. The temporal shape and duration of the short pulses at the input of the dispersion stretching device 40 and at the output of the dispersion compensating devise 50 are approximately the same. The dispersion stretching and dispersion compensating devises 40 and 50 may be implemented as segments of optical fibers with large positive or negative group velocity dispersions, chirped fiber Bragg gratings or other devices exhibiting large positive or negative group velocity dispersions. The range of the dispersion can be from ±100 ps/nm to ± 2 ns/nm.

The receiver 60 detects the optical pulse sequence and converts it back to an electrical signal. Preferably, the receiver is optimized for the pulse detection and has slightly larger detection bandwidth than a tandard NRZ receiver at the same data rate.

The transmission line A-B, being a dispersion compensated fiber transmission line, comprises one or more fiber spans of a requested type of fiber. For example, each fiber span as shown in Figure 2 comprises single mode (SM) fiber 11, a dispersion compensation unit 22, and an optical amplifier 33. The dispersion compensation unit 22 has the same amount of total dispersion over the predetermined wavelength range as the SM fiber 11, but with opposite sign.

One dispersion compensation unit may be utilized for dispersion compensation within several fiber spans, each having only a segment of SM fiber and an optical amplifier.

An advantage of the present invention is that it enables linear transmission of RZ signals with average signal power similar to that in standard NRZ systems, which results in improved SNR and reduced timing jitter, and hence makes possible higher performance transmission systems with high capacity and longer reach.

While there have been shown and described what are at present considered preferred embodiments of the present invention, it will be obvious to those skilled in the art that various changes and its implementations in the transmission line as well as modification of the laser source may be made to achieve a non-soliton transmission of short pulse signals via an optical fiber without departing from the scope of the invention as defined by the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A method of non-soliton transmission of short pulse optical signals via an optical network comprising the steps of:

₅ modulating retum-to-zero ( RZ) optical pulses by a transmission signal, each said RZ optical pulse having duration shorter than a Vi bit period of said transmission signal; stretching said RZ modulated optical pulses to duration of said transmission signal, linearly propagating said stretched optical pulses via a transmission line for suppressing non-linear effects within said transmission line; and 0 restoring duration of said modulated RZ optical pulses by compressing said stretched pulses.

2. The method of non-soliton transmission of claim 1 , wherein said duration of stretched RZ modulated optical signals is equal to or greater than !/₂ bit period of said transmission signal. 5

3. The method of non-soliton transmission of claim 2, wherein said modulated RZ optical pulses and restored optical pulses have approximately the same temporal shape.

4. The method of non-soliton transmission of claim 3, further comprising the step of o compensating chromatic dispersion of said transmission line.

5. A method of non-soliton transmission of short pulse signals via an optical fiber network comprising the steps of : generating short return-to-zero (RZ) optical pulses having duration equal to or shorter 5 than a 1/2 bit period of said signal; modulating said generated optical pulses by a digital signal; transmitting said digital signal via a transmission line within said optical fiber, stretching said modulated optical pulses to obtain stretched optical pulses having duration comparable to bit period; 0 linearly propagating said stretched optical pulses via said transmission line for suppressing non-linear effects within said transmission line; compressing said propagated pulses to obtain compressed optical pulses having duration comparable to said generated optical pulses; and detecting said compressed optical pulses for obtaining said transmitted digital signal.

6. The method of non-soliton transmission of claim 5, further comprising the step of compensating chromatic dispersion of said transmission line over a predetermined range of

5 wavelengths within an optical fiber.

7. The method of non-soliton transmission of claim 6, wherein an integral dispersion of said transmission line is approximately zero.

0 8. The method of non-soliton transmission of claim 7, further comprising the step of external modulating of a laser source for generating said short optical pulses.

9. The method of non-soliton transmission of claim 7, further comprising the step of internal modulating of a laser source for generating said short optical pulses. 5

10. The method of non-soliton transmission of claim 7, wherein bit period of said transmission signal is in a range of about 100 ps to 400 ps, and bit period of said short pulses is in a range of about 20 ps to 200 ps.

0 11. A system for non-soliton transmission of short pulse signals via an optical fiber comprising: an optical source for generating short optical pulses having duration equal to or shorter than a Vτ bit period of said signal; a modulator for RZ modulation of said generated optical pulses by a digital signal; 5 a stretcher for stretching said generated optical pulses, said stretched optical pulses having duration comparable to the bit period of said digital signal. a transmission line for propagating said stretched optical pulses and digital signal; a compressor for compressing said stretched laser pulses, duration and shape of said compressed pulses being comparable to said duration of said generated short pulses; and _Q a receiver for receiving said compressed pulses and detecting said propagated digital signal.

12. The system for non-soliton transmission of claim 11, wherein said optical source is a mode-locked laser.

13. The system for non-soliton transmission of claim 11, wherein said optical source is a CW laser coupled to a short pulse modulator.

14. The system for non-soliton transmission of claim 13, wherein said stretcher and compressor are dispersion devices having identical value and opposite sign of group-velocity dispersion and dispersion slope over the predetermined wavelength range.

15. The system for non-soliton transmission of claim 13, wherein said stretcher and said compressor are formed by respective portions of said optical fiber.

16. The system for non-soliton transmission of claim 13, wherein said stretcher and said compressor are formed by respective portions of a multi-mode optical fiber.

17. The system for non-soliton transmission of claim 13, wherein said stretcher and said compressor are fiber Bragg gratings.

18. The system for non-soliton transmission of claim 17, wherein said transmission line further comprises at least one fiber link, at least one dispersion compensation module and at least one optical amplifier.

19. The system for non-soliton transmission of claim 18, wherein a length of said at least one fiber link is in a range of about 50 km to 120 km.

20. The system for non-soliton transmission of claim 19, wherein a bit period of said signal is in a range of about 100 ps to 400 ps, and duration of said short generated pulses is in a range of about 20 ps to 200 ps.