JP2013041291A - Optical modulator and optical modulating method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To allow an optical modulator to generate an optical 8-phase PSK signal of high quality, even when it is driven using a binary signal having some waveform distortion.SOLUTION: A first Mach-Zehnder modulator, a second Mach-Zehnder modulator, and a third Mach-Zehnder modulator are driven by binary signals. a phase shifter is applied with voltage, having phase difference of (180×n+90)°, where n is an integer, between output light of the first Mach-Zehnder modulator and output light of the second Mach-Zehnder modulator. An α parameter of the third Mach-Zehnder modulator is +0.25 or -0.25.

Description

  The present invention relates to an optical modulator and an optical modulation method for generating a multi-level modulation modulated optical signal for optical transmission of a large capacity signal.

  An optical fiber communication system is an important technology having an advantage that a large amount of information can be efficiently communicated. There are two main reasons why an optical fiber communication system can efficiently transmit information. The first reason is that the attenuation loss of the signal accompanying the transmission is reduced due to the low loss characteristic of the optical fiber serving as the transmission path. As a result, there is an advantage that the number of relay devices necessary for long distance transmission can be reduced. The second reason is that hardware required for transmission / reception of optical signals can be reduced by performing time division multiplexing. As a result, there is an advantage that the cost necessary for transmitting the same capacity can be reduced. The advantage for the second reason is particularly effective for the increase in communication capacity due to the spread of the Internet. For example, as a commercial system, a system that transmits 10 Gb / s per wavelength is operated, and recently, a system that transmits 40 Gb / s per wavelength has been started.

  In order to cope with the further increase in capacity of information communication, there is a need for an ultra-high-speed optical signal transmission technique capable of multiplexing more signals. In response to this demand, research and development relating to signal transmission at a speed of 100 Gbps (about 100 Gb / s) or higher has been actively promoted.

  In an optical fiber communication system that transmits a signal of up to 10 Gbps per wavelength, an NRZ (Non-Return-to Zero) method that allows easy signal generation and detection is widely used. The NRZ system is a system in which a digital value (binary value) is encoded and transmitted as light ON (on) and OFF (off).

  There are two main problems when performing ultrahigh-speed transmission by the NRZ method. The first problem is that waveform degradation occurs due to dispersion of the optical fiber. Waveform distortion due to the effects of chromatic dispersion and polarization dispersion of an optical fiber becomes more serious as the signal speed increases. Transmission distance is limited by chromatic dispersion and polarization dispersion. Specifically, the transmission distance limited by chromatic dispersion becomes shorter in inverse proportion to the square of the increase in signal speed. Further, the transmittable distance limited by the polarization dispersion becomes shorter in inverse proportion to the signal speed.

  The second problem is an operating speed limit of an electric circuit that performs signal multiplexing. For example, in order to perform signal transmission of 100 Gbps, an electronic circuit having a bandwidth of 100 GHz class (an electronic circuit having a frequency of a signal that can be normally processed is a 100 GHz class, that is, an electronic circuit that can normally process a signal of about 100 GHz) ) Is required. However, with the current technology, it is difficult to realize such a circuit with silicon CMOS which is excellent in economic efficiency. To achieve this, electronic device technology using InP-based materials is indispensable, but even if such technology is used, it is close to the device operating speed limit. There is a need to overcome the challenges.

  Therefore, attention has been focused on multilevel modulation technology of light. Multi-level phase modulation is a modulation technique in which information of a plurality of bits is assigned to three or more states of light. For example, in 40 Gbps optical transmission or the like, four-phase differential phase modulation (hereinafter referred to as QPSK) has attracted attention as an important technology.

  In DQPSK, two digital bits (00, 01, 10, 11) are assigned to the four phases of light, 0 °, 90 °, 180 °, and 270 ° (hereinafter referred to as symbols). In this method, since 2 bits are transmitted in one symbol, the symbol rate is half of the bit rate. As a result, the bandwidth required for the electronic circuit that processes the symbols can be about half the bit rate. For example, when a 100 Gb / s signal is transmitted using DQPSK, the symbol rate is 50 Gsymbol / s.

  As a result, the band of an electronic circuit that processes a signal of 50 G symbol / s, such as a multiplexing circuit or a modulator driver, may be up to about 50 GHz.

  Further, since the symbol rate becomes smaller than the bit rate, the time slot occupied by one symbol becomes twice the bit slot time. As a result, there is an advantage that the transmission distance limited by dispersion can be made longer than binary modulation such as NRZ because it is less susceptible to waveform distortion due to dispersion. Taking advantage of the above advantages, technological development relating to generation, transmission, and detection of DQPSK optical signals is being advanced.

  Further, the 8-level modulation method is an effective method as a modulation method for increasing the multi-level number of optical signals. In particular, the 8-phase phase shift keying modulation method (hereinafter referred to as 8-phase PSK) is considered to be effective as a method that enables simple reception by differential detection.

  Most simply, an 8-phase PSK signal can be generated by applying an 8-level voltage signal to the optical phase modulator. Since the optical phase modulator gives the light a phase shift proportional to the applied voltage, a drive signal having eight voltage levels and equal voltage differences between the levels may be applied to the optical phase modulator. This generation method is a simple method that can be realized by a single modulator. However, there is a problem that a complicated drive signal such as an eight-value voltage signal must be generated, and when the voltage difference between levels of the generated signal varies due to waveform distortion or the like, the optical phase also varies and the quality deteriorates. It is said that it is difficult to use in an ultra-high speed region exceeding several tens of Gbps.

  A non-patent document 1 discloses a method for generating an 8-phase PSK signal for solving this problem. FIG. 10 is an explanatory diagram showing a method of generating an 8-phase PSK signal shown in Non-Patent Document 1. 10 corresponds to FIG. 1 in Non-Patent Document 1. The light incident from the incident end 1010 is branched into two with the same amount of light. One of the branches is subjected to two-phase PSK modulation of phase 0 or π by the first phase modulator 1011. On the other hand, after the binary modulation of the phase 0 or π is also performed by the second phase modulator 1012, a phase shift of 3π / 2 is given by the phase shifter 1013. The two lights to which the phase modulation is applied are combined and output from the output end 1014. At the output end 1014, a signal in which the light phase is in four states of 0, π / 2, π, and 3π / 2 is generated. Further, the phase modulator 1015 modulates the phase of the light so as to shift by 0 or π / 4 according to the input electric signal. As a result, at the output terminal 1016, a PSK signal having eight states of light phases of 0, π / 4, π / 2, 3π / 4, π, 5π / 4, 3π / 2, and 7π / 4 is generated. Can do.

  FIG. 11 is an explanatory diagram showing a constellation display of the PSK signal generated by the above method (a display method in which the real part of the complex optical electric field is on the horizontal axis and the imaginary part is on the vertical axis). The phase modulation given by the first phase modulator 1011 shown in FIG. 10 is shown as phase modulation (specifically, an optical signal state) 1111 in FIG. 11 which is a complex plane display of the optical electric field. The phase modulation given by the second phase modulator 1012 shown in FIG. 10 is shown as phase modulation 1112 in FIG. 11 in consideration of the phase shift of 3π / 2 given by the phase shifter 1013 at the subsequent stage. When these are combined, four signal states (phase states) 1113 are generated. Furthermore, by applying PSK modulation by the phase modulator 1015, four phase states 1114 are added, and an optical 8-phase PSK signal is generated at the output 1016 of the modulator. For example, as shown in Patent Document 1, it is known that two-phase PSK can be performed more easily and accurately by using Mach-Zehnder modulators as the two phase modulators 1011 and 1012 shown in FIG.

  A method different from the modulation method for generating the optical 8-phase PSK signal is disclosed in Patent Document 2. In the system disclosed in Patent Document 2, a binary data signal is D / A converted and output as two sets of parallel binary data signals of m series and n series (m, n ≧ 2). Then, 2m level and 2n level analog signals are generated from the m-sequence and n-sequence parallel binary data signals, respectively. Further, the generated analog signal is supplied as a drive signal to a Mach-Zehnder type amplitude modulation unit that superimposes the in-phase component of amplitude modulation on the optical signal and a Mach-Zehnder type amplitude modulation unit that superimposes the orthogonal component of amplitude modulation on the optical signal. At this time, an arbitrary optical signal can be generated by changing the analog signal to be generated to an appropriate 2m level and 2n level analog signal. That is, an 8-phase PSK signal can be generated. Such a method has the advantage of reducing optical loss because only one modulator is required to generate an 8-phase PSK signal.

Japanese Patent No. 2760856 JP 2007-288591 A

Ly-Gagnon, D.-S .; Katoh, K .; Kikuchi, K.;, “Coherent demodulation of differential 8-phase-shift keying with optical phase diversity and digital signal processing”, Lasers and Electro-Optics Society, 2004 .LEOS 2004. The 17th Annual Meeting of the IEEE, Volume 2, 7-11 Nov. 2004 Page (s): 607-608 Vol.2

  However, the above background art has the following problems. The first problem is that the quality of the generated optical signal easily deteriorates due to the amplitude distortion of the signal that drives the modulator. For example, in the method for generating an optical 8-phase PSK signal disclosed in Patent Document 2, it is necessary that the signals applied to the in-phase component and the quadrature component are 2m level and 2n level analog signals. When the waveform of the analog signal is distorted, the distortion appears as it is in the in-phase component and the quadrature component of the optical signal. As a result, the distance between the multilevel signals is narrowed, and the quality is degraded. In general, the voltage for driving the optical modulator is as high as several volts or more, and it is known that it is difficult to amplify an analog signal having a symbol rate of 10 Gbps without distortion up to this amplitude. Therefore, it becomes difficult to suppress degradation of optical signal quality due to amplitude distortion.

  The second problem is that even when modulation is performed with a binary digital signal using an optical phase modulator, the quality of the generated optical signal is easily degraded due to amplitude distortion of the digital signal. is there. As described above, the first problem is that it is difficult to amplify a multi-value drive signal without distortion, so that the quality may deteriorate. In order to avoid quality degradation, a plurality of optical modulation units driven by binary digital signals may be arranged. In the configuration disclosed in Non-Patent Document 1 shown in FIG. 10, a plurality of light modulation units are arranged. The phase modulator 1015 performs PSK modulation with a phase fluctuation width of π / 4. However, in the optical modulator disclosed in Non-Patent Document 1, the modulation is realized using an optical phase modulator that gives a phase shift proportional to the applied voltage. In a modulator configured to give a phase shift proportional to the applied voltage, a deviation from the voltage giving the phase shift appears as a deviation of the phase shift amount as it is. The shift of the phase shift amount is a direct factor of quality deterioration of the generated optical 8-phase PSK signal. In order to generate a high-quality signal, a high-quality binary digital signal without overshoot or undershoot must be used, but a drive signal without such distortion is generated at a symbol rate exceeding 10 Gbps. It is not easy.

  Therefore, the present invention provides an optical modulator and an optical modulation method capable of generating a high-quality optical 8-phase PSK signal even when driven using a binary signal having some waveform distortion. Objective.

  An optical modulator according to the present invention includes a branching unit that splits incident light into two, a first Mach-Zehnder modulator for modulating the first light branched at the branching unit, and a branching unit. A second Mach-Zehnder modulator for modulating the second light, a phase shifter connected to the second Mach-Zehnder modulator for shifting the phase of the light, output light of the first Mach-Zehnder modulator, and output of the phase shifter A first Mach-Zehnder modulator, a second Mach-Zehnder modulator, and a third Mach-Zehnder modulator connected to the multiplexing unit for modulating the phase of the output light. The modulator and the third Mach-Zehnder modulator are driven with a binary signal, and the phase shifter has a phase difference of (180 × n) between the output light of the first Mach-Zehnder modulator and the output light of the second Mach-Zehnder modulator. (N is an integer +90) voltage becomes the degree is applied, alpha parameter of the third Mach-Zehnder modulator, characterized in that it is +0.25 or -0.25.

  The light modulation method according to the present invention includes a branching process for branching incident light into two, a first modulation process for modulating the first light branched by the branching process by a first Mach-Zehnder modulator, and a branching process. Second modulation processing for modulating the branched second light by the second Mach-Zehnder modulator, phase shift processing for shifting the phase of the light after the second modulation processing, and output light and phase shift by the first modulation processing A third Mach-Zehnder modulator that performs a multiplexing process that combines and outputs the output light by the processing, and a third modulation process that modulates the phase of the output light by the third-phase modulation by a third Mach-Zehnder modulator. Is set to +0.25 or −0.25, and the first Mach-Zehnder modulator, the second Mach-Zehnder modulator, and the third Mach-Zehnder modulator are driven with a binary signal, The phase shift processing is executed using a voltage for setting the phase difference between the output light of the Cach-Zehnder modulator and the output light of the second Mach-Zehnder modulator to (180 × n (n is an integer) +90) degrees. And

  According to the present invention, in an apparatus that generates an 8-phase PSK signal by driving with a plurality of binary signals, a high-quality 8-phase PSK signal can be obtained even if distortion is included in the binary signal.

It is a block diagram which shows the basic composition of the optical modulator by this invention which produces | generates an 8-phase PSK signal. It is explanatory drawing which shows the complex plane display of an optical electric field. 1 is a block diagram showing a first embodiment of an optical modulator according to the present invention. FIG. It is a block diagram which shows 2nd Embodiment of the optical modulator by this invention. It is a block diagram which shows 3rd Embodiment of the optical modulator by this invention. It is explanatory drawing which shows the complex plane display of an optical electric field. It is a block diagram which shows 4th Embodiment of the optical modulator by this invention. It is explanatory drawing which shows the complex plane display of an optical electric field. It is explanatory drawing which shows the complex plane display of an optical electric field. It is explanatory drawing which shows the production | generation method of the 8-phase PSK signal by background art. It is explanatory drawing which shows the complex plane display of an optical electric field.

  FIG. 1 is a block diagram showing a basic configuration of an optical modulator according to the present invention that generates an 8-phase PSK signal. In the present invention, as shown in Patent Document 1, by driving a Mach-Zehnder modulator with a binary signal, even if amplitude distortion occurs in the binary signal, an accurate phase 0 or π two-phase PSK signal is obtained. Using the feature that can generate the signal, three Mach-Zehnder modulators are used. The light incident on the incident end 100 is branched into two at the same intensity by the first branching unit 108. One of them is further branched to the same intensity by the second branching unit 106, and PSK-modulated by the first Mach-Zehnder modulator 112 and the second Mach-Zehnder modulator 113, respectively.

  The first phase shifter 118 has a first phase difference between the output of the first Mach-Zehnder modulator 112 and the output of the second Mach-Zehnder modulator 113 (specifically, the output of the first phase shifter 118). The phase of the output of the second Mach-Zehnder modulator 113 is shifted so that it becomes 90 ° in the optical multiplexer 107. The output of the first Mach-Zehnder modulator 112 and the output of the first phase shifter 118 are combined by the first optical multiplexer 107 and output. As a result, at the output of the first optical multiplexer 107, a four-phase PSK modulated signal having four phase states is generated. When the optical electric field at each bit of the output optical signal at this time is represented by a complex plane display of the optical electric field shown in FIG. 2, the four-phase PSK signal state is one of the four signal states 201, 202, 203, and 204. Become.

  The other light branched by the first branching unit 108 is two-phase PSK modulated by the third Mach-Zehnder modulator 114. The second phase shifter 119 has a phase difference between the output of the third Mach-Zehnder modulator 114 (specifically, the output of the second phase shifter 119) and the output of the first optical multiplexer 107. The phase of the output of the third Mach-Zehnder modulator 114 is shifted so as to be 45 °.

  The four-phase PSK signal and the two-phase PSK signal generated as described above are combined by the second optical multiplexer 109 and output from the output end 105. As described above, the optical phase relationship between the four-phase PSK signal and the two-phase PSK signal is set to a relative angle of 45 ° as shown in FIG. In FIG. 2, signal states 201-204 indicate four-phase PSK signal states, and signal states 211, 212 indicate two-phase PSK signal states. By combining two signals, eight optical phase states 221, 222, 223, 224, 225, 226, 227, and 228 shown in FIG. 2 are generated. Specifically, each optical phase state is generated in the following combinations.

Optical phase 0 ° (optical phase state 221): Synthesis of four-phase PSK signal state 201 and two-phase PSK signal state 211 Optical phase 45 ° (optical phase state 222): Four-phase PSK signal state 202 and two-phase PSK signal state 211 Optical phase 90 ° (optical phase state 223): synthesis of four-phase PSK signal state 201 and two-phase PSK signal state 212 optical phase 135 ° (optical phase state 224): four-phase PSK signal state 202 and two-phase PSK signal Combined state 212 Optical phase 180 ° (optical phase state 225): Combined 4-phase PSK signal state 203 and 2-phase PSK signal state 212 Optical phase 225 ° (optical phase state 226): 4-phase PSK signal state 204 and 2-phase Synthesis of PSK signal state 212 Optical phase 270 ° (optical phase state 227): Synthesis of 4-phase PSK signal state 203 and 2-phase PSK signal state 211 Optical phase 315 ° (optical phase state) 228): Synthesis of 4-phase PSK signal state 204 and 2-phase PSK signal state 211

  Note that the combined optical phase is an angle display of the phase angle of the vector representing the optical phase states 221 to 228 when the vector representing the optical phase state 221 is used as a reference. Specifically, it is a value obtained by subtracting 22.5 ° from the phase of each vector shown in FIG.

  An 8-phase PSK signal is generated by the above processing. The accuracy of the generated phase is determined by the quality of the PSK signal generated by each Mach-Zehnder modulator, the relative intensity when they are combined, and the phase. The relative intensity and phase at the time of multiplexing can be determined with high accuracy by appropriately controlling the branching ratio accuracy of the optical branching unit and the optical multiplexer, and the phase shifter. Even when distortion occurs, the phase accuracy of the two-phase PSK signal generated by the Mach-Zehnder modulator can be prevented from being affected. However, when the degree of distortion increases, there is a possibility that distortion in intensity change of the output signal occurs. However, due to the effect of the non-linear intensity modulation characteristic of the Mach-Zehnder modulator, the amount of intensity change is small, and the phase error caused by the intensity change is significantly smaller than when using a phase modulator according to the background art. That is, according to the present invention, a high-quality 8-phase PSK signal can be obtained even when waveform distortion is included in the drive signal.

Embodiment 1. FIG.
Next, embodiments of the present invention will be described with reference to the drawings.
FIG. 3 is a block diagram showing a first embodiment of an optical modulator according to the present invention. The optical modulator is formed on a lithium niobate (LiNbO ₃ ) crystal substrate (lithium niobate substrate) 300. The light incident from the incident fiber 301 (hereinafter referred to as the incident end 301) is coupled to the waveguide of the lithium niobate substrate 300 and branched into two by the first Y branch circuit 302. The branching ratio is 1: 1.

  The first output of the first Y branch circuit 302 is further branched into two by the second Y branch circuit 303. The branching ratio is 1: 1. The first output of the second Y branch circuit 303 is connected to the input of the first Mach-Zehnder modulator 304. The first Mach-Zehnder modulator 304 modulates input light. The second output of the second Y branch circuit 303 is connected to the input of the second Mach-Zehnder modulator 305. The second Mach-Zehnder modulator 305 modulates the input light. The output of the second Mach-Zehnder modulator 305 is connected to a phase shifter (first phase shifter) 308.

  An appropriate static voltage is applied to the phase shifter 308 so that the phase difference of the phases of the outputs of the two Mach-Zehnder modulators 304 and 305 in the first multiplexing circuit 309 is 90 °. The output of the first Mach-Zehnder modulator 304 and the output of the phase shifter 308 are connected to two inputs of the first multiplexing circuit 309 on the lithium niobate substrate 300, respectively. The first multiplexing circuit 309 multiplexes two input lights and outputs them.

  The second output of the first Y branch circuit 302 is connected to the input of the third Mach-Zehnder modulator 311. The third Mach-Zehnder modulator 311 modulates input light. The output of the third Mach-Zehnder modulator 311 is connected to a phase shifter (second phase shifter) 313. The phase shifter 313 is appropriately configured so that the phase difference between the output of the third Mach-Zehnder modulator 311 (specifically, the output of the phase shifter 313) and the output of the first multiplexing circuit 309 is 45 °. A static voltage is applied. The output of the first multiplexing circuit 309 and the output of the phase shifter 313 are connected to two inputs of the second multiplexing circuit 315 on the lithium niobate substrate 300, respectively. The second multiplexing circuit 315 multiplexes two input lights and outputs them from an output fiber (output end) 316.

  The encoder 350 appropriately separates a 3-bit input binary signal and outputs three binary signals. Three outputs of the encoder 350 are connected to a first amplifier 3511, a second amplifier 3512, and a third amplifier 3513. The first amplifier 3511, the second amplifier 3512, and the third amplifier 3513 amplify the output of the encoder 350. The output signals of the amplifiers 3511, 3512, and 3513 drive the first Mach-Zehnder modulator 304, the second Mach-Zehnder modulator 305, and the third Mach-Zehnder modulator 311.

  The first Mach-Zehnder modulator 304, the second Mach-Zehnder modulator 305, and the third Mach-Zehnder modulator 311 include DC bias ports 306, 307, and 312. A voltage is applied to the DC bias ports 306, 307, and 312 so that the input voltage from the first amplifier 3511, the second amplifier 3512, and the third amplifier 3513 is extinguished when the input voltage is zero.

  Next, the operation of the optical modulator shown in FIG. 3 will be described. The light input from the incident end 301 is split into two equal amounts of light by the first Y branch circuit 302. One light from the first Y branch circuit 302 is further branched by the second Y branch circuit 303. The first Mach-Zehnder modulator 304 modulates one light from the second Y branch circuit 303. The second Mach-Zehnder modulator 305 modulates the other light from the second Y branch circuit 303. The phase shifter 308 is configured so that one of the output light from the first Mach-Zehnder modulator 304 and the output light from the second Mach-Zehnder modulator 305 (specifically, the output light from the phase shifter 308) is relative to the other. The phase of the output light of the second Mach-Zehnder modulator 305 is adjusted so that the phase becomes 90 ° in the second multiplexing circuit 315. At this time, to the first Mach-Zehnder modulator 304, the drive signal obtained by amplifying the first signal from the encoder 350 by the amplifier 3511 to the peak-to-peak amplitude 2Vπ is input. The second Mach-Zehnder modulator 305 is supplied with a drive signal obtained by the amplifier 3512 amplifying the second signal from the encoder 350 to a peak-to-peak amplitude of 2Vπ. The DC bias ports 306 and 307 are applied with a voltage that is extinguished when the drive signals from the amplifiers 3511 and 3512 are zero. As a result, two-phase PSK signals corresponding to the respective drive signals are obtained as outputs of the first Mach-Zehnder modulator 304 and the second Mach-Zehnder modulator 305. Therefore, a 4-phase PSK signal is obtained at the output of the first multiplexing circuit 309.

  The other light from the first Y branch circuit 302 is modulated by the third Mach-Zehnder modulator 311. At this time, the third Mach-Zehnder modulator 311 receives the drive signal obtained by the amplifier 3513 amplifying the third signal from the encoder 350 to the peak-to-peak amplitude of 2Vπ. A voltage that is extinguished when the drive signal from the amplifier 3513 is 0 is applied to the DC bias port 312. As a result, a two-phase PSK signal corresponding to the drive signal is obtained as an output of the third Mach-Zehnder modulator 311. The two-phase PSK signal and the four-phase PSK signal from the first multiplexing circuit 309 are multiplexed by the second multiplexing circuit 515. At this time, the voltage applied to the phase shifter (second phase shifter) 313 is adjusted so that the phase difference between the two input lights in the second multiplexing circuit 515 is 45 °. As a result, the light combined by the second combining circuit 515 becomes a four-phase PSK signal having eight phase states.

  The voltage applied to the phase shifter (first phase shifter) 308 is not limited to a voltage that makes the phase difference 90 ° in the first multiplexing circuit 309. In the first multiplexing circuit 309, even when a voltage that makes the phase difference (180 × n (n is an integer other than 0) +90) ° is applied, a voltage that makes the phase difference 90 ° is applied. The same effect is obtained. Further, the phase shifter 313 is not limited to the one that makes the phase difference between the two lights 45 ° in the second multiplexing circuit 315. Even if the phase difference between the two lights in the second multiplexing circuit 315 is any phase difference of (90 × m (m is an integer other than 0) +45) °, the phase difference is set to 45 °. The same effect can be obtained.

Embodiment 2. FIG.
In the optical modulator of the first embodiment shown in FIG. 3, a method of generating an 8-phase PSK signal by only three Mach-Zehnder modulators is used. However, as can be seen from the optical phase states 221, 222, 223, 224, 225, 226, 227, and 228 shown in FIG. The output intensity is different. Such an intensity difference between symbols may cause deterioration of reception sensitivity due to the difference between symbols and waveform distortion during optical transmission.

  Therefore, in the second embodiment, a configuration for suppressing the intensity difference between symbols is added. FIG. 4 is a block diagram showing a second embodiment of the optical modulator according to the present invention. In the second embodiment shown in FIG. 4, the encoder 400 is provided with a fourth output as compared with the first embodiment shown in FIG. 3. The binary signal as the fourth output drives the intensity modulator (light intensity modulator) 401 via the amplifier 3515. The intensity modulator 401 modulates the intensity of the output light from the second multiplexing circuit 315. Specifically, when the symbol state is the optical phase state 221, 224, 225, 228 shown in FIG. In the case of the optical phase states 222, 223, 226, and 227, the transmission intensity is increased. By such an operation of the intensity modulator 401, the intensity difference between symbols can be minimized. In the second embodiment, an intensity modulator 401 or the like is added to the drive system of the optical modulator to complicate the configuration compared to the first embodiment, but the intensity between symbols is uniform and the characteristics are improved. Can be made.

  In the second embodiment, the intensity modulator 401 is used to suppress the intensity variation. However, any means other than the intensity modulator 401 may be used as long as the intensity of light can be controlled. For example, It is also possible to use an optical amplifier whose amplification factor is variable by a binary electric signal.

Embodiment 3. FIG.
FIG. 5 is a block diagram showing a third embodiment of the optical modulator according to the present invention. Similarly to the second embodiment, the third embodiment also has a configuration for suppressing the intensity difference between symbols. In the third embodiment, a third Mach-Zehnder modulator 502 and a fourth Mach-Zehnder modulator 503 are used in place of the third Mach-Zehnder modulator 311 in the first embodiment shown in FIG.

  The second output light divided into two equal amounts of light by the first Y branch circuit 302 is further branched by the third Y branch circuit 501. The third Mach-Zehnder modulator 502 modulates one light from the third Y branch circuit 501. The fourth Mach-Zehnder modulator 403 modulates the other light from the third Y branch circuit 501. A phase shifter (second phase shifter) 506 includes output light from the third Mach-Zehnder modulator 502 and output light from the fourth Mach-Zehnder modulator 503 (specifically, output light from the phase shifter 506). The other relative phase with respect to one phase is connected to the third combining circuit 507 (the second combining unit that combines and outputs the output light of the third Mach-Zehnder modulator and the output light of the second phase shifter). The phase of the output light of the fourth Mach-Zehnder modulator 503 is adjusted so that it becomes 90 °. That is, an appropriate voltage is applied to the phase shifter 506 so that the phase difference between the output light from the third Mach-Zehnder modulator 502 and the output light from the fourth Mach-Zehnder modulator 503 is 90 °.

  Further, the third Mach-Zehnder modulator 502 receives a drive signal obtained by amplifying the third signal from the encoder 500 by the amplifier 3513 to the peak-to-peak amplitude of 2Vπ. The fourth Mach-Zehnder modulator 503 receives a drive signal obtained by amplifying the fourth signal from the encoder 500 by the amplifier 3514 to the peak-to-peak amplitude of 2Vπ. The DC bias ports 504 and 505 are applied with a voltage that is extinguished when the drive signals from the amplifiers 3513 and 3514 are zero. As a result, as the outputs of the third Mach-Zehnder modulator 502 and the fourth Mach-Zehnder modulator 503, two-phase PSK signals corresponding to the drive signals are obtained. Therefore, a four-phase PSK signal is obtained as the output of the third multiplexing circuit 507.

  The four-phase PSK signal from the first multiplexing circuit 309 and the two-phase PSK signal from the third multiplexing circuit 507 are converted into the second combining circuit 315 (the output light of the first combining unit and the third combining signal). The output light of the phase shifter is combined and corresponds to a third combining unit that outputs the combined light. At this time, the relative relationship between the optical phases of the four-phase PSK signal and the two-phase PSK signal input to the second multiplexing circuit 315 is set to 45 °. For this purpose, an appropriate voltage (voltage for setting the relative phase relationship to 45 °) is applied to the third phase shifter 508 connected to the output of the third multiplexing circuit 507. Specifically, as shown in FIG. 6, the signal from the first multiplexing circuit 309 and the signal from the third multiplexing circuit 507 are combined with an angle difference of 45 °. The phase shift amount of the third phase shifter 508 is adjusted. That is, in the third phase shifter 50, the phase difference between the output light of the first multiplexing circuit 309 and the output light of the third multiplexing circuit 507 is 45 ° in the second multiplexing circuit 315. In addition, an appropriate voltage is applied.

  In the complex plane display of the optical electric field shown in FIG. 6, signal states 601 to 604 indicate the four-phase PSK signal states of the four-phase PSK signal from the first multiplexing circuit 309. Signal states 611 to 614 indicate the four-phase PSK signal states of the four-phase PSK signal from the third multiplexing circuit 507.

  When the phase shift amount of the third phase shifter 508 is adjusted and multiplexed by the second multiplexing circuit 315, the eight optical phase states 221, 222, 223, 224, 225, 226 shown in FIG. 227 and 228 are generated. Specifically, each optical phase state is generated in the following combinations. That is, an 8-phase PSK signal is obtained by the following combination.

Optical phase 0 ° (optical phase state 621): Combined 4-phase PSK signal state 601 and 4-phase PSK signal 611 Optical phase 45 ° (Optical phase state 622): Combined 4-phase PSK signal state 601 and 4-phase PSK signal 612 Optical phase 90 ° (Optical phase state 623): Combined 4-phase PSK signal state 602 and 4-phase PSK signal 612 Optical phase 135 ° (Optical phase state 624): Combined 4-phase PSK signal state 602 and 4-phase PSK signal 613 Optical phase 180 ° (optical phase state 625): combined 4-phase PSK signal state 603 and 4-phase PSK signal 613 Optical phase 225 ° (optical phase state 626): combined 4-phase PSK signal state 603 and 4-phase PSK signal 614 Optical phase 270 ° (optical phase state 627): 4-phase PSK signal state 604 and 4-phase PSK signal 614 combined optical phase 315 ° (optical phase state 628): 4-phase PSK signal state 6 4 Synthesis of 4-phase PSK signal 611

  Note that the voltage applied to the phase shifter 506 is not limited to a voltage that makes the phase difference 90 ° in the third multiplexing circuit 507. When a voltage that makes the phase difference 45 ° is applied to the third multiplexing circuit 507, even if the phase difference is (180 × m (m is an integer other than 0) +90) °. The same effect is obtained. In addition, the voltage applied to the third phase shifter 508 is not limited to a voltage that makes the phase difference 45 ° in the second multiplexing circuit 315. When a voltage that makes the phase difference 45 ° is applied to the second multiplexing circuit 315, even if the phase difference is any of (90 × k (k is an integer other than 0) +45) °. The same effect is obtained.

  The encoder 500 having four outputs has a function of generating a binary signal that realizes the above combination for each of the Mach-Zehnder modulators 304, 305, 502, and 503. Also, in the first embodiment, three Mach-Zehnder modulators are used, but in the third embodiment, a total of four Mach-Zehnder modulators are used to reduce the intensity variation between symbols. It is small.

Embodiment 4 FIG.
In the third embodiment, the intensity variation between symbols is reduced by using four Mach-Zehnder modulators. However, when four Mach-Zehnder modulators are used, the optical modulator can be reduced in size. There is a possibility that problems may arise in terms of optimization and performance stabilization.

  Therefore, in the fourth embodiment, a configuration is used in which three Mach-Zehnder modulators are used to generate an 8-phase PSK signal with less intensity variation between symbols. FIG. 7 is a block diagram showing a fourth embodiment of the optical modulator according to the present invention.

  In the fourth embodiment, a Mach-Zehnder modulator 701 that generates a predetermined chirp is used instead of a Mach-Zehnder modulator that does not generate a chirp (a phase modulation component generated along with intensity modulation) used for PSK modulation. Specifically, a Mach-Zehnder modulator 701 having an α parameter of 0.25 is used. The configuration on the incident end 301 side of the Mach-Zehnder modulator 701 is the same as the configuration from the first Y branch circuit 303 to the first multiplexing circuit 309 shown in FIG. When the Mach-Zehnder modulator 701 is driven by a binary signal with an amplitude of 2Vπ, a signal state 800 having a phase of 0 ° and a signal state 801 having a phase of 135 ° are shown in the complex plane display of the optical electric field shown in FIG. A PSK signal that takes two states is generated. In order to utilize this characteristic, the phase modulator 1015 shown in FIG. 10 is replaced with a Mach-Zehnder modulator 701.

  The light input from the incident end 301 is divided into two equal amounts of light by the first Y branch circuit 303. One light is modulated by the first Mach-Zehnder modulator 304. The other light is modulated by the second Mach-Zehnder modulator 305. Then, the phase shifter 308 is the other of the output light from the first Mach-Zehnder modulator 304 and the output light from the second Mach-Zehnder modulator 305 (specifically, the output light from the phase shifter 308). The phase of the output light of the second Mach-Zehnder modulator 305 is adjusted so that the relative phase of the second Mach-Zehnder modulator 305 becomes 90 ° in the first multiplexing circuit 309.

  At this time, the first Mach-Zehnder modulator 304 is supplied with a drive signal obtained by amplifying the first signal from the encoder 700 to an amplitude between peaks of 2Vπ by the amplifier 3511. Further, the second Mach-Zehnder modulator 305 receives a drive signal obtained by amplifying the second signal from the encoder 700 by the amplifier 3512 so that the peak-to-peak amplitude is 2Vπ. The DC bias ports 306 and 307 are applied with a voltage that is extinguished when the drive signals from the amplifiers 3511 and 3512 are zero. As a result, as the outputs of the first Mach-Zehnder modulator 304 and the second Mach-Zehnder modulator 305, two-phase PSK signals corresponding to the drive signals are obtained. Then, a 4-phase PSK signal is obtained as an output of the first multiplexing circuit 309.

  FIG. 9 is an explanatory diagram showing a complex plane display of the optical electric field representing the signal states 901, 902, 903, and 904 of the output of the first multiplexing circuit 309.

  When the output of the first multiplexing circuit 309 is modulated by an α = 0.25 Mach-Zehnder modulator 701 driven by a binary signal obtained by amplifying the output of the encoder 700, as shown in FIG. The signal is modulated into a signal state 911 according to the input. Note that the Mach-Zehnder modulator 701 receives a drive signal obtained by amplifying the third signal from the encoder 700 by the amplifier 3513 so that the peak-to-peak amplitude is 2Vπ. In addition, a voltage that is extinguished when the drive signal from the amplifier 3513 is 0 is applied to the DC bias port 702. A four-phase state is generated by combining the signal states 912, 913, and 914 generated from the signal states 902, 903, and 904 with the same modulation method, and an eight-phase PSK signal having a total of eight signal states is generated.

  In the modulation scheme of the fourth embodiment, as can be understood from FIGS. 8 and 9, in the two-phase PSK modulation by the Mach-Zehnder modulator 701, the locus of phase change is not linear, and the intensity change Thus, a phase state (signal state) deviated from 0 ° or 135 ° is generated. As a result, the phase quality of the generated signal is more likely to deteriorate than in the first to third embodiments with respect to distortion included in the drive waveform of the modulator. However, since the optical modulator includes only three Mach-Zehnder modulators 304, 305, and 701, when modulated with a binary signal without distortion, there is an advantage that a signal having no intensity variation between symbols is generated. is there.

  In the fourth embodiment, a case where two-phase PSK modulation of 0 ° and 135 ° is obtained using the Mach-Zehnder modulator 701 having an α parameter of 0.25 is shown, but the α parameter is −0.25. Even if a certain Mach-Zehnder modulator is used, two-phase PSK modulation of 0 ° and −135 ° is obtained, and an equivalent effect is obtained.

DESCRIPTION OF SYMBOLS 100 Incidence end 105 Outlet end 106 2nd branch part 107 1st optical multiplexer 108 1st branch part 109 2nd optical multiplexer 112 1st Mach-Zehnder modulator 113 2nd Mach-Zehnder modulator 114 3rd Mach-Zehnder modulator 118 First phase shifter 119 Second phase shifter 201, 202, 203, 204 Signal state (four-phase PSK signal state)
211,212 Signal state (two-phase PSK signal state)
221, 222, 223, 224, 225, 226, 227, 228 Optical phase state 300 Lithium niobate substrate 301 Incident fiber (incident end)
302 First Y branch circuit 303 Second Y branch circuit 304 First Mach-Zehnder modulator 305 Second Mach-Zehnder modulator 306 DC bias port 307 DC bias port 308 Phase shifter (first phase shifter)
309 First multiplexer 311 Third Mach-Zehnder modulator 312 DC bias port 313 Phase shifter (second phase shifter)
315 Second multiplexing circuit 316 Output fiber (output end)
350, 400, 500, 700 Encoder 3511, 3512, 3513, 3514, 3515 Amplifier 401 Intensity modulator 501 Third Y branch circuit 502 Third Mach-Zehnder modulator 503 Fourth Mach-Zehnder modulator 504 DC bias port 505 DC Bias port 506 Phase shifter 507 Third multiplexing circuit 508 Third phase shifter 601-604 Signal state (four-phase PSK signal state)
611-614 Signal state (4-phase PSK signal state)
621, 622, 623, 624, 625, 626, 627, 628 Optical phase state 701 Mach-Zehnder modulator (Mach-Zehnder modulator producing chirp)
702 DC bias port 800, 801 Signal state 901, 902, 903, 904, 911, 912, 913, 914 Signal state 1010 Incident end 1011 First phase modulator 1012 Second phase modulator 1013 Phase shifter 1013 Emission end 1015 Phase modulator 1111, 1112 Phase modulation (optical signal state)
1113, 1114 Signal state (phase state)

Claims (2)

A branching portion for splitting the incident light into two;
A first Mach-Zehnder modulator for modulating the first light branched by the branch unit;
A second Mach-Zehnder modulator for modulating the second light branched by the branching unit;
A phase shifter connected to the second Mach-Zehnder modulator for shifting the phase of light;
A multiplexing unit that multiplexes and outputs the output light of the first Mach-Zehnder modulator and the output light of the phase shifter;
A third Mach-Zehnder modulator connected to the multiplexing unit for modulating the phase of the output light;
The first Mach-Zehnder modulator, the second Mach-Zehnder modulator, and the third Mach-Zehnder modulator are driven by a binary signal;
A voltage is applied to the phase shifter so that the phase difference between the output light of the first Mach-Zehnder modulator and the output light of the second Mach-Zehnder modulator is (180 × n (n is an integer) +90) degrees. ,
The α parameter of the third Mach-Zehnder modulator is +0.25 or −0.25. A branching process for splitting the incident light into two;
A first modulation process for modulating the first light branched by the branch process by a first Mach-Zehnder modulator;
A second modulation process for modulating the second light branched by the branch process by a second Mach-Zehnder modulator;
A phase shift process for shifting the phase of the light after the second modulation process;
A multiplexing process for combining and outputting the output light from the first modulation process and the output light from the phase shift process;
Performing a third modulation process for modulating the phase of the output light by the multiplexing process by a third Mach-Zehnder modulator;
The α parameter of the third Mach-Zehnder modulator is set to +0.25 or −0.25;
Driving the first Mach-Zehnder modulator, the second Mach-Zehnder modulator, and the third Mach-Zehnder modulator with a binary signal;
The phase shift processing using a voltage for setting the phase difference between the output light of the first Mach-Zehnder modulator and the output light of the second Mach-Zehnder modulator to (180 × n (n is an integer) +90) degrees. An optical modulation method characterized in that

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