JP2006251570A - Optical transmitter and phase modulating method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a high-performance RZ-DPSK signal waveform by using a small-sized Mach-Zehnder modulator with a low driving voltage while munimizing waveform distortion of a generated optical signal even when an electric circuit has nonlinear amplification characteristics. <P>SOLUTION: This optical transmitter has an electronic circuit which generates 1st and 2nd RZ pulse trains made to correspond to 1st and 2nd values of transmitted data, and further has the Mach-Zehnder modulator which is equipped with 1st and 2nd modulation sections which have two arms branching and guiding input light and are at physically different positions on the two arms and modulates the input light by the 1st modulation section with the 1st RZ pulse train and by the 2nd modulation section with the 2nd RZ pulse train and wherein the 1st and 2nd modulation sections apply the RZ pulse trains to respectively different arms. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an optical transmitter suitable for long-distance high-speed optical communication and a phase modulation method suitable for application to the optical transmitter.

  The optical fiber communication system has become an important technology for realizing long-distance and large-capacity communication. In the optical fiber communication system currently commercialized, the intensity modulation method is used. In the intensity modulation method, for example, digital signals “1” and “0” are assigned to “present” and “not present” light pulses, respectively, and transmission is performed. The intensity modulation system has the features that it is easy to generate and detect a modulated signal and a modulated signal and enables long-distance transmission.

  On the other hand, with an increase in capacity of information transmission in recent years, higher speed is required for optical fiber communication. Although the data transfer rate currently in practical use is 10 Gbps at the maximum, research and development with a data transfer rate of 40 Gbps is being advanced for use in next-generation optical fiber communications. Furthermore, there is a strong demand for cost reduction by extending the transmission distance. For example, research on optical transmission technology over a distance exceeding 1000 km is underway.

  There are two problems in broadly classifying the realization of such high-speed and long-distance optical fiber communications.

  The first problem is countermeasures against optical noise accumulation. In the intensity modulation method, when the transmission speed is increased, the signal band to be used increases, and the amount of noise received by the system also increases. As a result, since the value of the signal-to-noise ratio at the receiving end becomes small, code errors increase and quality degradation occurs. In addition, as the transmission distance becomes longer, it is necessary to increase the number of optical amplifying repeaters for loss compensation, and the signal-to-noise ratio at the receiving end also deteriorates due to the accumulation of optical noise generated in the repeater for optical amplification. .

  Therefore, in order to realize high speed and long distance, it is necessary to reduce the accumulation of optical noise or develop a transmission system that is strong against optical noise accumulation. In recent years, in order to solve such a problem of optical noise accumulation, a phase modulation method, particularly a DPSK (Differential Phase Shift Keying) method has been attracting attention. The DPSK system is a system in which the phase of light is changed by 180 degrees when signal values of adjacent bit slots are different in order to transmit “1” and “0” of a digital signal. In particular, a system in which the DPSK method is combined with a 1-bit delay detection receiving method is attracting attention because of its advantages of high performance and simple configuration.

  At the transmission end of this system, with respect to the transmission data, the phase of the light in the bit slot is changed by 180 degrees when “1”, and the optical phase remains unchanged when “0”. At the receiving end, the received optical signal is branched, and after placing a 1-bit delay element on one of the branches, the two optical signals are caused to interfere. As a result, if the phase of the optical signal in a certain bit slot is the same as that of the optical signal one bit slot before, the intensity of the optical signal in that bit slot is maximized, and the light is extinguished when the phase difference reaches 180 degrees. . Using this principle, information applied to the phase change is converted into intensity information and received.

  By using the DPSK system, it is possible to perform communication with fewer errors even in a reception state with a signal-to-noise ratio lower than that of the intensity modulation system. The reason is as follows.

  FIG. 8A shows the arrangement in the complex electric field plane with the logical values “1” and “0” in the intensity modulation system, and FIG. 8B shows the arrangement in the DPSK system. From the figure, the distance between the logical values “1” and “0” is double in the DPSK method compared to the intensity modulation method. Since the DPSK scheme has such an arrangement relationship, the amount of noise is doubled, that is, the signal-to-noise ratio may be halved in order to obtain the same code error rate as that of the intensity modulation scheme. . Thus, the DPSK system is a transmission system that is resistant to noise, and is a transmission system that is suitable for increasing the speed and distance of optical fiber communication.

  The second problem is countermeasures against distortion of the optical waveform. In optical fiber communication, the main factor that distorts the optical waveform is the nonlinear optical effect of the optical fiber. In the case of the intensity modulation method, it is known that the distortion due to this effect increases as the transmission speed increases, and it is also known that the waveform distortion due to the nonlinear optical effect accumulates even in long-distance transmission. ing. Therefore, in order to realize high speed and long distance, it is necessary to use an optical fiber having a small nonlinear optical effect or to use a transmission system strong against the nonlinear optical effect.

  In order to deal with this problem, Patent Document 1 proposes a method in which each bit of a DPSK modulated signal is transmitted in the form of RZ (Return-to-Zero) pulses. This method is called the RZ-DPSK method. The RZ-DPSK method suppresses waveform distortion from two aspects by converting the signal of each bit of the DPSK signal into an RZ pulse. One is that, by using RZ, the peak light intensity can be made higher than the same average light intensity, and as a result, a signal-to-noise ratio can be obtained, so that transmission with lower power can be achieved. The other effect is that the pulse interference between bits can be suppressed by using the RZ type. As a result, the RZ-DPSK system has been rapidly recognized in recent years as a system that enables ultra-long distance transmission in 40 Gbps transmission.

  By the way, in the RZ-DPSK system, as shown in Non-Patent Document 1, two modulators are used for signal generation at the transmission end. The configuration of the transmitter disclosed in Non-Patent Document 1 is shown in FIG.

  The first Mach-Zehnder modulator 10 is an RZ pulse, and modulates the clock signal by amplifying it to a predetermined voltage by the driver 12. The second Mach-Zehnder modulator 14 performs DPSK modulation. The driver 16 amplifies the transmission data by amplifying it to twice the half-wave voltage of the Mach-Zehnder modulator. Here, the half-wave voltage of the Mach-Zehnder modulator is the half-wavelength of the optical pulse output from the modulator (the interval from the pulse value “1” to the next “0”) or “0” to the next “ The voltage required to obtain a predetermined light intensity in the section "1"). By the way, in order to improve the quality of the RZ-DPSK signal, there is no distortion in the RZ pulse shape, and the extra phase change (chirping) in 180 degree phase shift in DPSK modulation is extremely small. Desired. For this reason, a Mach-Zehnder modulator that does not generate chirp is suitable for the two modulators 10 and 14.

  The mechanism of DPSK signal generation using a Mach-Zehnder modulator will be described with reference to FIG. As the Mach-Zehnder modulator, an X-cut type or push-pull drive type is used to generate a signal without extra chirp. In a chirp-free Mach-Zehnder modulator, the phase of light is shifted by 180 degrees between adjacent signals of different values. Using this effect, the Mach-Zehnder modulator drives (modulates) transmission data amplified to a predetermined amplitude by matching the center value 18 of the signal amplitude with the bottom portion of the extinction curve 20 of the Mach-Zehnder modulator. ).

  As a result, the phase of the output light 22 becomes π and 0 according to the HIGH / LOW of the input data. At this time, the phase is digitally switched with the bottom of the extinction curve 20 as a boundary, and a high-quality DPSK signal without a transient phase change is generated.

  Although this RZ-DPSK signal generation method has an advantage that a high-quality signal can be obtained, Non-Patent Document 1 performs enlargement by using two Mach-Zehnder modulators and performs DPSK modulation and chirp-free RZ modulation. For this reason, there is a problem that the drive amplitude is very large.

  On the other hand, Non-Patent Document 2 shows a method of generating an RZ-DPSK signal by one Mach-Zehnder modulator. With reference to FIG. 11, the RZ-DPSK signal generation method in Non-Patent Document 2 will be described.

  11 has two differential amplifiers 30 and 32 in the electric input section. Transmission data and a clock signal are input to one differential amplifier 30, and inverted transmission data and a clock signal are input to the other differential amplifier 32, from which a differential signal is output. The output of the second differential amplifier 32 is inverted by the inversion circuit 34. The output of the inverting circuit 34 and the output of the first differential amplifier 30 are combined by the power combiner 36, amplified to a predetermined voltage by the driver 38, and input to the Mach-Zehnder modulator 40.

In the optical transmitter shown in FIG. 11, the transmission data and the clock signal are differentially synthesized by the two differential amplifiers 30 and 32 and the power combiner 36, whereby the transmission data is converted into RZ pulses. Further, the output of the first differential amplifier 30 drives the light emitting part on the right side of the extinction curve 20 in FIG. 10 to generate light of “phase π”, whereas the output of the second differential amplifier 32. Has a role of generating “phase 0” light. By combining these two signals in an electrical stage in advance, an RZ-converted DPSK signal can be generated by a single modulator. Since only one modulator is required, there is an advantage that the configuration can be reduced in size.
Japanese Patent Laid-Open No. 2003-60580 AH Gnauck, S. Chandrasekhar, J. Leuthold, L. Stulz, “Demonstration of 42.7-Gb / sDPSK Receiver With 45 Photons / Bit Sensitivity”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 15, NO. 1, pp.99-101 , JANUARY 2003 ： Xiang Liu, Yuan-Hua Kao, “GENERATION OF RZ-DPSK USING A SINGLE MACH-ZEHNDER MODULATOR AND NOVELDRIVER ELECTRONICS”, 30th European Conference on optical Communications (ECOC 2004), paper We 3.4.2

  However, the RZ-DPSK signal generation method shown in the prior art has several problems.

  The first problem is that it is difficult to keep the quality of the transmitted optical signal high. The reason is that in the electronic circuit of Non-Patent Document 2 shown in FIG. 11, the nonlinear amplification characteristics in the power combiner 36 and the driver 38 have an adverse effect. The power combiner 36 that synthesizes the pulse on the phase 0 side and the pulse on the phase π side needs to maintain the pulse shape, and therefore must perform an analog amplification operation. The waveform quality of this analog amplification operation (analog addition) is greatly influenced by the linear characteristics of the circuit.

  For example, assuming that the linear characteristic is not good, an addition characteristic curve 50 as shown in FIG. 12 is assumed. In the waveform after the addition processing, even if the phase 0 side pulse 52 and the phase π side pulse 54 are input with the same height, the waveform 56 has a different pulse height after the addition.

  The waveform when the waveform 56 of FIG. 12 is input to the Mach-Zehnder modulator 40 of FIG. 11 is as shown in FIG. FIG. 13A shows a case where the setting is such that the extinction of the pulse is high. In this case, the operating point is set so that the boundary between the phase 0 side pulse and the phase π side pulse coincides with the bottom of the extinction curve 20. In such a setting, since the voltage that gives the peak of light emission differs between the phase 0 side and the phase π side, the height of the generated modulation voltage pulse is different. This waveform distortion causes quality degradation.

  On the other hand, FIG. 13 (b) shows a case where the emission peak of the pulse is set to be aligned with the phase 0 side pulse and the phase π side pulse. In this case, the boundary between the phase 0 side pulse and the phase π side pulse is shifted from the bottom of the extinction characteristic. As a result, the waveform is not quenched between RZ-DPSK pulses, resulting in transmission quality degradation.

  As described above, if the waveform is asymmetrical due to the nonlinear characteristics of the power combiner 36, it is difficult to generate a high-quality RZ-DPSK optical signal.

  Deterioration of the quality of the generated signal due to circuit nonlinear characteristics becomes more serious in the driver 38. At present, a lithium niobate optical modulator widely used as a Mach-Zehnder optical modulator requires a voltage of several volts, particularly at a high speed of about 40 Gbps. When a drive signal of several volts is generated, it is very difficult to perform in the linear amplification region of the current transistor, so that the amplification characteristic tends to be nonlinear. For this reason, non-linear distortion occurs in the drive signal described above, resulting in quality degradation of the generated signal.

  The second problem is that the power consumption becomes high. The reason is that, in the prior art, since all the signals for driving the optical modulator must be synthesized in the electric circuit stage, the circuit scale increases. Furthermore, as pointed out in the first problem, since the final stage analog adder circuit and the drive circuit require high linearity, it is necessary to design a circuit using a linear portion of a transistor having a large capacity. . As a result, the current and voltage required for driving increase, which leads to an increase in power consumption.

  Therefore, an object of the present invention is to suppress the problems of deterioration of waveform quality and increase in power consumption, which are problems in the generation of RZ-DPSK signals, and realize generation of high-performance signals with high waveform quality, and an optical modulator. It is to provide a reduction in the driving voltage and miniaturization by integration of the light modulation section.

  According to the embodiment of the first optical transmitter of the present invention, as described in claim 1, the first and second RZs corresponding to the first and second logical values of the transmission data are described. An electronic circuit for generating a pulse train, and two arms for branching and guiding input light, and first and second modulators having different physical positions on the two arms, the first modulator The input light is modulated by the first RZ pulse train in the second modulation unit, the input light is modulated by the second RZ pulse train in the second modulation unit, and the RZ pulse trains are respectively applied to different arms in the first and second modulation units. And a Mach-Zehnder modulator to be added.

  Further, as described in claim 2, in the above-described first optical transmitter embodiment, the electronic circuit includes one transmission data obtained by bifurcating the transmission data and one of two transmission signals obtained by bifurcating the clock signal. A first product circuit that takes a product with a clock signal and outputs the first RZ pulse train; an inversion circuit that inverts the logical value of the other transmission data that is divided into two parts of the transmission data signal; and And a second product circuit that outputs the second RZ pulse train by taking the product of the transmission data that is output and the other two-branched clock signal.

  Furthermore, as described in claim 3, in the above-described first optical transmitter embodiment, the amplitudes of the output signals of the first and second product circuits are set to the half-wave voltage of the Mach-Zehnder modulator, respectively. And a first modulation unit of the Mach-Zehnder modulator is driven by an output of the first driver, and a second modulation unit is output of the second driver. And the input light is modulated.

  Furthermore, as described in claim 4, the first optical transmitter according to the embodiment includes an inverting circuit for inverting the output of one of the first and second product circuits. Instead of adding an RZ pulse train to different arms in the first modulation unit and the second modulation unit, an RZ pulse train is added to the same arm in the first modulation unit and the second modulation unit. ing.

  On the other hand, according to the embodiment of the second optical transmitter of the present invention, as described in claim 5, RZ pulse trains of the normal phase signal and the negative phase signal corresponding to the first logical value of the transmission data. And an electronic circuit for generating an RZ pulse train of a normal phase signal and a reverse phase signal corresponding to the second logical value of the transmission data, and first and second arms for branching and guiding the input light A positive phase corresponding to the first logic value in the first arm of the first modulation unit, the first and second modulation units having different physical positions on the first and second arms. A signal electrode to which a signal is applied, and a signal electrode to which a negative-phase signal corresponding to the first logic value is applied to the second arm of the first modulation unit; A signal electrode to which a negative phase signal corresponding to the first logic value is applied to the arm of And a Mach-Zehnder modulator having a second of said second signal electrode positive phase signal corresponding to the logic value is applied to the second arm of the modulator portion.

  Furthermore, in the embodiment of the second optical transmitter described above, the electronic circuit includes one transmission data obtained by bifurcating transmission data and one clock obtained by bifurcating a clock signal. A first product circuit that takes a product with the signal and outputs a first RZ pulse train; and converts the first RZ signal into a differential signal to generate a positive-phase signal and an inverse signal corresponding to the first logic value A product of a first single-phase differential conversion circuit for obtaining a phase signal, an inverting circuit for inverting the logical value of the other data obtained by bifurcating the transmission data signal, and an output of the inverting circuit and the other clock signal obtained by bifurcating A second product circuit that outputs a second RZ pulse train and converts the second RZ pulse train into a differential signal to obtain a negative phase signal and a positive phase signal corresponding to the second logic value And a second single-phase differential conversion circuit.

  Furthermore, as described in claim 7, in the embodiment of the second optical transmitter described above, the positive phase signal and the negative phase signal output from the first single-phase differential conversion circuit are respectively converted into the Mach-Zehnder modulator. First and second drivers for amplifying the half-wave voltage of the Mach-Zehnder modulator to the half-wave voltage of the Mach-Zehnder modulator, respectively. And a fourth driver.

  On the other hand, according to the embodiment of the first phase modulation method of the present invention, as described in claim 8, the first and second logic values corresponding to the first and second logical values of the transmission data, respectively. RZ pulse trains of the first and second modulators provided at physically different positions on the two arms that branch and guide the input light to the Mach-Zehnder modulator using the first RZ pulse train. The first modulation unit modulates the input light, uses the second RZ pulse train, modulates the input light in the second modulation unit, and each of the first modulation unit and the second modulation unit. An RZ pulse train is added to different arms.

  Furthermore, in the embodiment of the first phase modulation method described above, the product of one transmission data obtained by branching the transmission data into two and one clock signal obtained by branching the clock signal into two branches is obtained. The first RZ pulse train is obtained, the logical value of the other transmission data obtained by bifurcating the transmission data signal is inverted, and the product of the inverted transmission data and the other bifurcated clock signal is obtained. The second RZ pulse train is obtained.

  Furthermore, as described in claim 10, in the first phase modulation method embodiment described above, the amplitudes of the signals of the first and second RZ pulse trains are each amplified to the half-wave voltage of the Mach-Zehnder modulator. The first modulation unit of the Mach-Zehnder modulator is driven with the amplified first RZ pulse train, and the second modulation unit is driven with the amplified second RZ pulse train to modulate the input light. ing.

  Furthermore, according to an embodiment of the first phase modulation method described above, the output of one of the first and second RZ pulse trains is inverted, and the first and second RZ pulse trains are inverted. Instead of adding the RZ pulse train to different arms in the two modulation units, the RZ pulse train is added to the same arm in the first and second modulation units.

  On the other hand, according to an embodiment of the second phase modulation method of the present invention, as described in claim 12, RZ pulse trains of a normal phase signal and a negative phase signal corresponding to the first logical value of transmission data The first and second arms for branching and guiding the input light by generating RZ pulse trains of the normal phase signal and the reverse phase signal corresponding to the second logical value of the transmission data. For the Mach-Zehnder modulator having the first and second modulation units provided on the first and second arms so as to be physically different from each other, each of the first and second arms of the first modulation unit is provided. Are added with a positive phase signal and a negative phase signal corresponding to the first logical value, respectively, and a negative phase signal and a positive phase signal corresponding to the second logical value of the first and second arms of the second modulation unit are respectively added. A phase signal is added.

  Further, according to the second phase modulation method described above, as described in claim 13, a product signal of one transmission data obtained by bifurcating transmission data and one clock signal obtained by bifurcating the clock signal is obtained. The product signal is converted into a differential signal to obtain a normal phase signal and a reverse phase signal corresponding to the first logical value of the transmission data, and the logical value of the other data obtained by bifurcating the transmission data signal is inverted. Then, a logical signal of the inverted signal and the other two-branched clock signal is obtained, the product signal is converted into a differential signal, and a normal phase signal and a negative phase signal corresponding to the second logical value of the transmission data are obtained. Seeking a signal.

  Furthermore, according to the above-described second phase modulation method embodiment, the positive phase signal and the negative phase signal corresponding to the first logical value of the transmission data are respectively subjected to Mach-Zehnder modulation. Amplifying the half-wave voltage of the Mach-Zehnder modulator and amplifying the positive-phase signal and the negative-phase signal corresponding to the first logic value of the transmission data to the half-wave voltage of the Mach-Zehnder modulator, respectively. It is added to the Mach-Zehnder modulator.

  The first effect is that it is possible to generate an RZ-DPSK signal that realizes high performance of the system. The reason is that the modulation signal is applied to the Mach-Zehnder modulator via a separate driver so that degradation of the generated optical signal is reduced even if there is waveform distortion due to nonlinear characteristics of the drive circuit. is there.

  The second effect is that downsizing is possible. The reason for this is that the structure is applied to the Mach-Zehnder modulator after RZ conversion is performed in the electric stage, so there is no need to use two Mach-Zehnder modulators, and two drive parts are provided on the arm of a single Mach-Zehnder modulator. This is because, although it is necessary to provide a (modulation part), it is not necessary to multiplex and demultiplex light compared to using two Mach-Zehnder modulators, so that the Mach-Zehnder modulator can be reduced in size.

  The third effect is that low power consumption is possible. The reason for this is that in the generation of a normal RZ-DPSK signal, the voltage required to drive the Mach-Zehnder modulator needs to be twice the half-wave voltage, but in this configuration, a pulse that gives phase 0 to the two arms This is because the pulse train giving the phase π may be applied with an amplitude equivalent to a half-wave voltage. Furthermore, in the case of a configuration in which the two arms are differentially driven, the voltage can be further halved.

  Embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 shows the configuration of a first embodiment of an optical transmitter according to the present invention. Transmission data 100 and a clock signal 102 which are digital signal trains are input from an external circuit (not shown). The transmission data 100 is an NRZ (Non-Return-to-Zero) signal, and the clock signal 102 is synchronized with the speed of the transmission data 100. An inversion circuit (INV) 104 that performs logic inversion of the digital signal inverts the logic values “0” and “1” of the digital signal to logic values “1” and “0”, respectively. The first product circuit 106 receives the branched transmission data 100 and the branched clock signal 102 and outputs the product of these two input signals, and the second product circuit 108 In response to the data obtained by inverting the transmission data 100 and the other branched clock signal 102, the product of these two input signals is output. According to the present embodiment, an RZ-DPSK optical output can be obtained with one Mach-Zehnder modulator by using these two product circuits 106 and 108.

  The driver circuits 110 and 112 are analog amplifiers having high-speed and wide-band characteristics, respectively, and amplify the output amplitude (for example, about 1 V) of the product circuits 106 and 108 to the half-wave voltage of the optical modulator at the subsequent stage (for example, about 6 V). ).

  In the present embodiment, the Mach-Zehnder modulator 114 is a modulator made of lithium niobate (hereinafter abbreviated as LN), and on the LN substrate, an optical branching unit 116, two optical waveguide arms 118, and 120 and an optical multiplexing unit 122 are provided. In this embodiment, the Mach-Zehnder modulator 114 is an X-cut type in which the refractive index of the LN changes with the application of an electric field perpendicular to the substrate. A light source 124 is provided at the input of the Mach-Zehnder modulator 114.

  Further, in the present embodiment, the first modulation unit 126 and the second modulation unit 128 are provided on the two arms 118 and 120 of the Mach-Zehnder modulator 114, and these modulation units are connected to the drivers 110 and 112, respectively. Signal electrodes 130 and 132 for applying the above-mentioned signal, and electrodes 134 and 136 for applying a ground potential. The signal electrode 130 of the first modulation unit 126 and the signal electrode 132 of the second modulation unit 128 are provided on the two arms 118 and 120 of the Mach-Zehnder modulator 114, respectively. Accordingly, the corresponding ground electrodes 134 and 136 are also provided. It is provided on a different arm side.

  Since the two modulation units 126 and 128 are arranged at positions separated in the longitudinal direction of the arm, even when a modulation signal is simultaneously applied to the two modulation units, the electromagnetic field between the electrodes of the two arms in each modulation unit. Interference can be suppressed. Therefore, since a chirp-free Mach-Zehnder modulator can be realized, a high-quality RZ-DPSK optical signal can be generated.

  The inverter circuit 104, the product circuits 106 and 108, and the driver circuits 110 and 112 are well known to those skilled in the art and are not directly related to the present invention.

  The operation of the optical transmitter shown in FIG. 1 will be described with reference to FIG.

  The waveforms of the transmission data 100 and the clock 102 are shown in (1) and (2) of FIG. In this case, the output of the inverting circuit 104 is as shown in (3) of FIG. 2, and the signal waveforms of the outputs of the product circuits 106 and 108 are as shown in (4) and (5) of FIG. That is, an RZ pulse is generated from the product circuit 106 when the transmission data 100 is “1”, and an RZ pulse is generated from the product circuit 108 when the transmission data 100 is “0”. As described above, the amplitude of the RZ pulse output from the product circuit is, for example, about 1V.

  Drivers 110 and 112, which are analog amplifiers having a bandwidth of about the bit rate of the transmission data 100, respectively amplify the outputs of the product circuits 106 and 108 to about 6V required to drive the subsequent Mach-Zehnder modulator 114. . Since the amplification band of the driver is finite, the output waveforms of the drivers 110 and 112 are not rectangular but a smooth waveform (RZ waveform) as shown in (6) and (7) of FIG. The vertical axes of (1) to (7) in FIG. 2 indicate the signal amplitude.

  The outputs of the drivers 110 and 112 are input to the Mach-Zehnder modulator 114. At this time, the refractive index of the optical waveguide (arm) of the Mach-Zehnder modulator 114 is changed by the output of the driver 110 applied to the signal electrode 130 of the first modulation unit 126, and the light passing through the first arm 118 is changed. The phase shift changes as indicated by the solid line in (8) of FIG. In the present embodiment, since the Mach-Zehnder modulator is an X-cut type LN modulator, the second arm 120 has the same amount of optical phase shift (indicated by a broken line) in the opposite direction. That is, when a pulse voltage 140 (see (6) in FIG. 2) is applied from the first driver circuit 110, a voltage is applied in the right direction of the extinction curve 20 shown in FIG. The light pulse is generated.

  On the other hand, the output of the driver 112 applied to the signal electrode 132 of the second modulator 128 also causes a light phase change (phase shift) in both arms 118 and 120 as described above. In this case, as shown in FIG. 2 (9), the optical phase shift in which the sign is inverted occurs in the first arm 118 as shown by the solid line, and the optical phase shift shown in the broken line occurs in the second arm 120. . That is, the pulse voltage 142 (see (7) in FIG. 2) from the second driver circuit 112 is applied to an arm different from the signal from the first driver circuit 110 to cause a reverse phase shift. The operation is equivalent to the application of the voltage to the left of the extinction curve 20 shown in FIG. As a result, an RZ type optical pulse with an optical phase of 0 is generated.

  The phase change at the optical multiplexing unit 122 of the arms 118 and 120 is the sum of the phase changes at the two modulation units 126 and 128. That is, in the first arm 118, the phase shift of light is as shown by the solid line in (10) of FIG. 2, and in the second arm 120, the phase shift of light is as shown by the broken line in (10) of FIG. become. Here, when the modulation signals, which are the outputs of the drivers 110 and 112 applied to the arms 118 and 120, simultaneously become zero level, the light output of the modulator 114 becomes zero (extinguishes). A DC bias is applied (see FIG. 1).

  As a result, as shown in (11) of FIG. 2, when the optical pulse has a logical value “0”, the phase difference between both arms becomes 0, and when the optical pulse has a logical value “1”, both arms The phase difference between them becomes 2π, and becomes π at the bottom line (zero level in (10) of FIG. 2). Therefore, when the lights passing through the two arms 118 and 120 are combined to interfere with each other, the light is intensified at the peak time of the light pulse and is extinguished at the bottom line portion. Further, the interference at the optical phase difference of 0 and the interference at 2π change the phase of the light by π as shown in FIG. 10, and therefore, as shown in (12) of FIG. Since the phase of the optical output becomes 0 and π corresponding to 1 ″, an RZ-DPSK optical signal is generated.

  As described above, in this embodiment, the signals from the product circuits 106 and 108 are separately amplified and applied to the Mach-Zehnder modulator. As a result, even if nonlinearity exists in the product circuits 106 and 108, if the degree of nonlinearity and the amplification factor are adjusted by two drivers, the optical pulse signal output from the Mach-Zehnder modulator is symmetric in phases 0 and π. It can be a waveform. For this reason, the problem of unbalanced waveform height and insufficient quenching as shown in FIGS. 12 and 13 does not occur. That is, according to the present embodiment, it is possible to realize a high-quality RZ-DPSK optical pulse even if the circuit is non-linear.

  Each of the digital product circuits 106 and 108 shown in the above embodiment may be an analog multiplication circuit.

  Further, as shown in FIG. 3, selector circuits 106 a and 108 a may be provided instead of the digital product circuits 106 and 108. In this case, the transmission data 100 is applied to one input terminal 140 of the selector circuit 106a, and a level (voltage) corresponding to the logical value “1” of the transmission data 100 is fixedly applied to the other input terminal 142. Further, the branched transmission data 100 is added to one input terminal of the other selector circuit 108a, and a level corresponding to the logical value “0” of the transmission data 100 is added to the other input terminal 144. The selector circuit 106 a selects and outputs the level of the input terminal 142 when the transmission data 100 is “1” in response to the clock signal 102, while the selector circuit 108 a responds to the clock signal 102 and transmits the transmission data 100. When “0” is “0”, the level of the input terminal 144 is selected and output. In this way, as in FIG. 1, the RZ pulse is applied to the driver 110 when the transmission data is “1”, and the RZ pulse is applied to the driver 112 when the transmission data is “0”. Can do.

  Further, as shown in FIG. 4, electrodes 130 and 132 for inverting the output of the product circuit 108 of FIG. 1 and applying signals by the two modulation units 126 and 128 of the Mach-Zehnder modulator 114 are placed on one arm, By placing the ground electrodes 134 and 136 on the other arm, the same light output as described in FIG. 1 can be obtained. Further, on the contrary, by inverting the output of the product circuit 106 in FIG. 1, the same optical output as described in FIG. 1 can be obtained. In FIG. 4, the newly added inverting circuit is indicated by 104a, and the other reference numerals are the same as those in FIG.

  FIG. 5 is a flowchart for explaining an embodiment of the first phase modulation method according to the present invention. Since the first phase modulation method is a phase modulation method applied to the above-described first optical transmitter according to the present invention, the reference numerals shown in FIG. 1 are used.

  As shown in FIG. 5, in step ST10, the first RZ pulse train is output from the product circuit 106 when the transmission data 100 is "1", and the second RZ is output from the product circuit 108 when the transmission data 100 is "0". Outputs a pulse train.

  Next, in step 12, the first and second RZ pulse trains are amplified by the driver circuits 110 and 12, respectively.

  Further, in step 14, the first and second modulators 126 and 128 provided in physically different positions on the two arms 118 and 120 for branching and guiding the input light to the Mach-Zehnder modulator 114 are guided. The first modulation unit 126 modulates the input light by using a first RZ pulse train, and the second modulation unit 128 modulates the input light by using a second RZ pulse train, so that the first modulation is performed. The RZ pulse train is added to different arms in the unit 126 and the second modulation unit 128 to output an RZ-DPSK optical signal.

  Although not shown in the drawings, as described in FIGS. 1 to 4, in the above-described first phase modulation method embodiment, the first RZ pulse train is one transmission data obtained by bifurcating the transmission data. And the second RZ pulse train is obtained by inverting the logical value of the other transmission data obtained by bifurcating the transmission data signal, and the inverted transmission data. What is necessary is just to obtain | require and obtain the product with the other clock signal branched in two.

  Furthermore, in the embodiment of the first phase modulation method described above, the amplitudes of the signals of the first and second RZ pulse trains are each amplified to the half-wave voltage of the Mach-Zehnder modulator, and the amplified first RZ pulse trains are used. The first modulation unit of the Mach-Zehnder modulator is driven, and the second modulation unit is driven by the amplified second RZ pulse train.

  Furthermore, in the embodiment of the first phase modulation method described above, the output of either one of the first and second RZ pulse trains is inverted, and the first and second modulation units are respectively connected to different arms. Instead of adding a pulse train, an RZ pulse train may be added to the same arm in the first and second modulators.

  In the first embodiment of the optical transmitter described above, an X-cut type Mach-Zehnder modulator made of LN is used as the Mach-Zehnder modulator. However, this X-cut type Mach-Zehnder modulator has a drawback that the voltage required for the phase shift must be increased because the electro-optic coefficient of LN is small.

  On the other hand, there is a so-called Z-Cut type Mach-Zehnder modulator made of LN. This Z-Cut type uses an electric field change in a direction parallel to the LN substrate surface, and has an advantage that a large electro-optic coefficient can be obtained and a driving voltage (modulation voltage) can be reduced.

  The second embodiment of the optical transmitter according to the present invention uses a Z-cut type Mach-Zehnder modulator made of the above-mentioned LN.

  The Z-cut type Mach-Zehnder modulator has a problem that if an electric field is applied to only one of two arms that are optical waveguides, the phase shift does not become symmetric and a chirp occurs. In order to solve this problem, in the second embodiment of the optical transmitter of the present invention, as shown in FIG. 6, each of the first and second modulators 214 and 220 is modulated with two signals. (So-called dual drive).

  In FIG. 6, when the confusion with FIG. 1 does not occur, the reference numbers used in FIG. 1 are used. As shown in FIG. 6, a single-phase differential conversion circuit 200 is provided next to the first digital product circuit 106. Output a signal. These positive and negative phase signals are amplified by the drivers 202 and 204 and then input to the Mach-Zehnder modulator 206. On the other hand, a single-phase differential conversion circuit 208 is provided next to the second digital product circuit 108, and the single-phase differential conversion circuit 208 outputs a normal phase signal and a negative phase signal output from the product circuit 108. These positive phase signals and negative phase signals are amplified by the drivers 210 and 212 and then input to the Mach-Zehnder modulator 206.

  Unlike the first embodiment, the first modulation unit 214 of the Mach-Zehnder modulator 206 is provided with signal electrodes 216 and 218 on the two arms 118 and 120, respectively. Similarly, unlike the first embodiment, the second modulation unit 220 of the Mach-Zehnder modulator 206 is provided with signal electrodes 222 and 224 on the two arms 118 and 120, respectively.

  The positive phase signal and the negative phase signal output from the product circuit 106 amplified by the drivers 202 and 204 are applied to the signal electrodes 216 and 218, respectively, and the positive phase signal output from the product circuit 108 amplified by the drivers 210 and 212, respectively. The signal and the negative phase signal are applied to signal electrodes 224 and 222, respectively.

  With the configuration and signal application as described above, the first modulation unit 214 and the second modulation unit 220 have the light phase shift as shown in (8) and (9) of FIG. 2 as in the embodiment of FIG. Thus, the occurrence of chirp after interference can be suppressed. The optical phase shift of the arms 118 and 120 in the optical combining unit 122 is the same as (11) in FIG. 2, and the optical output is also an RZ-DPSK signal, as in the first embodiment.

  FIG. 7 is a flowchart for explaining an embodiment of the second phase modulation method according to the present invention. Since the second phase modulation method is a phase modulation method applied to the second optical transmitter embodiment described with reference to FIG. 6, the reference numerals shown in FIG. 6 are used.

  As shown in FIG. 7, in step ST20, when the transmission data 100 is “1”, the first RZ pulse train having the positive phase and the second RZ pulse train having the opposite phase are generated, and the transmission data 100 is “0”. Sometimes a positive-phase third RZ pulse train and a negative-phase fourth RZ pulse train are generated.

In step ST22, the first to fourth RZ pulse trains generated in step ST20 described above are each amplified by a driver. In step ST24, the input light to the Mach-Zehnder modulator 206 is branched and guided on the two arms 118 and 120. In the first modulation section 214 of the first and second modulation sections 214 and 220 provided at physically different positions, the input light is modulated using the first and second RZ pulse trains, and The second modulator 220 modulates the input light using the third and fourth RZ pulse trains and outputs an RZ-DPSK optical signal.

  Although not shown, in the embodiment of the second phase modulation method described above with reference to FIG. 7, the first phase RZ pulse train and the reverse phase when the transmission data 100 is “1”. The second RZ pulse train of the phase obtains a product signal of one transmission data obtained by bifurcating the transmission data 100 and one clock signal obtained by bifurcating the clock signal 102, and converts the product signal into a differential signal. Seeking. On the other hand, when the transmission data 100 is “0”, the third-phase RZ pulse train in the normal phase and the fourth RZ-pulse sequence in the reverse phase invert the logical value of the other data that has branched the transmission data signal into two, and this inversion. The logic signal of the other signal and the other clock signal branched in two is obtained, and the product signal is obtained by converting it into a differential signal.

  Further, in the above-described second phase modulation method embodiment, the positive phase signal and the negative phase signal corresponding to “1” of the transmission data 100 are amplified to the half-wave voltage of the Mach-Zehnder modulator, respectively. In addition, the normal phase signal and the reverse phase signal corresponding to “1” of the transmission data are respectively amplified to the half-wave voltage of the Mach-Zehnder modulator and applied to the Mach-Zehnder modulator.

The figure which shows the optical transmitter of the RZ-DPSK system by embodiment of the 1st optical transmission which concerns on this invention. 2 is a time chart for explaining the operation of the RZ-DPSK optical transmitter shown in FIG. 1. The figure which shows the modification of the electronic circuit which comprises some optical transmitters of FIG. The figure which shows the modification of the optical transmitter shown in FIG. 3 is a flowchart for explaining an embodiment of the first phase modulation method according to the present invention, and this phase modulation method corresponds to the first embodiment of the optical transmitter of the present invention. The figure which shows the optical transmitter of the RZ-DPSK system by embodiment of the 2nd optical transmitter which concerns on this invention. It is a flowchart for describing embodiment of the 2nd phase modulation method which concerns on this invention, Comprising: This phase modulation method respond | corresponds to embodiment of the 2nd optical transmitter shown in FIG. The figure explaining the position of 0 and 1 of the digital signal which can be set | placed on an optical electric field complex plane with an intensity | strength modulation system and a DPSK system. The figure for demonstrating the prior art example which uses two Mach-Zehnder modulators. The figure for demonstrating the mechanism which generates a RZ-DPSK signal using a Mach-Zehnder modulator. The figure for demonstrating the other prior art example using a single Mach-Zehnder modulator. The figure for demonstrating the problem of the other prior art example shown in FIG. The figure for demonstrating giving distortion to an optical output when the electric signal which drives a Mach-Zehnder modulator is distorted.

Explanation of symbols

100 Transmission data 102 Clock signal 104, 104a Inversion circuit 106, 108 Product circuit (digital product circuit)
106a, 108a selector 110, 112 driver (amplifier)
114 Mach-Zehnder Modulator 118, 120 Arm 130, 132 of Mach-Zehnder Modulator Electrode 134, 136 Ground (Ground) Electrode 200, 208 Single-Phase Differential Conversion Circuit 206 Mach-Zehnder Modulator 216, 218, 222, 224 Electric Signal Applying electrode

Claims (14)

An electronic circuit for generating first and second RZ pulse trains corresponding to the first and second logical values of the transmission data, respectively;
Two arms for branching and guiding the input light are provided, and the first and second modulation units having different physical positions are provided on the two arms, and the input light is input to the first modulation unit by the first modulation unit. A Mach-Zehnder modulator that modulates the RZ pulse train, modulates the input light with the second RZ pulse train in the second modulator, and adds the RZ pulse train to different arms in the first and second modulators, respectively. An optical transmitter characterized by. The electronic circuit takes a product of one transmission data obtained by bifurcating the transmission data and one clock signal obtained by bifurcating a clock signal, and outputs the first RZ pulse train; The second RZ pulse train is obtained by taking the product of an inversion circuit that inverts the logical value of the other transmission data obtained by bifurcating the transmission data signal and the transmission data that is the output of the inversion circuit and the other clock signal obtained by bifurcation. The optical transmitter according to claim 1, further comprising: a second product circuit that outputs First and second drivers for amplifying the amplitudes of the output signals of the first and second product circuits to half-wave voltages of the Mach-Zehnder modulator, respectively;
The first modulation unit of the Mach-Zehnder modulator is driven by the output of the first driver, and the second modulation unit is driven by the output of the second driver to modulate the input light. The optical transmitter according to claim 2.   An inverting circuit for inverting the output of one of the first and second product circuits, and instead of adding an RZ pulse train to different arms in the first and second modulators, respectively. 4. The optical transmitter according to claim 2, wherein an RZ pulse train is added to the same arm in the first modulation unit and the second modulation unit. RZ pulse trains of normal phase signals and negative phase signals corresponding to the first logic value of the transmission data are generated, and RZ pulse trains of positive phase signals and negative phase signals corresponding to the second logic value of the transmission data are generated. Electronic circuit,
First and second arms having first and second arms for branching and guiding input light, the first and second modulators having different physical positions on the first and second arms, The first arm of the first modulation unit has a signal electrode to which a positive phase signal corresponding to the first logic value is applied, and the second arm of the first modulation unit has a negative phase signal corresponding to the first logic value. And a signal electrode to which a negative phase signal corresponding to the first logic value is applied to the first arm of the second modulation unit, and the second modulation unit second signal An optical transmitter, comprising: a Mach-Zehnder modulator having a signal electrode to which a positive phase signal corresponding to the second logical value is applied to the arm. The electronic circuit is
A first product circuit that outputs a first RZ pulse train by taking the product of one transmission data obtained by bifurcating transmission data and one clock signal obtained by bifurcating a clock signal;
A first single-phase differential conversion circuit for converting the first RZ signal into a differential signal to obtain a positive phase signal and a negative phase signal corresponding to the first logical value;
An inverting circuit for inverting the logical value of the other data obtained by bifurcating the transmission data signal;
A second product circuit that outputs a second RZ pulse train by taking the product of the output of the inverting circuit and the other branched clock signal;
2. A second single-phase differential conversion circuit for converting the second RZ pulse train into a differential signal to obtain a negative phase signal and a positive phase signal corresponding to the second logical value. 5. The optical transmitter according to 5. First and second drivers that amplify the positive and negative phase signals output by the first single-phase differential conversion circuit to the half-wave voltage of the Mach-Zehnder modulator, respectively;
A third driver and a fourth driver for amplifying a negative phase signal and a positive phase signal output from the second single-phase differential conversion circuit to a half-wave voltage of the Mach-Zehnder modulator, respectively;
The optical transmitter according to claim 6, further comprising: A method for phase-modulating input light to a Mach-Zehnder modulator,
First and second RZ pulse trains are obtained corresponding to the first and second logical values of the transmission data, respectively.
The first modulation unit of the first and second modulation units provided at physically different positions on the two arms using the first RZ pulse train and branching and guiding the input light to the Mach-Zehnder modulator Modulating the input light in
Using the second RZ pulse train, the second modulator modulates the input light,
A phase modulation method comprising: adding an RZ pulse train to different arms in each of the first modulation unit and the second modulation unit. The first RZ pulse train is obtained by taking the product of one transmission data obtained by bifurcating the transmission data and one clock signal obtained by bifurcating the clock signal,
The second RZ pulse train is obtained by inverting the logical value of the other transmission data obtained by bifurcating the transmission data signal, and taking the product of the inverted transmission data and the other clock signal obtained by bifurcation. The phase modulation method according to claim 8. Amplifying the amplitude of the signals of the first and second RZ pulse trains to a half-wave voltage of the Mach-Zehnder modulator, respectively;
A first modulation unit of a Mach-Zehnder modulator is driven by the amplified first RZ pulse train, and a second modulation unit is driven by the amplified second RZ pulse train to modulate the input light. The phase modulation method according to claim 8 or 9. Inverting the output of one of the first and second RZ pulse trains;
10. The RZ pulse train is added to the same arm in the first and second modulation sections instead of adding the RZ pulse train to different arms in the first and second modulation sections, respectively. The phase modulation method according to 1. RZ pulse trains of normal phase signals and negative phase signals corresponding to the first logical value of the transmission data are generated, and RZ pulse trains of positive phase signals and negative phase signals corresponding to the second logical value of the transmission data are generated. ,
Mach-Zehnder modulation having first and second modulation units that have first and second arms for branching and guiding input light, and are provided on the first and second arms so that their positions are physically different from each other. A positive phase signal and a negative phase signal corresponding to the first logic value are respectively added to the first and second arms of the first modulation unit,
A phase modulation method comprising adding a negative phase signal and a normal phase signal corresponding to the second logical value of each of the first and second arms of the second modulation unit. A product signal of one transmission data obtained by bifurcating the transmission data and one clock signal obtained by bifurcating the clock signal is obtained, and the product signal is converted into a differential signal to correspond to the first logical value of the transmission data. Obtained normal phase signal and negative phase signal,
The logic value of the other data obtained by bifurcating the transmission data signal is inverted, the logic signal of the inverted signal and the other clock signal bifurcated is obtained, and the product signal is converted into a differential signal to transmit data. A positive phase signal and a negative phase signal corresponding to the second logical value of
The phase modulation method according to claim 12. Amplifying a normal phase signal and a negative phase signal corresponding to the first logical value of the transmission data to a half-wave voltage of the Mach-Zehnder modulator, respectively, and adding to the Mach-Zehnder modulator,
Amplifying a positive phase signal and a negative phase signal corresponding to the first logical value of the transmission data to a half-wave voltage of the Mach-Zehnder modulator, respectively, and adding to the Mach-Zehnder modulator,
The phase modulation method according to claim 12 or 13, characterized in that: