JP5724792B2 - Optical transmitter, optical communication system, and optical transmission method - Google Patents

Description

  The present invention relates to an optical communication technique, and more particularly to an optical transmitter, an optical communication system, and an optical transmission method that perform bias control of an optical modulator that modulates light.

  Conventionally, it has been known that the optimum bias voltage of an MZ (Mach-Zehnder) type optical modulator drifts due to temperature and aging fluctuations. In order to maintain the quality of the transmitted optical signal, the optimum bias value is set. Control methods to follow are being studied. For example, in an optical transmitter that performs automatic bias control (ABC: Automatic Bias Control) of an I / Q modulator, three bias voltages of I-ch, Q-ch, and Phase are set with an arbitrary electrical waveform input. In this case, the low-speed alternating signal (Dither) is superimposed on the I-ch and Q-ch bias voltages, and the Dither error signal detected from the monitor PD (Photo-Diode) current becomes zero. A device that performs feedback control on a point is known (for example, Non-Patent Document 1).

Yoshida et al., "A Study on Modulator Automatic Bias Control for Arbitrary Optical Waveform Generation", IEICE General Conference B-10-56, March 2, 2010

  In the conventional optical transmitter disclosed in Non-Patent Document 1, ABC is performed in a state where an arbitrary electric waveform is input to the I / Q modulator. There was a problem that it took a long time for the voltage to stabilize.

  The present invention has been made to solve the above-described problems. In the bias control of the optical modulator, the time for drawing in to the convergence point of the optimum bias voltage by preventing the modulation signal from being input in the initial state. The purpose is to speed up.

An optical transmitter according to the present invention is based on a modulation signal driving unit that outputs a modulation signal driven based on an input data series signal, an applied bias voltage, and the modulation signal input from the modulation signal driving unit. An optical modulation unit that modulates light and outputs the modulated optical signal; and after the bias voltage is controlled as an initial state in which the modulation signal is not input to the optical modulation unit from the modulation signal driving unit A control unit that controls the bias voltage as a normal state in which the modulation signal is input to the optical modulation unit from the modulation signal driving unit based on the control result of the initial state, and the control unit As the initial pull-in state as the initial state, the bias voltage is pulled to the target value based on a low-frequency signal having a frequency lower than that of the modulation signal, and then Examples condition and performs control of the bias voltage based on the low frequency signal.

  According to the present invention, in the optical transmitter, by not allowing the modulation signal to be input to the optical modulation unit in the initial state, it is possible to shorten the time taken to the convergence point of the bias voltage control.

1 is a block diagram showing an optical transmitter according to Embodiment 1 of the present invention. Explanatory drawing for demonstrating the optical transmitter by Embodiment 1 of this invention Explanatory drawing for demonstrating the optical transmitter by Embodiment 1 of this invention Configuration diagram showing an optical transmitter according to a second embodiment of the present invention Explanatory drawing for demonstrating the optical transmitter by Embodiment 2 of this invention Explanatory drawing for demonstrating the optical transmitter by Embodiment 2 of this invention Explanatory drawing for demonstrating the optical transmitter by Embodiment 2 of this invention Explanatory drawing for demonstrating the optical transmitter by Embodiment 2 of this invention Explanatory drawing for demonstrating the optical transmitter by Embodiment 2 of this invention

Embodiment 1 FIG.
1 is a block diagram showing an optical transmitter according to Embodiment 1 of the present invention. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 1, 1 is a light source, 2a and 2b are modulation signal drive units, 3a and 3b are bias drive units, 4 is a current-voltage conversion unit, 5a and 5b are minute signal amplification units as amplification units, 6 is a control unit, 7 is a synchronous detection unit, 8 is a modulation signal drive control unit, 9a, 9b and 9c are bias application units, 10a and 10b are monitoring units, 11a and 11b are minute modulation signal generation units as low frequency signal generation units, and 12 is Average modulation degree detector, 13a and 13b are adders, 14 is an optical modulator, 15a is a first MZ (Mach-Zehnder) type optical modulator, 15b is a second MZ type optical modulator, and 16 is an optical phase. An adjustment unit, 17 is a light intensity detection unit, and 18a to 18g are optical waveguides.

In FIG. 1, the light modulation unit 14 includes a first MZ type light modulation unit 15a, a second MZ type light modulation unit 15b, an optical phase adjustment unit 16, a light intensity detection unit 17, and optical waveguides 18a to 18g. For example, they are formed on a LiNbO ₃ substrate and are optically connected to each other. That is, in the light modulation unit 14, the optical waveguide 18a is connected to the optical waveguide 18b and the optical waveguide 18c. A first MZ-type light modulator 15a is connected between the optical waveguide 18b and the optical waveguide 18d, and a second MZ-type light modulator 15b is connected between the optical waveguide 18c and the optical waveguide 18e. An optical phase adjusting unit 16 is connected between 18e and the optical waveguide 18f. The optical waveguide 18d and the optical waveguide 18f are connected to the optical waveguide 18g, and the light intensity detector 17 is also connected.

  In FIG. 1, the light source 1 and the light modulation part 14 are optically connected by, for example, an optical fiber.

  In FIG. 1, the control unit 6 includes a synchronous detection unit 7, a modulation signal drive control unit 8, bias application units 9a, 9b, 9c, monitor units 10a, 10b, minute modulation signal generation units 11a, 11b, and average modulation degree detection. It is comprised by the part 12, and is each electrically connected. In FIG. 1, the control unit 6, the modulation signal drive units 2a and 2b, the bias drive units 3a and 3b, the current-voltage conversion unit 4, the minute signal amplification units 5a and 5b, the addition units 13a and 13b, and the light intensity detection unit. 17 are each electrically connected.

  The light source 1 is a semiconductor laser having, for example, an InP compound semiconductor mixed crystal as a constituent material, and emits and outputs a laser beam as a continuous wave in a 1.5 μm wavelength band. The configuration of the light source 1 is not limited to this. For example, a 1.3 μm wavelength band, a pulse wave, a solid-state laser, or the like may be used. In short, light for realizing desired optical communication is used. Can be applied as an embodiment of the present invention.

The first MZ light modulator 15a and the second MZ light modulator 15b are composed of, for example, a LiNbO ₃ crystal, and change in refractive index by applying an electric field, that is, MZ light modulation utilizing a so-called electro-optic effect. It is a vessel. The MZ type optical modulator is configured as a so-called MZ interferometer by connecting two optical waveguides having electrodes formed in parallel between two Y branch optical waveguides. The MZ-type optical modulator is configured such that, for light passing through the MZ interferometer, a modulation signal input to the modulation electrode and a refractive index change caused by a bias voltage applied to the bias electrode between the two optical waveguides. A light intensity change corresponding to the phase difference is given and output. The MZ type optical modulator is an optical modulator capable of achieving both high signal quality such as low chirp and high speed.

  The optical modulation unit 14 connects two MZ type optical modulators as the first MZ type optical modulation unit 15a and the second MZ type optical modulation unit 15b in parallel as an MZ interferometer, A complex optical electric field is generated by combining a certain Ich (In-phase channel) optical signal and a Qch (Quadrature-phase channel) optical signal, which is an imaginary part, with a carrier phase difference of π / 2. It is a parallel MZ type optical modulator (DP-MZM: also called a dual-parallel Mach-Zehnder Modulator, I / Q modulator). The optical modulation unit 14 includes a modulation electrode and a bias electrode in each MZ type optical modulator, and a phase control electrode as an optical phase adjustment unit 16 on one side of an optical waveguide connecting the MZ type optical modulators in parallel. It has.

  The configurations of the light modulation unit 14, the first MZ type light modulation unit 15a, and the second MZ type light modulation unit 15b are not limited to this. For example, an InP compound semiconductor mixed crystal or the like is used as a constituent material. In addition, an optical modulator in which an optimum bias voltage drifts due to temperature and aging fluctuations can be applied as an embodiment of the present invention.

  Next, the operation will be described. In FIG. 1, input data series signals Ich and Qch as data series signals are input to modulation signal driving units 2a and 2b, and in accordance with a control signal from the modulation signal driving control unit 8, a first MZ type optical modulation unit 15a. , 15b, respectively. The input data series signals Ich and Qch are, for example, differential quadrature phase shift keying (DQPSK) in which 2-bit information having a binary code of 1 or 0 is put on four phase differences at a transmission rate of 20 Gbit / s. : Differential Quadrature Phase Shift Keying) is an analog signal for the optical modulator 14 to perform.

  The current signal corresponding to the light intensity from the light intensity detector 17 is converted into a voltage signal by the current-voltage converter 4, and the output voltage signal is input to the minute signal amplifiers 5a and 5b. The amplified voltage signals are input to the synchronous detection unit 7 via the monitor units 10a and 10b in the control unit 6, respectively. The synchronous detector 7 extracts an error signal from the input voltage signal, and obtains a bias voltage by which the error signal becomes zero by calculation. The bias application units 9a and 9b output a bias voltage based on the error signal extracted by the synchronous detection unit 7 via the monitor unit 10a. The bias application unit 9c outputs a bias voltage based on the error signal extracted by the synchronous detection unit 7 via the monitor unit 10b.

  The adder 13a, 13b outputs the bias voltage driven by the bias driver 3a, 3b and the minute modulation signal as the low frequency signal from the minute modulation signal generator 11a, 11b from the bias applying unit 9a, 9b. The signals are added and input to the first MZ light modulator 15a and the second MZ light modulator 15b, respectively. The bias voltage obtained by driving the output from the bias applying unit 9 c by the bias driving unit 3 c is input to the optical phase adjusting unit 16.

  Further, light having a wavelength of 1.55 μm is output from the light source 1, and this light output is input to the light modulator 14. The light input to the light modulator 14 passes through the optical waveguide 18a and is demultiplexed at the same optical level into the optical waveguide 18b and the optical waveguide 18c. Input data series signals Ich and Qch, which are arbitrary analog signals, are driven by the modulation signal driving units 2a and 2b, and Ich and Qch analog modulation signals as modulation signals having sufficient strength to drive the MZ type optical modulator are obtained. Is output.

  The first MZ type optical modulation unit 15a modulates the light from the optical waveguide 18b as an Ich optical signal by the bias voltage including the minute modulation signal from the addition unit 13a and the Ich analog modulation signal from the modulation signal driving unit 2a. . The second MZ type optical modulation unit 15b modulates the light from the optical waveguide 18c as a Qch optical signal by the bias voltage including the minute modulation signal from the addition unit 13b and the Qch analog modulation signal from the modulation signal driving unit 2b. . The Qch optical signal modulated by the second MZ modulation unit 15b is phase-shifted by π / 2 by the optical phase adjustment unit 16, and becomes orthogonal to the Ich optical signal from the first MZ modulation unit. By adjusting the Ich and Qch optical signals to an orthogonal relationship, the present invention can be applied to a DQPSK optical transmitter and a dispersion pre-equalized optical transmitter that generates a complex optical electric field having an orthogonal relationship from an arbitrary drive waveform.

  In dispersion pre-equalization, waveform distortion due to chromatic dispersion and / or polarization dispersion of an optical transmission line is electrically equalized and transmitted in advance by digital signal processing on the transmission side. This pre-equalization technique is superior in that the system configuration can be simplified compared to optical dispersion compensation and the amount of equalization can be increased compared to dispersion compensation on the reception side using a conventional optical dispersion compensation device. Therefore, it is an effective compensation means even in a high-speed optical transmission technology in which the influence of waveform distortion in the optical transmission path becomes large, such as 40 Gbps or 100 Gbps or more.

  The Ich optical signal from the optical waveguide 18d and the Qch optical signal from the optical waveguide 18f are combined by the optical waveguide 18g, and the optical signal from the optical waveguide 18g becomes the optical output of the optical transmitter. The light intensity at this time is detected by a monitor PD (Photo-Diode) in the light intensity detector 17. The minute signal amplifying unit 5a amplifies the AC component by the low frequency minute modulation signal from the minute modulation signal generating units 11a and 11b included in the detected light intensity. The minute signal amplifying unit 5b amplifies the AC component by the low frequency minute modulation signal from the minute modulation signal generating units 11a and 11b included in the detected light intensity. At this time, the amplification levels of the minute signal amplification unit 5a and the minute signal amplification unit 5b are different.

  FIG. 2 is an explanatory diagram for explaining the optical transmitter according to the first embodiment of the present invention. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 2A, in the bias control of the first MZ type optical modulator 15a and the second MZ type optical modulator 15b, the bias voltage is 0.01 Vπ (Vπ: half-wave voltage of the modulator) from the optimum point. An example of the relationship between the detection error with respect to the average drive amplitude ratio m at the time of deviation, that is, the modulation factor is shown. Here, the modulation degree is defined as the ratio of the half-wave voltage of the MZ type optical modulator to the average drive amplitude of the analog signal.

  In FIG. 2A, the sign of the detection error is inverted at the modulation degree of 0.5, and the detection error increases as the modulation degree approaches 0 or 1. For this reason, the first MZ light modulator 15a and the second MZ light modulator 15b perform bias control by inverting the control polarity of bias control with a modulation degree of 0.5 as a boundary.

  FIG. 2B shows an example of the relationship between the average drive amplitude ratio m when the bias voltage is shifted by 0.01 Vπ from the optimum point in the bias control of the optical phase adjustment unit 16, that is, the relationship between the detection error and the modulation degree. The error signal obtained at this time is different from the error signal obtained in the bias control of the first MZ type optical modulator 15a and the second MZ type optical modulator 15b. As the degree approaches 1, the error signal becomes smaller.

  Here, as shown in FIGS. 2A and 2B, the bias control of the first MZ type optical modulation unit 15a and the second MZ type optical modulation unit 15b and the bias control of the optical phase adjustment unit 16 are as follows. Even if the bias deviation from the optimum point is the same, the error amount of the obtained error signal is greatly different. In this example, in the bias control of the optical phase adjustment unit 16, the first MZ type optical modulation unit 15a and the second MZ This is about 1/30 of the bias control of the type optical modulator 15b.

  Thus, the error signal obtained during bias control with the same bias deviation amount is large in the first MZ type optical modulation unit 15a and the second MZ type optical modulation unit 15b, but small in the optical phase adjustment unit 16. Therefore, the minute signal amplification unit 5a is used for bias control of the first MZ type light modulation unit 15a and the second MZ type light modulation unit 15b, and the minute signal amplification unit 5a is used for bias control of the optical phase adjustment unit 16. The minute signal amplifying unit 5b having a higher degree of amplification than that of FIG.

  In addition, as shown in FIGS. 2A and 2B, in the first MZ type optical modulation unit 15a, the second MZ type optical modulation unit 15b, and the optical phase adjustment unit 16, even with the same bias deviation amount, bias is applied. The error signal obtained at the time of control becomes larger as the average drive amplitude ratio m, that is, the modulation factor is 0.5 or less, becomes larger, and takes a maximum value when the modulation factor is 0, that is, when there is no modulation. Accordingly, the analog modulation signals Ich and Qch are not input to the first MZ type optical modulation unit 15a and the second MZ type optical modulation unit 15b, and are detected from the optical output of the optical modulator 14 by making them unmodulated. The error signal to be generated can be increased, whereby the time required for the first MZ type optical modulation unit 15a, the second MZ type optical modulation unit 15b, and the optical phase adjustment unit 16 to be pulled to the convergence point is controlled. There is an effect that can be accelerated. In particular, the effect is high in the bias control of the optical phase adjuster 16 having a small error signal.

  In FIG. 1, since the processing in the control unit 6 is digital signal processing, the monitor units 10a and 10b use an ADC (Analog-to-Digital Converter) to digitally convert an input voltage signal from an analog signal. Convert to signal and monitor. Similarly, the bias application units 9a, 9b, and 9c output a bias voltage converted from a digital signal to an analog signal using a DAC (Digital-to-Analog Converter). The synchronous detection unit 7 performs control in time series for each bias control for the first MZ light modulation unit 15 a, the second MZ type light modulation unit 15 b, and the optical phase adjustment unit 16. The procedure will be described later.

In the bias control of the first MZ type optical modulator 15a, the minute modulation signal generator 11a outputs a low frequency minute modulation signal whose polarity is inverted. The error signal at this time is defined as e _I = I (H, 0) −I (L, 0). I (D _I , D _Q ) indicates a monitor PD current in the light intensity detection unit 17, D _I indicates a low frequency minute modulation signal of the first MZ type light modulation unit 15 a, and D _Q indicates the second The low frequency minute modulation signal of the MZ type light modulation part 15b is shown. H indicates that the low frequency minute modulation signal has a positive polarity, L indicates a negative polarity, and 0 indicates that there is no low frequency minute modulation signal.

This performs feedback control so that the e _I is 0 in the first bias control of the MZ-type optical modulator section 15a. The convergence point that becomes 0 includes a Null point that is a point at which the light from the MZ type optical modulator is at the minimum level and a Peak point that is a point at which the light from the MZ type optical modulator is at the maximum level. . Therefore, when the average modulation degree detection unit 12 detects that the modulation degree of the input data series signal Ich is 0.5 or more, the polarity of the control is notified to the synchronous detection unit 7 so that the null point is controlled, and 0 When 0.5 or less is detected, the synchronous detection unit 7 is notified to reverse the polarity of the control when 0.5 or more, and the control is converged to the null point.

Similarly, the error signal is defined as e _Q = I (0, H) −I (0, L) in the bias control of the second MZ type modulation unit 15b. Also e _Q at this time is the feedback control is performed such that 0. The low frequency frequency of the minute modulation signal generation unit 11b is an integral multiple of the low frequency frequency of the minute modulation signal generation unit 11a, for example, twice. This is because these low-frequency minute modulation signals are also used for bias control of the optical phase adjustment unit 16. Note that the frequency of these minute modulation signals as the low frequency signals is lower than the frequency of the Ich and Qch analog modulation signals as the modulation signals. The operation of detecting the modulation degree of 0.5 or more and 0.5 or less by the average modulation degree detector 12 and inverting the polarity of the control is the same.

In the bias control of the optical phase adjustment unit 16, the minute modulation signal generation unit 11a and the minute modulation signal generation unit 11b simultaneously output a low frequency minute modulation signal. The low frequency is the same as the bias control frequency described above. The error signal at this time is defined as e _P = I (H, H) −I (H, L) − [I (L, H) −I (L, L)]. It performs feedback control so that e _P is zero in this control bias. The convergence point to be 0 is either a point where the optical phase difference between the Ich optical signal and the Qch optical signal is π / 2 or −π / 2. It may be converged to either of these, and can be switched by reversing the polarity of control.

  Next, the bias control procedure of this optical transmitter will be described in detail. FIG. 3 is an explanatory diagram for explaining the optical transmitter according to the first embodiment of the present invention, and is a flowchart showing a bias control procedure. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 3, after the light source 1 is turned on and a normal light level is input to the light modulation unit 14, the control unit 6 starts bias control. Note that the convergence point is not known at all before the start of bias control.

  First, as an initial pull-in state, the modulation signal drive units 2a and 2b are turned off by setting the drive gain to 0 by the control signal from the modulation signal drive control unit 8, and the Ich and Qch analog modulation signals are input to the optical modulation unit 14. As a result, the light modulation unit 14 is not modulated (step S1).

  Next, the minute modulation signal generation unit 11a is turned on, and the low frequency minute modulation signal is superimposed on the bias voltage applied to the first MZ type modulation unit 15a. As described above, the first MZ type modulation unit The bias control of 15a is performed (step S2). This is called Ich bias control. After a predetermined time has elapsed, the process proceeds to bias control of the next second MZ type modulator 15b.

  That is, the minute modulation signal generation unit 11a is turned off, the minute modulation signal generation unit 11b is turned on, and the low frequency minute modulation signal is superimposed on the bias voltage applied to the second MZ type modulation unit 15b, as described above. Next, the bias control of the second MZ type modulator 15b is performed (step S3). This is called Qch bias control. Similarly, after a predetermined time has elapsed, the process proceeds to the bias control of the next optical phase modulation unit 16.

  That is, by turning on the minute modulation signal generation unit 11a, the minute modulation signal generation unit 11b and the low frequency minute modulation signal are superimposed on each bias voltage, and the bias control of the optical phase modulation unit 16 is performed as described above. Perform (step S4). This is called Pch bias control.

After a predetermined time has elapsed, a convergence determination is made as to whether or not all the Ich, Qch, and Pch biases have converged on the target (step S5). Conditions of convergence judgment at this time is, for example, the Ich bias control, when the output value of the bias applying portion 9a falls within a certain range of variation, or error signal e _I was falls within a predetermined range close to 0 Cases. If not converged (No in step S5), the process returns to the Ich bias control again (step S2). If it has converged (Yes in step S5), the process proceeds to the next normal state.

  In the normal state, the modulation signal drive units 2a and 2b are turned on by setting the drive gain by the control signal from the modulation signal drive control unit 8, and the Ich and Qch analog modulation signals are supplied to the first MZ type modulation unit 15a and the first modulation signal. Each of the signals is input to the second MZ type modulation unit 15b, whereby the light modulation unit 14 is in a modulation state (step S6). At this time, the modulation signal drive control unit 8 adjusts the modulation degree to a preset modulation degree based on the modulation degree detected by the average modulation degree detection unit 12. As described above, since the polarity of the control may be reversed, the modulation degree in the optical modulation unit 14 is set so as not to cross 0.5 according to an arbitrary signal to be handled.

  As described above, during modulation in which an arbitrary modulation signal is input to the modulation electrode, Ich bias control (step S7), Qch bias control (step S8), and Pch bias control (step S9) are sequentially performed as in the case of no modulation. repeat. Thereby, it is possible to always optimally control each bias voltage of the light modulation unit 14.

  With the configuration and procedure of the optical transmitter described above, the initial pull-in control can be performed under the non-modulation condition in which the error signal is the largest, so the time from the start of the optical transmitter to the convergence of the bias control can be achieved. Shorter than conventional methods.

  In particular, when this optical transmitter is used as a dispersion pre-equalization optical transmitter, the dispersion pre-equalization amount is adjusted when searching for the dispersion pre-equalization amount and adjusting the optimum pre-equalization amount when installing the apparatus. The bias adjustment time to be performed each time is set is shortened, and a significant time shortening effect is expected.

  As described above, in the optical transmitter according to the first embodiment of the present invention, in the Ich, Qch, and Pch bias control of the optical modulation unit 14 as the two parallel MZ type optical modulator, the initial pull-in states are Ich, Qch The bias voltage is controlled in order at the time of no modulation when the analog modulation signal is not input to the optical modulation unit 14. As a result, by increasing the error signal detected from the light output of the light modulation unit 14, it is possible to shorten the time taken to bring the optimum bias voltage to the convergence point.

  As described above, in the optical transmitter according to Embodiment 1 of the present invention, the control unit 6 has been described as a digital signal processing circuit. However, the control function is controlled by a microcomputer or the like provided in the optical transmitter. You may make it implement | achieve by a software process using the computer program which performs a method.

  Further, the optical signal transmitted from the optical transmitter according to Embodiment 1 of the present invention may be applied to an optical communication system in which an optical fiber is transmitted and received by an optical receiver. Also, two or more optical transmitters according to Embodiment 1 of the present invention are provided, and the optical signals transmitted from the two or more optical transmitters are wavelength-multiplexed to transmit the optical fiber, and the wavelength is separated on the receiving side. You may make it apply to the WDM (Wavelength Division Multiplexing) optical communication system made to receive with two or more optical receivers for every wavelength.

Embodiment 2. FIG.
As described above, in the optical transmitter according to the first embodiment of the present invention, the initial state in which the modulation signal is not input to the optical modulation unit is biased as the initial pull-in state by the low frequency signal of the output monitor of the optical modulation unit. Although the voltage control is performed, the optical transmitter according to the second embodiment of the present invention has an average voltage of the output monitor of the optical modulation unit as an initial state in which the modulation signal is not input to the optical modulation unit. Thus, the initial value search state is obtained by searching for the initial value of the bias voltage. The bias voltage may be controlled in the normal state in which the modulation signal is input to the optical modulation unit, starting from the initial value of the bias voltage in the initial value search state, or a modification of the first embodiment. As an example, the bias voltage may be controlled as an initial pull-in state starting from an initial value of the bias voltage in the initial value search state.

  FIG. 4 is a block diagram showing an optical transmitter according to Embodiment 2 of the present invention. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 4, the configuration is the same as that of the optical transmitter according to the first embodiment shown in FIG. 1, except that the second monitor unit 19 and the initial value search unit 20 are added to the control unit 6. The description of the similar configuration and the similar operation is omitted. The second monitor unit 19 and the initial value search unit 20 may be configured to be added outside the control unit 6. In addition, the monitor units 10a and 10b shown in the first embodiment and FIG. 1 are clearly distinguished from the second monitor unit 19, so in the second and fourth embodiments, the first monitor units 10a and 10b Although called, the configuration and operation are the same.

  Next, the operation will be described. The second monitor unit 19 monitors the average value of the voltage signal output from the current-voltage conversion unit 4 by converting the analog signal into a digital signal using the ADC. The first monitoring units 10a and 10b monitor a low-frequency minute modulation signal as a low-frequency signal included in the voltage signal output from the current-voltage conversion unit 4 and amplified by the minute signal amplification units 5a and 5b. On the other hand, the second monitor unit 19 is different in that the average value of the voltage signal is monitored. As will be described later, the initial value search unit 20 searches and obtains the initial values of the Ich and Qch bias voltages based on the average value of the voltage signal from the second monitor unit 19, and outputs them to the bias application units 9a and 9b, respectively. To do.

  Next, the bias control procedure of this optical transmitter will be described in detail. FIG. 5 is an explanatory diagram for explaining an optical transmitter according to the second embodiment of the present invention, and is a flowchart showing a bias control procedure. In each figure, the same numerals indicate the same or corresponding parts. In FIG. 5, after the light source 1 is turned on and a normal light level is input to the light modulation unit 14, the control unit 6 starts bias control.

  First, as an initial value search state as an initial state, the modulation signal drive units 2a and 2b are turned off by setting the drive gain to 0 by the control signal from the modulation signal drive control unit 8, and the Ich and Qch analog modulation signals are optically transmitted. The fine modulation signal generators 11a and 11b are turned off so that the low frequency fine modulation signal is not inputted to the light modulation unit 14, so that the light modulation unit 14 is completely turned off. An unmodulated state is entered (step S11).

  Next, in order to monitor the CW (Continuous Wave) light output level of the light modulator 14, the second monitor 19 monitors the average value of the voltage signal output from the current-voltage converter 4. The initial value search unit 20 starts an initial value search, and changes the Ich, Qch, and Pch biases in a range of ± Vπ while monitoring the CW light output of the optical modulation unit 14 with the second monitor unit 19. Then, the bias voltage is controlled (step S12). Then, the initial value search unit 20 searches for a combination of Ich and Qch bias that causes the CW light output of the light modulation unit 14 to be most extinguished regardless of the Pch bias (step S13). As a result, the search result of the Ich and Qch bias becomes the Null point of the extinction curve of the optical modulator that is the control target.

Next, it is determined whether or not a BOL (Begging of Life) condition is satisfied (step S14). The BOL condition means a drive voltage range obtained by subtracting a bias variation due to aging from the maximum drive voltage of the bias terminal. LiNbO ₃ is used for the optical modulation unit 14 that is a two-parallel MZ modulator, and it is desirable to determine a bias value to be given initially in consideration of a bias fluctuation due to deterioration over time. When it is determined that the search result of the Ich and Qch bias satisfies the BOL condition (Yes in step S14), the initial value search is terminated, and the search result of the Ich and Qch bias is set to the normal state in the bias voltage control. The initial value of the bias voltage is set (step S15).

  By performing the initial value search and the BOL condition determination of the initial value, the Ich and Qch biases can be obtained with good reproducibility even when temperature and aging degradation are taken into account. A stable start-up operation can be realized even under operating environment conditions such as START and HOT START.

  When it is determined that the Ich and Qch bias search results do not satisfy the BOL condition, the process returns to the initial value search in step S12 again (No in step S14).

  After the initial value search is completed, the control operation of the bias voltage as a normal state is the same as steps S6 to S9 described in the first embodiment, and the description is omitted.

  With the configuration and procedure of the optical transmitter described above, the control time until the optimum bias voltage is converged is shortened by starting the control after searching for the initial value before starting the control operation in the normal state. In addition, the control range of the bias voltage in the normal state can be narrowed. If the control range can be narrowed, the accuracy of bias setting with respect to the control target can be increased, and the control error can be minimized.

  In particular, when this optical transmitter is used as a dispersion pre-equalization optical transmitter, the dispersion pre-equalization amount is adjusted when searching for the dispersion pre-equalization amount and adjusting the optimum pre-equalization amount when installing the apparatus. The bias adjustment time to be performed each time is set is shortened, and a significant time shortening effect is expected. Furthermore, in order to minimize degradation of optical performance as a dispersion pre-equalization optical transmitter, high bias control accuracy is required, and the control method of the second embodiment is effective in increasing bias accuracy. Become a method.

  Next, the operation of the control unit 6 including the second monitor unit 19 and the initial value search unit 20 will be described in detail. FIG. 6 is an explanatory diagram for explaining the optical transmitter according to the second embodiment of the present invention. When the optical output of the optical modulation unit 14 is in the CW state, the Pch bias is set within a range of π / 2 ± π. The average maximum output voltage of the current-voltage conversion unit 4 when varied (in FIG. 6, the average maximum output voltage of the current-voltage conversion unit at the time of Phase π / 2 ± π variation, the unit is V), and the Ich bias This shows the relationship with the amount of deviation from the control target (hereinafter referred to as Ich bias deviation, the unit of which is Vπ). In each figure, the same numerals indicate the same or corresponding parts. In FIG. 6, regarding the three biases of Ich, Qch, and Pch, when considering any bias combination within a range of ± Vπ for each bias, the range of the average maximum output voltage of the current-voltage conversion unit 4 is indicated by a thick arrow. As shown, it is in the range of 0-30V.

  On the other hand, in FIG. 6, when the control is started with the bias voltage value for which the Ich and Qch bias is controlled in advance set to the initial value, the control range becomes the minimum range necessary for normal operation in the normal state. The Ich bias deviation is within a range of ± 0.3 Vπ from the control target indicated by the circle, and the average maximum output voltage of the current-voltage converter 4 is suppressed to 5 V or less as indicated by the thin arrow. In this way, by narrowing the control range, it is possible to increase the bias setting accuracy with respect to the control target. Therefore, this method is effective when high bias control accuracy such as a pre-equalized optical transmitter is required. In addition, by narrowing the control range, the low-voltage operation of the circuits constituting the current-voltage conversion unit 4, the first monitor units 10a and 10b, and the second monitor unit 19 is also possible, which is advantageous for low-power operation. .

  FIG. 7 is an explanatory diagram for explaining an optical transmitter according to Embodiment 2 of the present invention, in which a minute modulation signal (hereinafter referred to as a first monitor) detected by a first monitor unit 10a used for control in a normal state. This is a relationship between the Ich bias deviation with respect to the three types of Qch bias and the minute modulation signal detected by the unit. In each figure, the same numerals indicate the same or corresponding parts. The Pch bias is in-phase condition (in FIG. 7, the phase bias is called in-phase condition). In FIG. 7, the control target indicated by a circle is a point where both the minute modulation signal detected by the first monitor unit and the Ich bias deviation become zero. However, as indicated by the two curves that do not pass through the circle, when the Qch bias deviates from the control target Null point, the point at which the minute modulation signal detected by the first monitor unit at the Ich bias becomes zero is controlled. You can see that it is off target.

  This is because, in a 2-parallel MZ type optical modulator, when the same light intensity detector 17 is used to control three different biases of Ich, Qch, and Pch, the bias used for each bias control and the minute modulation signal It means that the relationship is sensitive to other biases. In the conventional control circuit, the control range of each bias is set to be wider than ± Vπ so that the convergence operation can be performed on the control target even when the control operation is started with an arbitrary combination of biases. For this reason, the setting error with respect to the control target becomes large, and there is a limit to increase the control accuracy. On the other hand, in the control unit 6 of the second embodiment, the control target is searched for the Ich and Qch bias in advance, and the control is started with the search result as an initial value. The range can be narrowed, and the control operation can be realized with high control accuracy.

  FIG. 8 is an explanatory diagram for explaining the optical transmitter according to the second embodiment of the present invention, and is a flowchart showing a bias control procedure. In each figure, the same numerals indicate the same or corresponding parts. In order to realize a more reliable control operation, a control method based on a flowchart showing a bias control procedure in FIG. 8 that combines FIG. 3 and FIG. 5 is also an effective method. In the initial value search state, the control target of the Ich and Qch bias can be obtained by using the average value of the light intensity of the optical signal, but the Pch bias is driven to the control target π / 2 bias by using the minute modulation signal. There is a need. As shown in FIG. 2B, it is desirable that the average drive amplitude ratio at which the maximum detection error is obtained is zero, and that the modulation signal is not input to the optical modulation unit. Therefore, by inserting the initial pull-in state control operation after the initial value search state and before performing the normal state control operation, the Pch bias can be reliably driven in a short time.

  That is, in FIG. 8, in steps S11 to S15, the control target of Ich and Qch bias is obtained in the initial value search state, and in steps S1 to S5, the control target of Pch bias is pulled in to the initial pulling state. After setting all the bias voltage values of the Qch and Pch biases as control targets, the control operation in the normal state is executed in steps S6 to S9. Thereby, as a control operation in the normal state, it is possible to control the bias voltage reliably and with high accuracy in a short time as compared with the case where there is no initial search state and initial pull-in state.

  FIG. 9 is an explanatory diagram for explaining the optical transmitter according to the second embodiment of the present invention, and shows the relationship between the modulation loss degradation and the modulation factor (average drive amplitude ratio). In each figure, the same numerals indicate the same or corresponding parts. In FIG. 9, since the average output voltage monitored by the second monitor unit 19 is a value equivalent to the modulation loss degradation shown on the vertical axis in FIG. 9, when this average output voltage is used, the horizontal axis in FIG. It is possible to calculate the degree of modulation with the average drive amplitude ratio shown in FIG. Therefore, the second monitor unit 19 may be used for modulation degree detection instead of the average modulation degree detection unit 12.

  As described above, in the optical transmitter according to the second embodiment of the present invention, in the Ich, Qch, and Pch bias control of the optical modulation unit 14 as the 2-parallel MZ type optical modulator, Search for initial values of Ich and Qch bias control based on the average value of the light intensity output from the optical modulation unit 14 when the Qch analog modulation signal and the low frequency minute modulation signal are not input to the optical modulation unit 14 without complete modulation. And ask for it. As a result, it is possible to shorten the time taken to reach the optimum bias voltage convergence point, to narrow the control range of the bias voltage in the normal state, and to improve the bias voltage setting accuracy. Further, after the initial value search state, the control operation in the initial pull-in state may be inserted before performing the control operation in the normal state. As a result, there is an effect that the Pch bias can be reliably driven in a short time.

  As described above, in the optical transmitter according to the second embodiment of the present invention, the second monitor unit 19 and the initial value search unit 20 in the control unit 6 have been described as digital signal processing circuits. The above may be realized by software processing using a computer program that causes a microcomputer or the like provided in the optical transmitter to execute a control method.

  Further, the optical signal transmitted from the optical transmitter according to the second embodiment of the present invention may be applied to an optical communication system in which an optical fiber is transmitted and received by an optical receiver. Also, two or more optical transmitters according to the second embodiment of the present invention are provided, and the optical signals transmitted from the two or more optical transmitters are wavelength-multiplexed to transmit the optical fiber, and the wavelength is separated on the receiving side. You may make it apply to the WDM (Wavelength Division Multiplexing) optical communication system made to receive with two or more optical receivers for every wavelength.

DESCRIPTION OF SYMBOLS 1 Light source 2a, 2b Modulation signal drive part 4 Current voltage conversion part 5a, 5b Minute signal amplification part 6 Control part 7 Synchronous detection part 8 Modulation signal drive control part 9a, 9b, 9c Bias application part 10a, 10b 1st monitor part 11a, 11b Minute modulation signal generator 12 Average modulation degree detector 13a, 13b Adder 14 Light modulator 15a First MZ light modulator 15b Second MZ light modulator 16 Optical phase adjuster 17 Light intensity detection Units 18a to 18g Optical waveguide 19 Second monitor unit 20 Initial value search unit

Claims (8)

A modulation signal driving unit that outputs a modulation signal driven based on the input data series signal;
An optical modulation unit that modulates light based on the applied bias voltage and the modulation signal input from the modulation signal driving unit, and outputs the modulated optical signal;
After controlling the bias voltage as an initial state in which the modulation signal is not input to the optical modulation unit from the modulation signal driving unit, the modulation signal driving unit performs the modulation based on the control result of the initial state. A control unit that controls the bias voltage as a normal state in which a signal is input to the light modulation unit, and
The controller, as the initial pull-in state as the initial state, pulls the bias voltage to the target value based on a low-frequency signal having a frequency lower than that of the modulation signal, and then performs the low-frequency operation as the normal state. Controlling the bias voltage based on a signal;
An optical transmitter characterized by.   The control unit searches and obtains an initial value of the bias voltage based on an average value of the light intensity of the optical signal as an initial value search state as the initial state, and from the obtained initial value of the bias voltage. For the first time, as the initial pull-in state as the initial state, after the pull-in to the target value of the bias voltage based on a low-frequency signal having a frequency lower than that of the modulation signal, the normal state is based on the low-frequency signal. The optical transmitter according to claim 1, wherein the bias voltage is controlled. A light source for inputting emitted light to the light modulator;
A light intensity detector that detects the light intensity of the optical signal output by the light modulator;
A current-voltage converter that converts a current signal indicating the light intensity detected by the light intensity detector into a voltage signal;
An amplifier for amplifying the voltage signal converted by the current-voltage converter;
An adder for adding the low frequency signal to the bias voltage,
The controller is
A first monitor for monitoring the low frequency signal contained in the voltage signal from the amplifier;
A bias applying unit that applies the bias voltage to the light modulation unit;
A low-frequency signal generator for generating the low-frequency signal;
An average modulation degree detecting unit for detecting an average modulation degree of the data series signal;
A synchronous detector that calculates a value of the bias voltage based on the low-frequency signal monitored by the first monitor and the average modulation degree;
In the initial state, the modulation signal is not output from the modulation signal driving unit, and in the normal state, the modulation signal driving unit outputs the modulation signal based on the average modulation degree from the modulation signal driving unit. A modulation signal drive controller for controlling the drive gain;
The optical transmitter according to claim 1 , further comprising: The controller is
A second monitoring unit for monitoring an average value of the voltage signal converted by the current-voltage conversion unit;
An initial value search unit for searching for an initial value of the bias voltage based on an average value of the voltage signal monitored by the second monitor unit;
The optical transmitter according to claim 3 , further comprising: The light modulation unit, MZ optical transmitter according to any one of claims 1 to 4, characterized in that it comprises a (Mach-Zehnder) type optical modulator. The optical modulation unit is a 2-parallel MZ type optical modulator,
The control unit controls the three bias voltages of Ich bias, Qch bias, and Pch bias,
The amplifying unit, the optical transmitter according to claim 3 or claim 4, characterized in that the high amplification degree of control of the bias voltage of the Pch bias compared to Ich bias and Qch bias. An optical communication system, wherein an optical signal transmitted from the optical transmitter according to any one of claims 1 to 6 is transmitted through an optical fiber and received by an optical receiver. A modulation signal driving step for outputting a modulation signal driven based on the input data series signal;
An optical modulation step of modulating light by an optical modulation unit based on the applied bias voltage and the modulation signal by the modulation signal driving step, and outputting the modulated optical signal;
After controlling the bias voltage as an initial state in which the modulation signal by the modulation signal driving step is not input to the optical modulation unit, based on the control result of the initial state, the modulation by the modulation signal driving step A control step for controlling the bias voltage as a normal state in which a signal is input to the optical modulation unit, and
In the control step, after the pull-in to the target value of the bias voltage based on a low-frequency signal having a frequency lower than that of the modulation signal as the initial pull-in state as the initial state, the low-frequency is set as the normal state. Controlling the bias voltage based on a signal;
An optical transmission method characterized by the above.

Images