1. Field of the Invention
The present invention relates to a drive control technique for an optical modulator used in optical communication. In particular, the invention relates to a drive control apparatus and drive control method, which can stably control the drive of an optical modulator used in ultra high speed optical transmission and the like.
2. Description of the Related Art
Recently, in optical transmission systems, with the progress of wavelength division multiplexing (WDM) technology and higher transmission speed per wavelength, systems enabling of large capacity optical transmission are being realized. Also, for the modulation code form for transmission used in such systems, in order to enlarge the transmission capacity and transmission distance, forms such as optical duo binary code and carrier suppressed RZ (CS-RZ) code have been proposed, instead of conventional NRZ (non return to zero) code or RZ (return to zero) code (refer to Japanese Unexamined Patent Publication No. 2001-119344). In particular, the CS-RZ code is said to be the most promising modulation code form for the next generation, since it enables enlargement of transmission distance and transmission capacity by a simple method.
Also, in optical modulation for ultra high speed transmission, in order to suppress wavelength fluctuations (chirping) during modulation, an external modulation method has been used, for allowing a semiconductor laser to continuously emit light, to switch the light on and off by an external modulator. For the external modulator, a Mach-Zehnder optical modulator (to be referred to as MZ modulator hereunder) is prevailing.
FIG. 31 shows an example of optical output characteristics of an MZ optical modulator with respect to a drive signal. In the conventional NRZ modulation method, on/off modulation of the optical output is performed by matching a high level and a low level of a drive signal, whose level fluctuates at a period T0, with an apex A of optical emission and an apex B of optical extinction in the optical output characteristics. In the following explanation, a difference of bias voltages corresponding to the apexes A and B of emission/extinction in the optical output characteristics which fluctuate periodically is represented by Vπ. According to this, an amplitude of the drive signal in the NRZ modulation method described above becomes Vπ.
For the MZ optical modulator as described above, the advantage is less chirping. However, there is a problem in that the optical output characteristics with respect to the drive signal drift timewise due to temperature change and a change with time, resulting in that interference between the codes occurs in the on/off level of the optical output. In order to solve this problem to stably control an operating point of the MZ optical modulator, a control at the operating point is necessary, such that, as shown in FIG. 31, in the case where a curve “a” showing the optical output characteristics with respect to the drive voltage moves to the left to become a curve “b”, a bias voltage by the drive signal is made lower according to the such a change, and in the case where the curve “a” moves to the right to become a curve “c”, the bias voltage is made higher according to such a change. In the conventional modulation method using the NRZ code or the RZ code, a compensation technique has been proposed in Japanese Patent Publication No. 3-251815, for superimposing a low frequency signal onto the drive signal to detect an fluctuation amount and a fluctuation direction of the operating point, and then controlling the bias voltage by feedback to hold the operating point normal.
Under the background as described above, also for the new modulation methods such as the optical duo binary modulation method and the CS-RZ modulation method and the like, an operating point control technique for the MZ optical modulator which can correspond to these modulation methods is required.
As an operating point control technique for an MZ optical modulator corresponding to the CS-RZ modulation method, there is the one disclosed in Japanese Unexamined Patent Publication No. 2001-119344. In this conventional technique, as shown in FIG. 32, a transmission section of the CS-RZ modulation method comprises: a transmission light source 110 that generates continuous light; an MZ optical modulator 120 on a data modulation side, that is driven in accordance with a drive signal obtained by adjusting, by a drive circuit 121, a data signal corresponding to the NRZ modulation method so that the amplitude thereof becomes Vπ, and an MZ optical modulator 130 on a clock modulation side, that is driven in accordance with a drive signal obtained by adjusting, by a drive circuit 131, a clock signal having a frequency of ½ the bit rate of the data signal so that the amplitude thereof becomes 2Vπ.
In such a transmission section of the CS-RZ modulation method, by adjusting a crossing point in an output from the optical modulator 120 on the data modulation side, to an optical extinction phase of the optical modulator 130 on the clock modulation side, an optical signal having the same waveform as for when the RZ code is used is output as an optical output. Therefore, for the optical modulator 120 on the data modulation side, since the drive signal of the NRZ code form with amplitude Vπ is used in the same manner as for the aforementioned case shown in FIG. 31, the operating point can be optimized by applying the conventional operating point control method. On the other hand, for the optical modulator 130 on the clock modulation side, for example as shown in FIG. 33, since the waveform of the drive signal is a sine wave with amplitude 2Vπ, it becomes necessary to control the operating point so that the amplitude of the drive signal at the time of optical extinction becomes zero.
In addition, for the operating point control method of the MZ optical modulator corresponding to the optical duo binary modulation method, the technique disclosed in Japanese Unexamined Patent Publication No. 2000-162563 is known. In this conventional technique, for example as shown in FIG. 34, different to the case of the modulation method using the NRZ code or the RZ code, a drive signal corresponding to the optical duo binary code, being a differential code with amplitude 2Vπ, is supplied to an MZ optical modulator 210. This drive signal is superimposed with an operating point control signal of low frequency generated by an oscillator 212. A part of the optical output from the MZ optical modulator 210 is branched by an optical coupler 220, and then sent to a phase comparator 223 via an optical receiver 221 and an I/V converter 222. In the phase comparator 223, a phase comparison is performed on the superimposed component contained in the optical output, and the operating point control signal, to thereby detect the fluctuation amount and the fluctuation direction of the operating point. Then, the operating point is optimized by feeding back the result of the phase comparison to a control of a bias supply circuit 225. Furthermore, in the conventional technique described above, for example as shown in FIG. 35, a compensation method has also been proposed, for performing a control such that the frequency of the superimposed component contained in the optical output becomes a maximum, by utilizing the phenomenon where the frequency of the superimposed component becomes twice the frequency of the operating point control signal when the bias voltage coincides with the operating point, to hold the operating point normal.
Considering the control of the operating point of the MZ optical modulator on the clock modulation side in the CS-RZ modulation method by applying the conventional operating point control technique as described above, even if an operating point control signal with a low frequency is superimposed on the clock signal, since the waveform of the clock signal is a sine wave, the optical output from the optical modulator only becomes a “1” level (apex A of optical emission) temporally, and there is no point where a “1” level is continuous for the optical output, as in the case where an operating point control signal is superimposed on a rectangular wave data signal. Therefore, the superimposed component with a low frequency corresponding to the operating point control signal rarely appears in the optical output from the optical modulator. Consequently, in the conventional method of superimposing the operating point control signal, it is difficult to detect the fluctuation amount and the fluctuation direction of the operating point in the MZ optical modulator on the clock modulation side.
Also, in the method which controls the operating point by detecting that the frequency of the superimposed component contained in the output from the optical modulator becomes twice the frequency of the operating point control signal, in the MZ light modulator-on the clock modulation side, since the point where the optical output becomes a maximum is only temporary as described above, the detection signal becomes faint so that it is difficult to determine the frequency of the superimposed component. Moreover, in the conventional method, there is also a problem in that when the amplitude of the drive signal is shifted from 2Vπ, then in the optical output, two bias points exist which generate a superimposed component with twice the frequency of the operating point control signal, and in the case where the operating point is controlled based on either of the bias points as a reference, only output waveforms with deviated duty are obtained, so that normal modulation operation cannot be realized.
Furthermore, in the transmission section of the CS-RZ modulation method as shown in FIG. 32, in order to obtain a desired optical output, an adjustment or a control is required at several points such as the amplitude and the phase between the two drive signals of differential form corresponding to the clock signal, and the phases between the clock signal and the data signal. Therefore, there is a disadvantage in that when providing the additional configurations for the conventional drive control, this invites a higher cost and a larger size for the transmission section.