JP2004118060A - Driving gear - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving gear controlled so as to output a stable output light signal having little deterioration of a quenching rate from an optical modulator by stabilizing an operation point of the optical modulator and having little fluctuation of a light waveform. <P>SOLUTION: The driving gear for giving an adjusting drive signal for the optical modulator on the basis of drive voltage in response to an input drive signal comprises a low frequency oscillating means; a superposing means for superposing a low frequency signal to an electric input signal; a compositing means for compositing the low frequency signal to a bias signal in response to a phase difference between the low frequency component of an output light signal of the light modulator and the original low frequency signal of the low frequency oscillating means; and a drive means for giving the drive voltage in which an upper limit waveform and a lower limit wave for to the optical modulator are asymmetric to each other on the basis of a superposing signal of the superposing means and a composite signal of an adding means. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a driving device, and can be applied to, for example, a device that adjusts a voltage applied to an external optical modulator such as a Mach-Zehnder type optical modulator and stabilizes an operating point of the external optical modulator.
[0002]
[Prior art]
[0003]
For example, a Mach-Zehnder (MZ) type optical modulator represented by an LN optical modulator (lithium niobate modulator) using lithium niobate (LiNbO3) is applied with It is known that the transmission characteristic of light changes sinusoidally with respect to voltage, but this transmission characteristic is sensitive to disturbances such as temperature and changes with time (bias point drift).
[0004]
Here, the “bias point drift (operating point drift)” refers to the characteristic curve (curve indicating the light transmission characteristic of the external modulator) representing the relationship between the applied voltage and the light intensity. That is.
[0005]
There are various methods for compensating this, and the method (low frequency superposition method) disclosed in Patent Document 1 is widely used because it is simple.
[0006]
FIG. 2 is a circuit diagram showing a low-frequency superposition method according to Patent Document 1. In the low-frequency superposition method disclosed in Patent Document 1, an external light modulator 2 (for example, a Mach-Zehnder (MZ) type optical modulator) is driven by a superposition signal in which a low-frequency signal 4 is superimposed on an electric signal 3 so that external light The low-frequency signal is also superimposed on the output optical signal output from the modulator 2. The output optical signal is optically branched, a low-frequency signal is detected from the output optical signal, a phase difference between the detected low-frequency signal and the original low-frequency signal is detected, and a bias point is feedback-controlled. That is.
[0007]
[Problems to be solved by the invention]
However, the low-frequency superposition method disclosed in Patent Document 1 described above has a problem that the transmission light waveform is deteriorated, particularly the extinction ratio is deteriorated because the low-frequency signal is directly superimposed on the optical signal used for transmission.
[0008]
FIG. 3 is a diagram showing a waveform of an output optical signal according to the above-described conventional low-frequency superposition method. As shown in FIG. 3, it can be seen that the optical power (light intensity) of the waveform of the output optical signal (“optical waveform” in the figure) fluctuates vertically symmetrically due to the superposition of the low-frequency signal. Since this waveform is similarly modulated at both the 1 level and the 0 level, the symmetry of the optical waveform is good, but deterioration from the viewpoint of the extinction ratio is inevitable.
[0009]
Therefore, there is a need for a drive device that stabilizes the operating point of the optical modulator and controls the optical modulator to output a stable output optical signal with less fluctuation of the optical waveform and less deterioration of the extinction ratio.
[0010]
[Means for Solving the Problems]
In order to solve such a problem, a driving device of the present invention provides a driving device that provides an adjusted driving signal to an optical modulator that optically modulates light from a light source based on a driving voltage according to an input driving signal. A low-frequency oscillating means for oscillating the high-frequency signal, a superimposing means for superposing the low-frequency signal on the electric input signal, a low-frequency component contained in the optical signal output from the optical modulator, A synthesizing means for synthesizing the low-frequency signal with a bias signal corresponding to a phase difference with the low-frequency signal, a superimposed signal superimposed by the superimposing means, and a synthesized signal synthesized by the adding means. And a drive unit for applying a drive voltage to the optical modulator in which the lower waveform and the lower waveform are asymmetric.
[0011]
BEST MODE FOR CARRYING OUT THE INVENTION
(A) First Embodiment Hereinafter, a first embodiment of a driving device according to the present invention will be described in detail with reference to the drawings.
[0012]
In the first embodiment, a driving voltage input to a Mach-Zehnder optical modulator (MZ optical modulator) is adjusted, a deviation of an operating point of the MZ optical modulator is compensated, and the operating point is stabilized. A case where the present invention is applied to a driving device will be described.
[0013]
(A-1) Configuration of First Embodiment FIG. 1 is a circuit diagram showing a configuration example of an optical modulation device including a drive device of the present embodiment. In FIG. 1, a thick line indicates a path of an optical signal, and a thin line indicates a path of an electric signal. The optical modulator shown in FIG. 1 performs feedback control on a low-frequency signal, and adjusts a drive voltage based on feedback information.
[0014]
As shown in FIG. 1, the optical modulation device includes a light source 1, a Mach-Zehnder (MZ) type optical modulator 2, an optical branch 9, and a driving device 100 according to the present embodiment. Reference numeral 100 denotes an electric signal (modulated electric signal) 3, a low-frequency oscillator 4, an amplifier 5, a capacitor 6, a bias tee 7, a terminating resistor 8, a photodiode (PD) 10, and a band-pass filter (BPF). 11), an amplifier 12, a multiplier 13, a low-pass filter (LPF) 14, a differential amplifier 15, a comparison voltage 16, an adder 17, an amplifier 18, a voltage dividing resistor 19, and an inverting switch 20. Is provided.
[0015]
The low-frequency oscillator 4 oscillates a low-frequency signal of a predetermined frequency (f0), and supplies the oscillated low-frequency signal to the amplifier 5 and also to the inverting switch 20 and the multiplier 13.
[0016]
The amplifier 5 amplifies the electric signal (modulated electric input signal) 3 to an amplitude suitable for modulation. The amplifier 5 receives a low-frequency signal from the low-frequency oscillator 4, superimposes the low-frequency signal on the electric signal 3, and supplies the superimposed signal to the MZ optical modulator 2 via the capacitor 6. is there. When amplifying the electric signal 3 to an amplitude suitable for modulation, the amplifier 5 performs amplitude modulation with a low-frequency signal to generate a superimposed signal.
[0017]
In the present embodiment, the amplifier 5 is described as having an amplitude amplification function and a superposition function, but these functions may be realized by different modules.
[0018]
FIG. 4 is a diagram illustrating a waveform of an electric signal input to the Mach-Zehnder optical modulator 2 according to the first embodiment. FIG. 4A shows a waveform of an electric signal applied from the capacitor 6 to the MZ optical modulator 2.
[0019]
As shown in FIG. 4A, the electric signal output from the amplifier 5 is an AC signal (AC signal) whose envelope cycle is 1 / f0 because the electric signal is superimposed on the low-frequency signal. Since the electric signal passes through the capacitor 6 after the superposition of the low-frequency signal, the electric signal has a waveform which is narrowed to zero and oscillated vertically.
[0020]
The Mach-Zehnder (MZ) optical modulator 2 has one modulation input terminal receiving a superimposed signal in which the electric signal 3 and the low frequency signal are superimposed from the amplifier 5 via the capacitor 6, and the other modulation input terminal. A terminal receives an output signal (a superimposed signal in which a bias signal and a low-frequency signal are superimposed) from the directly connected bias tee 7 and a driving voltage corresponding to these driving signals (the superimposed signal and the output signal of the bias tee 7). , A continuous light (for example, a coherent light such as a laser light) from the light source 1 is modulated, an input signal is converted into an optical signal, and an output optical signal is provided to the optical branch 9.
[0021]
The MZ type optical modulator 2 is an example of an optical modulator, and may be applied to any other optical modulator having an electric input signal terminal and a bias signal input terminal.
[0022]
The output signal from the bias tee 7 input to the MZ type optical modulator 2 will be described later.
[0023]
The optical branch 9 branches the output optical signal from the MZ type optical modulator 2 into two, and gives one of the two to a photodiode (PD) 10. The other of the two branches is supplied to an external device (not shown) as an output optical signal.
[0024]
The photodiode (PD) 10 is a unit having an optical-electrical conversion function of converting a received output optical signal into an electric signal. The PD 10 supplies the converted electric signal to a band-pass filter (BPF) 11.
[0025]
The band-pass filter (BPF) 11 is a means for extracting a predetermined frequency component (f0). The band-pass filter (BPF) 11 extracts a low-frequency component from the electrical signal obtained by electrically converting the output optical signal by the PD 10 and outputs the low-frequency component. The frequency component is provided to the multiplier 13 via the amplifier 12. Therefore, the BPF 11 does not extract frequency components other than the predetermined frequency.
[0026]
The low frequency component extracted by the BPF 11 corresponds to the frequency component of the low frequency signal oscillated by the low frequency oscillator 4. That is, the BPF 11 extracts the frequency component of the low-frequency signal included in the output optical signal.
[0027]
The multiplier 13 receives a low frequency component included in the electric signal extracted by the BPF 11 via the amplifier 12, receives a low frequency signal (original low frequency signal) from the low frequency oscillator 4, and This is to multiply the low frequency signal.
[0028]
The LPF 14 is means for passing a signal having a frequency equal to or lower than a predetermined frequency.
[0029]
When the output signal obtained by the multiplication by the multiplier 13 is given to the LPF 14, the LPF 14 passes a signal having a frequency equal to or lower than a predetermined frequency. The LPF 14 compares the phases of these two signals, and responds to the phase difference. A DC voltage (DC voltage) is output.
[0030]
Therefore, the direction of the operating point drift of the MZ type optical modulator 2 can be detected based on the DC voltage (DC voltage) output from the LPF 14.
[0031]
The differential amplifier 15 receives the DC voltage from the LPF 14 and calculates an output corresponding to a difference from the comparison voltage 16 based on the DC voltage. By making the comparison voltage 16 variable, it can be used for adjusting the bias control point. Of course, the comparison resistor 16 may be set to a constant voltage.
[0032]
The adder 17 receives the output signal from the differential amplifier 15, receives the low-frequency signal whose gain has been adjusted by the amplifier 18 and the voltage-dividing resistor 19, and biases the added signal obtained by adding the low-frequency signal to the output signal. 7 is given.
[0033]
FIG. 4B is a diagram illustrating a waveform of an addition signal of the adder 17.
[0034]
The adder 17 receives a DC signal (DC signal) corresponding to the difference from the comparison voltage 16 from the differential amplifier 15 and adds a low-frequency signal to the DC signal, so that the addition signal shown in FIG. Shows a waveform having fluctuation of a predetermined cycle (1 / f0) for one polarity.
[0035]
The inverting switch 20 receives the low-frequency signal from the low-frequency oscillator 4, inverts the low-frequency signal, and supplies the inverted signal to the voltage dividing resistor 19.
[0036]
The inversion switch 20 has a function of switching the polarity of the low-frequency signal, and can cope with a case where the logic of the MZ type optical modulator 2 is a positive logic and a case where the logic is a negative logic.
[0037]
In the present embodiment, the inversion switch 20 is provided between the low-frequency oscillator 4 and the voltage dividing resistor 19, but the inversion switch 20 is superimposed on the electric signal 3 input to the MZ optical modulator 2. Since this is a means for finally switching either the polarity of the low-frequency signal and the low-frequency signal superimposed on the bias signal, for example, the inverting switch 20 is connected between the low-frequency oscillator 4 and the amplifier 5. May be provided.
[0038]
The voltage dividing resistor 19 receives a low frequency signal from the inverting switch 20 and divides the electric field (voltage) of the low frequency signal. The voltage dividing resistor 19 supplies the divided low frequency signal to the amplifier 18. This voltage dividing resistor 19 may be variable.
[0039]
The amplifier 18 receives the divided low-frequency signal from the voltage dividing resistor 19 and adjusts the gain. The amplifier 18 supplies the gain-adjusted low-frequency signal to the adder 17.
[0040]
The bias tee 7 has a low-frequency port and a high-frequency port. The low-frequency port is connected to the adder 17, and the high-frequency port is connected to the terminating resistor 8. The bias tee 7 is connected to the MZ type optical modulator 2, receives the addition signal from the adder 17, and supplies the addition signal to the MZ type optical modulator 2. The addition signal functions as a bias point voltage to the MZ optical modulator 2, and the bias tee 7 supplies a bias signal corresponding to the addition signal from the adder 17 to the MZ optical modulator 2. .
[0041]
FIG. 4C is a diagram showing a waveform of a drive signal that the MZ type optical modulator 2 takes in as a drive voltage. In FIG. 1, the MZ optical modulator 2 has a configuration in which the superimposed signal from the capacitor 6 and the added signal from the bias tee 7 are separately captured, respectively. The driving signal to be captured is a signal having a waveform shown in FIG.
[0042]
The waveform of the drive signal shown in FIG. 4C is obtained by adding the superimposed signal from the capacitor 6 of FIG. 4A and the addition signal of the adder 17 of FIG. 4B. . In this drive signal, the fluctuation of one polarity is canceled, and the fluctuation of the logic of the other polarity is doubled in amplitude. It should be noted that, except when the characteristic of superimposing the low-frequency signal by the amplifier 5 is linear, it cannot be completely canceled, but in most cases, it can be canceled to a level that hardly causes a problem.
[0043]
Further, in FIG. 4C, the fluctuation on the 0 level side of the drive signal is canceled, but the fluctuation on the 1 level side may of course be canceled. In this case, it is necessary to reverse the polarity of the output signal of the adder 17 shown in FIG. Here, the amplitude and the polarity of the output signal shown in FIG. 4B can be freely adjusted by the amplifier 18, the voltage dividing resistor 19 and the inverting switch 20.
[0044]
(A-2) Operation of First Embodiment Next, the operation of the first embodiment will be described.
[0045]
The continuous light emitted by the light source 1 enters the MZ type optical modulator 2.
[0046]
The electric signal 3 is amplitude-amplified by the amplifier 5 and superimposed on the low-frequency signal oscillated from the low-frequency oscillator 4, and the superimposed signal is supplied to the MZ optical modulator 2 via the capacitor 6. .
[0047]
The output optical signal output from the MZ type optical modulator 2 is split by an optical splitter 9, applied to a photodiode 10, and subjected to opto-electric conversion by the photodiode 10. The converted electric signal is provided to the BPF 11.
[0048]
In the BPF 11, when the electric signal from the photodiode 10 includes a predetermined frequency component (f0), the frequency component is extracted and given to the multiplier 13. The multiplier 13 also receives a low-frequency signal (original low-frequency signal) from the low-frequency oscillator 4, and compares the phase of the extracted frequency component with the phase of the original low-frequency signal by the multiplier 13 and the LPF 14. Then, a DC voltage corresponding to the phase difference is supplied to the differential amplifier 15.
[0049]
That is, when a predetermined frequency component is extracted by the BPF 11, the operating point drift of the MZ optical modulator 2 is determined based on the DC voltage from the LPF 14 corresponding to the phase difference between the frequency component and the original low-frequency signal. Can be detected.
[0050]
The difference between the DC voltage output from the LPF 14 and the comparison voltage 16 is amplified in the differential amplifier 15, and the output signal of the differential amplifier 15 is provided to the adder 17.
[0051]
Accordingly, the differential amplifier 15 detects the direction of the operating point drift, and adjusts the phase so that the output optical signal does not include the low frequency signal in consideration of the operating point drift direction.
[0052]
The output signal from the differential amplifier 15 is added to the low-frequency signal whose amplitude has been adjusted in the adder 17, and the added signal is supplied to the MZ optical modulator 2 via the bias tee 7.
[0053]
At this time, the drive voltage for driving the MZ type optical modulator 2 is a voltage corresponding to a drive signal obtained by adding the superimposed signal from the input capacitor 6 and the addition signal from the bias tee 7. One of the fluctuations is canceled. As a result of the addition of the driving signals shown in FIG. 4C, fluctuations on the lower limit side (0 level side) of the driving signals are canceled.
[0054]
FIG. 5 shows the relationship between the driving voltage (applied voltage in FIG. 5) based on the driving signal and the light intensity (optical power) of the output optical signal (optical waveform in FIG. 5) in the MZ optical modulator 2. FIG.
[0055]
The applied voltage shown in FIG. 5 is a voltage according to the drive signal shown in FIG. 4C, and the direction in which the fluctuation of the drive voltage is canceled out is the minimum transmission of the waveform of the transmission characteristic of the MZ optical modulator 2. The applied voltage is applied to the MZ type optical modulator 2 in accordance with the point. At this time, the other fluctuation of the drive voltage has a periodicity of a predetermined period 1 / f0.
[0056]
When an applied voltage is applied to the MZ optical modulator 2, an output optical signal is output based on the applied voltage. One of the fluctuations of the applied voltage is canceled and the canceled one is matched to the transmission characteristic of the MZ optical modulator 2, so that the lower limit waveform (0 level) of the output optical signal does not fluctuate. Further, since the position of the middle point between the upper and lower limits of the other fluctuation of the applied voltage matches the maximum transmission point of the transmission characteristic of the MZ optical modulator 2, the upper limit side waveform (one level) of the output optical signal is predetermined. It will have a periodicity of 1/2 f0. That is, since the lower limit waveform (0 level) of the output optical signal is stable, the extinction ratio is improved as compared with the conventional case of FIG.
[0057]
Further, since the fluctuation of the upper limit side waveform of the output optical signal has a period of 1 / 2f0, the frequency f0 component is not extracted even if the output optical signal is given to the BPF 11 after the optical-electrical conversion, so that the multiplication is performed. The low-frequency signal component is not detected by the detector 13 and the LPF 14.
[0058]
FIG. 6 shows the relationship between the drive voltage (applied voltage in FIG. 6) based on the drive signal when the operating point drift occurs in the positive direction and the light intensity (light power) of the output light signal (light waveform in FIG. 6). FIG.
[0059]
As shown in FIG. 6, when the operating point drift occurs in the positive direction, the output optical signal intensity has a periodicity of 1 / f0, and its phase is the same as that of the original low-frequency signal.
[0060]
That is, when the DC voltage corresponding to the phase difference between the low-frequency signals is positive by the multiplier 13 and the LPF 14, it is possible to detect that the operating point is drifting in the positive direction.
[0061]
FIG. 7 shows the relationship between the drive voltage (applied voltage in FIG. 7) based on the drive signal when the operating point drift occurs in the negative direction and the light intensity (light power) of the output light signal (light waveform in FIG. 7). FIG.
[0062]
As shown in FIG. 7, when the operating point drift occurs in the negative direction, the output optical signal intensity has a periodicity of 1 / f0, and its phase is opposite to that of the original low frequency signal.
[0063]
That is, when the DC voltage corresponding to the phase difference between the low-frequency signals is negative by the multiplier 13 and the LPF 14, it is possible to detect that the operating point is drifting in the negative direction.
[0064]
(A-3) Effects of the First Embodiment As described above, according to the present embodiment, the adder 17 adds the low frequency signal to the output signal from the differential amplifier 15 for compensating the bias point. Is generated, and the added signal and the superimposed signal obtained by superimposing the low-frequency signal on the modulated electric signal are added, so that the fluctuation of one waveform of the drive signal can be canceled, and this drive signal is used. One level of the optical waveform of the output optical signal can also be stabilized, so that the extinction ratio can be improved.
[0065]
(B) Second Embodiment Next, a second embodiment of the driving device according to the present invention will be described with reference to the drawings.
[0066]
The second embodiment is applied to the first embodiment, that is, a drive device that cancels fluctuation of one side level of a drive signal input to an MZ optical modulator and stabilizes one level of an output optical signal. is there.
[0067]
(B-1) Configuration and Operation of Second Embodiment FIG. 8 is a circuit diagram showing a configuration of an optical modulation device including a drive device according to the second embodiment.
[0068]
As shown in FIG. 8, the optical modulation device includes a light source 1, an MZ type optical modulator 2, an optical branch 9, an optical amplifier 21, and a driving device 200 according to the second embodiment. The device 200 includes an electric signal (modulated electric signal) 3, a low-frequency oscillator 4, an amplifier 5, a capacitor 6, a bias tee 7, a terminating resistor 8, a photodiode (PD) 10, and a band-pass filter ( (BPF) 11, an amplifier 12, a multiplier 13, a low-pass filter (LPF) 14, a differential amplifier 15, a comparison voltage 16, an adder 17, an amplifier 18, and a voltage dividing resistor 19. It is.
[0069]
When the configuration of the second embodiment has the configuration requirements corresponding to those of the first embodiment, the corresponding reference numerals are given, and the description of the functions thereof will be omitted.
[0070]
The second embodiment is characterized in that an optical amplifier 21 is provided downstream of the optical branch 9.
[0071]
The optical amplifier 21 is a means for receiving the output optical signal split by the optical splitter 9, making the output optical signal correspond to a predetermined gain, and compressing the fluctuation of one side level of the output optical signal.
[0072]
This predetermined gain will be described with reference to the relationship diagram of the input / output characteristics of the optical amplifier 21 with respect to the low frequency signal shown in FIG.
[0073]
As shown in FIG. 9, the gain of the optical amplifier 21 with respect to the low-frequency signal has a so-called saturated state in which when the value of the input light exceeds a predetermined value, the value of the output light becomes substantially constant. ing.
[0074]
Therefore, when the output optical signal from the optical branch 9 is input, the optical amplifier 21 can constantly compress the fluctuation of the output optical signal according to the relationship of the saturation state.
[0075]
For example, as shown in FIG. 5, in the first embodiment, the output optical signal output from the MZ type optical modulator 2 can stabilize by canceling the fluctuation at the 0 level, but can stabilize the output optical signal at the 1 level. The amplitude is made twice as large as that of the conventional method without canceling the fluctuation. The optical amplifier 21 can compress this one-level fluctuation and output it.
[0076]
(B-3) Effects of the Second Embodiment As described above, according to the present embodiment, by providing the optical amplifier 21 having the predetermined input / output characteristics, the output optical signal waveform fluctuates according to the input / output characteristics. Can be compressed.
[0077]
Further, according to the present embodiment, when the light intensity of only one side level is stabilized as in the first embodiment, the fluctuation of the opposite side level can be compressed to a constant level. As a result, it is possible to obtain an optical modulation waveform of an output optical signal having small fluctuations at both levels.
[0078]
(C) Other Embodiments In the above-described first and second embodiments, as shown in FIG. 5 (C), the fluctuation of the waveform of the drive signal on the 0 level side is canceled and stabilized. The fluctuation on the one level side may be canceled. Further, since it is sufficient to prevent the deterioration of the extinction ratio, the invention is not limited to canceling the one-sided waveform of the drive signal, and the waveforms on both sides of the drive signal may be adjusted so as to improve the extinction ratio. . Therefore, the adjustment may be performed so that the upper limit waveform and the lower limit waveform of the drive signal are asymmetric in a broad sense.
[0079]
In the first and second embodiments, the drive voltage is supplied via the bias tee 7, but if the waveform of the drive signal modulated at a low frequency can be adjusted, the capacitor 6 and the bias tee 7 are provided. It may not be necessary.
[0080]
In the second embodiment, the fluctuation of the output light is compressed by using the saturation characteristic of the optical amplifier 21. However, the optical amplifier 21 has a time constant that is sufficiently faster than the period (1 / f0) of the low-frequency signal, and The same effect can be obtained by performing the constant output control.
[0081]
【The invention's effect】
As described above, according to the present invention, in a driving device that provides an adjusted drive signal to an optical modulator that optically modulates a light source light based on a drive voltage corresponding to an input drive signal, a low-frequency oscillator that oscillates a low-frequency signal Oscillating means, superimposing means for superimposing a low-frequency signal on the electrical input signal, low-frequency components included in the optical signal output from the optical modulator, and the position of the original low-frequency signal oscillated by the low-frequency oscillating means. Based on the synthesizing means for synthesizing the low-frequency signal with the bias signal according to the phase difference, the superimposed signal superimposed by the superimposing means, and the synthesized signal synthesized by the adding means, And a driving means for applying a driving voltage to the optical modulator to provide an asymmetrical driving voltage, thereby stabilizing the operating point of the optical modulator, reducing the fluctuation of the optical waveform, and reducing the extinction ratio. The light modulator It can be controlled so as to output.
[Brief description of the drawings]
FIG. 1 is a circuit diagram illustrating a configuration of an optical modulator according to a first embodiment.
FIG. 2 is a circuit diagram showing a configuration of a conventional optical modulator.
FIG. 3 is a diagram showing an output optical signal waveform according to a conventional low-frequency superposition method.
FIG. 4 is a diagram illustrating a waveform of an electric signal input to the MZ-type optical modulator according to the first embodiment.
FIG. 5 is a diagram illustrating a relationship between a driving voltage based on a driving signal and a light intensity of an output optical signal when an operating point drift does not occur in the MZ optical modulator according to the first embodiment.
FIG. 6 is a diagram illustrating an output optical signal waveform when the operating point drift of the first embodiment occurs in the positive direction.
FIG. 7 is a diagram illustrating an output optical signal waveform when the operating point drift occurs in the negative direction according to the first embodiment.
FIG. 8 is a circuit diagram illustrating a configuration of an optical modulator according to a second embodiment.
FIG. 9 is a diagram showing a relationship between input and output characteristics of an optical amplifier with respect to a low-frequency signal according to the second embodiment.
[Explanation of symbols]
2 Mach-Zehnder (MZ) optical modulator 3 Electric signal (modulated electric signal)
4 low frequency oscillator, 5 amplifier, 7 bias tee,
10: photodiode, 11: band-pass filter (BPF),
12 amplifier, 13 multiplier, 14 low-pass filter (LPF)
15: differential amplifier, 17: adder, 18: amplifier, 19: voltage dividing resistor,
20: reversing switch, 21: optical amplifier.

Claims (2)

For a light modulator that optically modulates light source light based on a drive voltage according to an input drive signal, a drive device that provides the adjusted drive signal,
Low frequency oscillation means for oscillating a low frequency signal;
Superimposing means for superimposing the low-frequency signal on the electric input signal;
Combining the low-frequency signal with a bias signal corresponding to the phase difference between the low-frequency component included in the output optical signal from the optical modulator and the original low-frequency signal oscillated by the low-frequency oscillator. Means,
A driving unit that supplies a drive voltage to the optical modulator, in which an upper limit waveform and a lower limit waveform are asymmetric based on the superimposed signal superimposed by the superimposing unit and the synthesized signal synthesized by the adding unit. A driving device, comprising: 2. The driving device according to claim 1, further comprising an optical amplifying unit for compressing an optical intensity width of an output optical signal from the optical modulator.

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