JP3822548B2 - Optical modulator controller - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical modulator control device used in an optical communication system or the like, and in particular, a control for stabilizing an optical signal output from an electrical / optical converter having characteristics of bias voltage input and optical signal output having periodicity. The present invention relates to technology, and is used, for example, in a long-distance high-speed optical fiber communication network using an optical duobinary modulation system.
[0002]
[Prior art]
In an optical communication system using a high-speed optical fiber, an NRZ modulation method that performs optical modulation using a NRZ (Non Return to Zero) signal, which is a binary digital signal, is generally used, but the time division method (TDM) is used. When increasing the capacity, waveform degradation due to chromatic dispersion (GVD) becomes a factor that limits the transmission distance. Also, since the dispersion tolerance is inversely proportional to the square of the data transmission rate (bit rate), assuming that the dispersion tolerance is about 800 ps / nm in a 10 Gb / s system, the dispersion tolerance is in a 40 Gb / s system with a transmission rate of 4 times. Decreases to about 50 ps / nm of 1/16 and becomes severe.
[0003]
An optical duobinary modulation method has been proposed as one of the methods for reducing the above-described waveform degradation due to wavelength dispersion (see, for example, Non-Patent Document 1).
[0004]
This optical duobinary modulation scheme reduces the influence of chromatic dispersion by reducing the bandwidth of the optical signal spectrum by about half compared to the NRZ modulation scheme. For example, the bandwidth of the optical signal spectrum in a 10 Gb / s system is 10 GHz in frequency and 0.2 nm in wavelength in the case of NRZ modulation, whereas 5 GHz in wavelength in the case of optical duobinary modulation. 0.1nm, which is half that of NRZ modulation.
[0005]
The propagation speed of light differs depending on the wavelength, and the greater the bandwidth of the optical signal spectrum, the greater the change in propagation speed and the greater the waveform distortion caused by long-distance transmission. If the bandwidth can be reduced, the fluctuation width of the propagation speed is reduced, and the dispersion tolerance is increased.
[0006]
Here, an outline of the optical duobinary modulation method will be described.
[0007]
FIG. 5 shows a configuration of a modulation unit based on the optical duobinary modulation method.
[0008]
FIG. 6 shows a signal waveform and an eye pattern of each part in FIG.
[0009]
FIG. 7 shows the principle of the optical duobinary modulation method.
[0010]
In FIG. 5, 101 is a semiconductor laser, and 102 is a Mach-Zehnder type optical modulator (hereinafter referred to as MZ type modulator). 103 is a precoder that encodes a binary NRZ input signal of 40 Gb / s, 104 is a modulator driver that functions as an amplitude adjuster, and 105 is a passband for low-frequency signals about 1/4 of the bit rate (BR). A low-pass filter (LPF) 106 has a bias adjusting circuit (bias tee), and 107 is a terminator.
[0011]
After the binary NRZ signal input is encoded by the precoder 103, the amplitude is adjusted by the modulator driver, and after passing through the low-pass filter 105, it is converted into a ternary signal and the signal of the MZ modulator 102 Applied to the electrode.
[0012]
In the optical duobinary modulation method, the drive voltage of the MZ type modulator 102 is doubled compared to the NRZ modulation method, so that the MZ type modulator 102 is modulated with a drive amplitude (= 2Vπ) that is twice Vπ. In addition, the DC bias voltage (the center of the drive voltage) is set so as to be driven between two adjacent vertices among the periodic light emission vertices on the drive voltage versus light output characteristic curve.
[0013]
6, (a) is a binary NRZ input signal and its eye pattern, (b) is an output signal and its eye pattern of the precoder 103, (c) is an output signal and its eye pattern of the low-pass filter 105, (D) is a diagram showing an output optical signal of the MZ type modulator 102 and its eye pattern.
[0014]
As can be seen from FIG. 6, the output optical signal of the MZ modulator is an optical signal having exactly the same logic as the binary NRZ signal input, and therefore a receiver (not shown) that receives this optical signal is It can be converted into a binary NRZ signal without the need for a decoder.
[0015]
The optical duobinary modulation method as described above is characterized in that the bandwidth of the optical signal spectrum is about half that of the conventional NRZ modulation method, and the influence of chromatic dispersion can be reduced. In the WDM system, channels can be arranged with higher density. In other words, when increasing the capacity by wavelength multiplexing technology, the wavelength band that can be amplified by the optical amplifier is one of the limiting factors, but by utilizing the narrow bandwidth of the optical signal spectrum of the optical duobinary modulation method. The channels can be arranged at a higher density within the amplification bandwidth of the optical amplifier.
[0016]
By the way, the conventional direct modulation method (method of increasing / decreasing the driving current of the semiconductor laser in accordance with the modulation input) is affected by wavelength fluctuation (chirping) of the output optical signal and dispersion in the optical fiber as the transmission speed increases. As a result, long-distance transmission has become difficult.
[0017]
On the other hand, the MZ-type modulator can perform electro-optical conversion without causing wavelength fluctuation in principle, and has the advantage that the fluctuation in wavelength of transmitted light is small when used in an optical transmission device. is there.
[0018]
However, in the MZ type modulator, the operating point of the drive voltage vs. optical output characteristics fluctuates with time (DC drift) as the temperature of LiNbO3 substrate changes with time, etc., and the output (optical signal) is not good. There is a problem that it becomes stable.
[0019]
[Non-Patent Document 1]
AJPrice et al., "Reduced Bandwidth Optical Digital Intensity Modulation with Improved Chromatic Dispersion Tolerance", Electron. Lett., Vol.31, No.1, pp.58-59, 1995
[0020]
[Problems to be solved by the invention]
As described above, the optical duobinary modulation method using the conventional MZ type modulator has a problem that the output optical signal becomes unstable due to the characteristic variation of the MZ type modulator. For this reason, the optical duobinary modulation optical communication system that uses the MZ-type modulator always operates stably and the bias voltage is controlled according to the operating point fluctuation so that the transmission output (optical signal) is stabilized. It becomes necessary to do.
[0021]
The present invention has been made in view of the above circumstances, and a voltage having a signal amplitude between two adjacent light emission vertices or between two adjacent light emission extinction points in the periodic characteristic of the optical modulator voltage versus light output. When using a modulation system that drives an MZ type modulator, the change in voltage vs. optical output characteristics of the MZ type modulator due to temperature fluctuations, aging fluctuations, etc. can be compensated for with a simple configuration, which reduces the size. It is an object of the present invention to provide an optical modulator control device that is easy.
[0023]
[Means for Solving the Problems]
This onset bright light modulator control device has a driving voltage versus light output characteristic represented by the curve emitting vertices or quenching vertex repeated periodically, around the drive voltage is a DC bias voltage corresponding to the three-value signal An optical modulator that modulates input light in accordance with the drive voltage and outputs a binary optical signal, a DC bias generation circuit that generates the DC bias voltage, and a binary NRZ signal input in three values The signal is converted into a signal, and based on this ternary signal, a drive voltage having a signal amplitude corresponding to the two adjacent points of the light emission extinction vertex or the light extinction vertex of the drive voltage versus the optical output characteristic of the optical modulator is generated. A drive circuit for supplying to the optical modulator in consideration of the DC bias voltage generated by the DC bias generation circuit, and a light detection means for detecting an average light output level representing a time average of the light output of the light modulator And the DC bar Control means for determining the DC bias voltage based on a difference in average optical output power of the optical modulator before and after slightly increasing or decreasing the bias voltage.
[0029]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0030]
<First Embodiment>
FIG. 1 is a block diagram showing an optical transmission device using an optical modulator control device for optical fiber communication according to the first embodiment of the present invention. In the figure, the optical modulator control device uses an optical duobinary modulation system, and the circuit portion of the optical modulator control device is constituted by, for example, a multichip module including a plurality of semiconductor devices.
[0031]
In FIG. 1, light emitted from a semiconductor laser 101 that is a light source is input to an MZ modulator 102 that is an external modulator. On the other hand, a binary NRZ signal input from the outside is input to the precoder 103 and encoded. The output signal of the precoder 103 is amplified by the modulator driver 104 and then converted to a ternary digital signal by the low-pass filter (LPF) 105. The ternary digital signal adjusts the bias voltage with a bias tee to generate a drive signal for the MZ modulator 102.
[0032]
The MZ type modulator 102 modulates the output light from the semiconductor laser 101 according to the drive signal. The output light of the MZ type modulator 102 is bifurcated by an optical branching unit 108, one of which is output as an optical signal, and the other is input to a photodiode (PD) 109 for optical signal monitoring. The monitor PD 109 converts the input optical signal into a current proportional to the optical power and supplies it to the current / voltage conversion type amplifier 110. The amplifier 110 converts the output current of the monitor PD 109 into a voltage and outputs an optical output monitor voltage Vav. Here, the monitor PD 109 and the amplifier 110 detect the time average value Pav of the optical output power of the MZ type modulator 102, and do not require high-speed response characteristics that are often expensive.
[0033]
The output voltage Vav of the amplifier 110 is sent to the control unit 111. In this example, the control unit 111 includes an MCU (micro control unit) 113, an A / D converter 112, and a D / A converter 114.
[0034]
The MCU 113 converts the input voltage value Vav into a digital signal by the A / D converter 112, and then stores it in a memory area built in the MCU, for example. Thereafter, the voltage input to the bias tee 106 via the D / A converter 114 and the differential amplifier 115 is increased or decreased so as to give a slight change to the bias voltage of the MZ type modulator 102 (Vav stored in the memory area). . Then, referring to the voltage value Vav ′ after increasing / decreasing the bias voltage as described above, a difference ΔVav (= Vav′−Vav) from the voltage value Vav before increasing / decreasing is obtained, and bias control is performed based on the ΔVav. A signal is generated, converted into an analog signal by the D / A converter 114, and sent to the differential amplifier 115. The bias control signal is amplified by the differential amplifier 115 and applied as the bias voltage Vb of the MZ modulator 102 via the bias tee 106.
[0035]
The operation principle of the bias control method of the optical duobinary MZ modulator 102 shown in FIG. 1 will be described below with reference to FIGS.
[0036]
FIG. 2 is a characteristic diagram showing an example of the relationship (light transmission characteristics) between the input drive voltage and the output optical signal when the magnitude of the DC bias voltage Vb of the MZ type modulator 102 shown in FIG. 1 is changed. .
[0037]
FIG. 3 is a characteristic diagram showing the relationship between the DC bias voltage Vb of the MZ modulator 102 shown in FIG. 1 and the difference ΔPav between the average power outputs of the output optical signals.
[0038]
In FIG. 2, the signal amplitude Vpp of the input voltage of the MZ type modulator 102 is in a state equal to 2Vπ. Vπ is the difference between the drive voltage when the light transmittance is the maximum value (peak) and the drive voltage when the light transmittance is the minimum value (zero). In FIG. 2, + QUAD is a point where the light transmittance is an intermediate value between the maximum value and the minimum value, and the slope is positive, and -QUAD is a point where the light transmittance is an intermediate value between the maximum value and the minimum value, and the slope is negative. Is shown.
[0039]
When the bias voltage Vb of the MZ modulator 102 is at the optimum operating point Vnull (the bias voltage that matches the extinction peak of the curve of the drive voltage versus the optical output characteristic), the ternary input signal (the low-pass filter 105) A binary output optical signal is obtained.
[0040]
However, the bias voltage Vb of the MZ type modulator 102 shifts its light transmission characteristics in the horizontal axis direction in FIG. 2 due to DC drift (operating point drift).
[0041]
In FIG. 2, (1) and (1) ′ indicate the input voltage waveform and the output light waveform when the bias voltage Vb of the MZ modulator 102 is increased by a minute amount ΔV (ΔV> 0) from the immediately preceding value. The relationship is shown, and the time average value of the output optical signal power in this case is indicated by Pav (1).
[0042]
Also, (2) and (2) ′ show the relationship between the input voltage waveform and the output light waveform when the bias voltage Vb of the MZ type modulator 102 is reduced by a minute amount ΔV (ΔV> 0) from the previous value. The time average value of the output optical signal power in this case is indicated by Pav (2).
[0043]
And the difference (Pav (2) -Pav (1)) of the time average value of output optical signal power is shown as (DELTA) Pav. In this case, (1) and (2) in FIG. 2 indicate the case where the bias voltage Vb is shifted to the positive side (rightward on the horizontal axis in the figure) with respect to the optimum operating point Vnull due to DC drift. It can be seen that Pav (1) when the drift amount to the positive side with respect to Vnull is large is lower than Pav (2) when the drift amount is small.
[0044]
Contrary to the above, even when the bias voltage Vb is shifted to the negative side (leftward on the horizontal axis in the figure) with respect to the optimum operating point Vnull due to DC drift (not shown), It can be seen that Pav (1) when the drift amount is large is lower than Pav (2) when the drift amount is small.
[0045]
As described above, when DC drift occurs, the amplitude of the output optical signals {circle around (1)}, {circle around (2)} decreases compared to the case where the bias voltage Vb is at the optimum operating point Vnull, thereby reducing the extinction ratio and the optical power. Since degradation occurs, it is necessary to compensate for DC drift.
[0046]
That is, when a DC drift occurs, it is necessary to compensate for the DC drift by considering the drift amount as a drive voltage change amount and changing the value of the drive voltage by the voltage change amount ΔVb. This compensation can be equivalently realized by changing Vb by ΔVb.
[0047]
In FIG. 3, a bias voltage Vbopt at which the average power difference ΔPav = 0 is a bias value when the relationship between the light transmission characteristics of the MZ modulator 102 and the input signal is in an optimal relationship, and is Vnull (voltage vs. light). The bias voltage to match the extinction peak of the output characteristic curve).
[0048]
Here, as shown in FIG. 2, when Vb is positioned on the positive side from the optimum value Vnull due to DC drift (when the light transmission characteristic of the MZ modulator 102 has shifted to the left from the optimum state). , Vb is increased or decreased by ΔV (ΔV> 0) from the immediately preceding value (when moving away from or closer to Vnull), ΔPav <0. Contrary to the above, when Vb is positioned on the negative side from the optimum value Vnull due to DC drift (when the light transmission characteristics of the MZ modulator 102 are shifted to the right from the optimum state), Vb is set to just before it. ΔPav> 0 when ΔV (ΔV> 0) is increased / decreased from the value (when approaching or moving away from Vnull).
[0049]
Therefore, the difference ΔPav between the average power before and after increasing / decreasing Vb is detected, the amount and direction of the DC drift deviation are determined based on the absolute value | ΔPav | and the polarity, and ΔPav is set to zero by changing Vb. If the control is performed, Vb can be matched with the optimum bias voltage Vnull.
[0050]
FIG. 4 is a flowchart showing an example of a control procedure by the control unit 111 in the optical modulator control device shown in FIG. This control is realized by writing a control program in the program ROM of the MCU 113 and executing it.
[0051]
Next, a control procedure will be described with reference to FIG.
[0052]
(1) In the first step S501, Vb is set to an initial value (usually 0V).
[0053]
(2) In the second step S502, constants (ΔVb, Vπ, etc.) necessary for control are read from the external memory.
[0054]
(3) In the third step S503, Vb is increased by a slight change ΔV (> 0) with respect to the initial value (Vb = Vb + ΔV).
[0055]
(4) In a fourth step S504, the time average value of the optical output power (actually the voltage value Vav) is referred to and stored as Pav (1) in a memory area built in the MCU 113.
[0056]
(5) In a fifth step S505, Vb is decreased by a slight change ΔV with respect to the initial value (Vb = Vb−2ΔV).
[0057]
(6) In the sixth step S506, the time average value of the optical output power (actually the voltage value Vav) is referred to and stored as Pav (2) in the memory area built in the MCU 113.
[0058]
(7) In the seventh step S507, Vb is returned to the initial value (Vb = Vb + ΔV).
[0059]
(8) In the eighth step S508, ΔPav = Pav (2) −Pav (1) is calculated, the determination process is performed on the value as follows, and the branch process is performed according to the result (condition). .
[0060]
(9) When ΔPav is larger than the allowable error ε (positive value), Vb is decreased and set to Vb−ΔVb × | ΔPav | (9th step S509). That is, Vb is greatly decreased as | ΔPav | increases.
[0061]
(10) When ΔPav is smaller than the allowable error −ε, Vb is increased and set to Vb + ΔVb × | ΔPav | (tenth step S510). That is, Vb is greatly increased as | ΔPav | increases.
[0062]
(11) When ΔPav is equal to or smaller than ε and equal to or larger than −ε, Vb is considered to be at the optimum operating point, and the value of Vb is not changed (11th step S511), and control is waited for T1 seconds (first). 12 step S512), and then the process returns to the third step S503, and the above-described controls (3) to (11) are repeated.
[0063]
According to the control procedure described above, even if the light transmission characteristics of the MZ type modulator 102 change due to changes in ambient temperature or changes over time, the Vb of the MZ type modulator 102 is corrected and controlled accordingly, so that it always operates optimally. Can be maintained.
[0064]
If the order of increase / decrease of ΔV is reversed from the above example, that is, Vb is decreased by ΔV from the initial value in the third step S503, and Vb is decreased by ΔV from the initial value in the fifth step S505. When ΔPav is larger than ε, Vb is increased by ΔVb × | ΔPav |, and when ΔPav is smaller than −ε, Vb is decreased by ΔVb × | ΔPav |. .
[0065]
That is, the optical modulator control device of the present invention includes an optical modulator that outputs a binary optical signal by supplying a driving voltage corresponding to a ternary signal as a modulation input centered on a DC bias voltage, and the optical modulator. A bias voltage determining unit that determines the DC bias voltage based on average light output information representing a time average of the light outputs. The bias voltage determination unit determines the DC bias voltage based on a difference in average optical output power between the optical modulators before and after the DC bias voltage is slightly increased or decreased.
[0066]
As a specific example, the optical modulator control device of the first embodiment has a drive voltage versus light output characteristic indicated by a curve in which the light emission peak or the extinction peak repeats periodically, and the drive voltage corresponding to the ternary signal is An optical modulator that is given around a DC bias voltage, modulates the input light according to the drive voltage, and outputs a binary optical signal, a DC bias generation circuit 106 that generates a DC bias voltage, and a binary NRZ The signal input is converted into a ternary signal, and based on this ternary signal, a signal amplitude corresponding to two adjacent points of the periodic light emission peak (or extinction peak) of the driving voltage versus the optical output characteristic of the optical modulator is obtained. And a drive circuit (103, 104, 105, 106) that generates a drive voltage having the same voltage and supplies the drive voltage to the optical modulator in a superimposed manner. Here, the signal amplitude of the drive voltage of the optical modulator (for example, MZ type modulator 102) is set to twice the difference Vπ of the drive voltage when the light transmittance of the optical modulator is the maximum value and the minimum value. By controlling the DC bias voltage to match a specific extinction vertex (or light emission vertex) of the driving voltage versus the optical output characteristic of the optical modulator based on the detection result of the difference in the average optical output level, an optical duo Realizes binary modulation.
[0067]
Further, in the optical modulator control device of the first embodiment, the light detection means 109 for detecting the average optical output level representing the time average of the optical output of the optical modulator, and the light before and after the DC bias voltage is slightly increased or decreased And a control means 111 for determining a DC bias voltage based on a difference in average optical output power of the modulator.
[0068]
Here, the control means 111 selects a control signal for controlling the DC bias generation circuit so that the current value of the DC bias voltage is centered on the current value of the DC bias voltage and is binary increased or decreased at a frequency sufficiently lower than the ternary signal. The difference between the average light output levels detected by the light detection means before and after the binary change of the DC bias voltage is detected, and the direction of the DC bias voltage is determined based on the detection result. Therefore, control is performed so as to correct only an appropriate amount.
[0069]
As a specific example of the control means 111, a micro control unit is used. The micro control unit includes a first control means for initially setting the DC bias voltage Vb to 0 V, a second control means for reading a constant necessary for control from an external memory, and the DC bias voltage Vb from the immediately preceding value. The difference ΔPav between the time average values of the respective optical output powers when increasing / decreasing by the unit change amount ΔVb is obtained, and when the absolute value of ΔPav is greater than or equal to a predetermined value, the change direction according to the sign of ΔPav depending on the magnitude And a third control means for controlling so that the bias voltage Vb is changed, and waiting for a predetermined time when the absolute value of ΔPav is less than a predetermined value, and repeatedly performing the control by the third control means. Control means.
[0070]
According to the optical modulator control device having the above-described configuration, the MZ modulator can be controlled according to the operating point drift of the MZ modulator 102 due to a DC bias voltage applied to the MZ optical modulator 102, an environmental temperature, a change with time, and the like. It is possible to compensate for the operating point drift of 102 and perform control so as to operate at the optimum operating point, and to prevent deterioration of the extinction ratio of the output optical signal due to operating point drift.
[0071]
Further, since the magnitude of the DC bias voltage is changed in a binary manner using the differential amplifier 115, an expensive variable gain amplifier having a wide dynamic range is not required, and simplification and miniaturization are facilitated. In addition, since synchronous detection is not performed, the circuit configuration is simplified in this respect, the number of parts is reduced, and miniaturization is facilitated.
[0072]
The optical transmission device of the present invention includes the optical modulator control device of the present invention as described above, a light source that emits input light of the optical modulator, and an optical signal output from the optical modulator to an optical communication fiber. And means for transmitting. This makes it possible to provide the features of the optical modulator control device described above.
[0073]
Also, the control method of the optical modulator of the present invention is to increase or decrease the DC bias voltage of the drive voltage according to the ternary signal supplied as the modulation input of the optical modulator, and to change before and after the increase and decrease change. The DC bias voltage is controlled based on the difference in the average optical output power of the optical modulator. This makes it possible to realize an optical modulator control device and an optical transmission device having the above-described features.
[0074]
Also, the optical modulator control method of the present invention provides a first bias voltage to the optical modulator along with the driving voltage corresponding to the ternary signal to modulate the input light according to the driving voltage and output the optical signal. A first step, a second step of detecting an average optical output power representing a time average of the optical output of the optical modulator, and a binary change of the DC bias voltage at a frequency sufficiently lower than that of the ternary signal. The difference between the average optical output power detected by the second step before and after changing the DC bias voltage in a binary manner is detected, and the DC bias voltage is calculated based on the detection result. And a third step of controlling. This makes it possible to realize an optical modulator control device and an optical transmission device having the above-described features.
[0075]
Further, the control program for the optical modulator of the present invention is based on the difference in the optical output average power of the optical modulator before and after the DC bias voltage of the optical modulator is changed binary with respect to the micro control unit. The present invention realizes a function of controlling the DC bias voltage of the optical modulator. As a result, it is possible to realize an optical modulator control method having the above-described features.
[0076]
The optical modulator control program of the present invention is an optical signal output from an optical modulator in which a drive voltage corresponding to a ternary signal is supplied as a modulation input centered on a DC bias voltage to a micro control unit. The function of detecting the average optical output level representing the time average of the current, the function of generating a control signal for binary change of the DC bias voltage at a frequency sufficiently lower than the input signal, and the magnitude of the DC bias voltage The difference between the average optical output levels detected before and after the binary change is detected, and the function of controlling the DC bias voltage of the optical modulator based on the detection result is realized. . As a result, it is possible to realize an optical modulator control method having the above-described features.
[0077]
【The invention's effect】
As described above, according to the optical modulator control device of the present invention, for example, when using the optical duobinary modulation method, even if the characteristics of the optical modulator fluctuate due to ambient temperature and changes with time, with a simple device configuration, It becomes possible to stabilize the output optical signal.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an optical transmission apparatus using an optical modulator control apparatus for optical fiber communication according to a first embodiment of the present invention.
2 is a characteristic diagram showing an example of a relationship (light transmission characteristic) between an input drive voltage and an output optical signal when the magnitude of the DC bias voltage Vb of the MZ type modulator shown in FIG. 1 is changed.
3 is a characteristic diagram showing a relationship between a direct current bias voltage Vb of the MZ modulator shown in FIG. 1 and a difference ΔPav between average power outputs of output optical signals.
4 is a flowchart showing an example of a control procedure by a control unit in the optical modulator control device shown in FIG. 1;
FIG. 5 is a block diagram illustrating a modulation unit using an optical duobinary modulation method.
6 is a diagram showing signal waveforms and eye patterns of respective parts in FIG. 5;
FIG. 7 is a view for explaining the principle of the optical duobinary modulation method;
[Explanation of symbols]
101 ... Semiconductor laser (light source),
102… MZ type optical modulator,
103… Precoder,
104 ... modulator driver,
105… Low-pass LPF
106… Biasty,
107… Terminator,
108… light splitting means,
109… photodiode for monitoring,
110… amplifier,
111… Control unit,
112… A / D converter,
113… MCU,
114… D / A converter,
115… differential amplifier.

Claims (6)

Drive voltage vs. light output characteristics indicated by a curve in which the light emission peak or the light extinction peak repeats periodically, and the drive voltage corresponding to the ternary signal is given around the DC bias voltage, and the input light according to the drive voltage An optical modulator that modulates and outputs a binary optical signal;
A DC bias generation circuit for generating the DC bias voltage;
A binary NRZ signal input is converted into a ternary signal, and based on this ternary signal, it corresponds to between two adjacent points of the periodic light emission extinction vertex or extinction vertex of the driving voltage versus optical output characteristic of the optical modulator. A drive circuit that generates a drive voltage having a signal amplitude, and supplies the drive voltage to the optical modulator in superposition with the DC bias voltage generated by the DC bias generation circuit;
Light detecting means for detecting an average light output level representing a time average of the light output of the light modulator;
And a control means for determining the DC bias voltage based on a difference in average optical output power of the optical modulator before and after the DC bias voltage is slightly increased or decreased. The control means includes
A function of selectively generating a control signal for controlling the DC bias generating circuit so that the current value of the DC bias voltage is changed in a binary manner at a frequency sufficiently lower than the ternary signal around the current value of the DC bias voltage. And detecting a difference in average light output level detected by the light detection means before and after changing the DC bias voltage in a binary manner, and setting the DC bias voltage in an appropriate direction based on the detection result optical modulator control device of the amount by claim 1, wherein the controller controls so as to correct such. The drive circuit sets the signal amplitude of the drive voltage to twice the difference Vπ of the drive voltage when the light transmittance of the optical modulator is the maximum value and the minimum value,
The control means controls the DC bias voltage to coincide with a specific extinction apex or emission apex of the driving voltage versus the optical output characteristic of the optical modulator based on the detection result of the difference in the average optical output level. The optical modulator control device according to claim 1 . The control means uses a micro control unit, the micro control unit,
First control means for initially setting the DC bias voltage Vb to 0V;
Second control means for reading constants necessary for control from an external memory;
The difference ΔPav between the time average values of the respective optical output powers when the DC bias voltage Vb is increased or decreased from the previous value by the unit change amount ΔVb is obtained. If the absolute value of ΔPav is greater than or equal to a predetermined value, the magnitude is increased. In response, third control means for controlling the bias voltage Vb to change in a changing direction according to whether ΔPav is positive or negative, and controlling to wait for a predetermined time when the absolute value of ΔPav is less than a predetermined value;
Optical modulator control device according to any one of claims 1 to 3, characterized in that it comprises a repetition control means for repeatedly control by the third control means. The third control means includes
Fourth control means for setting the DC bias voltage Vb higher by a unit change amount ΔVb than the previous value;
The fifth control means for referring to the time average value of the optical output power after the control by the fourth control means and storing it in the memory area built in the micro control unit as the first average optical output power Pav (1). When,
Sixth control means for setting the DC bias voltage Vb lower by 2ΔVb than the set value by the fourth control means;
A seventh control means for referring to the time average value of the optical output power after the control by the sixth control means and storing it as a second average optical output power Pav (2) in a memory area built in the micro control unit. When,
The DC bias voltage Vb is returned to the value before setting by the fourth control means, and the difference ΔPav (= Pav (2) −Pav (1)) between the two average optical output powers is calculated, An eighth control means for conditional branching;
When ΔPav is larger than the allowable error ε (positive value), Vb is controlled to decrease and set to Vb−ΔVb × ΔPav. When ΔPav is smaller than the allowable error −ε, Vb is increased. Control is performed to set Vb + ΔVb × ΔPav. When ΔPav is equal to or smaller than ε and equal to or larger than −ε, Vb is considered to be at the optimum operating point, and the control waits for a predetermined time without changing the value of the bias voltage Vb. The optical modulator control device according to claim 4 , further comprising: ninth control means. The third control means includes
Fourth control means for setting the DC bias voltage Vb lower by a unit change amount ΔVb than the previous value;
The fifth control means for referring to the time average value of the optical output power after the control by the fourth control means and storing it in the memory area built in the micro control unit as the first average optical output power Pav (1). When,
Sixth control means for setting the DC bias voltage Vb higher by 2ΔVb than the set value by the fourth control means;
A seventh control means for referring to the time average value of the optical output power after the control by the sixth control means and storing it as a second average optical output power Pav (2) in a memory area built in the micro control unit. When,
The DC bias voltage Vb is returned to the value before setting by the fourth control means, and the difference ΔPav (= Pav (2) −Pav (1)) between the two average optical output powers is calculated, An eighth control means for conditional branching;
When ΔPav is larger than the allowable error ε (positive value), Vb is controlled to increase by ΔVb × ΔPav, and when ΔPav is smaller than the allowable error −ε, Vb is decreased by ΔVb × ΔPav. And when the ΔPav is equal to or less than ε and equal to or greater than −ε, Vb is regarded as being at the optimum operating point, and the ninth control means waits for a predetermined time without changing the value of the bias voltage Vb. The optical modulator control device according to claim 4 .

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