JP2004117729A - Optical modulator controlling device, optical transmitting device, and controlling method and controlling program for optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To stabilize an output optical signal by a simple device system even if characteristics of an optical modulator are fluctuated by the change of ambient temperature and the secular change when an optical duo binary modulation system is utilized. <P>SOLUTION: In the optical duo binary modulation system, DC bias voltage is increased and decreased so as to compensate DC drift of the optical modulator by the change of ambient temperature and the secular change and a bias controlling signal is formed based on the difference between time average power values of the output optical signals when the DC bias voltage is increased and decreased to correct the DC bias voltage to be the optimum value. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an optical modulator control device, an optical transmission device, an optical modulator control method and a program for use in an optical communication system or the like, and more particularly, to an electric / electrical device having a characteristic in which a bias voltage input and an optical signal output have periodicity. The present invention relates to a control technique for stabilizing an optical signal output from an optical converter, and is used, for example, in a long-distance high-speed optical fiber communication network using an optical duobinary modulation method.
[0002]
[Prior art]
In an optical communication system using a high-speed optical fiber, an NRZ (Non Return to Zero) signal, which is a binary digital signal, is generally used to perform optical modulation, but an NRZ modulation method is used. When increasing the capacity, waveform degradation due to chromatic dispersion (GVD) is 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), if the dispersion tolerance is about 800 ps / nm in a 10 Gb / s system, the dispersion tolerance in a 40 Gb / s system whose transmission speed is quadrupled. Is reduced to 1/16, about 50 ps / nm, and becomes severe.
[0003]
An optical duobinary modulation method has been proposed as one of the methods for reducing the waveform deterioration due to the chromatic dispersion (for example, see Non-Patent Document 1).
[0004]
This optical duobinary modulation method reduces the influence of chromatic dispersion by making the bandwidth of the optical signal spectrum approximately half as compared with the NRZ modulation method. For example, the bandwidth of the optical signal spectrum in the 10 Gb / s system is 10 GHz in frequency and 0.2 nm in wavelength in the case of the NRZ modulation system, whereas it is 5 GHz in the case of the optical duobinary modulation system. The wavelength is 0.1 nm, which is half that of the NRZ modulation method.
[0005]
Since the propagation speed of light differs depending on the wavelength, the larger the bandwidth of the optical signal spectrum, the greater the change in the propagation speed and the greater the collapse of the waveform due to long-distance transmission. Can be reduced, the fluctuation width of the propagation speed becomes smaller, and the dispersion tolerance increases.
[0006]
Here, an outline of the optical duobinary modulation method will be described.
[0007]
FIG. 5 shows a configuration of a modulation unit using the optical duobinary modulation method.
[0008]
FIG. 6 shows a signal waveform of each part in FIG. 5 and an eye pattern thereof.
[0009]
FIG. 7 shows the principle of the optical duobinary modulation method.
[0010]
In FIG. 5, reference numeral 101 denotes a semiconductor laser, and 102 denotes a Mach-Zehnder optical modulator (hereinafter, referred to as an MZ modulator). Reference numeral 103 denotes a precoder that encodes a 40 Gb / s binary NRZ input signal, 104 denotes a modulator driver that functions as an amplitude adjustment unit, and 105 denotes a low-band signal pass band of about 1/4 of a bit rate (BR). A low-pass filter (LPF) 106 is a bias adjustment 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 a modulator driver, and further passed through a low-pass filter 105 to be converted into a ternary signal. Applied to the electrodes.
[0012]
In the optical duobinary modulation method, the driving voltage of the MZ modulator 102 is twice as large as that in the NRZ modulation method, so that the MZ modulator 102 is modulated with a driving amplitude twice as large as Vπ (= 2Vπ). Further, the DC bias voltage (center of the driving voltage) is set so that the driving is performed between two adjacent vertexes of the periodic light emitting peaks on the driving voltage versus light output characteristic curve.
[0013]
6A shows a binary NRZ input signal and its eye pattern, FIG. 6B shows an output signal of the precoder 103 and its eye pattern, FIG. 6C shows an output signal of the low-pass filter 105 and its eye pattern, (D) is a diagram showing the output optical signal of the MZ 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, so that a receiver (not shown) for receiving this optical signal It can be converted to a binary NRZ signal without the need for a decoder.
[0015]
The optical duobinary modulation method as described above has a feature that the bandwidth of the optical signal spectrum is reduced to about half as compared with the conventional NRZ modulation method, and the influence of chromatic dispersion can be reduced. In the WDM) system, channels can be arranged at 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 using the narrow band property of the optical signal spectrum of the optical duobinary modulation method. In addition, 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 (a method in which the drive current of the semiconductor laser is adjusted in accordance with the modulation input) is affected by the wavelength fluctuation (chirping) of the output optical signal and the 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 modulator is capable of performing electro-optical conversion without causing a wavelength fluctuation in principle, and has an advantage in that the fluctuation of the wavelength of the transmission light is small by using the optical transmission apparatus. is there.
[0018]
However, in the MZ modulator, the operating point of the drive voltage versus light output characteristic fluctuates with time (DC drift) due to a change in temperature or aging of LiNbO3 as a substrate material, and the output (light signal) is not good. There is a problem that it becomes stable.
[0019]
[Non-patent document 1]
A. J. Price et al. , “Reduced Bandwidth Optical Digital 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 modulator has a problem that the output optical signal becomes unstable due to the characteristic fluctuation of the MZ modulator. For this reason, the optical communication system of the optical duobinary modulation method using the MZ type modulator controls the bias voltage according to the operating point fluctuation so that the operation always stably operates and the transmission output (optical signal) is stabilized. Need to be done.
[0021]
The present invention has been made in view of the above circumstances, and a voltage having a signal amplitude between two adjacent light emitting peaks or between two adjacent light emitting vanishing points in a periodic characteristic of a voltage versus an optical output of an optical modulator. When a modulation method that drives an MZ modulator is used, it is possible to compensate for changes in the voltage-optical output characteristics of the MZ modulator due to temperature fluctuations, aging fluctuations, and the like with a simple configuration, thereby achieving miniaturization. An object of the present invention is to provide an optical modulator control device, an optical transmission device, an optical modulator control method, and a program thereof, which can be easily performed.
[0022]
[Means for Solving the Problems]
A first optical modulator control device according to the present invention includes: an optical modulator to which a drive voltage corresponding to a ternary signal is supplied as a modulation input centering on a DC bias voltage and outputs a binary optical signal; A bias voltage determining unit that determines the DC bias voltage based on average light output information representing a time average of the light output of the device.
[0023]
The second optical modulator control device of the present invention has a drive voltage versus light output characteristic indicated by a curve in which a light emission peak or an extinction peak periodically repeats, and a drive voltage corresponding to a ternary signal has a DC bias voltage. An optical modulator that is provided as a center, modulates input light according to 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 The driving signal is converted into a ternary signal, and based on the ternary signal, a driving voltage having a signal amplitude corresponding to a period between two adjacent light emitting peaks or extinction peaks of the driving voltage versus light output characteristic of the optical modulator. A driving circuit for generating and superimposing the DC bias voltage generated by the DC bias generation circuit and supplying the same to the optical modulator; and a light for detecting an average optical output level representing a time average of the optical output of the optical modulator. Detecting means; And a controller configured to determine the DC bias voltage based on a difference between an average optical output power of the optical modulator before and after the DC bias voltage is slightly increased / decreased.
[0024]
The optical transmitter of the present invention is an optical modulator control device of the present invention, a light source that emits input light of the optical modulator, and a unit that transmits an optical signal output from the optical modulator to an optical communication fiber. It is characterized by having.
[0025]
According to a first control method of an optical modulator of the present invention, a DC bias voltage of a drive voltage corresponding to a ternary signal supplied as a modulation input of an optical modulator is increased or decreased in a binary manner, and before and after the increase or decrease. The DC bias voltage is controlled based on the difference in the average optical output power of the optical modulator.
[0026]
According to a second control method of the optical modulator of the present invention, a DC bias voltage is applied to the optical modulator together with a drive voltage corresponding to a ternary signal, so that input light is modulated according to the drive voltage to output an optical signal. A first step, a second step of detecting an average optical output power representing a time average of an optical output of the optical modulator, and a step of converting the DC bias voltage into a binary signal at a frequency sufficiently lower than the ternary signal. Including a function of generating a control signal for changing the DC bias voltage, detecting a difference between the average optical output powers detected in the second step before and after the DC bias voltage is changed in a binary manner. And controlling the DC bias voltage based on the third step.
[0027]
The control program of the first optical modulator control device according to the present invention provides a control program for a micro control unit, wherein the difference between the optical output average power of the optical modulator before and after the DC bias voltage of the optical modulator is binary-changed. A function of controlling the DC bias voltage of the optical modulator based on
[0028]
According to the control program of the second optical modulator control device of the present invention, the drive voltage corresponding to the ternary signal is output from the optical modulator supplied as a modulation input centering on the DC bias voltage to the micro control unit. A function of detecting an average optical output level representing a time average of an optical signal, a function of generating a control signal for binary-changing the DC bias voltage at a frequency sufficiently lower than an input signal, and a function of generating the control signal. A function of detecting a difference between the detected average optical output levels before and after changing the magnitude of the bias voltage in a binary manner, and controlling a DC bias voltage of the optical modulator based on the detection result. It is characterized by.
[0029]
BEST MODE FOR CARRYING OUT 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 a first embodiment of the present invention. In the figure, an optical modulator control device uses an optical duobinary modulation method, and a circuit portion of the optical modulator control device is configured by, for example, a multi-chip module including a plurality of semiconductor devices.
[0031]
In FIG. 1, light emitted from a semiconductor laser 101 as a light source is input to an MZ modulator 102 as 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 amplitude-amplified by a modulator driver 104, and then converted into a ternary digital signal by a low-pass filter (LPF) 105. The ternary digital signal generates a drive signal for the MZ modulator 102 by adjusting a bias voltage with a bias tee.
[0032]
The MZ modulator 102 modulates the output light from the semiconductor laser 101 according to the drive signal. The output light of the MZ modulator 102 is split into two by an optical splitter 108, one is output as an optical signal, and the other is input to a photodiode (PD) 109 for monitoring an optical signal. The monitor PD 109 converts the input optical signal into a current proportional to the optical power and supplies the current 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 modulator 102, and do not require a high-speed response characteristic that is 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 stores the digital signal in, for example, a memory area built in the MCU. Thereafter, as a voltage input to the bias tee 106 via the D / A converter 114 and the differential amplifier 115, the bias voltage is increased or decreased so as to give a slight change to the bias voltage of the MZ modulator 102 (Vav stored in the memory area). . Then, the difference ΔVav (= Vav′−Vav) from the voltage value Vav before the increase / decrease is obtained with reference to the voltage value Vav ′ after the increase / decrease of the bias voltage as described above, and the bias control is performed based on the ΔVav. A signal is generated, converted into an analog signal by a D / A converter 114, and sent to a differential amplifier 115. The bias control signal is amplified by the differential amplifier 115 and applied as a bias voltage Vb of the MZ modulator 102 via the bias tee 106.
[0035]
The operation principle of the bias control system of the MZ modulator 102 for the optical duobinary system of FIG. 1 will be described below with reference to FIGS.
[0036]
FIG. 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 modulator 102 shown in FIG. 1 is changed. .
[0037]
FIG. 3 is a characteristic diagram showing a relationship between the DC bias voltage Vb of the MZ modulator 102 shown in FIG. 1 and the difference ΔPav between the average power output of the output optical signal.
[0038]
FIG. 2 shows a state where the signal amplitude Vpp of the input voltage of the MZ modulator 102 is equal to 2Vπ. Vπ is the difference between the drive voltage when the light transmittance is at the maximum value (peak) and the drive voltage when the light transmittance is at the minimum value (zero). In FIG. 2, + QUAD indicates 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 indicates 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 matching the extinction peak of the curve of the driving voltage versus the optical output characteristic), the ternary input signal (the low-pass filter 105) is used. (Depending on the ternary signal output from the above).
[0040]
However, the bias voltage Vb of the MZ modulator 102 has a light transmission characteristic shifted in the horizontal axis direction in FIG. 2 due to a DC drift (operating point drift).
[0041]
In FIG. 2, (1) and (1) 'represent the input voltage waveform and the output light waveform when the bias voltage Vb of the MZ modulator 102 is increased by a small 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]
(2) and (2) 'show the relationship between the input voltage waveform and the output light waveform when the bias voltage Vb of the MZ modulator 102 is reduced by a small amount ΔV (ΔV> 0) from the immediately preceding value. The time average of the output optical signal power in this case is indicated by Pav (2).
[0043]
The difference between the time average values of the output optical signal powers (Pav (2) -Pav (1)) is shown as ΔPav. In this case, {circle around (1)} and {circle around (2)} in FIG. 2 show 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]
Conversely, 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), the bias voltage Vb is also shifted to the negative side with respect to Vnull. 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]
When the DC drift occurs as described above, the extinction ratio and the optical power are reduced because the amplitudes of the output optical signals (1) 'and (2)' are lower than when the bias voltage Vb is at the optimum operating point Vnull. Since degradation occurs, it is necessary to compensate for DC drift.
[0046]
That is, when a DC drift occurs, it is necessary to consider the drift amount as a drive voltage change amount and compensate for the DC drift by 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, the bias voltage Vbopt at which the average power difference ΔPav = 0 is a bias value when the relationship between the light transmission characteristic of the MZ modulator 102 and the input signal is in an optimal relationship, and is Vnull (voltage vs. light). (The bias voltage that matches the extinction peak of the curve of the output characteristic).
[0048]
Here, as shown in FIG. 2, when Vb is located on the positive side from the optimum value Vnull due to DC drift (when the light transmission characteristic of the MZ modulator 102 is shifted leftward from the optimum state). , Vb is increased or decreased by ΔV (ΔV> 0) from the immediately preceding value (in the case of moving away from or approaching Vnull), ΔPav <0. Conversely, when Vb is located on the negative side from the optimum value Vnull due to DC drift (when the light transmission characteristic of the MZ modulator 102 is shifted to the right from the optimum state), Vb is set immediately before that. When the value is increased or decreased by ΔV (ΔV> 0) from the value of (i.e., when approaching or moving away from Vnull), ΔPav> 0.
[0049]
Therefore, the difference ΔPav between the average powers before and after increasing or 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 Vb is changed to make ΔPav zero. With such control, Vb can be made to match 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 a program ROM of the MCU 113 and executing the control program.
[0051]
Next, a control procedure will be described with reference to FIG.
[0052]
(1) In a first step S501, Vb is set to an initial value (normally 0 V).
[0053]
(2) In a second step S502, constants (ΔVb, Vπ, etc.) necessary for control are read from an external memory.
[0054]
(3) In the third step S503, Vb is increased by a small change ΔV (> 0) from 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 reduced by a small change ΔV from the initial value (Vb = Vb−2ΔV).
[0057]
(6) In a 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 a 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 processing is performed on the value as follows, and the branch processing is performed according to the result (condition). .
[0060]
(9) When ΔPav is larger than the allowable error ε (positive value), Vb is reduced and set to Vb−ΔVb × | ΔPav | (ninth step S509). That is, Vb is greatly reduced as | ΔPav | is larger.
[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 increased greatly as | ΔPav |
[0062]
(11) When ΔPav is equal to or smaller than ε and equal to or larger than −ε, it is considered that Vb is at the optimum operating point, the value of Vb is not changed (eleventh step S511), and control is waited for T1 seconds (Wait) (No. Twelve steps S512), and then return to the third step S503 to repeat the above controls (3) to (11).
[0063]
According to the control procedure described above, even when the light transmission characteristic of the MZ modulator 102 changes due to a change in the ambient temperature or a change over time, the Vb of the MZ modulator 102 is corrected and controlled accordingly, and the optimum operation is always performed. Can be maintained.
[0064]
When the order of increase and 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 greater than ε, Vb may be increased by ΔVb × | ΔPav |, and when ΔPav is less than −ε, Vb may be decreased by ΔVb × | ΔPav |. .
[0065]
That is, the optical modulator control device of the present invention includes: an optical modulator that supplies a drive voltage corresponding to a ternary signal as a modulation input around a DC bias voltage and outputs a binary optical signal; And 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 of the first and second light sources. The bias voltage determination unit determines the DC bias voltage based on the difference between the average optical output power of the optical modulator before and after the DC bias voltage is slightly increased or decreased.
[0066]
As a specific example, the optical modulator control device according to the first embodiment has a drive voltage vs. optical output characteristic indicated by a curve in which a light emission peak or an extinction peak periodically repeats, and a drive voltage corresponding to a ternary signal is generated. An optical modulator that is provided with a DC bias voltage as a center, modulates input light according to a 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 the ternary signal, a signal amplitude corresponding to an interval between two adjacent light emitting peaks (or extinction peaks) of the driving voltage versus light output characteristic of the optical modulator is calculated. And a drive circuit (103, 104, 105, 106) for generating a drive voltage having the drive voltage and supplying the drive voltage to the optical modulator while overlapping the DC bias voltage. Here, the signal amplitude of the driving voltage of the optical modulator (for example, the MZ modulator 102) is set to twice the difference Vπ between the driving voltages when the light transmittance of the optical modulator is the maximum value and the minimum value. And controlling the DC bias voltage to match a specific extinction peak (or light emission peak) of the driving voltage versus light output characteristic of the optical modulator based on the detection result of the difference between the average light output levels. It realizes a binary modulation method.
[0067]
Further, in the optical modulator control device of the first embodiment, the optical modulator 109 detects 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 controller 111 for determining a DC bias voltage based on a difference between the average optical output powers of the modulators.
[0068]
Here, the control unit 111 selects a control signal for controlling the DC bias generation circuit so that the DC bias voltage is increased or decreased at a frequency sufficiently lower than the ternary signal around the current value of the DC bias voltage. The DC bias voltage includes a function of generating the DC bias voltage, and detects a difference between the average light output levels detected by the light detection means before and after the DC bias voltage is changed in a binary manner. Based on the detection result, the DC bias voltage is adjusted in an appropriate direction. Is controlled so as to correct by 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 unit for initializing the DC bias voltage Vb to 0 V, a second control unit for reading a constant required for control from an external memory, and a DC bias voltage Vb from the immediately preceding value. The difference ΔPav between the time average values of the respective optical output powers when the amount of change ΔVb is increased or decreased by the unit change amount ΔVb is obtained. A third control means for controlling the bias voltage Vb to be changed at a time, and controlling to wait for a predetermined time when the absolute value of ΔPav is less than a predetermined value; and repeating the control by the third control means repeatedly. Control means.
[0070]
According to the optical modulator control device having the above-described configuration, the MZ modulator is controlled in accordance with the DC bias voltage applied to the MZ optical modulator, the environmental temperature, the operating point drift of the MZ modulator due to aging, and the like. The operation point drift is compensated for so that the operation can be performed at the optimum operation point, and the deterioration of the extinction ratio of the output optical signal due to the operation point drift can be prevented.
[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 components is reduced, and downsizing is facilitated.
[0072]
Further, the optical transmission device of the present invention includes an optical modulator control device of the present invention as described above, a light source for emitting input light of the optical modulator, and an optical signal output from the optical modulator to an optical communication fiber. Transmission means. This makes it possible to provide the features of the optical modulator control device described above.
[0073]
Further, in the control method of the optical modulator according to the present invention, the DC bias voltage of the drive voltage corresponding to the ternary signal supplied as the modulation input of the optical modulator is increased or decreased in a binary manner, and before and after the increase or decrease. The DC bias voltage is controlled based on the difference between the optical output average powers of the optical modulator. This makes it possible to realize an optical modulator control device and an optical transmission device having the above-mentioned features.
[0074]
Further, the control method of the optical modulator according to the present invention is such that the input light is modulated according to the drive voltage to output the optical signal by applying a DC bias voltage to the optical modulator together with the drive voltage corresponding to the ternary 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 step of binary-changing the DC bias voltage at a frequency sufficiently lower than the ternary signal. A function of generating a control signal for detecting the difference in the average optical output power detected in the second step before and after the DC bias voltage is changed in a binary manner, and based on the detection result, the DC bias voltage is determined. 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-mentioned features.
[0075]
Further, the control program of the optical modulator of the present invention is based on the difference between the optical output average power of the optical modulator before and after changing the DC bias voltage of the optical modulator to binary in the micro control unit. It is characterized by realizing a function of controlling the DC bias voltage of the optical modulator. This makes it possible to realize a method for controlling an optical modulator having the above-described features.
[0076]
Further, the control program for the optical modulator of the present invention provides an optical signal output from an optical modulator supplied as a modulation input with a drive voltage corresponding to a ternary signal centered on a DC bias voltage to a micro control unit. A function of detecting an average optical output level representing a time average of the DC bias voltage, a function of generating a control signal for binary-changing the DC bias voltage at a frequency sufficiently lower than the input signal, and a magnitude of the DC bias voltage. And a function of controlling a DC bias voltage of the optical modulator based on a result of the detection. . This makes it possible to realize a method for controlling an optical modulator having the above-described features.
[0077]
【The invention's effect】
As described above, according to the optical modulator control device, the optical transmitter, the optical modulator control method, and the control program of the present invention, for example, when the optical duobinary modulation method is used, the characteristics of the optical modulator are changed to the ambient temperature. Also, even if it fluctuates due to aging, the output optical signal can be stabilized with a simple device configuration.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an optical transmission device using an optical modulator control device for optical fiber communication according to a first embodiment of the present invention.
FIG. 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 modulator shown in FIG. 1 is changed.
3 is a characteristic diagram showing a relationship between a DC bias voltage Vb of the MZ modulator shown in FIG. 1 and a difference ΔPav between an average power output of an output optical signal and FIG.
FIG. 4 is a flowchart showing an example of a control procedure by a control unit in the optical modulator control device shown in FIG.
FIG. 5 is a block diagram showing a modulation unit using an optical duobinary modulation method.
FIG. 6 is a view showing signal waveforms of respective parts in FIG. 5 and eye patterns thereof.
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 ... bias tee,
107 ... terminator,
108 optical branching means,
109: Photodiode for monitor,
110 ... an amplifier,
111 ... control unit,
112 A / D converter,
113… MCU,
114… D / A converter,
115 ... Differential amplifier.

Claims (15)

An optical modulator that supplies a drive voltage corresponding to a ternary signal as a modulation input centering on a DC bias voltage and outputs a binary optical signal,
An optical modulator control device, comprising: a bias voltage determining unit that determines the DC bias voltage based on average optical output information indicating a time average of the optical output of the optical modulator. 2. The light according to claim 1, wherein the bias voltage determination unit determines the DC bias voltage based on a difference between an average optical output power of the optical modulator before and after the DC bias voltage is slightly increased / decreased. Modulator controller. The light emitting apex or the extinction apex has a drive voltage vs. light output characteristic indicated by a curve that repeats periodically, and a drive voltage according to a ternary signal is provided with a DC bias voltage as a center, and an input light according to the drive voltage. An optical modulator that modulates and outputs a binary optical signal;
A DC bias generation circuit that generates the DC bias voltage;
The binary NRZ signal input is converted into a ternary signal, and based on the ternary signal, the driving voltage vs. light output characteristic of the optical modulator corresponds to a point between two adjacent light emitting peaks or extinction peaks. A drive circuit that generates a drive voltage having a signal amplitude, and supplies the drive voltage to the optical modulator while overlapping the DC bias voltage generated by the DC bias generation circuit;
Light detection means for detecting an average light output level representing a time average of the light output of the light modulator,
An optical modulator control device comprising: a control unit that determines the DC bias voltage based on a difference between an average optical output power of the optical modulator before and after the DC bias voltage is slightly increased / decreased. The control means,
A function of selectively generating a control signal for controlling the DC bias generation circuit so as to increase or decrease in a binary manner at a frequency sufficiently lower than the ternary signal around the current value of the DC bias voltage Detecting the difference between the average light output levels detected by the light detection means before and after changing the DC bias voltage to a binary value, and adjusting the DC bias voltage in an appropriate direction based on the detection result. An optical modulator control device, which controls so as to correct by a small amount. The drive circuit sets the signal amplitude of the drive voltage to twice the difference Vπ between the drive voltage when the light transmittance of the optical modulator is the maximum value and the light transmittance when the light transmittance is the minimum value,
The control means controls the DC bias voltage to match a specific extinction peak or a light emission peak of the drive voltage versus light output characteristic of the optical modulator based on the detection result of the difference between the average light output levels. An optical modulator control device using an optical duobinary modulation method. The optical modulator control device according to any one of claims 1 to 5, wherein the modulator is a Mach-Zehnder modulator. 7. The optical modulator control device according to claim 3, wherein a micro control unit is used as the control unit. The micro control unit,
First control means for initially setting the DC bias voltage Vb to 0 V;
Second control means for reading constants necessary for control from an external memory;
When the DC bias voltage Vb is increased or decreased by the unit change amount ΔVb from the immediately preceding value, a difference ΔPav between the time average values of the respective optical output powers is obtained. Third control means for controlling the bias voltage Vb to change in the direction of change depending on the sign of ΔPav, and controlling to wait for a predetermined time when the absolute value of ΔPav is less than a predetermined value;
8. The optical modulator control device according to claim 7, further comprising: repetition control means for repeatedly performing control by said third control means. The third control means includes:
Fourth control means for setting the DC bias voltage Vb higher than the immediately preceding value by a unit change amount ΔVb;
Fifth control means for storing a time average value of the optical output power after control by the fourth control means as a first average optical output power Pav (1) in a memory area built in the micro control unit. 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 a time average value of the optical output power after the control by the sixth control means and storing it in a memory area built in the micro control unit as a second average optical output power Pav (2). When,
The DC bias voltage Vb is returned to a value before being set by the fourth control means, and a difference ΔPav (= Pav (2) -Pav (1)) between the two average optical output powers is calculated. Eighth control means for conditional branching,
When ΔPav is larger than the allowable error ε (positive value), Vb is controlled to be reduced and set to Vb−ΔVb × ΔPav, and when ΔPav is smaller than the allowable error −ε, Vb is increased. Vb + ΔVb × ΔPav is controlled to be set. When ΔPav is equal to or smaller than ε and equal to or larger than −ε, Vb is regarded as being 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 7, further comprising ninth control means. The third control means includes:
Fourth control means for setting the DC bias voltage Vb lower than the immediately preceding value by a unit change amount ΔVb;
Fifth control means for storing a time average value of the optical output power after control by the fourth control means as a first average optical output power Pav (1) in a memory area built in the micro control unit. When,
Sixth control means for setting the DC bias voltage Vb higher than the value set by the fourth control means by 2ΔVb;
A seventh control means for referring to a time average value of the optical output power after the control by the sixth control means and storing it in a memory area built in the micro control unit as a second average optical output power Pav (2). When,
The DC bias voltage Vb is returned to a value before being set by the fourth control means, and a difference ΔPav (= Pav ▲ 2 ▼ -Pav ▲ 1) between the two average optical output powers is calculated. 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 reduced by ΔVb × ΔPav. And a ninth control unit that waits for a predetermined time without changing the value of the bias voltage Vb, considering that Vb is at the optimum operating point when the ΔPav is equal to or smaller than ε and equal to or larger than −ε. The optical modulator control device according to claim 7, wherein: An optical modulator control device according to any one of claims 1 to 10,
A light source that emits input light of the optical modulator;
Means for transmitting an optical signal output from the optical modulator to an optical communication fiber. The DC bias voltage of the drive voltage according to the ternary signal supplied as the modulation input of the optical modulator is increased or decreased in a binary manner, and based on the difference between the average optical output power of the optical modulator before and after the increase or decrease. Controlling the DC bias voltage by controlling the DC bias voltage. A first step of modulating input light according to the drive voltage to output an optical signal by applying a DC bias voltage to the optical modulator together with a drive voltage corresponding to the ternary signal;
A second step of detecting an average optical output power representing a time average of the optical output of the optical modulator;
A function of generating a control signal for changing the DC bias voltage into a binary signal at a frequency sufficiently lower than the ternary signal, wherein the second control signal is generated before and after changing the DC bias voltage to a binary signal. A step of detecting a difference between the average optical output powers detected in the steps, and controlling the DC bias voltage based on the detection result. For the micro control unit,
A function of controlling the DC bias voltage of the optical modulator based on a difference between the optical output average powers of the optical modulator before and after changing the DC bias voltage of the optical modulator in a binary manner is realized. Control program for optical modulator. For the micro control unit,
A function of detecting an average optical output level representing a time average of an optical signal output from an optical modulator supplied as a modulation input with a driving voltage corresponding to a ternary signal being centered on a DC bias voltage;
A function of generating a control signal for changing the DC bias voltage into a binary value at a frequency sufficiently lower than the input signal;
A function of detecting a difference between the detected average optical output levels before and after changing the magnitude of the DC bias voltage in a binary manner, and controlling a DC bias voltage of the optical modulator based on the detection result is realized. A control program for an optical modulator.

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