JP2005091517A - Method for controlling bias voltage of optical modulation element and stabilization optical modulator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform an operation point control of an optical modulator such as an LN modulator without using a low-frequency signal like a conventional one, and to hold operation at an arbitrary operation point. <P>SOLUTION: Signal light from a signal light input end 11 and inspection light from an inspection light source 12 are multiplexed with a multiplexer 13 to be input to the LN modulation element 14. The inspection light from the output light is taken out with a demultiplexer 16 to electrically convert the inspection light with an opto-electrical converter 18 so as to monitor the output voltage. When a fluctuation occurs in the output voltage, a processing circuit 19 generates a bias voltage control signal in response to the voltage fluctuation and performs a voltage control of a control voltage source 1E generating bias voltage on the basis of the control signal. Thereby even when an operation point drift occurs in the LN modulation element 14, an operation point is held. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical modulator, and more particularly to a bias voltage control technique for stabilizing the operating characteristics of an optical modulator.

  In optical communication and optical signal processing, it is indispensable to convert changes in electrical signals into light intensity, phase and frequency changes, and optical modulators are widely used as a means for such conversion. Yes. In general, an optical modulator is provided with an input end to which light from a light source is input to an optical element, an output end thereof, and an input end of an electric signal to be a modulation signal, and light input from the optical input end. Is modulated by the electric signal. Typical examples of such an optical modulator include a lithium niobate optical modulator (hereinafter referred to as an LN modulator), an absorption optical modulator, and an acousto-optic device optical modulator. In the LN modulator and the absorption type optical modulator, an electric signal (hereinafter referred to as an RF signal) that is normally input is a high-frequency AC signal, which is applied to the optical element in a form in which a certain DC bias voltage is superimposed thereon.

  Incidentally, the characteristics of the optical modulator vary greatly depending on the bias voltage. FIG. 5 shows the static characteristics of the LN modulator. The horizontal axis represents the DC bias voltage to be applied, and the vertical axis represents the amount of light transmitted through the LN element. As shown in the figure, the transmitted light amount (P0) has a periodic characteristic with respect to the DC bias voltage (Vb), and it can be seen that when Vb varies, P0 varies greatly. Such a static characteristic curve is referred to as an S-characteristic. Hereinafter, one point of the S-characteristic is referred to as an operating point, the voltage at the maximum operating point at which the light intensity is maximum is Vmax, and the light intensity is minimum. The voltage at the minimum operating point is referred to as Vmin, and the voltage at the intermediate operating point located at the midpoint between the maximum operating point and the minimum operating point is referred to as Vquad. The voltage difference between Vmax and Vmin is called half-wave voltage Vpai, and the voltage difference between adjacent Vmax and Vmin is called one-wavelength voltage V2pai.

  FIG. 6 is a graph showing an electrical waveform and a change in light intensity when the DC bias voltage is set to the intermediate operating point voltage Vquad when a digital electrical signal is converted into a digital optical signal. In the figure, a solid line 301 indicates an S-characteristic, a solid line 302 indicates an electric signal waveform, and a solid line 303 indicates an optical signal waveform. When the electric signal waveform 302 changes with a rectangular time, the optical signal waveform 303 also changes with a rectangular time.

  It is known that the static characteristics shown in FIG. 5 change over time due to factors such as temperature changes. A dotted line 304 shown in the graph of FIG. 6 indicates a change in static characteristics when such a change with time occurs. That is, as shown in the figure, Vpai and V2pai do not change, but the S-characteristic is translated (operating point drift) with respect to the bias voltage. When such an operating point drift phenomenon occurs, even if a rectangular electric signal waveform 302 is input, the output optical signal waveform 303 is not rectangular, and the intended output light cannot be obtained.

  Conventionally, an automatic bias control system has been known as a method for compensating for a change in static characteristics of an LN modulator with time. In this method, when the static characteristic changes, the bias voltage is controlled in accordance with the change, and the operation point of the optical modulator is corrected so as to keep the operation intermediate point or the maximum / minimum operation point. FIG. 4 is a block diagram showing an example of an optical modulator employing such an automatic bias control system. In the figure, an optical signal is input to an LN modulation element 402 from an input terminal 401, and this optical signal is modulated by an electrical signal applied to the LN modulator 402. This electric signal is supplied from the amplifier 403, and the output signal of the amplifier 403 is input to the RF signal input terminal 404 to the amplifier 403 and the low frequency oscillator 405 to the gain control voltage terminal of the amplifier 403. In addition, a low frequency signal of about 1000 Hz is superimposed on the signal. The signal light modulated by passing through the LN modulation element 402 is directed to the output terminal 407, but a part of the signal light is branched by the optical branching device 406 and input to the photoelectric converter 408, and the output thereof is bias voltage. To the control circuit 409.

  The control circuit 409 extracts a low frequency component from the input electric signal, analyzes the low frequency signal, and corrects the operating point drift by changing the bias voltage by the following method. That is, for example, when the bias voltage is maintained at the maximum operating point, the voltage component of the frequency of the low frequency signal is detected, and the bias voltage is sequentially changed so that the component becomes the maximum, and the voltage component The so-called “chase control” method of the maximum point is used, in which a critical point where the phenomenon changes from an increase to a phenomenon is found. In this case, the doubled frequency component exhibits an extreme value even at the minimum operating point of the LN modulation element 402. Therefore, depending on whether the direct current component of light increases or decreases when the bias voltage is changed in the increasing direction, It is determined whether the operating point is the maximum operating point or the minimum operating point.

Further, when the bias voltage is maintained at the intermediate operating point, control is performed so that the voltage component of the low frequency signal is minimized. Whether the slope of the S-characteristic is positive or negative is determined by whether the direct current component of light increases or decreases when the bias voltage is changed in the increasing direction. Further, when the bias voltage deviates from the intermediate point, the generation of harmonics of 2 or 3 times changes from the LN modulator, so that these can be detected and controlled.
JP 2000-171766 A

  As described above, there is a method of keeping the operating point of the LN modulation element 402 at the maximum operating point, the minimum operating point, or the intermediate operating point based on the harmonic component of the low-frequency signal superimposed on the electric signal to be modulated. A control method for maintaining the operating point at an arbitrary point has not yet been proposed. In other words, the conventional method is a chasing control method based on the frequency component of the low-frequency signal and its harmonic components, so the maximum and minimum points can be detected, but any other operating point can be detected. There is no reference, so the operating point cannot be kept at an arbitrary point.

  In optical communication and optical signal processing, it may be necessary to keep the operating point at a point other than the maximum operating point, the minimum operating point, or the intermediate operating point. For example, when an optical signal is modulated with a digital electrical signal, if the amplitude of the electrical signal is a half-wave voltage Vpai, ideal optical modulation is performed by operating the intermediate operating point as a bias point. However, in the case where the amplitude of the electric signal is smaller than the half-wave voltage Vpai, if the intermediate operating point is set as the bias point, an output waveform including the bias is generated. There is a disadvantage that the intended output cannot be obtained without falling to zero.

  Further, for controlling the bias voltage of the LN modulation element, various disadvantages occur in applying low frequency modulation to the signal light using the low frequency generator 405 itself. That is, when performing such low-frequency modulation, it is necessary to use a special amplifier having a gain control voltage terminal as the amplifier 403 that generates an electrical signal for modulation. Further, when it is necessary to use two or more LN modulators in series or to apply low frequency modulation to signal light for purposes other than controlling the LN modulators, these modulated low frequency signals are mutually connected. A filter function must be provided so as not to interfere, and there is a problem that the device configuration of the optical modulator becomes complicated.

  The present invention has been made in view of such circumstances, and can control the operating point of an optical modulator such as an LN modulator without using a low-frequency signal as in the prior art, and at any operating point. An object of the present invention is to provide a bias voltage control method for a light modulation element and a stabilized light modulator.

  In order to achieve the above object, a bias voltage control method for an optical modulation element according to claim 1 is directed to an optical modulation element operating by being supplied with a bias voltage, and an inspection of a signal light and a wavelength different from the signal light. Superimposing and inputting light, taking out the signal light from the output light of the light modulation element, monitoring the light intensity of the inspection light according to the operating point of the light modulation element to be held, When a change occurs in the intensity, a voltage change corresponding to the change is applied to the bias voltage of the light modulation element.

  According to a second aspect of the present invention, there is provided a stabilized optical modulator comprising: an optical modulation element that receives a signal light, operates with a bias voltage, and outputs a modulated signal light; and a bias voltage source that generates the bias voltage. An inspection light source that emits inspection light having a wavelength different from that of the signal light, optical multiplexing means that superimposes the inspection light on the signal light and inputs the light to the light modulation element, and output light of the light modulation element And an optical demultiplexing means for extracting the inspection light from the optical demultiplexing means, and a photoelectric conversion means for photoelectrically converting the inspection light extracted by the optical demultiplexing means, and further monitoring the output voltage of the photoelectric conversion means to obtain the output voltage. It is characterized by comprising control means for generating a bias voltage control signal corresponding to the voltage fluctuation when the fluctuation occurs and performing voltage control of the bias voltage source based on the control signal.

  Further, the stabilized optical modulator according to claim 3 is different from the signal light in an optical modulator having an optical modulation element that receives the signal light, operates with a bias voltage, and outputs the modulated signal light. An inspection light source that emits inspection light of a wavelength, optical multiplexing means that superimposes the inspection light on signal light and inputs the light to the light modulation element, optical demultiplexing means that extracts the inspection light from the output light of the light modulation element, Photoelectric conversion means for photoelectrically converting the inspection light taken out by the optical demultiplexing means, and further comprising a differential amplifier for applying a bias voltage to the optical modulation element, and the differential voltage amplified by the differential amplifier is The reference voltage source is configured to be generated based on a difference between an output voltage of a reference voltage source that generates a predetermined reference voltage and an output voltage of the photoelectric conversion means.

  According to the bias voltage control method according to claim 1, rather than taking out the low frequency component superimposed from the modulated signal light itself and controlling the bias voltage, the inspection light different from the signal light is superimposed, Since the bias voltage is controlled based on the modulation state of the inspection light, the bias voltage can be controlled completely independently of the signal light. Therefore, control can be performed without being affected by the waveform and amplitude of the electric signal for modulating the signal light, and the operating point can be kept at an arbitrary point. It is also possible.

  According to the stabilized optical modulator of the second aspect, since the control unit is configured to generate an appropriate bias voltage control signal according to the output fluctuation of the inspection light, the static characteristic of the optical modulation element has changed over time. As a result, when a change occurs in the output power of the inspection light, the bias voltage is controlled so as to keep the operating point constant immediately. In this control, a table of what voltage variation should be applied to the shift of the static characteristics is stored in advance, and an optimum bias voltage control signal is considered in consideration of the output voltage monitoring result performed by the photoelectric conversion means. By setting a program that selects, it is implemented in software.

  According to the stabilized optical modulator of the third aspect, the bias voltage can be controlled by a hardware configuration called a differential amplifier. That is, the differential voltage based on the difference between the output voltage of the reference voltage source and the output voltage of the photoelectric conversion means is nothing but the fluctuation of the inspection light power, that is, the fluctuation of the static characteristics of the light modulation element. Therefore, if the bias voltage is controlled by the differential voltage, the operating point can be maintained.

    As described above, according to the bias voltage control method and the stabilized optical modulator of the light modulation element of the present invention, even if the static characteristic (S-characteristic) of the light modulation element drifts due to a change with time, the operating point is practically effective. The bias voltage can be automatically controlled so as not to change. In addition, since the inspection light output combined with the signal light and similarly modulated is used as a control base, the bias voltage is controlled to be held at an arbitrary operation point instead of the limited operation point as in the prior art. Can be easily performed.

  Further, it is not necessary to apply low frequency modulation to an amplifier that generates an electrical signal for modulation as in the prior art, and it is not necessary to use a special amplifier having a gain control voltage terminal as a modulation amplifier. There is no need to consider the signal interference problem.

  Accordingly, the device configuration of the optical modulator can be simplified and cost reduction can be expected, and since the operation can be maintained at an arbitrary operating point, the adjustment of the optical modulator is facilitated, and further, there is a problem of interference of low frequency signals. Therefore, the present invention has excellent effects such as using two or more LN modulators in series or applying low frequency modulation to signal light for purposes other than controlling the LN modulator. is there.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
(Embodiment 1)
FIG. 1 is a block diagram showing a configuration example of a stabilized optical modulator 1 according to the present invention. In the figure, an electrical signal for optical modulation can be applied to the LN modulation element 14 from the RF signal input end 15, and the signal light incident from the signal light input end 11 is optically modulated in the LN modulation element 14. After that, it has a basic structure such that it is output from the signal light output end 17.

  In the present invention, in addition to the basic configuration, an inspection light source 12 that generates inspection light is provided. The inspection light emitted from the inspection light source 12 is combined with the signal light by a multiplexer 13 such as an optical coupler, and is input to the LN modulation element 14 together with the signal light. A semiconductor laser can be used as the inspection light source 12, and a wavelength different from that of the signal light is selected so that the emission wavelength does not interfere with the signal light input from the signal light input terminal 11. For example, if the signal light is in the 1.5 μm band, a semiconductor laser having a central emission wavelength of 1.48 μm may be selected as the inspection light source 12. It is desirable that the inspection light source 12 be capable of modulating the light intensity at a low frequency as necessary so that various controls can be performed.

  Both the signal light and the inspection light input to the LN modulation element 14 are modulated by the electrical signal input from the RF signal input terminal 15. Output light (modulated signal light and inspection light) of the LN modulation element 14 is input to the duplexer 16. The demultiplexer 16 serves to demultiplex the signal light and the inspection light, whereby the signal light is separated and sent to the signal light output end 17 and the inspection light toward the photoelectric converter 18.

  The inspection light input to the photoelectric converter 18 is converted into an electrical signal here and input to the signal processing circuit 19. The signal processing circuit 19 analyzes an electrical signal derived from the inspection light, generates an optimum bias voltage control signal by a method described later, and sends it to the control voltage source 1E. The control voltage source 1E generates a bias voltage determined by the control signal and applies it to the LN modulation element 14.

  FIG. 2 is a functional block diagram for explaining a mode of processing in the signal processing circuit 19. The output electrical signal of the photoelectric converter 18 is first input to the voltage monitor unit 191. By monitoring the voltage fluctuation, the state in which the inspection light is being modulated is replaced with an electrical signal. Monitored at Next, the voltage fluctuation detector 192 obtains the voltage fluctuation Δv by comparing the reference voltage stored in the reference voltage storage unit 193 with the monitor voltage. The reference voltage is a voltage corresponding to an operating point to be held. For example, when a bias is held at an intermediate operating point of S-characteristics, an output voltage (photoelectric converter) that should be originally obtained at the intermediate operating point. 18 output voltage).

  Here, if the operating point drift does not occur in the LN modulation element 14, the voltage monitor unit 191 detects the voltage according to the reference voltage, and in this case, the voltage fluctuation detection unit 192 outputs Δv = 0. On the other hand, when the operating point drift occurs, a phase shift occurs between the reference voltage and the voltage detected by the voltage monitor unit 191. The voltage fluctuation detecting unit 192 detects the amount corresponding to the shift as Δv and outputs this. To do.

  The output information of Δv determined by the voltage fluctuation detector 192 is converted into a digital signal by the AD converter 194. The digital signal is input from the AD converter 194 to the arithmetic unit 195 as a signal corresponding to Δv.

  The data storage unit 196 stores a data table relating to changes in Δv and bias voltage. That is, a data table is stored in which the output voltage fluctuation Δv of the photoelectric converter 18 caused by the static characteristic change of the LN modulation element 14 and the bias voltage fluctuation Δq corresponding to the voltage fluctuation Δv are stored as parameters. For example, when information indicating that the voltage fluctuation is Δv1 is given by a request from the unit 195, it is possible to extract the bias voltage fluctuation (Δq1) corresponding to the data table by referring to the data table. It is configured. As is apparent from the graph of the time-dependent change of the static characteristics shown in FIG. 6, such a data table is parallel-moved while maintaining the S-characteristic form itself, although the static characteristics are displaced by the change with time. Therefore, it can be created in advance and stored in the data storage unit 196.

  The arithmetic unit 195 performs arithmetic processing based on information related to Δv input from the AD converter 194 and information related to Δq extracted from the data storage unit 196, and is necessary for maintaining the operating point. The change ΔVb in the bias voltage is determined. Information on the bias voltage change ΔVb is sent to the control signal generator 197.

  The control signal generator 197 has a function of generating a control signal for controlling the output voltage of the control voltage source 1E that actually generates a bias voltage. When information related to the bias voltage change ΔVb is input, a control signal corresponding to the ΔVb is given to the control voltage source 1E, and the output voltage of the control voltage source 1E is changed by ΔVb.

  The operation of the stabilized optical modulator 1 of the present invention configured as described above will be described. From the signal light input terminal 11, light having a wavelength used in general optical communication, for example, signal light in a 1.5 μm band is input. On the other hand, as the inspection light emitted from the inspection light source 12, light having a different wavelength is selected so that interference with the signal light does not occur. For example, 1.48 μm light is emitted from the inspection light source 12. These signal light and inspection light are combined by the multiplexer 13 and input to the LN modulation element 14. Then, both the signal light and the inspection light are modulated by the electric signal input from the RF signal input terminal 15. For example, if a rectangular wave-shaped electrical signal is input from the RF signal input terminal 15, the signal light and the inspection light are also modulated into a rectangular wave-shaped optical signal accordingly and output to the demultiplexer 16.

  The demultiplexer 16 separates the 1.5 μm band signal light and the 1.48 μm inspection light, and the signal light is output to the outside from the signal light output end 18 for optical communication. On the other hand, the inspection light is branched and output to the photoelectric converter 18 and is monitored by the voltage monitor unit 191 in a form in which the optical power is converted into an electrical signal.

  Here, when a change with time occurs in the LN modulation element 14, as described above, an operating point drift as shown by a dotted line in FIG. 6 occurs in the signal light. Since it is modulated by the element 14, the operating point drift similarly occurs. Therefore, if the operating point drift of the inspection light is corrected, the operating point drift of the signal light is also corrected.

  When a change with time actually occurs in the LN modulation element 14, as described above, the voltage fluctuation detection unit 192 of the processing circuit 19 responds to the change with time as compared with the reference voltage stored in the reference voltage storage unit 193. The voltage fluctuation Δv is determined. For example, when the operating point is to be held at the midpoint of the S-characteristic, the reference voltage is the output voltage vq of the photoelectric converter 18 corresponding to the bias voltage Vquad at the intermediate operating point, and with respect to this vq It is determined how much voltage fluctuation Δv has occurred.

  When such a change with time is viewed from the result, if the voltage fluctuation is + Δvn, the bias voltage to be applied by the voltage source 1E at the intermediate operating point is the bias voltage (Vquad) corresponding to vq. Regardless, a bias voltage corresponding to vq + Δvn is applied to the LN modulation element 14. Therefore, in order to maintain the operating point at the midpoint of the S-characteristic, if the output voltage of vq + Δvn currently detected by the voltage monitor unit 191 is controlled to become the output voltage of vq, a change with time will occur. However, the operating point is maintained.

  The voltage fluctuation detection unit 192 sends a voltage fluctuation signal such as Δvn to the AD converter 194, which converts the signal into a digital signal and inputs the digital signal to the arithmetic unit 195. This voltage variation Δvn is a voltage variation of the voltage derived from the inspection light and output from the photoelectric converter 18 and must be replaced with an actual bias voltage variation. As described above, the data storage unit 196 stores the data table for this replacement, and the calculation unit 195 determines the change ΔVb of the bias voltage necessary for holding the operating point with reference to this data table. To do. Then, the control signal generator 197 generates a control signal necessary for changing the bias voltage generated by the control voltage source 1E by ΔVb, and sends this control signal to the control voltage source 1E for application to the LN modulation element 14. The bias voltage to be changed is changed.

  By such control, the operating point drift due to the change with time is compensated, and the light modulation according to the original static characteristics of the LN modulation element 14 is performed. Therefore, in the above-described example, the output voltage of the photoelectric converter 18 also returns from vq + Δvn to the original vq. In the same manner, the output voltage of the photoelectric converter 18 is continuously monitored by the voltage monitor unit 191. If the voltage fluctuation Δv is detected, the same procedure is repeated and the operating point is kept constant. Is.

  The above has mainly described the case where the bias is held at the intermediate operating point of the S-characteristic. However, when the bias is held at any other operating point, the voltage originally output by the photoelectric converter 18 at the operating point. Can be stored in the reference voltage storage unit 193, Δv can be obtained by a similar operation, and the bias voltage can be automatically adjusted. That is, the operating point holding is not limited to the maximum / minimum operating point and the intermediate operating point as in the conventional method, and the bias can be held at any intended operating point.

  Note that the inspection light emitted from the inspection light source 12 may be continuous light, or light subjected to low-frequency intensity modulation may be used as the inspection light. In this case, a low-frequency filter used for modulation may be built in the photoelectric converter 18 and the effective value of the AC signal that has passed through the filter may be sent to the processing circuit 19.

(Embodiment 2)
FIG. 3 is a block diagram showing a configuration example of the stabilized optical modulator 2 that achieves the object of the present invention by a hardware configuration. Similarly to the first embodiment, an electrical signal for optical modulation can be applied to the LN modulation element 24 from the RF signal input end 25, and the signal light incident from the signal light input end 21 is LN modulation element 24. It has a basic structure such that it is optically modulated and then outputted from the signal light output end 27. Further, an inspection light source 22 that generates inspection light is provided. The inspection light is combined with the signal light by the multiplexer 23 and is input to the LN modulation element 24. The LN modulation element 24 generates the signal light and the inspection light. After being modulated together, the signal light and the inspection light are demultiplexed by the demultiplexer 26, the signal light is demultiplexed to the signal light output terminal 27, and the inspection light is demultiplexed to the photoelectric converter 28. The same as in the first embodiment.

  In the present embodiment, the processing method of the output voltage of the photoelectric converter 28 is different from that of the first embodiment. A voltage amplifier 291 is connected to the output terminal of the photoelectric converter 28. The photoelectric converter 28 photoelectrically converts the inspection light input from the branching filter 26 to generate a voltage. The voltage amplifier 291 amplifies the output voltage and superimposes a predetermined DC voltage on the differential amplifier. To 292. The reference voltage source 29E generates a predetermined bias voltage and drives the LN modulation element 24. Therefore, this voltage is a voltage corresponding to the operating point of the LN modulation element 24, and when the operating point is set at the midpoint of the S-shaped characteristic, it is a voltage Vm corresponding thereto. The voltage amplifier 291 amplifies the output voltage of the photoelectric converter 28 and superimposes the DC voltage in order to adjust the scale to the generated voltage (for example, Vm) of the reference voltage source 29E.

  The output of the reference voltage source 29E and the output of the voltage amplifier 291 are input to the differential amplifier 292. The differential amplifier 292 converts the output voltage of the reference voltage source 29E into a predetermined value based on the difference between the outputs of the two, and supplies this to the LN modulation element 24 as a bias voltage. That is, when there is no difference between the output of the reference voltage source 29E and the output of the voltage amplifier 291, the voltage generated by the reference voltage source 29E is supplied to the LN modulation element 24 as it is, and there is an output voltage difference between the two. In this case, the voltage is increased or decreased by the differential amplifier 292 in accordance with the voltage difference, and then supplied to the LN modulation element 24.

  The operation of the stabilized optical modulator 1 of the present invention configured as described above will be described in the case where the operating point of the LN modulator 24 is held at the midpoint of the S-characteristic. Since the process until the photoelectric converter 28 photoelectrically converts the inspection light is the same as that in the first embodiment, the description thereof is omitted.

  First, the case where the LN modulation element 24 does not change with time will be described. The output voltage v1 of the photoelectric converter 28 is amplified by the voltage amplifier 291 and a predetermined DC voltage is superimposed thereon, and then the differential amplifier 292. Is input. On the other hand, in the reference voltage source 29E, a voltage Vm corresponding to the intermediate operating point of the S-shaped characteristic of the LN modulation element 24 is generated and input to the differential amplifier 292. When no change with time has occurred, no voltage is input from the voltage amplifier 291, and the voltage Vm is applied as a bias voltage to the LN modulation element 24 without being differentially converted by the special differential amplifier 292. That is, the DC voltage superimposed by the voltage amplifier 291 cancels the output voltage v1 from the photoelectric converter 28 in the initial state where no operating point drift occurs, and sets the output voltage of the voltage amplifier 291 to zero. DC voltage.

  On the other hand, when a change with time occurs in the LN modulation element 24, the output voltage v2 input from the photoelectric converter 28 does not become 0 even if the voltage amplifier 291 performs a DC voltage superimposition process. The voltage amplifier 291 outputs only the voltage fluctuation Δv corresponding to the point drift. Here, although the S-characteristic is a curve characteristic, it can be approximated by a straight line in the vicinity of the intermediate operating point. Therefore, if the slope constant is k, the voltage fluctuation Δv and the voltage Δq corresponding to the drift from the intermediate operating point The relationship can be expressed linearly as Δv = kΔq. Further, if the gain of the voltage amplifier 291 is adjusted to k = 1, it can be handled as Δv = Δq.

  Now, it is assumed that a drift occurs in which the S-characteristic of the LN modulation element 24 is reduced by Δq in terms of a bias voltage. Then, the amount of light received by the photoelectric converter 28 also decreases, and the output voltage also decreases. On the other hand, in the voltage amplifier 291, since the DC voltage corresponding to the bias voltage before the S-characteristic drifts continues to be superimposed, the output voltage of the voltage amplifier 291 is not 0 but is generated by Δv = Δq. That is, the voltage amplifier 291 outputs the voltage Δq to the differential amplifier 292.

  The reference voltage source 29E does not change and generates a voltage Vm corresponding to the intermediate operating point of the S-characteristic and outputs it to the differential amplifier 292. As described above, when the voltage amplifier 291 receives a voltage of Δq, Due to the differential action of the differential amplifier 292, the voltage supplied from the differential amplifier 292 to the LN modulation element 24 at this time becomes Vm−Δq. This bias voltage corresponds to the midpoint after the operating point drift occurs, and as a result, the bias voltage of the LN modulation element 24 is held at the midpoint.

  In the above description, the relationship between the voltage fluctuation Δv and the voltage Δq corresponding to the drift from the intermediate operating point is linearly assumed to be Δv = kΔq. However, since the S-characteristic is generally a sine function, this assumption is not necessarily valid. It can be said that it does not. However, the S-characteristic rarely drifts greatly, and in many cases, the drift voltage Δq is also small. Therefore, the above linear approximation is practically acceptable. Therefore, according to this embodiment, it is possible to hold the bias voltage at an arbitrary operating point. However, since the slope varies depending on the operating point, it is desirable to adjust the slope constant k according to the operating point.

  Even when the S-characteristic drifts drastically, it is possible to cope with this by approximating the linear approximation into several sections and giving an appropriate constant k for each linear approximation section.

  In the first and second embodiments described above, the case where the LN modulation element is used as the light modulation element has been described. However, other light modulation elements such as an absorption light modulation element and an acousto-optic light modulation element may be used. However, it is possible to hold the operating point at any operating point in the same manner.

1 is a block diagram showing an embodiment of a stabilized optical modulator according to the present invention. It is a functional block diagram for demonstrating operation | movement of the processing circuit shown in FIG. It is a block diagram which shows other embodiment of the stabilization optical modulator concerning this invention. It is a block diagram which shows an example of the conventional optical modulator. It is a graph which shows the static characteristic (S-characteristic) of the relationship between the bias voltage of a LN modulation element, and the transmitted light amount. It is a graph which shows the relationship when signal light is modulated with an electric signal by the LN modulation element, and the curve shown with the dotted line in the figure represents the situation of the operating point drift.

Explanation of symbols

1, 2 Optical modulator 11, 21, 401 Signal light input terminal 12, 22 Inspection light source 13, 23 Multiplexer 14, 24, 402 LN modulation element 15, 25, 404 RF signal input terminal 16, 26, 406 Demultiplexing 17, 27, 407 Signal light output 18, 28, 408 Photoelectric converter 19 Processing circuit 191 Voltage monitor unit 192 Voltage fluctuation detection unit 193 Reference voltage storage unit 194 AD converter 195 Calculation unit 196 Data storage unit 197 Control signal Generator 1E Control voltage source 291 Voltage amplifier 292 Differential amplifier 29E Reference voltage source 301 S-characteristic 302 Electric signal waveform 303 Optical signal waveform 304 Static characteristic change 403 Amplifier 405 Low frequency oscillator 409 Bias voltage control circuit

Claims (3)

A signal light and inspection light having a wavelength different from that of the signal light are superimposed and input to an optical modulation element that operates with a bias voltage applied.
Taking out the signal light from the output light of the light modulation element, and monitoring the light intensity of the inspection light according to the operating point of the light modulation element to be held,
A bias voltage control method for an optical modulation element, wherein when a change occurs in the light intensity of the inspection light, a voltage change corresponding to the change is applied to a bias voltage of the light modulation element. In an optical modulator having a light modulation element that receives a signal light, operates with a bias voltage and outputs a modulated signal light, and a bias voltage source that generates the bias voltage,
An inspection light source that emits inspection light having a wavelength different from that of the signal light, an optical multiplexing unit that superimposes the inspection light on the signal light and inputs the light to the light modulation element, and light that extracts the inspection light from the output light of the light modulation element Demultiplexing means, and photoelectric conversion means for photoelectrically converting the inspection light taken out by the light demultiplexing means,
Further, the output voltage of the photoelectric conversion means is monitored, and when a fluctuation occurs in the output voltage, a bias voltage control signal corresponding to the voltage fluctuation is generated, and voltage control of the bias voltage source is performed based on the control signal. A stabilized optical modulator comprising control means for performing the following. In an optical modulator having an optical modulation element that receives signal light, operates with a bias voltage, and outputs modulated signal light.
An inspection light source that emits inspection light having a wavelength different from that of the signal light, an optical multiplexing unit that superimposes the inspection light on the signal light and inputs the light to the light modulation element, and light that extracts the inspection light from the output light of the light modulation element Demultiplexing means, and photoelectric conversion means for photoelectrically converting the inspection light taken out by the light demultiplexing means,
And a differential amplifier for applying a bias voltage to the light modulation element, wherein the differential voltage amplified by the differential amplifier includes an output voltage of a reference voltage source for generating a predetermined reference voltage, and the photoelectric conversion means. A stabilized optical modulator configured to be generated based on a difference between the output voltage and the output voltage.

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