JP4026823B2 - Light modulator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a technique for setting a desired operating point in an initial operation or the like in an optical modulator having an optical modulator whose operating point is set by a bias voltage.
[0002]
[Prior art]
In an optical modulation device that modulates the intensity of light by an input signal, an optical modulator that modulates light by an electro-optic effect or a magneto-optic effect is often used.
[0003]
The operating point of these optical modulators is determined by a DC bias voltage. However, even if the operating point is set by applying a constant bias voltage to the optical modulator, the modulation characteristics of the optical modulator itself may change due to the influence of temperature and changes over time, and the operating point may be shifted. .
[0004]
For this reason, in an optical modulation device using this type of optical modulator, the deviation of the operating point is detected based on the light emitted from the optical modulator, and the bias voltage is varied in the direction in which the deviation is reduced, so that In many cases, feedback control is performed so that the optical modulator operates at a set operating point.
[0005]
FIG. 8 shows a configuration of a conventional optical modulation device 10 that stabilizes the operating point by such feedback control.
[0006]
In FIG. 8, the light P emitted from the laser diode 11 is incident on the optical modulator 12 utilizing the electro-optic effect of lithium niobate (LiNbO ₃ ).
[0007]
The drive amplifier 13 amplifies the input data signal D, generates a modulation signal M on which the output signal A of the low frequency oscillator 16 is superimposed, and inputs the modulation signal M to the optical modulator 12.
[0008]
The optical modulator 12 modulates the intensity of the incident light P with the modulation signal M, and emits the intensity-modulated light Pm to the optical coupler 17 of the operating point detection means 15.
[0009]
The operating point detection means 15 is for detecting a shift of the operating point of the optical modulator 12, and the incident light Pm is branched into two by the optical coupler 17, while Pm1 is emitted to the outside as outgoing light of the optical modulator. On the other hand, the other Pm2 is received by the light receiving element 18 made of a pin (PIN) photodiode, for example.
[0010]
The light receiving element 18 converts the incident light Pm2 into an electric signal E whose voltage changes according to the intensity change, and outputs the electric signal E to a BPF (band pass filter) 19.
[0011]
The BPF 19 extracts a signal A ′ having the same frequency component as the output signal A of the low frequency oscillator 16 from the output signal E of the light receiving element 18 and outputs the signal A ′ to the phase detector 20.
[0012]
The phase detector 20 compares the phases of the output signal A of the low-frequency oscillator 16 and the output signal A ′ of the BPF 17, and a signal Q whose voltage changes according to the phase difference is a signal corresponding to the deviation of the operating point. To the DC amplifier 21.
[0013]
The DC amplifier 21 constitutes a bias voltage supply means, and supplies the output signal Q of the phase detector 20 to the optical modulator 12 as a bias voltage B that changes in a direction that eliminates the deviation of the operating point. Here, the range of the bias voltage B that can be supplied to the optical modulator 12 by the DC amplifier 21 is the range of the power supply voltages V + and V− supplied to the DC amplifier 21. The actual range of the bias voltage B is narrower than the range of the power supply voltage, but is assumed to be the same for explanation.
[0014]
On the other hand, as shown in FIGS. 9A and 9B, the optical modulator 12 has a modulation characteristic F in which the intensity of emitted light periodically changes sinusoidally with respect to a monotonous change in input voltage. However, this modulation characteristic varies, for example, as a characteristic F ′ in FIG. 9A and a characteristic F ″ in FIG.
[0015]
In general, the operating point of the optical modulator is preferably the point where the dynamic range with respect to the input signal is the widest, and this point is the intermediate point between the maximum and minimum of the sinusoidal modulation characteristic F (the bias voltage B can be taken). If there is no range of V + to V−, S1 to Sn). Hereinafter, these points are called stable points.
[0016]
When the operating point deviates from the stable point, the phase of the signal A ′ with respect to the signal A changes according to the amount and direction of the deviation, and a signal corresponding to the phase difference is output from the phase detector 20 to the DC amplifier 21. The DC amplifier 21 changes the bias voltage B in a direction that eliminates the deviation of the operating point, and maintains the state where the operating point coincides with the stable point.
[0017]
[Problems to be solved by the invention]
However, in the case of an optical modulation device that controls the operating point of the optical modulator 12 to coincide with the stable point as described above, the operating point of the optical modulator 12 is any in a transient state such as when the power is turned on. There is a possibility that the stable points S1 to Sn also coincide with each other. When the operating point coincides with a stable point close to the lower limit or upper limit of the input voltage, the bias voltage cannot follow the drift of the modulation characteristic, and the operating point Incurs a gap.
[0018]
For example, in the initial operation when the power is turned on, if the modulation characteristic fluctuates as F ′ in the state where the operating point matches the stable point S1 close to the lower limit with respect to the characteristic F in FIG. The bias voltage cannot be lowered below the lower limit (V−) of the voltage, and an operating point shift occurs.
[0019]
On the other hand, if the modulation characteristic fluctuates as F ″ with the operating point matching the stable point Sn close to the upper limit with respect to the characteristic F in FIG. 9B, the upper limit (V + ), The bias voltage cannot be increased more and the operating point shift occurs.
[0020]
As described in Patent Document 1 below, this problem is obtained by obtaining the average power of two outgoing lights with reversed phases obtained from a Mach-Zehnder type optical modulator, and generating a monitor voltage corresponding to the difference between them. The monitor voltage when the operating point of the optical modulator coincides with the point to be set is used as a reference value, and the deviation of the monitor voltage with respect to the reference value is detected as a shift of the operating point to reduce the shift of the operating point. The same occurs for an optical modulator configured to supply a bias voltage to the optical modulator.
[0021]
[Patent Document 1]
JP-A-4-294318
An object of the present invention is to provide an optical modulation device that solves this problem.
[0023]
[Means for Solving the Problems]
In order to achieve the above object, an optical modulation device according to claim 1 of the present invention includes:
An optical modulator (12) whose operating point is set by a bias voltage;
An operating point detecting means (15) for detecting a deviation of the operating point from the point to be set based on the light emitted from the optical modulator;
An optical modulation device comprising bias voltage supply means (21) for supplying the optical modulator with a bias voltage within a predetermined voltage range that changes in a direction to reduce the deviation of the operating point based on the detection result of the operating point detection means. In
By limiting the range that the bias voltage supplied to the optical modulator can take within a certain period of time from the initial operation so as to be a range narrower than the predetermined voltage range, the light within the limited voltage range. A voltage range control means (31) is provided for setting an initial operating point of the modulator and controlling so that a range that the bias voltage can take after the predetermined time has passed is the predetermined voltage range.
[0024]
The optical modulation device according to claim 2 of the present invention is the optical modulation device according to claim 1,
The voltage range control unit controls a power supply voltage supplied to the bias voltage supply unit, and a range that can be taken by the bias voltage supplied to the optical modulator during the predetermined time from the initial operation time , by limiting the narrower range than the previous SL predetermined voltage range, and sets the initial operating point of the optical modulator in a voltage range which is the limit, the possible range of the bias voltage after the elapse of the predetermined time is the predetermined voltage It is characterized by controlling it to be within a range .
[0025]
According to a third aspect of the present invention, there is provided the optical modulation device according to the first aspect,
The voltage range control means inserts a limiter circuit (35) between the bias voltage supply means and the optical modulator, and is supplied to the optical modulator during the predetermined time from the initial operation time. the possible range of the bias voltage by limiting the narrower range than the previous SL predetermined voltage range, and sets the initial operating point of the optical modulator in a voltage range which is the limit, the bias voltage after the elapse of the predetermined time It is characterized by controlling so that the range which can be taken becomes the said predetermined voltage range .
[0026]
Moreover, the light modulation device according to claim 4 of the present invention is the light modulation device according to any one of claims 1 to 3,
The voltage range control means, the possible range of the bias voltage supplied to the optical modulator, during said predetermined time from the time of the initial operation, the predetermined voltage narrow constant range next than the range, after the elapse of the predetermined time The voltage is changed stepwise to the predetermined voltage range.
[0027]
The light modulation device according to claim 5 of the present invention is the light modulation device according to claim 1 or 2 ,
The voltage range control means has a range that can be taken by the bias voltage supplied to the optical modulator , which is narrower than the predetermined voltage range during the predetermined time from the initial operation, and the predetermined voltage after the predetermined time has elapsed. It is characterized by being continuously increased and changed so as to reach the voltage range.
[0028]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 shows a configuration of an optical modulation device 30 to which the present invention is applied.
[0029]
The laser diode 11, the optical modulator 12, the drive amplifier 13, and the low-frequency oscillator 16, the optical coupler 17, the light receiving element 18, the BPF 19, the phase detector 20, and the bias that configure the optical modulator 30. Since the DC amplifier 21 as the voltage supply means is the same as that shown in the light modulation device 10 of FIG. 8, the same reference numerals are given and description thereof is omitted.
[0030]
In this optical modulation device 30, the range that can be taken by the bias voltage B supplied to the optical modulator 12 at a predetermined timing such as during initial operation is limited to a range narrower than the predetermined voltage range that the DC amplifier 21 can output. Then, within the limited voltage range, the operating point of the optical modulator 12 is made to coincide with the point to be set (one of the stable points or the vicinity thereof), and the operating point coincides with the point to be set or the vicinity thereof. Voltage range control means 31 is provided for controlling the range that can be taken later by the bias voltage B to be a predetermined voltage range.
[0031]
If the bias voltage falls within the predetermined voltage range when the operating point is not a stable point but in the vicinity thereof, there is no problem because the operating point changes from the vicinity of the stable point to the stable point.
[0032]
As a specific configuration of the voltage range control means 31, the power supply voltage supplied to the DC amplifier 21 is controlled to restrict the possible range of the bias voltage B supplied to the optical modulator 12 to a narrow range indirectly. In some cases, the bias voltage signal input to the optical modulator 12 is directly restricted.
[0033]
FIG. 2 is a configuration example of the voltage range control means 31 that controls the power source of the DC amplifier 21.
[0034]
The voltage range control means 31 sets the power supply voltage supplied to the DC amplifier 21 for a certain time T as Va and -Va (the voltage width corresponding to the half period of the modulation characteristic of the optical modulator 12 is Va). Then, after the predetermined time T has elapsed, the power supply voltage is set to a predetermined voltage range V +, V−.
[0035]
When the voltage range control means 31 configured in this way is used, the range that the bias voltage B can take with respect to the modulation characteristic F of the modulator 12 is as shown in FIG. ) Until a predetermined time T elapses, the normal output voltage setting range V + to V− of the DC amplifier 21 is limited to a range of Va to −Va, and the initial operating point is also set to a range of Va to −Va. The
[0036]
In other words, the operating point of the optical modulator 12 can be matched with the only stable point Sk in the voltage range Va to -Va or in the vicinity thereof within this fixed time T.
[0037]
In addition, since the regular power supply voltages V + and V− are supplied to the DC amplifier 21 after the lapse of the predetermined time T, the fluctuation of the modulation characteristic F of the optical modulator 12 due to a temperature change or a change with time is reduced. Since the bias voltage B can be followed in a wide voltage range, it can be operated for a long time.
[0038]
FIG. 4 shows a configuration example of the voltage range control unit 31 that directly limits the bias voltage signal input to the optical modulator 12.
[0039]
This voltage range control means 31 has a limiter circuit 35 that regulates a voltage higher than Va to Va, regulates a voltage lower than −Va to −Va, and outputs a voltage in a range of Va to −Va as it is. The limiter circuit 35 is connected to the bias voltage supply line between the DC amplifier 21 and the optical modulator 12 via the switch 32, and the switch 32 is opened and closed by the timer circuit 33.
[0040]
In the case of the voltage range control means 31 having this configuration, the switch circuit 32 is closed by the timer circuit 33 until the predetermined time T has elapsed since the initial operation, and is controlled to be opened after the predetermined time T has elapsed. The operation shown in FIG. 3 is performed in exactly the same manner as in the second configuration example.
[0041]
In addition to the configuration in which the limiter circuit 35 is inserted as described above, examples of the configuration of the voltage range control unit 31 that directly limits the bias voltage signal input to the optical modulator 12 include those shown in FIG. The A / D converter 36 for A / D converting the output Q, the CPU 37 programmed to adjust the bias voltage B according to the A / D conversion output, and the output of the CPU 37 for D / A conversion In the bias voltage supply means constituted by the D / A converter 38 and the DC amplifier 21 for amplifying the D / A converted output, the CPU 37 has a function of limiting the bias voltage range for a predetermined time. Thus, the range that the bias voltage supplied to the optical modulator 12 can take during the initial operation or the like may be limited to a narrow range. Note that the D / A conversion output may be the bias voltage B without using the DC amplifier 21.
[0042]
In addition, each voltage range control unit 31 described above gradually changes the range that the bias voltage supplied to the optical modulator 12 can take, but the range that the bias voltage supplied to the optical modulator 12 can take. May be gradually increased from a narrow range to a wide range over time.
[0043]
For example, as shown in FIG. 6, by adding a slow start function 39 between the power supply terminal of the DC amplifier 21 and the power supplies V + and V−, the power supply voltage supplied to the DC amplifier 21 is gradually increased during the initial operation. It can be increased continuously.
[0044]
In this case, as shown in FIG. 7, at a timing when a certain time T elapses from the initial operation time, control is performed so that the range in which the bias voltage can be taken is approximately in the range of Va to -Va. Thus, the range in which the bias voltage can be taken is controlled to reach the normal voltage ranges V + and V−. The slow start function 39 can be realized by a simple integration circuit as long as the stable point is near 0 volts.
[0045]
When the voltage range control means 31 having such a configuration is used, the range that the bias voltage B can take with respect to the modulation characteristic F of the modulator 12 is as shown in FIG. 0) until a certain time T elapses, it gradually and continuously expands from a range narrower than the voltages Va to -Va to the voltages Va to -Va.
[0046]
From the initial operation to the elapse of a predetermined time T, the operation point detection means 15 detects the operation point deviation and the operation of supplying the bias voltage B to the optical modulator 12 according to the deviation is a direct current amplifier. The voltage range that the bias voltage B can take during this time is limited to a narrow range of Va to -Va or less, so that the transient fluctuation of the operating point is also within this voltage range.
[0047]
That is, the supply operation of the bias voltage B by the DC amplifier 21 is normally performed within the voltage range Va to −Va within the fixed time T, and the operating point of the optical modulator 12 is at the timing when the fixed time T elapses. It corresponds to the only stable point Sk in the voltage range Va to -Va or the vicinity thereof.
[0048]
As described above, the light modulation device 30 according to the embodiment restricts the range of the bias voltage supplied to the light modulator 12 during the initial operation so that the range can be narrower than the predetermined voltage range, and is within the restricted voltage range. Thus, the operating point of the optical modulator 12 is made to coincide with the stable point to be set or the vicinity thereof, and the voltage that controls the range that the bias voltage can take after the operating point coincides with the stable point or the vicinity thereof becomes a predetermined voltage range. Range control means 31 is provided.
[0049]
For this reason, it becomes possible to set the operating point of the initial bias voltage to an arbitrary stable point.For example, by setting the stable point at the center of the predetermined voltage range, the movable range of the stable point can be increased. Long-term continuous operation is possible.
[0050]
In the above description, the range in which the bias voltage supplied to the optical modulator can be taken during the initial operation when the voltage range control unit 31 is turned on is limited to a narrow range. For example, the range of the bias voltage supplied to the optical modulator 12 may be limited to a narrow range when a manual switch operation or a predetermined time elapses.
[0051]
Further, in the above description, the range of the bias voltage that can be taken during the initial operation is set to a range that has a width corresponding to one period of the modulation characteristic of the optical modulator 12 with the center being 0 volt, and the operating point is set to 0 volt. Although it was matched with a specific point in the vicinity, this does not limit the present invention, and the center and width of the possible range of the bias voltage limited to a predetermined timing by the voltage range control means 31 is determined by the optical modulation. It can be arbitrarily set according to the modulation characteristics of the device and its fluctuation tendency.
[0052]
For example, when only the positive side (or negative side) is used as the bias voltage, the range that the bias voltage can take at a predetermined timing is set to V + / 2 (or to the normal voltage range 0 to V + (or 0 to V−). What is necessary is just to set to the range of voltage width Va mentioned above centering on V- / 2).
[0053]
In the above description, the center point of the region having a positive slope with respect to the monotonic increase of the voltage is used as the stable point in the modulation characteristics of the optical modulator 12, but the center point of the region having a negative slope is stable. It is good also as a point.
[0054]
Further, in the optical modulation device 30 described above, the operating point detection means 15 for detecting the deviation of the operating point of the optical modulator 12 includes the low frequency oscillator 16, the optical coupler 17, the light receiving element 18, the BPF 19, and the phase detector 20. However, this is not intended to limit the present invention, and other configurations of the light modulation device, such as the light modulation device disclosed in Patent Document 1 described above, are described. As described above, the average power of the two outgoing lights emitted from the optical modulator is obtained, the monitor voltage corresponding to the difference is generated, and the monitor when the operating point of the optical modulator matches the point to be set An optical modulator that uses a voltage as a reference value, detects a deviation of a monitor voltage with respect to the reference value as an operating point deviation, and supplies a bias voltage to the optical modulator to reduce the operating point deviation. The present invention can be applied.
[0055]
【The invention's effect】
As described above, the light modulation device of the present invention limits the possible range of the bias voltage supplied to the light modulator for a fixed time from the initial operation time to be a range narrower than the predetermined voltage range, Voltage range control means is provided for setting an initial operating point of the optical modulator within the limited voltage range and controlling the range that the bias voltage can take after the predetermined time has elapsed to be a predetermined voltage range. .
[0056]
For this reason, it becomes possible to set the operating point of the initial bias voltage to an arbitrary stable point.For example, by setting the stable point at the center of the predetermined voltage range, the movable range of the stable point can be increased. Long-term continuous operation is possible.
[Brief description of the drawings]
FIG. 1 is a block diagram showing a configuration of an embodiment of the present invention. FIG. 2 is a diagram showing a configuration example of a main part of the embodiment. FIG. 3 is a diagram for explaining an operation of the embodiment. FIG. 5 is a diagram showing another configuration example of the main part of the embodiment. FIG. 6 is a diagram showing another configuration example of the main part of the embodiment. FIG. FIG. 8 is a block diagram illustrating a configuration of a conventional apparatus. FIG. 9 is a diagram illustrating a modulation characteristic of an optical modulator.
DESCRIPTION OF SYMBOLS 11 ... Laser diode, 12 ... Optical modulator, 13 ... Drive amplifier, 15 ... Operating point detection means, 16 ... Low frequency oscillator, 17 ... Optical coupler, 18 ... Light receiving element, 19 ... BPF , 20 ... Phase detector, 21 ... DC amplifier, 30 ... Optical modulator, 31 ... Voltage range control means, 32 ... Switch, 33 ... Timer circuit, 35 ... Limiter circuit, 36 ... A / D converter, 37 ... CPU, 38 ... D / A converter, 39 ... slow start function

Claims (5)

An optical modulator (12) whose operating point is set by a bias voltage;
An operating point detecting means (15) for detecting a deviation of the operating point from the point to be set based on the light emitted from the optical modulator;
An optical modulation device comprising bias voltage supply means (21) for supplying the optical modulator with a bias voltage within a predetermined voltage range that changes in a direction to reduce the deviation of the operating point based on the detection result of the operating point detection means. In
By limiting the range that the bias voltage supplied to the optical modulator can take within a certain period of time from the initial operation so as to be a range narrower than the predetermined voltage range, the light within the limited voltage range. Optical modulation characterized by comprising voltage range control means (31) for setting an initial operating point of the modulator and controlling so that a range that the bias voltage can take after the predetermined time has passed is the predetermined voltage range apparatus. The voltage range control unit controls a power supply voltage supplied to the bias voltage supply unit, and a range that can be taken by the bias voltage supplied to the optical modulator during the predetermined time from the initial operation time , by limiting the narrower range than the previous SL predetermined voltage range, and sets the initial operating point of the optical modulator in a voltage range which is the limit, the possible range of the bias voltage after the elapse of the predetermined time is the predetermined voltage The light modulation device according to claim 1, wherein the light modulation device is controlled to be within a range . The voltage range control means inserts a limiter circuit (35) between the bias voltage supply means and the optical modulator, and is supplied to the optical modulator during the predetermined time from the initial operation time. the possible range of the bias voltage by limiting the narrower range than the previous SL predetermined voltage range, and sets the initial operating point of the optical modulator in a voltage range which is the limit, the bias voltage after the elapse of the predetermined time The light modulation device according to claim 1, wherein the range that can be taken is controlled to be the predetermined voltage range . The voltage range control means, the possible range of the bias voltage supplied to the optical modulator, during said predetermined time from the time of the initial operation, the predetermined voltage narrow constant range next than the range, after the elapse of the predetermined time 4. The light modulation device according to claim 1, wherein the light modulation device is changed stepwise to the predetermined voltage range. The voltage range control means has a range that can be taken by the bias voltage supplied to the optical modulator , which is narrower than the predetermined voltage range during the predetermined time from the initial operation, and the predetermined voltage after the predetermined time has elapsed. claim 1 or claim 2 Symbol placement of the light modulation device, characterized in that to continuously increasing change to reach the voltage range.

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