JP5003586B2 - Semiconductor laser drive circuit - Google Patents

Description

  The present invention relates to a semiconductor laser driving circuit for driving a semiconductor laser.

  As a drive circuit for driving a semiconductor laser, a shunt drive type semiconductor laser drive circuit is known (for example, see Patent Document 1). A shunt drive type semiconductor laser drive circuit includes a transistor (corresponding to a switching current source in Patent Document 1) connected in parallel to the semiconductor laser, and supplies a bias current source (patent) to the semiconductor laser by switching the transistor. The output light of the semiconductor laser is modulated by branching a bias current from the first current source in Document 1).

Incidentally, an optical transmitter used for optical communication or the like is required to have a constant optical power and extinction ratio of output light. However, a semiconductor laser used as a light source in an optical transmitter has a lower luminous efficiency as the temperature becomes higher. Therefore, a semiconductor laser driving circuit used in this type of optical transmitter includes an APC circuit (auto power control circuit) in order to keep the optical power and extinction ratio of the output light of the semiconductor laser constant. This APC circuit controls so that a larger bias current and modulation current are supplied to the semiconductor laser as the temperature of the semiconductor laser becomes higher.
JP-A-6-152027

  In the shunt drive type semiconductor laser drive circuit, the gate bias voltage (or base bias voltage) of the transistor is set large in order to increase the amplitude of the modulation current at a high temperature. On the other hand, since the amplitude of the modulation current can be reduced at low temperatures, it is desirable to reduce the bias current from the bias current source in order to reduce power consumption. The flowing current cannot be reduced, and the bias current cannot be appropriately reduced. As a result, it has been difficult to reduce power consumption.

  Therefore, an object of the present invention is to provide a semiconductor laser driving circuit capable of reducing power consumption.

  The semiconductor laser drive circuit according to the present invention includes (a) a bias current source for generating a bias current to be supplied to the semiconductor laser and changing the bias current according to the temperature of the semiconductor laser, and (b) a control terminal and a pair. The semiconductor laser is connected in parallel to the semiconductor laser and switches the current flowing between the pair of current terminals in accordance with the modulation signal applied to the control terminal to shunt the bias current. And (c) a bias voltage generating circuit that supplies a bias voltage to the control terminal of the transistor and changes the bias voltage according to the temperature of the semiconductor laser.

  According to this semiconductor laser drive circuit, since the bias voltage generation circuit changes the bias voltage of the transistor according to the temperature of the semiconductor laser, the bias voltage of the transistor can be lowered when the temperature of the semiconductor laser is relatively low. . As a result, the bias current can be appropriately reduced, and the power consumption can be reduced.

  The bias voltage generation circuit described above preferably further changes the bias voltage in response to variations and variations in the threshold voltage of the transistor.

  Since the threshold voltage of the transistor also varies or varies depending on the temperature, conventionally, it has been necessary to further increase the bias voltage of the transistor so that the LOW level of the modulation signal does not become smaller than the threshold voltage of the transistor. . Therefore, the bias current from the bias current source cannot be reduced appropriately, and it has been difficult to reduce power consumption.

  However, according to this semiconductor laser drive circuit, since the bias voltage generation circuit further changes the bias voltage in response to fluctuations and variations in the threshold voltage of the transistor, the bias voltage of the transistor can be appropriately reduced. As a result, the bias current can be reduced more appropriately, and the power consumption can be reduced more appropriately.

  The semiconductor laser drive circuit described above further includes a bias current detection circuit that detects a bias current and generates a detection signal corresponding to the bias current, and the bias voltage generation circuit changes the bias voltage according to the detection signal. Is preferred.

  According to this semiconductor laser drive circuit, the bias voltage of the transistor is changed according to the bias current by the bias current detection circuit and the bias voltage generation circuit. Since this bias current is changed according to the temperature of the semiconductor laser by the bias current source, the bias voltage of the transistor is changed according to the temperature of the semiconductor laser. Thereby, when the temperature of the semiconductor laser is relatively low, the bias voltage of the transistor can be lowered, and the bias current can be appropriately reduced. Therefore, power consumption can be reduced.

  The semiconductor laser driving circuit described above further includes a bias current detection circuit that detects a bias current and generates a current corresponding to the bias current as a detection signal. The bias voltage generation circuit converts the detection signal into a voltage. A conversion circuit and a series circuit of a semiconductor element having fluctuations and fluctuations in threshold voltage corresponding to fluctuations and fluctuations in the threshold voltage of the transistor, and a bias voltage corresponding to the fluctuations and fluctuations in the detection signal and the threshold voltage of the semiconductor elements Is preferably changed.

  According to this semiconductor laser drive circuit, the bias voltage of the transistor is changed according to the bias current by the bias current detection circuit and the current-voltage converter in the bias voltage generation circuit. Since this bias current is changed according to the temperature of the semiconductor laser by the bias current source, the bias voltage of the transistor is changed according to the temperature of the semiconductor laser. Further, the bias voltage generation circuit includes a semiconductor element connected in series to the current-voltage conversion unit, and the semiconductor element has a variation and variation in the threshold voltage corresponding to the variation and variation in the threshold voltage of the transistor. The bias voltage is changed according to variations and variations in the threshold voltage of the transistor. As a result, the bias voltage of the transistor can be appropriately reduced, and the bias current can be more appropriately reduced. Therefore, the power consumption can be more appropriately reduced.

  According to the present invention, the power consumption of the semiconductor laser driving circuit can be reduced.

  DESCRIPTION OF EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are denoted by the same reference numerals.

  FIG. 1 is a circuit diagram showing a semiconductor laser drive circuit according to an embodiment of the present invention. A semiconductor laser drive circuit 1 shown in FIG. 1 is a circuit for driving a semiconductor laser 5. The semiconductor laser drive circuit 1 includes a driver unit 10, a bias current source 20, a transistor 30, a photodiode 40, an APC circuit (auto power control circuit) 50, a bias current detection circuit 60, and a bias voltage generation circuit 70. And DC-cut capacitance elements 81, 82, 83 and termination resistance elements 84, 85.

  The driver unit 10 receives the input modulation signal Sin and generates a modulation signal Sm. The driver unit 10 includes an amplifier 11 and a differential amplifier circuit 12. The amplifier 11 amplifies the input modulation signal Sin to generate a differential modulation signal and outputs it to the differential amplifier circuit 12. The differential amplifier circuit 12 generates a modulation signal Sm according to the differential modulation signal. The differential amplifier circuit 12 includes transistors 13 and 14 that constitute a differential pair, load resistance elements 15 and 16, and a modulation current source 17.

  In the present embodiment, the transistors 13 and 14 are NPN bipolar transistors, respectively. The differential modulation signals from the amplifier 11 are input to the bases of the transistors 13 and 14, respectively. The collector of the transistor 13 is connected to a power source on the high potential side via the load resistive element 15 and is grounded at high frequency by the terminating resistive element 84 via the DC-cut capacitive element 82. On the other hand, the collector of the transistor 14 is connected to the power source on the high potential side through the load resistive element 16 and is connected to the gate of the transistor 30 through the DC cut capacitive element 81. The sources of the transistors 13 and 14 are both connected to the modulation current source 17, and the modulation current source 17 is grounded to the power source on the low potential side.

  The modulation current source 17 supplies the modulation current Im to the transistors 13 and 14, the load resistance elements 15 and 16, and the termination resistance element 84. The modulation current source 17 changes the value of the modulation current Im according to the value of the modulation current control signal Vm from the APC circuit 50.

  The bias current source 20 is connected to the anode of the semiconductor laser 5 via the bias current detection circuit 60, and the cathode of the semiconductor laser 5 is grounded to the power source on the low potential side. The bias current source 20 generates a bias current Ib to be supplied to the semiconductor laser 5. The bias current source 20 changes the value of the bias current Ib according to the bias current control signal Vb from the APC circuit 50.

  The transistor 30 is an N-type MOSFET in this embodiment, and is connected to the semiconductor laser 5 in parallel. Specifically, the drain (current terminal) of the transistor 30 is connected to the anode of the semiconductor laser 5, and the source (current terminal) of the transistor 30 is grounded to the power source on the low potential side. The modulation signal Sm from the driver unit 10 is input to the gate (control terminal) of the transistor 30. In addition, the gate bias voltage Vg from the bias voltage generation circuit 70 is input to the gate of the transistor 30 via the terminating resistance element 85. A capacitive element 83 for cutting off a direct current component is connected between a node between the bias voltage generation circuit 70 and the terminating resistance element 85 and a power source on the low potential side. The transistor 30 is switched according to the modulation signal Sm, and modulates the output light of the semiconductor laser 5 by diverting the bias current Ib.

  Part of the output light of the semiconductor laser 5 is input to the photodiode 40. The anode of the photodiode 40 is grounded to the power source on the low potential side, and the cathode is connected to the APC circuit 50. The photodiode 40 generates a current signal Si having a value corresponding to the magnitude of the output light from the semiconductor laser 5 and supplies it to the APC circuit 50.

  The APC circuit 50 supplies the bias current control signal Vb to the bias current source 20, and changes the bias current control signal Vb according to the value of the current signal Si. In other words, the APC circuit 50 changes the value of the bias current Ib so that the value of the current signal Si is constant, that is, the optical power of the output light of the semiconductor laser 5 is constant. Further, the APC circuit 50 supplies the modulation current control signal Vm to the modulation current source 17 of the differential amplifier circuit 12 in the driver unit 10, and changes the value of the modulation current control signal Vm according to the value of the current signal Si. . In other words, the APC circuit 50 changes the value of the modulation current control signal Vm so that the value of the current signal Si is constant, that is, the extinction ratio of the output light of the semiconductor laser 5 is constant. Thus, the APC circuit 50 performs APC control so that the optical power and extinction ratio of the output light of the semiconductor laser 5 are kept constant.

  Here, the emission efficiency of the semiconductor laser 5 decreases as the temperature increases. Since the APC circuit 50 detects the output light of the semiconductor laser 5 through the photodiode 40 and performs APC control so as to keep the optical power and extinction ratio of the output light of the semiconductor laser 5 constant, the semiconductor laser 5 The bias current control signal Vb and the modulation current control signal Vm, that is, the bias current Ib and the modulation current Im are set so as to keep the optical power and extinction ratio of the output light of the semiconductor laser 5 constant even if the temperature of the laser beam fluctuates. Will be changed.

  The bias current detection circuit 60 is connected between the semiconductor laser 5 and the transistor 30 and the bias current source 20, detects the bias current Ib, and has a detection signal Sd having a value corresponding to the value of the bias current Ib. Is generated. In the present embodiment, the bias current detection circuit 60 constitutes a current mirror circuit composed of transistors 61 and 62.

  In the present embodiment, the transistors 61 and 62 are P-type MOSFETs. The transistor 61 is connected between the semiconductor laser 5 and the transistor 30 and the bias current source 20. Specifically, the source of the transistor 61 is connected to the bias current source 20, and the drain of the transistor 61 is connected to the cathode of the semiconductor laser 5 and the drain of the transistor 30. The gate of the transistor 61 is connected to its own drain and the gate of the transistor 62. The source of the transistor 62 is connected to the bias current source 20, and the drain of the transistor 62 is connected to the bias voltage generation circuit 70.

  In this way, the bias current detection circuit 60 supplies the mirror current Id of the bias current Ib to the bias voltage generation circuit 70 as the detection signal Sd.

  The bias voltage generation circuit 70 generates a gate bias voltage Vg according to the value of the detection signal Sd and supplies it to the gate of the transistor 30. The bias voltage generation circuit 70 includes a resistance element (current / voltage conversion unit) 71, a transistor (semiconductor element) 72, and a reference voltage source 73.

  The resistance element 71, the transistor 72, and the reference voltage source 73 are connected in series. Specifically, one end of the resistance element 71 is connected to the drain of the transistor 62 in the bias current detection circuit 60, and the other end is connected to the drain of the transistor 72. The gate of the transistor 72 is connected to the drain, and the source is connected to the reference voltage source 73. That is, the transistor 72 is diode-connected. The reference voltage source 73 is grounded to the power source on the low potential side.

  The resistance element 71 generates a voltage according to the detection signal Sd. That is, the resistance element 71 converts the mirror current Id into a voltage. Specifically, the resistance element 71 generates a voltage Id × R71, where R71 is the resistance value of the resistance element 71. Since the mirror current Id is proportional to the bias current Ib, and the bias current Ib is changed according to the temperature of the semiconductor laser 5 as described above, the voltage Id × R71 is changed according to the temperature of the semiconductor laser 5. It becomes.

  Since the transistor 72 is diode-connected and Id is very small, the forward voltage is approximately the gate threshold voltage Vth. In the present embodiment, the transistor 72 is the same type N-type MOSFET as the transistor 30. As a result, the transistor 72 has the variation and variation of the gate threshold voltage Vth corresponding to the variation and variation of the gate threshold voltage of the transistor 30, and the forward voltage of the diode-connected transistor 72 is the gate threshold voltage of the transistor 30. It will be changed in response to voltage fluctuations and variations.

  Since the resistance element 71, the transistor 72, and the reference voltage source 73 are connected in series, the bias voltage generation circuit 70 includes the voltage Id × R71 of the resistance element 71 and the gate threshold voltage Vth of the transistor 72 described above. Then, a gate bias voltage Vg = Id × R71 + Vth + Vref, which is an added value of the reference voltage Vref of the reference voltage source 73, is generated. Therefore, as described above, the bias voltage generation circuit 70 changes the gate bias voltage Vg in accordance with the temperature of the semiconductor laser 5 and in response to fluctuations and variations in the gate threshold voltage Vth of the transistor 30. Become.

  Next, the effects of the semiconductor laser drive circuit 1 of the present embodiment will be described in comparison with the semiconductor laser drive circuit of the comparative example. FIG. 2 is a circuit diagram showing a semiconductor laser driving circuit according to a comparative example of the present invention. The semiconductor laser drive circuit 1X of the comparative example shown in FIG. 2 does not include the bias current detection circuit 60 in the semiconductor laser drive circuit 1, and includes a bias voltage generation circuit 70X instead of the bias voltage generation circuit 70. This is different from the present embodiment.

  The bias voltage generation circuit 70X supplies a gate bias voltage Vg having a constant voltage value to the gate of the transistor 30. That is, in the semiconductor laser drive circuit 1X, the constant gate bias voltage Vg is supplied to the transistor 30 without depending on the temperature fluctuation of the semiconductor laser 5 and without depending on the fluctuation and variation of the gate threshold voltage of the transistor 30. The Rukoto.

  FIG. 3 shows the drain-source current with respect to the gate-source voltage of the transistor in the semiconductor laser driving circuit of the comparative example. FIG. 3A shows the drain-source current Ids characteristics with respect to the gate-source voltage Vgs of the transistor 30 when the temperature of the semiconductor laser 5 and the semiconductor laser driving circuit 1X is high (for example, 85 ° C.). FIG. 3B shows the gate-source region of the transistor 30 when the temperature of the semiconductor laser 5 and the semiconductor laser driving circuit 1X is lower than that of FIG. 3A (for example, room temperature 25 ° C.). A drain-source current Ids characteristic with respect to the voltage Vgs is shown.

  FIG. 4 shows the drive current of the semiconductor laser with respect to the gate-source voltage of the transistor in the semiconductor laser drive circuit of the comparative example. FIG. 4 shows the drive current Ild characteristics of the semiconductor laser 5 with respect to the gate-source voltage Vgs of the transistor 30 under the temperature condition of FIG.

  According to FIG. 3A, in order to keep the optical power and extinction ratio of the output light constant, it is necessary to supply a modulation current Ildm having a large amplitude to the semiconductor laser 5 at a high temperature. It can be seen that the amplitude Vgsm of the inter-voltage Vgs needs to be increased. It can be seen that the gate bias voltage Vg needs to be relatively large so that the voltage value on the low potential (LOW) side in the amplitude Vgsm of the gate-source voltage Vgs is equal to or higher than the gate threshold voltage of the transistor 30.

  On the other hand, according to FIG. 3B, in order to keep the optical power and extinction ratio of the output light constant, the amplitude of the modulation current Ildm supplied to the semiconductor laser 5 may be small at a low temperature, and the gate − The amplitude Vgsm of the source voltage Vgs may be small. However, since the gate bias voltage Vg is constant, the current A always flows through the transistor 30.

  In this regard, referring to FIG. 4, the bias current Ib of the bias current source 20 is set to 60 mA or more because of the large modulation current Ildm at high temperature. However, the amplitude of the modulation current Ildm may be small at a low temperature, but the current A always flows through the transistor 30 because the gate bias voltage Vg is constant.

  Therefore, in the semiconductor laser drive circuit 1X of the comparative example, the bias current Ib cannot be greatly reduced, and it is difficult to reduce power consumption.

  Therefore, the semiconductor laser drive circuit 1 according to the present embodiment is configured so that the gate bias of the transistor 30 corresponds to the temperature of the semiconductor laser 5 and the semiconductor laser drive circuit 1 and to the fluctuation and variation of the gate threshold voltage Vth of the transistor 30. The voltage Vg is changed.

  Specifically, the current signal from the photodiode 40 indicates that the temperature of the semiconductor laser 5 and the semiconductor laser drive circuit 1 has become high, and the optical power and extinction ratio of the output light have been reduced due to the reduction in the light emission efficiency of the semiconductor laser 5. When detected by a decrease in Si, the APC circuit 50 changes the bias current control signal Vb and the modulation current control signal Vm, the bias current source 20 increases the bias current Ib, and the modulation current source 17 increases the modulation current Im. . Then, the detection signal Sd, that is, the mirror current Id is increased by the bias current detection circuit 60, and the gate bias voltage Vg is increased by the resistance element 71 in the bias voltage generation circuit 70. Thereby, even if the temperature of the semiconductor laser 5 and the semiconductor laser drive circuit 1 becomes high, the optical power and extinction ratio of the semiconductor laser 5 can be kept constant.

  On the other hand, the temperature of the semiconductor laser 5 and the semiconductor laser drive circuit 1 is decreased, and the increase in the light power and extinction ratio of the output light due to the increase in the emission efficiency of the semiconductor laser 5 is due to the increase in the current signal Si from the photodiode 40 When detected, the APC circuit 50 changes the bias current control signal Vb and the modulation current control signal Vm, the bias current source 20 decreases the bias current Ib, and the modulation current source 17 decreases the modulation current Im. Then, the detection signal Sd, that is, the mirror current Id is reduced by the bias current detection circuit 60, and the gate bias voltage Vg is reduced by the resistance element 71 in the bias voltage generation circuit 70. Thereby, even if the temperature of the semiconductor laser 5 and the semiconductor laser drive circuit 1 is lowered, the optical power and extinction ratio of the semiconductor laser 5 can be kept constant. Further, by reducing the gate bias voltage Vg, the amount of reduction of the bias current Ib can be increased, and power consumption can be reduced.

  FIG. 5 shows the drain-source current with respect to the gate-source voltage of the transistor in the semiconductor laser driving circuit of this embodiment. FIG. 5 shows current-voltage characteristics when the temperatures of the semiconductor laser 5 and the semiconductor laser driving circuit 1 are relatively low (for example, normal temperature 25 ° C.).

  According to FIGS. 5 and 3B, in the semiconductor laser driving circuit 1 of the present embodiment, the current A that always flows through the transistor 30 is reduced when the bias voltage generation circuit 70 lowers the gate bias voltage Vg at a low temperature. It can be seen that it has been reduced. As a result, in the semiconductor laser drive circuit 1 of the present embodiment, the bias current Ib can be greatly reduced.

  By the way, the gate threshold voltage of the transistor 30 varies and varies. Further, in the drain-source current Ids with respect to the gate-source voltage Vgs, the influence of the secondary characteristics is large in the vicinity of the gate threshold voltage (see B portion in FIG. 6), so that the output light waveform of the semiconductor laser 5 is distorted. . Therefore, in the semiconductor laser drive circuit 1X of the comparative example, the gate is set so that the LOW level of the amplitude Vgsm of the gate-source voltage Vgs of the transistor 30 is in the linear region (see C portion in FIG. 6) even at high temperature. The bias voltage Vg is set larger. For this reason, the bias current Ib cannot be greatly reduced even at low temperatures, and a large amount of current always flows through the transistor 30, making it difficult to reduce power consumption.

  Therefore, in the semiconductor laser drive circuit 1 of the present embodiment, the gate bias voltage Vg of the transistor 30 is changed in accordance with the variation and variation in the gate threshold voltage Vth of the transistor 30.

  Specifically, when the temperature of the semiconductor laser 5 and the semiconductor laser drive circuit 1 becomes high and the gate threshold voltage of the transistor 30 decreases (see FIG. 6), the forward direction of the diode-connected transistor 72 in the bias voltage generation circuit 70 Since the voltage Vth also decreases, the gate bias voltage Vg decreases.

  On the other hand, when the temperature of the semiconductor laser 5 and the semiconductor laser drive circuit 1 becomes low and the gate threshold voltage of the transistor 30 increases (see FIG. 6), the forward voltage Vth of the diode-connected transistor 72 in the bias voltage generation circuit 70 also increases. Since it increases, the gate bias voltage Vg increases.

  Even when the gate threshold voltage of the transistor 30 varies, the forward voltage Vth of the diode-connected transistor 72 in the bias voltage generation circuit 70 varies in the same manner, so that the gate bias voltage Vg can be changed.

  In this way, the diode-connected transistor 72 in the bias voltage generation circuit 70 can change the gate bias voltage Vg corresponding to the variation and variation in the gate threshold voltage of the transistor 30, so The gate bias voltage Vg can be set so that the LOW level of the amplitude Vgsm of the voltage Vgs is set to the minimum value in the linear region. As a result, the bias current Ib can be reduced so that the current that always flows through the transistor 30 is minimized.

  FIG. 7 is a diagram illustrating voltages at various parts in the bias voltage generation circuit with respect to the environmental temperature. In FIG. 7, the voltage H indicates the voltage Id × R71 of the resistance element 71, and the voltage I indicates the forward voltage Vth of the diode-connected transistor 72. The voltage J indicates the gate bias voltage Vg. As described above, the bias voltage generation circuit 70 adds the voltage Id × R71 that is changed according to the temperature of the semiconductor laser 5 and the voltage Vth that is changed according to the variation and variation of the gate threshold voltage of the transistor 30. By generating the gate bias voltage Vg, the gate bias voltage Vg is changed in accordance with the temperature of the semiconductor laser 5 and in response to fluctuations and variations in the gate threshold voltage of the transistor 30.

  As described above, according to the semiconductor laser driving circuit 1 of the present embodiment, the bias voltage generation circuit 70 changes the gate bias voltage Vg of the transistor 30 according to the temperature of the semiconductor laser 5, so that the temperature of the semiconductor laser 5 is compared. When the voltage is low, the gate bias voltage Vg of the transistor 30 can be lowered. As a result, the bias current Ib can be appropriately reduced, and the power consumption can be reduced.

  Further, according to the semiconductor laser drive circuit 1 of the present embodiment, the bias voltage generation circuit 70 further changes the gate bias voltage Vg corresponding to the variation and variation in the gate threshold voltage of the transistor 30, so that the gate bias of the transistor 30 The voltage Vg can be appropriately reduced. As a result, the bias current Ib can be reduced more appropriately, and the power consumption can be reduced more appropriately.

  The present invention is not limited to the above-described embodiment, and various modifications can be made.

1 is a circuit diagram showing a semiconductor laser drive circuit according to an embodiment of the present invention. It is a circuit diagram which shows the semiconductor laser drive circuit which concerns on the comparative example of this invention. It is a figure which shows the drain-source current with respect to the gate-source voltage of the transistor in the semiconductor laser drive circuit of a comparative example. It is a figure which shows the drive current of the semiconductor laser with respect to the gate-source voltage of the transistor in the semiconductor laser drive circuit of a comparative example. It is a figure which shows the drain-source current with respect to the gate-source voltage of the transistor in the semiconductor laser drive circuit of this embodiment. It is a figure which shows the drain-source current with respect to the gate-source voltage of the transistor in the semiconductor laser drive circuit of this embodiment. It is a figure which shows the voltage of each part in the bias voltage generation circuit with respect to environmental temperature.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Semiconductor laser drive circuit, 5 ... Semiconductor laser, 10 ... Driver part, 11 ... Amplifier, 12 ... Differential amplifier circuit, 13, 14 ... Transistor, 15, 16 ... Resistance element for load, 17 ... Modulation current source, 20 DESCRIPTION OF SYMBOLS ... Bias current source, 30 ... Transistor, 40 ... Photodiode, 50 ... APC circuit, 60 ... Bias current detection circuit, 61, 62 ... Transistor, 70 ... Bias voltage generation circuit, 71 ... Resistance element (current voltage conversion part), 72 ... transistor (semiconductor element), 73 ... reference voltage source, 81, 82, 83 ... DC cut capacitor element, 84,85 ... termination resistor element.

Claims (4)

A bias current source for generating a bias current to be supplied to the semiconductor laser and changing the bias current according to the temperature of the semiconductor laser;
The bias current is provided with a control terminal and a pair of current terminals, connected in parallel to the semiconductor laser, and switches a current flowing between the pair of current terminals in accordance with a modulation signal applied to the control terminal. A transistor for modulating the output light of the semiconductor laser,
A bias voltage generation circuit that supplies a bias voltage to the control terminal of the transistor and changes the bias voltage according to the temperature of the semiconductor laser;
A semiconductor laser drive circuit comprising:   2. The semiconductor laser driving circuit according to claim 1, wherein the bias voltage generation circuit further changes the bias voltage in response to a variation and variation in a threshold voltage of the transistor. A bias current detection circuit that detects the bias current and generates a current corresponding to the bias current as a detection signal;
The bias voltage generation circuit includes a series circuit of a current-voltage conversion unit that converts the detection signal into a voltage, and a semiconductor element having a threshold voltage variation and variation corresponding to the threshold voltage variation and variation of the transistor. Changing the bias voltage according to fluctuations and variations in the detection signal and the threshold voltage of the semiconductor element;
The semiconductor laser drive circuit according to claim 2. A bias current detection circuit that detects the bias current and generates a detection signal corresponding to the bias current;
The bias voltage generation circuit changes the bias voltage according to the detection signal;
The semiconductor laser driving circuit according to claim 1.

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