JP5120969B2 - Low power laser drive circuit - Google Patents

Description

  The present invention relates to a laser drive circuit used in a transmitter for optical communication, and more particularly to a technique for reducing power consumption without hindering high-speed operation.

  FIG. 11 shows a configuration example of a conventional ultrafast laser driving circuit (see, for example, Non-Patent Document 1). This laser drive circuit takes in the differential burst data signal DATA input to the input terminals INP and INN from the input buffer circuit 41 to the cross point control unit 42 in which the cross point of the eye pattern is set by the control voltages VB1 and VB2. The signal is amplified by the buffer circuits 43 and 44 and input to the drive circuit 45, and the drive circuit 45 drives the laser element LD. VC1 is a control voltage of the drive circuit 45, and VC2 is a control voltage of the current source I40 that supplies a bias current to the laser element LD. L40 is an inductor for high frequency cut.

  In this configuration, there is a problem that a constant current is consumed regardless of the operation of the laser element LD, and the power consumption is high.

  FIG. 12 shows a configuration example of another conventional laser driving circuit with higher functionality realized by the SiGe process or the like. This laser drive circuit has a laser element LD having high-frequency cut inductors L1 and L2 connected to the anode side and the cathode side, and inputs differential burst data DATA from input terminals INP and INN to the laser element LD. A modulation circuit 10 that outputs a modulation signal according to data, a bias circuit 20 that supplies a bias current to the laser element LD, an output light of the laser element LD is monitored, and bias control signals VCSP and VCSM are supplied to the modulation circuit 10 In addition, an APC (Automatic Power Control) circuit 30 for supplying a bias control signal VCSB to the bias circuit 20 is provided. A transmission enable signal TX_EN is input to the modulation circuit 10, the bias circuit 20, and the APC circuit 30. In the transmission enable signal TX_EN, ON indicates enable and OFF indicates non-enable.

  The modulation circuit 10 is composed of a gate circuit 11 for gating the input burst data DATA by turning on / off the transmission enable signal TX_EN, an amplification pre-buffer circuit 12, and an emitter follower, and converts the amplitude of the burst data DATA into a large amplitude. The level conversion circuit 13 and the output buffer circuit 14 functioning as a drive circuit. The pre-buffer circuit 12, the level conversion circuit 13, and the output buffer circuit 14 are each provided with their current sources I11, I12, and I13, and each of the pre-buffer circuit 12, the level conversion circuit 13, and the output buffer circuit 14 is supplied with the bias control signal VCSP or VCSM output from the APC circuit 20. The output optical power is controlled to be constant. The bias circuit 20 is also controlled by the bias control signal VCSB output from the APC circuit 20 so that the output optical power of the laser element LD becomes constant.

  In this configuration, the optical output can be turned on / off by the gate circuit 11 in accordance with the on / off of the transmission enable signal TX_EN, but it does not have a function of reducing power consumption when the burst operation is not performed. That is, there is a problem that a constant current is consumed regardless of the operation of the laser element LD, and the power consumption is high.

  6A shows the current flowing through the laser element LD of the conventional circuit shown in FIG. 12, and FIG. 7A shows the current flowing through the laser driving circuit of the conventional circuit shown in FIG. .

  In FIG. 6A, the transmission enable signal TX_EN is turned on (high level) in synchronization with the input of the burst data signal, and the current that flows through the laser element LD after the gate circuit 11 is turned on in response thereto. Rise time Ton required for stabilization is 20 ns, the transmission enable signal TX_EN is turned off (low level), and in response to this, the current flowing through the laser element LD is cut off after the gate circuit 11 is turned off. The fall time Toff required for this is several ns, which is high speed. However, as shown in FIG. 7A, even when the transmission enable signal TX_EN is off, a current equivalent to that at the time of on flows in the modulation circuit 10 and the bias circuit 20, and the power supply of the laser driving circuit is 3.3 V, When the laser power supply VDDLD is 5 V, it can be seen that about 1 W of power consumption is always consumed.

“OKI Semiconductor Co., Ltd. Online Catalog Data Sheet, 10G Direct Modulation Laser Driver KGA4185” [Search December 24, 2009], Internet <URL: http://www.okisemi.com/jp/dbps_data/datasheet/ optical / ODHKGA4185-05.pdf>

  As described above, in the laser drive circuit of FIG. 11, the current consumption basically does not change when the laser element LD is turned on / off. This is because the circuit current is determined by the amount of current that the current source transistor flows, and this amount of current is determined by the control voltages VCSP, VCSM, and VCSB that the APC circuit 30 continues to output. This problem can be realized by cutting off the current of the current source, but according to this method, a certain amount of time is required for the circuit to rise during the process of recovering from the cut-off state to the energized state. The delay causes an increase in the time during which the laser element is turned on, which causes a problem when operating at high speed.

  An object of the present invention is to provide a laser drive circuit that can reduce power consumption during laser extinction without interfering with high-speed operation of the laser drive circuit.

In order to achieve the above object, a laser driving circuit according to a first aspect of the present invention includes a modulation current generation circuit that generates a modulation current in a burst mode according to an input signal and outputs the modulation current to a laser element, A laser driving circuit comprising: a modulation circuit having a first current source for supplying an operating current; and a bias circuit for applying a bias current to the laser element by a second current source, wherein the modulation circuit includes an anode of the laser element A potential of a terminal connected to the cathode is set higher than a potential of a differential output terminal connected to the cathode and a terminal connected to the anode among the differential output terminals when the transmission enable signal is turned off. A bias circuit, the collector or drain of which is connected to the cathode of the laser element. And the second current source connected to the emitters or sources of the first and second transistors, and the transmission enable signal. The first transistor is turned on when the second transistor is turned on and the second transistor is turned off to output the bias current of the second current source to the laser element. When the transmission enable signal is turned off, the first transistor is turned on. There as a bias control signal the second transistor is turned on and configured to set the cathode side of the laser device in a high impedance, adjusts the current before Symbol second current source turns off, the transmission a first control voltage for outputting a predetermined current to the second current source when the enable signal on said transmission Providing a second control voltage to cut off the second current source current when off enable signal, and, Vh said first control voltage, said second control voltage when the Vl, wherein The Vh and Vl are set to have a relationship of Vh>Von>Vl> 0V with respect to the minimum operating voltage Von of the second current source.
According to a second aspect of the present invention, in the laser driving circuit according to the first aspect, the minimum operating voltage Von is a threshold voltage between a base and an emitter of a transistor constituting the second current source or between a gate and a source. It is a voltage to be applied.
According to a third aspect of the present invention, in the laser drive circuit according to the first or second aspect, the first control voltage is set when the transmission enable signal is on, and the second control voltage is set when the transmission enable signal is off. And a switch circuit for outputting to the second current source.
According to a fourth aspect of the present invention, in the laser drive circuit according to the third aspect , a voltage follower circuit is connected between the output side of the switch circuit and the second current source.
According to a fifth aspect of the present invention, in the laser drive circuit according to the third aspect , the first control voltage and the second control voltage are input to the switch circuit, and an input path for the first control voltage. In addition, a voltage follower circuit is connected.
The invention according to claim 6 is the laser drive circuit according to claim 4 or 5, wherein the voltage follower circuit is replaced with an emitter follower circuit or a source follower circuit.

  According to the present invention, as the bias control signal for adjusting the current of the current source of the modulation current generation circuit and / or the bias circuit, the control voltage Vh for outputting a predetermined current to the current source when the transmission enable signal is turned on and the off time Since the control voltage Vl for cutting off the current of the current source is set to have a relationship of Vh> Von> Vl> 0V with respect to the minimum operating voltage Von of the current source, the current source is turned on / off at high speed and simultaneously The power consumption can be reduced, and the power can be reduced when the laser is extinguished without hindering the high-speed operation of the laser driving circuit.

It is a circuit diagram of the laser drive circuit of the 1st example of the present invention. FIG. 2 is a circuit diagram of an output buffer circuit and a bias circuit applicable to the laser driving circuit of FIG. It is a circuit diagram of the laser drive circuit of the 2nd Example of this invention. It is a circuit diagram of the laser drive circuit of the 3rd Example of this invention. FIG. 10 is a circuit diagram of a laser drive circuit according to a modification in which the connection of the voltage follower circuit and the switch circuit is reversed to the configuration of FIG. 4 in the third embodiment of the present invention. It is a characteristic view of the burst response by ON / OFF of the transmission enable signal of the laser drive circuit of the 3rd Example of this invention. It is a characteristic view of the circuit current by ON / OFF of the transmission enable signal of the laser drive circuit of the 3rd Example of this invention. It is a special drawing of the difference of the rise time Ton of the laser element by the difference of the voltage Vl of the laser drive circuit of the 3rd Example of this invention. It is a circuit diagram which shows the example of the generation circuit of the voltage Vl. It is a circuit diagram of the laser drive circuit of the 4th Example of this invention. It is a circuit diagram of a laser drive circuit for 10 Gb / s that is currently most used. It is a circuit diagram of a typical laser drive circuit using SiGe.

  The basic configuration of the present invention includes means for switching a bias control signal of a current source included in each of a modulation current generation circuit and a bias circuit in the modulation circuit, in addition to the conventional modulation circuit, bias circuit, and APC circuit, and bias control thereof Means for generating a high-potential-side control voltage Vh and a low-potential-side control voltage Vl as signals. Here, the modulation current generation circuit of the present application is a general term for all functional blocks necessary for generating a modulation signal, for example, shown in the modulation circuit 10 of FIG. However, the gate circuit 11 or a circuit corresponding thereto is not included. In another example, it corresponds to the input buffer circuit 41, the cross point control unit 42, the buffer circuits 43 and 44, and the drive circuit 45 illustrated in FIG. The control voltage Vh is a voltage for adjusting the current of the current source to an appropriate value when the laser element (laser diode) is on, and the control voltage Vl is a voltage for turning off the current source.

  When the transistor constituting the current source is a bipolar transistor, the base-emitter voltage Vbe (threshold voltage) at which the transistor is turned on is about 0.65 V although there is a slight difference depending on the process. Therefore, if the control voltage Vl is a voltage that makes the base-emitter voltage Vbe equal to or lower than the threshold voltage, the current flowing through the current source transistor decreases rapidly.

  Therefore, if the value of the control voltage Vl is set not to the ground potential (0V) but to a value slightly lower than the value at which the base-emitter voltage Vbe of the current source transistor becomes the threshold voltage, the control voltage Vl is set to the ground potential. Since the pre-bias effect is achieved compared to the case where the bias control signal is switched to the control voltage Vh, the laser element can be turned on at a high speed, and both high speed and a power saving mode by current interruption are possible. It becomes. As described above, the control voltage Vl applied to the current source is set to a potential higher than the ground potential in synchronization with the transmission enable signal TX_EN being turned off.

  The control voltage Vh applied to the current source transistor is controlled by an APC circuit so that an appropriate light output and extinction ratio can be obtained. Since the APC circuit performs control based on the amount of current generated by a monitor photodiode (not shown) that receives laser light from the laser element and the temperature detected by the temperature sensor, a bias control signal is output regardless of the presence or absence of a burst signal. Continue to output. On the other hand, ON / OFF of the laser element is controlled by ON / OFF of the transmission enable signal TX_EN output from the PON-MAC IC (PON control IC). Therefore, the laser element is synchronized with ON / OFF of the transmission enable signal TX_EN. If the bias control signal is set to the control voltage Vl when the transmission enable signal TX_EN is turned off, the power at the time of extinction can be reduced.

  Therefore, in the present invention, in synchronization with the ON / OFF of the transmission enable signal TX_EN, the bias control signal of the current source is set to the control level (control voltage Vh) at the time of light emission, and the current cutoff level (control voltage Vl) at the time of extinction. It was set as the structure to do. Although the laser element can be turned off at the same time by cutting off the current from the current source, the level of the control voltage Vl can be set within a range in which the switching time does not become long by combining with the conventional circuit configuration for turning on / off the laser element. It is possible to set a value that can achieve power saving (current interruption) as much as possible, and the stability of the control voltage Vl does not have to be strict.

  With these configurations, it is possible to reduce the power consumption by turning off the current source when the burst operation is not emitting light, and at the same time, the high-speed operation of the rise time Ton and the fall time Toff of the optical output is compatible. .

<First embodiment>
A first embodiment of the present invention will be described with reference to FIG. In the first embodiment, in the laser drive circuit, the modulation current generation circuit 15 and the bias circuit 20 of the modulation circuit 10 have current sources I10 and I20 controlled by bias control signals Vcsi_M and Vcsi_B, respectively. As the bias control signals Vcsi_M and Vcsi_B, control voltages Vh and Vl having a relationship of Vh> Vl are used. The values of the bias control signals Vcsi_M and Vcsi_B are switched to the control voltage Vh / Vl in accordance with on / off of the transmission enable signal TX_EN synchronized with the presence / absence of the burst data signal, thereby the modulation current generation circuit 15 and the bias circuit The currents of the 20 current sources I10 and I20 increase or decrease.

  Here, the values of the control voltages Vh and Vl may be different from the bias control signals Vcsi_M and Vcsi_B. The value of the control voltage Vh on the high potential side is a value necessary for supplying a predetermined laser modulation current and bias current to the laser element LD during laser emission, that is, during burst communication, and the APC circuit 30 emits laser light. It is a value that is automatically controlled so that the intensity becomes a predetermined value. A monitor photodiode for monitoring the laser emission intensity, a temperature sensor for detecting the temperature, and the like are connected to the APC circuit 30 (not shown).

  The gate circuit 11 is a data cutoff circuit composed of an AND circuit and a switch circuit. For example, the modulation circuit 10 and the bias circuit 20 are configured by a differential circuit, and when the transmission enable signal TX_EN is off, the gate circuit 11 is controlled so that the differential of the output buffer circuit in the modulation current generation circuit 15 is performed. Of the output terminals LDP and LDN, the terminal LDP connected to the anode side of the laser element LD is fixed to the Low level, and the terminal LDN connected to the cathode side is fixed to the High level. At this time, in the bias circuit 20 as well, the terminal to which the cathode LDK of the differential laser element LD is connected is fixed to a high impedance.

  FIG. 2 shows an example of a typical output buffer circuit and bias circuit applicable to the present invention. The output buffer circuit shown in FIG. 2A includes transistors Q1 to Q3 and resistors R1 to R5. The differential laser drive signals ISPB and ISPN are input to the bases of the transistors Q1 and Q2, and the bias is applied to the base of the transistor Q3. A control signal Vcsi_M is input. 2B includes transistors Q4 to Q6, resistors R6 to R9, and an inductor L1. The positive phase signal TX_EN_P and the negative phase signal of the differential transmission enable signal are provided at the bases of the transistors Q4 and Q5. TX_EN_N is input, and the bias control signal Vcsi_B is input to the base of the transistor Q6.

  When the transmission enable signal TX_EN is turned from on to off, the output buffer circuit shown in FIG. 2A has the laser drive signal ISPB fixed to Low and ISPN fixed to High by the operation of the gate circuit 11, and the anode of the laser element LD. The side terminal LDP is fixed at a low potential and the cathode side terminal LDN is fixed at a high potential, so that the current path of the laser element is interrupted. At this time, in the bias circuit shown in FIG. 2B, since the positive phase signal TX_EN_P of the transmission enable signal is fixed to Low and the negative phase signal TX_EN_N is fixed to High, the bias current path is switched, and the laser element The cathode terminal LDK of the LD is set to high impedance.

  Thus, the laser element LD is quenched. However, at this time, if the potentials of the bias control signals Vcsi_M and Vcsi_B do not change, the current divided by the resistors R5 and R9 is obtained by subtracting the base-emitter voltage Vbe of the current source transistors Q3 and Q6 from these potentials. Will continue to flow.

  In contrast, in the present embodiment, these bias control signals Vcsi_M and Vcsi_B are controlled in synchronization with on / off of the transmission enable signal TX_EN, so that the current flowing through the resistors R5 and R9 can be reduced during laser extinction. is there.

  In the present embodiment, both the bias control signals Vcsi_M and Vcsi_B are controlled. However, even when either one of them is controlled, the effect of the present invention is partially exhibited. Further, the gate circuit 11 is not necessarily required when the data signal is previously burst and input to the laser driving circuit. Of course, even in this case, there is no problem that the gate circuit 11 is provided. In general, there are many methods in which data is bursted by the action of the gate circuit 11, and the input data in this case is a continuous signal.

  Since the light emission / extinction of the laser element LD is controlled by the gate circuit 11 in accordance with the on / off of the transmission enable signal TX_EN, the circuit current does not necessarily need to be completely cut off by the control of the bias control signals Vcsi_M and Vcsi_B. . In other words, the extinction of the laser element LD is realized by the action of the gate circuit 11, and therefore the communication quality is not affected even if the control voltage Vl varies somewhat.

<Second embodiment>
FIG. 3 shows a second embodiment of the present invention. The difference from the first embodiment is that the modulation current generation circuit 15 and the bias circuit 20 have a modulation current switch circuit 16 and a bias switch circuit 21 connected to the current sources I10 and I20. is there. The switch circuits 16 and 21 are switched according to on / off of the transmission enable signal TX_EN. The switch circuits 16 and 21 have a control voltage Vl for turning off the current sources I10 and I20 higher than 0V and lower than a voltage Von at which the currents of the current sources I10 and I20 are effectively turned on (Von>Vl>). 0V) is input from a circuit (not shown). In FIG. 3, this control voltage is indicated by Vl_M and Vl_B. As a result, the power consumption is reduced, and the light output on / off switching time is shortened to enable high-speed response.

  The APC circuit 30 of this embodiment does not need to change its output voltage in conjunction with the on / off of the transmission enable signal TX_EN, and may always output a predetermined control voltage. This voltage is indicated by Vh_M and Vh_B.

  The switch circuits 16 and 21 desirably have a short switching response time to the on / off control signal (TX_EN in this embodiment) and a small on-resistance Ron. Specifically, the response time is preferably several ns or less, and Ron is preferably several tens of Ω or less. This is because in a switch circuit with a long response time, the rise time when the laser element LD is turned on becomes long, resulting in a decrease in communication efficiency, and when the resistance Ron is large, the output voltage of the APC circuit 30 is dropped in the switch circuit. This is because an error occurs in the control. Also in this embodiment, the role of the gate circuit 11 is the same as that of the first embodiment.

<Third embodiment>
FIG. 4 shows a third embodiment of the present invention. In the third embodiment, in addition to the configuration of the second embodiment, the bias between the output side of the switch circuit 16 and the input terminal of the bias control signal Vcsi_M of the current source I10, and between the output side of the switch circuit 21 and the current source I20. The drive power for the bias control signals Vcsi_M and Vcsi_B is increased by inserting the voltage follower circuits 17 and 22 between the input terminals of the control signal Vcsi_B, respectively.

  In this embodiment, the step response time (Srew Rate) of the operational amplifiers constituting the voltage follower circuits 17 and 22 is preferably as short as possible. However, the step response of the operational amplifier is generally about several tens V / μs and changes by 1V. Tens of ns are required.

  Therefore, in order to avoid the operation delay due to this, the voltage follower circuits 17 and 22 are connected to the front stage as shown in FIG. 5 and the switch circuits 16 and 21 are connected to the rear stage so that the control voltage Vh_M is passed through the voltage follower circuit 17. Thus, the control voltage Vh_B may be supplied to the switch circuit 21 via the voltage follower circuit 22 to the switch circuit 16. In this configuration, the control voltages Vh_M and Vh_M that are the outputs of the voltage follower circuits 17 and 22 do not change depending on whether the transmission enable signal TX_EN is turned on or off, so that the step response performance is irrelevant. Of course, the control voltages Vl_M and Vl_B may also be supplied via another voltage follower circuit, but the control voltages Vl_M and Vl_B are extinguished while the control voltages Vh_M and Vh_M affect the light emission characteristics of the laser element LD. This is a control voltage at the time, and even if it fluctuates somewhat, the communication quality itself is not affected, so the effect is small.

  FIG. 6 is a characteristic diagram comparing and comparing the burst response in the prior art (configuration of FIG. 12) and the third embodiment (configuration of FIG. 4), and FIG. 7 is a diagram comparing and simulating changes in circuit current. It is. 6 and 7, (a) is the prior art, and (b) is the third embodiment.

  The prior art of FIG. 12 has a configuration in which the bias control signals VCSM (= Vcsi_M) and VCSB (= Vcsi_B) are constant voltages, and the light emission / extinction of the laser element LD is controlled only by the operation of the gate circuit 11. Although slightly described above, the rise time Ton and fall time Toff of the laser element LD are short as shown in FIG. 6A, but the time during which the laser element LD is extinguished as shown in FIG. 7A. Even in the band, a current close to that at the time of light emission flows.

  On the other hand, in the third embodiment, as shown in FIG. 6 (b), the rise time Ton becomes slightly longer, but as shown in FIG. 7 (b), the current during extinction is greatly reduced. Yes. In the simulation in this embodiment, the rise time of the switch circuits 16 and 21 is set to 15 ns, which is a speed that can be realized by a general analog switch regardless of the amount of change in voltage. The control voltage Vh_M was 1.1V, the control voltage Vh_B was 0.9V, and the control voltages Vl_M and Vl_B were 0.5V.

  FIG. 8 is a simulation result showing how the rise time Ton during which the laser element LD is turned on is affected by the selection of the value of the control voltage Vl (= Vl_M, Vl_B). Also in this simulation, the transition time from Vh to Vl is set to 15 ns, which is a speed that can be realized by a general analog switch, regardless of the value of the control voltage Vl. When the control voltage Vl is 0V, the Vl generating circuit is unnecessary (suffices to be grounded), but the rise time Ton becomes long. As the control voltage Vl is increased, the rise time Ton can be shortened. As a result, if the base-emitter voltage Vbe of the transistor exceeds the threshold voltage (usually about 0.65 V), the current blocking effect is obtained. That is, the low power effect is reduced. If the voltage is lower than the threshold voltage, the current rapidly decreases. Therefore, the control voltage Vl is preferably set so that the base-emitter voltage Vbe is 0.6 V or less. The control voltage Vl can be optimized according to the characteristics of the devices constituting the circuit.

  FIGS. 9A to 9D are examples of a circuit for generating a control voltage Vl (= Vl_M, Vl_B) that can be used in each embodiment of the present invention. R11 to R18 are resistors, Q11 to Q13 are transistors, and ZD is a Zener diode. Although a constant voltage generating circuit as shown in FIG. 6D is preferable, in the case of a monolithic configuration with a laser driving circuit, simple circuits such as (a) to (c) may be used. However, the circuit (c) is designed so that the control voltage Vl is as close as possible to the threshold voltage of the transistor Q12 because the constant current characteristic of the transistor Q12 may deteriorate due to the collector voltage being lower than the base voltage. There is a need. This is because, as described above, the control voltage Vl is a voltage applied at the time of extinction, and it is one of the features of the present invention that even if the control voltage Vl slightly varies, it does not affect the communication itself. The configuration as shown in (d) is an example of the simplest constant voltage generation circuit. However, when the configuration is a discrete (external) circuit apart from the IC of the drive circuit, there is an advantage that the number of components can be reduced. is there.

<Fourth embodiment>
FIG. 10 shows a fourth embodiment of the present invention. In the fourth embodiment, instead of the voltage follower circuits 17 and 22 in the third embodiment (FIG. 4), a switch circuit is provided between the output side of the switch circuit 16 and the input terminal of the bias control signal Vcsi_M of the current source I10. The emitter follower circuits 18 and 23 are respectively inserted between the output side of 21 and the input terminal of the bias control signal Vcsi_B of the current source I20. Similar to the third embodiment, the driving force for the bias control signals Vcsi_M and Vcsi_B is increased. However, the operation can be performed faster than the voltage follower circuits 17 and 22 using the operational amplifier. This is because the step response time of the emitter follower circuit is faster than that of the voltage follower circuit limited by the operational amplifier. As another form of the present embodiment, there is one in which the emitter follower circuit is replaced with a source follower circuit. In this case, the same effect as that of the emitter follower circuit can be obtained.

  In each of the above embodiments, the modulation current generation circuit 10 has been described as an example having one current source I10. However, the modulation current generation circuit 10 has the pre-buffer circuit 12, the level conversion circuit 13, and the output shown in FIG. When the buffer circuit 14 is provided and the current sources I11, I12, and I13 are individually provided, the current sources I11, I12, and I13 are turned on / off by individual switch circuits, or The sources I11, I12, and I13 are controlled by the same control voltage so as to be turned on / off via one switch circuit 16, or at least the current source I13 of the output buffer circuit 14 is turned on / off via the switch circuit 16. To be turned off.

10: modulation circuit, 11: gate circuit, 12: pre-buffer circuit, 13: level conversion circuit, 14: output buffer circuit, 15: modulation current generation circuit, 16: switch circuit, 17: voltage follower circuit, 18: emitter follower Circuit 20: Bias circuit, 21: Switch circuit, 22: Voltage follower circuit, 23: Emitter follower circuit 30: APC circuit LD: Laser element L1, L2: Inductor Vcsi_M: Bias control signal for modulation circuit Vcsi_B: For bias circuit Bias control signal Vh, Vh_M, Vh_B: High potential side control voltage Vl, Vl_M, Vl_B: Low potential side control voltage TX_EN: Transmission enable signal

Claims (6)

A modulation current generating circuit that generates a modulation current in a burst mode according to an input signal and outputs the modulation current to a laser element, a modulation circuit having a first current source that supplies an operating current to the modulation current generating circuit, and a second current source A laser drive circuit comprising a bias circuit for applying a bias current to the laser element according to
The modulation circuit includes a differential output terminal connected to an anode and a cathode of the laser element, and the cathode more than a potential of a terminal connected to the anode among the differential output terminals when the transmission enable signal is turned off. And a gate circuit for setting the potential of the terminal connected to the high,
The bias circuit includes a first transistor whose collector or drain is connected to the cathode of the laser element, a second transistor whose collector or drain is connected to a power supply terminal, and the first and second transistors. And a second current source connected to an emitter or a source, and when the transmission enable signal is turned on, the first transistor is turned on and the second transistor is turned off to cause the laser element to A bias current of a current source is output, and when the transmission enable signal is turned off, the first transistor is turned off and the second transistor is turned on to set the cathode side of the laser element to high impedance. And
As the bias control signal for adjusting the current before Symbol second current source, a first control voltage to output a predetermined current to the second current source during on of the transmission enable signal, off of the transmission enable signal A second control voltage for interrupting the current of the second current source sometimes.
When the first control voltage is Vh and the second control voltage is Vl, the Vh and Vl are set to Vh>Von>Vl> with respect to the lowest operating voltage Von of the second current source. A laser driving circuit characterized in that the relationship is set to 0V. The laser driving circuit according to claim 1,
The minimum operating voltage Von is a voltage that gives a threshold voltage between a base and an emitter or between a gate and a source of a transistor constituting the second current source. In the laser drive circuit according to claim 1 or 2,
A switch circuit is provided that outputs the first control voltage to the second current source when the transmission enable signal is on and the second control voltage when the transmission enable signal is off. Laser drive circuit. In the laser drive circuit according to claim 3 ,
A laser drive circuit, wherein a voltage follower circuit is connected between an output side of the switch circuit and the second current source. In the laser drive circuit according to claim 3 ,
A laser driving circuit, wherein the first control voltage and the second control voltage are input to the switch circuit, and a voltage follower circuit is connected to an input path of the first control voltage. In the laser drive circuit according to claim 4 or 5,
A laser driving circuit, wherein the voltage follower circuit is replaced with an emitter follower circuit or a source follower circuit.

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