Laser driving circuit and laser driving method

Abstract

To provide a laser driving circuit and a laser driving method that enable instantaneous switching of current sources and prevents unnecessary power consumption when light emission is stopped.SOLUTION: A laser driving circuit includes a laser, a first transistor connected as a current source to the anode side of the laser, and a second transistor connected as a current source to the cathode side of the laser, where the first transistor is a PNP transistor and the second transistor is an NPN transistor, and inputs a differential drive signal to the anode and cathode of the laser.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a technique for a laser driving circuit and a laser driving method. [Background technology]

[0002] In an access network, bidirectional communication (single-core bidirectional communication) is realized with one optical fiber by multiplexing and demultiplexing a transmission optical signal and a reception optical signal with different wavelengths inside an optical transmission and reception module. For this reason, as shown in FIG. 9, a transmission module (TOSA) and a reception module (ROSA) are structured close to each other, and "crosstalk" occurs in which a high-frequency electric signal leaks from the TOSA side, which drives a laser with a large current, to the ROSA side, which receives a very sensitive and minute electric signal, via a common ground. FIG. 9 is a diagram showing an example of the configuration of a module of the conventional technology and an example of crosstalk. To suppress crosstalk, it is necessary to suppress fluctuations in the ground potential caused by high-frequency signals. It has been disclosed that crosstalk can be suppressed by connecting a laser (LD) and a laser driver circuit (LDD) that drives the LD with a differential interface, since a signal electric field neutral point (virtual ground) is generated between the differential signals (see Non-Patent Document 1).

[0003] Driving a laser using such a differential interface is relatively easy to achieve in optical transmitters that use typical "continuous signals." However, it is extremely difficult for "burst signal" transmitters used for upstream transmission in access networks for several reasons. In a typical single-phase drive optical transmitter as shown in Figure 10, the cathode terminal of the LD is connected to the inverse phase output terminal of the LDD. Figure 10 is a diagram showing an example of the circuit configuration of a single-phase drive optical transmitter of the prior art. To achieve differential drive, the positive phase output terminal of the LDD would be connected to the anode terminal, but since the anode and cathode do not have the same DC potential, the differential output of the LDD cannot be directly connected. This requires capacitive coupling using a sufficiently large capacitance, but since there is no path for the DC current flowing from the LD to the LDD, a circuit for supplying a DC bias is required on the LD side. To transmit a differential signal to both the anode and cathode terminals, a power supply (V dd ) or ground, a choke coil or a constant current source circuit is required, as shown in Figure 11. Figure 11 is a diagram showing an example of the circuit configuration of a differential drive type optical transmitter according to the prior art.

[0004] In addition, an optical transmitter that transmits a burst signal (burst transmitter) transmits an optical signal only while permission to transmit is granted, so it must have the ability to instantly turn on and off the laser. To stop the LD from emitting light, the bias current must be cut off, but the amount of current cannot be changed instantly due to the influence of the choke coil attached to the anode terminal side. In order to achieve the light emission and stop functions, a differential amplifier configuration is used with the LD as a load on one side, as shown in Figure 12, and a transmission stop signal (Tx disabled A technique has been proposed in which a current is switched to a path on the dummy resistor side using a resistor R1 to stop light emission (see Patent Document 1). Fig. 12 is a diagram showing an example of the circuit configuration of a burst-type optical transmitter according to the prior art.

[0005] In addition, the burst transmitter must also have the ability to instantly output and stop the LDD output signal so that the signal is not transmitted to the laser when the emission stops, causing erroneous emission. The commonly used circuit configuration is shown in Figure 13, where a logical product (AND) circuit is connected to the input of the LDD, and the Tx disabled Fig. 13 is a diagram showing a transient response due to coupling capacitance of a burst-type optical transmitter of the prior art.

[0006] Also, when the LD starts to emit light, a current flows through the LD, and the cathode potential is significantly lowered. At the same time, when the input of a signal starts, the transient response occurring at both terminals of the coupling capacitance has the same polarity (downward to the right) at the anode terminal and the cathode terminal, as shown by the signals g921 and g931 in FIG. 14. FIG. 14 is a diagram showing an example of the transient response of a laser drive circuit of the prior art. The diagram of the symbol g900 shows an example of the transient response of the capacitor C901, and the diagram of the symbol g910 shows an example of the transient response of the capacitor C902. Moreover, the diagram of the symbol g920 shows an example of the transient response of the anode of the LD, and the diagram of the symbol g930 shows an example of the transient response of the cathode of the LD. Moreover, the symbols g901, g911, g921, and g931 each show the transient response when the AOLC (automatic off-level compensation circuit) is in the off state, and the symbols g902 and g922 each show the transient response when the AOLC is in the on state. If the curves of this transient response can be made to match, the curves can be eliminated by differential driving of the LD, and instantaneous stability of the emission intensity can be obtained. To achieve this, the change in the voltage between both terminals of the coupling capacitance must be equal on the anode and cathode sides, and the time constants of the transient response must also match.

[0007] In general, to suppress relaxation oscillation and turn-on delay of the LD, the DC bias voltage of the LD is designed not to fall below the threshold voltage depending on the signal amplitude, so the change in the voltage between both terminals of the coupling capacitance does not match. disable By intentionally lowering the off level when emitting light is stopped in accordance with the signal, like the signal g902 in Fig. 14, the change in the voltage between both terminals of the coupling capacitance can be adjusted to be equal (see Non-Patent Document 2 and Patent Document 2). On the other hand, in order to match the time constants of the transient response, it is necessary to adjust the impedance balance of the choke coil and constant current source circuit connected above and below the LD. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2011-182108 A [Patent Document 2] International Publication No. 2009 / 104746 [Non-patent literature]

[0009] [Non-Patent Document 1] Tomoaki Yoshida, Shunji Kimura, et al., “First single-fibre bi-directional XFP transceiver for optical metro / access networks”, Paper We4.P.021, European Conference on Optical Communication, ECOC 2005. [Non-Patent Document 2] Hirotaka Nakamura, Shunji Kimura, et al., “AC-Coupled Burst-Mode Transmitter Using Baseline-Wander Common-Mode-Rejection Technique for 10-Gbit / s-Class PON Systems”, IEEE J. Lightw. Technol., vol.27, No.1-4, pp.336-342, Jan.-Feb. 2009 Summary of the Invention [Problem to be solved by the invention]

[0010] In conventional laser driving using a differential interface, a coupling capacitance or a choke coil is required to achieve differential driving. These elements are generally very large, resulting in a long transient response and degrading the instantaneous response performance of the burst transmitter. Furthermore, the technology described in Patent Document 1 has the problem that a certain amount of power is consumed because a direct current flows into the dummy side even when light emission is stopped. In addition, in the conventional technology shown in Figure 13, the average potential of the LDD output signal fluctuates between when output is on and when output is off, so the average potential of the anode and cathode terminals on the LD side experiences a transient response due to charging and discharging of the coupling capacitance. The red line in the figure shows the transient response of the average potential when both terminals of the coupling capacitance on the positive and negative phase sides are equally charged and discharged. It can be seen that a large voltage is applied between the two terminals of the LD when light emission begins, causing excessive light emission. The response time of the transient response becomes longer as the coupling capacitance value increases, so a long overhead is required, which poses the issue of causing a fatal decrease in transmission efficiency. Furthermore, in the techniques described in Non-Patent Document 2 and Patent Document 2, in order to increase the impedance of the constant current source circuit, a vertical stacked transistor configuration like the differential amplification configuration described above is required, which tends to increase power consumption.

[0011] In view of the above circumstances, an object of the present invention is to provide a laser driving circuit and a laser driving method that are capable of instantaneously switching current sources and preventing unnecessary power consumption when light emission is stopped. [Means for solving the problem]

[0012] One aspect of the present invention is a laser drive circuit comprising a laser, a first transistor connected as a current source to an anode side of the laser, and a second transistor connected as a current source to a cathode side of the laser, wherein the first transistor is a PNP transistor and the second transistor is an NPN transistor, and which inputs a differential drive signal to the anode and cathode of the laser.

[0013] One aspect of the present invention is a laser drive control method in which, when a laser stops emitting light, a control unit controls a first control signal to a first transistor connected to an anode side of the laser to a power supply voltage value, and controls a second control signal to a second transistor connected to a cathode of the laser to a reference voltage value, and, when the laser emits light, controls the first control signal to be lowered below the power supply voltage value, and controls the second control signal to be higher than the reference voltage value. Effect of the Invention

[0014] According to the present invention, it is possible to instantaneously switch the current source, and it is possible to prevent unnecessary power consumption when light emission is stopped. [Brief description of the drawings]

[0015] [Figure 1] FIG. 2 is a diagram illustrating an example of the configuration of a laser drive circuit according to the first embodiment. [Diagram 2] 5A to 5C are diagrams illustrating control signals and the like during light emission in the first embodiment. [Diagram 3] 5A to 5C are diagrams illustrating an example of changes in current flowing through a laser LD and voltages at various parts during light emission in the first embodiment. [Figure 4] FIG. 11 is a diagram illustrating an example of the configuration of a laser drive circuit according to a second embodiment. [Diagram 5] FIG. 13 is a diagram illustrating an example of the configuration of a laser drive circuit according to a third embodiment. [Figure 6] 11 is a diagram showing waveforms of an anode voltage Va and a cathode voltage Vc, and an example of a light emission threshold value. [Figure 7] 11 is a diagram showing an example of characteristics when a control signal Vgsp is changed during light emission. FIG. [Figure 8] 11 is a flowchart of a processing procedure performed by a control unit of the embodiment. [Figure 9] 1A and 1B are diagrams illustrating an example of a configuration of a module and an example of crosstalk according to a conventional technique. [Figure 10] FIG. 1 is a diagram showing an example of a circuit configuration of a single-phase drive type optical transmitter according to the prior art; [Figure 11] FIG. 1 is a diagram illustrating an example of a circuit configuration of a differentially driven optical transmitter according to a conventional technique. [Figure 12] FIG. 1 is a diagram illustrating an example of a circuit configuration of a burst-type optical transmitter according to a conventional technique. [Figure 13] FIG. 1 is a diagram showing a transient response due to coupling capacitance of a conventional burst-type optical transmitter. [Figure 14] FIG. 1 illustrates an example of a transient response of a prior art laser driver circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail with reference to the drawings.

[0017] [First embodiment] Fig. 1 is a diagram showing a configuration example of a laser driver circuit according to the present embodiment. As shown in Fig. 1, the laser driver circuit 1 according to the present embodiment includes a laser LD, a transistor Q1 (first transistor), and a transistor Q2 (second transistor). Capacitors C1 and C2 are capacitively coupled.

[0018] The laser LD has an anode connected to the drain of a transistor Q1 and one end of a capacitor C1, and a cathode connected to the drain of a transistor Q2 and one end of the capacitor C1. The transistor Q1 is a P-channel MOSFET (metal-oxide semiconductor field-effect transistor). The source of the transistor Q1 is connected to the power supply V dd (power supply voltage) is supplied, and the control signal V gsp (First control signal) is input. The transistor Q2 is an N-channel FET. The source of the transistor Q2 is grounded (reference voltage) and the gate is connected to a control signal V gsn (Second control signal) is input. The laser driving circuit further includes an LDD (laser driver circuit), an AOLC (automatic off-level compensation circuit), a control unit, and the like, as shown in FIG. 5 or FIG.

[0019] The laser driver circuit 1 is a circuit on the transmitter side, and can be applied to the transmission side of an optical transceiver module.

[0020] When the laser LD stops emitting light, the control signal V gsp "V dd " and the control signal V gsn is controlled to “GND”. In addition, when the laser LD starts emitting light, the control signal V gsp V dd At the same time, the control signal Vgsn The voltage value is raised from the GND level to control current to flow through the laser LD.

[0021] FIG. 2 is a diagram showing control signals and the like during light emission in this embodiment. As shown in FIG. 2, the voltage on the anode side of the laser LD (anode voltage) is V a The voltage on the cathode side of the laser LD (cathode voltage) is V c Let us assume that. As shown in FIG. 2, a differential drive signal is input to the anode and cathode of the laser LD via capacitors C1 and C2.

[0022] 3 is a diagram showing an example of changes in the current flowing through the laser LD and the voltage of each part during light emission in this embodiment. The horizontal axis is time (ns), the vertical axis on the left side is the current (mA) flowing through the laser LD, and the vertical axis on the right side is the voltage (V). a ,V c ,V gsp ,V gsn Each represents the voltage change from the start of light emission. LD is the change in current flowing through the laser LD from the start of light emission.

[0023] In the conventional circuit, when the laser LD is in the light emitting state, the anode voltage V a Since the cathode voltage V was fixed to the power supply voltage by the choke, c Only goes down. In contrast, in the laser driver circuit of this embodiment, the gate width of the P-channel transistor Q1 is set to W p , the gate width of the N-channel transistor Q2 is W n With these gate widths and control signal V gsp and V gsn By appropriately setting the amount of change in the anode voltage V a and the cathode voltage V c The changes in can be set with a high degree of freedom.

[0024] In addition, when the light emission stops, no current flows through the laser LD, so the V a =V cHowever, when current flows as shown in Figure 3, V a V on the high potential side c It can be seen that the potentials are shifted to the lower potential side at almost equal intervals. According to this embodiment, if it is designed to be equal to the shift amount of the average potential of the positive and negative phase outputs of the LDD (as shown by symbols g11 to g14 in FIG. 2), the voltage between the terminals of the coupling capacitance will not change. That is, in this embodiment, the average voltage of the positive and negative phase outputs of the differential drive signal to the anode and cathode of the laser LD is set to be equal to the change amount (shift amount) between when no current flows through the laser LD and when it flows through the laser LD.

[0025] As a result, according to this embodiment, no transient response occurs, and light emission can be started without excessive light emission caused by the transient response of the coupling capacitance.

[0026] In this manner, in the laser driver circuit 1 of this embodiment, a P-channel transistor and an N-channel transistor are used as a current source, instead of the choke coil on the power supply side used in the prior art.

[0027] According to this embodiment, this configuration eliminates the need for a choke coil, so the amount of current can be switched instantly, and it is possible to prevent unnecessary power consumption when the light emission is stopped. Also, according to this embodiment, since it can be configured with an integrated device, it can be realized monolithically.

[0028] [Second embodiment] Fig. 4 is a diagram showing a configuration example of the laser driver circuit of this embodiment. As shown in Fig. 4, the laser driver circuit 1A of this embodiment includes a laser LD, a transistor Q1, a transistor Q2, and a differential amplifier 101. Capacitors C1 and C2 are capacitively coupled.

[0029] A differential amplifier 101 has a first output terminal connected to the gate of a transistor Q1, a second output terminal connected to the gate of a transistor Q2, and an input terminal Tx disableA control signal is input. Note that the control signal of the first terminal and the control signal of the second terminal are inverted to each other, for example, symbols g12 and g14 in FIG.

[0030] The configuration of FIG. 4 is a control signal V gsp and V gsn This is applicable when the change amounts of the transistors Q1 and Q2 are equal, and the gate of the transistor Q1 is connected to the gate of the transistor Q2 by the differential amplifier 101. disable The control signal can be used for control in the same manner as in the first embodiment.

[0031] [Third embodiment] Fig. 5 is a diagram showing a configuration example of the laser driver circuit of this embodiment. As shown in Fig. 5, the laser driver circuit 1 of this embodiment includes, for example, a laser LD, a transistor Q1, a transistor Q2, an LDD 102, a current source 103, an automatic off-level compensation circuit 104, a differential conversion circuit 105, a logical product (AND) circuit 106, and a control unit 107. Capacitors C1 and C2 are capacitively coupled.

[0032] The LDD 102 has a positive phase output terminal connected to the other end of the capacitor C1, a negative phase output terminal connected to the other end of the capacitor C2, an input terminal connected to the positive logic output terminal and the negative logic output terminal of the AND circuit 106, and one end of the current source 103 connected to the off level control terminal.

[0033] The other end of the current source 103 is grounded, and the output of an automatic off level compensation circuit 104 is input to a control terminal.

[0034] The automatic off level compensation circuit 104 receives the Tx disable A control signal is input.

[0035] The differential conversion circuit 105 outputs a normal signal and an inverted signal of the input signal. disable A control signal is input, a non-inverted signal is output to a first terminal of the AND circuit 106, and an inverted signal is output to a second terminal of the AND circuit 106.

[0036] The logical product circuit 106 has a third input terminal D T A control signal is input from the control unit 107 and the fourth input terminal is connected to D c A control signal is input from the control unit 107. The AND circuit 106 outputs a positive-phase output and a negative-phase output to the LDD 102 in response to the AND of the four input signals. T , D c is the same as the input signal to the LDD 102 shown in FIG. 11, FIG. 12, etc.

[0037] The control unit 107 controls the laser LD to start and stop emitting light, and outputs control signals DT, Dc, and Tx. disable Generate and control.

[0038] Figure 6 shows the anode voltage V a and the cathode voltage V c The waveform g21 is an example of the anode voltage V a The waveform g22 is the cathode voltage V c This is the waveform of V TH is the emission threshold of the laser LD.

[0039] The control unit 107 is a -V c V TH After increasing the current value by the amount of V TH Only V gsp Adjust the value of V a The control unit 107 then uses the automatic off-level compensation circuit 104 to shift the off-level when light emission is stopped to V TH If you lower it by just this amount, the operation will be as shown in Figures 5 and 6, and no transient response will occur.

[0040] Figure 7 shows the control signal V gsp 6 is a diagram showing an example of characteristics when the control signal V during light emission is changed. In FIG. 6, the horizontal axis is time (ns), the vertical axis on the left side is the current (mA) flowing through the laser LD, and the vertical axis on the right side is the voltage (V). The characteristic diagram of reference symbol g31 is the first characteristic example, and the characteristic diagram of reference symbol g32 is the first characteristic example, and the characteristic diagram of reference symbol g33 is the first characteristic example, and the control signal V during light emission is changed. gspThis is an example of characteristics when slightly lowered.

[0041] Since the current of the P-channel transistor Q1 and the current of the N-channel transistor Q2 must be equal, the drain-source voltage V of the transistor Q1 ds is low, and the drain-source voltage V ds Therefore, the control signal V gsp If you lower it a little, as shown by arrows g33 and g24 in Figure 5, V a , V c Both can be shifted to the high potential side.

[0042] As mentioned above, in order to suppress relaxation oscillation and turn-on delay of the laser LD, V a -V c >V spp (peak to peak value of the signal voltage). In this case, the control unit 107 a The automatic off-level compensation circuit 104 controls the LDD 102 so that the average potential fluctuation on the positive phase output side is increased.

[0043] (Control Processing) Next, an example of a control procedure performed by the control unit 107 will be described. Fig. 9 is a flowchart of a processing procedure performed by the control unit of the embodiment. Note that this processing and processing procedure are applicable to each embodiment.

[0044] (Step S1) The control unit 107 determines whether to start or stop light emission.

[0045] (Step S2) When the light emission is stopped, the control unit 107 gsp =V dd , V gsn After the process, the control unit 107 returns to the process of step S1.

[0046] (Step S3) When light emission starts, the control unit 107gsp V dd Lower (e.g. GND), V gsn above GND (for example, V dd After the process, the control unit 107 returns the process to step S1.

[0047] In each of the above-mentioned embodiments, the transistors Q1 and Q2 are not limited to MOSFETs, and the same effects can be obtained even if other FETs or bipolar transistors are used.

[0048] The control unit 107 is configured using a processor such as a CPU (Central Processing Unit) and a memory. All or part of the functions of the control unit 107 may be realized using hardware such as an ASIC (Application Specific Integrated Circuit), a PLD (Programmable Logic Device), or an FPGA (Field Programmable Gate Array). The above program may be recorded on a computer-readable recording medium. Examples of computer-readable recording media include portable media such as flexible disks, magneto-optical disks, ROMs, CD-ROMs, and semiconductor storage devices (e.g., SSDs: Solid State Drives), and storage devices such as hard disks and semiconductor storage devices built into computer systems. The above program may be transmitted via an electric communication line.

[0049] In the laser driver circuit 1 (or 1A, 1B) configured in this manner, a choke coil is not required, the current source can be switched instantly, and no unnecessary power consumption occurs when the light emission is stopped. In addition, the laser driver circuit 1 (or 1A, 1B) configured in this embodiment can be configured with an integrated device, so that it can be realized monolithically.

[0050] Although an embodiment of the present invention has been described in detail above with reference to the drawings, the specific configuration is not limited to this embodiment, and designs that do not deviate from the gist of the present invention are also included. [Industrial Applicability]

[0051] The present invention is applicable to, for example, optical transceiver modules, optical transmitters, etc. in access networks. [Explanation of symbols]

[0052] LD... laser, Q1... transistor, Q2 transistor, C1... capacitor, C2 capacitor, 101... differential amplifier, 102... LDD, 103... current source, 104... automatic off level compensation circuit, 105... transceiver, 106... logical product circuit, 107... control unit

Claims

1. A laser, a first transistor connected as a current source to an anode side of the laser; a second transistor connected as a current source to the cathode side of the laser; Equipped with the first transistor is a PNP transistor, the second transistor is an NPN transistor, applying differential drive signals to the anode and the cathode of the laser; Laser driver circuit.

2. the first transistor has one end connected to the anode and the other end connected to a power supply voltage; the second transistor has one end connected to the cathode and the other end connected to a reference voltage; 2. The laser drive circuit of claim 1.

3. 3. The laser drive circuit according to claim 2, further comprising a differential amplifier that controls the first transistor and the second transistor, which are the current sources, by outputting a control signal to the first transistor and a control signal to the second transistor using a transmission stop signal that stops the laser from emitting light.

4. an automatic off-level compensation circuit for lowering the differential drive signal by a predetermined voltage when the laser is off-level and the laser stops emitting light; 3. The laser drive circuit according to claim 2.

5. increasing a current value so that the difference between the voltage value on the anode side and the voltage value on the cathode side becomes larger by the predetermined voltage; A first control signal to the first transistor when the laser is at an off level is reduced by the predetermined voltage.

5. The laser drive circuit according to claim 4.

6. A control unit is provided, The control unit is When the laser stops emitting light, a first control signal to the first transistor is controlled to a power supply voltage value, and a second control signal to the second transistor is controlled to a reference voltage value; During emission of the laser, the first control signal is controlled to be lower than the power supply voltage value, and the second control signal is controlled to be higher than a reference voltage value.

3. The laser drive circuit according to claim 2.

7. The average voltage of the positive and negative phase outputs of the differential drive signal to the anode and cathode of the laser is equal to the conversion between when no current flows through the laser and when it flows through the laser.

3. The laser drive circuit according to claim 2.

8. The control unit: When the laser stops emitting light, a first control signal to a first transistor connected to an anode side of the laser is controlled to a power supply voltage value, and a second control signal to a second transistor connected to a cathode of the laser is controlled to a reference voltage value; During emission of the laser, the first control signal is controlled to be lower than the power supply voltage value, and the second control signal is controlled to be higher than a reference voltage value. Laser drive control method.

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