JP2009032736A - Bias circuit for laser driver - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a more suitable profile of a bias circuit for a laser driver for controlling voltage level of a driver output signal using a transistor. <P>SOLUTION: Collectors of a transistor pair 14 are connected to a driver output transmission path 6 coupling an output terminal of the laser driver 4 and a laser 2. Emitters of a transistor pair 14 are connected to the power source. A voltage level of an output signal of the laser driver 4 is detected with reference to a potential at the neutral point of the resistors R<SB>1</SB>and R<SB>2</SB>. A base potential of the transistor pair 14 is adjusted in accordance with the detected voltage level. A pair of inductors 20, 22 are inserted between the driver output transmission path 6 and the emitters of the transistor pair 14. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a bias circuit used in a laser driver that drives a laser.

  Conventionally, for example, as disclosed in Patent Document 1 below, a bias circuit that connects a circuit output unit and a power source via a transistor and controls the transistor to adjust a bias applied to the circuit output unit It has been known. In the conventional bias circuit, the DC voltage level of the output signal is detected, and the detection result is fed back to the base voltage (or gate voltage) of the transistor. According to such a method, an appropriate bias can be applied so as to keep the DC voltage level of the output signal at a desired value.

JP 2003-318721 A JP 2000-354055 A JP 2001-111167 A JP 2003-163413 A

  When the laser diode is driven by the laser drive driver, the overall DC voltage level of the output of the laser drive driver due to the change in the mark ratio of the output pattern of the laser drive driver (the ratio at which the high level is output per unit time) Can occur. Such a change in voltage level is not preferable because it may cause waveform deterioration.

  In this regard, by using the conventional bias circuit (specifically, the description of paragraphs 0030, 0031, and 0032 of JP-A-2003-318721 and the bias circuit disclosed in FIGS. 5 and 6), Voltage level variations can be detected and an appropriate bias applied to maintain the voltage level of the laser driver output at a desired value.

  However, for example, the conventional bias circuit uses an open collector transistor circuit. In this case, there is a possibility that an impedance mismatch occurs in a high frequency region due to a parasitic capacitance of an open collector transistor. From this point, the present inventor has found that there are still matters to be solved in the conventional bias circuit.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a more preferable aspect of a bias circuit for a laser driving driver that controls the voltage level of a driver output signal.

In order to achieve the above object, a first invention is a laser drive driver bias circuit, comprising:
A transistor whose collector terminal or drain terminal is connected to the output terminal of the laser driver and whose emitter terminal or source terminal is connected to the power supply;
A detection circuit for detecting a voltage level of an output signal of the laser drive driver;
An adjustment circuit that adjusts the potential of the base terminal or gate terminal of the transistor according to the voltage level detected by the detection circuit;
An inductor element inserted between the output terminal of the laser driver and the collector terminal or the drain terminal of the transistor;
It is characterized by providing.

In order to achieve the above object, a second invention is a laser drive driver bias circuit,
A transistor whose collector terminal or drain terminal is connected to the output terminal of the laser drive driver;
A detection circuit for detecting a voltage level of an output signal of the laser drive driver;
An adjustment circuit that adjusts the potential of the base terminal or gate terminal of the transistor according to the voltage level detected by the detection circuit;
The power supply terminal of the laser drive driver and the emitter terminal or source terminal of the transistor are interposed and connected, and the power supply potential supplied from the power supply terminal to the laser drive driver is converted to the emitter terminal or A DCDC converter to be supplied to the source terminal;
It is characterized by providing.

A third invention is a bias circuit for a laser driving driver in order to achieve the above object,
A current source connected to the output terminal of the laser driver,
A photodiode that receives light emitted from a laser driven by the laser driver and changes the output according to the intensity of the emitted light; and
A control circuit for controlling a supply current amount of the current source according to the output of the photodiode;
It is characterized by providing.

  According to the first invention, since the bias circuit is configured by inserting the inductor element between the collector terminal or drain terminal of the transistor and the power supply terminal of the laser drive driver, impedance mismatch in the high frequency region is alleviated. Meanwhile, the voltage level of the output signal of the laser driver can be maintained at a desired value.

  According to the second aspect, the power supply voltage of the laser driver can be converted by the DCDC converter and supplied to the transistor. As a result, there is an advantage that it is not necessary to prepare a new power supply system for connection to the emitter terminal or the source terminal of the transistor when configuring the bias circuit of the laser driver.

  According to the third aspect of the invention, the information on the light output from the laser can be directly reflected in the control of the output level of the laser driver. Therefore, it is possible to realize a bias circuit capable of feedback control including variations in laser characteristics.

Embodiment 1 FIG.
[Configuration of Embodiment 1]
FIG. 1 is a diagram for explaining a laser drive driver bias circuit according to the first embodiment of the present invention. FIG. 1 is a circuit diagram showing a laser drive driver bias circuit 10, a laser 2 and a laser drive driver 4 of the present embodiment. Capacitors C ₁ and C ₂ are inserted in a driver output transmission path 6 that connects the laser 2 and the laser drive driver 4.

  A laser driving driver bias circuit 10 (hereinafter also simply referred to as a circuit 10) according to the present embodiment is a circuit that provides a bias point of a driver output using an open collector transistor as a current source. At that time, in order to set the bias point to a fixed voltage, a negative feedback circuit using an operational amplifier is configured. In the present embodiment, an open drain transistor may be used as a current source.

Specifically, the circuit 10 includes an operational amplifier 12, a transistor pair 14 including transistors Tr ₁ and Tr ₂ , resistors R ₁ and R ₂ , and inductors L _{1 to} L _{4 as} shown in the circuit diagram of FIG. Connected and configured. That is, the collector of the transistor pair 14 is connected to the driver output transmission line 6 that connects the output terminal of the laser driver 4 and the laser 2, and the emitter of the transistor pair 14 is connected to the power source.

In the present embodiment, the inductors L _{1 to} L ₄ are inserted between the collector of the transistor pair 14 and the driver output transmission line 6 (that is, inserted in series with an open collector current source). ). In the following description, as shown in FIG. 1, L ₁ and L ₂ are called an inductor pair 20, and L ₃ and L ₄ are called an inductor pair 22 corresponding to each insertion position.

In the circuit 10, as shown in FIG. 1, resistors R ₁ and R ₂ connected in series connect a midpoint between the inductors L ₁ and L ₃ and a midpoint between the inductors L ₂ and L _4. Yes. The midpoint potential of the resistors R ₁ and R ₂ is input to the operational amplifier 12. The midpoint potential is considered to be the DC level of the output of the laser driver 4. The operational amplifier 12 receives a fixed potential (Vx). In such a configuration, the base voltage of the transistor pair 14 is adjusted based on the result of comparison by the operational amplifier 12 between the midpoint potential and the fixed potential Vx.

In the circuit 10, the configuration excluding the inductors L _{1 to} L ₄ is the same as the known circuit configuration disclosed in Japanese Patent Laid-Open No. 2003-318721. Therefore, the detailed connection relationship and circuit operation will not be described. In the present embodiment, the midpoint potential of the resistors R ₁ and R ₂ is detected, and the detected potential is used as the voltage level of the output of the laser drive driver 4. However, the voltage level of the output of the laser driver 4 is not limited to this. For example, the voltage level of the output of the laser drive driver 4 can be detected using various known methods as disclosed in Japanese Patent Application Laid-Open No. 2003-318721.

[Operation and Effect of Embodiment 1]
Hereinafter, the operation and effect of the first embodiment will be described in detail. In the following description, first, the prior art will be described using FIGS. 2 to 6 and the like, and the superiority of this embodiment over this will be described.

FIG. 2 is an example of an electric circuit for driving a conventional laser diode. As shown in FIG. 2, in the conventional circuit, in order to separate the voltage level between the laser drive driver 104 and the laser diode 102, C ₁₀₁ and C ₁₀₂ are inserted and connection by capacitive coupling is often performed. .

In that case, in order to maintain the output level of the drive driver, the inductors L ₁₀₁ and L ₁₀₂ are used to connect to a fixed potential (= power supply). For high frequency signal components, since the inductance maintains a high impedance, the output amplitude is hardly affected by the inductance connection. However, if the signal pattern of the output of the laser driver is continuously high or low, or if the appearance probability of high and low is not the same over a long period of time, the driver output level itself varies. Resulting in. In such a case, the waveform may be deteriorated, which is not preferable.

  FIG. 3 is a diagram showing an example in which the output level fluctuates in a laser drive driver having a bias circuit in which an inductor is connected to a fixed potential as shown in FIG. FIG. 3 shows the output of the laser drive driver in the circuit as shown in FIG. 2 observed with an oscilloscope. When the ratio at which the High level is output in the output pattern per unit time (also referred to as “mark ratio”) varies, it can be seen that the overall level of the output varies in the vertical direction of the voltage. This is a phenomenon that occurs because the impedance of the inductor is not sufficiently large with respect to the period in which the data mark rate varies.

FIG. 4 is a diagram showing a simulation waveform in which a specific pattern is output by the conventional bias circuit as shown in FIG. 2 (eye pattern evaluation using a CID pattern (High, Low unevenness)). However, 100 nH inductors corresponding to L ₁₀₁ and ₁₀₂ in the circuit of FIG. 2 are used. The output data pattern is “00010100”, and the probability of High is intentionally reduced. As a result, a level shift of several hundred mV was observed in 15 cycles.

  FIG. 5 shows the simulation waveform of FIG. 4 in an eye pattern (eye pattern evaluation using a CID pattern (High, Low unevenness)). As shown in FIG. 5, the waveform has a large noise in the vertical direction.

  In order to suppress this level shift, it is necessary to use a larger-sized inductor. For example, FIG. 6 shows the result of a simulation similar to FIG. 5 with the inductor value set to 1 μH. From the simulation results of FIG. 6, it can be seen that in the current 15 cycles, the level shift will be at a level where there is no problem if the inductor is 10 times 1 μH.

  Actually, it is considered that fluctuations in High and Low occur over a longer period. In order to cope with this, it is necessary to use a larger-sized inductor.

  In such a case, for example, there is a technique using a chip inductor. Generally, the chip inductor has a larger mounting area as the inductor value becomes larger. In addition, the allowable amount of current is reduced and the parasitic resistance is increased. In a large amplitude application such as driving a laser driver, a chip inductor is selected at the expense of mounting area.

  However, it is becoming difficult to mount a large-sized inductor on a small-sized wiring board used in a compact optical transceiver.

  In order to solve this problem, it is possible to configure a bias circuit that applies current to the laser drive driver output terminal using an open collector or open drain transistor pair as a current source. In order to fix the output level of the laser driver using this transistor pair, the midpoint voltage of the output of the laser driver is monitored, and the base or gate potential of the transistor pair is fixed using a negative feedback circuit. It can be set as the structure to do.

  According to the above method, there is an advantage that an inductor having a large size is certainly unnecessary. However, when the configuration described above is simply used, there is a possibility that an impedance mismatch at a high frequency may occur due to the parasitic capacitance of the open collector transistor.

  Therefore, in the first embodiment, inductor pairs 20 and 22 are inserted between the output of the laser driver 4 and the transistor pair 14. The inductor pair 20 can mitigate impedance mismatch in the high frequency region caused by the parasitic capacitance of the transistor.

The inductor pair 22 can also suppress impedance mismatch due to insertion of the resistors R ₁ and R ₂ . Both inductor pairs 20 and 22 may be inserted, or one of them may be inserted. An inductor that can maintain a high impedance at a desired frequency can be used.

  Also, the high impedance characteristic at a very low frequency near DC can be covered by an open collector transistor. Therefore, a relatively small size of the inductor pair 20 or the inductor pair 22 is sufficient. Thereby, high impedance can be maintained from low frequency to high frequency without increasing the mounting area. In other words, the inductor pair 20, 22 has a high impedance in a specific high frequency region higher than a specific low frequency region in the vicinity of DC where high impedance is obtained by the open collector (or open drain) transistor pair 14. It is preferable to have frequency characteristics that can be obtained.

  As described above, according to the present embodiment, the bias circuit is configured by inserting the inductor pairs 20 and 24 between the collector terminal of the transistor pair 14 and the power supply terminal of the laser drive driver 4. The voltage level of the output of the laser driver 4 can be maintained at a desired value while relaxing the impedance mismatch in the region. In the present embodiment, the same effect can be obtained by using an open drain transistor pair instead of the open collector transistor pair 14.

Embodiment 2. FIG.
FIG. 7 is a diagram illustrating a laser driving bias circuit according to the second embodiment. As shown in FIG. 7, in this embodiment, a DCDC converter is used in combination to constitute a laser drive driver bias circuit (hereinafter also simply referred to as “bias circuit”). In addition, the same code | symbol is attached | subjected to the structure same as FIG. 1, and the description is abbreviate | omitted.

  The emitter (or source) potential of the transistor pair 14 of the bias circuit needs to be set higher than the power supply voltage of the laser drive driver 4 because the output of the laser drive driver 4 needs to be pulled up.

  Therefore, the power supply of the laser driver 4 is used as an input, and the voltage boosted using the DCDC converter 30 is used as an output to be the emitter potential of the transistor pair 14. Further, by supplying a fixed potential (Vx) to be compared by the operational amplifier 12 from a power supply common to the laser drive driver 4, the DC level of the output of the laser drive driver 4 can be set to the power supply voltage.

  By using the DCDC converter 30, it is not necessary to prepare a power supply with a voltage different from the power supply of the laser drive driver 4, and a bias power supply can be created with a single power supply of the laser drive driver 4 (the power supply can be shared). ).

Embodiment 3 FIG.
FIG. 8 is a circuit diagram showing a laser driver bias circuit according to the third embodiment. In addition, the same code | symbol is attached | subjected to the structure same as FIG. 1, and the description is abbreviate | omitted.

  The laser driver bias circuit according to the present embodiment is a circuit that uses feedback by optical output. In order to set the DC level of the laser drive driver 4 so as to optimize the laser output, the optical output of the laser is branched by the branching mechanism 40 and this light is supplied to the photodiode 42. The photodiode 42 is connected to a current circuit 50 for current monitoring. That is, the branched laser light is used for current control of the photodiode 42.

  For the branching of the laser beam, a method using a photocoupler, a method of directly irradiating the photodiode 42 with the back light of the laser 2, and the like can be considered. The current flowing in the photodiode 42 is reflected in the current values of the current circuits 52 and 54 connected to the output of the laser driver 4 by using current replication by the current mirror circuit 44. In addition, the impedance mismatch of the transmission path of a driver output can also be suppressed by designing the output impedance of the current circuits 52 and 54 high.

  According to the third embodiment described above, feedback control including variation in laser characteristics can be realized by directly controlling the driver output level from the laser output.

  The first to third embodiments described above can be used not only individually but also in combination. As a result, a more effective laser driver bias circuit can be configured.

It is a circuit diagram of the bias circuit for laser drive drivers of Embodiment 1 of this invention. It is a figure used for description of the effect of Embodiment 1, Comprising: It is a connection diagram between the conventional drive driver and a laser diode. It is a figure used for description of the effect of Embodiment 1, Comprising: It is a figure which shows the example of the fluctuation | variation of the output level of a driver when the conventional bias circuit is used. It is a figure used for description of the effect of Embodiment 1, Comprising: It is a simulation waveform when a specific pattern is output with the conventional bias circuit. FIG. 6 is a diagram used for explaining the effect of the first embodiment, and shows the simulation waveform of FIG. 4 by an eye pattern. It is a figure used for description of the effect of Embodiment 1, Comprising: It is the result of having performed the simulation similar to FIG. 5, changing an inductor value. It is a circuit diagram of the bias circuit for laser drive drivers of Embodiment 2 of this invention. It is a circuit diagram of the bias circuit for laser drive drivers of Embodiment 3 of this invention.

Explanation of symbols

2 Laser 4 Laser drive driver 6 Driver output transmission line 10 Laser drive driver bias circuit 12 Operational amplifier 14 Transistor pair 20, 22 Inductor 30 DCDC converter 40 Branch mechanism 42 Photo diode 44 Current mirror circuit 50, 52, 54 Current circuit

Claims (7)

A transistor whose collector terminal or drain terminal is connected to the output terminal of the laser driver and whose emitter terminal or source terminal is connected to the power supply;
A detection circuit for detecting a voltage level of an output signal of the laser drive driver;
An adjustment circuit that adjusts the potential of the base terminal or gate terminal of the transistor according to the voltage level detected by the detection circuit;
An inductor element inserted between the output terminal of the laser driver and the collector terminal or the drain terminal of the transistor;
A bias circuit for a laser driving driver, comprising: A DCDC converter connected to a power supply terminal of the laser drive driver and converting and outputting a power supply potential supplied from the power supply terminal to the laser drive driver;
2. The laser drive driver bias circuit according to claim 1, wherein the power source connected to the emitter terminal or the source terminal is the DCDC converter. The power supply terminal of the laser driver and the adjustment circuit are connected,
3. The adjustment circuit according to claim 1, wherein the adjustment circuit adjusts the potential of the base terminal or the gate terminal based on a comparison between the voltage level detected by the detection circuit and the potential of the power supply terminal. Laser drive driver bias circuit. A photodiode that receives light emitted from a laser driven by the laser driver and changes the output according to the intensity of the emitted light; and
A control circuit for controlling the potential of the base terminal or the gate terminal of the transistor according to the output of the photodiode;
The bias circuit for a laser driving driver according to any one of claims 1 to 3, further comprising: A transistor whose collector terminal or drain terminal is connected to the output terminal of the laser drive driver;
A detection circuit for detecting a voltage level of an output signal of the laser drive driver;
An adjustment circuit that adjusts the potential of the base terminal or gate terminal of the transistor according to the voltage level detected by the detection circuit;
The power supply terminal of the laser drive driver and the emitter terminal or source terminal of the transistor are interposed and connected, and the power supply potential supplied from the power supply terminal to the laser drive driver is converted to the emitter terminal or A DCDC converter to be supplied to the source terminal;
A bias circuit for a laser driving driver, comprising: The power supply terminal of the laser driver and the adjustment circuit are connected,
6. The laser drive according to claim 5, wherein the adjustment circuit adjusts the potential of the base terminal or the gate terminal based on a comparison between the voltage level detected by the detection circuit and the potential of the power supply terminal. Driver bias circuit. A current source connected to the output terminal of the laser driver,
A photodiode that receives light emitted from a laser driven by the laser driver and changes the output according to the intensity of the emitted light; and
A control circuit for controlling a supply current amount of the current source according to the output of the photodiode;
A bias circuit for a laser driving driver, comprising:

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