JP2001007436A - Laser diode driver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent generation of a delay not only in a continuous signal but also a burst signal by an arrangement wherein an automatic quantity of light control circuit shunts a current corresponding to the output of a monitoring photodetector from a current supplied from a constant current power supply circuit. SOLUTION: The laser diode driver comprises a laser drive circuit 10, an LD module 11 incorporating an LD and a photodiode PD, and a constant current power supply circuit 13. An automatic quantity of light control circuit 15 shunts a current supplied from the constant current power supply circuit 13 depending on the light receiving current from the photodiode PD. The laser drive circuit 10 drives the LD based on the pulse of an input signal and supplies a drive current from the constant current power supply circuit 13 to the LD. When the light receiving current from the photodiode PD is varied from a specified level, the automatic quantity of light control circuit 15 shunts a drive current corresponding to the difference forcibly to other passage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode driving device, and more particularly to a laser diode driving device which keeps its output constant by following a burst signal and a continuous signal as an input signal (pulse) even when its output fluctuates due to temperature or the like. The present invention relates to a laser diode driving device to be controlled.

[0002]

2. Description of the Related Art In general, the characteristics (threshold current, slope efficiency, etc.) of a laser diode (hereinafter referred to as an LD) greatly change due to a temperature change.

[0003] Therefore, in order to obtain a stable optical output even if there is a temperature change, when driving the LD with a pulse, A
PC (Automatic Power Controller)
ol) The circuit was controlled so as to have a stable output waveform.

For example, in the conventional laser diode driving device 1 shown in FIG. 4, transistors Qa and Qb and a resistor Ra are connected in series on the cathode side of an LD, and a buffer for inputting an input signal (pulse) to the base of the transistor Qa. 2 and APC is connected to the base of transistor Qb.
The circuit 7 is connected.

The APC circuit 7 includes an amplifier 4 for amplifying an output signal of a photodiode PD for receiving an LD laser pulse light, and a filter circuit 5 connected to the amplifier 4. Photodiode PD
Is input as a feedback signal.

Further, the APC circuit 7 amplifies the difference between the signal inputted through the amplifier 4 and the filter circuit 5 from the feedback signal from the photodiode PD and the reference voltage 3, and controls the transistor Qb with this difference signal. A differential amplifier 6 is provided.

In the conventional laser diode driving device 1 configured as described above, the transistor Qa is turned on when the input signal via the buffer 2 is High, and the LD is turned on.
To cause light emission (at this time, the transistor Q
b is turned on). The transistor Qa is turned off when the input signal is at the low level, and stops the light emission of the LD.

The laser pulse light from the LD is received by a photodiode PD, and the output of the photodiode PD is amplified by an amplifier 4 and then averaged by a filter circuit 5 to provide an APC circuit 7 as a LD feedback signal. Is input to

The APC circuit 7 sends the difference between the feedback signal and a reference voltage (a reference voltage for obtaining a stable output) to the transistor Qb so that the average light quantity of the light pulse from the LD is always at a constant level. Had been controlled.

However, in the method of stabilizing the output of the LD by feeding back the average level of the monitor light of the LD, there is a delay in the time until the control of the fluctuation of the light emission level is started. When emitting light,
This time delay is not a problem, but when a burst signal (a signal whose data is not continuous) is converted to an optical signal and emitted light, the response of the APC circuit 7 does not follow at the start of light emission and normal data transmission is performed. Could not be performed.

For these reasons, in recent years, APC circuits 7
In some cases, a method corresponding to the burst signal shown in FIG.

In this method, a resistor Rb and a transistor Qc are connected in series to an LD, and a resistor Rc is connected in series to a monitoring photodiode PD.

Further, one of the resistors Rd is connected to the anode of the monitoring photodiode PD, and the other of the resistors Rd is connected to one of the resistors Re. The other end of the resistor Re is connected to the output of the differential amplifier 9.

The differential amplifier 9 has a negative input terminal connected to the connection point h of the resistors Rd and Re, a positive input terminal connected to the reference voltage VREF, and an output terminal connected to the base of the transistor Qc via the resistor Rf. Connected to

Further, a buffer 7 is connected to the base of the transistor Qc via a diode 8.

That is, in this method, a laser pulse light of an LD is received by a photodiode PD, a light receiving current of the photodiode PD is input as a feedback signal via a resistor Rd, and is compared with a reference voltage VREF. Amplify based on Re, and send this signal to the base of transistor Qc via resistor Rf.

At this time, the light output is instantaneously controlled for each pulse of the input signal input through the buffer 7 and the diode 8, so that a stable light output is always obtained.

[0018]

SUMMARY OF THE INVENTION As shown in FIG.
In the conventional APC circuit system, there is a delay in the time until the control of the fluctuation of the light emission level starts, and when a continuous signal is emitted, this delay is not a problem, but the burst signal (data is In the case of converting a non-continuous signal into an optical signal and emitting light, there is a problem that the response of the APC circuit 7 does not follow at the start of emission or at the stop of emission, preventing normal data transmission.

A method corresponding to a burst signal as shown in FIG. 5 can cope with a delay at the start of light emission, but requires a high-speed operational amplifier and requires a positive / negative operation for operating the operational amplifier. Two power supplies are required, and an IC circuit for generating a reference power supply (VREF) for the operational amplifier is required. That is, there has been a problem that the circuit size is increased and the cost is increased.

The present invention has been made to solve the above problems, and has as its object to provide a laser diode driving circuit which does not cause a delay not only for a continuous signal but also for a burst signal.

[0021]

According to the present invention, there is provided a laser driving apparatus comprising: a laser for emitting laser light; a laser driving circuit for driving the laser; a light receiving element for monitoring for receiving the laser light; A constant current power supply circuit for supplying a drive current to the laser, and the automatic light amount control circuit shunts a current corresponding to an output from the monitor light receiving element from a current supplied from the constant current power supply circuit. I do.

Thus, when the current received by the monitoring light-receiving element changes from a predetermined value, the drive current corresponding to the difference is forced to flow to another path by the automatic light amount control circuit.

[0023]

FIG. 1 is a schematic configuration diagram of a laser diode driving device according to the present embodiment. The laser diode driving device shown in FIG. 1 includes a laser driving circuit 10, an LD module 11 having a built-in LD and a photodiode PD.
And a constant current power supply circuit 13.

The LD of the LD module 11 emits laser light, and the photodiode PD (also referred to as a monitoring light receiving element) receives the laser light emitted by the LD and sends out a light receiving current corresponding to the amount of light received. .

The laser drive circuit 10 drives the LD at intervals according to the pulse of the input signal.

The constant current power supply circuit 13 includes the laser drive circuit 1
Each time the LD is driven by 0, a constant drive current is supplied.

The automatic light quantity control circuit 15 shunts the current supplied from the current power supply circuit 13 according to the light receiving current from the photodiode PD.

Thus, the laser drive circuit 10 drives the LD based on the pulse of the input signal to drive the constant current power supply circuit 1
3 is supplied to the LD to emit light.

The laser beam is received by the photodiode PD, and a light receiving current corresponding to the amount of received light is sent to the automatic light amount control circuit 15.

When the light receiving current from the photodiode PD changes from a predetermined value, the automatic light amount control circuit 15 forcibly shunts the driving current according to the difference to another path.

Therefore, it is possible to emit a stable laser beam even for a burst signal.

FIG. 2 is a schematic configuration diagram of a laser diode driving device according to another embodiment. The laser driving circuit 10 in the laser diode driving device shown in FIG. 2 includes at least a switching circuit including a switching element Q1 and a switching element Q2, inputs a data signal to the switching element Q1, and outputs the data signal to the switching element Q2. Is input.

The automatic light quantity control circuit 15 is connected in parallel to the laser drive circuit 10, and increases or decreases the shunt current according to the increase or decrease when the output of the photodiode PD increases or decreases from a predetermined output.

That is, while the data signal is at the high level, the switching element Q1 is turned on, and the switching element Q1 is turned on.
A drive current is supplied to the device to emit light.

The switching element Q1 is turned off while the data signal is at the low level to cut off the drive current and extinguish the light.

On the other hand, the switching element Q2 receives an inverted signal of the data signal, and while the inverted signal is at the low level, the switching element Q2 is turned off (the data signal is at the high level) and the drive current flows to the LD side. Let it.

The switching element Q2 has a high level.
During this period, the transistor is turned on (the data signal is low and Q1 is off), and the current flows to the switching element Q2 without flowing the current to the LD. As a result, no charge remains in the LD at the time of light extinction, so that waveform distortion at the time of light extinction can be suppressed, and switching between light emission and light extinction of the LD can be accelerated.
Such control is particularly effective when driving the LD with a high-speed pulse signal.

FIG. 3 is a detailed circuit diagram of the laser diode driving device of the present embodiment. 1 and 2, the laser diode driving device of the present embodiment includes a laser driving circuit 10, an automatic light amount control circuit 15, a constant current power supply circuit 13, an LD module 11 having a built-in LD and a photodiode PD. You. The laser drive circuit 10 and the automatic light amount control circuit 15 are connected in parallel with the constant current power supply circuit 13 at a common connection point e.

Further, the laser drive circuit 10 is provided with a switching circuit (Q1, Q2, R1, R3 to R5) for controlling the LD of the LD module 11 excluding the range of the dashed line composed of the shunt circuit 15a and the photodiode PD shown in FIG.
(Excluding R2)), the collector of the transistor Q1 is connected to the cathode of the LD, and the anode of the LD is connected to one end of the resistor R3. The other end of the resistor R3 is connected to Vcc. An anode of the LD and a diode D1 for preventing application of a reverse voltage to the LD of the transistor Q1 are connected in parallel to the LD.

The base of the above-mentioned transistor Q1 is connected to a resistor R5 for inputting data A as a pulse signal, the emitter is connected to the emitters of transistors Q4 and Q2, which will be described later, and to the common connection point e. It is connected.

That is, when the data A is at the high level, the transistor Q1 is turned on, and the drive current Ia1 of the LD flows through the resistor R3.

The other end of the collector of the transistor Q2 is V
A resistor R1 connected to cc is connected, a common connection point e of a constant current power supply circuit 13 described later is connected to the emitter,
The other end of the base is connected to a resistor R4 connected to a terminal for inputting data B which is a pulse signal.

The laser diode driving device includes a constant current power supply circuit 13 and an automatic light amount control circuit 15,
The output of D is controlled to be constant.

The constant current power supply circuit 13 includes a transistor Q5
A resistor R9 having one end connected to the emitter of the transistor Q5 and the other end grounded, a resistor R7 and a variable resistor VR.
1 and a series circuit 13a in which a resistor R8 is connected in series. One end of the resistor R7 of the series circuit 13a is connected to Vcc
And one end of the resistor R8 is connected to the ground, and the variable point h of the variable resistor VR1 is connected to the base of the transistor Q5.

The collector of the transistor Q5 is connected to a common connection point e of the constant current power supply circuit 13, and the common connection point e is connected via the laser diode drive circuit 10 or a shunt circuit 15a of an automatic light quantity control circuit 15 described later. It is connected to the Vcc power supply.

That is, the constant current power supply circuit 13 turns on the transistor Q5 with a predetermined voltage by the resistor R7, the variable resistor VR1, and the resistor R8 in accordance with the supply of the Vcc power via the laser diode drive circuit 10. This allows a constant current to flow through transistor Q5.

As a result, the laser diode driving circuit 10
Transistor Q2 is on (when data B is at a high level), transistor Q1 is on (when data A is at a high level), or transistor Q4 of shunt circuit 15a of automatic light quantity control circuit 15 described later is on. , The current flowing into the common connection point e is kept constant.

Further, the automatic light amount control circuit 15 includes a shunt circuit 15a and a control voltage generation circuit 15b for generating a control voltage for controlling the shunt circuit 15a.

The shunt circuit 15a connects the transistors Q4 and Q3 in Darlington connection, and connects the collectors of both transistors to a resistor R2 having one end connected to Vcc.
Is connected to the other end. The emitter of the transistor Q4 is connected to the common connection point e.

The control voltage generation circuit 15b includes a transistor Q6 having a base connected to the photodiode PD of the LD module 11, a resistor R10 having one end connected to the collector of the transistor Q6 and the other end connected to Vcc, and a base connected to the base of the transistor Q6. A resistor R11 having one end commonly connected to the photodiode PD is provided. The emitter of the transistor Q6 is connected to ground.

The other end of the resistor R11 is connected to a shunt circuit 15a.
The resistor R6 connected to the base of the transistor Q3 and the collector of the transistor Q6 are commonly connected (hereinafter referred to as a connection point k).

That is, since the photodiode PD is applied with the potential of Vcc in the reverse direction via the resistors R10 and R11, the monitor current Ipd flows when the laser beam from the LD is received, and the connection point k Of the current Ik flowing through the shunt circuit 15a by raising or lowering the voltage Vk of
a2 is controlled.

The operation of the laser diode driving circuit 10 configured as described above will be described below.

The laser drive circuit 10 of the present embodiment has
It is assumed that Vcc is supplied and transistor Q5 is turned on.

The control voltage generating circuit 15b constituting the automatic light quantity control circuit 15 sets the control voltage Vk at the voltage dividing point k to a predetermined level (a level at which the laser light is stabilized) with the supply of Vcc.

In such a state, for example, the data A goes high and is input to the base of the transistor Q1 through the resistor R5, and the data B obtained by inverting the data A becomes the transistor B through the resistor R4. When input to Q2, when data A is high and data B is low, transistor Q2 is turned off and transistor Q1 is turned on, causing current Ia1 to flow through the LD of LD module 11 to emit light. This current Ia1 is Vcc, resistance R
3, flows through the route of LD, transistor Q1, transistor Q5, resistor R9, and ground. The light emission amount of the LD is based on the collector current of the transistor Q5.

At this time, the constant current flowing through the transistor Q5 is determined by the base voltage Vb and the base-emitter voltage Vbe5 of the transistor Q5 determined by the resistor R7, the variable resistor VR1, and the resistor R8.

That is, current Ia = (Vb-Vbe5) / R9.

Therefore, by adjusting the variable resistor VR1, the voltage Vb changes, and the constant current Ia supplied from the constant current power supply circuit 13 can be adjusted.

At the same time, the resistance R1 of the control voltage generation circuit 15
To 0, Vcc is added, supplying the current Ib to the base of the transistor Q6 via the resistor R11 and supplying the current Ic to the transistor Q6 via the resistor R10. The monitor current Ipd according to the light output of the LD flows through the photodiode PD.

For this reason, when the data A is High,
The light emission pulse emitted from the LD of the LD module 11 is received by the photodiode PD and the monitor current Ip
When d is removed, the voltage at the connection point k increases, the base voltage of the transistor Q3 increases, the collector current of the transistor Q4 increases, and the current flowing through the LD of the LD module 11 decreases.

That is, the current Ipd by the photodiode PD is added to the base potential of the transistor Q3. In other words, when the amount of light emitted from the LD increases due to temperature or the like (the amount of received light increases), the control voltage Vk is immediately applied.
Rises, the base voltage of Q3 rises, and transistor Q3
4 is increased.

Therefore, when the light emission amount of the LD is increasing, the shunt amount of the current Ia2 to the route of the resistor R2 and the transistor Q4 is increased with respect to the current Ia flowing through the constant current power supply circuit 13. Therefore, the drive current Ia1 flowing through the LD decreases, and as a result, the amount of light emitted from the LD decreases.

The above-described control voltage Vk is represented by VK = R11 (Ipd + Ic / hfe) + Vbe6. That is, when the current Ipd increases, the control voltage Vk also increases. Conversely, when the current Ipd decreases, the control voltage Vk also decreases.

When the amount of light received by the photodiode PD is small (the amount of light emitted from the LD is small), the current Ipd decreases and the control voltage Vk decreases, so that the base voltage of the transistor Q3 decreases and the current flowing through the transistor Q4 decreases. Ia
2 also decreases.

Therefore, the shunt current Ia2 from the current Ia flowing through the constant current power supply circuit 13 decreases, so that the drive current Ia
a1 increases, and the amount of light emitted from the LD increases.

That is, by adjusting the variable resistor VR1 of the constant current power supply circuit 13 in advance, the drive current Ia1 of the LD is adjusted to set the light emission amount to a predetermined level. When the light emission amount from the LD increases to a predetermined level or more, the shunt current Ia2 flowing through the transistors Q4 and Q5 and the ground route is increased, and the drive current Ia1 flowing through the LD is reduced to reduce the light emission amount.

Conversely, when the amount of light emission falls below a predetermined level, the shunt current Ia2 flowing through the transistors Q4 and Q5 and the ground route is reduced, and the drive current Ia flowing through the LD is reduced.
By increasing the light emission amount by increasing a1, a constant laser output is obtained.

The data B is high and the data A is L
When it becomes OW, the transistor Q1 is turned off, the light emission from the LD is stopped, and the transistor Q2 is turned on, and V
cc, resistor R1, transistor Q2, transistor Q
5, a current flows through the route of the resistor R9 and the ground.

That is, even when the data is inverted, the current amount of the constant current power supply circuit 13 does not become “0” and the current flows stably, so that an abnormal current flows in the LD when switching between normal rotation and inversion of data. Does not flow.

Accordingly, in the present embodiment, the automatic light quantity control circuit 15 makes the potential Vk at the connection point k immediately change the drive current of the LD in accordance with the amount of the monitor current Ia1 of the photodiode PD, and immediately executes the shunt circuit. 15a is diverted as another path.

As a result, APC starts from the first bit of the data.
The control is applied, and it is possible to maintain a stable light emitting state not only for a continuous signal but also for a burst signal.

Further, the automatic light amount control circuit 1 of this embodiment
5 is operable only with Vcc (single power supply) and does not require an IC such as an operational amplifier as a component, so that only transistors, resistors, and variable resistors are required, so that it is possible to reduce the cost and reduce the circuit scale.

[0074]

As described above, according to the present invention, when the light receiving current changes from a predetermined value, the driving current is forcibly divided into another path in accordance with the difference, thereby immediately responding to the fluctuation. be able to.

That is, when the light receiving current corresponding to the amount of light received by the light receiving element exceeds a predetermined value, the shunt current can be immediately increased to reduce the amount of light emission, and the amount of light emission decreases. In some cases, the shunt current can be immediately reduced to increase the light emission amount.

As a result, the APC control is started from the first bit of the input data, and an effect that a stable light emitting state can be maintained not only for a continuous signal but also for a burst signal is obtained. .

In addition, since an expensive circuit such as an operational amplifier is not used for the circuit configuration and all the circuits are controlled by a single power supply (main power supply), the circuit scale is reduced and the cost can be reduced. The effect has been obtained.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a laser diode driving device according to the present embodiment.

FIG. 2 is a schematic configuration diagram of a laser diode driving device according to another embodiment.

FIG. 3 is a detailed circuit diagram of the laser driving device of the present embodiment.

FIG. 4 is a schematic configuration diagram of a laser diode drive circuit using a conventional APC method.

FIG. 5 is a schematic configuration diagram of a conventional laser diode drive circuit corresponding to a burst signal.

[Explanation of symbols]

 Reference Signs List 10 laser diode drive circuit LD laser diode PD photodiode 11 LD module 13 constant current power supply circuit 15 automatic light quantity control circuit 15a shunt circuit 15b control voltage generation circuit

Claims (3)

[Claims] A laser that emits laser light; a laser drive circuit that drives the laser; a light receiving element for monitoring that receives the laser light; an automatic light amount control circuit; A laser diode driving device, comprising: a current power supply circuit; wherein the automatic light amount control circuit shunts a current corresponding to an output from the monitoring light receiving element from a current supplied from the constant current power supply circuit. 2. The laser driving circuit includes a switching circuit that inputs an input signal and an inverted signal of the input signal to drive the laser, and wherein the automatic light amount control circuit is provided for the constant current power supply circuit. 2. The laser diode driving device according to claim 1, wherein the shunt current is increased or decreased when an output from the monitor light receiving element increases or decreases from a predetermined output by being connected in parallel with the laser driving circuit. 3. The automatic light quantity control circuit is connected in parallel to the constant current power supply circuit, the laser, and the laser drive circuit, and immediately increases and decreases the shunt current according to a control voltage based on a current from the monitor light receiving element. The laser diode driving device according to claim 1, further comprising: a shunt circuit; and a control voltage generation circuit that changes the control voltage based on a current according to an output from the monitoring light receiving element.