JPH01179378A - Control system of laser diode - Google Patents

Abstract

PURPOSE:To realize an ON/OFF control system by a simple circuit formation at a low cost, by connecting resistance to a laser diode in parallel and turning switching elements ON or OFF, thereby controlling a differential current through bypass operations after charging a drive current of the laser diode into a constant current which is required to obtain a prescribed quantity of light. CONSTITUTION:Switches SW 1 and 2 are in a state of ON immediately before a video signal VIDEO (binary signal) is given in a driver 17 and a feedback loop is formed. Input voltage Vi of a driver 16 is controlled by comparator 18 and the driver 16 allows a drive current Is (oscillation starting current) + Id to flow into a laser diode LD according to input voltage Vi of a differential current and then, its diode emits light at a constant intensity of light. Just before a video signal VIDEO is inputted to the driver 17, the switches SW 1 and 2 turned to OFF and allow the input side of a sampling hold circuit 19 to be opened and the output side to be connected to the input line of the driver 16.

Description

[Detailed Description of the Invention] This invention provides a laser beam printer, a digital copying machine,
The present invention relates to a control method for a laser diode used in a writing system of a facsimile machine, an optical disk device, etc.

In general, as a writing system in the various devices mentioned above, a laser diode is used to generate a laser beam, which is modulated according to the written data and scanned over a photoreceptor or recording medium. There are some that perform image formation or data recording.

In such a laser writing device, it is necessary to maintain the light intensity of the laser beam generated by the laser diode at a certain value, and to control the on/off state (emit or cut the laser beam) according to the writing data.

Laser diodes currently on the market generally have a photodiode optically coupled to them for monitoring their emission intensity, and the emission intensity of the laser diode is controlled by controlling the output of this monitoring photodiode. .

However, with a simple analog feedback method, the on/off control frequency is high, on the order of megahertz (MHz), and cannot be followed.

For this reason, the laser diode is forcibly controlled on and off using a digital element capable of high-speed switching while applying analog feedback. However, if the entire current flowing to the laser diode is controlled on and off, the amount of change in current will be reduced. The larger the current value, the worse the frequency characteristics of a switching element. In general, the larger the current value, the worse the frequency characteristics of a switching element. When the laser diode's current is turned on and off completely, the temperature of the laser diode will fluctuate due to fluctuations in power consumption. Therefore, since drift is likely to occur, a control method is used in which only the current necessary to take advantage of the characteristics of the laser diode is changed.

FIG. 4 shows the general input/output characteristics of a laser diode; there is an oscillation start current Is at which the laser diode begins to oscillate as the current is increased, and a predetermined optical output P. To obtain , it is necessary to flow a difference current Id.

FIG. 4 shows an example of a circuit using a conventional control method for controlling this differential current Id.

In this example, the laser diode LD and the monitoring photodiode PD are optically coupled. In addition, PDI in the figure is the monitor current by photodiode PD,
LDB is the bias current of the laser diode LD, that is, the oscillation starting current, and LDP further corresponds to the differential current (Id in FIG. 4) for causing the laser diode to emit light.

VH2 is a variable resistor for adjusting monitor current, and VH2 is a variable resistor for adjusting bias current.

The control circuit 1 is composed of a pulse current output circuit 2, a bias input current output circuit 3, an amplifier 4, a reference signal/bias setting circuit 5, and a bias current control circuit 6, and the bias current output circuit 3 is controlled while monitoring the monitor current PDI. oscillation starting current is applied to the laser diode LD, and the pulse current output circuit 3 outputs a difference current L according to the video signal (VIDEO).
Driving DP.

However, in this conventional control method, the required current value (difference current) is turned on and off after detecting the oscillation starting current of the laser diode, which makes the circuit complicated and expensive. There was a problem.

This invention has been made in view of the above points, and uses a simple circuit to control on/off the difference current between the energizing current necessary to obtain a predetermined amount of light from the laser diode and the oscillation start current. The purpose is to realize this configuration at a low cost.

L-bottom In order to achieve the above object, the present invention provides a laser diode control method that controls the difference current between the current flowing through the laser diode necessary to obtain a predetermined amount of laser diode light and the oscillation starting current of the laser diode. After connecting a resistor in parallel with the laser diode and setting the driving current of the laser diode to a constant current necessary to obtain a predetermined amount of light, the switching element is turned on and off to bypass control the difference current. This is how it was done.

Hereinafter, a detailed explanation will be given based on one embodiment of the present invention.

FIG. 1 is a block circuit diagram showing one embodiment of the present invention, which is a laser diode drive control circuit used in, for example, a writing system of a laser beam printer, which implements a laser diode control method according to the present invention.

In this circuit, a laser diode LD and a photodiode PD that monitors its emission intensity are optically coupled.

This circuit includes a driver 16 that causes a drive current to flow through the laser diode LD to cause it to emit light, a driver 17 that functions as a switching element that controls on/off the light emission according to the video signal VIDEO, and a photodiode PD. The monitor voltage Vm is set to a constant value of $
A comparator 18 that controls the input voltage Vi of the driver 16 by comparing it with the voltage Vr, a sampling and hold circuit that samples and holds the driver input voltage output from the comparator 18, and an electronic that switches its input and output. Switch S Wl, S by analog control switch etc.
It is composed of W2 and the like.

In addition, the laser diode LD includes a photodiode P.
A resistor R1 is connected in parallel via D, and driver 1
6. On the output side of 17, there is also a resistor R4 for current limiting,
R6 is inserted.

FIG. 2 shows the video signal VIDEO and the switch SWl,
3 is a time chart showing the relationship between ON and OFF of SW2.

Just before the video signal VIDEO (binary signal) is input to the driver 17, the switches SWl and SW2 are in the ON state as shown in FIG. 1, forming a feedback loop, and the photodiode The comparator 18 controls the input voltage Vi of the driver 16 so that the monitor voltage Vm by the PD matches the reference voltage Vr, and the driver 16 controls the drive current Is according to the input voltage Vi.
+Id is passed through the laser diode LD, causing it to emit light with a constant emission intensity. Here, Is is the oscillation start current, and Id is the difference current.

At this time, the sampling hold circuit 1 is sampling the driver input voltage Vi.

Immediately before the video signal VIDEO is input to the driver 17, the switches SWl and SW2 are turned off (OFF) by a switching signal from a control section (not shown).
The input side of the sampling hold circuit 1 is open, but the output side is connected to the input line of the driver 16.

At this time, the feedback loop becomes open, but the previous driver input voltage Vi is held by the sampling hold circuit 1st and is output as the driver 1st input voltage, so it appears that feedback is being applied. It is maintained.

Therefore, the laser diode LD maintains the peak value of its emission intensity constant, and when the video signal VIDEO is at a high level °H °, the output side of the driver 17 is opened, and the drive current Is+Id from the driver 16 flows and emits light. Driver 17 when low level °L°
Since the output of the laser diode L becomes the ground level, the difference current Id flows to GND via the resistor R6.
Only the oscillation starting current Is flows through D.

Laser diode LD does not emit light.

In this way, according to this embodiment, with a simple circuit configuration,
While keeping the light intensity of the laser beam by the laser diode constant, it is turned on and off in response to a high-frequency video signal.
Can be turned off.

FIG. 3 shows an example of a circuit that further embodies the embodiment shown in FIG. The circuit 19 includes a hold capacitor C2 and an operational amplifier OP2 for impedance conversion.

Resistor R2 and reference voltage setting variable resistor VRI.

A circuit that divides a power supply voltage of 10 V to generate a reference voltage Vr is configured.

In this circuit, first, switches SWl, SW2
We will explain the operation when it is turned on.

The driver 17 is an open collector IC driven by the video signal VIDEO.
When O is not input, the input remains at H° and the output is in an open state. Then, analog feedback is applied between the laser diode LD and the photodiode PD.

First, when the power +V of the laser diode LD and the circuit power are turned on, the output of the photodiode PD is zero at first, and the reference voltage Vr is applied to the negative input terminal of the operational amplifier ○P1 between the constant voltage power supply +V and the ground (GND). Since a positive potential divided by the resistor R2 and the variable resistor VRI is applied, the output of the operational amplifier ○P1 becomes a low potential, and a base current flows to the transistor Q1 through the switch SW2 and the base resistor R3.

Corresponding to this base current, the collector current of the transistor Q1 is supplied as a drive current Is+Id to the laser diode LD through a current limiting limiter resistor R4.

As a result, the laser diode LD emits light, and in response to the light, a monitor current Im flows through the photodiode PD, and a monitor voltage Vm generated between the terminals of the resistor R connected in series with this is applied to the Applied to the input terminal.

When the positive and negative input terminals of the operational amplifier OPI become approximately the same potential, this circuit becomes balanced and the laser diode LD
The output of the photodiode PD and the output of the photodiode PD are kept constant.

In this state, the switch SWI is ON, and the output of the operational amplifier OPI is connected to the protection resistor R5 and the switch SWI.
The capacitor C2 is charged through the capacitor C2, and its potential is sampled. The operational amplifier OP2 converts the sampling voltage into impedance and outputs it. Therefore, in this state, the ON terminal and OFF terminal of the switch SW2 are at the same potential.

Next, as shown in the time chart of FIG. 2, just before the video signal VIDEO is generated, the switches SWl,
When both SW2 are switched to the OFF side, a constant base current is supplied to the transistor Q1 from the operational amplifier OP2 regardless of the amount of light emitted by the laser diode LD and the output of the photodiode PD.

Therefore, regardless of the poor responsiveness of the photodiode PD and operational amplifier ○P1, the laser diode LD responds to the °H"'L" of the video signal VIDEO by the driver 17 while keeping the peak value of the emission intensity constant. On/off control is performed.

As described above, according to the present invention, the emission intensity of a laser diode and the on/off control at high frequency can be realized at low cost with a relatively simple circuit.

[Brief explanation of the drawing]

Fig. 1 is a block circuit diagram showing an embodiment of the present invention, and Fig. 2 shows the video signal VIDEO and switch S in Fig. 1.
Figure 3 is a circuit diagram showing a more specific circuit example of the embodiment shown in Figure 1. Figure 4 is a typical input circuit for a laser diode. Figure 5 is a block circuit diagram showing an example of a circuit using a conventional laser diode control method. 16... Driver for light emission intensity control 17... Driver for on/off control 18... Comparator 19... Sampling hold circuit LD... Laser diode PD... Photodiode R1... Parallel resistance i Engineering〉 Figure 4

Claims (1)

[Claims] 1. A laser diode control method that controls the difference current between the laser diode current necessary to obtain a predetermined amount of laser diode light and the oscillation starting current of the laser diode, which includes a resistor connected in parallel with the laser diode. . A laser diode control method, characterized in that after the driving current of the laser diode is set to a constant current necessary to obtain a predetermined amount of light, the differential current is bypass-controlled by turning on and off a switching element.