JPH0567825A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To realize a stable relation independent to the temperature in the relation between an input digital and generating light intensity by compensating the temperature characteristics of dark current in a monitor photodiode provided with a temperature compensation element in a reference voltage circuit. CONSTITUTION:This is composed of a semiconductor laser 1, drive circuit 2, monitor photodiode 3, amplifier circuit 4, reference voltage circuit 5, comparison circuit 6, and threshold information circuit 7, and generates a laser light of modulated light intensity proportional to the inputted digital data. The circuit 5 is composed of a constant current circuit 51, resistor 52 and posistor 53, so that it is added with the temperature characteristics to cancel the beam intensity changes in the laser 1 due to the changes of dark current in the diode 3, thereby the beam intensity does not vary depending on the temperature. Even when the amount of dark current varies based on the temperature, the relation between the inputted data and beam intensity does not change and as a result, stable relation is realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser driving circuit for generating a laser beam having an intensity corresponding to an input signal inputted from the outside, and more particularly to a semiconductor laser driving circuit having a temperature compensating element.

[0002]

2. Description of the Related Art In a laser printer or the like, a photosensitive member is scanned and exposed by modulated light from a semiconductor laser to write an image. As is well known, the semiconductor laser exceeds a current value called a threshold current. It has a characteristic of emitting a laser beam when driven by an electric current. This threshold current varies greatly depending on each element and temperature, so the light intensity output by the semiconductor laser varies greatly depending on each element even with the same drive current, and is extremely unstable with respect to temperature. (See FIG. 9).

When an image is formed by exposing with such an unstable laser beam, the output image is crushed or blurred, and the image quality is significantly impaired. Therefore, in an environment where the ambient temperature of the semiconductor laser changes, it is necessary to stabilize the light intensity of the semiconductor laser by an output control device of the semiconductor laser or the like.

Therefore, conventionally, in order to prevent the light intensity of the semiconductor laser from changing due to the change in the threshold current, in the semiconductor laser drive circuit, the threshold current means is set to a predetermined value between the original modulation operations. The monitor diode monitors the light intensity of the laser light generated by the semiconductor laser at time intervals, and controls the bias current flowing to the semiconductor laser so that the light intensity corresponding to a predetermined input signal (hereinafter referred to as a control signal) becomes constant. The control operation was performed. Alternatively, a Peltier element or the like is used to drive the semiconductor laser while keeping the temperature constant.

[0005]

However, depending on the type of monitor diode, as shown in FIG. 2, the dark current increases as the temperature rises. Then, there is a problem that the threshold current means reduces the drive current as if the light intensity of the laser light becomes large, and as a result, the light intensity decreases.

Further, the method using the Peltier element or the like has a problem that the apparatus becomes large in scale and there is a risk of dew condensation.

The present invention has been made in order to solve the above-mentioned problems, and an object thereof is to input an input even if the temperature changes and the magnitude of dark current changes without using a Peltier element or the like. An object of the present invention is to provide a semiconductor laser drive circuit in which the relationship between data and light intensity does not change.

[0008]

In order to achieve this object, a semiconductor laser drive circuit of the present invention receives a semiconductor laser which is current-driven to generate a laser beam and a part of the laser beam of the semiconductor laser. Means for converting the photocurrent into a photocurrent, an amplifier for amplifying the photocurrent and outputting a light intensity signal, a reference voltage means for outputting a reference voltage as a reference of the light intensity of the laser light, and the light intensity signal. A comparator that compares the magnitude with the reference voltage and a threshold value that outputs threshold information corresponding to the threshold current of the semiconductor laser based on the comparison result of the comparator and keeps the light intensity constant. A semiconductor laser drive circuit comprising a current means and a drive means for outputting a drive current based on a result of addition of an input signal inputted from the outside and the threshold value information, wherein the reference voltage means is provided with a monitor means. Temperature characteristics And a temperature compensating element having a temperature characteristic extinguish Chi.

[0009]

In the semiconductor laser drive circuit of the present invention having the above structure, the relationship between the input signal and the light intensity is kept constant by the control operation described below.

That is, if the control operation is started between the original modulation operations, the input signal is switched to a predetermined control signal, and the threshold value information generated by the threshold current means is added to the drive means. The semiconductor laser is current-driven based on the addition result, and the semiconductor laser emits laser light.

The monitor means is composed of, for example, a photodiode, receives a part of the laser light, and generates a photocurrent proportional to the light intensity. The photocurrent is amplified by an amplifier into a light intensity signal, which is compared with a reference voltage output from a reference voltage means by a comparator.

The threshold current means reduces the threshold information because the comparison result is larger than the predetermined light intensity if the light intensity signal is larger than the reference voltage. If the light intensity signal is smaller than the reference voltage, the light intensity signal is smaller than the predetermined light intensity, so the threshold value information is increased.

By repeating the control operation, the light intensity of the laser light with respect to the control signal is kept constant.

At this time, the threshold information corresponds to the magnitude of the threshold current of the semiconductor laser, and even if the threshold current changes due to temperature change, the threshold information also changes accordingly. The light intensity of the laser is kept constant.

The current generated by the monitor means is composed of a photocurrent caused by laser light and a dark current not caused by the laser light. A change in dark current due to a change in temperature results in a change in the corresponding light intensity signal, but the reference voltage to be compared is Because the temperature compensating element of the reference voltage means changes and cancels out, that is, when the temperature rises and the dark current increases, the light intensity signal apparently increases, but the reference voltage also increases, so the actual light intensity does not change. However, the light intensity does not change even when the temperature becomes low.

Therefore, the relationship between the input digital data and the generated light intensity is stable regardless of the temperature.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

The structure of the semiconductor laser drive circuit of this embodiment will be described with reference to FIGS.

As shown in FIG. 1, the semiconductor laser drive circuit of the present invention includes a semiconductor laser 1, a drive circuit 2, a monitor photodiode 3, an amplifier circuit 4, a reference voltage circuit 5, a comparison circuit 6, and a threshold value information circuit 7. And generates a laser beam having a light intensity modulated in proportion to the input digital data of, for example, 8 bits.

The drive circuit 2 is composed of an adder circuit 21, a D / A conversion circuit 22 and a drive circuit 23 as shown in FIG. 3, and adds D / A by adding input digital data and threshold data. A conversion is performed and a current proportional to the added value is output to the semiconductor laser 1.

The monitor photodiode 3 is usually housed in the same container as the semiconductor laser 1, to which a reverse bias voltage of a predetermined constant voltage is applied, and receives the back light of the semiconductor laser 1 to generate a photocurrent. is there. As described above, a dark current whose magnitude changes depending on temperature is also generated regardless of light reception. The two currents are collectively called a monitor current.

As shown in FIG. 4, the amplifier circuit 4 comprises an operational amplifier 41 and a resistor 42, and is a circuit for amplifying a monitor current and converting it into a monitor voltage.

As shown in FIG. 5, the reference voltage circuit 5 is composed of a constant current circuit 51, a resistor 52 and a posistor 53 which is a kind of temperature sensitive element. The reference voltage circuit 5 serves as a reference voltage for the light intensity of laser light. It occurs.

The comparison circuit 6 is composed of a comparator 61 as shown in FIG. 6, and compares the monitor voltage with the reference voltage to judge whether or not the light intensity of the laser light is larger than the reference light intensity. It outputs a judgment signal.

As shown in FIG. 7, the threshold value information circuit 7 comprises a control circuit 71 and an up / down counter (hereinafter, counter) 72. The counter 72 counts by a control operation based on a determination signal as described later. The value is increased / decreased and output as threshold data. The threshold data corresponds to the threshold current of the semiconductor laser 1, and increases / decreases with the increase / decrease of the threshold current to keep the relationship between the input digital data and the light intensity of the semiconductor laser constant. It is a thing.

Next, a control operation for keeping the relationship between the input digital data of the semiconductor laser driving circuit configured as described above and the light intensity of the semiconductor laser 1 constant will be described.

That is, when a control signal, not shown and described, is input to the control circuit 71 between the original modulation timings, the input digital data is switched to the control data and becomes, for example, FFh and the count value of the counter 72. Is reduced by a predetermined number N. (N is a positive integer) The back light of the semiconductor laser 1 enters the monitor photodiode 3 and generates a photocurrent. The monitor current, which is the sum of the photocurrent and the dark current, is amplified by the amplifier circuit 4 and compared in magnitude with the reference voltage by the comparator circuit 6. The comparison output is connected to the counter 72, and the clock signal CLK whose description is omitted until the comparison circuit 6 determines that the monitored light intensity is higher than the reference voltage 53 corresponding to the predetermined light emission intensity. The counter 72 is counted up with a limit of 2N.

In one control operation, the counter 72 can change the count value by ± N. When N is increased, the control gain is increased, and when N is decreased, the control gain is decreased.

The control signal is generated at a predetermined appropriate time interval, and even if the threshold current of the semiconductor laser 1 changes, the control signal follows the change and the count value of the counter 72 of the threshold circuit 5 (that is, the threshold value). (Value data) changes, the threshold value information circuit 7 absorbs the change in the threshold current of the semiconductor laser 1, and keeps the light intensity at Pff when the input digital data is FFh.

As shown in FIG. 2, the semiconductor laser 1 has a characteristic that the light intensity increases linearly after the drive current exceeds the threshold current, and the above control operation is performed.
The relationship between the input digital data and the light intensity is constant.

Next, the operation of the reference voltage circuit 5 which is a characteristic part of the present invention will be described.

Since the monitor current generated by the monitor photodiode 3 is the sum of the photocurrent and the dark current due to the laser light, it can be expressed by the following equation 1. Here, Im is a monitor current, Il is a photocurrent when the light intensity of the semiconductor laser 1 is P, and Id is a dark current at a temperature t.

Im = Il + Id (Equation 1) In the monitor photodiode 3, the dark current Id has a characteristic of increasing with temperature rise. Since the reverse bias voltage of the dark current Id at the temperature t is constant, the constant K1 is used. It can be expressed as the following Expression 2.

Id = K1 · exp (t / 10.9) (Equation 2) The monitor current Im is amplified and converted into a monitor voltage Vm by the amplifier circuit 4. Assuming that the conversion constant is K2,
Can be expressed as

Vm = K2 · Im (Equation 3) The posistor 53 has a temperature characteristic that the logarithm of its resistance value is proportional to the temperature, as shown in FIG. 8, for example, and at the temperature t. The resistance value Rth of the posistor 53 is as shown in the following Expression 4. (Rp and D are constants) Rth = Rp · exp (t / D) (Equation 4) The reference voltage Vref output from the reference voltage circuit 5 is Ic which is the magnitude of the constant current output from the constant current circuit 51. If the size of the resistor 52 is represented by R, it is as shown in the following Expression 5.

Vref = Ic · (R + Rth) (Equation 5) Since the control operation for keeping the light intensity of the semiconductor laser 1 constant is performed as described above, the monitor voltage Vm expressed by Equation 3 is It becomes equal to the reference voltage Vref represented by 5. Therefore, the following Equation 6 is obtained.

K2 · {Il + K1 · exp (t / 10.9)} = Ic · {R + Rp · exp (t / D)} (Equation 6) The photocurrent Il due to the light intensity of the semiconductor laser 1 influences the temperature t. In order to avoid this, it is necessary that the term of the temperature t does not exist in the equation 6. This is expressed by the following Equation 7, and the light intensity of the semiconductor laser 1 is P at this time.

K1 · K2 · exp (t / 10.9) = Ic · Rp · exp (t / D) (Equation 7) Equations 7 to 8 and 9 are obtained.

D = 10.9 (Equation 8) Rp = K1 · K2 / Ic (Equation 9) That is, if the posistor 53 is selected so as to satisfy the equations 8 and 9, the temperature characteristic of the monitor photodiode 3 becomes the reference voltage. By canceling each other out, the light intensity of the semiconductor laser 1 does not change regardless of the temperature.

As described above in detail, the reference voltage output from the reference voltage circuit 5 is the posistor 53 which is a temperature compensation element.
As a result, since the temperature characteristic is added so as to cancel the variation of the light intensity of the semiconductor laser 1 due to the change of the dark current of the monitor photodiode 3, the light intensity does not vary with the temperature.

Therefore, the relationship between the input digital data and the generated light intensity is stable regardless of the temperature.

[0042]

As is apparent from the above description, the semiconductor laser driving circuit of the present invention includes a temperature compensating element in the reference voltage circuit so that the reference voltage has a temperature characteristic, and the temperature characteristic of the dark current of the monitor photodiode is increased. Therefore, even if the temperature changes, the relationship between the input digital data and the light intensity of the laser light generated by the semiconductor laser is stable.

[Brief description of drawings]

FIG. 1 is a diagram showing an overall configuration of a semiconductor laser drive circuit of the present invention.

FIG. 2 is a diagram showing temperature characteristics of dark current of a monitor diode.

FIG. 3 is a diagram showing a configuration of a drive circuit.

FIG. 4 is a diagram showing an amplifier circuit.

FIG. 5 is a diagram showing a reference voltage circuit.

FIG. 6 is a diagram showing a comparison circuit.

FIG. 7 is a block diagram showing a configuration of a threshold information circuit.

FIG. 8 is a diagram showing a temperature characteristic of a posistor.

FIG. 9 is a diagram showing a relationship between a drive current of a semiconductor laser and light intensity.

[Explanation of symbols]

 1 semiconductor laser 2 drive circuit 3 monitor photodiode 4 amplifier circuit 5 reference voltage circuit 6 comparison circuit 7 control circuit 53 posistor

Claims (1)

[Claims] 1. A semiconductor laser which is current-driven to generate a laser beam, a monitor means for receiving a part of the laser beam of the semiconductor laser and converting it into a photocurrent, and an optical intensity signal for amplifying the photocurrent. A reference voltage means for outputting a reference voltage serving as a reference for the light intensity of the laser light, a comparator for comparing the magnitude of the light intensity signal and the reference voltage, and a comparison result of the comparator. , The threshold information corresponding to the threshold current of the semiconductor laser is output, and the threshold current means for keeping the light intensity constant, the input signal inputted from the outside and the threshold information are added. In a semiconductor laser drive circuit comprising drive means for outputting a drive current based on the result, the reference voltage means is provided with a temperature compensating element having a predetermined temperature characteristic, and the temperature characteristic of the monitor means is canceled. The semiconductor laser drive circuit, characterized in that.