JPH0541553A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To make it possible to detect accurately excessive optical output by adjusting said output with the slope efficiency of a semiconductor laser with a control means and generating constant current related with a threshold value current of the semiconductor laser at a specified temperature. CONSTITUTION:There is provided with a piezoelectric current means 7 which is installed in parallel to a detection means 8 which detects an excessive state of laser light, a control means which is adjusted with differential quantum efficiency (slope efficiency) and a drive means and generates piezoelectric current whose magnitude is associated with a threshold value current of the semiconductor 1 at a specified temperature. Especially, the piezoelectric current means 7 is adjusted to measure preliminarily the magnitude of a threshold value current of each semiconductor laser 1 at the coldest temperature out of service environment temperatures and generate equivalent piezoelectric currents. This construction makes it possible to prevent excessive optical strength without damaging the semiconductor laser 1 even when the slop efficiency or the threshold value current of the individual semiconductor lasers 1 are subjected to variability.

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 specifically to a threshold current and a differential quantum of each semiconductor laser. The present invention relates to a semiconductor laser drive circuit capable of detecting an excessive output corresponding to a variation in efficiency (hereinafter referred to as slope efficiency).

[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. 2).

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 in which the ambient temperature of the semiconductor laser changes, it is necessary to stabilize the optical output intensity of the semiconductor laser with an output control device of the semiconductor laser or the like.

Therefore, conventionally, a current corresponding to a threshold current is added as an offset current to a modulation signal to drive the semiconductor laser with current, and a control signal is input instead of the modulation signal during a non-image signal modulation period. The optical output is monitored by the optical monitor circuit and fed back to the drive circuit, thereby providing a control loop for controlling the offset current corresponding to the threshold current and stabilizing the optical output of the semiconductor laser. The output light intensity of the semiconductor laser was stabilized.

In addition to the above characteristics, semiconductor lasers also have the characteristics that they are extremely weak against excessive output and are prone to deterioration. Therefore, the magnitude of the offset current is monitored, and when the offset current is equal to or larger than a predetermined magnitude, it is detected as an over-output, and the operation of the drive circuit is stopped.

[0006]

However, in the conventional method of detecting an excessive output, when the reference current for determining an excessive output is set small, the threshold current becomes large,
Alternatively, a semiconductor laser with a low slope efficiency may be erroneously determined to be an excessive output even though it is not an excessive output, or if the reference current for determining an excessive output is set to a large value, the threshold current small,
Alternatively, a semiconductor laser having a large slope efficiency has a drawback in that even if the output is excessive, it is erroneously determined that the output is not excessive.

The present invention has been made to solve the above-mentioned problems, and an object thereof is to accurately detect an excessive optical output even if the threshold currents and slope efficiencies of individual semiconductor lasers are different. Another object of the present invention is to provide a semiconductor laser drive circuit capable of detecting the above.

[0008]

In order to achieve this object, a semiconductor laser drive circuit of the present invention comprises a conversion means for converting an input signal input from the outside into a physical quantity, and a drive based on the converted physical quantity. Drive means for outputting a current, a semiconductor laser that is current driven by the drive current to generate laser light, and threshold current means for outputting threshold information related to the threshold current of the semiconductor laser, A semiconductor laser drive circuit comprising a detecting means for detecting an excessive state of the laser light by threshold information, the adjusting means being provided in the converting means and adjusted in accordance with the slope efficiency of the semiconductor laser, And a constant current means provided in parallel with the drive means for generating a constant current having a magnitude related to the threshold current of the semiconductor laser at a predetermined temperature.

[0009]

In the semiconductor laser drive circuit of the present invention having the above-mentioned structure, the constant current means measures the threshold current of each semiconductor laser at the lowest temperature in the operating environment temperature in advance and obtains an equal constant current. Is adjusted to occur.

The threshold data generated by the threshold current means is added to the digital data input from the outside.

The conversion means converts the added digital data into an analog current. The converted analog current is converted into an analog voltage by the adjusting means. Here, the adjusting means is composed of a variable resistance element or the like, and performs conversion and adjustment by adjusting the efficiency of conversion into an analog voltage by the size of the variable resistance.

The size of the variable resistor is adjusted in advance according to the magnitude of the slope efficiency of the mounted semiconductor laser, and the output analog voltage is adjusted according to the slope efficiency of the individual mounted semiconductor lasers. It has been corrected.

The drive means current-drives the semiconductor laser based on the analog voltage. Here, since the constant current generated by the constant current means also flows in the semiconductor laser, the semiconductor laser emits a laser beam based on the current obtained by summing these.

A current corresponding to the difference between the threshold current at the ambient temperature of the semiconductor laser and the threshold current at the lowest temperature is generated by the threshold information output from the threshold current means. The change in the light intensity due to the change in the threshold current due to the change in temperature is canceled by the change in the threshold information output by the threshold current means.

As described above, since the analog voltage output by the adjusting means has a characteristic corrected by the variable resistance so as to cancel the variation in the light intensity due to the variation in the slope efficiency of the semiconductor laser, the analog voltage and the input digital data are The relationship of the obtained light intensities is not affected by the variation of the slope efficiency of each semiconductor laser and has a predetermined constant relationship.

Therefore, since the relationship between the threshold information and the light intensity is also the same, when the excessive output is detected by the threshold information, the relationship is constant regardless of the variation of the slope efficiency, and it is accurate. Excessive output can be detected, and the constant current means absorbs variations in the threshold current of individual semiconductor lasers, so the threshold information can be accurately output regardless of the threshold current of each semiconductor laser. Can be detected.

[0017]

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

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

The semiconductor laser drive circuit of the present invention comprises an overcurrent detection circuit 8, a constant current circuit 7, an addition circuit 6, a threshold current circuit 5, a D / A conversion circuit 4, an I / V conversion circuit 3 and a drive circuit 2. It is composed of a semiconductor laser 1.

The constant current circuit 7 is a circuit having the structure shown in FIG. 6, and is composed of a variable resistor 71, a transistor 72 and a resistor 73. The size is determined, and the variable resistor 71 is adjusted as described below.

That is, the threshold current of the semiconductor laser 1 at the lowest temperature (for example, 0 ° C.) in the operating environment temperature is measured in advance and the constant current is adjusted so as to be equal to the current. Once adjusted, the adjusted current value need not be readjusted unless the semiconductor laser 1 is replaced.

The threshold current circuit 5 is constructed as shown in FIG. 4 in detail, and is composed of a monitor diode 51, an amplifier circuit 52, a reference voltage 53, a comparator 54 and an 8-bit counter 55. To FFh (hexadecimal,
The same holds true for the following), which outputs the threshold data th corresponding to the threshold current.

The overcurrent detection circuit 8 is configured as shown in FIG. 7 in detail, and is composed of a digital comparator 81 and a dip switch 82. The count value of the counter 55 of the threshold current circuit 5 is dip switched. 82
The counter 55 is compared with the reference count value set in
If the count value of exceeds the reference count value, the overcurrent is detected as an overcurrent.

The adder circuit 6 adds and outputs the above-mentioned threshold value data th and the input digital data d input from the outside and having any value from 0 to FFh. Since the threshold data th and the input digital data d are 8-bit data that can take values from 0 to FFh, the addition result is 9-bit digital data that takes any value from 0 to 1FEh.

The D / A conversion circuit 4 has the configuration shown in FIG. 5, and outputs a current proportional to any digital data from 0 to 1FFh that is input.

The I / V conversion circuit 3 converts the current output from the D / A conversion circuit 4 into a voltage value by means of a resistor, and a variable resistor 33 for adjusting the conversion gain (the resistance value is Vr.
Represented by)). The variable resistor 33 is adjusted by the slope efficiency of the semiconductor laser 1 as described later in detail. Once adjusted, the adjusted resistance value need not be readjusted unless the semiconductor laser is replaced.

The drive circuit 2 includes transistors 21 and 23.
And resistors 22 and 24, and outputs a current proportional to the input voltage.

A semiconductor laser 1 is connected to the drive circuit 2, and the semiconductor laser 1 is driven by a current obtained by adding a current output by the drive circuit 2 and a constant current output by the constant current circuit 7.

Next, the operation of the semiconductor laser drive circuit configured as described above will be described.

The input digital data d is added to the threshold data th generated by the threshold current circuit 5 and then converted into an analog voltage by the D / A conversion circuit 4.

The threshold current circuit 5 performs the control operation described in detail below. That is, when a control signal not shown and described is generated, the input digital data d is switched to the control data and becomes, for example, FFh, and the counter 55
The count value of is reduced by a predetermined number N.

A part of the laser light emitted from the semiconductor laser 1 enters the monitor diode 51 to generate a photocurrent, which is amplified by the amplifier circuit 52 and compared in magnitude by the reference voltage 53 and the comparator 54. .. The comparison output is the counter 5
The counter 55 is limited to 2N by the clock signal CLK, which is connected to No. 5 and is not described until the comparator 54 determines that the monitored light intensity is higher than the reference voltage 53 corresponding to the predetermined light emission intensity. Is counted up as.

That is, in one control operation, the counter 5
5 can change the count value by ± N, and the count value of the counter 55 is the threshold value data th described above.
It becomes.

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 count value of the counter 55 of the threshold current circuit 5 (that is, The threshold value data th) changes, the threshold current circuit 5 absorbs the change in the threshold current of the semiconductor laser 1, and the light intensity is kept at Pff when the input digital data is FFh.

The threshold data th and the digital data d input from the outside are added to be d + th, and D /
The A conversion circuit 4 converts the analog current i. With the conversion efficiency as K1, the analog current i can be expressed by the following equation 1 (digital data is used after being normalized).

I = K1 (d + th) (Equation 1) The current i is converted into an analog voltage v by the I / V conversion circuit 3. Since the I / V conversion circuit 3 is a circuit including the variable resistor 33 as shown in FIG. 1, it can be expressed by the following expression 2.

V = Vr × i (Formula 2) The drive circuit 2 drives the semiconductor laser 1 with the current id proportional to the above-mentioned analog voltage v. If the resistance value of the resistor 24 is Re, the drive current id can be expressed by the following expression 3.

Id = v / Re (Equation 3) If the current value by the constant current circuit 7 is represented by ib, the light intensity p when the semiconductor laser 1 is driven with a current of id is It can be expressed as shown in Expression 4. Note that it
h is the threshold current of the semiconductor laser 1, and η is the slope efficiency of the semiconductor laser 1.

P = η (id + ib-ith) (Equation 4) From the above Equations 1 to 4, the light intensity p of the semiconductor laser 1 is calculated.
Can be expressed as in Equation 5 below.

[0040]

[Equation 1]

Here, due to the function of the threshold current circuit 5, when the input digital data d is 0, the light intensity is 0 regardless of the slope efficiency η of the semiconductor laser 1, and therefore the following equation 6 is obtained. can get.

[0042]

[Equation 2]

If the light intensity obtained when the input digital data d is D, for example, is PD, then equations 6 and 5 above are used.
Then, the following equation 7 is obtained.

[0044]

[Equation 3]

Here, Vr = K2 × Re / (K1 × η)
If the variable resistor 33 of the I / V conversion circuit 3 is adjusted so that the following equation 8 is obtained.

PD = K2 × D (Equation 8) That is, the I / V conversion circuit 3 calculates Vr = K2 × Re /
If the light intensity is adjusted so as to satisfy (K1 × η), the obtained light intensity does not depend on the slope efficiency η, and when the input digital data d is D, the light intensity is as shown in Equation 8,
It is not affected by variations in the slope efficiency η of the semiconductor laser 1.

As described above in detail, the I / V conversion circuit 3
The analog voltage output by is added with a characteristic to cancel the fluctuation of the light intensity due to the slope efficiency η of the semiconductor laser 1 by the variable resistor 33 as the adjusting means, so that the light intensity varies depending on the slope efficiency η ( (Shown in FIG. 3).

Therefore, the relationship between the input digital data and the generated light intensity is constant regardless of the semiconductor laser 1.

Next, the operation of the overcurrent detection circuit 8 of the present invention will be described with reference to FIG.

In the unlikely event that the threshold current circuit 5 fails to detect the light intensity (such as disconnection of the monitor diode 51), the threshold current circuit 5 increases the drive current. Then it works to increase the light intensity. That is, the count value of the counter 55 sequentially increases. If the count value increases, the drive current of the semiconductor laser 1 increases correspondingly, and in the worst case, the semiconductor laser 1 will be destroyed. It works to prevent the destruction of.

That is, when the count value increases, the digital comparator 81 eventually detects that the reference count value set in the DIP switch 82 is exceeded. The digital comparator 81 generates an overcurrent detection signal (OVER signal), and the control circuit (not shown) which receives this signal stops the operation of the semiconductor laser drive circuit.

If the reference count value is set to a value of ER, the light intensity Per of the semiconductor laser 1 when the overcurrent detection circuit 8 generates the OVER signal is expressed by the following expression 9 from expression 5 to expression 8. It

[0053]

[Equation 4]

That is, K2 is higher than the light intensity during normal operation.
The operation is stopped when it increases by x (ER-th). th is threshold data output from the counter 55,
It changes depending on the change of the threshold current due to temperature change or the like regardless of the individual semiconductor laser 1, and the value becomes 0 at the lowest temperature in the operating environment temperature, and the light intensity at that time is K2.
Since it becomes × (D + ER), even if the input digital data has the maximum size FFh, if the reference count value ER is selected so as not to exceed the light intensity Pmax determined by the absolute maximum constant of the semiconductor laser 1, the semiconductor laser 1 is selected. There is no fear of destroying it. The condition is expressed by the following equation 10.

ER <Pmax / K2-FFh (Equation 10) As described in detail above, according to the semiconductor laser drive circuit of the present invention, the semiconductor laser drive circuit of the present invention is affected by variations in threshold current and slope efficiency of individual semiconductor lasers. Excessive output can be detected accurately.

The present invention is not limited to the embodiments described in detail above, and various modifications and improvements are possible.

For example, although the input digital data is 8 bits in this embodiment, it may be 10 bits.

Although the threshold information is added to the input digital data and then D / A converted, each of them is D / A converted.
It may be configured such that the converted information is converted into analog information and then added.

[0059]

As is apparent from the above description, in the semiconductor laser driving circuit of the present invention, the adjusting means is adjusted so as to correct and absorb the variation in slope efficiency of each semiconductor laser, and the constant current circuit is Since it is configured to adjust and absorb variations in threshold current, it detects excessive light intensity without damaging the semiconductor laser even if the slope efficiency and threshold current of individual semiconductor lasers vary. it can.

[Brief description of drawings]

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

FIG. 2 is a diagram showing a relationship between a driving current of a semiconductor laser and a light intensity.

FIG. 3 is a diagram showing a relationship between input digital data and light intensity when a variation in slope efficiency is not corrected.

FIG. 4 is a circuit diagram showing a configuration of a threshold current circuit.

FIG. 5 is a diagram showing a configuration of a D / A conversion circuit.

FIG. 6 is a diagram showing a configuration of a constant current circuit.

FIG. 7 is a diagram showing a configuration of an overcurrent detection circuit.

[Explanation of symbols]

 1 semiconductor laser 2 drive circuit 3 I / V conversion circuit 4 D / A conversion circuit 5 threshold current circuit 6 adder circuit 7 constant current circuit 8 overcurrent detection circuit

Claims (2)

[Claims] 1. A conversion means for converting an input signal input from the outside into a physical quantity, a drive means for outputting a drive current based on the converted physical quantity, and a current drive by the drive current to generate a laser beam. A semiconductor laser driving circuit comprising: a semiconductor laser for driving the semiconductor laser; and a threshold current means for outputting threshold information relating to a threshold current of the semiconductor laser; Adjusting means adjusted in accordance with the differential quantum efficiency of the semiconductor laser; constant current means provided in parallel with the drive means for generating a constant current of a magnitude related to the threshold current of the semiconductor laser at a predetermined temperature; A semiconductor laser drive circuit comprising: 2. The semiconductor laser drive circuit according to claim 1, wherein the detection means detects an excessive state of the laser light based on threshold information.