JPS62237785A - Semiconductor laser driving device - Google Patents

Abstract

PURPOSE:To emit laser light having an excellent extinction ratio by providing current control means for correcting a threshold bias current value by a predetermined amount to apply it to a semiconductor laser, and applying a bias current near the threshold value to the laser to improve frequency and thermal characteristics. CONSTITUTION:When a controller 5 which is used also as correcting means calculates nonlight emitting bias current value IBIAS, it feeds the bias current value IBIAS to a latch 7, and applies the value IBIAS to a semiconductor laser 1. The controller 5 feeds data for increasing a light emitting current to a latch 8 until a predetermined quantity of emitted light is obtained. In other words, the current of a light emitting constant current circuit 4 added to the bias current of a bias constant-current circuit 2 is applied to the laser 1. When the laser 1 emits a predetermined light quantity by the added current, the controller 5 stops transferring data, holds the value at the latch 8 and instructs a laser OFF signal to a switching circuit 3.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser driving device, and more particularly to a laser driving device that applies a bias current.

[Conventional technology]

It is well known that in order to improve the frequency characteristics of a semiconductor laser, a bias current is applied even when no light is emitted. It is also desirable to apply a bias current to improve so-called thermal characteristics, a phenomenon in which the amount of light varies depending on the thermal history when the laser is turned on continuously and when it is turned on in a pulsed manner. In order to increase the effect, it is desirable to apply a bias current as close to the emission threshold as possible.

As such a conventional bias current drive device, one shown in FIG. 4 has been proposed.

FIG. 4 is a circuit diagram illustrating an example of a conventional bias current control device. The structure and operation will be explained below.

Signal BIAS for applying bias current to terminal 31
When input, a bias current determined by the resistor 32 is applied to the semiconductor laser 33.

Then, when the laser-on signal LON to turn on the laser is input to the terminal 34, the current for light emission determined by the resistor 35 is added to the bias current, and the semiconductor laser 33
is input, and the semiconductor laser 33 starts emitting light.

By the way, it is well known that the current value at which light emission starts, that is, the threshold current of the semiconductor laser 33 changes depending on the ambient temperature. Therefore, in order to apply a bias with a fixed current as shown in FIG. 4, either the semiconductor laser 33 is controlled to a constant temperature, or the temperature of the semiconductor laser 33 is detected and the bias current value is automatically controlled. There must be. However, since the threshold current of each semiconductor laser 33 is different, it is necessary to adjust the bias value for each semiconductor laser 33. In addition, in the method of automatically controlling the bias current value by detecting the temperature of the semiconductor laser 33, the initial characteristics of the laser are different from the characteristics when the efficiency decreases due to aging, etc., so thermal characteristics can be improved by applying a bias current. The effect weakens over time.

In order to improve this drawback, a proposal has been made to monitor the amount of light emitted from the semiconductor laser 33 and to sample and hold the amount of light emitted.

FIG. 5 is a block diagram illustrating a conventional sample and hold circuit for monitoring the amount of emitted light. The structure and operation will be explained below.

The amount of light emitted from the semiconductor laser 33 is monitored by an optical sensor 41 composed of a photodiode or the like. The monitored amount of emitted light is sampled and held by a low level sample and hold circuit 42 when the amount of light is at a low level, and sampled and held by a high level sample and hold circuit 43 when the amount of light is at a high level, which is the operating light amount. The light amount value sampled and held at the low level is compared with a reference light amount value, and is driven by the bias constant current source 44 so as to reach a predetermined low level light amount.

On the other hand, when high-level light emission is instructed by the control circuit 45, the laser lighting changeover switch circuit (SW) 46 operates so that the current from the light emission constant current source 47 is added to the semiconductor laser 33 in addition to the bias constant current drive. do. At this time, the light emission constant current source 47 compares the light amount value sampled and held by the high level sample and hold circuit 43 with a predetermined high level reference light amount value, and controls the light amount to a predetermined high level light amount. Sampling timing control circuit 4
8 is set to low level by the control circuit 45.

The timing of sample and hold is controlled according to the timing of high-level light amount control.

[Problem that the invention seeks to solve]

By the way, with the method of controlling the bias current value by sampling and holding the amount of monitor light as shown in FIG. 5, it is not possible to make the amount of low-level light completely equal to NrQJ. Therefore, when performing high/low laser drive,
Although this is fine, it is desirable that the low level light amount be as small as possible when used in a laser drive where the low level light amount is a problem, such as an electrophotographic laser beam printer. This is because in the image exposure method, in which laser irradiation is applied to recorded areas, low-level light intensity causes fogging, and in the case of background exposure, in which laser irradiation is applied to areas that are not recorded, dark areas are not sufficiently covered. However, there is a problem in that images with low contrast are obtained.

In addition, in the example shown in Figure 5, differential amplification is performed by comparing the sampled and held light amount with the reference level to obtain the specified low-level light amount and high-level light amount, so the relationship between the current and the light amount, that is, the slope It is good if the efficiency is constant, but if the slope efficiency changes due to changes in temperature or changes over time, the control of the amount of light emitted will vary depending on the temperature of the laser and changes over time, making it impossible to achieve satisfactory control. .

This invention was made to solve the above problems, and by applying a bias current near the threshold to a semiconductor laser, the frequency characteristics and thermal characteristics are improved, resulting in a laser with an excellent extinction ratio. The object of the present invention is to obtain a semiconductor laser driving device that can emit light.

[Means for solving problems]

The semiconductor laser driving device according to the present invention provides first and second emitted light quantities outputted from the detection means when a bias current is applied to the semiconductor laser in a stepwise manner, and the corresponding first and second light quantities. Threshold bias current calculation means for calculating a threshold bias current value based on the bias current, and current control means for correcting the threshold bias current value calculated by the threshold bias current calculation means by a predetermined amount and applying it to the semiconductor laser. It has been established that

[Effect]

In this invention, the threshold bias current calculation means applies a bias current to the semiconductor laser in a stepwise manner, and calculates a threshold bias current value based on the first and second light emission amounts obtained from the detection means. Calculate.

Then, the current control means corrects this threshold bias current value by a predetermined amount and applies it to the semiconductor laser.

〔Example〕

FIG. 1 is a diagram illustrating the configuration of a semiconductor laser driving device showing an embodiment of the present invention, in which 1 is a semiconductor laser, which is connected to a bias constant current circuit 2 on the one hand, and is driven by a laser lighting signal on the other hand. It is connected to a light emitting constant current circuit 4 through a switching circuit 3 whose conduction is switched. Reference numeral 5 denotes a control unit composed of, for example, a CPU, which sends data for controlling the bias current and light emitting current to a latch 7 for the bias current and a latch 8 for the light emitting current via a multiplexer (MPX) 6. . The multiplexer 6 switches the data from the control unit 5 to the latch 7 for bias current control and transfers it, and for the light emission current control it switches to the latch 8 for transfer. The control section 5 also serves as threshold bias current calculation means of the present invention. 9,104f D/A converter (D/A) The D/A converter 9 converts the digital data held in the latch 7 into analog data, and the D/A converter 1o
converts the digital data held in the latch 8 into analog data and supplies it as data for controlling the output of the bias constant current circuit 29 and the light emission constant current circuit 4. 11
is an optical sensor that monitors the amount of laser beam emitted by the semiconductor laser 1. A volume 12 is adjusted so that a predetermined monitor output is obtained from the optical sensor 11 when the semiconductor laser 1 outputs a predetermined amount of light. Reference numeral 13 denotes an amplifier circuit that amplifies the output of the optical sensor 1 and sends a monitor output to the control section 5.

Next, the operation when FIG. 1 is applied to a laser beam printer will be described with reference to FIG. 2.

FIG. 2 is a slope efficiency characteristic diagram showing the relationship between laser applied current and laser emission power, where the vertical axis shows the laser emission power and the horizontal axis shows the laser applied current value.

First, when a print preparation command is issued from the outside, the control unit 5 sends data to the multiplexer 6, latch 7, and D/A converter 9 to increase the bias current. When the amount of light emitted from the semiconductor laser 1 reaches a predetermined first light emission level P1, the control unit 5 sets the bias current control data value II at that time.
is stored in the internal memory, the amount of light emission is further increased, the bias current value is increased to the second light emission level P2, and the bias current control data E2 at the time when the second light emission level P2 is reached is stored in the internal memory. Then,
The control unit 5 temporarily turns off bias current control and resets the data in the latch 7. Next, the control unit 5 sets the bias current control data value I+ for the first light emission level PI and the second
The threshold current value (threshold bias current value) Ith and non-emission bias current value I BIAS of the semiconductor laser 1 are calculated from the bias current control data E2 for the light emission level P2 using the following equations (1) and (2).
Calculate based on the formula.

However, even when the threshold current value Ith obtained from the above equation (1) is applied to the semiconductor laser 1, the semiconductor laser 1 is not sufficiently quenched and the light amount P decreases as shown in FIG.
It has been empirically found that the light of th is emitted. Therefore, since there is a possibility that image unevenness may occur due to this light emission, in this embodiment, the non-light emission bias current value I8
1AS is determined by the control unit 5, which also serves as the correction means of the present invention, based on the following equation (2).

I BIAS= I thXα・Fist-
1-1(2) However, α is a numerical value between 0 and 1, and is set to 0.8 in this embodiment. This is because if the numerical value α is close to O, the quenching property will improve, but the thermal property 9 frequency property will deteriorate, and if α is close to 1, the thermal property 9 frequency property will improve but the quenching property will deteriorate. This is because
It is desirable to select from 0.7≦α≦0,85.

The control unit 5 sets the non-emission bias current value I BIAS to the second (second
), the non-emission bias current value IB
Send IAS to latch 7 and set the non-emission bias current value I B
IAS is applied to the semiconductor laser 1. The control unit 5 sends data for increasing the light emitting current to the latch 8 until a predetermined amount of emitted light is obtained. That is, it is added to the bias current by the bias constant current circuit 2 and the light emitting constant current circuit 4
A current of is applied to the semiconductor laser 1. When the semiconductor laser 1 emits a predetermined amount of light due to the added current, the control unit 5 stops data transfer, holds the value in the latch 8, and issues a laser off signal to the switching circuit 3. As a result, only the non-emission bias current value IBI^S is applied to the semiconductor laser 1, and the semiconductor laser 1 becomes ready for printing. The above operation makes it possible to start printing, and thereafter the laser-on signal LON is issued from the control unit 5 based on the print signal, and the semiconductor laser 1 changes the input print signal based on the current obtained by adding the emission current to the bias current. It emits light accordingly. In the above embodiment, the bias current and the light emitting current are controlled between pages, but if the temperature change of the semiconductor laser 1 is small,
The bias current may be controlled only at the start of printing or once every several pages, and the light emitting current may be controlled between each page.

FIG. 283 is a block diagram illustrating the configuration of a laser drive control device showing another embodiment of the present invention.

Note that the light emitting constant current circuit 4 shown in FIG. Bias constant current circuit 2. Since the switching circuit 3 is the same, it is omitted.

In this figure, 2] is a CPU that sets a count signal E1 to a HIGH level so that an up/down counter (UD counter) 22 is incremented in accordance with an input print preparation command. The UD counter 22 functions as a down counter when the count signal E1 becomes LOW level. The UD counter 22 is chip-enabled by the enable signal E2 sent from the CPU 21 and starts counting the clock signal CKI, and is reset by the reset signal R1. A counter (CNT) 23 is chip enabled by the enable signal E3, starts counting the clock signal CKI, and is reset by the reset signal R2. 24 is a D/A converter (D/A), and UD counter 22
The bias current data determined by is converted into an analog bias current value ADI and supplied to the bias constant current circuit 2. 25 is a D/A converter (D/A), and counter 23
The count value corresponding to the light emitting current value counted by is converted into an analog light emitting current value AD2, and the analog light emitting current value AD2 is supplied to the light emitting constant current circuit 4.

When the print preparation signal is input to the CPU 21, the CPU
21 sets the count signal El (up count instruction) to HIGH level to the UD counter 22 and sends out an enable signal E2. As a result, UD counter 2
2 is a clock signal CKI until the light intensity of the semiconductor laser 1 shown in FIG. 1 reaches the first light intensity level P1 shown in FIG.
Count up and go. Further, the UD counter 22 counts up to the second light amount level P2, but the CPU 21 counts the number of clocks from the first light amount level P1 to the second light amount level P2. In this embodiment, the second light level P2 is set to twice the first light level P1, so when the second light level P2 is detected,
The PU21 receives the count signal E sent to the counter 22.
1 to a LOW level to function as a down count. 1. Next, count down from the second light level P2 by a number of clocks that is twice the number of clocks counted from the first light level P1 to the second light level P2,
Controls the threshold current value Ith. Further, in order to bring the semiconductor laser 1 into a non-emitting state, n clocks are decreased to a bias current control value. This is the same effect as multiplying the threshold current value Ith by the value α explained in the embodiment shown in FIG. The value is, for example, an amount that changes by 10 mA. Once the bias current value is determined in this way, C
The PU 21 sends an enable signal E3 to the counter 23.

In response, the counter 23 counts up until a predetermined amount of light is reached, and the output from the counter 23 is converted into an analog light emission current value AD2 by the D/A converter 25 and supplied to the light emission constant current circuit 4. When the control of the bias current value and the light emitting current value is completed in this way, the printer becomes ready for printing and emits light according to the input print signal.

Note that in the above embodiment, the threshold current value Ithe
Although the case where the calculation is performed by counting down the UD counter 22 has been explained, the UD counter 22 and the D/A converter 2
A subtraction circuit is provided between the first light intensity level PI and the second light intensity level P2 to subtract twice the number of clocks from the first light intensity level PI to the second light intensity level P2 from the count value of the UD counter 22 that has counted up to the second light intensity level P2. At the same time, n-clock subtraction may be performed to cause the semiconductor laser 1 to emit no light. Also, the case where the second light intensity level is counted as twice the first light intensity level has been explained;
It doesn't matter if the value is an integer multiple.

〔Effect of the invention〕

As explained above, the present invention provides the first and second emitted light quantities outputted from the detection means when a bias current is applied to the semiconductor laser in a stepwise manner, and the first and second biases corresponding thereto. Threshold bias current calculation means for calculating a threshold bias current value based on the current; and current control means for correcting the threshold bias current value calculated by the threshold bias current calculation means by a predetermined amount and applying it to the semiconductor laser. Even if the threshold current value, slope efficiency, and other characteristics of the semiconductor laser change due to changes in ambient temperature or deterioration of durability, a bias current near the threshold value can always be applied to the semiconductor laser, and the frequency It has the advantage of being able to significantly improve the characteristics and thermal characteristics, and also being able to always emit laser light with an excellent extinction ratio.

[Brief explanation of drawings]

FIG. 1 is a diagram explaining the configuration of a semiconductor laser driving device showing an embodiment of the present invention, FIG. 2 is a slope efficiency characteristic diagram showing the relationship between laser applied current and laser emission power, and FIG.
The figure is a block diagram explaining the configuration of a laser drive control device showing another embodiment of the present invention, FIG. 4 is a circuit diagram explaining an example of a conventional bias current control device, and FIG. FIG. 2 is a block diagram illustrating a sample and hold circuit for monitoring. In the figure, 1 is a semiconductor laser, 2 is a bias constant current circuit, and 3 is a semiconductor laser.
is a switching circuit, 4 is a light emitting constant current circuit, 5 is a control unit, 6 is a multiplexer, 7.8 is a latch, 9 and 10 are D
/A converter, 11 is an optical sensor, and 12 is a volume. Fig. 1 12

Claims (2)

[Claims] (1) In a semiconductor laser driving device that controls a bias current and an emission current applied to the semiconductor laser according to an output from a detection means that detects the amount of light emitted from the semiconductor laser, the bias current is applied to the semiconductor laser in stages. The amounts of first and second emitted light output from the detection means when variablely applied and the first amount of light corresponding thereto.
and a threshold bias current calculation means for calculating a threshold bias current value based on the second bias current, and a threshold bias current calculation means for correcting the threshold bias current value calculated by the threshold bias current calculation means by a predetermined amount to 1. A semiconductor laser driving device comprising: current control means for applying a current to the semiconductor laser. (2) The semiconductor laser driving device according to claim (1), wherein the second bias current applied to the semiconductor laser by the bias current calculation means is an integral multiple of the first bias current.