JPH0738478B2 - Semiconductor laser drive device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention 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]

Conventionally, it is well known that a method of applying a bias current even when light is not emitted is used to improve the frequency characteristics of a semiconductor laser. In addition, it is desirable to apply the bias current from the viewpoint of improving the so-called thermal characteristics, which is a phenomenon in which the amount of light differs depending on the thermal history when the laser is continuously turned on and when it is turned on in a pulsed manner. Then, in order to maximize the effect, it is desirable to apply a bias current as close to the light emission threshold value as possible.

As such a conventional bias current driver, the 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 described below.

When the signal BIAS for applying the bias current is input to the terminal 31, the bias current determined by the resistor 32 is applied to the semiconductor laser 33. Then, when the laser-on signal LON for turning 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 input to the semiconductor laser 33, and the semiconductor laser 33 starts emitting light. To do.

By the way, it is well known that the semiconductor laser 33 has a current value for starting light emission, that is, a threshold current, which changes depending on the ambient temperature. Therefore, in order to apply a bias with a fixed current as shown in FIG.
Must be controlled to a constant temperature, or the temperature of the semiconductor laser 33 must be detected to automatically control the bias current value. However, since the threshold currents of the semiconductor lasers 33 are 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 and the characteristics when the efficiency is lowered due to changes over time, etc. are different, so the thermal characteristics are improved by applying the bias current. The effect of will weaken over time.

In order to improve the drawback, it has been proposed to monitor the emitted light amount of the semiconductor laser 33 and sample-hold the emitted light amount.

FIG. 5 is a block diagram for explaining a conventional sample and hold circuit for monitoring the amount of emitted light. The structure and operation will be described 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 the low level sample and hold circuit 42 when the amount of light is low level, and is sampled and held by the high level sample and hold circuit 43 when the amount of operating light is high. The low-level sample-and-hold light amount value is compared with the reference light amount value, and is driven by the bias constant current source 44 so as to have 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. To do. At this time, the light emitting constant current source 47 compares the light quantity value sampled and held by the high level sample and hold circuit 43 with a predetermined high level reference light quantity value, and controls so as to obtain a predetermined high level light quantity. The sampling timing control circuit 48 controls the timing for performing sample hold according to the timing of the low level and high level light amount control by the control circuit 45.

[Problems to be Solved by the Invention] By the way, in the method of controlling the bias current value by sample-holding the monitor light amount as shown in FIG. 5, it is possible to completely set the low-level light amount to “0”. Can not. Therefore, when performing high / low laser drive,
However, the low-level light amount is preferably as small as possible when used in a laser drive in which a low-level light amount is a problem, for example, in an electrophotographic laser beam printer. This is because in the image exposure method in which the laser irradiation is applied to the recorded area, the low level light amount causes fog, and in the case of the background exposure in which the laser irradiation is applied to the non-recorded area, the dark area cannot be sufficiently removed. There was a problem that an image with low contrast was obtained.

Further, in the example shown in FIG. 5, since the differential amplification is performed by comparing the sampled and held light quantity with the reference level in order to obtain the specified low level light quantity and high level light quantity, the relationship between the current and the light quantity, that is, the slope. It is good if the efficiency is constant, but if the slope efficiency changes due to changes in temperature, aging, etc., the emission light amount control will vary depending on the temperature and aging of the laser, and there is a problem that satisfactory control cannot be performed. . Further, as shown in FIG. 6, spontaneous light emission (so-called LED mode) occurs even though laser oscillation does not occur in the vicinity of the threshold current, and depending on the configuration of the optical system, the light due to this natural emission deteriorates the extinction ratio. It was a factor. Therefore, the semiconductor laser 33 having the characteristics shown by the solid line in FIG. 6 and the semiconductor laser 33 having the characteristics shown by the dotted line are shown.
When compared with, a semiconductor laser having the characteristics shown by the solid line
Since 33 has a larger threshold current level, it has a problem that it is more likely to spontaneously emit light and the extinction ratio is significantly deteriorated. In FIG. 6, the vertical axis represents the laser emission power and the horizontal axis represents the laser applied current value.

The present invention has been made to solve the above problems, and improves the frequency characteristic and the thermal characteristic by applying a bias current to the semiconductor laser to the extent that laser oscillation does not occur in the vicinity of the threshold value, thereby extinguishing light. An object of the present invention is to obtain a semiconductor laser driving device capable of emitting laser light with excellent ratio.

[Means for solving problems]

The semiconductor laser driving device according to the present invention provides the first and second emitted light amounts and the first and second bias currents corresponding to the first and second emitted light amounts output from the detection means when the bias current is applied to the semiconductor laser stepwise. Threshold bias current calculating means for calculating the threshold bias current value based on the threshold bias current value, and current control for controlling the bias current applied when the semiconductor laser is extinguished based on the threshold bias current value calculated by the threshold bias current calculating means. And means are provided.

[Action]

In the present invention, the threshold bias current calculation means applies the bias current to the semiconductor laser in a stepwise manner, and at that time, the first and second emitted light amounts obtained from the detection means and the corresponding first and second emitted light quantities. The threshold bias current value applied to the semiconductor laser is calculated based on the bias current of 2. Then, the current control means controls the bias current applied when the semiconductor laser is extinguished, based on the threshold bias current value.

〔Example〕

FIG. 1 is a diagram for explaining the configuration of a semiconductor laser driving device showing an embodiment of the present invention. Reference numeral 1 is a semiconductor laser, which is connected to a bias constant current circuit 2 on the one hand, and by a laser lighting signal on the other hand. It is connected to the light emission 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 the emission current to a latch 7 for the bias current and a latch 8 for the emission current via a multiplexer (MPX) 6. . The multiplexer 6 switches and transfers the data from the control unit 5 to the latch 7 in the case of bias current control and to the latch 8 in the case of light emission current control. The control section 5 also serves as the threshold bias current calculation means of the present invention. 9 and 10 are D / A converters (D / A), the D / A converter 9 converts the digital data held in the latch 7 into analog data, and the D / A converter 10 is held in the latch 8. The converted digital data is converted into analog data and supplied as data for controlling the output of the bias constant current circuit 2 and the light emission constant current circuit 4. 11 is an optical sensor, a semiconductor laser 1
The amount of light of the laser beam emitted by is monitored.
Reference numeral 12 denotes a volume, which is adjusted so that a predetermined monitor output can be obtained from the optical sensor 11 when the semiconductor laser 1 outputs a predetermined amount of light. An amplifier circuit 13 amplifies the output of the optical sensor 11 and sends a monitor output to the control unit 5.

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

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

First, when a print preparation command is issued from the outside, the control unit 5 sends data for increasing the bias current to the multiplexer 6, the latch 7 and the D / A converter 9. When the light emission amount of the semiconductor laser 1 reaches the predetermined first light emission level P ₁ , the control unit 5 stores the bias current control data value I ₁ at that time in the internal memory and further increases the light emission amount to the second light emission level P 1. _The bias current value is increased to ₂ and the bias current control data I ₂ at the time when the second light emission level P ₂ is reached is stored in the internal memory. Next, the control unit 5 once turns off the bias current control and resets the data in the latch 7. Next, the control unit 5 is a bias current control data values I ₁ and the bias current control data I ₂ Metropolitan threshold current value of semiconductor laser 1 from the relative second light emission level P ₂ with respect to the first light emission level P ₁ (threshold bias current value) Ith
And the non-emission bias current value I _BIAS is _calculated based on the following equations (1) and (2).

However, even if 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 extinguished, and the light quantity Pth as shown in FIG.
It has been empirically known to emit light. Therefore,
Since image unevenness may occur due to this light emission, in this embodiment, the control unit 5 determines the non-emission bias current value I _BIAS based on the following equation (2).

I _BIAS = Ith × α (2) However, α is a numerical value between 0 and 1, and is 0.8 in this embodiment. When the numerical value α is close to 0, the extinction property is improved, but the thermal characteristics and frequency characteristics are deteriorated. When the numerical value α is close to 1, the thermal characteristics and frequency characteristics are improved but the extinction property is deteriorated. 0.7 ≦ α ≦ 0.85
It is desirable to select from.

When the control unit 5 calculates the non-light emission bias current I _BIAS, based on the equation (2), sends a non-light emission bias current I _BIAS to the latch 7, the non-light emission bias current I _BIAS is applied to the semiconductor laser 1 . The control unit 5 latches the data for increasing the light emission current until the predetermined light emission amount is obtained.
Send to. That is, the current of the light emitting constant current circuit 4 is added to the bias current of the bias constant current circuit 2 and applied to the semiconductor laser 1. When the semiconductor laser 1 emits a predetermined amount of light by the added current, the control unit 5 stops the 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 I _BIAS is applied to the semiconductor laser 1 and the semiconductor laser 1 becomes ready for printing. With the above operation, the printing can be started, and thereafter, the laser ON signal LON is issued from the control unit 5 based on the print signal, and the semiconductor laser 1 is input to the print signal based on the current obtained by adding the emission current to the bias current. It emits light in response. In the above embodiment, the bias current and the light emission current are controlled between each page, but when the temperature change of the semiconductor laser 1 is small, the bias current is controlled only at the print start time or on several pages. Done once,
The emission current may be controlled between pages.

FIG. 3 is a block diagram for explaining the configuration of a laser drive control device showing another embodiment of the present invention. The light emission constant current circuit 4, the bias constant current circuit 2, and the switching circuit 3 shown in FIG.

In this figure, reference numeral 21 denotes a CPU, which sets a count signal E1 to a high level so that an up / down counter (UD counter) 22 counts up in response to an input print preparation command. The UD counter 22 functions as a down counter when the count signal E1 goes low. The UD counter 22 is chip-enabled by the enable signal E2 sent from the CPU 21, starts counting the clock signal CK1, and is reset by the reset signal R1. Reference numeral 23 denotes a counter (CNT), which is chip enable by the enable signal E3, starts counting the clock signal CK1, and is reset by the reset signal R2. 24 is a D / A converter (D / A) that converts the bias current data determined by the UD counter 22 into an analog bias current value.
It is converted into AD1 and supplied to the bias constant current circuit 2. 25 is D
The / A converter (D / A) converts the count value corresponding to the light emission current value counted by the counter 23 into an analog light emission current value AD2, and supplies the analog light emission current value 4 to the light emission constant current circuit 4.

When the print preparation signal is input to the CPU 21, the CPU 21 sends the count signal E1 (up count instruction) to the UD counter 22 to HI.
At the same time as GH level, enable signal E2 is sent out. As a result, the UD counter 22 counts up the clock signal CK1 until the light quantity of the semiconductor laser 1 shown in FIG. 1 reaches the first light quantity level P ₁ shown in FIG. Although further UD counter 22 counts up to the second light intensity level P _2, the number of clocks from the first light level P ₁ to a second light level P ₂ is CPU21 is counting. In this embodiment, the second light quantity level P ₂ is set to be twice the first light quantity level P ₁ , so when the second light quantity level P ₂ is detected,
The CPU 21 sets the count signal E1 sent to the counter 22 to LOW
Set it to a level and let it function as a downcount. Then, the first light level P ₁ only the second light intensity level P times the number of clocks of the clock number counted until ₂ to count down from the second light level P _2, and controls the threshold current value Ith. Further, in order to bring the semiconductor laser 1 into the non-light emitting state, the value obtained by clocking down the down amount n is controlled by the bias current I _BIAS based on the following expression (3).

I _BIAS = Ith−I _D (3) This is the same effect as multiplying the threshold current value Ith described in the embodiment shown in FIG. 1 by the value α, and the D / A converter 24 Although it depends on the number of bits and the drive range of the bias current, the drive range of the bias current is 100 mA and the D / A converter 24 is composed of 8 bits in this embodiment. Is 0.39mA. Then, when the threshold current value Ith is 50 mA or less, the weight reduction value _ID is set to 10 mA, and the threshold current value Ith
If the value exceeds 50 and is 70 mA or less, the weight reduction value _ID is set to 15 mA,
When the threshold current value Ith exceeds 70 mA, the reduction value _ID is controlled to be 20 mA. In this case, the down amounts n from the threshold current are 25, 38 and 51, respectively.
When the bias current value is determined in this way, the CPU 21 sends the enable signal E3 to the counter 23 to control the light emission amount.
Is sent. In response to this, the counter 23 counts up until the predetermined light amount is reached, and the output from the counter 23 is D /
The A converter 25 converts the analog light emission current value AD2 and supplies it to the light emission constant current circuit 4. When the control of the bias current value and the light emission current value is completed in this way, printing is enabled and light emission is performed according to the input print signal.

In the above embodiment, the threshold current value Ith is set to UD
The case where the count is obtained by counting down the counter 22 has been described. However, a subtracting circuit is provided between the UD counter 22 and the D / A converter 24, and the count value of the UD counter 22 counted up to the second light level P ₂ together subtracted from 1 light level P1 the number of clocks twice the number of clocks to the second light intensity level P _2, it may be performed further n clock subtraction for non-emission of the semiconductor laser 1. Also, the case where the second light amount level is counted as twice the first light amount level has been described, but any numerical value that is an integral multiple may be used.

〔The invention's effect〕

As described above, according to the present invention, the first and second emitted light amounts output from the detection means when the bias current is applied stepwise to the semiconductor laser and the corresponding first light amounts.
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 applying the threshold bias current value at the time of extinction of the semiconductor laser based on the calculated threshold bias current value. Since the current control means for controlling the bias current is provided, even if the characteristics such as the threshold current value and the slope efficiency of the semiconductor laser change due to ambient temperature change and durability deterioration, the bias near the threshold value is always maintained. A current can be applied to the semiconductor laser, frequency characteristics and thermal characteristics can be significantly improved, and laser light emission with an excellent extinction ratio can always be performed. Further, since the bias current applied to the semiconductor laser can be varied based on the variation of the threshold current, there is an advantage that the extinction ratio can be prevented from lowering due to the variation of the threshold current.

[Brief description of drawings]

FIG. 1 is a diagram for 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.
FIG. 4 is a block diagram for explaining the configuration of a laser driving device showing another embodiment of the present invention, FIG. 4 is a circuit diagram for explaining an example of a conventional bias current control device, and FIG. 5 is a conventional emission light amount monitor. FIG. 6 is a block diagram for explaining a sample hold circuit for doing so, and FIG. 6 is a characteristic diagram showing the relationship between the laser applied current and the laser emission power. In the figure, 1 is a semiconductor laser, 2 is a bias constant current circuit, 3
Is a switching circuit, 4 is a light emitting constant current circuit, 5 is a control unit, 6 is a multiplexer, 7 and 8 are latches, 9 and 10 are D / A converters, 11 is an optical sensor, and 12 is a volume.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Yasumasa Otsuka 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Koichi Okuda 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Incorporated (72) Inventor Keiji Okano 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc.

Claims (1)

[Claims] 1. A semiconductor laser drive device for controlling a bias current and a light emission current applied to the semiconductor laser according to an output from a detection means for detecting the amount of light emitted from the semiconductor laser, wherein a bias current is applied to the semiconductor laser. Threshold bias current calculation for calculating the threshold bias current value based on the first and second emitted light amounts output from the detection means and the corresponding first and second bias currents when applied stepwise. Means and a current control means for controlling a bias current applied when the semiconductor laser is extinguished based on the threshold bias current value calculated by the threshold bias current calculation means. apparatus.