JPH05145154A - Driver for semiconductor laser - Google Patents

Abstract

PURPOSE:To emit a light at high speed properly by calculating the drive current required for emitting it with a specified value based on the quantity of the emitted light of a semiconductor laser and differential efficiency. CONSTITUTION:CPU 6 seeks the respective quantities of lights emitted by driving a semiconductor laser 1 with two current values, and calculates the differential efficiency as the difference of quantity of light the difference of current. This differential efficiency itself does not change temporally. Next, the semiconductor laser 1 is driven with the specified current value, that is, the emitter current suited to the base current of a transistor 3. And, if one applies the relation of the current and the quantity of light emitted in this case to the differential efficiency, the temporal deviation quantity of the property of the current/quantity of light emitted is sought, whereby the value of the current in case of getting the specified quantity of light emitted can be calculated. Thereby, proper emission can be performed at high speed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor laser drive device, and more particularly to a drive device with an APC (automatic power control).

[0002]

2. Description of the Related Art Semiconductor lasers are often used by arranging photodiodes in parallel to detect the amount of emitted light and control the drive current so that the detected amount of light reaches a predetermined value. Can be avoided, and a stable amount of emitted light can be obtained. Such drive current control is AP
Called C.

In a laser beam printer, such an APC is used to drive a semiconductor laser and suppresses a change in print quality over time.

[0004]

When APC is performed by a digital signal, the driving current is often increased or decreased step by step until it reaches the predetermined value. However, until the predetermined value is reached, It may take a long time, and the semiconductor laser needs to be driven only for APC, not for actual printing, which not only shortens the life of the semiconductor laser but also the life of the photoconductor in a laser beam printer. There was a point.

In order to solve this problem, there has been proposed a device in which the drive current is increased / decreased in a plurality of steps according to the situation (Japanese Patent Laid-Open No. 63-56059), but further speedup is desired depending on the application. The present invention has been made in view of such circumstances, and an object of the present invention is to provide a semiconductor laser driving device that can obtain a current value that can immediately obtain a predetermined amount of emitted light by one preliminary drive.

[0006]

SUMMARY OF THE INVENTION A semiconductor laser driving device of the present invention is a semiconductor laser driving device that detects a light emission amount of a semiconductor laser and controls a driving current so that the light emitting amount becomes a predetermined value. Means for driving the semiconductor laser in advance by the value, the detected light emission amount in each case, and the above-mentioned 2
Means for calculating and storing the differential efficiency of the semiconductor laser with one current value, means for driving the semiconductor laser with a predetermined current value prior to emitting light with the predetermined value, and the amount of emitted light detected in this case and the And a means for calculating a drive current required to emit light at the predetermined value based on the differential efficiency.

[0007]

Operation: First, driving is performed with two current values to obtain respective emitted light amounts, and the differential efficiency is calculated as a light amount difference / current difference. This differential efficiency itself does not change over time. Next, the semiconductor laser is driven with the predetermined current value. When the relationship between the current and the emitted light amount in this case is applied to the differential efficiency, the time-dependent shift amount of the current / emitted light amount characteristic is obtained, and the current value for obtaining the predetermined emitted light amount can be calculated from this.

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing the embodiments. FIG. 1 is a circuit diagram of a semiconductor laser driving device of the present invention, in which 1 is a semiconductor laser to be driven.
A photodiode 2 for detecting the amount of emitted light is arranged in parallel with the semiconductor laser 1. Generally, the photodiode 2 is provided in the direction opposite to the light output direction of the semiconductor laser 1.

The CPU 6 outputs a digital signal indicating a semiconductor laser drive current value to a D / A (digital / analog) converter 9, and the analog output is amplified by an operational amplifier 5 and given to the base of the transistor 3. Transistor 3
Is interposed between the power supply line and the semiconductor laser 1, and the semiconductor laser 1 is driven by the emitter current according to the base current of the transistor 3.

The collector of a transistor 11 whose emitter is grounded is connected to the connection point between the transistor 3 and the semiconductor laser 1, and the base of the transistor 11 becomes "L" level at the timing of driving the semiconductor laser 1.
A switch signal S _S for turning off is given. The photodiode 2 is reversely connected to a power supply line via a resistor 4, its anode is grounded, and its cathode is connected to an A / D (analog / digital) converter 10. The cathode potential is determined by the amount of light received by the photodiode 2 or the amount of light emitted by the semiconductor laser 1.

The digital output of the D / A converter 9 is input to the CPU 6 and used for the calculation described below. Other 7
Is a ROM (read-only memory) for storing programs executed by the CPU 6, and 8 is a RAM (random read / write memory) for storing calculation results and the like. 2 is an explanatory view of the operation principle of the device of the present invention, and FIGS. 3 to 5 are flowcharts showing the control procedure in the CPU 6. In FIG. 2, the horizontal axis represents the drive current of the semiconductor laser 1 and the vertical axis represents the amount of emitted light.
FIG. 6 is a characteristic diagram of emitted light quantity, in which the temperature T of the semiconductor laser 1 is T =
In the case of T ₁ , it is indicated by a solid line, and in the case of T = T ₂ (T ₁ <T ₂ ), it is indicated by a two-dot chain line.

The CPU 6 also calculates the differential efficiency η according to the procedure shown in FIG. 3 and stores it in the RAM 8. That is, the two preset current values D ₀₁ and D ₁₁ are read from the RAM 8 and set in an appropriate register (# 1). Where D ₀₁ <D ₁₁
In the case of a laser beam printer, D ₀₁ is a current value that gives an amount of emitted light equivalent to bias light below the development threshold of the photosensitive drum, and D ₁₁ is a current value that gives an amount of emitted light suitable for development. Then, the data of the current value D ₀₁ is output to the D / A converter 9, and the semiconductor laser 1 is driven by this (# 2). CPU
Reference numeral 6 captures the emitted light amount L ₀₁ during this emission as the output of the A / D converter 10 and stores it in the register as appropriate (# 3).

Similarly, the semiconductor laser 1 is driven by the current value D ₁₁ (# 4), and the emitted light amount L ₁₁ is captured and stored (# 5). Then, the differential efficiency η is calculated according to the following equation (1) and stored in the register or RAM 8 as appropriate (# 6). η = (L ₁₁ −L ₀₁ ) / (D ₁₁ −D ₀₁ ) ... (1) This slope or η does not change depending on the temperature change,
As shown in FIG. 2, the characteristics of drive current / emission light amount shift to the right while keeping the inclination. This η corresponds to the inclination of the portion on the right side of the solid line portion in FIG.

In the laser beam printer, the above-described calculation and storage of η are performed once when the laser beam printer is started. Thus, when printing is performed by the printer, the CPU 6 determines the drive current according to the procedure shown in FIG. First, the semiconductor laser 1 is driven by the current D ₁₁ (# 11). Then, the output of the A / D converter 10 at this time is taken in as the emitted light amount L at this time (# 12). Then, using this L and the previously stored L ₁₁ and η, the drive current D is calculated by the following equation (2).

D = D ₁₁ + (L-L ₁₁ ) / η (2) That is, the current for obtaining the same amount of light emission of L ₁₁ at the time of startup is as described above.
It is obtained by the equation (2). Assuming that the temperature T of the semiconductor laser 1 when executing # 11 and # 12 is T ₂ , a light emission amount L smaller than L ₁₁ is obtained at the current D _{11 as} shown in FIG. In order to obtain the amount L ₁₁ of emitted light under the condition of the temperature T = T ₂ , from FIG.
It is necessary to increase the current. [Delta] I is (3) _{ΔI = (L-L 11)} / η ... from the formula (3), which is driving current value D for obtaining the amount of emitted light L ₁₁ by adding the D ₁₁ (2 ) Equation is required.

The D thus obtained is appropriately set in the register (# 14) and is output to the D / A converter 9, so that a desired amount of emitted light can be obtained from the switch signal S _S. In the laser beam printer, it goes without saying that the semiconductor laser emission (# 11) is performed at a timing unrelated to printing, such as between sheets. FIG. 5 shows another embodiment of the drive current setting procedure. In this case, drive the laser diode 1 with low current D ₀₁ (# 21),
The light quantity L'in this case is detected (# 22). Since η has a linear relationship as shown in FIG. 2, the current correction amount ΔI associated with the temperature change T ₁ → T ₂ is also ΔI = (L′−L ₀₁ ) / η (4) even in the vicinity of the current value D _01. Therefore, by adding this to D ₁₁ , the drive current D for obtaining the emitted light amount L ₁₁ can be obtained by the following equation (5).

D = D ₁₁ + (L'-L ₀₁ ) / η (5) In the case of the laser beam printer, the current value D is as described above.
_{Since 01} indicates only the amount of emitted light below the development threshold, this emission is irrelevant to printing. Therefore step
# 21 can be executed at any time.

[0018]

According to the present invention as described above, the semiconductor laser emits light only once when the drive current D is determined, and the calculation for determining the drive current D can be performed instantaneously, so that appropriate light emission can be performed at extremely high speed. .. Further, since the light emission for determining the drive current is a minimum of once, the lifetime deterioration of the semiconductor laser can be minimized. When the present invention is applied to a laser beam printer, deterioration of the life of the photoconductor is minimized.

[Brief description of drawings]

FIG. 1 is a block diagram of a device of the present invention.

FIG. 2 is a drive current / emission light amount characteristic diagram.

FIG. 3 is a flowchart showing a procedure for η calculation.

FIG. 4 is a flowchart showing a procedure for determining a drive current.

FIG. 5 is a flowchart showing a procedure for determining a drive current.

[Explanation of symbols]

 1 Semiconductor laser 2 Photo diode 6 CPU

Claims (1)

[Claims] 1. A device for driving a semiconductor laser, which detects the amount of light emitted from a semiconductor laser and controls a drive current so that this amount becomes a predetermined value, a means for driving the semiconductor laser in advance with two current values, and in each case Means for calculating and storing the differential efficiency of the semiconductor laser based on the detected light emission amount and the two current values; and means for driving the semiconductor laser at a predetermined current value before emitting light at the predetermined value. And a means for calculating a drive current required to emit light at the predetermined value based on the detected light emission amount and the differential efficiency.