JPS60263486A - Semiconductor laser drive circuit - Google Patents

Abstract

PURPOSE:To contrive to always obtain the stable light output even if the case temperature of a semiconductor laser may change and information signals for driving may be high speed pulses, by a method wherein the instantaneous value of the light output is detected while the light output is not utilized, for instance, just before it is utilized, and this value is controlled so as to be kept always constant. CONSTITUTION:The semiconductor laser 12 irradiates laser beam L1 and L2 corresponding to the information signal input, and the beam L2 irradiates a light detector 20, and wave signals of the information signal 1 are obtained at a terminal M1. In the mean while, a light detector 17 outputs the pulse 2 in the period T0, and this pulse is amplified and arranged, and this is applied to an input terminal 55 as a sample signal 3. The output of the light detector 20 corresponding to the information signal wave form is applied to a sample and hold circuit 52, and this output also keeps constant value until the next signal is applied, even this output may change corresponding to a printed image during the period T2. When the light output of the semiconductor laser 12 increases while the period T0, because of the temperature change of the case, the output of the light detector 20 also increases. The output of an amplifier 53 becomes low voltage being inverting amplified, and the current and the light output of the laser 12 become small determined values. Hereby, the change of the light output is removed and the stable light output can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a semi-moving laser drive circuit that is small in size and capable of constantly obtaining stable optical output through high-speed pulse operation. In particular, it relates to a semiconductor laser drive circuit used as a light source for an optical printer.

(Prior Art) Semiconductor lasers have conventionally been used as one of the light sources of optical printers. In such printers, the light output from the semiconductor laser is amplitude-modulated according to the information signal input to the printer, and this exposes the photoreceptor to form a latent image due to static electricity on its surface, which is then developed. The structure is such that a visible image can be obtained by doing this. It is desirable that this visible image always have a uniform image density depending on the information signal input to the semiconductor laser. However, it was difficult to obtain uniform image density, and the situation was extremely unsatisfactory.

That is, the characteristics of a semiconductor laser change as the ambient temperature, particularly the case temperature changes, resulting in fluctuations in optical output, which ultimately has a great effect on image density.

The information signals input to the semiconductor laser are high-speed pulses 18,
Ah, 615, ov-fka. , --A/l/7, but there is no compensation for variations in optical output due to changes in case temperature, or the semiconductor laser is driven by continuous wave oscillation. Compensation would only be provided if the person was forced to do so.

(Problems to be Solved by the Invention) In the present invention, even if the temperature of the case of the semiconductor laser changes, even if the information signal to be driven is a high-speed pulse, it is possible to always obtain a stable optical output. That is.

(Means for solving the problem) In order to stabilize the optical output, the present invention detects the instantaneous value of the optical output during a period when the optical output is not used, for example, just before it is used, and By controlling the value so as to always keep it constant, it is intended to stabilize the light output during the usage period of the light output.

(Function) In other words, in a laser printer, when a laser beam scans the surface of a photoreceptor, a photodetector is installed at the end of the photoreceptor, and the laser beam is activated immediately before the laser beam scans the surface of the photoreceptor. The detector is irradiated, the intensity of the laser beam at that moment is detected and held for a certain period of time, and the semiconductor laser is driven so that the value of the holding signal corresponding to the intensity of the held laser beam is kept at a constant value. It is something.

The operation of this laser printer will be explained with reference to FIG. Reference numeral 11 denotes a semi-moving laser drive circuit including a semiconductor laser 12 (not shown), to which an information signal to be printed is applied from a circuit (not shown), and a laser beam L1 corresponding to the signal is emitted.

The laser beam L1 passes through the optical system 13 and passes through the rotating polygon mirror 14.
The light is deflected by a deflection device consisting of the following, and is incident on the surface of a drum-shaped photoreceptor 16 through an optical system 15.

The laser beam L1 is directed by an arrow P on the surface of the charged photoreceptor.
direction (main scanning direction), and the photoreceptor 16 also rotates gently in the direction of arrow Q (sub-scanning direction).
A latent image consisting of pixels corresponding to the applied information signal is formed on the surface of the latent image 6, which is developed by a developing device (not shown), transferred onto plain paper, and made into a visible image. Here,
A photodetector 17 that detects one pulse per main scan is arranged at the end of the photoreceptor 16 on the converging surface of the laser beam L1 that has passed through the optical system 15. When the laser beam L1 is scanned and a light 17 made of, for example, a photodiode is irradiated for a short period of time (To), the photodetector 17 outputs a pulse as shown in FIG. 3(2). Here, the waveform shown in FIG. 3(1) is the waveform of the information signal applied to the semiconductor laser 12. Laser beam L is applied to the photodetector 17
1 is irradiated for a short period of time, the information signal to be printed is not applied until the period T1, and in the subsequent period T2, the information signal to be printed is applied as shown in FIG. 3 (1). be done.

Here is the drive current ID and optical output P of the semiconductor laser.

The relationship between . That is, even if the drive current ID is constant, the optical output Po varies greatly depending on the change in case temperature. As a result, print quality had to deteriorate.

The configuration of the printer using the semiconductor laser shown in FIG. Knowing the exact position of beam L1,
It was also used as a control standard.

In contrast, in the present invention, in addition to the effect of this conventional photodetector, the output of the photodetector 17 is also used to stabilize the optical output.

(Example) An embodiment of the present invention will be described below with reference to FIG.

Here, 12 is a semiconductor laser, which emits a laser beam L1. 20 is a part of the laser beam L1 from a photodiode or a laser beam L emitted toward the rear of the semiconductor laser in the opposite direction to the laser beam L1.
The circuit 18, which includes a photodetector that constantly receives the irradiation of 2, constitutes a non-saturated current switch circuit, and includes a laser diode 12 connected in parallel with a protective diode 51 for preventing the application of a reverse voltage between the collector of the transistor 21 and the ground. It is connected. The emitters of the transistors 21 and 22 are connected through a variable resistor 32, and a resistor 34 corresponding to the laser diode 12 is connected to the collector of the transistor 22 between the ground and the transistor 23.
is a constant current circuit that limits the current to a constant value so that the non-saturated current switch circuit can perform high-speed switching operations in a non-saturated state.

The circuit including the transistors 24 to 27 is a bias circuit for effectively applying the information signal applied to the information signal input terminal 54 to the non-saturated current switch circuit 18. 19 is an output stabilization circuit for stabilizing the optical output, which includes a photodetector 20, a sample-and-hold circuit 52, a hold capacitor 49, and compares and amplifies the constant voltage vO and the output of the sample-and-hold circuit 52. The output of the photodetector 20 is applied to the sample-and-hold circuit 52, and the output of the photodetector 20 is applied to the sample-and-hold circuit 52.
A sample signal shown in FIG. 3(3) generated based on the detection signal of the photodetector 17 shown in FIG. 3 is applied through the sample signal input terminal 55. The output of this output stabilizing circuit 19 is applied to the base of a transistor 23 forming a constant current circuit to control the value of the constant current.

If an information signal is now input to the information signal input terminal 54,
Through the transistor 24 and the resistors 35 and 37, the information signal biased to the negative voltage side by the operation of the transistor 26 which has a constant current function is transferred to the non-saturated current switch circuit 1.
It is applied to the base of the transistor 210 of No. 8, and switches the collector current of the transistor 21, that is, the current of the semiconductor laser 12 in a non-saturated manner.

On the other hand, transistor 25. Resistor 40, transistor 27,
The resistor 41 is connected symmetrically to the transistor 24, the resistor 35, the transistor 26, and the resistor 36, and is a transistor connected to perform differential operation with the transistor 21 of the non-saturated current switch circuit 18 by adjusting the resistor 39. The bias of No. 22 is set to the optimum value. By adjusting the variable resistor 32, the balance between the transistors 21 and 22, which operate as a differential amplifier, can be set to an optimum value.

The current of the transistor 23 forming the constant current circuit is determined by its base voltage and the resistor 33, and this current value is determined by the transistors 21 and 2 of the non-saturated current switch circuit 18.
The transistors 21 and 22 are set to a value equal to the sum of the currents flowing through the transistors 2 and 2, so that the transistors 21 and 22 are always never saturated, so that both transistors can perform high-speed non-saturated switching operation. The current waveform during switching can be monitored by the resistor 34.

Thus, the semiconductor laser 12 emits laser beams L1 and L2, which are optical outputs corresponding to the input information signal, and the laser beam L2 irradiates the photodetector 20, which is made of, for example, a photodiode, as shown in FIG. 3(1). A signal corresponding to the waveform of the information signal is obtained at the terminal M1.

On the other hand, the photodetector 17 shown in FIG. 2, which is made of a photodiode card, for example, outputs a pulse as a detection signal shown in FIG. The signal is applied to the sample signal input terminal 55 as a sample signal shown in FIG.

52 is a sample fist-and-hold circuit (for example, μPC649D-NECLINEARIC198
1P, 311 to 314), the output of the photodetector 20 corresponding to the information signal waveform shown in FIG. Then continue to hold that output value.

Therefore, this sample-and-hold circuit 52
The output of the information signal waveform is as shown in Fig. 3 (1),
After period To, a certain period T1 passes, and in period T2, even if it changes depending on the image, it remains at a constant value until the next sample signal is applied.

The output of the sample-and-hold circuit 52 is inputted to one input terminal of the input terminal 53 through the resistor 31. From that output terminal there is a resistor of 4
4 and capacitor 48 are connected in parallel as input (1
1) It is fed back to the terminal, and the reference voltage Vo is applied to the other input terminal to form a comparison circuit.

The output of the operational amplifier 53 is divided by resistors 45.46° and 47 and applied to the base of the transistor 23 which serves as a constant current circuit. Here, a constant voltage diode 5 is connected between the connection point of resistors 45 and 46 and the power supply -V.
0 is for overvoltage prevention and is normally in an off state.

Since it operates in this way, when the optical output of the semiconductor laser 12 during the current period To increases due to a change in its case temperature, the output of the photodetector 20 also increases in the positive direction, and the output of the operational amplifier 53 is inverted and amplified. Therefore, the voltage becomes low and the base voltage of the transistor 23 is lowered, so the value of the constant voltage flowing through the transistor 23 is reduced. Therefore, the current of the non-saturated current switch circuit 18 is also reduced, and in order to reduce the collector current of the transistor 21, the current of the semiconductor laser 12 is suppressed to a small value, and the optical output of the semiconductor laser 12 is also reduced to a predetermined value.

C2) Now, looking at the voltage relationship of each part, the voltage (1) with respect to the power supply -V at the terminal M1 is Vl, where the resistance values of the resistors 42 and 43 are R42 and R43, respectively, and the current of the photodetector 20 is I20. -(R420R43) I20...(1). Further, the voltage of the output terminal M2 of the sample-and-hold circuit 52 with respect to the power supply -V is set to v2. The voltage of the output terminal M3 of the operational amplifier with respect to the power supply -V is 3, the reference voltage applied to the other input terminal of the operational amplifier is the voltage Vo with respect to the power supply -V, and the resistor 3
If the resistance values of 1 and 44 are R11 and R44, respectively, then the following equations are obtained. Further, if the base voltage of the transistor 23 with respect to the power supply -■ is v4, and the resistance values of the resistors 45.46 and 47 are R4s, R46 and R47,
V4 = (R47/(R45+R46+R47))
Va...(3).

This voltage v4 is the base ψ em of the transistor 23 -
In this way, the value of the current flowing through the transistor 23 is determined, the current of the non-saturated current switch circuit 18 is determined, and the current of the semiconductor laser 12 is determined.

The above operation can also be satisfactorily accomplished by using the output of the photodetector 17 instead of using the output of the photodetector 20 in FIG. This is because the output signal of the photodetector 20 is used only during the sample signal period shown in FIG. 3(3). Therefore, the output signal of the photodetector 17 shown in FIG. 3(2) is used instead of the output signal of the photodetector 20, and at the same time, the output signal of the photodetector 17 shown in FIG. 3(2) is amplified and shaped. It will be clear from the above description that a sample signal may be created and applied to the sample signal input terminal 55.

(Effects of the Invention) As explained above, from changes in the case temperature of the semiconductor laser, that is, changes in the environmental temperature, temperature changes caused by increases and decreases in the average current of the semiconductor laser depending on the content of the information input signal, etc. Fluctuations in the light output that occur can be removed with a simple circuit, and a more stable light output can be obtained at a lower cost than when the case temperature is kept at a constant temperature by cooling etc. Since it can also be driven in high-speed pulses, it has the unique feature of consistently obtaining stable printing quality when used as a light source for laser printers and the like.

[Brief explanation of drawings]

FIG. 1 is a diagram showing an embodiment of the present invention, FIG. 2 is a configuration diagram of a printer to which the present invention is applied, and FIG. 3 is a waveform diagram for explaining the operation of FIGS. 1 and 2. FIG. 4 is a diagram showing the temperature characteristics of a semiconductor laser. DESCRIPTION OF SYMBOLS 11... Semiconductor laser drive circuit, 12... Semiconductor laser, 13, 15... Optical system, 14... Rotating polygon mirror, 16... Photoreceptor, 17.20... Photodetector, 18
...Non-saturation current switch-path, 19...Output stabilization circuit, 21-27.1°) 57-) Z fi.3
1 to 47°°°11・°2...Sample Φ and hold circuit, 5311・Self operational amplifier, 5
4 ” ” ” Information signal (15) input terminal, 55...sample signal input terminal, Ll,
L2・e・laser beam. Agent Kozo Uchida α6) Fig. 3 (1)...Information signal (2)...Pulse output of photodetector 17 (3)...Sample signal Fig. 4 EF Drive current ID

Claims (6)

[Claims] (1) A semiconductor laser that emits a laser beam, a non-saturated current switch means for driving the semiconductor laser, a constant current means for supplying a constant current to the non-saturated current switch means, and the non-saturated current switch means. bias means for applying an information signal and a bias to the constant current means; and output stabilization means for holding the intensity of the laser beam immediately before the laser beam is used, comparing it with a reference voltage, and applying the output to the constant current means. A semiconductor laser drive circuit comprising: (2) The semiconductor laser drive circuit according to claim 1, wherein the non-saturated current switch means is a differential amplifier. (3) The semiconductor laser drive according to claim 1, wherein the output stabilizing means includes a sample-and-hold circuit and an operational amplifier for comparing the output of the sample-and-hold circuit with a reference voltage. circuit. (4) The semiconductor laser drive circuit according to claim 1, wherein the output stabilizing means includes a first photodetector for detecting the intensity of the laser beam. (5) the output stabilizing means characterizes the intensity of the laser beam in response to a sample signal created by the output of a second photodetector for detecting a point immediately before the laser beam is utilized; A semiconductor laser drive circuit according to claim 1. (6) The semiconductor laser drive circuit according to claim 5, wherein the intensity of the laser beam is obtained by the output of the second photodetector.