JPH0951137A - Semiconductor-laser driving apparatus - Google Patents

Abstract

(57) [Summary] [Purpose] To stabilize the laser output without being affected by the returning light. The front emission light of the semiconductor laser 21 is made into a light flux L substantially parallel to the optical axis by a SELFOC lens 22,
The return light on the exit surface Z of the SELFOC lens 22 is monitored by the rear monitor 26. The return light variable mechanism 25 adjusts the return light, and the laser control circuit 2 uses the fixed point power.
By driving the semiconductor laser 21 with a constant current by means of 7, the semiconductor laser 21 operates stably with a constant output even if the returning light changes.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical device using a semiconductor laser as a light source, and more particularly to a semiconductor laser driving device which solves the problem of returning light from an optical system to the semiconductor laser.

[0002]

2. Description of the Related Art A laser beam printer or the like is well known as an optical device using a semiconductor laser as a light source. FIG. 6 is a configuration diagram of a conventional embodiment of a laser scanning optical system, in which a laser unit 1 including a semiconductor laser and a collimator lens for converting a divergent light flux from the semiconductor laser into a parallel light flux has a cylindrical shape. A lens 2 and a polygon mirror 3 are sequentially arranged, and the polygon mirror 3 is rotated by a polygon motor 4. An imaging lens 5, a photodetector 6, a bending mirror 7 and a photosensitive drum 8 are sequentially arranged in the reflection direction of the polygon mirror 3.

The image forming lens 5 has a reflecting surface of the polygon mirror 3 and a surface of the photosensitive drum 8 in a conjugate relationship in the sub-scan section, has a function of tilt correction of the polygon mirror 3 and an fθ lens, and detects light. The device 6 is adapted to detect the timing of scanning the laser light.

The parallel light beam from the laser unit 1 is condensed in the sub-scanning direction by the cylindrical lens 2 and a line image is formed on the reflecting surface of the polygon mirror 3. The formed line image is deflected in the main scanning direction by the polygon mirror 3 rotated in the direction of arrow A by the polygon motor 4, passes through the imaging lens 5, is reflected by the bending mirror 7, and is scanned on the photosensitive drum 8. To do.

Here, if the optical path is set so that the laser light reflected by the folding mirror 7 passes through the rotation axis of the photosensitive drum 8, the laser light passing near the optical axis of the imaging lens 5 will be the photosensitive drum. The light is specularly reflected on the surface of No. 8 and the returned light is re-incident on the light emitting portion of the semiconductor laser of the laser unit 1 to reduce the laser output. The scanning area where the laser output is reduced causes defective exposure on the photosensitive drum 8. With respect to such return light, conventionally, the return light due to specular reflection is avoided by setting the optical path so that the laser light reflected by the folding mirror 7 deviates from the rotation axis of the photosensitive drum 8. .

By the way, in recent years, in order to reduce the cost of a laser beam printer, a plastic lens is often used in a laser scanning optical system. inside that,
In the collimator lens included in the laser unit 1, a SELFOC lens is often used instead of the conventional plano-convex glass lens.

FIG. 7 is an explanatory view of a laser unit 1 using a SELFOC lens. A SELFOC lens 12 is arranged as a cylindrical collimator lens so as to face the semiconductor laser 11. In addition, the semiconductor laser 11
A rear monitor 13 for detecting laser light emitted backward from the semiconductor laser 11 by a photodiode incorporated in the laser element is arranged at the rear of the semiconductor laser 11, and the semiconductor laser 11 and the rear monitor 13 are driven at a constant output. The laser control circuit 14 is connected.

The front emission light of the semiconductor laser 11 is incident on the SELFOC lens 12 as a divergent light flux, and is emitted as a light flux L substantially parallel to the optical axis due to the refractive index distribution action of the SELFOC lens 12. Since the SELFOC lens 12 has a cylindrical shape, the laser light is emitted perpendicularly on the emission surface Z, and the laser light internally reflected on the emission surface Z returns to the laser emitting portion of the semiconductor laser 11 through the same optical path.

In this case, the situation is different from the problem of specular reflection of the photosensitive drum 8, and the generation of return light is unavoidable. When this return light is re-incident on the laser emitting portion, the laser output becomes unstable, and the photosensitive drum 8 is caused. Similar to the case of the regular reflection, there is a problem that it causes defective exposure of the photosensitive drum 8.

[0010]

Conventionally, the following two methods are well known as methods for enhancing the characteristics of a semiconductor laser against returned light. One is a technique applied to a semiconductor laser such as a compact disc (CD). Since this semiconductor laser is used only for reproducing compact discs, the laser output is low at a rated value of 3 mW or less, and the semiconductor laser is structurally modulated at a relaxation oscillation frequency of about 1 GHz and is called a self-excited oscillation type laser. There is.

When operated at such a high frequency, the spectrum of the semiconductor laser becomes multimode (longitudinal multimode), the coherence is remarkably reduced, and the semiconductor laser cannot be coupled with the returning light. However, since the self-excited oscillation type laser cannot structurally increase the output, it cannot be used as a light source of a laser scanning optical system requiring an optical output of about 5 mW due to insufficient output.

Another method of strengthening the return light is known as a technique applied to a semiconductor laser such as a recordable optical disk device. Since an output of at least 20 mW or more is required for recording on an optical disc, a self-excited oscillation structure cannot be used, but at the time of reproduction, it is necessary to strengthen against return light like a compact disc.

For this reason, a high frequency of about several hundred MHz is superimposed on the laser drive current by an external oscillator so that the spectrum of the semiconductor laser becomes pseudo multimode. In this method, an external oscillator is newly required for high frequency modulation of the semiconductor laser, and the cost of the laser scanning optical system increases due to an increase in the number of parts.

Further, in the case of using a multi-beam laser, in the conventional method of driving a semiconductor laser at a high frequency, a beat signal is generated due to a subtle difference in the high frequency oscillation frequency of each semiconductor laser, and the laser output becomes unstable. There is a problem.

An object of the present invention is to solve the above problems,
It is an object of the present invention to provide a semiconductor laser driving device such as a laser printer which stabilizes the laser output by an economical method against return light generated by a SELFOC lens or the like.

[0016]

A semiconductor laser driving device according to a first aspect of the present invention for achieving the above object is a semiconductor laser that emits light in a single mode, and a sensor that receives a part of light emitted from the semiconductor laser. And a laser control circuit for controlling the semiconductor laser, wherein the semiconductor laser is controlled by a predetermined light emission output such that the output fluctuation is 10% or less even when combined with the return light from the external optical system. And

A semiconductor laser driving device according to a second aspect of the present invention includes a semiconductor laser that emits light in a single mode, a sensor that receives a part of light emitted from the semiconductor laser, and a laser control circuit that controls the semiconductor laser. Then
The semiconductor laser driving device and the optical device using the semiconductor laser as a light source unit are provided with a light quantity adjusting means, and the semiconductor laser has a predetermined light emission in which the output fluctuation is 10% or less even when combined with the return light from the external optical system. It is characterized by being controlled by output.

[0018]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in detail with reference to the embodiments shown in FIGS. FIG. 1 shows a configuration diagram of a first embodiment relating to a laser unit. In front of a semiconductor laser 21, a cylindrical SELFOC lens 22 as a collimator lens, a variable ND filter 23, and a corner cube 24 are sequentially arranged, and variable. ND filter 23,
The corner cube 24 constitutes a return light variable mechanism 25 that changes the return light.

A rear monitor 26 for detecting the amount of laser light emitted backward from the semiconductor laser 21 is arranged behind the semiconductor laser 21 by a photodiode incorporated in the laser element, and these semiconductor laser 21 and rear monitor 26 are arranged in the rear monitor 26. A laser control circuit 27 for driving the semiconductor laser 21 at a constant output is connected.

The forward emission light of the semiconductor laser 21 enters the SELFOC lens 22 as a divergent light flux, and is emitted as a light flux L substantially parallel to the optical axis due to the refractive index distribution action of the SELFOC lens 22. Since the SELFOC lens 22 is a cylindrical type, the laser light is emitted vertically on the emission surface Z,
The laser light internally reflected by the emission surface Z returns to the laser emitting portion of the semiconductor laser 21 through the same optical path. When the returning light is re-incident on the laser emitting portion, the P-I characteristic (optical output-current characteristic) of the semiconductor laser 21 changes as shown by the broken line in FIG.

In FIG. 2, the horizontal axis represents the laser drive current and the vertical axis represents the laser output. Also, the solid line X
Indicates qualitatively the state when the returning light is small, and the dotted line Y indicates the state when the returning light is large. When the return light is re-incident on the laser emitting portion, stimulated emission is generated inside the laser resonator in proportion to the light amount of the return light, and contributes to the oscillation start current of the semiconductor laser 21. The current becomes smaller. Further, the fact that the returning light is re-incident on the laser emitting portion means that the effective reflectance of the laser light emitting end face increases. That is,
At this laser light emitting end face, the amount of light that is regarded as internal reflection is increased by the amount of return light with respect to the transmitted light that is the output, so the effective reflectance increases and the laser emission efficiency decreases.

Therefore, when the solid line X and the dotted line Y are compared, the dotted line Y in the state of a large amount of returning light is inferior in laser emission efficiency to the solid line X in a state of a small amount of returning light, and the slope of the straight line becomes smaller, resulting in laser oscillation. The threshold current, which is the starting current, is Ib, which is smaller than the threshold current Ic on the dotted line X. Focusing on such a laser characteristic, the PI characteristic always crosses at a certain point even if the return light changes. This point is called fixed point power Po. If the semiconductor laser 21 is caused to emit light in the vicinity of the fixed point power Po, the laser output becomes substantially constant even if the return light changes, and the output power can be stabilized at 10% or less, which is allowed.

Next, how to obtain the fixed point power will be explained. When the return light is in the state of the solid line X in FIG. 2, when the laser control circuit 27 operates the semiconductor laser 21 at the constant current Ia, the laser output is obtained. Pa. When the transmittance of the variable ND filter 23 is increased and the returned light is increased to the state of the dotted line Y, the laser output drops to Pb. When the laser output decreases as the return light increases, the sign of the laser output difference (Pb-Pa) becomes negative, and it is necessary to reduce the operating current to Io to bring it closer to the fixed point power Po.

On the contrary, when the laser output increases as the return light increases, the sign of the laser output difference (Pb-Pa) becomes positive, and the operating current is raised to Io to increase the fixed point power Po.
Need to be closer to. Thus, focusing on the sign and absolute value of the laser output difference (Pb-Pa), the fixed point power
Po can be easily found and the semiconductor laser 21 can be controlled by the fixed point power.

In order to keep the output of the laser unit composed of the semiconductor laser 21 and the SELFOC lens 22 constant at the fixed point power even with environmental temperature changes, the output of the rear monitor 26 corresponding to the fixed point power becomes constant. Thus, the laser control circuit 27 is operated.

In this way, by detecting the fixed point power Po of the semiconductor laser 21 and controlling the laser with this predetermined output, the semiconductor laser 21 is always operated even if there is a returning light or the environmental temperature changes. It can be operated with a constant output and stabilized.

FIG. 3 is a block diagram in which the laser unit shown in FIG. 1 is applied to a laser scanning optical system. In the emitting direction of the laser unit 31, a light quantity adjusting means 32 such as a variable ND filter 23, a cylindrical lens 33 and a polygon. The mirrors 34 are arranged in sequence and the polygon mirror 34
Is rotated by a motor 35. Further, in the reflection direction of the polygon mirror 34, the imaging lens 36 and the photodetector 3
7, a bending mirror 38, and a photosensitive drum 39 are sequentially arranged, and the imaging lens 36 makes the reflecting surface of the polygon mirror 34 and the surface of the photosensitive drum 39 a conjugate relationship in the sub-scanning cross section to correct the tilt of the polygon mirror 34. And the function of the fθ lens, the photodetector 37 detects the timing of scanning the laser light.

The parallel luminous flux from the laser unit 31 is
It is condensed in the sub-scanning direction by the cylindrical lens 33, and a line image is formed on the reflecting surface of the polygon mirror 34. The formed line image is deflected by the motor 35 in the main scanning direction by the polygon mirror 34 rotating in the direction of arrow A, passes through the imaging lens 36 and the photodetector 37, is reflected by the bending mirror 38, and is reflected by the photosensitive drum. Scan over 39.

The laser light emitted from the laser unit 31 is controlled at a fixed point power by the laser control circuit 27 shown in FIG. 1 and maintains a stable output with respect to the returning light. It will exceed the output required on 39. Therefore, when the laser light controlled by the fixed point power is adjusted to an appropriate output on the photosensitive drum 39 by the light amount adjusting means 32, a stable and desired output for the return light can be supplied to the photosensitive drum 39. .

The light quantity adjusting means 32 may be any means for attenuating the light quantity in the optical path from the laser unit 31 to the photosensitive drum 39. For example, if a polarization filter is used which exhibits the same effect as an ND filter, a variable ND can be used.
The effect is the same as that of the filter 23, and the degree of freedom in adjusting the light amount increases.

If the amount of light to be adjusted is substantially determined, it is not necessary to add new parts by, for example, lowering the transmittance of other optical elements in advance. Normally, a non-reflection coating is applied to the optical components to secure a transmissivity of 95% or more. However, in the present embodiment, the optical amount is adjusted by arranging the optical components having a transmissivity of approximately 90% or less. Can be realized.

FIG. 4 is a block diagram of the second embodiment of the eraser unit, which has almost the same configuration as that of the first embodiment, except that a half-space is provided between the SELFOC lens 22 and the return light variable mechanism 25. Front mirrors 4 such as photodiodes are provided with mirrors 41 that are arranged to divide the front light flux into intensity divisions.
It is designed to detect and monitor in 2. The output of the front monitor 42 is connected to the laser control circuit 27, and the rear monitor 26 shown in FIG. 1 is not used.

In the first embodiment, when the returning light is re-incident on the laser emitting portion, the effective reflectance of the front emission end face changes, but the effective reflectance of the rear emission end face does not change, and the laser emission The ratio of the amount of emitted light in the front and the rear of the part changes depending on the amount of returned light. Therefore, even if the amount of light behind the laser emitting portion is monitored, it cannot be said that the amount of light emitted ahead is accurately detected. However, as shown in FIG.
With this, it is possible to realize more accurate control of the laser output as compared with the rear monitor 26 shown in FIG.

Further, in the laser scanning optical system shown in FIG. 3, if the laser unit using the front monitor 42 is arranged as the laser unit 31, the front emission light of the semiconductor laser 21 can be directly monitored, so that the laser scanning is performed. The laser output of the optical system can be further stabilized.

The light emitting point of the semiconductor laser 21 is adjusted so as to be at the focal position of the SELFOC lens 22, but as a simple method for examining the fixed point power, the SELFOC lens 22 is positioned at the optical axis direction. It is good to shift it slightly. If out of focus, SELFOC lens 2
The emitted light flux of No. 2 becomes a divergent light flux or a convergent light flux, and the amount of light returned from the exit surface Z of the SELFOC lens 22 suddenly changes.

When the constant current operation is performed and the returning light is decreased due to the defocus, and the laser output is increased, the operation current is decreased to approach the fixed point power. When the laser output decreases, the operating current is increased to bring it closer to the fixed point power. In this method, the return light variable mechanism 25 for changing the return light is not required, and the position adjustment of the SELFOC lens 22 can be performed simultaneously.
In particular, this method is effective when the return light from the exit surface Z of the SELFOC lens 22 is sufficiently large.

FIG. 5 is a block diagram of a third embodiment of a laser unit using a two-beam multi-beam laser 51 as a semiconductor laser.
1, a selfoc lens 52, which is a cylindrical collimator lens, and a half mirror 53 are sequentially arranged. In the reflection direction of the half mirror 53, a condenser lens 54 and two parallel-divided photodiodes 55a and 55b are arranged. It is arranged. In addition, the two-division photodiode 55a,
An array laser control circuit 56 is connected to the 55b and the multi-beam laser 51. The multi-beam laser 51 is a two-beam multi-beam laser in which two semiconductor lasers are monolithically formed on the same substrate, and can be individually operated by providing independent electrodes.

2 emitted from the multi-beam laser 51
The two laser beams are collimated by the SELFOC lens 52 into two parallel light fluxes L, reflected by the half mirror 53, and condensed by the condenser lens 54 on the light receiving surfaces 55a and 55b of the two-divided photodiodes. The output signals of the two-divided photodiodes 55a and 55b are output to the array laser control circuit 56 as monitor outputs of the two laser beams, and controlled by the respective APC circuits.

Since the two laser lights do not interfere with each other, the respective return lights are re-incident on the respective laser emitting portions of the multi-beam laser 51, the respective P-I characteristics are changed, and the fixed point power is changed. Can be found. Therefore, the fixed point powers of the two laser beams can be obtained by the same method as in the first embodiment.

Further, since the two semiconductor lasers are formed on the same substrate, they are manufactured in the same process and it is considered that the variations are extremely small. The characteristics of the two semiconductor lasers are well matched and the fixed point power is almost the same. It can be regarded as the same value.

As the laser unit 31 shown in FIG.
When the laser unit shown in FIG. 5 is arranged, two spots are formed on the photosensitive drum 39, and two lines can be processed by one deflection scan. For example, in a laser printer, the speed can be doubled. it can. Since the two semiconductor lasers are controlled with almost the same fixed point power, they can be adjusted to a desired output on the photosensitive drum 39 via the common light amount adjusting means 32.

By thus controlling the multi-beam laser with the fixed point power and using it together with the light quantity adjusting means, the output required by the optical device can be stabilized irrespective of the presence of the returning light. Moreover, since the semiconductor laser is not driven at a high frequency, new parts such as a high-frequency oscillator are not required, and the problem of the high frequency of two beams causing a beat, which is a problem peculiar to a multi-beam laser, is eliminated, and an economically excellent laser Control can be realized.

[0043]

As described above, the semiconductor laser driving device according to the first aspect of the invention uses the change in the P-I characteristic of the semiconductor laser with respect to the returning light, and controls the semiconductor laser with a predetermined light emission output which is not affected by the returning light. Therefore, even if the SELFOC lens is used, the laser output can be stabilized without requiring any special means.

The semiconductor laser driving device according to the second aspect of the present invention can provide a desired laser output by being combined with a light quantity adjusting means when applied to an optical device, so that all laser optical systems that generate return light can be provided. Achieved stable control of semiconductor laser at low cost,
The performance and reliability of the optical device can be improved.

Further, by applying the laser control circuit to the multi-beam laser, there is no need for a high-frequency generating means, and there is no problem inherent in the multi-beam laser such as generation of a beat signal due to high frequency between lasers. It is possible to realize highly reliable laser control at low cost.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a first embodiment.

FIG. 2 is a PI characteristic diagram of a semiconductor laser with respect to return light.

FIG. 3 is an explanatory diagram when a laser driving device is applied to an optical device.

FIG. 4 is a configuration diagram of a second embodiment.

FIG. 5 is a configuration diagram of a third embodiment.

FIG. 6 is a configuration diagram of a conventional laser scanning optical system.

FIG. 7 is an explanatory diagram of a laser driving device.

[Explanation of symbols]

 21 semiconductor laser 22, 52 selfoc lens 23 variable ND filter 24 corner cube 25 return light variable mechanism 26 rear monitor 27 laser control circuit 31 laser unit 33 light quantity adjusting means 34 polygon mirror 39 photosensitive drum 42 front monitor 51 multi-beam laser 55a, 55b 2-division photodiode 56 array laser control circuit

Claims (4)

[Claims] 1. A semiconductor laser that emits light in a single mode, a sensor that receives a part of the emitted light of the semiconductor laser, and a laser control circuit that controls the semiconductor laser, wherein the semiconductor laser is an external optical system. A semiconductor laser driving device characterized in that it is controlled by a predetermined light emission output such that the output fluctuation is 10% or less even when combined with the return light from. 2. The laser driving device according to claim 1, wherein the semiconductor laser is a multi-beam laser in which a plurality of lasers are formed on the same substrate and which can be independently driven. 3. A semiconductor laser driving device comprising: a semiconductor laser that emits light in a single mode; a sensor that receives a part of light emitted from the semiconductor laser; and a laser control circuit that controls the semiconductor laser. An optical device using a semiconductor laser as a light source section is provided with a light quantity adjusting means, and the semiconductor laser is controlled by a predetermined light emission output such that an output fluctuation is 10% or less even when combined with a return light from an external optical system. Semiconductor laser drive device. 4. The laser driving device according to claim 3, wherein the semiconductor laser is a multi-beam laser in which a plurality of lasers are formed on the same substrate and which can be independently driven.