JPS62243469A - Semiconductor laser driver - Google Patents

Abstract

PURPOSE:To obtain a halftone image without contrast distortion by controlling a control circuit of a drive current provided separately with a resistance control circuit depending on the optical output of a semiconductor laser to stabilize the optical output when the characteristic of the laser is changed. CONSTITUTION:A prescribed optical output P1 is obtained by a few laser drive current IA at the low temperature of the semiconductor laser 1 and the output is reduced more than the output P1 at the current IA at high temperature. The reduction in the optical output is detected by a quantity-of-light monitor 13 and a quantity-of-light signal in reducing the internal impedance in supplied from a quantity-of-light detection circuit 14 to a voltage control element 12. The applied voltage to a series circuit comprising the laser 1 and a resistance control circuit 8 is increased by the decrease in the internal impedance of the element 12, the current of the laser 1 is increased creased and the same output P1 as that at low temperature is obtained. Thus, the stable optical output is obtained independently of temperature changes.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a semiconductor laser drive device used in a laser printer or the like.

[Conventional technology]

In recent years, laser printers have been used as non-impact printers.

In a typical laser printer, as shown in FIG. 4, a laser beam R is generated from a semiconductor laser 1, made parallel by a collimator lens 2, and then irradiated onto a rotating polygon mirror 3.

The laser beam R reflected by the rotating polygon mirror 3 is irradiated onto the surface of a drum-shaped photoreceptor 5 via an fθ lens 4 that corrects the scanning speed, and scans the surface of the photoreceptor 5 by rotation of the rotating polygon mirror 3. .

At this time, the laser beam R is modulated by an image signal synchronized with a signal from a photosensor 6 placed near the photoreceptor 5, so a latent image corresponding to the image signal is formed on the surface of the photoreceptor 5. be done. Therefore, by reversing and developing this latent image, a predetermined image can be obtained.

Usually, the image signal is a binary signal, and the image is either white or black.
It is expressed in stages of brightness, but to express intermediate tones,
It is necessary to modulate the semiconductor laser 1 with a multilevel signal. For this reason, as shown in Japanese Unexamined Patent Publication No. 58-91474, it is known that multiple weights selectively driven by switching elements use resistors to express intermediate tones. ing.

FIG. 5 shows a conventional semiconductor laser driving device described in the above publication.

Semiconductor laser 1 and resistance control circuit 8 are connected in series to power supply 7 . In the resistance control circuit 8, a resistor 9
8 to 9d and switching elements 10a to 10d are connected in series, and these series circuits are connected in parallel to each other. The switching elements 10a to 10c receive an image signal altamta having 3-bit gradation data.
m are supplied respectively, and these image signals a+
+at+as is supplied to the switching element 10d via the OR circuit 11.

Image signal al+8! Switching element 1 according to +83
0a to 10c are conductive, but since the resistors 98 to 9C are each weighted, eight gradation currents flow through the semiconductor laser l, making it possible to express halftones. Note that the OR circuit 11 receives the image information signal al+affi+
82 exists, switching element 1
The resistor 9d is provided to cause a dark value current to flow through the semiconductor laser 1 by making the resistor 0d conductive.

[Problem that the invention seeks to solve]

However, in the drive device described in the above publication, the semiconductor laser 1
Since the current flowing through the semiconductor laser 1 is uniquely determined by the voltage of the power source 7 and the resistance values of the resistors 98 to 9d, there is a problem in that the optical output characteristics of the semiconductor laser 1 change significantly due to temperature changes.

For example, assume that predetermined switching elements 10a to 10c are made conductive so that a current of 5 A is passed through the semiconductor laser 1, and an optical output of 3+mW is obtained. However, as the temperature rises, the light emitting efficiency of the semiconductor laser 1 decreases, so the optical output decreases to, for example, 2 mW. Therefore, there was a problem in that the amount of light irradiated onto the photoreceptor 5 changed greatly, which adversely affected the image quality. In particular, in the case of multilevel modulation, fluctuations in optical output cause gradation distortion and degrade image quality.

Further, in order to stabilize the optical output, it is known from Japanese Patent Application Laid-Open No. 187961/1987 that the optical semiconductor laser 1 is cooled by an electronic cooling element such as a Peltier element. However, in this case, there were problems in that the device became complicated and the stability level was low despite the high cost.

The present invention was devised in order to solve the above-mentioned problems, and an object of the present invention is to provide a driving device that can obtain a stable amount of light even when a semiconductor laser is multilevel modulated.

[Means for solving problems]

The present invention provides a semiconductor laser drive device that connects a resistance control circuit that can be switched to a predetermined resistance value according to an image signal to a semiconductor laser and multi-value modulates the laser drive current of the semiconductor laser. Separately, the present invention is characterized in that a circuit for detecting the optical output of the semiconductor laser and controlling the driving current so that the optical output is constant is provided in the laser driving current path.

〔Example〕

DETAILED DESCRIPTION OF THE INVENTION Hereinafter, features of the present invention will be specifically described based on examples with reference to the drawings.

FIG. 1 shows a circuit diagram of a semiconductor laser driving device according to an embodiment of the present invention, and parts corresponding to those of the conventional example shown in FIG. 5 are given the same reference numerals and redundant explanation will be omitted.

In the embodiment of the present invention, the emitters of a plurality of switching elements 10a to 10d provided in a resistance control circuit 8 are connected in common, and a voltage control element 12 such as a transistor
grounded through. Further, a light amount monitor 13 such as a photodiode is provided near the light emitting portion of the semiconductor laser 1, and the output of the light amount monitor 13 is supplied to the voltage control element 12 via a light amount detection circuit 14. The light amount detection circuit 14 and the voltage control element 12 constitute a light amount control circuit 15.

Next, the operation of the semiconductor laser driving device according to the embodiment of the present invention will be explained.

3-bit image data al+8! :a3 is supplied to the base of each of the switching elements 10a to 10c of the resistance control circuit 8, and predetermined switching elements 10a to 10c are rendered conductive according to the content of the image data al+a mushroom L'al. Therefore, from the power supply 7, the semiconductor laser 1. Resistor 98~9d
, switching elements 16a to 10d and voltage control element 1
Current flows through 2. Each switching element 10a
Since the resistors 98 to 9C connected in series with =10c are each weighted, eight gray levels of current can be passed through the semiconductor laser 1 according to the image data al, az+a3.

Next, the operation of stabilizing the optical output will be explained.

First, a constant image signal for obtaining a predetermined optical output, for example, an image signal in which only al and a are at a high level, is sent to each of the switching elements 10a to 10c, and the semiconductor laser 1 is turned on. The amount of light at this time is detected by a light amount monitor 13, and the voltage control element 12 is controlled by the output of the light amount detection circuit 14, thereby stabilizing the amount of light from the semiconductor laser 1.

When the semiconductor laser l is at a low temperature, it has an optical output characteristic as shown by the curve wAA in FIG. 2, so that a predetermined optical output P+ can be obtained with a relatively small laser drive current IA. However, at high temperatures, the semiconductor laser l has an optical output characteristic as shown by curve B, so the laser drive current IA remains unchanged.
In this case, the optical output becomes P2, resulting in a significant decrease in the optical output.

This decrease in light output is detected by the light intensity monitor 13, and a light intensity correction signal is supplied from the light intensity detection circuit 14 to the voltage control element 12 in a direction that reduces its internal impedance. The voltage of the power supply 7 is the voltage of the semiconductor laser 1. Since the voltage is divided by the internal impedance of each of the resistance control circuit 8 and the voltage control element 12, when the internal impedance of the voltage control element 12 decreases, the voltage applied across the series circuit of the semiconductor laser l and the resistance control circuit 8 decreases. To increase. Therefore, the current flowing through the semiconductor laser I increases to I3 as shown in FIG. 2, and the same optical output P1 as at low temperature can be obtained. That is, stable optical output can be obtained regardless of temperature changes.

At this time, as shown in curves A and B in Figure 2, the slopes of the characteristics are almost constant and parallel at low and high temperatures, so if the light intensity is corrected at only one point, the optical output for the laser drive current is The adhesiveness is maintained.

Further, the dark value current of the semiconductor laser 1 also changes depending on the temperature, but since the current flowing through the resistor 9d that controls this dark value current is also controlled by the voltage control element 12, the semiconductor laser 1
It can be set to a value suitable for the operation.

However, if the above-mentioned light amount correction is performed while the semiconductor laser 1 is being modulated by the image signal to be printed, the change in light amount due to modulation will be suppressed, so the amount to be corrected is detected in advance and stored. The amount of light is corrected based on this.

This correction may be performed each time a print is made or during a non-image portion of one scan of the laser beam.

In the above embodiment, a transistor was used as the voltage control element 12 and connected in series to the ground side of the resistance control circuit 8, but the present invention is not limited to this, and other active elements such as an FET may also be used. can. Further, the voltage control element 12 is not limited to the arrangement in the embodiment, and may be provided on the anode side of the semiconductor laser 1, for example. That is,
The voltage control element 12 may be of any type as long as it can control the voltage applied from the anode of the semiconductor laser l to the resistors 98 to 9d and the switching elements 10a to 10d.

FIG. 3 shows another embodiment according to the invention in which a semiconductor laser has a built-in photodiode.

Note that parts corresponding to those in the embodiment shown in FIG. 1 are given the same reference numerals, and redundant explanation will be omitted.

In the figure, 16 indicates a drive circuit, which corresponds to the resistance control circuit 8 and OR circuit 11 in FIG. An image signal from an external image processing device or the like is supplied to the drive circuit 16, and the semiconductor laser l is modulated. The semiconductor laser l has a built-in photodiode and a photodiode 13a.
The current flowing through the photodiode 13a also changes depending on the optical output of the photodiode 13a. This change in current is amplified by an amplifier 17, then converted into a voltage, and compared with a predetermined reference voltage in a comparison circuit 18. The comparison circuit 18 outputs a signal of 1 or 0 depending on the level of the input voltage, and this signal is supplied to the up/down counter 19 as a down signal or an up signal. The up/down counter 19 is supplied with a signal obtained by shaping a signal from a photosensor 6 (see FIG. 4) provided close to the photoreceptor 5 by a waveform shaping circuit 22 . At the timing of this signal, the comparator circuit 1
When the output of 8 is 0, the up/down counter 19 becomes an up counter and the counter 19 is incremented by +1, and when the output is 1, it becomes a down counter and the counter 19 becomes -1.
1 will be given.

The digital signal of this counter 19 is converted into a voltage by a D/A converter 20, and after being amplified by an amplifier circuit 21,
The signal is supplied to the laser driving device 16 as a light amount correction signal.

Therefore, the semiconductor laser l is not only modulated by the image signal, but also changes depending on the current flowing through the photodiode 13a.

There is a proportional relationship between the optical output of the semiconductor laser 1 and the current flowing through the photodiode 13a built into the semiconductor laser 1, and the temperature coefficient is sufficiently small compared to the stability of the light amount.
By amplifying the current flowing through the photodiode 13a and feeding it back to the laser driving device 16, the output of the semiconductor laser 1 can be controlled to be constant even when the temperature changes.

〔Effect of the invention〕

As described above, according to the present invention, in a semiconductor laser driving device that multi-value modulates the current flowing through a semiconductor laser, a circuit for controlling the drive current is provided separately from the resistance control circuit for multi-value modulation, The control circuit is controlled according to the optical output of the semiconductor laser. This makes it possible to stabilize the optical output even if the characteristics of the semiconductor laser change.

Therefore, the amount of light of the laser beam obtained from the semiconductor laser multilevel modulated by the image signal is stabilized, and when the semiconductor laser is used in a laser printer, it is possible to obtain a halftone image without distortion of density.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram showing a semiconductor laser driving device according to an embodiment of the present invention, FIG. 2 is a graph showing optical output characteristics of the semiconductor laser, FIG. 3 is a block diagram of another embodiment of the present invention, and FIG. The figure is a schematic perspective view showing the optical system of a general laser printer, and FIG. 5 is a circuit diagram showing a conventional semiconductor laser driving device. 1: Semiconductor laser 2: Collimator lens 3: Rotating polygon mirror 4: fθ lens 5: Photoconductor 6: Photo sensor 7: Power supply
8: Resistance control circuit 98-9d: Resistor 10a-10d Niswitching element 11: OR circuit 12: Voltage control element 13: Light amount monitor 13a: Photodiode 14: Light amount detection circuit 15: Light amount control circuit 16: Laser drive circuit
17.21 F amplifier 18: Comparator 19 Near-tub down counter 20 F D/A converter 22: Waveform shaping circuit R:
Laser beam patent applicant Fuji Xerox Co., Ltd. agent
Masu Tebori (and 2 others) Figure 1 Figure 2

Claims (1)

[Claims] 1. In a semiconductor laser drive device that connects a resistance control circuit that can be switched to a predetermined resistance value according to an image signal to a semiconductor laser and multivalue modulates the laser drive current of the semiconductor laser, what is the resistance control circuit? Separately, a semiconductor laser driving device characterized in that a circuit for detecting the optical output of the semiconductor laser and controlling the driving current so that the optical output is constant is provided in the laser driving current path.