JPH02151173A - Semiconductor laser control circuit - Google Patents

Abstract

PURPOSE:To attain the gradation control of multi-stage with excellent linearity without the occurrence of deficient luminous quantity or excess luminous quantity by adopting constant current source using a comparator, an up-down counter and a D/A converter so as to decide a drive current for a prescribed minimum optical intensity and a maximum overlap drive current for maximum luminous intensity. CONSTITUTION:A driving current (current stimulating a semiconductor laser 1 at a prescribed minimum luminous intensity Lmin) of a 1st constant current source 2 is controlled and decided by using a 1st feedback control loop 5 and an overlap driving current (current stimulating the semiconductor laser 1 at a prescribed maximum luminous intensity Lmax) of a 2nd constant current source 8 is controlled and decided by using a 2nd feedback control loop 6. The control operation by the 1st and 2nd feedback control loops 5, 6 as above is implemented between pages or between main scanning lines to stop the count of 1st and 2nd up-down counters 8, 12 during the write period thereby preserving the count at that time.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a semiconductor laser control circuit in a laser printer or the like capable of recording with multi-gradation by changing the intensity of laser light from a semiconductor laser.

2. Description of the Related Art In recent years, digital copying machines that read original images using solid-state scanning elements such as CCD line sensors and output the read signals to a laser printer or the like to make copies have become popular. In such a device, target document images include not only text and graphic images such as texts but also images with gradation such as photographs and pictures. When outputting the read image data of an image with such gradation using a laser printer, the following methods have been implemented or proposed in the past in order to reproduce the shading of the image (gradation reproduction). There is.

Firstly, there is a method based on the dither method that uses a plurality of pixels to produce shading in a condensed manner.

Second, as shown in Japanese Patent Application Laid-Open No. 62-181575, there is a method of varying the writing pulse width of a light source (semiconductor laser) to create shading.

Thirdly, there is a method proposed by the applicant of the present invention in which the light intensity of the write pulse from a light source (semiconductor laser) is varied to produce 'a-lightness'. In other words, there are multiple levels of light intensity modulation signals for the semiconductor laser, and in order to stably output light at each light intensity level, multiple constant current sources are switched via a modulation switching element. It is connected to a laser and changes the light intensity output of the semiconductor laser according to the connected constant current source.

More specifically, one of the plurality of constant current sources is set near the bias current of the semiconductor laser, and the rest are set so that their current values have a 2°:1 relationship. There is.

Problems to be Solved by the Invention The first method is in principle unsuitable for fine images, and is not suitable for high-quality, high-quality laser printers and the like.

In the case of the second method, it becomes more difficult to form a reference triangular wave for varying the writing pulse width to produce shading as the image clock becomes faster, and is not suitable for high speeds.

In the case of the third method, when the variable level of light intensity is set to 2'', as many as (n+1) constant current sources are required, which increases the size of one circuit.In addition, the current flowing through each of these constant current sources In order to set the value, it is necessary to adjust the current of each constant current source (for example, adjust the volume or set the power by monitoring and feedback of the semiconductor laser light).

Means for Solving the Problems In the invention as set forth in claim 1, in addition to providing a semiconductor laser and a monitoring photoelectric conversion element for monitoring the light intensity of the light emitted from the semiconductor laser, first, from this monitoring photoelectric conversion element, a first comparator that compares the monitor output of the monitor with a first reference value corresponding to a predetermined minimum light intensity level; and a comparison output from the first comparator when the drive current is controlled by the first timing control means. A first up/down counter that performs a counting operation and holds a counted value when a match occurs, and a first D that converts the counted value of this first up/down counter into an analog signal.
/A converter, and a second constant current source that causes a drive current of a magnitude corresponding to the analog signal from the first D/A converter to flow through the semiconductor laser. Further, a second comparator that compares a monitor output from the monitor photoelectric conversion element with a second reference value corresponding to a predetermined maximum light intensity level;
When controlling the superimposed current by the timing control means, this second
a second up/down counter that counts the comparison output from the comparator and holds the counted value when a match occurs;
a second D/A converter that converts the count value of the up/down counter into an analog signal;

The analog signal from this second D/A converter is input to the reference voltage signal terminal that regulates the maximum output, and the third D/A converter outputs an analog signal of a magnitude corresponding to the digital image density signal for this maximum output. an A converter, and a second constant current source that is connected in parallel to the first constant current source and flows a superimposed drive current to the semiconductor laser according to an analog signal from the third D/A converter. establish.

Further, in the invention according to claim 2, the first constant current source and the second constant current source in the invention according to claim 1 are one constant current source, and the analog output from the first D/A converter is An adder is provided for inputting the signal and the analog signal from the third D/A converter to this constant current source.

In the invention described in claim 1, first, the monitor output of the semiconductor laser monitored by the monitoring photoelectric conversion element is compared with the first reference value corresponding to the minimum light intensity level by the first comparator. The first up/down counter performs an up or down counting operation in accordance with this comparison output, and the counted value is converted into an analog signal by the first D/A converter. A drive current having a magnitude corresponding to this analog signal is caused to flow through the semiconductor laser by the first constant current source, and is controlled so that the comparison outputs of the first comparator match. When the comparison outputs of the first comparator match, the counting operation of the first up-down counter is stopped, the count value is held, and the drive current value of the first constant current source is also adjusted until the light intensity of the semiconductor laser reaches a predetermined value. The state is determined to have the minimum light intensity. In the semiconductor lighting state with the drive current determined by the first constant current source, the monitor output of the semiconductor laser monitored by the monitoring photoelectric conversion element is then measured by the second comparator to determine the maximum light intensity. It is compared with a second reference value corresponding to the level. The second up/down counter performs up or down counting operation according to this comparison output, and the counted value is determined by the second D/D/
The A converter converts it into an analog signal. This second D/
The analog signal from the A converter is input to the reference voltage signal terminal of the third D/A converter, and is used to regulate the maximum output of this third D/A converter. Here, at the stage where the outputs of the second comparator do not match, the maximum output of the third D/A converter has not been determined either.

In addition, the digital image density signal is set to, for example, the maximum density level, and based on the analog signal from the third D/A converter, the drive current applied to the semiconductor laser by the second constant current source also increases, The comparison outputs of the two comparators are controlled so that they match. When the comparison outputs of the second comparator match.

The counting operation of the second up/down counter is stopped, the counted value is held, and the counted value is passed through the second D/A converter to the third D/A converter.
The maximum output of the /A converter is determined.

That is, the superimposed drive current value by the second constant current source to be superimposed on the already determined drive current value by the first constant current source is also determined in such a state that the light intensity of the semiconductor laser reaches a predetermined maximum light intensity. Ru. After such control and determination, the third D
When a digital image density signal is input to the /A converter, an analog signal having a magnitude corresponding to the digital image density signal relative to the maximum output is outputted to the second constant current source, and the superimposed drive current is variably controlled. In other words, if the digital image density signal is an n-bit signal, by simply using the 1.2 constant current source, the light intensity can be adjusted in 2'' steps from the minimum light intensity level to the minimum light intensity level depending on the data of the digital image density signal. The level can be outputted arbitrarily.At this time, since the drive current for the predetermined minimum light intensity and maximum light intensity is controlled and determined, no matter which light intensity level there is, there will be no light intensity shortage or overshoot. Furthermore, since the minimum light intensity level - the minimum light intensity level is divided by n-bit data, the light intensity of each stepwise level also has good linearity.

Further, in the invention as claimed in claim 2, since the adder is added, it is possible to use only one constant current source as the first and second constant current sources.

Embodiment An embodiment of the invention set forth in claim 1 will be described with reference to FIGS. 1 and 2. First, a semiconductor laser 1 for optical writing in a laser printer or the like is provided and connected to a power source. This semiconductor laser 1 has, for example, a driving current-optical output characteristic as shown in FIG. 2, and II!
The light intensity changes depending on the dynamic current. That is, as is well known, it is divided into two linear regions, a spontaneous emission region and a laser oscillation region, with the bias current air □ as a boundary.

A parallel circuit of a first constant current source 2 and a second constant current source 3 is connected to the semiconductor laser 1 having such characteristics. Further, a monitoring photoelectric conversion element, for example, a photodiode 4, is provided to receive a portion of the laser light emitted from the semiconductor laser 1 and monitor the intensity of the light.

A first feedback control loop 5 is provided between the photodiode 4 and the first constant current source 2,
A second feedback control loop 6 is provided between the photodiode 4 and the second constant current source 3.

The first feedback control loop 5 includes a first comparator 7
, a first up/down counter 8 , and a first D/A converter 9 . First, the photodiode output (monitor output) is compared with a reference voltage VaxF+e+u+ as a reference value in the first comparator 7. Here, the reference voltage ■. F+1al is a voltage value that is equal to the output of the photodiode 7 when the predetermined minimum light intensity L min in the laser oscillation region is in the drive current-light intensity output characteristic of the semiconductor laser 1 shown in FIG. . First comparator 7
The comparison result is input to the first up/down counter 8 as an H level or L level digital binary signal. The first up/down counter 8 counts the output from the first comparator 7 (up or down) according to the timing of the clock signal from the clock generator 10.

The count value output from this counter 8 is converted into an analog signal by the first D/A converter 9, and then fed back to the first constant current source 2, and the first constant current g2 is generated according to the analog signal. A driving current is applied to the semiconductor laser 1. In such feedback control, the first comparator 7
When the polarity of the output from is reversed (and therefore matched)
The counting operation by the first up/down counter 8 is stopped and the counted value is held. As a result, the first D
The magnitude of the analog signal from the /A converter 9 is also fixed,
The driving current of the first constant "f3.flow"rX2 is determined.

The drive current at this time is a current for causing the semiconductor laser 1 to emit light at a predetermined minimum light intensity. This is the operation of controlling and determining the drive current of the first constant current source 2 by the first feedback control loop 5.

The second feedback control loop 6 connects the second comparator 1
1, a second up/down counter 12, a second D/A converter 13, and a third D/A converter 14. first,
A constant drive current determined by a first constant current source 2 is applied to the semiconductor laser 1 .

In this state, the photodiode output (monitor output) is
This time, the second comparator 11 uses the base $ as the reference value.
It is compared with the voltage Vi+iF+eax+. Here, the reference voltage V RRF +Il@xl corresponds to a predetermined maximum light intensity L68. in the laser oscillation region in the drive current-light intensity output characteristic of the semiconductor laser 1 shown in FIG. This voltage value is equal to the output of the photodiode 7 when .

The comparison result by the second comparator 11 is also input to the second up/down counter 12 as an H level or L level digital binary signal. This second up/down counter 12 also counts the output from the second comparator 11 according to the timing of the clock signal from the clock generator 10. The count value output from this counter 12 is converted into an analog signal by the second D/A converter 13, and then converted into an analog signal by the third D/A converter 13.
Reference voltage signal terminal (reference terminal) V of converter 14
Input to RIIF. This third D/A converter 14 has an input terminal to which an n-bit digital image density signal for optical writing is input.
The analog signal output from the A converter 14 is digital data that is the maximum. Therefore, the maximum output of the third D/A converter 14 is the maximum output of the second D/A converter 1 at that point.
3, the value corresponds to the analog signal input manually. At this point, if the comparison output from the second comparator 11 is no longer a coincidence output, control is continued and the second constant current #3 is passed through the second and third D/A converters 13 and 14. The drive current applied to the semiconductor laser 1 is controlled to increase. At this time, since the drive current for the minimum light intensity variation is applied by the first constant current source 2, the drive current by the second constant current source 3 becomes a superimposed drive current. Therefore, when the light intensity of the semiconductor laser 1 reaches the predetermined maximum light intensity level LIIIax and the polarity of the output from the second comparator 11 is reversed (therefore, when -. The counting operation by the down counter 12 is stopped and the counted value is held. Thereby, the magnitude of the analog signal from the second D/A converter 13 is also fixed, and the maximum output of the third D/A converter 14 is determined.

The maximum output at this time is the maximum data of the digital image density signal, and the semiconductor laser 1 is set to a predetermined maximum light intensity Lma.
This is to regulate the superimposed drive current to be superimposed by the second constant current source 3 in order to emit light at x. This is the second
A second constant current source 3 with a feedback control loop 6 of
This is the control/determination operation of the superimposed drive current.

Such first and second feedback control loops 5 and 6
For example, in the case of a laser printer, the control operation is performed every page or every main scanning line, and during the writing period, the first and second up/down counters 8, 12
By stopping counting and holding the count value at that time, the drive current value of the first constant current source 2 and the third D/D/
The maximum output of the A converter 14 is also maintained at the maximum value. That is, during the writing period, the minimum light intensity L m
A state is reached in which the drive current for in and the drive current for maximum light intensity L max have been determined. In this condition.

If an n-bit digital image density signal for optical writing is input to the third D/A converter 14, an analog signal of a magnitude corresponding to the digital data for the maximum output in the third D/A converter 14 is generated. is output to the second constant current source 3,
A drive current of that magnitude is applied to the semiconductor laser 1. The first constant current source 2 supplies the determined drive current. Therefore, by simply inputting a digital image density signal corresponding to an n-bit gray image to the third D/A converter 14, the minimum light intensity L can be adjusted.
Light can be emitted at any light intensity level of 2'' between min and maximum light intensity Lmax.

Note that the timing of the control operations by the first and second feedback control loops 5 and 6 as described above may be performed both between pages and between each main scanning line, but such control may be performed between each main scanning line. If it is performed every line, it is not suitable for high-speed writing. Therefore, in practice, control by the first feedback control loop 5 for the minimum light intensity L min, which has a greater influence on light intensity fluctuations, is performed every main scanning line, while the control by the first feedback control loop 5 for the maximum light intensity L 1 lax is performed for each main scanning line. Even if the control by the feedback control loop 6 is performed every page (or every half page opened or every two pages), sufficient control accuracy can be maintained. In order to control the timing of such control operations, a signal C0NTl is inputted to the enable terminal of the first up/down counter 8 from a first timing control means (not shown), and the timing for every main scanning line is inputted to the enable terminal of the first up/down counter 8. It is controlled to perform a control operation (counting operation). Further, a signal C0NT2 is also input to the enable terminal of the second up/down counter 8 from the second timing control means (not shown).

For example, control operation (counting operation) at the timing between pages
controlled to do so.

Next, an embodiment of the invention according to claim 2 will be described with reference to FIG. In this embodiment, instead of the first and second constant current sources 2 and 3, only one constant current source 15 is provided connected to the semiconductor laser 1, and this constant current source 15 is used for the first D/A conversion. Vessel 9
The control is performed by the output of an adder 16 which receives an analog signal from the D/A converter 14 and an analog signal from the third D/A converter 14 as inputs. According to this, for the semiconductor laser 1, the constant current source 15 secures the drive current for the minimum light intensity Lmin based on the analog signal from the second D/A converter 9, and the third D/A Based on the analog signal from the converter 14, the drive current to be superimposed is secured in accordance with the data of the digital image density signal.

Effects of the Invention Since the present invention is configured as described above, according to the invention described in claim 1, the first comparator, the first up/down counter, and the first D/A converter are used. A drive current for a predetermined minimum light intensity is determined by the constant current source control of the first constant current source, and a second comparator, a second up/down counter, and a second D/D/
The maximum value of the superimposed drive current for a predetermined maximum light intensity is determined by the second constant current source control using the A converter and the third D/A converter, and the Since the light intensity of the semiconductor laser is variably controlled in multiple stages using a digital image density signal corresponding to the input image density, it is possible to perform multi-stage gradation control under the advantageous situation of implementation using only two constant current sources. It is possible to achieve this with good linearity without causing insufficient or excessive light intensity, and even if there are multiple gradations, control can be done only for the minimum and maximum light intensity, and the control system can be configured. Furthermore, according to the second aspect of the invention, the number of constant current sources can be reduced to only one by simply adding an adder.

device, 12... second up/down counter, 13...
・First D/A converter, 14... Third D/A converter, 15... Constant current source, 16... Adder quantity Request
Rico Co., Ltd.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the invention as claimed in claim 1, FIG. 2 is a drive current-light intensity output characteristic diagram of a semiconductor laser, and FIG. 3 is a block diagram showing an embodiment of the invention as claimed in claim 2. FIG. DESCRIPTION OF SYMBOLS 1... Semiconductor laser, 2... First constant current source, 3...
... second constant current source, 4... photoelectric conversion element for monitor,
7... First comparator, 8... First up/down counter, 9... First D/A converter, 11... Second
Comparison, 5) ■ Electric breasts 1-

Claims (1)

[Claims] 1. A semiconductor laser, a monitoring photoelectric conversion element that monitors the light intensity of the light emitted from the semiconductor laser, and a monitor output from the monitoring photoelectric conversion element that corresponds to a predetermined minimum light intensity level. a first comparator that compares the comparison output with a first reference value, and a first timing control means that counts the comparison output from the first comparator when the drive current is controlled and holds a counted value when a match is made; a first up/down counter; a first D/A converter that converts the counted value of the first up/down counter into an analog signal; a first constant current source that causes a drive current of a certain magnitude to flow through the semiconductor laser; and a second constant current source that compares a monitor output from the monitor photoelectric conversion element with a second reference value corresponding to a predetermined maximum light intensity level. a comparator; a second up/down counter that counts the comparison output from the second comparator and holds a counted value when a match occurs during superimposed current control by the second timing control means;
A second D/A converter converts the count value of the up/down counter into an analog signal, and the analog signal from this second D/A converter is input to a reference voltage signal terminal that regulates the maximum output. a third D/D outputting an analog signal having a magnitude corresponding to the digital image density signal with respect to the maximum output;
an A converter, and a second constant current source that is connected in parallel to the first constant current source and flows a superimposed drive current to the semiconductor laser according to an analog signal from the third D/A converter. A semiconductor laser control circuit characterized by: 2. The first constant current source and the second constant current source are one constant current source, and the analog signal from the first D/A converter and the analog signal from the third D/A converter Claim 1 further comprising an adder for inputting the constant current source to the constant current source.
The semiconductor laser control circuit described.