JPH06334248A - Drive circuit for laser diode - Google Patents

Abstract

PURPOSE:To achieve stabilized halftone through a simple structure employing no pulse width modulation circuit. CONSTITUTION:A sample & hold circuit 29 detects a minimum level S14 of signals S12 proportional to the quantity of light outputted from a laser diode 17 and a comparator 32 and an adder 15 control the magnitude of a current signal S7 being fed to the laser diode 17 such that the minimum level S14 matches a reference lowest level Srmin. A sample & hold circuit 28 detects a maximum level S13 of the signals S12 and a comparator 31 and a D/A converter 14 control the magnitude of the current signal S7 such that the maximum level S13 matches a reference maximum level Srmax. Consequently, the upper and lower limit levels of optical output from the laser diode 17 are made constant and an optical output proportional to the amplitude of video signal can be obtained within that range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser diode drive circuit suitable for application to a laser printer such as a video printer.

[0002]

2. Description of the Related Art Since a laser printer is a non-contact type printer, it has characteristics such as high printing speed and high printing quality. Therefore, it is widely used, for example, as a video printer. It has become to.

Recently, a laser diode, which is a semiconductor laser, is often used as a laser light source for this laser printer. This is because it is advantageous for downsizing and weight reduction.

[0004]

By the way, since the light output vs. forward current characteristic of a laser diode has temperature dependence, it is generally necessary to keep the light output constant even if the case temperature changes. , APC (Automatic Power Control)
It is working.

This APC operation is performed by the laser light amount (light output).
In order to keep the maximum value of the output constant, the reference DC voltage value is compared with the output current value of the photodiode that monitors the light intensity of the laser diode, and feedback control is performed so that the output current value of the photodiode becomes a constant value. To do. Then, to obtain a pseudo halftone, P
A laser diode was driven by a WM (pulse width modulation) circuit.

However, the above-mentioned conventional laser diode drive circuit has various problems described below.

Since the pulse width modulation circuit includes an A / D converter and the like as an indispensable constituent element, the circuit structure becomes relatively complicated, a wide space for the circuit is required, and it is expensive. . Since an image obtained by using the pulse width modulation circuit is a halftone dot image, a moire image, so-called dot interference is likely to occur, and an image with high definition cannot be obtained. If the frequency of the pulse width modulation circuit is increased, it becomes expensive.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a laser diode drive circuit which has a simple structure and is inexpensive and does not cause dot interference.

[0009]

The present invention is described in, for example, FIG.
As shown in FIG. 3, the image signal S6 having the no-signal level portion 41, the minimum value level portion 42, the maximum value level portion 43, the sync signal portion 44, and the video signal portion 45 is the current signal S7.
And the light quantity proportional signal S1 corresponding to the light quantity L emitted from the laser diode 17.
1 (S12) is output to the monitor means 26, and the level S14 of the minimum value level portion 42 in the light amount proportional signal S12 is detected so that the detected minimum value level S14 and the predetermined minimum value level Srmin match. The first feedback means 29, 32, 15 for controlling the magnitude of the current signal S7 supplied to the laser diode 17 and the level S13 of the maximum value level section 43 in the light quantity proportional signal S12 are detected, and the detected maximum is detected. The second feedback means 28, 31, 14 for controlling the magnitude of the current signal S7 supplied to the laser diode 17 is provided so that the value level S13 and the predetermined maximum value level Srmax match. .

Further, according to the present invention, the second feedback means 28, 31, 14 are means having the D / A conversion circuit 14.

[0011]

According to the present invention, the first feedback means 2
The level S of the minimum value level section 42 in the light amount proportional signal S12 proportional to the light emission amount (light output) L of the laser diode 17 supplied from the monitor means 26 by 9, 32, and 15.
14 is detected, and the magnitude of the current signal S7 supplied to the laser diode 17 is controlled so that the detected minimum value level S14 and the predetermined minimum value level Srmin match, and the second feedback means 28 is also provided. , 3
1, 14 detects the level S13 of the maximum value level section 42 in the light amount proportional signal S12, and the detected maximum value level S
The magnitude of the current signal S7 supplied to the laser diode S17 is controlled so that 13 and the predetermined maximum value level Srmax match. For this reason,
Upper and lower limit values Lmax, L of the optical output of the laser diode 17
Since min is made constant, an optical output proportional to the amplitude of the video signal 45 can be obtained in that range. In this way, the structure is simple, the cost is low, and the dot interference does not occur.

Further, the second feedback means 28, 3
When the reference voltage of the D / A conversion circuit 14 is changed by configuring the circuits 1 and 14 to have the D / A conversion circuit 14, the upper limit value Lmax of the light output can be easily controlled.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a laser diode driving circuit of the present invention will be described below with reference to the drawings.

FIG. 1 shows the configuration of a laser diode drive circuit according to this embodiment. The laser diode drive circuit according to the example of FIG. 1 includes a digital video signal generation circuit 11 which is an image memory. An 8-bit digital video signal S1 which is an image signal output from the digital video signal generation circuit 11 is added to the digital addition circuit 12
Is supplied to one of the input terminals. Digital adder circuit 12
The minimum value maximum value signal S2 is supplied from the minimum value maximum value generating circuit 13 which is a memory to the other input terminal of the.

The digital video signal S3 which is the output signal of the digital addition circuit 12 is converted into an analog video signal S4 through the D / A conversion circuit 14 and supplied to one input terminal of the addition circuit 15. The bias signal S5 is supplied to the other input terminal of the adder circuit 15.

The video signal S6 as a voltage signal which is an output signal of the adder circuit 15 is converted into a current signal S7 by the drive amplifier circuit 16 and supplied to the laser diode 17.

FIG. 2 shows a light output characteristic 18 of the laser diode 17. In FIG. 2, the horizontal axis represents the current signal S7 which is the drive current, and the vertical axis represents the optical output. As can be understood from the light output characteristic 18, the laser diode 17 has a laser oscillation point that is an inflection point, and is supplied with a drive current that exceeds the laser oscillation point to emit the laser light L. Output.

The laser light L output from the laser diode 17 is a polygon mirror 2 that rotates in the direction of arrow A.
The image is reflected and deflected by 0, and an arrow B is drawn on the OPC (Organic Photo-Conductive) 21 as a photoconductor which is a recording medium.
Scan in the direction. On the scanning line of the OPC 21 in the direction of arrow B,
Reference symbol X indicates an image forming range.

Static electricity is stored in advance in the OPC 21. In this case, the OPC 21 is moved in the direction orthogonal to the paper surface, and the charge of the portion of the OPC 21 irradiated with the laser beam L moves, so that a two-dimensional electrostatic latent image is formed on the OPC 21. Toner is attached to the electrostatic latent image, transferred to the printing paper, and then fixed, whereby a hard copy image is obtained.

Reference numeral 22 is a sync signal light receiving circuit,
When the laser light L moves in the direction of arrow B, the sync signal light receiving circuit 22 receives a sync signal portion 44 (see FIG. 3) in the laser light L, which will be described later, and outputs the sync signal S8 to the timing generation circuit 23. Supply.

The timing generation circuit 23 basically has a counter and generates various timing signals.

That is, the timing generation circuit 23, based on the sync signal S8, the data read signal S9 of the digital video signal generation circuit 11 which is an image memory and the data read signal S1 of the minimum value maximum value generation circuit 13 which is a memory.
0, and sample and hold pulses P1 and P2 are generated.

The laser diode 17 has a case 2
5, and is integrated with the photodiode 26.

The photodiode 26 receives the monitor light Lm which is proportional to the amount of the laser light L emitted from the laser diode 17 toward the polygon mirror 20. From the photodiode 26, a light amount proportional signal S11, which is a current signal proportional to the emitted light amount of the laser light L, is supplied to the current-voltage conversion amplifier circuit 27.

The light quantity proportional signal S12, which is a voltage signal output from the current conversion amplifier circuit 27, is supplied to the sample hold circuits 28 and 29.

From the sample and hold circuits 28 and 29,
The maximum value sample hold signal S13 and the minimum value sample hold signal S14 are supplied to the comparison input terminals of the comparison circuits 31 and 32, respectively.

Predetermined reference maximum value level Srmax and reference minimum value level Srmin, which are target values, are supplied from terminals 33 and 34 to the reference input terminals of the comparison circuits 31 and 32, respectively.

The comparison circuit 31 determines the reference maximum value level Srm.
A gain control signal S15 is generated according to the comparison result between the ax and the maximum value sample hold signal S13, and is supplied to the reference voltage input terminal of the D / A conversion circuit 14. The comparison circuit 32 outputs the bias signal S5 according to the comparison result of the reference minimum value level Srmin and the minimum value sample hold signal S14.

Next, regarding the operation of the main part of the above embodiment,
The description will be made also with reference to FIG.

FIG. 3A shows a time axis. Figure 3B
Shows the waveform of the video signal S6 supplied to the drive amplifier circuit 16. The video signal S6, the current signal S7 converted by the drive amplifier circuit 16, and the laser beam L
The respective waveforms of the monitor light Lm and the light quantity proportional signals S11 and S12 are similar waveforms. Therefore, in the following description, those signals will be described with reference to the waveform of FIG. 3B as necessary.

As shown in FIG. 3B, the video signal S6 corresponding to one scanning line on the OPC 21 has a minimum value in the non-signal level portion 41 (the level at which the laser diode 17 is operated below the laser oscillation point). Level part 42, maximum value level part 43, sync signal part 44 and video signal part 45
have. The cycle between the adjacent sync signal units 44, 44 is constant.

At time t1 to time t2, when the sync signal S8 is received by the sync signal light receiving circuit 22 from the laser light L deflected by the polygon mirror 20, the timing generating circuit 23 causes the sync signal S8, that is, The data read signal S9 is supplied to the digital video signal generation circuit 11 during a period from a time point t3 to a time point t4 after a lapse of a fixed time after starting time measurement from the falling point (time point t2) of the sync signal section 44.

Video data is read from the digital video signal generation circuit 11 by the data read signal S9. The read video data appears as the video signal portion 45 (see FIG. 3B) in the video signal S6. Although not shown, the sync signal S8 is transmitted to the polygon mirror 20.
It has a function of synchronizing the rotation of the image with the reading of the video data.

By scanning the OPC 21 in the direction of arrow B with the laser beam L whose intensity is modulated by the video signal section 45, the electrostatic latent image corresponding to the shading (halftone) of the original image is displayed in the image forming range X on the OPC 21. A latent image (electrostatic charging pattern) is formed.

As described above, toner (not shown) is adhered to the electrostatic latent image for development, and the printing paper is brought into close contact with the OPC 21 to transfer the image to the printing paper. You get a hard copy.

Further, the data read signal S output from the timing generation circuit 23 from time t5 to time t6 and from time t7 to time t8 after a lapse of a fixed time from time t2.
The minimum value data and the maximum value data are read from the minimum value / maximum value generation circuit 13 by 10 and added to the digital video signal S1 by the addition circuit 12.

The minimum value data and the maximum value data added to the digital video signal S1 are the light amount proportional signal S12,
Appear as the minimum value level part 42 and the maximum value level part 43 (see FIG. 3B).

The respective levels of the maximum value level section 43 and the minimum value level section 42 are sample hold pulses P1 and P.
2, the sample and hold circuits 28 and 29 respectively hold the maximum and minimum value sample and hold signals S13 and S14.

The maximum value sample hold signal S13 and the reference maximum value level Srmax are compared by the comparison circuit 31,
By supplying the gain control signal S15 according to the comparison result as the reference voltage of the D / A conversion circuit 14,
Analog video signal S output from the A / A conversion circuit 14
The amplitude of 4 is changed. That is, the gain control signal S15
Is a signal for controlling the gain of the D / A conversion circuit 14.
Since the reference voltage of the D / A conversion circuit 14 is changed to control the gain, the gain can be easily controlled. Further, the minimum value sample hold signal S14 and the reference minimum value level Srmin are compared by the comparison circuit 32, and the bias signal S5 corresponding to the comparison result is supplied to the addition circuit 15, whereby the no-signal level of the video signal S6 is obtained. Part 4
The overall level including 1, that is, the bias level is changed.

In this case, in the comparison circuit 31, the maximum value level portion 43 in the light amount proportional signal S11 corresponding to the monitor light Lm of the laser light L has a constant value (reference maximum value level Srmax).
Feedback is applied so that
In No. 2, feedback is applied so that the minimum value level portion 42 in the light amount proportional signal S11 becomes a constant value (reference minimum value level Srmin). Therefore, by setting the reference maximum value level Srmax and the reference minimum value level Srmin to values corresponding to the maximum value Lmax (see FIG. 2) and the minimum value Lmin of the optical output of the laser light L, respectively, The light output of the laser light L corresponding to the portion 45 can be changed between the maximum value Lmax and the minimum value Lmin. The minimum value Lmin of optical output
The reference minimum value level Srmin for determining is set such that the minimum value Lmin of the optical output is higher than the laser oscillation point. At that time, the no-signal level section 41
Then, the laser diode 17 is set to a level at which laser oscillation does not occur (a level below the laser oscillation point).

Here, consider a case where the light output characteristic 18 of the laser diode 17 changes due to a change in ambient temperature or the like. In this case, the minimum value Lmin and the maximum value Lm of the optical output of the current signal S7 supplied to the laser diode 17
Minimum value current signal S7min corresponding to ax (see FIG. 2)
And the value of the maximum value current signal S7max respectively changes, but the light-current conversion characteristic of the photodiode 26 hardly changes when compared with the laser diode 17 with respect to the change of the ambient temperature, so the minimum value of the optical output. Lmin
And the maximum value Lmax is maintained at a substantially constant value.

As described above, according to the above embodiment, the sample hold circuit 29 causes the level S14 of the minimum value level section 42 in the light quantity proportional signal S12 to be generated at regular intervals (every time when the sync signal section 44 is detected in this embodiment). Is detected, and the magnitude of the current signal S7 supplied to the laser diode 17 by the comparison circuit 32 is set to the minimum value Lmin of the optical output so that the detected minimum value level S14 matches the predetermined reference minimum value level Srmin. And a value corresponding to the light quantity proportional signal S by the sample hold circuit 28.
The level S13 of the maximum value level section 43 in 12 is detected,
The magnitude of the current signal S7 supplied to the laser diode 17 by the comparison circuit 33 is set to a value corresponding to the maximum value Lmax of the optical output so that the detected maximum value level S13 matches the predetermined reference maximum value level Srmax. I'm trying to control.

For this reason, the upper and lower limit values (the maximum value L _max and the minimum value L _min ) of the optical output of the laser diode 17 are made constant, and the light output proportional to the amplitude of the video signal section 45 is within that range. It becomes possible to obtain an output, that is, an optical output corresponding to a halftone. By doing this, A
Since the pulse width modulation circuit that includes the / D converter and the like as an essential requirement is not used, the laser printer can be configured at a low cost and at which a high-precision image without dot interference can be obtained. In this case, as can be seen from FIG. 3, the minimum value level section 42, the maximum value level section 43, and the sync signal section 44 have the laser light L in the image forming range X on the OPC 21.
Since it is inserted by using a part of the period other than the period of scanning, the video signal section 44 is not affected.

In the above embodiment, the D / A conversion circuit 14 is used as the gain varying means for handling the digital video signal S1 as the input signal, but when the input signal is an analog video signal. In the D
Of course, the / A conversion circuit 14 may be replaced with a variable gain amplifier circuit.

Further, according to this embodiment, since the pulse width modulation circuit is not used, it is easy to widen the band and the circuit space can be reduced.

Furthermore, the present invention is not limited to the above-mentioned embodiments, and it goes without saying that various configurations can be adopted without departing from the gist of the present invention.

[0047]

As described above, according to the present invention,
The first feedback means detects the level of the minimum value level portion in the light quantity proportional signal proportional to the light emission quantity (light output) of the laser diode supplied from the monitor means, and detects the detected minimum value level and a predetermined minimum value. The magnitude of the current signal supplied to the laser diode is controlled so as to match the value level, and the level of the maximum value level portion in the light amount proportional signal is detected by the second feedback means to detect the maximum value. The magnitude of the current signal supplied to the laser diode is controlled so that the level and the predetermined maximum value level match.
Therefore, the upper and lower limit values of the optical output of the laser diode are made constant, and within that range, an optical output proportional to the amplitude of the video signal, that is, an optical output corresponding to a halftone can be obtained. . By doing so, it is possible to obtain the effect that the dot configuration is inexpensive and the dot interference does not occur with a simple configuration.

Further, the second feedback means is D / A
This D /
By changing the reference voltage of the A conversion circuit, it is possible to easily control the upper limit value of the light output.

[Brief description of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of a laser diode drive circuit according to the present invention.

FIG. 2 is a characteristic diagram showing an optical output of a laser diode.

FIG. 3 is a waveform diagram provided for explaining the operation of the example in FIG.

[Explanation of symbols]

 14 D / A conversion circuit 15 Addition circuit 17 Laser diode 26 Photodiode 31, 32 Comparison circuit L Laser light Lm Monitor light

Claims (2)

[Claims] 1. A laser diode to which an image signal having a no-signal level portion, a minimum value level portion, a maximum value level portion, a sync signal portion, and a video signal portion is supplied as a current signal, and the amount of light emitted from this laser diode is changed. A monitor means for outputting a proportional light amount signal and a level of the minimum value level portion in the light amount proportional signal, and the laser so that the detected minimum value level and a predetermined minimum value level coincide with each other. First feedback means for controlling the magnitude of the current signal supplied to the diode, and the level of the maximum value level portion in the light quantity proportional signal is detected, and the detected maximum value level and a predetermined maximum value are detected. And a second feedback means for controlling the magnitude of the current signal supplied to the laser diode so that the level of the laser diode and the level of the laser diode match. Drive circuit. 2. The second feedback means is D /
The laser diode drive circuit according to claim 1, which is a means having an A conversion circuit.