JPS63217764A - Laser recorder - Google Patents

Abstract

PURPOSE:To make the loop gain of an APC circuit constant by applying the gain adjustment so as to compensate the differentiation quantization efficiency of a semiconductor laser and the sensitivity difference of a photodetector. CONSTITUTION:A picture signal S1 is converted into a luminous level command signal 55 by a correction table 40, subjected to D/A conversion 16 and inputted to a summing point 2 of an APC circuit 8 as a command signal Vref. Then a light beam 4 having an intensity corresponding to the signal Vref is radiated from the semiconductor laser 1. A gain regulating means 50 and an LPF 51 are provided in a path from the summing point 2 to a V/I convertion amplifier 3 and a gain regulating means 52 is provided in a path from an I/V conversion amplifier 7 to the summing point 2. Then the nonlinearity of the lighting level command signal versus laser optical output characteristic is corrected linear by a V-P characteristic correction table to make the loop gain of the APC circuit 8 constant.

Description

Detailed Description of the Invention (Industrial Application Field) The present invention relates to a laser recording device that records a continuous tone image by scanning a photosensitive material with a laser beam modulated based on an image signal. The present invention relates to a laser recording device capable of recording high-gradation images by modulating the light intensity of a laser beam in an analog manner.

(Prior Art) Conventionally, optical scanning recording apparatuses have been widely put into practical use, which record an image on a photosensitive material by deflecting a light beam using an optical deflector and scanning the photosensitive material.

A semiconductor laser has conventionally been used as one of the means for generating a light beam in such an optical scanning recording device. This semiconductor laser has many advantages, such as being smaller, cheaper, and consumes less power than gas lasers, and can be directly modulated by changing the drive current.

However, on the other hand, as shown in Figure 2, the optical output characteristics of this semiconductor laser with respect to the drive current vary drastically between the LED region (natural light emitting region) and the laser oscillation region, so it is not suitable for continuous tone image recording. The problem is that it is difficult. In other words, if intensity modulation is performed using only the laser oscillation region where the drive current vs. optical output characteristic is linear,
The dynamic range of optical output is only about 2 digits at most. As is well known, it is impossible to obtain a high quality continuous tone image with this level of dynamic range.

For example, JP-A-56-115077 and JP-A-56-1
As shown in No. 52372, etc., the optical output of the semiconductor laser is kept constant, and the semiconductor laser is continuously switched to zero.
Attempts have also been made to record a continuous tone image by turning off the scanning beam to make the scanning beam pulsed light, and controlling the number or width of the pulses for each pixel to change the amount of scanning light.

However, when performing pulse number modulation or pulse width modulation as described above, for example, if the pixel clock frequency is IM
In the case of Hz, if the resolution of the density scale, that is, the amount of scanning light is to be maintained at 10 bits (approximately 3 digits), the pulse frequency must be set extremely high, at least 1 Gf-1 z. The semiconductor laser itself can be turned on and off at this frequency, but pulse count circuits for pulse number control or pulse width control cannot operate at such high frequencies, and in the end, requires the pixel clock frequency to be significantly lower than the above values. Therefore, the recording speed of the device has to be significantly reduced.

Furthermore, in the above method, the amount of heat generated by the semiconductor laser chip changes depending on the number or width of pulses output during the recording period of each pixel, and as a result, the drive current versus light output characteristics of the semiconductor laser changes. The exposure amount per pulse may vary. If this happens, the gradation of the recorded image will shift, making it impossible to obtain a high-quality continuous tone image.

On the other hand, as shown in, for example, Japanese Patent Application Laid-open No. 71374/1983, a method has also been proposed in which a high-gradation image is recorded by combining the above-mentioned pulse number modulation or pulse width modulation with the above-mentioned light intensity modulation. However, in this case as well, the problem arises that the amount of heat generated by the semiconductor laser chip changes depending on the number or width of pulses as described above, and as a result, the amount of exposure per pulse changes.

Considering the above, for example, the density scale is 10 bits.
In other words, to record a high gradation image of about 1024 gradations,
It is desirable to perform optical intensity modulation across the LED region and the laser oscillation region shown in FIG. 2 described above to ensure a dynamic range of optical output of approximately three ranges. However, in the above two regions, the driving current vs. optical output characteristic of the semiconductor laser naturally becomes non-linear, so in order to easily and accurately record high l@i tone images, it is necessary to In order to be able to control the image density at density intervals, it is necessary to compensate for the above characteristics by some method to linearly change the relationship between the light emission level command signal of the semiconductor laser and the optical output.

Conventionally, as a circuit that linearizes the relationship between the light emission level command signal of the semiconductor laser and the optical output, it detects the light intensity of the laser beam and sends a feedback signal corresponding to the detected light intensity to the semiconductor laser light emission level command. An optical output stabilization circuit (hereinafter referred to as an APC circuit) that feeds back a signal is known. FIG. 3 shows an example of this APC circuit, and the APC circuit will be explained below with reference to FIG. A light emission level command signal Vref that commands the light emission intensity of the semiconductor laser 1 is input to the voltage-current conversion amplifier 3 through the addition point 2, and the amplifier 3 supplies a drive current proportional to this command signal Vref to the semiconductor laser 1.
supply to. A light beam 4 emitted forward from the semiconductor laser 1 is used to scan a photosensitive material through a scanning optical system (not shown).

On the other hand, the intensity of the light beam 5 emitted to the rear side of the semiconductor laser 1 is detected by, for example, a bin photodiode 6 for monitoring the amount of light installed inside the case of the semiconductor laser. The intensity of the light beam 5 thus detected is proportional to the intensity of the light beam 4 actually used for image recording. The intensity of the light beam 5, that is, the output current of the photodiode 6 indicating the intensity of the light beam 4, is converted into a feedback signal (voltage signal) Vpd by a current-voltage conversion amplifier 7, and the feedback signal Vpd is input to the above-mentioned addition point. 2 is input. From this addition point 2, the light emission level command signal ■re
A deviation signal Ve indicating the deviation between f and the feedback signal Vpd is output, and the deviation signal Ve is converted into a current by the voltage-current conversion amplifier 3 to drive the semiconductor laser 1.

(Problems to be Solved by the Invention) From the above-mentioned addition point 2 to the voltage-current conversion amplifier 3, the semiconductor laser 1, the photodiode 6, and the current-voltage conversion amplifier 7
If the loop gain of the A10 circuit, which includes a loop that returns to addition point 2 through , is ensured to be sufficiently large, the relationship between the light emission level command signal and the semiconductor laser light output becomes linear.

The loop gain of the A10 circuit is determined by the gain of the amplifier included therein, the photodetector, and the gain of the semiconductor laser itself, but this loop gain varies for each APC circuit even if the same circuit elements are used. It often varies. In other words, although it is easy to keep the gain of the amplifier constant, there are relatively large individual differences in the differential quantum efficiency, which is the gain of semiconductors, and the sensitivity of photodetectors such as photodiodes, so the loop gain of the A10 circuit is It will vary.

If there is variation in the loop gain of the A10 circuit in this way, the sharpness of the recorded image will vary from one laser recording device to another. In other words, when the loop gain of the A10 circuit is high, the light emission response of the semiconductor laser improves and the image sharpness increases, but when this loop gain is low, the light emission response worsens and the image sharpness decreases. Become. Further, if there are variations in the loop gain of the A10 circuit, the response of the optical output of the semiconductor laser to the light emission level command signal will also vary, making it impossible to control the light amount with high precision.

Furthermore, there are variations in the loop gain of the A10 circuit,
If the value is too high than the designed value, the operation of the A10 circuit may become unstable and oscillation may occur.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a laser recording device that can solve the various problems mentioned above by eliminating variations in the loop gain of the A10 circuit. It is.

(Means for Solving the Problems) The laser recording device of the present invention includes a semiconductor loop, a beam scanning system that scans a light beam emitted from the semiconductor laser onto a photosensitive material, and a light emission level corresponding to an image signal. A laser recording device comprising: a laser operation control circuit that generates a command signal, controls the drive current of the semiconductor laser based on the signal, and modulates the light intensity of the laser beam; This APC circuit is characterized in that it is provided with an A10 circuit and a gain adjustment means is provided within this APC circuit.

(Function) If the gain adjustment means as described above is provided, the A10 circuit can be adjusted by setting the gain of the gain adjustment means to compensate for the differential quantum efficiency of the semiconductor laser and the sensitivity difference of the photodetector. It is possible to keep the loop gains constant.

(Example) Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 shows a laser recording apparatus according to an embodiment of the present invention. Image signal generator 10 generates an image signal S1 carrying a continuous tone image. This image signal S1 is, for example, a digital signal representing a continuous tone image with a density scale of 1Qbit. The image signal generator 10 switches signals for one main scanning line based on a line clock S2, which will be described later, and outputs an image signal S1 for each pixel based on a pixel clock S3. In this example, the pixel clock frequency is IMH21. In other words, the recording time for one pixel is 1 μsec.
(seconds).

The above-mentioned image signal S1 passes through the multiplexer 11 and is sent to the RA
After being subjected to a correction described later in a correction table 40 consisting of M, it is converted into, for example, a 16-bit light emission level command signal S5. This light emission level command signal S5 is inputted to the D/A converter 16 via the multiplexer 15, and here the light emission level command signal Vref consisting of an analog voltage signal is inputted to the D/A converter 16.
is converted to This light emission level command signal V ref is
It is input to the addition point 2 of the APC circuit 8. The voltage-current conversion amplifier 3, semiconductor laser 1, photodiode 6, and current-voltage conversion amplifier 7 of the APC circuit 8 are respectively the voltage-current conversion amplifier 3 and the voltage-current conversion amplifier 3 in the circuit of FIG.
It operates in the same way as the semiconductor laser 1, the photodiode 6, and the current-voltage conversion amplifier 7, and therefore, the semiconductor laser 1 emits light with an intensity corresponding to the light emission level command signal Vref (that is, corresponding to the image signal S1). Beam 4 is emitted.

In this device, unlike the circuit shown in FIG. 3, a gain adjustment means 50 and a low-pass filter 51 are arranged in this order on the path from the addition point 2 to the voltage-current conversion amplifier 3.
A gain adjustment means 52 is also arranged on the path from the current-voltage conversion amplifier 7 to the summing point 2, and the effects thereof will be explained later.

The light beam 4 is passed through a collimator lens 17 to become a parallel beam, and then enters an optical deflector 18, such as a polygon mirror, where it is reflected and deflected.

The light beam 4 thus deflected is passed through a focusing lens 19, which is usually an fθ lens, focused on a minute spot on the photosensitive material 20, and scans the photosensitive material 20 in the X direction (main scan). The photosensitive material 20 is transported by a transport means (not shown) in the Y direction substantially perpendicular to the main scanning direction X, thereby causing the light beam 4 to perform sub-scanning. In this way, the photosensitive material 20 is two-dimensionally scanned by the light beam 4 and exposed. As described above, since the light beam 4 is intensity-modulated based on the image signal S1, a continuous tone image carried by the image signal S1 is recorded on the photosensitive material 20 as a photographic latent image. Note that when the light beam 4 scans the photosensitive material 20 as described above, the photodetector 21 detects that the beam 4 has passed through the starting point of the main scan.
The start point detection signal @S6 output from the photodetector 21 is input to the clock generator 36. The clock generator 36 generates the aforementioned line clock S2 and pixel clock S in synchronization with the input timing of this start point detection signal $6.
Output.

The photosensitive material 20 is then passed through a developer@22 where it undergoes a development process. As a result, the continuous tone image is recorded on the photosensitive material 20 as a visible image.

Here, the image signal @S1 in the above-mentioned correction table 40
The correction will be explained below. The correction table 40 includes a gradation correction table 12, an inverse log conversion table 13, and a correction table (hereinafter referred to as a V-P characteristic correction table) 14 for linearly correcting the light output characteristic of the semiconductor laser 1 relative to the light emission level command signal. Consisting of

The gradation correction table 12 is a known one for correcting the gradation characteristics of the photosensitive material 20 and its development processing system. Although this gradation correction table 12 may have fixed correction characteristics, in this embodiment, the gradation correction table 12 has fixed correction characteristics.
gradation characteristics change from lot to lot, or development@2
In consideration of the fact that the characteristics of the developing solution in 2 change with time, etc., the correction characteristics are configured to be able to be modified as appropriate in accordance with the actual gradation characteristics.

That is, the test pattern generation circuit 26 outputs a test pattern signal S4 carrying several levels (for example, 16 levels) of image density on the photosensitive material 20, and this signal S4 is input to the multiplexer 11. At this time, the multiplexer 11 is switched from the image recording state in which the image signal S1 is input to the correction table 40 as described above, and the test pattern signal S4 is input to the correction table 40.
The state is such that the input is made. The semiconductor laser 1 is driven as described above based on this test pattern signal S4,
The light beam 4 is therefore intensity modulated. As a result, the density changes stepwise on the photosensitive material 20, for example, 16
step wedges (test patterns) are recorded as photographic latent images. This photosensitive material 20 is sent to a developing machine 22, and the step wedge is developed. After development, the photosensitive material 20 is placed in a densitometer 23, and the optical density of each of the step wedges is measured. The optical densities thus measured are input to the density value input means 24 in association with each step wedge, and the density value input means 24 outputs a density signal S7 indicating the optical density of each step wedge. This concentration signal S7 is generated by the table creation means 37.
and the table creating means 37 receives this concentration signal S.
7 and the test pattern signal S4, a gradation correction table is created that allows a predetermined image density to be obtained with the value of the predetermined image signal S1.

As described above, this gradation correction table associates approximately 16 levels of image signal values with respective predetermined image density values. Data $8 indicating this gradation correction table is input to the data interpolation means 38, where interpolation processing is performed to obtain a gradation correction table that can correspond to the 1024-step (-10 bit) image signal S1. The aforementioned gradation correction table 12 is formed based on the data S9 indicating this gradation correction table.

When recording an image based on the image signal S1, the image signal S1 input to the tone correction table 12 via the multiplexer 11 is converted into a signal 81 by the tone correction table 12.
', and then converted into a light emission level command signal S1'' by the inverse log conversion table 13.

Next, the VP characteristic correction table 14 will be explained. A
In the PC circuit 8, even if the feedback signal Vpd is fed back to the addition point 2, the light emission level command signal and the light beam 4
It is difficult to establish an ideal relationship between the intensity and the intensity (the relationship indicated by the solid line in FIG. 4). That is, in order to obtain this ideal relationship, the loop gain of the APC circuit 8 must be set to 70.
Although it is necessary to set the loop gain extremely high, on the order of dB, it is currently extremely difficult to achieve such a high loop gain. The V-P characteristic correction table 14 is provided to obtain the above-mentioned ideal relationship. That is, if the ideal relationship between the light emission level command signal Vref and the optical output of the semiconductor laser 1 is a straight line shown as a in FIG. 5, and the actual relationship is a curve shown as b in FIG. The characteristic correction table 14 is formed so as to convert the voltage value Vin into a value V, assuming that the voltage value when the light emission level command signal 81'' is directly D/A converted is Vin. In other words, the light emission level command signal ■re
If the value of f is Vin, only the light intensity of P' can be obtained, but if the above conversion is done, the voltage value ■i
The light intensity of Po is obtained for n. In other words, the voltage value ■in corresponding to the light emission level command signal S1'' and the optical output Pf
The relationship is linear.

With this configuration, the density in the photosensitive material 20 can be controlled at equal intervals by changing the image signal S1 by a predetermined amount. The characteristic curve in FIG. 5 is obtained when the semiconductor laser 1 is driven across its LED region and laser oscillation region as described above. Since this is ensured, a high gradation image of about 1024 levels can be recorded easily and with high precision as described above.

As described above, the nonlinearity of the light emission level command signal versus laser light output characteristic caused by the nonlinear drive current versus light output characteristic of the semiconductor laser 1 is linearly corrected by the V-P characteristic correction table 14. For example, voltage-current conversion amplifier 3, semiconductor laser 1, foot diode 6, current-
A consisting of a system returning from the voltage conversion amplifier 7 to the addition point 2
The loop gain of the PC circuit 8 no longer needs to include the gain necessary to correct the nonlinearity. That is, this loop gain is used to correct deviations from the drive current vs. optical output characteristics of the semiconductor laser 1 due to transient temperature changes that occur during the operation of the semiconductor laser 1 or errors in the case temperature constant control of the semiconductor laser 1. Furthermore, it is only necessary to secure as much as necessary to correct the drift of the amplifier, etc. Specifically, for example, when the pixel clock frequency is IMHz and the semiconductor laser 1 is operating with an optical output of 3 mW, it is sufficient that the loop gain is maintained at about 30 dB. This level of loop gain can be easily achieved with the current state of the art.

Here, the creation of the VP characteristic correction table 14 will be explained. A table creation device 35 can be connected to the device shown in FIG. 1 as appropriate.

This table creation device @35 includes the test signal generation circuit 27
, a table creation circuit 28 and a memory 29.

When creating the V-P characteristic correction table 14, a level-variable digital test signal S10 is output from the test signal generation circuit 27 and input to the multiplexer 15. At this time, the multiplexer 15 is switched from the image recording state in which it sends the light emission level command signal S5 to the D/A converter 16 as described above, to the state in which it sends the test signal $10 to the D/A converter 16. Ru. Also table creation circuit 2
8 is connected so that the feedback signal Vpd output from the current-voltage conversion amplifier 7 of the APC circuit 8 is input. The test signal 810 is output so that the level increases or decreases in steps. At this time, the table creation circuit 28 first generates a reference signal corresponding to the lowest optical output from the built-in variable level signal generator, and compares the reference signal with the feedback signal Vpd. This reference signal has a voltage value ■in in FIG. Then, the table creation circuit 28 latches the value of the test signal 810 when these two signals match. Since the voltage value indicated by this latched test signal 310 corresponds to the voltage value ■ in FIG. 5, the relationship between the voltage value ■in and V can be seen. The table creation circuit 28 changes the value of the reference signal in 1024 ways and finds the relationship between the voltage value Vin and ■ in each case. As a result, as described above, a correction table is created that converts the voltage value Vin of 1024 steps into ■. The correction table created in this way is temporarily stored in the memory 29 and then set as the VP characteristic correction table 14.

After creating the VP characteristic correction table 14 in this manner, the table creation device 35 is disconnected from the APC circuit 8.

Next, the point of setting the loop gain of the APC circuit 8 to be constant will be explained. The gain adjustment means 50 and 52 are attenuators made of trimmer resistors, for example, and by adjusting these gain adjustment means 50 and 52, the loop gain of the APC circuit 8 can be changed. In this embodiment, in particular, variations in sensitivity of the photodiode 6 are eliminated by the gain adjustment means 52. When adjusting this sensitivity, the light emission level command signal V
Instead of ref, a variable level test signal V tes
is input. This test signal V tes is input using, for example, the test signal generation circuit 27 .

At this time, the intensity of the light beam 4 is measured using, for example, an optical power meter, and the value of the test signal y tes is set so that a predetermined light intensity P tes is obtained. Then, while monitoring the feedback signal Vpd output from the gain adjustment means 52, the gain adjustment means 52 is adjusted so that the signal Vl)d and the light intensity p tes have a predetermined relationship. By doing this, variations in sensitivity of the photodiode 6 are compensated for.

After performing the above adjustment, the gain adjustment means 50
Adjustment is performed to set the loop gain of the PC circuit 8 to a predetermined value. When making this adjustment, a predetermined light emission level command signal V
ref is input to addition point 2, and the output from addition point 2, that is, the deviation signal Ve, is monitored. If the loop gain of the APC circuit 8 is Go, then Ve = 'J ref / (1+Go), so the gain adjustment means 50 is adjusted so that the deviation signal Ve that satisfies the above equation is obtained. Also, the light emission level command signal y
Adjustment can also be made by giving a step input as rer and observing the overshoot amount and overshoot time.

Note that the loop gain adjustment described above must be performed in a laser oscillation region where the semiconductor laser 1 has a high differential quantum efficiency.

In order to more accurately adjust the loop gain to a predetermined value, after compensating for the sensitivity variation of the photodiode 6 described above, the APC
Opening the loop of circuit 8, the light emission level command signal V ref
A sine wave is superimposed on the feedback signal Vpd, and the amplitude of the sine wave appearing in the feedback signal Vpd is compared with the amplitude of the sine wave, and the gain adjustment means 50 is adjusted so that the ratio thereof becomes equal to the desired loop gain. Good too.

In the above embodiment, the light emission level command signal 81''
V-P correction table 1 that makes the relationship of optical output Pf linear
4 is provided, but if the gain of the APC circuit 8 can be secured to be sufficiently large, for example, about 70 dB, this APC
Since the ideal relationship shown by the solid line in FIG. 4 can be obtained only by the circuit 8, especially the V-P correction table 14 as described above
There is no need to provide

In addition, the beam scanning system that scans the light beam 4 may be provided with an optical element whose incident light intensity/output light transmittance characteristic is nonlinear, such as a polarizing filter, an interference filter, or an aperture limiting plate. In this case, it is preferable to form the VP correction table 14 so as to also compensate for the above-mentioned nonlinearity.

Further, in the above embodiment, two gain adjustment means 50
．． 52, however, the laser recording apparatus of the present invention may be provided with one or three or more gain adjustment means.

(Effects of the Invention) As explained in detail above, in the laser recording device of the present invention, the loop gain of the APC circuit can be adjusted to a desired value, so the relationship between the light emission level command signal and the semiconductor laser light output can be maintained without individual differences. By setting it constant, it is possible to control the amount of light with high precision, and it is also possible to prevent the loop gain of the APC circuit from becoming too high than the designed value and causing the circuit to oscillate. Furthermore, if the loop gain of the APC circuit is the same between each laser recording device, the light emission response of the semiconductor laser will be uniform among the laser recording devices, and therefore the sharpness of the recorded image will be maintained at a desirable constant value. This makes it possible to increase the commercial value of the laser recording device.

[Brief explanation of the drawing]

Fig. 1 is a schematic diagram showing a laser recording device according to an embodiment of the present invention, Fig. 2 is a graph showing the driving current versus optical output characteristics of a semiconductor laser, and Fig. 3 is an example of a semiconductor laser optical output stabilizing circuit. FIG. 4 is a graph showing the relationship between the light emission level command signal and the semiconductor laser light output, and FIG. 5 is a graph explaining the effect of the V-P characteristic correction table in the apparatus of the above embodiment. 1... Semiconductor laser 2... Summing point 3... Voltage-current conversion amplifier 4.5... Light beam 6... Photodiode 7... Current-voltage conversion amplifier 8... APC circuit 10... Image signal generator 14... V-P characteristic correction table 16... D/A converter 17... Collimator lens 18... Light deflector 19... Converging lens 20... Photosensitive material 35...Table creation device 50.52...Gain adjustment means Sl...Image signal S5, V ref...Emission level command signal Vp
d...Feedback signal ■e...Difference signal Figure 2 Figure 4 Figure 5 Figure 3

Claims (1)

[Scope of Claims] A semiconductor laser that emits a light beam; a beam scanning system that scans the light beam onto a photosensitive material; and a beam scanning system that generates a light emission level command signal corresponding to an image signal and controls the semiconductor laser based on the signal. and a laser operation control circuit that modulates the intensity of the light beam by controlling a drive current of the laser, the laser operation control circuit detects the intensity of the light beam and modulates the intensity of the light beam. What is claimed is: 1. A laser recording apparatus comprising: a light output stabilizing circuit that feeds back a corresponding feedback signal to the light emission level command signal; and a gain adjusting means is provided within the light output stabilizing circuit.