JP2692984B2 - Laser recording device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a laser recording apparatus for recording a light beam on a recording medium using a semiconductor laser.

[Prior Art] A laser beam printer has been widely used in the past as a device for recording a digitally represented halftone image on a photosensitive recording medium. This is because the laser beam whose intensity is modulated in proportion to the image density is deflected by an optical deflector for main scanning, and a recording medium such as a film or a drum is moved perpendicularly to the main scanning direction to perform sub-scanning. The image is recorded on.

As a means for generating the above-mentioned laser beam, a semiconductor laser is currently the cheapest and the smallest, and has a feature that the intensity can be directly modulated by a driving current.

However, the disadvantage of the semiconductor laser is that the drive current
It can be said that the light output characteristic has a remarkable temperature negative characteristic. 12 is an example of the drive current-optical output characteristics of the semiconductor laser of FIG. 12, the horizontal axis of the semiconductor laser drive current [mA],
The vertical axis is the optical output [mW] and the case temperature is 0 ℃, 25 ℃,
It is measured at 50 ° C. When read from the graph, a temperature negative characteristic of about -0.1 mW / ° C can be found. This means that the light output fluctuates greatly due to fluctuations in the outside temperature. Further, the semiconductor laser chip raises its temperature due to its own light emission loss, which causes a decrease in optical output.

Since the laser printer records at a speed of 1 to 2 msec for main scanning and a few seconds for one page, the ambient temperature does not change during at least one main scanning period, and the change in light output due to temperature changes during that period is due to the emission loss of the semiconductor laser itself. Only given due to heat rise.

As a means for compensating the optical output fluctuation due to the above temperature change, a circuit that feed-backs to the drive current by monitoring the emission level of the semiconductor laser to a predetermined level (constant during the irradiation time) successively by using a photodiode (hereinafter APC) Circuit) is known.

Figure 13 is a block diagram of the basic APC circuit configuration.
1 is a set voltage for making the amount of light emission proportional, 2 is a voltage addition circuit, 3 is a semiconductor laser drive voltage Vd, 4 is a voltage / current conversion circuit for converting the drive voltage of 3 into an actual drive current Id,
5 is a drive current Id, 6 is a semiconductor laser, 7 is a PIN photodiode for monitoring the laser emission amount, and 8 is a monitor current I.
m and 9 are current / voltage conversion circuits for converting the monitor current Im into the monitor voltage Vm of 10. The optical output of the semiconductor laser 6 is set to PI
Since it is not shown in the figure because it is monitored by the N photo diode 7, the PIN photo diode 7 monitors the amount of light emitted from the rear surface of the semiconductor laser at the rear end of the laser chip, or a beam splitter is provided in front of the laser chip. It is configured to monitor the dispersed light. FIG. 13 shows a typical single loop feedback control system, in which the error between the set voltage Vs of 1 and the monitor voltage Vm becomes the drive voltage Vd of 3, so that the optical output changes due to the temperature change. In order not to do so, control is always performed so as to be proportional to the set voltage Vs.

[Problems to be Solved by the Invention] However, in the above-mentioned conventional example, since the semiconductor laser is driven and oscillated by the input current having the rectangular waveform, the optical output level changes up and down due to the temperature change, and it is necessary to quickly compensate for this. The APC circuit is very difficult to design as described later.

That is, in order to increase the extinction ratio (dynamic range) of the light output, a method of performing intensity modulation by changing the pulse width / number of one pixel while keeping the light output constant (pulse width / number modulation), or pulse width / number Considering the adoption of a system that combines modulation and light output change (amplitude modulation), the recording speed (pixel clock frequency) per pixel of the laser beam printer is as high as several MHz, and for example, a gradation of 8 bits. If pulse width modulation with is performed, the pulse width will be the minimum and a very short number of nsec. In order to accurately adjust the intensity of the semiconductor laser emitting such a short pulse width with the APC circuit, the control speed must be extremely high, several tens GHz. It can be said that such high-speed control is extremely difficult and unrealistic.

Also, if you use a normal stable control speed APC circuit,
The drive speed of the entire semiconductor laser drive circuit is reduced, and high-speed pulse width / number modulation cannot be performed.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-mentioned problems of the conventional example, and to provide a laser recording apparatus which has a simple structure and can accurately perform exposure control by a semiconductor laser without being affected by temperature fluctuations.

[Means for Solving the Problems] The gist of the present invention for achieving the above object is to provide a semiconductor laser light source that exposes a recording medium for each pixel for information recording, and a light emitted from the semiconductor laser light source. 1 for each pixel
It has a control means for controlling so as to increase the output gradually within the range of the pixel, and a detection means for separating and detecting a part of the emitted light, and the control means partially outputs the emitted light whose output gradually increases. Based on the detection result of the detection means that has detected,
The laser recording apparatus is characterized in that the gradual output increase of the emitted light is stopped according to the data of each pixel.

Description of Embodiments First, the principle of each embodiment of the present invention will be described below.

[Means and Actions for Solving the Problem] In the present invention, the semiconductor laser is driven by a current having a waveform that gradually rises with time, and its optical output is monitored to respond to a temperature change.

Looking at the current-light output characteristics of the semiconductor laser shown in FIG. 12, the slope efficiency η [mW / mA] hardly changes even if the temperature changes. (The graph moves in parallel due to temperature fluctuations.) The slope efficiency may vary slightly depending on the type of semiconductor laser, but it can be used as long as it can be regarded as invariable within a range where temperature fluctuations are small.

Now, let P ₀ be the minimum optical output of the laser oscillation of the semiconductor laser. The light output in the LED oscillation region below P ₀ shall be negligible. If you can't ignore it, LE with poor coherence
The light in the D area can be reduced by an interference filter or a polarization filter. The driving current for outputting the minimum laser oscillation output P ₀ at a certain temperature T ₁ is i ₀ . The drive current of the semiconductor laser is linearly increased from i ₀ and the optical output is monitored. It is assumed that the time i is t [sec] and the current i increases in proportion to time as shown in the following equation.

i = i ₀ + kt (1) k is a constant.

The drive current is cut off when the light output rises from P ₀ by Ps. This is shown in FIG. In FIG. 6, 11 is a drive current that linearly increases, and 12 is an optical output when the temperature is T ₁ . The integrated value of the light output 12 is the exposure amount E, which is obtained by the following equation.

Here, consider the case where the temperature of the semiconductor laser chip rises from T ₁ to T ₂ . As described above, it is considered that the slope efficiency does not change in the temperature characteristics of the semiconductor laser light output and only the current-light output characteristics move in parallel. In this case, the minimum laser oscillation light output P ₀ does not change, but the drive current i ₀ for outputting P ₀ changes. This is approximately a current at which the optical output of the current-optical output characteristic at the temperature T ₂ after the parallel movement becomes P ₀ . The drive current for outputting P ₀ when the temperature of the semiconductor laser chip becomes T ₂ (> T ₁ ) is i ₀ ′ (> i ₀ ). As in the previous case, the change in light output when the current is linearly increased as shown in equation (1) is shown in FIG. 7 at 13 (broken line). Laser oscillation occurs from i ₀ ′ and cuts off the drive current when the light output rises by Ps. As shown in FIG. 7, since the slope efficiency does not change, the shape of the light output of 13 is exactly the same as when the temperature is T ₁ , and therefore the exposure amount E is almost the same as the expression (2). That is, considering the exposure amount of the equation (2) as the exposure amount of one pixel, the exposure amount does not change due to the temperature change of the semiconductor laser, and only the exposure position changes due to the temperature change. The position movement due to this temperature fluctuation amount is within the range of one pixel, and since it is minute, it falls below the resolving power of the human eye, which means that the exposure amount change due to temperature change is substantially corrected.

Next, the extinction ratio when the semiconductor laser is exposed to the sawtooth wave in this way will be obtained. In equation (2), the set light output Ps is
Theoretically, it can be 0, but in reality it cannot be 0 due to factors such as the delay of the control system, and can be set from the minimum value Ps ≠ 0. If the maximum value is Ps ₁ , the extinction ratio is as follows.

_{Ps 0 (P 0 + Ps 0} /2): Ps 1 (P 0 + Ps 1/2) ... (3) This value is proportional to the square of the set value. For example, if P ₀ = 1 mW and the maximum light output of the semiconductor laser is 15 mW, the extinction ratio will be 1:15 if simply intensity-modulated. When Ps ₀ = 1 mW and Ps ₁ = 15 mW in the equation (3), the extinction ratio when exposed by the sawtooth wave of the present invention is significantly increased to 1:85.

The control system of the present invention is relatively simple because the pulse width modulation and the exposure amount control can be configured by the same circuit. Further, since the control is performed only by turning on once and turning off once, a stable control system can be easily configured.

In order to further change the exposure amount E, in Eq. (2), Ps may be changed to change E. Alternatively, the slope k may be changed with Ps kept constant, or k and
It is also possible to change E by changing both of Ps.

Further, in a multi-beam type semiconductor laser having two laser oscillation mechanisms on the same chip or in the housing, one semiconductor laser is driven in a sawtooth shape and the other is pulse width modulated by a constant current, which is a conventional method. The temperature variation correction of the exposure amount of the pulse width modulation can be performed by the constant current drive.

FIG. 8 shows the optical output in this case.
Is the optical output of a semiconductor laser driven in a sawtooth wave, the optical output B
Indicates the optical output of a semiconductor laser driven by a constant current. The temperatures of the two semiconductor lasers are almost the same because they are on the same chip. When the light output A rises by Ps, the light output B is shut off. When the temperature rises from T ₁ to T ₂ , the optical output B decreases by the amount of temperature rise as shown in the figure, but when it reaches the temperature T ₂ , the timing of interruption is delayed as shown in the figure,
As a result, the exposure amount (time integrated value) of the light output B becomes equal at any temperature.

Example 1 Hereinafter, an example applied to a laser recording apparatus for recording a halftone image will be described in detail with reference to the drawings. FIG. 1 is a block diagram of a semiconductor laser drive circuit of the present invention, and 22 is a semiconductor laser. Reference numeral 23 denotes a PIN photodiode for monitoring the optical output of the semiconductor laser 22, which monitors the rear surface emission amount at the rear end of the chip inside the semiconductor laser 22 or provides a beam splitter outside the semiconductor laser 22 to provide the front surface. It is configured to monitor a part of the light emission amount.

The laser beam emitted from the semiconductor laser 22 is main-scanned by an optical system including an optical deflector such as a polygon mirror (not shown), and is irradiated onto a recording medium such as a photosensitive film or a photosensitive drum. The recording medium is gradually moved in the sub-scanning direction, so that a halftone image is two-dimensionally recorded on the recording medium.

Reference numeral 30 is a light emission amount setting value that is intended to be proportional to the light emission amount of the semiconductor laser 22, and is digital data. A look-up table 27 for converting pixel data is a means for correcting that the exposure amount becomes proportional to the square of the set value. 28 is a digital / analog converter for converting the corrected pixel data into an analog value. 26 is a comparator for comparing the detected light output with pixel data. Reference numeral 25 is a flip-flop for performing set / reset at the rising edge of the input, which is set by the pixel clock and reset by the output of the comparator 26. Reference numeral 20 is a sawtooth wave generation circuit that outputs a sawtooth wave synchronized with the pixel clock. 21 is a sawtooth wave semiconductor laser 22
, A voltage / current conversion circuit for converting the driving current into a drive current of the flip-flop 25, and a switch means 31 which is turned on / off by the output Q of the flip-flop 25. Reference numeral 24 is a current / voltage converter for converting the detection current of the PIN photodiode 23 into a voltage.

Next, the operation of the above configuration will be described according to the timing chart shown in FIG. In FIG. 2, C represents a pixel clock. "e" indicates an analog value obtained by converting the pixel data input in synchronization with the pixel clock by the digital / analog converter 28. Q is the output of flip-flop 25, which controls the on / off of switch 31. Vd is a sawtooth generator
The output of 20 is the input of the voltage / current converter 21, and v ₀ is the offset corresponding to the minimum current for laser oscillation of the semiconductor laser. Id is a drive current of the semiconductor laser, and when Q is on, a current according to Vd flows, and when it is off, no current flows. L is the optical output of the semiconductor laser.
Rs is the output of the comparator 26 and is an analog / digital converter
A current / voltage converter 24 in which the output e of 28 is the detected value of the optical output
When it is larger than the output of, it becomes high level, and when it is smaller, it becomes low level. Rs rising edge flip-flop
The output Q of 25 is reset.

In FIG. 2, the rising edge of 29 causes the output Q of the flip-flop 25 to go high and the switch 31 to turn on. The semiconductor laser starts laser oscillation at the same time when the switch 31 is turned on. When the optical output L becomes larger than the output of the analog / digital converter, Rs becomes high level, and at the rising edge 30, the flip-flop is reset and the optical output is cut off. By this operation, as described above, the exposure amount of the light output does not depend on the temperature change,
It depends only on the output e of the digital converter.

The exposure amount is the value obtained by replacing Ps with e in equation (2),
It is a form of a quadratic equation for e. Therefore, a lookup table for trying to make the exposure amount proportional to the pixel data 30.
27 is obtained from equation (2). Fig. 3 shows the shape of the lookup table, where 32 is the shape of the exposure dose (2).
Equation 31 is the lookup table.

[Embodiment 2] FIG. 4 is a block diagram of a second embodiment of the present invention. Since it is almost the same as the block diagram of the first embodiment shown in FIG. 1, only different points will be described. Reference numeral 33 denotes a semiconductor laser, which is formed on the same chip as the semiconductor laser 22. The semiconductor laser 22 is directly optically coupled to the PIN photodiode 23. The laser beam generated from the semiconductor laser 33 is scanned by using an optical system, an optical deflector, etc. not shown here, and an image is recorded on the photosensitive recording medium. The switch 31 used in FIG. 1 is used to turn on / off the drive current of the semiconductor laser 33. 34
Is a constant current source for driving the semiconductor laser 33 with a constant current.

The output current of the constant current source 34 is preset so that the semiconductor laser 33 emits an appropriate light output. The optical output of the semiconductor laser is Q in the timing chart of FIG.
Similarly, the pulse width modulation within one pixel is performed. As described above, even if the temperature of the semiconductor laser chip changes and the light output changes, the exposure amount can be controlled by adjusting the pulse width without being affected by the temperature change. In this case, the pixel data and the exposure amount are proportional to each other, and the look-up table as shown in FIG. 1 is unnecessary.

[Embodiment 3] FIG. 5 is a block diagram of a third embodiment of the present invention. In this embodiment, a triangular waveform having the same rising and falling speed is used as the laser drive current.

22 is a semiconductor laser. Reference numeral 23 denotes a PIN photodiode for monitoring the optical output of the semiconductor laser 22, which monitors the rear surface emission amount at the rear end of the chip inside the semiconductor laser 22 or provides a beam splitter outside the semiconductor laser 22. It is configured to monitor a part of the front surface emission amount. The laser beam generated from the semiconductor laser 22 is scanned by using an optical system, an optical deflector, or the like not shown here, and an image is recorded on the photosensitive material. 30 is a semiconductor laser
It is the light emission amount setting value that is going to be proportional to the light emission amount of 22.
It is digital data. A look-up table 27 for converting pixel data is a means for correcting that the exposure amount becomes proportional to the square of the set value. 28 is a digital / analog converter for converting the corrected pixel data into an analog value. 26 is a comparator for comparing the detected light output with pixel data. A flip-flop 25 is set at the rising edge of the set / reset input, is set by the pixel clock, and is reset by the output of the comparator 26. An AND gate 35 is used to control the duty of the square wave output Vs. 36 is an integrating circuit for shaping a square wave into a triangular wave Vt, 37 is a comparator provided to output a voltage v ₀ for flowing a minimum current for causing laser oscillation of a semiconductor laser, and 38 is a triangular wave Vt and v ₀ Is an adder that outputs the drive voltage Vd. Reference numeral 21 is a voltage / current conversion circuit for converting the drive voltage Vd into the drive current of the semiconductor laser 22, and 24 is a current / voltage converter for converting the detection current of the PIN photodiode 23 into a voltage.

Next, the operation of the above configuration will be described according to the timing chart shown in FIG. In FIG. 6, C represents a pixel clock. e represents an analog value obtained by converting the pixel data input in synchronization with the pixel clock by the digital / analog converter 28.

Vs is the output of the AND gate 35, which is a square wave whose duty is controlled by the output of the flip-flop 25 in synchronization with the pixel clock. Vt is a triangular wave output from the integrating circuit 36. Vd
Is an input voltage of the voltage / current converter 21, and is obtained by adding v ₀ to Vt when Vt> 0 and ₀ when Vt = 0. By adding the output of the comparator 37, the semiconductor laser 22 oscillates from the beginning of the rising of the triangular wave. L is the optical output of the semiconductor laser. Rs is the output of the comparator 26, which becomes high level when the output e of the analog / digital converter 28 is larger than the output of the current / voltage converter 24 which is the detected value of the optical output, and becomes low level when smaller. .
The output Q of the flip-flop 25 is reset at the rising edge of Rs.

In FIG. 6, the rising edge of 39 causes the output Q of the flip-flop 25 to go high, generating a triangular wave Vt. When a triangular wave is generated, the comparator 37 adds the voltage v ₀ to the triangular wave to generate Vd. A light output L of the semiconductor laser is generated as shown by the flow of a drive current proportional to Vd. L is detected and a digital / analog converter 28
Exceeds the output e of the comparator 26, the output of the comparator 26 becomes a high level, and the rising edge thereof resets the flip-flop 25. Then, the integrator 36 starts discharging and the triangular wave Vt
Begins to descend. This descending speed is the same as the ascending speed, and the light output also has a symmetrical shape to that at the rising time. The amount of exposure due to this light output also has no temperature dependence as in the first embodiment. The look-up table is the same as in the first embodiment.

[Embodiment 4] FIG. 9 is a block diagram of a fourth embodiment of the present invention.
Since it is almost the same as the block diagram of the first embodiment shown in FIG. 1, only different points will be described.

50 is a sawtooth wave generator, but the slope of the sawtooth wave generated can be set by an external electrical input. The inclination is obtained by converting the pixel data into an analog value by the digital / analog converter 28. The comparator 26 is configured to compare the optical output detected by the photodiode 23 with a predetermined constant value Ps. With this configuration, the drive current can be cut off when the light output exceeds the constant value Ps. The exposure amount E (the integrated value of the light output) at this time is given by the above-mentioned equation (2). Equation (2) is described again for explanation.

Therefore, η is the slope efficiency of the semiconductor laser, k is the slope of the driving current, and P ₀ is the minimum optical output of the semiconductor laser.

In the above-described first embodiment, the exposure amount E is controlled by changing Ps while keeping k in the equation (2) constant, but in the present embodiment, Ps is kept constant and k is changed corresponding to pixel data. The exposure amount E is controlled.

A means for changing the slope of the sawtooth wave by an electric input can be realized by a circuit configuration shown in FIG. 10, for example.
In FIG. 10, the pixel clock is input to the monostable multivibrator to fix the duty ratio, and the pixel clock is input to the triangular wave generator using the operational amplifier. The slope of the sawtooth wave is changed by changing the resistor used in the triangular wave generator by an electronic volume using an FET.

In this embodiment, since the light output is linearly increased and cut off at a certain light output, the effect of almost eliminating the change in the exposure amount due to the temperature change is the same as in the first embodiment.

It goes without saying that the driving current of the semiconductor laser may be a triangular wave as shown in the third embodiment, instead of the sawtooth wave.

[Fifth Embodiment] FIG. 11 is a block diagram of a fifth embodiment of the present invention.
The configuration is similar to that of FIG. 9 described above, but is characterized in that the input of the comparator 26 is also connected to the pixel data. In the present embodiment, the light amount setting value Ps also changes in proportion to the slope k, so that the pulse width is constant (one pixel) regardless of the pixel data value. This indicates that the exposure amount E is controlled by simultaneously changing the gradient k of the drive current and the light amount setting value Ps in the equation (2).

Now, the above-described first to fourth embodiments are a kind of pulse width modulation, and the pulse width, that is, the exposure time is changed according to the value of the pixel data. Therefore, when this is applied to the image recording apparatus, the pulse width within one pixel changes according to the value of the pixel data, and the exposure area within one pixel is biased. There was a problem that the degree of bias also fluctuated.

On the other hand, in the present embodiment, the pulse width, that is, the exposure time can be kept substantially constant by simultaneously changing the slope k and the light amount setting value Ps while associating them.
In other words, almost all the area within one pixel is exposed, so that the bias and fluctuation of the exposure area, which is a drawback of pulse width modulation, is eliminated. As a result, an image with high recording accuracy can be obtained.

In this embodiment also, the drive current is not a sawtooth wave,
As a matter of course, the triangular wave as shown in the third embodiment may be used.

By using the triangular wave, the density center of each pixel is aligned with the center of one pixel, and the density distribution within one pixel is made more uniform, so that a more accurate and high-quality halftone image can be obtained.

Now, in all the embodiments described above, 1
Although the pulse width modulation is performed once in the pixel,
The same modulation may be repeated multiple times within one pixel. This can be achieved by multiplying the frequency of the sawtooth wave or the triangular wave by an integer in the configuration of the embodiment. As a result, the density distribution within one pixel is subdivided and equalized, and a higher quality image is obtained.

[Advantages of the Invention] As described above, according to the present invention, the exposure amount control by the optical output of the semiconductor laser can be performed at high speed and at low cost without being affected by the temperature variation.

[Brief description of the drawings]

FIG. 1 is a block diagram of a semiconductor laser driving device embodying the present invention, FIG. 2 is a timing chart for explaining the operation of FIG. 1, and FIG. 3 shows the shape of the look-up table in FIG. FIG. 4, FIG. 4 is a block diagram of the second embodiment, FIG. 5 is a block diagram of the third embodiment, FIG. 6 is a timing chart for explaining the fourth operation, and FIG. FIG. 8 is a diagram for explaining the concept of the invention, FIG. 8 is a diagram for explaining the concept of the present invention, FIGS. 9 and 10 are configuration diagrams of a fourth embodiment of the present invention, and FIG. FIG. 12 is a configuration diagram of a fifth embodiment of the invention, FIG. 12 is an example of a driving current-optical output characteristic of a semiconductor laser, and FIG. 13 is a block diagram of a conventional APC circuit. The main symbols in the figure are: 20 …… Sawtooth generation means, 21 …… Voltage / current conversion means, 22 …… Semiconductor laser, 23 …… PIN photodiode, 25 …… Set-up reset flip-flop that operates on the rising edge, 31 ... Switch for cutting off the semiconductor laser drive current, 26 ... Comparator that compares the detected value of optical output with the set value, 27 ... Lookup table,

Claims (3)

(57) [Claims] 1. A semiconductor laser light source that exposes a recording medium for each pixel for information recording, and control so that light emitted from the semiconductor laser light source is gradually increased within a range of one pixel for each pixel. And a detection means for separating and detecting a part of the emitted light, wherein the control means detects a part of the emitted light whose output gradually increases based on the detection result of the detection means. A laser recording device, characterized in that the gradual increase in output of the emitted light is stopped according to the data of each pixel. 2. The laser recording apparatus according to claim 1, wherein the gradual stop of the output increase of the emitted light is a cutoff of the optical output. 3. The laser recording apparatus according to claim 1, wherein the gradual output increase of the emitted light is executed by repeating the same modulation a plurality of times within one pixel.