JPH03105368A - laser recording device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an apparatus for modulating and driving a semiconductor laser, and a laser recording apparatus for recording a light beam onto a recording medium using a semiconductor laser.

[Prior Art] Laser beam printers have been widely used as devices for recording digitally expressed halftone images onto photosensitive recording media. In this method, a laser beam whose intensity is modulated in proportion to the image density is deflected by an optical deflector to perform main scanning, and a recording medium such as a film or drum is moved perpendicular to the main scanning direction to perform sub-scanning. This is to record images.

As a means for generating the above-mentioned laser beam, semiconductor lasers are currently the cheapest and smallest, and have the advantage of being able to directly modulate the intensity with a drive current.

However, a drawback of semiconductor lasers is that the drive current/light output characteristic has a significant temperature-negative characteristic. Figure 12 is an example of the driving current vs. optical output characteristic of a semiconductor laser, where the horizontal axis shows the driving current (mA) of the semiconductor laser.
, the vertical axis shows the optical output (mW), and the case temperature is O℃,
Measurements were taken at 25°C and 50°C. Reading from the graph, a temperature negative characteristic of about -0.1 mW/'e can be found. This means that the light output fluctuates greatly due to fluctuations in outside temperature. Furthermore, the temperature of the semiconductor laser chip increases due to its own emission loss, resulting in a decrease in optical output.

Laser printers record at a speed of 1 to 2 msec main scanning and several seconds per page, so the outside temperature does not change during at least one main scanning period, and changes in optical output due to temperature changes during that time are due to light emission loss of the semiconductor laser itself. It is given only by those due to heat increase.

As a means of compensating for the optical output fluctuation due to the temperature change,
Conventionally, a circuit (hereinafter referred to as AP) sequentially monitors whether the emission level of a semiconductor laser matches a predetermined level (constant during the irradiation time) using a photodiode and feeds it back to the drive current.
C circuit) is known.

Fig. 13 is a configuration block diagram of a basic APC circuit, where 1 is a set voltage to make it proportional to the amount of light emitted, 2 is a voltage addition circuit, 3 is a semiconductor laser drive voltage Vd, and 4 is a drive voltage for 3. A voltage/current conversion circuit that converts the actual drive current to ■d, 5 is the drive current Id, 6 is the semiconductor laser, 7 is the PIN photodiode that monitors the amount of laser light emission, 8 is the monitor current m I m s 9 is the monitor current The current that converts Im into a monitor voltage Vm of 10/? This is an f-pressure conversion circuit.

The optical output of the semiconductor laser 6 is monitored by a PIN photodiode 7, so although it is omitted in the figure, the PIN photodiode 7 monitors the amount of light emitted from the back surface of the semiconductor laser at the rear end of the laser chip, or A beam splitter is provided in front of the beam splitter, and the beam splitter is configured to monitor the split light. FIG. 13 shows a typical single-loop feedback control system, with a set voltage of 1 V
The error between s and the monitor voltage Vm becomes the drive voltage Vd of 3, so that the optical output does not fluctuate due to temperature changes.
It is controlled so that it is always proportional to the set voltage Vs.

[Problems to be Solved by the Invention] However, in the conventional example described above, since the semiconductor laser is driven and oscillated with a rectangular waveform input current, the optical output level changes up and down due to temperature changes, and it is difficult to quickly compensate for this. APC circuits are extremely difficult to design, as will be described later.

In other words, in order to increase the extinction ratio (dynamic range) of the optical output, the pulse width of one pixel is
Considering the use of a method that modulates the intensity by changing the number of pulses (pulse width/number modulation) or a method that combines pulse width/number modulation and optical output variation (amplitude modulation), one pixel of a laser beam printer The average recording speed (pixel clock frequency) is as high as several MHz, and for example, if pulse width modulation with 8-bit gradation is performed, the minimum pulse width will be as short as several nanoseconds. The intensity adjustment of a semiconductor laser that emits such a short pulse width is
In order to perform the C circuit accurately, the control speed must be several tens of GHz.
z must be extremely high. Performing such high-speed control is extremely difficult and unrealistic.

Also, if you use a normal APC circuit with stable control speed,
This reduces the driving speed of the entire semiconductor laser driving circuit, making it impossible to perform high-speed pulse width number modulation.

SUMMARY OF THE INVENTION An object of the present invention is to solve the problems of the prior art described above and to provide a device that can easily cope with temperature changes.

[Means and Effects for Solving the Problems] According to the present invention, a semiconductor laser is driven with a current having a waveform that gradually increases over time, and its optical output is monitored to respond to temperature changes.

Looking at the current vs. optical output characteristics of the semiconductor laser shown in Figure 12, we can see that the slope efficiency η(m W /
mA) has hardly changed. (The graph moves in parallel due to temperature fluctuations.) Although the slope efficiency may vary slightly depending on the type of semiconductor laser, it can be used as long as it can be considered to remain unchanged within a small temperature fluctuation range.

Now, let P0 be the minimum optical output of the laser oscillation of the semiconductor laser. It is assumed that the light output of the LED oscillation region below P0 can be ignored. If it cannot be ignored, the light in the poorly coherent LED region can be reduced by interference filters or polarizing filters. At a certain temperature T1,
The drive current for outputting the lowest laser oscillation light output P0 is assumed to be 10. The driving current of the semiconductor laser is increased linearly from to, and the optical output is monitored. Time t(s
ee), the current i is assumed to increase in proportion to time as shown in the following equation.

i=io +kt ・-・(1) k is a constant.

The drive current is cut off when the optical output increases by Ps from P0. This situation is shown in FIG. 11 in Figure 6
is the driving current that increases linearly, and 12 is the optical output when the temperature is T. The integral value of the light output 12 is the exposure amount E, which is determined by the following equation.

Let us now consider the case where the temperature of the semiconductor laser chip increases from T1 to T2. As mentioned 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 shift in parallel. In this case, the minimum laser oscillation light output P. does not change, but the drive current 10 for outputting Po changes. Approximately, this means that the optical output of the current vs. optical output characteristic at the parallel-shifted temperature T2 is P
The temperature of the 6 semiconductor laser chip, which is the current of G, is T2
P when it becomes (>T+). The drive current to output i.゜(〉1.). As before, 13 (broken line) in FIG. 7 shows the change in optical output when the current is increased linearly as shown in equation (1).

Laser oscillation occurs from 10', and the drive current is cut off when the optical output increases by Ps. As shown in FIG. 7, since the slope efficiency does not change, the shape of the optical output of 13 is exactly the same as when the temperature is T1, and therefore the exposure amount E is (
2) It is almost the same as formula. That is, if the exposure amount in equation (2) is considered as the exposure amount for one pixel, the exposure amount will not change due to temperature changes of the semiconductor laser, and only the exposed position will change due to temperature changes. This positional movement due to temperature fluctuation is within the range of one pixel, and is minute, so if it falls below the resolution of the large eye, it means that the exposure change due to temperature change has been substantially corrected. Next, let's find the extinction ratio when a semiconductor laser is exposed with a sawtooth wave as described above. In equation (2), the set optical output Ps is:
Theoretically, it can be 0, but in reality, due to factors such as delays in the control system, O (can be set from the minimum possible value Ps≠0.If the maximum value is Ps., the extinction ratio is given by the following formula.P S(+ (P0+P56 /2): PS+
(Po 1 Ps, /2) (3) This value is proportional to the square of the set value.

For example, if p0 = 1 mW and the maximum optical output of the semiconductor laser is 15 mW, the extinction ratio will be 1:15 if the intensity is simply modulated. In equation (3), Pso = 1mW
, Psl=15 mW, the extinction ratio when exposed with the sawtooth wave of the present invention significantly increases to 1:85.

The control system of the present invention is relatively simple because pulse width modulation and exposure control can be configured in the same circuit. Furthermore, since control is performed by only turning on once and turning off once, it is easy to construct a stable control system.

In order to further change the exposure amount E, in equation (2),
It is sufficient to change E by changing Ps, but instead, it is also possible to change the slope k while keeping Ps constant, or to change E by changing both k and Ps.

In addition, on the same chip or within the housing (in a multi-beam type semiconductor laser with two laser oscillation mechanisms, one semiconductor laser is driven in a sawtooth waveform and the other is pulse width modulated by constant current) Temperature fluctuation correction of the exposure amount of pulse width modulation can be performed by constant current driving. Figure 8 shows the optical output in this case, where optical output A is the optical output of the semiconductor laser driven in a sawtooth waveform. B indicates the optical output of a semiconductor laser driven by a constant current.The temperatures of both semiconductor lasers are approximately the same since they are on the same chip.When the optical output A increases by Ps, the optical output B is cut off. When the temperature rises from T to T2, the optical output B decreases by the amount of temperature rise as shown in the figure, but when the temperature reaches T2, the timing of the cutoff is delayed as shown in the figure, and as a result the optical output decreases. The exposure amount (time integral value) of B is the same at any temperature.

[Example 1] Hereinafter, an example applied to a laser recording apparatus for recording halftone images will be described in detail with reference to the drawings. FIG. 1 shows a block diagram of a semiconductor laser driving circuit according to the present invention, and 22 is a semiconductor laser. 23 is a PIN photodiode for monitoring the optical output of the semiconductor laser 22, and the amount of light emitted from the rear side can be monitored at the rear end of the chip inside the semiconductor laser 22, or a beam splitter can be provided outside the semiconductor laser 22 to monitor the amount of light emitted from the front side. It is configured to monitor a portion of the amount of light emitted.

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

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

Next, the operation in the above structure will be explained according to the timing chart shown in FIG. In FIG. 2, C represents a pixel clock. e indicates an analog value obtained by converting pixel data inputted in synchronization with the pixel clock by the digital/analog converter 28. Q controls the on/off of the switch 31 by the output of the flipflop 25. Vd is the output of the sawtooth generator 20, and Vd is the input of the voltage/current converter 21. is an offset corresponding to the minimum current for causing the semiconductor laser to oscillate; Id is the drive current of the semiconductor laser; when Q is on, a current according to Vd flows; when Q is off, no current flows. L is the optical output of the semiconductor laser. Rs is the output of the comparator 26, and 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 it is smaller. The output Q of the flip-flop 25 is reset at the rising edge of Rs.

In FIG. 2, the output Q of the flipflop 25 becomes high level due to the rising edge of the signal 29, and the switch 3
1 is turned on. The semiconductor laser starts laser oscillation at the same time that 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 its rising edge 30, the flip-flop is reset and the optical output is cut off. With this operation, as described above, the exposure amount of the optical output does not depend on temperature changes, but only on the output e of the analog/digital converter.

The exposure amount is obtained by replacing Ps with e in equation (2),
It is in the form of a quadratic equation regarding e. Therefore, the lookup table 27 for making the exposure amount proportional to the pixel data 30 can be obtained from equation (2). Figure 3 shows the shape of the lookup table, where 32 is the shape of the exposure amount (
2) Equation 31 is a 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 FIG. 1, which is a block diagram of the first embodiment, only the different parts will be explained. 33 is a semiconductor laser, which is constructed on the same chip as the semiconductor laser 22. The semiconductor laser 22 is a PIN photodiode 23
is directly optically coupled to the The laser beam generated from the semiconductor laser 33 is scanned using an optical system, a light polarizer, etc. (not shown here), and an image is recorded on a photosensitive recording medium. And switch 3 used in Figure 1
1 is used to turn on and off the drive current of the semiconductor laser 33. 34 is a constant current source that drives the semiconductor laser 33 with a constant current.

The output current of the constant current source 34 is set in advance so that the semiconductor laser 33 emits an appropriate optical output. The optical output of the semiconductor laser becomes the same as Q in the tying chart of FIG. 2, and pulse width modulation within one pixel is performed.

As mentioned above, even if the temperature of the semiconductor laser chip changes and the optical output changes, the exposure amount can be controlled without being affected by temperature fluctuations by adjusting the pulse width. In this case, the pixel data and the exposure amount are proportional, and a look-up table as shown in FIG. 1 is not required.

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

22 is a semiconductor laser. 23 is a PIN photodiode for monitoring the optical output of the semiconductor laser 22, and the amount of light emitted from the back surface is monitored at the rear end of the chip inside the semiconductor laser 22, or a beam splitter is provided at the outer edge of the semiconductor laser 22. It is configured to monitor a portion of the amount of light emitted from the front surface. The laser beam generated from the semiconductor laser 22 is scanned using an optical system, a light deflector, etc. (not shown here), and an image is recorded on the photosensitive material. 30
is a light emission amount setting value to be made proportional to the light emission amount of the semiconductor laser 22, and is digital data. Reference numeral 27 denotes a look-up table for converting pixel data, and is means for correcting the exposure amount from being proportional to the square of the set value. 28 is a digital/analog converter that converts the corrected pixel data into analog values. 26 is a comparator that compares the detected light output and pixel data. Reference numeral 25 denotes a flip-flop which is activated at the rising edge of the set/reset input, and is set by the pixel clock and is connected to the comparator 26.
is reset by the output of

35 is an AND gate, which is used to control the duty of the square wave output Vs. 36 converts the square wave into a triangular wave Vt
37 is a voltage V for flowing the minimum current to cause the semiconductor laser to oscillate. A comparator 38 is provided to output triangular waves Vt and V. This is an adder that adds up and outputs the drive voltage Vd. 21 is a voltage/current conversion circuit that converts the driving voltage Vd into an IR current of the semiconductor laser 22; 24 is a current/voltage converter that converts the detected current of the PIN photodiode 23 into a voltage.

Next, the operation of the above configuration C will be explained according to the timing chart shown in FIG. In FIG. 6, C represents a pixel clock. e indicates an analog value obtained by converting 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 synchronized with the pixel clock and whose duty is controlled by the output of the flipflop 25. Vt is a triangular wave output from the integrating circuit 36. Vd is the input voltage of the voltage/current converter 21,
If Vt>O, set Vo; if Vt=O, set 0 to V
This is in addition to t. By adding the output of the comparator 37, the semiconductor laser 22 oscillates from the beginning of the rise of the triangular wave. L is the optical output of the semiconductor laser. Rs is the output of the comparator 26, and the output e of the analog/digital converter 28 is the current /
When the output is larger than the output of the voltage converter 24, it becomes a high level, and when it is smaller, it becomes a low level. At the rising edge of Rs, the output Q of the flipflop 25 is reset.

In FIG. 6, the rising edge of 39 causes the output Q of the flipflop 25 to become high level, resulting in a triangular wave Vt.
occurs. When a triangular wave is generated, the comparator 37
Vd is created by adding voltage v0 to the triangular wave. As a driving current proportional to Vde flows, an optical output L of the semiconductor laser is generated as shown in the figure. Detect L, digital/
When the output e of the analog converter 28 is exceeded, the output of the comparator 26 becomes high level and the flipflop 25 is reset by its rising edge. Then integrator 3
6 starts discharging and the triangular wave Vt starts falling. This downward speed is the same as the upward speed, and the optical output is also symmetrical to that during the upward movement. The amount of exposure due to this optical output is also independent of temperature, as in the first embodiment. The lookup table is also 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 FIG. 1, which is a block diagram of the first embodiment, only the different parts will be explained.

50 is a sawtooth wave generator, and the slope of the generated sawtooth wave can be set by external electrical input. The slope uses pixel data converted 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 optical output exceeds a certain value Ps.

The exposure amount E (integral value of light output) at this time is as described above (2)
It is given by Eq. For the sake of explanation, equation (2) is written again.

Here, η is the slope efficiency of the semiconductor laser, k is the slope of the drive current, and Po is the minimum optical output of the semiconductor laser.

In the previous embodiment 1, the exposure amount E was controlled by changing Ps while keeping k in equation (2) constant, but in this embodiment, Ps
The exposure amount E is controlled by keeping k constant and changing k in accordance with pixel data.

The means to change the slope of the sawtooth wave by electrical input is as follows:
For example, it can be realized with a circuit configuration as shown in FIG. In FIG. 10, a pixel clock is input to a monostable multivibrator to fix the duty ratio, and is input to a triangular wave generator using an operational amplifier. The slope of the sawtooth wave is changed by making the resistor used in the triangular wave generator variable using an electronic volume using an FET.

In this embodiment, since the optical output is increased linearly and cut off at a certain optical output, there is an effect that almost no change in exposure amount due to temperature change is achieved, as in the first embodiment.

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

[Example 5] FIG. 11 is a block diagram of a fifth embodiment of the present invention.

The configuration is similar to that of FIG. 9 above, but is characterized in that the input of the comparator 26 is also connected to pixel data. In this embodiment, the light amount set value Ps also changes in proportion to the slope k, and is designed to have a constant pulse width (one pixel) regardless of the value of pixel data. This means that in equation (2), the exposure amount E is controlled by simultaneously changing the slope k of the drive'ia current and the light amount setting value Ps.

Now, in the first to fourth embodiments described above, a type of pulse width modulation is used, and Barth's cap, that is, the exposure time is changed depending on the value of pixel data. Therefore, when this is applied to an image recording device, the pulse radiation changes within one pixel according to the value of pixel data, causing bias in the exposure area within one pixel, and furthermore, depending on the pixel data. There was a problem that the degree of bias also fluctuated.

On the other hand, in this embodiment, the pulse width, that is, the exposure time, can be kept almost constant at all times by correlating the slope k and the light amount setting value Ps and changing them simultaneously. In other words, since almost the entire area within one pixel is always exposed, the bias and fluctuation of the exposed area, which are disadvantages of pulse width modulation, are eliminated. As a result, an image with high recording accuracy can be obtained.

It goes without saying that in this embodiment as well, the drive current may be a triangular wave as shown in the third embodiment, instead of a sawtooth wave.

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

Now, in all the embodiments explained above, pulse width modulation is performed once within one pixel, but once pulse width modulation is performed within one pixel.
Similar modulation may be repeated multiple times within a pixel. This can be achieved by multiplying the frequency of the sawtooth wave or triangular wave by an integer in the structure of the embodiment. As a result, the density distribution within a pixel is subdivided and equalized, resulting in a higher quality image.

In addition to the laser recording device described in the above embodiment, the present invention can be widely applied to various devices using semiconductor lasers in various fields, such as laser reading devices, laser processing machines, and medical laser treatment devices. It is possible to do so.

[Effects of the Invention] According to the present invention, exposure amount control using the optical output of a semiconductor laser can be performed at high speed and at low cost without being affected by temperature fluctuations.

[Brief explanation of drawings]

Figure 1 is a configuration diagram of a semiconductor laser driving device embodying the present invention, Figure 2 is a timing chart for explaining the operation of Figure 1, and Figure 3 shows the shape of the lookup table in Figure 1. 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 operation of the fourth embodiment, and FIG. 7 is a block diagram of the second embodiment. Figure 8 is a diagram for explaining the idea of the invention, Figures 9 and 10 are diagrams for explaining the idea of the invention.
The figure is a block diagram of the fourth embodiment of the present invention, Figure 11 is a block diagram of the fifth embodiment of the present invention, Figure 12 is an example of the driving current vs. optical output characteristic of a semiconductor laser, and Figure 13 is a diagram of the configuration of the fifth embodiment of the present invention. This is a block diagram of a conventional APC circuit, and the main symbols in the diagram are: 20...Sawtooth wave generation means, 21...Voltage/current conversion means, 22...Semiconductor laser, 23. ...PIN photodiode, 25...Set/reset flip-flop that operates on rising edge, 31...Switch for cutting off drive current of semiconductor laser, 26...Optical output. Comparator for comparing detected value and set value, 7... Lookup table,

Claims (8)

[Claims] (1) a semiconductor laser; means for setting an optical output setting value of the semiconductor laser; means for driving the semiconductor laser with a predetermined input waveform so that the optical output gradually increases over time; and the driven semiconductor laser. means for detecting the optical output of the semiconductor laser; means for comparing the detected optical output with the set value; and controlling the optical output of the semiconductor laser at the predetermined input waveform until the optical output reaches the set value. A semiconductor laser driving device comprising: means for forming a semiconductor laser. (2) The semiconductor laser driving device according to claim 1, wherein the optical output setting value is variable depending on the exposure amount, and the slope of the predetermined input waveform that gradually increases the optical output is constant. (3) The semiconductor laser driving device according to claim 1, wherein the optical output setting value is a constant value, and the slope of the predetermined input waveform that gradually increases the optical output is variable depending on the exposure amount. (4) The semiconductor laser driving device according to claim (1), wherein the optical output setting value and the slope of the predetermined input waveform that gradually increases the optical output are variable according to the exposure amount, and the exposure time is substantially constant. . (5) The semiconductor laser driving device according to claim 1, wherein when the gradually increasing optical output reaches the set value, the optical output is gradually decreased using an input waveform having an opposite slope to the predetermined input waveform. (6) The semiconductor laser is capable of independently outputting two laser beams from within the same housing, one of which is driven so that the optical output increases gradually, and its output is detected;
2. The semiconductor laser driving device according to claim 1, wherein the other is driven to have a constant output, and the output of the latter is controlled by comparing the detected output value of the former with the set value. (7) a semiconductor laser; means for setting an optical output setting value of the semiconductor laser; means for driving the semiconductor laser with a predetermined input waveform so that the optical output gradually increases over time; and the driven semiconductor laser. means for detecting the optical output of the semiconductor laser; means for comparing the detected optical output with the set value; and controlling the optical output of the semiconductor laser at the predetermined input waveform until the optical output reaches the set value. A laser recording apparatus comprising: means for forming a semiconductor laser; and means for irradiating a recording medium with the output of the semiconductor laser. (8) The laser recording apparatus according to claim (7), further comprising means for relatively moving the output beam of the semiconductor laser and the recording medium, and forming an image on the recording medium.