JPS6354273B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a recording device that uses a semiconductor laser as a light source and performs recording by scanning an image recording body with a laser beam modulated based on image information; The present invention relates to a laser beam scanning device used in a reading device.

Since the optical output of a semiconductor laser has a significant temperature dependence, it is necessary to stabilize the optical output in order to use a semiconductor laser as a light source for a recording device or a reading device. Conventionally, as a method for this stabilization, a method of controlling the temperature of a laser diode (hereinafter abbreviated as LD) has been known. However, since there is a temperature difference (this is not constant) between the case (mounting part), which is the temperature detection point, and the LD chip, the optical output fluctuates greatly, and this method alone can reduce the optical output within the allowable fluctuation range. It was difficult to keep it within myself. In addition, in recording devices such as laser printers, the average output of the LD during recording is not constant, so
It has also been impossible to use a stabilization method for optical communications lasers that controls the LD drive current based on the LD's average output.

Under these circumstances, recently, a sampling time synchronized with the beam position detection signal has been established within the recording time, and a part of the laser beam is sampled through a beam splitter, and the intensity of the laser beam is detected and the LD A method was proposed to control the drive current. However, this method requires a sampling time within the recording time and for each scanning line, so there is a problem that the sampling time is extremely short and the frequency response of the feedback system must be extremely high. For this reason, there is a limit to speeding up image forming processing. Also, a method has been attempted in which the LD drive current is controlled by sampling not every row but every page. In this case as well, the voltage for determining the drive current is obtained from the hold capacitor of the sample and hold circuit, but capacitors generally have leakage current, so it is impossible to hold the same voltage for a long time. There was a problem in that the drive current could not be kept constant throughout recording.

The present invention has been made to solve the above problems, and its purpose is to provide a laser beam scanning device that is capable of high-speed image reading and image formation, and that can also stably control light output.

The basic configuration of the present invention that achieves this objective is: a comparator that compares the optical output level of the LD with a reference value;
A counter that counts the clock input according to the output of this comparator, a DA converter that converts the output of this counter into analog, an adder that adds a fixed amount of offset to the output of this DA converter, and this Outputs the current corresponding to the output voltage of the adder and
This device is characterized by being configured with an LD driver that drives the LD.

Hereinafter, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a schematic diagram showing the main parts of an embodiment of a laser beam scanning device (applied to a laser printer) according to the present invention. In the figure, 1 is
A semiconductor laser device 2 having an LD inside is a rotating polygon mirror for reflecting a laser beam that comes out from the semiconductor laser device 1 and enters the optical system 8. This rotating polygon mirror 2 rotates at a constant speed in the direction of the arrow in the figure in order to cause the laser beam to run in the main scanning direction X. Reference numeral 3 denotes a photosensitive drum which rotates in the sub-scanning direction Y, and is constructed by, for example, depositing a photoconductive insulating layer containing selenium as a main component on the surface of a hollow aluminum cylinder. 4
is a charger for uniformly charging the surface of the photoreceptor drum 3. The surface of the photoreceptor drum 3 charged by the charger 4 is irradiated with a laser beam reflected by the rotating polygon mirror 2, and the surface of the photoreceptor drum 3 is statically charged. Forms an electrolatent image.
Note that, in this figure, a developing device, a heat fixing device, a paper feed/discharge mechanism, etc. for outputting the latent image as a hard copy are not shown. 5 is a photodetector such as a phototransistor for detecting the optical output of the LD at the scanning start end; 6 is an electric circuit section having various functions such as supplying a driving current to the semiconductor laser device 1;
Reference numeral 7 denotes an f/θ lens that focuses the laser beam onto the surface of the photosensitive drum 3.

Fig. 2 is an electrical configuration diagram showing a part of the apparatus shown in Fig. 1, mainly showing the details of the electric circuit section 6 in Fig. (omitted). In Figure 2, 21
2 is a light output detector having a photodetector 5 that detects the intensity of the laser beam for a certain period of time before the start of one page recording;
2 is an amplifier that appropriately amplifies a voltage signal (monitor signal) corresponding to the laser beam intensity obtained from the optical output detector 21, and 23 and 24 are comparators, respectively. The comparator 23 compares the output Vp of the amplifier 22 with the reference voltage Vref, and outputs "0" when Vp>Vref.
When Vp≦Vref, “1” is output. On the other hand, the comparator 24 compares Vp with Vref+ΔV, and
＞Vref+ΔV, set “1”, Vp≦Vref+ΔV
When , "0" is output. 2
5 is an up/down counter which increments when the clock is input to the up terminal U, and subtracts when the clock is input to the down terminal D. 26 and 27 are AND gates for guiding a clock to the up terminal and down terminal, to which a gate signal G is commonly applied. This gate signal G
is given by a control circuit (not shown), and is "1" during the period when laser beam irradiation is possible when outputting a modulation signal based on image information or sampling the laser beam output at the start of scanning, and "0" during other periods. ” is the signal. Further, the output of the comparator 23 is input to the gate 26, and the clock is passed when the output A of the comparator 23 and the gate signal G are both "1". On the other hand, gate 27
is inputted with the output B of the comparator 24, and allows the clock to pass when both B and G are "1". The clock in this case has a sufficiently high frequency compared to the scanning speed of the laser beam. When the up/down counter 25 that counts such clocks overflows (addition,
overflow may occur regardless of subtraction),
The light emitting diode _D1 of the error display circuit 28 lights up and at the same time sends an error signal to the control circuit to give a warning to the user. The error display circuit 28 is shown as being composed of a light emitting diode _D1 pulled up to the +5V power supply by a resistor R.
Other configurations may also be used. Note that a microcomputer or the like is used as a control circuit (not shown). 29 is a digital-to-analog converter (hereinafter abbreviated as DA converter) that converts the output value of the up/down counter 25 into analog; 30 is a DA converter 29
An adder 31 adds an offset voltage Vos to the output Vc of the adder 30, and an adder 31 drives the LD with a current corresponding to the output voltage Vos + Vc of the adder 30, and performs ON/OFF modulation of the current in synchronization with a modulation signal based on image information. It is an LD driver that can. Also, 3
3 is an OR gate that takes the logical sum of the outputs A and B of the comparators 23 and 24, 34 is an AND gate that takes the logical product of the output of the OR gate 33 and gate signal G, and 35 is a logical sum of the output of the AND gate 34 and the modulation signal. This is an OR gate that takes the signal and outputs it to the LD driver 31.

The operation of the apparatus of this embodiment configured as described above will be explained next. The optical output characteristics of the LD are shown in Figure 3. That is, when the laser light output intensity exceeds a certain LD drive current value, it rapidly increases in proportion to the current. However, this relationship depends on temperature, and exhibits a characteristic that the higher the LD temperature for the same current, the lower the laser intensity. Now, since the gate signal G becomes "1" during the sampling period before the start of recording one page, and the output A of the comparator 23 becomes "1", the LD
lights up. At this time, the optical output level of the LD is detected by the photodetector 5, and a voltage Vp corresponding to the laser intensity is obtained via the amplifier 22. If this Vp is lower than Vref, the output of the comparator 23 is "1" and the output of the comparator 24 is "0", so the OR gate 33
The output is “1” and the LD is in a lit state,
A clock is input to the up terminal U of the up/down counter 25. The output of the up/down counter 25, which increases by counting the input clock, is simultaneously converted into an analog signal by the DA converter 29 and applied to the adder 30. The LD driver 31 increases its output current in response to the gradually increasing output Vos+Vc of the adder. When the optical output intensity of the LD rises to Vref in terms of the output of the amplifier 22 due to an increase in the drive current, the output of the comparator 23 is inverted to "0". As a result, the gate 26 prohibits the passage of the clock, and at the same time, the output of the OR gate 33 becomes "0" and the LD is turned off regardless of the gate signal G. Further, the up/down counter 25 holds the count value up to that point. In this way LD
The light output intensity of is raised and fixed to a predetermined level. Further, when the laser beam intensity is greater than Vref+ΔV in terms of the output of the amplifier 22, the output of only the comparator 24 becomes "1", and the OR gate 3
The output of the counter 3 also becomes "1", the LD is in a lit state, and a clock is input to the down terminal D of the counter 25 via the gate 27. This results in
The output of counter 25 decreases and the output of adder 30
Vos+Vc decreases contrary to the case described above. Accordingly, the output current of the LD driver 31 also decreases, and the LD
The light output of is also reduced. When the output of the amplifier 22 corresponding to the optical output intensity of the LD becomes Vref+ΔV, the clock counting of the counter 25 stops, and at the same time, the output of the OR gate 33 becomes "0" and the LD turns off.
The light output intensity of is fixed at that level. At the same time as the predetermined sampling time before recording one page ends, the gate signal G applied from the control circuit becomes "0", and the clock input to the up/down counter 25 is controlled regardless of the outputs of the comparators 23 and 24. prohibit. Of course, the sampling time is LD
The up/down counter 25 will not exceed the timing clock within this time.
In this way, the optical output intensity of the LD is set to Vref for each page.
It can be automatically adjusted to within a corresponding level in the range of Vref+ΔV.

Note that when the laser beam is detected, Vp has already reached
If it is between Vref and Vref+ΔV, the counter 25 does not count the clocks and the same state is maintained.

Specific numerical examples in the above embodiment are as follows. The LD driver 31 is selected so that the output current is 25 mA per 1 V of input voltage. Since the LD drive current fluctuation is approximately 30 mA for a temperature fluctuation of 10 to 40°C, the 6-bit DA
The converter corresponds to 0.5mA per bit.
Vos will be below the rated output at the lowest temperature
Set it to 4.4V, Vref to 3V, ΔV to 0.1V, and 1.2V as the Vc variable range.

In the above embodiment, the optical output intensity level is controlled by the width of ΔV, but in order to further reduce the width of ΔV, a configuration as shown in FIG. 4 may be used. The difference in Fig. 4 from Fig. 2 is that the comparator uses Vref.
The only difference is that only a comparator 23 is used to compare Vp and Vp, and a clock is input through a gate 26 so that the counter 25 only performs addition and counting.
This counter 41 is reset by a command from the control circuit every time one page is scanned. Therefore, the LD driver 31 is given a voltage that gradually increases from the initial value Vos each time, and each time the driven LD is controlled, the intensity increases from the voltage corresponding to Vos.
It is designed to be fixed when the strength increases to correspond to Vref.

Although the above explanation was about a laser printer using a photosensitive drum, the present invention can also be applied to other types of laser recording or laser reading. Therefore, the surface to be scanned in the apparatus of the present invention is not limited to a photoreceptor having a photoconductive insulator (selenium, OPC, ZnO, amorphous silicon, etc.), but also a silver salt photosensitive material, a photosensitive resin, a thermoplastic material, etc. It will be formed from various materials such as heat-sensitive recording material.

As explained in detail above, according to the present invention,
You can get the following effects:

(1) To maintain Vc with a digital counter and DA converter, it is not possible to use a conventional capacitor.
Compared to the method of holding Vc, there is no attenuation related to elapsed time, and the optical output can be controlled at a constant level for a long time.

(2) In the case of the conventional analog type, the response time of the feedback system is long and requires a settling time of about 20 ms, but in the present invention, the response time is as short as 6.4 ms at maximum even when the clock is 10 KHz, and the implementation of Fig. 2 is possible. In the case of the example, from the second time onwards, it is only necessary to correct the deviation from the previous value, so it can be settled in just a few hundred μs.

(3) In conventional analog feedback control, the rate of fluctuation in the optical output of the LD is compressed to a fraction of the loop gain, and the larger the loop gain, the smaller the fluctuation rate, but increasing the loop gain causes oscillation. However, according to the present invention, there is no problem of oscillation and the fluctuation rate can be easily suppressed.

(4) Since the drive current is varied to an appropriate value using a sufficiently high-speed clock, the settling time is short and high-speed recording can be achieved.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing the main parts of an embodiment of the laser beam scanning device according to the present invention, FIG. 2 is an electrical configuration diagram of the device shown in FIG. 1, and FIG. 3 is an explanation of the optical output characteristics of the LD. 4 are explanatory diagrams showing other embodiments of the present invention. 5...Photodetector, 21...Light output detector, 22
...Amplifier, 23, 24...Comparator, 25...Up/down counter, 26,27,34...
AND gate, 29...DA converter, 30...Adder, 31...LD driver, 33, 35...OR gate.

Claims (1)

[Claims] 1. Detecting part of the optical output of the laser diode,
A laser beam scanning device using a laser beam whose optical output is stabilized by comparing the detection output with a reference value includes a comparator that compares a voltage Vp corresponding to the optical output intensity of the laser diode with the reference value; A counter that counts the clock input according to the output of the comparator, a digital-analog converter that converts the output of this counter into analog, and an adder that adds a fixed amount of offset to the output of this digital-analog converter. and a laser diode driver that outputs a current corresponding to the output voltage of the adder to drive the laser diode, and the optical output intensity reaches a predetermined value within a sampling time before the laser beam starts scanning one page. A laser beam that controls input pulses to the counter, causes the counter to hold the count value at the end of the sampling time, and outputs the count value to the digital-to-analog converter until one page scanning is completed. scanning device. 2. The comparator is a ratio of two comparators that compare the voltage Vp with two different reference values, and the counter is used to add or subtract input clocks based on the outputs of the two comparators. 2. A laser beam scanning device according to claim 1, wherein the laser beam scanning device is an up-down counter.