JPS62247671A - Method for controlling density of laser beam printer - Google Patents

Abstract

PURPOSE:To preform the control of density of a laser beam printer without using a movable part by varying the pulse width of the analog signal which drives directly a laser diode. CONSTITUTION:The picture data on an address designated by an address decod er 23 is read out of a dynamic memory 24 synchronously with the pixel clock of 2MHz and applied to AND gates 25a-25h respectively as the output of 8 bits (256 gradations). These gates 25a-25h are controlled by a data selector 29 and the pulse of the analog signal is modulated by opening/closing said gates. Here the pluses having seven different widths are outputted in parallel from a shift register 28 and sent to the selector 29. The selector 29 selects the optional one of those seven different pulses according to the states of data inputs A-C and send it to gates 25a-25h respectively.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a density adjustment method in a laser beam printer of the type in which a laser diode is directly driven by an analog input signal.

Conventional Technique 1r A conventional method for adjusting the density of a laser beam printer is, for example, by rotating a beam splitter. FIG. 7 shows a laser beam printer that executes the above method, in which 1 is an analog input signal applied to the LD drive circuit 2, 3 is a laser diode (hereinafter abbreviated as LD), 4
is the laser light emitted from the LD, and the beam splitter 5
The beam is separated into an imaging beam 6 and a monitoring beam 7. The imaging beam 6 passes through another beam splitter 8° optical system 9 and forms an image on the imaging surface. The monitoring beam 7 is photoelectrically converted by the optical sensor 1) and negatively fed back to the LD drive circuit 2.

The one beam splitter 8 is rotatable around the optical axis. When the rotation angle of the beam splitter 8 from a predetermined position is θ, the power of the laser beam on the image forming surface 10 changes by cos 2θ as shown in FIG. This is because the transmittance changes when the polarizing beam splitter is rotated since the LD is P-polarized due to its structure. The rotation of this beam splitter 8 is controlled by software, thereby switching the power range on the imaging surface.
Concentration adjustment becomes possible.

By the way, the conventional concentration adjustment method described above uses a movable structure in which the beam splitter is rotated, so the material of the movable part
Problems such as rust and wear may occur depending on environmental conditions.

Furthermore, due to the eccentricity of the beam splitter, there is a problem in that when the beam splitter is rotated, the optical axis shifts and the image becomes distorted. This problem is particularly noticeable in color printers that form images by superimposing a plurality of beams.

In view of these problems, it is an object of the present invention to provide a useful density adjustment method that can adjust the density of a laser beam printer without using a movable part.

Means for Solving the 5 Problems In order to achieve the above object, the present invention shortens the light emission of the laser diode by pulse modulating the analog signal that directly drives the laser diode at a frequency corresponding to one pixel on the image. The laser beam printer is characterized in that the pulse width of the analog signal is varied.

Function: In the present invention, an LD drive signal, which is an analog signal, is pulse-modulated at a frequency corresponding to one pixel on an image.
It uses the D drive method. This method prevents LD mode hopping noise that tends to occur due to temperature fluctuations,
This is a new LD driving method devised to reproduce beautiful images.

Since the density adjustment method of the present invention is basically based on this new LD driving method, it also has the aspect of being able to prevent mode hopping noise. Therefore, in this section, the reason why mode hopping noise can be prevented by the above-described novel LD driving method will be explained.

One pixel corresponds to one dot, which is the lowest unit for expressing an image. The frequency corresponding to one pixel cannot be uniformly determined because it is related to the scanning speed of the laser beam, the sensitivity of the sensitive material on the image forming surface, etc., but is set to about several MHz, for example. However, there is a frequency limit due to printing technology, and a high frequency such as 50 MHz cannot be used in practice.

FIG. 4 shows a laser beam printer used to test whether mode hopping noise can be prevented by pulse modulating an analog signal at what frequency. In the figure, the same components as in FIG. 7 are given the same numbers and the explanations are omitted.

Reference numeral 12 denotes an optical sensor that detects the density of the image on the image forming surface lo, and its detection signal is observed by an oscilloscope 14 via an amplifier 13.

In this printer, a 100 Hz rectangular wave signal shown in FIG. 5(a) was added in place of the analog signal 1, and the image on the imaging surface 10 at that time was observed with an oscilloscope 14. The observation results are shown in Figure (b). However, this data is
This data is obtained when the minimum light emitting power of D is 0 milliwatts, the maximum light emitting power is 70% of the maximum rating, and the ambient temperature of the LD is artificially varied. It can be seen from Figure (b) that the optical power is not flat but randomly modulated. This random modulation is caused by mode hopping noise, which is induced not only by fluctuations in the LD ambient temperature but also by the LD's self-heating due to light emission, and tends to be amplified by the return light from external optical components. .

Although the negative feedback circuit of the LD drive circuit 2 has a function of preventing fluctuations in optical power and has a frequency response of about 20 MHz, it has been found that it is completely powerless against mode hopping noise. Figure 6(a) is the same as Figure 5(b) above.
) is a diagram reproducing an image on the image forming surface 10 when observation data of . As can be seen from a comparison with the uniform image without mode-hopping noise shown in FIG. 6(b), the mode-hopping noise causes shading in the image and the image quality is significantly degraded.

Regarding the waveform in Fig. 5(b), if the rising part of the LD light emission is expanded 10 times on the time axis, Fig. 5(c)(d)
)(e)(f).

From these figures (C) to (f), it can be seen that almost no mode hopping noise occurs, if not zero, until about 0.5 μs from the rise of LD light emission.

Therefore, by pulse modulating the analog signal with a period of M [(z order), mode hopping noise can be prevented and image quality can be improved.

In this case, if the analog signal is pulse-modulated irrespective of the imaging speed, even if mode hopping noise can be prevented, image density will be uneven. That is, for example, if the analog signal is turned off for a short time once in units of 2 pixels, 2 pixels on the image
Since the optical power stops once while the laser beam irradiates the dot, one dot of the two dots is irradiated during a period in which the optical power does not stop, and one dot is irradiated during a period in which the optical power stops for a short time. There is an inconvenience that there is a difference in density from one dot to another. On the other hand, 1 pixel unit is 1
If the analog signal is turned off for a short period of time, the above disadvantages will not occur, and as long as the intensity of the optical power is constant, the density of each dot will be uniform.

The present invention employs the above-mentioned LD driving method and uses the method to vary the pulse width of the analog signal that is pulse-modulated, thereby forming a beautiful image in which the image density depends only on the pulse width. be able to.

Figure 1 shows a circuit that can change the pulse width of an analog signal in seven steps from 178 to 778 as an example of implementing the method of the present invention, and 21 is a 32 MHz clock pulse generator, for example. , 22 are, for example, 4-bit binary counters as frequency dividers. Since the frequency of the clock pulse is divided by 1/16 by the frequency divider 22, a pulse (pixel clock) of 2 MHz, which is appropriate as a frequency corresponding to one pixel, is obtained. 23 is an address decoder, 2
4 is a dynamic memory that stores image data in a binarized state, 25a...25h are AND gates,
26 is a DA converter.

27 is an amplifier, the output of which is applied to an LD drive circuit (not shown). 28 is an 8-bit parallel output shift register, 29 is a data selector, and the pulse width of the analog signal is determined by the angle between these two.

According to the above configuration, the image data at the address specified by the address decoder 23 is read out from the dynamic memory 24 in synchronization with the 2 MHz pixel clock, and
AND gate 25a as bit (256 gradation) output
~25h added. The AND gates 25a to 25h are opened and closed by a data selector 29. Since the output data becomes zero when the gate is closed, the analog signal after DA conversion also becomes zero level during that period. Therefore, pulse modulation of the analog signal is performed by opening and closing the AND gates 25a to 25h. The frequency of this pulse modulation is 2 MHz, which is equal to the pixel clock.

The AND gates 25a to 25h are opened and closed by both the shift register 28 and the data selector 29. That is, when pulses with seven different widths are output in parallel from the shift register 28 and sent to the data selector 29, the data selector 29 selects pulses with seven different widths depending on the state of data inputs A-C. any one of them
Only one pulse is selected and sent to AND gates 25a to 25h. The AND gates 25a to 25h open the gates for a predetermined time (an arbitrary time among 178 to 7/8 pixels) by this pulse, and close the gates for the remaining time.

As described above, while performing pulse modulation of the analog signal that drives the LD, the pulse width can be varied in seven steps. Figures 2(a), (b) and (c) show modulated analog signals with different pulse widths. here,
Since the pulse width and the average power of the laser beam on the imaging surface are linearly proportional as shown in FIG. 3, density adjustment can also be performed linearly.

It goes without saying that the present invention is not limited to the above embodiments. For example, the method of the present invention can be applied to a color printer that drives three LDs with different wavelengths simultaneously and reproduces colors using a window material based on the same subtractive color mixing principle as in ordinary color photography. By varying the pulse width to control the power on the imaging surface, color balance can be corrected. On the other hand, in a color printer, changing the image density while maintaining color balance can be achieved by combining this method with a conventional method.

Since the density adjustment method of the present invention is carried out as described above, it has the following effects.

■ Density can be adjusted using an electrical method that varies the pulse width when pulse modulating the analog input signal of the LD, making it possible to eliminate the need for a movable structure, eliminate the disadvantages associated with it, and improve printer performance. Reliability can be significantly improved.

■ Since there is a linear relationship between the pulse width and the average power on the imaging surface, density adjustment can be performed linearly from low density images to high density images.

■ Since the analog signal is pulse modulated at a frequency corresponding to one pixel, the LD can emit light while preventing mode hopping noise, and therefore the density can be adjusted while preventing unevenness in image density caused by such noise. A laser beam printer that can be adjusted and has truly excellent image quality can be provided.

[Brief explanation of drawings]

Figure 1 is a diagram showing a circuit for carrying out the method of the present invention, and Figure 2 (
a), (b), and (c) are waveform diagrams showing several examples of varying pulse widths, Fig. 3 is a graph showing the relationship between pulse width and average power on the imaging surface, Figs. 4 and 5, FIG. 6 is a diagram explaining the reason why mode hopping noise can be eliminated by the new LD driving method basically adopted in the present invention, and FIG. 4 is a diagram showing a laser beam printer used to confirm mode hopping noise. Figure 5 is a waveform diagram showing mode hopping noise, Figure 6 is a diagram showing printer states with and without mode hopping noise, and Figure 7 is a conventional laser beam printer. A schematic configuration diagram showing
FIG. 8 is a diagram showing the rotation angle of the beam splinter and the amount of laser beam transmission. Engineering: Analog signal, 3: Laser diode. Patent applicant: Minolta Camera Co., Ltd. Figure 2 Figure 3

Claims (1)

[Claims] (1) In a laser beam printer in which the analog signal that directly drives the laser diode is pulse-modulated at a frequency corresponding to one pixel on the image to stop the laser diode from emitting light for a short time, the pulse width of the analog signal is A method for adjusting the density of a laser beam printer, characterized in that the density is variable.