JPH0643373A - Optical scanner - Google Patents

Abstract

PURPOSE:To prevent an image-forming position from being deviated by compensating the fluctuation of a mirror surface at an initial position by detecting the light quantity per unit area of a light beam on a scanning line, and applying offset to the preceding focusing position of a convergence optical system corresponding to the light quantity. CONSTITUTION:A control circuit 30 outputs a piezo driving signal of radial shape waveform whose center part is lowered against both terminal parts of the scanning line on a photosensitive drum setting the peak position of a light detection signal outputted from an optical detector 21 as reference when the peak value of the signal exceeds a prescribed threshold value. A piezo element 20 separates a mirror 14 by a collimator lens 13 by furthermost distance at the center part of the scanning line by the piezo driving signal. Also, the control circuit 30 outputs the piezo driving signal of radial shape by adding a prescribed offset value when the peak value of the light detection signal is less than a power value. Thereby, the piezo element 20 separates the mirror 14 by distance in accordance with the offset value through a scanning period by the collimator lens 13. Thereby, a beam waist position can approach the collimator lens 13 as a whole.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning device, and more particularly to an optical scanning device for scanning a laser light beam.

A laser scanner as an optical scanning device applied to a POS scanner, a laser printer, optical measurement, etc.
Depending on each purpose, required performance such as scanning speed, resolution, and linearity of scanning line is required. Particularly, in recent years, the price of laser printers has been reduced, and the spread of OA laser printers has been remarkable.

In the laser scanner, it is desired that the basic optical performance, that is, the functions of deflection scanning, beam focusing, and light focusing be improved. Particularly in recent laser printers, higher resolution is desired due to the demand for more beautiful graphic functions. Further, it is desired that the optical system be cheaper and more compact.

[0004]

2. Description of the Related Art A conventional laser scanner deflects and scans a laser light beam by using a polygon or a hologram disk, and converges the light by using an f-Θ lens or the like. The high resolution is dealt with by increasing the number of lenses or making the optical lens aspherical to reduce the beam spot diameter on the scanning line. Therefore, there is a problem that the lens system is expensive. On the other hand, although the lens is made of plastic for the purpose of an inexpensive lens system, there is a problem that miniaturization is difficult due to the spatial arrangement of the lens system. Therefore, the present applicant has proposed a patent application for an optical system using a piezo element and capable of varying the focal length. This is to move the front focus of the lens and change the back focus by the displacement of the mirror surface. The displacement of the mirror surface is performed by electric drive.

[0005]

In the above conventional device,
There is a problem that the initial position of the mirror surface when the piezo element is not driven changes due to the deterioration of the piezo element and the expansion of the optical component due to the temperature, and the imaging position of the light beam deviates from the scanning surface.

The present invention has been made in view of the above points, and it is an object of the present invention to provide an optical scanning device that compensates for fluctuations in the initial position of a mirror surface and prevents deviation of the imaging position of a light beam. .

[0007]

The optical scanning device of the present invention comprises:
After irradiating a polygon mirror or a hologram disk, which converges and rotates a light beam with a converging optical system, and scans, the focal position changing means changes the front focal position of the converging optical system in synchronism with scanning, and after the converging optical system. In an optical scanning device having a focus on a scanning line, a photodetector provided on the scanning line for detecting a light amount per unit area of a light beam, and a focus position varying means for the light amount detected by the photodetector. Offset means for adding an offset to the front focus position of the variable converging optical system.

[0008]

In the present invention, since the front focus position is offset in accordance with the detected light amount, the front focus position is changed and the rear focus position is deviated from the scanning line. When the detected light amount is decreased, the front focus position is changed by the offset. The back focal position is compensated so as to be on the scanning line.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 shows a schematic block diagram of the device of the present invention. In the figure, the laser light beam emitted from the laser diode 10 is collimated by the collimator lens 11 and then reflected by the half mirror 12. The reflected light beam is reflected by the mirror 14 through the collimator lens 13, passes through the collimator lens 13 again, passes through the half mirror 12, and is applied to the polygon mirror 16. The light beam reflected by the polygon that rotates at a constant speed scans the photosensitive drum 17 in the axial direction.

The back surface of the mirror 14 is fixed to the piezo element 20, and the distance between the mirror 14 and the collimator lens 13 is variable according to the voltage applied to the piezo element 20, and the collimator lens 13 causes the light beam to travel. Is converged on the position of the photosensitive drum 17 to form an image.

Further, the scanning line 1 is adjacent to the photosensitive drum 17.
8 is provided with a photodetector 21. The photodetector 21 has a diameter 8 smaller than the beam waist diameter in front of the photodiodes 21a and 22 shown in FIG.
Light-shielding mask 2 having a pinhole 23a of about 0 μm
It is composed of 3 and 3. That is, the photodetector 21 detects the amount of incident light per opening area of the pinhole 23a.
Since the light beam is converged by the collimator lens 13 so that the beam waist is on the photosensitive drum 17,
When the light beam scans on the photodetector 21, the amount of light entering the photodiode 22 from the pinhole 23a when the photodetector 21 is located at a position A before the beam waist position of the light beam shown in FIG. The amount of light is smaller than the amount of light incident on the photodiode 22 from the pinhole 23a when the photodetector 21 is near the waist position. Therefore, in the positional relationship of position A in FIG. 4 in which the beam waist position is behind the fixed photodetector 21, the output of the photodetector 21 becomes as shown in FIG. 5A due to the scanning of the light beam. Further, in the positional relationship of the position B in FIG. 4 where the beam waist position is near the position of the photodetector 21, the output of the photodetector 21 is as shown in FIG. 5 (B).

Here, at the time of manufacture, the distance between the mirror 14 and the collimator lens 13 is set to be slightly shorter than the optimum value, and the beam waist position is the photosensitive drum even if the piezo element 20 is deteriorated or the optical components are expanded in temperature. It is arranged so as not to be behind the peripheral surface of 17.

FIG. 1 is a block diagram showing an embodiment of the main part of the device of the present invention. In the figure, the photodetection signal output from the photodetector 21 is supplied to the control circuit 30. This light detection signal is shown in FIG.
As shown in (A), a peak occurs each time the light beam scans the photodetector 21. The control circuit 30 compares the peak value of the light detection signal with a predetermined threshold value V _TH . If the peak value exceeds the threshold value V _TH , both ends of the scanning line 18 on the photosensitive drum 17 with the peak position as a reference. A piezo drive signal having a waveform shown in FIG. 6B having a substantially parabolic shape in which the central portion is lower than the central portion is generated and output. The piezo drive circuit 31 amplifies the power of the piezo drive signal and outputs the piezo element 2
Apply 0. The piezo element 20 displaces the mirror 14 away from the collimator lens 13 as the voltage of the piezo drive signal is lower, that is, in the central portion of the scanning line 18. As a result, the front focus of the collimator lens 13 becomes longer, and the beam waist position, which is the rear focus position, approaches the collimator lens 13 and is on the outer peripheral surface of the photosensitive drum 17.

Further, when the peak value of the light detection signal is less than the threshold value V _TH , the control circuit 30 adds a predetermined offset value α to the substantially parabolic piezo drive signal output with this peak as a reference, and outputs it. As a result, the piezo element 20 separates the mirror 14 from the collimator lens 13 by a distance corresponding to the offset value α throughout the scanning cycle, so that the beam waist position is entirely covered by the collimator lens 13.
To compensate for fluctuations in the mirror surface due to deterioration of the piezo element, temperature expansion of optical components, and the like.

In the control circuit 30, if the peak value of the light detection signal does not exceed the threshold value V _TH in the state where the offset value α is added to the piezo drive signal, the offset value α of the next scanning cycle is increased by a predetermined amount. It may be configured as follows.

FIG. 7 is a block diagram showing a modification of the essential part of the device of the present invention. In the figure, the same parts as those in FIG.
The description is omitted. In FIG. 7, a piezo element 40 is provided between the piezo element 20 and the mirror 14, and the piezo elements 20 and 40 are individually driven by the piezo drive devices 31 and 41.

The photodetection signal as shown in FIG. 8A output from the photodetector 21 is supplied to the control circuit 42 and the comparison circuit 43. The control circuit 42 generates a substantially parabolic piezoelectric drive signal shown in FIG. 8B in which the central portion is lower than both ends of the scanning line 18 on the basis of the peak position of the light detection signal, and the piezoelectric drive circuit 31 is generated. Supply. The piezo element 2
The operations of 0, the piezo drive circuit 31, and the control circuit 42 are the same as when the peak value of the photodetection signal in FIG.

The comparison circuit 43 uses the peak value of the photodetection signal as a threshold.
Value V_YHAnd the peak value is the threshold value V _THWhen less than Figure 8
Generate a piezo drive signal with a constant value α as shown in (C)
It is supplied to the piezo drive circuit 41. Piezo drive times
The path 41 power-amplifies the piezo drive signal from the comparison circuit 43.
Then, the voltage is applied to the piezo element 40. This makes mirror 1
4 is a collimator at a distance corresponding to the value α throughout the scanning cycle
The lens 13 is separated from the lens 13 to compensate for variations in the mirror surface.

In the above embodiment, the polygon mirror 16 is used.
Although the scanning of the light beam is performed by this, the scanning of the light beam may be performed by using a hologram disk.

[0020]

As described above, according to the optical scanning device of the present invention, it is possible to compensate the fluctuation of the initial position of the mirror surface and prevent the deviation of the image forming position of the light beam, which is extremely useful in practice.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a main part of a device of the present invention.

FIG. 2 is a schematic configuration diagram of the device of the present invention.

FIG. 3 is a diagram showing a photodetector.

FIG. 4 is a diagram showing a relationship between a photodetector and a light beam.

FIG. 5 is a waveform diagram of a light detection signal.

FIG. 6 is a waveform diagram of a light detection signal and a piezo drive signal.

FIG. 7 is a configuration diagram of a main part of the device of the present invention.

FIG. 8 is a waveform diagram of a light detection signal and a piezo drive signal.

[Explanation of symbols]

 10 Laser Diode 11, 13 Collimator Lens 14 Mirror 16 Polygon Mirror 17 Photosensitive Drum 18 Scan Line 20 Piezo Element 21 Photo Detector 30 Control Circuit 31 Piezo Drive Circuit

Claims (1)

[Claims] 1. A focal point variable means (20, 3) for irradiating a polygon mirror (16) or a hologram disk which is converged and rotated by a converging optical system (13) with a light beam for scanning, and for changing a focal position.
0, 31) In an optical scanning device in which the front focus position of the converging optical system is changed in synchronization with scanning and the rear focal point of the converging optical system is on the scanning line, a unit area of a light beam provided on the scanning line It has a photodetector (21) for detecting the amount of light, and an offset means (30) for adding an offset to the front focus position of the converging optical system that is varied by the focus position varying means according to the amount of light detected by the photodetector. An optical scanning device characterized by the above.