JPH0679919A - Laser scanning device - Google Patents

Abstract

PURPOSE:To equalize laser beam power on a surface of a photosensitive material to a predetermined level over the whole scanning width and record a high- quality image with less shading distortion. CONSTITUTION:A laser beam is detected by a plurality of photodiodes 6, 7, 8 disposed behind a semitransmission type folded mirror in a scanning direction and is also detected by a photodiode 9 provided on the same plane as a surface 10 of a photosensitive material, and the operation of a laser diode 1 is controlled on the basis of outputs from the photodiodes 6 to 9, whereby laser beam power on the surface of the photosensitive material is stabilized to a predetermined level over the whole scanning width.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser for scanning a photoconductor surface with a laser beam modulated by an image signal and forming a latent image of an image on the photoconductor surface in a recording section of a facsimile machine, a copying machine, a printer or the like. Regarding scanning device.

[0002]

2. Description of the Related Art In this type of laser scanning device, a semiconductor laser (hereinafter referred to as a laser diode) outputs laser light, which passes through a collimator lens and is then scanned by a polygon mirror rotating at a high speed. The image is swung at a high speed in the image height direction, and further reaches the photoconductor surface via a lens system such as an fθ lens and a folding mirror (a normal mirror that does not transmit light) in order, and scans the photoconductor surface. At the time of image recording, the laser beam is modulated by the image signal to form a latent image of the image on the surface of the photoconductor.

Control of laser power is performed by a laser diode (LD) as shown as a flow chart in FIG.
During a certain period, the current value Im of the monitor photodiode (PD) built in the laser diode is detected and compared with a specified value, and the laser diode of the laser diode is maintained so as to keep Im at the specified value. This is performed by a method of repeating the control for increasing / decreasing the current value Iop (steps 52 to 56).

[0004]

However, in the conventional configuration in which the laser power is controlled by detecting and feeding back the power at the laser light emission position, the power of the laser light on the surface of the photoconductor is scanned by a scan width. However, it is difficult to make it uniform and uniform, and there is a problem that deterioration of image quality easily occurs due to so-called shading distortion.

An object of the present invention is to improve image quality in a laser scanning apparatus by stabilizing the laser light power on the surface of the photosensitive member to a predetermined level over the entire scanning width.

[0006]

According to the present invention, a laser beam emitted from a laser light source is swung in a scanning direction by a polycogon mirror, and is then folded back to a photosensitive body surface by a folding mirror to scan the photosensitive body surface with the laser light. In the laser scanning device described above, the folding mirror is a semi-transmissive mirror, and a plurality of first photoelectric conversion elements for detecting the laser light transmitted through the folding mirror at different positions in the scanning direction, Based on the detection output of the second photoelectric conversion element for detecting the laser light folded back by the folding mirror on the same surface as the photoreceptor surface, and the first photoelectric conversion element and the second photoelectric conversion element, The laser light power on the surface of the photoconductor is set to a predetermined level within the scan width for the laser light source And it is characterized in providing a means for performing a control for uniform.

[0007]

The detection output of each first photoelectric conversion element indicates the laser light power transmitted through the folding mirror, and does not directly indicate the laser light power on the photosensitive member surface. However, since the laser light power on the surface of the photoconductor is detected by the second photoelectric conversion element, the detection output of the first photoelectric conversion element is used as a reference to detect the output of each first photoelectric conversion element on the surface of the photoconductor at the corresponding scanning position. The laser light power can be estimated.

Therefore, if an appropriate number of first photoelectric conversion elements are arranged at appropriate positions within the scan width, based on the detection output of each first photoelectric conversion element and the second photoelectric conversion element,
By controlling the laser light source so that the estimated laser light power on the surface of the photoconductor at the scan position corresponding to each first photoelectric conversion element is set to a predetermined level, the laser light power on the surface of the photoconductor is changed to the scan width. Can be stabilized at a predetermined level over the entire range.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic structure of an optical portion of a laser scanning device according to the present invention. Reference numeral 1 is a laser diode as a laser light source, and 10 is a photoconductor surface scanned with a laser beam. A collimator lens 2, a high-speed rotating polygon mirror 3, an optical system 4 such as an fθ lens, and a semi-transmissive folding mirror are sequentially arranged in an optical path from the laser diode 1 to the photosensitive member surface 10 in a predetermined relative positional relationship. There is.

At the time of recording an image, laser light modulated by an image signal is emitted from the laser diode 1, and this laser light is collimated by the collimator lens 2 and enters the polygon mirror 3, and the mirror surface of the rotating polygon mirror 3 is reflected. By being reflected by, it is swung in the scanning direction at a high speed by a certain width. The laser beam shaken by the polygon mirror 3 passes through the optical system 4 and reaches a semi-transmissive folding mirror (half mirror) 5, and a part of the laser beam is bent almost right below to the photoconductor surface 10. Goes through and goes straight.

The structure described up to this point is the same as the conventional device except that the folding mirror 5 is of a semi-transmissive type. The characteristic features of the present invention are that the folding mirror 5 is a semi-transmissive mirror and that it is described below.

In the present embodiment, three photodiodes 6, 7 and 8 are provided on the back side of the folding mirror 5 side by side in the scanning direction as photoelectric conversion elements for detecting the laser light transmitted through the folding mirror 5. Has been.
The positional relationship of the photodiodes 6, 7, and 8 in the can direction is as follows. The photodiode 7 is arranged at a position corresponding to the center of the scan width of the laser light (image height = 0 mm) on the photoconductor surface 10. The photodiode 6 is provided at a position corresponding to the scan start side end (image height = + 128 mm) of the scan width, and the other end (image height = −128 mm).
The photodiode 8 is provided at a position corresponding to (mm). However, the scanning direction is from left to right in the figure.

The reason why the laser light power can be detected at an arbitrary position within the scan width is that the folding mirror 5 is of a semi-transmissive type, which cannot be easily achieved by a non-transmissive mirror.

Further, a photodiode 9 for detecting the laser light reflected by the folding mirror 5 on the same surface as the photoconductor surface 10 is provided in the vicinity of the photodiode 6. The photodiode 9 serves both as a means for detecting the power of the laser light on the photoconductor surface 10 and a means for detecting the synchronization signal of the laser light.

As shown in FIG. 2, a slit member 12 is arranged in front of the light-receiving surface of each photodiode 6, 7, 8, 9. A slit 13 having a small width in the scanning direction of the laser beam is opened in each slit member 12, and the laser beam is incident on the light receiving surface of each photodiode through this slit 13 to thereby scan each photodiode 6, 7, The output waveforms of 8 and 9 are sharpened (see FIG. 3 for output waveforms).

During the scanning operation of the photoconductor surface 10, laser light is sequentially incident on the photodiodes 6, 7, 8, and 9. An example of the output (current) waveform of each photodiode (PD) at this time is shown in FIG. The peak value (P6, P7, P8, P9) of the output waveform of each photodiode corresponds to the power of the incident laser light.

A photodiode 9 as shown in FIG.
The output waveform of is also used as a synchronization waveform for synchronizing the image signals, and is further used as a reference of the power of the laser light on the photoconductor surface 10. That is, as shown in FIG. 4, the control board (driving control circuit) 11 of the laser diode 1 sets the peak value P9 of the output waveform of the photodiode 9 to the laser light power P ₊ on the surface of the photoconductor at the image height +128 mm. _128, and the power P ₀ on the photoconductor surface at the position where the image height is 0 mm and the power P ₋₁₂₈ on the photoconductor surface at the position where the image height is ₋₁₂₈ mm are expressed by the following formula P ₊₁₂₈ = P9 P ₀ = P9 × P7 / P6 P _-128 = P9 × P8 / P6, and P ₊₁₂₈ = P ₀ = P _-128 = Ps (Ps
Control the drive current of the laser diode 1 so that it becomes a set value. As a result, the laser light power on the surface of the photoconductor is kept at the set value Ps over the entire scan width, and high-quality image recording with less shading distortion is possible. Further, the synchronization control of the laser light is performed by using the output waveform of the photodiode 9 as the synchronization waveform, but this is the same as the conventional one, and the description thereof will be omitted.

It is also possible to provide a photodiode dedicated to synchronization detection in addition to the photodiode 9 as in the conventional case, and to obtain a synchronization waveform by injecting a laser beam into the photodiode through a dedicated lens and a folding mirror. The configuration of this embodiment has an advantage that the number of parts is reduced.

Further, a larger number of photodiodes having the same purpose as the photodiodes 6, 7 and 8 are arranged at arbitrary positions within the scan width, and similar laser power correction is further performed by the output of each photodiode. It can also be performed with high precision.

[0020]

As can be understood from the above description, according to the present invention, the laser light power on the surface of the photosensitive member is stabilized at a predetermined level over the entire scanning width, and a high quality image in which shading distortion is corrected is obtained. Can be recorded.

[Brief description of drawings]

FIG. 1 shows a configuration of an optical portion of a laser scanning device according to the present invention.

FIG. 2 shows a slit member on the front surface of a photodiode.

FIG. 3 shows an example of output waveforms of four photodiodes.

FIG. 4 shows a laser power control system of a laser scanning device.

FIG. 5 shows a conventional laser power control flow.

[Explanation of symbols]

 1 Laser diode (LD) 2 Collimating lens 3 Polygon mirror 4 Lens system 5 Semi-transmissive folding mirror 6-9 Photodiode (PD) 10 Photosensitive surface 11 Control board 12 Slit member 13 Slit

Claims (1)

[Claims] 1. A laser scanning device that scans the surface of a photoconductor with laser light by swinging a laser beam output from a laser light source in a scanning direction by a polygon mirror and folding it back toward a surface of the photoconductor by a folding mirror. The mirror is a semi-transmissive mirror, and a plurality of first photoelectric conversion elements for detecting the laser light transmitted through the folding mirror at different positions in the scanning direction, and on the same surface as the photoconductor surface. A second photoelectric conversion element for detecting the laser light folded back by the folding mirror, and based on the detection output of each of the first photoelectric conversion element and the second photoelectric conversion element, to the laser light source, For uniformizing the laser light power on the photoconductor surface to a predetermined level within the scan width A laser scanning device comprising: means for controlling.