JPS6367074A - Laser scanner - Google Patents

Abstract

PURPOSE:To eliminate an influence of a return light caused by a rotary polygon mirror, and to improve the durability of a laser by constituting the titled scanner so that a semiconductor laser is not allowed to emit a light beam, at the time of a state that a luminous flux from an optical deflecting means returns to the semiconductor laser. CONSTITUTION:When a laser is turned on in an initial state first, and a photodetector detects the laser, the laser is turned off, and after a prescribed clock, ON and OFF of the laser by image information are started. Subsequently, after a clock corresponding to an image area of a one line portion, the laser is turned off, and turned off continuously during a prescribed clock including the time when a return light is generated. After its prescribed clock, the laser is turned on immediately before scanning the photodetector part. By turning on the laser by such a sequence, the time when the laser is turned on per the number of print sheets is decreased naturally, and when a life of the laser is as prescribed, the number of print sheets per the laser can be increased.

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field] The present invention relates to a laser scanning device that scans a light beam from a semiconductor laser to a desired image forming position.

[Prior art]

2. Description of the Related Art In recent years, laser beam printers (hereinafter simply referred to as LBP) have attracted much attention due to their advantages such as high image quality and high-speed printout.

The heart of this LBP is the laser scanning device, and the performance of this laser scanning device greatly influences the image quality of the LBP.

FIG. 2 shows an outline of this laser scanning device.
In the laser unit], the light emitted from the semiconductor laser is converted into parallel light by a collimator lens. This parallel light beam is deflected and scanned by two rotating polygon mirrors, which are light deflecting elements, and then imaged by an imaging optical system 3 onto a light receiving surface (not shown) arranged at an image forming position.

Normally, the LBP has a photosensitive I/ram placed at this image forming position, and starts a certain clock from the time when a photodetector placed above the image forming position or at an equivalent position outside the image area detects the laser beam. Place it and start drawing the image. Then, the semiconductor laser is turned ON-OFF in accordance with the image information for one line, and when the drawing for one line is completed, the laser is temporarily turned off (extinguished), or the laser is applied to the photodetector in the next scan. Immediately turn it on again to let the light in.

A time chart I diagram representing this timing is shown in FIG. The shaded area here is an image area for forming an image, and during this period, the laser is QN-OFF in accordance with the image information. As shown in FIG. 3, when the laser is turned ON and OFF at such timing, the laser usually remains lit for most of the non-image area.

The semiconductor laser is placed in the non-image area, but as shown in FIG. Also, reflected light from a rotating polygon mirror (hereinafter simply referred to as return light) directly returns to the semiconductor laser that is being used. Such time corresponds to between clock a and clock b in FIG. Since the returned light passes through the optical path from the semiconductor laser to the rotating polygon mirror in the completely opposite direction, it is imaged at the light emitting point of the laser chip. Such returned light is transmitted to the laser and has the following effects.

(r) The driving method of the semiconductor laser is based on sight detection of a bin photodiode arranged inside the semiconductor laser element.
If an auto power control method was used to keep the light intensity constant, the photodiode would falsely detect a light intensity higher than the actual output due to the returned light, so the light intensity would be reduced. In the case of the return light from the rotating polygon mirror, almost the same light as the light emitted by the laser is returned, so the degree of false detection is severe and the output is significantly reduced.

(I+) If the driving method of a semiconductor laser is an auto-current control method that tries to keep the output constant by keeping the current constant, the laser oscillation will be affected by the light focused on the laser chip. There will be fluctuations in power.

Therefore, regardless of the laser driving method, fluctuations in laser power that cause fluctuations in laser power naturally result in changes in the image. The returned light enters the laser in the non-image area,
If the influence of the returned light is strong, it may even affect the image area. In particular, when the laser output is large, when the laser scans very fast, or when the laser and the image area are very close to each other, the image is likely to be affected.

[Purpose of the invention]

An object of the present invention is to provide a laser scanning device that eliminates the influence of return light from a rotating polygon mirror, improves the durability of a laser, and does not adversely affect an image area.

In the laser scanning device according to the present invention, the above object is achieved by not causing the semiconductor laser to emit light when the light beam from the optical deflection means returns to the semiconductor laser.

(Description of Structure and Effects of the Invention) FIG. 1 is a diagram showing the timing of laser modulation employed in the laser scanning device according to the invention.

First, in the initial state, the laser is turned on, and when the photodetector detects the laser, the laser is turned off, and after a certain clock, the laser is turned on and off based on image information, and after a clock in charge of the image area for one line, the light is turned off, and the return light is turned off. The light will remain off for a certain period of time, including the time when this happens.

After locking for a certain period of time, the light is turned on immediately before the fluorodetector section is scanned. Of course, this lighting must be turned on for an extra period of time to account for error factors such as the division accuracy of the rotating polygon mirror and uneven rotation, but the time is sufficiently short compared to the time it takes to scan the laser between the laser unit and the image area. It is possible to set the lighting time.

Also, if the sequence is set like this, the laser lighting time for the number of prints will inevitably be reduced.Conversely, for example, if the laser life time is constant, the number of prints per laser will increase. do. In other words, the life of the laser is substantially extended.

In the above embodiment, the light deflecting means was a rotating polygon mirror, but it goes without saying that other deflecting means such as a gabarno mirror or a reflective porodaram scanner are also effective.

Further, when an optical system is disposed between the laser and the optical deflection means, the same problem occurs because the reflected light returns along the same optical path by the same optical path length. It is also valid in the case of a follow-up lever.

As explained above, in the present invention, after the image writing is completed and immediately before the image writing start signal forming means, the laser is turned off at least for a period of time during which the return light is generated by the laser light deflection element, and the laser light intensity is changed due to the laser return light. This method not only prevents the occurrence of image density unevenness, but also lowers the ratio of laser lighting time to the number of prints, and has the effect of substantially extending the life of the laser.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the laser modulation timing of the laser scanning device of the present invention, FIG. 2 is a diagram showing an example of the laser scanning device,
FIG. 3 is a diagram showing laser modulation timing in a conventional laser scanning device, and FIG. 4 is a diagram showing how return light is generated in the laser scanning device. 1-laser unit, 2-rotating polygon mirror, 3-imaging optical system.

Claims (1)

[Claims] (1) A laser scanning device characterized in that the semiconductor laser does not emit light when the light beam from the optical deflection means returns to the semiconductor laser.