JPH1020225A - Laser beam scanning optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser beam scanning optical device capable of accurately adjusting a beam condensing position even though a beam condensing condition detecting means having a simple constitution is used and high-speed scanning is performed. SOLUTION: This device condenses a laser light beam radiated from a laser diode 1 on a photoreceptor 40 through a focusing lens 3, a polygon mirror 6, and an fθ lens 7, and also performs scanning in a direction shown by an arrow (b). The laser beam to perform scanning is detected by an optical sensor 17 for SOS, and the pulse light emission of the laser diode 1 is controlled specified time later from the detection signal. The pulse beam is made incident on a condensing condition detecting device 10 by means of a knife edge method, and defocusing amount is detected. The lens 3 moves on an optical axis so as to adjust the laser beam condensing position based on the detected defocusing amount.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser beam scanning optical device, and more particularly to a laser beam scanning optical device incorporated as an image printing means in a laser printer or a digital copying machine.

[0002]

2. Description of the Related Art In recent years, laser beam scanning optical devices incorporated as image printing means in laser printers and digital copying machines have been capable of printing at high density in order to improve image quality. For this reason, the spot diameter of the laser beam on the surface to be scanned (photoreceptor) becomes smaller, and the allowable depth of focus becomes smaller. When the environment changes, particularly when the optical device generates heat during use and the optical element and its holder undergo thermal expansion, the light condensing position shifts in the front-back direction of the surface to be scanned, and the beam spot diameter increases. It has become unacceptable in maintaining image quality.

To cope with such a problem, Japanese Patent Application Laid-Open No. 2-58016 discloses a laser beam condensing state detected at a position optically equivalent to a photosensitive member, and moving a focusing lens to collect the beam. It is disclosed to correct the light position.

[0004]

However, recently,
As the print speed is set to be high, the scanning speed of the beam spot has become extremely high. Therefore, in the conventional beam condensing state detecting means, it is necessary to increase the response speed of the detecting circuit, and it is necessary to use an element having a high responsiveness capable of sampling the output of the light receiving element at intervals of MHz and GHz order. , Which led to increased costs.

Therefore, an object of the present invention is to be able to cope with high-speed scanning despite the use of a beam condensing state detecting means having a simple structure, to adjust the beam condensing position accurately, and to obtain a high quality image. It is an object of the present invention to provide a laser beam scanning optical device capable of performing the above.

[0006]

SUMMARY OF THE INVENTION In order to achieve the above object, a laser beam scanning optical device according to the present invention includes an optical element for adjusting a condensing position of a laser beam emitted from a laser light source, and an optical element for scanning. Detecting means for detecting that the laser beam has passed and generating a detection signal; pulse light emitting means for causing the laser light source to emit pulse light a predetermined time after the generation of the detection signal; and an optically substantially equivalent position to the surface to be scanned. And a beam condensing state detecting means for receiving a pulsed laser beam, and control means for driving the optical element based on a detection result of the detecting means and adjusting a condensing position of the laser beam. Have.

In the above arrangement, the laser beam being scanned emits a pulse at a fixed point, and the pulse beam is incident on the focusing state detecting means. Therefore, even if the laser beam is scanned at a high speed, the focused state of the beam can be detected as a stationary beam. That is, according to the present invention, since the laser light source emits a pulse at a fixed point and detects the focused state of the pulse beam, the focused state of the laser beam can be detected by the detecting means having a simple configuration even at high speed scanning. A high-quality image can be obtained at all times by adjusting the focus position of the laser beam by driving the optical element based on the focus state detection result.

Further, in the present invention, it is preferable that the beam condensing state detecting means includes a charge accumulation type sensor and an integrating circuit for integrating a detection value detected by a plurality of scans. The detection accuracy of the beam focusing state is improved, and the effect of adjusting the focusing position can be improved.

[0009]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiments of a laser beam scanning optical device according to the present invention will be described below with reference to the accompanying drawings.

In FIG. 1, a laser beam scanning optical device includes a laser diode 1, a collimator lens 2, a focusing lens 3, a cylindrical lens 4,
Plane mirror 5, polygon mirror 6, fθ lens 7 (constituted of lenses 7a, 7b, 7c), plane mirror 8, plane mirror 15 for SOS, cylindrical lens 16 for SOS, and optical sensor 17 for SOS And a beam condensing state detector 10.

The laser diode 1 is modulated (ON / OFF) based on image data input to a drive circuit (not shown).
It is controlled and emits a laser beam when turned on. This laser beam is converged substantially parallel by the collimator lens 2, the focusing position is adjusted by the focusing lens 3 (described in detail below), and reaches the polygon mirror 6 from the cylindrical lens 4 via the plane mirror 5.

The polygon mirror 6 is driven to rotate at a constant speed in the direction of arrow a. The laser beam is deflected at a constant angular velocity on each deflecting surface based on the rotation of the polygon mirror 6 and enters the fθ lens 7. The laser beam transmitted through the fθ lens 7 is reflected by the plane mirror 8 and then condensed on the photosensitive drum 40, and scans the photosensitive drum 40 in the direction of arrow b. lens 7 has a function of correcting the laser beam deflected mainly by the polygon mirror 6 at a constant angular velocity so that the main scanning speed on the surface to be scanned (photosensitive drum 40) is constant, that is, a function of correcting distortion. Have.

The photosensitive drum 40 is driven to rotate at a constant speed in the direction of arrow c. The main scanning in the direction of arrow b by the polygon mirror 6 and the fθ lens 7 and the sub-scanning of the drum 40 in the direction of arrow c cause the photosensitive drum 40 to move on the drum 40. An image (electrostatic latent image) is formed.

The laser beam at the leading end in the main scanning direction of the laser beam is reflected by the plane mirror 15, passes through the cylindrical lens 16, and enters the SOS optical sensor 17. The beam detection signal output from the SOS optical sensor 17 generates a vertical synchronization signal for determining a printing start position for each scanning line.

The focusing lens 3 is mounted on a base plate 20, and an output pinion 22 of a stepping motor 21 is mounted on a rack 20a formed on the side surface of the base plate 20.
Are engaged. By rotating the stepping motor 21 forward or backward in response to a signal from the control unit 30, the lens 3 can move forward and backward on the optical axis, and this movement causes the laser beam to condense on the photosensitive drum 40. Is adjusted.

The beam condensing state detector 10 is disposed on one side of the photosensitive drum 40 and at an optically equivalent position to the surface to be scanned, and detects the converging state of the laser beam on the surface to be scanned. I do. Specifically, the knife edge method is used, and the knife 11 and two charge storage type photoelectric conversion sensors 12A and 12A are used.
B. As shown in FIG.
Cuts off half of the laser beam L whose edge is incident on the sensors 12A and 12B. When the laser beam L is in a focused state, as shown in FIG.
The divided portion of B is irradiated with a pinpoint. When the laser beam L is in the front focus state, as shown in FIG. 2B, the ratio of irradiating the sensor 12A increases. On the other hand, when the laser beam L is in the back focus state, as shown in FIG.
The ratio of irradiating the sensor 12B increases.

FIG. 3 shows the relationship between the defocus amount of the laser beam L and the output voltages of the sensors 12A and 12B. Outputs from the sensors 12A and 12B are stored in a capacitor, and by reading out the output, the defocus amount can be detected without increasing the response speed.

Next, the procedure for adjusting the focusing position will be described. At the time of image formation, the laser diode 1 is first turned on continuously for each scanning line, and the SOS is first turned on.
When the laser beam is detected by the optical sensor for
A signal is emitted. After a predetermined time from the generation of the SOS signal, the laser diode 1 emits pulse light. The predetermined time is a time during which the laser beam moves from the SOS optical sensor 17 to the detector 10. The pulse emission time is desirably such that the laser beam L can be regarded as not moving on the sensors 12A and 12B. Specifically, it is desirable that the amount of beam movement on the surface to be scanned is about 1/10 or less of the beam spot diameter. In the present embodiment, since the knife edge method is used, the laser diode 1 emits pulse light at the timing when the center of the laser beam L irradiates the edge of the knife 11. In addition, in order to stabilize the detection signal, it is only necessary to emit pulses in a plurality of scans and average the detection values. Since the pulse emission is detected, a HOE (Holograhic Optical Element) conventionally known as the detector 10 is used.
Can be used to detect the light collection state.

The defocus amount (see FIG. 3) of the laser beam detected from the output voltages of the sensors 12A and 12B as described above is input to the control unit 30, and the stepping motor 21 is driven to move the focusing lens 3 to the optical axis. To adjust the focused position of the laser beam on the surface to be scanned. Lens 3 corresponding to the detected defocus amount
Of course is stored in the control unit 30 in advance.

Next, the circuit configuration of the control unit 30 will be described with reference to FIG. The control unit 30 is configured around a CPU 31, and includes a timer circuit 32 and an LD drive circuit 33.
It has. The SOS signal d from the SOS optical sensor 17 is input to the CPU 31 and the timer circuit 32. When a predetermined time elapses in synchronization with the SOS signal d, the timer circuit 32 generates a pulse signal e for a time that can be regarded as the laser beam not moving to the LD drive circuit 33 as described above. The count value of the timer circuit 32 is set by the CPU 31. This count value corresponds to the scanning time from when the laser beam irradiates the SOS optical sensor 17 to when it irradiates the beam focusing state detector 10.

On the other hand, the CPU 31 counts the number of beam incidences on the detector 10 by counting the SOS signal d. The LD drive circuit 33 outputs the pulse signal e
The light emission signal f is transmitted to the laser diode 1 in response to.
Further, the LD drive circuit 33 outputs a light emission signal f in accordance with image data g transferred from an image controller (not shown).
Is sent. The pulse beam emitted from the laser diode 1 by the pulse signal e irradiates the sensors 12A and 12B of the detector 10.

The laser beams incident on the sensors 12A and 12B are photoelectrically converted in accordance with the amounts of light, and input to the amplifier circuits 34A and 34B as current signals h _A and h _B , and the amplified signals i _A and i _B are integrated into the integration circuits 35A and 34B. Integrated at 35B. Here, the integrated signals j _A and j _B are used as sample-and-hold circuits 36.
Samples are held at A and 36B. The hold timing is controlled by a hold signal k output from the CPU 31. That is, the CPU 31 counts the number of times of beam incidence on the sensors 12A and 12B based on the SOS signal d, and sends a hold signal k to the sample and hold circuits 36A and 36B when a predetermined count value is reached. At the same time as sending out the hold signal k, the CPU 31 takes in the output signals m _A and m _B from the sample hold circuits 36A and 36B, and calculates the defocus amount based on the characteristics shown in FIG. The movement of the focusing lens 3 based on the defocus amount is as described above.

The laser beam scanning optical device according to the present invention is not limited to the above embodiment, but can be variously modified within the scope of the invention. In particular, the type, shape, and arrangement of the optical element such as the fθ lens are arbitrary. Also,
In addition to the knife edge method, various devices such as a device using a grating filter and a device using moire fringes can be used as the beam condensing state detecting means. further,
As a means for deflecting the laser beam, a scanning device using an acousto-optic effect can be used other than the polygon mirror.

[Brief description of the drawings]

FIG. 1 is a perspective view showing a laser beam scanning optical device according to an embodiment of the present invention.

FIG. 2 is an explanatory diagram of a detection state of a beam focusing state detector used in the optical device.

FIG. 3 is a graph showing output voltage characteristics from each sensor of the detector.

FIG. 4 is a block diagram showing a control unit of the optical device.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Laser diode 3 ... Focusing lens 10 ... Beam focusing state detector 11 ... Knife 12A, 12B ... Photoelectric conversion sensor 21 ... Focusing position adjustment stepping motor 30 ... Control part 31 ... CPU 35A, 35B ... Integration circuit 40 ... Photoconductor drum

Claims (2)

[Claims] 1. A laser beam scanning optical device which focuses a laser beam emitted from a laser light source on a minute point and scans a surface to be scanned in a line at a substantially constant speed. An optical element for adjusting the focusing position of the laser beam, a detection unit for detecting that the scanned laser beam has passed and generating a detection signal, and the laser light source a predetermined time after the generation of the detection signal Pulse light emitting means for emitting a pulse of light; a beam condensing state detecting means arranged at an optically substantially equivalent position to the surface to be scanned; and the optical element is driven based on a detection result of the beam condensing state detecting means. And a control means for adjusting a focusing position of the laser beam. 2. A laser according to claim 1, wherein said beam condensing state detecting means includes a charge storage type sensor and an integrating circuit for integrating detection values detected by a plurality of scans. Beam scanning optics.