JP2005215188A - Scanning optical apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a compact scanning optical apparatus suitable for inexpensive and highly precise printing, in which contradictory characteristics are compatible with each other and image irregularity and color shifts accompanying dislocated imaging position are prevented. <P>SOLUTION: The scanning optical apparatus, in which an image is formed by scanning with a laser luminous flux emitted from a semiconductor laser with a deflector and scanning an image carrier with the luminous flux with a refraction element and a diffraction element, is characterized by that the variation in the imaging position in a main scanning direction on the image carrier due to an environmental variation of the scanning optical apparatus is corrected by the variation in power of the refraction element and the refraction element in the scanning optical apparatus and the wavelength variation of the semiconductor laser, and that a surface light emission semiconductor laser is used as a light source having a plurality of light fluxes. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a scanning optical device used in an image forming apparatus such as a laser beam printer or a laser facsimile.

  Conventionally, in a scanning optical device used in an image forming apparatus of a laser beam printer or a digital copying machine, a light beam which is light-modulated from a light source means in accordance with an image signal and is emitted periodically by, for example, an optical deflector composed of a polygon mirror. Image recording is performed by deflecting, focusing in a spot shape on the surface of a photosensitive recording medium by an imaging optical system having an fθ characteristic, and performing optical scanning.

  FIG. 7 is a schematic diagram of a conventional scanning optical device disclosed in Patent Document 1. In FIG.

  In FIG. 7, the divergent light beam emitted from the light source means 91 is converted into substantially parallel light by the collimator lens 92, and the light beam is limited by the diaphragm 93 and is incident on the cylindrical lens 94 having a predetermined refractive power only in the sub-scanning direction. . Of the substantially parallel light beam incident on the cylindrical lens 94, the light beam exits in the state of the substantially parallel light beam as it is within the main scanning plane. In the sub-scanning plane, the light beam is focused and formed as a substantially linear image on the reflection surface of the optical deflector 95 composed of a polygon mirror.

  Then, the light beam deflected and reflected by the reflecting surface of the optical deflector 95 is guided to the scanned surface 98 via the scanning optical system (fθ lens) 96 having the fθ characteristic, and the optical deflector 95 is rotated in the direction of the arrow. By doing so, the image information is recorded by optically scanning the photosensitive drum surface 98 in the direction of the arrow 9B.

  In addition, due to the recent increase in speed of devices, a multi-beam that emits a plurality of light beams is often used as a light source.

Japanese Patent Application Laid-Open No. 11-223783

  FIG. 8 shows an apparatus for forming a color image by simultaneously using a plurality of the scanning optical devices 11 to 14 and recording image information for each color on different photosensitive drums 21 to 24, respectively. In such an image forming apparatus, since a plurality of scanning lines are superimposed to form an image, it is particularly important to reduce scanning line deviation (hereinafter referred to as registration deviation) between colors. for that reason. This scanning optical device is required to have the following two performances.

  1) Compensation for variations in the imaging position (spot misalignment) in the main scanning direction due to variations in the wavelength of the semiconductor laser as the light source and variations in the laser wavelength for each of the plurality of scanning optical devices.

  Furthermore, when using a plurality of light sources having a plurality of emission points in one element typified by a multi-beam laser, jitter due to a plurality of light beams occurs due to the wavelength difference between the light sources, and the image quality is significantly reduced. It is more important to perform wavelength compensation of the spot imaging position.

  As a light source for a laser beam, an inexpensive edge-emitting laser of a Fabry-Perot resonator type is mainly used. In general, a semiconductor laser has a gain distribution over a wide wavelength range of about several tens of nm. doing. Further, the Fabry-Perot resonator has many resonance peaks at intervals of about 3 mm when the oscillation wavelength is 0.78 μm and the resonator length is 300 μm, for example. For these two reasons, the Fabry-Perot laser easily oscillates in a multimode even in the CW drive state. Further, in the case of direct modulation, the multimode oscillation is increased by the change in carrier density during relaxation oscillation. As a result, the wavelength spectrum broadens to several nanometers and has excellent dynamic single mode characteristics, such as DFB (Distributed Feedback) laser and DBR (Distributed Bragg Reflector) laser. However, the cost of the laser becomes high.

  Further, another required performance is as follows.

  2) A change in magnification (spot position deviation) in the main scanning direction accompanying an environmental change (temperature change) such as a temperature rise is compensated.

  In recent years, plastic lenses that can be mass-produced are often used as fθ lenses, but they have a larger coefficient of thermal expansion than glass lenses, and it is also important to solve the above problem 2).

  Japanese Patent Laid-Open No. 10-197820 describes an example of a configuration for correcting the position deviation of main scanning due to wavelength variation due to a change in the light source over time by combining the refractive lens and the diffractive lens with respect to the problem 1). The change in the shape of the optical element and the change in refraction / diffraction efficiency due to the temperature change in the above 2) will be further adversely affected.

  Further, in Japanese Patent Laid-Open No. 11-223783, in response to the above problem 2), a combination of a refractive lens and a diffractive lens allows for a change in refractive index and a change in shape due to a change in environmental temperature. An example of a configuration is described in which the amount of focus movement caused by a change in environmental temperature is used positively to cancel the amount of focus movement caused by chromatic aberration of magnification by actively using a wavelength change due to mode hopping. With respect to the change in the wavelength and the variation in the wavelength between the light sources, the imaging position shifts in the main scanning direction.

  The present invention has been made in view of the above-described problems of the prior art, can achieve both conflicting characteristics, can prevent image unevenness and color misregistration due to image formation position deviation, and is inexpensive and highly accurate. An object is to provide a compact scanning optical device suitable for printing.

  To achieve the above object, the present invention provides a scanning optical device that scans a laser beam emitted from a semiconductor laser with a deflector, scans the beam with an refracting element and a diffractive element, and forms an image. The change in the imaging position in the main scanning direction on the surface of the image carrier due to the environmental variation of the scanning optical device is corrected by the power variation of the refracting element and the diffractive element in the scanning optical device and the wavelength variation of the semiconductor laser. In addition, a surface emitting semiconductor laser is used as a light source having a plurality of light speeds.

  Further, in a scanning optical device for forming a color image by scanning light beams from a plurality of scanning optical devices onto a plurality of image carriers, imaging in the main scanning direction on the surface of the image carrier due to environmental fluctuations of the scanning optical devices The position change is corrected by the power change of the refracting element and the diffractive element in the scanning optical device and the wavelength variation of the half-body laser, and a surface emitting semiconductor laser is used as the light source having the plurality of light speeds. Yes.

  A plurality of surface emitting semiconductor lasers may be arrayed.

  The refractive element is preferably made of a plastic material.

  According to the present invention, since the surface emitting semiconductor laser is used as the light source, the change in aberration associated with the environmental variation of the scanning optical device can be detected without worrying about the potential wavelength difference of each device. By correcting the power change between the refracting portion and the diffracting portion and the wavelength variation of the semiconductor laser, it is possible to suppress registration deviation between colors and image density unevenness between colors with a simple configuration, A compact scanning optical device suitable for high-definition printing can be obtained.

  Embodiments of the present invention will be described below with reference to the accompanying drawings.

<Embodiment 1>
FIG. 1 is a schematic diagram of a main part of a color image forming apparatus including a scanning optical apparatus according to Embodiment 1 of the present invention.

  In the figure, 11, 12, 13, and 14 are scanning optical devices, 21, 22, 23, and 24 are photosensitive drums as image carriers, 31, 32, 33, and 34 are developing units, and 41 is a conveyor belt. is there. As will be described later, the color image forming apparatus according to this embodiment includes four scanning optical devices (11, 12, 13, and 14) that suppress aberration fluctuations due to environmental fluctuations (temperature fluctuations). ), M (magenta), Y (yellow), and B (black), and image signals (image information) are recorded on the surfaces of the photosensitive drums 21, 22, 23, and 24 in parallel. Is printed at high speed.

  Next, a method for satisfactorily correcting the aberration change accompanying the environmental fluctuation and the aberration accompanying the wavelength fluctuation of the scanning optical apparatus, which is a feature of the present invention, and its optical element will be described. Since the scanning optical devices 11, 12, 13, and 14 constituting the color image forming apparatus have the same configuration and optical action, the scanning optical device 11 will be described below as a representative.

  FIG. 2 is a schematic view of the principal part showing the scanning optical device 11 and the image carrier 21 corresponding thereto, FIG. 3 is a sectional view of the principal part in the main scanning direction of the optical system shown in FIG. 2, and FIG. 1 is a main conceptual diagram of a semiconductor laser 1. FIG.

  The structure of the surface emitting laser 1 will be briefly described with reference to FIG.

  On the n-type GaAs substrate 601, an n-type semiconductor multilayer reflective film 602, an n-type AlGaAs cladding layer 603, an i (intrinsic) -InGaAs active layer 604, a p-type AlGaAs cladding layer, a p-type semiconductor multilayer reflective film 606, MBE, etc. It is the one that grew up sequentially by the method of. Then, current is injected into the active layer 604 by the two electrodes 610 and 611 to excite light, resonate by the upper and lower reflective films 602 and 606, and light is extracted in a direction perpendicular to the substrate 601. Thus, when a surface emitting laser is used as a light source, the active layer has a gain distribution over a wide wavelength range of about several tens of nm, but the resonator length is as short as several hundreds of nm to several μm. For example, when the oscillation wavelength is 0.78 μm and the resonator length is 0.23 μm, the resonance peak interval is several hundred nm, and there can be substantially only one resonance mode. Therefore, single mode oscillation can be achieved even during modulation. Further, since the resonator is constituted by a mirror having a reflectivity of 99% or more, the spectral line width (full width at half maximum) of 1 nm or less can be obtained even with a laser having a large finesse and a very small resonator length. Therefore, the scanning magnification as designed can be obtained, and no difference occurs for each scanning optical device. Also, the scanning magnification for each color image forming apparatus is almost as designed.

  2 and 3, reference numeral 2 denotes a collimator lens as a first optical element, which converts a divergent light beam (light beam) emitted from the light source means 1 into a substantially parallel light beam. Reference numeral 3 denotes an aperture stop, which limits a passing light beam (light quantity). Reference numeral 4 denotes a cylindrical lens (cylinder lens) as a second optical element, which has a predetermined refractive power only in the sub-scanning direction, and the light deflection which will be described later in the sub-scan section in the sub-scan section. An image is formed on the deflection surface 5a of the vessel 5 as a substantially line image.

  Reference numeral 5 denotes an optical deflector composed of, for example, a polygon mirror (rotating polygonal mirror) as a deflecting element, and is rotated at a constant speed in the direction of arrow A in the figure by a driving means (not shown) such as a motor.

  Reference numeral 6 denotes a scanning optical element as a third optical element having an fθ characteristic, which has a refracting part and a diffractive part. The refracting portion is composed of a single plastic toric lens 61 having different powers in the main scanning direction and the sub-scanning direction, and both lens surfaces of the toric lens 61 in the main scanning direction are aspherical. The diffractive portion is composed of a long diffractive optical element 62 having different powers in the main scanning direction and the sub-scanning direction.

  In the present embodiment, a toric lens 61 is provided on the optical deflector 5 side and a diffractive optical element 62 is provided on the photosensitive drum 21 surface side from the midpoint between the rotation axis of the optical deflector 5 and the surface of the photosensitive drum 21 (scanned surface). Arranged. The scanning optical element 6 forms an image of a light beam based on the image information deflected by the optical deflector 5 on the surface of the photosensitive drum 21, and corrects the surface tilt of the deflecting surface 5a of the optical deflector 5 in the sub-scanning section. ing. The diffractive optical element 62 in the present embodiment is made of plastic made by injection molding. However, the present invention is not limited to this, and for example, the same effect can be obtained even if a diffraction grating is produced by a replica on a glass substrate. Is obtained.

  As described above, the color image forming apparatus according to the present embodiment uses the four scanning optical devices 11, 12, 13, and 14 to use the light beams based on the respective modulation signals, and the corresponding photosensitive drums 21, 22 and 22 correspond to the latent images. 23, 24 on the surface. For example, latent images of C (cyan), M (magenta), Y (yellow), and B (black) are formed on the corresponding photosensitive drums 21, 22, 23, and 24, and then multiple transferred onto a recording material. One full-color image is formed.

  In the scanning optical device 11 according to the present embodiment, a plurality of divergent light beams emitted from the semiconductor laser 1 are converted into substantially parallel light beams by the collimator lens 2, and the light beam (light quantity) is limited by the aperture stop 3 to form the cylindrical lens 4. Incident. Of the substantially parallel light beam incident on the cylindrical lens 4, the light beam is emitted as it is in the main scanning section. In the sub-scan section, the light beam converges and forms a substantially linear image (a linear image longitudinal in the main scanning direction) on the deflecting surface 5a of the optical deflector 5. The light beam deflected by the deflecting surface 5a of the optical deflector 5 is guided onto the surface of the photosensitive drum 21 through the toric lens 61 and the diffractive optical element 62, and the optical deflector 5 is rotated in the direction of arrow A. By doing so, the surface of the photosensitive drum 21 is optically scanned in the direction of arrow B. Then, as described above, for example, C (cyan), M (magenta), Y (yellow), and B (black) latent images are formed on the corresponding photosensitive drums 21, 22, 23, and 24, and then recorded. A full color image is formed by multiple transfer onto the material.

  In the present embodiment, the aberration change in the main scanning direction on the surface of the photosensitive drum 21 due to the environmental fluctuation (temperature fluctuation) of the scanning optical device 11 is the power change between the toric lens 61 and the diffractive optical element 62 and the semiconductor laser 1. It is corrected by wavelength variation. This aberration change in the main scanning direction is a magnification change or / and a focus change.

  FIG. 5 is an explanatory diagram showing paraxial aberration (field curvature in the main scanning direction and magnification (position) deviation, etc.) before and after the environmental change in the present embodiment, and the solid line shows the characteristic (design value) before the environmental change. The broken line is a characteristic (effective value) when the scanning optical device is heated by + 25 ° C.

  Generally, when image information for each color is recorded on a plurality of photosensitive drum surfaces from a plurality of scanning optical devices and a color image is formed, registration deviation between colors and image density unevenness between colors are made visually inconspicuous. In each scanning optical device, it is necessary that the magnification deviation due to environmental fluctuations (temperature fluctuations) is 40 μm or less and the focal deviation in the main scanning direction is ± 1.0 mm or less.

  In the present embodiment, as shown in FIG. 5, the magnification (position) deviation in the main scanning direction due to the + 25 ° C. temperature rise is 31 μm, which is, for example, within about 3/4 pixels in the case of a printer with a resolution of 600 dpi. Pixel (position) shift can be suppressed. Further, the focus deviation in the main scanning direction is +0.7 mm, and both of them can be suppressed to a level causing no visual problem.

  In the present embodiment, the behavior at the time of temperature increase has been mainly described. However, for example, the same effect as described above can be obtained in other environmental fluctuations such as a temperature drop.

  In this embodiment, all these environmental fluctuations are compensated by a plastic optical element, thereby reducing the manufacturing cost by molding, and shortening the optical path length by correcting the high angle of view using an aspherical surface. Can be achieved simultaneously.

  As described above, in this embodiment, the toric lens 61 and the diffractive optical element 62 are used as the scanning optical element 6 of the scanning optical device 11 as described above, and a plurality of scanning optical devices 11, 12, 13, and 14 are used. By recording an image on the photosensitive drums 21, 22, 23, and 24, a color image forming apparatus in which there is little registration shift between colors due to environmental fluctuations such as temperature rise, and there is little unevenness in image density between colors. Can be realized with a simple configuration and at a low cost.

  Also, by using a surface emitting laser as the light source, it is possible to suppress the wavelength difference between the light sources of a plurality of deflection scanning devices, so that it is optically combined with an optical element having a strong wavelength dependency such as a diffraction grating type condensing element. Even when the system is formed, an optical system having good characteristics can be obtained without causing a decrease in light collection efficiency and an increase in spot diameter.

<Embodiment 2>
The second embodiment of the present invention is a case where a plurality of semiconductor lasers as light sources are arrayed as shown in FIG. 6 in order to increase the speed of the apparatus. Since the end face of the Fabry-Perot laser is formed by cleavage, when a plurality of arrays are used as a multi-beam light source, the resonator length varies from laser to laser, and the reflectivity varies depending on the state of the cleavage end face. This causes a difference in characteristics (threshold current, operating current, voltage, etc.) for each laser, and there is a drawback in that it is necessary to screen the laser array in order to obtain a laser array with uniform characteristics, which is expensive. In addition, since the operating current is different for each laser, a difference occurs in the temperature of each laser, resulting in a difference in oscillation wavelength. This is also disadvantageous when used in combination with a diffraction grating type optical element.

  When DFB lasers and DBR lasers are arrayed, they are more sensitive to the state of the end face than the Fabry-Perot laser, and the oscillation wavelength differs due to the difference in the reflection phase at the end face. Since it is very difficult to match the reflection phase, it is not realistic.

  On the other hand, in the case of a surface-emitting semiconductor laser, the lower mirror layer, the active layer, and the upper mirror layer are collectively manufactured by epitaxial growth on the original substrate, and the resonator is configured without performing a cleavage process. . Therefore, the variation of the oscillation wavelength in the case of arraying depends on the variation of the resonance wavelength, that is, the variation of the composition and the film thickness due to the in-plane distribution during growth. The current epitaxial growth technique is very stable. When a surface emitting semiconductor laser structure is manufactured, the difference in resonance wavelength at a point 30 mm away in the growth wafer is several nm. When four laser arrays are arranged with a pitch of 60 μm, the distance between both ends is 180 μm. In this range, the resonance wavelength is stable with a variation of 1 mm or less. When a surface emitting laser array is actually manufactured, even if a difference in element characteristics due to variations in manufacturing processes is taken into consideration, the surface emitting laser array can be suppressed to a few variations. Therefore, when an optical system is formed by combining a surface emitting laser array and a diffraction grating type condensing element, performance as designed can be obtained for each laser.

  Therefore, since the wavelength difference of the laser beam for each light emitting point is small, it can be designed on the magnification compensation side when the environment changes without worrying about chromatic aberration of magnification.

  Although the color image forming apparatus according to the present embodiment includes a plurality of scanning optical devices and a plurality of photosensitive drums corresponding thereto, the photosensitive drum may be configured as a single unit. In this case, a plurality of light beams emitted from a plurality of scanning optical devices are respectively guided to different regions on a single photosensitive drum surface, and a color image is formed by scanning the photosensitive drum surface with the plurality of light beams. You should do it.

  The present invention is applicable to a scanning optical device used in an image forming apparatus such as a laser beam printer or a laser facsimile.

It is a schematic sectional drawing of the principal part of a color image forming apparatus provided with the scanning optical apparatus concerning Embodiment 1 of this invention. FIG. 2 is a perspective view showing the scanning optical device shown in FIG. 1 and an image carrier corresponding thereto. FIG. 3 is a cross-sectional view of a main part in the main scanning direction of the optical system shown in FIG. 2. It is a typical sectional view of a surface emitting semiconductor laser. It is a paraxial aberration diagram of the scanning optical apparatus according to the first embodiment of the present invention. It is a perspective view of the arrayed semiconductor laser. It is a block diagram of the conventional scanning optical apparatus. It is sectional drawing of a color image forming apparatus provided with the conventional scanning optical apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source means 2 1st optical element (collimator lens)
3 Aperture stop 4 Second optical element (cylinder lens)
5 Deflection element (optical deflector)
6 Third optical element (scanning optical element)
61 toric lens 62 diffractive optical element 11-14 scanning optical device 21-24 image carrier (photosensitive drum)
31-34 Developer 41 Conveyor belt

Claims (4)

In a scanning optical device that deflects a laser light beam emitted from a semiconductor laser by a deflector and scans the light beam on an image carrier by a refracting element and a diffractive element to form an image,
The change in the imaging position in the main scanning direction on the surface of the image carrier due to the environmental variation of the scanning optical device is corrected by the power variation of the refractive element and the diffractive element in the scanning optical device and the wavelength variation of the semiconductor laser, A scanning optical apparatus using a surface emitting semiconductor laser as a light source of a light beam. In a scanning optical device that forms a color image by scanning light beams from a plurality of scanning optical devices on a plurality of image carriers,
The change in the imaging position in the main scanning direction on the surface of the image carrier due to the environmental variation of the scanning optical device is corrected by the power variation of the refractive element and the diffractive element in the scanning optical device and the wavelength variation of the semiconductor laser, A scanning optical apparatus using a surface emitting semiconductor laser as the light source of the double light beam.   3. The scanning optical device according to claim 1, wherein a plurality of the surface emitting semiconductor lasers are arrayed.   The scanning optical device according to claim 1, wherein the refractive element is made of a plastic material.

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