JP2005241850A - Optical scanner and image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical scanner which is capable of reducing a beam spot diameter by considering even a temperature variation, moreover, is capable of securing the high precision beam spot positional accuracy, and is low cost, and to provide an image forming apparatus capable of the high quality image outputting using the optical scanner. <P>SOLUTION: The optical scanner comprises a light source, a deflector for deflecting a light beam from the light source, a coupling lens for coupling the light beam from the light source, a scanning optical system which guides the optical beam deflected by the deflector to a surface to be scanned, and a liquid crystal element capable of phase shift, wherein at least one of the optical elements disposed in the optical path has a surface of positive power and is made of resin. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical scanning device using an optical writing unit of a writing optical system using a light beam, and an image forming apparatus using them.

  In recent years, in an image forming apparatus such as a laser beam printer or a digital copying machine, an image to be formed has been improved in image quality, speed, and color, and the print quality requested by a user has also increased.

  Multi-beam scanning that scans with a plurality of light beams is effective for the demand for high-speed formation images. However, in that case, it is necessary to adjust the pitch (scanning line interval in the sub-scanning direction) between the plurality of beams. As a method of adjusting the pitch between a plurality of beams, there are a method of rotating a multi-beam light source unit around the optical axis and a method of using an optical element for pitch adjustment. (For example, refer to Patent Document 1).

In addition, in order to realize high image quality of the formed image, it is necessary to reduce the beam spot diameter. Several inventions have been disclosed so far for reducing the beam spot diameter (see, for example, Patent Document 2, Patent Document 3, and Patent Document 4).
JP-A-9-131920 Japanese Patent Laid-Open No. 3-116112 Japanese Patent Laid-Open No. 5-19190 JP 2001-166237 A

  However, there is a technical problem to reduce the beam spot diameter. In particular, it is necessary to reduce the beam spot diameter in consideration of the temperature rise in the apparatus due to many heat sources such as a polygon scanner provided in the image forming apparatus and the optical scanning apparatus, and the temperature environment change due to the temperature fluctuation of the use environment.

However, in the conventional example in which the light source unit is rotated around the optical axis, since the light source unit itself is moved, the reliability of the electrical components becomes a problem. Further, in the conventional example using the optical element for pitch adjustment, a high-precision optical element made of glass is required, leading to an increase in cost.
Even if the beam spot diameter is initially adjusted with high accuracy, the beam spot position shifts with the change with time such as temperature fluctuation.

  Therefore, a “liquid crystal element” driven by an electric signal has been proposed as a pitch adjusting means (see Japanese Patent Application Laid-Open No. Hei 6-214177). By detecting the beam pitch by beam pitch detection separately provided and driving the liquid crystal element based on the detection result, it is possible to correct the change with time of the beam pitch. The liquid crystal element is a beam pitch adjusting means having features such as low voltage driving, no heat generation, no noise, no vibration, small size and light weight.

  The liquid crystal element used in the pitch adjusting means has a cell structure in which a liquid crystal layer of about several [μm] to several tens [μm] is sealed with two glass substrates. For this reason, when the ambient temperature changes, the liquid crystal layer having a relatively high expansion coefficient thermally expands as the ambient temperature rises, and the central portion of the liquid crystal element may swell, resulting in a positive power lens effect. there were. As described above, when the liquid crystal element exhibits a lens effect due to an increase in the ambient temperature of the liquid crystal element, the beam waist position is changed, and the beam spot diameter may be deteriorated (increased).

The present invention has been made to solve the above-described problems of the prior art, and it is possible to reduce the beam spot diameter in consideration of temperature fluctuations, and also to ensure high precision beam spot position accuracy. An object of the present invention is to provide a possible and low-cost optical scanning device.
It is another object of the present invention to provide an image forming apparatus capable of outputting a high-quality image using the optical scanning device.

  In order to achieve the above object, the invention described in claim 1 includes a light source, a deflector for deflecting a light beam from the light source, a coupling lens for coupling the light beam from the light source, and the deflector. An optical scanning device having a scanning optical system that guides a light beam deflected by a scanning surface to a surface to be scanned and a liquid crystal element capable of phase modulation, wherein at least one of the optical elements disposed in the optical path has a positive power And is made of resin.

  According to a second aspect of the invention, in the first aspect of the invention, the resin optical element having positive power is an optical element constituting a scanning optical system.

  The invention described in claim 3 is the invention described in claim 1, characterized in that the resin optical element having positive power is an optical element disposed between a coupling lens and a deflector.

  According to a fourth aspect of the invention, in the first aspect of the invention, the liquid crystal element capable of phase modulation has a function of deflecting an optical path of an incident light beam.

  According to a fifth aspect of the invention, in the fourth aspect of the invention, the liquid crystal element capable of phase modulation has a focus adjustment function.

  A sixth aspect of the present invention is the liquid crystal device according to any one of the first to fifth aspects, wherein the liquid crystal element has a plurality of light sources and is disposed in at least one of optical paths of a plurality of light beams emitted from the plurality of light sources. And at least two light beams are guided to a common surface to be scanned.

  A seventh aspect of the present invention is the liquid crystal device according to any one of the first to sixth aspects, wherein the liquid crystal element has a plurality of light sources and at least one of optical paths of a plurality of light beams emitted from the plurality of light sources. , And at least one of the plurality of light beams is guided to a surface to be scanned different from the other light beams.

  According to an eighth aspect of the present invention, in the first aspect of the present invention, an optical system including at least two optical elements is provided between the coupling lens and the deflector, and the optical system rotates at least at a positive power. It is comprised from the resin-made optical elements which have a symmetrical surface.

  According to a ninth aspect of the present invention, in the image forming apparatus for forming an image on the image carrier by executing the electrophotographic process, the exposure process of the electrophotographic process is performed as the means for performing the exposure process of the electrophotographic process. The optical scanning device described is used.

  According to the first aspect of the present invention, in the optical scanning device including the liquid crystal element capable of phase modulation, the power change caused by the liquid crystal element accompanying the temperature change can be canceled by the power change of the resin optical element, It is possible to correct a beam waist position shift on the surface to be scanned.

  According to the second aspect of the invention, the scanning optical lens having a complicated aspherical shape is made of resin, so that it can be mass-produced using a mold.

  According to the third aspect of the invention, the optical system disposed between the coupling lens and the deflector is provided with a beam waist displacement correction function, so that the scanning optical system is provided with a correction function. Can be expanded. Further, by configuring the optical system provided between the coupling lens and the deflector with two or more optical elements, the beam waist position can be corrected more effectively.

  According to the fourth aspect of the present invention, the beam spot position on the surface to be scanned can be varied by deflecting the optical path of the laser beam by a small angle by the liquid crystal element.

  According to the invention described in claim 5, by controlling the divergence, convergence, and parallel state of the laser beam (light beam) by the liquid crystal element, the beam waist position deviation in the vicinity of the surface to be scanned is intentionally generated by an external electric signal. Can be controlled.

  The invention described in claim 6 provides a multi-beam scanning device that scans one surface to be scanned with a plurality of laser beams. When this is used as an exposure device of an image forming apparatus, a single laser beam scanning device and In comparison, it is possible to reduce the number of rotations of the polygon scanner when outputting a predetermined number of prints, thereby reducing heat generation, noise and power consumption, and energy saving. In this case, since the beam spot position on the photoreceptor surface can be corrected by the deflection function of the liquid crystal element, the scanning line interval can be maintained with high accuracy, and a high-quality output image can be obtained.

  According to the seventh aspect of the present invention, there is provided a multi-beam scanning device capable of correcting the relative positions between a plurality of beam spots that scan on a plurality of scanned surfaces, which is tandem (color). By using it as an exposure means of an image forming apparatus, it is possible to obtain a print output image with little color misregistration between a plurality of photosensitive members (scanned surfaces). This makes it possible to reduce the timing (frequency) of detecting color misregistration between the photoconductors. In the case of an image forming apparatus that forms a toner image for color misregistration detection, the amount of toner required is small. As a result, wasteful toner discharge can be reduced.

  According to the eighth aspect of the invention, not only can the correction of the beam waist position shift at the time of temperature change be performed, but also the temperature correction lens is arranged so as to be movable and adjustable in the optical axis direction. The beam waist position shift can be corrected at. Further, by adopting a rotationally symmetric surface, it is possible to suppress the occurrence of attachment errors (in the rotational direction) during movement adjustment.

  According to the ninth aspect of the present invention, since the optical scanning device capable of scanning the beam spot at a desired position on the surface to be scanned (photosensitive member) is used as the exposure unit of the image forming apparatus, it is high speed and high density. High-quality print output images can be acquired.

  Embodiments of an optical scanning device and an image forming apparatus according to the present invention will be described below with reference to the drawings.

  FIG. 1 is a diagram for explaining an embodiment of an optical scanning device 20 used in the present invention. In FIG. 1, an optical scanning device 20 includes a semiconductor laser 11, a coupling lens 12, and a light source unit 18 having a liquid crystal element, a cylindrical lens 13, a polygon mirror 14 as an optical deflector, and a first scanning lens. 15a and a scanning optical system 15 including a second scanning lens 15b.

  The optical scanning device 20 is an optical scanning device that scans the scanning surface 16 with one laser beam 21 emitted from one light source, that is, a light source including the semiconductor laser 11 and the coupling lens 12. The light source composed of the semiconductor laser 11 and the coupling lens 12 constitutes a light source unit 18 shown in FIG. As shown in FIG. 2, the present invention can be applied to a multi-beam optical scanning apparatus that simultaneously scans a plurality of laser beams emitted from a plurality of light sources (for example, semiconductor laser arrays).

In FIG. 1, a laser beam 21 emitted from a semiconductor laser 11 and made into a substantially parallel light beam by a coupling lens 12 is applied to a deflecting reflection surface of a polygon mirror 14 as a deflector by the action of a cylindrical lens 13 in the sub-scanning direction. Is formed only as a line image that is long in the main scanning direction. When the polygon mirror 14 is rotationally driven at a constant speed, the laser beam 21 is deflected at an equal angular velocity by the deflection reflection surface. The deflected laser beam 21 is imaged as a beam spot on the scanned surface 16 which is a photosensitive member by the scanning optical system 15, and on the scanned surface 16 by the fθ function provided in the scanning optical system 15. Scanned in speed. An image is written on the scanned surface 16 by the scanning of the laser beam 21. The write start timing is determined based on a synchronization detection signal obtained when the laser beam is incident on the synchronization detection sensor 19.
In general, each of the optical elements described above is housed in an optical housing 17 as shown in FIG.

An optical scanning device, particularly a multi-beam scanning device, has an optical beam position correction means for initial adjustment of the beam spot position on the surface to be scanned, and for correction of environmental change and beam spot position variation over time. It is equipped.
As the light beam position correcting means, optical path deflecting means for deflecting the laser beam by a minute angle is used. As a conventional method for deflecting an optical path by an optical path deflecting means, “rotating a folding mirror”, “shifting or rotating a cylindrical lens”, “shifting or rotating a prism”, or “electro-optic element (AOM)” is used. And “rotate a parallel plate disposed between the semiconductor laser and the coupling lens”.
However, the above-described conventional small angle deflection method has problems such as an increase in size of the apparatus, power consumption, heat generation, and noise.

  In the present invention, as shown in FIG. 1, a liquid crystal element 43 is employed as a minute angle deflection means for the optical path. Liquid crystal elements are generally small and light, have low energy consumption, and have characteristics such as no noise and no heat generation.

  The liquid crystal element has a phase modulation function capable of changing the phase of the incident laser beam. Specifically, it is possible to change the phase in the liquid crystal layer by applying an electrical signal to the liquid crystal element from the outside, and thus a liquid crystal element that forms a gradient in the sub-scanning direction can be configured. . Thus, the liquid crystal element can be used as an optical path deflecting unit that deflects a laser beam by a small angle (in the sub-scanning direction), that is, as a deflecting element. In the optical scanning device as shown in FIG. 1, the beam spot position on the scanned surface (photosensitive drum surface) 16 can be moved in the sub-scanning direction by using the liquid crystal element 43 as an optical path deflecting element.

  The change of the beam waist position due to the temperature of the optical path of the laser beam 21 from the semiconductor laser 11 to the scanned surface 16 in FIG. 1, particularly the temperature (atmosphere temperature) in the optical housing 17, will be described with reference to FIGS. To do.

4 to 6 show the components arranged on the optical path through which the laser beam 21 passes from the semiconductor laser 11 to the scanned surface 16 in the optical scanning device 20 according to the above embodiment. It is a mimetic diagram at the time of arranging on a straight line along.
In FIG. 4 to FIG. 6, each element is described using a common symbol. That is, a laser beam emitted from the semiconductor laser 11 exits the coupling lens 12, passes through the liquid crystal element 43, passes through the cylindrical lens 13, is deflected by the polygon mirror 14, and passes through the scanning optical system 15. The beam 21 is configured to form an image of a beam spot 27 on the scanned surface 16.

  (A), (b), and (c) in FIGS. 4 to 6 represent differences in focus due to differences in ambient temperature in the optical housing in each configuration. The optical path schematic diagram shown in the upper stage of FIGS. 4 to 6 shows the optical path in the main scanning direction, and the optical path schematic diagram shown in the lower stage shows the optical path in the sub-scanning direction. 4 to 6 schematically show the optical path reflected by the polygon mirror 14 in a developed manner. That is, among the optical paths deflected and scanned by the polygon mirror 14, for example, the optical path and the optical element that reach the central image height are shown side by side on a straight line. That is, the schematic diagrams of the optical paths shown in FIGS. 4 to 6 show the thickness of the laser beam 21 (flux) caused by the deformation of the resin lens accompanying the temperature change necessary for the description of the embodiment of the present invention and the change in the refractive index. Only the increase / decrease in the width of the light beam, and the increase / decrease in the width of the light flux due to the deformation of the glass lens, the change in the refractive index, the change in the optical element spacing, etc. which are not necessary for the description of the embodiment of the present invention. Is not shown. Further, since the width of the light flux is enlarged and illustrated as appropriate (changing the magnification for each figure), a relative comparison cannot be made.

FIG. 4 is a schematic diagram of an optical path in the case where all of the optical elements inside the optical housing, that is, the cylindrical lens 13 and the scanning optical system 15, are made of glass for explaining the effect of the present invention.
FIG. 5 is a schematic diagram of an optical path in the case where all of the cylindrical lens 13 and the scanning optical system 15 inside the optical housing shown for explaining the embodiment according to the present invention are made of resin.
FIG. 6 shows that all of the cylindrical lens 13 and the scanning optical system 15 inside the optical housing shown for explaining another embodiment of the present invention are made of resin, and further, a temperature correction lens 22 made of resin is added. It is a schematic diagram of the optical path in the case.
FIGS. 5 and 6 illustrating the embodiment of the present invention show a state where the beam waist position is ideally corrected.

Before describing the embodiment of the present invention and the effects thereof with reference to FIGS. 4 to 6, elements that cause the temperature change in the embodiment of the present invention will be described with reference to FIG. 7. As shown in FIG. 7, the optical scanning device 20 includes a light source unit 18, a liquid crystal element 43, a cylindrical lens 13, a polygon mirror (deflector) 14 and a scanning optical system 15 in an optical housing 17. The light source unit 18 includes the semiconductor laser 11 and the coupling lens 12.
The polygon mirror 14 is assembled to a polygon motor (not shown) and is driven to rotate at a rotational speed of tens of thousands of rpm. At this time, the temperature inside the optical housing 17 rises due to the influence of heat generated from the driving IC of the polygon motor or heat generated by friction with the air due to rotation of the polygon mirror. Further, when the optical scanning device 20 is mounted on a laser printer or the like to which an electrophotographic process is applied, an external heat source such as heat generated from a fixing device that fixes the toner after transferring the toner onto a recording paper is used. May also affect.

  Next, the effect of heat in the optical housing will be described with reference to FIG. In FIG. 4, the optical elements (cylindrical lens 13 and scanning optical system 15) having at least a positive power surface housed in the optical housing 17 are made of glass with small thermal expansion coefficient and refractive index temperature fluctuation.

  When the ambient temperature shown in FIG. 4B is normal temperature (for example, 25 ° C.), the laser beam 21 converted into a parallel light beam by the coupling lens 12 passes through the liquid crystal element 13 and is in a state of a parallel light beam on the cylindrical lens 13. Incident at. Therefore, the laser beam 21 incident on the deflecting / reflecting surface of the polygon mirror 14 is a parallel light beam in the main scanning direction, and forms a long line image in the main scanning direction by the action of the cylindrical lens 13 in the sub scanning direction. Further, the scanning optical system 15 forms an image on the surface to be scanned (photosensitive drum surface) 16 in the main scanning direction on the surface to be scanned (photosensitive drum surface) 16 and converted into a polygon in the sub scanning direction. Since the deflecting / reflecting surface of the mirror 14 and the scanned surface 16 are conjugate, a line image formed on the deflecting / reflecting surface is formed on the scanned surface 16 by the action of the cylindrical lens 13. Therefore, when the ambient temperature is room temperature, an image is formed on the surface of the photosensitive drum 16 in both the main scanning direction and the sub-scanning direction, and beam waist position deviation does not occur.

  However, since the liquid crystal element 43 has a cell structure in which a liquid crystal layer of about several μm to several tens of μm is sealed with two glass substrates, the temperature in the optical housing 17 rises to a high temperature (for example, 45 ° C.). In this case, the central portion of the liquid crystal element 43 swells to generate a lens effect (positive power). This will be described with reference to FIG. When the liquid crystal element 43 exhibits a lens effect, the laser beam 21 incident on the polygon mirror 14 in the main scanning direction becomes a weak convergent light beam, and a line image is formed on the cylindrical lens 13 side from the polygon mirror 14 in the sub scanning direction. On the other hand, the scanning optical system 15 composed of a glass optical lens having a low coefficient of thermal expansion and a refractive index due to temperature fluctuation is hardly affected by the temperature rise. As a result, the beam waist position near the scanned surface 16 is shifted toward the polygon mirror 14 in both the main scanning direction and the sub-scanning direction, and the beam spot diameter increases.

  On the other hand, the case where the inside of the optical housing 17 becomes low temperature (for example, 5 ° C.) will be described with reference to FIG. Contrary to the temperature rise, the liquid crystal element 43 exhibits a negative power lens effect, so that the beam waist position in the vicinity of the surface to be scanned 16 moves away from the polygon mirror 14. As a result, the beam waist position in the vicinity of the scanned surface 16 is shifted in a direction away from the polygon mirror 14 in both the main scanning direction and the sub-scanning direction. This increases the beam spot diameter.

Next, a case where the scanning optical system 15 is made of a resin having a large thermal expansion coefficient and refractive index temperature variation will be described with reference to a schematic diagram shown in FIG.
FIG. 5B shows a case where the ambient temperature is normal temperature (for example, 25 ° C.), and the laser beam 21 converted into a parallel light beam by the coupling lens 12 is the same as in the case of FIG. The lens 13 and the scanning optical system 15 form an image on the scanned surface (photosensitive drum) 16 in both the main scanning direction and the sub-scanning direction.
FIG. 5A shows a case where the inside of the optical housing 17 is at a high temperature (for example, 45 ° C.). The liquid crystal element 43 generates positive power, and the laser beam 21 emitted from the liquid crystal element 43 is Both the scanning direction and the sub-scanning direction become “weak convergent light flux”. On the other hand, the curvature radii of the first scanning lens 15a and the second scanning lens 15b made of resin (having positive power) constituting the optical scanning system 15 increase as the temperature rises. Power is weakened.
In this way, when the temperature inside the optical housing 17 rises to a high temperature, the effects of the positive power generated in the liquid crystal element 43 and the positive power that weakens in the entire scanning optical system 15 are contradictory to each other. The effect is cancelled. As a result, the occurrence of a beam waist position shift near the scanned surface 16 is suppressed, and a stable small-diameter beam spot can be obtained.

Similarly, FIG. 5C shows a case where the inside of the optical housing 17 is at a low temperature (for example, 5 ° C.), and the liquid crystal element 43 exhibits a negative power lens effect, but the positive power of the entire scanning optical system 15 is Become stronger. For this reason, the effect is canceled by the mutual action, and the occurrence of the beam waist position deviation in the vicinity of the scanned surface 16 can be suppressed.
In FIG. 5, the beam waist position correction is performed only in the main scanning direction. The reason will be described later.

  The resin-made scanning lens constituting the scanning optical system 15 shown in FIG. 5 can be produced in large quantities at low cost using a molding die. That is, it is easy to create an aspheric shape with a molding die. On the other hand, the glass lenses constituting the scanning optical system 15 shown in FIG. 4 need to be ground and polished one by one by machining, which is not suitable for mass production.

As another embodiment of the present invention, a configuration in which a resin temperature correction lens 22 is added to a glass cylindrical lens 13 and a scanning optical system 15 will be described with reference to a schematic diagram shown in FIG. Unlike the configuration before the polygon mirror shown in FIG. 1, the optical scanning device 20 used in the second embodiment uses a temperature correction lens 22 between the liquid crystal element 43 and the cylindrical lens 13 as shown in FIG.
In FIG. 6, a resin temperature correction lens 22 having a positive power surface on the left incident surface and a glass cylindrical lens 13 are arranged between the coupling lens 12 and the polygon mirror 14. In order to further improve the correction effect, the temperature correction optical system may be composed of three or more optical elements. As an alternative to the combination of the temperature correction lens 22 and the cylindrical lens 13, a single lens structure having a positive power in both the main scanning direction and the sub-scanning direction may be used.

  Unlike the embodiment shown in FIGS. 4 and 5, in the embodiment shown in FIG. 6, the laser light 21 emitted from the semiconductor laser 11 is coupled to a weak divergent light beam by the coupling lens 12, and the liquid crystal element 43. Is converted into a parallel light beam by the function of the temperature correction lens 22 having positive power. The collimated laser beam 21 enters the cylindrical lens 13, and the subsequent behavior is the same in the main scanning direction and the sub-scanning direction due to the action of the cylindrical lens 13 and the scanning optical system 15, as in the embodiment of FIG. An image is formed on the scanned surface 16 (at room temperature, for example, 25 ° C.).

When the inside of the optical housing 17 becomes high temperature (for example, 45 ° C.) (see FIG. 6A), positive power is generated in the liquid crystal element 43, but since the positive power of the temperature correction lens 22 is reduced, The effect can be canceled, and as a result, the beam waist position shift can be corrected.
When the inside of the optical housing 17 becomes a low temperature (for example, 5 ° C.) (see FIG. 6C), negative power is generated in the liquid crystal element 43. However, since the positive power of the temperature correction lens 22 increases, The effect can be canceled. As a result, the beam waist position shift can be corrected.

  In addition to the first and second embodiments described above, the optical system disposed between the scanning optical system 15 and the coupling lens 12 and the deflector 14 may be made of resin. Furthermore, by adding an aspherical coefficient to the surface having power, it becomes possible to suppress the wavefront aberration deterioration and stabilize the beam spot shape. With a resin lens, even a lens with an aspherical coefficient added can be easily mass-produced using a molding die.

  When a resin lens is manufactured using a molding die, the surface accuracy of the optical surface (incident surface and / or outgoing surface) is affected by the influence of variations in molding conditions and cooling conditions after removal from the mold. In many cases, it is difficult to maintain the internal refractive index well, that is, as designed. Thus, if the surface accuracy of the optical surface cannot be maintained as calculated, a beam waist position shift may occur when the optical scanning device is assembled, and the beam spot diameter may increase. In order to avoid such an increase in beam spot diameter, the optical system disposed between the coupling lens 12 and the deflector 14 is composed of two or more optical elements, one of which is a positive power rotationally symmetric surface. It can be set as the resin-made lens which has.

  In this way, the resin lens may function as a temperature correction lens and be movable and adjustable in the optical axis direction. By moving and adjusting the temperature correction lens during assembly adjustment of the optical scanning device, it is possible to correct a beam waist position variation caused by molding variations of the temperature correction lens. By making the surface of the positive power rotationally symmetric, it is possible to reduce the influence on the optical performance even when the temperature correction lens rotates around the optical axis due to an assembly error or the like during movement adjustment.

  When adjusting the beam waist position, it is desirable to preferentially correct the beam waist position in the main scanning direction as in the numerical example shown in FIG. 9. The cylindrical lens 13 having power only in the direction may be configured to be movable and adjustable.

  As already described, the temperature in the optical housing 17 changes due to the influence of a heat source inside the optical scanning device 20, such as a polygon motor that rotates a polygon mirror, and an external heat source such as a fixing device. A change in the beam waist position when the temperature in the optical housing 17 changes from 25 ° C. to 45 ° C. will be described with reference to numerical examples in the table shown in FIGS. In this description, it is assumed that the temperature of the light source unit 18, the liquid crystal element 43, the cylindrical lens 13, the polygon mirror 14, the first scanning lens 15a, and the second scanning lens 15b in the optical housing 17 has changed.

  As a premise of the numerical example, the configuration shown in FIG. 4 is used in the configuration shown in FIG. The oscillation wavelength of the semiconductor laser 11 is 655 nm, the focal length fcol of the coupling lens 12 is 15 mm (when the internal temperature of the optical scanning device is 25 ° C.), and the first and second surfaces of the coupling lens 12 are coaxial aspheric surfaces. Although the numerical value is not shown, the wavefront aberration due to the coupling lens 12 is corrected well.

  The light beam emitted from the coupling lens 12 is coupled to a parallel light beam. The laser beam emitted from the coupling lens 12 is shaped by an aperture (opening) (not shown), passes through the liquid crystal element 43, and then enters the correction lens 22. The distance from the second surface (exit-side surface) of the coupling lens 12 to the aperture is 10 mm, the distance from the aperture to the liquid crystal element 43 is 8.3 mm, and the distance from the liquid crystal element 43 to the first surface of the cylindrical lens 13 is It was 29 mm.

  FIG. 8 shows optical system data from the coupling lens 12 to the scanned surface 16 in the above configuration. The cylindrical lens 13, the first scanning lens 15a, and the second scanning lens 15b are made of glass, and the linear expansion coefficient (α) thereof is 7.5E-06 (1 / ° C.). In FIG. 8B, the radius of curvature in the main scanning direction is Rm, the radius of curvature in the sub-scanning direction is Rs, and the refractive index at the working wavelength is N. Here, the incident side and the emission side of the cylindrical lens 13 are surface number 3 and surface number 4, the deflection reflection surface of the polygon mirror 14 is surface number 5, and the incident side and the emission side of the first scanning lens 15a are surface number 6 and surface. No. 7, surface number 8 and surface number 9 are the incident side and emission side of the second scanning lens 15b, and surface number 10 is the surface 16 to be scanned. The angle between the incident beam and the reflected beam with respect to the polygon mirror 14 of the laser beam reaching the image height H = 0 is 60 degrees.

  As already described, the liquid crystal element 43 has a cell structure in which a liquid crystal layer is sealed with two glass substrates, and the central portion swells as the temperature rises, producing a positive power lens effect. For example, experimentally, in the case of a liquid crystal element in which a liquid crystal layer having a thickness of several tens of μm is sealed with two glass substrates each having a length × width of 16 × 16 mm (a thickness of 0.5 mm), a temperature of 20 ° C. Due to the rise (rise from 25 ° C. to 45 ° C.), the incident surface (or exit surface) of the liquid crystal element, which was flat at 25 ° C., changes to a surface shape corresponding to R of 80,000 mm at 45 ° C. (transmitted wavefront aberration) Converted to λ / 0.8 spherical surface (λ = 655 nm).

Due to this influence, when the cylindrical lens 13 and the scanning optical system 15 (the first scanning lens 15a and the second scanning lens 15b) are composed of glass lenses, the amount of thermal expansion is small, so that the beam waist position is corrected. The beam waist position moves to the polygon mirror side.
Thus, in the case of the configuration of FIG. 4, the beam waist position shift when the temperature changes from 25 ° C. to 45 ° C. is −1.17 mm in the main scanning direction (the negative sign indicates the direction approaching the polygon mirror 14), It was -0.36 mm in the sub-scanning direction.

  Next, the case where the scanning optical system 15 is made of a resin having a large temperature fluctuation of the thermal expansion coefficient and the refractive index, that is, the structure of FIG. 5 will be described. As the temperature changes, the radius of curvature of each optical surface and the refractive index of the optical element change as in the numerical data shown in FIG. The configuration of FIG. 9 is omitted because it is the same as that of FIG. Further, the arrangement of the optical elements is the same as in the case of FIG.

  In the above configuration, the positive power of the scanning optical system 15 is reduced due to changes in the radius of curvature (and aspherical coefficient) and the refractive index of each optical surface. This acts to move the beam waist position away from the polygon mirror 14. This action cancels out the positive power (transmitted wavefront aberration is λ / 0.8) generated in the liquid crystal element, and the beam waist position fluctuation can be corrected.

  When the configuration of the embodiment according to the present invention, that is, the configuration shown in FIG. 5 is used, the beam waist position shift when the temperature in the optical housing changes from 25 ° C. to 45 ° C. is −0 in the main scanning direction. .06 mm (the negative sign (−) indicates the direction approaching the polygon mirror 14), the sub-scanning direction is +0.87 mm (the positive sign (+) indicates the direction moving away from the polygon mirror 14), and the main scanning It is possible to correct the beam waist position deviation in the direction.

  On the other hand, regarding the beam waist position in the sub-scanning direction, correction by the scanning optical system 15 becomes excessive as compared with the waist position shift by the liquid crystal element 43, and as a result, the beam waist position cannot be corrected well. In such a case, by controlling the refractive index distribution in the liquid crystal layer by phase modulation of the liquid crystal element, a lens effect (an effect of correcting the imaging position) is generated and the beam waist position in the sub-scanning direction is corrected. Can be configured. In the case of this configuration example shown in FIG. 5, it is possible to correct the beam waist position deviation in the main scanning direction.

  In the optical scanning device for an image forming apparatus, the surface of the photosensitive member is exposed while the beam spot moves in the main scanning direction. Therefore, the exposure beam in the main scanning direction is thicker (larger) than the main scanning beam spot diameter at rest. Become. Therefore, the beam spot diameter at rest needs to be set smaller in the main scanning direction than in the sub-scanning direction, and it is desirable to preferentially reduce changes in the beam waist position in the main scanning direction.

  Next, in the configuration of FIG. 1, in addition to the configuration shown in FIG. 6, that is, the glass cylindrical lens 13 and the scanning optical system 15 (the first scanning lens 15a and the second scanning lens 15b), a resin temperature correction lens. An example in which the optical system having the specifications shown in FIG.

  In the configuration of FIG. 6, the laser beam emitted from the coupling lens 12 is shaped by an aperture (opening) (not shown), passes through the liquid crystal element 43, and then enters the correction lens 22. The distance from the second surface of the coupling lens 12 to the aperture was 10 mm, the distance from the aperture to the liquid crystal element 43 was 8.3 mm, and the distance from the liquid crystal element 43 to the first surface of the cylindrical lens 13 was 13 mm. Note that the laser beam emitted from the semiconductor laser 11 and coupled by the coupling lens 12 is converted into a weak divergent light beam, and this divergent light beam is emitted from the second surface of the coupling lens 12 to the light emitting point side (to be scanned). The light is naturally condensed at a position 228.0 mm away on the opposite side of the surface.

According to the mechanism described above, even if the temperature inside the optical housing 17 changes and positive power or negative power is generated in the liquid crystal element, it is canceled by the power change of the temperature correction lens 22, and as a result, the beam waist position is corrected. In the case of the configuration of FIG. 6, when a temperature change occurs from 25 ° C. to 45 ° C., a beam waist position shift is generated by −0.01 mm in the main scanning direction (a negative sign (−) indicates a direction approaching the polygon mirror 14. ), +0.11 mm is generated in the sub-scanning direction (a positive sign (+) indicates a direction away from the polygon mirror 14).
FIG. 11 summarizes beam waist position shifts that occur when the temperature inside the optical housing 17 changes from 25 ° C. to 45 ° C. in the configuration of the schematic diagrams shown in FIGS. According to FIG. 12, “at least one of the optical elements arranged in the optical path from the light source to the scanned surface and having a positive power surface is made of resin”, the main in accordance with the temperature change inside the optical housing. It has become possible to correct variations in beam waist position in the scanning direction and / or sub-scanning direction.

The embodiment of the present invention has been described above with respect to the single beam optical scanning device that scans one laser beam. In recent years, there has been an increasing demand for higher printing speed and higher print density in laser printers and digital copiers. As an optical scanning device that achieves these demands, a “multi-beam optical scanning device” that simultaneously scans multiple laser beams. Has become mainstream.
Next, an embodiment of a multi-beam optical scanning device according to the present invention will be described.

  FIG. 2 is a perspective view showing an embodiment of a multi-beam optical scanning device 20a that simultaneously scans two laser beams. In FIG. 2, two laser beams 21 a and 21 b emitted from two semiconductor lasers 11 a and 11 b serving as light sources and emitted from coupling lenses 12 a and 12 b are polygon mirrors that are deflectors by the action of a common cylindrical lens 13. 14 is converged only in the sub-scanning direction and formed as a long line image in the main scanning direction on the deflecting / reflecting surface 14, and is scanned by the scanning optical system (scanning lens) 15 on the scanned surface 16 that is the surface of the photosensitive drum. Are scanned as beam spots. The multi-beam optical scanning device 20 a is configured such that the two laser beams 21 a and 21 b are guided to a common scanned surface (photosensitive member surface) 16.

  In such a multi-beam optical scanning device that scans a common surface to be scanned with a plurality of laser beams, a liquid crystal element having a deflection function disposed in at least one optical path of the laser beam is driven and controlled. Thus, the interval (scan line interval) between the plurality of beams on the surface to be scanned can be corrected to a predetermined value. Accordingly, it is possible to provide a multi-beam optical scanning device capable of scanning a plurality of beams while maintaining the scanning line interval with high accuracy.

  When the multi-beam optical scanning device is used as an exposure device for an image forming apparatus, it is possible to reduce the number of rotations of the polygon scanner when printing a predetermined number of prints compared to a single-beam optical scanning device, Noise and power consumption can be reduced to save energy. In addition, the beam spot position on the photoreceptor surface can be corrected by the deflection function of the liquid crystal element, so that the scanning line spacing can be maintained with high accuracy and the beam waist position deviation, that is, the beam spot diameter fluctuation is small, and a high-quality output image. Can be obtained.

  Further, when the multi-beam optical scanning device is used as a means for executing an exposure process of an image forming apparatus that executes an electrophotographic process to form an image, the scanning density is set according to the request of an operator (user). It is also possible to cope with switching between high speed and high density by switching. Further, unlike the configuration of FIG. 2 in which all the liquid crystal elements are disposed in the optical path of laser beams (for example, two), the number of liquid crystal elements used may be reduced as necessary.

Unlike the above-described multi-beam optical scanning apparatus, a configuration in which a plurality of laser beams emitted from a plurality of light sources are guided to different scanning surfaces as in another embodiment shown in FIG. .
The example shown in FIG. 3 is an example of a four-drum tandem image forming apparatus to which an electrophotographic process is applied, and has the optical scanning apparatus described so far as an exposure apparatus as an apparatus for executing an exposure process. Yes. In FIG. 3, there are four light source sections 22a, 22b, 22c, and 22d respectively corresponding to the four photosensitive drums 16Y, 16M, 16C, and 16K. A ring lens 12 is provided. Each laser beam transmitted through each coupling lens is individually transmitted through each cylindrical lens 13 and is formed as a line image near the deflection reflection surface of the polygon mirror 14. Thereafter, each laser beam is deflected and reflected by the polygon mirror 14 as described above, and is formed as a beam spot on the surface of each photosensitive drum by the first scanning lens 15a common to each laser beam and the individual second scanning lens. An image is formed and the surface of each photosensitive drum is scanned.

  FIG. 3 is a perspective view showing an example of an image forming apparatus to which an electrophotographic process is applied using the optical scanning apparatus having the above configuration as an exposure apparatus, and an example of a 4-drum tandem image forming apparatus. Hereinafter, a four-drum tandem image forming apparatus according to another embodiment of the present invention will be described with reference to FIG. An image forming technique based on an electrophotographic process including charging, exposure, development, transfer, fixing, cleaning, and the like is a known technique, and a description of processes not directly related to the present invention is omitted.

  First, in a tandem type full-color copying machine, four photosensitive drums 16Y, 16M, 16C, and 16K are transferred corresponding to each color of cyan (C), magenta (M), yellow (Y), and black (K). Along the conveying surface of the belt 31, a laser beam provided corresponding to each photosensitive drum is scanned by an optical scanning device to form an electrostatic latent image on the circumferential surface of each photosensitive drum, A corresponding color toner is visualized, and this is sequentially transferred onto a recording paper (sheet) conveyed by a transfer belt 31 to form a multicolor image. Accordingly, if the scanning position shifts in the sub-scanning corresponding directions are different for each color, the image quality is deteriorated and the color shift is caused.

In order to correct the color misregistration thus generated, in FIG. 3, a color misregistration detection sensor 56 is used as the color misregistration detection means. A predetermined toner mark (color misregistration detection toner image 55) is formed on each color (each photoconductor) between output sheets (not shown) conveyed on the transfer belt 31, and this is detected by the color misregistration detection sensor 56. By detecting it, the color shift can be grasped quantitatively. Based on the detection result, a liquid crystal element (not shown) can be controlled to correct color misregistration.
As described above, an output image with little color misregistration can be obtained by driving the liquid crystal element, so that it is possible to reduce the timing (frequency) of detecting the color misregistration between the photoconductors. The amount of toner required to form the mark for use can be reduced, and wasteful toner discharge can be reduced. Further, the beam waist position shift (that is, the beam spot diameter fluctuation) is small, and the output image quality can be maintained at high quality.

1 is a perspective view showing an embodiment of an optical scanning device according to the present invention. It is a perspective view which shows another embodiment of the optical scanning device concerning this invention. 1 is a perspective view showing an embodiment of an image forming apparatus according to the present invention. It is a schematic diagram which shows embodiment of the general optical scanning device for contrast with this invention. It is a schematic diagram which shows embodiment of the optical scanning device concerning this invention. It is a schematic diagram which shows another embodiment of the optical scanning device concerning this invention. It is a figure which shows embodiment of the optical scanning device concerning this invention. It is a table | surface figure which shows the optical system data in embodiment of the optical scanning device concerning this invention. It is a table | surface figure which shows the optical system data in embodiment of the optical scanning device concerning this invention. It is a table | surface figure which shows the optical system data in embodiment of the optical scanning device concerning this invention. It is a table | surface figure which shows the effect of embodiment of the optical scanning device concerning this invention. It is a figure which shows the structural example which added the correction lens of embodiment of the optical scanning device concerning this invention.

Explanation of symbols

11 Semiconductor laser 12 Coupling lens 13 Cylindrical lens 14 Polygon mirror (deflector)
15 scanning optical system 15a first scanning lens 15b second scanning lens 16 surface to be scanned (photosensitive drum surface)
DESCRIPTION OF SYMBOLS 17 Optical housing 18 Light source unit 19 Synchronization detection sensor 20 Optical scanning device 21 Laser beam 22 Temperature correction lens 25 Optical axis 27 Beam spot 31 Transfer belt 55 Color shift detection toner image 56 Color shift detection sensor

Claims (9)

A light source and a deflector for deflecting the light beam from the light source;
A coupling lens for coupling a light beam from the light source;
A scanning optical system for guiding the light beam deflected by the deflector to the surface to be scanned;
An optical scanning device having a phase-modulable liquid crystal element,
At least one of the optical elements provided in the optical path has a positive power surface and is made of resin.   2. The optical scanning device according to claim 1, wherein the resin optical element having positive power is an optical element constituting a scanning optical system.   2. The optical scanning device according to claim 1, wherein the resin optical element having positive power is an optical element disposed between a coupling lens and a deflector.   2. The optical scanning device according to claim 1, wherein the liquid crystal element capable of phase modulation has a function of deflecting an optical path of an incident light beam.   5. The optical scanning device according to claim 4, wherein the liquid crystal element capable of phase modulation has a focus adjustment function. In the optical scanning device according to any one of claims 1 to 5,
Having multiple light sources,
The liquid crystal element is disposed in at least one of optical paths of a plurality of light beams emitted from the plurality of light sources;
An optical scanning device characterized in that at least two light beams are guided to a common surface to be scanned. The optical scanning device according to claim 1,
Having multiple light sources,
The liquid crystal element is disposed in at least one of optical paths of a plurality of light beams emitted from the plurality of light sources,
At least one of the plurality of light beams is guided to a surface to be scanned different from the other light beams. An optical system composed of at least two optical elements is disposed between the coupling lens and the deflector,
2. The optical scanning device according to claim 1, wherein the optical system is composed of a resin optical element having at least a positive power rotationally symmetric surface.   9. An image forming apparatus for forming an image on an image carrier by executing an electrophotographic process, wherein the optical scanning device according to claim 1 is used as means for performing an exposure process of the electrophotographic process. An image forming apparatus.

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