Scanning optical apparatus

Abstract: A scanning optical apparatus is provided with a light source, a deflector and an aperture. A light beam generated from the light source is a linearly polarized light the polarized direction of which is inclined by 45° with respect to a plane of deflection of the light beam which is defined to the light beam plane formed by the light beam deflected by the deflectively reflection surface of the deflector with the elapse of time. The light beam is scanned to be deflected by the deflector. The aperture for limiting a diameter of the light beam is arranged between the light source and the deflector.

Description: The invention relates to a scanning optical apparatus which is used in a laser beam printer, a laser facsimile apparatus, or the like. In a scanning optical apparatus which is used in the laser beam printer, laser facsimile apparatus, or the like, a photo sensitive material is scanned by a light beam which has been deflected and scanned by a deflector, thereby forming an electrostatic latent image. The electrostatic latent image is developed to a toner image by a developing apparatus and the toner image is copy transferred onto a recording paper. After that, the toner is thermally fixed to the recording paper by a fixing apparatus after completion of the copy transfer of the toner image, so that the image is printed. As a light source which is used in such an apparatus, a laser light source is used. In particular, a semiconductor laser is often used to reduce the costs and size of the apparatus. Since a light emitted from the semiconductor laser is a divergent bundle of rays, the emitted light is converted into the parallel laser beam by a collimator lens. The laser beam transmitted through the collimator lens is deflected and scanned by a rotary polygon mirror and, further, passes through several scanning lenses and is formed as an image onto the photo sensitive material as a surface to be scanned. Some of such scanning lenses have not only the image forming function to form the parallel laser beam as an image onto the photo sensitive material but also the f- function to move an image point (light spot) onto the photo sensitive material at an equal velocity for the laser beam which is deflected at an equal angular velocity and enters the scanning lenses. Although the laser beam passes through the several lens surfaces and is refracted on its optical path. However, a loss of laser beam also occurs due to the reflection and an energy of the refracted light is reduced by only such an amount of loss. Generally, since the amount of loss by the reflection varies depending on an incident angle at which the laser beam enters the lens surface, in the laser beam printer, the energy of laser beam fluctuates due to the scanning angle of the laser beam, namely, the position of the scanning line. In the scanning optical system which is used in the conventional laser beam printer, accordingly, in order to eliminate the angle dependency of a reflectivity of each lens surface, thin optical films of a few layers are evaporation deposited, coated, or the like onto the lens surface, thereby solving such a problem. However, a thickness of each layer of the thin optical films is extremely thin to be 100 nm or less, an advanced technique is needed to stably coat such a thin optical film onto the lens surface, and there is also a large problem with respect to the stability due to the environment, costs, and the like. If such a thin film coating is abandoned, unevenness of exposure amounts occurs in the scanned region-because of the reasons mentioned above. In the output image, such unevenness results in a blur of a part of a character, so that an image quality is remarkably deteriorated. A prior art arrangement according to the preamble of claim 1 is known from document US-A-4 878 066. However, this prior art arrangement is specifically designed for distortion correction and does not provide any possibility for the compensation of deficiencies of the used optical system owing to physical properties thereof, i.e. the variation of the amount of reflection loss depending on an incident angle at which the light beam enters the lens surface can not be compensated. It is therefore an object of the present invention to provide a scanning optical apparatus capable of compensating the above mentioned variation of the amount of reflection loss without providing a thin optical film coated to the optical system. This object is achieved by a scanning optical apparatus according to claim 1. According to the invention, by setting the polarizing direction of a light source for generating a linearly polarized light to an angle of 45° for the plane of deflection (light beam plane which is formed with the elapse of time by a laser beam that is deflected by the deflection reflecting surface of a deflector), transmittance/field angle characteristics of the image forming lens to which the light beam which has been deflected and scanned by the deflector enters are lightened, thereby uniforming an exposure distribution on the photo sensitive material as a scanned surface. Further, by arranging an aperture to limit a diameter of the light beam to a position between the light source and the deflector, an image forming spot of a good shape is formed onto the photo sensitive material. Fig. 1 is a perspective view for explaining a construction of an embodiment of a scanning optical apparatus of the invention; Fig. 2 is a diagram for explaining a laser beam which is emitted from a semiconductor laser; Fig. 3 is a graph showing a reflectivity in case of a refractive index of 1.5; Fig. 4 is a diagram showing an intensity of laser beam on an aperture diaphragm; Fig. 5 is a diagram showing a spot shape; Fig. 6 is a graph showing an intensity and a spot ellipticity on a circular aperture diaphragm; Fig. 7 is a diagram showing a laser intensity distribution on an elliptic aperture diaphragm; Fig. 8 is a diagram showing a spot shape; and Fig. 9 is a diagram for explaining a state in case of using a polarizing plate. Fig. 1 is a schematic constructional diagram of a laser recording apparatus using a scanning optical apparatus of the invention. Reference numeral 1 denotes a semiconductor laser serving as a light source for generating a linearly polarized light beam; 2 a collimator lens for converting the divergent bundle of rays from the light source into almost the parallel light beam; 3 an aperture diaphragm to limit a diameter of the parallel light beam; 4 a rotary polygon mirror as a light deflector which rotates at a constant velocity in the direction indicated by an arrow A; and 5 an f- lens as image forming means for image forming the light beam deflected by the deflection reflecting surface of the polygon mirror 4 onto a photo sensitive material as a recording medium, which will be explained hereinlater, so as to have a spot-like shape. The f- lens 5 is designed so as to move the light spot at an equal velocity in accordance with a deflecting angle  of the polygon mirror. The ON/OFF operation of the semiconductor laser 1 is controlled by a laser driving system (not shown) so that an exposure distribution, which will be explained hereinlater, on the photo sensitive material exhibits characters, an image, or the like. Reference numeral 7 denotes a photo sensitive material as a surface to be scanned. The photo sensitive material 7 is optically scanned by the foregoing scanning system and rotates at an equal velocity in the B direction around a rotary shaft, thereby forming a two-dimensional exposure distribution onto the surface. An electrostatic copy processing mechanism (not shown) is arranged around the photo sensitive material and an image corresponding to the exposure distribution of the photo sensitive material is formed as a visible image onto a copy transfer material such as a paper or the like in accordance with a well-known process. Fig. 2 shows general characteristics of the output light from the semiconductor laser. An output light beam C of the semiconductor laser is a divergent bundle of rays having a divergent angle which is narrow in the direction (//) that is horizontal to a cleavage plane of a laser chip and is wide in the direction (┴) perpendicular to the laser chip cleavage plane. The light beam C is also a linearly polarized light whose polarizing direction (direction E of the electric field) is located in the // direction. In the invention, the semiconductor laser 1 in Fig. 1 is arranged so that the polarizing direction E is inclined at 45° () for the deflecting surface (light beam plane on which the laser beam that is deflected by the deflection reflecting surface of the deflector is formed with the elapse of time) as shown in Fig. 2. In the laser beam printer (LBP), it is desirable that the light scan is executed so as to form a uniform exposure distribution in the whole scanning region. When unevenness of the exposure distribution occurs, it appears as a blur of a part of a character in the output image, so that an image quality is remarkably deteriorated. Therefore, in the case such that the light spot is scanned at an equal velocity by the f- lens, it is necessary that an intensity of the laser beam on the photo sensitive material is constant irrespective of the location on the scanning line. A main factor of the change in intensity of the laser beam which is deflected is that in the image forming lens into which the laser beam enters after it was deflected, namely, in the f- lens in the embodiment, the angle between the incident laser beam and the lens transmitting surface is largely changed by the deflecting direction (hereinafter, referred to as a field angle). Generally, a reflectivity of glass or the like changes in dependence on a refractive index, an incident angle, or a polarizing direction of the incident light. Fig. 3 shows changes in incident angle and reflection loss when the laser beam enters the transmitting surface of a refractive index of 1.5. The reflection loss differs in accordance with whether the incident light is the S-polarized light or the P-polarized light. In the diagram, R s shows characteristics for the S-polarized light and R p indicates characteristics for the P-polarized light. When the incident light beam at the lens surface is the S-polarized light, that is, when the polarizing direction (//) is perpendicular to the reflection cross section (plane in which the incident light, reflected light, and refracted light are included), the reflection loss increases as the field angle is large. On the contrary, when the incident light beam is the P-polarized light, namely, when the polarizing direction is parallel to the reflection cross section, the reflection loss decreases until an incident angle called as Brewster angle at which the loss is equal to 0, and the reflection loss starts to increase in a manner similar to the S-polarized light when the incident angle is larger than the Brewster angle. Since the image forming optical system has at least two or more refractive planes, those losses are added at the respective planes. In the light beam which is emitted from the final plane of the image forming optical system, an intensity change of about 10 to 20% ordinarily occurs on the axis (scanning angle 0) and in the edge portion of the scanning line. From the deflecting angle and reflectivity angle dependencies mentioned above, in case of the S-polarized light, the exposure amount decreases in the image peripheral portion of a large field angle, and in case of the P-polarized light, the exposure amount increases. In the invention, as shown in Fig. 1, since the semiconductor laser 1 is arranged so that the polarizing direction is inclined by 45° with respect to the plane of deflection, the reflection loss exhibits angle characteristics as shown in R a in Fig. 3. There is a relation of R a = (R s + R p )/2 among R s , R p , and R a . Thus, the angle characteristics of the reflection loss at each surface of the lenses are almost flat in a range of the incident angle of 0 to 50°, for instance, in case of n = 1.5 and the exposure distribution on the photo sensitive material can be made uniform even when there is no coating. Therefore, a film to prevent the reflection is not coated to at least one or more transmission surfaces among the construction surfaces constructing the f- lens 5. Fig. 4 shows an intensity distribution of the laser beam at the aperture diaphragm 3 in Fig. 1. Reference numeral 3 denotes the aperture diaphragm member and 41 indicates an equal intensity line of the laser beam from the collimator lens 2. A hatched portion is a circular aperture portion. When the intensity of the laser beam is uniform in the whole region in the aperture portion, a spot occurring due to it has a circular shape. However, the light beam which contributes to actually form an image is the light beam from the collimator lens 2. The equal intensity line 41 of the light beam shows an ellipse having a major axis in the ⊥ direction and a minor axis in the // direction, namely, the polarizing direction as described in Fig. 2. Thus, laser beam intensities I p and I q at peripheral points of the aperture, for instance, at points P and Q are different. The equal intensity line 41 indicates a region of 1/2 of the intensity at the center of the aperture. Fig. 5 shows an image forming spot (major axis D//, minor axis D┴) on the photo sensitive material in such the case and the spot has an elliptic shape which is inclined for the scanning direction. The ellipse of Fig. 5 has an almost circular shape and when the ellipticity is defined to (D// - D┴)/D┴, there is no practical problem when the ellipticity lies within a range of ±10%. Fig. 6 is a graph showing the intensities I p and I q at points P and Q in Fig. 4 when they are normalized by the maximum intensity at the center of the laser beam and also showing the ellipticity of the image forming spot in Fig. 5. In Fig. 6, solid lines and broken lines represent laser light intensity when varying the ellipticity from 0% to 25%. When both of I p and I q are equal to or larger than 0.5, the ellipticity lies within a range of ±10% and no practical problem occurs. Even when the aperture has an elliptic shape, by setting such that the aperture lies within a region of an intensity of 0.5 or more as shown in Fig. 7, an ellipse which is vertically long is obtained in the direction perpendicular to the scanning direction as a moving direction of the scanning spot as shown in Fig. 8. That is, it is sufficient that the intensity of light beam at the edge of the aperture of the aperture diaphragm 3 is equal to or larger than 1/2 of the intensity at the center of the aperture. According to the embodiment as described above, in order to incline the polarizing direction of the laser beam, the semiconductor laser is rotated by 45° by using the optical axis as a rotational axis. Also, the polarized direction can be inclined by inserting an optical rotatory element or a polarizing plate into the emission light beam or the light beam after it was transmitted through the collimator lens without rotating the semiconductor laser. Fig. 9 is a diagram showing a light beam polarized state in the case where a polarizing plate is provided behind the collimator lens 2. Reference numeral 2 denotes the collimator lens and 81 indicates an electric field vector of the laser beam just after being emitted from the collimator lens. The direction of the electric field vector, namely, the polarizing direction E coincides with the // direction. Reference numeral 82 denotes a polarizing plate which is arranged so as to pass only the polarization component in the direction indicated by an arrow O which is inclined by 45° for the direction //. Reference numeral 83 denotes an electric field vector of the laser beam just after having passed through the polarizing plate. The polarizing direction of the laser beam after it was emitted from the collimator lens coincides with the // direction because of the characteristics of the semiconductor laser mentioned above. However, when the laser beam passes through the polarizing plate, only the polarization component in the direction of the O axis remains. Thus, a polarizing direction E' of the laser beam after it passed through the polarizing plate is also inclined by 45° with respect to the // direction in a manner similar to the case of O axis. As described above, it is possible to provide a laser beam printer using the scanning optical apparatus of low costs in which by inclining the polarizing direction of the laser beam which enters the scanning lens by 45° for the plane of deflection, even when the nonreflective coating of the lens is eliminated or the structure is simplified, the exposure intensity of the scanning line on the photo sensitive material as a surface to be scanned can be uniformed irrespective of the field angle, and by further arranging the aperture to limit the diameter of the light beam to a position between the light source and the deflector, an image forming spot of a good shape can be formed onto the photo sensitive material.

Claims: A scanning optical apparatus comprising: a light source (1) for generating a light beam; a deflector (4) for deflecting and scanning the light beam from said light source (1); an optical system (5) for converging said deflectively scanned light beam to a surface (7) to be scanned; and an aperture (3), arranged between said light source and said deflector, for limiting a diameter of said light beam; characterized in that said light beam consists of linear polarized light; the polarizing direction thereof being inclined by 45° with respect to the plane of deflection, to thereby obtain an almost flat angle characteristic of the reflection loss at each surface of said optical system (5) in a range of the incident angle of said optical system. An apparatus according to claim 1, characterized in that said light source (1) is a semiconductor laser and the inclination of the polarizing direction is realized by inclining a junction plane of the semiconductor laser with respect to the plane of deflection. An apparatus according to claim 1, characterized in that an intensity of the light beam at an aperture edge of said aperture (3) is equal or larger than 1/2 of an intensity at the center of the aperture. An apparatus according to claim 1, characterized in that said optical system is a f- lens (5) located between said deflector (4) and said surface (7) to be scanned, the transmission surfaces of said lens being not coated with a film to prevent reflection. An apparatus according to claim 1, characterized in that said surface to be scanned is the surface of a recording medium (7) for receiving said deflectively scanned light beam.

Inventors: ANDO TOSHINORI, ANDO, TOSHINORI, Ando, Toshinori, c/o Canon K.K.
Applicants: CANON KK, CANON KABUSHIKI KAISHA
Application year: 1992
Main classification: H04N1
IPC codes: G02B27, G03G15, G02B26, H04N1, B41J2