JP2000131632A - Optical scanning system and optical scanning device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To realize optical scanning by which high density scanning is made possible while securing the back length of a scanning optical system. SOLUTION: This is an optical system condensing a beam deflected by an optical deflector 5 on a surface to be scanned 9 as a light spot, and is constituted of two lenses 6 and 7, and the lens 6 on the side of the optical deflector 5 possesses positive refracting power in a horizontal scanning direction and negative refracting power in a vertical scanning direction, and the lens 7 on the side of the surface to be scanned possesses the positive refracting power in the vertical scanning direction. The lateral magnification β2 of the center image height of the scanning optical system in the vertical scanning direction satisfies the condition of 0.5<=|β2|<=1.0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a scanning optical system and an optical scanning device.

[0002]

2. Description of the Related Art Optical scanning devices are widely known in connection with digital copying machines and laser printers. The scanning optical system is an optical system that focuses a beam (which means “light beam” in this specification) deflected by an optical deflector as a light spot on a surface to be scanned. Such optical scanning devices are required to have higher density and higher speed of scanning. In addition, for a layout of an image forming apparatus using an optical scanning device, a “long back length” is often required for a scanning optical system. The “back length” is a distance from an imaging element (a lens, a concave mirror, or the like) included in the scanning optical system, which is provided closest to the surface to be scanned, to the surface to be scanned.
Generally, in an image forming apparatus using an optical scanning device, a photoconductive photoreceptor is uniformly charged by a charging unit, and information is written by optical scanning by the optical scanning device to form an electrostatic latent image. Is developed by developing means to obtain a toner image, and the toner image is transferred and fixed on a recording medium to obtain a desired recorded image. In such an image forming apparatus, the charging unit and the developing unit have a limited mechanical positional relationship with respect to the photoreceptor due to the mechanical configuration thereof, and therefore, there is little freedom in the arrangement position. On the other hand, the optical system of the optical scanning device has a considerable degree of freedom in the optical arrangement from the light source to the surface to be scanned (actually, the photosensitive surface of the photosensitive member). Since the optical design can be performed according to the layout of the means, a scanning optical system having a long back length as described above may be required according to a layout requirement.

For example, FIG. 11 schematically shows an example of an image forming apparatus using an optical scanning device 114. The photoconductive photoconductor 100 is formed in a cylindrical shape, rotates at a constant speed in the direction of the arrow, and is charged by a charging means (a corona discharge type is shown, but a contact type such as a charging roller may be used). The image is uniformly charged by 112, and an electrostatic latent image is formed by writing by the optical scanning device 114. The electrostatic latent image is developed by a developing unit 116, and the visible image obtained by the development is transferred to a transfer unit (a roller type is shown, but a transfer / separation charger type may be used) 120. Is transferred to a recording medium (transfer paper, plastic sheet for overhead projector, etc.) S. The transferred visible image is fixed on the recording medium S by the fixing unit 122 and is discharged out of the apparatus. In FIG. 11, reference numeral 118 denotes a toner hopper. The toner hopper 118 replenishes the stored toner to the developing unit 116 as needed.
The whole hopper can be replaced. Optical scanning device 1
Reference numeral 14 denotes a portion after the optical deflector 1141. The beam deflected by the optical deflector 1141 is converted into a lens 1142
The optical scanning device 1 transmits through the optical scanning device 1143, is bent in the optical path by the mirrors 1144 and 1145, and transmits through the lens 1146.
Then, the photosensitive member 100 is optically scanned. In this example, the lenses 1142, 1143, and 1146 form the actual state of the scanning optical system, and the back length is the photosensitive member 1 of the lens 1146.
This is the distance from the surface on the 00 side to the photoconductor 100. In order to reduce the burden on the user who frequently changes the toner hopper 118, the amount of toner stored in the toner hopper is increased to extend the toner hopper replacement period.
A long back length is required.

Recently, the “scanning density” of optical scanning has been increased to 1200 d.
A high density such as 2400 dpi or 2400 dpi is required. In order to increase the density of optical scanning, it is necessary to reduce the spot diameter of a light spot focused on the surface to be scanned. For this purpose, it is preferable that the imaging magnification of the scanning optical system be as small as possible. However, in order to reduce the scanning imaging magnification, it is usually necessary to bring the imaging element closest to the surface to be scanned closer to the surface to be scanned, and in such a case, it is difficult to realize the long back length. . Further, the length of the imaging element close to the surface to be scanned in the main scanning direction is increased, so that the manufacturing is not easy and the manufacturing cost is likely to be high. As a method that can speed up optical scanning, a “multi-beam method” that simultaneously scans a plurality of scanning lines on a surface to be scanned has attracted attention, and an LD array method in which light emitting sources of a monolithic semiconductor laser array are arranged in the sub-scanning direction, An optical scanning device of a multi-beam system using a light source of a beam combining system for combining beams from the above semiconductor lasers is being realized. When such an LD array type or beam combining type light source is used, similar to the case of the single beam type optical scanning device,
Since the optical system on the optical path from the light source to the surface to be scanned can be shared by a plurality of beams and used, a multi-beam optical scanning device having good stability against mechanical fluctuations can be provided. 1
To realize high-density optical scanning such as 200 dpi or 2400 dpi by multi-beam optical scanning,
The distance between light emitting sources in the light source (in the LD array method, LD
The distance between each light source in the array, the beam combining method,
It is necessary to reduce the virtual light source spacing of the combined beam).

For example, when the pitch of a plurality of scanning lines simultaneously scanned by a plurality of beams is equal to one scanning line, that is, in the case of so-called adjacent scanning, to achieve a scanning density of 2400 dpi, a light emitting source in a light source is used. Is generally 10 μm
Will be smaller than Consider the case where a monolithic semiconductor laser array is used as a light source.
If it is smaller than m, the blinking of one light emitting source will affect the blinking of an adjacent light emitting source “thermally and electrically”, making it difficult to control the modulation of each light emitting source independently. Also, in the case of a beam combining type light source, it is necessary to make the distance between the virtual light emitting sources of the combined beam in the sub-scanning direction extremely small and to adjust it with high accuracy. It is troublesome. In order to increase the distance between the light-emitting sources in the light source to some extent and to realize high-density multi-beam scanning, adjacent beams perform scanning on the surface to be scanned with an interval of one scanning line or more. What is necessary is just to perform "scan". However, as the number of scanning lines (interlacing order) over which adjacent beams jump, the “position at which the beam passes through the scanning optical system” greatly differs in the sub-scanning direction for each beam. In that case, the optical action of the scanning optical system will not be the same for each beam,
In particular, the magnification in the sub-scanning direction changes with the image height of the light spot, and the scanning line pitch "changes greatly with the image height". Therefore, the interlace order in the interlaced scan needs to be an “appropriate order” that is not so large. In order to realize high-density optical scanning, it is natural that even in the multi-beam scanning method, it is necessary to reduce the spot diameter of each optical spot formed on the surface to be scanned. It is necessary to reduce the lateral magnification of the optical system provided between the scanning surface and the scanning surface, and also minimize the magnification of the scanning optical system for condensing each deflection beam on the scanning surface. Is no different from the case of the single beam system. 1200 dpi
In order to realize high-density optical scanning such as or 2400 dpi by multi-beam optical scanning, by performing interlaced scanning, the interleaving order is appropriately adjusted while preventing the distance between the light emitting sources in the light source from becoming extremely narrow. It is preferable to suppress a large fluctuation of the scanning line pitch due to the image height. Regardless of the multi-beam system or the single-beam system, it is preferable that the scanning optical system has as small a lateral magnification as possible and a long back length. In addition, in the case of a multi-beam, it is desirable that an optical system provided between the light source and the optical deflector is shared by a plurality of beams from the viewpoint of stability of optical scanning with respect to mechanical fluctuation.

[0006]

SUMMARY OF THE INVENTION The present invention reduces the lateral magnification in the sub-scanning direction while securing the back length of the scanning optical system irrespective of the single beam system or the multi-beam system.
It is an object to realize a scanning optical system capable of realizing a small diameter light spot. Another object of the present invention is to realize a single-beam type and a multi-beam type optical scanning device adaptable to high density by using the above scanning optical system.

[0007]

A scanning optical system according to the present invention is an "optical system for condensing a beam deflected by an optical deflector as a light spot on a surface to be scanned" and has the following features. (Claim 1). That is, the scanning optical system is composed of two lenses. The lens on the optical deflector side has "positive refractive power in the main scanning direction and negative refractive power in the sub-scanning direction", and the lens on the scanned surface side has "positive refractive power in the sub-scanning direction". Is the thing. The lateral magnification in the sub-scanning direction of the center image height of the scanning optical system: β ₂ satisfies the condition: (1) 0.5 ≦ | β ₂ | ≦ 1.0.

In the scanning optical system according to the first aspect,
At least two lens surfaces have a curvature in the sub-scanning direction that changes in the main scanning direction, and at least one of the at least two lens surfaces has a curvature change in the sub-scanning direction that is asymmetric in the main scanning direction; Lateral magnification of the center image height in the sub-scanning direction: β
_{2. The} lateral magnification in the sub-scanning direction at an arbitrary image height: βh preferably satisfies the condition: (2) 0.93 ≦ | βh / β ₂ | ≦ 1.07. In the scanning optical system according to the second aspect, the curvature change on the surface where the curvature in the sub-scanning direction changes in the main scanning direction has “an extreme value of 2 or more”, and at least one extreme value is the main scanning direction. Direction position: he
Preferably satisfies the condition: (3) | he / hmax | ≧ 0.5 with respect to the effective lens height hmax from the optical axis on the + image height side or the −image height side. ). The “curvature in the sub-scanning direction” is defined as “a virtual plane cross section perpendicular to the main scanning direction near the lens surface (hereinafter,“ sub-scanning cross section ”).
), The curvature of the lens surface in the sub-scan section. Therefore, "the curvature in the sub-scanning direction changes in the main scanning direction" means that when the position in the sub-scanning cross section is changed in the main scanning direction, the curvature in the sub-scanning direction depends on the position in the sub-scanning cross section. Changes. In the multi-beam scanning optical system according to the third aspect, it is preferable that at least two lens surfaces whose curvature in the sub-scanning direction changes in the main scanning direction have "an air gap between the surfaces" (claim 4). The scanning optical system according to any one of claims 1 to 4 can be used in a single-beam optical scanning device as well as a single-beam optical scanning device. It can also be used to "collect light" (claim 5).

The optical scanning device according to claim 6 is a single-beam type optical scanning device. That is, the optical scanning device `` coupling the beam from the light source to the subsequent optical system by the coupling lens, and the coupled beam by the line image forming optical system, near the deflection reflection surface of the optical deflector,
An image is formed as a long linear image in the main scanning direction, deflected at an equal angular velocity by an optical deflector, and the deflected beam is condensed as a light spot on a surface to be scanned by a scanning optical system to scan the surface to be scanned. In the beam type optical scanning device, the scanning optical system according to any one of the first to fourth aspects is used as a scanning optical system. An optical scanning device according to a seventh aspect is a multi-beam optical scanning device.

In other words, this optical scanning apparatus "couples beams from a plurality of light-emitting sources to a subsequent optical system by a coupling lens, and couples the coupled plurality of beams by a common line image forming optical system. A plurality of linear images that are long in the main scanning direction and separated in the sub-scanning direction are formed near the position of the deflecting reflection surface of the deflector, and are simultaneously deflected at an equal angular velocity by an optical deflector. In a multi-beam type optical scanning device, a plurality of light spots are condensed on the surface to be scanned on the surface to be scanned in the sub-scanning direction, and a plurality of scanning lines are simultaneously scanned by the plurality of light spots. A multi-beam scanning optical system according to claim 5 is used as a system, and the coupling lens may be used individually or in common for a plurality of beams. The optical scanning unit of a multi-beam system according to claim 7, it can be used to "monolithic semiconductor laser array in which a plurality of light-emitting sources are arranged in the sub-scanning direction" as a light source (claim 8). In this case, it is preferable that the interval between the light emitting sources of the semiconductor laser array is “10 μm or more”.

In both the single beam system and the multi-beam system, the "lateral magnification in the sub-scanning direction (lateral magnification in the sub-scanning direction of all optical systems provided on the optical path from the light source to the surface to be scanned)" of the optical scanning device. Is the lateral magnification in the sub-scanning direction of the optical system disposed between the light source and the optical deflector: | β ₁ | and the lateral magnification in the sub-scanning direction of the scanning optical system disposed after the optical deflector: | β ₂ | | Β ₁ | is the ratio of the focal length of the coupling lens: fcup to the focal length of the cylindrical lens (line image forming optical system): fcyl. Generally, | fcyl / fcup | ≧ 4 is required. . That is, when | β ₁ | is smaller than 4, the focal length of the coupling lens increases, and the numerical aperture must be increased in order to secure the light use efficiency, which makes it difficult to design and manufacture the coupling lens. Become. In addition, the focal length of the line image forming optical system is shortened, and mutual interference of optical arrangements around the optical deflector is likely to occur, making optical layout difficult. Above |
As for β ₂ |, | β ₂ | ≧ 0.5 is the “practically usable range”. When | β ₂ | is smaller than 0.5, the lens on the surface to be scanned is generally too close to the photoreceptor, and the overall length of the lens in the main scanning direction becomes longer. Costs are also high. It is also difficult to secure a long back length. Therefore, the sub-scanning lateral magnification of the entire system: | beta | _{The, | β 1 | · | β} 2 | = 4 × 0.5 = 2 times is the lower limit. In addition, the scanning optical system has an enlargement magnification (| β ₂ |>
In the case of 1), it is difficult to reduce the diameter of the light spot. That is, the scanning optical system has a magnification in the sub-scanning direction: | β ₂ |
In the range of 5, it is desirable to be as small as possible, that is, “(0.5 ≦ | β ₂ | ≦ 1.0)”.

In the multi-beam scanning optical system according to the first aspect, in the main scanning direction, a positive refractive power is given to the lens on the optical deflector side to secure "constant velocity characteristics" such as fθ characteristics. In the sub-scanning direction, a so-called “retro focus type” is obtained by sequentially setting “negative / positive refractive power distribution” from the optical deflector side. For this reason, the "back-side principal point in the sub-scanning direction" can be arranged closer to the surface to be scanned than in the actual lens arrangement, and the back length can be increased. It becomes possible. By setting the lateral magnification in the sub-scanning direction; | β ₂ | in the range of the above equation (1), it is possible to effectively suppress the “lens lengthening in the main scanning direction” of the scanned surface side lens. . Further, since the scanning optical system according to the first aspect is the "retrofocus type in the sub-scanning direction" as described above, the "FNo. (F number)" can be reduced and the sub-scanning direction can be reduced. Therefore, the diameter of the light spot can be “reduced to a smaller size”, and it is possible to easily cope with high-density optical scanning. Further, as the sub-scanning direction is reduced, the "effect of surface tilt of the deflecting / reflecting surface of the optical deflector" is reduced, and "scanning line pitch unevenness" due to the surface tilt is reduced.

In the case where the optical system between the light source and the optical deflector is the same, the use of the scanning optical system according to claim 1 can reduce the lateral magnification of the entire system. When the system is used in a multi-beam optical scanning device using an LD array type light source, the pitch of the light emitting sources of the semiconductor laser array can be increased, and thermal and electrical interference between the light emitting sources can be reduced. Further, in the multi-beam system using the light source of the beam combining system, the interval between the virtual light emitting sources of the combined beams can be increased to some extent, so that the accuracy required for the beam combining is eased. For example, as the aforementioned “lateral magnification in the sub-scanning direction of the entire system”, | β
Assuming that | = 2, the relationship between the scanning density and the light emitting source pitch (unit: μm) of the semiconductor laser array is as follows.
The following list is obtained for each of 00 dpi and 2400 dpi. 1200 dpi adjacent scan 3rd jump 5th jump 7th jump Simultaneous scan pitch 21.17 63.5 105.85 148.19 Required laser array pitch 10.59 31.75 52.93 74.10 2400 dpi adjacent scan 3rd jump 5th jump 7th jump Simultaneous scanning pitch 10.58 31.75 52.92 74.08 Required laser array pitch 5.29 15.88 26.46 37.04 In the above description, for example, "tertiary jump" is when adjacent beams that scan the surface to be scanned perform "simultaneous scanning via two scanning lines". The same is true.

In general, it is better to set the "lateral magnification in the sub-scanning direction of the whole system: | β |"
Although the scanning optical system is easy to manufacture, on the other hand, the light emitting source pitch of the semiconductor laser array is smaller than the above, and
When a semiconductor laser array is used as a light source, its manufacture becomes difficult. In particular, in a semiconductor laser array having a light emitting source interval of 10 μm or less, thermal and electrical interference between light emitting sources is large,
It is extremely difficult to manufacture such that the individual light sources function as "independent light sources", and the use of such semiconductor laser arrays is not practical. 10 μm or more,
Preferably, a semiconductor laser array having a light emitting source spacing of 15 μm or more can be used in a range where thermal and electrical interference is small, and as shown above, an interlaced scanning can be performed even with a light emitting source spacing of 15 μm or more. Can realize 2400 dpi. By the way, in order to perform good optical scanning, the diameter of the light spot on the surface to be scanned (the diameter in the main scanning direction can be dealt with to some extent by electrical correction of the signal, but the diameter in the sub-scanning direction can be corrected by such correction). In particular, it is important that the spot diameter in the sub-scanning direction does not change significantly with the image height. This becomes particularly important in high-density optical scanning. In order for “the diameter of the light spot on the surface to be scanned in the sub-scanning direction to not change significantly with the image height”, it is necessary that the lateral magnification of the scanning optical system in the sub-scanning direction does not change significantly with the image height. Further, the variation in the lateral magnification of the scanning optical system in the sub-scanning direction due to the image height is considered to be a problem in the multi-beam scanning method that “the pitch of the scanning lines scanned at the same time (referred to as the scanning line pitch) changes with the image height”. Appears. Therefore, in the multi-beam optical scanning, it is necessary to "correct the lateral magnification in the sub-scanning direction of the multi-beam scanning optical system to be constant between image heights" in order to suppress "variation due to image height" of the scanning line pitch. It is. This means that the curvature in the sub-scanning direction is changed in the main scanning direction on at least two of the lens surfaces of the two lenses constituting the scanning optical system, and the bending in the sub-scanning direction causes the curvature in the “sub-scanning direction”. Is adjusted according to the image height ". In general, the center of rotation of a polygon mirror, which is an optical deflector, is set so as to be "shifted by h" from the optical axis of the scanning optical system. The lateral magnification in the scanning direction changes “asymmetrically in the main scanning direction”. This asymmetric lateral magnification change can be corrected by setting at least one of the two lens surfaces as an “asymmetrical surface in the sub-scanning curvature change”. At this time, in both the single beam method and the multi-beam method, the “change in the lateral magnification in the sub-scanning direction” in the effective main scanning area is 10%.
Or less, more preferably 7% or less. According to the second aspect of the present invention, the sub-scanning direction lateral magnification change of 7% or less is realized by satisfying the condition (2). In the case of optical scanning in the multi-beam system, if the change in the lateral magnification in the sub-scanning direction is 7% or less, "1200 dp
In the case of performing the “7th interlaced scanning with i”, the pitch varies by 10.37 μm from the simultaneous scanning pitch of 148.19 μm, and the adjacent pitch at 1200 dpi: 21.1
It can be suppressed to approximately half of 7 μm. In the case of the interlaced scanning, the fact that the pitch variation is “approximately half of the adjacent pitch” is an allowable limit for the scanning line pitch variation, and it is possible to further reduce the pitch variation in the case of the fifth or third jump. it can.

In general, when the magnification is to be kept constant, a high-order curved field curvature is likely to be generated. Particularly, in an optical system having a small number of lenses, aH ² + bH ⁴ (a and b are coefficients, and H is High) easily causes sagittal (corresponding to the sub-scanning direction) field curvature. This sagittal curvature of field,
The displacement of the rotation center of the polygon mirror: "sag due to deflection" due to h results in an asymmetric field curvature in the main scanning direction.
The higher-order curved field curvature can be corrected by giving a plurality of extreme values to the change in curvature in the sub-scanning direction, thereby changing the power on the lens surface in a higher-order function.
"Maximum bulging position" of field curvature in the sub-scanning direction at this time
The image height Hn is represented by the following equation when the effective writing height is “Hm”. (Reference: Fumio Kondo, “Lens Design Techniques (Optical Industrial Technology Association)”, p146-P148) Hn = (1 / √2) × Hm = 0.71 × Hm Image height: “field curvature near 0.71Hm” In order to correct the "bulge", it is effective to provide an extreme value of the curvature change in the sub-scanning direction near the lens surface position corresponding to the position. Further, in consideration of correcting even a higher-order field curvature exceeding the fourth order, in the invention according to the third aspect, the position of the extreme value: he is determined by the condition (3) together with the effective height of the lens: hmax. ).

As in the multi-beam scanning optical system according to the second aspect, at least two lens surfaces change the curvature in the sub-scanning direction in the main scanning direction and bend in the sub-scanning direction to cause the main scanning in the sub-scanning direction. In the case of “adjusting the point position”, the wider the distance between these two lens surfaces, the larger the amount of change in the principal point position, and the wider the range in which the lateral magnification in the sub-scanning direction can be adjusted by bending. Therefore, in the invention according to claim 4, in the scanning optical system, an air space is provided between the two lens surfaces so that the space between the two lens surfaces can be increased. In the embodiment described later, the air gap is set between the second surface and the third surface counted from the optical deflector side, but the two surfaces are referred to as the “first surface and the third surface”.
A surface may be used, and an optical material and an air space may be interposed therebetween. Further, as in the multi-beam type optical scanning device according to claim 7, for each beam subjected to the coupling lens, from the line image forming optical system to the scanning optical system is shared by a plurality of beams, The line image forming optical system and the subsequent components can be configured in the same manner as the single-beam optical scanning device, and a multi-beam scanning device with extremely high stability against mechanical fluctuations can be realized. In the case of a multi-beam optical scanning device, an LD array type light source or a beam combining type light source can be used as a light source. L
In the case of using a D array type light source, the distance between the light emitting sources of the semiconductor laser array is set to 10 μm.
By setting m or more, it is possible to effectively reduce thermal and electrical influences between the light emitting sources and perform good multi-beam scanning.

[0017]

FIG. 8 shows only an essential part of an optical scanning device according to an embodiment of the present invention. The optical scanning device shown in FIG. 8 is of a single beam type. The beam emitted from the light source 1A, which is a semiconductor laser, is a divergent light beam, and is coupled by the coupling lens 2 to the subsequent optical system. The form of the coupled beam can be a weakly divergent light beam, a weakly convergent light beam, or a parallel light beam, depending on the optical characteristics of the optical system thereafter. When passing through the aperture of the aperture 3, the beam transmitted through the coupling lens 2 is blocked at the periphery of the light beam, "beam-shaped", and enters the cylindrical lens 4 which is a "line image forming optical system". The cylindrical lens 4
A direction having no power is directed to the main scanning direction, a positive power is provided in the sub-scanning direction, and an incident beam is focused in the sub-scanning direction. To collect light. The beam reflected by the deflecting / reflecting surface passes through the two lenses 6 and 7 forming the “scanning optical system” while being deflected at a constant angular velocity with the rotation of the polygon mirror 5 at a constant speed. Is bent, and the photoconductive photoreceptor 9 forming the substance of the “scanned surface”
The light is condensed on the upper surface as a light spot, and the surface to be scanned is optically scanned.
Note that the beam is incident on the mirror 10 prior to optical scanning, and is focused on the light receiving element 12 by the lens 11. The write start timing of the optical scanning is determined based on the output of the light receiving element 12.

The "scanning optical system" is an optical system for condensing a beam deflected by the optical deflector 5 as a light spot on the surface 9 to be scanned, and is constituted by two lenses 6 and 7. . The lens 6 on the optical deflector 5 side has a positive refractive power in the main scanning direction and a negative refractive power in the sub-scanning direction.
The lens 7 on the surface 9 to be scanned has a positive refractive power in the sub-scanning direction. The scanning optical system has a lateral magnification in the sub-scanning direction at the center image height:
β ₂ satisfies the condition “0.5 ≦ | β ₂ | ≦ 1.0” (claim 1). Also, in this embodiment, at least two of the four lens surfaces of the lenses 6 and 7 have a curvature in the sub-scanning direction that changes in the main scanning direction, and at least two of the at least two lens surfaces. One side is
Curvature change in the sub-scanning direction asymmetrically in the main scanning direction, the sub-scanning direction lateral magnification of the central image height: beta _2, the sub-scanning direction lateral magnification of any image height: Betah condition "0.93 ≦ | βh / β ₂ | ≦ 1.0
7 "(claim 2), the curvature change asymmetric in the main scanning direction has two or more extreme values, and at least one of the extreme values is such that the position in the main scanning direction: he is + Effective lens height from the optical axis on the image height side or -image height side: hma
The condition “| he / hmax | ≧ 0.5” is satisfied for x (claim 3). Then, at least two lens surfaces whose curvature in the sub-scanning direction changes in the main scanning direction have an air gap between the surfaces. That is, the single-beam optical scanning device according to the embodiment shown in FIG. 8 couples a beam from a light source to a subsequent optical system by a coupling lens 2 and converts the coupled beam into a linear image forming optical system. 4, a linear image is formed in the vicinity of the deflecting and reflecting surface of the optical deflector 5 as a long line image in the main scanning direction, the light is deflected at an equal angular velocity by the optical deflector 5, and the deflection beam An optical scanning device that condenses a light spot on the scanning surface 9 and optically scans the surface 9 to be scanned.
The present invention uses a scanning optical system according to claims 1, 2, 3, and 4 (claim 6). FIG. 9 shows an embodiment of a multi-beam optical scanning device according to the present invention. In order to avoid complication, the same reference numerals as those in FIG. The light source 1 is a semiconductor laser array and includes four light emission sources ch1 to ch4.
Are arranged at equal intervals in the sub-scanning direction. Light source c
The interval between h1 to ch4 is 10 μm or more. 4
Four beams emitted from the two light emission sources ch1 to ch4 are:
As shown in the figure, "elliptical far field pattern"
Is a divergent light beam whose main axis is directed in the main scanning direction, but is coupled to the subsequent optical system by a coupling lens 2 common to the four beams. The form of each coupled beam can be a weakly divergent light beam, a weakly convergent light beam, or a parallel light beam, depending on the optical characteristics of the optical system thereafter. The four beams transmitted through the coupling lens 2 are beam-shaped by the aperture 3, respectively, are converged in the sub-scanning direction by the operation of the cylindrical lens 4 which is a "common line image forming optical system", and the polygon mirror 5 In the vicinity of the deflecting / reflecting surface, they are separated from each other in the sub-scanning direction and formed as linear images long in the main scanning direction. The four beams deflected at a constant angular velocity by the deflecting reflection surface are used as two scanning lenses,
7, the optical path is bent by the bending mirror 8, and the light is focused on the photosensitive member 9 as four light spots separated in the sub-scanning direction, and four scanning lines on the surface to be scanned are simultaneously optically scanned. One of the beams is moved to mirror 10 prior to light scanning.
And is focused on the light receiving element 12 by the lens 11. Based on the output of the light receiving element 12, the write start timing of the four-beam optical scanning is determined.

The "scanning optical system" is an optical system that focuses four beams simultaneously deflected by the optical deflector 5 on the surface 9 to be scanned as four light spots. It consists of. These lenses 5 and 6 are shown in FIG.
The lateral magnification in the sub-scanning direction of the center image height: β ₂ is the same as that described under the condition “0.5 ≦ | β ₂ | ≦
1.0 "(claim 1). As in the embodiment of FIG. 8, at least two of the four lens surfaces of the lenses 6 and 7 have a curvature in the sub-scanning direction that changes in the main scanning direction. At least one of the surfaces has a curvature change in the sub-scanning direction that is asymmetric in the main scanning direction, and a lateral magnification of the center image height in the sub-scanning direction: β ₂ ,
The horizontal magnification in the sub-scanning direction at an arbitrary image height: βh satisfies the condition (2) (claim 2), and the asymmetrical curvature change has two or more extreme values, at least one of which has at least one extreme value. The position in the main scanning direction: he satisfies the condition (3) with respect to the effective lens height from the optical axis: hmax on the + image height side or the-image height side (claim 3). That is, the multi-beam optical scanning device according to the embodiment shown in FIG.
The beam from ch4 is coupled to the subsequent optical system by the common coupling lens 2, and the coupled plural beams are converted by the common line image forming optical system 4 to the vicinity of the deflecting and reflecting surface of the optical deflector 5. Are formed as a plurality of linear images which are long in the main scanning direction and are separated in the sub-scanning direction, are simultaneously deflected at an equal angular velocity by the optical deflector 5, and the deflection beams are scanned by the common scanning optical systems 6 and 7. A multi-beam scanning device that converges on a surface 9 as a plurality of light spots separated in the sub-scanning direction and simultaneously scans a plurality of scanning lines with the plurality of light spots. The multi-beam scanning optical system according to claim 5 is used (claim 7), and a plurality of light-emitting sources ch are used as light sources.
A monolithic semiconductor laser array 1 in which 1 to ch4 are arranged in the sub-scanning direction is used (claim 8), and the distance between the light emitting sources ch1 to ch4 of the semiconductor laser array 1 is 10 μm.
That is all (claim 9).

FIG. 10 shows an embodiment of a multi-beam optical scanning apparatus according to the present invention. This optical scanning device,
The light source is a beam combining type light source.
The light sources 1-1 and 1-2 are semiconductor laser arrays, each having a single light emitting source. The beams emitted from the light sources 1-1 and 1-2 are coupled to coupling lenses 2-1 and 2-2-1, respectively.
2 coupled. The form of each coupled beam can be a weakly divergent light beam, a weakly convergent light beam, or a parallel light beam, depending on the optical characteristics of the optical system thereafter. Coupling lens 2-1, 2-
2 are transmitted through apertures 3-1 and 3-2.
And is incident on the beam combining prism 20. The beam combining prism 20 has a reflection surface, a polarization separation film, and a half-wave plate. The beam from the light source 1-2 is reflected by the reflection surface of the beam combining prism 20 and the polarization splitting film and exits the beam combining prism 20. The beam from the light source 1-1 has a polarization plane of 90 by a half-wave plate.
The beam is rotated by an angle and transmitted through the polarization splitting film, and is emitted from the beam combining prism 20. Thus, two beams are combined. By adjusting the positional relationship between the light-emitting portions of the light sources 1-1 and 1-2 with respect to the optical axes of the coupling lenses 2-1 and 2-2, the two beams that have been combined have a small angle in the sub-scanning direction. . The two combined beams are separated by a cylindrical lens 4 which is a common line image forming optical system, near the deflecting and reflecting surface of the polygon mirror 5 as a line image long in the main scanning direction and separated from each other in the sub scanning direction. To form an image. The two beams deflected at an equal angular velocity by the polygon mirror 5 pass through the two lenses 6 and 7 forming the scanning optical system, and the optical path is bent by the bending mirror 8, and the light is transferred onto the photoconductive photosensitive member 9. The light is condensed as two light spots separated in the sub-scanning direction, and two scanning lines on the surface to be scanned are optically scanned simultaneously. Note that the synchronous detection of one of the beams is the same as in the embodiment of FIG.

The “scanning optical system” is an optical system that focuses the two beams, which are simultaneously deflected by the optical deflector 5, on the surface 9 to be scanned as two light spots. The lens 6 has a positive refractive power in the main scanning direction and a negative refractive power in the sub-scanning direction.
Has a positive refractive power in the sub-scanning direction. Lateral magnification of the center image height in the sub-scanning direction of the scanning optical system by the lenses 5 and 6: β ₂
Satisfies the condition (1) (claim 1). Also in this embodiment, at least two of the four lens surfaces of the lenses 6 and 7 have a curvature in the sub-scanning direction that changes in the main scanning direction, and at least one of the at least two lens surfaces. The surface has a curvature change in the sub-scanning direction that is asymmetric in the main scanning direction, and a lateral magnification in the sub-scanning direction of the center image height: β ₂ ,
The horizontal magnification in the sub-scanning direction at an arbitrary image height: βh satisfies the condition (2) (claim 2), and the asymmetrical curvature change has two or more extrema, at least one of which is the main extremum. The position in the scanning direction: he satisfies the condition (3) with respect to the effective lens height from the optical axis: hmax on the + image height side or the-image height side (claim 3). In other words, the multi-beam scanning apparatus shown in the embodiment in FIG. 10 couples beams from a plurality of light-emitting sources 1-1 and 1-2 to subsequent optical systems by coupling lenses 2-1 and 2-2. Then, the plurality of coupled beams are formed as a plurality of line images which are long in the main scanning direction and separated in the sub-scanning direction near the deflecting reflection surface of the optical deflector 5 by the common line image forming optical system 4. Then, the light is deflected simultaneously at the same angular velocity by the optical deflector 5, and the deflected beams are condensed on the surface 9 to be scanned as a plurality of light spots separated in the sub-scanning direction by the common scanning optical systems 6 and 7. A multi-beam scanning apparatus for simultaneously scanning a plurality of scanning lines with these plurality of light spots, wherein the multi-beam scanning optical system according to claim 5 is used as the common scanning optical systems 6 and 7 (claim). Item 7).

[0022]

EXAMPLE A specific example of a scanning optical system commonly used in the above-described three embodiments using the LD array type light source shown in FIG. 9 will be described. FIG. 1 shows a “layout in the main scanning direction” of the optical system of the embodiment. The optical path of the beam from the light source is bent by the mirror 41 between the cylindrical lens 4 and the polygon mirror 5. Further, the polygon mirror 5 blocks noise such as “wind noise” from the outside,
The soundproof housing is provided in the soundproof housing, and the soundproof glass 15 is provided in the soundproof housing. The four beams from the light source side enter the deflecting / reflecting surface of the polygon mirror 5 via the soundproof glass 15, and the four beams (deflected beams) reflected by the deflecting / reflecting surface enter the lens 6 via the soundproof glass 15. I do. These optical systems are housed in a casing, and “windows” for emitting four beams from the casing.
Is provided with a dustproof glass 16 for preventing dust from entering the casing. Therefore, the four beams are emitted through the dustproof glass 16 and optically scan the surface 9 to be scanned. The scanned surface 9 is, of course, actually the surface of the photoconductor.

Hereinafter, specific data will be described. The semiconductor laser array as the light source 1 has four light emission sources ch1 and ch
2, ch3 and ch4. The attitude of the semiconductor laser array is set so that these light emitting sources are arranged in the sub-scanning direction. The semiconductor laser array has a cover glass, and the distance from each light source to the incident side of the cover glass is 0.4 mm.
49 mm. The multi-beam scanning is “fifth interlaced scanning (four scanning lines are interposed between two adjacent light spots)” using four light spots. “Semiconductor laser array” Number of light emitting sources: 4 (ch1 to ch4) Light emitting source pitch (distance between adjacent light emitting sources): 30.4 μ
The m-coupling lens 2 has a flat incident side surface and a concentric aspherical exit side surface, and converts each incident beam into a “parallel light beam”. That is, “coupling lens” focal length: 14.85 mm Coupling action: collimating action “cylindrical lens” focal length in the sub-scanning direction: 70.18 mm “polygon mirror” number of deflecting and reflecting surfaces: 5 inscribed circle radius: 13 mm rotation Distance between the axis and the optical axis of the multi-beam scanning optical system: h =
5.22 mm Angle between the incident direction of the beam from the light source side (the incident direction in the state of projection onto a plane orthogonal to the sub-scanning direction) and the optical axis of the multi-beam scanning optical system: 60 degrees. I will. Explaining the symbols in the data notation, the radius of curvature is expressed as “Rm” in the main scanning direction,
In the sub-scanning direction, “Rs” is represented, and the refractive index is represented by “n”. Note that “Rm, Rs” in the following data is “paraxial radius of curvature” except for the arc shape.

“Data of optical system between light source 1 and polygon mirror 5” Surface number Rm Rs L n Surface shape Cover glass 1 ∞ 0.3 1.514 plane 2 平面 平面 11.5884 plane Coupling lens 3 レ ン ズ ∞ 3.8 1.51436 plane 4-7.638 -7.638 14.8643 Coaxial aspherical aperture 5 ∞ ∞ 99 plane Cylindrical lens 6 3 36.1 3 1.51436 Cylinder plane 7 ∞ ∞ 27.361 plane Bending mirror 8 ∞ 音 17 plane Soundproof glass 9 ∞ ∞ 2 1.51433 plane 10 ∞. 20.716 plane In this data, L is “surface interval”.

“Data of the optical system between the polygon mirror and the surface to be scanned” Surface number Rm Rs xy αβn Deflection / reflection surface 0 ∞ ∞ 25.44 1.588 Soundproof glass 1 ∞ 0 2.01 08 -2 1.51433 2 ∞ ∞ 25.42 0 8 -2 Lens 6 3 -312.60 -312.60 31.40 0 1.52716 4 -82.95 104.02 78.00 0 Lens 7 5 -500.00 -63.50 3.50 0 1.52716 6 -1000.0 -23.38 44.5 0 Dustproof glass 7 ∞ ∞ 3.0 -2 1.51433 8 ∞ ∞ 95.88 -2 In this data, x is the surface interval on the optical axis of the multi-beam scanning optical system, “y” is the main scanning direction, and the beam incident side to the polygon mirror is +. “Α” is the tilt angle in the main scanning direction, “β” is the tilt angle in the sub-scanning direction, and the units of α and β are “degrees”. The surface shape of the lens 6 is such that the incident side (surface number 3) is “coaxial aspherical surface”, and the exit side (surface number 4) is “non-circular in the main scanning direction and circular in the sub-scanning direction”. is there. The surface shape of the lens 7 is such that the incident side (surface number 5 above) is “non-circular in both the main and sub-scanning directions” and the exit side (surface number 6 above) is “toroidal in both main and sub-scanning directions. Plane ".

The “coaxial aspherical surface” is represented by a depth difference X in the optical axis direction from the lens surface at the optical axis position (H = 0) with respect to the lens height H from the optical axis. That is, the yen number constant:
K, paraxial radius of curvature: R (Rm = Rs), higher order coefficients: A ₄ , A ₆
Is represented by the following equation (4) using. X = (H ² / R) / [1 + √ {1- (1 + K) (H / R) ² } + A ₄ · H ⁴ + A ₆ · H ⁶ + A ₈ · H ⁸ ··· (4)

At the exit side surface (surface number 4) of the lens 6 and the entrance side surface (surface number 5) of the lens 7, the curvature in the sub-scanning direction (the curvature in the sub-scanning section) changes in the main scanning direction. . In addition, these surfaces have a “non-arc shape” in a plane cross section including the optical axis and parallel to the main scanning direction (hereinafter, referred to as “main scanning cross section”). The paraxial radius of curvature in the main scanning section on the optical axis: Rm, the distance from the optical axis in the main scanning direction is Y, the conical constant in the main scanning section: K, and the higher order coefficients are A ₁ , A ₂ , A ₃ , A ₄ , A ₅ , A ₆ ,..., The lens height from the main scanning section in the sub scanning direction is Z, the radius of curvature in the sub scanning section including the optical axis is Rs _0, and the coordinates in the main scanning direction are: The curvature in the sub-scanning cross section at Y is represented by Cs, and the surface shapes of the surface numbers 4 and 5 are represented by the following expression (5), where X is the depth in the optical axis direction. ^{X = (Y 2 / Rm)} / [1 + √ {1- (1 + K) (Y / Rm) 2} + + A 1 · Y + A 2 · Y 2 + A 3 · Y 3 + A 4 · Y 4 + A 5 · Y 5 + A ₆ · Y ⁶ + ·· + (Z ² · Cs) / [1 + {{1- (Z · Cs) ² } (5) where, the coordinates in the main scanning direction: the curvature in the sub-scanning cross section at the position of Y : Cs is represented by the following equation (6). Cs = (1 / Rs ₀ ) + B ₁ · Y + B ₂ · Y ² + B ₃ · Y ³ + B ₄ · Y ⁴ + B ₅ · Y ⁵ + ··· (6) where B ₁ , B ₃ , B ₅ ··· When a value other than zero is assigned to the sub-scanning direction, the curvature in the sub-scanning section becomes asymmetric in the main scanning direction. In this case, since this lens surface does not have a rotationally symmetric axis, the optical axis means “X axis passing through the coordinate origin representing the shape of the lens surface”. Table 1 shows coefficients for specifying the surface shapes of the surface numbers 3, 4, and 5 in the multi-beam scanning optical system.

[0028]

[Table 1]

The exit side surface of the lens 6 (the above surface number 4)
A surface in which the curvature in the sub-scanning direction changes symmetrically in the main scanning direction as shown in FIG. 2 (this is apparent from the fact that, as shown in Table 1, 0 is an odd number of coefficients Bi: 0). Is). The incident side surface (the surface number 5) of the lens 7 is a surface in which the change in curvature in the sub-scanning direction is asymmetric in the main scanning direction. As shown in FIG. 3, the change in curvature in the main scanning direction is "3". Two extremes. " By performing “bending in the sub-scanning direction” using the two lens surfaces of surface numbers 4 and 5, the lateral magnification in the sub-scanning direction is made uniform as shown in FIG.

In this embodiment, the conjugate length in the sub-scanning direction of the multi-beam scanning optical system (the distance from the deflecting reflection surface of the polygon mirror to the surface to be scanned) is 309.15 mm, and the length almost half of that is the back length. : 143.38.
When the multi-beam scanning optical system is composed of two lenses,
If the refractive power in the sub-scanning direction of each lens is “combination of positive and positive” as in the prior art, the “back length near half the conjugate length in the sub-scanning direction is secured” The side principal point cannot be set to “the surface to be scanned beyond the intermediate position of the conjugate length in the sub-scanning direction”, so that the multi-beam scanning optical system cannot be reduced in the sub-scanning direction and is close to 1 ×. It was limited to an enlarged type (| β ₂ |｜ 1.1 times). On the other hand, the scanning optical system of the present invention has a so-called retrofocus type by making the refractive power in the sub-scanning direction negative for the lens on the optical deflector side and positive for the lens on the scanning surface side. The rear principal point in the scanning direction can be set to “more to be scanned”, and the image formation in the sub-scanning direction can be reduced.

FIG. 6 shows an optical path diagram in the sub-scanning direction. The lateral magnification: | β ₂ | in the sub-scanning direction of the multi-beam scanning optical system after the optical deflector is 0.73 times.

The lateral magnification of the optical system closer to the light source than the optical deflector in the sub-scanning direction: | β ₁ | is the focal length of the coupling lens and the cylindrical lens: 14.85 mm, 7
Using 0.18 mm, 70.18 / 14.85 = 4.
73 times, and therefore the lateral magnification | β |
Is approximately 3.45, and if this is accurately determined by ray tracing, | β | = 3.48. Since the light emitting source pitch in the semiconductor laser array is 30.4 μm, the arrangement pitch of the light spots on the surface to be scanned in the sub-scanning direction is 105.
8 μm. At a scanning density of 1200 dpi, the interval between adjacent scanning lines is 21.17 μm, but the arrangement pitch of light spots in the sub-scanning direction in this embodiment is 105.8.
μm, which is five times the above-mentioned value of 21.17 μm, so that fifth-order interlaced scanning can be realized. FIG.
This shows the state of the scanning lines when “5th interlaced scanning” is performed with four beams at 1200 dpi. The “empty scanning line” in the upper part of the figure is a gap that is vacant at the start of writing in interlaced scanning.
As can be seen from the figure, the written “interval: 21.17
The “μm adjacent scanning line” has the same bending direction (projecting upward in the drawing) in the sub-scanning direction, and has a bending amount of 9 μm. Further, the pitch fluctuation is as good as 0.17 μm at the maximum. FIG. 7 shows light emission sources ch1 and c in the light source.
Curvature of field (solid line: sub-scanning direction, broken line: main scanning direction) and uniform velocity characteristics (solid line: linearity, broken line: fθ) for beams emitted from h2, ch3, and ch4, respectively.
Characteristics). (A), (b), (c), and (d) correspond to the beams from the light emitting sources ch1, ch2, ch3, and ch4, respectively. For each beam, the field curvature in the main and sub-scanning directions is suppressed to 0.2 mm or less, and the linearity is about 0.2%, and the uniform velocity characteristics are also good.

As described above, since | β ₂ | = 0.73, equation (1) is satisfied. The lens surface whose curvature in the sub-scanning direction “changes asymmetrically in the main scanning direction” (the incident side surface of the lens 7: the above-described surface number 5) has three extreme values of the curvature change. The two extreme values located at the position in the main scanning direction are as follows: He is the effective lens height from the optical axis on the + image height side or the -image height side: hmax: | (he) / (hmax) | = | ± 102 / ± 110 | = 0.92
7, which satisfies the expression (3). Furthermore, | βh / β ₂ |
Is in the range of 0.998 to 1.0077 and satisfies the expression (2).

In the above embodiment, the FNo. Is 31.7, and FNo. Of a multi-beam scanning optical system proposed in, for example, Japanese Patent Application Laid-Open No. 8-297256 is conventionally known. : Brighter than 52 to 73.5,
The beam spot diameter can be "narrowed down", and the density can be increased.

In the above embodiment, the two lenses 6 and 7 of the scanning optical system are made of a plastic material.
Of course, a glass material may be used, or a plastic material and a glass material may be combined.

[0036]

As described above, according to the present invention, a novel scanning optical system and optical scanning device can be realized. The scanning optical system of the present invention is a “retrofocus type” in which negative and positive refractive powers are sequentially distributed from the optical deflector side in the sub-scanning direction. Direction, so that the scanning optical system can be "reduced in the sub-scanning direction" while maintaining a long back length, regardless of the single beam method or multi-beam method. By setting the lateral magnification: | β ₂ | in the range of the expression (1), it is possible to effectively suppress an increase in the overall length of the lens to be scanned in the main scanning direction and to reduce the diameter of the light spot. The use of such a scanning optical system in a multi-beam optical scanning device stabilizes the scanning line pitch against mechanical fluctuations and realizes a good multi-beam optical scanning that can be adapted to high density. It is possible to do.

[Brief description of the drawings]

FIG. 1 is a diagram illustrating an optical system layout in a main scanning direction according to an embodiment.

FIG. 2 is a diagram illustrating a change in curvature in the sub-scanning direction of the exit-side lens surface of the lens 6 in the main scanning direction in the embodiment.

FIG. 3 is a diagram showing a change in curvature in the sub-scanning direction of the lens surface on the incident side of the lens 7 in the main scanning direction in the embodiment.

FIG. 4 is a diagram illustrating a change in horizontal magnification in the sub-scanning direction according to an image height in the embodiment.

FIG. 5 is a diagram showing scanning lines written in fifth-order interlaced scanning according to the embodiment.

FIG. 6 is an optical path diagram in a sub-scanning direction in the embodiment.

FIG. 7 is a diagram illustrating field curvature and constant velocity characteristics with respect to a beam from each light emitting source in the example.

FIG. 8 is a diagram for explaining an embodiment of the optical scanning device of the single beam type according to the present invention.

FIG. 9 is a diagram for explaining an embodiment of a multi-beam optical scanning device according to the present invention.

FIG. 10 is a diagram for explaining another embodiment of the multi-beam optical scanning device according to the present invention.

FIG. 11 is a diagram illustrating a scanning optical system that requires a long back length.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Light source 2 Coupling lens 3 Beam shaping aperture 4 Cylindrical lens as a line image imaging optical system 5 Polygon mirror as an optical deflector 6 Lens constituting a multi-beam scanning optical system 7 Configuring a multi-beam scanning optical system Lens 9 Scanned surface (substantially photoconductive photoconductor)

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) 2H045 BA02 BA22 BA23 BA32 CA55 CA65 2H087 KA19 LA22 PA02 PA17 PB02 QA03 QA07 QA12 QA19 QA21 QA22 QA32 QA37 QA41 QA42 RA05 RA07 RA13 RA45 5C072 AA03 HA02

Claims (9)

[Claims] An optical system for converging a beam deflected by an optical deflector as a light spot on a surface to be scanned, comprising two lenses, a lens on the optical deflector side in a main scanning direction. The lens on the surface to be scanned has a positive refractive power in the sub-scanning direction, and the lateral magnification in the sub-scanning direction at the center image height: beta ₂ is the condition: (1) 0.5 ≦ | β 2 | scanning optical system that satisfies the ≦ 1.0. 2. The scanning optical system according to claim 1, wherein at least two lens surfaces have a curvature in a sub-scanning direction that changes in a main scanning direction, and at least one of said at least two lens surfaces has a sub-scanning direction. The curvature change in the scanning direction is asymmetric in the main scanning direction, and the horizontal magnification in the sub-scanning direction at the center image height: β ₂ , and the horizontal magnification in the sub-scanning direction at an arbitrary image height: βh, Condition: (2) 0.93 ≦ | Βh / β ₂ | ≦ 1.07. 3. The scanning optical system according to claim 2, wherein the curvature change on the surface where the curvature in the sub-scanning direction changes asymmetrically in the main scanning direction has two or more extreme values, and at least one extreme value is , Its position in the main scanning direction: he
Satisfies the condition: (3) | he / hmax | ≧ 0.5 with respect to the effective lens height hmax from the optical axis on the + image height side or the −image height side. system. 4. The multi-beam scanning optical system according to claim 3, wherein the curvature in the sub-scanning direction changes in the main scanning direction.
A scanning optical system, wherein two lens surfaces have an air gap between the surfaces. 5. The scanning optical system according to claim 1, wherein the scanning optical system is used to collect a plurality of beams simultaneously deflected as a plurality of light spots on a surface to be scanned. Scanning optical system. 6. A beam from a light source is coupled to a subsequent optical system by a coupling lens, and the coupled beam is moved to a position near a deflecting reflection surface of an optical deflector by a line image forming optical system in a main scanning direction. A single beam that scans the surface to be scanned by converging a deflected beam as a light spot on the surface to be scanned by the scanning optical system, An optical scanning device of the system, wherein the scanning optical system according to any one of claims 1 to 4 is used as a scanning optical system. 7. A beam from a plurality of light sources is coupled to a subsequent optical system by a coupling lens, and the coupled plurality of beams are combined by a common line image forming optical system.
A plurality of linear images long in the main scanning direction and separated in the sub-scanning direction are formed in the vicinity of the position of the deflecting reflection surface of the optical deflector, and are simultaneously deflected at the same angular velocity by the above-mentioned optical deflector. In a multi-beam optical scanning device that converges on a surface to be scanned as a plurality of light spots separated in a sub-scanning direction by an optical system and simultaneously scans a plurality of scanning lines with the plurality of light spots, An optical scanning device of a multi-beam system, wherein the scanning optical system according to claim 5 is used as a scanning optical system. 8. A multi-beam optical scanning device according to claim 7, wherein a monolithic semiconductor laser array in which a plurality of light emitting sources are arranged in a sub-scanning direction is used as a light source. Optical scanning device. 9. The multi-beam optical scanning device according to claim 8, wherein the distance between the light emitting sources of the semiconductor laser array is 10 μm or more.