JP3432178B2 - Optical scanning optical system and image forming apparatus using the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical scanning optical system and an image forming apparatus using the optical scanning optical system, and in particular, a light beam emitted from a light source means is incident on a deflection surface from approximately the center of the deflection angle of an optical deflector (front incidence). Laser beam printer having an electrophotographic process, such as an electrophotographic process, in which image information is recorded by optically scanning the surface to be scanned through a scanning lens system having an fθ characteristic. It is suitable for an image forming apparatus such as an LBP) or a digital copying machine.

[0002]

2. Description of the Related Art Conventionally, in an optical scanning optical system used in an image forming apparatus such as a laser beam printer or a digital copying machine, a light beam which is optically modulated from a light source means in accordance with an image signal and is emitted, for example, a rotating polygon mirror (polygon). mirror)
Is periodically deflected by an optical deflector composed of the following, and is focused in a spot shape on the surface of a photosensitive recording medium (photosensitive drum) by an imaging optical system (scanning lens system) having the fθ characteristic, and the surface is optically scanned. Then, the image is recorded.

In recent years, in order to make the optical scanning optical system compact, an optical scanning optical system of a type in which a light beam emitted from a light source means is made incident on the deflecting surface from approximately the center of the deflection angle of the optical deflector (front incidence). Various proposals have been made.

FIG. 4 is a schematic view of a main part of an optical scanning optical system proposed in, for example, Japanese Patent Laid-Open No. 8-122676. In the figure, a light flux (laser light) emitted from the light source 41 in the main scanning direction substantially parallel to the bottom surface of the casing is converted into a substantially parallel light flux by the collimator lens 42, and a cylindrical lens 43 having power only in the sub-scanning direction and a plane surface. The light is incident on an optical deflector (polygon mirror) 45 via a mirror 44. The optical deflector 45 causes the first separation angle θ1 in the sub-scanning direction.
The light beam reflected and deflected by the curved mirror 46 is reflected back to the optical deflector 45 side in the sub-scanning direction at the second separation angle θ2, and mainly passes through the anamorphic lens 47 having a power in the sub-scanning direction. The light flux transmitted through the anamorphic lens 47 is reflected by the optical path refraction mirror 48, and forms a spot on the photosensitive drum surface 49 for scanning in the main scanning direction which is the generatrix direction of the photosensitive drum. In the sub-scanning direction, the light flux is once imaged by the cylindrical lens 43 in the vicinity of the deflection surface of the optical deflector 45,
An image is re-formed on the photosensitive drum surface 49 mainly by the power of the anamorphic lens 47.

In the figure, since a curved mirror 46 is used instead of the scanning lens, the light beam emitted from the light source 41 is emitted between the curved mirror 46 and the optical deflector 45 in the main scanning direction, The plane mirror 44 is bent at 90 °.

On the other hand, an optical scanning optical system using a scanning lens has been proposed in, for example, Japanese Patent Laid-Open No. 9-96773. FIG. 5 is a schematic view of a main part of the optical scanning optical system proposed in the publication. The figure shows a general example of the configuration of a front incidence (axial incidence) optical system using a scanning lens.

In the figure, the light beam emitted from the light source 51 is converted into a substantially parallel light beam by the collimator lens 52, and the light beam is limited by the aperture stop 53 and is incident on the cylindrical lens 54. Here, of the substantially parallel light beams incident on the cylindrical lens 54, the light beams converge in the sub-scanning cross section, pass through the second and first scanning lenses 57 and 56 via the folding mirror 55, and are deflected by an optical deflector (polygon). The light enters the deflecting surface 58a of the mirror 58 and is almost a line image near the deflecting surface 58a (a line image long in the main scanning direction).
Is imaged as. At this time, the light beam incident on the deflecting surface 58a is in a sub-scanning cross section including the rotation axis of the optical deflector 58 and the optical axis of the scanning lens system, and is a plane perpendicular to the rotation axis of the optical deflector 58 (of the optical deflector). The light enters from an oblique direction with respect to the rotation plane.

On the other hand, in the main scanning cross section, the light flux remains as it is, passes through the second and first scanning lenses 57 and 56 via the folding mirror 55, and is deflected from the approximate center of the deflection angle of the optical deflector 58. It is incident on the surface 58a (front incidence).

The light beam deflected by the deflecting surface 58a of the optical deflector 58 is guided to the photosensitive drum surface 61 via the first and second scanning lenses 56 and 57, and the folding mirror 59 and the cylindrical mirror 60. The optical deflector 58
Is rotated in the direction of arrow A to optically scan the photosensitive drum surface 61 in the direction of arrow B (main scanning direction). As a result, an image is recorded on the photosensitive drum surface 61 as a recording medium.

At this time, before the optical scanning on the photosensitive drum surface 61, the BD luminous flux (writing) which is a part of the luminous flux deflected by the optical deflector for adjusting the timing of the scanning start position on the photosensitive drum surface 61. A position detection light beam 62 is reflected by a BD mirror (synchronization detection mirror) 63 and guided to a BD sensor (photodetector) 64. Then, the BD signal (synchronization signal) obtained by detecting the output signal from the BD sensor 64 is used to adjust the timing of the scanning start position of image recording on the photosensitive drum surface 61.

Since the photodetector 64 in the figure can be arranged on the side of the light source 61, the wiring of the photodetector and the light source and the substrate circuit can be easily put together, and it is not necessary to draw the wiring unnecessarily, so that it is excellent in noise and response characteristics. There are advantages such as.

[0012]

In the conventional optical scanning optical system (Japanese Unexamined Patent Publication No. 8-122676), since the light beam emitted from the light source 41 is directed in the main scanning direction, the plane mirror 44 is used.
It is bent at 0 ° and is incident on the optical deflector 45. The angle of separation in the sub-scanning direction is formed by inclining the reflecting surface of the light deflector 45, so that the light source 41 to the light deflector 4 is inclined.
The light flux going to 5 must be parallel to the bottom of the casing. When the light beam traveling from the light source 41 to the plane mirror 44 or the light beam traveling from the plane mirror 44 to the light deflector 45 makes an angle with the bottom surface, a long focal line formed in the vicinity of the light deflector 45 in the main scanning direction with respect to the bottom surface. There arises a problem that the tilt, the curved mirror 46 and the cylindrical mirror 48 significantly deteriorate the imaging performance. Since it is inevitable that an angular error will occur due to assembly tolerances and component tolerances, it is desirable to suppress the sensitivity of the focal line tilt as low as possible, and the angle at which the plane mirror 44 bends should be as sharp as possible.

On the other hand, in the configuration using the conventional scanning lens (Japanese Patent Laid-Open No. 9-96773), the light source 51 and the light beam 62 for detecting the writing start position are located on the opposite sides of the optical axis of the scanning lens system 71. This can be read from FIG. 5, but there is no description of the positional relationship regarding the shape of the scanning lens or the components of the incident system, particularly the positional relationship between the light source 51, the folding mirror 55 and the optical deflector 58, and the folding angle of the folding mirror 55. Nothing is disclosed about the structure to make it compact and its effect.

An object of the present invention is to provide an optical scanning optical system and an image forming apparatus using the optical scanning optical system, which can make the entire apparatus compact by appropriately setting each element constituting the optical scanning optical system. To do.

[0015]

According to another aspect of the invention, there is provided an optical scanning optical system, wherein a light beam emitted from a light source unit including a light source means is passed through an incident optical system including a folding mirror so that the deflection angle of an optical deflector is substantially the same. Incident from the center on the deflection surface of the optical deflector,
In the optical scanning optical system for forming an image of the light beam deflected by the optical deflector on the surface to be scanned by an imaging optical system including a scanning lens system and optically scanning the surface to be scanned, the optical deflector A part of the deflected light beam is passed through the scanning lens system as a light beam for detecting the writing start position, guided to the writing start position detecting means, and the output from the writing start position detecting means is used to scan the surface to be scanned. There is control means for controlling the timing of the start position, and the position where the light beam for writing position detection passes through the scanning lens system and the position of the light source means are mutually relative to the optical axis of the imaging optical system. It is characterized in that at least a part of optical elements arranged on the opposite side and constituting the scanning lens system are formed in a shape asymmetric with respect to the optical axis of the imaging optical system in the main scanning direction.

The invention of claim 2 is characterized in that in the invention of claim 1, the distance from the light source means to the folding mirror is shorter than the distance from the folding mirror to the deflecting surface of the optical deflector.

According to a third aspect of the present invention, in the first aspect of the invention, an angle at which the light flux emitted from the light source unit is returned to the optical deflector side by the folding mirror is less than 45 °.

According to a fourth aspect of the present invention, in the first aspect of the present invention, a line segment from the reflection point of the folding mirror to the farthest end of the light source unit and the light deflection from the reflection point of the folding mirror. The distance when the line segment to the farthest end of the container is projected on the sub-scan section is L in order.
When L1 and L2 are satisfied, the condition of L1 <L2 is satisfied.

According to a fifth aspect of the present invention, in the first aspect of the invention, the light beam for detecting the writing start position is 1/2 the scanning angle of view.
It is characterized by passing a larger range.

According to a sixth aspect of the invention, in the first aspect of the invention, the scanning lens system also constitutes a part of the incident optical system.

According to a seventh aspect of the invention, in the first aspect of the invention, the light beam emitted from the incident optical system is incident on the deflection surface of the optical deflector in an oblique direction in the sub-scan section.

An image forming apparatus according to an eighth aspect of the present invention is characterized in that an image is formed using the optical scanning optical system according to any one of the first to seventh aspects.

[0023]

[0024]

[0025]

[0026]

[0027]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] FIG. 1 is a cross-sectional view of a main part in a main scanning direction when the scanning optical apparatus according to Embodiment 1 of the present invention is applied to an image forming apparatus such as a laser beam printer or a digital copying machine. FIG. 2 is a main-scan sectional view, and FIG. 2 is a main-portion cross-sectional view in the sub-scanning direction of FIG.

In FIGS. 1 and 2, 1 is a light source means,
For example, it consists of a semiconductor laser. Reference numeral 2 denotes a converging lens, which converts the divergent light flux emitted from the light source means 1 into a substantially divergent light flux. The light source means 1 and the converging lens 2 are integrated and housed in the same light source unit 3.

An aperture stop 4 shapes the light beam emitted from the converging lens 2 into a desired optimum beam shape.
Reference numeral 5 denotes an incident system cylindrical lens having a predetermined refracting power only in the sub-scanning direction, and a light beam passing through the aperture stop 4 is deflected by a deflecting surface (reflecting surface) of an optical deflector 12 described later in the sub-scanning cross section. ) 11 is formed as a substantially line image. Reference numeral 6 denotes a folding mirror, and in this embodiment, an angle α at which the luminous flux emitted from the light source means 1 is folded back to the optical deflector 12 side by the folding mirror 6 is set to 40 °. It should be noted that the folding back angle α may be less than 45 °.

The light beam reflected by the folding mirror 6 is transmitted through the second and first scanning lenses 8 and 7, is converted into a substantially parallel light beam in the main scanning section, and a plurality of light deflectors are arranged from substantially the center of the deflection angle. Irradiate the deflection surface (reflection surface).

Each element of the light source means 1, the converging lens 2, the aperture stop 4, the cylindrical lens 5, the folding mirror 6, and the first and second scanning lenses 7 and 8 constitutes one element of the incident optical system 21. is doing.

Reference numeral 12 denotes an optical deflector, which is composed of, for example, a polygon mirror (rotary polygon mirror), and is rotated at a constant speed in the direction of arrow A in the figure by a driving means (not shown) such as a motor.

Reference numeral 22 denotes an image forming optical system having a light condensing function and fθ characteristics, which has a predetermined power in the main scanning direction.
The optical deflector 12 has a scanning lens system (fθ lens system) 23 having second scanning lenses 7 and 8 and a cylindrical lens (long lens) 9 having a predetermined power only in the sub-scanning direction. An image of the deflected light beam is formed on the surface to be scanned 10, and the deflection surface 11 of the optical deflector 12 and the surface to be scanned 10 are made substantially conjugate with each other in the sub-scanning cross section. Correcting the fall. Further, in the present embodiment, the shape of the second scanning lens 8 is formed so as to be asymmetric with respect to the optical axis M of the imaging optical system 22 in the main scanning direction.

Reference numeral 10 is a photosensitive drum surface as a surface to be scanned.

Reference numeral 13 denotes a synchronization detection mirror (hereinafter also referred to as "BD mirror"), which is a light-starting position detection light beam (hereinafter "BD light flux") for adjusting the timing of the scanning start position on the photosensitive drum surface 10. 14) is reflected to the synchronization detecting means side.

Reference numeral 15 denotes an optical sensor (hereinafter also referred to as "BD sensor") as a synchronization detecting means, and in the present embodiment, a synchronization signal (hereinafter referred to as "BD" obtained by detecting an output signal from the BD sensor 15). (Also referred to as “signal”), the control unit 16 adjusts the timing of the scanning start position of image recording on the photosensitive drum surface 10.

In the present embodiment, the position where the BD light beam 14 passes through the scanning lens system 23 and the position of the light source means 1 are arranged on the opposite sides with respect to the optical axis M of the imaging optical system 22. ing.

In this embodiment, the light flux emitted from the semiconductor laser 1 is converted into a substantially divergent light flux by the converging lens 2, and the light flux is limited by the aperture stop 4 and is incident on the cylindrical lens 5. Here, of the substantially divergent light fluxes that have entered the cylindrical lens 5, the light fluxes converge in the sub-scanning cross section, and the second and first light fluxes pass through the folding mirror 6.
Of the deflecting surface 1 of the optical deflector 12 after passing through the scanning lenses 8 and 7.
It is incident on the beam No. 1 and is formed as a substantially line image (a line image elongated in the main scanning direction) in the vicinity of the deflection surface 11. Deflection surface 1 at this time
In the sub-scan section including the rotation axis of the optical deflector 12 and the optical axis M of the imaging optical system 22, the light beam incident on the plane 1 is a plane perpendicular to the rotation axis of the optical deflector 12 (rotation plane of the optical deflector). ) With a small angle of incidence of β / 2.
That is, the light beam emitted from the incident optical system 21 is obliquely incident on the deflecting surface 11 of the optical deflector 12 within the sub-scanning section by an angle β /
It is incident with 2.

Then, the light beam deflected by the deflecting surface 11 of the optical deflector 12 is deflected at the same angle β / 2 as when it was incident,
The light is guided onto the photosensitive drum surface 10 through the first and second scanning lenses 7 and 8 and the cylindrical lens 9, and the optical deflector 12 is rotated in the direction of arrow A, so that the photosensitive drum surface 10 is rotated. Is optically scanned in the direction of arrow B (main scanning direction). As a result, an image is recorded on the photosensitive drum surface 10 as a recording medium.

At this time, before the optical scanning on the photosensitive drum surface 10, the BD luminous flux 14 which is a part of the luminous flux deflected by the optical deflector 12 in order to adjust the timing of the scanning start position on the photosensitive drum surface 10. Passes through the first and second scanning lenses 7 and 8, is reflected by the BD mirror 13, and is guided to the BD sensor 15. Then, using the BD signal obtained by detecting the output signal from the BD sensor 15, the control unit 16 adjusts the timing of the scanning start position of image recording on the photosensitive drum surface 10.

Since the optical scanning optical system of the present embodiment is front incidence (axial human radiation) as described above, the scanning range on the photosensitive drum surface 10 is symmetrical about the optical axis M of the imaging optical system 22. Become. In the present embodiment, the first and second scanning lenses 7, before the light beam emitted from the semiconductor laser 1 is incident on the optical deflector 12 and twice after being deflected by the optical deflector 12,
It has a double-pass structure that passes eight.

In the present embodiment, as described above, the position where the BD light beam 14 passes through the scanning lens system 23 and the position of the semiconductor laser 1 are located on the opposite sides of the optical axis M of the imaging optical system 22. The BD light flux 14 is set so as to pass through a range larger than 1/2 of the scanning field angle.

Since the second scanning lens 8 allows the BD light flux 14 to pass through, it requires an effective width a larger than the effective width b required for scanning as shown in FIG. For this reason, even if the light beam of the incident optical system 21 passes through the vicinity of the second scanning lens 8 and does not interfere with it, and the folding angle α by the folding mirror 6 is made as small as possible, as described above.
The scanning lens 8 is formed so as to be asymmetric with respect to the optical axis M of the imaging optical system 22 in the main scanning direction. That is, the length a from the optical axis M to the end of the second scanning lens 8 on the side where the BD light flux 14 passes is the end of the second scanning lens 8 on the side where the light flux at the end of the writing scan passes. The shape is asymmetric with respect to the optical axis M so as to be longer than the length b up to.

Further, in this embodiment, the folding mirror 6 is arranged at a position closer to the semiconductor laser 1 than the intermediate position of the optical path from the semiconductor laser 1 to the deflection surface 11 of the optical deflector 12. That is, the distance d2 from the semiconductor laser 1 to the folding mirror 6 is shorter than the distance d1 from the folding mirror 6 to the deflection surface 11 of the optical deflector 12.

By arranging the folding mirror 6 in this way, the semiconductor laser 1 can be arranged in the main scanning direction without unnecessarily increasing the area of the optical box, thereby making the entire device a compand. be able to.

Further, in this embodiment, the angle α at which the light beam emitted from the semiconductor laser 1 is folded back by the folding mirror 6 is set to 40 ° as described above. This angle α can be further reduced by arranging the folding mirror 6 further than the semiconductor laser 1 or by making the shape of the second scanning lens 8 asymmetrical, which contributes to downsizing.

Also, to reduce the folding angle α, the image forming performance on the surface of the photosensitive drum 10 caused by the angle error of the oblique incident angle β / 2 in the sub-scan section and the ray angle error caused by the tilting of the folding mirror 6 is caused. Sensitivity to deterioration can be reduced. This makes it possible to stabilize the optical performance.

As described above, in the present embodiment, the position where the BD light beam 14 passes through the scanning lens system 23 as described above,
The position of the semiconductor laser 1 and the position of the semiconductor laser 1 are arranged on opposite sides to the optical axis M of the imaging optical system 22, and
By forming the shape of the scanning lens 8 so as to be asymmetric with respect to the optical axis M, the light flux passing through the incident optical system 21 does not interfere with the second scanning lens 8 and the light flux is reflected by the folding mirror 6. It is also possible to reduce the angle α at which the lens is folded back, thereby reducing the size of the entire lens system and making the entire device compact.

In this embodiment, the shape of the second scanning lens 8 is formed to be asymmetric with respect to the optical axis M.
If necessary, the shape of the first scanning lens 7 may also be formed so as to be asymmetric with respect to the optical axis M.

[Second Embodiment] FIG. 3 shows a second embodiment of the present invention.
FIG. 8 is a cross-sectional view (main scanning cross-sectional view) of main parts in the main scanning direction when the scanning optical device of 1 is applied to an image forming apparatus such as a laser beam printer or a digital copying machine. In the figure, the same elements as those shown in FIG. 1 are designated by the same reference numerals.

The present embodiment differs from the first embodiment in that the line segment from the reflection point 6a of the folding mirror 6 to the farthest end 3a of the light source unit 3 and the reflection point 6a of the folding mirror 6 are different. When the line segments from the light source to the end 12a at the farthest position of the optical deflector 12 are projected on the sub-scan section in the order of L1 and L2, respectively, L1 <L2. . Other configurations and optical actions are substantially the same as those of the first embodiment, and similar effects are obtained.

That is, in this embodiment, the line segment from the reflection point 6a of the folding mirror 6 to the end 3a at the farthest position of the light source unit 3 and the reflection point 6 of the folding mirror 6 are included.
The distance when the line segment from a to the end portion 12a at the farthest position of the optical deflector 12 is projected on the sub-scan section is L in order.
When L1 and L2 are satisfied, the condition L1 <L2 is satisfied.

Further, in the present embodiment, the semiconductor laser 1 and the converging lens 2 are configured as the light source unit 3 which is adjusted to a predetermined value and integrated as in the case of the first embodiment, and the interchangeability of the light source as a service part is improved. Making it easy.

As described above, in this embodiment, since the light source unit 3 is arranged closer to the folding mirror 6 than the outer shape of the light deflector 12 as described above, the size of the optical box is not limited, so that the main scanning cross section is obtained. The size of the optical box in the direction orthogonal to the main scanning direction is not increased, and the arrangement of the optical system including the mechanical structure can be made compact in size.

The constituent features of the first embodiment and the constituent features of the second embodiment described above may be combined.

[Other Embodiments] As another embodiment of the present invention, the light beam emitted from the light source means is deflected from approximately the center of the deflection angle of the optical deflector through an incident optical system including a folding mirror. Scanning optics for making a light beam deflected by the optical deflector incident on a deflecting surface of the optical device and forming an image on a surface to be scanned by an imaging optical system including a scanning lens system, and optically scanning the surface to be scanned. In the system, by using at least one means out of a plurality of means shown below, the overall size of the apparatus is made compact.

The distance d2 from the light source means to the folding mirror is shorter than the distance d1 from the folding mirror to the deflection surface of the optical deflector.

The angle α at which the light flux emitted from the light source means is folded back by the folding mirror is set to less than 45 °.

Sub-scanning is performed for a line segment from the reflection point of the folding mirror to the farthest end of the light source unit and a line segment from the reflection point of the folding mirror to the farthest end of the optical deflector. The distance when projected onto the cross section is L1,
When L2 is set, L1 <L2.

[0060]

As described above, according to the present invention, in the optical scanning optical system and the image forming apparatus using the same, the position where the light beam for writing position detection passes through the scanning lens system and the position of the light source means are connected. The scanning lens system is arranged on opposite sides of the optical axis of the image optical system, and at least a part of the optical elements constituting the scanning lens system is asymmetric with respect to the optical axis of the imaging optical system in the main scanning direction. By doing so, it is possible to achieve an optical scanning optical system and an image forming apparatus using the optical scanning optical system, which is capable of downsizing the entire lens system and downsizing the entire apparatus.

Further, according to the present invention, as described above, the distance from the light source means to the folding mirror is shorter than the distance from the folding mirror to the deflection surface of the optical deflector, and the light is emitted from the light source means. Set the angle at which the light beam is reflected by the folding mirror to less than 45 °. The line segment from the reflection point of the folding mirror to the end of the light source unit farthest from the reflection point of the reflection mirror to the farthest point of the optical deflector. When the distances when projecting the line segment to the end at the position on the sub-scanning cross section are set to L1 and L2, respectively, L1 <L2 is configured, whereby the overall size of the apparatus can be made compact. It is possible to achieve an optical scanning optical system and an image forming apparatus using the optical scanning optical system.

[Brief description of drawings]

FIG. 1 is a main-scan sectional view of a first embodiment of the present invention.

FIG. 2 is a sub-scan sectional view of the first embodiment of the present invention.

FIG. 3 is a main-scan sectional view of a second embodiment of the present invention.

FIG. 4 is a schematic view of a main part of a conventional scanning optical device.

FIG. 5 is a schematic view of a main part of a conventional scanning optical device.

[Explanation of symbols]

1 Light source means (semiconductor laser) 2 Converging lens 3 light source unit 4 aperture stop 5 Cylindrical lens 6 folding mirror 7 First scanning lens 8 Second scanning lens 9 Cylindrical lens 10 Scanned surface (photosensitive drum surface) 11 Deflection surface 12 Optical deflector (rotating polygon mirror) 13 Mirror for synchronization detection 14 Luminous flux for writing position detection 15 Synchronization detection means 16 Control means 21 Incident optical system 22 Imaging optical system 23 Scanning lens system

Claims (8)

(57) [Claims] 1. A light beam emitted from a light source unit including a light source means is made incident on a deflection surface of the light deflector from approximately the center of a deflection angle of the light deflector through an incident optical system including a folding mirror, and the light deflection is performed. In the optical scanning optical system that forms an image on the surface to be scanned by the imaging optical system including the scanning lens system and optically scans the surface to be scanned, the light beam deflected by the optical deflector is deflected by the optical deflector. A part of the luminous flux is passed through the scanning lens system as a luminous flux for detecting the writing start position, guided to the writing start position detecting means, and the output from the writing start position detecting means is used to determine the scanning start position on the surface to be scanned. There is control means for controlling the timing, and the position where the light beam for detecting the writing start position passes through the scanning lens system and the position of the light source means are on opposite sides to the optical axis of the imaging optical system. Are arranged to configure the scanning lens system. Ku and a part of the optical element in the main scanning direction,
An optical scanning optical system, which is formed in a shape asymmetric with respect to the optical axis of the imaging optical system. 2. The optical scanning optical system according to claim 1, wherein the distance from the light source means to the folding mirror is shorter than the distance from the folding mirror to the deflection surface of the optical deflector. 3. The angle at which the light flux emitted from the light source unit is returned to the optical deflector side by the folding mirror is 4
The optical scanning optical system according to claim 1, wherein the optical scanning optical system is less than 5 °. 4. A line segment from the reflection point of the folding mirror to the farthest end of the light source unit, and from the reflection point of the folding mirror to the farthest end of the optical deflector. The optical scanning optical system according to claim 1, wherein a condition of L1 <L2 is satisfied, where L1 and L2 are distances when line segments are projected on the sub-scanning cross section, respectively. 5. The optical scanning optical system according to claim 1, wherein the light beam for detecting the writing position passes through a range larger than ½ of a scanning field angle. 6. The optical scanning optical system according to claim 1, wherein the scanning lens system also constitutes a part of the incident optical system. 7. The optical scanning optical system according to claim 1, wherein the light beam emitted from the incident optical system is incident on the deflection surface of the optical deflector in an oblique direction in the sub-scan section. 8. An image forming apparatus, wherein an image is formed by using the optical scanning optical system according to any one of claims 1 to 7.