JP2002040340A - Laser beam scanner - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a laser beam scanner which can be miniaturized while maintaining accuracy of detection of a beam for synchronism detection in an optical detection element. SOLUTION: A free surface mirror 8 is used as an optical element which reflects and condenses the synchronism detection beam Ld emitted from a semiconductor laser 2 and deflected by a polygon mirror 5a on an SOS sensor 9. The free surface mirror 8 has a positive power at least in a main scanning direction and is formed so that the curvature of the cross-sectional form of the reflecting surface in the principal scanning direction gradually varies in the direction and does not have a symmetrical axis thus an aberration on the surface of the SOS sensor 9 is not generated.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a laser scanning device, and more particularly to an improvement in an optical system for obtaining a synchronization signal in a main scanning direction.

[0002]

2. Description of the Related Art In an electrophotographic digital copying machine or laser printer, a laser scanning device exposes and scans the surface of a photosensitive drum to form an electrostatic latent image, to which a toner image is supplied from a developing device. And visualize it.
Such a laser scanning device is configured to drive a laser diode based on image data, deflect the laser beam by a polygon mirror, and scan the photosensitive drum one scanning line at a time through a plurality of scanning lenses. Is done.

In this type of laser scanning apparatus, the timing of starting scanning in each scanning line is made uniform.
Before the start of scanning of each line, a laser diode is caused to emit light for a very short time at a deflection angle that does not impinge on the photosensitive drum (the laser beam emitted at this time is hereinafter referred to as a "synchronous detection beam"), and this is a photoelectric sensor ( Hereinafter, it is referred to as “SOS sensor.” “SOS” stands for “Start Of S”
abbreviation for "can". ), And after counting a predetermined number of clocks from the timing of the detection, the laser diode is optically modulated based on the image data for the corresponding one scanning line to render the laser diode, whereby the writing position in the main scanning direction is aligned and the jitter is reduced. Is prevented from occurring.

Therefore, the detection accuracy of the synchronization detection beam by the SOS sensor (more precisely, the detection accuracy of the timing at which the synchronization detection beam passes through the sensor surface of the SOS sensor) greatly affects image accuracy. . For this purpose, it is desirable that the synchronization detection beam be condensed on the sensor surface of the SOS sensor at least in the main scanning direction. Since the focal length is set to be large so that an image is formed on the photosensitive drum, the distance S
The distance to the OS sensor must be set large,
The size of the laser scanning device cannot be reduced.

In order to solve this problem, for example, Japanese Patent Application Laid-Open No.
In the laser scanning device disclosed in Japanese Patent No. 20457, a condensing lens having a short focal length is disposed immediately before the SOS sensor separately from the scanning lens, thereby shortening the distance to the SOS sensor. The size is reduced. However, according to this configuration, there is a problem that the detection accuracy of the passage timing of the synchronization detection beam is deteriorated. That is, when the focal length of the condenser lens 40 is long as shown in FIG.
When the inclination of the incident light to the light changes by θ1, the image point becomes P1
However, if the condenser lens 41 having a short focal length is used as shown in FIG. 9B, the distance to the image point is also shortened. On the other hand, the distance d2 (d2
Only <d1) moves. The shorter the focal length of the condenser lens, the smaller the amount of movement of the focal point on the sensor surface with respect to the amount of inclination of the synchronization detection beam due to the inclination of the reflection surface of the polygon mirror, and the longer the synchronous detection. Since the spot of the use beam stays on the sensor surface of the SOS sensor, the accuracy of timing detection deteriorates.
In addition, the F value on the sensor surface becomes smaller as the focal length becomes shorter, the range of the focal point becomes narrower, and the defocus when an error occurs in the position of each optical element becomes larger, so that the defocus on the sensor surface becomes larger. The spot shape collapses, which also deteriorates the detection accuracy.

FIG. 10 shows an example of an arrangement of optical elements of a laser scanning device which can be considered to solve the above-mentioned problem. As shown in the drawing, the synchronization detection beam Ld emitted from the laser diode 51 is reflected by the reflecting surface of the rotating polygon mirror 52, and is reflected by the scanning lens 53 and the cylindrical lens 54 having power in the main scanning direction. The light enters the mirror 55, where the light path is changed in a direction substantially parallel to the photosensitive drum 58, and then enters the sensor surface of the SOS sensor 57 disposed near the opposite end of the scanning lens 56. By doing so, the cylindrical lens 5
Since the distance from the SOS sensor 57 to the SOS sensor 57 can be increased, and the focal length of the cylindrical lens 54 can be increased to form an image of the laser beam on the sensor surface, the timing detection in the main scanning direction can be deteriorated or the spot due to defocus can occur. It is difficult for the shape to collapse.

[0007]

However, according to the configuration shown in FIG. 10, there is a problem that the size of the apparatus cannot be sufficiently reduced. FIG. 11A is an enlarged plan view of the periphery of the reflection mirror 55. As shown in FIG. 11A, a laser beam for scanning the photosensitive drum by the cylindrical lens 54 (hereinafter, referred to as a “scanning beam”). In order to prevent interference with Ls, the angle θ2 formed between the synchronization detection beam Ld and the scanning beam Ls at the start of scanning is increased, or the position of the reflection mirror 55 is set farther from the scanning lens 53, and synchronization detection is performed. The condensing lens 5 at a position where the gap between the scanning beam Ls and the scanning beam Ls is widened.
4 must be provided, and in any case, a space larger than a space required for arranging the scanning optical system is originally required, so that the apparatus cannot be sufficiently reduced in size.

Further, even when the cylindrical lens 54 is disposed downstream of the reflection mirror 55 as shown in FIG. 11B, the synchronous detection beam Ld is prevented so that the cylindrical lens 54 does not interfere with the scanning beam Ls. Therefore, it is necessary to increase the angle θ2 of the scanning beam Ls at the start of scanning or to make the position of the reflection mirror 55 farther from the scanning lens 53, so that the apparatus cannot be downsized as in the case of FIG.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems, and has as its object to provide a laser scanning apparatus capable of achieving downsizing of the apparatus while maintaining the detection accuracy of the synchronous detection beam. And

[0010]

In order to achieve the above object, a laser scanning device according to the present invention deflects a laser beam emitted from a light source in a main scanning direction by a deflecting device, and passes the laser beam through a scanning optical system. A laser scanning device having a configuration for scanning on a surface to be scanned, wherein a laser beam deflected to a predetermined position is guided to a photodetector via a reflection mirror to acquire a write synchronization signal in a main scanning direction, The mirror is characterized in that the mirror has power at least in the main scanning direction, and that the curvature of the cross-sectional shape of the reflecting surface in the main scanning direction gradually changes in the main scanning direction and does not have a symmetry axis.

Further, according to the present invention, at least one of the optical elements included in the scanning optical system is provided in the optical path of the laser beam deflected in a predetermined direction by the deflecting device until the laser beam reaches the reflection mirror. It is characterized in that a part is interposed. Further, according to the present invention, the reflection mirror is preferably
In the sub-scanning direction, it has a stronger positive power than the positive power in the main scanning direction.

Further, the present invention is characterized in that a detection position adjusting means for holding the photodetector movably in a direction along an image plane formed by the reflection mirror is provided.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, embodiments of the laser scanning device according to the present invention will be described with reference to the drawings. Figure 1
1 is a schematic perspective view showing an overall configuration of a laser scanning device 100 according to an embodiment of the present invention.

As shown in FIG.
Comprises a light source unit 1, a deflecting unit 5, a scanning optical system 6, an SOS sensor 9, and the like. The light source unit 1 includes a semiconductor laser 2, a collimator lens 3, and a cylindrical lens 4, and a laser beam LB emitted from the semiconductor laser 2.
Is converted into parallel light via the collimator lens 3 and then condensed only in the sub-scanning direction by the cylindrical lens 4 for so-called surface tilt correction.

The deflecting unit 5 is configured to rotate the polygon mirror 5a at a constant angular velocity in a direction indicated by an arrow by a polygon motor (not shown), and the laser reflected and deflected by the reflection surface on the side surface of the polygon mirror 5a. Beam LB is
The surface of the photosensitive drum 7, which is driven to rotate in the direction of the arrow in the drawing, is exposed and scanned through a scanning optical system 6 including an fθ lens 6a, a cylindrical lens 6b, and a folding mirror 6c.

A free-form mirror 8 is located near the front end of the cylindrical lens 6b and at a position not obstructing the optical path of the scanning beam Ls for scanning the surface of the photosensitive drum 7.
Is provided. Also, the cylindrical lens 6b
The SOS sensor 9 is disposed near the end opposite to the free-form surface mirror 8 and at a position that does not interfere with the scanning beam Ls.

The free-form surface mirror 8 outputs the synchronization detection beam Ld emitted from the semiconductor laser 2 before the start of scanning of each scanning line out of the laser beam LB reflected by the reflection surface of the polygon mirror 51, using the SOS sensor. 9 and the synchronization detection beam Ld is just S
It has such power as to condense light on the sensor surface of the OS sensor 9. In the present specification, “to converge” a laser beam at a specific position means that the laser beam converges and becomes a beam waist at the specific position. The “main scanning direction” indicates the direction in which the deflected laser beam moves with respect to each optical element, and the “sub-scanning direction” is orthogonal to a plane including the trajectory of the deflected laser beam. Means direction.

FIG. 2A schematically shows the cross-sectional shape of the reflecting surface of the free-form surface mirror 8 in the main scanning direction and the condensing state of the synchronization detection beam reflected by the reflecting surface in the main scanning direction. FIG. In FIG.
Indicates the sensor surface of the SOS sensor 9. As shown in the figure, the cross-sectional shape of the reflection surface 8a of the free-form surface mirror 8 in the main scanning direction is concave, and its curvature gradually changes along the main scanning direction. Has an asymmetric curve having no axis of symmetry, so that the synchronous detection beam Ld is
The light is condensed on the sensor surface of the sensor 9 without aberration.

That is, since the angle of the reflection surface of the polygon mirror 5a when reflecting the synchronization detection beam, the angle of the laser beam with respect to the direction of the principal ray, and the angle of the sensor surface of the SOS sensor 9 are different, FIG. If a normal concave mirror having an axis of symmetry is used as in (2), there will be a difference in the optical path length between the beams passing through the left and right positions separated in the light beam width direction from the position of the principal ray, causing coma aberration and the like. As a result, the spot shape on the sensor surface 9a collapses. If the spot shape collapses, the detection accuracy of the synchronization detection beam Ld deteriorates. Therefore, as shown in FIG. 2A, the cross-sectional shape of the reflecting surface in the main scanning direction is formed by a free-form surface having no symmetric axis. This is to prevent aberration from occurring on the sensor surface 9a.

The shape of the reflecting surface of the free-form surface mirror 8 is generally expressed by the following equation 1 in xyz three-dimensional coordinates. Here, z is the vertical direction in FIG. 1 = sub-scanning direction, and y is the main scanning direction of the scanning optical system 6 in FIG. The direction headed is the + direction. X is a direction orthogonal to both the z direction and the y direction.

[0021]

(Equation 1)

The following Table 1 shows each order i and y of y and z.
This shows the value of the coefficient aij set according to j.

[0023]

[Table 1]

The value of the coefficient aij when z is the zeroth order (j = 0) indicates the sectional shape of the free-form surface mirror 8 in the main scanning direction.
Further, the value when z is quadratic indicates the cross-sectional shape of the free-form surface mirror 8 in the sub-scanning direction. In Table 1, since the value of the coefficient aij is not 0 when the order is y3 and z0, it can be seen that the cross-sectional shape of the free-form surface mirror 8 in the main scanning direction is asymmetric. The coefficient value is designed so that coma does not occur on the sensor surface.
Further, since there is no coefficient value corresponding to the y0th order z3th order or the y0th order z1st order, and the coefficient in the y0th order z2nd order is not 0, the cross-sectional shape in the sub-scanning direction is formed so as to have a symmetric axis in the direction. However, since the first-order and second-order coefficients are not 0, the cross-sectional shape of the reflection surface in the sub-scanning direction is formed to be different depending on the value of y, that is, the position in the main scanning direction.

The optical path length from the converging point in the sub-scanning direction near the reflection surface of the polygon mirror 5a to the reflection surface of the free-form surface mirror 8 and the optical path length from the reflection surface of the free-form surface mirror 8 to the SOS sensor 9 Is different between the beams passing through the left and right positions separated from the position of the principal ray in the light beam width direction, so that the reflection surface in the sub-scanning direction suitable for focusing on the SOS sensor 9 in the sub-scanning direction The cross-sectional shape is
This is because it differs depending on the position in the main scanning direction. If the cross-sectional shape of the reflecting surface in the sub-scanning direction is made constant, the light condensing position in the sub-scanning direction differs at the position in the main scanning direction in the light beam, and a groove-like aberration or the like occurs to deteriorate the imaging state. .

Further, from the coefficients aij shown in Table 1, the curvature of the cross-sectional shape of the reflecting surface of the free-form surface mirror 8 in the main scanning direction becomes larger as going in the + direction of y, and the cross-sectional shape in the sub-scanning direction becomes larger. It can be seen that the curvature also increases as going in the + direction of y. The positive power of the free-form surface mirror 8 in the sub-scanning direction is set to be larger than the positive power in the main scanning direction. This is because the beam converged in the sub-scanning direction by the cylindrical lens 4 for the surface tilt correction is reflected by the reflecting surface of the polygon mirror 5a and then diffuses in the direction, and the beam is converged in the main scanning direction. This is because, after being converted into parallel light by the collimator lens 3, the light is already converged in the main scanning direction by the scanning lens 6a.

FIG. 3 is a graph showing the results of a comparative experiment to show the effect of the present embodiment. In the figure, the synchronous detection beam L focused on the SOS sensor 9 is shown.
4 shows an intensity distribution of the energy d (hereinafter, referred to as “beam intensity”) in the main scanning direction. The vertical axis represents the beam intensity when the peak value is 1, and the horizontal axis represents the main beam of the synchronization detection beam Ld. The position (unit: μm) of the sensor surface in the main scanning direction when the position of the light beam is set as the origin is shown.

This comparative experiment was performed on the following three types of reflecting mirrors. Although the reflecting mirror having the free-form surface in the present embodiment is asymmetric in the main scanning direction as described above, the expression 1
Where there is no y1-order z2 term, that is, a reflecting mirror in which the cross-sectional shape of the reflecting surface in the sub-scanning direction is symmetric and the shape is constant at any position in the main-scanning direction. The mirror mirror has a reflecting surface whose cross-sectional shape in the main scanning direction is symmetrical, although it differs depending on the position in the main scanning direction.
(No tertiary z0 term) The intensity distribution when the free-form surface mirror of (Experimental result) is used is a graph G1 shown by a black square in FIG. 3 and is almost symmetrical about the position of the principal ray. I have. This indicates that the spot shape of the synchronization detection beam is hardly distorted.

However, the cross-sectional shape in the sub-scanning direction differs depending on the position in the main scanning direction as described above, and a reflective mirror having a symmetrical cross-sectional shape in the main scanning direction is indicated by a white rhombus in the figure. As shown in the graph G2, the peak position is largely shifted leftward, and a small mountain (sub-peak) is formed at the position of -50, and the symmetry is significantly deteriorated.
This is due to the occurrence of coma and the like.

In the case where the cross-sectional shape of the reflecting surface is asymmetric in the main scanning direction and has the same symmetrical shape in the sub-scanning direction regardless of the position in the main scanning direction as described above,
The intensity distribution of the graph G3 indicated by a white triangle in the figure is obtained, and it can be seen that it is slightly shifted to the left as compared with the case of. This is considered to be caused by the occurrence of groove aberration. Therefore, it is most preferable that the shape of the reflecting surface 8a of the free-form surface mirror 8 be formed as in the present embodiment.
The synchronous detection beam can be focused on the sensor surface of the S sensor 9 without any spot collapse, and the detection accuracy can be improved. In addition, even in the case of the reflection mirror, the intensity distribution maintains sufficient symmetry as compared with that of the reflection mirror, and the reflection mirror can be used.

With the above configuration, the size of the apparatus can be reduced as compared with the related art. FIG. 4 is a plan view showing the vicinity of the free-form surface mirror 8 of the laser scanning device according to the present embodiment. As shown in FIG. 4, according to the present embodiment, the synchronization detection beam Ld is focused in the main scanning direction. This eliminates the need for a lens, and can eliminate the need for a cylindrical lens as compared with a conventional case where a cylindrical lens and a flat plate reflection mirror are used together (FIGS. 11A and 11B).
Thereby, the cylindrical lens and the scanning beam L
It is no longer necessary to consider interference with s, and the angle θ3 between the synchronization detection beam Ld and the scanning beam Ls at the start of scanning can be reduced, or the distance from the polygon mirror to the reflection mirror can be shortened. The device can be miniaturized. In addition, since the cross-sectional shape of the free-form surface mirror 8 in the main scanning direction is formed so as not to cause aberration as described above, the detection accuracy can be sufficiently ensured. In addition, since a cylindrical lens is unnecessary, the number of parts is reduced, and the cost of parts and the cost of assembly and adjustment can be reduced.

The sensor surface 9a of the SOS sensor 9 is
It is installed so as to be displaceable at a predetermined angle with respect to the direction orthogonal to the principal ray of the synchronization detection beam Ld.
FIG. 5A is a perspective view showing a shape of a sensor holding member 10 for installing the SOS sensor 9 on an apparatus frame (not shown). As shown in FIG.
0 has an L-shape having a vertical portion 10a and a horizontal portion 10b. The back surface of the SOS sensor 9 is
It is held by the holding member 10 by being attached to four pedestals 10c provided on the surface of the base member via an adhesive or the like.

FIG. 5B is a schematic view of a state where the SOS sensor 9 is installed in the apparatus via the sensor holding member 10 when viewed from directly above FIG. In FIG.
Indicates the position of the image plane of the synchronization detection beam Ld reflected and condensed by the free-form surface mirror 8, and in the case of the present embodiment, the polygon mirror 5a reflects the synchronization detection beam Ld when reflected. Due to the relationship with the inclination of the reflection surface, it is inclined by about 57 ° with respect to the principal ray of the synchronization detection beam Ld.

As shown in the figure, three long holes 10d, 10e and 10f are formed on the bottom surface of the sensor holding member 10 so that the longitudinal direction thereof forms an angle of 45 ° with the sensor surface 9a of the SOS sensor 9. The longitudinal direction of the long hole is substantially parallel to the image surface S formed by the free-form surface mirror 8 and at a position where the image surface S and the sensor surface 9a of the SOS sensor 9 intersect. The sensor holding member 10 is attached.

[0035] Pins 10g and 10h, which are erected at predetermined positions of an apparatus main body frame (not shown), are fitted with elongated holes 10 of the holding member 10.
d and 10f are engaged with each other and guided, and are fixed at a position where the synchronization detection beam Ld is condensed almost at the center of the sensor surface 9a via the screw 10i. As a result, the position of the sensor surface 9a of the SOS sensor 9 is changed to the image plane S
, And even if the installation angle of the free-form surface mirror 8 in the main scanning direction slightly deviates from the normal direction, the position of the SOS sensor 9 is displaced by the adjustment mechanism, The synchronous detection beam Ld can be always incident on the sensor surface 9a in a condensed state, and the adjustment at the time of assembly can be made extremely easy.

The sensor surface 9a of the SOS sensor 9 is
The inclination of the elongated holes 10d to 10f with respect to the vertical portion 10a is set to 45 ° so that the normal line is inclined in the direction opposite to the cylindrical lens 6b, whereby the reflected light of the synchronization detection beam Ld on the sensor surface 9a is reduced. The light does not directly enter the photosensitive drum 7 and does not adversely affect exposure scanning.

(Modification) Although the laser scanning device according to the present invention has been described based on the embodiment, it goes without saying that the content of the present invention is not limited to the above-described embodiment. Such a modification can also be implemented. (1) First Modification In the above-described embodiment, the synchronization detection beam Ld is incident on the free-form surface mirror 8 after passing through the fθ lens 6a. However, as shown in FIG. L
d may enter the free-form surface mirror 8 after passing through both the fθ lens 6a and the cylindrical lens 6b. In this case, since the synchronization detection beam Ld has already been converged in the sub-scanning direction by passing through the cylindrical lens 6b, the power of the free-form surface mirror 8 in the sub-scanning direction is smaller than that in FIG. Good. The following Table 2 shows Equation 1 showing the shape of the reflecting surface of the free-form surface mirror 8.
Shows the value of the coefficient aij applied to.

[0038]

[Table 2]

From the values of the coefficients aij shown in Table 2, the curvature of the cross-sectional shape in the main scanning direction of the reflecting surface of the free-form surface mirror 8 increases in the negative y-direction, and the curvature of the cross-sectional shape in the sub-scanning direction. It can be seen that becomes larger as going in the + direction of y. When the free-form mirror 8 is provided behind the cylindrical lens 6b as shown in FIG. 6, the optical path length between the free-form mirror 8 and the SOS sensor 9 can be made longer than in the embodiment of FIG. The scanning speed of the synchronization detection beam Ld on the sensor surface 9a of the SOS sensor 9 is increased, and further improvement in detection accuracy can be expected.

(2) Second Modification In the above embodiment, the beam Ld for synchronization detection is focused also in the sub-scanning direction only by the free-form surface mirror 8, but as shown in FIG. A cylindrical lens 11 having a power in the sub-scanning direction may be provided immediately before 9, and this may be combined with the free-form surface mirror 8 to focus the synchronization detection beam Ld on the sensor surface of the SOS sensor 9. . Also in this case, the free-form surface mirror 8
The power in the sub-scanning direction is set to be weaker than in the embodiment of FIG. In the present modification, the synchronization detection beam Ld is designed to be substantially parallel light in the sub-scanning direction due to reflection on the free-form surface mirror 8. Table 3 below shows an example of the value of the coefficient aij for specifying the shape of the reflecting surface of the free-form surface mirror 8 in this case.

[0041]

[Table 3]

From the values of the coefficients aij shown in Table 3, the curvature of the cross-sectional shape of the reflecting surface of the free-form surface mirror 8 in the main scanning direction increases in the + y direction, and the curvature of the cross-sectional shape in the sub-scanning direction. Is larger in the negative y direction. By converging the synchronization detection beam Ld in the sub-scanning direction in the vicinity of the SOS sensor 9 as in the present modification, even if a slight error occurs in the installation angle of the free-form surface mirror 8 in the sub-scanning direction. Since the synchronous detection beam Ld is refracted by the cylindrical lens 11 so as to approach the optical axis, there is an advantage that the beam can be incident on the sensor surface of the SOS sensor 9. Further, since the cylindrical lens 11 has no power in the main scanning direction, even if it is arranged near the SOS sensor 9, it does not affect the scanning speed of the SOS sensor 9 on the sensor surface.

Although the present modification requires a little space for the arrangement of the cylindrical lens 11 in comparison with the embodiment of FIG. In order not to be affected by the error of the installation angle in the scanning direction, S
It is desirable to dispose a cylindrical lens having power in the sub-scanning direction immediately before the OS sensor. On the other hand, as compared with the present modification, the apparatus can be omitted as much as the cylindrical lens in the main scanning direction can be omitted using the free-form surface mirror 8 as in the present modification. Can be downsized.

(3) Third Modification In the embodiment of FIG. 1 or the first and second modifications, the optical element of the scanning optical system 6 is not used until the synchronization detection beam Ld reaches the free-form surface mirror 8. However, since the free-form surface mirror 8 can freely set the power in the main scanning direction and the sub-scanning direction by design, the free-form mirror 8 scans with the synchronization detection beam Ld as shown in FIG. Free-form mirror 8 without any optical element of optical system 6
It is also possible to make the incident light. In Table 4 below,
An example of the value of a coefficient aij in Expression 1 showing the surface shape of the free-form surface mirror 8 applied to the present modification is shown.

[0045]

[Table 4]

From the coefficients aij values shown in Table 4, the curvature of the cross-sectional shape in the main scanning direction of the reflecting surface of the free-form surface mirror 8 increases in the + y direction of y as in the case of the second modification. , The curvature of the cross-sectional shape in the sub-scanning direction is y
It can be seen that it becomes larger as going in the minus direction. With such a configuration, not only the degree of freedom in designing the apparatus is increased, but also the length of the scanning optical system 6 in the main scanning direction is reduced by the scanning beam L.
Since only the deflection width of s needs to be set, the cost can be reduced.

[0047]

As described above, according to the present invention, a laser beam deflected in a predetermined direction by a deflecting device is guided to a photodetector via a reflection mirror to acquire a write synchronization signal in the main scanning direction. In the laser scanning device, the reflection mirror has power at least in the main scanning direction, and the curvature of the cross-sectional shape of the reflection surface in the main scanning direction gradually changes in the main scanning direction to change the axis of symmetry. It is formed so as not to have. By changing the optical path of the laser beam deflected in a predetermined direction by a reflection mirror, a certain length of optical path length can be secured to the photodetector using the space of the original device, and the beam spot on the detection surface of the photodetector can be secured. In addition to increasing the moving speed of the laser beam and providing the reflecting mirror with power at least in the main scanning direction, a lens for condensing light in that direction becomes unnecessary, and the detection accuracy of the laser beam for synchronization detection is maintained at a certain level or more. While securing, the number of parts can be reduced and the size of the device can be reduced. In addition, since the curvature of the cross-sectional shape of the reflection surface of the reflection mirror in the main scanning direction is gradually changed in the main scanning direction so as not to have a symmetric axis, coma aberration occurs on the detection element surface. Can be prevented, and the detection accuracy of the laser beam for synchronization detection can be further improved.

Also, part of at least one of the optical elements included in the scanning optical system is interposed in the optical path until the laser beam deflected in a predetermined direction by the deflection device reaches the reflection mirror. By doing so, the deflection angle between the laser beam for synchronization detection and the laser beam for scanning can be made smaller, and the reflection mirror can be installed more inside, contributing to the miniaturization of the device and the detection. The power of the reflecting mirror can be reduced by the condensing power of the optical element of the scanning optical system through which the scanning laser beam has passed.

Further, the reflecting mirror has a positive power in the sub-scanning direction that is higher than the positive power in the main scanning direction, so that the laser beam is previously adjusted to correct surface tilt in the deflecting device. Even when the light is focused in the scanning direction, the laser beam for detection can be focused in the sub-scanning direction on the detection surface of the light detection element without providing a separate cylindrical lens. The change in the light intensity of the laser beam when scanning the surface becomes steeper, and the detection accuracy can be improved.

Further, the present invention further comprises detection position adjusting means for holding the photodetector movably in a direction along the image plane formed by the reflection mirror. If the installation angle of the mirror is set to a direction in which an error is likely to occur, even if the installation error occurs, the detection laser beam is incident on the detection surface of the photodetector while maintaining the focused state. Can be easily adjusted.

[Brief description of the drawings]

FIG. 1 is a schematic perspective view showing an overall configuration of a laser scanning device according to an embodiment of the present invention.

FIG. 2 is a schematic diagram showing a beam condensing state in a main scanning direction when a free-form surface mirror is used as a reflection mirror of a synchronization detection beam and when a concave mirror is used.

FIG. 3 is a graph showing a result of a comparative experiment for showing an effect in the embodiment of the present invention.

FIG. 4 is a plan view showing the vicinity of a free-form surface mirror of the laser scanning device according to the embodiment of the present invention.

FIG. 5 is a diagram showing a shape and an attached state of a sensor holding member for holding an SOS sensor in an apparatus frame in the laser scanning device.

FIG. 6 is a schematic perspective view showing an overall configuration of a laser scanning device according to a first modification of the present invention.

FIG. 7 is a schematic perspective view showing the overall configuration of a laser scanning device according to a second modification of the present invention.

FIG. 8 is a schematic perspective view showing an overall configuration of a laser scanning device according to a third modification of the present invention.

FIG. 9 is an explanatory diagram illustrating a difference in a moving speed of a synchronization detection beam in the SOS sensor due to a difference in focal length of the condenser lens.

FIG. 10 is a schematic perspective view showing an example of an arrangement of optical elements of a laser scanning device which can be considered to solve the problem regarding the focal length of the condenser lens.

FIG. 11 is a plan view showing an arrangement in which a plane mirror and a cylindrical lens are used in combination for condensing a synchronous detection beam on an SOS sensor.

[Explanation of symbols]

 2 Semiconductor laser 5a Polygon mirror 6a fθ lens 6b Cylindrical lens 8 Free-form surface mirror 9 SOS sensor 10 Sensor holding member 11 Cylindrical lens

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 2C362 AA48 BA87 BA90 BB29 BB30 BB31 DA03 DA06 2H045 AA01 BA02 CA63 CA89 DA02 2H087 KA19 RA06 RA13 TA01 TA03 TA06 5C072 AA03 BA01 DA02 DA04 DA21 HA02 HA09 HA13 XA01 XA05

Claims (4)

[Claims] A deflecting device for deflecting a laser beam emitted from a light source in a main scanning direction and scanning the laser beam on a surface to be scanned via a scanning optical system; What is claimed is: 1. A laser scanning device which guides a light detection element via a reflection mirror to obtain a write synchronization signal in a main scanning direction, wherein the reflection mirror has at least a positive power in the main scanning direction and has a main surface having a main surface having a positive power. A laser scanning device, wherein a curvature of a cross-sectional shape in a scanning direction gradually changes in the main scanning direction and has no axis of symmetry. 2. A part of at least one of the optical elements included in the scanning optical system is located in the middle of the optical path of a laser beam deflected in a predetermined direction by the deflecting device to reach the reflection mirror. The laser scanning device according to claim 1, wherein: 3. The laser scanning device according to claim 1, wherein the reflection mirror has a higher positive power in the sub-scanning direction than in the main scanning direction. 4. The apparatus according to claim 1, further comprising a detection position adjusting unit that holds the photodetector movably in a direction along an image plane formed by the reflection mirror. The laser scanning device according to claim 1.