KR100854805B1 - Optical scanning system and image forming apparatus using the same - Google Patents

Abstract

Disclosed is a compact optical scanning device and an image forming apparatus capable of outputting a high quality image, wherein the optical scanning device is deflected by a deflection device for scanning deflection of the light beam from the light source in the main scanning direction, and a deflection surface of the deflecting device. An imaging optical system for imaging the light beam on the scan surface; The deflection surface reciprocates to reciprocally scan the scan surface in the main scanning direction of the luminous flux deflected by the deflection surface; The first direction means the direction of one phase difference of wavefront aberration in the main scanning direction between the main beam and the last light beam of the beam reflected by the deflection plane at the effective deflection angle of the deflection plane corresponding to the maximum scanning position of the effective scan area. In this case, the phase difference is generated as a result of reflection of the light beam by the deflection plane, while the second direction is a wavefront in the main scanning direction between the last light beam and the main beam of the light beam reflected by the deflection plane at the effective deflection angle of the deflection plane. Means the direction of the phase difference on the other side of the aberration, wherein the phase difference occurs as a result of the luminous flux passing through the imaging optical system; At least one optical system inside the imaging optical system is provided with at least one non-circular arc-shaped optical surface in the main scanning cross section so as to ensure that the first direction and the second direction are opposite to each other.

Description

Optical scanning device and image forming device using the same {OPTICAL SCANNING SYSTEM AND IMAGE FORMING APPARATUS USING THE SAME}

1 is a cross-sectional view along the main scanning cross section for explaining the first embodiment of the present invention

2 is a schematic view for explaining a state of the light beam reflected by the deflection surface according to the first embodiment of the present invention;

Fig. 3 is a schematic diagram showing the shape of the light spot at the scanning end according to the first embodiment of the present invention.

Fig. 4 is a schematic diagram showing the arrangement of the lenses of the imaging optical system in the sub-scanning direction according to the first embodiment of the present invention.

5 is a schematic diagram showing details of the optical deflector according to the first embodiment of the present invention;

6A is a cross-sectional view of a movable plate of the optical deflector according to the first embodiment of the present invention.

6B is a schematic view for explaining a deformation of the movable plate of the optical deflector according to the first embodiment of the present invention.

Fig. 7 is a schematic diagram showing an approximation model manufactured in consideration of the deformation of the movable plate in Example 1 of the present invention.

Fig. 8 is a graph showing the results of calculating the deformation of the movable plate of Example 1 of the present invention by the finite element method.

FIG. 9 is a graph for explaining the deformation of the movable plate when the inclination at the origin in FIG. 8 is taken as 0; FIG.

FIG. 10 is a schematic diagram showing a profile of a spot at each scanning position on a scan surface in Example 1 of the present invention. FIG.

11 is a schematic diagram showing a profile of a spot at each scanning position on a scan surface in a comparative example;

12 is a schematic diagram for explaining the shape of a wavefront (equal phase surface) in the main scanning direction of the light beam after being reflected by the deflected and distorted deflection surface;

Fig. 13 is a schematic diagram for explaining the shape of a wavefront (isophase) formed after the parallel light beam (plane wave) passes through the f-θ lens system.

It is a schematic diagram which shows the profile of the spot in each dice on the scanning surface in Example 1 of this invention.

15 is a graph for explaining the wave front aberration generated by the deformation of the deflection plane in Example 1 of the present invention.

16 is a graph for explaining wavefront aberration generated by the f-θ lens according to the first embodiment of the present invention.

Fig. 17 is a graph for explaining the wave front aberration that can be provided by correcting the wave front aberration generated in the deformation of the deflection plane in Embodiment 1 of the present invention.

18 is a cross-sectional view along the main scanning cross section for explaining the second embodiment of the present invention.

Fig. 19 is a schematic diagram showing details of the optical deflector according to the second embodiment of the present invention.

20 is a schematic view for explaining the principle of the optical deflector according to the second embodiment of the present invention.

Fig. 21 is a schematic diagram showing a model for explaining a resonator type optical deflector having two movable plates.

Fig. 22 is a graph for explaining the vibration angle (deflection angle) of the movable plate of the optical deflector according to the second embodiment of the present invention.

Fig. 23 is a graph for explaining the angular velocity of the movable plate of the optical deflector according to the second embodiment of the present invention.

24 is a graph for explaining the angular velocity of the movable plate in the comparative example when there is only mode 1;

Fig. 25 shows the ideal image height when scanning is performed by using the ideal f-θ lens in Example 2 of the present invention, and the actual image height when scanning is performed using the same f-θ lens. A graph

FIG. 26 is a graph showing the difference (f-θ error) between two curves in FIG.

Fig. 27 is a graph showing the f-θ error of the f-θ lens according to the second embodiment of the present invention.

28 is a graph for explaining the angular acceleration of the movable plate of the optical deflector according to the second embodiment of the present invention.

29 is a graph for explaining the angular acceleration of the movable plate in the comparative example when there is only mode 1;

30 is a graph showing the results of calculating the deformation of the movable plate of Example 2 of the present invention by the finite element method;

Fig. 31 is a schematic diagram showing a profile of a spot at each scanning position on a scan surface in Example 2 of the present invention.

32 is a schematic diagram showing a profile of a spot at each scanning position on a scan surface in Example 2 of the present invention.

Fig. 33 is a graph showing the spot diameter in the main scanning direction on the photosensitive drum surface in Example 2 of the present invention.

34 is a schematic diagram for explaining a state of a scanning line on a photosensitive drum in Example 2 of the present invention.

35 is a cross-sectional view along the main scanning cross section for explaining the third embodiment of the present invention.

Fig. 36 is a graph showing the results of calculating the deformation of the movable plate of the third embodiment of the present invention by the finite element method;

Fig. 37 is a graph showing the results of calculating the deformation of the movable plate of the third embodiment of the present invention by the finite element method;

Fig. 38 is a schematic perspective view three-dimensionally illustrating the amount of deformation of the movable plate in the third embodiment of the present invention.

Fig. 39 is a schematic diagram showing a profile of a spot at each scanning position on the scan surface in the third embodiment of the present invention.

Fig. 40 is a schematic diagram showing a profile of a spot at each scanning position on the scan surface in Example 3 of the present invention.

Fig. 41 is a schematic diagram for explaining the magnitude relationship of the wavefront with respect to the sub-scan direction formed after the parallel light beam passes through the f-θ lens system.

42 is a schematic cross-sectional view along a sub-scan section of the image forming apparatus according to an embodiment of the present invention.

Figure 43 is a schematic cross-sectional view along the sub-scan section of the color image forming apparatus according to an embodiment of the present invention

Fig. 44 is a schematic diagram for explaining wavefront aberration generated by the f-θ lens system according to the present invention.

<Description of the symbols for the main parts of the drawings>

1: light source means 2: aperture stop

3: condensing optical system (collimator lens)

4: lens system 5: mirror

6, 166: optical deflector 6a, 166a: deflection surface

7, 167, 187: imaging optical system (f-θ lens system) 8: surface to be scanned (photosensitive drum surface)

11, 12, 13, 14: Optical scanning device 21, 22, 23, 24, 101: Photosensitive drum

26, 173, 174: torsion springs 31, 32, 33, 34: developer

41, 42, 43, 44, 103: luminous flux 51: conveyance belt

52, 117: external device

53, 111: printer controller

60: color image forming apparatus 67, 171, 172: movable plate

100: optical scanning unit 102: charging roller

104: image forming apparatus 107: developing apparatus

109: paper cassette 112: transfer paper

113: fixing roller (fixing device)

114: pressure roller 115: motor

116: discharge roller 175, 176: support

The present invention relates to an optical scanning apparatus and an image forming apparatus using the same. In particular, the present invention relates to an optical scanning device that can be suitably used, for example, in a laser beam printer (LBP), a digital copier or a multifunction printer with an electrophotographic process.

As for the optical scanning device having a reciprocating optical deflector as an optical deflector (deflection means) for reflecting light beams, for example, Japanese Patent Application Laid-Open No. 2004-191416 (Patent Document 1) and Japanese Patent Application Laid-Open No. It has already been proposed in 2005-173082 (patent document 2).

In the said patent document 1, scanning of the said light beam is made by making multiple reflections of a light beam (light beam) between a sine-vibration mirror (deflection surface) and two fixed mirrors arrange | positioned facing this vibration mirror. To increase the angle.

In addition, in Patent Document 1, when the scanning angle of the light beam is increased, the multi-reflection is performed by using a combination of the small vibration mirror and the two fixed mirrors, which were originally composed of only one vibration mirror. It is very complicated. Therefore, it is not preferable about the viewpoint of miniaturization.

Moreover, in the said patent document 1, since the light beam is multireflected, it is necessary to enlarge the magnitude | size of a vibration mirror (deflection surface) in a scanning direction, and it is unpreferable for high speed scanning. In addition, deformation of the vibration mirror surface due to angular acceleration or air resistance during sine vibration cannot be avoided.

In order to cope with this, in Patent Document 1, as the vibration angle of the vibration mirror is increased, the focus error caused by the deformation of the vibration mirror surface is corrected by micro-vibration of the coupling lens in synchronization with the vibration period.

In the configuration in which the scanning angle of the light beam is increased by the multiple reflection method, the light beam passes through the end portion of the vibration mirror as the vibration angle of the vibration mirror increases. This means that the influence of the deformation amount of the vibration mirror increases as the deflection angle (vibration angle) increases.

Therefore, as the deflection angle (vibration angle) increases, the focus error amount also increases. This is the reason that the structure of patent document 1 requires quite complicated control which micro-vibrates a coupling lens in synchronization with a vibration period.

In addition, in the reciprocating optical deflector, the dynamic deformation of the deflection surface in the main scanning direction which comes after the reciprocating motion cannot be avoided.

When the deflection plane of the optical deflector is deformed in the main scanning direction, the luminous flux reflected by the deflection plane is affected by wavefront aberration twice the amount of deformation of the deflection plane. This seriously degrades the imaging performance.

On the other hand, in Patent Literature 2, as an attempt to reduce the deformation in the main scanning direction of the deflection mirror surface, slots are formed on the rear surface of the deflection mirror, and the area and placement density of these slots are changed depending on the position in the main scanning direction. have.

In addition, the Y-shaped support beam for axially supporting the deflection mirror is used at two different locations with respect to the main scanning direction of the deflection surface to reduce the deformation in the main scanning direction of the deflection mirror surface.

On the other hand, some optical scanning devices having a reciprocating optical deflector take advantage of only one deflection surface, and do not use the planar tilt correction optical system in their imaging optical system.

However, such an apparatus involves a problem that the image forming performance deteriorates when the deformation in the main scanning direction of the deflection mirror is changed by the position in the sub scanning direction of the deflection mirror.

Accordingly, it is an object of the present invention to provide a high-resolution image output and a compact optical scanning device and an image forming apparatus having such optical scanning device.

According to one aspect of the invention, in order to achieve the above object, the light source means; Deflection means configured to scan deflect the light beam from the light source means in the main scanning direction; And an imaging optical system configured to image light beams deflected by the deflection surface of the deflection means on the scan surface; The deflection surface is configured to reciprocally scan the scan surface in the main scanning direction at a light beam deflected by the deflection surface of the deflection means by performing a reciprocating movement; The first direction indicates the direction of one phase difference of wavefront aberration in the main scanning direction between the main light beam and the last light beam of the beam reflected by the deflection surface at the effective deflection angle of the deflection surface corresponding to the maximum scanning position of the effective scanning area. Wherein the phase difference is generated as a result of reflection of the light beam by the deflection surface; The second direction represents the direction of the phase difference on the other side of the wavefront aberration in the main scanning direction between the last light beam and the main light beam of the light beam reflected by the deflecting surface at the effective scanning angle of the deflection surface, wherein the phase difference is that the light flux is the image formation. Occurs as a result of passing through an optical system; In order to ensure that the first direction and the second direction are opposite to each other, at least one optical system inside the imaging optical system is provided with at least one optical surface having a non-arc shape in the main scanning cross section. Chi is provided.

In summary, according to the present invention, an optical scanning device and an image forming apparatus having such optical scanning device can be achieved that can significantly reduce deterioration of the focusing spot on the scanned surface even when using a reciprocating optical deflector.

These and other objects, features and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments taken in conjunction with the accompanying drawings.

EMBODIMENT OF THE INVENTION Hereinafter, preferred embodiment of this invention is described with reference to an accompanying drawing.

[ Example  One]

Fig. 1 illustrates a cross section (main scanning cross section) of a main part in the main scanning direction in Embodiment 1 of the present invention.

In the present specification, the term "scanning direction" is a direction perpendicular to the deflection axis of the optical deflector and the optical axis of the imaging optical system, that is, the direction in which the light beam is scanned by the optical deflector. The term "sub-scan direction" is a direction parallel to the deflection axis of the optical deflector.

The term "main scanning cross section" is a plane including the main scanning direction and the optical axis of the imaging optical system. The term "sub scanning cross section" is a cross section parallel to the optical axis of the imaging optical system and perpendicular to the main scanning cross section.

In Fig. 1, reference numeral 1 denotes a light source means made of, for example, a semiconductor laser. (2) is an aperture stop which determines the beam diameter by limiting the width of the light beam passing therethrough.

Denoted at 3 is a condensing optical system (collimator lens) having a function of converting the divergent light beam from the light source means 1 into a parallel light beam. Reference numeral 4 denotes a lens system (cylindrical lens) having a predetermined power (refractive power) only in the sub-scan section (sub-scan direction).

The lens system 4 forms an image as a substantially linear image on the deflection surface 6a of the optical deflector (deflection means), which will be described later, with respect to the sub-scan section of the light beam converted by the collimator lens 3 into parallel light flux. do.

Reference numeral 5 denotes a mirror which deflects the light beam passing through the cylindrical lens 4 with respect to the main scanning direction and guides the optical beam to the optical deflector 6.

Here, the collimator lens 3 and the cylindrical lens 4 are components of the input (light incidence) optical system LA. The collimator lens 3 and the cylindrical lens 4 may be replaced by an integral structure of a single optical element (anamorphic lens).

The optical deflector (deflecting means) 6 is composed of a resonance type optical deflector configured such that the deflection surface 6a performs reciprocating sine motion based on resonance. In this embodiment, the deflection surface 6a of the optical deflector 6 reciprocates. Through this reciprocating motion, the scan surface 8 is reciprocally scanned in the main scanning direction by the luminous flux provided by the incident optical system LA. have.

The reciprocating motion of the deflection surface 6a of the optical deflector 6 is performed based on the resonance drive and according to the sinusoidal vibration.

Reference numeral 7 denotes an imaging optical system (f-θ lens system) including first and second imaging lenses (f-θ lenses) 71 and 72. This serves to image the luminous flux generated on the basis of the photosensitive drum (scanning surface) 8 while being deflected by the optical deflector 6 and generated based on the image information.

The first and second f-theta lenses 71 and 72 constituting the imaging optical system 7 of this embodiment are formed when the deflection surface 6a of the optical deflector 6 is deformed in the main scanning cross section during the reciprocating motion. In addition, the wave front aberration of the light beam in the main scanning cross section generated in accordance with the deformation amount thereof is reduced.

(8) is a photosensitive drum surface to be scanned.

In the present embodiment, the light beam width and the cross-sectional shape are adjusted by the aperture stop 2 to the divergent light beam which is modulated and emitted from the semiconductor laser 1 in accordance with the image information, and the collimator lens 3 is used as the parallel light beam. Is converted.

Then, the light beam is projected (front incidence) from the center of the oscillation angle (deflection angle) of the optical deflector 6 with respect to the main scanning cross section through the cylindrical lens 4 and the mirror 5.

On the other hand, with respect to the sub-scan section, the light beam is incident (diagonal incidence) on the deflection surface 6a with a finite angle with respect to the sub-scan direction.

Then, by the reciprocating motion of the deflecting surface 6a of the optical deflector 6, the light beam deflected and reflected in the main scanning direction is guided onto the photosensitive drum surface 8 through the f-? Lens system 7. Therefore, by reciprocating the deflection surface 6a of the optical deflector 6, the photosensitive drum surface 8 is scanned in the main scanning direction at the speed of light. By this process, image recording on the photosensitive drum (recording medium) is performed.

The optical deflector 6 in this embodiment consists of a resonant optical deflector configured such that the deflection surface 6a performs reciprocating sine vibration based on resonance.

In general, in the optical deflector configured to perform sinusoidal vibration, when the area of the deflection surface is enlarged, it becomes difficult to achieve high speed vibration. For this reason, when embedding such a deflector in a laser beam printer or a digital copier, for example, it is necessary to make the dimension of the deflection surface as small as possible.

In this embodiment, in this respect, the luminous flux is projected as a frontal incident on the deflection surface 6a of the optical deflector, that is, the luminous flux is deflected from the right upper part (f-theta lens system 7) in FIG. Is projected towards the front of the. In other words, in the main scanning cross section, the light beam is projected on the front face of the deflection surface 6a of the optical deflector 6 in the optical axis direction of the imaging optical system 7.

By the above-mentioned frontal incidence, the size (width in the main scanning direction) of the deflection surface 6a of the optical deflector 6 can be made smallest, so that vibration at high speed is facilitated.

On the other hand, when the light incidence method is used, the light beam incident on the deflection surface 6a of the optical deflector and the light beam deflected and reflected by the deflection surface 6a may interfere with each other. To avoid this, the light beam projected on the deflection surface 6a is incident at a finite angle of incidence in the sub-scanning direction with respect to the plane normal to the deflection surface 6a (that is, an oblique incident optical system is formed).

Specifically, in this embodiment, the incidence angle of 2 degrees is made in the sub scanning direction with respect to the plane normal of the deflection surface 6a, and the deflection surface 6a is viewed from the lower side (downward from the ground in Fig. 1) in the sub scanning direction. The light beam is incident on.

As a result, the light velocity deflected and reflected by the deflection surface 6a is incident at an angle of 2 degrees with respect to the plane normal of the deflection surface 6a in the sub-scanning direction and upwards (in the plane of FIG. 1 above) in the sub-scanning direction. .

The f- [theta] lens system 7, which is an imaging optical system, is disposed upwards at a predetermined distance in the sub-scanning direction to ensure that the deflected light beam reflected by the upward deflection is incident. Therefore, the deflection light beam incident on the f-? Lens system (imaging optical system) 7 is imaged on the photosensitive drum surface 8 as a light spot.

The deflection surface 6a of the optical deflector 6 is reciprocated in the main scanning direction within the range of the maximum amplitude (maximum deflection angle) ± φ max as described above. More specifically, the deflection surface 6a performs a sinusoidal vibration in which the deflection angle (vibration angle) φ can be expressed by the angular frequency ω and the time t as follows:

 φ = φmax · sin ωt.

In the optical deflector 6 in this embodiment, the maximum amplitude phi max of the deflection surface 6a is ± 36 degrees. Of these amplitudes, a range of ± 22.5 degrees is selected as the effective deflection angle and used for image recording.

In general, an arcsin lens is often used as an imaging lens for converting a light beam deflected and reflected by a sinusoidal oscillation optical deflector into a light beam having a uniform movement on a scan surface. This arc sine lens has an optical characteristic that the F-No. (F number) of the scanning end portion in the main scanning direction of the scan surface tends to be larger than that of the scan center of the scan surface. This causes a problem that the spot diameter in the main scanning direction at the scanning end of the scan surface becomes larger than the spot diameter in the main scanning direction at the scanning center of the scan surface.

This is a phenomenon caused by scanning a light beam whose angular velocity changes sine at a constant speed on the surface to be scanned. As described above, if the spot diameter in the scanning direction is uneven on the scan surface, various inconveniences such as deterioration of the gray scale reproducibility of the half-tone image and deterioration of the line width reproducibility of the thin lines depending on the place are caused.

In the present embodiment, in order to cope with these, the imaging lens has a characteristic that the spot diameter in the main scanning direction on the scan surface can be kept constant within the effective scanning area. It is installed by.

On the other hand, if the f-θ lens is simply used as an imaging lens in combination with the sinusoidal oscillation optical deflector 6, the scanning end portion is scanned on the photosensitive drum surface 8 as compared with the scanning center portion (the optical axis of the f-θ lens system 7). The problem is that the speed is slowed and shrinking of the image in the main scanning direction occurs.

In this embodiment, in order to cope with this problem, the modulation clock of the semiconductor laser 1 is continuously changed in synchronization with the dice in the main scanning direction on the photosensitive drum surface 8. This eliminates the above inconvenience.

According to the above configuration, undesirable unevenness of the spot diameter in the main scanning direction as described above on the photosensitive drum surface 8 can be completely avoided. As a result, inconveniences such as deterioration of the gradation reproducibility of the halftone image and deterioration of the linewidth reproducibility of the thin lines with the place are reliably eliminated.

Further, the slower scanning speed at the scanning end (maximum image height) on the photosensitive drum surface 8 as compared to the scanning center portion on the photosensitive drum surface 8 is that on the photosensitive drum surface 8 at the scanning end. It means that the exposure energy is increased. From this, it can be seen that the gray scale reproducibility of the halftone image can be improved by controlling to continuously reduce the amount of emitted light of the semiconductor laser 1 at the scanning end portion (maximum image height area).

In the present embodiment, as described above, the light beam deflected and reflected by the deflection surface 6a forms an angle of 2 degrees in the sub-scan direction with respect to the plane normal of the deflection surface 6a, and upwards in the sub-scan direction (Fig. 1). Deflected to the top of the ground). 2 illustrates this schematically.

As can be seen from FIG. 2, it can be seen that the light beam deflected and reflected by the deflection surface 6a forms a conical surface whose peak is the deflection reflection point 6b on the deflection surface 6a. Therefore, on the surface where the light beam is incident on the lens, the deflected and reflected light beam forms a curved path in the sub-scanning direction.

When such a light beam enters the f-θ lens system 7, the scanning line on the photosensitive drum surface 8 will be curved in the sub-scanning direction. In addition, when the light beam scanning along the conical surface enters the f-? Lens system 7, the light beam can be focused in a spot shape at the scanning center portion normally; As the light beam approaches the scanning end, there is a problem in that the shape of the focused spot (meaning "image spot") deteriorates as shown in FIG.

Fig. 3 shows the contour line converted into the intensity distribution of the deteriorated focusing spot at the scanning end portion on the scan surface.

The contour lines in Fig. 3 are 0.02 and 0.05, respectively (from the outside) when the peak intensity of the focus spot is normalized to one. The intensity | strength sliced at the level of 0.1 0.1353, 0.3679, 0.5, 0.75, and 0.9 is shown. 3, the horizontal direction corresponds to the main scanning direction scanned by the focusing spot, and the vertical direction corresponds to the sub scanning direction orthogonal to the main scanning direction.

In the present embodiment, as shown in FIG. 4, the optical axis 71a of the first f-θ lens 71 among the first and second f-θ lenses 71 and 72 is the deflection surface 6a. Is arranged at an angle of 2 degrees upwards so as to coincide with the chief ray of the light beam deflected and reflected toward the scanning center on the scan surface. That is, it is eccentrically rotated 2 degrees (2 degrees) with respect to the normal line of the deflection surface 6a in the subscan cross section centering on the axis | shaft of the main scanning direction.

On the other hand, the optical axis 72a of the second f-θ lens 72 faces downward in the direction opposite to the first f-θ lens 71 in the sub-scan section with respect to the plane orthogonal to the rotation axis of the deflection surface 6a. It is arranged to be inclined by 1.83383 degrees. That is, about the axis of the main scanning direction, it is eccentrically rotated by 1.83383 degrees downward with respect to the normal to the deflection surface 6a in the sub-scan section.

In addition, the second f-θ lens 72 ensures that the light beam is incident to a position above the vertex 72b in the sub-scan section of the first surface (incident surface) of the second f-θ lens 72. Are arranged in a sub-scan direction by a predetermined amount.

With this arrangement, both the curvature in the sub-scan direction of the scanning line on the photosensitive drum surface and the deterioration of the focusing spot at the scanning end portion on the scan surface are corrected.

Next, the optical deflector 6 of the present embodiment will be described in more detail. As described above, the optical deflector 6 is composed of a resonant optical deflector whose deflection surface 6a is configured to perform reciprocating sine vibration based on resonance.

5 shows the details of the optical deflector 6 in the present embodiment. As shown in FIG. 5, the optical deflector 6 is comprised by the movable plate 67 and the torsion spring 26 which elastically supports the movable plate 67 and the mechanical ground support part 25. As shown in FIG. All these components are torsionally vibrated by the drive stage 16 about the torsion axis C (an axis parallel to the sub-scanning direction). The driving means 16 may include, for example, a fixed magnet coil and a movable magnet installed on the movable plate 67.

Moreover, the movable plate 67 has a deflection surface (not shown) for deflecting the luminous flux, and deflects and scans the luminous flux from the light source means 1 based on the torsional vibration of the movable plate 67.

In general, in an optical deflector requiring high speed operation, the deflection surface undergoes a large angular acceleration because the deflection surface is torsionally vibrated within a specific angle. Therefore, during driving, the deflection surface is greatly deformed by receiving an inertial force by its own weight.

FIG. 6A is a cross-sectional view taken along the line A-A when the movable plate 67 in FIG. 5 is made of a flat plate (cuboid).

The optical deflector 6 of this embodiment is driven near the resonance frequency and is torsionally vibrated. Therefore, the deflection angle of the movable plate 67 with respect to time changes to a sine wave shape. Therefore, at the moment when the maximum angular velocity (e.g., the maximum deflection angle in the case of sinusoidal vibration) is applied, the greatest deformation occurs.

6B shows the deformation of the movable plate 67 at that time. As shown in FIG. 6B, when the movable plate 67 is deformed, the deflection surface 6a formed on the movable plate 67 is also deformed.

When the movable plate 67 consists of a rectangular parallelepiped, it is possible to demonstrate the deformation | transformation of the movable plate 67 at the time of a torsional vibration using the approximation model shown in FIG.

What is shown in FIG. 7 is corresponded especially to the right half part of sectional drawing of the movable plate 67 in FIG. 6A. The deformation of the movable plate 67 is point symmetrical about the torsion axis C, and can be approximated to the deformation of the structural beam with the fixed end support as the center portion as shown in the figure.

When the angular acceleration θ × (2πf) ² (θ is a deflection angle and f is a torsional vibration frequency) is applied to the movable plate 67 by torsional vibration, the deformation amount (distortion) of the structural beam shown in FIG. y is represented by the following formula [1]:

.....[1]

.....[One]

(In the meal,

x is the distance from the torsion axis C shown in FIG. 7;

ρ is the density of the movable plate 67;

E is the Young's modulus of the movable plate 67;

t is the thickness of the movable plate 67;

W _h is half the width D in the main scanning direction of the deflection plane).

From the formula (1), deformation (distortion) y is the deflection is proportional to the angle θ, W _h 5 V and the square of the frequency f, the greater is the main scanning direction of a deflecting surface width (that is, a large deflecting surface opening) In this case, when the deflection angle is large, when high frequency driving is required, it can be seen that the influence of deformation of the movable plate 67 due to its own weight becomes remarkable.

The optical deflector 6 of this embodiment has a natural vibration frequency of torsional vibration of 2 kHz, a width in the main scanning direction of the movable plate 67 (value of W) of 3 mm, a width of the sub scanning direction of 1 mm, a thickness t. Is 200 μm. As described above, the movable plate 67 receives an inertial force by its own weight at the time of vibration and causes deformation.

8 is a graph showing the results of calculating the deformation of the movable plate 67 by the finite element method. The deformation | transformation of the A-A cross section in FIG. 5 in the case where a mechanical effective deflection angle is +22.5 degree at the time of 2 microsecond drive is shown. Moreover, the inclination of the connection part (part B of FIG. 5) between the torsion spring 26 and the movable plate 67 is taken as zero.

Here, the definitions of the scan angle and deflection angle are given as follows.

The scanning angle can be specified as an angle formed by the optical axis of the imaging optical system 7 in the main scanning cross section and the chief ray of the light beam deflected and scanned by the deflection surface of the optical deflector 6. Therefore, the scan angle is twice the deflection angle (vibration angle).

Here, the scanning line recording start position on the scanning surface (upper side of the paper in Fig. 1 and at the same time as the center of the scanning center of the scanning line on the scanning surface) is taken as the center of the incident optical system LA. The deflection angle of the opposite side has a positive sign.

On the other hand, the scanning line recording end position on the scanning surface (in Fig. 1, the lower side of the paper and the incident optical system LA side) while taking the center of the scanning line (the optical axis of the imaging optical system 7) on the scanning surface as the center. ) Deflection angle has a negative sign.

The + sign direction of y in FIG. 8 corresponds to the advancing direction (right side of the drawing) of the light beam reflected by the deflecting surface 6a in FIG. 1, while the + sign direction of x is the deflection surface in FIG. 1. Corresponding to the scanning line recording start position (6a) (upper side in the page and opposite to the incident optical system LA in FIG. 1).

FIG. 9 is a graph showing the deformation amount (also referred to as a “displacement amount”) of the A-A cross section in FIG. 5 when the inclination at the origin of the graph of FIG. 8 is taken as zero. In FIG. 9, it can be seen that a deformation similar to the deformation amount y represented by the above formula [1] is obtained, and the movable plate 67 is deformed by the torsional vibration.

Here, when the deflection surface 6a of the optical deflector 6 is causing the deformation as shown in Fig. 9, the luminous flux reflected by the deflection surface 6a is twice the amount of deformation y shown in Fig. 9. Wavefront aberration occurs. Therefore, the focusing spot on the photosensitive drum surface 8 will be adversely affected.

In fact, it can be seen from FIG. 9 that coma of wavefront aberration occurs.

In the optical scanning device using the rotating face mirror as the optical deflector 6, since the rotating face mirror rotates at a constant angular velocity, the angular acceleration is always zero. Therefore, it does not receive much angular acceleration compared to the optical deflector using sinusoidal vibration. Therefore, wavefront aberration as described above will not normally occur.

For these reasons, deformation of the deflection surface is often not particularly considered when designing an imaging lens for use in an optical scanning device having a rotating multifaceted mirror.

However, when using an optical deflector that performs sinusoidal vibration in combination with an imaging lens designed as described above (that is, without considering deformation of the deflection surface), focusing spots are caused by wavefront aberration caused by deformation of the deflection surface 6a. This will be degraded.

Fig. 10 uses an imaging lens designed on the premise of using a rotating mirror, while the optical deflector of this embodiment is used as an optical deflector (the natural vibration frequency of torsional vibration is 2 kHz, and the width of the movable plate in the main scanning direction is 3 mm. 1 mm in width in the sub-scanning direction and 200 µm in thickness t are used.

10 illustrates the shapes of the spots on the photosensitive drum surface 8 when the mechanical deflection angle is +22.5 degrees, +21.028 degrees, +16.822 degrees, +12.617 degrees, +8.411 degrees, +4.206 degrees and 0.0 degrees, respectively. It is.

3, the contour line of the intensity distribution of each spot is also illustrated. These contour lines correspond to the intensity sliced to the levels of 0.02, 0.05, 0.1, 0.1353, 0.3679, 0.5, 0.75 and 0.9 (from the outside) when the peak intensity of the focus spot is normalized to 1, respectively.

As a comparative example, FIG. 11 shows the spot shape on the photosensitive drum surface 8 when the same imaging lens is used and the deflection surface 6a is not deformed at all. Also in this figure, similarly to FIG. 3, the horizontal direction is the main scanning direction in which the spot scans the surface, and the vertical direction is the sub scanning direction orthogonal to the main scanning direction.

As can be seen from FIGS. 10 and 11, the spot shape of FIG. 10 when the deflection surface 6a is deformed is compared with the shape of the focus spot of FIG. 11 when the deflection surface 6a is not deformed at all. Thus, a large sidelobe is generated in the main scanning direction.

In addition, the appearance itself of the focus spot deforms asymmetrically, and the shape of the focus spot deteriorates considerably. In the case of an effective deflection angle of +22.5 degrees, the peak intensity of the side lobes exceeds 0.05 (that is, 5% of the peak intensity of the main spot).

It is well known that the image quality decreases as the peak intensity of the side lobe increases. In particular, when the peak intensity of the side lobe exceeds 5% with respect to the peak intensity of the main spot, the deterioration of the image quality becomes considerably large. This is undesirable for an optical scanning device or an image forming apparatus that requires high quality image output.

It is also understood that the larger the deflection angle, the greater the deterioration of the spot shape of the focusing spot. This is because, as described above, the larger the maximum deflection angle of the deflection surface, the larger the deformation of the deflection surface.

In this embodiment, in order to cope with this, the effective deflection angle of the deflection surface corresponding to the end (maximum image height) of the scanning line inside the effective image region on the scan surface is ± 22.5 degrees.

In order to suppress the deterioration of the spot shape of the focusing spot, it is preferable to set the effective deflection angle of the deflection surface to ± 30 degrees or less.

If the optical deflector based on sine vibration is used in combination with an imaging lens designed without considering the deformation of the deflection surface as described above, the focusing spot will be deteriorated by the wave front aberration caused by the deformation of the deflection surface 6a. will be. If this occurs, it becomes very difficult to achieve a light scanning apparatus or an image forming apparatus that requires high quality image output.

The first and second f-theta lenses 71 and 72 of this embodiment shown in FIG. 1 are caused by the distorted deflection surface 6a as shown in FIG. 9 due to the application of a large angular acceleration as a result of sinusoidal vibration. It is configured to reduce the amount of wavefront aberration generated.

Here, in the present embodiment, the "first direction" is one phase difference of wavefront aberration in the main scanning direction between the main light beam and the last light beam of the light beam reflected by the deflecting surface 6a at the effective deflection angle of the deflection surface 6a. The phase difference is generated as a result of reflection by the deflection surface of the luminous flux. Further, the "second direction" indicates the direction of the phase difference on the other side of the wavefront aberration in the main scanning direction between the last light beam and the main light beam of the light beam reflected by the deflection surface 6a at the effective deflection angle of the deflection surface 6a. The phase difference at this time is generated as a result of passing the light beam through the imaging optical system 7.

At this time, in this embodiment, in order to ensure that the first direction and the second direction are opposite to each other, at least one non-circular optical surface in the main scanning cross section in at least one optical system of the imaging optical system 7 is provided. Are installing.

Here, the "light flux reflected by the deflection plane at the effective deflection angle of the deflection plane" means the light flux that reaches the scanning end (maximum image height) of the scanning line inside the effective image area on the scan surface.

Hereinafter, the optical principle is demonstrated.

FIG. 12 is a schematic diagram showing the shape W1 of the wavefront (isotope) in the main scanning direction of the light beam after the incident parallel light beam (plane wave) is reflected by the distorted deflection surface 6a as shown in FIG. 9.

The y direction in FIG. 12 corresponds to the direction of the displacement amount y taken along the vertical axis of the graph of FIG. Moreover, there is also shown the traveling state of the light beam reflected in the (+) direction of the displacement amount y. The (+) direction along the x direction in FIG. 12 corresponds to the (+) direction along the x axis of the graph of FIG. 9.

In this embodiment, a light beam having a width corresponding to 2.4 mm in width of the effective reflection surface is incident on a width of 3 mm in the main scanning direction (x direction) of the deflection surface 6a.

As is apparent from Fig. 12, the shape of the wavefront (equal phase) in the main scanning direction of the light beam after being reflected by the distorted deflection surface 6a is deformed by an amount twice the distorted shape of the deflection surface 6a.

More specifically, a difference of? L1 ₊ and? L1 ₋ is generated in the optical path length with respect to the beam center (the main beam of the beam) in the beam end (the final beam) in the main scanning direction (the x direction in FIG. 12).

Here,? L1 ₊ means the optical path length difference on the + side (upper side) in the main scanning direction, and δL1 ₋ means the optical path length difference on the − side (bottom) in the main scanning direction. The last light beam on the + side (up side) in the main scanning direction is the last light beam on the scanning line recording start position side (upper side in Fig. 3 and opposite to the incident optical system LA) with respect to the main light beam. In addition, in the main scanning direction, the last light beam on the − side (bottom side) is the last light beam on the scanning line recording start position side (the bottom side in Fig. 1 and the incident optical system LA side).

Next, the shape of the wavefront (equal phase surface) after the parallel light beam (plane wave) has passed through the f-θ lens system 7 is considered.

Fig. 13 shows the shape of the wavefront (equal phase) after the parallel light beam (plane wave) when the effective deflection angle of the deflection surface 6a is +22.5 degrees (toward the scanning line recording start position) passes through the f-? Lens system 7. It is a schematic diagram showing. When the f-θ lens system 7 is an ideal aberration-free lens, the wavefront (isophase) after passing through the f-θ lens system 7 forms a spherical wave S (solid line). Here, the wavefront (equivalent phase) after passing through the f-θ lens system 7 has a shape as indicated by (W2) (dotted line) in FIG. 13, that is, the luminous flux in the main scanning direction (x direction in FIG. 13). A difference of δL2 ₊ and δL2 ₋ is generated in the optical path length with respect to the light beam center portion (the main ray of light) at the end (the last ray). In addition, δL2 ₊ means the optical path length difference of + side (up) (+ x direction in FIG. 12) of a main scanning direction, and δL2 <_-> (down side) (-x direction in FIG. 12) of a main scanning direction Means the optical path length difference.

As can be seen from FIG. 13, based on the optical path difference with respect to the spherical wave S (solid line) of the wavefront (isophase) after passing through the f-θ lens system 7, it is distorted as shown in FIG. 12. The wavefront (equal phase surface) in the main scanning direction of the light beam reflected by the deflection surface 6a is reduced. In other words, the wave front aberration generated by the distorted deflection surface 6a is corrected by passing the f-? Lens system 7.

Here, in order to ensure that the wave front aberration generated by the distorted deflection surface 6a is well corrected by passing through the f-? Lens system 7, it is preferable to satisfy the following conditional formula [2]:

.....[2].

.....[2].

More preferably, the following conditional formula [3] is satisfied:

.....[3].

..... [3].

Fig. 14 shows the shape of the focusing spot on the photosensitive drum surface 8 according to this embodiment of the present invention.

According to this embodiment, the wavefront aberration generated by the deflection surface 6a distorted by the large angular acceleration in response to the sinusoidal vibration is reduced by the f-? Lens system 7. As a result, it can be seen from FIG. 14 that the side lobe is reduced in comparison with the shape of the focusing spot shown in FIG. 10, and the appearance itself of the focusing spot is also improved. In particular, at an effective deflection angle of +22.5 degrees, a side lobe having a peak intensity of 5% as shown in Fig. 9 is completely corrected.

FIG. 15 shows wavefront aberration (here, the difference between the ideal plane wave and the actual wavefront caused by the deformation) caused by the deformation at the effective deflection angle +22.5 degrees of the deflection surface 6a in the present embodiment. The horizontal axis (unit: mm) of the graph is a pupil coordinate in the main scanning direction at the entrance pupil position of the optical system, and the pupil radius 1.2 mm is normalized to 1 here.

It is determined from FIG. 15 that there is a coma of wavefront aberration.

The vertical axis represents the amount of wave front aberration, and the unit is lambda (780 nm). As for the direction of the wavefront aberration, the direction in which the actual wavefront is delayed with respect to the traveling direction of the wavefront with respect to the ideal wavefront is taken as negative.

Since the deflection surface 6a is deformed as shown in Fig. 9, it can be seen that a large wavefront aberration occurs there.

Fig. 16 shows wavefront aberration (spherical wave S described above) and actual wavefront which occur after the parallel light beam (plane wave) passes through the f-θ lens system 7 when the effective deflection angle is +22.5 degrees in this embodiment. ).

As in Fig. 15, the horizontal axis of the graph corresponds to the pupil coordinate in the main scanning direction to the incident pupil position of the optical system, and the pupil radius 1.2 mm is normalized to one. The vertical axis represents the amount of wave front aberration, and the unit is lambda (780 nm). As for the direction of the wavefront aberration, all directions in which the actual wavefront is delayed with respect to the traveling direction of the wavefront with respect to the ideal wavefront are taken as negative.

The f-θ lens system 7 according to the present embodiment actively generates a wavefront aberration equal to the wavefront aberration generated by the deformation of the deflection surface 6a shown in FIG. 15 in the reverse direction (offset direction). have.

Fig. 17 is a graph showing the wave front aberration (the difference between the spherical wave S described above and the actual wave front) after the light beam having the wave front aberration generated as a result of the deformation of the deflection plane 6a passes through the f-θ lens system 7. to be. As in Fig. 15, the horizontal axis of the graph corresponds to the pupil coordinate in the main scanning direction at the entrance pupil position of the optical system, and the pupil radius of 1.2 mm is normalized to one. The vertical axis represents the amount of wave front aberration, and the unit is lambda (780 nm). As for the direction of the wavefront aberration, the direction in which the actual wavefront is delayed with respect to the traveling direction of the wavefront with respect to the ideal wavefront is taken as negative.

As can be seen from FIG. 17, the wave front aberration generated by the deformation of the deflection surface 6a shown in FIG. 15 is generated by the deformation of the deflection surface 6a through the f-θ lens system as shown in FIG. 16. Compensation is made by actively generating the wavefront aberration equivalent to the generated wavefront aberration in the reverse direction (offset direction). As a result, good wave front aberration is achieved.

In the present embodiment, the first wave front aberration caused by the deformation of the deflection plane 6a of the optical deflector 6 based on the sine vibration is equal to the first wave front aberration through the f-θ lens system 7. Compensation is made by actively generating reverse wavefront aberration.

In this embodiment, first,? L1 ₊ denotes an optical path length difference between one last light beam (upper light beam) of the light beam reflected by the deflection plane and the main light beam of the light beam at the effective deflection angle +22.5 degrees of the deflection plane. The optical path length difference at this time is generated as a result of the light beam being reflected by the deflection surface. Secondly, δL1 ₋ is used to represent the optical path length difference between the other last light beam (lower light beam) and the main light beam of the light beam reflected by the deflection plane at an effective deflection angle of +22.5 degrees of the deflection plane. The optical path length difference is generated as a result of the luminous flux reflected by the deflection surface.

Third, δL2 ₊ is used to represent the optical path length difference between one last light beam (upper light beam) and the main light beam of the light beam reflected by the deflection plane at the effective deflection angle +22.5 degrees of the deflection plane. The optical path length difference is generated as a result of passing the light beam through the imaging optical system. Fourth, δL2 ₋ is the distance between the other last light beam (lower light beam) of the light beam generated when the light beam reflected by the deflection plane passes through the deflection plane at an effective deflection angle of +22.5 degrees of the deflection plane, and the main light beam of the light beam. The optical path length difference is generated as a result of passing the light beam through the imaging optical system.

At this time, the imaging optical system satisfies the following relation [4]:

...[4]

...[4]

In the present embodiment, the present embodiment satisfies the above formula [4], taking the case where the effective deflection angle of the deflection surface is +22.5 degrees as an example. However, the present embodiment has the above equation even when the effective deflection angle of the deflection surface is -22.5 degrees. Satisfies [4]. Incidentally, in this embodiment, the above formula [4] is, of course, satisfied at the total deflection angle within an effective deflection angle of ± 22.5 degrees.

Here, the "light beam reflected by the deflection plane at the effective deflection angle of the deflection plane" means the light beam reaching the scanning end (maximum image height) of the scanning line in the effective image area on the scan surface.

From the above description, it can be seen that the amount of wavefront aberration generated by the deformation of the deflection surface 6a can be considerably reduced.

As described above, according to the embodiment of the present invention, it is possible to generate a higher quality image output while using the optical deflector 6 based on sine vibration, and to reduce or avoid deterioration of image quality. An optical scanning device or an image forming apparatus is achieved.

Tables 1a and 1b below show the specifications of the optical system of the optical scanning device in this embodiment of the present invention.

Reference wavelength used λ (nm) 780 Number of flash points n One Position of light emitting point x0 (mm) -29.38709 y0 (mm) -75.99937 z0 (mm) -3.57057 Semiconductor Laser Cover Glass Refractive Index n0 1.51072 Semiconductor laser cover glass thickness deg (mm) 0.25 Aperture Position x1 (mm) -17.80914 y1 (mm) -55.94578 z1 (mm) -2.76195 Shape of aperture Oval Main scan 2.4 mm x Sub scan 1.72 mm Distance between light emitting point and collimator lens first surface d0 (mm) 23.67000 Collimator Lens First Side Position x2 (mm) -17.55930 y2 (mm) -55.51303 z2 (mm) -2.74450 Collimator Lens Second Surface Position x3 (mm) -16.55991 y3 (mm) -53.78204 z3 (mm) -2.67470 Collimator lens thickness d1 (mm) 2.00000 Collimator Lens Refractive Index n1 1.76203 Collimator lens first surface curvature radius R1 (mm) 182.21200 Collimator Lens Second Surface Curvature Radius R2 (mm) -20.83080 Distance between collimator lens second surface to cylindrical lens first surface d2 (mm) 19.76000 Cylindrical lens first side position x4 (mm) -6.68592 y4 (mm) -36.67980 z4 (mm) -1.98508 Cylindrical lens second side position x5 (mm) -3.68775 y5 (mm) -31.48681 z5 (mm) -1.77569 Cylindrical lens thickness d3 (mm) 6.00000 Cylindrical lens refractive index n2 1.51072 Cylindrical lens first face sub-scan direction curvature radius Rs3 (mm) 26.99300 Cylindrical lens surface curvature radius Rm3 (mm) infinity Cylindrical lens curvature radius R4 (mm) infinity Distance between cylindrical lens second surface to optical path folding mirror d4 (mm) 36.38000 Fiber path folding mirror position x6 (mm) 14.49117 y6 (mm) 0.00000 z6 (mm) -0.50604 Optical Fiber Folding Mirror Curvature Radius R5 (mm) infinity Optical path folding mirror to deflection reflecting distance d5 (mm) 14.50000 Deflection Reflective Position x6 (mm) 0.00000 y6 (mm) 0.00000 z6 (mm) 0.00000 Deflection reflecting surface to distance between first f-θ lens first surface d6 (mm) 24.50000 First f-θ lens first surface position x6 (mm) 24.48508 y6 (mm) 0.00000 z6 (mm) 0.85504 First f-θ lens second surface position x7 (mm) 32.48020 y7 (mm) 0.00000 z7 (mm) 1.13423 First f-θ lens thickness d7 (mm) 8.00000 First f-θ Lens Refractive Index n3 1.52420 Distance between the first surface of the first f-θ lens and the first surface of the second f-θ lens d8 (mm) 15.00000 2nd f-θ lens first surface position x8 (mm) 47.47106 y8 (mm) 0.00000 z8 (mm) 1.11088 2nd f-θ lens second surface position x9 (mm) 54.46748 y9 (mm) 0.00000 z9 (mm) 0.88685 2nd f-θ lens thickness d9 (mm) 7.00000 2nd f-θ lens refractive index n4 1.52420 Distance between the second surface of the second f-θ lens and the surface to be scanned d10 (mm) 173.72622 Pitch Position x10 (mm) 173.70276 y10 (mm) 0.00000 z10 (mm) 2.85489 f-θ lens main scanning direction focal length f (mm) 135.75817 Incident angle of incidence optical system (scanning cross section) γ (degrees) 120.00000 Incident optical system diagonal inclination angle (sub-scan section) β (degrees) 2.00000 1st f-θ lens upward angle (sub-scan section) δ (degrees) 2.00000 1st f-θ lens downward angle (sub-scan section) η (degrees) 1.83383 Optical deflector maximum scanning angle ζ (degrees) 36.00000 Optical Deflector Effective Injection Angle ξ (degrees) 22.50000 Optical deflector resonant frequency f0 (㎑) 2.00000 Optical deflector deflection reflector size Rectangle Main scan 3mm x Sub scan 1mm (thickness 0.2mm)

First f-θ Lens Shape Front page Page 2 R -61.83626 R -35.48526 k -5.87163E + 00 k -2.45242E + 00 B4 3.76675E-06 B4 -3.86823E-07 B6 -1.30464E-10 B6 2.56668E-09 B8 -1.27161E-13 B8 3.57959E-13 B10 5.42375E-18 B10 0.00000E + 00 r -62.21270 r -59.18750 D2 2.50064E-03 D2 -9.00838E-05 D4 4.47165E-06 D4 -1.99468E-06 D6 -3.38261E-09 D6 3.02931E-09 D8 -2.44356E-12 D8 3.76989E-13 D10 2.15635E-14 D10 6.61287E-16 2nd f-θ lens shape Front page Page 2 R 77.24019 R 77.21539 k -1.11058E + 00 k -1.40572E + 01 B4 -4.81789E-06 B4 -3.14620E-06 B6 2.28668E-09 B6 1.22437E-09 B8 -7.53262E-13 B8 -3.42569E-13 B10 9.70410E-17 B10 2.45104E-17 r -37.74570 r -13.93790 D2 3.67322E-03 D2 1.33995E-03 D4 3.51750E-06 D4 -1.12369E-06 D6 8.37822E-10 D6 6.90229E-10 D8 -4.71373E-13 D8 -2.56901E-13 D10 -9.72956E-17 D10 3.91913E-17

As for the aspherical shape of the main scanning cross section of the f-θ lens, the intersection of each lens surface and the optical axis is taken as the origin. The optical axis direction is taken as the X axis and the axis perpendicular to the optical axis in the main scanning cross section is taken as the Y axis, and the axis perpendicular to the optical axis in the sub scanning cross section is taken as the Z axis.

Here, the following relation [5] is given:

...[5]

... [5]

(Wherein R is the radius of curvature and k and B ₄ to B ₁₀ are aspherical coefficients).

In addition, the shape of the sub-scan section is such that the radius of curvature r 'when the lens plane coordinate in the main scanning direction is Y can be given by the following formula [6]:

....[6]

.... [6]

(Wherein r is the radius of curvature on the optical axis and D ₂ to D ₁₀ are the respective coefficients).

For the non-circular shape of the main scanning sections of the f-θ lenses 71 and 72, the number of surfaces of the optical surface (lens surface) constituting the f-θ lens system is m, and the surface shape in the main scanning section of each optical surface is m. The following formula [7]:

...[7]

... [7]

When expressed by, the following conditional expression [8] is satisfied:

...[8]

...[8]

Wherein U _j takes U _j = -1 when the optical plane is a transmissive plane and a light incidence plane, U _j = +1 when the optical plane is a transmissive plane and a light exit plane, and U _j = +1 when the optical plane is a reflective plane and the optical plane is a reflective plane If U _j = +1. N _j is the same as the refractive index of the glass material when the optical surface is a transmissive surface, and N _j = 2 when the optical surface is a reflective surface.

Further, in the formula [8], dX / dY _{(out) j} is the main scanning cross section, where the outer scanning last light beam of the luminous flux reaching the maximum scanning position in the effective scanning area on the scan surface is the j surface. Inclination of the scanning end with respect to the optical axis of the optical surface at a position passing therethrough; dX / dY _{(in) j} is the main surface of the optical surface at the position where the inner scanning last light beam of the light beam that reaches the maximum scanning position in the effective scanning area on the scan surface passes through the j-th surface. Inclination of the scanning center with respect to the optical axis; dX / dY _{(p) j} is the inclination of the optical plane of the optical plane at the position where the principal ray of the luminous flux reaching the maximum scanning position of the effective scanning area on the scan plane in the main scan section passes through the j-th plane. .

The conditional expression [8] is (i) an asymmetric component of wavefront aberration with respect to the principal ray of the luminous flux received when the luminous flux reaching the maximum dice in the effective scanning area on the scan surface passes through the f-θ lens system (7); ii) The correlation with the surface shape of each surface of the f-theta lens system 7 is shown.

As shown in FIG. 16, the f-θ lens system 7 according to the present embodiment aggressively reverses the amount of wavefront aberration equal to the wavefront aberration generated by the deformation of the deflection surface 6a in the reverse direction (offset direction). It is configured to generate.

Fig. 44 shows an arbitrary lens surface of the chief ray and the final ray and the f-θ lens system 7 (here, for example, the final plane) reaching the maximum scanning position (here Y> 0) of the effective scanning area on the surface to be scanned; It is a diagram showing.

In order to generate wavefront aberration as shown in Fig. 16, in the main scanning cross section, (i) the angle formed by the main light beam and the main light beam on the scanning end side exiting from the lens surface, and (ii) the final light beam on the scanning center side, An angle difference needs to exist between the angles of the chief rays of light.

More specifically, as shown in Fig. 44, both the final light beam on the scanning end side and the final light beam on the scanning center portion must reach the scanning center side with respect to the arrival position of the chief ray on the scan surface.

In order to meet this, it is necessary to satisfy the following conditions.

First, in the main scanning cross section, (i) the final light beam toward the scanning end of the luminous flux reaching the maximum scanning position (here Y> 0) of the effective scanning area on the scanning surface passing through the lens surface, and (ii) f α _(out) is used to represent the angle formed by the optical axis of the -θ lens system 7. Secondly, in the main scanning cross section, (i) the final light beam toward the scanning center of the luminous flux reaching the maximum scanning position (here Y> 0) of the effective scanning area on the scanning surface passing through the lens surface, and (ii) f α _(in) is used to represent the angle formed by the optical axis of the -θ lens system 7. Third, in the main scanning cross section, (i) the chief ray of luminous flux reaching the maximum scanning position (here Y> 0) of the effective scanning area on the scanned surface passing through the lens surface, and (ii) the f-θ lens system (7). Α _(p) is used to represent the angle formed by the optical axis of. In this way, the following conditional expression is satisfied:

(α _(out) -α _(p) )-(α _(p) -α _(in) )> 0

In other words,

α _(out) + α _(in) -2α _(p) > 0 .... [9].

Here, dx / dy _(out) is a main beam cross section where a final light beam toward the scanning end of the luminous flux that reaches the maximum scanning position (here, Y> 0) in the effective scanning area on the scan surface passes through the lens surface. It is an inclination with respect to the optical axis of the f-theta lens system 7 of the lens surface at the position. In addition, dx / dy _(in) indicates that in the main scan section, the last light beam toward the scanning center of the luminous flux that reaches the maximum scanning position (here, Y> 0) of the effective scanning area on the scan surface passes through the lens surface. It is the inclination with respect to the optical axis of the f-θ lens system 7 of the lens surface at the position. Further, dx / dy _(p) is the f-θ at the position where the main beam of the luminous flux that reaches the maximum scanning position (here, Y> 0) of the effective scanning area on the scan surface in the main scanning cross section passes through the lens surface. It is the inclination with respect to the optical axis of the lens system 7. Moreover, the refractive index of the light incident side of the lens surface is represented by N, and the refractive index of the light exit side of the lens surface is represented by 1.

In this way, Equation [9] can be rewritten as in Equation [10]:

....[10]

.... [10]

In the present description, for the sake of simplicity, explanation has been made with reference to an example in the case of one optical surface. However, in the case of plural optical surfaces, the sum of the inclination relationships of the lens surfaces needs to satisfy the above formula [10].

When there are a plurality of optical surfaces, U _j takes U _j = -1 when the optical plane is a transmissive plane and a light incidence plane, and U _j = +1 when the optical plane is a transmissive plane and a light exit plane, If the face is a reflective face, then U _j = +1. N _j is the same as the refractive index of the glass material when the optical surface is a transmissive surface, and N _j = 2 when the optical surface is a reflective surface. Therefore, it is necessary to satisfy the following conditional formula [11] instead of the above formula [10]:

....[11]

.... [11]

Tables 1c and 1d below show the numerical values in the present Example and the numerical values on the left side of the formula [11].

 Y> 0 Front page Page 2 Page 3 Page 4 Scanning end marginal light-pass Y coordinate 23.5078 25.5376 43.6122 45.4727 Y-pass Y coordinate 21.9026 24.1839 42.1086 44.0691 Marginal Y-coordinate at the center of scan 20.3084 22.8352 40.5895 42.6369 dx / dy (out) -0.10503 -0.38188 -0.13347 -0.40257 dx / dy (up) -0.12708 -0.40728 -0.09675 -0.33802 dx / dy (in) -0.14383 -0.42204 -0.06310 -0.28078 U -One One -One -One N 1.52420 1.52420 1.52420 1.52420 U (N-1) (dx / dy (out) + dy / dy (in) -2dx / dy (p)) -0.00278 0.00558 0.00161 0.00383 Conditional Expression [11] Left Side 0.00824

Y <0 Front page Page 2 Page 3 Page 4 Scanning end marginal light-pass Y coordinate -23.5078 -25.5376 -43.6122 -45.4727 Y-pass Y coordinate -21.9026 -24.1839 -42.1086 -44.0691 Marginal Y-coordinate at the center of scan -20.3084 -22.8352 -40.5895 -42.6369 dx / dy (out) 0.10503 0.38188 0.13347 0.40257 dx / dy (up) 0.12708 0.40728 0.09675 0.33802 dx / dy (in) 0.14383 0.42204 0.06310 0.28078 U -One One -One -One N 1.52420 1.52420 1.52420 1.52420 U (N-1) (dx / dy (out) + dy / dy (in) -2dx / dy (p)) 0.00278 -0.00558 -0.00161 -0.00383 Conditional Expression [11] Left Side -0.00824

From these tables, it can be seen that according to this embodiment, the value of the left side of the conditional expression [11] is positive when Y> 0, and becomes negative when Y <0, thereby reliably satisfying the conditional expression [11].

According to this embodiment, by satisfying the conditional expression [11], as shown in Fig. 16, positively generated wavefront aberration equivalent to wavefront aberration caused by the deformation of the deflection surface 6a is actively generated in the reverse direction (offset direction). Let's do it. According to this configuration, the wave front aberration caused by the deformation of the deflection surface 6a is effectively reduced, and image output of high quality is achieved.

[ Example  2]

FIG. 18: shows the cross section (main scanning cross section) of the principal part of the main scanning direction of Example 2 of this invention. In Fig. 18, elements corresponding to those shown in Fig. 1 are given the same reference numerals.

This embodiment is different from the first embodiment in that an optical deflector 166 having a structure different from that of the optical deflector 6 used in the first embodiment is used as the deflecting means. In addition, the first and second imaging lenses (f-theta lenses) 161 and 162 constituting the f-theta lens system 167 have different shapes. The remaining part has the same construction and optical action as in Example 1, giving similar advantageous results.

In Fig. 18, reference numeral 166 denotes an optical deflector (deflection means), and has a structure shown in Fig. 19.

In Fig. 19, the optical deflector 166 is provided with a plurality of movable plates 171, 172, a plurality of torsion springs 173, and 174 integrally formed of one plate, and the torsion spring 173 and 173 are fixed to the support part 175 and 176, respectively. The optical deflector 166 also has a deflection surface formed on one of the movable plates 171 of the plurality of movable plates.

As can be seen from FIG. 19, the plurality of torsion springs 173 and 174 are arranged in a straight line along one same axis. By connecting the plurality of movable plates 171 and 172 in a unitary structure in series, these movable plates 171 and 172 become the axes of the torsion springs 173 and 174 (that is, in the sub-scan direction). Can be oscillated around the deflected axis).

On the movable plate 171, a deflection surface (not shown) for deflecting the light beam is formed. Through torsional vibration of the movable plate 171, the light beam from the light source means can be deflected and scanned in the main scanning direction.

Next, the principle of the optical deflector 166 having the above configuration will be described with reference to FIG.

20 is a schematic diagram illustrating the principle of the optical deflector 166 of this embodiment. In Fig. 20, 1801 to 1803 are n movable plates. 1811 to 1813 are torsion springs of n torsion springs 1811 to 1813, and 1821 is a support portion.

The torsion springs 1811 to 1813 are arranged along a straight line, and the movable plates 1801 to 1803 are arranged to be swingable around the torsion axis of the torsion springs 1811 to 1813.

The equation of free vibration of the system would be given by the following equation [12]:

... [12]

However, I _k is the moment of inertia of the movable plate, K _k is the spring constant of the torsion spring, and Q _k is the torsion angle of the movable plate (k = 1 ... n).

When the eigenvalue of M k ^-1 of the system λ _k d (k = 1 ··· n), angular frequency ω _k of the natural mode are given as shown in the following formula [13]:

....[13]

.... [13]

In the optical deflector 166 of this embodiment, the angular frequency ω _k in these eigenmodes contains a frequency corresponding to a reference frequency and an integer multiple of the reference frequency.

That is, the reciprocating motion of the deflection surface of the optical deflector 166 in this embodiment has a plurality of separate natural vibration modes. These separated natural vibration modes include a reference vibration mode that is a natural vibration mode at a reference frequency and an integer vibration mode that is a natural vibration mode at a frequency corresponding to an integer multiple of two or more times of the reference frequency.

Here, as an example, a resonance type optical deflector 166 having two movable plates as shown in FIG. 21 will be described.

The optical deflector 166 shown in FIG. 21 is disposed on one coaxial axis connecting two movable plates 1901 and 1902 and two movable plates 1901 and 1902 in series. Two torsion springs 1901 and 1902 are provided.

In addition, there is a support portion 1921 that supports a part of two torsion springs 1901 and 1902. Further, there are driving means 1941 for applying torque to at least one of the two movable plates 1901 and 1902 and driving control means 1951 for controlling the driving means 1941.

Here, the following formula [14] is assumed:

....[14]

.... [14]

Where the eigenvalue of M ⁻¹ k

λ _One  = 1.5790 × 10 ⁸

λ ₂  = 6.3166 × 10 ⁸

Therefore, the corresponding natural frequency is given by:

ω _One  = 2π × 2000 [ H ₂ ]

ω ₂ = 2π × 4000 [ H ₂ ] .

That is, ω ₂ = 2ω ₁ . Hereinafter, these vibration modes are referred to as "mode 1" (reference vibration mode) and "mode 2" (integer vibration mode).

In the optical deflector 166 of this embodiment, a system composed of two movable plates 1901, 1902, two torsion springs 1911, and 1912 can simultaneously vibrate at a reference frequency and an integer multiple of the frequency thereof. The drive control means 1951 controls the drive means 1194 so as to control the drive means.

At that time, it is possible to drive in various ways by changing the amplitude and phase of the movable plate at various frequencies at a reference frequency and a frequency corresponding to an integer multiple thereof.

In this embodiment, the drive control means 1951 controls the drive means 1941 to set the following conditions. That is, in FIG. 19,

1) The maximum vibration amplitude φ ₁ of the movable plate 171 in the mode ₁ is

φ ₁ = 36.68757 °.

2) Angular frequency ω ₁

ω _One  = 2π × 2000 [H _z ].

3) The maximum vibration amplitude φ ₂ of the movable plate 171 in the mode ₂ is

φ ₂ = 5.61 180 °.

4) The angular frequency ω ₂ is

ω ₂ = 2π × 4000 [H _z ] .

5) Each phase has a difference of 180 °.

The size of the movable plate 171 is 3.0 mm in the vertical direction (main scanning direction) and 2.0 mm in the horizontal direction (sub-scan direction) in FIG.

Here, the vibration angle (deflection angle) θ ₁ of the movable plate 171 is given by the following equation [15]:

...[15].

... [15].

Since the deflection surface (not shown) is provided in the movable plate 171, the luminous flux from the semiconductor laser ₁ is deflected and scanned at an angle 2θ _{1 which} is twice the angle of the above formula [15].

On the other hand, the angular velocity dθ ₁ / dt and the angular acceleration d ² θ ₁ / dt ² of the movable plate are given by the following formulas [16] and [17], respectively:

....[16]

.... [16]

....[17].

[17].

FIG. 22 shows the vibration angle (deflection angle) θ ₁ of the movable plate 171 of the optical deflector 166 in the present embodiment. In Fig. 22, the horizontal axis represents the period (time), and the vertical axis represents the oscillation angle (deflection angle) θ ₁ (in this unit, ° (degree)).

In the present embodiment from FIG. 22, by simultaneously exciting the mode 1 and the mode 2, an area in which the vibration angle θ ₁ is substantially proportional to time compared to the normal sinusoidal vibration (that is, the vibration angle can be regarded as proportional to time). Area) is generated.

In the first embodiment, the optical deflector 6 is composed of a deflector based on sine vibration, and an f-? Lens is used as an imaging lens combined with this deflector.

When a f-theta lens is simply used as an imaging lens in combination with a sinusoidal vibration deflector, the scanning speed at the scanning end on the photosensitive drum surface 8 is delayed compared to the scanning center on the photosensitive drum surface 8. There is a problem that the image is reduced in the main scanning direction.

In the first embodiment, the problem is avoided by continuously changing the modulation clock of the semiconductor laser 1 in synchronization with the dice in the main scanning direction on the photosensitive drum surface 8.

On the other hand, in the optical deflector 166 of the present embodiment, the mode 1 and the mode 2 are simultaneously excited, whereby the vibration angle (deflection angle) θ ₁ is substantially proportional to the time (i.e., the vibration angle) compared to the normal sine vibration. An area that can be considered proportional to this time) has been created.

That is, there is an area in which the deflection plane at an isometric speed can be considered to be deflecting. Therefore, by using a normal f-? Lens as the imaging lens combined with this, it is possible to achieve scanning close to the constant velocity on the photosensitive drum surface 8.

This has the advantage that it is not necessary to continuously change the modulation clock of the semiconductor laser 1 in synchronization with the dice in the main scanning direction on the photosensitive drum surface 8 as in the first embodiment.

Fig. 23 is a graph for explaining the angular velocity dθ ₁ / dt of the movable plate 171 of the optical deflector of the present embodiment. Specifically, the angular velocity dθ ₁ / dt at the scanning center (cycle is 0) is 160.0 (deg / sec), and the angular velocity increases as the scanning position moves to the scan end, so the angular velocity dθ ₁ / at the period ± 0.098. dt is maximum, and the value at that time is 164.708 (deg / sec).

Thereafter, the angular velocity decreases as the dice become closer to the scanning end, and the angular velocity dθ ₁ / dt becomes 160.0 (deg / sec) at the maximum scanning position (period is ± 0.14) of the effective scanning region.

Uniformity of the angular velocity of the movable plate 171 in the effective scanning region indicates that the angular velocity of the deflection plane at any position in the effective scanning region is represented by dθ ₁ / dt, where the maximum value of dθ ₁ / dt is 164.708 (deg / sec). ) And the minimum value of dθ ₁ / dt is 160.0 (deg / sec). Therefore, the angular velocity uniformity can be set as follows:

(dθ ₁ / dt) (dθ ₀ /dt)=164.708/160.0=1.1294.

That is, it may be set to 2.94% or less.

For comparison, FIG. 24 shows an example of the angular velocity dθ ₁ / dt of the movable plate 171 with only mode 1.

In the case of only Mode 1 shown in Fig. 24, sinusoidal vibration is simply provided. Therefore, the angular velocity changes sinusoidally. Thus, as shown in Fig. 23, it is apparent that there is no region where the angular velocity can be considered constant.

On the other hand, although the angular velocity dθ ₁ / dt in the present embodiment is almost constant, it is not completely equiangular velocity.

FIG. 25 is a view for focusing the luminous flux deflected and scanned at a height of 107 mm up to a period of 0.14 by a deflection surface that is deflected at a completely isometric speed corresponding to the value of the angular velocity in the period 0 (t = 0) shown in FIG. It represents the ideal image height when scanning with an ideal f-theta lens with focal length.

FIG. 25 shows the actual light beam reflected by the deflection surface 166a having the angular velocity dθ ₁ / dt of the optical deflector 166 of the present embodiment and scanned by the same f-θ lens with a deflection scanning up to a period of 0.14. The height of the image is also shown.

The angular velocity dθ ₁ / dt of the optical deflector 166 of this embodiment gradually increases with time from time t = 0, and coincides with the angular velocity at t = 0 at the period 0.14.

Therefore, the ideal image height provided by scanning using an ideal f-? Lens is obtained by scanning an optical beam deflected and scanned by a period 0.14 by a reflecting surface deflected at full isotropic velocity corresponding to the value of the angular velocity in period 0 (t = 0). Compare with

By doing so, the luminous flux reflected by the deflection surface 166a of the angular velocity dθ ₁ / dt shown in FIG. 23 and deflected and scanned up to a period of 0.14 is obtained by the actual image height provided by scanning using an ideal f-θ lens. You can see that the page has a large value.

Fig. 26 shows this difference as an f-? Lens.

It can be seen that there is a maximum error of about 1.7 mm at the scanning end (period 0.14) on the scan surface. If the error can be neglected according to its specification in the image forming apparatus in which the scanning optical apparatus is incorporated, the deflector can be used in this state.

In the f-θ lens system 167 according to the present embodiment, the f-θ lens component 167 shown in FIG. 26 generated by the deviation from the isometric speed of the angular velocity dθ ₁ / dt is applied to the f-θ lens system 167. Corrected by

FIG. 27 is a diagram showing an f-θ error when the light beam reflected by the deflection surface 166a having the angular velocity dθ ₁ / dt shown in FIG. 23 is scanned by the f-θ lens system 167. Compared with FIG. 26, it can be seen that the f-θ error is significantly reduced.

Next, FIG. 28 shows the angular acceleration d ² θ ₁ / dt ² of the movable plate 171 of the optical deflector 166 in this embodiment, and as a comparative example, FIG. 29 shows the movable plate excited by mode 1 only ( Angular acceleration d ² θ ₁ / dt ² of 171).

28 and 29 show that the angular acceleration of the movable plate 171 of the optical deflector 166 in the period 0.14 (shortest end of the scan) is significantly smaller in FIG. 28. That is, it can be seen that the angular acceleration d ² θ ₁ / dt ² of the movable plate 171 of the optical deflector 166 at the period 0 to ± 0.14 (shortest end of the scan) is smaller in FIG. 28 than in FIG. 29.

In the optical deflector 166 according to the present embodiment, by simultaneously exciting the mode 1 and the mode 2, it is possible to considerably reduce the angular acceleration d ² θ ₁ / dt ² of the movable plate 171 as compared with the normal sine vibration. .

On the other hand, as described with reference to the first embodiment, the movable plate 171 may be deformed by the angular acceleration during vibration. However, since the angular acceleration of the movable plate 171 in this embodiment is significantly smaller than the angular acceleration at the time of sine vibration, the deformation of the movable plate 171 will be very small.

30 shows a graph of the results of calculating the deformation of the movable plate 171 of this embodiment by the finite element method. Here, the width | variety of the main scanning direction of the movable plate 171 is 3 mm, the width of the sub-scanning direction is 1 mm, and thickness is 200 micrometers. Compared with the deformation amount (FIG. 9) in the said Example 1, it turns out that deformation amount is 1/3 or less.

Therefore, the deflection surface 166a is similarly deformed in response to the deformation of the movable plate 171, but since the deformation is about 1/3 of the first embodiment, the wave front aberration generated by the deflection surface 166a is also the first embodiment. It will be about one third of it.

Therefore, in that case, the adverse effect given to the focusing spot on the photosensitive drum surface 8 will be very small.

Fig. 31 shows an example in which the f-? Lens designed assuming that the deflection surface 166a is not deformed, but the deflection surface 166a is actually deformed as in the present embodiment.

Specifically, FIG. 31 shows that the deflection angles of the deflection surface 166a are +22.856 degrees (corresponding to a period of 0.14), +21.599 degrees, +16.485 degrees, +12.628 degrees, +8.800 degrees, +3.754 degrees, and 0.0 degrees, respectively. The spot shape on the photosensitive drum surface 8 at the time is shown.

FIG. 31 shows contour lines of the spot intensity distribution similar to FIG. 14. In Fig. 31, the contour lines show the intensity sliced at the levels of 0.02, 0.05, 0.1, 0.1353, 0.3679, 0.5, 0.75 and 0.9 (from the outside) when the peak intensity of the focus spot is normalized to 1, respectively.

Although a large number of side lobes in the main scanning direction are generated at the scanning end portion (+22.856 degrees) on the scan surface, the spot shape is better than the spot shape shown in FIG.

In addition, since the peak intensity of the side lobes does not exceed 0.05 (i.e., 5% of the peak intensity of the main focusing spot), severe image degradation will not be caused. However, the spot shapes at deflection angles +22.856 degrees and +21.599 degrees are not very good. In consideration of this, in the present embodiment, as in the first embodiment, the amount of wave front aberration generated as a result of the deformation of the deflection surface 166a is corrected by the f-?

Here, in the present embodiment, first, δL1 ₊ denotes an optical path length difference between one main ray (upper ray) and the main ray of one of the beams reflected by the deflection plane at an effective deflection angle of +22.85 degrees of the deflection plane. The optical path length difference is generated as a result of the light beam being reflected by the deflection surface. Secondly, δL1 ₋ is used to represent the optical path length difference between the other last light beam (lower light beam) and the main light beam of the light beam reflected by the deflection plane at an effective deflection angle of +22.85 degrees of the deflection plane, and the optical path length difference is The luminous flux is generated as a result of being reflected by the deflection surface.

Third, δL2 ₊ is used to represent the optical path length difference between one last light beam (upper light beam) and the main light beam of the beam reflected by the deflection plane at the effective deflection angle +22.85 degrees of the deflection plane, and the optical path length difference is The luminous flux is generated as a result of passing through the imaging optical system.

Here, the marginal light beam on the + side (upper side) with respect to the main scanning direction is the marginal light beam on the scanning line recording start position side (upper side in the page and opposite to the incident optical system LA in FIG. 19) with respect to the main beam of light beams. to be. The marginal light on the negative side of the main scanning direction refers to the scanning line recording start position on the scanning surface (the lower side in the page and the incident optical system LA side in FIG. 19) with respect to the main light ray. It's board rays.

Fourth, δL2 ₋ is used to represent the optical path length difference between the other last light beam (lower light beam) and the main light beam of the light beam reflected by the deflection plane at an effective deflection angle of +22.85 degrees of the deflection plane, and the optical path length difference is The luminous flux is generated as a result of passing through the imaging optical system.

At this time, the imaging optical system satisfies the following relation [18]:

....[18].

.... [18].

In this embodiment, the case where the effective deflection angle of the deflection surface is +22.85 degrees is described as an example, but the above equation [18] is satisfied. However, even when the effective deflection angle of the deflection surface is -22.85 degrees, the embodiment expresses the above formula [18]. Satisfied. In addition, in this embodiment, it is a matter of course that the above formula [18] is satisfied at all deflection angles within an effective deflection angle of ± 22.85 degrees.

From the above, it can be seen that the amount of wave front aberration generated by the deformation of the deflection surface can be significantly reduced.

Here, the "light beam reflected by the deflection plane at the effective deflection angle of the deflection plane" means the light beam reaching the scanning end (maximum image height) of the scanning line in the effective image area on the scan surface.

32 shows the spot shape on the photosensitive drum surface 8 in this embodiment. Compared with FIG. 31, it can be seen that the side lobes of the focusing spot are particularly well corrected at +22.856 degrees and +22.599 degrees.

On the other hand, as shown in Fig. 23, the angular velocity dθ ₁ / dt of the movable plate 171 of the optical deflector 166 in this embodiment has a constant angular velocity in a period of ± 0.14, but it is completely constant. no.

In the present embodiment, as described above, the error is corrected by the f-θ lens system 167. However, when the luminous flux that is not deflected at an isometric speed is corrected to be at a constant speed on the photosensitive drum surface 8, the spot diameter in the main scanning direction will change.

This change in the spot diameter in the main scanning direction on the scan surface is inversely proportional to the angular velocity dθ ₁ / dt of the movable plate 171 of the optical deflector 166. In consideration of this, the maximum vibration amplitudes φ ₁ , φ ₂ of the movable plate 171 in the mode 1 and the mode 2, the angular frequencies ω ₁ , ω ₂ , the phase difference, and the like are optimally selected. Therefore, by setting the change in the angular velocity in the effective scanning area to a minimum, it is possible to minimize the change in the spot diameter in the main scanning direction.

In the present embodiment, the maximum vibration amplitude φ ₁ of the movable plate 171 in the mode 1 is φ ₁ = 36.68757 °, and the angular frequency ω ₁ is ω ₁ = 2π × 2000 [H _z ].

The maximum vibration amplitude phi ₂ of the movable plate 171 in the mode 2 is phi ₂ = 5.61180 degrees, and the angular frequency ω ₂ is ω ₂ = 2π × 4000 [H _z ].

By setting each phase so as to have a difference of 180 ° from each other, as shown in Fig. 23, the change in the angular velocity in the effective scanning area is reduced. Thereby, the change of the spot diameter in the main scanning direction can be made small.

The uniformity of the spot diameter in the effective scanning area becomes particularly remarkable when it exceeds 10%. In view of this, in order to ensure that the change in the angular velocity dθ ₁ / dt of the movable plate 171 of the optical deflector 166 is suppressed to 10% or less in the effective scanning area, the movable plates in Mode 1 and Mode 2 It is preferable to optimally select the maximum vibration amplitude φ ₁ , φ ₂ of 171, and the angular frequencies ω ₁ , ω ₂ , and phase difference.

That is, the maximum value of the angular velocity of the deflection plane at any position in the effective scanning area is represented by (dθ ₁ / dt) max, and the minimum value of the angular velocity of the deflection plane at any position in the effective scanning area is (dθ _1). When expressed as / dt) min, the following conditions are preferable:

(dθ ₁ / dt) max / (dθ ₁ / dt) min <1.1.

Fig. 33 shows the spot diameter in the main scanning direction on the photosensitive drum surface 8 in this embodiment. In this embodiment, the maximum value of the spot diameter in the main scanning direction of the focusing spot in the same scanning line in the effective scanning area on the scan surface 8 is represented by φn ₁ , and the focusing in the same scanning line in the effective scanning area on the scanning surface 8 is indicated. When the minimum value of the spot diameter in the main scanning direction of the spot is represented by φn ₀ , the following relational expression [19] is satisfied:

φn ₁ / φn ₀ <1.1 .... [19]

More preferably, it is necessary to satisfy the following relation [20]:

φn ₁ / φn ₀ <1.5 .... [20].

Therefore, the spot diameter in the main scanning direction of the focus spot in this embodiment is varied from 67.27 µm to 69.17 µm, and the change in diameter is maintained at about 2.8%. As a result, it is possible to achieve an optical scanning device or an image forming apparatus that requires high quality image output.

In addition, since the optical deflector having the reciprocating motion uses the optical deflector in this embodiment, when the image is formed in both the path and the path, the inclination of the scanning line on the photosensitive drum surface 8 as shown in FIG. Since it changes alternately, a pitch nonuniformity is caused in an image edge part. In consideration of this, in the optical scanning apparatus of the present embodiment, image formation can be performed during scanning of one of the reciprocating strokes.

In this case, however, the scanning efficiency is reduced by half. Therefore, in order to eliminate this, it is preferable to use a monolithic multiple beam semiconductor laser or the like having a plurality of light emitting points (light emitting portions) as the light source means.

Tables 2A and 2B below show the specifications of the scanning optical system of this embodiment.

Reference wavelength used l (nm) 780 Number of flash points n One Position of light emitting point x0 (mm) -29.38709 y0 (mm) -75.99937 z0 (mm) -3.57057 Semiconductor Laser Cover Glass Refractive Index n0 1.51072 Semiconductor laser cover glass thickness deg (mm) 0.25 Aperture Position x1 (mm) -17.80914 y1 (mm) -55.94578 z1 (mm) -2.76195 Shape of aperture Oval Main scan 2.4 mm x Sub scan 1.72 mm Distance between light emitting point and collimator lens first surface d0 (mm) 23.67000 Collimator Lens First Side Position x2 (mm) -17.55930 y2 (mm) -55.51303 z2 (mm) -2.74450 Collimator Lens Second Surface Position x3 (mm) -16.55991 y3 (mm) -53.78204 z3 (mm) -2.67470 Collimator lens thickness d1 (mm) 2.00000 Collimator Lens Refractive Index n1 1.76203 Collimator lens first surface curvature radius R1 (mm) 182.21200 Collimator Lens Second Surface Curvature Radius R2 (mm) -20.83080 Distance between collimator lens second surface to cylindrical lens first surface d2 (mm) 19.76000 Cylindrical lens first side position x4 (mm) -6.68592 y4 (mm) -36.67980 z4 (mm) -1.98508 Cylindrical lens second side position x5 (mm) -3.68775 y5 (mm) -31.48681 z5 (mm) -1.77569 Cylindrical lens thickness d3 (mm) 6.00000 Cylindrical lens refractive index n2 1.51072 Cylindrical lens first face sub-scan direction curvature radius Rs3 (mm) 26.99300 Cylindrical lens surface curvature radius Rm3 (mm) infinity Cylindrical lens curvature radius R4 (mm) infinity Distance between cylindrical lens second surface to optical path folding mirror d4 (mm) 36.38000 Fiber path folding mirror position x6 (mm) 14.49117 y6 (mm) 0.00000 z6 (mm) -0.50604 Optical Fiber Folding Mirror Curvature Radius R5 (mm) infinity Optical path folding mirror to deflection reflecting distance d5 (mm) 14.50000 Deflection Reflective Position x6 (mm) 0.00000 y6 (mm) 0.00000 z6 (mm) 0.00000 Deflection reflecting surface to distance between first f-θ lens first surface d6 (mm) 24.50000 First f-θ lens first surface position x6 (mm) 24.48508 y6 (mm) 0.00000 z6 (mm) 0.85504 First f-θ lens second surface position x7 (mm) 32.48020 y7 (mm) 0.00000 z7 (mm) 1.13423 First f-θ lens thickness d7 (mm) 8.00000 First f-θ Lens Refractive Index n3 1.52420 Distance between the first surface of the first f-θ lens and the first surface of the second f-θ lens d8 (mm) 15.00000 2nd f-θ lens first surface position x8 (mm) 47.47106 y8 (mm) 0.00000 z8 (mm) 1.16583 2nd f-θ lens second surface position x9 (mm) 54.46769 y9 (mm) 0.00000 z9 (mm) 0.94852 2nd f-θ lens thickness d9 (mm) 7.00000 2nd f-θ lens refractive index n4 1.52420 Distance between the second surface of the second f-θ lens and the surface to be scanned d10 (mm) 174.90596 Pitch Position x10 (mm) 174.87914 y10 (mm) 0.00000 z10 (mm) 3.06292 f-θ lens main scanning direction focal length f (mm) 135.81017 Incident angle of incidence optical system (scanning cross section) γ (degrees) 120.00000 Incident optical system diagonal inclination angle (sub-scan section) β (degrees) 2.00000 1st f-θ lens upward angle (sub-scan section) δ (degrees) 2.00000 1st f-θ lens downward angle (sub-scan section) η (degrees) 1.77899 Optical deflector maximum scanning angle ζ (degrees) 38.24400 Optical Deflector Effective Injection Angle ξ (degrees) 22.85590 Optical deflector resonant frequency f0 (㎑) 2.00000 Optical deflector deflection reflector size Rectangle Main scan 3mm x Sub scan 1mm (thickness 0.2mm)

First f-θ Lens Shape Front page Page 2 R -60.37653 R -35.88049 k -5.59958E + 00 k -2.83241E + 00 B4 3.39474E + 00 B4 -4.40767E-07 B6 -1.32326E-10 B6 1.94101E-09 B8 -1.08146E-13 B8 3.41840E-13 B10 0.00000E + 00 B10 0.00000E + 00 r -62.30060 r -59.33670 D2 2.59900E-03 D2 -1.54472E-04 D4 2.16896E-05 D4 -1.88223E-06 D6 -8.62574E-10 D6 2.55867E-09 D8 -1.85569E-11 D8 7.35046E-13 D10 8.53487E-14 D10 0.00000E + 00 2nd f-θ lens shape Front page Page 2 R 75.60110 R 80.18178 k -1.21062E + 00 k -1.67582E + 01 B4 -5.06980E-06 B4 -3.64676E-06 B6 2.31400E-09 B6 1.37576E-09 B8 -7.43803E-13 B8 -3.93865E-13 B10 9.66145E-17 B10 3.12718E-17 r -37.45820 r -13.93870 D2 3.76861E-03 D2 1.33908E-03 D4 3.09223E-06 D4 -1.17051E-06 D6 4.00640E-10 D6 7.47398E-10 D8 -4.29864E-13 D8 -2.66160E-13 D10 3.90865E-17 D10 3.74682E-17

Each coefficient in Table 2b has the same meaning as the various coefficients described in the first embodiment.

For the aspherical shape in the main scanning section of the f-θ lens, the intersection point between each lens surface and the optical axis is taken as the origin, and the axis perpendicular to the optical axis in the X axis and the main scanning section in the optical axis direction is provided in the Y and sub scanning sections. The axis orthogonal to the optical axis is taken as the Z axis.

Here, the following relation [21] is given:

...[21]

... [21]

(Wherein R is the radius of curvature and k and B ₄ to B ₁₀ are aspherical coefficients).

In addition, the shape of the sub-scan section is such that the radius of curvature r 'when the lens plane coordinate in the main scanning direction is Y can be given by the following formula [22]:

....[22]

.... [22]

(Wherein r is the radius of curvature on the optical axis and D ₂ to D ₁₀ are the respective coefficients).

As for the non-circular shape of the main scanning cross sections of the f-θ lenses 161 and 162, the number of surfaces of the optical surface (lens surface) constituting the f-θ lens system is m, and the surface shape in the main scanning cross section of each optical surface is m. Formula [23]:

...[23]

... [23]

Expressed by the following conditional expression [24]:

...[24]

... [24]

Wherein U _j takes U _j = -1 when the optical plane is a transmissive plane and a light incidence plane, U _j = +1 when the optical plane is a transmissive plane and a light exit plane, and U _j = +1 when the optical plane is a reflective plane and the optical plane is a reflective plane If U _j = +1. N _j is the same as the refractive index of the glass material when the optical surface is a transmissive surface, and N _j = 2 when the optical surface is a reflective surface.

Y> 0 Front page Page 2 Page 3 Page 4 Scanning end marginal light-pass Y coordinate 23.8256 26.0248 44.5711 45.7894 Y-pass Y coordinate 22.2286 24.6691 43.1176 44.5121 Marginal Y-coordinate at the center of scan 20.6504 23.3226 41.6516 43.2030 dx / dy (out) -0.12743 -0.39364 -0.19294 -0.56148 dx / dy (up) -0.14644 -0.41363 -0.15573 -0.49107 dx / dy (in) -0.16058 -0.42475 -0.12032 -0.42688 U -One One -One -One N 1.52420 1.52420 1.52420 1.52420 U (N-1) (dx / dy (out) + dy / dy (in) -2dx / dy (p)) -0.00255 0.00465 0.00095 0.00326 Conditional Expression [24] Left Side 0.00631

Y <0 Front page Page 2 Page 3 Page 4 Scanning end marginal light-pass Y coordinate -23.8256 -26.0248 -44.5711 -45.7894 Y-pass Y coordinate -22.2286 -24.6691 -43.1176 -44.5121 Marginal Y-coordinate at the center of scan -20.6504 -23.3226 -41.6516 -43.2030 dx / dy (out) 0.12743 0.39364 0.19294 0.56148 dx / dy (up) 0.14644 0.41363 0.15573 0.49107 dx / dy (in) 0.16058 0.42475 0.12032 0.42688 U -One One -One -One N 1.52420 1.52420 1.52420 1.52420 U (N-1) (dx / dy (out) + dy / dy (in) -2dx / dy (p)) 0.00255 -0.00465 -0.00095 -0.00326 Conditional Expression [24] Left Side -0.00631

From these tables, according to the present embodiment, when Y> 0, the value on the left side of the conditional expression [24] is positive (+), and when Y <0, it becomes negative (-) to ensure that the conditional expression [24] is satisfied. I can see that there is.

According to this embodiment, by satisfying the conditional expression [24], as shown in Fig. 16, positively generated wavefront aberration equal to wavefront aberration generated by the deformation of the deflection surface 6a is actively generated in the reverse direction (offset direction). Let's do it. According to this configuration, the wave front aberration generated as a result of the deformation of the deflection surface 6a is effectively reduced, and high quality image output is achieved.

[ Example  3]

Fig. 35 shows a cross section (main scanning cross section) of the main part in the main scanning direction of the third embodiment of the present invention. In Fig. 35, elements corresponding to those shown in Fig. 18 are denoted by the same reference numerals.

This embodiment is different from the second embodiment in that the incident optical system LA is configured without the cylindrical lens 4. In addition, in the first and second imaging lenses (f-θ lenses) 181 and 182 constituting the f-θ lens system 187, the deformation in the main scanning direction of the deflection surface 166a is deflected surface 166a. It is designed in consideration of being changeable by the position in the sub-scan direction of ().

The remaining part has the same construction and optical action as in Example 2, giving similar advantageous results.

As described above, the optical deflector 166 based on the reciprocating motion as in the present embodiment does not need to adopt a planar tilt correction optical system because there is only one deflection surface 166a.

Therefore, in the present embodiment, the deflection surface 166a and the photosensitive drum surface 8 are conjugated to each other by the f-θ lens system 187 in the sub-scan section. That is, no planar tilt correction optical system is used here. In addition, no cylindrical lens or the like is disposed between the deflection surface 166a and the light source means 1.

The configuration and principle of the optical deflector 166 of this embodiment are as described with reference to the second embodiment.

The deflection surface 166a of the optical deflector 166 is deformed in the main scanning cross section (main scanning direction) due to the angular acceleration due to the reciprocating motion, and the amount of deformation depends on the position in the sub scanning direction of the deflection surface 166a. Is changing.

36 is a graph showing the results of calculating the deformation of the movable plate 171 of the present embodiment by the finite element method.

Specifically, FIG. 36 is a diagram where the movable plate 171 constituting the optical deflector 166 has a period of 0.14 (45 degrees at the scanning angle of the light beam) when the movable plate 171 constituting the optical deflector 166 is resonantly driven according to the above formula [15]. The deformation | transformation of AA cross section (center part of sub-scanning direction) in 19 is shown.

Here, the inclination of the connecting portion (part (C) of FIG. 19) between the torsion spring 173 and the movable plate 171 is zero. The horizontal axis represents the position coordinates of the movable plate 171 (unit is μm), and the vertical axis represents the deformation amount y (unit is μm) of the movable plate 171.

Similarly, FIG. 37 is a graph of the result of calculating the deformation | transformation of the B-B cross section (position 0.9 mm away from the center part of a subscanning direction toward a subscanning direction) in FIG. Similarly, the inclination of the connecting portion (part (C) of FIG. 19) between the torsion spring 173 and the movable plate 171 is zero.

36 and 37 show that the deformation amounts of the A-A cross section and the B-B cross section are different.

38 is a schematic perspective view of the deformation amount three-dimensionally. The direction of the movable plate position x in FIG. 37 corresponds to the main scanning direction, and the direction of the movable plate position z corresponds to the sub scanning direction.

As can be seen from FIG. 38, the cross section along the main scanning direction is deformed as shown in FIGS. 36 and 37. In addition, since the deformation amount is changed depending on the position in the sub-scan direction of the movable plate 171, there is also a deformation in the sub-scan section.

In FIG. 38, if the negative direction of the movable plate position x and the upper side of the deflection surface 166a in FIG. 35 coincide, the dice shown in FIG. 38 generate | occur | produce the dice shown in FIG. It corresponds to the maximum effective scanning position on the side far from the light source side. That is, the dice having the deformation shown in Fig. 38 correspond to the maximum effective scanning position on the opposite side to the side on which the light source means 1 is disposed around the optical axis of the imaging optical system 187 in the main scanning cross section. This corresponds to the deformation of the deflection plane at the effective deflection angle of +22.5 degrees.

As can be seen from FIG. 38, the deflection surface 166a is deformed into a concave surface shape in the z-direction cross section (sub-scan section) on the negative side of the movable plate position x, and the positive position of the movable plate position x In the z-direction cross section (sub-scan section) on the (+) side, it is deformed into a convex surface shape.

In addition, although not shown about the deformation | transformation of the deflection surface at effective deflection angle -22.5 degree | times, the deflection surface 166a is a convex surface in the z direction cross section (sub-scan section) of the negative side of the movable plate position x. On the contrary, in the z-direction cross section (sub-scan section) on both sides of the movable plate position x, it is deformed into a concave surface shape.

In the present embodiment, the deflection surface 166a and the photosensitive drum surface 8 are not conjugated by the f-θ lens system 187 in the sub-scan section. That is, no tilt correction optical system is provided. Therefore, the luminous flux incident on the deflection surface 166a has a desired luminous flux width in the main and sub-scanning directions, respectively.

In the case of using a normal tilt correction optical system, the light beam converged in the sub-scanning direction is incident on the deflection surface. In this case, the light beam width in the sub-scan direction on the deflection surface is generally about 0.1 mm or less.

On the other hand, when no inclination correction optical system is provided, the luminous flux width in the sub-scan direction on the deflection surface requires a luminous flux width determined by the spot diameter on the photosensitive drum surface 8. In the present embodiment, the light beam width on the deflection surface 166a is 2.4 mm in the main scanning direction and 1.72 mm in the sub scanning direction.

In the present embodiment, as shown in Figs. 36 to 38, the movable plate 171 is deformed by its own weight. As a result, the light beam reflected by the deflection surface 166a generates wavefront aberration twice as large as the deformation amount y shown in FIGS. 36 to 38. Therefore, the focusing spot on the photosensitive drum surface 8 will be adversely affected.

In the optical scanning device having a rotating polygonal mirror as an optical deflector, since the rotating polygonal mirror rotates at a constant angular velocity, no deformation as shown in Figs. 36 to 38 occurs, so that such wavefront aberration does not usually occur.

For these reasons, deformation of the deflection surface is often not particularly taken into consideration when designing an imaging lens used in a light scanning apparatus having a rotating multifaceted mirror.

However, when the optical deflector 166 based on resonance driving is used in combination with the imaging lens designed in this way (that is, without considering the deformation of the deflection surface), the wave front aberration generated by the deformation of the deflection surface 166a is used. This causes the focus spot to deteriorate.

Fig. 39 shows the spot shape on the photosensitive drum surface 8 when the deflection surface 166a is actually deformed in the deflection surface 166a using an imaging lens designed on the assumption that the deflection surface 166a is not deformed. .

Specifically, FIG. 39 shows on the photosensitive drum surface 8 when the respective deflection angles of the light beams deflected by the deflection surface 166a are +22.5 degrees, +21.028 degrees, 0 degrees, -21.028 degrees, and -22.5 degrees, respectively. The shape of the spot is shown. Each contour line in FIG. 39 corresponds to the intensity sliced to the levels of 0.02, 0.05, 0.1, 0.1353, 03679, 0.5, 0.75 and 0.9 (from the outside), respectively, when the peak intensity of the focus spot was normalized to 1.

In Fig. 39, the horizontal direction corresponds to the main scanning direction in which the spot scans the surface, and the vertical direction corresponds to the sub scanning direction orthogonal to the main scanning direction.

As can be seen from FIG. 39, the side lobe is generated in the main scanning direction and the diagonal direction in the shape of the spot when the deflection surface 166a is deformed. In addition to this, the outer shape itself of the focusing spot is also deformed into a barrel shape, and the shape of the focusing spot is severely deteriorated.

It is well known that the image quality decreases as the peak intensity of the side lobe increases. In particular, when the shape of the focusing spot is in the shape of a barrel like the shape of both ends in Fig. 39, the reproducibility of the diagonal lines is severely deteriorated. This is undesirable for an optical scanning device or an image forming apparatus that requires high quality image output.

As described above, when the optical deflector based on the resonance driving is used in combination with the imaging lens designed without considering the deformation of the deflection surface, the focus spot is deteriorated by the wave front aberration caused by the deformation of the deflection surface 166a. Will be. As a result, it becomes very difficult to achieve an optical scanning device or an image forming apparatus that requires high quality image output.

In view of the above problem, in this embodiment, the amount of wavefront aberration generated by the distorted deflection surface 166a as shown in Figs. 36 to 38 due to the resonance driving of the f-? Lens system 187 is f. The amount is reduced after passing through the -θ lens system 187.

Fig. 40 shows the shape of the spot formed on the photosensitive drum surface 8 in this embodiment of the present invention.

In this embodiment, the f-? Lens system 187 reduces the amount of wave front aberration generated by the deflection plane 166a distorted under angular acceleration in response to the resonance drive. Therefore, it can be seen from FIG. 40 that the side lobe is reduced compared to the spot shape shown in FIG. 39 and the spot outline itself is improved.

In the present embodiment, " first direction " is one phase difference of wavefront aberration in the main scanning direction between the main light beam and the last light beam of the light beam reflected by the deflection surface 166a at the effective deflection angle of the deflection surface 166a. The phase difference at this time is generated as a result of the light beam reflected by the deflection surface. Alternatively, the "first direction" may mean the direction of the optical path length difference of the last light beam in the main scanning direction with respect to the main light beam of the luminous flux reflected by the deflection surface 166a, wherein the difference in the optical path length is that the luminous flux is the deflection. It is generated as a result of reflecting the surface.

Further, the "second direction" is the phase difference of the other phase of the wavefront aberration in the main scanning direction between the last light beam and the main light beam of the light beam reflected by the deflection surface 6a at the effective deflection angle of the deflection surface 6a. The phase difference at this time is generated as a result of the light beam passing through the f-θ lens system 187. Alternatively, the "second direction" may mean the direction of the optical path length difference in the main scanning direction of the final beam relative to the main beam of the luminous flux reflected by the deflection surface 166a, wherein the optical path length difference is It is generated as a result of passing through the imaging optical system.

At this time, in the present embodiment, in order to ensure that the first direction and the second direction are opposite to each other, the non-circular optical surface is formed in the main scanning cross section in at least one optical system of the f-θ lens system 187. At least one is installed.

The optical principle is as shown in Example 1 described above (see Figs. 12 to 17). As a result, in this embodiment, it is possible to reduce the amount of wave front aberration caused by the deformation of the deflection surface 166a.

As described above, in this embodiment, the wave front aberration generated by the deformation of the deflection surface 166a of the optical deflector 166 based on the sinusoidal driving as described above is caused by the deformation by the f-θ lens system 187. It is actively generating wave aberration equivalent to wave front aberration.

Here, in the present embodiment, firstly, δL1 ₊ denotes an optical path length difference between one last light beam (upper light beam) and the main light beam of the light beam reflected by the deflection plane at the effective deflection angle +22.5 degrees of the deflection angle. The optical path length difference at this time is generated as a result of the light beam being reflected by the deflection surface. Second, δL1 ₋ is used to indicate the optical path length difference between the other last light beam (lower light beam) and the main light beam of the light beam reflected by the deflection plane at the effective deflection angle of +22.5 degrees of the deflection plane. The difference is generated as a result of the luminous flux reflected by the deflection plane.

Thirdly, δL2 ₊ is used to represent the optical path length difference between one last light beam (upper light beam) and the main light beam of the beam reflected by the deflection plane at the effective deflection angle +22.5 degrees of the deflection plane, and the optical path length at this time The difference is generated as a result of the luminous flux passing through the imaging optical system. Fourth, δL2 ₋ is used to represent the optical path length difference between the other last light beam (lower light beam) and the main light beam of the light beam reflected by the deflection plane at the effective deflection angle of +22.5 degrees of the deflection plane. The difference is generated as a result of the luminous flux passing through the imaging optical system.

At this time, the imaging optical system satisfies the following relation [25]:

....[25].

.... [25].

According to the above configuration, the amount of wavefront aberration generated by the deformation of the deflection surface 166a can be significantly reduced.

In the present embodiment, the case where the effective deflection angle of the deflection surface is +22.5 degrees is described as an example, but the above formula [25] is satisfied. However, even when the effective deflection angle of the deflection surface is -22.5 degrees, the embodiment expresses the above formula [25]. Satisfy. Incidentally, in the present embodiment, the above equation [25] is, of course, satisfied at the total deflection angle within an effective deflection angle of ± 22.5 degrees.

Next, the wavefront aberration in the sub-scan section will be described in more detail.

FIG. 41: is a figure which shows typically the state of the wavefront after the parallel light beam which entered the dice corresponding to the light beam passed through the f-theta lens system 187. FIG.

The luminous flux incident on the f-θ lens system 187 is parallel in the main scanning cross section and in the sub scanning cross section.

The x-axis of FIG. 41 corresponds to the x-axis of FIG. 38, the y-axis of FIG. 41 corresponds to the y-axis of FIG. 38, and the z-axis of FIG. 41 corresponds to the z-axis of FIG. This also applies to the x and y axes of FIG. 35 as well.

Specifically, FIG. 41 shows a wavefront in the sub-scanning direction after the parallel light flux when the sign of the deflection angle is + has passed through the f-θ lens system 187. That is, in the main scanning cross section, the light passes through the f-θ lens system 187 of the luminous flux reaching the dice between the scanning center on the scan surface and the maximum effective scanning position (maximum image height) on the scan surface toward the recording start position. The wavefront in the sub-scanning direction is shown.

The scanning center and the recording start position on the scan surface mean the upper side of FIG. 35 and the opposite side of the incident optical system LA.

In FIG. 41, ro represents the radius of curvature of the wavefront in the sub-scanning direction at the position of the light ray (main-light ray) in the center of the main scanning direction after the parallel light flux passes through the f-θ lens system 187. Similarly, ru is the radius of curvature of the wavefront in the sub-scanning direction at the light beam position on the scanning end side (x direction is + side) with respect to the main light beam in the main scanning direction after the parallel light beam passes through the f-θ lens system 187. It is shown. Similarly, rl is the radius of curvature of the wavefront in the sub-scan direction at the light beam position on the scanning center side (x direction is-side) with respect to the main light beam in the main scanning direction after the parallel light beam passes through the f-θ lens system 187. It is shown.

In this embodiment, as shown in FIG. 41, the curvature of the wavefront in the sub-scan direction of the light beam at the light transmissive position on the scanning end side with respect to the main light beam of the light beam after the parallel light flux passes through the f-θ lens system 187 The radius ru is set as follows.

That is, the curvature radius ru of the wavefront in the sub-scan direction of the light beam at the light beam passing position on the scanning end side with respect to the main beam of the light beam It is larger than the curvature radius rl of the wavefront. That is, rl <ru.

Here, the "scanning end side" means the + side in the x direction, and the "scanning center side" means the-side in the x direction.

By making the f-θ lens system 187 in such a shape, the wavefront is deformed into a concave shape in the z-direction cross section (sub-scan section) of the movable plate position x as shown in FIG. Moreover, in the z direction cross section (sub-scan section) of the + side of the movable plate position x, it deform | transforms into convex surface shape conversely. As a result of these deformations, the wavefront aberrations generated can be effectively corrected.

On the other hand, the wavefront in the sub-scanning direction after the parallel light flux when the sign of the deflection angle is-passes through the f-θ lens system 187 is as follows.

The radius of curvature ru 'of the wavefront in the sub-scan direction of the light beam at the beam passing position on the scanning center with respect to the main beam of the light beam Is smaller than the radius of curvature rl '. Rl '> ru'.

That is, in the main scanning section, the luminous flux reaching the dice between the scanning center on the scanning surface and the maximum effective scanning position (maximum image height) on the scanning surface toward the recording start position passes through the f-θ lens system 187. The cross section in the subsequent sub-scanning direction satisfies the relation of rl '> ru'.

Here, the "scanning center side" means the x direction + side of FIG. 41, and the "scanning end side" means the x direction-side of FIG.

As described above, according to this embodiment of the present invention, even if the optical deflector 166 based on the resonance driving is used, the optical scanning that requires high-quality image output and high quality image output without deterioration of image quality is required. It is possible to achieve an optical apparatus or an image forming apparatus.

Tables 3A and 3B below show the specifications of the scanning optical system in this example.

Reference wavelength used l (nm) 780 Number of flash points n One Position of light emitting point x0 (mm) -29.38709 y0 (mm) -75.99937 z0 (mm) -3.57057 Semiconductor Laser Cover Glass Refractive Index n1 1.51072 Semiconductor laser cover glass thickness deg (mm) 0.25 Aperture Position x1 (mm) -17.80914 y1 (mm) -55.94578 z1 (mm) -2.76195 Shape of aperture Oval Main scan 2.4 mm x Sub scan 1.72 mm Distance between light emitting point and collimator lens first surface d1 (mm) 23.67000 Collimator Lens First Side Position x2 (mm) -17.55930 y2 (mm) -55.51303 z2 (mm) -2.74450 Collimator Lens Second Surface Position x3 (mm) -16.55991 y3 (mm) -53.78204 z3 (mm) -2.67470 Collimator lens thickness d2 (mm) 2.00000 Collimator Lens Refractive Index n2 1.76203 Collimator lens first surface curvature radius R2 (mm) 182.21200 Collimator Lens Second Surface Curvature Radius R3 (mm) -20.83080 Distance between collimator lens second side to optical path folding mirror d3 (mm) 62.14000 Fiber path folding mirror position x4 (mm) 14.49117 y6 (mm) 0.00000 z6 (mm) -0.50604 Optical Fiber Folding Mirror Curvature Radius R4 (mm) infinity Optical path folding mirror to deflection reflecting distance d4 (mm) 14.50000 Deflection Reflective Position x5 (mm) 0.00000 y6 (mm) 0.00000 z6 (mm) 0.00000 Deflection reflecting surface to distance between first f-θ lens first surface d5 (mm) 24.50000 First f-θ lens first surface position x6 (mm) 24.48508 y6 (mm) 0.00000 z6 (mm) 0.85504 First f-θ lens second surface position x7 (mm) 32.48020 y7 (mm) 0.00000 z7 (mm) 1.13423 First f-θ lens thickness d6 (mm) 8.00000 First f-θ Lens Refractive Index n6 1.52420 Distance between the first surface of the first f-θ lens and the first surface of the second f-θ lens d7 (mm) 15.00000 2nd f-θ lens first surface position x8 (mm) 47.47106 y8 (mm) 0.00000 z8 (mm) 0.67495 2nd f-θ lens second surface position x9 (mm) 54.46881 y9 (mm) 0.00000 z9 (mm) 0.49723 2nd f-θ lens thickness d8 (mm) 7.00000 2nd f-θ lens refractive index n8 1.52420 Distance between the second surface of the second f-θ lens and the surface to be scanned d9 (mm) 119.93561 Pitch Position x10 (mm) 173.74492 y10 (mm) 0.00000 z10 (mm) 4.26525 f-θ lens main scanning direction focal length f (mm) 136.23663 Incident angle of incidence optical system (scanning cross section) γ (degrees) 120.00000 Incident optical system diagonal inclination angle (sub-scan section) β (degrees) 2.00000 1st f-θ lens upward angle (sub-scan section) δ (degrees) 2.00000 2nd f-θ lens downward angle (sub-scan section) η (degrees) 1.45477 Optical deflector maximum scanning angle ζ (degrees) 38.24400 Optical Deflector Effective Injection Angle ξ (degrees) 22.50000 Optical deflector resonant frequency f0 (㎑) 2.00000 Optical deflector deflection reflector size Rectangle Main scan 3.2mm x Sub scan 2mm (thickness 0.2mm)

First f-θ Lens Shape Front page Page 2 R -62.03226 R -35.51636 k -5.72820E + 00 k -2.52967E + 00 B4 3.85813E-06 B4 -3.83325E-07 B6 -1.16907E-10 B6 2.55048E-09 B8 -1.52913E-13 B8 3.79417E-13 B10 3.97146E-19 B10 0.00000E + 00 r -60.13640 r -50.24600 D2 -2.13334E-05 D2 2.62162E-04 D4 2.24995E-07 D4 -5.70578E-08 D6 0.00000E + 00 D6 -1.71022E-10 D8 1.95034E-12 D8 3.16804E-12 D10 0.00000E + 00 D10 -1.43556E-15 2nd f-θ lens shape Front page Page 2 R 77.38909 R 77.00424 k -1.06823E + 00 k -1.44697E + 01 B4 -4.82366E-06 B4 -3.14157E-06 B6 2.28120E-09 B6 1.22380E-09 B8 -7.52588E-13 B8 -3.43288E-13 B10 9.70343E-17 B10 2.44857E-17 r -78.36720 r -40.90430 D2 -5.14090E-06 D2 2.66485E-06 D4 2.18649E-07 D4 -6.86574E-09 D6 6.49609E-12 D6 1.38227E-14 D8 -2.66003E-14 D8 -2.25877E-16 D10 -2.40929E-18 D10 -3.16679E-19

Each coefficient in Table 3b has the same meaning as the various coefficients described in the first embodiment.

As for the aspherical surface shape of the main scanning sections of the f-θ lenses 181 and 182, the intersection point of each lens surface and the optical axis is taken as the origin, and the optical axis direction is the X axis, and the axis perpendicular to the optical axis in the main scanning section is the Y axis, An axis perpendicular to the optical axis is taken as the Z axis in the sub-scan section.

Here, the following relation [26] is given:

...[26]

... [26]

(Wherein R is the radius of curvature and k and B ₄ to B ₁₀ are aspherical coefficients).

In addition, the shape of the sub-scan section is such that the radius of curvature r 'when the lens plane coordinate in the main scanning direction is Y can be given by the following equation [27]:

....[22]

.... [22]

(Wherein r is the radius of curvature on the optical axis and D ₂ to D ₁₀ are the respective coefficients).

For the non-circular shape of the main scanning cross section of the f-θ lens, the number of planes of the optical surface (lens surface) constituting the f-θ lens system is m, and the surface shape in the main scanning cross section of each optical surface is expressed by the following formula [28]:

...[28]

... [28]

When expressed by the following conditional expression [29] is satisfied:

...[29]

... [29]

Wherein U _j takes U _j = -1 when the optical plane is a transmissive plane and a light incidence plane, U _j = +1 when the optical plane is a transmissive plane and a light exit plane, and U _j = +1 when the optical plane is a reflective plane and the optical plane is a reflective plane If U _j = +1. N _j is the same as the refractive index of the glass material when the optical surface is a transmissive surface, and N _j = 2 when the optical surface is a reflective surface.

Tables 3c and 3d below show the numerical values of the present Example and the left side of the formula [29].

Y> 0 Front page Page 2 Page 3 Page 4 Scanning end marginal light-pass Y coordinate 23.5380 25.5722 43.6421 45.5000 Y-pass Y coordinate 21.9195 24.2074 42.1213 44.0806 Marginal Y-coordinate at the center of scan 20.3295 22.8617 40.6177 42.6643 dx / dy (out) -0.10139 -0.37430 -0.13716 -0.40843 dx / dy (up) -0.12419 -0.40088 -0.09961 -0.34251 dx / dy (in) -0.14142 -0.41644 -0.06593 -0.28538 U -One One -One -One N 1.52420 1.52420 1.52420 1.52420 U (N-1) (dx / dy (out) + dy / dy (in) -2dx / dy (p)) -0.00292 0.00578 0.00203 0.00460 Conditional Expression [29] Left Side 0.00949

Y <0 Front page Page 2 Page 3 Page 4 Scanning end marginal light-pass Y coordinate -23.5380 -25.5722 -43.6421 -45.5000 Y-pass Y coordinate -21.9195 -24.2074 -42.1213 -44.0806 Marginal Y-coordinate at the center of scan -20.3295 -22.8617 -40.6177 -42.6643 dx / dy (out) 0.10139 0.37430 0.13716 0.40843 dx / dy (up) 0.12419 0.40088 0.09961 0.34251 dx / dy (in) 0.14142 0.41644 0.06593 0.28538 U -One One -One -One N 1.52420 1.52420 1.52420 1.52420 U (N-1) (dx / dy (out) + dy / dy (in) -2dx / dy (p)) 0.00292 -0.00578 -0.00203 -0.00460 Conditional Expression [29] Left Side -0.00949

From these tables, it can be seen that according to the present embodiment, when Y> 0, the value on the left side of equation [29] is positive, and when Y <0, it becomes negative, which satisfactorily satisfies equation [29].

According to this embodiment, by satisfying the formula [29], the positive wavefront aberration equivalent to the wavefront aberration generated by the deformation of the deflection surface 166a is actively generated in the reverse direction (offset direction). According to this configuration, the wavefront aberration caused by the deformation of the deflection surface 166a is effectively reduced, and image output of high quality is achieved.

In addition, in the present embodiment, as the optical deflector, as described above, a resonant optical deflector in which a system composed of a plurality of movable plates and a torsion spring is driven and controlled to simultaneously oscillate at a frequency of a reference frequency and an integer multiple thereof is used. Is not limited to this.

For example, this invention is effective also in the system using the optical deflector based on simple shaping | vibration.

Incidentally, in the present embodiment, the imaging optical system 187 is composed of two lenses. However, the imaging optical system 187 is not limited to this. The diffraction optical element may also be included in the imaging optical system.

[ Of image forming apparatus Example ]

42 is a cross-sectional view of an essential part of a sub scanning direction of an image forming apparatus according to an embodiment of the present invention. In the figure, numeral 104 denotes an image forming apparatus.

Code data Dc is input to the image forming apparatus 104 from an external device 117 such as a personal computer. This code data Dc is converted into image data (dot data) Di by the printer controller 111 in the apparatus.

Subsequently, this image data Di is input to the optical scanning unit (i.e., optical scanning device) 100 constructed in accordance with any one of the above embodiments. From the optical scanning unit 100, a light beam (i.e., a light beam) 103 modulated in accordance with the image data Di is generated, and the photosensitive surface of the photosensitive drum 101 is moved in the main scanning direction by the light beam 103. Is injected.

The photosensitive drum 101 which is an electrostatic latent image bearing member (photosensitive member) is rotated in the clockwise direction by the motor 115. Through this rotation, the photosensitive surface of the photosensitive drum 101 is relatively moved with respect to the light beam 103 in the sub-scan direction perpendicular to the main scanning direction.

Directly above the photosensitive drum 101, a charging roller 102 for uniformly charging the surface of the photosensitive drum 101 is provided to contact the surface of the photosensitive drum. The light beam 103 scanned by the optical scanning unit 100 is projected onto the surface of the photosensitive drum 101 charged by the charging roller 102.

As described above, the light beam 103 is modulated according to the image data Di. The luminous flux 103 is irradiated to the photosensitive drum 101 to form an electrostatic latent image on the surface of the photosensitive drum 101. Next, the electrostatic latent image thus formed is developed as a toner image by the developing apparatus 107 in contact with the photosensitive drum 101 downstream of the rotation direction of the photosensitive drum 101 more than the irradiation position of the light beam 103. do.

The toner image developed by the developing apparatus 107 in this way is transferred paper (transfer material) 112 by a transfer roller (transfer device) 108 provided to face the photosensitive drum 101 at the bottom of the photosensitive drum 101. Is transferred to.

The transfer paper 112 is housed in the paper cassette 109 in front of the photosensitive drum 101 (right side in FIG. 42), but they may be supplied manually. A paper feed roller 110 is provided at the end of the paper cassette 109, and the paper 112 in the paper cassette 109 is supplied to the paper conveying path.

In this way, the paper 112 on which the unfixed toner image has been transferred is further conveyed to the fixing apparatus behind the photosensitive drum 101 (left side in FIG. 42). The fixing device includes a fixing roller 113 having a built-in fixing heater (not shown) and a pressure roller 114 installed to be pressed against the fixing roller 113. Then, the transfer paper 112 conveyed from the image transfer section is heated under pressure at the pressure-contacting portion between the fixing roller 114 and the pressure roller 114 to fix the unfixed toner image on the transfer paper 112.

In addition, a discharge roller 116 is provided behind the fixing roller 113, and the paper 112 on which the image is fixed is discharged out of the image forming apparatus.

Although not shown in FIG. 42, the printer controller 111 controls each component in the motor 115 or other image forming apparatus, an optical deflector in the optical scanning unit described later, etc. in addition to the data converting function described above. It has various functions.

The recording density of the image forming apparatus used in the present invention is not particularly limited. However, considering that the higher the recording density is, the higher quality is required, the configuration of Embodiments 1 to 3 of the present invention will be more effective when introduced into an image forming apparatus of 1200 dpi or higher.

[color Of image forming apparatus Example ]

43 is a schematic diagram of principal parts of a color image forming apparatus according to an embodiment of the present invention. This embodiment is a tandem type color image forming apparatus in which four optical scanning devices are arranged so as to record image information on the surface of a photosensitive drum (the support member) corresponding to each other in parallel.

In Fig. 43, reference numeral 60 denotes a color image forming apparatus, and 11, 12, 13, and 14 are optical scanning devices each having a configuration according to any one of the preceding embodiments, ( 21, 22, 23 and 24 are photosensitive drums or photosensitive members (supporting member), 31, 32, 33 and 34, respectively, developing apparatus, 51 Is the conveyance belt.

In FIG. 43, color signals of R (red), G (green), and B (blue) are input to the color image forming apparatus 60 from an external device 52 such as a personal computer. These color signals are converted into image data (dot data) corresponding to C (cyan), M (magenta), Y (yellow), and K (black) by the printer controller 53 in the apparatus.

These image data are input to the optical scanning devices 11, 12, 13, and 14, respectively. In response, the light beams 41, 42, 43 and 44 modulated according to the respective image data are generated from these optical scanning devices. By these light beams, the photosensitive surfaces of the photosensitive drums 21, 22, 23, and 24 are scanned in the main scanning direction.

The color image forming apparatus of this embodiment is provided with four optical scanning devices 11, 12, 13, and 14, which are C (cyan), M (magenta), Y (yellow), and K, respectively. It corresponds to each color of (black). These scanning devices operate in parallel with each other to record image signals on the photosensitive drums 21, 22, 23, and 24, so that color images can be printed at high speed.

In the color image forming apparatus of the present embodiment, as described above, the four optical scanning devices 11, 12, 13, and 14 use the luminous flux based on the respective image data to produce latent images of each color. They are formed on the corresponding photosensitive drums 21, 22, 23, and 24, respectively. Thereafter, these images are multiplied on the recording sheet to form one full color image.

As the external device 52, for example, a color image reading device equipped with a CCD sensor may be used. In this case, this color image reading apparatus and the color image forming apparatus 60 will provide a color digital copier.

According to the present invention, even when a reciprocating optical deflector is used, it is possible to provide an optical scanning device capable of significantly reducing deterioration of a focusing spot on a scanned surface, and an image forming apparatus having such optical scanning device.

In the above description, the present invention has been described with reference to the configuration disclosed in the present specification, but the present invention is not limited to the above-described detailed description, and the present application may be modified or changed within the scope of the following claims. It is intended to be covered.

Claims (21)

A light source means, deflection means configured to scan deflect the light flux from the light source means in the main scanning direction, and an imaging optical system configured to image the light beam deflected by the deflection surface of the deflection means onto the scan surface; In the optical scanning device, the deflection surface is configured to reciprocally scan the scan surface in the main scanning direction at a light beam deflected by the deflection surface of the deflection means by performing a reciprocating movement. As a result of reflection of the luminous flux by the deflection surface, between the main beam and the last light beam of the luminous flux reflected by the deflection surface at the effective deflection angle of the deflection surface corresponding to the maximum scanning position of the effective scanning area on the scan surface A first phase difference of wavefront aberration occurs in the main scanning direction; As a result of the passage of the light beam through the imaging optical system, a second phase difference of wavefront aberration occurs in the main scanning direction between the last light beam and the main light beam of the light beam reflected by the deflection plane at the effective deflection angle of the deflection plane; At least one optical surface constituting the imaging optical system is non-arc within the main scanning cross section so that the first phase difference and the second phase difference are opposite to each other. δ L1 ₊ is used to represent the optical path difference between the main beam of light beam reflected by the deflection plane and the last light beam at the scanning end side of the beam in the main scanning direction, and the optical path difference at this time is Generated as a result of reflection of the light beam by the deflection surface; δL1 ₋ is used to represent the optical path difference between the main beam of light beam reflected by the deflection plane and the final light beam toward the scanning center portion of the light beam at the effective deflection angle of the deflection plane, wherein the optical path difference is Occurs as a result of reflection of the light beam; [Delta] L2 ₊ is used to represent the optical path difference between the principal ray of the light beam reflected by the deflection surface at the effective deflection angle of the deflection surface and the final light beam at the scanning end side of the light beam, wherein the optical path difference is the image speed of the image formation. Generated as a result of passing through the optical system; δL2 ₋ is used to represent the optical path difference between the chief ray of the light beam reflected by the deflection surface at the effective deflection angle of the deflection surface and the final light beam toward the scanning center portion of the light beam, wherein the light path difference is the image speed of the image formation Occurs as a result of passing through an optical system; The optical imaging system characterized in that the imaging optical system satisfies the following relation:

. delete The number of optical surfaces constituting the imaging optical system is m, and the surface shape in the main scanning cross section of each optical surface is expressed as X = f (Y) (X is a function of Y). When the point of intersection between the optical plane and the optical axis of the imaging optical system is taken as the origin, the direction of the optical axis is taken as the X axis, and the axis orthogonal to the optical axis in the main scanning cross section is taken as the Y axis, the effective deflection angle of the deflection plane is as follows. Optical scanning device characterized by satisfying the conditions:

In the formula, U _j is the case of the optical surface is a transmission surface if the light input surface U _j = takes -1, the optical surface is a transmission surface when the light exit surface U = _j + 1 takes the optical surface is a reflective surface U _j = Coefficient taking +1; N _j is the same as the refractive index of the glass material when the optical surface is the transmissive surface, and N _j = 2 when the optical surface is the reflective surface; dX / dY _{(out) j} is the optical surface at the position where the last light beam on the scanning end side of the light beam that reaches the maximum scanning position of the effective scanning area on the scan surface passes through the j-th surface in the main scanning section The slope of the scan end with respect to the optical axis of; dX / dY _{(in) j} is the optical surface at the position where the last light beam at the scanning center side of the light beam that reaches the maximum scanning position of the effective scanning area on the scan surface passes through the j-th surface in the main scanning section The slope of the scan center with respect to the optical axis of; dX / dY _{(p) j} is a slope with respect to the optical axis of the optical plane at a position where the main beam of light beams reaching the maximum scanning position of the effective scanning area on the scan plane passes through the j-th plane in the main scan section . The optical scanning device of claim 1, wherein the reciprocating motion of the deflection surface is performed based on a resonance drive. The optical scanning device of claim 1, wherein the reciprocating motion of the deflection surface is performed based on sine vibration. 5. The reference vibration mode according to claim 4, wherein the reciprocating motion of the deflection surface based on the resonance drive has a plurality of separate natural vibration modes, and the plurality of separate natural vibration modes are natural vibration modes based on a reference frequency. And a multiple vibration mode, which is a natural vibration mode based on a frequency corresponding to an integer multiple of two or more times of the reference frequency. The apparatus of claim 6, wherein the biasing means comprises: a plurality of movable plates; A plurality of torsion springs disposed on the shaft to connect the plurality of movable plates in series; A support for locally supporting the plurality of torsion springs; Drive means for applying torque to at least one of said plurality of movable plates; And a drive control means for controlling the drive means. The optical scanning device according to claim 7, wherein the deflection surface is formed in one of the plurality of movable plates, and the plurality of movable plates and the plurality of torsion springs are provided in an integral structure. The optical scanning device of claim 8, wherein the drive control means controls the drive means to simultaneously excite the reference vibration mode and the drain vibration mode. 7. The light beam that is scanned and deflected by the deflection surface reciprocating in the main scanning direction is scanning deflected at an angular velocity different from the isotropic velocity in the effective scanning region, and satisfies the following conditions. Optical scanning device: (dθ ₁ / dt) _max / (dθ ₁ / dt) _min <1.1 [Wherein, (dθ ₁ / dt) _max is the maximum value of the angular velocity of the deflection surface in any dice in the effective scanning area; (dθ ₁ / dt) _min is the minimum value of the angular velocity of the deflection plane at any dice in the effective scanning region. 11. An optical scanning apparatus according to claim 10, wherein said imaging optical system converts a light beam scanned and deflected by said deflection means at a speed different from an isometric speed onto a constant speed light beam on said scan surface. 2. The main scanning direction of the focused spot along the same scanning line in the effective scanning area on the scanning surface as φm1, the maximum value of the spot diameter in the main scanning direction of the focusing spot along the same scanning line in the effective scanning area on the scanning surface. A light scanning device characterized in that the following relationship is satisfied when the minimum value of the spot diameter is? M2: φm1 / φm2 <1.1. The optical scanning device of claim 1, wherein the light source means has at least two light emitting points. The deflection surface of the deflection means is deformed in the main scanning cross section due to the angular acceleration caused by the reciprocating motion, and the deformation amount of the deflection surface is changeable depending on the position in the sub-scan direction of the deflection surface. Optical scanning device, characterized in that. 15. The optical scanning apparatus of claim 14, wherein the imaging optical system does not function to scan the deflection surface and the surface to be scanned in a conjugated relationship in a sub-scan section. 15. The shape of the sub-scanning cross section at the position in the main scanning direction of the deflection surface corresponding to the scanning end side with respect to the axis of the reciprocating motion of the deflection surface at the effective deflection angle of the deflection surface. The shape of the sub-scan section at the position in the main scanning direction of the deflection surface corresponding to the scanning center portion with respect to the axis of the reciprocating motion of the deflection surface is convex with respect to the scan surface. Optical scanning device characterized in that the deformation. 17. The image forming apparatus according to claim 16, wherein the parallel light beam passes through the imaging optical system at a position where the light beam reflected by the deflection surface passes through the imaging optical system at the effective deflection angle of the deflection surface. The radius of curvature of the wavefront in the sub-scan direction of the luminous flux at the position where the last light beam on the scanning end side passes with respect to the main light beam of the luminous flux is the luminous flux at the position where the last light beam on the scanning center side passes with respect to the main light beam of the luminous flux Optical scanning device, characterized in that greater than the radius of curvature of the wavefront in the sub-scan direction. An optical scanning device according to any one of claims 1 to 17; A photosensitive material disposed on the scan surface; A developing device for generating a toner image by developing an electrostatic latent image formed on the photosensitive material surface by the light beam scanned by the optical scanning device; A transfer device for transferring the developed toner image to a transfer material; And Fixing apparatus for fixing the transferred toner image to the transfer material Image forming apparatus comprising a. An optical scanning device according to any one of claims 1 to 17; And The printer controller converts the code data supplied from the external device into an image signal and inputs it to the optical scanning device. Image forming apparatus comprising a. A plurality of optical scanning device according to any one of claims 1 to 17; And A plurality of image bearing members each disposed on a scanning surface of a corresponding optical scanning device among the optical scanning devices to form images of different colors; Color image forming apparatus comprising a. 21. The color image forming apparatus according to claim 20, further comprising a printer controller for converting a color signal supplied from an external device into image data of a color different from the color of the color signal, and inputting the image data to a corresponding optical scanning device. .

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