JPH02166415A - Optical scanning method - Google Patents

Abstract

PURPOSE:To compensate curvature of field in a subscanning direction and to make a high-density optical scan by varying the length of the optical path between a 1st image formation optical system and a rotary polygon mirror by an optical path length varying device. CONSTITUTION:The optical path length varying device is arranged between the 1st image formation optical system 2 and rotary polygon mirror 3. The optical path length varying device consists of a fixed part 8a, a movable part 8f, and a driving part, and the fixed part 8a and movable part 8b are composed of combinations of roof prisms and/or roof mirrors. The movable part 8b of the optical path length varying device is displaced by a driving part by being driven by a piezoelectric element or magnetostrictive element according to the curvature of field in the subscanning direction by 2nd image formation systems 5 and 6 every time luminous flux is deflected, thereby making an optical scan while compensating the curvature of field in the subscanning direction. Therefore, the light source device 1 and 1st image formation optical system 2 may be fixed as they are and their position accuracy is held high. Consequently, the high-density optical scan is made excellently.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an optical scanning method.

[Prior Art] Optical scanning is performed for the purpose of writing and reading information, and various methods have been known for use in laser printers, laser facsimiles, digital copying machines, laser engraving machines, etc. There is.

A well-known optical scanning system is one in which a light beam from a light source device is imaged as a long line image in a direction corresponding to the main scanning by a first imaging optical system, and the line image is formed. The light beam is deflected by a rotating polygon mirror having a deflection reflecting surface near the image position.

The deflected light beam is imaged into a spot on the scanned surface by a second imaging optical system that establishes a geometrically optically substantially conjugate relationship between the starting point of deflection by the deflection reflecting surface and the scanned surface in the sub-scanning direction. There is an optical scanning method that optically scans the surface to be scanned.

This optical scanning method is a well-known optical scanning method that corrects the surface tilt of a rotating polygon mirror, and even if the surface tilt of the rotating polygon mirror occurs, the main scanning position hardly changes in the sub-scanning direction.

However, this optical scanning method uses an anamorphic lens system with stronger power in the sub-scanning direction than in the main scanning direction as the second imaging optical system.

Therefore, large curvature of field tends to occur in the sub-scanning direction.

Further, since the position of the deflecting reflection surface of the rotating polygon mirror changes in the optical axis direction as the rotating polygon mirror rotates, this also causes field curvature in the sub-scanning direction.

When a strong curvature of field occurs in the sub-scanning direction, the spot diameter in the sub-scanning direction of the imaging spot that scans the scanned surface varies depending on the scanning position. This makes it difficult to realize high-density optical scanning, which requires a high degree of stability in the spot diameter.

Conventionally, the following two methods are known as methods for solving the problem of field curvature in the sub-scanning direction as described above. The first method is to correct field curvature by designing the lens of the second imaging optical system.

Second, in synchronization with the deflection of the deflected light beam by the rotating polygon mirror,
This is a method of correcting field curvature by displacing the light source and the first imaging optical system in the optical axis direction.

[Problems to be Solved by the Invention] However, the first method described above is not easy in practice, especially within the framework of demands for low cost and compactness of lenses.
There are naturally limits to the correction of field curvature. The second method is theoretically quite effective, but the light source and first imaging optical system are required to have fairly high positional accuracy due to the configuration of the optical scanning device. If it is displaced, there is a possibility that the positional accuracy required for the light source etc. will be disturbed due to the displacement movement.

The present invention was made in view of the above-mentioned circumstances, and
The purpose is to provide a novel optical scanning method that enables high-density optical scanning by properly correcting field curvature in the sub-scanning direction.

[Means for Solving the Problems] The present invention will be explained below. Six optical scanning methods of the present invention are as follows. Statue.

The light flux is deflected by a rotating polygon mirror having a deflection reflection surface in the vicinity of the imaging position of the line image, and the starting point of deflection by the deflection reflection surface and the scanned surface are approximately conjugate in terms of geometric optics in the sub-scanning direction. An optical scanning method for optically scanning the surface to be scanned by forming a spot-like image of the deflected light beam on the surface to be scanned by a second related imaging optical system, and its characteristics are as follows. It is at the point of

That is, between the first imaging optical system and the rotating polygon mirror,
An optical path length changing device is distributed.

The optical path length changing device consists of a fixed part, a movable part, and a driving part.

The fixed part and the movable part are a combination of a roof prism and/or a roof mirror.

The movable part of this optical path length changing device is opened and displaced by a piezoelectric element or a magnetostrictive element by a drive part in accordance with the curvature of field in the sub-scanning direction by the second imaging optical system for each deflection of the deflected light beam. , performs optical scanning while correcting field curvature in the sub-scanning direction.

As described above, the fixed part and the movable part are a combination of roof prisms, roof mirrors, or roof prisms.

As described above, the movable part is displaced using a piezoelectric element or a magnetostrictive element, and an example of a driving method using a piezoelectric element is a method using a piezoelectric actuator.

For displacement by the magnetostrictive element, a voice coil actuator system known in connection with focusing of optical pickups can be used.

In addition, in order to correct the curvature of field in the sub-scanning direction by the displacement of the movable part, the relationship between the curvature of field and the amount of displacement of the movable part to correct it must be determined and stored for each scanning position. The amount of displacement may be controlled according to the scanning position, or the amount of curvature of field in the sub-scanning direction may be detected, and the amount of displacement of the movable portion may be feedback-controlled based on it.

[Function] As described above, in the present invention, the optical path length changing device
The curvature of field in the sub-scanning direction is corrected by changing the optical path length between the imaging optical system and the rotating polygon mirror.

Therefore, the light source device and the first imaging optical system can remain fixed, and their positional accuracy can be maintained at a high level.

[Example] Hereinafter, description will be given based on specific examples.

FIG. 1 shows only the essential parts of an embodiment of the present invention.

Reference numeral 1 indicates a light source such as a laser or a light source device consisting of a light source and a condensing device. A substantially parallel light beam emitted from the light source device 1 enters a cylinder lens 2, which is a first imaging optical system.

The light beam transmitted through the cylinder lens 2 becomes unidirectional focused light due to the focusing action of the lens 2.

The light enters the rotating polygon mirror 3 through an optical path length changing device shown by a fixed part 8a and a movable part 8b. Then, a line image LI is formed near the deflection reflection surface 4 of the rotating polygon [3. This line image L! The longitudinal direction corresponds to the main scanning direction.

The light beam is then reflected by the deflection reflection surface 4 and enters the scanned surface 7 via the second imaging optical system.

The second imaging optical system is constituted by optical scanning lenses 5 and 6, and forms a spot image of the light beam on the scanned surface 7. As the rotating polygon tIA3 rotates, the imaging spot scans the surface to be scanned 7.

Figure 2 is a light source bag! The optical path from 1 to the scanned surface 7 is developed and shown as viewed from the direction corresponding to main scanning. In FIG. 2, the vertical direction corresponds to the sub-scanning direction. The upper diagram (a) in FIG. 2 shows a state in which the imaging position of the second imaging optical system is shifted from the scanned surface 7 by ΔX'' corresponding to the amount of field curvature. The amount ΔX' can be corrected by shifting the imaging position of the line image by ΔX from the normal position, as shown in the lower diagram (b) of FIG.

The relationship between ΔX and Δx1 in this correction is expressed as ΔX'=β using the lateral magnification β of the second imaging optical system.
It is given by 2 ・ΔX.

In the present invention, the above-mentioned ΔX is achieved by changing the distance between the first imaging optical system and the deflection reflection surface.

Here, the specifications of the second imaging optical system will be specifically given.

The lens surfaces of the lenses 5 and 6 are the first to fourth surfaces from the rotating polygon mirror side toward the surface to be scanned.

The radius of curvature of the first surface is the radius of curvature in the main scanning direction (rotated polygon j
! 3, the radius of curvature on the cross section of the deflection surface formed by the deflection of the optical axis ray of the ideal deflected light beam is r
i, the radius of curvature in the sub-scanning direction (the radius of curvature on the cross section of the plane passing through the optical axis and perpendicular to the deflection plane orthogonal to the deflection plane) is ri'
, the distance between the first surface and the (i+1)th surface is di1 rotation polygon! ! Let nj be the refractive index of the fifth lens from the 3rd side.

i ri ri' di j
njl -107,774cXl) 5.8?2 1
1.712212 co oo 10
．． 96J33 (-) -52,565
6,80721,8754-45,569-12,05
2 Note that the above values are values when the composite focal length Mf amount in the main scanning direction is standardized to 100.

At this time, the brightness in the main scanning direction is F/No = 54.7,
Effective deflection angle 2θ = 67.8", angle α between the optical axis ray of the light beam incident on the rotating polygon t//13 and the optical axis of the second imaging optical system: 60°, the inside of the rotating polygon mirror 3 Tangential circle radius R and the above fm
Ratio R/fa=0.132, composite focal length in sub-scanning direction 1
1fs=22.698, lateral magnification β=-4.12.

The curvature of field when the deflected light beam is incident on this second imaging optical system is as shown in FIG.

The broken line is for the main scanning direction, and the solid line is for the sub-scanning direction.

Now, the fixed part 8a of the optical path length changing device is a convex roof mirror as shown in FIG. 1, and the movable part 8b is a concave roof mirror as shown in the same figure.

Since the angle formed by the mirror surface of each roof mirror is set to 90°, the light flux from the cylinder lens 2 toward the deflection reflection surface 4 of the rotating polygon mirror is directed to the fixed part 8a as shown in FIG.
The optical path can be bent by the movable part 8b so as to form three sides of a rectangle.

As shown in FIG. 4, the movable part 8b is Δ
When the lens is displaced by a, the optical path length between the cylinder lens 2 and the deflection/reflection surface 4 changes by 2Δa.

Now, at a certain scanning position, if the distance by which the imaging position of the line image by the cylinder lens 2 should be displaced is ΔX in order to correct the curvature of field in the sub-scanning direction, then this distance is shown in Figure 2. That is, ΔX'/β2 is calculated for ΔX' as described above, and if this is performed by displacing the movable portion 8b, the displacement Δa may be set to Δx'/2β2.

FIG. 5 is a diagram showing how the movable portion should be displaced along with the scanning position in order to correct the curvature of field (actual curve) in the sub-scanning direction shown in FIG. 3.

That is, the right side of the figure means moving the movable part 8b so that the position of the line image is displaced toward the image side, and the left side means moving the movable part 8b so that the position of the line image is displaced toward the light source side. means. That is, in the former case, the displacement of the movable part 8b may be controlled to shorten the distance between the fixed part 8a and the movable part 8b, and in the latter case, the displacement of the movable part 8b may be controlled to increase the distance.

When the displacement of the movable J18b is controlled for each deflection as shown in FIG. 5 in this manner, the curvature of field in the sub-scanning direction is almost completely corrected as shown in FIG. 6.

Furthermore, the curvature of field in the main scanning direction is hardly affected by the above correction action.

Therefore, the diameter of the imaged spot in the sub-scanning direction on the surface to be scanned becomes extremely constant regardless of the scanning position. Furthermore, fluctuations in the diameter of the imaged spot in the main scanning direction due to curvature of field in the main scanning direction can be corrected by adjusting the light amount in the light source device 11. Therefore, high-density optical scanning becomes possible.

In this embodiment, displacement control of the movable part is performed as follows.

That is, the movable portion 8b is displaced by a voice coil type displacement mechanism known in connection with focusing of optical pickups.

Further, the relationship between the scanning position and the displacement amount Δa is as shown in FIG. 5, and this relationship is stored in the storage device of the wobbling section. The scanning position by the deflected light beam is detected in relation to the clock, and the movable portion 8a is displaced in accordance with the detected scanning position so as to correct the curvature of field in the sub-scanning direction.

Correction of field curvature may also be performed in a feedback manner.

To do this, for example:

That is, a semi-transparent mirror is arranged between the second imaging optical system and the surface to be scanned, and a part of the deflected light beam is separated as a separated light beam and guided to a surface equivalent to the surface to be scanned, and this surface is scanned. let A plurality of image sensors such as CCDs are provided on this surface in the scanning direction. The direction in which the light-receiving elements are arranged in each sensor is
Orthogonal to the direction in which the equivalent surface is scanned. Then, when the separated light flux passes through each sensor position, if the diameter of the spot of the separated light flux is detected by the output of the sensor, it is possible to detect the field curvature in the sub-scanning direction from this result. The displacement of the movable part of the optical path length changing device may be feedback-controlled so that the imaging spot diameter becomes a predetermined value. An LD that uses a regular mirror instead of a semi-transparent mirror and has two light sources as light sources.
An array may be used, and the light beam from one of the light emitting sources may be guided to a surface equivalent to the surface to be scanned.

[Effect of the invention] According to the present invention, a novel optical scanning method can be provided.

Since this method has the above-mentioned configuration, the second imaging optical system, which has a considerable field curvature in the sub-scanning direction, does not move the light source or the first imaging optical system. Good high-density optical scanning can also be achieved using the following. Note that the roof angle of the roof mirror or roof prism of the optical path length changing device is not limited to 90° as described in the embodiment, but may be set to other angles.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining a device according to an embodiment of the present invention.
FIGS. 2 to 6 are diagrams for explaining the present invention in relation to the above-mentioned embodiments. 160. Light source device, 200. Cylinder lens as first imaging optical system, 3661 rotating polygon mirror, 421. Polarized reflective surface, 5, 8. ,, a lens constituting the second imaging optical system, 706. Surface to be scanned, 8a, Fixed part of optical path length changing device. ab,..., the movable part of the optical path length changing device 40.

Claims (1)

[Scope of Claims] A rotary polygon that forms a long line image in a direction corresponding to the main scanning by a first imaging optical system, and has a deflection reflection surface in the vicinity of the position where the line image is formed. The deflected light beam is scanned by a second imaging optical system which deflects the light beam with a mirror and makes the starting point of deflection by the deflection reflecting surface and the surface to be scanned in a substantially conjugate relationship in terms of geometrical optics in the sub-scanning direction. In an optical scanning method in which a spot image is formed on a surface and the surface to be scanned is optically scanned, a fixed part, a movable part, and a driving part are arranged between the first imaging optical system and the rotating polygon mirror. An optical path length changing device is provided in which the above-mentioned fixed part and movable part are formed by a combination of a roof prism and/or a roof mirror, and the movable part of this optical path length changing device is controlled by the above-mentioned moving part to each deflection of the deflected light beam. In each case, a piezoelectric element or a magnetostrictive element is driven and displaced according to the curvature of field in the sub-scanning direction by the second imaging optical system, and optical scanning is performed while correcting the curvature of field in the sub-scanning direction. An optical scanning method characterized by: