JPH02238423A - Optical scanner - Google Patents

Abstract

PURPOSE:To effectively eliminate ununiformity of the diameter of a light spot in the sub- scanning direction which remains behind after eliminating a curvature of image and to execute a high density optical scan by controlling a very small oscillation of a cylindrical lens executed by an oscillating means by a second control means, and equalizing the diameter in the sub-scanning direction of the light spot by synchronizing with an optical scan executed by a deflection luminous flux. CONSTITUTION:A relation of an image forming position in the main scanning direction and the diameter of a light spot in the sub-scanning direction by a residual fluctuation is checked in advance, and an aperture, etc. are designed so that the maximum light spot diameter in the sub-scanning direction in the residual fluctuation becomes a target spot diameter 2W3. In this state, a cylindrical lens 2 is brought to very small oscillation by an oscillating means 9 so as to eliminate a fluctuation of the light spot diameter in the sub-scanning direction caused by the residual fluctuation. The oscillation quantity is related in advance to a synchronizing clock and stored in advance in a second control means 11, and the residual fluctuation is eliminated by controlling the oscillation of the cylindrical lens 2 by synchronizing with an optical scan by the oscillating means 9 in accordance with its storage contents. In such a way, since the light spot diameter in the sub-scanning direction can be equalized with high accuracy, the optical scan of a good quality can be executed.

Description

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

[Prior Art] A substantially parallel light beam from a laser light source device is imaged by a cylindrical lens as a long line image in a direction corresponding to the main scanning direction, and a rotating polygon mirror having a deflection reflection surface near the imaging position of this line image is used to form an image. Optical scanning devices that perform optical scanning by deflecting a beam of light and forming an optical spot on a surface to be scanned using an imaging optical system have traditionally been associated with laser printers, laser facsimile machines, digital copying machines, laser engraving machines, etc. It is well known.

As is well known, optical scanning devices that use a rotating polygon mirror have the problem of "plane tilt," and in order to correct this surface tilt, the above device uses a cylindrical lens to convert the substantially parallel light beam from the laser light source device into a rotating polygon mirror. A long line image is formed in the direction corresponding to the main scanning direction near the deflection reflection surface of the mirror, and an imaging optical system is used to make the starting point of deflection by the deflection reflection surface and the scanned surface approximately conjugate geometrically in the sub-scanning direction. The relationship is Therefore, the imaging optical system becomes an anamorphic optical system in which the power in the sub-scanning direction is stronger than the power in the main-scanning direction, and strong curvature of field tends to occur in the sub-scanning direction.

[Problems to be Solved by the Invention] The curvature of field in the sub-scanning direction as described above causes the diameter of the light spot in the sub-scanning direction that scans the scanned surface to vary with the imaging position, and the main scanning area In this case, the diameter of the optical spot in the sub-scanning direction becomes non-uniform, which becomes a major obstacle in realizing high-density optical scanning.

Various attempts have been made to correct the above-mentioned curvature of field in the sub-scanning direction using the performance of the imaging optical system, but in order to satisfy the demand for higher density, it is necessary to design the imaging optical system. The problem was that it was extremely difficult.

The present invention was made in view of the above-mentioned circumstances, and
The purpose of this is to be able to easily and reliably remove the curvature of field in the sub-scanning direction, and to effectively remove the non-uniformity of the light spot diameter in the sub-scanning direction that remains even after the curvature of field has been removed. Therefore, an object of the present invention is to provide a novel optical scanning device that enables high-density optical scanning.

[Means to solve the problem] The present invention will be explained below.

The optical scanning device of the present invention can be applied to a laser printer, a laser facsimile, a digital copying machine, a laser plate making machine, etc., and includes "a laser light source device, a line image forming device, a rotating polygon mirror, and an imaging optical system." .

A substantially parallel laser beam is emitted from the "laser light source device."

The "r-ray image forming device" images a substantially parallel light beam from a laser light source device into a long line image in a direction corresponding to the main scanning direction. The "rotating polygon mirror" has a deflection reflection surface near the imaging position of the line image, and reflects the light beam incident from the laser light source device via the line image forming device. The reflected light flux becomes a deflected light flux as the rotating polygon mirror rotates and enters the imaging optical system.

The "imaging optical system" images the deflected light beam as a light spot on the surface to be scanned, but at this time, the origin of deflection by the deflection reflection surface and the surface to be scanned are approximately conjugate in geometrical optics with respect to the sub-scanning direction. The effect of surface tilt on a rotating polygon mirror is corrected.

Now, the feature of the present invention lies in the "r-ray image forming device".

The "r-ray image forming device" includes a cylindrical lens, a displacement means, a swinging means, and first and second control means. The "displacement means" is means for displacing the cylindrical lens in the optical axis direction.

The "swinging means" is a means for slightly swinging the cylindrical lens around the optical axis.

The "first control means" is means for controlling the displacement of the cylindrical lens by the displacement means so as to remove the field curvature of the imaging optical system in the sub-scanning direction in synchronization with the optical scanning by the deflected light beam. be.

The "second control means" is a means for controlling the slight rocking of the cylindrical lens by the rocking means so as to equalize the diameter of the light spot in the sub-scanning direction in synchronization with the optical scanning by the deflected light beam. be.

Note that a known appropriate parallel movement mechanism may be used as the displacement means, and an electrostrictive element or a magnetostrictive element may be used as the drive source. Further, as the swinging means, for example, a swinging mechanism using a four-bar link mechanism may be constructed using the holding frame of the cylindrical lens as a driven link, and the drive link may be swinged using an electrostrictive element or a magnetostrictive element.

[Function] The "imaging optical system" establishes a geometrically optically approximately conjugate relationship between the starting point of deflection by the deflection reflecting surface and the surface to be scanned in the sub-scanning direction. What is imaged is an image formed by the above-mentioned imaging optical system of the "r-ray image produced by the line image imaging device". When the imaging position of the line image by the line image imaging device changes on the optical path, the imaging position of the line image by the imaging optical system also shifts in accordance with the vertical magnification of the imaging optical system. By utilizing this fact, field curvature in the sub-scanning direction can be removed.

Therefore, the cylindrical lens of the line image forming device is displaced in the optical axis direction by the displacement means, and this movement displaces the imaging position of the line image on the optical path. In order to eliminate the curvature of field in the sub-scanning direction associated with the optical system, it is synchronized with the optical scanning by the deflected light beam.
It's about controlling.

In this way, fluctuations in the optical spot diameter due to curvature of field in the sub-scanning direction can be eliminated. However, this alone cannot completely eliminate fluctuations in the optical spot diameter. The diameter of a light spot obtained by converging a laser beam is a finite one determined by the so-called beam waist diameter, and this is the same for the above-mentioned "line image". Therefore, the diameter of the light spot in the sub-scanning direction on the surface to be scanned is the diameter of the beam of the line image produced by the cylindrical lens.
The beam waist diameter is linked to the lateral magnification in the sub-scanning direction of the imaging optical system, and when the imaging position of the line image is displaced with the displacement of the cylindrical lens, the lateral magnification changes accordingly, resulting in a change in the light spot diameter. occurs. This variation is called "residual variation" in the optical spot diameter. This residual variation is a higher order and minute amount compared to the variation in the optical spot diameter due to the field curvature mentioned above, and can be ignored in many cases, but it is still necessary to improve the quality of printing by optical scanning. cannot be ignored.

Therefore, in the present invention, the minute swing of the cylindrical lens by the "oscillation means" is controlled by the second control means so as to "uniform the diameter of the light spot in the sub-scanning direction in synchronization with the optical scanning by the deflected light beam."It's about controlling. When the generatrix direction of the cylindrical lens is tilted from the normal direction due to rocking, the power in the sub-scanning direction becomes smaller than the normal state, so this can be used to control the sub-scanning of the light spot on the scanned surface. The diameter of the direction can be changed.

[Embodiments] Hereinafter, specific embodiments will be described with reference to the drawings.

FIG. 1 schematically shows only the parts necessary for explaining one embodiment of the present invention.

A laser light source device designated by reference numeral 1 is composed of a light source or a light source and a collimating lens system, and emits a substantially parallel light beam. Note that in this embodiment, a semiconductor laser is assumed as the light source.

The parallel light beam from the light source 1 is imaged by the cylindrical lens 2 of the line image forming device as a long line image in the direction corresponding to the main scanning in the vicinity of the deflection reflection surface 4 of the rotating polygon mirror 3.

When the rotating polygon mirror 3 rotates around the axis 3A, the light beam reflected by the deflection reflecting surface 4 becomes a deflected light beam and enters the imaging optical system.

The imaging optical system is a so-called fθ lens composed of two lenses 5 and 6, and images the deflected light beam as a light spot on the scanned surface 7. This light spot optically scans the scanned surface 7. The lens 5 is a single lens consisting of a cylindrical surface and a flat surface, and the lens 6 is a single lens consisting of a cylindrical surface and a toric surface.

FIG. 2 shows the optical system of FIG. 1 developed along the optical path so that the sub-scanning direction is the vertical direction.

FIG. 2(a) shows a state in which light from the laser light source device 1 is imaged as a line image at the position of the deflection reflection surface 4 due to the action of the cylindrical lens 2 of the line image forming device. The lens 5.6 constituting the imaging optical system has a geometrically optically substantially conjugate relationship between the starting point of deflection by the deflection reflecting surface 4 and the surface to be scanned 7 in the sub-scanning direction. is imaged on the scanned surface 7.

However, as shown in FIG. 2(b), when the imaging position of the line image shifts by ΔX, the imaging position in the sub-scanning direction by the imaging optical system shifts by ΔX', and these displacements ΔX and ΔX' As is well known, the relationship Δx=β2·ΔX holds between them, where β is the lateral magnification of the imaging optical system in the sub-scanning direction.

Section 22 lists specific data regarding the "imaging optical system."

From the side of the deflection reflection surface 4 toward the scanned surface side, the radius of curvature of the i-th lens surface in the main scanning direction (radius of curvature seen from the sub-scanning direction) is R41, and the radius of curvature in the sub-scanning direction (radius of curvature in the main scanning direction) is When RiY is the radius of curvature (as viewed from the direction), d is the distance between the i-th lens from the deflection-reflection surface 4 side, and n is the refractive index of the j-th lens from the deflection-reflection surface, these are as follows. Given.

l R i X R i Y d i
J n j1 -107.774 c
o 5.672 1 1.712212
ω ω 10. 9663Co
-52.565 8.807 2 1.67
54 -45.569 -12.052 Composite focal length fM in main scanning direction = 100, brightness F/No.54.
7. Deflection angle 2θ=67. 8 degrees, horizontal magnification β in the sub-scanning direction
= -4.12, composite focal length in the sub-scanning direction fs = 22.
698, the ratio of the focal length 9 to the radius R of the inscribed circle of the rotating polygon mirror 3 R/fM=0. It is 132. Further, the above refractive index is for light having a wavelength of 780 nm from the laser light source device 1.

The above-mentioned "imaging optical system" in the case where the cylindrical lens 2 of the line image imaging device forms an image of the light beam from the laser light source device 1 very close to the deflection reflection surface 4 as shown in FIG. 2(a). Figure 3 shows the curvature of field due to The broken line indicates the curvature of field in the main scanning direction, and the solid line indicates the curvature of field in the sub-scanning direction.

In FIG. 1, the displacement means 8 displaces the cylindrical lens 2 in the optical axis direction.

When a voltage according to a control signal is applied to the displacement means 8 from the first control means 10, the cylindrical lens 2 is displaced in the optical axis direction according to the applied voltage by the aforementioned electrostrictive element or the like.

The amount of movement of this cylindrical lens 2 is the amount of displacement Δ of the line image.
Equal to X.

This displacement ΔX causes a displacement ΔX′ in the imaging position of the light spot. These two are connected by the relationship Δx°=β2·ΔX explained earlier.

Therefore, if the amount of field curvature in the sub-scanning direction of the imaging optical system (see Figure 3) is set as W(z) according to the imaging position 2 in the main scanning direction, the cylindrical lens 2 in the optical axis direction, and the displacement ΔX of the imaging position due to this displacement
In order to cancel the field curvature W(z) in the sub-scanning direction, it is sufficient to set ΔX' : -W(z), and the cylindrical lens 2 should be adjusted by -W(z)/β2. All you have to do is displace it. This amount of displacement is shown in Figure 4. On the other hand, the relationship between the amount of displacement of the cylindrical lens 2 and the applied voltage can be determined in advance.

Since the imaging position 2 of the light spot is determined temporally by the correspondence with optical scanning, the imaging position 2 is determined in correspondence with the synchronization clock that synchronizes the optical scanning, and each imaging position and the cylindrical lens 2 are aligned. The voltage V(z) that can be displaced by W(z)/β2
) is set in advance in the first control means 10 shown in FIG. Field curvature can be removed.

FIG. 5 shows the curvature of field state after the curvature of field in the sub-scanning direction has been removed as described above. In the above method, since no change occurs in the main scanning direction, the curvature of field in the main scanning direction remains in the same state as in FIG. 3. However, if only the field curvature in the main scanning direction is present, it can be removed by designing the imaging optical system, and variations in the light spot diameter in the main scanning direction due to the degree of field curvature shown in Figure 5 are hardly a problem. .

Now, as is well known, the laser beam has a Gaussian intensity distribution, and when this is imaged as a light spot on the scanned surface 7, this light spot has a Gaussian intensity distribution as shown in FIG. have

In an optical scanning device, printing is performed using a photosensitive recording medium on the surface to be scanned.If the threshold value of the photosensitive recording medium that responds to light energy is set as the image forming level as shown in Fig. 7, The light spot diameter itself is 2w in FIG.

In Figure 6, curve 3-1 indicates that the laser beam is 2 times in the sub-scanning direction.
A state is shown in which the light spot is focused on the scanned surface 7 as a light spot with a diameter of w2. From this state, if the generatrix direction (direction of no curvature) of the cylindrical lens 2 is tilted from the normal direction, that is, the direction corresponding to the main scanning, the power of the cylindrical lens 2 in the sub-scanning direction becomes smaller, and this causes the surface to be scanned 7 to The intensity distribution of the light spot in the sub-scanning direction is the sixth
It will look like curve 3-2 in the figure, and the light spot diameter will be 2w3.
become. Therefore, in the present invention, the "residual variation" in the diameter of the light spot in the sub-scanning direction is effectively made uniform by swinging the cylindrical lens 2.

That is, specifically, the relationship between the imaging position 2 in the main scanning direction and the light spot diameter in the sub-scanning direction due to the residual variation is investigated in advance, and the maximum light spot diameter in the sub-scanning direction due to the residual variation is determined to be the target spot diameter. The aperture (generally provided closer to the light source than the cylindrical lens to regulate the light spot diameter) is designed so that the diameter of the light spot is 2W3, and the swing The cylindrical lens 2 is slightly oscillated by means 9 (see FIG. 1). The amount of oscillation is stored in advance in the second control means l1 in relation to the synchronization clock, and the oscillation means 9 controls the oscillation of the cylindrical lens in synchronization with the optical scanning according to the stored contents to control the remaining fluctuation. can be removed.

The first and second control means can be configured by a common microcomputer.

Further, since the swing angle of the cylindrical lens 2 is about 1 degree at most, the swing does not affect the main scanning direction.

[Effects of invention As described above, according to the present invention, a novel optical scanning device can be provided.

Since this device is constructed as described above, the diameter of the light spot in the sub-scanning direction can be made uniform with extremely high precision, and therefore, extremely high-quality light scanning can be performed.

[Brief explanation of drawings]

FIG. 1 is a diagram showing one embodiment of the present invention, and FIGS. 2 to 7 are diagrams for explaining the principle of the present invention in relation to the embodiment.

Claims (1)

[Scope of Claims] A laser light source device, a line image forming device that images a substantially parallel light beam from the laser light source device as a long line image in a direction corresponding to main scanning, and a vicinity of the imaging position of the line image. A rotating polygonal mirror having a deflection-reflecting surface is disposed between the rotating polygon mirror and the scanned surface, and the deflection origin by the deflection-reflection surface and the scanned surface are arranged so that they are substantially conjugate in terms of geometrical optics with respect to the sub-scanning direction. and an imaging optical system that images the deflected light beam by the deflection reflecting surface as a light spot on the scanned surface, and the line image forming device includes a cylindrical lens and an imaging optical system that focuses the cylindrical lens with light. Displacement means for displacing in the axial direction; and first control means for controlling the displacement by the displacement means so as to remove field curvature of the imaging optical system in the sub-scanning direction in synchronization with optical scanning by the deflected light beam. a rocking means for slightly rocking the cylindrical lens around the optical axis; and a rocking means for making the diameter of the light spot uniform in the sub-scanning direction in synchronization with the optical scanning by the deflected light beam. 1. An optical scanning device comprising: second control means for controlling minute fluctuations.