JPH02254409A - Scanning optical system and laser beam printer using its system - Google Patents

Abstract

PURPOSE:To control the spot diameter brought to image formation on the surface to be scanned by applying an electric field to a plane-parallel element consisting of a ferroelectric substance crystal and varying a refractive index of the plane-parallel element by an electro- optical effect, and making the optical path length variable. CONSTITUTION:A roughly parallel luminous flux emitted from a light source part 1 consisting of a semiconductor laser and a collimator lens passes through a cylindrical lens 2 having refracting power only in the sub-scanning direction for constituting a first image forming optical system and a parallel plane element 14 consisting of a ferroelectric substance crystal, is deflected by the deflecting/reflecting surface of a rotary polygon mirror 3 for rotating in the direction as indicated with an arrow 8, and its luminous flux forms an image on the surface 6 to be scanned by a second image forming optical system 5, and also, allowed to execute a scan in the direction as indicated with an arrow 7. On the other hand, in the sub-scanning direction, the luminous flux from the light source part 1 forms an image linearly in the vicinity of the deflecting/reflecting surface 4 by a first image forming optical system constituted of the lens 2 and the element 14. A second image forming optical system 5 forms an image point on the surface 6 to be scanned being in an optical conjugate relation by setting an image forming point as an object point. In such a way, by varying an electric field applied to the element 14 and varying a refractive index of the element 14, a position of a focal line by the lens 2 can be varied.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a scanning optical system that keeps the spot diameter in the sub-scanning direction substantially constant on a surface to be scanned, and a laser beam printer using the same.

Conventional technology scanning optical systems are used in laser beam printers, etc.
A laser beam is imaged into a spot on a surface to be scanned, such as a photosensitive drum, and then scanned.

A rotating polygon mirror is generally used as a means for deflecting a laser beam. However, in a rotating polygon mirror made up of a plurality of reflecting surfaces, each reflecting surface has a so-called surface tilt due to processing errors, processing distortions, etc., and pitch unevenness occurs in the interval between main scanning lines, that is, in the sub-scanning direction. A technique for optically correcting this surface tilt is known. As a method having a high correction ability, a method in which the reflecting surface and the surface to be scanned have an optically conjugate relationship in the sub-scanning direction is well known. In order to realize such an optical power arrangement, a method using a long cylindrical lens (Japanese Unexamined Patent Publication No. 50-9
3720) or a method using a toric lens (Japanese Unexamined Patent Publication No. 172317/1982).

Problems to be Solved by the Invention In conventional scanning optical systems that use long cylindrical lenses or toric lenses, when the deflection angle is large, the amount of field curvature becomes large, which poses a problem when obtaining a minute spot. . In particular, there is a problem that astigmatism, which contributes to the spot diameter in the sub-scanning direction, is not sufficiently corrected. Furthermore, as the rotating polygon mirror rotates, the deflection point moves in the optical axis direction and in a direction perpendicular to the optical axis. Among these, the movement of the deflection point in the optical axis direction has a large influence on the variation of the spot diameter in the sub-scanning direction. In addition, since the amount of movement of the deflection point on both the left and right sides of the deflection is asymmetrical, it is extremely difficult to properly correct the aberrations, even if the number of lenses constituting the optical system is increased. Therefore, when performing high-density recording, it is necessary to use a narrow range of deflection angles, and when a large scanning width is required, the optical path length must be increased, leading to an increase in the size of the device. .

An object of the present invention is to provide a parallel plane element made of a ferroelectric crystal in the configuration of a first imaging optical system that forms a linear image of a light beam from a light source in the vicinity of a reflecting surface of a deflector. By applying an electric field to a parallel plane element made of a dielectric crystal and changing the refractive index by the electro-optic effect, the imaging position of the focal line formed by the first imaging optical system is changed, and the spot diameter in the sub-scanning direction is changed. The objective is to satisfactorily correct the factors contributing to this, that is, the fluctuations in the image plane position in the sub-scanning direction.

! ! ! Means for Solving the Problems The scanning optical system of the present invention includes a light source section, a first imaging optical system, a deflector, and a second imaging optical system. The light source unit emits a substantially parallel light beam from a light source such as a semiconductor laser and a collimator lens. The first imaging optical system consists of at least one cylindrical lens and a ferroelectric crystal that has a variable optical path length and a variable focal line imaging position to form a linear image of the light beam from the light source. Consists of planar elements. The deflector consists of a rotating polygon mirror, and its reflecting surface is the first
It is located near the linear image formed by the imaging optical system. The second imaging optical system forms an image of the light beam deflected by the deflector on the surface to be scanned and scans the surface.

Furthermore, it has means for applying an electric field to the parallel plane element made of ferroelectric crystal constituting the first imaging optical system to change the refractive index of the parallel plane element by an electro-optic effect.

In the present invention, in order to control the spot diameter on the scanned surface to be almost constant, especially in the sub-scanning direction, an electric field is applied to the parallel plane element made of ferroelectric crystal of the first imaging optical system to produce an electro-optic effect. The refractive index of the plane-parallel element made of ferroelectric crystal is changed by changing the imaging position of the first imaging optical system, which is the object point of the second imaging optical system, in the sub-scanning direction.

This corrects fluctuations in the image plane position in the sub-scanning direction,
To maintain a good spot diameter formed on the surface to be scanned.

EXAMPLE Hereinafter, an example of the present invention will be described in detail with reference to the drawings.

1 and 2 show an embodiment of the present invention. FIG. 1 shows the scanning optical system viewed from a direction parallel to the deflection plane, and FIG. 2 shows the scanning optical system of FIG. 1. unfolds along the optical path,
It is viewed from a direction parallel to the sub-scanning direction. Therefore,
The vertical direction in FIG. 2 corresponds to the sub-scanning direction. In FIG. 1, a substantially parallel light beam emitted from a light source section 1 consisting of a semiconductor laser and a collimator lens is composed of a cylindrical lens 2 having refractive power only in the sub-scanning direction and a ferroelectric lens 2, which constitutes a first imaging optical system. The light passes through a plane-parallel element 14 made of crystal, and is deflected by the deflecting reflection surface 4 of the rotating polygon mirror 3 rotating in the direction of the arrow 8. The deflected light beam forms an image on the scanning surface 6 by the second imaging optical system 5, and is scanned in the direction of the arrow 7.

On the other hand, in the sub-scanning direction, as shown in FIG. A linear image is formed near the reflective surface 4. The second imaging optical system 5 forms an image point on the scanned surface 6 having an optically conjugate relationship with the image forming point as an object point.

Here, when the rotating polygon mirror that is the deflector rotates and a deflection effect is performed, a change in the deflection point position of the deflection reflection surface occurs.
This causes a shift Δ in the position of the object point in the sub-scanning direction of the second imaging optical system.

This deviation Δ is determined by the relationship δ=Δ・β2, where β is the vertical magnification of the second imaging optical system in the sub-scanning direction, and the deviation δ of the imaging position on the scanned surface in the sub-scanning direction is calculated by the following relationship: δ=Δ・β2 expressed. Therefore, in order to correct this deviation δ and further correct the curvature of field in the sub-scanning direction of the second imaging optical system, parallel planes made of ferroelectric crystals constituting the first imaging optical system are used. By changing the electric field applied to the element 14, the parallel plane element 1 made of ferroelectric crystal
Change the refractive index of 4. That is, by changing the refractive index, the optical path length can be changed, and the position of the focal line formed by the cylindrical lens 2 can be changed. As a result, fluctuations in the object point position of the second imaging optical system in the sub-scanning direction caused by the rotation of the rotating polygon mirror can be strengthened by controlling the electric field applied to the parallel plane element 14 made of the ferroelectric crystal. By controlling the refractive index of the plane-parallel element 14 made of dielectric crystal, it is possible to change and cancel the image position by the first imaging optical system.

FIG. 3 shows the imaging position of the first imaging optical system, that is, the object point position of the second imaging optical system, due to the change in optical path length when the refractive index of the parallel plane element 14 made of ferroelectric crystal changes. The principle of movement is shown from the sub-scanning direction. A substantially parallel light beam from a light source is converted into a convergent light beam by a cylindrical lens 2 having refractive power only in the sub-scanning direction and is incident on a parallel plane element 14 made of a ferroelectric crystal. When the refractive index of the ferroelectric crystal is low, the incident light beam moves along the optical axis 13 as shown by the solid line 19.
Focus a focal line on top. On the other hand, when the refractive index of the ferroelectric crystal is high, the incident light beam forms a focal line as shown by the broken line 20. Therefore, the imaging position of the focal line of the first imaging optical system can be arbitrarily controlled in accordance with the change in the value of the refractive index of the parallel plane element made of ferroelectric crystal. This suppresses fluctuations in the diameter of the spores due to movement of the image plane position in the sub-scanning direction, which has been a particular problem in the past with increasing density.

Incidentally, since the change in the refractive index of the parallel plane element made of the ferroelectric crystal is repeated for each deflection period in the main scanning direction, the amount of change is stored in advance in a storage device as a function of time. , the electric field applied to the parallel plane element made of ferroelectric crystal can be controlled by sequentially reading out the information.

Furthermore, in the above example, the first imaging optical system is composed of a parallel plane element consisting of one cylindrical lens and one ferroelectric crystal, but it may be composed of a single cylindrical lens or a plurality of each element to correct aberrations. It is also possible to increase the degree of freedom and the variable range of the optical path length.

FIG. 4 shows the configuration of a parallel plane element 14 made of a ferroelectric crystal 15 provided with a transparent electrode 910 on a plane perpendicular to the optical axis 13. Terminals 11.12 are connected to the transparent electrodes 9,10. When a predetermined voltage is applied to the terminals 11 and 12, an electric field is applied in a direction parallel to the optical axis 13, and the refractive index of the parallel plane element 14 made of the ferroelectric crystal 15 changes due to the electro-optic effect, thereby adjusting the optical path length to the desired length. can be the value of FIG. 5 shows the configuration of a plane-parallel element 14 made of a ferroelectric crystal 15 with electrodes 16, 17 provided on a plane parallel to the optical axis 13. Terminals 11.12 are connected to the electrodes 16.17. When a predetermined voltage is applied to the terminals 11 and 12, an electric field is applied in a direction perpendicular to the optical axis 13, and the refractive index of the parallel plane element 14 made of the ferroelectric crystal 15 changes due to the electro-optic effect.
Similarly, the optical path length can be set to a desired value. Due to these, the movement of the image formation position of the linear image of the first imaging optical system caused by the movement of the deflection point caused by the rotation of the rotating polygon mirror;
In other words, the movement of the object point position of the second imaging optical system is controlled by the terminal 11.
．． By controlling the voltage applied to 12, the refractive index of the plane-parallel element made of ferroelectric crystal is changed, and by changing the optical path length, the position of the linear image formed by the first imaging optical system can be adjusted to the above-mentioned position. Changes can be controlled so as to offset the movement. The change in the refractive index of a ferroelectric crystal is caused by a first-order electro-optic effect proportional to the electric field, also known as the Pockels effect, a second-order electro-optic effect proportional to the square of the electric field, also known as the Kerr effect, and higher-order effects. be. In any case, it is desirable that the refractive index changes as much as possible depending on changes in the electric field; for example, L 1Nbo3. KH2PO,,KD2P
O4゜BaTi0. , LiTa0. Crystals made of materials such as

The scanning optical system using a plane-parallel element made of ferroelectric crystal according to the present invention can reduce image plane movement in the sub-scanning direction, which was previously particularly difficult to correct and was an impediment to higher resolution. Laser beam printers using this technology can achieve high image quality. FIG. 6 shows the main structure of the optical system-related portion of a laser beam printer using a parallel plane element made of ferroelectric crystal as the first imaging optical system.

The laser beam, which is a substantially parallel beam from the light source 1, forms a linear image in the vicinity of the rotating polygon mirror 3 by a cylindrical lens 2 and a plane-parallel element 14 made of a ferroelectric crystal, and is further deflected into a second image. Angry light drum 2 by image optical system 5
1, a scanning line 18 is recorded as a latent image. The recorded latent image is visualized by a normal electrostatic development process and transferred and fixed onto paper.

Effects of the Invention As described above, according to the present invention, it is possible to highly correct the movement of the image plane in the sub-scanning direction, and it is particularly suitable for laser beam printers that require scanning with a high-density spot. It becomes possible to realize a scanning optical system even at a large deflection angle, and this has great industrial value.

[Brief explanation of drawings]

Fig. 1 shows an embodiment of the present invention, and is a configuration diagram of the scanning optical system viewed from a direction parallel to the deflection plane. Fig. 2 shows an embodiment of the invention, and shows the configuration of the scanning optical system viewed from a direction parallel to the deflection plane. The configuration diagram seen from a parallel direction, Figure 3 is a diagram seen from a direction parallel to the sub-scanning direction.
FIG. 4 is an explanatory diagram showing changes in the imaging position of the focal line when the refractive index of the ferroelectric crystal constituting the first imaging optical system changes;
FIG. 5 is a perspective view showing the configuration of a parallel plane element using a ferroelectric crystal constituting the first imaging optical system, and FIG. 6 is a schematic configuration diagram of a laser beam printer using the scanning optical system of the present invention. It is. DESCRIPTION OF SYMBOLS 1...Light source section, 2...Cylindrical lens, 3...Rotating polygon mirror, 5...Second imaging optical system, 6... ...Scanned surface, 14...
...Parallel plane element. Name of agent: Patent attorney Shigetaka Awano

Claims (4)

[Claims] (1) A light source unit, a first imaging optical system that forms a linear image of the light beam from the light source unit, and a deflector that has a reflective surface near the linear image that is formed by the first imaging optical system. and a second imaging optical system that images the light beam deflected by the deflector onto a scanned surface, and the first imaging optical system has a parallel plane made of at least one ferroelectric crystal. An image is formed on the surface to be scanned by applying an electric field to the plane-parallel element made of the ferroelectric crystal and changing the refractive index of the plane-parallel element by the electro-optic effect to make the optical path length variable. A scanning optical system that controls the spot diameter. (2) A transparent electrode is provided on each plane perpendicular to the optical axis of a plane-parallel element made of a ferroelectric crystal, and a voltage is applied to the transparent electrode to change the refractive index of the plane-parallel element made of a ferroelectric crystal. 2. A scanning optical system according to claim 1, which uses a plane-parallel element made of a ferroelectric crystal with variable optical path length. (3) An electrode is provided on each plane parallel to the optical axis of a plane-parallel element made of a ferroelectric crystal, and a voltage is applied to the electrodes to change the refractive index of the plane-parallel element made of a ferroelectric crystal, thereby changing the optical path length. 2. A scanning optical system according to claim 1, which uses a plane-parallel element made of a ferroelectric crystal whose oscillation is variable. (4) A laser beam printer using the scanning optical system according to claim (1).