JPH03210519A - scanning optical device - Google Patents

Abstract

PURPOSE:To reduce the loss of light quantity by arranging a fixed reflecting mirror oppositely to one reflecting face out of a pair of reflecting faces constituting a rotary polygon mirror and arranging an incident beam, a Fresnel rhomb prism and a polarized beam splitter oppositely to the other reflecting face. CONSTITUTION:Respective mirror faces of the rotary polygon mirror 9 consist of pairs of reflecting faces 9a, 9b inclined in the rotational center direction of the mirror 9 and intersecting with each other at right angles, the reflecting mirror 15 having at least one reflecting face 9b for returning a beam to the other reflecting face 9a is fixed oppositely to one reflecting face out of the reflecting faces 9a, 9b forming a pair and the Fresnel rhomb prism 13 and the polarized beam splitter 11 are arranged oppositely to the other reflecting face. Concretely, the fixed reflecting mirror 15 is used for a plane mirror having one reflecting face 9a or a roof type reflecting mirror 9. Consequently, the compact and comparatively inexpensive scanning optical device can be manufactured and the loss of light quantity can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a scanning optical device that performs deflection scanning by reflecting a light beam emitted from a light source on the mirror surface of a rotating polygon mirror.

[Prior Art] Conventionally, as a scanning optical device, for example, as shown in FIG.
The laser beam LB emitted from the laser light source 30 is modulated based on predetermined image information, and the modulated laser beam LB is made into parallel light by a collimator lens 32, and then input into a cylindrical lens 33, which rotates at a constant speed. The laser beam LB is reflected by the rotating polygon mirror 34, which scans the laser beam in an equal daily motion, and the fθ lens 36 converts the equal daily motion into scanning in a linear motion, and also changes the diameter of the laser beam LB. It is known that an image is recorded by narrowing down the light to form a minute spot on the photosensitive drum 38.

In this type of scanning optical device, in order to scan the laser beam LB at high speed, it is conceivable to rotate the rotating polygon 1134 at high speed or increase the number of mirror surfaces of the rotating polygon mirror 34, but due to the mechanical structure of the rotation mechanism, This is because there is a limit to the increase in rotational speed, and the rotating polygon 1134 becomes larger due to the increase in mirror surfaces.

Scanning at higher speeds was difficult. Also, in order to improve the quality of the recorded image, the diameter of the spot on the photosensitive drum 38 can be reduced by decreasing the exit F number of the fθ lens 36; It is necessary to increase the beam diameter of the laser beam LB. However, in order to increase the beam diameter, the rotating polygon mirror 34
It is necessary to enlarge the mirror surface of the rotating polygon 11.
34 becomes large, causing problems such as the need to suppress the rotational speed of the rotating polygon [134], making high-speed scanning difficult.

In order to solve the above problems, as shown in FIG. 7, a fixed reflection tIA 50 having a roof-type rotating polygon mirror 48 and at least one plane mirror disposed opposite to one of the reflection surfaces 48b, A 174-wavelength plate 52 and a polarizing beam splitter 54 are arranged to face the other reflective surface 48a.
A scanning optical device 40 is conceivable. In this device, the laser beam LB from the semiconductor laser 42 is transmitted through a collimator lens 44 and a cylindrical lens 46.
is incident on one reflective surface 48a through the other reflective surface 4
Since the laser beam LB is emitted from the fixed reflecting mirror 50 and re-entered the same rotating polygon mirror 48 for deflection scanning, the laser beam LB
The scanning angle is twice as wide as that of the device shown in FIG. Therefore, when realizing the same scanning, the outer diameter of the roof-type rotating polygon mirror 48 can be made smaller and more compact than the device shown in FIG. The polygon mirror 48 can be rotated at high speed.

Furthermore, the scanning angle of the reflecting surface 48a or 48b in the same plane perpendicular to the rotation axis (shown in FIG. By making the exit F number from the lens sufficiently small, the diameter of the spot on the photosensitive drum (not shown) can be made sufficiently small, and a high-quality image can be recorded on the photosensitive drum.

By the way, in the above optical configuration, the roof-type rotating polygon!
Since the incident optical path and the output optical path of the laser beam LB in 148 may be the same optical path, it is necessary to separate the output optical paths somewhere. For example, as shown in FIG. It is conceivable to make the laser beam LB obliquely incident on the main scanning cross section of 11148 to separate the input optical path and the output optical path. However, in this case, the amount of lateral shift of the laser beam LB during scanning increases,
Since the roof-type rotating polygon mirror 48 becomes larger (the 11 sides have to be made wider), there is no merit in making it roof-type.
Problems such as this occur. Also, as shown in Figure 9,
It is also possible to make the laser beam LB obliquely incident on the sub-scanning section of the roof-type rotating polygon mirror 1148, but although the roof-type rotating polygon mirror 48 will not become large, the laser beam B will be twisted and the scanning line will be bent. At the same time, the imaging performance deteriorates.

Therefore, in the above device 40, the 4/4 plate 52 and the polarizing beam splitter 54 are used to separate the input optical path and the output optical path. That is, the laser beam L emitted from the semiconductor laser 42 and transmitted through the polarizing beam splitter 54
B is converted from 2 linearly polarized light on 52 L/4 plates to circularly polarized light and then converted into a roof-type rotating polygon! 148. And fixed reflection with the reflecting surfaces 48a and 48b of the roof-type rotating polygon mirror 48! After being reflected by the I50 an odd number of times and converted into circularly polarized light in the opposite direction, it enters the E/4 plate 52 again and is converted into linearly polarized light whose polarization plane is perpendicular to the linearly polarized light at the time of incidence. , which is now reflected by the polarizing beam splitter 54,
The light is further reflected by a reflecting mirror 56, takes an optical path parallel to the incident optical path in the sub-scanning direction cross section, passes through an fθ lens (not shown), and forms an image on a photosensitive drum (not shown).

[Problems to be Solved by the Invention] However, in the above device, the laser beam LB deflected and scanned by the roof-type rotating polygon II 48 is
Since the beam enters the polarizing beam splitter 54 with an angle of view, an E/4 plate 52 and a polarizing beam splitter 54 of a size that can cover the area during scanning are required, and these members 5
A problem arises in that 2.54 becomes larger.

Furthermore, 'L/' of the deflected and scanned beam on-axis and off-axis
Since the angle of incidence on the fourth plate 52 changes, the circularly polarized light reflected by the roof-type rotating polygon mirror 48 becomes completely circularly polarized light.
There is also the problem that the light does not enter the fourth plate 52, so when it is converted into linearly polarized light by the fourth plate 52, it does not become completely linearly polarized light, and when it is reflected by the polarizing beam splitter 54, a loss in light amount occurs. . Of course, it is conceivable to use a laminated type crystal λ/4 plate with less dependence on the incident angle, but it is expensive and therefore the manufacturing cost will increase significantly.

SUMMARY OF THE INVENTION Therefore, an object of the present invention is to provide a scanning optical device that can be manufactured in a small size and at a relatively low cost, and that exhibits less fluctuation in light amount.

[Means for Solving the Problems] In the present invention that achieves the above object, in a scanning optical device that deflects and scans a light beam emitted from a light source by reflecting it on a mirror surface of a rotating polygon mirror, each mirror surface of the rotating polygon mirror is , consisting of a pair of reflective surfaces inclined toward the center of rotation of a rotating polygon mirror and orthogonal to each other, and having at least one reflective surface facing one of the pair of reflective surfaces and returning to the one reflective surface. A mirror is fixed and opposite the other of said pair of reflective surfaces.

A Fresnel-Rom prism and a polarizing beam splitter are provided.

Specifically, the fixed reflector is a plane mirror with one reflective surface or a roof-type reflector, and from the light source to the rotating polygon mirror is a polarizing beam splitter that is tilted with respect to the optical axis in the main scanning plane. Alternatively, the light beam is guided through a Fresnel-Rhom prism tilted with respect to the optical axis in the main scanning plane.

[Example] Embodiments to which the present invention is applied will be described below with reference to the drawings.

First, a first embodiment of the present invention will be described.

FIG. 1 is a front view (a cross-sectional view in the sub-scanning direction) of the first embodiment, and FIG. 2 is a plan view thereof (a view in a main-scanning cross-section).

As shown in the figure, the scanning optical device 1 includes a semiconductor laser 3 as a light source that emits a laser beam LB, a collimator lens 5 that collimates the laser beam LB incident from the semiconductor laser 3, and a collimator lens 5 that emits light only in a predetermined direction. Cylindrical lens 7 that focuses
a roof-type rotating polygon mirror (hereinafter simply referred to as a polygon mirror) 9 that encloses a plurality of mirror surfaces formed by two reflecting surfaces 9a and 9b;
A polarizing beam splitter 11 disposed facing one of the reflective surfaces 9a, a Fresnel-Rom prism (Fresnel rhomboid) 13 disposed between the polarizing beam splitter 11 and the polygon mirror 9, and a polygon The mirror 9 is arranged to face the other reflecting surface 9b of the mirror 9, and is configured mainly with a fixed reflecting mirror 15 consisting of one plane mirror.

The cylindrical lens 7 has power only in the sub-scanning direction, which is a direction perpendicular to the main-scanning section, and images the parallel light beam LB incident from the collimator lens 5 onto a line on the fixed reflecting mirror 15.

The polarized beam input splitter 11 is disposed at a predetermined angle (that is, inclined with respect to the optical axis) with respect to the laser beam LB incident from the semiconductor laser 3 in the main scanning section. Therefore, the linearly polarized laser beam LB whose polarization direction has a predetermined angle with respect to the incident plane passes through the polarizing beam splitter 11 without loss of light quantity and enters the Fresnel-Rom prism 13.

As shown in FIG. 3, the Fresnel-Rom prism 13 is made of a prismatic member made of a transparent isotropic body with a parallelogram cross section.
A polarizing prism converts the linearly polarized laser beam LB that enters from the semiconductor laser 3 via the polarizing beam splitter 11 into circularly polarized light, and converts the polarized and scanned circularly polarized laser beam LB that enters from the polygon mirror 9 into linearly polarized light. be. In the Fresnel-Rom prism 13, the incident linearly polarized laser beam LB is totally reflected on the prism surface and is divided into a P-wave component whose electric vector is polarized parallel to the plane of incidence and an S-wave component whose electric vector is polarized light perpendicular to the plane of incidence. It is separated into At that time, the phase jump δS of the S-polarized light and the phase jump δP of the P-polarized light are determined by the refractive index of the Fresnel-Rom prism 13 being n, and the incident angle being θ.
Then, as is clear from Fresnel's equation, it is expressed by the following equation.

δ5=2tan-'((n"s i n"-θ-
1) l/1/n-cos θ) δP=2jan-'(n (n"sin" θ
-1)l/a/cosθ) Here, the angular phase difference of the Fresnel-Rom prism 13 (δP
An example of the correlation between the angle of incidence of the laser beam LB on the Fresnel-Lomb prism 13 is shown in FIG. 4(A). The Fresnel-Rom prism 13 is attached to the laser beam LB so that the angle is in the range of 45° to 50°.
By adjusting the arrangement angle of , the angular phase difference becomes about 45°. When the laser beam LB passes through the Fresnel-Lomb prism 13, it is totally reflected twice on the prism surface of the Fresnel-Lomb prism 13, so the angular phase difference thereof is about 90°. Therefore, the laser beam LB is converted from linearly polarized light to circularly polarized light.

In the scanning optical device 1, the laser beam LB emitted from the semiconductor laser 3 passes through the collimator lens 5, becomes a parallel beam, passes through the polarizing beam splitter 11, passes through the Fresnel-Rom prism 13, and enters the polygon mirror 9. The light is incident on one reflecting surface 9a. At this time, the linearly polarized laser beam LB having a predetermined angle with respect to the optical axis passes through the polarizing beam splitter 11 and then passes through the Fresnel-Rom prism 13.
is converted into circularly polarized light.

Then, the laser beam LB is reflected by one reflecting surface 9a of the polygon mirror 9 and enters the other reflecting surface 9b, and is further reflected there, thereby changing the direction in which it enters the one reflecting surface 9a. travels in the opposite direction, then the laser beam L
B is a fixed reflection placed so as to intersect with the direction of travel!
115 and is turned back, the light enters the reflecting surface 9b again and takes an optical path of reflecting surface 9b - reflecting surface 9a - Fresnel-Rom prism 13. Since it is reflected an odd number of times on the reflecting surface, it is different from the time of incidence. The laser beam LB, which is 0 circularly polarized light that becomes reversely circularly polarized light and re-enters the polarization beam splitter 11 from the Fresnel-Lomb prism 13, enters the Fresnel-Lomb prism 13, and the plane of polarization is at right angles to the linearly polarized light upon incidence. Since the light is converted into linearly polarized light, it is then reflected by the polarizing beam splitter 11. and auxiliary reflection [117
It is reflected by the fθ lens (not shown) and passes through the photosensitive drum (
(not shown). At this time, as shown in Figure 2, there are many faces! 19 rotates, the laser beam LB that has passed through the polarizing beam splitter 11 is scanned in the main scanning cross section in an equal daily motion, but is converted into a linear motion by the fθ lens and scanned on the photosensitive drum.

The laser beam LB travels along the optical path described above, but at that time, the angular phase difference between the P-polarized light and the S-polarized light is 90@
If the laser beam LB is distorted, the Fresnel-Lomb prism 13 does not convert the linearly polarized light into completely circularly polarized light, but instead becomes elliptically polarized light. If the laser beam LB is elliptically polarized light, a loss in light quantity occurs when it is reflected by the polarizing beam splitter 13.

Therefore, in order to prevent this loss, the prism angle of the Fresnel-Rom prism 13 is set to 48°, and the scanning angle range of the laser beam LB is set to 0° to 34°. In this way, Figure 4 (B)
The polarization degree (Ellipticity, ωX100
= tan δ/2x100) and the prism incident angle, the angle of incidence on the prism reflective surface falls within the range of 48 to 56.3 degrees, and the variation in light amount at all scanning angles is within 5%. becomes.

As described above, in this embodiment, one reflective surface 9a of the polygon mirror 9
By arranging the Fresnel-Rom prism 13 and the polarizing beam splitter 11 facing each other, the laser beam LB scanned by the polygonal mirror 9 can be scanned by the polygonal mirror 9! It is possible to reduce a loss in the amount of light caused by a change in the angle of emission from the laser beam 119 (angle of incidence on the Fresnel-Rom prism 13). In addition, a λ/4 plate with strong incidence angle dependence or an expensive crystal plate/
Since there is no need to use four plates (there is no need to make the light beam obliquely incident on the polygon mirror 9 (see FIG. 8), the device can be manufactured in a small size and at low cost.

Further, in this embodiment, a fixed mirror 11115 is disposed opposite to one reflecting surface 9b of the rotating polygon mirror 9, and two of the rotating polygon mirrors 9
The laser beam LB reflected by the two reflecting surfaces 9a and 9b and incident on the fixed reflecting mirror 15 is reflected by the fixed reflecting mirror 15 and reflected again by the two reflecting surfaces 9a and 9b. Therefore, the scanning angle of the laser beam LB is expanded to twice that of the conventional example shown in FIG. Therefore, the outer diameter of the roof-type rotary polygon mirror 9 can be reduced and the size can be reduced, so that the roof-type rotary polygon mirror 7 can be rotated at high speed, and the laser beam LB can be scanned at high speed. Alternatively, it is possible to increase the number of mirror surfaces of the polygon mirror 9 even if the size is the same, and it becomes possible to scan the laser beam LB at higher speed without increasing the rotation speed.

In this embodiment, since the phase difference between the P-polarized light and the S-polarized light due to total reflection of the Fresnel-Lomb prism 13 is utilized, the laser beam L traveling straight through the reflective surface of the polarizing beam splitter 11 is
By tilting the scanning cross section at a predetermined angle with respect to B (that is, with respect to the optical axis) as shown in Figure 2, the laser beam LB becomes a Fresnel beam as a beam tilted at 45° including a P polarization component and an S polarization component. Since the laser beam LB is configured to be incident on the Romme prism 13, the cross section of the laser beam LB is elliptical, and the imaging spot is tilted on the photosensitive drum, but as shown in FIG. By tilting the laser beam LB and shaping the cross section of the laser beam LB into a circular shape, a good image can be recorded.

[Effects of the Invention] As explained above, according to the present invention, a fixed reflecting mirror is disposed opposite to one reflecting surface of a plurality of pairs of rotating polygon mirrors, and an incident beam is arranged opposite to the other reflecting surface. A Fresnel-Rom prism and a polarizing beam splitter are installed as a means for separating the optical path of the output beam and the output beam, reducing the loss of light quantity and eliminating the need for the conventional 4/4 plate or expensive There is no need to use a crystal λ/4 plate. That is,
It is possible to provide an inexpensive and compact scanning optical system that has little difference in polarization retardation over all scanning angles and has little light loss.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the configuration of the first embodiment in a cross-section in the sub-scanning direction, FIG. 2 is a diagram in the main-scanning cross-section, FIG. 3 is an explanatory diagram of a Fresnel ROM prism, and FIG. 4(A) is a diagram showing a Fresnel ROM prism. A graph showing the correlation between the angular phase difference of the prism and the prism incidence angle, FIG. 4 (B) is a graph showing the correlation between the angle of incidence and the degree of polarization, and FIG. FIG. 6 is a perspective view of a conventional example, FIG. 7 is a cross-section in the sub-scanning direction of another conventional example, and FIG. 8 is a main-scanning cross-section of another conventional example. FIG. 9 is a diagram showing the configuration of another conventional example in a cross-section in the sub-scanning direction. 1...Scanning optical device, 3...Semiconductor laser, 9...Roof type rotating polygon mirror, 11. Polarizing beam splitter, 3. Fresnel-Rom prism, - Fixed reflecting mirror

Claims (1)

[Scope of Claims] 1. In a scanning optical device that deflects and scans a light beam emitted from a light source by reflecting it on a mirror surface of a rotating polygon mirror, each mirror surface of the rotating polygon mirror is inclined toward the rotation center of the rotating polygon mirror. a reflecting mirror comprising a pair of reflecting surfaces orthogonal to each other, and having at least one reflecting surface facing one of the pair of reflecting surfaces and returning to the one reflecting surface is fixed; A scanning optical device in which a Fresnel-Rom prism and a polarizing beam splitter are arranged opposite to each other on the other surface. 2. Claim 1, wherein the polarizing beam splitter is arranged at a predetermined angle with respect to the optical axis in the scanning plane of the light beam.
Scanning optical device as described. 3. Claim 1, wherein the Fresnel-Rom prism is arranged at a predetermined angle with respect to the optical axis in the scanning plane of the light beam.
Scanning optical device as described.