Optical scanner

Details Optical scanner Optical scanner

2001-05-29

2004-08-31

Phan, James (Department: 2872)

Optical: systems and elements

Deflection using a moving element

C359S204200, C359S205100, C359S208100, C359S216100, C359S488010

Reexamination Certificate

active

06785029

ABSTRACT:

RELATED APPLICATION
This application is based on application No.
2000-157609
filed in Japan, the content of which is incorporated herein by reference.
FIELD OF THE INVENTION
The present invention relates to an optical scanner provided in an image forming apparatus such as a laser beam printer, and more particularly, to an optical scanner that scans a surface by means of a plurality of light beams.
DESCRIPTION OF THE PRIOR ART
In an image forming apparatus such as a laser beam printer, a surface to be scanned (for example, the surface of a photoconductor drum) is scanned by a light beam to thereby form an image on the surface.
Typically, scanning is performed by deflecting the light beam from a light source in one direction by a deflector such as a rotating polygon mirror, and varying the relative positions of the light beam and the surface to be scanned in a direction perpendicular to the direction of the deflection. Deflecting the light beam is called main scanning, and the direction thereof is called the main scanning direction. Varying the relative positions of the light beam and the surface to be scanned in a direction perpendicular to the main scanning direction is called sub-scanning, and the direction thereof is called the sub-scanning direction.
In recent years, to increase the speed of image formation, scanning is performed by means of a plurality of light beams spaced a minute distance apart in the sub-scanning direction. There have been proposed various methods for obtaining light beams spaced a minute distance apart, and they are broadly divided into those using a single light source and those using a plurality of light sources.
For example, in Japanese Laid-open Patent Application No. 9-281420, a plurality of light beams spaced a minute distance apart are directly obtained from a single surface emitting laser having a plurality of light emitting portions, and in Japanese Laid-open Patent Application No. H8-338957, a plurality of lasers and a plurality of optical fibers are used, and the light beams from the lasers are conducted by the optical fibers so as to be spaced a minute distance apart at the scanned surface. Moreover, in Japanese Laid-open Patent Application No. 9-218363, two lasers and a polarization beam splitter are used, and two light beams spaced a minute distance apart and traveling substantially in the same direction are obtained by transmission and reflection by the polarization beam splitter.
Generally, an optical scanner requires several optical elements such as a collimator lens for collimating light beams and an imaging lens for converging the collimated light beams on the surface to be scanned, in addition to the deflector for the main scanning. To prevent contamination and damage, the deflector is housed in a case having a glass window that allows light beams to pass therethrough. Further, a mirror for bending the optical paths of the light beams is sometimes provided to reduce the overall size.
In many cases, a thin film for enhancing reflectance or transmittance is provided on the surface of each optical element. Typical film structures of reflection surfaces and transmission surfaces of optical elements of a conventional optical scanner are shown in Tables 1 to 6. The refractive index n, the optical film thickness nd and the absorption coefficient k shown in these tables are for a light beam of a wavelength &lgr; of 780 nm.
TABLE 1
Reflection surface RX (wavelength &lgr; = 780 nm)
Refractive
Absorption
Material
Index n
Coefficient k
Al
1.92
7.08
TABLE 2
Reflection surface RY (wavelength &lgr; = 780 nm)
Refractive
Optical
Absorption
Material
Index n
Thickness nd
Coefficient k
First Layer
TiO
2
2.35
0.25&lgr;
Second Layer
MgF
2
1.38
0.25&lgr;
Al
1.92
7.08
TABLE 3
Reflection surface RZ (wavelength &lgr; = 780 nm)
Refractive
Optical
Absorption
Material
Index n
Thickness nd
Coefficient k
First Layer
TiO
2
2.35
0.25&lgr;
Second Layer
MgF
2
1.38
0.25&lgr;
Third Layer
TiO
2
2.35
0.25&lgr;
Fourth Layer
MgF
2
1.38
0.25&lgr;
Al
1.92
7.08
TABLE 4
Transmission surface TX (wavelength &lgr; = 780 nm)
Refractive
Material
Index n
BK7
1.51
TABLE 5
Transmission surface TY (wavelength &lgr; = 780 nm)
Refractive
Optical
Material
Index n
Thickness nd
First Layer
MgF
2
1.38
0.25&lgr;
BK7
1.51
TABLE 6
Transmission surface TZ (wavelength &lgr; = 780 nm)
Refractive
Optical
Material
Index n
Thickness nd
First Layer
MgF
2
1.38
0.25&lgr;
Second Layer
ZrO
2
2.10
0.50&lgr;
Third Layer
Al
2
O
3
1.62
0.25&lgr;
BK7
1.51
The reflection surface RX of Table 1, which has the simplest structure, comprises only a reflection film of Al. The reflection surface RY of Table 2 has two layers of thin films of MgF
2
and TiO
2
on a reflection film of Al. For the layer numbering of the thin films in the various surfaces, the outermost layer is numbered 1. The reflection surface RZ of Table 3 has four layers of thin films of MgF
2
and TiO
2
on a reflection film of Al.
The transmission surface TX of Table 4 having the simplest structure as only BK7 glass which is the base material of the element. The transmission surface TY of Table 5 has one layer of thin film of MgF
2
on the base material of BK7 glass. The transmission surface TZ of Table 6 has three layers of thin films of Al
2
O
3
, ZrO
2
and MgF
2
on the base material of BK7 glass.
The refractive indices n of the thin films provided on the reflection surfaces RY and RZ and the transmission surfaces TY and TZ depend on the material, and the optical film thickness nd of the thin films are ¼ or ½ of the wavelength &lgr;.
Reflection surfaces and transmission surfaces of the above-described structures which have been used in optical scanners using a single light beam are also used in optical scanners which employ a plurality of light beams. However, when reflection surfaces and transmission surfaces of optical elements of optical scanners using a plurality of light beams are disposed as described below, the light quantities of the light beams on the surface to be scanned vary due to a disagreement in polarization direction among the light beams, so that the densities of the images formed by the light beams vary. This is because, when the polarization directions of the light beams are different, the reflectance and the transmittance vary according to the incident angle.
The relationships between the reflectances for s-polarized light and for p-polarized light and the incident angle on the reflection surfaces RX, RY and RZ of Tables 1 to 3 are shown in
FIGS. 11
to
13
. The relationships between the transmittances for s-polarized light and for p-polarized light and the incident angle on the transmission surfaces TX, TY and TZ of Tables 4 to 6 are shown in
FIGS. 14
to
16
. As is apparent from these figures, s-polarized light is more easily reflected than p-polarized light, and the differences in reflectance and transmittance between these two types of polarized light increases as the incident angle increases.
Generally, the angle of incidence of light beams on the deflector such as a rotating polygon mirror performing the main scanning is at most approximately 60° and the angle of incidence on the window of the case of the deflector is also at most approximately 60°. When the simple reflection surface RX is used as the reflection surfaces of the deflector, the difference between the reflectance for s-polarized light and the reflectance for p-polarized light at an incident angle of 60° is as large as 17.1%. Even when the reflection surface RY or RZ having a thin film is used as the reflection surface of the deflector, the difference between the reflectances for the two types of polarized light at an incident angle of 60° is 11.1% or 7.4%, which is still large. When the simple transmission surface TX is used for the window of the case, the difference between the transmittance for s-polarized light and the transmittance for p-polarized light at an incident angle of 60° is 17.8%, and even when the transmission surface TY or TZ is used for the window, the difference between the

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