Optical: systems and elements – Prism – With refracting surface
Reexamination Certificate
2000-07-31
2001-04-03
Phan, James (Department: 2872)
Optical: systems and elements
Prism
With refracting surface
C359S831000
Reexamination Certificate
active
06212022
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a beam shaping optical system that affects a cross-sectional shape of a light beam, and a multi-beam optical system employing the beam shaping optical system.
A laser diode emits a beam having an elliptical-shaped far-field pattern. To convert the elliptical-shaped light beam to a beam having a circular cross section, conventionally, a beam shaping optical system has been used. The conventional beam shaping optical system is provided with a single or a pair of wedge prisms, which are aligned such that a principal section thereof is in parallel with a direction of a semiminor axis of the elliptical-shape of the beam. It should be noted that the principal section of a wedge prism is defined as a plane perpendicular to both of the two refractive surfaces thereof through which a light beam passes. By the above arrangement, the semiminor axis of the elliptical beam is magnified by the wedge prism to have substantially the same length of the semimajor axis thereof.
This type of beam shaping optical system is generally included in an optical system of a laser beam printer or the like in order to control light output by a light source such as a laser diode effectively to form a symmetrical beam spot on an object surface.
Further, in order to increase the scanning efficiency of such devices (laser beam printers and the like), a plurality of light beams may be simultaneously used to read or write information. In such a case, a multi-beam laser diode is generally used as a light source. A multi-beam laser diode includes a plurality of light emitting points arranged in a single element.
However, if the conventional beam shaping optical system which is designed for converting the cross-sectional shape of a single light beam is used for the multi-beam optical system, a problem arises in that, due to a variation in incident angles onto the wedge prism among the beams emitted from the multiple emitting points of the multi-beam laser diode and collimated by a collimating lens, intervals between the spots formed on the objective surface (i.e., a surface on which the spots are to be formed) cannot be maintained constant. Specifically, the light emitting points of the multi-beam laser diode are aligned in a direction in parallel with the semiminor axes of the elliptical shape of the emitted beams. Therefore, in order to magnify the semiminor axis of the elliptical cross section of each beam, it is preferable that the light emitting points of the multi-beam laser diode are aligned in a direction parallel to the principal section of the wedge prism. However, since an angular magnification by the beam shaping optical system in the principal section varies depending on the incident angle of an incident beam, emitting angles of the plurality of beams are different from incident angles thereof. Therefore, when the conventional beam shaping optical system is used for a multi-beam optical system employing a multi-beam laser diode, beam spots formed on the objective surface may be positioned at irregular intervals even if the emitting points of the light beams are positioned at predetermined regular intervals.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an improved beam shaping optical system with which elliptical-shaped light beams emitted from light emitting points aligned at regular intervals are converted to circular-shaped light beams which are aligned at regular intervals.
According to an aspect of the invention, there is provided a multi-beam optical system, comprising: a light source having a plurality of light emitting portions from which a plurality of light beams are emitted, respectively; a collimating lens which collimates the plurality of light beams into a plurality of parallel light beams; a beam shaping optical system including at least one wedge prism, the beam shaping optical system shaping cross section of the plurality of parallel beams; an image forming optical system which converges the plurality of parallel beams transmitted through the beam shaping optical system, wherein the plurality of light emitting portions are aligned in a direction perpendicular to a principal section of the wedge prism.
Since the plurality of light emitting portions are aligned in a direction perpendicular to the principal section of the wedge prism, the wedge prism functions as a parallel plate in the direction in which the light emitting portions are aligned, if the angle between each adjacent beams are even, the spots formed on the objective surface have regular intervals.
Optionally, each of the plurality of light emitting portions may emit an elliptical light beam which has an elliptical cross section having a semimajor axis and semiminor axis, and the plurality of light emitting portions can be aligned in a direction where the semiminor axis extends.
Further, the light source comprises a multi-beam laser diode having the plurality of light emitting portions aligned in a direction parallel to an active layer of the laser diode.
Furthermore, the beam shaping optical system comprises a first and a second wedge prisms, a principal section of the first wedge prism and a principal section of the second wedge prism are parallel to each other.
Still optionally, when a reference ray is incident on the first wedge prism and a skew ray, which has a same incident angle as the reference ray if projected on the principal section and inclined by an angle V with respect to the reference ray in a plane perpendicular to the principal section, a condition:
|&thgr;
8
−&thgr;′
8
|<0.100°
is satisfied,
where, &thgr;8 is an emission angle of the reference ray emitted from the second wedge prism in the principal section, and &thgr;′8 is an emission angle of the skew ray emitted from the second wedge prism projected on the principal section, and |V|<10°.
Further, the image forming optical system may include a deflector which deflects the plurality of light beams emerged from the beam shaping optical system, and a scanning lens which converges the plurality of light beams deflected by the deflector, and the plurality of light emitting portions may be arranged such that a plurality of spots formed on a surface to be scanned are apart from each other in an auxiliary scanning direction.
According to another aspect of the invention, there is provided a beam shaping optical system, comprising a first and a second wedge prism, principal sections of the first and second wedge prisms being parallel with each other, wherein when a reference ray is incident on the first wedge prism and a skew ray, which has a same incident angle as the reference ray if projected on the principal section and inclined by an angle V with respect to the reference ray in a plane perpendicular to the principal section, a condition:
|&thgr;
8
−&thgr;′
8
|<0.100°
is satisfied,
where, &thgr;
8
is an emission angle of the reference ray emitted from the second wedge prism in the principal section, and &thgr;′′
8
is an emission angle of the skew ray emitted from the second wedge prism projected on the principal section, and |V|<10°.
Optionally, an incident angle of the reference ray onto the first prism may be zero degree.
Further, it is preferable that an anamorphic ratio Mv/Mh satisfies a condition:
1.00
<Mv/Mh<
3.00
where, Mv is a magnification (i.e., a diameter of an emitted beam/a diameter of an incident beam) of the beam shaping optical system within a plane perpendicular to the principal section, and Mh is a magnification (i.e., a diameter of an emitted beam/a diameter of an incident beam) of the beam shaping optical system in a plane perpendicular to the principal section.
REFERENCES:
patent: 4768184 (1988-08-01), Reno
patent: 4770507 (1988-09-01), Arimoto et al.
patent: 4872747 (1989-10-01), Jalkio et al.
patent: 4904068 (1990-02-01), Tatsuno et al.
patent: 5130840 (1992-07-01), Iima et al.
patent: 5189546 (1993-02-01), Iizuka
patent:
Asahi Kogaku Kogyo Kabushiki Kaisha
Greenblum & Bernstein P.L.C.
Phan James
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