Optical: systems and elements – Prism – With refracting surface
Reexamination Certificate
2000-09-22
2003-01-07
Spyrou, Cassandra (Department: 2872)
Optical: systems and elements
Prism
With refracting surface
C359S833000, C359S834000
Reexamination Certificate
active
06504660
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a prism which may be incorporated, for example, in an optical head used for recording/reproducing signals on an optical disk, and a method for producing the prism. The present invention further relates to an optical beam shaping apparatus using the above-mentioned prism for shaping a spatial light intensity distribution (for example, for shaping from an oval distribution to a circular distribution) of a light beam such as laser light, and an optical head device employing such an optical beam shaping apparatus. The present invention further relates to a method for shaping a light beam.
2. Description of the Related Art
FIG. 1
is a schematic cross-sectional view showing an optical beam shaping apparatus disclosed in Japanese Laid-Open Publication No. 62-187321 as an example of a conventional optical beam shaping apparatus. Herein, a refractive index of air is referred to as n
0
=1.
In the optical beam shaping apparatus shown in
FIG. 1
, a laser light
2
is emitted from a semiconductor laser
1
, transmitted through a collimating lens
3
, and is thereby converted into a parallel light
4
. The parallel light
4
, in turn, is incident on a surface
5
A of a prism
5
made of a glass material (with a refractive index of n
1
) at an incident angle &psgr;
1
(wherein &psgr;
1
is an angle defined by the incident light
4
and a normal
5
A′ to the surface
5
A of the prism
5
). The incident light
4
is refracted at the surface
5
A of the prism
5
, and becomes a refracted light
4
a
having a refractive angle &psgr;
1
′ with respect to the normal
5
A′ and an angle &psgr;
2
(not shown) with respect to the incident light
4
.
The refracted light
4
a
is then incident on a surface
5
B (or “a bottom surface
5
B” which opposes the surface
5
A of the prism
5
) at an incident angle &psgr;
2
(wherein &psgr;
2
is an angle defined by the light
4
a
and a normal
5
B′ to the surface
5
B). The refracted light
4
a
reflects off the surface
5
B and becomes a reflected light
4
a
′. An angle between the reflected light
4
a
′ and the incident light
4
is referred to as an angle &thgr;
2
′ (not shown).
The reflected light
4
a
′ is incident on the surface
5
A at an incident angle &psgr;
21
(wherein &psgr;
21
is an angle defined by the light
4
a
′ and the normal
5
A′), thereby being refracted by a refractive angle &psgr;
21
′ to the normal
5
A′ and becomes emitting light
6
. An angle between the refracted light
6
(i.e., the emitting light
6
) and the original incident light
4
is referred to as an azimuth angle &thgr;
21
′ (not shown).
When the surface
5
A of the prism
5
is inclined by &agr;
1
to the incident light
4
, the incident angle &psgr;
1
is characterized as follows:
&psgr;
1
=&pgr;/2−&agr;
1
Formula (1)
Further, the following Formula (2) is derived from Snell's Law at the surface
5
A:
sin &psgr;
1
=n
1
sin &psgr;
1
′ Formula (2)
Due to this refraction, the incident light
4
is either magnified or reduced by a factor of (cos &psgr;
1
′/cos &psgr;
1
) within the refracting plane (i.e., within the plane of the drawing). The azimuth angle &thgr;
2
of the refracted light
4
a
is given by the following Formula (3):
&thgr;
2
=−&pgr;2+&agr;
1
+&psgr;
1
′ Formula (3)
When the bottom surface
5
B is inclined by &agr;
2
to the incident light
4
, the incident angle &psgr;
2
is characterized as follows:
&psgr;
2
=&pgr;/2−&agr;
2
+&thgr;
2
Formula (4)
Further, the following Formula (5) is derived from the Law of Reflection at the bottom surface
5
B:
&thgr;
2
′=&pgr;/2+&agr;
2
−&psgr;
2
Formula (5)
The incident angle &psgr;
21
of the light
4
a
′ to the surface
5
A is given by the following Formula (6):
&psgr;
21
=&pgr;/2+&agr;
1
−&thgr;
2
′ Formula (6)
Further, the following Formula (7) is derived from Snell's Law at the surface
5
A:
n
1
sin &psgr;
21
=sin &psgr;
21
′ Formula (7)
Due to this refraction, the light is further magnified or reduced by a factor of (cos &psgr;
21
′/cos &psgr;
21
) within the refracting plane.
The azimuth angle &thgr;
21
′ of the emitting light
6
is given by the following Formula (8):
&thgr;
21
′=&pgr;/2+&agr;
1
−&psgr;
21
′ Formula (8)
Due to the two refractions at the surface
5
A, the emitting light
6
is either magnified or reduced by a factor of m within the refracting plane, where m is given by the following Formula (9):
m
=(cos &psgr;
1
′/cos &psgr;
1
)·(cos &psgr;
21
′/cos &psgr;
21
) Formula (9)
By sequentially applying the above-mentioned Formulae (1) through (9), for example, when BK7 is selected as a glass material for forming the prism
5
under the following conditions: an oscillation wavelength of the semiconductor laser
1
=0.64385 &mgr;m (where n
1
=1.51425); &agr;
1
=17.59°; and &agr;
2
=31.34°, an azimuth angle &thgr;
21
′ of the emitting light
6
of 89.9963° and a magnification ratio m of 2.501 are obtained. The traveling direction of the emitting light
6
is bent by an angle of about 90° with respect to that of the incident light
4
and the beam is magnified about 2.5 times within the refracting plane.
In general, the parallel light
4
derived from the light
2
emitted from the semiconductor laser
1
has an oval spatial light intensity distribution (an oval cross-sectional intensity with an ellipticity of about 2.5). The above-described prism
5
magnifies the spatial light intensity distribution in a direction along a minor axis of the oval distribution, thereby obtaining the parallel light having a circular spatial light intensity distribution (a circular cross-sectional intensity).
However, such a conventional light beam shaping apparatus has the following problems.
A glass material forming the prism
5
always has a wavelength dependency of the refractive index (i.e., “dispersion”). Specifically, the refractive index of the light becomes smaller as the wavelength of the light becomes longer. For example, in the case where the prism
5
is made of BK7 under the conditions where an oscillation wavelength of the semiconductor laser
1
is 0.70652 &mgr;m, the refractive index n
1
of the prism
5
is 1.51243. Under this circumstance, the azimuth angle (emitting angle) &thgr;
21
′ of the emitting light
6
is 89.9313° which is smaller by 0.065° than that in the above-described case where the oscillation wavelength of the semiconductor laser
1
is 0.64385 &mgr;m.
Generally, due to variation in the output of the semiconductor laser
1
, the oscillation wavelength is momentarily fluctuated several nanometers. When the oscillation wavelength is fluctuated, for example, by 10 nm in the above-described conventional optical beam shaping apparatus which employs the prism
5
made of BK 7, the azimuth angle &thgr;
21
′ of the emitting light
6
changes by 0.0104°.
In the case where the emitting light
6
is focused by an objective lens (e.g., with a focal length of 3 mm) so as to be used in an optical head for recording/reproducing signals in an optical disk, the above-mentioned change in the angle of 0.0104° will result in a spot displacement of 0.54 &mgr;m. This spot displacement of 0.54 &mgr;m is not negligible when the optical head is used for reproducing signals recorded in signal pits of the optical disk on the order of submicrons, and may result in a fatal defect.
SUMMARY OF THE INVENTION
A prism of the present invention includes: a first portion made of a first material having a wavelength dependency in a refractive index; and a second portion abutting to the first portion, the second portion being made of a second material having a wavelength dependency in a refractive in
Nishiwaki Seiji
Saimi Tetsuo
Fineman Lee A.
Matsushita Electric - Industrial Co., Ltd.
Renner , Otto, Boisselle & Sklar, LLP
Spyrou Cassandra
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