1987-04-27
1988-06-14
Corbin, John K.
G02B 1310
Patent
active
047508191
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to an anamorphic prism for correcting an anisotropy in beam radiation angles.
BACKGROUND ART
For example, in an optical head for a programmable optical recording medium, a collimator lens having a high numerical aperture is normally used in order to improve utilization efficiency of a beam. When a semiconductor laser is used as a light source, an anisotropy in beam irradiation angles is generally corrected using an anamorphic prism.
The semiconductor laser causes a wavelength fluctuation .DELTA..lambda. of 10 to 20 nm due to a change in temperature, output power, and the like, or upon incidence of a return beam from the optical recording medium. A refractive index of the anamorphic prism changes due to the wavelength fluctuation .DELTA..lambda., and as a result, an angle of an exit beam from the anamorphic prism may be changed. However, the conventional anamorphic prism cannot cope with this respect.
In a conventional anamorphic prism which consists of a single prism using an optical glass SF13 and has an enlargement magnification ratio .beta..apprxeq.1.7, if .DELTA..lambda..apprxeq.10 nm, an exit angle change of .DELTA..xi..apprxeq.0.02.degree. occurs. Therefore, when an objective lens having a focal length f=4.5 mm is used, a beam spot is shifted by .DELTA.x=f.multidot..DELTA..xi.=1.5 .mu.m in a lateral direction on the optical recording medium.
The wavelength fluctuation of about .DELTA..lambda..apprxeq.10 nm easily occurs due to various conditions, and cannot be controlled. As a result, it is difficult to perform positional control of the beam spot for a recording or reproducing operation, and a stable and accurate recording or reproducing operation cannot be performed.
DISCLOSURE OF INVENTION
An anamorphic prism according to the present invention comprises a first prism which has a refractive index of n.sub.1, a refractive index change of .DELTA.n.sub.1 due to a wavelength fluctuation, and a vertex angle of .theta..sub.1, and a second prism which has the refractive index of n.sub.2, the refractive index change of .DELTA.n.sub.2, and a vertex angle of .theta..sub.2, and is bonded to the first prism. If an incident angle on the second prism of the beam that is incident on the first prism at an incident angle .phi..sub.1 is given as .phi..sub.2 and the incident angle .phi..sub.2 <the vertex angle .phi..sub.1, the first and second prisms and the incident angle .phi..sub.1 satisfy the following relation: /(n.sub.1.sup.2 -sin.sup.2 .phi..sub.1) satisfy: /(n.sub.1.sup.2 -sin.sup.2 .phi..sub.1) 0.degree. with respect to the vertex angle .theta..sub.2.
With this structure, the anamorphic prism according to the present invention has no dependency with respect to a wavelength, and can have an achromatic structure.
For this reason, when the anamorphic prism according to the present invention is applied to an optical head in which a semiconductor laser is used as a light source and which is used for a recording or reproducing operation of information with respect to an optical recording medium such as an optical disk, even if a wavelength fluctuation occurs in the semiconductor laser due to a change in temperature, output power or the like of the semiconductor laser or upon incidence of the return beam from the optical recording medium, an angle of a beam emerged from the anamorphic prism will not be changed. Therefore, even if the wavelength fluctuation occurs, the irradiation beam spot cannot be fluctuated on the optical recording medium, and a stable and accurate recording or reproducing operation can be performed.
BRIEF DESCRIPTION OF DRAWINGS
FIGS. 1 and 2 are side views for explaining the principle of the present invention, respectively;
FIGS. 3 to 6 are side views showing first to fourth embodiments of the present invention; and
FIG. 7 is a graph showing the relationship between a wavelength and an exit angle change in the first embodiment and the two prior arts of the present invention.
BEST MODE OF CARRYING OUT THE INVENTION
First to fourth emb
REFERENCES:
patent: 4017160 (1977-04-01), Betensky
Corbin John K.
Gass Rebecca D.
Sony Corporation
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