Optical: systems and elements – Lens – With light limiting or controlling means
Reexamination Certificate
2001-06-21
2002-08-27
Epps, Georgia (Department: 2873)
Optical: systems and elements
Lens
With light limiting or controlling means
C359S328000, C359S332000, C385S129000, C385S132000
Reexamination Certificate
active
06441970
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical waveguide device used for optical information processing and optical measurement control performed utilizing a coherent light source, and a light source device and an optical apparatus including the optical waveguide device.
2. Description of the Related Art
In the fields of optical information recording and reproduction, a higher density of recording and reproduction is realized by using a light source for emitting light having a shorter wavelength. For example, whereas conventional compact disk apparatuses use near infrared light having a wavelength of about 780 nm, digital versatile disk (DVD) apparatuses for recording and reproducing information at a higher density use red semiconductor laser light having a wavelength of about 650 nm. In order to realize a next-generation optical disk apparatus for recording and reproducing information at a still higher density, development of blue laser light source devices have been actively developed. For example, a wavelength conversion element using a non-linear optical substance has been developed to be included in a compact and stable blue laser light source device.
FIG. 14
is a schematic view illustrating an exemplary blue light source device using a second harmonic generation element (hereinafter, referred to as an “SHG element”)
117
. With reference to
FIG. 14
, the SHG element
117
will be described.
The SHG element
117
includes a dielectric substrate
114
and a high refractive index area having a width of about 3 &mgr;m and a depth of about 2 &mgr;m formed by a proton exchange method. The high refractive index area acts as an optical waveguide
115
. Infrared light emitted from a semiconductor laser
111
having a wavelength of about 850 nm is collected on an incident surface
139
of the SHG element
117
through a collection lens
112
and then propagated through the optical waveguide
115
in the SHG element
117
to form a fundamental guided wave.
Lithium niobate crystals forming the dielectric substrate
114
have a non-linear optical constant. As a result of sufficiently large non-linear optical constant, a harmonic guided wave having a wavelength of about 425 nm is obtained by wavelength conversion of the infrared light, and excited from the electric field of the fundamental guided wave.
In order to compensate for a propagation constant difference between the fundamental guided wave and the harmonic guided wave, domain invention areas
116
are periodically formed in the optical waveguide
115
. The harmonic guided waves which are excited throughout the optical waveguide
115
are coherently added together and then come out from an outgoing surface
138
of the SHG element
117
.
In order to correctly compensate for the propagation constant difference between the fundamental guided wave and the harmonic guided wave, the wavelength of the fundamental guided wave needs to be maintained at a certain value. Accordingly, as the semiconductor laser
111
, a DBR laser is used for its very small wavelength fluctuation in accordance with the temperature or the like. A DBR laser has another feature in that since light is oscillated at a single wavelength, the light has a satisfactorily high coherency and a satisfactorily low RIN (relative intensity noise).
FIG. 15
is a schematic view of an optical disk pickup including the SHG element
117
shown in
FIG. 14
for providing blue light. With reference to
FIG. 15
, an operation of the optical disk pickup will be described.
Harmonic blue light output by the SHG element
117
passes through a collimator lens
113
, a polarization beam splitter
120
, a 1/4 wave plate
121
and an objective lens
122
and then is collected to an optical disk
124
.
The light modulated by the optical disk
124
is reflected by the polarization beam splitter
120
and guided to a light detector
125
by a collection lens
123
. Thus, a reproduction signal is obtained.
The SHG element
117
outputs linearly polarized light in a direction parallel to the page. This light passes through the 1/4 wave plate
121
and returns through the 1/4 wave plate
121
to become a polarized light which is in a direction perpendicular to the page. Thus, the light reflected by the optical disk
124
is all reflected by the polarization beam splitter
120
and does not return toward the SHG element
117
theoretically.
However, the optical disk
124
includes a material having a birefringence. Accordingly, in actuality, an unnecessary polarized component returns toward the SHG element
117
through the polarization beam splitter
120
.
While data stored in the optical disk
124
is reproduced, the objective lens
122
is positionally controlled to focus the light accurately to the optical disk
124
. Accordingly, the outgoing surface
138
of the SHG element
117
and the optical disk
124
form a confocal optical system. Thus, the light reflected by the optical disk
124
is accurately collected at the optical waveguide
115
on the outgoing surface
138
of the SHG element
117
.
In an optical system including a semiconductor laser as a light source, the light component which returns toward a light source after being reflected induces noise (mode hop noise). Conventionally, various proposals have been made for avoiding the mode hop noise.
For example, oscillation in a plurality of longitudinal modes is caused by modulating light from the semiconductor laser with a harmonic signal or by causing self-oscillation of the semiconductor laser.
In the field of optical communication, for collecting light from a semiconductor laser to an optical fiber, a light isolator utilizing a magneto-optical effect is commonly inserted between the semiconductor laser and the optical fiber.
Japanese Laid-Open Publication No. 5-323404 discloses a method, by which an incident surface of an optical fiber or an optical waveguide is obliquely polished, so that the returning light is obliquely reflected and does not return to the semiconductor laser.
These technologies are for reducing the mode hop noise induced by the light returning to inside the semiconductor laser as a light source.
The present inventors performed experiments on data reproduction by an optical pickup including the SHG element
117
shown in FIG.
15
. As a result, the present inventors found a noise which is generated by the following mechanism, which is different from induction by the returning light.
The returning light collected at the optical waveguide
115
on the outgoing surface
138
of the SHG element
117
is reflected by the outgoing surface
138
and interferes with the light coming out from the optical waveguide
115
. Thus, an interference noise is generated.
Due to such an interference noise, the optical power output from the SHG element
117
appears to have been changed from the optical disk
124
, and thus a reproduction signal from the optical disk
124
is modulated with a low frequency noise, resulting in signal deterioration.
Whereas noise induced by the returning light is generated by the interaction of the light inside the semiconductor laser
111
and the returning light reflected by the incident surface
139
of the SHG element
117
, the interference noise is generated by the interference of the light from the SHG element
117
and the returning light reflected by the outgoing surface
138
of the SHG element
117
.
The present inventors found another cause of the interference noise as a result of a further research. A portion of the returning light from an external optical system external to the optical waveguide device (including, for example, collimator lens
113
) is re-excited in the optical waveguide
115
as a guided wave and reflected by the incident surface
139
of the SHG element
117
. The light reflected by the incident surface
139
is interfered with the light from the semiconductor laser
111
. Such an interference also causes the interference noise.
As described above, an optical system including an optical waveguide device involv
Kasazumi Ken'ichi
Kitaoka Yasuo
Mizuuchi Kiminori
Yamamoto Kazuhisa
Epps Georgia
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
Spector David N.
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