Optical waveguides – Planar optical waveguide
Reexamination Certificate
2002-08-01
2004-08-31
Sanghavi, Hemang (Department: 2874)
Optical waveguides
Planar optical waveguide
C385S122000, C359S328000
Reexamination Certificate
active
06785457
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical waveguide device used, e.g., in the fields of optical information processing and optical application measurement and control, and to a coherent light source and an optical apparatus using the same.
2. Description of the Related Art
Optical information recording/reproducing apparatuses can achieve higher density by using a shorter-wavelength light source. For example, a widespread compact disk (CD) apparatus uses near-infrared light having a wavelength of 780 nm, while a digital versatile disk (DVD) apparatus that can reproduce information with higher density uses a red semiconductor laser having a wavelength of 650 nm. To achieve a next-generation optical disk apparatus with even higher density, a blue laser source with even shorter wavelength has been under active development. For example, to provide a small and stable blue laser source, a second harmonic generation (hereinafter, referred to as “SHG”) device has been developed by using a nonlinear optical material.
FIG. 6
is a schematic view showing an example of the configuration of an optical apparatus that includes a SHG blue light source using a SHG device.
First, the SHG blue light source will be described by referring to FIG.
6
.
As shown in
FIG. 6
, a SHG blue light source
101
includes a SHG device
103
and a semiconductor laser
104
. The semiconductor laser
104
is connected directly to the SHG device
103
.
The SHG device (optical waveguide device)
103
includes an optical material substrate
105
. A high refractive index region with a width of about 3 &mgr;m and a depth of about 2 &mgr;m is formed on the optical material substrate
105
by a proton-exchange method. This high refractive index region functions as an optical waveguide
106
. Infrared light having a wavelength of 850 nm is emitted from the semiconductor laser
104
, focused on an entrance end face
106
a
of the optical waveguide
106
on the SHG device
13
, and propagates in the optical waveguide
106
so as to be a fundamental guided wave. LiNbO
3
crystals, which are used as a substrate material for the optical material substrate
105
, have a large nonlinear optical constant. Therefore, a harmonic guided wave having half the wavelength of the fundamental light (425 nm) is excited from the electric field of the fundamental light. To compensate for a difference in propagation constant between the fundamental light and the harmonic light, a periodic polarization inversion region
107
is formed on the optical waveguide
106
. The harmonic light that is excited over the entire region of the optical waveguide
106
is added coherently, which then exits from an exit end face
106
of the optical waveguide
106
.
It is necessary to maintain the wavelength of the fundamental light precisely constant to ensure accurate compensation for the difference in propagation constant between the fundamental light and the harmonic light. Therefore, a distributed Bragg reflection (hereinafter, referred to as “DBR”) semiconductor laser is used as the semiconductor laser
104
. The DBR semiconductor laser includes a DBR region and shows extremely small wavelength variations with respect to temperature or the like.
Next, the operation of an optical pickup system that includes the SHG blue light source using the SHG device will be described by referring to FIG.
6
.
As shown in
FIG. 6
, an optical apparatus
102
includes the SHG blue light source (coherent light source)
101
, a focusing optical system, and a photodetector
112
. The SHG blue light source
101
includes the SHG device
103
and the semiconductor laser
104
. The focusing optical system includes a collimator lens
108
, polarizing beam splitter
109
, a quarter-wave plate
110
, and an objective lens
111
.
The harmonic blue light emitted from the SHG device
103
passes through the collimator lens
108
, the polarizing beam splitter
109
, the quarter-wave plate
110
, and the objective lens
111
in sequence, and thus is focused on an optical disk
113
. The light modulated by the optical disk
113
is reflected from the polarizing beam splitter
109
and directed to the photodetector
112
through a focusing lens (not shown), thereby providing a reproduction signal. At this time, linearly polarized light emitted from the SHG device
103
in the direction parallel to the sheet of the drawing is polarized in the direction perpendicular thereto by passing through and returning to the quarter-wave plate
110
. All the reflected light from the optical disk
113
is deflected by the polarizing beam splitter
109
and does not return to the side of the SHG blue light source
101
.
However, the base material for the actual optical disk
113
has a birefringent property. Thus, undesired polarized components generated in the optical disk
113
may pass through the polarizing beam splitter
109
and return to the side of the SHG blue light source
101
, which is referred to as return light. During reproduction of the optical disk
113
, the position of the objective lens
111
is controlled so as to ensure precise focusing on the optical disk
113
. Therefore, the exit end face
106
b
and the optical disk
113
constitute a confocal optical system, and the reflected light from the optical disk
113
is focused precisely on the exit end face
106
b.
When the reflected light from the optical disk
113
returns to the side of the SHG blue light source
101
as described above, noise is caused. To avoid this noise, various techniques have been proposed. Examples of such techniques include a method for generating a plurality of longitudinal modes by modulating a semiconductor laser with a high frequency signal and a method for also generating a plurality of longitudinal modes by causing self-oscillation in a semiconductor laser. In the field of optical communication, an optical isolator that has a magneto-optical effect generally is located between a semiconductor laser and an optical fiber so that light from the semiconductor laser is focused on the optical fiber. Moreover, another method has been proposed that prevents reflected light from returning to a semiconductor laser by cutting the entrance end face of an optical fiber or an optical waveguide so as to reflect the reflected light obliquely (JP 5(1993)-323404 A or the like).
These techniques reduce noise caused by light returning to the semiconductor laser. As a result of experiments on reproduction of the optical pickup that includes the optical waveguide type SHG device
103
shown in
FIG. 6
, the present inventors found noise caused by a different mechanism from that of the conventional noise induced by return light. This noise is interference noise generated when the return light focused on the exit end face
106
b
is reflected and interferes with light emitted from the optical waveguide
106
. The output power of the SHG blue light source
101
appears to change due to this interference effect when observed from the optical disk side, and a reproduction signal of the optical disk
113
is modulated by low frequency noise, which leads to degradation of the reproduction signal. The noise induced by the return light in the semiconductor laser
104
is generated by the interaction between light inside the semiconductor laser
104
and the return light. On the other hand, the interference noise is generated by the interference between light emitted from the SHG blue light source
101
and the return light.
As described above, there are two different types of noise in the optical system that uses the optical waveguide device (the SHG device
103
): low frequency interference noise and mode hopping noise. The low frequency interference noise occurs when light emitted from the SHG blue light source
101
is reflected and returns to the exit end face of the SHG blue light source
101
to cause interference in the optical system outside the SHG blue light source. The mode hopping noise results from the inside of the semiconductor laser
104
. Various techn
Kasazumi Ken'ichi
Kitaoka Yasuo
Mizuuchi Kiminori
Yamamoto Kazuhisa
Knauss Scott Alan
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P.C.
Sanghavi Hemang
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