Optical: systems and elements – Optical frequency converter – Dielectric optical waveguide type
Reexamination Certificate
2002-04-16
2004-12-07
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Dielectric optical waveguide type
C359S328000, C385S122000
Reexamination Certificate
active
06829080
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 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. For convenience, harmonic light that is produced by the SHG device also referred to as “SHG light” in the following.
FIG. 22
is a schematic view showing an example of an SHG blue light source including an SHG device.
First, the SHG blue light source will be described by referring to FIG.
22
.
As shown in
FIG. 22
, a high refractive index region with a width of about 3 &mgr;m and a depth of about 2 &mgr;m is formed on an optical material substrate
114
by a proton-exchange method. This high refractive index region functions as an optical waveguide
110
. Infrared light with a wavelength of 850 nm emitted from a semiconductor laser
111
is focused on the entrance end face of an SHG device
117
and propagates in the optical waveguide
110
on the SHG device
117
so as to be a fundamental guided wave. LiNbO
3
crystals, which are used as a substrate material for the optical material substrate
114
, have a large nonlinear optical constant. Therefore, a harmonic guided wave having half the wavelength of the fundamental light (i.e., 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
112
is formed on the optical waveguide
110
. The harmonic light that is excited over the entire region of the optical waveguide
110
is added coherently, which then exits from the exit end face of the optical waveguide
110
.
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
111
. The DBR semiconductor laser includes a DBR region and shows extremely small wavelength variations with respect to temperature or the like. In addition to such small wavelength variations, the DBR semiconductor laser also is characterized by high coherence and small noise because it oscillates with a single wavelength.
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.
22
.
As shown in
FIG. 22
, harmonic blue light emitted from the SHG device
117
passes through a collimator lens
119
, a polarizing beam splitter
120
, a quarter-wave plate
121
, and an objective lens
122
in sequence, and then is focused on an optical disk
124
. The light modulated by the optical disk
124
is reflected from the polarizing beam splitter
120
and directed to a photodetector
125
through a focusing lens
123
, thus providing a reproduction signal. At this time, linearly polarized light emitted from the SHG device
117
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
121
. All the reflected light from the optical disk
124
is reflected by the polarizing beam splitter
120
and does not return to the light source side.
In the aforementioned conventional technique, a configuration in which all the reflected light from the optical disk
124
is reflected by the polarizing beam splitter
120
and does not return to the light source side has been described. However, the base material for actual optical disks has a birefringent property. Thus, undesired polarized components generated in the optical disk
124
may pass through the polarizing beam splitter
120
and return to the light source side, as indicated by return light
126
. During reproduction of the optical disk
124
, the position of the objective lens
122
is controlled so as to ensure precise focusing on the optical disk
124
. Therefore, the exit end face of the SHG device
117
and the optical disk
124
constitute a confocal optical system, and the reflected light from the optical disk
124
is focused precisely on the exit end face of the SHG device
117
(i.e., the exit end face of the optical waveguide
110
).
As described above, the reflected light that returns from the optical disk to the light source side becomes return light to induce noise in the optical system using a semiconductor laser as the light source, and various techniques for avoiding this 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 that returns from the optical disk to the light source side obliquely (JP 5(1993)-323404 A or the like).
These techniques reduce noise caused by return light that returns to the inside of a semiconductor laser. As a result of experiments on reproduction of the optical pickup that includes the optical waveguide type SHG device shown in
FIG. 22
, the present inventors found noise caused by a different mechanism from that of the conventional noise induced by the return light. This noise is interference noise to be generated when the return light focused on the exit end face of the optical waveguide is reflected therefrom and interferes with light emitted from the optical waveguide. The output power of the light source appears to change due to this interference effect from the optical disk side, and a reproduction signal of the optical disk 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 is generated by the interaction between light inside the semiconductor laser and the return light. On the other hand, the interference noise is generated by the interference between light emitted from the light source and the return light. More detailed studies conducted by the present inventors showed that a portion of the return light from an external optical system is excited again in the optical waveguide of the SHG device (the optical waveguide device) as a guided wave, then is reflected from the entrance end face of the optical waveguide, and also causes interference noise.
As described above, there are two different types of noise in the optical system that uses the optical waveguide device: low frequency interference noise and mode hopping noise. The low frequency interference noise occurs when light emitted from a light source is reflected and re
Kasazumi Ken'ichi
Kitaoka Yasuo
Mizuuchi Kiminori
Morikawa Akihiro
Yamamoto Kazuhisa
Knauss Scott A
Lee John D.
Merchant & Gould P.C.
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