Coherent light generators – Particular operating compensation means
Reexamination Certificate
2000-01-10
2001-10-02
Leung, Quyen P. (Department: 2881)
Coherent light generators
Particular operating compensation means
C372S038020
Reexamination Certificate
active
06298075
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to an optical apparatus and a method for producing the same. Specifically, the present invention relates to a light generator including a semiconductor laser and a waveguide type optical function device, and a method for producing the same. The present invention also relates to an oscillation wavelength stabilizer for a light source such as a semiconductor laser having a distributed Bragg-reflector (DBR) region, a harmonic output stabilizer for a short wavelength light source which includes a semiconductor laser having a DBR region and a wavelength converting device, and further to an optical disk system including the same.
2. Description of the Related Art
The entire disclosure of U.S. patent application Ser. No. 08/910,097 filed on Aug. 12, 1997 is expressly incorporated by reference herein. In the optical information processing field, optical functional devices for modulating light output from a semiconductor laser a high speeds or halving the wavelength of laser light have been vigorously developed. In particular, waveguide type optical functional devices are promising for realizing the modulation of laser light at a frequency of several gigahertz or more or obtaining 1 mW or more of short-wavelength laser light. Hereinbelow, a waveguide type second harmonic generation (SHG) device (Mizuuchi et al., IEEE, Journal of Quantum Electronics, 30 (1994), pp. 1596) and an optical modulation device will be briefly described.
A typical SHG device
50
will be explained referring to
FIG. 16
, which is a perspective view of the SHG device
50
.
The SHG device
50
includes a z-cut LiTaO
3
crystal substrate
31
. A waveguide
32
and periodical domain inverted regions
33
extending perpendicular to the waveguide
32
are formed on the z-cut LiTaO
3
crystal substrate
31
. The SHG device
50
allows harmonics to be generated with high efficiency by compensating the unmatching of the propagation constant between a fundamental wave and a generated harmonic with the periodical structure of the domain inverted regions
33
.
Such an SHG device
50
is fabricated in the following manner.
A Ta electrode pattern is formed on the z-cut LiTaO
3
crystal substrate
31
(made of nonlinear optical crystal) by evaporation and photolithography having a periodic spacing of several micrometers. A voltage of 2 kV/mm is then applied to the resultant substrate to form the periodic domain inverted regions
33
. Thereafter, a stripe made of Ta, extending perpendicular to the periodic domain inverted regions
33
, is formed on the substrate. The resultant substrate is heat-treated with pyrophosphoric acid for about 16 minutes, subjected to proton exchange, and annealed at about 420° C. for about one minute, to form the waveguide
32
.
The proton-exchanged waveguide
32
allows only light having a polarized component in the z direction to propagate therethrough. In general, the SHG device using the z-cut LiTaO
3
crystal substrate has a higher conversion efficiency into a harmonic, though the SHG device can also be fabricated using an x-cut LiTaO
3
crystal substrate.
A conventional light generator including such a waveguide type SHG device and a semiconductor laser will now be described.
Referring to
FIG. 17
, light emitted from a semiconductor laser
34
is guided into a waveguide
40
having a waveguide type SHG device
39
via two coupling lenses
35
and
38
. More specifically, light emitted from a semiconductor laser
34
is collimated by a collimator lens
35
, passes through a half-wave plate
36
and a bandpass filter
37
, and is focused on the waveguide
40
of the waveguide type SHG device
39
by a focusing lens
38
. The semiconductor laser
34
oscillates at a TE mode, while the waveguide
40
allows only a light component polarized in the z direction to propagate therethrough. The half-wave plate
36
is used to obtain the maximum overlap between the light emitted from the semiconductor laser
34
and the light propagating through the waveguide
40
. The laser light emitted from the semiconductor laser
34
is converted into harmonic light while propagating in the waveguide
40
and output from the emergent end face thereof.
This conventional light generator generates about a 2.8 mW blue light with a wavelength of 430 nm from 120 mW laser light with a wavelength of 860 nm emitted from an AlGaAs semiconductor laser as the fundamental wave. The module volume of this light generator using the above two coupling lenses
35
and
38
, is about 3 cc (Kitaoka et al, The Review of Laser Engineering, 23 (1995), pp. 787).
As optical disk systems and optical communication systems have been generally and widely used, demands for reducing the size and cost of relevant components have increased. In order to reduce the size and cost of the light generator including the semiconductor laser and the waveguide type optical functional device, it is required to simplify the optical coupling adjustment (i.e., the alignment adjustment for obtaining the optical coupling) and omit any coupling lens. In the conventional module structure, as shown in
FIG. 17
, using two coupling lenses
35
and
38
for guiding laser light of the semiconductor laser
34
to the waveguide
40
, there are required four axial adjustments: i.e., adjustments of the focusing lens
38
and the collimator lens
35
both along the optical axis (direction y in FIG.
17
); and adjustments of the semiconductor laser
34
in directions x and z in FIG.
17
. Thus, a certain period of time is required for realizing the adjustment, and fabrication costs is increased. Further, the module structure including two coupling lenses has a relatively large volume of about 3 cc and occupies a relatively large space. The time and cost required for the optical coupling adjustment and the module volume of about 3 cc of the conventional optical functional device are disadvantageous in the application of the device to consumer appliances such as optical disk system.
A direct-coupling module including no coupling lens has been proposed to achieve size reduction (Japanese Patent Publication No. 5-29892). This type of module, however, still requires alignment adjustments for optical coupling along two or three axes, requiring time and cost for the optical coupling.
Another problem of the conventional light generator is that presently it is difficult to optically couple the waveguide formed on the z-cut crystal substrate by proton exchange and the semiconductor laser light oscillating at the TE mode on one submount in a simple manner. The waveguide formed on the z-cut crystal substrate by proton exchange allows only light of a TM mode to propagate therethrough. Therefore, a half-wave plate is required to mount both the waveguide and the semiconductor laser emitting light of a TE mode on one submount.
By the way, optical disk systems using a near infrared semiconductor laser with a wavelength of a 780 nm band or a red semiconductor laser with a wavelength of 670 nm have been vigorously developed. In order to enhance the density of an optical disk, it is required to reproduce smaller spots. To reproduce smaller spots, a higher numerical aperture (NA) of a focusing lens and a shorter-wavelength of a light source are required. One of the conventional techniques for shortening the wavelength is second harmonic generation (SHG), where a near infrared semiconductor laser and a domain inverted type waveguide device of a quasi-phase matching (QPM) method is used (see Yamamoto et al., Optics Letters, Vol. 16, No. 15, 1156 (1991)).
FIG. 34
is a schematic structural view of a blue light source using a domain inverted type waveguide device (the SHG blue laser).
Referring to
FIG. 34
, the light source includes a 0.85 &mgr;m-band, 100 mW-class AlGaAs DBR semiconductor laser
123
, a collimator lens
124
with an NA of 0.5, a half-wave plate
125
, a focusing lens
126
with an NA of 0.5 and domain inverted type waveguide device
127
. The DBR semiconductor laser
123
includes a DBR
Kato Makoto
Kitaoka Yasuo
Mizuuchi Kiminori
Nishiuchi Ken'ichi
Yamamoto Kazuhisa
Leung Quyen P.
Matsushita Electric - Industrial Co., Ltd.
Ratner & Prestia
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