Optical: systems and elements – Optical frequency converter – Harmonic generator
Reexamination Certificate
1999-02-10
2001-11-27
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
Harmonic generator
C372S022000
Reexamination Certificate
active
06323990
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for stabilizing an output of higher harmonic waves used in the fields of optical information processing, optical application measurement control, and the like utilizing coherent light, and a short wavelength laser beam source using the method.
2. Description of the Related Art
In the field of optical information processing, short wavelength laser beam sources for optical recording require an output of several mW or more. As blue laser beam sources, the combination of a semiconductor laser emitting fundamental waves and a light wavelength conversion device generating higher harmonic waves of the fundamental waves is promising.
FIG. 22
is a cross-sectional view showing a structure of a conventional short wavelength laser beam source
5000
emitting blue light. Fundamental waves P
1
emitted by a semiconductor laser
121
are collimated by a collimator lens
124
and focused onto an optical waveguide
102
formed inside of a light wavelength conversion device
122
by a focus lens
125
. The fundamental waves P
1
are converted into higher harmonic waves P
2
in the optical waveguide
102
and output. Each component of the short wavelength laser beam source
5000
is mounted on a base member
120
made of Al. The light wavelength conversion device
122
is positioned on a quartz plate
123
with its face having the optical waveguide
102
down.
Next, the light wavelength conversion device
122
used in the conventional short wavelength laser beam source
5000
will be described.
FIG. 23A
is a perspective view of the conventional light wavelength conversion device
122
;
FIG. 23B
is a cross-sectional view taken along a line
23
B—
23
B of FIG.
23
A. Hereinafter, the operation of the light wavelength conversion device
122
will be described by illustrating the generation of higher harmonic waves (wavelength: 437 nm) from fundamental waves (wavelength: 873 nm) (see K. Yamamoto and K. Mizuuchi, “Blue light generation by frequency doubling of a laser diode in a periodically-domain inverted LiTaO
3
waveguide”, IEEE Photonics Technology Letters, Vol. 4, No. 5, pp. 435-437, 1992).
As shown in
FIGS. 23A and 23B
, the light wavelength conversion device
122
includes the optical waveguide
102
formed in a LiTaO
3
substrate
101
. The optical waveguide
102
is provided with periodically domain-inverted layers (domain-inverted regions)
103
. The mismatch in propagation constant between the fundamental waves P
1
and the higher harmonic waves P
2
to be generated is compensated by a periodic structure composed of the domain-inverted regions
103
and non-domain-inverted regions
104
. This allows the fundamental waves P
1
to be converted into the higher harmonic waves P
2
at high efficiency so as to be output. The arrows in
FIG. 23B
represents the direction of a domain in each region.
Next, the principle of amplification of the higher harmonic waves in the light wavelength conversion device
122
will be described with reference to
FIGS. 24A and 24B
.
FIG. 24A
schematically shows inner structures, namely, the direction of domains of a device
131
which have no domain-inverted regions and of a device
132
which has domain-inverted regions. The arrows in
FIG. 24A
represent the direction of a domain in each region.
In the device
131
, domain-inverted regions are not formed and the directions of domains are aligned in one direction. When fundamental waves pass through the device
131
, the waves are partially converted into higher harmonic waves. However, in the structure of the device
131
, an output of higher harmonic waves
131
a
merely repeats increasing and decreasing along the passing direction of the optical waveguide, as shown in FIG.
24
B.
On the other hand, in the device
132
which has first-order periodically domain-inverted regions, an output of higher harmonic waves
132
a
increases in proportion to the square of length L of the optical wavelength as shown in FIG.
24
B. It should be noted that only when a quasi-phase match is established, the output of the higher harmonic waves P
2
can be obtained from the incident fundamental waves P
1
in the domain-inverted structure. The quasi-phase match is established only when a period &Lgr;
1
of the domain-inverted region is identical with &lgr;/(2(N
2
&ohgr;−N&ohgr;)), where N&ohgr; is an effective refractive index of the fundamental waves (wavelength: &lgr;), and N
2
&ohgr; is an effective refractive index of the higher harmonic waves (wavelength: &lgr;/2).
A method for producing a conventional light wavelength conversion device
5000
having the above-mentioned domain-inverted structure as a fundamental structure component will be described.
First, a periodic Ta film pattern with a width of several &mgr;m is formed on the LiTaO
3
substrate
101
made of non-linear optical crystal by vapor deposition and photolithography. The Ta film pattern is subjected to a proton-exchange treatment at 260° C., followed by being heat treated at around 550° C. Thus, the domain-inverted regions
103
are formed in the LiTaO
3
substrate
101
. Then, a Ta film slit is formed on the LiTaO
3
substrate
101
, heat treated in pyrophosphoric acid at 260° C. for 12 minutes, and subjected to an anneal treatment at 420° C. for one minute. Thus, the optical waveguide
102
is formed.
When the optical waveguide
102
has a length of 10 mm and the fundamental waves P
1
having a power of 37 mW with respect to a wavelength of 873 nm is input to the light wavelength conversion device
122
produced as described above, higher harmonic waves P
2
having a power of 1.1 mW can be output.
However, allowable width of the light wavelength conversion device
122
with respect to the wavelength of the fundamental waves is generally as small as 0.1 nm. For this reason, the light wavelength conversion device
122
cannot allow mode hopping of a semiconductor laser and spreading of the wavelength of output light.
For example, in the conventional light wavelength conversion device
122
having the above-mentioned domain-inverted regions, the allowance with respect to the wavelength fluctuation of a fundamental wave laser beam at a device length of 10 mm is very narrow; typically, an allowable wavelength half value width of around 0.1 nm. The allowable change with respect to temperature is typically as small as 3° C. Because of this, when a light wavelength conversion device is combined with a semiconductor laser, the following problems arise: The output of the semiconductor laser is likely to be affected by the change in temperature and consequently wavelength fluctuation occurs in output light; as a result, fundamental waves are not converted into higher harmonic waves or the output of higher harmonic waves converted from fundamental waves greatly fluctuates.
The above-mentioned problems will be described in detail below.
Typically, when the wavelength of a semiconductor laser shifts by only 0.05 nm, the output of higher harmonic waves to be obtained becomes half of an intended value. The allowability with respect to the change in wavelength of a semiconductor laser is small. For example, when the ambient temperature during the operation of a semiconductor laser shifts from 20° C. to 21° C., the vertical mode of the semiconductor laser shifts by one and the oscillation wavelength shifts from 820.0 nm to 820.2 nm. Because of this, the output of higher harmonic waves becomes zero.
Regarding the allowable width of the light wavelength conversion device
122
with respect to the change in temperature, when the ambient temperature changes, the output of higher harmonic waves cannot be obtained even if the oscillating wavelength of the semiconductor laser is stable. Furthermore, frequent occurrence of mode hopping causes noise leading to problems in reading from optical disks.
SUMMARY OF THE INVENTION
According to one aspect of the present invention, a method for stabilizing an output of higher harmonic waves includes the steps of: converting fundamental
Kato Makoto
Kitaoka Yasuo
Mizuuchi Kiminori
Yamamoto Kazuhisa
Lee John D.
Matsushita Electric - Industrial Co., Ltd.
Ratner & Prestia
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