Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
1999-05-18
2004-03-23
Scott, Jr., Leon (Department: 2828)
Coherent light generators
Particular beam control device
Nonlinear device
C372S020000, C372S023000, C372S050121, C372S092000, C372S025000, C372S026000, C372S108000
Reexamination Certificate
active
06711183
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an optical wavelength conversion device used for optical information processing and optical measurement fields utilizing a coherent light source, a coherent light generator using such an optical wavelength conversion device, and an optical information processing apparatus using such a coherent light generator.
The present invention also relates to a short-wavelength light source including a semiconductor laser and an optical wavelength conversion device, and an optical disk system using such a short-wavelength light source.
2. Description of the Related Art
An optical wavelength conversion device utilizing a nonlinear optical effect has been used in a variety of fields because such a device serves to widen the range of wavelengths usable by a laser light source by converting the wavelength of light output from the laser light source.
For example, an optical wavelength conversion device utilizing second harmonic generation (SHG) converts laser light into second harmonic wave light having a half wavelength, thereby enabling to realize generation of short-wavelength light which is otherwise difficult. When parametric oscillation is utilized, it is possible to generate light beams having different wavelengths continuously from a single-wavelength light source, thereby realizing a wavelength tunable light source. When a sum frequency is utilized, it is possible to convert two light beams having different wavelengths into a light beam having a third wavelength.
In the above optical wavelength conversion utilizing a nonlinear optical effect, phase matching conditions must be satisfied between the fundamental wave light before conversion and the second harmonic wave light after conversion. Techniques for this phase match includes a birefringence method in which the propagation velocities of the fundamental wave light and the second harmonic wave light in a crystal are made identical to each other by utilizing the birefringence of the crystal, and a quasi phase match method in which a nonlinear grating is used to achieve phase match.
In reality, however, the wavelength allowance range satisfying the phase matching conditions is extremely narrow. It is therefore necessary to control the wavelength of the fundamental wave light with markedly high precision, and thus it is difficult to stabilize the output power.
Studies have been done to widen the wavelength allowance range to increase the stability of optical wavelength conversion.
FIG. 33
illustrates a conventional optical wavelength conversion device for widening the wavelength allowance range (see Japanese Application No. 3-16198). Hereinbelow, generation of second harmonic wave light P
2
having a wavelength of 0.42 &mgr;m from fundamental wave light P
1
having a wavelength of 0.84 &mgr;m will be described in detail with reference to FIG.
33
.
The optical wavelength conversion device shown in
FIG. 33
includes an LiNbO
3
substrate
1101
, an optical waveguide
1102
formed on the substrate
1101
, and a layer
1103
in which polarization is periodically inverted (a domain-inverted layer) formed for the optical waveguide
1102
. The propagation constant of the fundamental wave light P
1
is not matched with that of the second harmonic wave light P
2
to be generated. This mismatch in propagation constant is compensated for by the periodic structure of the domain-inverted layer
1103
, thereby enabling to generate the second harmonic wave light P
2
with high efficiency.
Although the above conventional optical wavelength conversion device which performs wavelength conversion using the periodic domain-inverted layer
1103
exhibits high conversion efficiency, the allowance range of phase match wavelength in which wavelength conversion is possible is extremely narrow. In order to overcome this problem, the optical waveguide
1102
of the optical wavelength conversion device shown in
FIG. 33
has portions having different propagation constants so as to widen the wavelength allowance range.
When the propagation constant of the optical waveguide
1102
is varied, the phase match wavelength in the optical waveguide
1102
varies. The phase matching conditions as used herein are conditions under which the optical wavelength conversion device can perform wavelength conversion. The phase match wavelength as used herein is a wavelength of incident light which satisfies the phase matching conditions. By dividing the optical waveguide
1102
into regions A, B, C, and D having different widths as shown in
FIG. 33
, the phase match wavelength varies depending on the width of the optical waveguide
1102
in the respective regions. By this construction, the phase matching conditions can be satisfied in any of the regions A, B, C, and D having different widths of the optical waveguide even when the wavelength of incident light is changed. Thus, the number of phase match wavelengths increases in the entire device. As a result, the wavelength allowance range for the optical wavelength conversion device is widened and thus a stable wavelength conversion device can be fabricated.
The phase matching conditions for the regions A, B, C, and D can also be established by varying the depth of the optical waveguide
1102
among the regions A, B, C, and D, or varying the period of the domain-inverted layer
1103
among the regions A, B, C, and D. In these cases, also, an optical wavelength conversion device having a wide wavelength allowance range is obtained.
A construction combining a periodic domain-inverted structure and a phase control section is also disclosed (see Japanese Application No. 4-070726).
FIG. 34
illustrates a conventional optical wavelength conversion device having such construction for widening the wavelength allowance range.
The optical wavelength conversion device shown in
FIG. 34
includes a plurality of domain-inverted regions
1105
and phase control sections
1106
between the adjacent domain-inverted regions
1105
, which are both formed on a nonlinear optical crystal
1101
. In this device, it is attempted to widen the allowance range of phase match wavelengths by utilizing the difference in the phase matching conditions of the respective domain-inverted regions
1105
. It is also attempted to reduce a variation of the output power of second harmonic wave light P
2
(i.e., the SHG output power) caused by a wavelength variation of fundamental wave light P
1
by adjusting phase mismatch generated between the domain-inverted regions
1105
by the phase control sections
1106
.
It is possible to further widen the allowance range of phase match wavelength by increasing the number of the domain-inverted regions
1105
. For example,
FIGS. 35A and 35B
show tuning curves representing the relationship between the wavelength of fundamental wave light and the SHG output power when the domain-inverted layer is segmented into three regions and four regions, respectively. It is observed from these tuning curves that the wavelength allowance range can be greatly widened by increasing the number of segmented regions.
A method for widening the allowance range of phase match wavelength by modulating a periodic structure of domain-inverted is also disclosed.
For example, Suhara et al., IEEE Journal of Quantum Electronics, vol. 26 (1990) pp. 1265-1276 discloses a method for widening the allowance range of phase match wavelength by changing a periodic structure of domain-inverted in a chirping fashion. Specifically, this method uses a linear chirping structure where the period of domain-inverted is increased in proportion to the distance. In this method, since phase displacement linearly changes in the domain-inverted structure, the allowance range of phase match wavelength can be widely increased.
On the other hand, in recent years, optical disk systems using a near-infrared semiconductor laser having a 780 nm wavelength band and a red semiconductor laser having a wavelength of 670 nm have been vigorously developed. On the other hand, in order to realize hi
Kitaoka Yasuo
Mizuuchi Kiminori
Yamamoto Kazuhisa
Jr. Leon Scott
Matsushita Electric - Industrial Co., Ltd.
RatnerPrestia
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