Optical: systems and elements – Optical frequency converter
Reexamination Certificate
1998-09-28
2001-01-30
Lee, John D. (Department: 2874)
Optical: systems and elements
Optical frequency converter
C372S022000, C372S098000, C372S103000
Reexamination Certificate
active
06181461
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an optical system having a stable resonator and more particularly to an optical system having a resonator which is used as an optical system for irradiating or generating ultraviolet rays (for example, ultraviolet light having a wavelength of 400 nm or shorter).
2. Description of Related Art
Heretofore, optical systems having a stable resonator have been involved in the problem of high order transverse mode generation. In detail, the existence of, for example, a scattering source in a resonator results in generation of a high order transverse mode in the resonator due to slight scattering from the scattering source. Because such high order transverse mode competes with the longitudinal mode, such generation of a high order transverse mode results in reduced longitudinal mode output in the resonator. Irregular generation of a high order transverse mode results in unstable longitudinal mode output in the resonator. Therefore, such generation of a high order transverse mode has been a problem in obtaining a stable longitudinal mode, particularly a constant stable longitudinal mode with time. Any optical system having at least one optical part other than mirrors in a stable resonator has been involved in such problem.
This problem is particularly more serious for wavelength conversion optical systems having an external resonator. In wavelength conversion using an external resonator (refer to M. Oka and S. Kubota, Jpn. J. Appl. Phys. Vol. 31 (1992) pp. 513 and M. Oka et. al., in Digest of Conference on Laser and Electron-Optics (OSA. Washington D.C. 1992), paper CWQ7), an external resonator is provided with an optical crystal for functioning as a wavelength conversion element, slight scattering due to impurities contained in the optical crystal and surface causes generation of a high order transverse mode in the resonator. The high order transverse mode results in reduced or unstable fundamental wave output in the resonator, and results in a reduced or unstable wavelength-converted harmonic. As described herein above, the above-mentioned generation of a transverse mode has been a problem in obtaining stable harmonic output in wavelength conversion optical systems having an external resonator.
The above-mentioned problem of the prior art will be described hereinunder with reference to drawings. For example, when a fundamental wave having a wavelength of 532 nm is converted to ultraviolet light having a wavelength of 266 nm using an external resonator, an external resonator having the structure shown in
FIG. 7
has been used.
In
FIG. 7
, characters
1
to
3
represent high reflectance mirrors having ultra high reflectance, for example, as high as 99.95% or higher at the wavelength of 532 nm, character
4
represents an input (incident) mirror having a reflectance of, for example, 99% at the wavelength of 532 nm, and character
5
represents a non-linear optical crystal BBO, namely a wavelength conversion element provided with a low reflectance film of, for example, as low as 0.1% or lower at the wavelength of 532 nm. The high reflectance mirror
3
is placed on a VCM (refer to in the above-mentioned J. J. A. P) namely a positioning device though not shown in the drawing, and controlled by, for example, a servo driving system. As described herein above, the external resonator is composed of components shown with the characters
1
to
5
.
A fundamental wave (wavelength of 532 nm) is irradiated onto the external resonator, amplified between the mirrors, and the amplified fundamental wave is converted to a second harmonic (wavelength of 266 nm in this case) by the non-linear optical crystal
5
(BBO). The second harmonic is shown with the arrow
7
in FIG.
7
.
When the wavelength is converted as described herein above, a transverse mode due to scattering can be generated, particularly in the case of an external resonator as shown in
FIG. 7
, slight scattering due to photorefractive characteristics of the non-linear optical crystal
5
(BBO) increases to result in generation of the high transverse mode. As a result, the output of the fundamental wave having a wavelength of 532 which contributes to wavelength conversion is reduced.
The output of the second harmonic generated by wavelength conversion is formulated by the following equation:
P
2
&ohgr;=&ggr;
SH
(
P
&ohgr;)
2
Wherein, P&ohgr; represents the output of the fundamental wave irradiated onto the non-linear optical crystal
5
(BBO), P
2
&ohgr; represents the output of the second harmonic generated by wavelength conversion using the non-linear optical crystal
5
, and &ggr;
SH
represents a constant called a non-linear conversion factor determined from the crystal length of the non-linear optical crystal
5
, the wavelength of the fundamental wave, the spot size, and a focusing parameter.
From this equation it is obvious that the output of the second harmonic P
2
&ohgr; decreases with decreasing output of the fundamental wave P&ohgr;, and the output of the second harmonic becomes unstable with irregular generation of a high order transverse mode.
Actually in wavelength conversion using the external resonator shown in
FIG. 8
, the second harmonic output (axis of ordinate, mW) exhibits unstable behavior with time (abscissa).
SUMMARY OF THE INVENTION
The present invention is accomplished to solve the above-mentioned problem of the prior art, and it is the object of the present invention to provide an optical system having a resonator (for example, a wavelength conversion optical system having an external resonator) which supplies a stable output (stable wavelength conversion output) without a transverse mode generation problem due to the existence of a scattering source and optical crystal used.
To attain the above-mentioned object, an optical system having a resonator according to the present invention has at least one optical part other than mirrors in the stable resonator, wherein the optical system is provided with, on at least one point in the stable resonator, an aperture having a circular hole with a diameter 1 to 10 times the beam diameter at each point.
In this patent specification, the diameter of a beam is defined as twice the distance between the point where the beam intensity decreases to 1/e
2
of that at the center (the point where the beam intensity is maximum) and the center.
To attain the above-mentioned object, an optical system having a resonator which is an optical system having at least one optical part other than mirrors in a stable resonator is provided with, on at least one point in the stable resonator, a slit having a width 1 to 10 times the beam diameter at each point.
An optical system having a resonator which is a wavelength conversion optical system having a plurality of mirrors and an optical crystal for functioning as a wavelength conversion element in a resonator is provided with, on an arbitrary space in the stable resonator comprising the plurality of mirrors and optical crystal, an aperture having an edge of arbitrary shape.
An optical system having a resonator which is an optical system having at least one optical part other than mirrors in a stable resonator is provided with, on at least one point in the stable resonator, a knife edge having at least one linear or curved edge so that the nearest distance between the beam and the knife edge is 1 to 10 times the beam radius from the center of the beam.
According to the present invention, because on at least one point in a stable resonator an aperture having a circular hole with a diameter 1 to 10 times the beam diameter at each point, or a slit having a width 1 to 10 times, or an aperture having an edge of an arbitrary shape or at least one knife edge having a linear or curved edge provided so that the nearest distance from the beam is 1 to 10 times the beam radius from the beam center is provided, stable output is obtained likely due to suppressed transverse mode generation as the result of application of this structure.
In Japanese Patent Laid-open No.
Kaneda Yuushi
Kondo Kenji
Masuda Hisashi
Oka Michio
Wada Hiroyuki
Lee John D.
Limbach & Limbach L.L.P.
Sony Corporation
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