Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
2000-04-05
2002-06-11
Ip, Paul (Department: 2828)
Coherent light generators
Particular beam control device
Nonlinear device
C372S021000, C372S092000, C372S095000, C372S098000, C372S101000
Reexamination Certificate
active
06404786
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a laser beam generating apparatus, and more particularly to a laser beam generating apparatus wherein a resonator, provided outside of a laser oscillator, contains a barium borate crystal and a laser beam in the ultraviolet region is supplied, with harmonic content extracted from the laser beam generated by the laser oscillator. In further detail, the invention relates to an optical system for irradiating optical components with an ultraviolet beam of not more than 400 nm in wavelength or a laser beam generating apparatus for generating an ultraviolet beam of not more than 400 nm in wavelength.
2. Description of the Related Art
If, in the field of semiconductor manufacturing for example, a laser beam in the ultraviolet region can be used in a stepper (a sequentially shifting exposure system), finer processing than what is currently done will be made possible, enabling large-capacity memory elements which are further enhanced in the level of integration to be manufactured. A laser beam in the ultraviolet region can be applied not only for this purpose but also to photochemical reactions and biotechnology, and therefore practical availability of ultraviolet lasers in many different fields is awaited.
By a method according to the related art with high potential for practical application to generate a laser beam in the ultraviolet region, a barium borate crystal, which is a nonlinear optical crystal, is disposed in a resonator provided outside the laser oscillator, and secondary harmonic content is extracted from the laser beam generator by the laser oscillator.
Where a laser beam in the ultraviolet region is to be generated by this method, a harmonic content of the required intensity, i.e. an ultraviolet laser beam, is obtained by squeezing the waist of the laser beam (i.e. the radius of the cross section of the beam) which is allowed to pass the barium borate crystal, because the nonlinear conversion coefficient of the barium borate crystal is generally small.
However, the squeezing of the waist of the laser beam results in a greater power density of the laser beam in the barium borate crystal, leading to the problem that they may be heavily damaged both on the surface and inside.
Therefore, such a laser beam generating apparatus according to the related art, although an ultraviolet laser beam is obtained in a high output during the early phase of its use, steeply drops in output with the lapse of time, making it difficult for a high output to be maintained for a long period.
Incidentally, by the conventional method, if the power of an ultraviolet laser beam is 100 mW, the output can last for not more than 400 hours, and the velocity of degradation (the velocity of power drop) is about 1.35×10
−4
[%/hour].
The damage to the barium borate crystal can be more clearly observed by microscope. FIG.
FIG. 1
is a schematic diagram showing the result of microscopic observation of the trace of a beam pattern formed in a barium borate crystal where the beam waist is 23 &mgr;m.
This diagram is a front view of the laser beam emitting end face of the barium borate crystal, in which the area surrounded by a dotted line
102
is the part damaged by the laser beam, looking more turbid than the surrounding normal part. Incidentally, it is because the generated harmonic content spreads at an angle of about 4° to the original laser beam that the damage is oblong laterally.
Furthermore, there is another problem that optical components deteriorate in performance characteristics when irradiated in the atmosphere with an ultraviolet ray of not more than 400 nm in wavelength, presumably because the optical losses of the optical components increase in such a situation. Such optical losses are presumed to occur as moisture and oily contents in the atmosphere on the surface of the optical components react and the reaction products and particles around them stick to the surface of the optical components.
When an ultraviolet beam of not more than 400 nm in wavelength is to be generated, in wavelength conversion using an external resonator (for information on which, see M. Oka and S. Kubota, Jpn. J. AppI. Phys. Vol. 31 (1992), pp. 513, and M. Oka et. al., in the Digest of Conference on Laser and Electro-Optics (OSA, Washington, D.C., 1992), paper CWQ7) or the like, the harmonic output is significantly reduced by intricate performance deterioration of a mirror or a nonlinear optical element arranged within the external resonator. This deterioration again, as the present inventor sees it, seems attributable to similar circumstances to what was described above. When, for instance, an ultraviolet beam of not more than 400 nm in wavelength formed by wavelength conversion passes an optical component, such as a mirror, it adversely affects the performance of the optical component (e.g. the mirror).
Therefore, for use where optical components are to be irradiated with an ultraviolet beam of not more than 400 nm in wavelength as well as where an ultraviolet beam of not more than 400 nm is to be generated, there is a keen call for the development of an optical system which can prevent the optical performance of optical components from being adversely affected by an increase in optical losses or their output performance and other attributes from being deteriorated.
Problems with the aforementioned related art will be described below with reference to drawings. For instance, where a dominant wave of 532 nm in wavelength is to be converted in wavelength into an ultraviolet beam of 266 nm in wavelength by using an external resonator, the structure of the external resonator—art will be as illustrated in FIG.
2
.
In
FIG. 2
, what are denoted by reference numerals
10
,
12
and
14
are highly reflective mirrors having an ultra-high reflectance at a wave-Length of 532 nm, e.g. a reflectance of 99. 95% or more; what is denoted by numeral
8
is an incidence mirror having a high reflectance, e.g. a reflectance of 99% at a, wavelength of 532 nm; and what is denoted by numeral
6
is a nonlinear optical crystal BBO, which is a wavelength converting element coated with a less reflective film having a low reflectance, e.g. a reflectance of not more than 0.1% at a wavelength of 532 nm., The highly reflective mirror
14
is installed over a VCM (see the above-cited SRF92 collection of preliminary papers), which is a positioning device (not shown), and can be controlled by, for instance, a servo drive system. The elements
6
, B,
10
,
12
and
24
referred to above constitute an external resonator section.
When a dominant Wave (of 532 nm in wavelength here) schematically indicated by an arrow
30
in
FIG. 2
is brought to incidence on this external resonator, it is amplified between the mirrors, and the amplified dominant wave is converted by the nonlinear optical crystal
6
(BBO) into a secondary harmonic (of 266 nm in wavelength here). This secondary harmonic is schematically indicated by an arrow
31
in FIG.
2
.
When such a wavelength conversion as described above is accomplished in the atmosphere, optical losses (to be specific, mainly scattering) of the mirrors (especially the mirror
10
) increase. The relationship between an optical loss and the power of the dominant wave of 532 nm in wavelength, amplified in the external resonator, can be represented by the following equation.
P&ohgr;={square root over ( )}(&dgr;cav
2
+4&ggr;
SH
Pi−&dgr;cav)2&ggr;
SH
Equation 1
Where &dgr;cav is the optical loss at a wavelength of 532=in the external resonator; P&ohgr;, the power of the amplified dominant wave; Pi, the power of the dominant wave of 532 nm in wavelength coming incident on the external resonator; and &ggr;
SH
, a constant known as a nonlinear conversion factor determined by the crystalline length of the nonlinear optical crystal
6
(BBO), wavelength of the dominant wave, spot size and focusing parameter.
Equation 1 given above reveals that, in the external resonator, the p
Kondo Kenji
Oka Michio
Wada Hiroyuki
Ip Paul
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Rodriguez Armando
Sony Corporation
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