Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
1999-10-09
2001-12-04
Arroyo, Teresa M. (Department: 2881)
Coherent light generators
Particular beam control device
Nonlinear device
C372S021000, C372S097000
Reexamination Certificate
active
06327281
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention concerns a Nth (N>2) harmonic generation device, and in a particular a method and apparatus for generating a harmonic beam.
BACKGROUND OF THE INVENTION
Optical harmonic generation using nonlinear medium such as a crystal, provides a method for doubling, tripling or more generally producing the Nth harmonic of the frequency of electromagnetic radiation emitted by a laser or other high intensity source. The 3rd harmonic is produced by sum frequency generation of the laser fundamental and its 2nd harmonic beams through 2nd order nonlinearities of the nonlinear medium (P( )=
0
(3)*P
0
( )*P
0
(2)). The common 3rd harmonic generation method, which is known in the art, combines the fundamental and the 2nd harmonic beam in a nonlinear crystal to generate its 3rd harmonic in the external laser cavity. For a high power laser, typical efficiency is limited to less than 25% of overall conversion efficiency, since unconverted fundamental beam and 2nd harmonic beams are wasted in that configuration. The overall conversion efficiency=P(3)/P
0
( )=
0
(3)*
1
(2)P
0
3
( )) is less than 25% for even very high power pulsed Nd:YAG laser (see Continuum Product brochures).
Recent intracavity 3rd harmonic has improved conversion efficiency substantially. See for example U.S. Pat. No. 5,898,717. But unconverted 2nd harmonic is still wasted in those configurations.
The theory of intracavity 2nd harmonic generation (SHG) has been discussed by R. G. Smith, in the
I.E.E.E. Journal of Quantrum Electr.
QE-6, 215-223 (1970). Harmonic generations have been demonstrated in crystals such as LiNbo
3
, BaNa (NbO
3
); Lio
3
, KDP, KTiOP
4
, BBO, LiB
3
O
5
, and CsLiB
6
O
10
. LiB
3
O
5
is also referred to as LBO crystal and CsLiB
6
O
10
is referred to as CLBO.
4th harmonic generation is known in the art. See U.S. Pat. No. 5,206,868. Resonance cavity is built to produce harmonics for single longitudinal mode has been demonstrated. However, the cavity is rather complicated and sometime the device is not practically useful. Multi-longitudinal mode lasers are more popular and easier to maintain. 5th harmonic generation is also known see: U.S. Pat. No. 5,742,626. Improved efficiency and more practical harmonic generations are sought for harmonic generation lasers.
SUMMARY OF THE INVENTION
According to the invention, a method and apparatus for generating an Nth harmonic frequency beam (N>2) is provided. In one aspect of the invention, a laser fundamental resonator cavity is provided for generating a preselected fundamental beam. The laser cavity includes a lazing medium, preferably a lazing crystal located between fundamental beam reflective surfaces preferably a first and second fundamental beam reflective mirrors which define the laser cavity. A 2nd or 3rd harmonic cavity is provided preferably a 2nd harmonic cavity optionally both a 2nd and 3rd harmonic cavity. The 2nd or 3rd harmonic cavity is at least partially separate from the laser fundamental cavity and at least partially overlaps the laser fundamental beam cavity. The harmonic cavity is formed between 2nd or 3rd harmonic beam reflective surfaces, preferably a first and second reflective mirrors preferably a first and second reflective surfaces having a second or 3rd harmonic generator preferably a harmonic nonlinear crystal located between reflective surfaces along the harmonic optical axis which extends between the first and second harmonic reflective surfaces. The fundamental beam from the laser fundamental cavity is directed into the harmonic cavity to incident on the harmonic nonlinear crystal for converting at least a portion of the fundamental beam to a harmonic beam. An Nth (N>2) harmonic generator preferably an Nth harmonic nonlinear crystal is in optical communication with the 2nd or 3rd harmonic nonlinear crystal. In the case of an Nth harmonic generator N>3, a 3rd harmonic cavity may be provided either alone or in combination with a 2nd harmonic cavity. When N=3 only a 2nd harmonic cavity is used. According to the invention, the harmonic beam is directed across the Nth (N>2) harmonic nonlinear crystal for conversion of the harmonic beam to a preselected Nth (N>2) harmonic beam. The resulting laser device efficiently converts the harmonic beam to a Nth (N>2) harmonic beam. Unconverted fundamental beam is directed back across the lazing medium for amplification. Desirably an output coupler for the Nth (N>2) harmonic beam is provided for directing Nth (N>2) harmonic beam outside the laser cavity. Typically, the Nth harmonic beam provided according to the invention is 3rd, 4th or 5th harmonic beam. According to the invention, improved conversion of fundamental and harmonic beam to Nth(N>2) is achieved. In addition, the power output of Nth(N>2) harmonic beam is enhanced.
According to the invention, the laser medium can be selected depending on the desired laser characteristics such as laser fluorescent lifetime, thermal Lansing, optical and mechanical properties. Preferably Nd:YAG, Nd:YLF; Nd:YVO
4
crystals are used. Optionally Nd, Er, Ho doped glasses or crystals are used as well as Ti:Sapphire, Cr:LiSaf or other lazing medium.
Useful in this invention are a type I or II phase matching nonlinear crystal for 2nd harmonic generation and type I or type II phase matching crystal for 3rd harmonic generation. In a type I phase matching crystal for 2nd harmonic generation, the fundamental beam is polarized orthogonal to that of resulting second harmonic beam. In a type I phase matching crystal for 3rd harmonic generation, the fundamental beam and 2nd harmonic beam incident on the type I crystal in parallel polarization to one another and produce a 3rd harmonic beam with perpendicular polarization to the 1st and 2nd harmonic beams. In a type II phase matching crystal for 3rd harmonic generation, the fundamental beam and 2nd harmonic beam are orthogonally polarized and produce a 3rd harmonic beam with its polarization parallel to the polarization of one of the input beams (for example in a type II LBO crystal the polarization of the fundamental beam (1064 nm) and the 3rd harmonic beam (355 nm) will be parallel). Also useful are 4th and 5th harmonic generation crystals.
The preferred embodiment of the present invention is illustrated in the drawings and examples. However, it should be expressly, understood that the present invention should not be limited solely to the illustrative embodiment.
REFERENCES:
patent: 5206868 (1993-04-01), Deacon
patent: 5742626 (1998-04-01), Mead et al.
patent: 5898717 (1999-04-01), Yin
patent: 6061370 (2000-05-01), Yin
patent: 6229829 (2001-05-01), Yin
Arroyo Teresa M.
Monbleau Davienne
Photonics Industries International, Inc.
Quinton, Esq. James A.
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