Coherent light generators – Particular beam control device – Nonlinear device
Reexamination Certificate
1999-07-28
2001-05-08
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular beam control device
Nonlinear device
C372S098000, C372S092000
Reexamination Certificate
active
06229829
ABSTRACT:
FIELD OF THE INVENTION
The field of the invention concerns a frequency quadrupled laser, and in particular a method and apparatus for generating a fourth harmonic beam.
BACKGROUND OF THE INVENTION
Optical harmonic generation using a non-linear medium such as a crystal, provides a method for doubling, tripling or quadrupling of the frequency of electromagnetic radiation emitted by a laser or other high intensity source. Harmonic generation and optical parametric oscillation have been demonstrated in crystals such as LiNbO, KDT, KTP, BBO, LBO and others. Blue, green and UV lasers have found industrial, medical and scientific applications.
UV lasers are used in drilling, microwelding and the like. Intracavity fourth harmonic generation has been suggested. See Tso Yee Fan and Bruce H. T. Chai, “Intracavity Fourth Harmonic Generation Using Three Pieces of LBO in a Nd:YAG Laser”,
OSA Proceeding on Advanced Solid State Lasers
1994 Vol. 20, pps. 377-380. However, such prior art fourth harmonic lasers are lower powered in the order of about 270 mw of fourth harmonic power. Other fourth harmonic lasers have been proposed see U.S. Pat. No. 5,206,868. There still exists a need for improved high powered fourth harmonic lasers.
SUMMARY OF THE INVENTION
According to the invention a fourth harmonic frequency generating method and apparatus is provided. The system includes within the optical cavity an active laser medium, a second harmonic generator for generating second harmonic frequency of the fundamental frequency emitted by the laser. According to the invention, fundamental beam is directed to a second harmonic generation crystal where a portion of the fundamental beam is converted to second harmonic beam. Both the second harmonic and unconverted fundamental beams are directed back across the second harmonic generator by reflective surfaces for a second pass prior to any conversion of second harmonic beam to a higher harmonic beam. Preferably the second harmonic and fundamental beams are reflected by one of the laser cavity reflective surfaces for the second pass. Optionally, there can be separate reflective surfaces to reflect each of the fundamental and second harmonic beams. A third harmonic generator for generating third harmonic frequency of the fundamental frequency, a fourth harmonic generator for generating a fourth harmonic frequency of the fundamental frequency and a wavelength selective coupling to facilitate the removal of the fourth harmonic beam from the optical cavity are provided in optical communication with the reflected second harmonic and fundamental beam. Optionally the unconverted second harmonic beam can be directed through the laser medium as additional pumping. In another aspect of the invention substantially all UV beams are removed from the laser cavity preferably 99% or greater. A UV beam separator is desirably provided between the fourth harmonic generator and the laser medium. The UV beam separator can be a separating mirror or a prism. Desirably, the UV beam separator is a prism for separating unconverted fundamental beam from UV third and fourth harmonic beam. The resulting laser has improved conversion efficiency and an improved mode quality.
Fourth harmonic output of Nd:YAG or Nd:YLF or Nd:YVO
4
is desirable in some applications such as in small hole drilling on multilayer circuit boards. The fourth harmonic wavelength is one fourth of its fundamental wavelength. The focus ability is proportional to the wavelength. What this means is the shorter the wavelength, the smaller the spot size to which the beam can be focused. In addition to its better focus ability, the higher photon energy of the fourth harmonic tends to have better interaction in certain material processing applications.
Useful in this invention are a type I and type II preferably type I phase matching crystal for second harmonic generation and type I or type II phase preferably type II matching crystal for third harmonic generation and a type I or type II preferably type I phase matching crystal for the fourth harmonic generation. In a type I phase matching crystal for a second harmonic generation, the polarization orientation of the fundamental beam is orthogonal to the one of the second harmonic beam. In a type II phase matching crystal for third harmonic generation, the polarization orientations of fundamental beam and second harmonic beam are orthogonal. Resulting polarization orientation of the third harmonic beam is parallel to the polarization orientation of one of two input beams (for example in a type II LBO crystal the polarization of the fundamental beam and the third harmonic beam will be parallel).
In a type I LBO fourth harmonic crystal, both the fundamental and third harmonic beam polarization orientations are parallel.
It is an object of the invention to provide efficient intracavity generation of optical fourth harmonic frequency of light or radiation, using several nonlinear crystals or other conversion means positioned within the optical cavity containing the active laser medium.
It is an object of the invention to provide laser frequency quadrupling apparatus by using LBO nonlinear crystals within the laser cavity.
It is an object of the invention to provide frequency quadrupling apparatus with a high conversion efficiency (quadrupled output power/total fundamental output power).
It is an object of the invention to provide high output of quadrupled frequency beam in ultraviolet.
It is further object of the invention to remove substantially all the fourth harmonic beam from the laser cavity.
A further object of the invention is to provide a fourth harmonic beam with improved mode quality.
A further object of the invention is to remove substantially all UV radiation from the laser cavity before it come in contact with the lasing medium.
Other and further objects will become apparent from the specifications, drawings and claims.
REFERENCES:
patent: 5898717 (1999-04-01), Yin
patent: 5936983 (1999-08-01), Yusong et al.
patent: 6061340 (2000-05-01), Yin
patent: 6078598 (2000-06-01), Ohtsuki et al.
Jr. Leon Scott
Photonics Industries International, Inc.
Quinton, Esq James A.
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