The field of the invention concerns harmonic lasers, and in particular a method and apparatus for generating frequency doubled, tripled and higher harmonic laser beams.
Optical harmonic generation is well known in the art. Intracavity second harmonic generators are known. See for example J. M. Yarborough, et al, xe2x80x9cEnhancement of Optical Second Harmonic Generation by Utilizing the Dispersion of Airxe2x80x9d, Vol. 18, No. 3., Applied Physics p 70-73. Third harmonic generators are also known in the art. Generally third harmonic generation requires the generation of a second harmonic beam. External third harmonic lasers, where a second harmonic nonlinear crystal and a third harmonic nonlinear crystal are located outside the cavity, are known in the art, see for example U.S. Pat. No. 5,835,513. Intracavity third harmonic lasers are also known. See: U.S. Pat. Nos. 5,898,717 and 6,002,695. It is desired to provide improved external third harmonic generation efficiency. Third harmonic generation requires one photon from each of the second harmonic and the fundamental to form a third harmonic photon. One second harmonic photon represents twice as much energy as the fundamental. For optimum third harmonic conversion, about twice as much second harmonic power is required as fundamental power. In prior art external second harmonic generation, that would mean 67% conversion from the fundamental to the second harmonic. With so high conversion efficiency, the fundamental beam intensity can be distorted substantially to prevent efficient third harmonic generation, as well as deteriorated transverse mode quality. There is still a need for more reliable second and third harmonic generators and for improved efficiency in external cavity third harmonic generators.
The present invention relates to an improved second harmonic laser. In addition, the invention relates to an improved higher order harmonic laser which provide an externally generated third harmonic beam fourth harmonic or fifth harmonic beam.
According to the invention, a second harmonic laser is provided. The laser includes a first high reflector and an output coupler defining a laser resonator cavity having an optical axis. The resonator cavity includes a laser medium for producing a fundamental beam of electromagnetic radiation. Desirably, the laser medium is Nd;YAG, Nd:YLF, Nd:YV04, although other laser mediums are also contemplated such as Ti:sapphire, Nd:YAB and the like. A second harmonic generator is located within the resonator cavity along the optical axis for generating a second harmonic beam from fundamental beam propagating from and to the laser medium. The first high reflector is reflective of fundamental beam. The output coupler is at least partially reflective of fundamental beam and transmissive, preferably highly transmissive of a second harmonic beam. The second harmonic generator is located between the output coupler and the laser medium, so that fundamental beam propagating from the laser medium makes a first pass through the second harmonic generator where a portion of the fundamental beam is converted to a second harmonic beam. The fundamental beam reflected by the output coupler passes through the second harmonic generator where another portion of the fundamental beam is converted into second harmonic beam. The fundamental and the second harmonic propagate toward the laser medium. A beam separator, desirably a dichroic mirror is located between the second harmonic generator and the laser medium. The dichroic mirror is highly reflective for second harmonic beam and highly transmissive for fundamental beam or highly transmissive for second harmonic beam and highly reflective for fundamental beam for fundamental beam, so that second harmonic beam is separated from the fundamental beam. A second high reflector is located in optical communication with the beam separator desirably to receive second harmonic beam separated by the dichroic mirror and reverse its direction of propagation, so that it will propagate through the second harmonic generator toward the output coupler.
In operation, a second harmonic laser according to the invention is provided. The laser medium is pumped by any desired pumping source such as laser, laser diode(s) or lamp(s). The laser resonator is formed between the high reflector and the output coupler. The fundamental beam propagating from the laser medium is directed through a second harmonic generator where a portion of the fundamental beam is converted to the first second harmonic beam. The fundamental and second harmonic beams propagating from the second harmonic generator are then directed to an output coupler, which is at least partially reflective of fundamental beam and transmissive, preferred highly transmissive of second harmonic beam and directs second harmonic beam outside the cavity. The fundamental beam reflected by the output coupler makes a second pass through the second harmonic generator and a second additional second harmonic beam is produced. The fundamental and second harmonic beams are then directed to a beam separator, desirably a dichroic mirror located between the second harmonic generator and the laser medium. The dichroic mirror is highly reflective for second harmonic beam and highly transmissive for fundamental beam or highly transmissive for second harmonic and highly reflective for fundamental beam. The separated second harmonic beam is then directed to a second high reflector in optical communication with the dichroic mirror to receive second harmonic beam separated by the dichroic mirror. The second high reflector is reflective for second harmonic beam. As a result the direction of propagation of the second harmonic beam produced on the second pass is reversed and second harmonic beam is returned along the optical axis to propagate toward the output coupler. Thus, the second harmonic beam incidents on the dichroic mirror and is directed along the optical axis through the second harmonic generator and then is transmitted through the output coupler. Desirably the two second harmonic beams are in substantially same propagation wave front and overlapping in transverse mode when they are combined.
In another aspect of the invention, a third harmonic laser or higher harmonic is provided. Such a laser includes a first high reflector and an output coupler forming a resonator cavity having an optical axis. The resonator cavity includes a laser medium for producing a fundamental beam. Desirably, the laser medium is Nd:YAG, Nd:YLF, Nd:YV04, although other laser mediums are also contemplated such as Ti:sapphire, Nd:YAB and the like. The laser medium can be pumped by any desired pumping source for example laser, laser diode, laser diode bar, fiber coupled laser diode bar or lamp which are well known in the art. The laser medium can be either end pumped or side pumped which are also well known. The first high reflector is reflective of a fundamental beam. A second harmonic generator is located within the cavity formed between the first high reflector and the output coupler for generating a second harmonic beam from the fundamental beam. The output coupler is highly transmissive for second harmonic beam and partially transmissive for a fundamental beam. Desirably the second harmonic and fundamental are provided in a predetermined power ratio most desirably about 2:1 second harmonic to fundamental. A third harmonic generator is positioned external to the resonator cavity and is located along the optical path from the output coupler, so that the fundamental and the second harmonic beams incident on the third harmonic generator where portions of the second and the fundamental are converted to third harmonic beam.
In another embodiment of the invention, the second harmonic generator is located between the output coupler and the laser medium, so that fundamental beam makes a first and second pass through the second harmonic generator. A beam separator desirably a dichoric mirror is located between the second harmonic generator and the laser medium, to reverse the direction of the propagation of the second harmonic beam generated on the second pass of the fundamental beam through the second harmonic generator, so that the second harmonic beam propagates along the optical axis toward the output coupler where it combines with the first second harmonic beam and is transmitted outside of the cavity and incident on the third harmonic generator.
In another aspect of the invention fourth and fifth harmonic lasers are provided. In the third harmonic generator, not all of the fundamental (1w) and second harmonic (2w) beams are converted to third harmonic (3w). Thus, along with the third harmonic beam, substantial amounts of fundamental and second harmonic beam are propagating from the third harmonic generator.
In the fourth harmonic laser, the third harmonic laser described above is provided. A fourth harmonic generator is located external to the resonator optical cavity in optical communication with the beams propagating from the third harmonic generator. The fourth harmonic generator is desirably a nonlinear crystal cut for fourth harmonic generation 1w+3w.
In operation the third harmonic laser is operated. The beams propagating from the third harmonic generator, desirable the fundamental beam 1w and third harmonic beam 3w are directed through to fourth harmonic generator where a portion of the 1w and 3w beams are converted to fourth harmonic beam (4w).
In the fifth harmonic laser, the third harmonic laser described above is provided. A fifth harmonic generator is located external to the resonator cavity in optical communication with the beams propagating from the third harmonic generator. Desirably the fifth harmonic generator is a fifth harmonic nonlinear crystal cut for fifth harmonic generation 2w and 3w. In operation a third harmonic laser is operated as described above. The second harmonic and third harmonic beams propagating from the third harmonic generator are directed through the fifth harmonic nonlinear generator for conversion of second harmonic (2w) and third harmonic (3w) to fifth harmonic beam (5w).
It is an object of the invention to provide an efficient intracavity second harmonic laser.
It is an object of the invention to provide an efficient external cavity third harmonic generation laser.
It is an object of the invention to provide an efficient method to generate a predetermined power ratio of the second harmonic and the fundamental in substantially the same propagation direction and having substantially overlapping transverse mode.
It is an object of the invention to provide an efficient method to generate second harmonic and fundamental in a power ratio of 2:1 in substantially the same propagation direction and substantially overlapping transverse mode.
It is an object of the invention to provide efficient external third harmonic generation with improved transverse mode.
It is an object of the invention to provide efficient external cavity third harmonic generation laser with a second harmonic generator located in the laser cavity and a third harmonic generator located external to the cavity.
It is an object of the invention to provide an efficient external cavity fourth or fifth harmonic generation laser with a second harmonic generator located in the laser cavity.
It is an object of the invention to provide an efficient external cavity third harmonic generation laser with a second harmonic generator located in the laser cavity and a third harmonic generator located external to the cavity and a LBO nonlinear crystal as the third harmonic generator.
It is an object of the invention to provide efficient external cavity third harmonic generation laser with a second harmonic generator located in the laser cavity and a third harmonic generator located external to the cavity and a LBO nonlinear crystal as the third harmonic generator oriented to partially compensate for the walk off generated from a CPM second harmonic generator.
Other and further objects will become apparent from the appended specification drawing and claims. It should be understood that there are numerous embodiments contemplated by the subject invention. Every embodiment of the invention does not necessarily achieve every object of the invention.
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.
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