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Laser solutions
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Harmonic Generation |
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| TABLE OF CONTENTS |
Harmonic Generator AssembliesThe U.S. Laser offers a series of harmonic generators for converting near infrared light into visible or u. v. light by generating the second, third, or fourth harmonics. Designed primarily for use with U.S. Laser's fundamental (TEM00) mode acousto-optically q-switched Nd:YAG lasrers. The systems are also applicable to other lasers operating similarly. If you are unsure as to the suitability of one of our products for your use, please contact our sales department for technical assistance. Our harmonic generators generaly fall into two main catagories, angle tuned, also known as critical phase matching, and temperature tuned, also known as non critical phase matching. With angle tuning, the harmonic generator crystal is oriented and cut at a spicific angle, and is fine tuned by tilting the crystal to achieve optimum phase matching. U.S. Laser's angle tuned systems are provided with a precision five axis mount. to provide the alignment control required. With temperature tuning, the crystal is mounted in a specially designed oven, and the temperature is elevated to the optimum phase match temperature. Once the optimum temperature is achieved, a precision temperature controllerregulates the temperature. For the second and third harmonic, the harmonic generator crystals are placed inside of the laser resonator cavity (intracavity) . This orientatioon is done to improve conversion efficiency, since the intracavity circulating power is significantly higher than the power coupled out of the cavity. When generating the second harmonic intracavity, a special front laser cavity mirror (PN3415) is provided with the SGH assembly. This mirror fully reflects the fundamental wavelength while transmitting thja second harmonic. The mirror effectively acts as a beam separator, and the laser output will be exclusively the second harmonic, with no residual fundamental radiation. In third and fourth harmonic generation, the output is a mixture of the fundamental and the harmonic, and requires the beams to be separated. To accomplish this, USL offers a dichroic beam separator assembly which transmits the harmonic wavelength while rejecting the unwanted wavelength. Unlike a beam separating prism, the beam separator does not deviate the beam path. SECOND HARMONIC GENERATORS: Second harmonic generators are used to convert the fundamental Nd:YAG wavelength of 1064 nm to the second harmonic at 532 nm. The SHG assembly is designed primarily for use intracavity with acousto-optically q-switched lasers operating in fundamental mode. Three SHG doubler assemblies are offered, two angle tuned, and one temperature tuned.
THREE MIRROR INLINE CAVITY : Conventional frequency doubler systems (such as the model 1007-1 thru -3) efficiently transmit 532 nm radiation that is generated in the forward direction thru the SHG crystal. Unconverted 1064nm radiation reflected from the front mirror returns thru the SHG crystal in the rearward direction, and is also converted to 532 nm. This energy may be reclaimed by using a third mirror in the cavity which reflects the rearward 532 nm radiation back to the laser output. Using this concept it is possible to extract as much as 25%-40% more 532 nm radiation than is possible with a conventional two mirror cavity system.
THIRD HARMONIC GENERATOR: The third harmonic of Nd:YAG at 355 nm is generated by sum-frequency mixing of the fundamental 1064 nm radiation with the second harmonic 532 nm. Two crystals are placed intracavity, both the second harmonic crystal, and the third harmonic crystal. The laser output is a mixture of the second harmonic at 532 nm, and the third harmonic at 355nm, and therefore, a beam separator must be used.
FOURTH HARMONIC GENERATOR: The fourth harmonic generator is used extracavity to convert the second harmonic output at 532 nm to the fourth harmonic at 266 nm. The output is a mixture of 532nm and 266 nm light, and therefore, a beam separator is required.
REPLACEMENT HARMONIC GENERATOR CELLS: Replacement cells provide an easy way to change or replace a damaged harmonic generator crystal. Handling and mounting crystals requires extreme care and cleanliness. USL takes the worry out of the job by providing complete cells with premounted crystals. Cells are directly compatible with corresponding USL angle tuned or temperature tuned mounts.
BEAM SEPARATOR ASSEMBLY: When generating the third and fourth harmonics of Nd:YAG, the harmonic generator output is a mixture of the two wavelengths. For the harmonic wavelength to be usable, it must be separated. A dispersion prism may be used, but the two beams are deviated at different angles to the normal beam path. To be useful, the beam must now be redirected back to the normal beam path. The USL beam separator design uses a series of four dichroic mirrors. As the beam travels thru the separator, the harmonic beam is fully reflected while >99% of the unwanted wavelength is transmitted and absorbed. Using this scheme the unwanted wavelength is attenuated by a factor of approximately 108. To allow for correction of any small walk off that may occur as the beam travels thru the separator, the last dichroic mirror is mounted in a kinematically adjustable mount.
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