Abstract

Neodymium based monolithic lasers are known to be efficient, in particular when they include intracavity frequency doubling (ref 1). They can provide emission with highly stable frequency (ref 2). Finally, since optical degradation is more likely to occur at crystal interfaces, reliability is expected to be improved by monolithic designs. The advent of high power blue diode lasers has allowed to pump other rare earths, such as Praseodymium, emitting directly in the visible range and potentially emitting in the UV range with appropriate intracavity frequency doubling stage. In order to optimize the performances of these new lasers, it is interesting to develop monolithic designs of the lasers.

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Compared to Nd:YAG based monolithic lasers, several new challenges are induced by the new crystals and shorter wavelengths. First challenges are related to the Pr:YLF crystal. It is a fragile crystal that can easily break under thermal constraints and its interfaces can be degraded by strong pump intensity at high temperature. In addition, thermal load induces a cylindrical thermal lens, which may strongly degrade polarization based filtering (such Lyot filtering). The use of undoped YLF crystal assembled to the input facet of Pr:YLF crystal may strongly reduce detrimental effects. This is currently under test. Cylindrical lensing impact can be eliminated by inserting the Pr:YLF crystal in the middle of a pair of 45° quarter wave plates. Improved circularization of the pumping beam is also an option because it reduces the pump intensity at beam waist.

Second challenges are related to UV generation. LBO or BBO frequency doublers have very narrow acceptance and cannot be thermally tuned. We have solved this issue by polishing large plates, measuring error and correcting the error by a second polishing. This process is however still to be industrialized. Fragility of BBO and large coefficient of thermal expansion (CTE) of CTE requires an elastic bonding. The challenge of elastic bonding is related to the UV transparency of the solgel. Both optical contacting and UV- proof solgels are being investigated. Results at 640nm, 607nm, 698nm, 720nm and 320nm will be presented at the conference. They will include slope efficiency, single frequency operation and reliability. In particular, long term operation of a 200mW single diode pumped 320nm laser will be discussed.

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