Doubled praseodymium offers route to UV

05 May 2006

Researchers in Germany and Italy unveil a simple way to generate continuous UV light which could be useful for fluorescence microscopy.

Intracavity frequency doubling of red-emitting praseodymium (Pr) laser crystals is an ideal way to produce continuous wave (CW) UV light, according to a research team from Germany and Italy. Starting with a fundamental wavelength of 640 nm, the team's setup generates 19 mW at 320 nm. (Optics Express 14 3282)

"We report CW UV generation at 320 nm by intracavity frequency-doubling of Pr:YLF and Pr:BYF lasers operating at 640 nm," says Andre Richter and co-workers. "This is the first intracavity second harmonic generation (SHG) demonstration resulting in CW UV laser radiation to the best of our knowledge."

Due to an energy level scheme that allows several transitions in the red, orange, green or blue spectral regions, trivalent Pr ions are possible route to visible laser emission.

The team used a V-type resonator to generate the UV output. In both cases, a semiconductor laser emitting around 310 mW at 480 nm is focussed into the Pr crystal to produce the characteristic 640 nm emission. This red light is then folded into the second arm of the resonator where it passes through a frequency-doubling lithium triborate (LBO) crystal.

Looking first at Pr:YLF, the highest CW output power of 19 mW at 320 nm was achieved using a 8-mm long LBO crystal. Richter and colleagues reported powers of 7.8 mW and 13.4 mW for 3- and 5-mm long LBO crystals respectively.

Similar results were recorded for Pr:BYF. "With the 8-mm long LBO we measured 18.9 mW at 320 nm," say the team. "CW UV radiation of 7.3 mW and 13.9 mW was obtained with the 3- and 5-mm thick LBO respectively. The conversion of the available fundamental power from the visible to the UV was 36%."

The team is now planning to exploit other transitions to generate wavelengths such as 360 nm, 303 nm and 261 nm. "The UV lasers are simple, robust, efficient and offer many wavelengths in the visible and UV spectral range," conclude the authors. "The application potential of these laser lies for instance in measurement techniques and fluorescence microscopy."

This work was carried out by researchers at the Institute of Physics, University of Hamburg; NEST at the University of Pisa; and Coherent Lübeck of Germany.