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Frequency converter accesses new wavelengths

14 Mar 2007

Using nonlinear crystals to convert the frequency of a laser gets a new look as an Italian researcher unveils a 3/2 frequency multiplier that could simplify RGB laser systems.

Multiplying the frequency of continuous wave radiation by a factor of 3/2 is now possible thanks to work being carried out in Italy. Researcher Gabriele Ferrari says the simple frequency converter scheme generates radiation that is useful for a number of applications including spectroscopy and metrology. (Optics Express 15 1672)

"This is the first experimental demonstration of 3/2 optical frequency multiplication," Ferrari, from the European Laboratory for Nonlinear Spectroscopy at the University of Florence, told optics.org. "Visible radiation is normally accessible only through optically pumped dye lasers. I wanted a simple way to produce visible light and that's how the idea of the 3/2 multiplier came about."

The converter is based on a multi-resonant OPO operating at frequency degeneracy. This means that the pump, signal and idler fields all resonate in the same cavity and the frequencies of the signal and idler fields coincide.

The radiation generated by the OPO has half the frequency of the pump. By using two nonlinear crystals, it is possible to sum the pump and OPO fields and generate radiation at 3/2 of the pump frequency.

"The first crystal splits the fundamental field into two identical OPO photons with double the wavelength," explained Ferrari. "The second satisfies the phase matching for the process where one fundamental and one OPO generated photon are summed into one photon of energy 3/2 of the initial fundamental."

To demonstrate the multiplier, Ferrari produced 40 mW of red light at 671 nm from a pump delivering 400 mW at 1006.5 nm. The pump is a MOPA system where the master laser is a diode laser stabilised within an extended cavity producing 30 mW at 1006.5 nm in single longitudinal mode. A taper amplifier raises this to 400 mW before it is coupled into the cavity.

"The multi-resonant cavity gives me the freedom to resonate optical fields with different wavelengths simultaneously in the same cavity, regardless to their precise frequency," added Ferrari. "Active stabilization of the cavity with respect to the pump field is possible and makes the use of highly stable cavity design unnecessary. It also allows me to tune the OPO to have the exact division by a factor two. Finally, I can reduce the oscillation threshold. I can now operate the system without the tapered amplifier."

Delighted with the initial results, Ferrari is now working on several refinements. "I am trying to improve the conversion efficiency, reduce the oscillation threshold and improve long term reliability," he concluded. "I am also working on a three times multiplier for CW lasers which should provide 308 nm light from 924 nm using a single resonator."

•  Ferrari would like to acknowledge support from Rudi Grimm from the University of Innsbruck, Austria; Laser & Electro-Optic Solutions of Italy; and Sacher Lasertechnik of Germany.

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