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Frequency quadrupling brings high-power UV within reach

04 May 2007

A 250 mW UV source developed by US researchers could suit quantum cryptography, spectroscopy or micromachining.

A team from MIT, Wright-Patterson Air Force Base and AdvR has demonstrated a narrowband pulsed UV laser delivering 250 mW that could be used to produce entangled photon pairs for quantum cryptography keys (Optics Letters 32 10 1290).

Laser sources for entangled photons have to meet some stringent criteria, and commercial UV lasers fall short of the requirements. "They are hard to operate in pulsed regimes and their output power range is limited," Onur Kuzucu of MIT told optics.org.

The team's solution was to take the output of a pulsed infrared source at 1560 nm and quadruple the frequency with periodically poled nonlinear crystals, resulting in usable pulsed power at UV wavelengths. "We calculated we would need to achieve around 100 mW of UV power to be of practical use as an entangled photon source," said Kuzucu.

The initial IR source, an erbium-doped fiber laser, had to be customized to avoid spectral broadening caused by the peak power levels inside the fiber. The team adjusted the fiber lengths and removed several passive components of the standard design.

The doubling crystals were also a challenge. "First we upconverted our starting laser to 780 nm with a frequency-doubling crystal of MgO-doped periodically poled lithium niobate, MgO:PPLN. Then we applied a second doubling with a periodically poled KTP crystal, PPKTP, to upconvert again from 780 to 390 nm, our UV wavelength," Kuzucu explained.

Unfortunately, almost all nonlinear crystals begin absorbing light at the UV end of the spectrum, at best affecting the output quality and at worst rendering the crystals unusable. "The growth technique used for a particular crystal can have a significant effect on these UV-related effects, and we found that hydrothermically grown PPKTP offered the best output performance at UV," observed Kuzucu. "With a better quality crystal and proper anti-reflection coating, we've actually now exceeded our published results and managed to reach 400 mW."

And even higher powers could be on the horizon by using so-called type-I phase matching in the PPKTP crystals. "Manufacturing this type of crystal needs more precision in the poling process used to imprint a periodical grating, but once that's done we expect that output powers could approach 1 W unless damage occurs," Kuzucu believes.

The potential uses in cryptography have already caught the eye of the quantum information community, but Kuzucu expects other end uses to open up too. "Enhanced feature resolution in laser micromachining should be helped by the shorter wavelengths, and it's in a spectral region where transparent materials start to absorb. The source could also prove useful in spectroscopy, where internal carrier dynamics of solid-state materials could be analyzed."

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