21 May 2008
A solid-state laser emitting 200 mW continuous-wave at 244 nm is set to replace power-hungry argon-ion sources.
Researchers from the US and Japan have unveiled an all-solid-state laser that produces 200 mW continuous-wave (CW) at 244 nm. Future iterations of the system are being touted as direct replacements for frequency-doubled argon-ion lasers that require huge amounts of power and cooling.
"There has not been any CW solid-state laser source at this wavelength," Yushi Kaneda from the University of Arizona's College of Optical Sciences told optics.org. "Efficient CW conversion is limited by the nonlinear coefficient of available materials, which makes a simple single-pass approach impractical."
The laser design can be broken down into two parts. The first step uses a folded resonator to generate 488 nm from an intracavity frequency-doubled optically pumped semiconductor laser (OPSL) chip. The 488 nm beam is then coupled into a bow-tie external resonator where a CLBO crystal produces emission at 244 nm.
"We were able to demonstrate single-frequency operation of the OPSL, which allows coupling to an external optical resonator," explained Kaneda. "Owing to good transparency, CLBO creates a high finesse (low loss) resonator, which enhances the optical power. 200 mW is similar to the powers of commercial frequency-doubled argon-ion lasers used to fabricate fibre Bragg gratings as well as in metrology applications."
While the research team at Arizona has perfected the OPSL technology, a group at Osaka University has been optimizing the CLBO crystal.
"CLBO can phase match for SHG at 238.5 nm or longer, has a reasonable nonlinear coefficient, very good transparency down to 180 nm as well as high damage threshold," commented Kaneda. "Our system has an efficiency of 33%, which is the input power at 488 nm to the measured 244 nm output power."
Kaneda and colleagues are now optimizing the OPSL and believe that higher CW output power at 244 nm is possible with increased 488 nm input power. In this preliminary experiment, the team was able to hold the reported performance levels for three hours, with no degradation to any of the optical components.
"The stability as well as the output power was limited by the 488 nm from the OPSL and I believe that this can be made much better," commented Kaneda. "The external resonator is an interferometer and air turbulence affects the servo loop to maintain the locking for example. This can be solved with a proper engineering effort."
As well as setting its sights on higher powers, the team will be changing the composition of the quantum wells in the OPSL chip to produce other wavelengths. Because the deep-UV output from this laser system is single frequency, it is well suited to spectroscopic applications.
This work was reported in the postdeadline session (paper CPDA2) at CLEO in May and is supported by a grant from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.
Jacqueline Hewett is editor of Optics & Laser Europe magazine.
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