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  • Bow Tie Is Key to New Blue Laser

Photonics Spectra
Nov 2000
Michael D. Wheeler

BOZEMAN, Mont. -- For years, researchers have sought to generate long-wavelength blue laser light inexpensively. Now a team from Montana State University has done just that by frequency doubling a Fabry-Perot diode laser in a bow-tie-shaped ring cavity.

The work may affect such commercial applications as optical data storage, spectroscopy and display systems.

Until now, researchers looking to generate coherent blue light have had limited options.

Using a bow-tie-shaped ring cavity, a commercially available near-IR laser diode and a nonlinear crystal, physicists at Montana State University have developed an economical source of coherent blue light.

One approach has been gallium- nitride-based diode lasers, which boast efficiency and promise economy but are limited to the 390- to 410-nm range.

Another approach, second-harmonic generation using a single-mode laser and a nonlinear crystal, is expensive because it requires the use of a distributed Bragg reflector laser or an external cavity laser.

The university physicists devised a simple setup that is based on a commercially available 200-mW InGaAs diode laser.

They inserted a KNbO3 nonlinear crystal in the bow-tie ring cavity formed by four mirrors and designed to prevent any direct backreflection into the diode laser.

The cavity focused the input beam into a tight circular spot at the 3 × 3 × 3-mm crystal's location to boost the conversion efficiency. Feedback from a 600-grooves-per-millimeter diffraction grating was used to lock the diode to the cavity's resonance

More efficient

"Our experiment setup is a simple and low-cost scheme for efficiently generating a tunable blue light source by the frequency doubling of a Fabry-Perot diode laser," said Xiaoguang Sun, a graduate student on the project. "We plan to increase the conversion efficiency by optimizing the system and increasing the wavelength scanning range." He said the bow-tie setup could be used with different diode lasers to generate other wavelengths.

The researchers have coaxed 16 mW from the laser, which is tunable from 484 to 488 nm and which displays near-diffraction-limited Gaussian TEM00 output. Details of the group's work were published in the Feb. 21 issue of Applied Physics Letters.

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