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Simple source reaches for the stars

10 Feb 2009

A tunable 589 nm source made from commercial parts offers a simple route to sodium guide stars.

A laser capable of emitting 5.5 W at 589 nm could be the ideal way to generate the artificial sodium guide stars required by modern telescopes that rely on adaptive optics, say researches in Australia. Based on a laser-driven optical parametric amplifier (OPA), the source is said to offer versatile pulse formats, power scaling and rapid tunability over previous 589 nm laser sources.(Optics Express 17 437)

"We wanted to make a simple system that could be built using commercial laser components and was readily scalable to higher power," Malte Duering of the Laser Physics Centre (LPC), part of The Australian National University's Research School of Physical Sciences and Engineering, told optics.org. "An important aspect is that the laser hardware is simple and contains no complex resonators or stabilization. Our system requires only a single high power neodymium solid-state pump laser and a small semiconductor laser."

A laser guide star (LGS) is an artificial star generated by shining a high-power laser into the upper atmosphere to create a spot of light. Adaptive optics systems then use this point as a reference to correct for the effects of the atmosphere on optical wavefronts in real time.

The proof-of-concept scheme simply uses a passively modelocked Nd:YVO4 MOPA to pump a multi-stage lithium triborate OPA seeded by a tunable CW semiconductor diode laser.

Duering believes that this straight-forward way of generating 589 nm has never been tried before, given that OPAs are conventionally used to create broadband or ultra-short pulses in combination with optical parametric generators or white light sources.

"The guide-star application requires relatively narrowband emission and most OPAs have not been used to produce such narrow linewidths," he explained. "We show however that by seeding the amplifier with a readily available grating-tuned semiconductor laser we can achieve transform limited output pulses and close to the required linewidths."

The current setup is said to produce 5.5 W at 589 nm and a slightly larger than desired linewidth of around 13 GHz. The group believes, however, that linewidths of around 3 GHz, spanning the Doppler width of the sodium D2 line, should be achievable by using an intracavity etalon to stretch the pulses from their 1064 nm pump laser to about 300 ps. It is also expected that a 150 W system capable of 15-20 W of 589 nm light could be obtained by simply adding another amplifier without significant loss of beam quality.

A special feature of the setup is that the output frequency can also be directly controlled by tuning the CW beam that seeds the OPA. Rapid (up to ~kHz) electronically controlled tuning of the output frequency on and off the sodium resonance line could reduce the impact of Rayleigh scattered light on the adaptive optics system.

Caryl Richards is features editor of Optics & Laser Europe magazine.

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