Compact fiber laser for 589 nm laser guide star generation
D.M. Pennington, J.W. Dawson, A. Drobshoff, S. Payne, D. Bonaccini, W. Hackenberg, L. Taylor
CLEO/Europe. 2005 Conference on Lasers and Electro-Optics Europe, 2005.
Laser guide stars are crucial to the broad use of astronomical adaptive optics, because they facilitate access to a large fraction of possible locations on the sky. Lasers tuned to the 589 nm atomic sodium resonance can create an artificial beacon at altitudes of 95-105 km, thus coming close to reproducing the light path of starlight. The deployment of multiconjugate adaptive optics on large aperture telescopes world-wide will require the use of three to nine sodium laser guide stars in order
... achieve uniform correction over the aperture with a high Strehl value. Current estimates place the minimum required laser power at > 10 W per laser for a continuous wave source, though a pulsed format, nominally 6 μs in length at ~ 16.7 kHz, is currently preferred as it would enable tracking the laser through the Na layer to mitigate spot elongation. The lasers also need to be compact, efficient, robust and turnkey. We are developing an all-fiber laser system for generating a 589 nm source for laser-guided adaptive optics. Fiber lasers are more compact and insensitive to alignment than their bulk laser counterparts, and the heat-dissipation characteristics of fibers, coupled with the high efficiencies demonstrated and excellent spatial mode characteristics, make them a preferred candidate for many high power applications. Our design is based on sum-frequency mixing an Er/Yb:doped fiber laser operating at 1583 nm with a 938 nm Nd:silica fiber laser in a periodically poled crystal to generate 589 nm. We have demonstrated 14 W at 1583 nm with an Er/Yb:doped fiber laser, based on a Koheras single frequency fiber oscillator amplified in an IPG Photonics fiber amplifier. The Nd:silica fiber laser is a somewhat more novel device, since the Nd 3+ ions must operate on the resonance transition (i.e. 4 F 3/2 -4 I 9/2 ), while suppressing ASE losses at the more conventional 1088 nm transition. Optimization of the ratio of the fiber core and cladding permits operation of the laser at room temperature by minimizing the 1088 nm gain, along with induced bend loss. A 938 nm seed beam is provided by a 0.2 W diode laser, frequency broadened to 400 MHz by DC modulating the diode. This seeds a two stage double-clad, Nd:doped fiber amplifier, producing 16 W of 938 nm light with M 2~ 1.05. Over 3.5 W at 589 nm in continuous wave (CW) format has been generated by sumfrequency mixing the two lasers in periodically poled potassium dihydrogen phosphate (PPKTP). To convert the system to a pulsed format, we added amplitude modulators after both the 1583 nm and 938 nm oscillators and a pre-amplifier in each line to restore the average power to the level prior to modulation. Frequency mixing is simplified by using a pulsed format as the higher peak power facilitates more efficient conversion. To date we have demonstrated 3.8 W at 589 nm in periodically poled stoichiometric lithium tantalate (PPSLT) using a 1 μs pulse length and a 10% duty cycle. Additional bandwidth, pre-compensation for square pulse distortion (SPD) and polarization maintaining amplifier fiber is currently being implemented to enable scaling to higher output power and lower repetition rate. Details of these experiments, system design and performance will be presented.