Multistage optical parametric amplifier for the generation of sodium guide star

ABSTRACT We report proof-of-concept experiments generating 5.5W of rapidly tuneable 589nm laser radiation through SHG of 1178nm radiation generated by optical parametric amplification in Lithium Triborate. The two stage OPA was pumped by a frequency doubled mode-locked Nd:YVO 4 laser (15ps, 1.5MHz, 50W) and seeded by a tuneable diode laser, allowing convenient tuning on and off the Na D2 line. Our findings imply that if scaled up in power and with modified pump pulse duration, this approach can lead to a versatile high-power Na guide star source which can be built from readily available commercial components. Keywords: non-linear optics, optical parametric amplification, guide star 1. INTRODUCTION Sodium laser guide stars in conjunction with adaptive optics systems have great potential to significantly improve the resolution of Earth based telescopes by cancelling out optical aberrations introduced by the atmosphere [1]. Light emitted at the Sodium D2 line, 589nm, is used to excite sodium fluorescence in a 10km thick sodium layer 90km above Earth’s surface. This technique allows the generation of artificial guide stars at will, rather than relying on the scarce number of natural stars with adequate emission characteristic s, greatly increasing sky cove rage. The output spectrum of an ideal laser guide star source covers the full manifold of the Na D2 lines § 3GHz wide, which requires a transform-limited Gaussian pulse with a duration § 146ps. Extremely large telescopes (ELT) require several guide star sources spread across the telescope’s aperture to obtain the necessary photon yield to effectively cancel out the wave front distortions. Hence, there is a growing need for improved guide star sources featuring higher power, variable pulse formats and tuneability. Since these laser sources have to be operated at the telescope site, often in remote areas, they have to be rugged and low in maintenance, making solid state and fibre lasers ideal candidates. Most solid state based approaches rely on sum frequency mixing of 1064nm and 1319nm, laser lines of Nd:YAG [3,4,5]. Disadvantage of this scheme is that it requires active cavity stabilization, tuneability is not easily achieved and the 1319nm gain in Nd:YAG is small. Hence, we used two-staged optical parametric amplification to generate 1178nm radiation, which can be frequency doubled with high efficiency to 589nm. The optical parametric amplifier (OPA) was pumped by a frequency doubled, passively mode-locked Nd:YVO