Abstract

Artificial laser guide stars based on the resonant excitation of the mesospheric sodium layer (at 90 km) are currently implemented on large telescopes in order to extend the sky coverage to sky regions where no intense natural star is present to pilot the adaptive optics loop. However because of the tip-tilt indetermination, the sky coverage is not complete yet. Polychromatic laser guide star is a potential solution to this limitation, based on the measurement of the differential tip-tilt by pointing the relative position of the different chromatic components of the sodium guide star. To produce the polychromatic LGS, we propose to use a high power (> 20 W) laser at 330 nm [1], based on the frequency mixing of a frequency doubled Nd:YAG laser at 532 nm and an amplified Ti:Sa laser at 870 nm. To solve the problem of saturation posed by the pulsed format we are developing a Ti:Sa frequency-shifted-feedback (FSF) laser delivering a broad continuous spectrum in order to cover (after frequency mixing) the hyperfine-Doppler width of the 3S_1/2➔4P_3/2 transition at 330 nm. Our Ti:Sa laser can operate either in cw or pulsed mode. To our knowledge this is the first pulsed solid-state frequency-shifted-feedback laser.

© 2009 IEEE