Laser Guide Stars

The two dominant types of laser guide stars are the sodium beacon and the Rayleigh beacon. The principle of the sodium guide star is to tune the wavelength of the laser radiation to a resonance of sodium atoms at 589.2 nm. This causes sodium atoms, naturally occurring in the mesosphere at an altitude of around 90 km, to absorb laser light and subsequently to emit fluorescence at the same wavelength. This approach has the nice feature of obtaining fluorescence light essentially only from a narrow range of high altitudes. Its disadvantage is that the required orange/yellow laser source with a power of e.g. 10 W or even 50 W and a small linewidth is not easy to construct and accordingly expensive. Available technological options for sodium beacons include

• a Raman laser based on a bulk crystal, pumped with a frequency-doubled Q-switched neodymium-based solid-state laser
• an 1178-nm Raman fiber laser (or Raman MOPA), pumped with an ytterbium-doped fiber laser, with subsequent frequency doubling e.g. in periodically poled KTP
• sources based on sum frequency mixing of two laser sources (continuous-wave or pulsed), e.g. at 1064 and 1319 nm, or at 938 and 1583 nm
• a pulsed dye laser, which may be made as a “modeless laser” (superluminescent source) for effectively exciting sodium ions with different longitudinal velocities [6].

In contrast, a Rayleigh guide star is based on Rayleigh scattering in the lower atmosphere. In order to use only the scattered light from the higher parts of the atmosphere (at roughly 30 km height), one uses a pulsed laser together with time-gating detection in the wavefront sensor. As the Rayleigh beacon is not based on a narrowband resonance, the chosen wavelength is not critical, except that it should be short because Rayleigh scattering is most efficient at short wavelengths. A common choice is that of a green laser source, such as a frequency-doubled solid-state laser, but a copper vapor laser (→ gas lasers) or an excimer laser can also be used. Such laser sources can be less complex than those of sodium guide stars, and at the same time more powerful, but the lower altitude of the backscattered light compromises the quality of the wavefront correction.

In many cases, laser guide star sources emit nanosecond pulses, rather than continuously. The pulsed format simplifies the nonlinear frequency conversion in the laser source, and it makes possible time-gated detection.

Although a number of different laser sources for laser beacons have been demonstrated, only a few observatories appear to be using laser guide stars so far: the Lick Observatory of the University of California, the Palomar Observatory of Caltech, and the Keck Observatory in Hawaii, all using sodium beacons. There is also the William Herschel Telescope of the Isaac Newton Group in La Palma, Canary Islands, using a Rayleigh guide star. Several other large observatories are currently developing laser guide stars and adaptive optics systems of various types. Examples are the Very Large Telescope of ESO, Gemini North, and the Multiple Mirror Observatory (MMTO) in Arizona.

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[13]	Keck Observatory in Hawaii, http://www.keckobservatory.org/
[14]	Lick Observatory of the University of California, http://mthamilton.ucolick.org/
[15]	Palomar Observatory of Caltech, http://www.astro.caltech.edu/palomar/
[16]	Isaac Newton Group of Telescopes of La Palma, http://www.ing.iac.es/

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