Because the
There are two main types of laser guide star system, known as sodium and Rayleigh beacon guide stars.
Sodium beacons are created by using a laser tuned to 589.2
Rayleigh beacons rely on the scattering of light by the molecules in the lower atmosphere. In contrast to sodium beacons, Rayleigh beacons are much simpler and less costly, but do not provide as good a wavefront reference, since the artificial beacon is generated much lower in the atmosphere. The lasers are often pulsed, with measurement of the atmosphere being time-gated (taking place several microseconds after the pulse has been launched, so that scattered light at ground level is ignored and only light that has traveled for several microseconds high up into the atmosphere and back is actually detected).
Dye lasers were the first laser sources used in laser guide star applications.[3][4][5][6] These tunable lasers have continued to play a significant role in this field.[7][8] However, the use of fluid gain media has been considered by some researchers as disadvantageous.[9] Second generation laser sources for sodium guide star applications include sum-frequency-mixed solid-state lasers.[10] New third generation laser systems based on tunable diode lasers with subsequent narrow-band Raman fiber amplification and resonant frequency conversion have been under development since 2005. Since 2014 fully engineered systems are commercially available.[11] Important output features of the tunable lasers mentioned here include diffraction-limited beam divergence and narrow-linewidth emission.[6]
The sodium laser guide star for use in adaptive optics to correct for atmospheric distortions is believed to have been invented by Princeton physicist
Laser guide star adaptive optics is still a very young field, with much effort currently invested in technology development. As of 2006, only two laser guide star AO systems were regularly used for science observations and have contributed to published results in
Since April 2016,[14] the 4 Laser Guide Star Facility (4LGSF) has been installed at the ESO's Very Large Telescope (VLT),[15] as a new subsystem of the Adaptive Optics Facility (AOF).[16] The 4LGSF is a complement of the VLT Laser Guide Star Facility (LGSF). Instead of a single laser beam, the 4LGSF propagates four laser beams into the skies of Paranal, in northern Chile, producing four artificial stars by illuminating sodium atoms located in the atmosphere at 90 km altitude. These four stars enable getting a better correction in a specific direction, or widening the field of view corrected by an adaptive optics. Each laser delivers 22 watts in a diameter of 30 cm (12 in). The 4LGSF Laser System is based on a fiber Raman laser technology, developed at ESO and transferred to industry.[17][18]
The upgrade to four lasers with fiber Raman laser technology is necessary to support the new instruments at Paranal Observatory,[15] like HAWK-I (with GRAAL) [19] and MUSE (with GALACSI).[20] Also with the 4LGSF the stability is increased, the amount of preventative maintenance support and the preparation of an observing run time will be considerably reduced compared to the LGSF, which currently uses still its original dye laser (planned to be replaced by a fiber laser). The 4LGSF helps astronomers to test devices for the
For sodium laser guide stars, there are three main challenges to overcome: Larmor precession, recoil, and transition saturation.[22] Larmor precession, which is the precession of the sodium atom in the geomagnetic field (precisely, it is the precession of the quantized total atomic angular momentum vector of the atom), decreases the atomic fluorescence of the laser guide star by changing the angular momentum of the atom before a two-level cycling transition can be established through optical pumping with circularly polarized light. Recoil from spontaneous emission, resulting in a momentum kick to the atom, causes a redshift in the laser light relative to the atom, rendering the atom unable to absorb the laser light and thus unable to fluoresce. Transition saturation is the depopulation of atoms from a state of higher angular momentum (F=2) to a state of lower angular momentum (F=1), resulting in a different absorption wavelength.[22]