Two techniques are available for creating laser guide stars suitable for use in astronomy. Rayleigh laser guide stars are based on simple atmospheric back-scattering of laser photons off air molecules. Sodium laser guide stars are based on resonance backscattering of laser light off sodium atoms suspended in the upper atmosphere. While sodium laser guide stars have the advantage of producing a laser guide star high in the atmosphere (at 95 km above mean sea level), they suffer from the need to develop, construct and maintain a complex laser system that -- in most instances -- costs more than $2 million. Rayleigh laser guide stars have the disadvantage of being produced at lower altitudes (the UnISIS laser guide star is located 18 km above the observatory or 20 km above mean sea level), but they have the distinct advantage of being relatively inexpensive to create since relatively simple and robust lasers can be used. When the UnISIS Excimer laser was purchased in 1990, the laser cost $70,000. A laser pulse is emitted by the system and the primary mirror of the telescope is used to focus the light to 18 km altitude. Scattered light that happens to return on a path back toward the telescope is detected by the telescope after a very fast range gating scheme is used to isolate the focused waist. The return laser light appears back at the telescope 120 microseconds after it is emitted. The return laser guide star signal is then used to determine how the deformable mirror has to be distorted to remove aberrations that the atmosphere has put into the wave front. This process is repeated as often as the laser pulses, thereby providing continuous corrections for atmospheric effects. The UnISIS laser can be operated at 167 or 333 pulses per second.
A pulsed Excimer laser provides the source of light for the UnISIS laser guide star. The laser light is generated at 351 nm (the middle of the standard photometric U-band) when an electrical discharge occurs between a pair of electrodes which are suspended in a pressurized chamber of gas. The electrodes are charged to high voltage by a network of large capacitors and discharge over a very short time interval initiating the pulsed laser light. If the pressurized chamber is loaded with xenon and fluorine (with neon as the buffer or filler gas) the Excimer laser emits at 351 nm. Excimer lasers also work at shorter wavelengths (308 nm, 248 nm, 157 nm) if loaded with other gas mixtures, but these are of little interest for laser guide star work because the high opacity of the Earth's atmosphere at the shorter wavelengths limits the height to which the laser light can penetrate. The 351 nm wavelength is most convenient because in the near-UV, the Earth's atmosphere is still relatively transparent. This 351 nm wavelength is also just short of the limit of human vision.
The pulse repetition rate of an Excimer laser is selected to maximize the laser output power. Each laser configuration (electrode structure and gas partial pressure) has an optimal operating range, and for the UnISIS Excimer laser working at 351 nm, the total output energy peaks at a repetition rate in the range between 100 to 300 pulses per second. For test purposes UnISIS is often run at 167 Hz, but for the best AO performance, the laser repetition rate is set to 333 Hz. Power per pulse is usually 100 mJoules at the output port of the laser, but only 60 mJoules makes it into the sky (the rest is not in the Gaussian beam core but in a diffuse surrounding halo). Projected into the sky in core of the laser beam at 333 Hz is approximately 20 Watts, even though the laser itself is working at 33 Watts.
Laser beam quality is crucial issue for laser guide star systems because a tightly focused laser guide star provides the best performance in Shack-Hartmann sensors. Excimer lasers do not ordinarily produce the highest quality beams output beams. In fact, with plane mirrors at the ends of the gas chamber, the output beam divergence is approximately 3 milliradians. To produce higher beam quality, the plane laser mirrors are removed and are replaced by two curved mirrors. In this configuration the laser is said to have an unstable resonator cavity. Astronomers will find it easy to visualize the optical design of the unstable resonator cavity because it is identical to that of a Cassegrain telescope with the rear laser mirror being the concave primary and the front laser mirror being a (semi-transparent) secondary. With unstable resonator optics, the beam divergence is reduced to 300 microradians. However, even this is insufficient to produce a tight laser guide star on the sky, so the last stage of beam improvement in UnISIS is to expand the laser output beam by a factor of 100 by using the 2.5-m primary mirror as the projector. In this way the final beam divergence is reduced to approximately 3 microradians ~ 0.6 arcsec.
The UnISIS Excimer laser is housed in a special air conditioned room located in the basement of the Coude spectrograph chamber at the Mt. Wilson 2.5-m Telescope dome. Appropriate electrical service was installed, and the room is equipped with gas cylinders of helium, neon, xenon, and fluorine. Special precautions were taken for the fluorine gas because of its toxicity.
The Excimer laser is shown in the picture below. The cabinet in the foreground (with the vent pipe running to the ceiling) holds the fluorine gas bottle, and the laser itself is next in line. The output laser beam emerges from the far side of the laser cabinet, reflects off a set of beam-steering mirrors, and eventually is sent through a portal in the ceiling to the beam combining optics that are located in the Coude Room some 10 meters overhead.
A blue line was superimposed on this image to show the path of the output laser beam
as it would leave the laser beam steering optics and head for the protective tube that
protects the beam as it travels to the Coude beam-combining-room 10 meters above.
Because 351 nm radiation is invisible to the eye, even when the laser is running,
the beam is actually invisible. Also notice that the blue beam seems to pierce the
plastic cover that protects the laser beam steering optics. When the laser is
actually operating, the plastic lid is opened.
Some of the internal components of the 33 Watt Excimer laser can be seen
in this image. Only the closer side of the capacitor banks are visible here
(the capacitors are brown-colored disks). The capacitor banks run the full 1-m length
of the gas chamber. The very rapid
high voltage discharge to the laser electrodes is controlled with a thyratron switch
located at the center of the capacitor banks (not visible in this image, but it sits
directly under the two cooling fans).
The output laser path is shown by the superimposed blue
line. Three beam-steering mirrors are used to achieve good alignment
of the laser output beam with the subsequent optics. The large mirror on
the right hand side is equipped with picomotors (the red devices on the
rear-side of the mirror mount) for remote-control of the beam position.
All mirrors have 99.9% reflectivity at 351 nm. As mentioned before, the
351 nm laser light is not visible to the eye, so the blue line was added
to this image for schematic purposes.