The final amplifier in the Gemini front end also has a bow-tie configuration, and as in the
previous amplifier the infra-red pulses pass through the crystal four times. The Titanium-Sapphire (TiS) crystal in this amplifier is 24 mm in diameter, 12 mm thick and pumped by four, frequency-doubled Q-switched Nd:YAG lasers, each capable of delivering up to 1.3 Joules of green light. However, the pump energy is restricted to a total of 4.5 Joules, to reduce the risk of optical damage in the amplifier. The beams from the four pump lasers are image-relayed to the TiS crystal, where they pump a region 18 mm in diameter.
The TiS crystal is mounted in water-cooled housing to remove excess heat (the average power deposited in the crystal is 25 Watts !). There are vacuum spatial filters after the first and third passes of the crystal, to clean up the spatial profile of the beam and provide image relaying. The infra-red pulses are amplified to an energy of around 1.2 Joules in this amplifier.
Rotating waveplate serves both target areas
After the amplifier, the beam is expanded to 31mm diameter in a beam-expanding telescope. At this point, a rotating half-wave plate and a polariser sends the pulses alternately to target area 2 and to Gemini. The waveplate rotates continuously at a rate of 1¼ revolutions per second, or 45 degrees every tenth of a second, so if one pulse has its polarization flipped by 90 degrees, the next will be unaffected. The polarizer after the waveplate reflects one polarization and transmits the other, thus sending a 5 Hz pulse train to each area. The rotation of the waveplate is synchronised to a 5 Hz trigger signal derived from the 10 Hz rate of the Gemini front end.
Delivering laser shots to the target areas
The energy in each beam can be controlled by means of slide-in attenuator mirrors that have 1% transmission, plus a combination of a rotatable waveplate and polarizer that provide variable attenuation over a range of 100:1. An expanded continuous-wave diode beam, used for pre-shot alignment, can be injected into the beam to target area 2 via a slide-in mirror. Attenuated pulses at 5Hz can also be used for diagnostics set-up and alignment if required. Shots fired at full energy are restricted to 1 Hz and for safety there must be nobody in the area when full energy shots are fired. A fast electromechanical shutter in the beam allows single pulses to be admitted to the target area on demand when the Fire button is pressed in the control room.
Super intense Gemini beam
The beam path to Gemini (link opens in a new window) has a similar set of attenuators, so that the energy levels associated with each power mode of the laser are the same in both Gemini target areas. The Gemini beam travels in a vacuum pipe in a trench under the floor, and is image-relayed with an expansion to 50 mm diameter by the time it enters the Gemini laser hall.
