The Gemini front end system, in common with other high-power lasers, has a MOPA (Master Oscillator - Power Amplifier) architecture. This means that a low energy, high-quality pulse is generated at the start of the laser chain, and amplified to successively greater energies in a sequence of amplifiers.
The source pulses are extremely short, so the technique of Chirped Pulse Amplification is used to avoid distortion of the pulses and damage to the laser during amplification.
The front end here consists of an ultra-short pulse oscillator that provides low-energy, high-quality seed pulses of around 12 femtoseconds (fs) duration. These are slightly stretched to around 7 picoseconds (ps) in a glass block before being amplified to millijoule energy in a kHz repetition rate preamplifier. After the preamplifier, individual pulses are selected by a fast Pockels cell at a repetition rate of 10 Hz.
A much greater stretch is then applied, using a wavelength-dependent optical delay line (pulse stretcher). The pulses are stretched to either 530 ps (if the pulse will be used in target area 2) or 1060 ps (if the pulse will be sent to Gemini). Pulses of this length can be amplified to multi-Joule energies without reaching intensities that would damage optical components or cause severe distortion of the pulse. If the pulses were amplified without stretching, the resulting intensity could severely damage or even shatter optical components in the laser.
Three amplifiers are used in sequence to reach a final energy of more than 1 Joule per pulse. Each amplifier consists of a titanium-sapphire (TiS) crystal that is pumped by pulses of green light from another laser. The green light excites the titanium ions in the crystal to a state of higher energy, a condition in which they are able to amplify the infra-red light in the stretched pulse as it passes through the crystal.
The infra-red beam is sent through each crystal several times, in order to extract as much energy as possible from the pumped region. As the energy in the beam increases, both the beam size and the crystals are made larger in successive amplifiers to keep the intensity of the light below the level where damage will occur.
All these lasers run at 10 pulses per second, and are accurately synchronized so both the pump and seed pulses arrive at the crystal at the correct times. At the output of the third amplifier, the beam with the 10 Hz pulse train is separated into two beams each with a 5 Hz pulse train. One beam is sent to the Gemini laser area, and the other to target Area 2, Attenuators are used to control the pulse energy in each beamline. For TA2, a fast-acting mechanical shutter allows the experimenters to use either the full pulse train for setup and alignment, or to select individual full-energy pulses on demand.
To aid alignment of the pulse compressors, a dual-wavelength CW diode can be injected into either beam line. The two wavelengths are 785nm and 810nm, which are near the extremes of the pulse spectrum. Used in conjunction with alignment mirrors and imaging, the dual-wavelength beam allows the grating alignment in the pulse compressors to be optimised rapidly.
In the target area, the pulses are recompressed to a duration of about 40 fs before use. The compressor is in a vacuum chamber, and once the pulse is compressed the beam remains in vacuum until it hits the target. Pulses can be used at repetition rates up to 2 Hz at full energy, or at 10 Hz with low energy for alignment purposes.
