The design of the amplifiers calls for a small-signal gain of around 4.2 per pass to achieve the design output energy of 25 joules. Modelling of the performance showed that this output could be achieved with a total of around 60 joules of pump energy in a 50 mm diameter beam, while keeping the energy density on the crystal at a safe level.

A common problem with large-aperture, high-power amplifiers is that energy can be lost to so-called parasitic processes, in which the light emitted spontaneously in the crystal competes with the laser beam to extract the stored energy of the pumped region. The higher the gain along a particular path, the greater this loss becomes, and in some configurations the faces of the crystal can reflect enough energy back into the gain region for lasing action to occur; a so-called ‘parasitic oscillation’.

Several novel techniques were used in the design of the Gemini amplifiers to minimise these losses, which has ensured the amplifiers are efficient and behave as predicted. The most important of these was to specify crystals that absorb only 90% of the pump energy as it passes through. This leads to a fairly even deposition of energy within the crystal, and avoids the formation of regions of very high gain at the crystal faces, which can cause the problems described. However, it would be wasteful to throw away 10% of the pump light, so the transmitted beams are returned to the crystal by mirrors, where a further 9% of the light is absorbed. In this way 99% of the pump energy is deposited as uniformly as possible in the gain region.

In addition to this, the crystal is mounted in a cell and surrounded by a liquid with a fairly high refractive index (1-bromonaphthalene) that has an absorbing dye dissolved in it. This traps the light that reaches the edge of the crystal and prevents it being scattered back into the gain region. Tests of the amplifier without the liquid present showed that this process caused the output to be clamped at a very low level, whereas when the liquid was added the performance of the amplifier agreed with the predictions of our modelling.

Output energies of 25 joules were measured with around 60 joules of pump energy, in good agreement with the modelling. At this energy, assuming a transmission efficiency of 60% in the compressor (which would be lower than average) the energy reaching the target area is 15 joules.