Disk laser

The thickness of the disk is considerably smaller than the laser beam diameter.[3] The disk laser concepts allow very high average and peak powers[4] due to its large area, leading to moderate power densities on the active material.Initially, disk lasers were called active mirrors, because the gain medium of a disk laser is essentially an optical mirror with reflection coefficient greater than unity.An active mirror is a thin disk-shaped double-pass optical amplifier.The first active mirrors were developed in the Laboratory for Laser Energetics (United States).[6] Scalable diode-end-pumped disk Nd:YAG laser had been proposed in [7] in Talbot active mirror configuration.[8] Then, the concept was developed in various research groups, in particular, the University of Stuttgart (Germany)[9] for Yb:doped glasses.In the disk laser, the heat sink does not have to be transparent, so, it can be extremely efficient even with large transverse sizeThe increase in size allows the power scaling to many kilowatts without significant modification of the design.Then, to avoid strong losses due to the exponential growth of the ASE, the transverse-trip gaindetermines the optical energy, which is output from the laser cavity at each round-trip).Then, at some critical size, the disk becomes too thick and cannot be pumped above the threshold without overheating.Roughly, the round-trip loss should scale inversely proportionally to the cubic root of the power required.Thin disk diode-pumped solid-state lasers may be scaled by means of transverse mode-locking in Talbot cavities.is due to randomly distributed phase distortions across an active mirror of the order[13] In order to reduce the impact of ASE, an anti-ASE cap consisting of undoped material on the surface of a disk laser has been suggested.[15][16] Such a cap allows spontaneously emitted photons to escape from the active layer and prevents them from resonating in the cavity.This could allow an order of magnitude increase in the maximum power achievable by a disk laser.[14] In both cases, the back reflection of the ASE from the edges of the disk should be suppressed.At operation close to the maximal power, a significant part of the energy goes into ASE; therefore, the absorbing layers also should be supplied with heat sinks, which are not shown in the figure.The thick dashed line represents the estimate for the uncovered disk.The thick solid line shows the same for the disk with undoped cap.All future experiments and numerical simulations and estimates are expected to give values of[17] In the vicinity of the curves mentioned, the efficiency of the disk laser is low; most of the pumping power goes to ASE, and is absorbed at the edges of the device.In these cases, the distribution of the pump energy available among several disks may significantly improve the performance of the lasers.In quasi continuous wave regime, the maximal mean power can be estimated by scaling the saturation intensity with the fill factor of the pump, and the product of the duration of pump to the repetition rate.At short duration pulses, more detailed analysis is required.[18] At moderate values of the repetition rate (say, higher than 1 Hz), the maximal energy of the output pulses is roughly inversely proportional to the cube of the background lossAt low repetition rate (and in the regime of single pulses) and sufficient pump power, there is no general limit of energy, but the required size of the device grows quickly with increase of the required pulse energy, setting the practical limit of energy; it is estimated that from a few joules to a few thousand joules can be extracted in an optical pulse from a single active element, dependently on the level of the background internal loss of the signal in the disk.