There have been a wide variety of laser architectures developed, aimed at ex- tending the power and brightness scalability of solid state lasers. One of the principal strategies for reducing thermal effects is to increase the ratio of cool- ing surface to pumped volume. Another approach that can be applied to a range of laser geometries is cryogenic cooling of the active medium [20]. The thermal properties of most laser materials have been shown to improve dra- matically at very low temperatures, such as that of liquid nitrogen. For ex- ample the thermal conductivity becomes much larger, making removal of heat much more efficient. Temperature gradients and therefore thermal effects can be made very small. The disadvantage however, is that the complexity be- comes prohibitively large for most laser systems. Not only does the cooling system itself add complexity, but also all components which are cooled to low temperature need to be placed in a partial vacuum to avoid problems with condensation. Some of the more popular architectures which can all be used in both laser and amplifier configurations are summarised as follows.
2.6.1.1 Side-Pumped Slab Lasers
The side-pumped slab laser uses a slab of the active medium which is cooled via its top and bottom surfaces which have the largest area. In this scheme, shown in 2.9, one or two of the remaining faces are pumped and the laser ra- diation is approximately orthogonal to the pump radiation. One particular advantage of side-pumping slabs like this is that the high aspect ratio of the pumping faces of the slab are ideally suited for pumping with high power diode bars which have a similarly high aspect ratio, as discussed in Section 2.7.2. This simplifies the design and means that very high efficiency of inci- dent pump power to available pump power can be achieved. One drawback of this technique, is that the pumped volume is generally not well matched to the laser mode. This implies that it can be difficult to achieve high extraction of stored energy. When a gain medium with a high absorption coefficient such as Nd:YVO4 is used, the inversion density is localised in the region closest to the
pumping face. By utilising a grazing incidence laser beam with a total internal reflection at the pump face, as shown in Figure 2.10 (a), the spatial overlap of
Figure 2.9: Single-bounce slab laser configuration with diode bar side-pumping
the laser and pump volumes is higher and hence the efficiency can be increased compared to a simple beam transit through the material. Moreover, one of the main attractions of this geometry is that the laser beam is inverted horizon- tally on reflection at the pumped face which means that the distortions, which result from the horizontal gain and temperature gradients, are equal and op- posite for each half of the transit through the material. This implies very good compensation for beam quality degradation since the horizontal distortions acquired on the input section of the transit are almost completely reversed on the output section. Another common technique for improving the extraction efficiency is to allow the laser beam to make multiple passes through the ma- terial, each one accessing a different region of the slab, as shown in Figure 2.10 (b). From a thermal point of view, slab lasers have some favourable properties such as large cooling area to pump volume ratio, and the pump distribution is more uniform than in end-pumped systems leading to reduced thermal lens- ing. However, due to the lack of cylindrical symmetry, the thermal lensing effects, when they do occur, can be more problematic with the strong astigma- tism of the thermal lens being one example.
Figure 2.10: Side-pumped slab configurations. (a) Grazing- incidence bounce geometry; (b) Multiple-pass geometry.
2.6.1.2 End-Pumped Rod Lasers
The laser and amplifier systems described later in this thesis are all based on end-pumped rod geometries. In this scheme the pump and laser beams are collinear and the heat removal is through the sides of the rod, as shown in Figure 2.11.
Figure 2.11: End-pumped rod laser configuration
The advantage of this scheme, compared to side pumping for example, is that the pump and laser beams can be very well matched spatially to allow effi- cient extraction of the stored energy and easy selection of TEM00 modes in a
resonator. The absorption efficiency can also be made large by choosing a rod of the appropriate length. The cylindrical symmetry of the system, compared to side pumped systems, also implies that the thermal effects are well defined so some of the associated problems can be reduced by simple adjustment of system parameters such as the beam spot sizes. Additionally, the cylindri- cal symmetry of the system implies that thermal lensing can be compensated for, to some extent, by simple optics. End-pumped rods do, however, suffer from particularly strong thermal lensing effects due the high pump deposition density which leads to strong aberrations in the wings of the pump distribu- tion. One of the most simple techniques for reducing the temperature rise in end-pumped laser materials, is to choose lower doping concentration in the material and longer length. This spreads the thermal load over a larger vol- ume and therefore allows the heat to be removed more efficiently. The main disadvantage is that this requires a pump beam with a better beam quality in order to confine the beam to a small volume in the gain medium. In effect, the pump beam quality limits the maximum length of the rod. Some recent
systems have incorporated undoped end-caps on the laser rods [16, 21]. This implies that pump radiation is only absorbed in the doped region away from the surface. The main advantage of this is to eliminate end-face bulging by minimising the temperature rise at the surface.
2.6.1.3 Thin-Disc Lasers
Figure 2.12: An example of a thin-disc laser set-up
One laser geometry, which has been proved to be very successful for power scaling, is the thin-disc laser. In this scheme, shown in Figure 2.12, a thin disc of the gain medium, of a few hundredµm thickness, which is HR coated on the back surface for pump and laser radiation and AR coated on the front surface, is bonded to a water cooled heat sink. A laser resonator is formed between a curved output coupler and the HR surface of the disc. In this scheme, the cooling surface is large compared to the pumped volume so high output power can be extracted from a small volume. Since the back surface of the disc is cooled, the heat flux and the laser beam axis are collinear to each other. As a consequence, thermal lensing effects are dramatically reduced. Due to the very short interaction length in the gain medium, large doping concentrations are required as well as multiple passes of the pump radiation in order to achieve high absorption efficiency. Using this kind of system with Yb:YAG as the gain medium, cw laser powers of up to 1kW have been achieved [22]. Nd3+ based
lasers materials have a larger quantum defect so thermal loading is typically almost three times higher than in Yb:YAG. One of the highest reported output powers for a thin disc laser based on Nd3+ was a Nd:GdVO
4 thin-disc laser
thermal effects in the gain medium which degrade the beam quality. Thermal distortions stem mainly from the bending of the crystal due to the difference in expansion of its front and back surfaces with additional aberrations resulting from a radial temperature gradient at the edge of the pumped area.
2.6.1.4 Fiber Lasers
Figure 2.13: Double-clad fibre laser
Power scaling of fibre lasers is a field attracting much attention in recent years. In many applications, fibre lasers are replacing bulk solid-state lasers as cheaper, more efficient and less cumbersome alternatives, often with better performance characteristics. As shown in Figure 2.13, a typical fibre laser consists of a dou- ble clad, fused silica optical fibre with a rare-earth doped core. The inner cladding is used to guide the pump radiation which is absorbed in the core over typically several metres. The core can be made single mode for the las- ing wavelength so that the output is diffraction limited even at high power. A laser cavity can be set up either externally, as shown in the Figure, or in- ternally using Fibre Bragg Gratings (FBGs). The very high surface to volume ratio of fibres allows for very high pump powers to be absorbed without ex- cessive heating. Thermal effects most commonly take the form of damage to the fibre end facets, or the outer polymer coating, caused by intense diode pumping. The glass host for the active medium gives broader absorption and emission bandwidths than crystal based lasers which allows broad wavelength tunability or potentially ultrashort pulse generation. Compared to bulk solid- state lasers, fibre lasers and amplifiers have superior thermal properties and potentially higher extraction efficiencies, however they suffer from a number of disadvantages. Many applications require polarised output and fibres do
not, in general, maintain the polarisation state of ingoing light. Additional complexity in fabrication is required to produce polarisation maintaining fi- bres. Moreover, in pulsed operation, the high laser intensities in the fibre core can lead to a number of undesirable nonlinear effects. The most common are Stimulated-Brillouin-Scattering (SBS) and Stimulated-Raman-Scattering (SRS) whereby some of the power becomes backscattered or forward-scattered by acoustic or optical phonons respectively [24]. Some problems can also arise from self-phase modulation caused by the Kerr nonlinearity. Increasing the core size of the fibre is necessary to reduce the nonlinear effects, but this makes single-mode operation more difficult to achieve.