What is a solid state laser?

This class of laser, which includes the first working ruby laser, does not encompass semiconductor devices (i.e. diode lasers). Certain atoms, when held in a suitable crystalline host, can be excited by light from an external source, producing lasing action. The crystal is usually a cylindrical rod which when mounted in an optical cavity results in a simple and versatile laser. The most common member of the solid state laser family, is the neodymium laser.

Glasses and yttrium aluminium garnet (YAG) are the common host materials for the neodymium solid state laser. The best known alternatives to YAG include yttrium lithium fluoride (YLiF₄), known by the acronym YLF, tungsten oxide-based mediums and sapphire crystals usually doped with titanium particles. Different output wavelengths are produced by these lasers as indicated below, with the most common wavelengths used for materials processing shown first.

Various options are also available for the pump source, the choice of which strongly influences laser characteristics. Continuous arc lamps, pulsed flashlamps and more recently diode lasers are used commercially. Solid state lasers can generate continuous beams of a few milliwatts to several kW, short pulses with peak powers in the gigawatt range, or pulsed beams with average powers in the kW range.

Accessories for these lasers can shift the output wavelength from the near infra-red to the visible (frequency multiplying) or produce high energy, short duration pulses (Q-switching or mode locking).

With the additional benefit of fibre optic beam delivery, it is fairly obvious why the Nd:YAG (in particular) laser has such an important role in materials processing applications. Fig.1, shows a schematic arrangement of a diode pumped Nd:YAG solid state laser, which offers higher efficiency, due to the almost ideal pumping wavelength of the GaAs laser diode (at 0.8µm), and smaller sizes. Diode pumped Nd:YAG laser efficiency (wall plug) can reach 10% compared with the typically 2-3% efficiency of the lamp pumped lasers.

One of the latest developments in solid state lasers is the thin disk laser, which has recently become commercially available in powers up to 1kW. The active medium in this laser consists of an ytterbium-doped YAG crystal lasing at 1030nm. The lasing medium takes the form of a thin disk rather than a rod, thus improving heat dissipation and reducing the 'thermal lens' effect. The net result is an improvement in beam quality.

Another outstanding recent development in the solid state laser field is the fibre laser. In one example of this laser, ytterbium-doped optical fibre is end-pumped with a diode laser, however, several different designs and fibre cladding technologies exist and compete in the market for materials processing. Close to 1kW of power has been achieved in single mode configuration and up to 10kW in multi-mode, by combining beams from several individual lasers. Wall-plug efficiencies typically exceed 20%.

The divergence of lamp pumped neodymium laser beams is usually a few milliradians with beams several mm in diameter. Beam divergences are generally smaller for diode pumped lasers as are the beam diameters, the latter brought about by the use of smaller diameter rods. The table below shows a comparison of typical beam qualities, for different commercially available solid state laser technologies at 1kW of power. In the near infra-red, glass, fused silica and quartz are all used as transmissive optical elements. Reflective mirrors can be polished copper or suitably coated glass substrates.

Note: For comparison purposes, the figures for beam quality in this table are given at 1kW. For some of these lasers, higher powers are available but at reduced beam quality.

Further information

What is a laser?
What are short pulse lasers?
Diode lasers
Nd:YAG laser welding
What is a fibre laser?