The common underlying principle of all lasers is stimulated emission. This term refers to a kind of “forced” emission, e.g. by light, as opposed to spontaneous emission which is the basis for fluorescence. Stimulated emission requires
(over-)population of an excited quantum state from which stimulated emission can occur. Under normal conditions of a Boltzmann distribution between ground state and excited state, stimulated emission is a very rare process. Thus, lasers require an inversion of the equilibrium population, i.e. the excited state has to be populated more than the ground state. In order to reach this non-equilibrium distribution, energy is transferred from a coupled higher energy level, e.g. in another material, a process known as pumping. Depending on the physical make of the laser, pumping can be achieved by light (optical pumping) or an electrical field. The emitted photons induce further emission that leads to self-amplification. Therefore, lasers generally produce coherent light (light being in-phase, since emitted at the same time). As a side note, stimulated emission is also the underlying principle of stimulated emission depletion (STED) applied to super-resolution microscopy. A general overview of lasers used for confocal microscopy is found on the Leica Science Lab.
By definition, the lasing medium (also known as active laser medium) in a solid state laser is a solid (as opposed to a gas, for example). Solid-state lasers are an established laser type. In fact, the first laser, built by Theodore Maiman in 1960, used an optically pumped ruby crystal as lasing material. If the pumping is done electrically in a semi-conductor, the laser is called a diode laser. In case the pumping is done through light (optical pumping), one refers to it as an optically pumped or diode pumped solid state laser.
There is a large variety of diode lasers available at a reasonable price which makes them ideal for entry-level confocal imaging. In the Leica TCS SP8 several kinds of solid state lasers are used. They are very stable, produce only little heat and therefore need no cooling. The Compact Supply Unit of the Leica TCS SP8 contains a set of different diode lasers and optically pumped solid state lasers and offers a small footprint, while the Flexible Supply Unit allows to combine gas lasers with optically pumped solid state lasers, but requires more space and better cooling.
At the opposite end of the scale, both technically and in terms of wavelength, are femtosecond pulsed infrared lasers such as Ti:Sapphire lasers using titanium doted sapphire crystals as lasing material (see photograph on Wikipedia). With their tunable emission wavelengths from around 680 – 1050 nm they are used for multiphoton imaging deeper within tissues as scattering of longer wavelengths is reduced. Also, Ti:Sapphire lasers are used for photoactivation and laser microdissection.
A special variant are optical parametric oscillators (