Solid state lasers

Solid state lasers are either semiconductor (or diode) lasers pumped electrically or those with a crystalline or glass matrix pumped optically.

Diode lasers use the recombinations between the “electron-hole” pairs found in the semiconductors to emit light in the form of stimulated emission. The pump source is electrical with an efficiency that can reach 60%. The wavelength can cover from the near UV to the near infrared depending on the materials chosen (GaN, GaAlInP, AlGaAs).

These are the most compact (the cavity uses the cleaved sides of the semiconductor and is barely 1 mm long) and the most efficient lasers available. The power can now reach several kilowatts by putting together hundreds of diode lasers and combining them in the same optic fibre. The only disadvantages of these diode lasers are the poor spatial quality of the emitted beam and that they cannot operate at a pulsed rate (Q-switching, see section IV).

Other solid state lasers can compensate for the disadvantages of diode lasers. They use matrices that cannot conduct current so cannot be pumped electrically. They are pumped optically by either diode lasers or arc lamps (flash lamps). The matrices are doped with ions whose transitions provide the laser effect (Nd3+,Yb3+, Er3+,Ti3+). In general, solid state lasers emit in the red and near infrared. Of particular interest is the wavelength of Nd3+:YAG(Y3Al5O12) with an emission at 1064 nm.

Following the host and the ions used, the emission spectra can be narrow (fraction of nm) or wide (undredth of nm). Tr3+ : sapphire is one of the material having the largest spectrum : for 700 nm to 1100 nm.

Thanks to non-linear optics, it is possible to convert the wavelength of solid state lasers into the visible and the ultraviolet. In fact, when the electric field intensity is very high, as is the case for laser waves, matter does not respond linearly to the electromagnetic excitation of light. It responds by emitting new frequencies. Figure 23 shows that it is possible to generate new frequencies in a water cell if the laser is intense enough.

Figure 24 illustrates another example of the non-linear effect created in a standard optic fibre when the peak power density exceeds GW/cm2: a green beam (532 nm) is injected into the fibre. New frequencies are generated in the orange and in the red by the Raman effect.

These non-linear effects vary according to the nature of the materials. To promote this effect, so-called non-linear crystals are used. Figure 25 shows another example of generating frequencies in the visible, this time from a non-linear crystal. The most commonly used non-linear effect is frequency doubling, particularly for the conversion of (emits in the green).

Solid state lasers differ in the geometry of their amplifying media: some are large (generally crystals) of millimetric dimensions and there are optic fibres that can be several metres long. The diode pumped solid state lasers, and particularly the fibre lasers, are extremely robust and have a lifetime longer than 10,000 hours. They are highly valued for their industrial applications (welding, marking). Their compactness is an added advantage.