Laser beam characteristics

xenon, krypton or mercury lamps and, due to the considerable difficulty of implementation and size, is now generally abandoned in favour of a similar process, used in dye or YAG lasers, which uses another laser, either a semiconductor or nitrogen laser, as source. Ä Using an electrical discharge: a system known as electrical pumping and is widely used in gas and semiconductor lasers. Ä Through a chemical reaction: system used in chemical lasers. Ä Through a process of rapid expansion of a gas: system used in gas-dynamic lasers. To obtain the operation of a laser, two essential conditions must be met. The first, as we have seen, is population inversion, in which most of the atoms in the active material are in an excited state rather than, as is normally the case, in the fundamental state. In order to achieve this, the atoms must first be supplied with energy by the process of pumping. The second is to maintain the population inversion for a spontaneous emission from the atom after excitation are very short, on the order of nanoseconds, but there are particular energy levels of the atom, called metastable, where an atom remains excited without decaying for much longer. Pumping must conveniently transport atoms to one of these metastable states, either directly or through intermediate decays.

Lasers in which the electrical discharge takes place continuously and the pumping continuously supplies the metastable state with excited atoms are called continuous wave laser. In pulsed lasers, on the other hand, the supply can be discontinuous: the phenomenon is triggered as soon as there is a population inversion, but is short-lived because it ends as soon as the metastable state is emptied. Continuous wave laser are characterised by the power of the beam, pulsed lasers by the energy of the pulse emitted.

In addition to lasers operating in the visible range, lasers emitting infrared radiation, particularly at a wavelength of 10 µm, are widely used, such as high-power lasers used in machining. There are also lasers operating in the ultraviolet and X-ray band.

The main characteristics of the output beam of a laser are as follows: Ä Monochromaticity: this is a consequence of two con-

comitant facts. Firstly, only an EM wave which has a characteristic atomic frequency can be amplified and, since the two mirrors constitute a resonant cavity, oscillation can occur at the characteristic resonant frequencies of the cavity. Secondly, the laser has a much smaller linewidth (up to 10 orders of magnitude) than the normal width of the E2 → E1 transition observed in spontaneous emission. Ä Coherence: understood as spatial and temporal coherence. > Spatial coherence: given two points on the wave front at time t0 of an EM wave, let E1 and E2 denote their respective electric fields. By definition of wave front, the phase difference between the two electric fields at time t0 will be zero. If this phase difference remains zero at any time t, the two points are said to be coherent. If this happens whatever the two points on the wave front are, the EM wave is said to have perfect spatial coherence. > Time coherence: consider the electric field on the

EM wave at time t and time t' close to each another.

If the phase difference between E(t) and E(t') remains constant, the EM wave is said to have perfect temporal coherence. Ä Directionality: this is a consequence of the fact that the active material is essentially placed in a resonant cavity consisting of two mirrors. In fact, only an EM wave propagating in the direction orthogonal to the mirrors (or in direction very close to it) will be able to oscillate. The directionality of the light beam emerging from a laser cavity increases with the length of the cavity for geometrical reasons. Ä Brilliance: defined as the power emitted by an EM wave source per unit area and per unit solid angle.

A laser beam has a very high brilliance as a result of being collimated. Ä Short pulse duration: implies a concentration of energy in time. It can be seen as a complement to monochromaticity which, in turn, implies a concentration of energy in terms of wavelength. However, while all lasers can, in theory, be made monochromaticity by operating on the cavity, only laser with a large linewidth can produce pulses of very short duration.

For gas lasers, this means being able to produce light pulses of 0.1-1 ns duration (not particularly short).

For liquid and solid-state lasers, which have 105 – 106 times larger linewidths, light pulses of 10-100 fs duration can be generated.