Chapter 26 Lasers

Principles

The increasing surgical use of lasers, with their inherent potential hazards to patients and operating room staff, mandates an understanding of their physical principles by anaesthetists.¹

The word laser is an acronym for ‘light amplification by stimulated emission of radiation’. The laser produces an intense beam of pure monochromatic light (one wavelength: one colour), in which all of the waves are in phase (coherent). The output beam is likely to be of a very small cross-sectional area and is virtually a non-divergent parallel (collimated) beam. These properties mean that energy may be delivered to very small areas of tissue with great accuracy, and the intense parallel beam of light constitutes a very large amount of power per unit area of tissue. The wavelength of a laser is determined by the lasing medium used. Although described as monochromatic, most laser media produce light within a narrow waveband consisting of a number of discrete frequencies. In order to come up with the concept of a laser, which was first described in 1958 and first demonstrated in 1960, scientists had to understand the notion of quantum physics and of Niels Bohr’s model of the atom, with its orbital discrete energy levels (see below).

Although there are many more complex laser systems,² the basic components of a laser are shown in Fig. 26.1. The lasing medium may be a solid, liquid or gas. The atoms of a lasing medium are excited to high energy levels by a ‘pumping’ source, which may be a high-voltage discharge in the case of a gas, an intense flash of light from a flashtube or the energy from a radio frequency power source. Fig. 26.2 shows the excitation and emission process possible in a gaseous lasing medium. A photon of energy from the pumping source may be absorbed by a stable atom in its so called ‘ground state’, which then becomes an atom in an excited state, with an electron or electrons in an orbital shell at a higher energy level. Spontaneous emission of a photon of energy occurs as the electrons fall back to shells of a lower energy state and the excited atom reverts to the ground state.

Figure 26.1 Basic components of a laser.

Figure 26.2 Absorption, excitation and emission processes.

If a further photon of pumping energy, at the correct wavelength, is applied to an atom in its excited state, it will fall to its ground state and two photons of energy will be emitted instead of one. This is known as stimulated emission, originally described by Einstein in 1917 as the basis for laser technology ³ and the inversion of the energy states is referred to as population inversion. The emitted photons thus produced are in phase with, have the same polarization, and travel in the same direction, as the stimulating radiation. This mechanism is amplified by many of the escaping photons being reflected back into the lasing medium by the mirrors. Thus a chain reaction occurs, and this can be thought of as a positive feedback system. The process produces an intense source of light energy, some of which is allowed to escape through the partially reflecting mirror at the output end of the lasing medium. The output beam of the laser is usually directed to the tissues through a fibre-optic light guide. However, the wavelength of the carbon dioxide laser is so long, at 10.6 µm, that there is no fibre currently available to transmit energy, so that it has to be directed by a series of mirrors instead.