Laser physics

Lasers are devices that generate light energy. A substance or medium is ‘pumped’ by an external energy source which moves electrons from a stable to an ‘excited’ state. Some of these excited electrons will spontaneously decay to a lower energy state. In the case of lasers, the excited molecule returns to a stable, lower energy state by emitting the extra energy as light in the form of a photon. A photon is a discrete bundle of energy with no mass. Once released, this photon cruises along in a population of like molecules which are also excited. When it strikes another excited molecule, the process of amplification occurs because the excited molecule is stimulated to release its extra energy as another photon of exactly the same wavelength and precisely in phase with the first. When all of this activity occurs in a tube with mirrors at both ends, the energy builds up as the light waves bounce back and forth. If one of the mirrors has a small central aperture, it can be opened in such a way as to release laser light.

Why ‘laser’? It’s an acronym formed from the first letters of the process described above; namely, Light Amplification by Stimulated Emission of Radiation. The concept of radiation in the form of wavelengths is derived from the dual nature of our understanding of light as being characterized by both photons and electromagnetic waves. There are unique features of laser light that make it useful for a variety of applications including medical ones. Laser light is collimated or highly directional which keeps the energy of the beam from dissipating over distance. Laser light is coherent with the peaks and troughs of the sinusoidal wave traveling through space in phase with one another. Coherence allows the energy within the beam to be concentrated with precision.

Finally, the light is monochromatic or emitted at a narrow band of the electromagnetic spectrum. It is this monochromatic property that is used in medicine to achieve specific goals in tissue. Once this laser light has been formed in the generator, it can be manipulated in several useful ways. The release can be controlled by a variety of gating mechanisms ranging from simple shutters to complex pulse generators. For medical uses, the beam is either released as a continuous beam or as a pulsed one. The power output is measured in watts or joules per second. The beam can be focused which is a useful property when cutting or resurfacing by scanning. It can be transmitted through mirrored conduction tubes, wave guides or fiber conductors.

If the beam is released in a pulse with the intent to conduct it to the target without significantly modifying its physical characteristics, the power of the laser must be high enough to produce an appropriate effect when it reaches the target. Once this emitted laser light strikes tissue, it can be reflected, scattered, absorbed or transmitted. While reflection is not a very useful property in medicine, there are some applications where scattering can be employed to achieve therapeutic goals. The most useful current medical applications are absorption and transmission.

Our current concept of what happens when laser light and tissue interact was articulated by Anderson and Parrish ¹ in 1984 in their landmark paper describing selective photothermolysis. Specific components in tissue absorb light more intensely at some wavelengths than at others. These components are known as chromophores. For example, absorption by water peaks at 2900 nm and at 10600 nm. The erbium laser emits at 2940 nm and the carbon dioxide laser emits at 10600 nm. The incident energy from both of these lasers is rapidly absorbed by water in the most superficial layers of skin where it is converted to thermal energy. If the laser energy is sufficiently high, it will vaporize the water and in doing so will thereby remove or ablate the tissue. By controlling the incident energy and the duration of application, precise amounts of skin can be removed with limited collateral thermal damage.

Another example of this concept of absorption can be seen in the choice of an argon laser emitting at 514 nm (in the green part of the visible spectrum) where water absorption is very low and melanin absorption is relatively high. Energy emitted from this laser passes through the fluid medium of the eye to be absorbed by the chromophore melanin in the retinal pigment epithelium. The thermal energy generated by this effort can be useful in treating some pathology such as diabetic retinopathy. Figure 22-1 shows the different chromophores and their wavelengths of absorption. The goal of laser skin resurfacing is to vaporize or ablate surface tissue while minimizing thermal damage adjacent to the site of application. The obvious first step is to build the laser with enough power to reach the heat of vaporization, which in the case of water is 100°C. How efficiently this process of vaporizing tissue proceeds will be determined largely by the shape of the laser beam and the time of application. Since most medical lasers emit in either a true Gaussian shape or something similar to it, the most useful bell-shaped configuration will have the energy of the beam highly concentrated in the central portion of the beam. The objective in ablative resurfacing lasers is to exceed the energy of vaporization for the broadest area of the beam possible and to minimize the undesirable formation of thermally devitalized but intact skin on the margins of the beam. In prosaic terms, a beam configuration that looks more like a ‘high top hat’ than a ‘beret’ is desirable.