Laser Technology—a surgical tool of the past, present, and future - Clinical

AORN Journal,  Nov, 2003  by Karen Andersen

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Albert Einstein is credited with developing the field of quantum electronics (ie, a fancy name for laser physics) with a paper on photoelectric effects. (1) Einstein postulated that light does not consist of continuous waves (ie, small, hard particles) as early theorists believed. Instead it consists of bundles of energy called photons.

Neils Bohr's model of the atom, set forth in a series of papers in 1913, described atoms as constantly in motion, continuously vibrating, and moving. (2) In simple terms, the atom is composed of a small, dense nucleus that contains positively charged protons and neutral neutrons. (3) (p132) Negatively charged electrons orbit the nucleus, similar to planets around the sun. This little family is held together by electrical attraction between the positive and the negative, like metal shavings to a magnet.

During an atom's resting state, electrons are close to the nucleus. In the atmosphere, however, electrons tend to absorb energy that occurs naturally. As electrons absorb this energy, they jump to a higher energy level or orbit, where the atom is said to be excited. The atom can exist in this excited, unstable state for only a short period of time before the electrons have to return to their resting orbit. As electrons fall back to their resting orbit, they spontaneously release a photon of energy--a tiny burst of surplus energy, a particle of light. (4) This burst of light, which is called a photon, heads off on a random pathway. This theory is called the spontaneous emission of radiation. It exists in the natural world as fluorescence. Ordinary light from a flashlight or from the sun is emitted spontaneously when atoms release excess energy with no outside intervention. Spontaneous emission is the source of virtually all the light we see in nature, and a photon is the basic unit of radiation that constitutes light. (5) (p2)

If a photon becomes too excited, it moves away from the atom in a process called ionization. This process occurs in wavelengths at the lower end of the electromagnetic spectrum, such as in x-rays. Photon ionization can harm tissue, so health care professionals need to wear protective lead aprons when they work with x-ray sources. (5) (p32)

Einstein took Bohr's theory and expanded it. He believed that if a photon released from an excited atom interacted with a second excited atom carrying the right amount of energy, two identical photons would be released and travel in the same direction. These photons would continue to multiply as they interacted with additional excited atoms through the process of stimulated emission. Two electrons would become four photons, four photons would become 16 photons, and 16 photons would become 256 photons. This multiplication of photons would occur when the atoms were exposed to an outside source of energy, which would stimulate them. Einstein's theory postulated that the outcome of this process would be transmitted as light energy, because all wavelengths produced would be identical.

THE FIRST LASERS

The term laser is an acronym for light amplification by the stimulated emission of radiation (Table 1). It describes the mechanism of producing light energy by the process described earlier, creating photons that travel in the same direction and that share exactly the same frequency, energy, and phase. In this context, the term radiation encompasses a wide range of light sources. Of these sources, only a few are dangerous forms (eg, nuclear radiation, x-rays).

No one realized the potential of the laser until 1954, when practical research turned the theory into reality. Arthur L Schawlow, PhD, a researcher at Bell Laboratories, Murray Hill, NJ, and Charles H Townes, PhD, a consultant to Bell Laboratories, proposed that if atoms are stimulated in a specific active medium inside a laser cavity (ie, a tube or chamber), photons will multiply at an accelerated rate. According to their theory, the active medium is energized by an excitation source, and the energy is amplified when two high reflectivity mirrors are placed on each end of the cavity, thereby enhancing the process of stimulation. Schawlow and Townes theorized that the stimulation alone would not be effective unless a condition called population inversion occurred. If out of several million atoms only a few atoms are excited, it is unlikely that these atoms can stimulate the others and produce an efficient or noticeable source of energy. When electrons in the excited state outnumber electrons in the resting, or ground state, however, population inversion has taken place.

Schawlow and Townes determined that the greater the percentage of electrons in the excited state, the greater the probability of stimulated emission. They published their idea in a scientific journal in 1958, applied for a patent, and received historical credit for the concept of the laser. Townes and Alexander Prochorov, who was conducting the same research in Moscow, received the Nobel Prize in Science in 1964 for their innovative work.