The classical theory of electromagnetic waves successfully explains “macroscopic” light phenomena, but fails to explain phenomena at the atomic and molecular levels. Quantum theory, as illustrated in Figure 2-3B, can explain such phenomena. For the following considerations, it therefore becomes crucial to consider the quantum behavior of light.
When an electron absorbs a photon and jumps to a higher energy level (stimulated absorption, see Fig 2-3B), usually it quickly drops back to the original level and emits a photon identical in frequency to the one it absorbed (spontaneous emission, see Fig 2-3B).
However, atoms of some elements have 2 energy levels that are close together. When a photon is absorbed, the electron jumps to the highest energy level. Instead of dropping back to the original level, the electron first transitions to the slightly lower level without emitting visible energy. Next, it drops to the initial energy level and emits a photon. The emitted photon has less energy than the stimulating, absorbed photon and, therefore, a lower frequency, or longer wavelength (and different perceived color). This phenomenon, called fluorescence, occurs only in materials possessing close spacing between energy levels. Its clinical utility derives from the essential feature that the absorbed and emitted photons have a different wavelength and is the basis of fluorescein angiography and macular autofluorescence (discussed in Chapter 8).
In most cases, electrons remain in elevated energy states for very short periods. However, the elevated state is sometimes metastable; that is, the electron may remain in the elevated state for several seconds or longer before dropping back down. Such a process describes how light is produced by phosphorescence, which is essentially identical to fluorescence except that the transitions take longer. Light resulting from fluorescence stops immediately after removal of the exciting energy, whereas light resulting from phosphorescence persists long after the exciting energy ceases.
A photon of appropriate frequency passing near an electron in a metastable state may stimulate the electron immediately to drop to a lower state and radiate an identical photon (Fig 2-12). Such stimulated emission is the basis of the light emission in lasers. In fact, the word laser originated as an acronym for Light Amplification by Stimulated Emission of Radiation.
Lasers use an active medium with an appropriate metastable state. Energy is introduced into the active medium in a variety of ways. For example, optical pumping uses a bright incoherent light source to excite a large number of electrons into the metastable state. The active medium is inside a resonator cavity, which typically has a fully reflecting mirror on one end and a partially reflecting one on the other; this design causes light to make numerous passes through the active medium, producing more and more photons by stimulated emission with each pass (Fig 2-13).
Contrary to common belief, lasers are not very efficient light sources. Compared with the amount of energy required to power a laser, the amount of energy produced is modest. The light produced, however, has unique and useful characteristics. Laser light has a very narrow bandwidth (ie, it is nearly a single wavelength or monochromatic) and, consequently, it has high temporal coherence. The coherence length is relatively long—about half the length of the resonator cavity—typically a few centimeters. Lasers are the most intense sources of monochromatic light available.
Although the total energy in laser light may be slight, it can be focused on a very small area to produce a very high energy density (ie, energy that is transferred per square centimeter, or fluence; see Table 2-2). Laser light is also highly directional and, depending on the design of the resonator, may also be polarized.
Figure 2-12 In a process called stimulated emission, which is the basis of light emission in lasers, a photon of appropriate frequency passing near an electron in an excited state may stimulate the electron to drop to a lower state and emit an identical photon.
(Reproduced with permission from Steinert RF, Puliafito CA. The Nd:YAG Laser in Ophthalmology: Principles and Clinical Applications of Photodisruption. Philadelphia: Saunders; 1985. Redrawn by C. H. Wooley.)
Figure 2-13 Simple schematic of a laser illustrating the active medium within the resonator cavity formed by the mirrors and the pump, which raises a majority of electrons to elevated states (population inversion) in the active medium. One mirror is fully reflective (100% R), whereas the other is partially transparent (<100% R). As drawn, the mirror is 66% reflective, and the average light wave makes 3 round-trips through the active medium before being emitted.
(Reproduced with permission from Steinert RF, Puliafito CA. The Nd:YAG Laser in Ophthalmology: Principles and Clinical Applications of Photodisruption. Philadelphia: Saunders; 1985. Redrawn by Jonathan Clark.)
Lasers are usually named after their active medium. The medium can be a gas (argon, krypton, carbon dioxide, argon with fluoride [excimer], or helium with neon), a liquid (dye), a solid (an active element supported by a crystal, such as neodymium: yttrium-aluminum-garnet [Nd:YAG] and titanium: sapphire [Ti:Sapphire]), or a semiconductor (diode).
Lasers may operate continuously (eg, an argon laser for photocoagulation), commonly referred to as continuous wave, or in pulses (eg, a Nd:YAG laser for capsulotomy). Mode-locking and Q-switching are 2 common methods of producing a pulsed output. The details of such methods are beyond the scope of this chapter. Note, however, that while Q-switching allows for the production of short pulses (mainly on the order of nanoseconds, ie, 10−9 s), it is mode-locking that can produce ultrashort pulses, with a duration on the order of picoseconds (ie, 10−12 s) or less, depending on the properties of the laser. In fact, the pulse duration is mainly dependent on the spectral bandwidth of the laser medium; the larger its spectral bandwidth, the shorter the pulse that can be realized. Thus, ultrashort laser pulses are not monochromatic, and for the generation of femtosecond pulses, a laser with a large spectral bandwidth is required. For a Ti:Sapphire laser with a 128-THz spectral bandwidth, for example, the shortest pulse that can be created would be around 3.4 femtoseconds (ie, 10−15 s).
We have learned previously that power is the amount of energy delivered in a given time period. A watt is 1 joule of energy delivered over 1 second. The same joule delivered over a nanosecond has a power of 1 billion watts; over a picosecond, 1 trillion watts; and over a femtosecond, 1 quadrillion watts. Pulsing is a way of increasing the power of a laser’s output by delivering a modest amount of energy over a very short period.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.