Figure 8-18 Simple schematic of a laser illustrating the active medium within the optical resonance 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 b y Jonathan Clark.)

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 color or wholly 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 per square centimeter, or irradiance). Laser light is also highly directional and, depending on the design of the resonator, may also be polarized.

Lasers may operate continuously (eg, an argon laser photocoagulator) or in pulses (eg, a 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.

Power is the amount of energy delivered in a given time period. A watt is 1 joule of energy delivered over 1 second, but the same joule delivered over a nanosecond has a power of 1 billion 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 time period.

Light–Tissue Interactions

Laser surgery involves 1 of 4 light–tissue interactions: photocoagulation, photoablation, photodisruption, or photoactivation.

Photocoagulation

Photocoagulation is the process by which heat generated by the absorption of light denatures proteins. Pigmented tissue absorbs light and converts it to heat, which denatures (coagulates) the pigmented and adjacent tissues.

Retinal photocoagulation was first performed by focusing sunlight onto the retina using a heliostat. Sunlight was replaced by a xenon light source, which was ultimately replaced by a variety of lasers. During retinal photocoagulation, laser light is absorbed by the retinal pigment epithelium (RPE), and the heat produced denatures (coagulates) the retinal proteins. The outer retinal layers are more affected than are the inner layers, a fact that has several clinical implications.

The more edematous the retina, the less heat reaches the inner layers and the less visible the laser burn. Accordingly, when photocoagulating an edematous retina, it is important to look for signs of photocoagulation occurring in the deeper retinal layers. The difficulty with coagulating the inner retinal layers is the reason laser photocoagulation is often ineffective in preventing the progress of retinoschisis, especially when only the innermost layers split. Controlling laser spot size and duration is crucial. Too much laser power concentrated into too small a spot and too short an exposure time