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“Laser” is an acronym for light amplification by stimulated emission of radiation. Most light sources radiate energy in all directions, with waves that are out of phase (incoherent), and with multiple wavelengths. By contrast, laser light has a single wavelength (monochromatic) and waves that are in phase (coherent) with very little tendency to spread out (collimated), so they can illuminate with extremely high power (irradiance). A 1-watt laser produces a retinal irradiance approximately 100 million times greater than a 100-watt light bulb.

Laser light is generated from a “gain” medium such as a transparent crystal rod, a semiconductor diode (solid-state laser), a gas, or a liquid dye (Figure 23–1). The gain medium is housed in a resonator cavity with a fully reflective mirror at one end and a partially reflective mirror at the other. An optical or electrical source “pumps” energy into the gain medium, raising the energy level of the atoms to a high and unstable level.

When a high-energy electron returns to a lower energy level, the excess energy is released as a photon of light (Figure 23–2). If this photon encounters another atom in the nonexcited ground state, it will be absorbed, and an electron of the recipient atom will be promoted to a higher energy level. If the photon encounters another atom that is already in a high-energy state, the photon will not be absorbed, but instead will stimulate the release of a second photon. Critically, the new photon will have the same wavelength, phase, and direction as the first photon.

Figure 23–2.

Photon absorption resulting in spontaneous or stimulated emission according to the level of electron excitation.

If the gain medium is excited to the point where more atoms are in an excited than a nonexcited (absorbing) state, “population inversion” is said to have occurred. In this unnatural state, photons encountering an atom are more likely to stimulate further photon emission than to be absorbed, resulting in an amplification cascade of exponentially increasing photon release. The presence of mirrors at either end of the resonator cavity, positioned a whole number of wavelengths apart, allows a standing wave of stimulated photon emission in the gain medium between the mirrors. A proportion of photons exits the resonator cavity through the partially reflective mirror, giving an output of laser light.

Pulsed Laser

Laser energy can be emitted continuously or in pulses, which usually have pulse durations of nanoseconds (1 ns = 10–9 s) or less.

Q-switching is a method of pulse generation in which the quality (Q) of the resonator is decreased by closing an optical switch between the mirrors of the ...

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