Laser Physics

Lasers are devices that produce light with properties very different than those of other light sources, e.g., incandescent bulbs or LEDs. These unique characteristics enable a remarkably wide range of applications. Laser light can travel large distances as a narrow beam without diverging, allowing it to be used in laser pointers, laser light shows, and even for communication between satellites. The light can also be focused to a very tight spot, enabling sub-cellular microscopic imaging, reading/writing large amounts of data to/from DVDs and Blu-ray discs, and photolithography, which is critical in the production of modern microelectronics. Furthermore, if this tightly-focused light is confined to very short bursts or pulses, high-intensity lasers can be used for a variety of micromachining applications, including cutting/marking materials such a ceramics, glass, and metals as well as safe ablation of human tissue. Finally, laser light can have a very narrow spectrum or singular color that enables high-resolution spectroscopy and optical fiber communication.

A laser is a source of coherent light. It contains an optical oscillator that increases the amplitude of an optical field while maintaining its phase. This coherent amplification is achieved through Light Amplification by Stimulated Emission of Radiation (LASER). The process of amplification is the result of stimulating an atom such that it emits an identical photon to one already present. This "clone" photon has the same phase, frequency, direction, and polarization as the original photon, meaning they are coherent. It is this coherent amplification of light that gives lasers their unique output characteristics.

Simplified diagram of a laser, including the gain medium, pump source, and optical resonator mirrors
Figure 1: Simplified diagram of a laser, including the gain medium, pump source, and optical resonator mirrors with cavity length (d).

The three key components of a laser are a gain medium, a pump source, and a resonator. Figure 1 shows a pictorial representation of an operational laser. The pump source is the mechanism by which a population inversion (necessary for generating stimulated emission) is produced in the gain medium. The gain medium is chosen to achieve efficient lasing operation as well the desired characteristics for the laser output. The resonator mirrors allow for selective reflection of incoherent photons produced by spontaneous emission back into the gain medium. Stimulated emission replicates these reflected photons and this sequential reflection/amplification process gives rise to a large number of coherent photons. Finally, a portion of these photons are transmitted through a partially reflective mirror, delivering the output laser beam.

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