lecture 5

MODES OF OSCILLATION
The amplitude of a light beam is increased in a laser by multiple passes of coherent light waves through the active medium. The process is accomplished by an active medium placed between a pair of mirrors that act as a feedback mechanism. During each round trip between the mirrors, the light waves are amplified by the active medium and reduced by internal losses and laser output. A number of different combinations of mirrors, such as plane and curved, have been utilized in practical lasers. The pair of mirrors, axially arranged around an intervening volume, sometimes is called an "optical cavity," or a "laser resonator." Only certain frequencies of EM radiation will set up standing waves within this volume. These allowed frequencies of oscillation are referred to as "axial," or "longitudinal," modes of the cavity.
This module discusses the optical cavity of a laser, gain and loss in optical cavities, cavity configurations, standing waves in optical cavities, and the effects of all these factors on laser operation and output. In the laboratory, the student will align the optical cavity of a He-Ne laser.
OPTICAL CAVITIES:
A laser is essentially an amplifier placed between two mirrors. The presence, shape, and separation of the mirror surfaces determine the spatial distributions of the electromagnetic fields inside the laser. An optical cavity is a volume bounded by two or more reflective surfaces. The optical cavity of a typical laser is depicted in Figure 1. The optical axis is a line perpendicular to the mirror surfaces at the center of the optical cavity. The aperture is the element within the cavity that limits the size of the beam. In most cases, the aperture is at the end of the active medium; but in some lasers, an additional aperture is installed in the cavity to limit the beam to a desired diameter.