Laser principle-L-5

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.

Fig. 1 The optical cavity of a laser
LOSS AND GAIN IN OPTICAL CAVITIES:
A laser contains an amplifying medium and an optical cavity. Spontaneous emission of photons, some of which takes place along the direction of the optical axis, begins the formation of the laser beam. The beam is reflected backward and forward between the two mirrors. During each round trip of the cavity, the beam passes through the active medium twice and is amplified; some of the light passes through the output coupler to form the output beam, and some of the light is removed from the beam due to losses in the cavity. The remaining portion of the light energy is reflected back into the optical cavity. All these factors must be considered in the design of a laser optical cavity.
LOSS IN OPTICAL CAVITIES:
The following factors contribute to losses within the optical cavities of lasers:
1-Misalignment of the mirrors: If the mirrors of the cavity are not aligned properly with the optical axis, the beam will not be contained within the cavity, but will move farther toward one edge of the cavity after each reflection.
2-Dirty optics: Dust, dirt, fingerprints, and scratches on optical surfaces scatter the laser light and cause permanent damage to the optical surfaces. Instructions for the cleaning of laser optics are presented later in this module.
3-Reflection losses: Whenever light is incident on a transparent surface, some portion of it always is reflected. Brewster windows and antireflection coatings greatly reduce this loss of light but cannot eliminate it entirely.
4-Diffraction loss: Part of the laser light may pass over the edges of the mirror or strike the edges of the aperture and be removed from the beam. This is the largest loss factor in