Page 157 - Sensors and Control Systems in Manufacturing

118
Cha p te r
T w o

military weapons and research. These lasers are attractive for military
applications because of the large power-to-weight ratio.
2.12.2.3 Feedback Mechanism
Mirrors at each end of the active medium are used as a feedback mech-
anism. The mirrors reflect the light produced in the active medium
back into the medium along its longitudinal axis. When the mirrors
are aligned parallel to each other, they form a resonant cavity for the
light waves produced within the laser. They reflect the light waves
back and forth through the active medium.
In order to keep stimulated emission at a maximum, light must be
kept within the amplifying medium for the greatest possible distance.
In effect, mirrors increase the distance traveled by the light through
the active medium. The path that the light takes through the active
medium is determined by the shape of the mirrors. Figure 2.111 shows
some of the possible mirror combinations. Curved mirrors are often
used to alter the direction in which the reflected light moves.

2.12.2.4 Output Coupler
The feedback mechanism keeps the light inside the laser cavity. In
order to produce an output beam, a portion of the light in the cavity
must be allowed to escape. However, this escape must be controlled.
This is most commonly accomplished by using a partially reflective
mirror in the feedback mechanism. The amount of reflectance varies
with the type of laser. A high-power laser may reflect as little as 35
percent, with the remaining 65 percent being transmitted through the

d PLANE PARALLEL

R 1
CONFOCAL

R 1 R 2
LONG RADIUS

R 1
HEMISPHERICAL

R
R 1 2
SPHERICAL

R 1 R 2
UNSTABLE

FIGURE 2.111 Mirror combinations for a feedback mechanism.