Page 322 - Tunable Lasers Handbook

282 Norman P. Barnes

source is injected into the resonator to control the wavelength. Using this tech-
nique, the spectral bandwidth of the laser is usually restricted to one or two lon-
gitudinal modes. If a single longitudinal mode is produced, the spectral band-
width is on the order of tens of femtometers.
Prisms are broad wavelength control devices that can achieve a low loss
when set at Brewster’s angle but they tend to polarize the laser. If an optical mate-
rial, such as fused silica, is fabricated into a prismatic shape, as shown in Fig. 32,
an incident ray is deviated by propagating through the prism. Deviation is charac-
terized by a deviation angle E. Deviation IS dependent on the incident angle and
the refractive index IZ [65]. For many applications, the incident angle is set at
Brewster’s angle to minimize losses; By selecting the apex angle of the prism, a,
Brewster’s angle can be achieved at both the input and output surfaces of the
prism. Although the reflection loss associated with a prism in this configuration
can be very low, the use of a prism usually polarizes the laser. If the laser is not
naturally polarized. restricting operation to a polarized mode can significantly
increase the losses. If the laser is naturally polarized, losses associated with the
use of a Brewster’s angle prism can be very small if aligned correctly.
Wavelength control by a prism is achieved because the angle of deviation
depends on the wavelength. If a Brewster’s angle prism is used, the angle of
deviation depends only on the refractive index. Since the refractive index
depends on wavelength, the angle of deviation depends on the wavelength. With
a prism in the laser resonator, the resonator will be aligned correctly only for one
wavelength. It is the dependence on the angle of deviation with wavelength that
allows the prism to tune the laser. Wavelength control can be achieved by vary-
ing the orientation of the resonator mirrors.
The spectral bandwidth of a single-pass prism can be estimated by calculat-
ing the variation of the angle of deviation with wavelength. To estimate the angle
of deviation. this quantity can be expanded in a Taylor series, that is

d&
E = E, + - Ah
dh

FIGURE 32 Dispersive prism.