Page 328 - Tunable Lasers Handbook

288 Norman P. Barnes

from noise can extract a significant amount of the stored energy. Power or
energy requirements depend critically on the net gain of the power oscillator. In
addition, the alignment of the seed laser to the power oscillator is critical. Espe-
cially critical are the transverse overlap of the seed with the mode of the power
oscillator and the direction of propagation of the seed with respect to the power
oscillator. A full analysis of the power required can be found in the literature as
well as an analysis of the critical alignment.
For single-wavelength operation of a solid-state laser, ring resonators are
often preferred to standing-wave resonators. Standing-wave resonators are
formed by two reflective surfaces facing each other, similar to a Fabry-Perot
etalon. As such, waves in a standing-wave resonator propagates both in a for-
ward and a reverse direction. If the propagation in the forward direction is char-
acterized by the propagation term exp(-jb), then the propagation in the reverse
direction is characterized by the propagation term exp(+jk-.). In these expres-
sions. j is the square root of -1, k is the wave vector, and z is the spatial coordi-
nate along the direction of propagation. Waves propagating in the forward and
reverse directions interfere to create an intensity pattern characterized by
cosl(k-.). If the laser operates at a single wavelength. the power density is zero at
the nulls of the cosine squared term. At these positions, the energy stored in the
active atoms will not be extracted. Unextracted stored energy will increase the
gain for wavelengths that do not have nulls at the same spatial position as the
first wavelength. Increased gain may be sufficient to overcome the effects of
homogeneous gain saturation and allow a second wavelength to lase. Con-
versely, no standing-wave patterns exist in a ring resonator. By eliminating the
standing-wave pattern, homogeneous broadening will help discriminate against
other wavelengths and thus promote laser operation at a single wavelength. For
this reason, ring resonators are often preferred for single-wavelength operation
of a solid-state laser.

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