Page 368 - Tunable Lasers Handbook

328 Norman P. Barnes

AgGaSe, has large nonlinear coefficients but suffered initially from limited
transmission% the near infrared. Absorption in the near infrared has been miti-
gated to a large extent by an annealing process. Because of the large vapor pres-
sure of Se. this material often grows Se deficient. To overcome this, grown
crystals have been annealed in Se-rich atmospheres. By doing this, the absorp-
tion in the near infrared is substantially reduced. Birefringence of this material
is sufficient to effect phase matching but not so large as to impose severe accep-
tance angle problems. Both optical parametric oscillators and amplifiers have
been demonstrated using this material.
ZnGeP, has an even larger nonlinearity than AgGaSe,. It too suffers from
absorption problems in the near infrared. As this material has a high vapor pres-
sure during growth, an absorption analogy with AgGaSe, is possible. Several
approaches to lowering this absorption have been tried with varying degrees of
success. Birefringence of this material allows phase matching of a wide variety
of nonlinearity interactions without incurring severe birefringence effects. In
addition, this material has better thermal characteristics than AgGaSe,.
TAS, or T13AsSe,, is a mid-infrared nonlinear crystal with sufficient bire-
fringence to allow phase matching of a wide variety of nonlinear interactions. It
has reasonably large nonlinear coefficients that have allowed its use as a nonlin-
ear crystal. However, as mid-infrared nonlinear crystals with even larger nonlin-
ear coefficients are available. this material also has seen somewhat limited use.

8. PHASE-MATCHING CALCULATIONS

Phase-matching curves are used to describe the orientation of the nonlinear
crystal for which phase matching will be achieved. In uniaxial crystals. the angle
for which phase matching is achieved is usually displayed as a function of the
interacting wavelengths. In biaxial crystals, two angles are needed to describe
the orientation of the nonlinear crystal. Consequently, phase matching can be
achieved at a locus of points. Thus. for a given set of interacting wavelengths.
the locus of the phase matching angles is usually described in terms of the polar
and azimuthal angles. To determine the phase-matching angle or angles. the
refractive indices at the interacting wavelengths must be determined.
A Sellmeier equation can be used to describe the variation of the refractive
indices with wavelength. Historically several equations have been used to
describe the variation of the refractive index as a function of wavelength. How-
ever, the Sellmeier equation has several advantages, including a physical basis
and the ability to describe accurately the refractive index over relatively large
wavelength intervals. Several forms of the Sellmeier equation have been
reported, but the form that is most usually associated with a physical basis is
expressed as