Page 367 - Tunable Lasers Handbook

7 Optical Parametric Oscillators 327

effect could be annealed out if the temperature were high enough. Annealing tem-
peratures range from about 100 to 2UO"C, depending on the purity of the nonlin-
ear crystal. Another option to avoid this effect was to confine operation to long
wavelengthls, roughly longer than 1 .O pm. LiNbO, displays a relatively large dif-
ference in the variation of the ordinary and extraordinary refractive indices with
temperature making temperature tuning of nonlinear devices practical.
KTP, or KTiOPO,, properties allow it to overcome many of the shortcom-
ings of the previous nonlinear crystals. KTP has large nonlinear coefficients. and
can be phase matched to have a large acceptance angle. It is a biaxial material,
unlike the previous materials. which are all uniaxial. Being biaxial allows a
greater variety of phase-matching conditions to be explored in order to find a
larger effective nonlinear coefficient. a larger acceptance angle, or both. Its ini-
tial acceptance was hindered by the availability of sufficiently large crystals, a
problem that has been largely ameliorated. Its ultraviolet absorption edge tends
to limit the use of this crystal in the visible region.
BBO. or the p phase of BaB,O,, is a nonlinear crystal that is finding appli-
cations in the visible and near infrared. It has relatively large nonlinear coeffi-
cients. good transmission in the visible region, and its large birefringence allows
phase matching throughout the visible region of the spectrum. However, this
large birefringence leads to birefringence angle and acceptance angle problems
in some cases. It does appear that this material is slightly hygroscopic.
LBO, or LiB,OS. is also a nonlinear crystal that will have applications in the
visible region of the spectrum. It has similar transmission as BBO, but it does
not display nonlinear coefficients as large as BBO. However, they are larger than
those available with the KDP isomorphs. It does not suffer from the large bnre-
frringent angle effects of BBO and its biaxial nature allows a wider range of
phase-matching conditions to be explored. It does not appear that this material is
hygroscopic.
CdSe has a wide range of transparency in the mid-infrared regim and is one
of the first of the mid-infrared nonlinear crystals to be useful for optical para-
metric oscillator applications. CdSe has large nonlinear coefficients that allou
efficient interactions to occur despite the fact that the interactions occur at longer
wavelengths. However, it has a relatively low birefringence that can allow long
interactions lengths, but not all desired interactions can be phase matched in this
material.
AgGaS, is an interesting crystal for several reasons other than its nonlinear
properties. Although it is birefringent, its birefringence vanishes at one particular
wavelength in the visible. Vanishing of the birefringence has led to other appli-
cations such as optical filters. If near-infrared as well as mid-infrared transmis-
sion is desired, this nonlinear crystal is a good choice. It has large nonlinear
coefficients, but not as large as AgGaSe,. Consequently, the latter crystal is often
selected in preference to this crystal except in cases where better visible and
near-infrared transmission is desired.