Page 366 - Tunable Lasers Handbook

326 Norman P. Barnes

phase matching to be effected with a concomitant increase in the acceptance
angle and possibly in the efficiency.
Several of the available nonlinear crystals can be evaluated by considering the
factors just outlined. Because of space limitations, such a survey cannot evaluate
all of the known nonlinear crystals. Consequently, only a few select nonlinear
ciystals are evaluated here. More nearly complete surveys can be found in the lit-
erature. In general. the nonlinear crystals can be divided into two categories,
depending on their range of transparency. Oxide crystals will generally transmit in
the visible and near infrared while the semiconductor materials can transmit from
the near infrared through much of the mid-infrared region. Tables 1 and 1 wmma-
rize the important properties of the select nonlinear crystals for facile reference.
ADP, or NH,H,PO,, was one of the earliest nonlinear crystals to be used.
ADP existed before lasers were invented and was useful because of its piezo-
electric properties. As such, nonlinear crystals large enough for practical devices
were available immediately. However. it does have relatively 101% nonlinear coef-
ficients, a somewhat limited acceptance angle, and is hygroscopic. To avoid
degrading the optical faces of a hygroscopic crystal by exposure to a humid
atmosphere, it is often kept in a sealed container that may be heated. Because of
the large difference in the variation of the refractive indices with temperature.
ADP can be temperature tuned 017er a relatively large range. Even though several
useful nonlinear devices have been demonstrated using this material, its use has
been declining, primarily because of the availability of better materials.
KDP. or KH,PO,. was also available before the invention of the laser. KD*P,
an isomorph where the hydrogen is replaced by deuterium. has nearly identical
nonlinear coefficients and refractive indices but better transmission in the near
infrared. especially beyond about 1.0 pm. As such, KD*P is often preferred in
cases where a high average power is required. Use of this material as a second
harmonic generator for Nd:YAG lasers is common. However, like ADP, this crys-
tal also has relatively low nonlinear coefficients and somewhat limited acceptance
angle. KDP is also hygroscopic and therefore often kept in a crystal oven.
CD*A. or CsD,AsO,, is an isomorph of KDP and was developed primarily
as a harmonic geneiator for Nd:YAG lasers. Its nonlinear coefficients are about
the same as the previous two nonlinear crystals. but this material can achieve
nearly noncritical phase matching for second harmonic generation of Nd:YAG
lasers. Noncritical phase matching provides for a significantly enhanced accep-
tance angle and negligible birefringence angle effects. As with other KDP iso-
morphs. CD*A is hygroscopic.
LiNbO, was the first nonlinear crystal to demonstrate optical parametric
oscillation. Nonlinear coefficients of this material are significantly larger than the
previous three materials. However, this material suffered from optically induced
refractive index inhomogeneities when irradiated with short-wavelength laser
radiation. This deleterious effect can be mitigated by growing very pure materi-
als, but it has not yet been eliminated. However. it has been discovered that this