Page 365 - Tunable Lasers Handbook

7 Optical Parametric Oscillators 325

TABLE 4 Nonlinear Coefficients for Selected Nonlinear Materialsa

Crgslal Point group Nonlinear Coefficients

ADP 42m d36 = 0.53
KDP 42m d36 = 0.44
CD*A 42m dj6 = 0.40
LiNbO, 3m d2> = 2.76 d;, = -5.44
BBO 3m dZ2 = 2.22. dj, = 0.16
KTP mmL d31 = 6.5 d72 = 5.0 d,, = 13.7
dzl = 7.6 dI5 = 6.1
LBO mm2 d,, = -1.09 d;, _- = 1.17 d33 = 0.065
4gGa4, 4.2m d36 = 13.4
AgGaSe? 42m d36 = 37.4
CdSe bmm d,,5 = 18.0
ZnGeP? 4?m li14 = 75.4
TI ,ASS?, 3m d222 = 16.0 d,, = 15.0
aUnits of the nonlinear coefficients are IOW? m/V.

merely permute the subscripts are equal. Conditions where this is valid can be
met in cases where the dispersion of the electronic polarizability is negligible.
Such conditions exist in a majority of practical crystals. Assumption of this sym-
metry condition simplifies the expressions for the nonlinear coefficient.
Birefringence must be sufficient to achieve phase matching and adequate
tuning but bleyond that more birefringence is not usually desirable. A large bire-
fringence usually indicates a restricted acceptance angle and a large birefrin-
gence angle, Both of these effects can limit the efficiency of the parametric inter-
action. However. there are instances where angular tuning rates can benefit from
a large birefringence.
Temperature sensitivity arises through the variation of the refractive indices
with temperature, Because, in general, the variations of the ordinary and the
extraordinary refractive index with temperature are different, the phase-matching
condition varies with temperature. If this difference is large, a small variation in
the ambient temperature changes the phase-matching condition and adversely
affects the efficiency. Thus, to maintain the efficiency, temperature control of the
nonlinear crystal may b'e required. Although temperature control is straight-
forward it adds complexity to the system. In high-power situations, a large dif-
ference in the variation of the refractive indices adversely affects the average
power limits of a given nonlinear interaction. On the other hand. a large differ-
ence in the variation of the refractive indices with temperature may allow 90"