Page 375 - Tunable Lasers Handbook

7 Optical Parametric Oscillators 335

\ 1.064~rn Pump

20 22 24 26 2%
Angle (degrees)
FIGURE 16 Phase-matching curve for BBO for 0.537- and 1.064-pm pumps.

Opticall quality of the nonlinear crystals has improved. Optical quality
improvements have occurred both in the form of decrcased absorption and
decreased distortion. For example, LiNbO, crystals were found to suffer from
optically induced refractive index inhomogeneities. It was found that, in part,
these probllems could be traced to Fe impurities. By decreasing the Fe impuri-
ties, the susceptibility of optically induced refractive index inhomogeneities was
decreased. Similarly. the short-wavelength absorption in AgGaSe, was corre-
lated with a deficiency of Se. By annealing these crystals in an atmosphere rich
in Se, the short-wavelength transmission of these crystals improved. Initially
some nonlinear crystals were deliberately doped with impurities to reduce
growth time and therefore cost. While some impurities are benign, others can
cause unwanted absorption. Increased absorption can limit the efficiency and
average power limit mailable with a given nonlinear crystal. In addition, some
crystals tended to grow multidomain. That is, not all of the nonlinear crystal was
oriented in the same manner. Multidomain crystals limit efficiency by limiting
the effective length of the nonlinear crystal. As growth technology improved,
many of these problems were resolved.