1. D C Tran, Proc. OFC; paper TUA1; Atlanta, Georgia (1986).
2. S Takahashi, SPIE Proceedings 618, Infrared Optical Materials and Fibers IV.
3. P W France, S F Carter, M W Moore and J R Williams, to be published.
4. M E Lines; J Appl Phys, 55, 4052 (1984).
5. T Izawa, N Sibata and A Takeda; Appl Phys Lett, 31, 33 (1977).