1. J. Streckert, Opt. Lett., 5, No.12, 505–506 (1980).
2. P. D. Lasay, Techn. Digest Sump. Opt. Fiber Measur., 93–95 (1980).
3. A. N. Gur’yanov, D. D. Gusovskii, E. M. Dianov, et al., Quant. Electron., 8, No.8, 1802 (1981).
4. A. V. Belov and A. S. Kurkov, Trudy IOFAN SSSR, 23, 49 (1990).
5. Y. Katsugama, M. Tokuda, N. Uchida, and M. Nakahara, Electron. Lett., 12, No.25 (1976).