1. S. Shiraishi, K. Fujiwara andS. Kurosaki, US Patent. 4082420 (1979).
2. K. Abe, in 2nd European Conference Optical Communication (IEE, Paris, 1976) p. 59.
3. A. Kawana, T. Miya, S. Araki andY. Furui,Trans. IECEJ E65 (1982) 529.
4. K. Rau, A. Muhlich andM. Treber, in Technical Digest, Topical Meeting of Optical Fiber Transmission (Williamberg, 1977) P. TuC4-1.
5. H. Yokota, H. Kanamori, G. Tanaka andK. Yano, in Proceeding of Eleventh European Conference Optical Communication (Venice, Italy, 1985) p. 77.