In an effort to strengthen its hold on the undersea fiber optic cable market, Tyco International Ltd. agreed to purchase AT&T Submarine Systems Inc., a manufacturer of undersea fiber optic telecommunications cable. The purchase price was $850 million. Tyco, based in Exeter, N.H., makes disposable medical products, packaging materials, and electrical and electronic components. Tyco has projected the submarine cable unit's revenues at about $1 billion for 1997. In March, the company bought home-security company ADT for $5.4 billion.