SpecTran Corp. of Sturbridge, Mass., a developer of glass optical fibers, has announced the termination of a supply agreement it had with Corning Inc. in Corning, N.Y. In a deal forged in January 1996, Corning agreed to purchase multimode optical fibers from SpecTran under a multiyear supply contract. In exchange for canceling the agreement, SpecTran will receive a series of cash payments totaling $4,056,000.