Research and development effort on a field deployable coherent lightwave system at Bell Laboratories is reviewed. Detailed features and design of a 1 . 7 Gb/s FSK single filter heterodyne detection system and its performance in a one-month transmission field trial are described. Long distance transmission experiments of such a system with Er-doped fiber amplifiers at 1 . 7 Gb/s and 2. 5 Gb/s are presented. I.