As a partial response to Fred Goldstein's questions in Saturday's Digest, the first field test of fiber technology in the Bell System was the Atlanta system in 1975. It used fiber to transfer DS-3 signals across the city. The commercial version of this was FT-3 (again using DS-3 signals) and was introduced in 1979. Fiber was used in 1980 to transmit voice, video, and TV from the winter Olympics at Lake Placid. That usage was greatly expanded at the LA Olympics in 1984.
The first long distance system was installed between New York City and Washington DC along the NE corridor in 1983, at a much higher rate than DS-3. In 1984 Long Lines was planning/installing about 3300 km of fiber network, again operating well above the DS-3 rate.
Based on that history, I would guess that fiber systems became "common" in the mid-80s. More recently transmission and switching systems have migrated to packet based. In fact, AT&T announced a few years ago that they would buy no more circuit based switches (but with
40 year life there are still a bunch of them around...).