American Telephone & Telegraph Corp. has announced that it is embarking on a one-year test of the economics and effectiveness of hair-thin glass fibres as a substitute for copper wires in transmitting telephone conversations and other telecommunication signals.

A 1 1/2-mile cable containing the revolutionary glass fiber will run under the streets of Chicago in the first test simulting actual operating conditions, AT&T chairman John D. Debuts told a Chicago business group Friday.

"We are optimistic about the potential of lightware communications for reducing coast and providing new future telecommunications capabilites," Debutts said.

Each pair of the hair-thin glass fibers, activated by a solid-state kaser or light-emitting diode, can carry as many as 600 separate telephone conversations, compared to 24 over a normal copper wire. The glass fibers can carry not only voices but also data and video signals, just like the wires currently used, but are both less bulky and able to carry more information.

If the experiment proves the effectiveness of the optical fibers, Debutts indicated that lightwave systems could be used to carry information between telephone switching centers by the 1980s and could evevtually supersede the present copper cable thoughout the Bell telecommunications system.