Nippon Telegraph and Telephone Corporation

Toshiyuki Tsuchiya

Nobuyuki Tokura

Jun Sato

Kazuyoshi Ohno

Ikutaro Kobayashi

Satoyuki Matsui

Bothway optical communication is carried out through a wavelength division multiplex system between two terminal stations (11a, 12a), each having a two-beam interference type filter (15) coupled with an optical cable (10) for multiplexing and/or de-multiplexing transmitted light and received light. The filter (15) has passbands and attenuation bands having periodic characteristics for the wavelength in the communication wavelength band. The wavelength of the transmitted light is essentially the same as the wavelength of the received light in each terminal station, with a small offset of the two wavelengths less than 5 nm. The oscillation wavelength of the laser (21) for transmission is adjusted so that it coincides with the passband of the filter, which doubles as a reference wavelength of the oscillation wavelength of the laser. Preferably, the two-beam interference type filter (15) is a Mach Zehnder type asymmetrical interferometer having a pair of directional couplers (F1, F2) connected to each other through a pair of optical fibers (f1, f2) so that the period of the passbands and/or the attenuation bands of the filter (15) is adjustable.

Wavelength division multiplex bothway optical communication system

A testing apparatus is presented which evaluates the performance of optical signal transmissions in the 1.3 .mu.m wavelength region, independently or simultaneously with signal transmission. An optical signal is transmitted in the network.fwdarw.user direction in a thermo-optic (TO) switch 11, and the signal is also looped in the direction A of an optical directional coupler 3. An optical time domain reflectometer (OTDR) 10 Generates a test beam at 1.55 .mu.m wavelength region, which is reflected by a wavelength selective filter 8 back to the OTDR 10 in the user-network direction. The apparatus is thus able to perform testing of the transmission cable 7 simultaneously with the signal transmission. When an operating problem is to be pinpointed in the system, the optical switch 11 is switched so as to permit the signal from a line terminal LT 1 to transmit in the network.fwdarw.user direction. The signal is attenuated by the same magnitude as the loss budget between the terminals NT 9 and LT1, and is looped back to LT 1, thereby enabling Loop 1 testing at the same signal strength level as in the actual signal transmission.

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Yokohama, Japan

Toshiyuki Tsuchiya