A fourth fiber window

A fourth fiber window

WILLIAM B. GARDNER, AT&T

Automated systems that use optical time-domain reflectometers to monitor fiber loss are beginning to appear. In principle, wavelength-division multiplexing technology could be used to probe in-service fibers at an unused spectral "window." This option is used by Japan`s Nippon Telegraph & Telephone Corp. in its automatic optical-fiber operations and support system, for example.

In some distribution systems, however, the second lightwave transmission window at 1310 nanometers and the third window at 1550 nm are both in use--one for telephony and the other for video. The first window at 850 nm is lossy and below cutoff. The high peak powers of the OTDR make it an unwelcome intruder in windows already in use.

This situation has driven system designers to explore the wavelength region between 1600 and 1650 nm. AET Telecommunication of Italy has measured the loss of 1740 installed fibers and 500 splices at 1550 and 1625 nm and has reported similar loss results at both wavelengths. Company researchers reported their results at the NIST Symposium on Optical Fiber Measurements in Boulder, CO, last September. They described a system that mechanically switches a 1625-nm OTDR among 240 in-service fibers. Approximately a dozen of these loss-measurement systems have been installed to date in Italy and Argentina.

The use of a fourth fiber-optic transmission window at 1600 to 1650 nm in commercial systems raises standards issues for both fibers and systems. AET Telecommunication presented some of these issues at the March 1995 meeting of Working Party 2 of the International Telecommunication Union`s Study Group 6. This working party, chaired by Dean Frey at AT&T Network Cable Systems, is responsible for standards that deal with the construction, installation and protection of fiber-optic cables.

Working Party 4 of ITU`s Study Group 15, chaired by Federico Tosco of Cselt, in Italy, has considered the use of the fourth window for the surveillance of fiber-optic cables in local access networks. However, neither Study Group 6 nor Study Group 15 has yet provided any sanction for the use of the fourth window.

In North America, discussions are taking place in the Telecommunications Industry Association`s Working Groups FO-6.6.5, chaired by Felix Kapron at Bell Communications Research, and FO-2.1/6.6, chaired by Allen Cherin at AT&T Network Cable Systems. At the association`s January 1995 meeting in Albuquerque, NM, Raj Dave at Teradyne presented data showing that 1650-nm signal losses in installed fiber-optic cables correlate, albeit imperfectly, with the losses at 1550 and 1310 nm. The TIA, however, like the ITU, has not endorsed the use of wavelengths beyond 1600 nm.

This new fourth window, however, introduces some performance obstacles that arise from fiber`s sensitivity to bending loss at long wavelengths. When fibers are measured on 15-centimeter-diameter spools, the 1600- to 1650-nm loss is usually low and stable. In the standard 100-turn macrobending test, where the bend diameter is reduced to 7.5 cm, the 1600- to 1650-nm loss increases and becomes sensitive to temperature, tension, vibration and other conditions.This test is designed to simulate fibers configured in splice applications. But it does not determine whether small perturbations could trigger false alarms in fourth-window monitoring systems.

Another obstacle could arise because fiber and cable manufacturers do not currently measure or control their products in the 1600- to 1650-nm window. The added losses incurred from bending, cabling and temperature cycling are less predictable at these longer wavelengths than in the other three windows.

If adequate long-wavelength loss cannot be guaranteed without serious economic impact, workable options could probably be found in the second and third windows. q

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