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 SANTA FE, N.M., June 25 /PRNewswire/ -- AT&T (NYSE: T) engineers have field-tested an experimental lightwave telecommunications system that operates at twice the capacity of today's highest-capacity systems. It uses wavelength division multiplexing ("WDM") and optical amplifiers.
 WDM involves transmission of light pulses over various wavelengths (colors) of light. Optical amplifiers are spliced-in segments of optical fiber containing the rare earth element erbium; they boost lightwave signals traveling through the fiber.
 In a 1991 field trial, the AT&T team operated the system at 6.8 gigabits (billion bits per second) by transmitting 1.7-gigabit signals over each of four wavelengths of light.
 Jonathan Nagel, of the Lightwave Hardware Development Department at AT&T Bell Laboratories, described the trial in an invited paper he presented here today at the Topical Meeting on Optical Amplifiers and Their Applications.
 Nagel's co-authors were Samia Bahsoun and Daniel Fishman of the same department and Donald R. Zimmerman of the Network Technology Performance Department, along with James Thomas and James Gallegher of AT&T Network Services Division.
 The field trial involved transmission of optical signals over 840 kilometers of dispersive fiber with the optical amplifiers placed 70 kilometers apart.
 An earlier AT&T trial involved four-channel WDM transmission equalling 6.8 gigabits over 70 kilometers, with optical amplifiers used to compensate for WDM splitter losses and increase system gain.
 "These trials are part of a feasibility study for next-generation lightwave systems," said Nagel. "We used standard single-mode fiber and off-the-shelf transmission equipment with in-line optical amplifiers."
 The trials were conducted in Roaring Creek, Pa., using installed lightguide cable, half of which was being used for commercial telecommunications service.
 The meeting on optical amplifiers was sponsored by the Optical Society of America and the Lasers and Electro-optics Society of the Institute of Electrical and Electronic Engineers.
 AT&T Optical Amplifier System Design and Field Trial
 -- Additional Technical Information
 On one channel, a lithium-niobate-controlled Mach-Zender interferometer was used to modulate externally a CW laser and provide a chirp-free signal. Three other lasers were biased above threshold and small-signal modulated to provide low-chirp FSK signals. These channels were used to compare dispersion penalties for FSK and externally modulated transmitters.
 Signals on the four 1.7 Gb/s channels were combined and sent through a chain of 13 amplifiers spaced 70 km apart through 840 km of fiber with zero dispersion at 1.3 microns, 14,200 ps/nm of dispersion at 1.55 microns and 264 dB of total loss.
 After the last amplifier in the chain, the four signals were passively split into four tunable fiber Fabry-Perot filters for channel selection, with each filter locked to a 5-to-11 kHz channel identifying tone superimposed on the signal at the transmit side.
 Components were chosen to be field compatible. Lasers and receivers were from the commercially available AT&T FT Series G transmission system. The external modulator was designed to be operated with the 1.5-volt peak-to-peak voltages supplied by existing regenerators. Rack- mounted optical amplifiers used the 48-volt office power supply, as did every other circuit pack. The low-loss fiber Fabry-Perot filter was set in a circuit pack containing electronics for tuning and locking to a given tone. Long-term unattended stability of the entire system was a key requirement for the field experiment.
 The concatenated gain profile of 13 amplifiers differs considerably from the small signal gain profile of a single amplifier. The 1530nm peak completely disappears, and the concatenated gain is largest near 1558nm. If signal wave-lengths in the 1530 to 1540nm region had been used, the signals would have decayed through the chain and emerged with low signal-to-noise ratios (SNR). Adequate SNR was maintained on all channels using wavelengths in the 1550 to 1560nm range.
 The SNR needed for stable system performance depends on the receiver, optical filter, transmitter and optical path degradations, as well as the system margins required for aging, temperature and other degradations. In this system, 20-22dB of SNR was enough for long-term error-free performance, as established using bit-error-rate measurements for varying system lengths and numbers of channels.
 The most important design parameter for optically amplified systems is optical SNR ratio per channel. When designing an amplified system, it is essential to maintain an adequate SNR in every channel, under all conditions. SNR is reduced by having low input signals into the optical amplifiers or by channel spacings that move some channels to low gain regions of the concatenated amplifier gain curve. With multiple channels, it is essential to use amplifiers with enough output power that adequate signal power can be maintained for all channels.
 Work on this project involved AT&T people in research, component development, system development and network planning areas. They include the authors of the technical paper and Tingye Li, Alan Gnauck, Steve Korotky, Adel Saleh, Bob Tkach, Randy Giles, Andy Chraplyvy, Anders Olsson, Terry Cline, Bob Tench, Kinichiro Ogawa, Detlef Gloge, Stan Lumish, Marty Sewell, Chungpeng Fan, Rod Luhn, Jack Matthews and Mohammed Fatehi.
 -0- 6/25/92
 /CONTACT: Donna C. Cunningham, 802-482-3748, or evenings, 802-482-2933; or Dan Lawlwer, 908-234-5254, or evenings, 908-537-6048, both of AT&T/
 (T) CO: AT&T ST: New Jersey, New Mexico IN: TLS SU:

TQ-LR -- NY002 -- 3576 06/25/92 08:01 EDT
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Date:Jun 25, 1992

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