Gigabit-transmission papers captivate attention at OFC post-deadline session

Gigabit-transmission papers captivate attention at OFC post-deadline session

George Kotelly

The large audience in Conference Room 6C of the San Diego Convention Center typified the immense interest in the post-deadline papers presented at the February 1995 Optical Fiber Conference. The event perennially attracts the biggest crowd of any session during the week-long gathering. The main theme of the 10 papers presented focused on gigabit-speed transmissions.

An additional 18 post-deadline papers were presented elsewhere that focused on a variety of late-breaking studies covering optical amplifiers, fiber gratings, wavelength-division multiplexing devices and lasers.

Of the 28 post-deadline papers delivered, U.S. companies offered 12, Japanese firms contributed eight, and the European and Australian companies presented a total of eight. These are summaries of some key papers.

A team of investigators from MIT Lincoln Laboratory, AT&T Bell Laboratories and Digital Equipment Corp. described a 20-channel, two-level, passive wavelength-routed, all-optical network test bed that spans 90 kilometers in the Boston metropolitan area. The 20 channels are carried on frequencies spaced by 50 megahertz. Channels are accessed through optical terminals that offer 155-Mbit/sec and 1.244- and 2.488 Gbit/sec circuit-switched "A" services or scheduled simultaneous time-division-multiplexing/frequency-division-multiplexing "B" services with an aggregate rate per channel of 1.344 Gbits/sec.

The B-services allow independent optically routed connections by differing bit rates to share network resources such as terminal equipment and wavelengths by dividing transmissions into 128 independently allocated 1.953-microsecond time slots. Wavelength-routed point-to-point and multicast services can be run among optical terminals without optical/electronic conversions. The test bed can handle multiple protocols such as fiber distributed data interface, Ethernet, synchronous optical network, asynchronous transfer mode, and digital and wireless video.

Engineers at NTT Optoelectronics Laboratories in Ibaraki, Japan, have fabricated a 16-wavelength-channel add/drop multiplexer on a planar lightwave circuit platform. The 85-by-60-millimeter device consists of three arrayed-waveguide gratings, 16 thermo-optic switches and four ports. It functions from 1542 to 1556 nm with 0.8-nm channel spacings.

Tests demonstrate that the on-off crosstalk for the main input port to the main output port and for the main input port to the drop port are -24 dB and -13 dB, respectively. The fiber-to-fiber insertion losses are 7 to 8 dB and 3 to 4 dB when signals are coupled to the main input port and to the add port, respectively.

Investigators at AT&T Bell Laboratories discussed an 8-channel wavelength-division multiplexing system that uses a single femtosecond erbium-doped fiber laser as a broadband (5-terahertz) source and was spectrally sliced and modulated by an integrated optical multichannel controller. Error-free operation is obtained at the laser`s 40-Mbit/sec rate centered at 1.56 microns.

A large study team from AT&T Bell Laboratories has made possible the transmission of eight 5-Gbit/sec non-return-to-zero channels over a distance of 8000 kilometers for a total transmission capacity of 40 Gbits/sec.

The transmission experiment used an eight-wavelength transmitter and a 1000-km erbium-doped fiber amplifier chain in a circulating group. Eight distributed feedback lasers spanning a wavelength range of 1556 to 1559.7 nm at a channel spacing of 0.53 nm were multiplexed in a series of directional couplers.

The amplifier chain employed 20 spans of 45-km lengths of negative dispersion fiber and two spans of positive dispersion fiber. No 4-wave mixing effects were observed in the optical spectrum. The average bit error ratios for all eight channels were better than 2䁾-10. The Q-factors ranged from 16.3 dB to 18.1 dB.

A large group of engineers at AT&T Bell Laboratories pooled their efforts to demonstrate the stable wavelength-division multiplexing transmission of eight 2.5-Gbit/sec channels in a circulating loop of 2400 km. Even without the use of frequency-guiding filters for all eight channels, the transmission was error-free (less than 10-10) over distances longer than 10 megameters; some channels exhibited error-free transmission to distances of 12 to 14.4 Mm.

A team effort

Two NTT teams, one from Optical Network Systems Laboratories in Kanagawa, Japan, and one from Opto-Electronics Laboratories in Ibaraki, Japan, combined efforts to transmit 16 channels of 10-Gbit/sec optical frequency-division-multiplexed signals over 1000 km of singlemode fiber using dispersion-compensating fiber and 980-nm pumped erbium-doped fiber amplifiers with gain equalizers. Channel wavelengths ranged from 1549.53 nm to 1559.13 nm. Tests showed no power penalties, and no bit-error-rate floors were observed on any channel.

Another AT&T Bell Laboratories group reported the transmission of eight 20-Gbit/sec channels over 232 km of conventional fiber. Centered at 1555 nm, with a channel spacing of 1.6 nm, the system employs amplifiers located every 80 km, which corresponds to more than 15 times the 1-dB dispersion limit. Compensating the dispersion slope allowed penalty-free operation for all channels.

Yet another AT&T Bell Laboratories team accomplished 2.5-Gbit/sec unrepeatered transmissions over 529 km with a transmission fiber loss of 93.8 dB using suppression techniques, high-power Raman pump lasers, forward error correction and dispersion compensation. High-power, 1.48-micron, diode-pumped Raman lasers with output powers to 1.3 watts were utilized to increase the power budget improvements obtained from the pumped amplifiers. A power penalty of only 0.2 dB and error-free transmission were observed.

In a similar transmission experiment, researchers from STC Submarine Systems Ltd. in London; Alcatel Submarcom in Nozay, France; and Alcatel Alsthom Recherche in Marcoussis, France, obtained data transmissions at 2.5 Gbits/sec and 622 Mbits/sec using non-return-to-zero encoding over fiber-optic cable spans of 511 and 531 km, respectively. The use of remotely pumped post- and preamplifiers, forward error correction and dispersion-compensating fiber contributed to achieving unrepeatered transmissions and overcoming loss limitations.

Australian engineers from the University of Sydney and Telecom Australia Research Laboratories obtained 10-Gbit/sec transmission at 1.54 microns over 270 km in non-dispersion-shifted fiber using an adjustable chirped 120-mm fiber Bragg grating dispersion compensator.

A Japanese group from NTT Optical Network Systems Laboratories in Kanagawa claimed the highest data rate. The investigators reported a single-channel, single-polarization, 200-Gbit/sec, time-division multiplexed optical transmission experiment using 2.1-picosecond optical pulses. They also recover a prescaled clock directly from the 200-Gbit/sec signal to drive an all-optical demultiplexer. The 100-km transmission fiber consists of two 40-km and one 20-km lengths of dispersion-shifted fibers with in-line amplifiers at 40 and 80 km. The zero-dispersion wavelength of the total fiber was 1.558 microns. q

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