OFC post-deadline talks tout terabit transmission
GEORGE KOTELLY
Living up to its well-earned reputation for presenting optical technology breakthroughs, the Optical Fiber Communication `96 conference held recently in San Jose offered post-deadline papers that mesmerized standing-room-only crowds. Three separate experiments achieved record-breaking terabit-per-second transmissions over fiber-optic cables using wavelength-division multiplexing (WDM) techniques.
Indeed, new records were being set throughout the conference: Attendance totaled 6700, exhibitors exceeded 250 and post-deadline papers reached 36. Besides the captivating terabit papers, other post-deadline papers covered fiber gratings, optical amplifiers, fibers, transmitters/ receivers, and lasers, but nearly 40% of the papers had WDM in their titles.
Of the 36 papers presented, U.S. companies offered 19, Japanese firms contributed 7, and European and Australian establishments delivered 10. Despite the high interest in all the post-deadline papers, the three terabit-transmission papers were the highlights of the session.
In one of the groundbreaking papers, a team of researchers from Fujitsu Laboratories Ltd. (Kawasaki, Japan) amazed the audience by describing the WDM transmission of 55 wavelengths, each at 20 gigabits per second, for an aggregate capacity of 1.1 Tbit/sec over 150 kilometers of 1.3-micron, zero-dispersion singlemode fiber (see figure). Using pre-emphasis techniques, wideband erbium-doped fiber amplifiers and dispersion-compensating fiber with a negative-dispersion slope, the group claimed that bit-error rate degradation was not observed in any channel, even without channel-by-channel dispersion adjustment. For optical sources, 46 distributed feedback lasers and 9 external cavity tunable lasers were used. The 55 channels ranged from 1531.70 nanometers (channel 1) to 1564.07 nm (channel 55); channel spacing was 75 gigahert¥(0.6 nm).
The signals were externally modulated by a lithium niobate (LiNbO3) Mach-Zehnder modulator to obtain 20-Gbit/sec, nonreturn-to-zero pulses. The optical amplifiers, which consisted of 1.48-micron, pumped, high-alumina, codoped, erbium-fiber amplifiers, served as postamplifiers, inline repeaters and common preamplifiers. The 0.5-decibel bandwidth was 19 nm, and the total launched power equaled +13 decibel relative to milliwatt. All signals were passed through singlemode fiber with 50-km amplifier spacings. Receiver sensitivity yielded -29.7 dBm at a bit-error rate of 10-11 for center channel 28. In response to a question from the audience, the presenter declared that transmission distance could probably be pushed to 200 to 250 km with current technology.
Terabit/sec transmission was also accomplished by a group of investigators from AT&T Research and Bell Laboratories at Lucent Technologies (Holmdel, NJ) by using WDM and polarization multiplexing. The group implemented 50 signal channels, each at 20 Gbits/sec, for an aggregate capacity of 1 Tbit/sec, over 55 km of nonzero-dispersion fiber. The 50 channels were generated by polarization multiplexing 24 external-cavity lasers and one distributed feedback laser (channel 16) using star couplers and waveguide grating routers. The 24 copolarized wavelengths were split by a 3-dB coupler, separately modulated by LiNbO3 Mach-Zehnder modulators, and then recombined with orthogonal polarizations in a polarization beam splitter.
The wavelengths ranged from 1542 (channel 1) to 1561.2 nm (channel 25) with a 100-GH¥channel spacing. Electronically multiplexing two 10-Gbit/sec, 215 -1 pseudorandom bit streams using a gallium arsenide multiplexer produced the 20-Gbit/sec nonreturn-to-zero drive signals. To decorrelate the bit patterns of the two polarization channels at each wavelength, the outputs of two modulators traversed different lengths of fiber before polarization multiplexing. To decorrelate the modulation on different WDM channels in each polarization, the bit streams were temporally dispersed after polarization multiplexing by being transmitted through a 3-km length of conventional step-index fiber with a dispersion of 17 picoseconds/nm-km.
In this approach, the 50 decorrelated 20-Gbit/sec signals propagated through 55 km of nondispersion fiber at a zero-dispersion wavelength of 1513 nm and a dispersion slope of 0.07 psec/nm2-km. The signals were detected by two pin-based optical-to-electrical converters. One converter was used for clock recovery. The output of the other converter was electronically demultiplexed to two Gbit/sec bit streams in a dual-gate field-effect transistor circuit.
Engineers at Nippon Telegraph & Telephone Optical Network Systems Laboratories in Kanagawa, Japan, also accomplished terabit transmission using a low-noise, single, supercontinuum WDM source that generated short pulses of less than 0.3 psec. Using this source, they transmitted 100-Gbit/sec signals in 10 channels, for an aggregate capacity of 1 Tbit/sec over a 40-km length of dispersion-shifted fiber. A special 400-GH¥channel-spaced arrayed-waveguide-grating WDM demultiplexer/ multiplexer was used to filter the source signals.
The 10 channels at the output of the WDM demultiplexer/multiplexer ranged from 1533.6 to 1562.0 nm with an average output power per channel of -10 dBm. These outputs were modulated by a common LiNbO3 intensity modulator and time-division multiplexed by a 10x planar lightwave circuit optical time-division multiplexed multiplexer to produce the 1-Tbit/sec signals. Next, the signals were amplified by a fluoride-based broadband erbium-doped fiber amplifier for delivery over a 40-km, dispersion-shifted fiber that provided a zero-dispersion wavelength of 1561.3 nm.
After fiber-compensation filtering, the 100-Gbit/sec per channel signals were fed into a prescaled phase-locked loop timing-extraction circuit and an all-optical time-domain demultiplexer based on four-wave mixing to derive error-free transmission. The presenter amazed the audience by claiming that 5-Tbit/sec transmission might be realized soon by extending the super-broad bandwidth of the supercontinuum generator from 80 to 200 nm.
Other outstanding experiments were reported outside the arena of terabit transmissions. For example, a team from Lucent Technologies demonstrated error-free, soliton, WDM transmissions from 1554 to 1558 nm using two to eight channels of 10 Gbits/sec each over paths ranging to 19 megameters, equal to transoceanic distances. Besides taking advantage of soliton and sliding-frequency guiding filter technologies, the team incorporated new technology in the form of "dispersion-tapered" fiber spans. A soliton pulse-shaper source, based on a LiNbO3 Mach-Zehnder-type modulator, generated the signals with a controlled chirp; a second modulator imposed the data--a 215-bit, random pattern. Delivery over a 4-km length of standard fiber compressed the pulses to -20 psec and separated the bits of adjacent channels by 40 psec. Then, a 3.7-meter length of polarization-maintaining fiber, used as a half-wave plate, enabled adjacent channels to be launched with orthogonal polarizations.
Next, the signals were passed through a recirculating loop that contained six spans of 33.3 km each between erbium-doped fiber amplifiers and piezo-driven, Fabry-Perot etalon filters, operating at 1557 nm. At the receiver, the desired 10-Gbit/sec channel was first selected by a wavelength filter, and then time-division multiplexed to 2.5 Gbits/sec by a polarization-insensitive, electro-optic modulator having a 3-dB bandwidth of 14 GH¥and driven by a locally recovered clock. Results indicated that the bit-error rate was better than 1 ¥ 10-9 on all channels.
In another WDM experiment, American researchers at Corning Inc. in Corning, NY, worked cooperatively with colleagues at Siemens AG in Munich to transmit 80-Gbit/sec signals, approximately equal to one million voice channels, over 360 km of a single nonzero dispersion-shifted fiber. The researchers claimed that the combination of data rate and distance over a single uncompensated optical fiber system in the 1550-nm window was a record. The relatively low group-velocity dispersion of the fiber (SMF-LS), which was less than -3.5 psec/nm-km over 1530 to 1560 nm, was large enough to minimize the four-wave mixing degradation but small enough to allow 10-Gbit/sec transmission over long-haul terrestrial distances.
In the experimental setup, eight distributed feedback lasers were combined in a fiber coupler and externally modulated with a 231 -1 pseudorandom bit stream by a Mach-Zehnder LiNbO3 modulator. The wavelengths varied from 1547.8 to 1559.1 nm, with a uniform channel spacing of 200 GH¥(ڥ.6 nm). Channel bits were decorrelated by passing the multiplexed signals through 4.4 km of standard singlemode fiber and then amplified. The transmission line consisted of three inline optical amplifiers and four 90-km spans of SMF-LS fiber. Input power to each fiber span was adjusted to 14 dBm. The corresponding bit-error rate achieved was 2.0 x 10-17. q