The latest and greatest reports at the annual Optical Fiber Communications Conference (OFC; San Diego, CA) traditionally come in the late Thursday postdeadline sessions. As revealed at OFC 2019, with single-fiber capacity edging toward the nonlinear Shannon limit, high-end hero experiments have moved to multicore fiber. But this year's late-paper session held on March 7 also broke new ground in other areas: radio over fiber transmission for 5G networks, direct detection of a terabit per second, and use of buried fiber carrying 400-Gbit/s signals to sense automobile traffic on a Texas highway.
The biggest bandwidth in a hero experiment was 0.715 petabits per second traveling 2009.6 km through a loop of 19-core fiber, reported by Benjamin Puttnam of the National Institute of Information and Communications Technology (Tokyo, Japan) and colleagues. Each core carried polarization-division multiplexed 16-QAM (quadrature amplitude modulated) signals at 24.5 gigabaud on 345 optical channels with wavelengths from 1530.52 to 1605.52 nm in the short (C) and long (L) bands of erbium-doped amplifiers.
At the end of the 31.4-km multicore fiber loop, couplers extracted signals from each core and split them into short- and long-wavelength bands for amplification in two parallel 19-core fiber amplifiers, one amplifying long wavelengths, the other short (see figure). After amplification, another set of couplers rearranged the amplified outputs into the other end of the multicore fiber loop. Splice and fiber loss limited the signal to 64 passes through the loop. That added up to a bit-rate-distance product of 1.44 exobit-kilometers per second. "We believe this represents the highest throughput of any transmission demonstration in the medium- to long-haul regime and the largest throughput-distance product for demonstration using [spatial-division-multiplexed] amplifiers," wrote Puttnam and nine colleagues from the National Institute of Information and Communications Technology (NICT), Furukawa Electric (Ichihara, Chiba, Japan), and the Royal Institute of Technology (Stockholm, Sweden).
To boost wireless bandwidth, 5G uses high microwave frequencies that propagate poorly in open air, so 5G base stations will need to be much closer to wireless users than in today's cellular networks. That creates demand for a generation of fiber-optic cables to carry signals between the network core and the wireless distribution nodes. OFC heard new results on a leading approach, "radio over fiber," in which fiber cables carry radio signals generated at or near data centers in the network core to wireless nodes close to users.
Joonyoung Kim of the Electronics and Telecommunications Research Institute (Daejon, Korea) described a radio over fiber system that provides a broadband connection to multiple arrays of antennas built to serve many users. Transmitters at the network edge generate outgoing radio signals at intermediate frequencies of 1.6 to 4.4 GHz and use them to modulate lasers at 1310, 1350, 1470 and 1550 nm that are wavelength-division multiplexed (WDM) into a single fiber. Equipment at the antenna site upconverts the radio signals to the 28-GHz band for 5G transmission. Return signals received in the 28-GHz band are downconverted to intermediate-frequency radio signals for transmission through fiber to the data center. Using coarse WDM avoids the need to cool laser sources at the antenna site.
They first tested the radio over fiber system in a 5G mobile network built by Korea Telecom for the Pyeongchang Winter Olympics in 2018. At OFC, they reported the first tests of their system with the MIMO (multiple-in, multiple-out) antenna arrays required for full 5G service. Their link carried the equivalent of 147.4 Gbit/s. It's an interesting parallel to old Bell System's test of a fiber-optic cable at the 1980 Winter Olympics in Lake Placid, NY, which wound up as the main video feed to television broadcasters because fiber withstood the bitter winter weather better than coaxial cable.
The late papers also included a revival for the direct-detection systems that were pushed into the background a decade ago, when three OFC 2009 postdeadline papers reported terabit-per-second coherent transmission on a single channel. Coherent system prices have dropped since then, and their data rates have soared. Yet direct detection retains an important operational advantage because it does not require costly management of laser wavelengths at both transmitters and receivers, Di Che of Nokia Bell Labs (Holmdel, NJ) said in the postdeadline session. He and colleagues at Nokia Bell Labs and the University of Melbourne (Australia) reached a net data rate of 1.02 Tbit/s by using 5-bit symbols and 64QAM modulation. They consider their approach very promising for reaching low operational costs for high-volume, short-reach links.
An interesting new twist was a report that fibers carrying live data at 400 Gbit/s could do double duty as fiber sensors of traffic on an adjacent highway. A one-inch cable containing 432 fibers was buried 36 to 48 inches deep in a two-inch duct 30 to 45 feet from two highways forming an 80-km loop near Richardson, TX. The 400-gig wavelength channels were spaced at 50 GHz intervals and modulated with dual-polarization 144-QAM so they carried 8.4 bits/second/per hertz over a 55-km span. Glenn Wellbrock of Verizon (Richardson, TX) and colleagues at Verizon and NEC Laboratories America (Princeton, NJ) sensed the auto traffic by directing pulses from a phase-sensitive optical time-domain reflectometer through the fiber in the direction opposite to the live data traffic. They measured road conditions, the number of vehicles, and their speed by the changes in Rayleigh scattering via interferometric phase beating in the fiber. They counted vehicles on the highway with 94.5% accuracy and measured their speed with 98.5% accuracy.
"I am not surprised we could measure it," said Wellbrock, "but what did surprise us was how accurate the measurements were for fiber buried so deep and that we did it on a fiber with active traffic. That had not been shown to date and I feel is really important because it effectively enables just about all fiber cables to be used as sensors if warranted." That makes it attractive for smart city and community applications.
