Faster, better, and higher-performance components and systems highlighted four parallel postdeadline sessions during the 25th annual Optical Fiber Communications conference (OFC; Baltimore, MD; March 2000). As regular papers have to be submitted months before the conference, the postdeadline work gives a sharper picture of the state of the fast-moving technology. Important trends included tunable laser sources, optical switching, better optical amplifiers, and more-practical versions of ultrahigh-performance systems (see Laser Focus World, April 2000, p. 9).
System makers and telecommunications operators want tunable lasers they can plug into any channel of wavelength-division-multiplexing (WDM) systems to avoid the need for huge inventories of spare lasers for each available wavelength and, potentially, in wavelength conversion and optical switching. Kenneth Knopp of CoreTek (Burlington, MA) described a new and promising approach—an optically pumped vertical-cavity surface-emitting laser (VCSEL) tunable continuously between 1527 and 1570 nm, with output to 6 mW. Electrical pumping traditionally has been preferred outside the laboratory but has proved difficult for 1550-nm VCSELs. CoreTek is betting that diode-laser pumping with a 980-nm source will be more practical.
The CoreTek laser has a curved top mirror mounted on a microelectromechanical system device that moves it vertically. Adjusting the drive voltage by 30 V changes cavity length enough to tune the laser across its entire tuning range. Together with a flat rear mirror, the curved mirror forms a stable cavity with round-trip loss less than 0.1%, which generates a 10-µm laser spot. The curved mirror both transmits pump light and serves as the output coupler, with its 99% reflectivity giving very high spectral purity. Tuning is continuous and free of mode hops, Knopp reported, with lowest output of 3.5 mW in its 43-nm range. Members of the OFC audience were not the only ones impressed; two weeks after the meeting, Nortel Networks (Brampton, Ontario, Canada) agreed to buy CoreTek for stock worth up to $1.43 billion (see Fiberoptics Industry Report, p. 111 this issue).
New views
Optical fiber amplifiers are spreading into new transmission windows. A team from NTT Photonics Laboratories (Tokai, Ibaraki, Japan) has taken an important step in developing thulium-doped fiber amplifiers for the otherwise unused S band at 1480 to 1530 nm. Earlier experiments showed that a complex two-wavelength pump scheme could shift thulium's gain to this region from shorter wavelengths. At OFC, T. Sakamoto and colleagues reported that a new technique for doping the fiber with high thulium concentrations could produce the desired wavelengths. They reported gain greater than 30 dB with a 5-dB noise factor at 1480 to 1510 nm and showed that their amplifiers could handle eight separate 10-Gbit/s channels at wavelengths between 1483 and 1505 nm.
The demands for pump power in the 1450- to 1480-nm range are rising with the number of channels in WDM systems and the growing interest in Raman amplification balancing gain across the transmission band. SDL (San Jose, CA) is cranking up the power of 1480-nm pump lasers to record levels to meet these demands, said developer Mehrdad Ziari. Using a laser with a single stripe that flared in width toward the output port, his group generated fiber-coupled output of 1 W. They also generated fiber-coupled output of 475 mW from a 1480-nm laser with a single uniform narrow stripe. Using their pump lasers, they demonstrated Raman gain of higher than 10 dB.
Hero experiments
At the traditional hero-experiments sessions, Lucent Technologies—Bell Labs (Holmdel, NJ) claimed a new record for the highest combined data rate sent through a single fiber—3.28 Tbit/s. The speed was only a modest increment over the 3 Tbit/s that NTT Network Innovation Laboratories (Yokosuka, Kanagawa, Japan) reported at last year's OFC, but the Bell demonstration made other important steps. It sent signals a total of 300 km compared to only 40 km in the NTT experiments and divided its link into three 100-km fiber spans, a length common in long-distance transmission.
T. N. Nielsen and colleagues transmitted 40-Gbit/s signals on each of 40 channels in the C band at 1530 to 1562 nm and 42 channels in the longer-wavelength L band at 1570 to 1605 nm, each with 100-GHz spacing. They combined three key features used in earlier terabit-transmission demonstrations: transmission in both erbium-doped fiber amplifier (EDFA) bands, distributed Raman amplification, and 40-Gbit/s channel speeds. Transmission was through a developmental Lucent fiber with very low dispersion slope of 0.037 ps/nm2/km. Their system included separate predispersion compensation stages for each of the two transmission bands, as well as a final stage of postdispersion compensation. Each in-line amplifier included distributed Raman amplification in the near end of the transmission fiber, plus separate parallel amplification stages for the two erbium-fiber amplifier bands, each with a length of high-dispersion-slope dispersion-compensating fiber.
In Japan, Toshiharu Ito and colleagues from NEC C&C Media Research Laboratories (Kawasaki), Sumitomo Electric Industries (Yokohama), and Oki Electric Industry (Tokyo) came close to Bell Labs in overall speed, reaching 3.2 Tbit/s. Their overall distance was longer—1500 km—but it came in 40 spans of 37.5 km, not in the longer spans that Bell Labs used. Ito's group also used parallel EDFAs for the 1550- and 1580-nm bands, but did not use Raman gain. They used polarization interleaving to pack a total of 160 channels, each transmitting 20 Gbit/s, with a narrow 0.4-nm (50-GHz) spacing.
Going the distance
Two papers from Tyco Submarine Systems (Eatontown, NJ) described impressive speeds over transoceanic distances. In one experiment, C. R. Davidson and colleagues packed 180 10-Gbit/s channels into the C-band at 1526 to 1568 nm. The shortest 22 wavelengths were 0.42 nm (50 GHz) apart; the others were separated by only 0.21 nm (25 GHz). This design yielded a spectral efficiency of 0.4 (bit/s) per hertz but required forward error correction that imposed a 23% overhead. This spectral efficiency allowed error-free transmission through 156 amplifiers and a total distance of 7000 km. In a separate demonstration, M. Nissov and colleagues of Tyco transmitted 32 separate 20-Gbit/s channels a total distance of 6200 km; careful optimization of reduced slope dispersion allowed the first transoceanic demonstration of 20-Gbit/s channel speeds.
In an eagerly anticipated report, a team from Qtera Corp. (Boca Raton, FL) described a field trial of their much-heralded ultralong-haul terrestrial wavelength-division multiplexing system operating at 10 Gbit/s. The 23-span route covered 2410 km in the Qwest network, with average spans of 105 km, typical of long-distance terrestrial systems. Ian Haxell and colleagues of Qtera attributed bit-error rates better than 10-15 after forward error correction to "a combination of nonlinear-dispersion-managed return-to-zero transmission and distributed Raman amplification." Unlike laboratory demonstrations, they used comparatively few channels, all in the standard 1550-nm EDFA band. They transmitted one group of four wavelengths spaced 50 GHz apart at 1547.32 to 1548.51 nm and one pair spaced 100 GHz apart at 1559.79-1560.61 nm.
