OFC Report: Pick of the Papers
ANAHEIM, Calif. -- OFC2001 -- The hubbub generated by a showcase for more than 800 vendors can obscure one of the fundamental purposes of the OFC conference: the presentation of technological innovations by researchers from both industry and academia.
Like the show, the technical program this year was the biggest ever. The papers in the program were chosen from over 900 submissions, only a handful of which were solicited by OFC organizers. According to OFC management, the total input of papers was up 50 percent this year compared to last.
Today, with booths struck and airline tickets at the ready, many attendees will linger for the final series of presentations. Called the Postdeadline Papers (PDs), these are generally considered to represent the top technical breakthroughs out of the entire conference program.
Papers in this session are submitted on the first day of the conference, unlike those in the rest of the program, which have to be submitted months in advance. This means one of two things: They represent the latest advances in experiments that have only just been completed; or the authors of a paper want the results kept secret until the very last moment because of its commercially sensitive nature.
Late on Wednesday afternoon, the OFC technical program committee issued a list of their favorite picks from among the PDs. Here is the list of their choices (with our Light Reading filter applied, of course):
T. Hasegawa and co-workers, Sumitomo Corp. and Hokkaido University
Microstructured fibers, which have holes running along the entire length, exhibit unusual properties that could be useful in communication systems design. But to date, such fibers have had high losses. Sumitomo has come up with a design with reduced guided optical power in the holes of the fiber, which has a loss of 0.82 dB/km, 300 times lower than values reported before.
N. Keil and co-workers, Heinrich Hertz Institute
Polymers attract interest because they can be manufactured more cheaply than alternative materials, like silica-on-silicon. Polymer has a thermo-optic coefficient (its change of refractive index with temperature) that's ten times greater than silica. That's great for tunable applications, but not so great when wavelength stability is required, as it requires the expense of a cooler. HHI has fabricated an 8x8 AWG wavelength router whose properties don't change with temperature. The key is to match the thermo-optic coefficient of the waveguide material with the thermal expansion of the polymer substrate.
D.A. Francis and co-workers, Genoa Corp.
Genoa has integrated a semiconductor optical amplifier (SOA) with a "ballast" laser, and it calls the resulting device a "linear optical amplifier" (see Genoa Amps Up). The laser stabilizes the gain of the SOA, so that it remains constant even when the power level through the amplifier changes. This makes it possible to use the SOA to amplify multiple channels. The idea of gain saturation (the technical term) is not new -- it's the integration of the laser that's innovative.
A. Mooradian, Novalux Inc.
Vertical cavity surface-emitting lasers (VCSELs) are usually low power. Novalux has designed a device based on a VCSEL that offers extremely high powers. For more details see Novalux Details Laser Advance.
J. Leuthold and co-workers, Bell Labs
Optical switch fabrics are under development to overcome the bottleneck of optical-electrical-optical (OEO) conversion. But most that have been developed so far are wavelength blocking: In other words, it's not possible to switch any wavelength on any fiber to any wavelength on any other fiber, only to the same wavelength on a different fiber. That puts optical switch fabrics at a disadvantage to electrical ones. In this work Bell Labs has combined a MEMS (micro-electro-mechanical switch) fabric with a wavelength converter, creating a non-blocking switch. Wavelengths that need changing are directed to the wavelength converter before being directed to the tiny tilting mirrors in a 100x100 MEMS fabric.
A liquid crystal switch is used to switch channels from primary to backup in less than 5 microseconds. This forms the basis of an optical add/drop multiplexer for a ring node that includes the protection switching function.
The S-band is a previously unused region of the spectrum between 1480 and 1510nm that offers an opportunity to increase total system capacity. But first there are several problems to be resolved, like the need for amplifiers that operate in the S-band (erbium-doped amplifiers don't work), and other issues to do with the transmission characteristics of the fiber plant, as this selection of papers shows:
S. Aozasa and co-workers, Photonics Labs, NTT Corp.
The rare earth metal thulium amplifies most readily at wavelengths below the S-band. To get amplification in the S-band itself, the researchers pumped the thulium-doped fiber with 1400nm light, instead of the 1000nm light that has been used before. This encourages the energy transitions that provide S-band amplification through a process called "cross relaxation." Another innovation was the use of a double-pass configuration using an isolator, mirror, and circulator, that allowed the signal to pass through the fiber in both directions, leading to a higher conversion efficiency.
F. Roy and co-workers, Alcatel SA; Nozay, France
The researchers report an alternate pumping scheme for S-band amplification using two wavelengths supplied by a single fiber laser source. Light from a 1117nm ytterbium fiber laser is converted into 1240nm and 1400nm wavelengths using a Raman resonator.
K. Fukuchi and co-workers, NEC Corp.
NEC added 85 channels in the S-band to 92 in the C-band and 96 in the L-band to achieve a world record 10.92 Tbit/s transmission over 115km (see Alcatel Holds World Record for a Day). A thulium-doped amplifier was needed in the middle of the span. Raman pumping was employed to boost the power of the shortest wavelength channels in the S-band, which are continually losing power to the longer wavelengths through Raman scattering.
A. Pue and co-workers, Xtera Communications Inc.
Xtera managed to transmit 20 channels at 10.67 Gbit/s each over 867km. The distance was made up from 10 spans of standard singlemode fiber, with lumped Raman amplifiers between the spans.
C.K. Madsen and co-workers, Bell Labs
Bell Labs showed that its fiber-based widget can compensate multiple 10-Gbit/s signals. The compensator is based on a fiber resonator, with a thermo-optic phase shifter to tune the response.
Direct monitoring of the signal quality of future high bit-rate systems is difficult. A collaborative effort has managed to measure eye diagrams at 300 Gbit/s. This was done with ultra-short optical pulses generated by a highly nonlinear fiber ring laser. The time resolution was 1.6 picoseconds.
IP Over WDM
J.Y. Wei and co-workers, Telcordia Technologies Inc.
Traffic engineering solutions that can reconfigure WDM are urgently needed by IP networks, say the authors. They describe a solution which was implemented on a testbed set up by the U.S. government.
Siemens/Optisphere describe how it ran audio, video, and data traffic over 80 channels at 40 Gbit/s on one fiber (three spans) using Raman amps and forward error correction (see Optisphere Claims Record in DWDM Trial).
— Pauline Rigby, senior editor, and Mary Jander, senior editor, Light Reading http://www.lightreading.com