Technical breakthroughs at giant Optical-Fiber Conference

Multicolored laser light, switchable photonic crystals, bandwidth on demand among the topics

SOME HIGHLIGHTS OF THE TECHNICAL SESSIONS INCLUDE:

BANDWIDTH ON DEMAND

As anyone who has priced Internet service providers knows, higher bandwidth (i.e. faster connection speed) costs more. In addition, high bandwidth may be wasted when high-speed connections are used for low-bandwidth applications, such as sending email. Ideally customers, and particularly large companies, would save money by paying only for the bandwidth they need at any given time, rather than paying premium rates for the high bandwidth that they only require on occasion. In the paper ThH3 ("The Value Proposition for Bandwidth on Demand"), Ronald Skoog of Telecordia Technologies ([email protected]) will ponder the fundamental forces that will drive future deployment of Bandwidth on Demand (BoD) services and the network design that will make them work. He will also explore how dynamic, flexible networking capabilities will most likely be used, what type of services will ultimately be available to BoD customers, and how this will change the industry.

FIBER TO THE HOME

Have you noticed that home Internet access speeds have not significantly increased over the last few years? High-quality video still can't be viewed in real time. Full-length movies take hours to download, even with the fastest home broadband connections. Part of the reason is that the optical fiber revolution has not yet truly hit home, at least most homes. Although optical fibers form the backbone of modern telecommunications networks, most home phone connections end in a string of copper, severely limiting the maximum speed at which data can be transmitted in a phone system. But an emerging movement, called fiber to the home (FTTH), is wiring new and existing homes with fiber-optic connections. Delivering fiber to the home faces economic, regulatory, and competitive obstacles; however, OFC speakers will present solutions that are making FTTH more technically, economically, and practically feasible--bringing the light-speed advantages of fiber closer to home.

Sample papers on this topic include:

--Examining a potentially low-cost FTTH infrastructure called the Ethernet passive optical network (EPON), in which multiple users in a local computer network share a single fiber optic cable line, Kazuho Ohara of KDDI R&D Laboratories in Japan ([email protected]) will show that EPON is feasible for delivering streaming video and voice in addition to data, even when many consumers on the network use these services at the same time (Paper ThAA2, "Traffic Analysis of Ethernet-PON in FTTH Trial Service").

--According to Karl Rookstool of ADC in Texas ([email protected]), FTTH providers could save significant amounts in fiber construction costs by designing a remote-terminal based FTTH architecture. By providing a remote terminal, FTTH can be deployed on existing fiber feeder routes using either a few spare fibers or the transmission technique known as Wave Division Multiplexing, as opposed to delivering fiber straight from a central office (Paper ThAA4, "Economic Considerations of Central Office (CO) Broadband Distribution Terminals vs. Remote Terminal (RT) Broadband Distribution Terminals for Deploying Fiber to the Home (FTTH)").

MULTICOLORED LASER LIGHT IS RED-HOT

When college professors and business executives brandish their laser pointers, they are inadvertently showing off the unique qualities of laser light. For example, lasers basically produce a single pure color, such as red, in stark contrast to light bulbs, which shine many colors that mix together to make white. But now, researchers are combining lasers and fiber optics to generate multicolored light sources. Launching intense laser light into a specially designed optical fiber generates "nonlinear" effects that convert single-colored light into a wide range of colors. The resulting "supercontinuum light" has all of laser light's benefits but many additional uses. For example, it could allow a single laser to generate the multiple colors of light that travel down fiber in modern transmission systems, such as WDM. It could serve as a basis for ultra-precise "optical" clocks that promise better global navigation. It could provide light with very broad bandwidths for a medical imaging technique known as "optical coherence tomography" which can yield detailed images of human tissue. In the past year, there has been a flurry of new techniques for producing supercontinuum light. You'll find most of these advances at OFC.

Sample papers on this topic:

----To generate supercontinuum light at the 1550-nm wavelength used in telecom applications, OFS researcher Jeff Nicholson ([email protected]) and his colleagues use an erbium-doped fiber laser, in which erbium atoms in an optical fiber amplify incoming laser light to the desired level. In their technique, OFS researchers send intense, 100-femtosecond pulses through several meters of special "HNLF" optical fiber, designed at OFS, which has highly nonlinear properties but can be made with traditional telecom technology. The result is relatively high peak power, low-noise supercontinuum light that spans an octave of bandwidth (the frequencies at the high end of its spectrum are twice that of the low-end). This is believed to be the broadest spectrum ever produced with an all-fiber device based on a mode-locked erbium-doped fiber laser (Paper ThK5, "A High Coherence Supercontinuum Source at 1550 nm"). In a different approach that employs continuous (CW) streams of laser light rather than short pulses, Akheelesh Abeeluck of OFS ([email protected]) and coworkers use a Raman fiber laser, in which an optical signal is amplified by a second, lower-wavelength light source. In their work, Abeeluck and co-workers splice the Raman fiber laser to an HNLF in order to produce supercontinuum light. With 821 mW of power launched into 4.5 km of HNLF, they achieved a continuum with a bandwidth greater than 247 nm. This is believed to be the broadest continuum to date reported with a CW Raman fiber laser. (Paper ThT1, "Supercontinuum Generation in a Highly Nonlinear Fiber Using a Continuous Wave Pump")

-- Zulfadzli Yusoff of the University of Southampton ([email protected]) and colleagues send intense picosecond pulses through a holey optical fiber, a fiber with a special geometric pattern of holes running along its length. This pattern causes light to interact with the fiber in a nonlinear fashion, converting the single-colored laser light into a broad spectrum of colors. This nonlinear interaction is especially efficient in a holey fiber due to the fact that light is confined to such a tiny volume within the fiber, giving rise to enormous optical field strengths. In their work, Yusoff and colleagues then carry out a "spectral slicing" method that uses a filter to separate these colors. The separated colors then travel through individual fibers. This approach potentially reduces the complexity of producing the multiple colors of light used in WDM telecom systems, leading to possible cost savings (Paper FH3, "24 Channels x 10GHz Spectrally Sliced Pulse Source Based on Spectral Broadening in a Highly Nonlinear Holey Fiber ").

SWITCHABLE PHOTONIC CRYSTALS

Photonic crystals can affect the flow of photons in much the same way that electronic devices control the flow of electrons. Most photonic crystals, however, have specific properties that cannot be varied once the crystals are made. A few types of photonic crystals, such as fluid suspensions of colloidal silica, can be modified on the fly, but the time required to switch configurations is inconveniently long. Researchers at Brown University have now made photonic crystals that can be modified in fractions of a second. The switchable photonic crystals consist of a class of material known as holographic-polymer dispersed liquid crystals (H-PDLCs). Structures are defined in the material by exposing it to an interference pattern produced by a set of four laser beams. Unlike many other types of photonic crystals, the H-PDLC crystals can be modified in a single step. In addition, the new photonic crystals are easily constructed at a wide range of scales, and can replicate sophisticated structures including diamond lattices as well as anisotropic lattices that affect light differently depending on the direction it propagates through the crystal. Jun Qi ([email protected]) will discuss switchable H-PDLC photonic crystal in sessions on Monday (paper MF38, "Switchable Infrared Reflectors Fabricated in Polymer-Dispersed Liquid Crystals") and Thursday (paper ThI7, "2D and 3D Tunable Photonic Crystals Fabricated in Liquid Crystal/Polymer Composites").

INCREASING TELECOMMUNICATIONS CAPACITY WITH POLARIZATION

To maximize the data capacity of fiber-optics lines, modern systems send multiple colors of light down an optical cable, with each color representing a "channel" that carries different data. To increase transmission capacity even further, one can add channels by using more colors within the existing spectrum of light. This is analogous to putting more radio stations on the same dial. But when one squeezes in more colors, there is a greater risk of "crosstalk" or interference between the channels. A few years ago, Neal S. Bergano and Carl Davidson of Tyco Telecommunications ([email protected]) invented a technique to reduce this crosstalk by giving adjacent channels orthogonal polarization, in which the electric field directions of the light waves associated with adjacent colors are perpendicular to one another. To achieve the ultimate in tight channel spacing using the orthogonal launch technique one needs to add the additional step of polarization tracking at the receiver, where the incoming electric field direction of a degrading signal is converted to a desired electric field direction. Now, Davidson and colleagues have demonstrated this technique by sending, over a distance of 6,500 kilometers, 10 Gigabits of data per second in each channel with a spacing of 15 Gigahertz in frequency (0.12 nanometer wavelength) between the colors in neighboring channels. In doing so, they achieved a record 66% level in an important quantity called "spectral efficiency," which represents the bit rate per channel divided by the channel spacing (TuF3, "Polarization Tracking Receiver Demonstration Over Transoceanic Distance").

MICROSTRUCTURED FIBER AMPLIFIERS

The first microstructure fiber-based optical parametric amplifier in the telecom band has been built by researchers at Northwestern University. Prior fiber amplifiers, with characteristics that make them feasible in the 1500 nm region of interest to telecom, have required lengths on the order of kilometers. The researchers found that their new microstructured fibers provide high gain with lengths on the order of tens of meters and modest pump powers of about 1 watt. In addition, the fibers are relatively insensitive to bending losses and can be wound tightly into compact packages, which makes them attractive for practical telecom applications (R. Tang, paper ThT2, "Microstructure-Fiber Based Optical Parametric Amplifier in the 1550nm Telecom Band," contact: Prem Kumar, [email protected]).

These items were assembled by Ben Stein and James Riordon of the American Institute of Physics in cooperation with the Optical Fiber Communication Conference and Exposition and the respective OFC speakers.