SilkRoad Proposes One-Wavelength Optical Networking
By Erik Kreifeldt
SilkRoad Corp. has appeared in the optical networking scene with a single-wavelength technique that the company claims will transmit more than 6 Tbps of data on a single fiber within the next year. Using a single wavelength for high bit rate transmission circumvents many of the problems associated with optical networking, the company says, including nonlinear effects and signal add/drop.
The technology is ultimately capable of transmitting 10 Tbps over one fiber, SilkRoad claims, regardless of protocol and bit rate. The company unveiled the technology recently in a demonstration that transmitted 840 video signals (93.1 Gbps) from a 144-monitor video wall to a second over one 100 km length of fiber.
First-generation capacity
SilkRoad cites a previous lab demonstration of 200 Gbps over 200 miles on a single wavelength without amplifiers or repeaters. The transmission required nearly 30 dB of attenuation to keep from frying the receivers, reports Robert Gorman, SilkRoad's v.p. of sales and marketing. He anticipates that SilkRoad will specify spans of at least 400 miles in its first product before optical amplifiers are needed.
Although SilkRoad heralds its technology as a major scientific advancement, researchers in the optical networking community are skeptical and reluctant to comment, citing a dearth of information in scientific literature about the company's transmission method. To avoid a backlash of criticism, Gorman says SilkRoad kept the technology under wraps until it had evaluation units ready to ship. The U.S. Patent Office accepted SilkRoad's initial application covering 52 claims, and more than 200 additional claims are pending.
SilkRoad plans to introduce its first commercial system next year, Gorman says, reporting that the company has five field trials lined up from now through the Spring of 1999, with the first evaluation unit shipping in December 1998. The company is not revealing its customers. Able to carry 80 OC-48 (2.5 Gbps) signals, according to Gorman, SilkRoad's first product will have capacity in line with what DWDM vendors plan to offer in the same time frame.
How it works
Unlike DWDM, SilkRoad relies on a single wavelength to carry multiple signals. The technique uses an off-the-shelf DFB laser, but without the tuned cavity found in DWDM lasers, to produce the signal. A proprietary sampling technique combines signals into one wavelength, which is subsequently fed to an external modulator.
To sample an OC-48 signal, for instance, the technique uses a frequency higher than the maximum frequency of the OC-48 signal as seen by a spectrum analyser (0-500 MHz). This creates an upper and lower sideband around the sampling frequency (the upper sideband is filtered out).
To combine another OC-48 signal, the system chooses another sampling frequency with harmonics and sidebands that do not interfere with the first one. Silkroad says its sampling method does not require as high a frequency (2.2 to 2.5 times the maximum) that engineers would otherwise calculate, which enables greater information density in a given modulator's frequency domain.
Using a 40 GHz modulator, Gorman says Silkroad's first system will handle 80 OC-48 signals. By aggregating the signals from multiple 980 nm lasers, he says subsequent versions of the system will scale up in capacity by orders of magnitude and still use a single wavelength.
The linewidth of the beam is 175 Hz, Gorman reports, compared with 280 kHz associated with DFB lasers used for DWDM. The narrow linewidth enables longer-distance transmission without suffering from nonlinear effects compared with DWDM transmission, he says.
New look at optical networking
"It's pretty different from using multiple lasers in a WDM environment," Gorman says of SilkRoad's technology, noting that dispersion compensation has become a cottage industry to enable high bit rate DWDM transmission on different varieties of fiber. "The problem is with WDM in the first place," he says. "If you're not dealing with multiple wavelengths, you don't have a problem."
The SilkRoad platform uses a beamsplitter to add and drop signals. "We don't have to screw around with 'lab rat' experiments with optical crossconnects," Gorman says, noting that optical switching is fast, but prohibitively expensive at $15 million for a four-wavelength lab device.