New multiplexing technologies are emerging to fill the need for fiber in metropolitan networks. Many of these networks still lack bandwidth, but it's costly and time-consuming for local telcos to lay new fiber links to buildings they wish to serve. Multiplexing technologies can help add capacity to these networks, and new methods are emerging that overcome some of the limitations of traditional approaches.
Carriers typically increase the bandwidth on an existing fiber-optic connection by multiplexing signals from several different transmitters into the fiber, then demultiplexing them back into individual channels at the receiving end of the network. But the 1,310-nanometer lasers used in metro networks can vary in wavelength-from 1,260 nm to 1,360 nm for Fabry-Perot lasers and 1,280 nm to 1,340 nm for distributed-feedback (DFB) lasers. This combining of channels into a cable results in crosstalk and difficulty in demultiplexing. Both factors hurt performance.
Another approach is to expand the fiber capacity by purchasing a whole new wavelength-division multiplexing (WDM) system. However, replacing existing transmitters and receivers with those on a regular wavelength grid is expensive. And coarse WDM systems, which use fewer wavelengths, are not cost-effective at 10 Gbits/second.
An alternative is wave-shift multiplexing (WSM), where the incoming channels of unknown wavelength around 1,310 nm are first shifted in frequency onto a regular grid of wavelengths using an all-optical wavelength-shifting technique. The channels may then be safely multiplexed into a single fiber and demultiplexed into separate channels at the receiver.
The wavelength shifting is performed by a semiconductor optical amplifier (SOA), a laser diode similar to those in a CD or DVD player, modified by antireflective coatings on the chip facets to prevent lasing. SOAs provide gain, so that there is typically no loss in signal strength. Since the input frequency is unknown, four-wave mixing is ruled out. As well, the range of the wavelength's shift must span 50 nm, which makes interferometric approaches difficult. Therefore, WSM systems use cross-gain modulation, in which the incoming signal causes the SOA to go into saturation, resulting in a loss of gain. A probe laser set to the desired output wavelength is modulated by the changing gain from the signal passing through the SOA.
Monitoring any all-optical system requires optical detectors and associated electronics for measuring the average optical power at strategic points in the system. In this case, a high-bandwidth optical detector with associated microwave electronics measures the signal-modulation power transmitted into the fiber to ensure that the bit-error rate at the receiver will meet specifications, even for the worst-case receiver sensitivity. A sophisticated software model of the optical system, located in the microprocessor on each channel card, sets parameters for initialization and maintenance of optimum performance. The system will automatically reconfigure the optics to optimum performance when a customer replaces a channel card with one having different power, light frequency, bit rate or protocol.
One benefit of an all-optical technology such as WSM is the availability of protocol- and bit-rate-transparent optical signals throughout. This allows the customer to change protocols or bit rate without reconfiguring the system hardware or software. A further advantage of using all-optical channel cards is that they can be inserted into or removed from an operating WSM system without any difficulty. Because photons are uncharged, unlike electrons, there are no voltage or current spikes on breaking or making optical connections. The absence of sparks could also be important in hazardous situations.
The initial versions of WSM systems will be constructed using discrete components, but as usual, integration is expected to reduce manufacturing costs over time. Optics are being compressed into hybrid holographic photonic integrated circuits (HPIC), an approach that promises to dramatically reduce the costs of providing broadband access in the metro market. An HPIC connects unpackaged components into one planar device. A patterned layer overlays a planar waveguide so as to steer light between discrete components. Carefully designed holograms contained in the pattern control directional changes in the light. Bending its direction with holograms in the waveguide plane can reduce chip length from that of conventional waveguides that rely on slow bending. Holograms also allow coupling in and out of the waveguide for connecting to discrete components.
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