Optical Transport Performance

Optical transport can be divided into ultra long haul, long haul, and metropolitan.
 Ultra long haul is up to 6000 kilometers, typically using a 40 channel × 10 Gbps
multiplex.
 Long haul is up to 600 km, typically 80 channels × 10 Gbps.
 Metropolitan is between 6 and 60 km and may use dense wavelength division
multiplexers (40 × 40 Gbps channels).
 Long haul and ultra long haul requires repeaters every 80 kilometers or so.
These are usually erbium-doped fiber amplifiers.
At either end of the optical pipe, photodiode detectors exploit the properties of iridium
phosphide and iridium gallium phosphide. Iridium phosphide is transparent to
photons. Iridium gallium phosphide (conveniently) absorbs photons. We can therefore
recover the bit stream from the optical domain, but we still have the problem of how to
process packets at optical transport speeds. A2.5 Gbps packet stream gives us 65 ns to
process a packet. A 40 Gbps packet stream gives us 4 ns.
Also in the optical domain, as bit rates increase (and bit duration decreases), the
shorter bits become more vulnerable to chromatic and polarization mode dispersion—
either in the fiber or in the amplifiers or filters. Iridium phosphide is used for devices
because it can deliver nanosecond response times.
In the optical frequency domain, we need to find mechanisms for improving laser
wavelength accuracy—for example, the use of phase lock loops to measure the output
from the laser and correct for temperature or current drift effects in order to improve
the control of optical frequency and phase. This may also include active trimming to
correct for device age drift. DCXO age drift characteristics can be determined over time
and correction characteristics applied (potentially over many years).