In fact, related
price-points for ROADM systems are even becoming competitive with static OADM
systems (owing to technological innovations, market competition, and intense pricing
pressures from carriers).
Commercial ROADM systems can provide remote automated add-drop of up to 40
wavelengths (any-wavelength-any-node) and use various technologies [2]. For example,
some vendors deploy wavelength selective switch (WSS) fabrics whereas others use
broadcast and select designs. In addition, tunable filters can also be used on input
trunks to drop selected channels, for example, fiber Bragg gratings. Nevertheless alloptical
ring transmission is quite challenging and requires some very specialized provisions.
Foremost is the need for rapid AGC to stabilize wavelength powers across all
links [3]. This is a crucial requirement as individual lightpath connections can experience
sizeable power fluctuations during transient events such as connection setup/
takedown or faults. AGC is achieved by coupling EDFA amplifier designs with attenuators
and modern subsystems to provide good gain flatness over wide input ranges with
millisecond timings.
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