Direct detection is the simplest form of optical signal recovery. The transmitter modulates the intensity of the optical carrier, and the receiver uses a photodetector (PIN diode or avalanche photodiode) that responds only to the optical power envelope. 100 Gb/s links (and higher) are now common between regional data centers, often stretching tens of kilometers. The receiver implements strong dispersion and delay lines on a compact chip. For much of the past fifteen years, the boundary between these two approaches was relatively clear.
[pdf] A passive optical network (PON) is a telecommunications network that uses only unpowered devices to carry signals, as opposed to electronic equipment. In practice, PONs are typically used for the between (ISP) and their customers. In this use, a PON has a topology in which an ISP uses a single device to serve many end-user sites using a system suc.
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