Direct detection uses the amplitude of the light to encode the information. The transmission distance
to achieve depends on the data rate of the signal that is used. At lower data rates, the link length can
achieve 100th of kilometers. However, as the data rate or the signal speed goes up, the link length
decrease exponentially.
In coherent transmission, rather than using the amplitude to encode information, properties of the
light, such as phase and polarization. Consequently, this increase the data rate while still being able
to transmit over long distances.
In direct detection, the laser on the transmitting side converts the electrical to optical On-Off
pulses and sends through the optical medium. The laser pulses are then received by the photodiode on
the receiving end, and converts back into electrical signals. As light pulses go through the fiber,
they get smeared out. Such phenomenon is called dispersion. At higher throughput, dispersion effects
become more problematic because pulses get closer together. As pulses start overlapping to each other,
it then becomes difficult, even impossible for the photodetector to correctly detect each individual
pulse at the received end.
To overcome dispersion effect and increase transmission speed, an alternative approach is needed.
Apart from just using the amplitude or intensity to encode information, phase can be used by changing
the delay between pulses and the polarization, which is the orientation of the signal.
By combining amplitude, phase, and polarization changes, we can encode three bits in the same time span.
A coherent detection technique can be used to received the signal, which uses a local oscillator
to generate a reference signal and to compare with the incoming signal. Mixing the two signals
coherently, the amplitude and phase information per polarization can be reconstructed.
The reconstructed signals are fed into the digital signal processing algorithms to compensate any
transmission impairments such as dispersion.