3 Advanced aspects of optical modulations in achieving high bit rates and immunity to signal degradation
3.1 Polarization multiplexing in QPSK for terabit transmission

The idea of multiplexing modulation states based on polarization

Polarization Division Multiplexing (PDM) has been widely denoted either by polarization multiplexing, polarization division multiplexing, dual polarization or orthogonal polarization. PDM-QPSK is primarily designed for 100 Gbps channel systems.

PDM-QPSK combined with coherent detection (demodulation). In case of coherent demodulation carrier used for demodulation purpose is in phase and frequency synchronism with carrier used for modulation purpose. For non-coherent demodulation it is not in synchronism. Coherent light is a light in which the electromagnetic waves maintain a fixed and predictable phase relationship with each other over a period of time.

Main benefits

Advantages:

  • spectral efficiency as it can halve the symbol rate
  • PDM-QPSK has been the main candidate for 100 Gbps transponders, due to its high tolerance against signal distortions
  • PDM-QPSK is better than DPQSK at the cost of implementation complexity
  • it can also be efficient at 100 Gbps with 50 GHz spacing, where it can enable hundreds of km by using appropriate dispersion compensation techniques, inline amplifiers, etc.

Disadvantages

Disadvantages and limits:

  • cost of implementation complexity
  • need for higher power consumption
  • faster digital signal processing circuits
  • analogue to digital converters

Transceivers

Transceiver’s construction:

  • In a PDM-QPSK transmitter we have to deal with four signal components for the in-phase respectively quadrature signals and for the two polarizations.
  • Similarly as in previous formats, the binary to electrical signal conversion is performed,
  • Low-pass Bessel filters are used again to consider the impact of a non-ideal conversion.
  • The four electrical signals are then launched to two QPSK modulators, two per each.
  • The output signal of one of the modulators travels through a polarization rotator and then it is combined with the output from the second QPSK modulator to obtain a single PDM-QPSK modulated signal.
  • The PDM-QPSK receiver includes many components such as a single ended 90° hybrid with local oscillator and four PIN photodiodes to enable the coherent detection; trans-impedance amplifiers, electrical filters, electronic dispersion compensator and in the end a memoryless blind receiver to separate the in-phase and quadrature components and orthogonal polarizations.
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40 Gbps transmission with 100 GHz channel spacing over sample 12-km long SMF. Notice the best performance of PDM-QPSK format [2].
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40 Gbps transmission with 50 GHz channel spacing over sample 12-km long SMF. Notice the best performance of PDM-QPSK format [2].