Polarization and Polarization Controllers - NTNU

5 Applications
Applications of polarization controllers in optical networks range from compensation of polarization
related effects that are detrimental to system performance, to useful exploitation of the polarization
phenomenon. Three cases are presented briefly: polarization optimization, PMD compensation and
polarization demultiplexing.
All cases involve dynamic (also called automatic) polarization control. Since the SOP changes
randomly with time in installed fiber cables, one needs an automatic control system that monitors the
system and continuously adjusts the controllers to ensure that the system performs optimally.
5.1 Polarization optimization
Figure 9: Optimizing polarization state of light entering a device with PDL (from [3]).
Tx: transmitter (laser), DPC: dynamic polarization controller, FBC:
feedback circuit.
Many components in optical communication networks, e.g. wavelength multiplexers, suffer from
polarization dependent loss (PDL). This means that the component absorbs different amounts of light
for different input states of polarization. Fig. 9 shows a control system that compensates this
polarization sensitivity by measuring the optical power and adjusting the input SOP so that output
power is always maximized and constant.
Another optimization example is optical wavelength conversion, in which the efficiency of the
converter depends on the input SOP.
5.2 Compensation of Polarization-Mode Dispersion
Figure 10: Compensation of PMD (from [3]). Tx: transmitter (laser), DPC: dynamic
polarization controller, FBC: feedback circuit, DGD: differential group
delay).
Polarization-mode dispersion (PMD) is one of the main obstacles to overcome when migrating from
10 Gbps per wavelength channel to 40 Gbps channels. The optical field of a data signal is a
superposition of two orthogonal polarizations that travel at slightly different speeds, thus causing pulse
spreading and interference between neighboring bits. This spread is denoted differential group delay
(DGD). An example of PMD compensation is shown in Fig. 10, in which the pulse spreading is
cancelled by delaying the fast polarization.
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