Polarization Isolation and Cross Coupling
Cross coupling is defined as the coupling of light from one axis to the orthogonal axis and can be expressed as a ratio, either a percentage or in dB (0.01 = 1% =-20dB) It occurs where polarization maintaining fiber has been stressed, modified or processed to form a device such as an end termination. Connector key misalignment to an axis is another cause of cross coupling as plane polarized light would be launched on both axes. The region over which the fiber is supported in any device can result in residual stress-induced birefringence which causes a small amount of cross coupling between the fast and slow axes. In an evanescent wave coupler, made with two fibers, any misalignment of the polarization axes will also contribute to cross coupling.
Usually, couplers are used with light launched on the slow axis. At each successive device in a chain the cross coupled components that are resolved along the fast axis are summed with respect to phase and amplitude to give resultants. The square of the resultant is the intensity. A set of components in series will therefore have a resultant state of cross coupling at each device that is dependent on the phase difference between the fast and slow axis and the magnitudes of cross coupling for each preceding device, including the input state of polarization.
As intensity is the square of amplitude and in coherent light, superposition of waves is performed with respect to amplitude and phase, the final intensity variations can be unexpectedly high, when all the contributing cross coupling components are in phase and add. This is an unusual condition and arrays kept at a nominally constant temperature usually display low overall cross coupling and low modulation effects. Temperature control of the individual paths connecting components allows a minimization of cross coupling to very low levels.
Wavelength dependency of coherent sources appears as a similar effect to phase dependence.
In incoherent light the output is simply the sum of the intensities of the individual cross couplings.