Cascaded PM Components

The effects of long path lengths of PM fiber in arrays or trees of PM components and the importance of high ER ( low cross coupling ) components

In general, when light from a source is divided and propagated along two separate paths, then recombined, optical interference takes place and the well known interference fringes are observed. The degree of interference depends upon the state of polarization of the two beams and the coherence length of the source. In the case of fiber optics as the propagation medium, a singlemode can be regarded as analogous to a free space collimated beam.

PM fiber allows the fiber to be orientated to obtain parallel interfering plane polarized states and allows interferometers to be made that are free from signal fading, however if both axes are illuminated, and there is cross coupling, then interference between fast and slow axis modes will occur. The phase difference is path length dependent, and therefore temperature and stress, dependent. Cross coupling, by itself, is not a complete description of the polarization state and a more general polarization representation is described in Stokes, Poincare and the Polarization Ellipse.

In general PM fiber is used to propagate plane polarized light along the slow axis. When we consider polarization effects in long lengths of PM fiber, the phase difference between cross coupled components from any source must be considered. This includes couplers, connectors, splices, polarizers and micro-bends.

Strings, or series of polarization sensitive components, in homogeneous free space do not exhibit the phase sensitive phenomena that occur with birefringent PM fiber. This birefringence property, that maintains plane polarization along the axis of the fiber, is the cause of the cross coupled phase variations. When devices are linked by PM fiber and a narrow band source is used, such that the coherence length is long compared to the fiber path, the interaction of cross coupled components is summed with respect to phase and amplitude at the last device output. This result is the optical interference of a series of coherent components having phase dependence on fiber path length.

Strings or trees of PM components along a PM fiber path are therefore subject to variations in the output state of polarization that are temperature dependent.

Connectors present problems when a -27dB coupler is required to have FC/PC or APC connectors. Connectors will degrade performance unless they have some rotational adjustment with respect to the key to allow optimum axis alignment as well as low stress construction.

Figure 6. shows a 2 x 8 array of splice free PM couplers. The linking fibers are controlled by two Peltier cooling devices on the first fiber links and micro heaters on the four remaining links. This arrangement is very convenient for a thermally controlled package, having adequate thermal gradients and using low power.

The fiber paths linking all 2 x 8 couplers within the array is typical of the path A to B to C, having a cooler followed by a heater. The 8 output paths must share common cooling and heating links, so there are 6 variables to determine 8 output ER values.

In practice to assemble a high ER/Lm device, a computer driven heating and cooling cycling algorithm is used to drive the coolers and heaters adn this gives an indication of the temperatures that will give the best average ER for all 8 channels at a specific wavelength.

The program Cascade V 2.0 shows the output from a string of PM components.

Selection of components with a high extinction ratio -25db to -30dB range is essential for splice free strings or trees with > -20dB performance/channel including input and output connectors.

A fixed temperature will ensure that the output state is fixed, in this case an output analyzer can be used to give plane polarized output and the

component is simply regarded as loss. Alternatively different temperatures can be set internally within a package to optimize all outputs for

and improve performance.