Splice free, cascaded assemblies, of polarization maintaining components, having very low extinction ratio and low loss, give superior performance to spliced components.   


Fiber optic signal paths that include splices, connectors, PM couplers, and input - output alignment devices, generally show serious problems of poor Extinction Ratio, with high loss and large signal intensity variations with respect to temperature. The latter is due to the temperature dependence of the birefringence of the fiber, changing the fast and slow axes refractive index difference. This causes cross coupled components to vary in phase along the fiber path.

The above problems are minimized by EOI “Splice Free” assembly methods, where coupler halves are manufactured on a continuous length of fiber. This allows strings of coupler halves to be assembled in various configurations. The use of the highest ER couplers (typically -24 to -27 dB), the lowest loss couplers (< 0.1dB) and short fiber connecting links gives a compact package. With a solid heat sinking base, temperature fluctuations are minimized and thermally controlled versions have been made for the most critical applications. 

Long coherence sources cause the addition of cross coupled components with respect to amplitude and phase and give surprisingly high ER values. (Linear addition of the ER of separate components is the case for low coherence sources).

The program "CASCADE"  is a model for analysis of cascaded strings of PM couplers and other polarization sensitive components.  It allows entry of any number of components, fiber link lengths and a range over which to vary the temperature of those links.  Output includes the Extinction Ratio defined as 10 log fast/slow, the phase difference of the fast and slow axis light and the state of polarization. Note that when the phase difference is zero, the polarization state is plane. This shows as equivalent ER spiking off the graph to infinity, but this is not aligned with the slow axis. 

Performance Example 

A very simple but informative example is two couplers at -20dB ER, i.e. 0.01 or 1% intensity cross coupling. 0.01 intensity is an amplitude of 0.1

 If the components are in the same phase, they are summed to 0.2 amplitude and squared to 0.04 intensity.

If pi out of phase this gives zero intensity, resulting in a 4% modulation of both fast and slow axis intensity with a temperature change of about 2 deg per meter of fiber or by an equivalent mechanical strain.


Array Examples

A 1 x 8 or 1X6 splitter composed of discrete couplers has (in-line) 3 couplers and 2 splices. This gives 5 cross coupled contributions controlled by 4 fiber links. Using typical values averaging -20dB per component the model Cascade demonstrates why a “Splice Free” assembly and three -25dB couplers are needed for performance substantially in the -20dB range.

The spliced case often results in -10dB.  

The performance of an array or cascaded assembly is a statistical result, determined by the tolerances of all contributing components.  Removing splices reduces that number, using very high ER couplers (-25dB to -27dB) shifts the average to a higher ER. Use of FC type connectors tends to degrade the ER as these devices have some internal stress as well as an alignment tolerance of the PM axes to the key.

Individual couplers made by EOI have a typical ratio tolerance at 50/50 of +/-3% The loss is nominally 2.5% for wavelengths of <1000nm and ER values are both fiber and wavelength dependent and are typically -22dB to -30dB

The problem in construction of cascaded assemblies is that couplers cannot be selected; they are whatever the statistical production gives.  We therefore state a mean coupler performance which can have a large variation for a particular array.

The inclusion of “fiber in line polarizers” after each coupler attenuates the cross coupled component by -10 to -15 dB, and makes a significant improvement to performance in the case of -20dB couplers, however, these devices have some cross coupling due to residual stress in manufacture that can be in the -27dB to -30 dB range and are less useful for high ER coupler arrays.

The advantage of in-line polarizers in an array of -25 dB couplers is where there might be one or two lower ER devices at -22dB.  There is therefore a price/performance/delivery time consideration for inclusion of in-line polarizers.