Product Data 954P-HI
HIGH ISOLATION COUPLERS
Polarization preserving fibers used at wavelengths well above cut off tend to show differential bend loss in the TE and TM modes (fast and slow axes). A fiber coil of a few turns can give as much as 10 dB differential loss with 0.5 dB or less loss in the slow axis. This effect can be used to make an in line polarizer on the input/output ports of a coupler. The penalty is launching into a smaller core fiber with lower guidance and increased bend loss sensitivity. Splicing to larger core fibers also requires more effort.
Fibers with a cut off in the 1 nm range can be used for 1.55 nm operation, 954P-P WDM Pump Couplers are an example. Combined with a long output fiber of about 7 turns in a nominally 50 mm diameter coil, the result is a splice free in line output polarizer with low loss.
Polarization isolations of -28 dB are typical with some less than -30dB with selected fiber.
If polarization isolation is more important than low loss then using a lower cut off fiber is a very effective means of removing cross coupled components.
This is especially important in arrays of PM couplers where modulation due to interference of cross coupled components is a major problem.