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 is possible as the mode field diameters are similar in both fibers.

Fibers with a cut off in the 1 nm range can be used for 1.55 nm operation, Couplers combined with a long output fiber of about 7 turns of nominally 50 mm diameter gives a splice-free "in line" output polarizer.

Using couplers with -27dB isolation improved polarization isolations of up to -37 dB are achieved, with the penalty of an extra 0.5 to 0.8 dB loss. Testing of assemblies or devices with such high isolation cannot be done with splices or connectors as they contribute to the cross-coupled components. Free space launching of well-aligned plane-polarized light and high isolation detection optics is essential. Any optical component in the light path must be strain free.

If polarization is more important than low loss, then using a lower cut-off fiber is an effective means of removing cross-coupled components.