A single mode in-line blocking filter using fiber lenses and a Rotary Splice.

An optical cutoff filter with single mode fiber pigtails is demonstrated for possible use as a blocking filter in a WDM upgrade to the FT Series G System. The blocking filter uses graded-index multimode fibers as collimating lenses between the single mode fiber ends. These fiber-lenses are fused to single mode fibers and inserted into rotary splice ferrules. A multilayer dielectric interference filter is cemented to one ferrule and the assembly is inserted into a rotary splice alignment sleeve. A second ferrule with a matching fiber-lens is inserted into the opposite end of the alignment sleeve. Rotating the splice optimizes the coupling. The insertion loss of the fiber-lens/rotary splice filter measured 0.8dB. The short-wave rejection is > 30dB. The performance of the filter was found to be stable over the measured temperature range of 0C to 65C.