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REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 69 NUMBER 10 OCTOBER 1998 A demountable cryogenic feedthrough for plastic optical fibers J S Butterworth a C R Brome P R Huffman C E H Mattoni b D N McKinsey and J M Doyle Department of Physics Harvard University Cambridge Massachusetts 02138 Received 16 June 1998 accepted for publication 8 July 1998 A superfluid helium tight optical fiber feedthrough has been developed Heat shrinkable Kynar provides a removable seal around a plastic optical fiber The seal preserves the continuity of the fiber and is reliable after repeated thermal cycles 1998 American Institute of Physics S0034 6748 98 00210 X wire with the desired diameter is chosen and inserted through the stainless steel tube protruding at least a few centimeters out of both sides A thin layer of Dow Corning vacuum grease9 is applied to both the wire and the tube to facilitate removal of the Kynar after heating A 25 mm length of Kynar tubing is slid over the assembly until it equally covers the tube and the wire The Kynar sleeve is formed by heating it with hot air and therefore shrinking it onto the wire The wire and tube are then removed Final assembly now takes place A small amount of vacuum grease is applied to both the fiber and the stainless tube The Kynar is slid along the fiber while continuously rotating the former to avoid binding The exact position of the fiber can be adjusted before sliding the Kynar onto the metal tube to complete the feedthrough The feedthroughs are found to work reliably after repeated thermal cycling from 100 mK to 300 K They have been disassembled and reused several times with no deterioration of performance provided that prior to each reassembly vacuum grease is applied to both the metal tube and fiber Although the feedthrough was originally designed to over A variety of experiments rely on the room temperature detection of light produced inside a low temperature apparatus Optical fibers offer a convenient method for transporting light

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