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A demountable cryogenic

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A demountable cryogenic feedthrough for plastic optical fibersJ. S. Butterworth,a)C. R. Brome, P. R. Huffman, C. E. H. Mattoni,b)D. N. McKinsey,and J. M. DoyleDepartment 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 Kynarprovides a removable seal around a plastic optical fiber. The seal preserves the continuity of the fiberand is reliable after repeated thermal cycles. © 1998 American Institute of Physics.@S0034-6748~98!00210-X#A variety of experiments rely on the room temperaturedetection of light produced inside a low temperature appara-tus. Optical fibers offer a convenient method for transportinglight from a cryogenic region to room temperature. Crucialto the success of such a technique is the development ofreliable feedthroughs. One such feedthrough which we findto work well at room temperature is described in theliterature.1In the case of glass optical fibers, cryogenicfeedthrough techniques using solder exist.2We have developed a fiber-based detection system for anexperiment to demonstrate the magnetic trapping of ultracoldneutrons in a 100 mK4He bath.3As part of that system, ademountable, superfluid-helium-tight vacuum feedthroughfor multiclad polystyrene optical fibers was necessary.4We began our development effort by trying an epoxy-based feedthrough. A common design for cryogenic electri-cal feedthroughs uses epoxy to form a seal between the wiresand the vacuum can through which they pass.5Cryogenicfiber feedthroughs of this type using Stycast 12666weretested but did not have long-term reliability. We have notfound a satisfactory alternative epoxy. Such an epoxy musthave a thermal contraction well matched to that of the fiberand must adhere well to the fluoropolymer cladding. Epoxy-sealed feedthroughs are also less desirable because once as-sembled they cannot subsequently be dismantled.We have developed a feedthrough that relies on the ther-mal contraction of Kynar @polyvinylidenefluoride ~PVDF!#to ensure a reliable seal to both the optical fiber and thevacuum can. We expect that fluoropolymers other than Ky-nar could be substituted.7The design of the feedthrough isshown in Fig. 1. Heat shrinkable Kynar tubing ~2 mm initialinner diameter!8is used to seal the optical fiber to the stain-less steel tube, which is brazed to the vacuum can. The metaltube ~1.6 mm outer diameter, 0.2 mm wall! is chosen suchthat the 0.8 mm fiber easily passes through.The Kynar sleeve is formed prior to making the seal.This avoids heating the polystyrene fiber which has a lowsoftening temperature. A copper wire is drawn to a diameterapproximately 5–10mm larger than that of the fiber bystretching a slightly oversized wire by hand. A section of thewire with the desired diameter is chosen and inserted throughthe stainless steel tube, protruding at least a few centimetersout of both sides. A thin layer of Dow Corning vacuumgrease9is applied to both the wire and the tube to facilitateremoval of the Kynar after heating. A 25 mm length of Ky-nar tubing is slid over the assembly until it equally covers thetube and the wire. The Kynar sleeve is formed by heating itwith hot air ~and therefore shrinking it onto the wire!. Thewire and tube are then removed.Final assembly now takes place. A small amount ofvacuum grease is applied to both the fiber and the stainlesstube. The Kynar is slid along the fiber, while continuouslyrotating the former to avoid binding. The exact position ofthe fiber can be adjusted before sliding the Kynar onto themetal tube to complete the feedthrough.The feedthroughs are found to work reliably after re-peated thermal cycling from 100 mK to 300 K. They havebeen disassembled and reused several times with no deterio-ration of performance, provided that prior to each reassemblyvacuum grease is applied to both the metal tube and fiber.Although the feedthrough was originally designed to over-a!Present address: Institut Laue—Langevin, B.P. 156, 38042 Grenoble,France.b!Electronic mail: [email protected] FIG. 1. Vacuum feedthrough assembly.REVIEW OF SCIENTIFIC INSTRUMENTS VOLUME 69, NUMBER 10 OCTOBER 199836970034-6748/98/69(10)/3697/2/$15.00 © 1998 American Institute of Physicscome problems associated with the large thermal contractionof polystyrene fibers, it is expected to work equally well withother kinds of optical fibers, metal wires, and coaxial leads.This work was supported in part by the National ScienceFoundation under Grant No. PHY-9424278.1E. R. I. Abraham and E. A. Cornell, Appl. Opt. 37, 1762 ~1998!.2G. A. Williams and R. E. Packard, Rev. Sci. Instrum. 45, 1029 ~1974!.3J. M. Doyle and S. K. Lamoreaux, Europhys. Lett. 26, 253 ~1994!.4S-type Y-11M fibers supplied by Kuraray International Corporation, NewYork, NY.5F. Mathu and H. C. Meijer, Cryogenics 22, 428 ~1982!.6Stycast 1266 manufactured by Emerson and Cuming, Woburn, MA.7One feedthrough using Teflon heat shrink instead of Kynar was made andtested. Teflon heat shrink, McMaster Carr Supply Company, New Brun-swick, NJ.8Voltrex brand KYS-004 manufactured by SPC Technology, Chicago, IL.9Dow Corning Corp., Midland, MI.3698 Rev. Sci. Instrum., Vol. 69, No. 10, October 1998 Butterworthet


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