Fiber Optic Feedthrough Design For Use In Cryogenic Dilution Refrigeration Systems

Abstract

Bose-Einstein condensation is a fundamental state of dilute gases of bosons. It was originally predicted in 1924 by S. Bose and A. Einstein, and it refers to a quantum configuration at low temperatures in which a large portion (the condensate fraction) of particles collapse into the ground state. As can be seen in Figure 1, Rb-87 has been shown to exhibit the properties of Bose-Einstein condensation at low temperatures. Our experiment focuses on studying superfluid Helium-4. A superfluid is a phase of matter with zero viscosity, infinite conductivity, and other unusual properties. It is generally accepted that the superfluid properties of supercooled Helium-4 is caused by the composite boson exhibiting behavior associated with Bose-Einstein condensation, but it has not yet been conclusively proven. The goal of our experiment is to analyze the transmission characteristics of a slab of Helium-4 superfluid. These transmission characteristics could hopefully be used to offer some evidence of Bose-Einstein condensation in superfluid Helium-4. To accomplish this task, we used a dilution refrigeration system to cool our experimental cell down to extremely low temperatures. Inside the cell, we use a laser pulse in a fiber optic cable to produce a pulse of atoms at the bottom of the slab of Helium-4. These atoms are then transmitted through the slab and shot out the other side. The transmitted atoms are then detected on a series of superconducting bolometers. With bolometers, we are able to accurately and quickly determine energy levels of the transmitted atoms when they strike the surface of each bolometer. The purpose of my research focused on the design of our internal vacuum can and cell. The aspect of the design that needed modifying was the optical fiber feed through, which can be seen in Figure 3. These feedthroughs are essential in transferring laser pulses from an external source into the experimental cell. The feedthroughs must be leak proof in order to allow the dilution refrigeration system to run smoothly as well as to maintain accuracy within the experimental cell.