Brillouin optical fiber sensor for cryogenic thermometry a Luc Thévenaz, a Alexandre Fellay, a Massimo Facchini, b Walter Scandale, c Marc Niklès, a Philippe A. Robert a EPFL Swiss Federal Institute of Technology, TOP-MET Laboratory of Metrology and Photonics, CH-1015 Lausanne, Switzerland b CERN, LHC/MMS Division, CH-1211 Geneva 23, Switzerland c Omnisens SA, EPFL-PSE, CH-1015 Lausanne, Switzerland ABSTRACT Supraconductive installations are now commonly used in large facilities, such as power plants and particle accelerators. This requires a permanent temperature control at very low temperature, but cryogenic temperature measurements in the 1-77K range requires expensive calibrated temperature probes. We report here the possibility to use stimulated Brillouin scattering in optical fibres for temperature sensing down to 1K. Such a technique offers the additional advantage to make possible distributed measurement, so that very large structures and systems can be controlled using a single fiber and a single analysing instrument. In addition only one by- pass for the fiber is required as input to the cryogenic vessel, that is definitely a key advantage for the design and the energy loss. Brillouin scattering in optical fibers has never been investigated so far at temperature below 77K (nitrogen boiling point). This absence of interest probably results from the constant decrease of scattering efficiency that was observed while cooling the fiber down to 77K. Our measurements show the unexpected feature that scattering efficiency is significantly raised below 50K and is even much better than observed at room temperature. The relevance and the feasibility of the technique is demonstrated in real scale on the supraconductive magnets for the future world largest particle accelerator, namely the large hadron collider (LHC) at CERN Laboratory in Geneva. Keywords: optical fiber, cryogenic temperature, distributed sensor, Brillouin scattering. 1. INTRODUCTION In the last twelve years, many papers dealing with Brillouin spectrum analysis for distributed strain or temperature sensing in optical fibres have been published. Different operational set-ups have been proposed so far, some of them being even commercially available. As far as temperature sensing is concerned, all these set-ups rely on the assumption of a linear dependence of the measured Brillouin frequency shift on the temperature. This assumption – though perfectly valid in the usual room temperature range – no longer holds in the domain discussed here. Basically the Brillouin effect is a scattering of an incident lightwave by the acoustic phonons of a medium, the spectrum of the scattered light containing key information about the vibrational properties of the considered medium. Practically the centre of the Brillouin spectrum is directly proportional to the velocity of a vibration mode, while its linewidth is related to its characteristic damping time. In the quasi-unidimensional geometry of single-mode optical fibres [1], the Brillouin spectrum of the backscattered light is by far dominated by the resonance peak corresponding to the fundamental longitudinal acoustic mode. As mentioned above, in a fairly wide temperature range (-25°C - 80°C) covering most of the usual applications, the Brillouin shift, that is the frequency difference between the incident and the scattered lightwaves, increases linearly, with a slope coefficient around 1.36 MHz/°C at 1319 nm [2]. Over the same temperature range the resonance linewidth steadily decreases. As far as we know the low temperature domain has been much less studied by the fibre optics community. Nevertheless, there are some large cryogenic installations where distributed temperature monitoring, and thus the use of Brillouin effect in fibres, can be a valuable alternative. A particle accelerator, like the large hadron collider (LHC) under Smart Structures and Materials 2002: Smart Sensor Technology and Measurement Systems Daniele Inaudi, Eric Udd, Editors, Proceedings of SPIE Vol. 4694 (2002) © 2002 SPIE · 0277-786X/02/$15.00 22 Downloaded from SPIE Digital Library on 22 Oct 2010 to 46.127.159.75. Terms of Use: http://spiedl.org/terms