Abstract: Because the thermal expansion coefficient ("TEC") of glass is small-to-negligible at low temperatures, traditional fiber Bragg grating ("FBG") sensors and long period grating ("LPG") sensors are not adequately sensitive to accurately measure cryogenic temperature changes. However, as demonstrated herein, the integration of a coating around an optical fiber (the coating having a TEC that is greater than that of the fiber alone) can allow the induction of additional thermal strain in the fiber. In this light, a new sensor for cryogenic temperature measurement has been invented that mechanically amplifies temperature transduction within fiber grating sensors. The invention has the benefit of being useful for measuring non-cryogenic temperature changes as well. The thickness of the coating can be optimized.

Abstract: A transducer has been invented that uses specially-oriented gratings in waveguide a manner that allows the simultaneous measurement of physical phenomena (such as shear force, strain and temperature) in a single sensing element. The invention has a highly sensitive, linear response and also has directional sensitivity with regard to strain. The transducer has a waveguide with a longitudinal axis as well as two Bragg gratings. The transducer has a first Bragg grating associated with the waveguide that has an angular orientation .theta..sub.a relative to a perpendicular to the longitudinal axis such that 0.degree.<.theta..sub.a <.theta..sub.max. The second Bragg grating is associated with the waveguide in such a way that the angular orientation .theta..sub.b of the grating relative to a perpendicular to the longitudinal axis is (360.degree.-.theta..sub.max)<.theta..sub.b <360.degree.. The first Bragg grating can have a periodicity .LAMBDA..sub.a and the second Bragg grating can have a periodicity .