Distributed and spatially averaged fiber optic temperature sensors and method using same

A device for sensing temperature changes at a plurality of locations, comprising

• (a) a light-transmitting optical fiber system along a path including said locations, said optical fiber system having a first end and a second end and a sensing element at each of said locations, each sensing element being so characterized that, when exposed to the temperature changes to be sensed and transmitting interrogating light of suitable wavelength or wavelengths .lambda..sub.v and an intensity P.sub.o, it converts a fraction .alpha.P.sub.o of the intensity of said interrogating light into a light separable from the interrogating light, at least part of the intensity of which is emitted from at least one of said two ends of the probe at wavelengths .lambda..sub.f different from .lambda..sub.v, where .alpha. is a temperature-dependent fraction smaller than unity, the value of which increases with increasing temperature within a temperature range, the intensity of said light of wavelengths .lambda..sub.f being substantially proportional to the value of .alpha.;
• (b) a source of said interrogating light of wavelength or wavelengths .lambda..sub.v ;
• (c) optical means for selectively directing a fraction of the intensity of said emitted light of wavelengths .lambda..sub.f to photodetector means;
• and
• (d) photodetector means for measuring the intensity of said pulses of optical radiation of wavelengths .lambda..sub.f.

An optical time domain reflectometry arrangement for sensing physical variables at a plurality of sensing points, including

• (a) an optical fiber having a first end and a second end and a glass core having a cross-sectional area perpendicular to the fiber length smaller than about 10.sup.-5 cm.sup.2 and comprised of laser material having an index of refraction n.sub.b, said core being disposed within a first cladding having an index of refraction n.sub.a lower than n.sub.b, said first cladding being surrounded by a second cladding having an index of refraction n.sub.c lower than n.sub.b, with substantially no space between said first cladding and said second cladding,
• wherein the cross-sectional area of said first cladding taken substantially transverse to the axial extent of the optical fiber has a first path across said area which passes substantially through the geometric center thereof whose length is substantially different from the length of a second path across said area which passes substantially through the geometric center thereof, said second path being substantially perpendicular to said first path, the length of said first path being not more than a few micrometers longer than the dimension of the core along a path parallel to said first path, the length of said second path being at least several times longer than the length of said first path,
• said fiber being disposed along a length including said plurality of sensing points and being additionally so characterized that, when optical pump radiation of suitable intensity and wavelengths .lambda..sub.s is injected into the cross-sectional area of said first cladding at said first end of the fiber and propagates along the length of the fiber, most of the energy of said pump radiation is absorbed by the laser material of said core at said sensing points, thereby generating a population inversion and luminescence radiation having wavelengths .lambda..sub.f different from the wavelengths of said pump radiation, the magnitude of the population inversion at each sensing point being a know function of the magnitude of the physical parameter;
• the arrangement additionally including
• (b) means for generating and launching said optical pump radiation into the cross-sectional area of said first cladding at said one end of the fiber, said pump radiation being in the form of pulses of submicrosecond duration and an intensity sufficiently high to generate a population inversion within the fiber core at said sensing points,
• (c) means for injecting radiation of said wavelengths .lambda..sub.f into said fiber core at said second fiber end, said radiation of said wavelengths .lambda..sub.f being pulse-amplified at said sensing points as it propagates along the fiber in the direction opposite to that of the pump radiation, the magnitude of the amplification at a sensing point being a known function of the magnitude of the population inversion and, hence, of the magnitude of the physical parameter at that point;
• (d) fiber optic means for directing a fraction of the intensity of said converted radiation of wavelengths .lambda..sub.f amplified at said sensing points along the fiber to photodetector means;
• and
• (e) photodetector means for measuring the intensities of said amplified radiation of wavelengths .lambda..sub.f at said sensing points.