Remote measurement of physical variables with fiber optic systems

A continuous optical fiber adapted to sense temperature changes at different points along its length, the fiber comprising a glass core A having an index of refraction n.sub.1, a first cladding B around and in contact with said core and having an index of refraction n.sub.2 lower than n.sub.1, the temperature coefficient of n.sub.2 being substantially different from the temperature coefficient of n.sub.1, and a second cladding C around and in contact with cladding B, said second cladding having an index of refraction n.sub.c lower than n.sub.2, the temperature coefficients of n.sub.2 and n.sub.c being substantially similar,

• the fiber being so characterized that, when interrogated with light pulses of submicrosecond duration and of suitable wavelength or wavelengths launched into core A and cladding B at the fiber launch end, the intensity of each interrogating light pulse propagating at any temperature sensing point along the fiber is distributed between said core A and said cladding B, the relative distribution at that point varying as a function of the fiber temperature at said point and being substantially independent of the fiber temperature at other points, the fiber being adapted to automatically process said relative distribution into pulses of backscattered light generated at the fiber core the intensity of which is an indicator of the fiber temperature at said sensing point, said pulses of backscattered light arriving at said fiber end separated in the time domain from optical signals generated at other temperature sensing points along the fiber.