Remote sensing of physical variables with fiber optic systems

DETAILED DESCRIPTION These and other objects are accomplished by using so-called intensity sensors in a new manner whereby, in a preferred embodiment of the invention, interrogating light is launched into the core of an optical fiber probe for a measurand, wherein a fraction .
alpha.
of its initial intensity is removed from the core, the value of .
alpha.
being an indicator of the value of the measurand.
Instead of discarding this removed fraction as in the prior art and estimating its value indirectly from the light intensity P.
sub.
o (1-.
alpha.
) transmitted by the probe, the removed light is captured and processed into an optical signal separable from the transmitted interrogating light and from signals from measurands acting simultaneously at any other point(s) on the fiber, but carried by the same fiber to a photodetection station, so that both the intensities of the transmitted interrogating light light beam and that of said removed fraction .
alpha.
, the value of .
alpha.
being an indicator of the value of the physical parameter, can be measured to give a ratiometric (referenced) reading of the value of the measurand, unaffected or only minimally affected by fluctuations of the intensity of the interrogating light, fiber losses or detector drift.
The processing of .
alpha.
into a separable, directly measurable signal, can be effected either in the optical spectral domain, by luminescence or Raman conversion into a light of different wavelengths from those of the interrogating light, or in the time domain, using pulsed or AC-modulated interrogating light and processing said fraction .
alpha.
into a signal which arrives at the photodetector either at a different time or with a phase shift relative to the interrogating light.
An important feature of the invention is that the sensing materials it uses can be homogeneously incorporated into components of long optical fibers, allowing the measurement of temperature and/or forces distributed over many locations, simultaneously and with a single fiber probe