A novel fiber optic sensor using 'BOF' has been developed by vapor deposition of dielectric BPF on an end face of a
single-mode fiber, as BPF on fiber-end (BOF), of which the reflection spectrum varies according to its temperature and
pressure. The spectrum shift is obtainable as the ratio of reflectance at both sides of the BOF center wavelength, which is
derived from the dual wavelength impulse responses of the sensor network obtained on the PN code correlation scheme. It
is called dual wavelength push-pull ratiometric reflectometry (DWPR). The resulting multipoint temperature and pressure
sensing operation will be demonstrated.
A new Time Division Multiplexing (TDM) optical fiber interrogation system for distributed Fiber Bragg Gratings (FBGs),
designated as Delayed Transmission/Reflection Ratiometric Reflectometry (DT3R), is presented. The Bragg wavelength shift of FBG,
which varies according to temperature or strain, is estimated as the ratio of transmittance and reflectance of a slope filter, to which the
reflected light from the FBGs is input. To obtain both simultaneously using a single detector, a definite propagation delay is given to
either transmission or reflection path, both of which are added to each other and detected using a common photodiode. The reflection
impulse response of the FBGs is analyzed using OTDR based on Pseudo-random Noise code (PN) correlation scheme. The hardware
structure of an interrogator is simple, resulting in low cost. The FBG topology is a "bus": multiple FBGs branch from the through line
started from the interrogator. Each FBG is distinguished by its time of flight between the interrogator and the FBG. Because of these
schemes, the FBGs can all be identical. A breakdown of any FBGs has no effect on the other sensors; moreover, "hot swapping" is
possible. Experimental evaluation results are presented herein.
This experimental temperature sensing system uses dual wavelength push-pull reflectometry and a thin-film band pass
filter deposited on an optical fiber end face. The system presents advantages over fiber Bragg grating sensors: it can use
the mature optical time domain reflectometry (OTDR) technology instead of expensive wavelength-selective technology;
it can probe the temperature in a small spot area; and it can be free from influences of disturbances along the optical fiber
or within the measuring system. Moreover, it preserves merits of optical fibers such as low transmission loss and
immunity to electromagnetic noise. The presented system has measurement accuracy of better than ±0.5°C.