Fiber Optic Sensors
The following is a very general and simplified overview to illustrate
the range and complexity of sensors.
Fiber optic sensors can be classified generally
as multimode and singlemode in operation. Extrinsic sensors are those
which use fiber to supply light to a sensing device and return signal
light to a detection system, intrinsic sensors use the fiber itself as
the transducer.
Multimode devices can depend on amplitude, spectral and mode to mode
conversion as the measurement method.
Amplitude effects can be demonstrated by transmission from fiber end to
fiber end where longitudinal or transverse displacement affect transmission.
Reflection from a surface using a fiber end(s)
as the transmission and receiving point gives amplitude dependence as
well as dependency on other surface characteristics.
Mode to mode energy conversion can occur by stressing a multimode fiber,
such that the refractive index profile is disturbed allowing mode to
mode coupling.
Singlemode devices
Bend loss can be used as a means of measurement or a means of
attenuation control. Similar amplitude effects to those of multimode
fiber can be used with singlemode fiber where displacement sensitivity
is increased at lower light levels.
Bragg Gratings written into the fiber core can be made to form very
narrow band reflectors. Changes in grating period due to strain allow
strain gauges to be fabricated along a continuous fiber as a distributed
sensing system.
Interferometric devices can be made as intrinsic sensors where the
optical path or phase difference in an interferometer is dependent on an
external physical effect. The fiber optic gyro is probably the most
important example where the rotation of a Sagnac Loop results in a phase
difference. All the traditional interferometers such as Michelson, Mach
Zehnder, Fizeau, Sagnac and Fabry Perot can be made in fiber as well as
the Resonant ring.
Optical properties of the fiber such as Faraday effect allow sensitivity
to magnetic fields using polarization rotation.
Polarization preserving fiber is used extensively in interferometric
arrangements to eliminate the effects of field rotation and polarization
state changes that occur in singlemode fiber.
Electronic, optical and mechanical sensors are abundant, if a fiber
optic sensor is to be cost effective it must have a particular advantage
over competing technologies. High noise immunity in the presence of
strong electric fields is an advantage of photons in a waveguide vs.
electrons in a conductor. In optical terms the singlemode fiber is the
equivalent of a perfectly collimated beam, but it is very narrow and can
bend to follow any path.
The Extrinsic Fizeau Interferometer
A non contact displacement sensor, the PD-1000,
based on this concept, allows accuracy of nanometers over a large dynamic range of several
millimeters. A fiber tip is used as the first reflector and the target as the second. The tip
receives the reflected/scattered light from the target and creates the interference signal
which is transmitted to the detection system which is made up of a fringe tracking servo
and a fringe counter. The servo allows interpolation of the displacement within a fringe to
less than 1/100 fringe.
The sensor can be configured for derivatives of displacement such as
acoustic, vibration and pressure sensing where deflection of a diaphragm
is the transducing means. Surface profiling uses an X,Y translation
stage to give a raster scan and three dimensional displays.
A good example of this application is the measurement of the ablated
PMMA plastic calibration sample from the Excimer laser used for
simulated PRK in ophthalmic surgery.
