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.