Fiber Optic Sensors

Properties, which can be sensed:

Displacement
Force
Liquid Level
Magnetic Fields
Motion
Temperature
Vibration

Position (linear, angular)
Pressure (fluid, gas, etc.)
Sound
Radiation
Speed
Weight
Pressure

How does a Fiber Optic Sensor work?

In a fiber optic sensor, one of the following characteristics of a propagating
lightwave is altered under an externally induced physical parameter:

Intensity
of light

Frequency
of light

Phase
of light

Polarization state
of light

Why are fiber optic sensors becoming so popular?

Small/lightweight
Allow access into normally inaccessible areas (often embedded)
Passive (non-electrical)
Highly sensitive
Easy to install

The Fabry-Perot fiber optic sensor.

Fabry-Perot Filter (interferometer)

The Fabry-Perot fiber optic sensor:

Principal applications:
Pressure, Strain, Temperature sensing etc.


Design of Fabry-Perot strain sensor:

Fiso, Inc. sensor: see www.fiso.com

Fiber optic Bragg grating temperature and strain sensors

Schematic diagram of fiber Bragg grating

Principle of operation of fiber Bragg grating:

Schematic of instrumentation for fiber optic Bragg

sensor.

Fiber optic Bragg sensors advantages:

High sensitivity
Low –cost sensors
Small/lightweight
Linear response

Drawbacks

Sensitive to more than one parameter
Require relatively expensive processing equipment

Fiber Optic Sensing by
Optical Time Domain Reflectometry (OTDR)

The Optical Time-Domain Reflectometer enables us to look at a fiber link
from inside the fiber. In reality it is just a radar system for looking at fiber.
High intensity pulses are sent into the fiber from a specialised laser and
when the pulse returns its strength is displayed on an oscilloscope screen in
the form of a trace. A schematic of such a display is shown in Figure

In the trace you see reflections coming from all along the fiber itself. This
is the result of Rayleigh scattering. Rayleigh scattering was mentioned in
the chapter on optical fiber as the major limiting factor in fiber attenuation.
This scattering occurs backwards towards the transmitter and we can
receive it and display the result.

Fiber optic sensing by OTDR.

Optical Time-Domain Reflectometer- Based
Strain Monitoring System

Fiber optics is emerging as a mainstream sensor technology capable of
measuring numerous physical parameters;



New low-cost sources, detectors, and processing hardware and industry
maturity are making FO sensors more cost effective.