A fiber optic sensor is a sensor that either uses optical fiber as a sensing element (“intrinsic sensors”) or as a means of transmitting signals from a remote sensor to the electronics that process the signals (“extrinsic sensors”).
Basic construction of Optic sensors:
The optical fiber sensors are an optical source coupled with an optical fiber transmission line that directs the radiation to the sensor head as shown below:
The light is then returned to a photodetector and subsequent electronic processing system, either in a reflective or transmission mode, after being modified in some way by the sensor interaction through the optical fiber.
The signal processing system then detects, demodulates, and analyzes the changes introduced in the optical signal by the sensor head and then relates this to a change in the measurand field of interest.
Type of sensor heads:
Point or Distributed sensor:
The sensor head can be either a point sensor or distributed sensors as shown below a & b respectively.
Point senor makes the measurement in a localized region in space, while distributed head has the ability to make measurements along a length of the optical fibre.
Also, the sensor head can either be an extrinsic sensor to the fiber and consist of large optical components configured into a sensing mechanism, or it can be an intrinsic sensor to the fiber where the measuring process takes place within the optical fiber medium. In the former case, the function of the optical fiber is to transmit the light from and to the sensor head.
Single mode or multimode
The optical fiber can be either single mode or multimode, largely depending on the application and the method of measurement used. Single-mode fiber is generally used for intrinsic optical fiber sensors such as interferometric methods, whereas multimode fibers tend to be used in extrinsic sensors and transmit light from and to the sensor head.
Modulation Parameters:
Fiber optic sensors operate by modulating an optical carrier signal through some optical mechanism present in the sensing region, which is itself responsive to the external measuring parametric field.
There are a limited number of such possible optical properties that can be modulated in an optical sensor system, that are:
- Intensity modulation.
- Wavelength/frequency modulation.
- Temporal modulation.
- Phase modulation.
- Polarization modulation
In optical metrology, all of these optical modulation parameters are well known. In optical fiber sensors, the purpose is to adopt and extend these methods for use with the optical fiber media.