Fundamentals of Fiber Optics Sensing Technology

Fiber optics sensors basically consist of an optical source that is coupled to an optical transmission line that directs the radiation to a sensor head as illustrated below:

The light is then returned after being modified in some way by the sensor interaction, via the optical fiber, in either a reflective or transmission mode, to a photo-detector and subsequent electronic processing system.  

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.

The sensor head can be either a ‘point sensor’, making the measurement in a localized region in space or it may be a ‘distributed sensor’ which has the ability to make measurements along the optical fiber.

Additionally, the sensor head may also be either an ‘extrinsic sensor’ to the fiber and consist of bulk optical components configured into a sensing mechanism, or it may be an ‘intrinsic sensor’ to the fiber where the measurement process takes place within the fiber optics medium. In the former case, the optical fiber forms the function of transmitting the light to and from the sensor head, whilst in the latter it actively takes part in the sensing process where the inherent optical properties of the fiber are utilized.

Another important feature about optical fibers is that they can either be single mode or multimode, the choice of which depends largely on the application and the measurement technique being utilized. Generally, single mode fibers are used for intrinsic fiber optics sensors such as interferometric methods whereas multimode fibers tend to be used in extrinsic sensor systems where they transmit the light to and from the sensor head. Nonetheless, there are notable exceptions to this generalization, such as micro-bend systems.

You may also read:

• Fiber Optic Temperature Sensors: Principle of Operation & Applications
• Power Measurement in Fiber Optics, How it is Done
• Features of Optical Modulators in Telecommunication Systems
• Optical Time Domain Reflectometer (OTDR)
• The Principle of Operation of Optical Encoders
• Types of Sensors used in Measurement and Process Control