Recent advances in fiber optic technology and optoelectronic devices led to emergence of fiber optic sensors. With the availability of highly sensitive detectors and material loss almost approaching to negligible, it is possible to sense even slight variations in intensity level, phase shift and wavelength from external distresses on the optical fiber itself. This is the fundamental concept of fiber optic sensors. The primary function of a fiber optic sensor is to measure or monitor a physical quantity such as temperature, pressure, corrosion, humidity, and similar environmental factors. The basis of measurement is the net effect on its intensity modulation, operating wavelength, phase angle of the incident optical ray, or polarization of the light propagating through the optical fiber.
Advantages of Fiber Optic Sensors
• Multifunctional sensing capabilities including current, electric field, position, vibration, strain, viscosity, chemicals, and acoustic signals.
• Multiplexing capability to form sensing networks.
• Immune to electromagnetic and radio frequency interference.
• High sensitivity.
• Robust under worst operating conditions.
• Remote sensing capability.
• Inability to conduct electric current.
• Generally cylindrical geometry.
• Small size.
• Easy integration into composite materials and different types of natural or man-made structures.
General Structure of Fiber Optic Sensor
In its simplest form, a fiber optic sensor system comprises of an appropriate sensing element (a transducer having capability of converting the measurand quantity into an equivalent optical signal), an optical source such as an LED or an Injection Laser Diode), an optical fiber for transmission of optical signal from source to destination, an optical detector such as photodiode, and measuring instruments such as an optical spectrum analyzer (OSA) or an analog/digital oscilloscope.
The main principle of working of an optical fiber sensor is that the output of the physical transducer is used to modulate light intensity, phase angle, operating wavelength, or polarization of the optical signal. This results into a corresponding change in the operational characteristics of the optical signal received at the optical detector after traveling through optical fiber.
Classification of Fiber Optic Sensors
There are different ways of classifying fiber optic sensors. For example, these can be categorized based on their principle of operation including the process of modulation and demodulation.