Optical fibre sensors represent a transformative technology in the field of measurement and sensing. Unlike traditional electronic sensors, they utilise light as the primary information carrier, transmitting it through thin strands of glass or plastic fibre. This fundamental principle grants them a unique set of advantages, including immunity to electromagnetic interference, suitability for harsh environments, high sensitivity, and the potential for distributed sensing over long distances. The core operation involves modifying a property of light—such as its intensity, phase, wavelength, or polarisation—in response to an external physical, chemical, or biological parameter. This modulation is then detected and interpreted to provide accurate measurements.
The classification of optical fibre sensors is primarily based on the location where the light interacts with the measured parameter. The two main categories are extrinsic and intrinsic sensors. Extrinsic Fibre Optic Sensors use the optical fibre solely as a conduit to transmit light to and from an external sensing element. In these systems, the actual modulation of light occurs outside the fibre itself. A common example is a Fabry-Perot interferometric sensor, where a tiny air gap or a specialised diaphragm at the fibre tip acts as the sensing cavity. Changes in pressure or temperature alter the cavity length, which in turn modulates the reflected light's interference pattern. These sensors are often used for precise point measurements of pressure, temperature, or vibration in industrial and medical settings.
Intrinsic Fibre Optic Sensors are more integrated. Here, the optical fibre itself acts as the sensing medium. The external parameter directly affects the light propagating within the fibre, altering its characteristics. This category is further divided based on the sensing mechanism. A prominent type is the Fibre Bragg Grating sensor. An FBG is a periodic modulation of the refractive index written into the core of the fibre. It acts as a wavelength-specific mirror, reflecting a narrow band of light while transmitting all others. When the fibre is stretched, compressed, or subjected to temperature changes, the grating period changes, causing a shift in the reflected wavelength. By monitoring this shift, precise measurements of strain and temperature can be obtained. FBGs are widely deployed in structural health monitoring of bridges, aircraft, wind turbines, and pipelines.
Another significant intrinsic type is based on scattering effects within the fibre. Distributed Optical Fibre Sensors exploit phenomena like Rayleigh, Brillouin, and Raman scattering. A pulse of light is sent down the fibre, and the backscattered light is analysed. Changes in temperature or strain along the entire length of the fibre affect the properties of this scattered light. By using sophisticated techniques like Optical Time-Domain Reflectometry, the system can not only measure the parameter but also pinpoint its location along kilometres of fibre. This makes DOFS ideal for monitoring large-scale infrastructure like power cables, railway embankments, and border security perimeters, providing a continuous profile rather than discrete point measurements.
Interferometric sensors form another crucial class, where sensing is achieved by comparing the phase of light in two paths. In configurations like Mach-Zehnder or Michelson interferometers, the sensing fibre is exposed to the external field, while a reference fibre is isolated. The parameter of interest induces a phase difference between the two light beams, creating an interference pattern whose shift correlates to the measurement. These sensors offer extremely high sensitivity, making them suitable for detecting acoustic waves, magnetic fields, and minute refractive index changes in chemical or biological sensing applications.
The applications of optical fibre sensors are vast and continually expanding. In the energy sector, they monitor temperature in high-voltage transformers and along subsea pipelines. In civil engineering, they provide real-time data on stress, corrosion, and load in structures, enabling predictive maintenance. The biomedical field uses miniature fibre sensors for in-vivo pressure monitoring and minimally invasive surgical guidance. Their intrinsic safety (no electrical sparks) and durability make them indispensable in hazardous environments like oil refineries and mining operations.
In conclusion, the diverse types of optical fibre sensors—from point-based extrinsic and FBG sensors to fully distributed sensing systems—offer tailored solutions for complex measurement challenges. Their unique advantages continue to drive innovation across industries, promising even greater integration into the fabric of smart infrastructure and advanced sensing networks in the future. The choice of sensor type depends fundamentally on the specific requirements of range, sensitivity, spatial resolution, and the environmental conditions of the application.
