How Fiber Optic Temperature Sensor Works
Fiber optic sensors are divided into intrinsic and extrinsic sensors. The difference is that the optical fiber itself acts as the sensing element for the intrinsic sensor whereby an extrinsic sensor uses an external transducer. In other words the fiber is directly affected by the measurand in an intrinsic sensor compared to an extrinsic fiber where the fiber simply guides the light to or from the sensing element. Intrinsic sensors are often used to measure strain, temperature and pressure. Extrinsic sensors on the other hand are often used to measure vibration, rotation, displacement, velocity, acceleration, torque, and temperature.
Today there are many types of — both intrinsic and extrinsic — fiber optic sensors used for measuring temperature. To further illustrate how fiber optic temperature sensors are working one has to make a distinction between principles of operation. Three major types exist: the intensity modulated sensors, the phase modulated sensors and the wavelength modulated sensors. Intensity modulated sensors are based on the principle of letting a physical disturbance such as temperature cause a change in the received light through an optical fiber. Phase modulated sensors is based on the principle of comparing the phase of light in the sensing fiber with a reference fiber in an interferometer. Wavelength modulated sensors are based on the principle that a physical disturbance such as temperature or strain changes the reflected wavelength of the light. In general, phase modulated and wavelength modulated sensors are providing much more accurate measurements than intensity modulated sensors but at the cost of much more expensive interrogators.
There is no simple yet precise description to the question what is a fiber optic temperature sensor since there is a large variety of different sensor types that can be characterized in several ways. One of the most frequently used optical fiber sensors is the Fiber Bragg Grating (FBG) based sensor. It is an intrinsic sensor based on the wavelength modulation principle. The basic principle of the sensor is that certain wavelengths — the ones that satisfy the Bragg condition — are reflected at certain positions while all other wavelengths are reflected. This is achieved by creating gratings inside the core of an optical fiber. When the temperature of the optical fiber changes both the spacing between the gratings and the refractive index will change. Therefore, any change in temperature will cause a shift in reflected wavelength. One of the main benefits of FBG-based sensors is that it can be made very sensitive to measure extremely small temperature changes.
The tremendous development within telecommunications is the main reason why fiber optic temperature sensors are working since many years back and that they possesses a number of advantageous over traditional electrical sensors such as: high accuracy, insensitivity to electromagnetic interference and have extraordinary resistance to mechanical fatigue. These properties has made fiber optical sensors, and especially fiber Bragg grating based sensors, widely used in numerous industries as for example: oil & gas, civil structures, transportation, aerospace and process industry. Today there are a few established companies that design and produce complete temperature systems based on FBG-technology of which Swedish based Proximion is one.
