Fiber optic sensors are evolving towards sensitivity, accuracy, adaptability, compactness and intelligence. In this process, fiber-optic sensors, a new member of the sensor family, has become popular. Optical fiber has many excellent properties, for example: has the performance of electromagnetic and atomic radiation interference, fine diameter, soft, light weight mechanical properties; insulation, non-inductive electrical properties; water, high temperature, corrosion-resistant chemical properties, it can be in places where people can not reach (such as high-temperature areas), or areas harmful to people (such as nuclear radiation areas), to play the role of the human ear, but also beyond the human physiological boundaries, receiving external information that is not perceived by the human senses.
Principle.
The basic working principle of the fiber optic sensor is the light from the light source through the optical fiber into the modulator, so that the parameters to be measured and the light into the modulation area after the interaction, resulting in the optical properties of light (such as light intensity, wavelength, frequency, phase, polarization state, etc.) changes, called the modulated signal light, and then use the measured on the light transmission characteristics of the influence exerted to complete the measurement.
There are two types of measurement principles for fiber optic sensors.
(1) Physical fiber optic sensor principle, physical fiber optic sensor is the use of optical fiber sensitivity to changes in the environment, the input physical quantity into a modulated optical signal. Its working principle is based on the optical modulation effect of optical fiber, that is, the optical fiber in the external environmental factors, such as temperature, pressure, electric field, magnetic field and so on change, its light transmission characteristics, such as phase and light intensity, will change the phenomenon.
Therefore, if we can measure the light phase and light intensity changes through the optical fiber, we can know the change of the physical quantity to be measured. This type of sensor is also known as sensitive element type or functional fiber optic sensor. The point source beam of the laser diffuses into a parallel wave, which is divided into two paths by a beam splitter, one for the reference light path and the other for the measurement light path. External parameters (temperature, pressure, vibration, etc.) cause changes in the length of the fiber and the phase of the optical phase changes, resulting in a different number of interference fringes, the counting of its mode-wise movement, you can measure the temperature or pressure, etc.
(2) structural fiber-optic sensor principle, structural fiber-optic sensor is a measurement system composed of optical detection element (sensitive element) and fiber optic transmission circuit and measurement circuit. The optical fiber is only used as a light transmission medium, so it is also called light transmission or non-functional fiber optic sensor.
