Understanding Fiber Optic Sensors

Fiber optics were initially used for the transmission of light, making them suitable for long-distance information transfer and serving as the cornerstone of modern information society’s fiber optic communication. The characteristic parameters of light waves propagating in fiber optics can change indirectly or directly due to external factors, allowing fiber optic sensors to analyze and detect changes in these physical, chemical, and biological quantities.

Fiber optic sensors consist of a light source, incident fiber, outgoing fiber, optical modulator, optical detector, and demodulator. The basic principle is to send the light from the light source through the incident fiber into the modulation area, where the light interacts with the external measured parameters, causing changes in the optical properties of the light (such as intensity, wavelength, frequency, phase, polarization state, etc.) to become the modulated signal light, which is then sent through the outgoing fiber to the optical detector and demodulator to obtain the measured parameters.

Fiber optic sensors can be divided into two main categories based on structural types: one category is functional (sensing) sensors; the other category is non-functional (light-transmitting) sensors.

These sensors use fiber optics (or special fibers) that are sensitive and capable of detecting external information as sensing elements, modulating the light transmitted through the fiber to change characteristics such as intensity, phase, frequency, or polarization state, and then demodulating the modulated signal to obtain the measured signal.

In this case, fiber optics not only serve as the light guiding medium but also as sensitive elements, typically using multimode fibers.

Advantages: compact structure, high sensitivity. Disadvantages: requires special fibers, high cost. Typical examples: fiber optic gyroscopes, fiber optic hydrophones, etc.

These sensors use other sensitive elements to sense the changes in the measured quantities, with fiber optics serving only as the medium for information transmission, often using single-mode fibers. In this case, fiber optics only play a role in guiding light, with the light being measured and modulated on the sensitive elements.

Advantages: does not require special fibers or other special technologies, relatively easy to implement, low cost. Disadvantages: lower sensitivity. Most practical applications are non-functional fiber optic sensors.

1) Intensity Modulated Fiber Optic Sensors

The basic principle is that the physical quantity to be measured causes a change in the intensity of the light transmitted in the fiber, and by detecting the change in light intensity, the measurement of the quantity can be achieved. A constant light source emits light of a certain intensity into the sensor head; within the sensor head, the intensity of light changes under the influence of the measured signal, being modulated by the external field, causing the output light intensity envelope to match the shape of the measured signal. The output current measured by the photodetector also undergoes the same modulation, and the signal processing circuit detects the modulated signal, thus obtaining the measured signal.

The advantages of this type of sensor are simple structure, low cost, and ease of implementation, leading to early development and application. It is now successfully used in measuring displacement, pressure, surface roughness, acceleration, gaps, force, liquid levels, vibration, radiation, etc. There are many intensity modulation methods, generally classified as reflective intensity modulation, transmissive intensity modulation, optical mode intensity modulation, and refractive index and absorption coefficient intensity modulation, etc.

Generally, reflective intensity modulation, transmissive intensity modulation, and refractive index intensity modulation are referred to as external modulation types, while optical mode is referred to as internal modulation types. However, due to the limitations of the principles, it is easily affected by fluctuations in the light source and connector loss changes; therefore, this type of sensor can only be used in environments with minimal interference sources.

2) Phase Modulated Fiber Optic Sensors

The basic principle is that under the influence of the energy field being measured, the phase of the light waves in the fiber changes, and then using interferometric measurement techniques, the change in phase is converted into a change in light intensity, thereby detecting the physical quantity to be measured. The advantages of phase-modulated fiber optic sensors include extremely high sensitivity, a large dynamic measurement range, and fast response speed, while the disadvantages are higher requirements for the light source and precision of the detection system, leading to correspondingly higher costs.

The main application areas currently include: acoustic, pressure, or vibration sensors using the photoelastic effect; current and magnetic field sensors using the magnetostrictive effect; electric field and voltage sensors using the electrostrictive effect; and angular velocity sensors (fiber optic gyroscopes) using the Sagnac effect, etc.

3) Frequency Modulated Fiber Optic Sensors

The basic principle utilizes the Doppler frequency shift effect of light reflected or scattered by moving objects to detect their speed, meaning that the frequency of light is related to the motion state between the light receiver and the light source. When they are relatively still, the oscillation frequency of the received light is constant; when there is relative motion, the frequency of the received light shifts, with the magnitude of the shift related to the speed and direction of the relative motion.

Therefore, this type of sensor is often used to measure the speed of moving objects. Frequency modulation also includes other methods, such as the frequency changes of certain materials’ absorption and fluorescence phenomena with external parameters, as well as Brillouin and Raman scattering caused by quantum interactions, which are also forms of frequency modulation. Its main applications are in measuring fluid flow, as well as gas sensors for measuring gas concentration or monitoring atmospheric pollution using Raman scattering caused by strong light irradiation; and temperature sensors using photoluminescence, etc.

4) Polarization State Modulated Fiber Optic Sensors

The basic principle is to use changes in the polarization state of light to convey information about the measured object.

Light waves are transverse waves, and their light vectors are perpendicular to the direction of propagation. If the direction of the light vector remains unchanged while its magnitude changes with phase, the light is called linearly polarized light. The plane formed by the light vector and the direction of propagation is the vibration plane of linearly polarized light.

If the magnitude of the light vector remains constant while its direction rotates uniformly around the propagation direction, the trajectory of the light vector’s endpoint is a circle, and such light is called circularly polarized light. If both the magnitude and direction of the light vector change in a regular manner, causing the endpoint of the light vector to rotate in an elliptical path, such light is called elliptically polarized light.

Using the polarization properties of light waves, polarization-modulated fiber optic sensors can be produced. In many fiber optic systems, especially those containing single-mode fibers, polarization plays an important role. Many physical effects can affect or change the polarization state of light, and some effects can cause birefringence phenomena. The so-called birefringence phenomenon refers to the phenomenon where a beam of incident light is split into two beams of refracted light in certain crystals whose optical properties vary with direction. The phase delay of light passing through birefringent media is a function of the input light’s polarization state.

Polarization state modulated fiber optic sensors have high detection sensitivity, can avoid the influence of light source intensity variations, and have a simpler structure and easier adjustment compared to phase-modulated fiber optic sensors. Their main application areas include current and magnetic field sensors using the Faraday effect; electric field and voltage sensors using the Pockels effect; pressure, vibration, or acoustic sensors using the photoelastic effect; and temperature, pressure, and vibration sensors utilizing birefringence. Currently, they are primarily used for monitoring high currents.

5) Wavelength Modulated Fiber Optic Sensors

Traditional wavelength-modulated fiber optic sensors utilize the spectral characteristics of the sensing probe that change with external physical quantities.

This type of sensor is mostly non-functional. In wavelength-modulated fiber optic probes, the fiber simply serves as a light guide, sending incident light to the measurement area, while the returned modulated light is sent to the analyzer. The key to fiber optic wavelength detection technology is the good performance of the light source and spectrum analyzer, which plays a decisive role in the stability and resolution of the sensing system.

Fiber optic wavelength modulation technology is mainly applied in fields such as medicine and chemistry. For example, it is used for analyzing blood gases in the human body, pH value detection, chemical analysis of indicator solution concentrations, analysis of phosphorescence and fluorescence phenomena, black body radiation analysis, and Fabry-Perot filter applications. The currently referred to wavelength-modulated fiber optic sensors mainly refer to Fiber Bragg Grating (FBG) sensors.

Fiber optic sensors possess extremely high sensitivity and precision, inherent safety, resistance to electromagnetic interference, high insulation strength, corrosion resistance, integration of sensing and transmission, and compatibility with digital communication systems. They can be summarized as follows:

(1) High sensitivity;

(2) Lightweight and flexible for easy installation;

(3) Electrical insulation and chemical stability. Fiber optics are a highly insulating and chemically stable material, suitable for environments requiring high voltage isolation and flammable or explosive conditions in electrical and chemical systems;

(4) Good safety. Fiber optic sensors are passive sensing elements, hence there are no safety hazards such as leakage or electric shock when used for measurement;

(5) Resistance to electromagnetic interference. Generally, the frequency of light waves is higher than that of electromagnetic radiation, so light propagating in fiber optics is not affected by electromagnetic noise;

(6) Distributed measurement. A single fiber can achieve long-distance continuous measurement and control, accurately measuring strain, damage, vibration, and temperature at any point, thereby forming a monitoring area over a large range, enhancing environmental detection levels;

(7) Long service life. The primary material of fiber optics is quartz glass, wrapped in a polymer material, giving it greater durability compared to metal sensors;

(8) Large transmission capacity. Using fiber optics as a backbone, large-capacity transmission fibers replace bulky multi-core underwater cables to collect and store information from various sensing points, and through multiplexing technology, achieve monitoring of distributed fiber optic sensors.

Distributed fiber optic sensing technology was proposed in the late 1970s, developing alongside the widely used Optical Time Domain Reflectometry (OTDR) technology in fiber optic engineering. Over the past decade, a series of distributed fiber optic sensing mechanisms and measurement systems have emerged and gradually applied across multiple fields. Currently, this technology has become one of the most promising technologies in fiber optic sensing.

Distributed fiber optic sensors employ unique distributed fiber detection technology to measure or monitor spatial distribution and time-varying information along the fiber optic transmission path. Utilizing the characteristics of light wave propagation in fiber optics, measurements of physical quantities (such as temperature, pressure, stress, and strain) can be continuously sensed along the fiber length, with the fiber serving as both the sensing medium and the transmission medium for the measured quantities. It allows for the spatial distribution and time-varying information of the measured field to be obtained simultaneously by arranging the sensing fiber along the field.

Distributed fiber optic sensors have the following characteristics:

1) The sensing elements in a distributed fiber optic sensing system are only the fiber optics;

2) A single measurement can obtain a one-dimensional distribution map of the measured quantity across the entire fiber area; by arranging the fiber in a grating pattern, the two-dimensional and three-dimensional distribution of the measured quantity can be determined;

3) The spatial resolution of the system is generally on the order of meters, thus for smaller changes in the measured quantity, only the average value can typically be observed;

4) The measurement accuracy of the system generally has a reciprocal relationship with spatial resolution;

5) The detected signals are generally weak, requiring the signal processing system to have a high signal-to-noise ratio;

6) Due to the need for extensive signal averaging, frequency scanning, and phase tracking during detection, a considerable amount of time is required to complete a single measurement.

Because fiber optic cables are not easily affected by electromagnetic interference, distributed fiber temperature sensing systems are commonly used for temperature monitoring and measurement in hotspot areas of power cables. The demand for understanding and managing harsh environments, as well as improving conditions for outdoor operations, is the primary reason for the stable growth of the distributed fiber temperature sensing system market. At the same time, the technical challenges of deploying sensor cables are also major obstacles faced by this market’s development.

As applications become increasingly widespread, distributed fiber optic sensors are now primarily used in six major fields, including structural monitoring of pipelines and offshore oil platforms; leakage detection in liquid pipelines and dams; road surface icing detection, railway monitoring; security system detection, power cable monitoring; monitoring of fiber optic communication production; and environmental monitoring and long-term temperature measurement.