Optical fiber was originally used for light transmission, suitable for long-distance information transmission, and is the cornerstone of fiber optic communication in the modern information society. The characteristic parameters of light waves propagating in optical fibers can change indirectly or directly due to external factors, and thus fiber optic sensors can analyze and detect these changes in physical, chemical, and biological quantities.
Fiber optic sensor
An optical fiber sensor consists of a light source, an incident optical fiber, an outgoing optical fiber, an optical modulator, a photodetector, and a demodulator. Its basic principle is to send light from the light source through the incident optical fiber into the modulation region. Within the modulation region, the light interacts with the external parameters to be measured, causing changes in the optical properties of the light (such as intensity, wavelength, frequency, phase, and polarization) to become modulated signal light. This modulated signal light is then sent through the outgoing optical fiber to the photodetector and demodulator to obtain the measured parameters.
Classification of fiber optic sensors
Fiber optic sensors can be divided into two main categories according to their structure: one is functional (sensing type) sensors; the other is non-functional (light transmission type) sensors.
Functional sensors
Using optical fibers (or special optical fibers) that are sensitive to and capable of detecting external information as sensing elements, the light transmitted within the optical fiber is modulated, causing changes in the intensity, phase, frequency, or polarization state of the transmitted light. The modulated signal is then demodulated to obtain the measured signal.
In this context, optical fiber is not only a light-guiding medium but also a sensitive element, and multimode optical fiber is often used.
Advantages: Compact structure, high sensitivity. Disadvantages: Requires special optical fibers, resulting in high cost. Typical examples: Fiber optic gyroscopes, fiber optic hydrophones, etc.
Non-functional sensor
This method utilizes other sensing elements to detect changes in the measured quantity, with the optical fiber serving only as a transmission medium; single-mode optical fiber is commonly used. The optical fiber acts solely as a light guide, and the light is modulated by the measurement as it falls on the fiber-optic sensing element.
Advantages: No special optical fibers or other special technologies are required, making it relatively easy to implement and low in cost. Disadvantages: Lower sensitivity. Most practical fiber optic sensors are non-functional.
Based on the different properties of the modulated light wave, both types of fiber optic sensors can be further divided into intensity-modulated fiber optic sensors, phase-modulated fiber optic sensors, frequency-modulated fiber optic sensors, polarization-modulated fiber optic sensors, and wavelength-modulated fiber optic sensors.
1) Intensity-modulated fiber optic sensor
The basic principle is that the physical quantity to be measured causes a change in the intensity of the light transmitted in the optical fiber, and the measurement is achieved by detecting the change in light intensity. A constant light source emits light of intensity , which is injected into the sensing head. Inside the sensing head, the intensity of the light changes under the influence of the measured signal, i.e., it is modulated by the external field. This causes the envelope of the output light intensity to have the same shape as the measured signal. The output current measured by the photodetector is also modulated in the same way. The signal processing circuit then detects the modulated signal to obtain the measured signal.
The advantages of this type of sensor are its simple structure, low cost, and ease of implementation. Therefore, it was developed and applied relatively early, and is now successfully used in the measurement of displacement, pressure, surface roughness, acceleration, gap, force, liquid level, vibration, radiation, etc. There are many intensity modulation methods, which can be broadly classified into 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 called external modulation, while optical modes are called internal modulation. However, due to the limitations of its principle, it is susceptible to the effects of light source fluctuations and connector loss variations, so this type of sensor can only be used in applications with relatively small interference sources.
2) Phase-modulated fiber optic sensor
The basic principle is that under the influence of the measured energy field, the phase of the light wave within the optical fiber changes. Interferometry is then used to convert this phase change into a change in light intensity, thereby detecting the physical quantity to be measured. The advantages of phase-modulated fiber optic sensors are extremely high sensitivity, a large dynamic measurement range, and fast response speed. Their disadvantages are relatively high requirements for the light source and the precision of the detection system, resulting in a correspondingly higher cost.
The main application areas at present are: sound, 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 rotational angular velocity sensors (fiber optic gyroscopes) using the Segnerk effect.
3) Frequency-modulated fiber optic sensor
The basic principle is to use the Doppler frequency shift effect of light reflected or scattered by a moving object to detect its speed. That is, the light frequency is related to the motion state between the light receiver and the light source. When they are relatively stationary, the received light oscillation frequency is used; when there is relative motion between them, the received light frequency shifts from its oscillation frequency, and the magnitude of the frequency shift is related to the magnitude and direction of the relative motion speed.
Therefore, this type of sensor is mostly used to measure the speed of moving objects. Frequency modulation also involves other methods, such as the frequency changes in absorption and fluorescence phenomena of certain materials depending on external parameters, and Brillouin and Raman scattering generated by quantum interactions, which are also frequency modulation phenomena. Its main application is measuring fluid flow; other applications include gas sensors that utilize Raman scattering when a substance is irradiated by strong light to measure gas concentration or monitor air pollution; and temperature sensors that utilize photoluminescence, etc.
4) Polarization-modulated fiber optic sensor
The basic principle is to use the change in the polarization state of light to transmit information about the object being measured.
Light waves are transverse waves, and their light vector is perpendicular to the direction of propagation. If the direction of the light vector remains constant, but its magnitude changes with the phase, such light is called linearly polarized light. The plane formed by the light vector and the direction of light propagation is the vibration plane of linearly polarized light.
If the magnitude of a light vector remains constant while its direction rotates uniformly around the direction of propagation, the trajectory of the light vector's end is a circle; such light is called circularly polarized light. If both the magnitude and direction of a light vector change regularly, and the end of the light vector rotates along an ellipse, such light is called elliptically polarized light.
The polarization properties of light waves can be used to create polarization-modulated fiber optic sensors. Polarization plays a crucial role in many fiber optic systems, especially those containing single-mode fibers. Many physical effects influence or alter the polarization state of light, some of which can cause birefringence. Birefringence is the phenomenon where, for crystals whose optical properties vary with direction, a beam of incident light is often decomposed into two refracted beams. The phase delay of light passing through a birefringent medium is a function of the polarization state of the input light.
Polarization-modulated fiber optic sensors offer high detection sensitivity, avoid the influence of light source intensity variations, and are simpler in structure and easier to adjust compared to phase-modulated fiber optic sensors. Their main applications include: current and magnetic field sensors utilizing the Faraday effect; electric field and voltage sensors utilizing the Pauli effect; pressure, vibration, or acoustic sensors utilizing the photoelastic effect; and temperature, pressure, and vibration sensors utilizing birefringence. Currently, their primary use is for monitoring strong currents.
5) Wavelength-modulated fiber optic sensor
Traditional wavelength-modulated fiber optic sensors utilize the property that the spectral characteristics of the sensing probe change with external physical quantities.
These types of sensors are mostly non-functional. In wavelength-modulated fiber optic probes, the fiber is simply used as a light guide, directing the incident light to the measurement area and sending the returning modulated light to the analyzer. The key to fiber optic wavelength detection technology is the good performance of the light source and the spectrum analyzer, which has a decisive impact on the stability and resolution of the sensing system.
Fiber optic wavelength modulation technology is mainly used in fields such as medicine and chemistry. Examples include the analysis of human blood gases, pH value detection, chemical analysis of indicator solution concentration, analysis of phosphorescence and fluorescence phenomena, blackbody radiation analysis, and Fabry-Perot filters. Currently, the term "wavelength modulation fiber optic sensor" primarily refers to fiber Bragg grating (FBG) sensors.
Features and advantages of fiber optic sensors
Fiber optic sensors offer advantages such as extremely high sensitivity and accuracy, inherent good security, resistance to electromagnetic interference, high insulation strength, corrosion resistance, integration of sensing and transmission, and compatibility with digital communication systems. These advantages are summarized below:
(1) High sensitivity;
(2) Lightweight, flexible and easy to install and embed;
(3) Electrical insulation and chemical stability. Optical fiber itself is a highly insulating and chemically stable material, suitable for harsh environments such as power systems and chemical systems requiring high-voltage isolation and flammable and explosive materials;
(4) Good safety. Fiber optic sensors are passive sensing elements, so there are no safety hazards such as leakage or electric shock when used in measurements;
(5) Electromagnetic interference resistance. Under normal circumstances, the frequency of light waves is higher than that of electromagnetic radiation , so the propagation of light in optical fibers is not affected by electromagnetic noise;
(6) Distributed measurement is possible. A single optical fiber can achieve long-distance continuous measurement and control, accurately measuring information such as strain, damage, vibration, and temperature at any point, thereby forming a monitoring area with a large range and improving the level of environmental monitoring;
(7) Long service life. The main material of optical fiber is quartz glass, with an outer cladding of polymer material, which makes it more durable than metal sensors;
(8) Large transmission capacity. Using optical fiber as the bus, high-capacity optical fiber replaces bulky multi-core underwater cables to collect and store information from various sensing points, and multiplexing technology is used to monitor distributed optical fiber sensors.
Distributed fiber optic sensors
Distributed fiber optic sensing technology was proposed in the late 1970s, developing alongside the emergence of Optical Time Domain Reflectometry (OTDR), a technology still widely used in fiber optic engineering. Over the past decade or so, a series of distributed fiber optic sensing mechanisms and measurement systems have been developed and gradually applied in various fields. Currently, this technology has become one of the most promising technologies in fiber optic sensing.
Distributed fiber optic sensors employ unique distributed fiber optic sensing technology to measure or monitor the spatial distribution and time-varying information along the fiber optic transmission path. Utilizing the properties of light propagation in optical fibers, the measured quantity (such as temperature, pressure, stress, and strain) can be continuously sensed along the fiber's length. The optical fiber serves as both the sensing medium and the transmission medium for the measured quantity. By arranging the sensing fibers along the field, it can simultaneously obtain information on the spatial distribution and time-varying changes of the measured field.
Distributed fiber optic sensors have the following characteristics:
1) The sensing element in a distributed optical fiber sensing system is only optical fiber;
2) A single measurement can obtain a one-dimensional distribution map of the measured area within the entire optical fiber region. By setting up the optical fiber in the shape of a grating, the two-dimensional and three-dimensional distribution of the measured area can be determined.
3) The spatial resolution of the system is generally on the order of meters, so for changes in the measured quantity over a narrower range, only the average value can usually be observed.
4) The measurement accuracy and spatial resolution of a system are generally mutually restrictive;
5) The detection signal is generally weak, therefore the signal processing system is required to have a high signal-to-noise ratio;
6) Because a large amount of signal addition averaging, frequency scanning, and phase tracking are required during the detection process, it takes a long time to complete a single measurement.
Because fiber optic cables are less susceptible to electromagnetic interference, distributed fiber optic temperature sensing systems are typically used for temperature monitoring and measurement in hotspot areas of power cables. The need to manage harsh environments and improve field operating conditions is a major driver of the stable growth of the distributed fiber optic temperature sensing system market. However, the technical challenges of deploying sensor cables remain a significant obstacle to this market's development.
With increasingly widespread applications, distributed fiber optic sensors are now mainly used in six major fields, including structural inspection of pipelines and offshore oil platforms; leakage detection of liquid pipelines and dams; road icing detection and railway monitoring; safety system detection and power cable monitoring; fiber optic communication production monitoring; environmental monitoring and long-term temperature measurement.
Fiber optic sensing technology is a novel sensing technology that has developed rapidly alongside fiber optic and optical communication technologies. In recent years, fiber optic sensing has achieved remarkable progress and widespread application in fields such as machinery, electronic instruments, aerospace, petroleum, chemical engineering, and food safety, including automatic control of production processes, online monitoring, and fault diagnosis.
