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This public account is a non-profit MEMS technology dynamic and industry reporting platform under the Guangdong Microtechnology Industrial Research Institute. We publish in-depth reports on MEMS, track industry dynamics, and provide technical popularization every week. This article is a technical popularization, being the 157th article of the public account.
In the second half of the 20th century, the invention of optical fibers opened a revolution in the field of communication. With technological advancements, the high sensitivity and electromagnetic interference resistance of optical fibers have been further explored, giving rise to the important branch of optical fiber sensors. Today, optical fiber sensors can measure various physical quantities such as vibration, pressure, temperature, and current, widely covering national economy, national defense, and daily life, especially demonstrating irreplaceable advantages in harsh environments, becoming a key technology driving the intelligent upgrade of multiple industries.
01
Basic Principles and Classification
The core performance of optical fiber sensors relies on the waveguide characteristics of optical fibers and their ability to modulate optical signals. Understanding their basic principles and classifications is key to mastering this technology.
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Optical Fiber Waveguide and Core Characteristics
An optical fiber waveguide, referred to as an optical fiber, is a light path made of dielectric materials with high optical transmittance, such as quartz, glass, and plastic. It has a double-layer concentric cylindrical structure, with the inner layer being the core (light-dense medium) with a higher refractive index, and the outer layer being the cladding (light-rare medium) with a lower refractive index. Light undergoes total internal reflection at the interface between the core and cladding, enabling long-distance transmission.
The transmission performance of optical fibers is mainly influenced by two factors.
Loss: It is divided into absorption loss and scattering loss. Absorption loss is the loss caused by the conversion of transmitted light energy into heat by the material; scattering loss is caused by the inhomogeneity of the optical fiber material, geometric size defects, or bending of the optical fiber. Among them, bending of the optical fiber changes the boundary conditions, disrupting total reflection, and the smaller the bending radius, the greater the loss.
Dispersion: During the transmission of input pulses in the optical fiber, the phenomenon of broadening occurs due to the different group velocities of light waves. Dispersion can lead to signal distortion, limiting transmission bandwidth, and in optical fiber communication and some sensing scenarios, it is necessary to focus on solving the signal distortion problem.
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Two Core Classifications
According to the role of optical fibers in sensors, optical fiber sensors are mainly divided into two categories.
Functional Optical Fiber Sensors (FF Type, Sensing Type): The optical fiber itself is the sensitive element, capable of both sensing and transmitting the measured information. These sensors are sensitive to external factors such as temperature and pressure, can maintain the characteristics of light such as phase and wavelength after sensing, have high detection sensitivity, but have strict requirements for optical components and are easily affected by environmental interference.
Non-Functional Optical Fiber Sensors (NF Type, Light Transmission Type): The optical fiber only serves as a medium for light transmission, with the measured object modulating the light signal through other sensitive elements, which is then transmitted to the output end for processing via the optical fiber. Their structure is simple, reliability is high, and the application range is wide, usually using multimode optical fibers with larger numerical apertures and core diameters to improve coupling efficiency with light-sensitive elements.
02
Optical Modulation and Demodulation
Optical modulation is the process of superimposing the measured information onto the carrier light wave, while demodulation is the process of converting the optical signal into a detectable electrical signal. Together, they determine the measurement accuracy of the sensor. Currently, there are four mainstream modulation methods.
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Optical Intensity Modulation and Demodulation
Optical intensity modulation detects changes in the carrier light intensity caused by the measured object, and the core of demodulation is to ensure a sufficient signal-to-noise ratio.
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Common Modulation Techniques
Micro-bending Effect: The optical fiber is clamped between two micro-bending plates with periodic ripples. When the optical fiber is micro-bent, the light intensity attenuates, reflecting the measured physical quantity through the amount of attenuation.
External Modulation: The modulation process occurs outside the optical fiber, dividing the optical fiber into sending and receiving parts, using reflectors or light-blocking screens to change the light transmission path and adjust the received light intensity.
Refractive Index Modulation: There are mainly three methods—using the measured physical quantity to change the refractive index of the medium, disrupting total internal reflection; coupling through evanescent fields (transmitted light waves outside the core transfer energy across media during total internal reflection); and creating physical property sensors using the refractive and reflective characteristics of different media.
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Key to Demodulation
It is necessary to reduce external interference, such as light source aging, changes in optical fiber coupling, and temperature drift of the modulator. The impact of these factors on measurement accuracy is far greater than that of signal noise and thermal noise, requiring optimization in structural design and manufacturing processes.
2
Polarization Modulation and Demodulation
Light waves are transverse waves, with their electric field vector and magnetic field vector orthogonal to the direction of propagation. According to the trajectory of the vibration vector in the plane perpendicular to the light beam, they can be divided into linearly polarized light, circularly polarized light, and elliptically polarized light. Polarization modulation utilizes electro-optic, magneto-optic, and photoelastic effects to change the polarization state of light, and during demodulation, a polarizer is used to convert polarization changes into light intensity changes for detection.
3
Phase Modulation and Demodulation
The principle of phase modulation is that the energy field being measured acts on the sensitive single-mode optical fiber, causing a phase change in the light wave propagating within the fiber. The phase change is then converted into amplitude change through interferometric measurement technology, and finally detected by a photoelectric detector. The accuracy of this type of sensor depends on the stability of the interference system and is commonly used for high-precision physical quantity measurements.
4
Frequency Modulation and Demodulation
Frequency modulation does not change the characteristics of the optical fiber; the optical fiber is only used to transmit the optical signal. Currently, it mainly utilizes the optical Doppler effect—motion of the measured object causes a shift in the light frequency, and by detecting frequency changes, information such as the speed of the measured object can be obtained.
03
Optical Fiber Bragg Grating Sensors
Optical fiber Bragg grating sensors are a new technology developed in the late 1990s, and with their high stability and multifunctionality, they have become representative products in the field of optical fiber sensing.
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Structure and Working Principle
The core of the optical fiber grating is doped with single-crystal germanium ions, and by irradiating the optical fiber with a UV light interference pattern, a spatial phase grating with periodic refractive index changes is formed in the core, known as the Fiber Bragg Grating (FBG). The core working principle is that changes in the measured parameters alter the grating period or effective refractive index, leading to a shift in the reflected light wavelength, which is detected to measure the parameters.
2
Main Types and Applications
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Optical Fiber Bragg Grating Temperature Sensors
These are among the earliest types applied. Temperature changes cause thermal expansion and contraction of the optical fiber (changing the grating period), while also triggering thermal optical effects (changing the effective refractive index), both leading to a shift in the center wavelength. To protect the fragile grating, it is usually encapsulated in a capillary steel tube, used for monitoring environmental temperature, equipment temperature, etc.
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Optical Fiber Bragg Grating Strain Sensors
This is the classic application. When the optical fiber is subjected to axial force, the photoelastic effect changes the grating period and effective refractive index, and the strain can be calculated from the wavelength shift. During use, the sensor needs to be bonded to the object being measured with adhesive, allowing both to deform together, widely used for strain monitoring in building structures and mechanical components.
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Optical Fiber Bragg Grating Pressure Sensors
Bare optical fiber gratings have low sensitivity to pressure and need to be enhanced through encapsulation. The mainstream solution is based on axial strain measurement, using adhesive or embedded packaging to convert pressure into strain in the optical fiber, used in scenarios such as pipeline pressure and equipment pressure.
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Optical Fiber Bragg Grating Acceleration Sensors
Using elastic elements (such as beam-type, spring-type, or hinge-type) to convert the displacement caused by acceleration into strain in the optical fiber grating, achieving wavelength modulation. The performance of the elastic elements directly affects the response accuracy of the sensor, commonly used for vibration monitoring and equipment status diagnosis.
Optical fiber Bragg grating sensors also have advantages such as small size, resistance to electromagnetic interference, and multiplexing capabilities, allowing for the formation of sensing networks, enabling long-term, distributed monitoring in aerospace, civil engineering, and mechanical fields.
04
Diverse Application Scenarios
The characteristics of optical fiber sensors make them suitable for various complex scenarios, from industrial monitoring to healthcare, from new energy to robotics, all demonstrating significant value.
1
Industrial and Engineering Monitoring
Level Monitoring: Optical fiber level sensors based on the principle of total internal reflection have a conical reflector at the probe end. When the probe is in the air, light is totally reflected back to the photodiode; after contacting the liquid surface, the change in the liquid’s refractive index disrupts total reflection, and when the light intensity suddenly decreases, it indicates the liquid level position.
Angular Velocity Monitoring (Optical Fiber Gyroscope): Utilizing the Sagnac effect, two beams of light propagating in opposite directions in the same closed optical path produce a light path difference due to system rotation, which is proportional to the angular velocity. By detecting the phase difference, the angular velocity can be calculated. Its accuracy far exceeds that of mechanical gyroscopes and laser gyroscopes, used for navigation and attitude control.
Current Monitoring: Based on the Faraday magneto-optic effect, the magnetic field generated by the current causes the linearly polarized light in the optical fiber to deflect, and the deflection angle can be detected to obtain the current value, suitable for non-contact monitoring of high-voltage power grids and large current equipment.
Engineering Safety Monitoring: The intelligent optical fiber early warning system from Zhongshan Precision detected bridge pier tilting and track displacement in the Shenzhen Metro project, triggering early warnings in time; in reservoir dam monitoring, it provides real-time perception of settlement and slope changes, offering a basis for drainage decisions during heavy rain. The optical fiber grating monitoring system from Jikang Technology participated in the downstream hydropower project of the Yarlung Tsangpo River, monitoring dam strain and tunnel deformation to ensure engineering safety.
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Healthcare Sector
Cancer Early Screening: The microwave photonic demodulation dual-wavelength optical fiber laser sensor developed by the team of Shao Liyang at Southern University of Science and Technology has improved the detection sensitivity of tumor markers by 2-3 orders of magnitude, with a detection limit as low as 0.076ng/mL, clinically validated in Shenzhen’s top-tier hospitals, suitable for large-scale screening.
Detection of Major Disease Markers: The team from Northeastern University invented a “gold film – gold nanorod – graphene oxide film” sandwich structure optical fiber sensor, enhancing sensitivity by 2 orders of magnitude, capable of detecting extremely low concentrations of cancer and other disease markers, aiding in early disease detection.
3
New Energy and Battery Monitoring
Optical fiber sensors play a key role in health monitoring of rechargeable batteries, mainly monitoring four types of parameters.
Temperature: FBG sensors are small and can be multiplexed, allowing them to be embedded inside batteries, with a measurement accuracy of 10pm/℃, reflecting the internal temperature distribution of the battery and avoiding thermal runaway.
Strain/Stress: During battery charging and discharging, the volume expands and contracts, and FBG sensors detect strain through wavelength shifts, reflecting the state of charge (SOC) and state of health (SOH), even providing warnings for overcharging risks.
Electrolyte Refractive Index: Changes in electrolyte concentration alter the refractive index, and tilted fiber Bragg gratings (TFBG) can detect this change, assessing battery SOH; gold film-coated SPR-TFBG can also analyze the ion intercalation process, aiding in battery mechanism research.
Spectral Features: Utilizing optical fiber infrared spectroscopy (IR-FEWS) and Raman scattering spectroscopy, tracking phase changes in electrodes and decomposition of electrolytes, revealing the internal electrochemical processes of batteries, optimizing battery performance.
4
Robotics and Bionic Technology
The Tsinghua University team combined distributed optical fiber sensing technology to develop multimodal tactile perception soft robotic fingers. These fingers simulate the sensory mechanisms of rodent whiskers and human fingertips, capable of recognizing contact forces as low as 0.01N, distinguishing sandpaper grit and sliding direction (CNN model recognition accuracy of 98.78%), and can also locate fingertip positions (accuracy of 97.79%) and identify material stiffness (accuracy of 98.38%), enabling dexterous grasping of fragile objects and underwater transparent objects, applicable in human-computer interaction and medical devices.
05
Domestic Industry and Research
China’s optical fiber sensor field has achieved a leap from technological breakthroughs to industrial implementation, with research teams and enterprises jointly promoting the localization process, breaking foreign monopolies.
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Technological Breakthroughs by Research Teams
Northeastern University: The intelligent perception and sensor team has spent over a decade solving the problems of “inability to measure, incomplete measurement, and inaccurate measurement.” They proposed high-order optical wave mode and plasmonic mode coupling theory to achieve trace substance detection; discovered the phenomenon of dispersion turning point control sensitivity, resolving the contradiction between high sensitivity and large measurement range; and broke through the quantum state scattering particle noise limit, enhancing the signal-to-noise ratio. Their deep-sea detection sensor has a temperature resolution of 0.001℃ and salinity resolution of 0.0016‰, providing security for marine safety.
Jinan University: In collaboration with China Shipbuilding Group, they developed the world’s first navigation-grade hollow-core optical fiber gyroscope, with zero bias instability of 0.0017°/h, reducing the existing record by 30 times, continuously operating stably for over 185 hours, and the temperature sensitivity is reduced by an order of magnitude compared to solid-core optical fiber gyroscopes, marking China’s leading position in the field of inertial navigation.
Academician Jiang Desheng’s Team: A pioneer in China’s optical fiber sensing technology, they developed the first optical fiber sensor (optical fiber wind pressure gauge), breaking foreign technological blockades. The developed optical fiber grating temperature fire alarm system has a response time of less than 20 seconds, with a cost per kilometer only one-third of foreign products, equipping 20,000 kilometers of tunnels and 90% of oil depots in China; they developed a large-capacity array system with over 100,000 optical fiber gratings on a single fiber, applied in major projects such as the C919 large aircraft and intelligent airport runways, promoting the transition of China’s technology from following to leading.
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Industrial Implementation by Enterprises
Heqi Optoelectronics: A leader in optical fiber sensors for power electronics and semiconductor equipment, having secured 150 million yuan in financing, with over 200 intellectual property rights, recognized as a national-level “specialized, refined, and innovative” small giant enterprise. Their integrated monitoring system for power equipment has been selected as a typical case of safety emergency equipment by the Ministry of Industry and Information Technology for 2024, and products in the semiconductor field have achieved mass delivery, serving leading domestic and foreign customers.
Baian Technology: Established for 21 years, with product lines including optical fiber sensors and spectral demodulation systems, applied in smart cities, industrial temperature measurement, rail transit, and other fields. In 2023, Baian Semiconductor, a subsidiary, invested in the first domestic 6-inch MEMS optical fiber sensor chip production line, with an annual capacity of 10,000 to 15,000 pieces, expected to expand to 30,000 pieces/year, filling domestic gaps.
Jikang Technology: A national-level “specialized, refined, and innovative” enterprise, serving the water conservancy and energy sectors with core technologies such as optical fiber grating monitoring systems. They participated in the downstream hydropower project of the Yarlung Tsangpo River, supplying monitoring equipment for scientific verification, with the related engineering monitoring equipment market expected to reach hundreds of billions; in 2024, they expect revenue of 357 million yuan, with net profit growing positively for seven consecutive years, with products applied in major projects such as the Three Gorges and Baihetan.
Guangge Technology: The leading domestic player in the distributed optical fiber sensing market, listed on the Science and Technology Innovation Board in 2023, possessing a complete set of demodulation solutions, with products serving the fields of power grids, oil and gas, and transportation.
In addition, the localization of core components is gradually being realized, with narrowband light sources from Xiamen Bige, lithium niobate waveguides from Shiwitong, and polarization-maintaining optical fibers from Changfei providing solid support for industrial development.
06
Summary and Future Trends
Optical fiber sensors, with their high sensitivity, resistance to electromagnetic interference, and corrosion resistance, have addressed the pain points of traditional sensors in harsh environments, high precision, and long-distance monitoring, becoming key technologies in the national economy and national defense. Through the collaborative efforts of research teams and enterprises, China has achieved a leap from following technology to partially leading, with an increasingly complete independent intellectual property system and a continuously expanding industrial scale.
In the future, optical fiber sensors will develop in three directions.
Chipization and Integration: Currently, demodulation instruments rely on discrete chips, which are large and costly. Future development of chipization will achieve miniaturization and cost reduction of demodulation instruments, promoting large-scale applications.
Multimodal and Intelligentization: Combining AI and big data technologies to achieve collaborative monitoring of multiple parameters (temperature, strain, refractive index), such as integrating spectral and strain data in battery monitoring to enhance state assessment accuracy; developing intelligent analysis systems for fault warning and predictive maintenance.
Deepening Application Scenarios: In special fields such as deep-sea exploration, aerospace, and quantum sensing, further enhancing the environmental adaptability of sensors; in the medical field, promoting the clinical implementation of minimally invasive detection and real-time monitoring technologies to assist precision medicine.
With the deepening of domestic substitution and the growth of downstream demand, China’s optical fiber sensor industry is expected to usher in a new round of explosion, providing stronger support for national strategies such as smart industry, healthy China, and energy security.
END
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