Excessively tilted fiber gratings (ExTFGs) are a type of special optical fiber grating device different from traditional fiber Bragg gratings, long period fiber gratings, and tilted fiber Bragg gratings. Due to the excessively tilted fiber fringe structure in the fiber core, ExTFGs could couple the light of the core mode into the high-order forward-propagating cladding modes, which would split into two sets of polarization dependent modes resulting in dual-peak resonances in the transmission spectrum. ExTFGs have the properties of the high refractive index sensitivity and low thermal crosstalk, which makes them very suitable for biochemical sensing applications. This paper will review the development of ExTFGs in terms of the mode coupling behavior, spectra characteristic, especially the refractive index sensitivity enhancement, biochemical modification methods of the sensor, and their applications in the bio-chemical sensing area, including pondus hydrogenii (pH) heavy metal ions, humidity, glucose, and immune sensing for various animal virus and biomarkers. Moreover, several composite sensing structures based on ExTFGs will be summarized.

Biochemical sensors have important applications in biology, chemistry, and medicine. Nevertheless, many biochemical sensors are hampered by intricate techniques, cumbersome procedures, and the need for labeling. In the past two decades, it has been discovered that liquid crystals can be used to achieve the optical amplification of biological interactions. By modifying recognition molecules, a variety of label-free biochemical sensors can be created. Consequently, biochemical sensors based on the amplification of liquid crystals have become one of the most promising sensors. This paper describes in detail the optical sensing principle of liquid crystals, sensing devices, and optical detection technologies. Meanwhile, the latest research findings are elucidated. Finally, the challenges and future research directions are discussed.

Localized surface plasmon resonance (LSPR) biosensors, which enable nanoscale confinement and manipulation of light, offer the enhanced sensitivity and electromagnetic energy localization. The integration of LSPR with the fiber-optic technology has led to the development of compact and versatile sensors for miniaturization and remote sensing. This comprehensive review explores various sensor configurations, fiber types, and geometric shapes, highlighting their benefits in terms of sensitivity, integration, and performance improvement. Fabrication techniques such as focused non-chemical bonding strategies and self-assembly of nanoparticles are discussed, providing control over nanostructure morphology and enhancing sensor performance. Bio-applications of fiber-optic LSPR (FOLSPR) sensors are detailed, specifically in biomolecular interactions and analysis of proteins, pathogens and cells, nucleic acids (DNA and RNA), and other small molecules (organic compounds and heavy metal ions). Surface modification and detection schemes are emphasized for their potential for label-free and real-time biosensing. The challenges and prospects of FOLSPR sensors are addressed, including the developments in sensitivity, fabrication techniques, and measurement reliability. Integration with emerging technologies such as nanomaterials is highlighted as a promising direction for future research. Overall, this review provides insights into the advancements and potential applications of FOLSPR sensors, paving the way for sensitive and versatile optical biosensing platforms in various fields.

Due to the benefits of the high sensitivity, real-time response, no labeling requirement, and good selectivity, fiber optic sensors based on surface plasmon resonance (SPR) have gained popularity in biochemical sensing in recent years. The current research on such sensors is hot in enhancing sensitivity, improving detection accuracy, and achieving the detection of biochemical molecules. The goal of this work is to present a thorough overview of recent developments in the optical fiber SPR biosensor research. Firstly, it explores the basic principles and sensing structures of optical fiber SPR biosensors, focusing on four aspects. Subsequently, this paper introduces three fiber optic surface plasmon biosensors: SPR, localized surface plasmon resonance (LSPR), and long-range surface plasmon resonance (LRSPR). Each concept is explained from the perspective of the basic principles of fiber optic SPR biosensors. Furthermore, a classification of fiber optic SPR biosensors in health monitoring, food safety, environmental monitoring, marine detection, and other applications is introduced and analyzed. Eventually, this paper summarizes the current research directions of SPR biosensors. Meanwhile, it provides a prospective outlook on how fiber optic SPR sensors will develop in the future.

Side polished fiber (SPF) has a controllable average roughness and length of the side-polishing region, which becomes a versatile platform for integrating multiple materials to interact with the evanescent field to fabricate all-fiber devices and sensors. It has been widely used in couplers, filters, polarizers, optical attenuators, photodetectors, modulators, and sensors for temperature, humidity, strain, biological molecules, chemical gas, and vector magnetic monitoring. In this article, an overview of the development history, fabrication techniques, fiber types, transmission characteristics, and varied recent applications of SPFs are reviewed. Firstly, the fabrication techniques of SPFs are reviewed, including the V-groove assisted polishing technique and wheel polishing technique. Then, the different types of SPFs and their characteristics are discussed. Finally, various applications of SPFs are discussed and concluded theoretically and experimentally, including their principles and structures. When designing the device, the residual thickness and polishing lengths of the SPF need to be appropriately selected in order to obtain the best performance. Developing all-fiber devices and sensors is aimed at practical usability under harsh environments and allows to avoid the high coupling loss between optical fibers and on-chip integrated devices.

Graphene as a truly 2-dimensional (2D) system is a promising candidate material for various optoelectronic applications. Implementing graphene as the main building material in ultra-broadband photodetectors has been the center of extensive research due to its unique absorption spectrum which covers most of the electro-magnetic spectra. However, one of the main challenges facing the wide application of pure graphene photodetectors has been the small optical absorption of monolayer graphene. Although novel designs were proposed to overcome this drawback, they often need complicated fabrication processes in order to integrate with the graphene photodetector. In this regard, fabrication of purely graphene photodetectors is a promising approach towards the manufacturing of simple, inexpensive, and high photosensitive devices. The fabrication of full graphene photodetectors (FGPDs) is mainly based on obtaining an optimal technique for the growth of high quality graphene, modification of electronic and optical properties of the graphene, appropriate techniques for transfer of graphene from the grown substrate to the desire position, and a proper design for photodetection. Therefore, the available states of the art techniques for each step of device fabrication, along with their pros and cons, are reviewed and the possible approaches for optimization of FGPDs have been proposed.

Fiber optofluidic laser (FOFL) integrates optical fiber microcavity and microfluidic channel and provides many unique advantages for sensing applications. FOFLs not only inherit the advantages of lasers such as high sensitivity, high signal-to-noise ratio, and narrow linewidth, but also hold the unique features of optical fiber, including ease of integration, high repeatability, and low cost. With the development of new fiber structures and fabrication technologies, FOFLs become an important branch of optical fiber sensors, especially for application in biochemical detection. In this paper, the recent progress on FOFL is reviewed. We focuse mainly on the optical fiber resonators, gain medium, and the emerging sensing applications. The prospects for FOFL are also discussed. We believe that the FOFL sensor provides a promising technology for biomedical analysis and environmental monitoring.

This paper reviews a wide variety of fiber-optic microstructure (FOM) sensors, such as fiber Bragg grating (FBG) sensors, long-period fiber grating (LPFG) sensors, Fabry-Perot interferometer (FPI) sensors, Mach-Zehnder interferometer (MZI) sensors, Michelson interferometer (MI) sensors, and Sagnac interferometer (SI) sensors. Each FOM sensor has been introduced in the terms of structure types, fabrication methods, and their sensing applications. In addition, the sensing characteristics of different structures under the same type of FOM sensor are compared, and the sensing characteristics of the all FOM sensors, including advantages, disadvantages, and main sensing parameters, are summarized. We also discuss the future development of FOM sensors.

Fiber Bragg grating (FBG) is the most widely used optical fiber sensor due to its compact size, high sensitivity, and easiness for multiplexing. Conventional FBGs fabricated by using an ultraviolet (UV) laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures. Recently, the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra. The femtosecond laser could also be used for inscribing various FBGs on almost all fiber types, even fibers without any photosensitivity. Such femtosecond-laser-induced FBGs exhibit excellent thermal stability, which is suitable for sensing in harsh environment. In this review, we present the historical developments and recent advances in the fabrication technologies and sensing applications of femtosecond-laser-inscribed FBGs. Firstly, the mechanism of femtosecond-laser-induced material modification is introduced. And then, three different fabrication technologies, i.e., femtosecond laser phase mask technology, femtosecond laser holographic interferometry, and femtosecond laser direct writing technology, are discussed. Finally, the advances in high-temperature sensing applications and vector bending sensing applications of various femtosecond-laser-inscribed FBGs are summarized. Such femtosecond-laser-inscribed FBGs are promising in many industrial areas, such as aerospace vehicles, nuclear plants, oil and gas explorations, and advanced robotics in harsh environments.

Accurate spectral measurement and wavelength determination are fundamental and vital for many fields. A compact spectrum analyzer with high performance is expected to meet the growing requirements, and speckle-based spectrum analyzer is a potential solution. The basic principle is based on using the random medium to establish a speckle-to-wavelength mapping relationship for spectrum reconstruction. This article introduces current speckle-based spectrum analyzers with different schemes and reviews recent advances in this field. Besides, some applications by using speckle-based spectrum analyzers are also introduced. Finally, the existing challenges and the future prospects of using speckle for spectrum recovery are discussed.

Optical fiber sensing technology has developed rapidly since the 1980s with the development of the optical fiber and fiber optical communication technology. It is a new type of sensing technology that uses light as a carrier and optical fiber as a medium to sense and transmit external signals (measurands). Distributed fiber optical sensors (DFOS) can continuously measure the external physical parameters distributed along the geometric path of the optical fiber. Meanwhile, the spatial distribution and change information of the measured physical parameters over time can be obtained. This technology has unmatched advantages over traditional point-wise and electrical measurement monitoring technologies. This paper summarizes the state-of-the-art research of the application of the distributed optical fiber sensing technology in geo-engineering in the past 10 years, mainly including the advantages of DFOS, the challenges in geo-engineering monitoring, related fundamental theoretical issues, sensing performance of the optical sensing cables, distributed optical fiber monitoring system for geo-engineering, and applications of optical fiber sensing technology in geo-engineering.

With micro- and nano-structured optical fibers, parts-per-million to parts-per-trillion level gas detection has been demonstrated for a range of gases such as methane, acetylene, ethane, carbon monoxide, hydrogen, and oxygen. We review the recent development in optical fiber gas cells and gas detection systems based on direct absorption, photothermal, photoacoustic, and stimulated Raman spectroscopies.

Fiber Bragg grating (FBG) array is a powerful technique for quasi-distributed sensing along the entire length of sensing fiber with fast response and high precision. It has been widely used for temperature, strain, and vibration monitoring. In this review work, an overview on the recent advances of FBG arrays is conducted. Firstly, the fabrication methods of FBG array are reviewed, which include femtosecond laser system and online writing technique. Then, the demodulation techniques for FBG arrays are presented and discussed. Distributed static sensing can be performed by demodulating wavelength shift of each FBG, while phase demodulation techniques with low noise are employed for dynamic vibration sensing. Simultaneous distributed dynamic and static sensing system based on FBG array is also outlined. Finally, possible future directions are discussed and concluded. It is believed that the FBG array has great development potential and application prospect.

This paper reviews the recent advances on the high-performance distributed Brillouin optical fiber sensing, which include the conventional distributed Brillouin optical fiber sensing based on backward stimulated Brillouin scattering and two other novel distributed sensing mechanisms based on Brillouin dynamic grating and forward stimulated Brillouin scattering, respectively. As for the conventional distributed Brillouin optical fiber sensing, the spatial resolution has been improved from meter to centimeter in the time-domain scheme and to millimeter in the correlation-domain scheme, respectively; the measurement time has been reduced from minute to millisecond and even to microsecond; the sensing range has reached more than 100 km. Brillouin dynamic grating can be used to measure the birefringence of a polarization-maintaining fiber, which has been explored to realize distributed measurement of temperature, strain, salinity, static pressure, and transverse pressure. More recently, forward stimulated Brillouin scattering has gained considerable interest because of its capacity to detect mechanical features of materials surrounding the optical fiber, and remarkable works using ingenious schemes have managed to realize distributed measurement, which opens a brand-new way to achieve position-resolved substance identification.

Recently, microfiber-optic sensors with high sensitivity, fast response times, and a compact size have become an area of interest that integrates fiber optics and nanotechnology. Distinct advantages of optical microfiber, such as large accessible evanescent fields and convenient configurability, provide attractive benefits for micro- and nano-scale optical sensing. Here, we review the basic principles of microfiber-optic sensors based on a broad range of microstructures, nanostructures, and functional materials. We also introduce the recent progress and state-of-the-art in this field and discuss the limitations and opportunities for future development.

Optical fiber sensor networks (OFSNs) provide powerful tools for large-scale buildings or long-distance sensing, and they can realize distributed or quasi-distributed measurement of temperature, strain, and other physical quantities. This article provides some optical fiber sensor network technologies based on the white light interference technology. We discuss the key issues in the fiber white light interference network, including the topology structure of white light interferometric fiber sensor network, the node connection components, and evaluation of the maximum number of sensors in the network. A final comment about further development prospects of fiber sensor network is presented.

RAO wuyu@uestc.edu.cn, and yjrao@uestc.edu.cn Abstract: Single atomically thick graphene, with unique structural flexibility, surface sensitivity, and effective light-mater interaction, has shown exceptional advances in optoelectronics. It opens a door for diverse functionalized photonic devices, ranging from passive polarizers to active lasers and parametric oscillators. Among them, graphene-fiber biochemical sensors combine the merits of both graphene and fiber structures, demonstrating impressively high performances, such as single-molecule detectability and fast responsibility. These graphene-fiber biochemical sensors can offer tools in various applications, such as gas tracing, chemical analysis, and medical testing. In this paper, we review the emerging graphene-fiber biochemical sensors comprehensively, including the sensing principles, device fabrications, systematic implementations, and advanced applications. Finally, we summarize the state-of-the-art graphene-fiber biochemical sensors and put forward our outlooks on the development in the future.

Fiber-optic hydrophone (FOH) is a significant type of acoustic sensor, which can be used in both military and civilian fields such as underwater target detection, oil and natural gas prospecting, and earthquake inspection. The recent progress of FOH is introduced from five aspects, including large-scale FOH array, very-low-frequency detection, fiber-optic vector hydrophone (FOVH), towed linear array, and deep-sea and long-haul transmission. The above five aspects indicate the future development trends in the FOH research field, and they also provide a guideline for the practical applications of FOH as well as its array.

Phase-sensitive optical time domain reflectometry (Ф-OTDR) is an effective way to detect vibrations and acoustic waves with high sensitivity, by interrogating coherent Rayleigh backscattering light in sensing fiber. In particular, fiber-optic distributed acoustic sensing (DAS) based on the Ф-OTDR with phase demodulation has been extensively studied and widely used in intrusion detection, borehole seismic acquisition, structure health monitoring, etc., in recent years, with superior advantages such as long sensing range, fast response speed, wide sensing bandwidth, low operation cost and long service lifetime. Significant advances in research and development (R&D) of Ф-OTDR have been made since 2014. In this review, we present a historical review of Ф-OTDR and then summarize the recent progress of Ф-OTDR in the Fiber Optics Research Center (FORC) at University of Electronic Science and Technology of China (UESTC), which is the first group to carry out R&D of Ф-OTDR and invent ultra-sensitive DAS (uDAS) seismometer in China which is elected as one of the ten most significant technology advances of PetroChina in 2019. It can be seen that the Ф-OTDR/DAS technology is currently under its rapid development stage and would reach its climax in the next 5 years.

During last decades, sensor elements based on the fiber Bragg grating (FBG) have been widely studied and developed due to the advantages of immunity to electromagnetic interference, compact size, high precision, and so on. The FBG itself is sensitive to axial strain and temperature variation directly and can indirectly measure these complex physical parameters, such as pressure, displacement, and vibration, by using some specially designed elastic structures to convert them into the axial strain of the FBG. Whether the FBG is fixed on the measured object to measure the strain directly or fixed on an elastic structure body to measure other physical quantities, these types of FBGs could be collectively called as strain sensing FBGs. The packaging of the FBG has important influence on FBG characteristics that directly affect the measurement accuracy, such as strain transfer, temperature characteristic, and spectral shape. This paper summarizes the packaging methods and corresponding temperature compensation methods of the currently reported strain sensing FBGs, focusing especially on fully pasted FBG, pre-stretched FBG with double-end fixed, and metallic packaging. Furthermore, the advantages and drawbacks of different packaging methods have been analyzed, which can provide a reference for future researches.

This paper reports the state of art in a variety of pressure and the detailed study of various matrix based pressure sensors. The performances of the bridges, buildings, etc. are threatened by earthquakes, material degradations, and other environmental effects. Structural health monitoring (SHM) is crucial to protect the people and also for assets planning. This study is a contribution in developing the knowledge about self-sensing smart materials and structures for the construction industry. It deals with the study of self-sensing as well as mechanical and electrical properties of different matrices based on pressure sensors. The relationships among the compression, tensile strain, and crack length with electrical resistance change are also reviewed.

This paper reviews the work on huge capacity fiber-optic sensing network based on ultra-weak draw tower gratings developed at the National Engineering Laboratory for Fiber Optic Sensing Technology (NEL-FOST), Wuhan University of Technology, China. A versatile drawing tower grating sensor network based on ultra-weak fiber Bragg gratings (FBGs) is firstly proposed and demonstrated. The sensing network is interrogated with time- and wavelength-division multiplexing method, which is very promising for the large-scale sensing network.

The recent research progress in the key device and technology of the fiber optic sensor network (FOSN) is introduced in this paper. An architecture of the sensor optical passive network (SPON), by employing hybrid wavelength division multiplexing/time division multiplexing (WDM/TDM) techniques similar to the fiber communication passive optical network (PON), is proposed. The network topology scheme of a hybrid TDM/WDM/FDM (frequency division multiplexing) three-dimension fiber optic sensing system for achieving ultra-large capacity, long distance, and high resolution sensing performance is performed and analyzed. As the most important device of the FOSN, several kinds of light source are developed, including the wideband multi-wavelength fiber laser operating at C band, switchable and tunable 2 μm multi-wavelength fiber lasers, ultra-fast mode-locked fiber laser, as well as the optical wideband chaos source, which have very good application prospects in the FOSN. Meanwhile, intelligent management techniques for the FOSN including wideband spectrum demodulation of the sensing signals and real-time fault monitoring of fiber links are presented. Moreover, several typical applications of the FOSN are also discussed, such as the fiber optic gas sensing network, fiber optic acoustic sensing network, and strain/dynamic strain sensing network.

This paper reviews high temperature sensing applications based on fiber Bragg gratings fabricated by use of femtosecond laser. Type II fiber Bragg gratings fabricated in the silica fiber can sustain up to 1200 ℃ while that fabricated in the sapphire fiber have the good thermal stability up to 1745 ℃.

The most relevant aspects related to the phase mask dithering/moving method for the fabrication of complex Bragg grating designs are reviewed. Details for experimental implementation of this technique is presented, including theoretical analysis of the calibration functions for the correct dither/displacement. Results from tailored Bragg grating structures fabricated by this method are shown. Apodized Bragg gratings with modeled spatial profiles were implemented, resulting in side mode suppression levels of more than 20 dB in gratings showing transmission filtering level higher than 30 dB. Chirped gratings with the spectral bandwidth up to 4 nm, π-shift and sampled Bragg gratings with equalized peaks equally spaced by 0.8 nm (100 GHz) were also fabricated.

This work presents a short review of the current research on the acousto-optic mechanism applied to optical fibers. The role of the piezoelectric element and the acousto-optic modulator in the excitation of flexural and longitudinal acoustic modes in the frequency range up to 1.2 MHz is highlighted. A combination of the finite elements and the transfer matrix methods is used to simulate the interaction of the waves with Bragg and long period gratings. Results show a very good agreement with experimental data. Recent applications such as the writing of gratings under the acoustic excitation and a novel viscometer sensor based on the acousto-optic mechanism are discussed.

The ability to operate despite failure will become increasingly important as the use of optical sensor networks grows, and the amount of sensing information to be handled by a sensor network is increasing, especially for safety and security applications. In this review, the four categories of protection to allow service to be reestablished after a failure (dedicated/shared and line/path) are thoroughly discussed. This paper also presents an overview of the most representative robust fiber-optic sensor systems, discussing their schemes, pros and cons.

A brief review on biconical tapered fiber sensors for biosensing applications is presented. A variety of configurations and formats of this sensor have been devised for label free biosensing based on measuring small refractive index changes. The biconical nonadiabatic tapered optical fiber offers a number of favorable properties for optical sensing, which have been exploited in several biosensing applications, including cell, protein, and DNA sensors. The types of these sensors present a low-cost fiber biosensor featuring a miniature sensing probe, label-free direct detection, and high sensitivity.

Fiber optic sensors have a set of properties that make them very attractive in biomechanics. However, they remain unknown to many who work in the field. Some possible causes are scarce information, few research groups using them in a routine basis, and even fewer companies offering turnkey and affordable solutions. Nevertheless, as optical fibers revolutionize the way of carrying data in telecommunications, a similar trend is detectable in the world of sensing. The present review aims to describe the most relevant contributions of fiber sensing in biomechanics since their introduction, from 1960s to the present, focusing on intensity-based configurations. An effort has been made to identify key researchers, research and development (R&D) groups and main applications.

The “lab-on-fiber” concept envisions novel and highly functionalized technological platforms completely integrated in a single optical fiber that would allow the development of advanced devices, components and sub-systems to be incorporated in modern optical systems for communication and sensing applications. The realization of integrated optical fiber devices requires that several structures and materials at nano- and micro-scale are constructed, embedded and connected all together to provide the necessary physical connections and light-matter interactions. This paper reviews the strategies, the main achievements and related devices in the lab-on-fiber roadmap discussing perspectives and challenges that lie ahead.

Consigned to the shadows of telecommunications, optical sensing has often taken a back seat in a young person’s mind when considering the importance of photonics, or optics, to the advancement of the society and of knowledge. Here, I touch on briefly how broad optical sensing and sensing generally has become and how and why it is becoming the catalyst for the convergence of many technologies and in the process raising significant philosophical questions about the transformation of our society and indeed ourselves. In doing so I touch on many of the complexities in real life that influence the breakthroughs we see today, including a healthy speculation and critique on our society and an awareness of the motivations to improve it that drive many of them.

This article reviews my new optical fiber sensing (OFS) research activities in China for the last ten years at Chongqing University and University of Electronic Science and Technology of China, since I returned from UK in 1999. The research progress in long period fiber gratings (LPFGs), distributed fiber sensing systems and microfiber sensors is introduced. For LPFGs, the processing method with high-frequency CO2 laser pulses types of LPFGs fabricated and the related applications for both optical sensing and optical communication are described. For distributed fiber sensing systems, the fiber-optic polarization optical time domain reflectometer (POTDR), fiber-optic phase-sensitive optical time domain reflectometer (Φ-OTDR) and Brillouin optical time-domain analyzer (BOTDA) are developed, respectively. For microfiber sensors, we mainly focus on the knot resonator and its application for sensing of the refractive index and acceleration, etc.

A brief review of recent progress in researches, productions and applications of full distributed fiber Raman photon sensors at China Jiliang University (CJLU) is presented. In order to improve the measurement distance, the accuracy, the space resolution, the ability of multi-parameter measurements, and the intelligence of full distributed fiber sensor systems, a new generation fiber sensor technology based on the optical fiber nonlinear scattering fusion principle is proposed. A series of new generation full distributed fiber sensors are investigated and designed, which consist of new generation ultra-long distance full distributed fiber Raman and Rayleigh scattering photon sensors integrated with a fiber Raman amplifier, auto-correction full distributed fiber Raman photon temperature sensors based on Raman correlation dual sources, full distributed fiber Raman photon temperature sensors based on a pulse coding source, full distributed fiber Raman photon temperature sensors using a fiber Raman wavelength shifter, a new type of Brillouin optical time domain analyzers (BOTDAs) integrated with a fiber Raman amplifier for replacing a fiber Brillouin amplifier, full distributed fiber Raman and Brillouin photon sensors integrated with a fiber Raman amplifier, and full distributed fiber Brillouin photon sensors integrated with a fiber Brillouin frequency shifter. The Internet of things is believed as one of candidates of the next technological revolution, which has driven hundreds of millions of class markets. Sensor networks are important components of the Internet of things. The full distributed optical fiber sensor network (Rayleigh, Raman, and Brillouin scattering) is a 3S (smart materials, smart structure, and smart skill) system, which is easy to construct smart fiber sensor networks. The distributed optical fiber sensor can be embedded in the power grids, railways, bridges, tunnels, roads, constructions, water supply systems, dams, oil and gas pipelines and other facilities, and can be integrated with wireless networks.

A brief review on suspended-core fibers for sensing applications is presented. A historical overview over the previous ten years about this special designed microstructure optical fiber is described. This fiber presents attractive optical properties for chemical/biological or gas measurement, but it can be further explored for alternative sensing solutions, namely, in-fiber interferometers based on the suspended-core or suspended-multi-core fiber, for physical parameter monitoring.

The authors review the recent advances in fabricating long-period gratings (LPGs) in photonic crystal fibers (PCFs). The novel light-guiding properties of the PCFs allow the demonstration of novel sensors and devices based on such LPGs. The sensitivity of these PCF LPGs to temperature, strain and refractive index is discussed and compared with LPGs made on conventional single-mode fibers. In-fiber devices such as tunable band rejection filters, Mach-Zehnder interferometers are discussed.

We proposed two schemes of generating and localizing dynamic gratings in optical fibers: one is based on the gain saturation in erbium-doped fiber; the other is based on Brillouin scattering in the fiber. By using these dynamic gratings, fully distributed strain/temperature sensors have been demonstrated. In this presentation, we review the principles, basic schemes, and experimental demonstrations of the novel dynamic grating techniques.

An overview of recent researches of surface plasmon resonance (SPR) sensing technology in Laboratory of Science and Technology of Micro-Nano Optics (LMNO), University of Science and Technology of China, is presented. Some novel SPR sensors, such as sensors based on metallic grating, metal-insulator-metal (MIM) nanoring and optical fiber, are designed or fabricated and tested. The sensor based on localized surface plasmon resonance (LSPR) of metallic nanoparticles is also be summarized. Because of the coupling of propagating surface plasmons and localized surface plasmons, the localized electromagnetic field is extremely enhanced, which is applied to surface-enhanced Raman scattering (SERS) and fluorenscence enhancement. Future prospects of SPR and/or LSPR sensing developments and applications are also discussed.

The combination of fiber optics with nano-structure technologies and sensitive thin films offers great potential for the realization of novel sensor concepts. Miniatured optical fiber sensors with thin films as sensitive elements could open new fields for optical fiber sensor applications. Thin films work as sensitive elements and transducer to get response and feedback from environments, in which optical fibers are employed to work as signal carrier. This article presents some research work conducted at the National Engineering Laboratory for Optical Fiber Sensing Technologies in recent years. Concrete examples are: Pd/WO3 co-sputtered coating as sensing material for optical hydrogen sensors shows robust mechanical stability and meanwhile good sensing performance; TbDyFe magnetostrictive coating directly deposited on fiber Bragg grating (FBG) demonstrates its possibility of miniature optical magnetic field/current sensors, and 40-pm shift of the FBG wavelength happens at a magnetic field order of 50mT.

The authors review their recent advances in the development of optical fiber Bragg grating (FBG) sensor technologies. After a brief review of the fiber grating sensors, several newly developed FBG sensors are described. With the continuous development of fiber materials, microstructures and post-processing technologies, FBG sensors are still creative after the first demonstration of permanent gratings thirty years ago.

This article reviews author’s research work on fiber-optic sensors over the last twenty years. It includes two aspects: low-coherence interferometric sensors (LCI) and fiber Bragg grating (FBG) sensors. For LCI sensors, author’s work mainly focuses on the interrogation and multiplexing methods for Fizeau and Fabry-Perot interferometric sensors at the University of Kent at Canterbury (UKC), UK, and study on novel Fabry-Perot interferometric sensors and their multiplexing methods at Chongqing University (CQU) and University of Electronic Science & Technology of China (UESTC), China, respectively. For FBG sensors, a number of multiplexing schemes are proposed and demonstrated at UKC, and then novel methods for realization of multi-parameter measurement and long-distance measurement based on the FBG sensor and its combination with other optical fiber sensors are also reported at CQU & UESTC. Thus, author’s study on these two topics can be divided into two periods, at UKC and at CQU & UESTC, China. This review is presented in such a time sequence.

The author’s research activities undertaken at the Applied Optics Group, the University of Kent at Canterbury are reviewed, during his time there from 1988-1992 and 1994-1996, followed by a summary of recent research. The areas of interest are high finesse ring resonators, tunable optical filters, novel optical fiber grating sensors in glass and polymer, femtosecond laser inscription and micromachining, environmental pollution monitoring, hydrogen activated Pd films on silicon and impurity measurement on silicon wafers.

In this paper the author describes research and development activities in the field of optical sensing undertaken since his time within the Applied Optics Group at the University of Kent. The main topics covered are laser vibrometry and optical range-finding techniques. The author also gives a summary of his research at University of Kent, covering 1982-1985.

In this paper, an overview of author’s research is presented, commencing at the University of Kent under Prof. David A. Jackson. Early research in short optical pulses and fiber-optic delay-line digital correlators led to optical communications research in code-division multiple access networking. This research was based on broadband incoherent light, and this theme continued with research into spectrum-sliced wavelength-division multiplexing. In shifting from photonics research to biomedical optics and biophotonics in the late 1990s, the emphasis on exploiting broadband light continued with research in optical coherence tomography, amongst other topics. In addition to the research outcomes, how these outcomes were attained is described, including mention of the exceptional contributions of many of my colleagues.

This paper provides a short overview of the time I spent as a member of the Applied Optics Group at the University of Kent (1985-1989) followed by a review of my research during my time at Cranfield University (1989 to date).

Different types of fiber-optic sensors based on glass or polymeric fibers are used to evaluate material behavior or to monitor the integrity and long-term stability of load-bearing structure components. Fiber-optic sensors have been established as a new and innovative measurement technology in very different fields, such as material science, civil engineering, light-weight structures, geotechnical areas as well as chemical and high-voltage substations. Very often, mechanical quantities such as deformation, strain or vibration are requested. However, measurement of chemical quantities in materials and structure components, such as pH value in steel reinforced concrete members also provides information about the integrity of concrete structures. A special fiber-optic chemical sensor for monitoring the alkaline state (pH value) of the cementitious matrix in steel-reinforced concrete structures with the purpose of early detection of corrosion-initiating factors is described. The paper presents the use of several fiber-optic sensor technologies in engineering. One example concerns the use of highly resolving concrete-embeddable fiber Fabry-Perot acoustic emission (AE) sensors for the assessment of the bearing behaviour of large concrete piles in existing foundations or during and after its installation. Another example concerns fiber Bragg grating (FBG) sensors attached to anchor steels (micro piles) to measure the strain distribution in loaded soil anchors. Polymer optical fibers (POF) can be — because of their high elasticity and high ultimate strain - well integrated into textiles to monitor their deformation behaviour. Such “intelligent” textiles are capable of monitoring displacement of soil or slopes, critical mechanical deformation in geotechnical structures (dikes, dams, and embankments) as well as in masonry structures during and after earthquakes.

Intensity-modulated fiber Bragg grating (FBG) sensors, compared with normal wavelength-encoding FBG sensors, can reduce the cost of sensor system significantly by using cost-efficient optical power detection devices, instead of expensive wavelength measurement instruments. Chirped-FBG (CFBG) based intensity-modulated sensors show potential applications in various sensing areas due to their many advantages, including inherent independence of temperature, high measurement speed, and low cost, in addition to the merits of all fiber-optic sensors. This paper theoretically studies the sensing principle of CFBG-based intensity-modulated sensors and briefly reviews their recent progress in measurement of displacement, acceleration, and tilt angle.

Oxygen and carbon dioxide sensors are involved in many chemical and biochemical reactions. Consequently, considerable efforts over years have been devoted to discover and improve suitable techniques for measuring gas concentrations by optical fiber sensors. Optical gas sensors consist of a gas-sensitive dye entrapped in a matrix with a high permeability to gas. With such sensors, gas concentration is evaluated based upon the reduction in luminescence intensity caused by gas quenching of the emitting state. However, the luminescence quenching effect of oxygen is highly sensitive to temperature. Thus, a simple, low-cost plastic optical fiber sensor for dual sensing of temperature and oxygen is presented. Also, a modified Stern-Volmer model is introduced to compensate for the temperature drift while the temperature is obtained by above dual sensor. Recently, we presented highly-sensitive oxygen and dissolved oxygen sensors comprising an optical fiber coated at one end with platinum (II) meso-tetrakis(pentafluorophenyl)porphyrin (PtTFPP) and PtTFPP entrapped core-shell silica nanoparticles embedded in an n-octyltriethoxysilane(Octyl-triEOS)/tetraethylorthosilane (TEOS) composite xerogel. Also, two-dimensional gas measurement for the distribution of chemical parameters in non-homogeneous samples is developed and is of interest in medical and biological researches.

Grating writing in structured optical fibers is reviewed. Various laser sources have been used including UV and near IR nanosecond and femtosecond lasers, each enabling different material processing regimes. The issue of scattering is modeled through simulation and compared with experiment. Good agreement has been established.

A review is presented of several technical solutions developed by the Applied Optics Group (AOG) in the field of low coherence interferometry applied to optical fiber sensors (OFS) that subsequently allowed AOG to quickly progress in the field of optical coherence tomography (OCT).

This paper summarizes research activities at University of Kent over the period from September 1981 through November 1984. Subsequent researches undertaken in the US Naval Research Laboratory and two start-up companies are also described.

In this paper I describe research activities in the field of optical fiber sensing undertaken by me after leaving the Applied Optics Group at the University of Kent. The main topics covered are long period gratings, neural network based signal processing, plasmonic sensors, and polymer fiber gratings. I also give a summary of my two periods of research at the University of Kent, covering 1985-1988 and 1991-2001.

The distributed sensor is proven to be a powerful tool for civil structural and material process monitoring. Brillouin scattering in fiber can be used as point sensors or distributed sensors for measurement of temperature, strain, birefringence and vibration over centimeters (Brillouin grating length) for point sensor or the pulse length for the distributed sensor. Simultaneous strain and temperature measurement with a spatial resolution of 20 cm is demonstrated in a Panda fiber using Brillouin grating technique with the temperature accuracy and strain accuracy of 0.4 ℃ and 9 με. This technique can also be used for distributed birefringence measurement. For Brillouin optical time domain analysis (BOTDA), we have developed a new technique to measure differential Brillouin gain instead of Brillouin gain itself. This technique allows high precision temperature and strain measurement over long sensing length with sub-meter spatial resolution: 50-cm spatial resolution for 50-km length, using return-to-zero coded optical pulses of BOTDA with the temperature resolution of 0.7 ℃, which is equivalent to strain accuracy of 12 με. For over 50-km sensing length, we proposed and demonstrated frequency-division-multiplexing (FDM) and time-division-multiplexing (TDM) based BOTDA technique for 75-km and 100-km sensing length without inline amplification within the sensing length. The spatial resolution of 2 m (100 km) and Brillouin frequency shift accuracy of 1.5 MHz have been obtained for TDM based BOTDA and 1-m resolution (75 km) with Brillouin frequency shift accuracy of 1 MHz using FDM based BOTDA. The civil structural health monitoring with BOTDA technique has been demonstrated.

The up to date progress of fiber sensing technologies in Tianjin University are proposed in this paper. Fiber-optic temperature sensor based on the interference of selective higher-order modes in circular optical fiber is developed. Parallel demodulation for extrinsic Fabry-Perot interferometer (EFPI) and fiber Bragg grating (FBG) sensors is realized based on white light interference. Gas concentration detection is realized based on intra-cavity fiber laser spectroscopy. Polarization maintaining fiber (PMF) is used for distributed position or displacement sensing. Based on the before work and results, we gained National Basic Research Program of China on optical fiber sensing technology and will develop further investigation in this area.

Fiber sensors have been developed for industry application with significant advantages. In this paper, Fiber sensors for oil field service and harsh environment monitoring which have been investigated in Tsinghua University are demonstrated. By discussing the requirements of practical applications, the key technologies of long-period fiber grating (LPFG) based fiber sensor, optical spectrum analyzer for oil detection, laser induced breakdown spectroscopy (LIBS) system for soil contamination monitoring, and seismic sensor arrays are described.

Pressure sensors based on fiber-optic extrinsic Fabry-Perot interferometer (EFPI) have been extensively applied in various industrial and biomedical fields. In this paper, some key improvements of EFPI-based pressure sensors such as the controlled thermal bonding technique, diaphragm-based EFPI sensors, and white light interference technology have been reviewed. Recent progress on signal demodulation method and applications of EFPI-based pressure sensors has been introduced. Signal demodulation algorithms based on the cross correlation and mean square error (MSE) estimation have been proposed for retrieving the cavity length of EFPI. Absolute measurement with a resolution of 0.08 nm over large dynamic range has been carried out. For downhole monitoring, an EFPI and a fiber Bragg grating (FBG) cascade multiplexing fiber-optic sensor system has been developed, which can operate in temperature 300 ℃ with a good long-term stability and extremely low temperature cross-sensitivity. Diaphragm-based EFPI pressure sensors have been successfully used for low pressure and acoustic wave detection. Experimental results show that a sensitivity of 31 mV/Pa in the frequency range of 100 Hz to 12.7 kHz for aeroacoustic wave detection has been obtained.

Recent developments in spectral white-light interferometry (WLI) are reviewed. Firstly, the techniques for obtaining optical spectrum are introduced. Secondly, some novel measurement techniques are reviewed, including the improved peak-to-peak WLI, improved wavelength-tracking WLI, Fourier transform WLI, and 3 × 3 coupler based WLI. Furthermore, a hybrid measurement for the intensity-type sensors, interferometric sensors, and fiber Bragg grating sensors is achieved. It is shown that these developments have assisted in the progress of WLI.

In this paper, the mode coupling mechanism of tilted fiber Bragg gratings (TFBGs) is briefly introduced at first. And a general review on the fabrication, theoretical and experimental research development of TFBGs is presented from a worldwide perspective, followed by an introduction of our current research work on TFBGs at the Institute of Modern Optics, Nankai University (IMONK), including TFBG sensors for single-parameter measurements, temperature cross sensitivity of TFBG sensors, and TFBG-based interrogation technique. Finally, we would make a summary of the related key techniques and a remark on prospects of the research and applications of TFBGs.

An overview of fiber laser sensing is presented. The design and the characteristics of distributed feedback (DFB) fiber lasers for high performance sensing applications are described. Demodulation techniques based on unbalanced fiber interferometer are discussed, especially for the noise level, the dynamic range, and the crosstalk in dense-wavelength-division multiplexing. Finally, the fiber laser sensing system configurations and field demonstrations for different applications are illustrated.

As a low-dimensional optical fiber with diameter close to or below the wavelength of light, optical micro/nanofiber (MNF) offers a number of favorable properties for optical sensing, which have been exploited in a variety of sensing applications, including physical, chemical, and biological sensors. In this paper we review the principles and applications of silica, glass, and polymer optical micro/nanofibers for physical and chemical sensing.

A brief review of recent progress made in a range of in-fiber integrated interferometers for measuring is presented, with particular attention paid to the multi-core based in-fiber integrated techniques, which have the potential to be exploited in a variety of wide applications.