•A novel fabrication method of in-fiber rectangular air Fabry-Perot (FP) strain sensor is proposed.•The length of FP cavity can be precisely controlled by a high-precision fiber cutting platform.•The in-fiber rectangular air FP cavity with different lengths have been fabricated and compared.•The cross-sensitivity between strain and temperature can be effectively overcome in the proposed sensor.An in-fiber rectangular air Fabry-Perot (FP) strain sensor based on a high-precision fiber cutting platform (HFCP) is proposed. The HFCP consisting of a CCD notation system, a micro-displacement platform, and an optical fiber cleaver can be used to precisely control the length of FP cavity. The microcavity of FP (even only tens of microns) with smooth reflective surface can be realized easily by using this system. The FP structures with different cavity lengths have been fabricated in this paper. Simulation and experimental results prove that the shorter length the cavity has, the higher strain sensitivity and the larger free spectral range (FSR) the sensor obtains. The strain sensitivity and FSR of in-fiber rectangular air FP sensor with a cavity length of 35 µm can be up to 2.23 pm/με and 28.5 nm respectively. Moreover, the proposed FP strain sensor has a negligible temperature sensitivity in the range of 25–75 °C. It is anticipated that such easy making, compact and low-cost fiber-optic strain sensors could find important applications in practice.

•A RI sensor based on an asymmetrical MZ interferometer with two step-like tapers is proposed.•The step-like taper is fabricated by fusion splicing two half tapers with an appropriate offset.•The sensor has a high sensitivity of −187.67 nm/RIU in the RI range of 1.3409–1.3736. A novel refractive index (RI) sensor based on an asymmetrical Mach–Zehnder interferometer (MZI) with two different step-like tapers is proposed. The step-like taper is fabricated by fusion splicing two half tapers with an appropriate offset. By further applying offset and discharging to the last fabricated step-like taper of MZI, influence of taper parameters on interference spectrum is investigated using only one device. This simple technique provides an on-line method to sweep parameters of step-like tapers and speeds up the optimization process of interference spectrum, meanwhile. In RI sensing experiment, the sensor has a high sensitivity of −185.79 nm/RIU (refractive index unit) in the RI range of 1.3333–1.3673.

•Developments of microfiber resonant ring sensors have been reviewed.•Preparation methods have been summarized.•Different sensing performances have been compared and analyzed.•Possible research content have been prospected.Due to its advantages of strong evanescent field, optical limiting, easily integrating with common single-mode fiber, the micro-nano optical fiber has gotten a worldwide attention in recent years. The sensing technology based on microfiber resonant ring has the advantages of anti-interference, quick response, high resolution, small size and stable measurement, which enable its potential applications in the food industry, manufacturing and environment monitoring to determine the temperature, humidity, refractive index, and current. In this paper, the developments and applications of the microfiber resonant ring sensors were reviewed from three aspects: the manufacture methods for different types of microfiber resonance ring were summarized; the applications were described and analyzed in detail; the possible research content in future was prospected.

•A fiber-optic sensor with an F-P Interferometer and an etched FBG is proposed.•The proposed sensor can measure strain and magnetic field simultaneously.•The etched FBG is sealed in a capillary with ferrofluids to detect magnetic field.•The sensitivities of the strain measurement is up to 1.41 pm/με.•The sensitivities of the magnetic field measurement is up to 5.11 pm/mT.A hybrid fiber-optic sensor consisting of a micro extrinsic Fabry-Perot Interferometer (MEFPI) and an etched fiber Bragg grating (FBG) is proposed, which can measure strain and magnetic field simultaneously. The etched FBG is sealed in a capillary with ferrofluids to detect the surrounding magnetic field. FBG with small diameter will be more sensitive to magnetic field is confirmed by simulation results. The MEFPI sensor that is prepared through welding a short section of hollow-core fiber (HCF) with single-mode fiber (SMF) is effective for strain detection. The experiment shows that strain and magnetic field can be successfully simultaneously detected based on hybrid MEFPI/FBG sensor. The sensitivities of the strain and magnetic field intensity are measured to be up to 1.41 pm/με and 5.11 pm/mT respectively. There is a negligible effect on each other, hence simultaneously measuring strain and magnetic field is feasible. It is anticipated that such easy preparation, compact and low-cost fiber-optic sensors for simultaneous measurement of strain and magnetic field could find important applications in practice.

•A novel refractive index sensor based on photonic crystal fiber Mach–Zehnder interferometer was proposed.•It was realized by cascading a section of PCF with halftaper collapse regions between two single mode fibers.•The relationship between RI sensitivity and interference length of the PCF-MZI was firstly investigated.•A high RI sensitivity of 181.96nm/refractive index unit (RIU) was achieved in the RI range of 1.33331.3574. A novel refractive index (RI) sensor based on photonic crystal fiber Mach–Zehnder interferometer (PCF-MZI) was proposed. It was realized by cascading a section of PCF with half-taper collapse regions (HTCRs) between two single mode fibers (SMFs). The relationship between RI sensitivity and interference length of the PCF-MZI was firstly investigated. Both simulation and experimental results showed that RI sensitivity increased with the increase of interference length. Afterwards, influence of HTCR parameters on RI sensitivity was experimentally investigated to further improve the sensitivity. With intensification of arc discharge intensity in HTCR fabrication process, HTCR with larger maximum taper diameter and longer collapsed region length was obtained, which enhanced evanescent field of the PCF-MZI and then generated higher RI sensitivity. Consequently, a high RI sensitivity of 181.96 nm/refractive index unit (RIU) was achieved in the RI range of 1.3333–1.3574. Increasing arc discharge intensity in HTCR fabrication process has the capacity to improve RI sensitivity of PCF-MZI and meanwhile provides higher mechanical strength and longer sensor life compared to the traditional method of tapering the fiber, which improves the RI sensitivity at the cost of reducing mechanical strength of the sensor. This PCF-MZI was characterized by high RI sensitivity, ease of fabrication, high mechanical strength, and robustness.

All-fiber Mach-Zehnder interferometers (MZIs) working as curvature sensors have been attracting increasing attention. However, previously reported MZI-based curvature sensors with offset or abrupt-taper structures suffer from weak mechanical strength and low curvature sensitivity. In this letter, a robust and highly-sensitive MZI-based curvature sensor formed by cascading two up-tapers in a single mode fiber (SMF) is proposed and experimentally demonstrated. By optimizing structural parameters of the MZI both in simulation and in experiment, a high quality interference spectrum with a maximum extinction ratio (ER) of around 20 dB is obtained, which is crucial in curvature sensing with spectrum intensity variation and interference dip identifiability being considered.Experimental results show that a high curvature sensitivity of −35.41 nm/m−1 is obtained in the range of 0.6846 m−1– 1.0606 m−1, which is 2.4 times higher than that of the interferometer with single-mode-multimode-single-mode (SMS) structure. The two up-tapers, featured by great mechanical strength, are fabricated by only fusion pushing together a whole SMF without cleaving procedure, contributing to excellent repeatability and consistency in the sensor fabrication process. Such an all-fiber sensor exhibits advantages of simple configuration, ease of fabrication, excellent mechanical strength, low cost, and high curvature sensitivity.

•A fiber ring cavity laser refractive index (RI) and temperature sensor is presented.•A core-offset Mach-Zehnder interferometer is utilized as a sensing element as well as a filter.•The Erbium doped fiber ring cavity laser setup is helpful to improve the spectral quality factor Q and the detection limit.•The calculated detection limits of 0.35 °C and 4.54 × 10−4 for temperature and RI measurement are 10 times higher than the individual MZ interferometer.In this paper, a fiber ring cavity laser refractive index (RI) and temperature sensor is presented. A core-offset Mach-Zehnder (MZ) interferometer is utilized as a sensing element as well as a filter. The core-offset distances of these two joints fabricated by a common fusion splicer are about 3.7 μm, which well ensure the robustness and tensile strength. The directions of these two offsets are unnecessary to be controlled since low fringe visibility is acceptable in this sensor. The Erbium doped fiber (EDF) ring cavity laser setup is helpful to improve the spectral quality factor Q and the detection limit (DL). The Q value of this laser sensor can be higher than that of ordinary core-offset sensing structures by two magnitudes. By combining the core-offset MZ interferometer with EDF ring cavity laser, the final calculated detection limits for temperature and RI measurement are 0.35 °C and 4.54 × 10−4, which are 10 times higher than those of the individual MZ interferometer. The proposed sensor possesses the advantages of low cost, simple operation and easy fabrication process.

•A composite interference structure for simultaneous measurement of RI and temperature is proposed and experimentally demonstrated.•The proposed structure was constructed with the combination of Sagnac loop mirror and balloon-like interferometer.•As the two sensing structures are based different interference principles, the two interference spectra do not interfere with each other.•Experimental results show that the sensitivities of the RI and temperature can reach 218.56 nm/RIU and 1.7 nm/ °C, respectively.A novel composite interference structure constructed with the combination of Sagnac loop mirror and balloon-like interferometer for simultaneous measurement of refractive index (RI) and temperature is proposed and experimentally demonstrated. A polarization maintaining fiber (PMF) is embedded into the Sagnac loop structure to form a Sagnac loop interferometer, which is highly sensitive to external temperature variations. The balloon-like structure is built with a bend single mode fiber (SMF) to form a modal interferometer, which is sensitive to both external RI and temperature variations. As the two sensing structures are based different interference principles, the two interference spectra do not interfere with each other. By optimizing the bending diameter of the balloon-like interferometer and the length of PMF in the Sagnac loop mirror, separate different resonance wavelengths can be well formed. Hence, the simultaneous measurement of RI and temperature can be realized by monitoring the shift of different resonance wavelengths. Experimental results show that the optimal sensitivities of the RI and temperature can reach up to 218.56 nm/RIU and 1.7 nm/°C, respectively.

Graphene as a novel material has laid a foundation for its applications in optical fiber sensors, due to its unique properties, especially the optical properties. On the other hand, optical fiber sensors have received world-wide attention due to their high sensitivity, small size, good anti-electromagnetism disturbance ability and other potential advantages. In this paper, the developments of graphene in the applications of optical fiber sensors were reviewed from four aspects. Firstly, the common preparation methods of graphene were introduced. Next, the optical properties of graphene have been concluded. And then, some typical optical fiber chemical and biological sensors based on graphene, such as temperature sensors, biological sensors and gas sensors, were reviewed. It was shown that graphene had a great potential in the optical fiber sensing technology. Furthermore, the deficiencies and challenges of the graphene in the applications of optical fiber sensors were analyzed. In a whole, the unique advantages of graphene have present their versatility and importance in the application fields of optical fiber sensors.

•A novel, highly sensitive and temperature insensitive tilt angle sensor based on an All-PCFI was proposed and demonstrated.•The All-PCFI was formed by a PCF with two collapse regions that were fabricated by fusion discharge technique.•In the measurement range of 0°–45°, the linear measurement sensitivity could be up to 55.67 pm/°.•In addition, the sensor could measure the tilt angle in all directions by demodulating signals of the four cantilever beams.A highly sensitive tilt angle sensor based on an all-photonic crystal fiber interferometer (All-PCFI) was proposed and demonstrated in this paper. The All-PCFI was formed by a PCF with two collapse regions that were fabricated by fusion discharge technique. Then a device formed by cantilever beams and iron ball was designed to transform the tilt angle to the strain, which would be detected by measuring the spectral responses of the All-PCFI. Experimental results showed that the spectrum would be red-shifted when the tilt angle changed from 0° to 90°. And in the measurement range of 0°–45°, the linear measurement sensitivity could be up to 55.67 pm/°. In addition, the sensor could measure the tilt angle in all directions by demodulating signals of the four cantilever beams.

A high-sensitive gas sensing method is proposed, in which miniaturized photonic crystal (PC) cavity is utilized as the sensing element, and cryptophane E molecular is infiltrated in defected holes of PC cavity as the sensitive material to methane gas. Particularly, slow light in side-coupled PC waveguide is optimized to enhance refractive index sensitivity of the PC cavity and fiber loop ring-down technology is used to demodulate output resonant spectrum of the sensing system. Simulation results demonstrate that a refractive index sensitivity of 450 nm/RIU and a quality factor of 1105 can be obtained in the slow light engineered PC cavity. For measurement of methane concentration, the theoretical sensitivity can be up to 4.21 μs/% and the minimum detectable concentration can be as accurate as 2.37 ppm.

•A new fiber optic micro bend sensor has been proposed for respiration monitoring.•The detected respiration rate was verified by counting the breathing manually.•This structure is a kind of seat-back type fiber optic micro bend sensor.•This sensors can be used to measure vital signs because it possess several advantages, such as real-time and accuracy, low cost and convenient operation.A new fiber optic micro bend sensor has been proposed in this paper for respiration monitoring. The detected respiration rate was verified by counting the breathing manually. Therefore the purpose of this paper is to find the appropriate gear numbers and cycles of the optical fiber micro bend modulator in order to enhance the accuracy of the measurement result. The relevant improvement measures will be applied to the medical field in the future. The number of the fiber optic micro bend sensor’s teeth has been determined by the simulation results and based on which the micro bend modulator tooth structure were designed. The size of the tooth is 6 mm, and the number of teeth is 15. This structure is a kind of seat-back type fiber optic micro bend sensor. This sensors can be used to measure vital signs because it possess several advantages, such as real-time and accuracy, low cost and convenient operation.

•A double photoconductive layers biochip focusing biological cells and forming specific pearl chains was studied.•It was composed of two photoconductive layers coated on the bottom and top of ITO-based glass.•The oscillating spatial electric field was employed to induce the motion of polarizable neutral particles.A typical double photoconductive layer biochip focusing biological cells and forming specific pearl chains has been studied theoretically in this paper. It was composed of two photoconductive layers coated on the bottom and top of ITO-based glass. A light pattern was used to create face-to-face virtual electrodes and the resulting oscillatory spatial electric field was employed to induce the motion of polarizable neutral particles. In order to estimate the behaviors of the suspended particles, a numerical model including dielectrophoretic forces, dipole–dipole forces and other forces, was implemented by means of the Monte Carlo method. The results indicated that steady-state chains could be formed in a uniform electric field owing to the dipole moment effect. In a non-uniform electric field created by the use of a light pattern, the positive DEP force created a more focused pattern of chains. The work concerning the numerical simulation indicated that this chip could form fixed-length particle chains in perpendicular alignment to satisfy the structured assembly of tissues in the histological engineering application.

•An all-fiber MZ interferometer for temperature and displacement sensor was proposed•It was fabricated by splicing three section of SMFs with a large core-offset.•Experiments showed the sensitivities were 25.26 pm/°C and 96.72 nW/mm.A novel, versatile, compact all-fiber Mach–Zehnder interferometer (MZI) for temperature and displacement sensing was proposed and demonstrated in this paper. It was fabricated by splicing a section of single mode fiber (SMF) between two SMFs with a large core-offset at two splicing joints, which were used to excite cladding modes and couple to the fiber core. And then the interference occurred between the cladding and external substances was utilized to measure temperature and displacement. Experimental results showed that the sensitivity of temperature measurement could be up to 25.26 pm/°C and the sensitivity of displacement measurement was 96.72 nW/mm. In addition, because the measurement of temperature and displacement could be implemented respectively by recording the light wavelength shifts and light intensity variation, temperature and displacement could be measured simultaneously with only one sensor probe. Meanwhile, there were some other advantages of the sensor, such as simple structure, small size, high sensitivity and low cost.

A simple, ultra compact and highly sensitive photonic crystal fiber interferometer (PCFI) for external refractive index (ERI) sensing was proposed and demonstrated in this paper. The PCFI was formed by splicing photonic crystal fiber (PCF) between two single mode fibers (SMFs) with a slight core-offset. The both joints were up-tapered joints which acted as mode splitter/combiner and were made by fusion tapering technique. The Mach–Zehnder interferometer (MZI) incorporated intermodal interference between core mode and cladding modes of the PCF. When the ERI changed, a RI variation of cladding modes would occur and the output interference spectrum would shift. By measuring the wavelength shift of the interference pattern, temperature-insensitive RI measurement could be achieved. In addition, the refractive index sensing properties with the different PCF diameters were also investigated experimentally. Experimental results showed that RI sensitivity could be up to 252 nm/RIU in the refractive index range of 1.333–1.379. And it could be anticipated that RI sensitivity could be improved if the PCF diameter continues to decrease. Meanwhile, the sensor had the advantages of simple structure, small size, high sensitivity, low cost and low temperature sensitivity.

•A new type of fiber Bragg grating (FBG) gas flow sensor was proposed.•Flow variation could be transformed into the deflection of the cantilever, resulting in the change of FBG wavelength and phase.•By detecting the change of the optical signal, the gas flow can be derived.•The proposed sensor can determine the flow of air within 1–6.5 m3/h and the sensitivity of the sensor approximately reaches 0.22 nm/(m3/h).Based on the structure of the traditional variable area flow meter and the principle of the optical fiber measurement, this article designed a new type of fiber Bragg grating (FBG) gas flow sensor. The tapered tube of the sensor is mounted vertically, and the float inside the tube moves up and down under the effect of the airflow. The longitudinal displacement of the float is converted to the deflection of the cantilever which bonded FBG under the action of magnetic force. Thus the flow variation could be transformed into the deflection of the cantilever, resulting in the change of FBG wavelength and phase. By detecting the change of the optical signal, the gas flow can be derived. The experimental results show that the sensor has the characteristics of good linearity and fast response time. The proposed sensor can determine the flow of air within 1–6.5 m3/h and the sensitivity of the sensor approximately reaches 0.22 nm/(m3/h). By optimizing the parameters of the sensor, the sensitivity and the accuracy can be improved accordingly.

This review covers photonic crystal cavities (PCCs) and their applications in optical sensors, with a particular focus on the structures of different PCCs. For each kind of optical sensor, the specific measurement principle, structure of PCC, and the corresponding sensing properties are all presented in detail. The summary of the reported works and the corresponding results demonstrate that it is possible to realize miniature and high-sensitive optical sensors due to the ultra-compact size, excellent resonant properties, and flexibility in structural design of PCCs. Finally, the key problems and new directions of PCCs for sensing applications are discussed.

•A novel optical fiber temperature sensor based on the principle of fiber loop mirror (FLM) was proposed.•An ethanol-filled high birefringence photonic crystal fiber (HB-PCF) is inserted into the FLM as a temperature sensing element.•Preliminary experimental results indicated that the temperature sensitivity of the two structures were 0.8833 nm/°C and 0.7896 nm/°C, respectively, for a 10 cm long HB-PCF.A compact optical fiber temperature sensor based on the principle of fiber loop mirror (FLM) was proposed. Different from the conventional ones, an ethanol-filled high birefringence photonic crystal fiber (HB-PCF) is inserted into the FLM as a temperature sensing element. Two structures based on the FLM were designed, one is to insert a short length of ethanol-filled HB-PCF in the FLM, and the other is to make a reflective probe with the ethanol-filled HB-PCF, since the independent probe is more suitable for practical application. The refractive index of the ethanol filled in the cladding air holes of HB-PCF would change versus the applied temperature, and the birefringence of the HB-PCF would change as well, which would affect the output wavelength shifts of the FLM. Experiments were carried out to verify the sensor principle. Preliminary experimental results indicated that the temperature sensitivity of the two structures were 0.8833 nm/°C and 0.7896 nm/°C, respectively, for a 10-cm-long HB-PCF.

•Researches on tunable photonic crystal were reviewed with various principles.•Relevant tuning principles were explained in detail.•Representative examples were listed with advantages and applications.•Development was predicted which was useful for further optical devices research.Tunable photonic crystal plays an important role for the fabrication of optical device. The photonic bandgap of it can be tuned by changing its crystal structural parameters or material dielectric properties. Research and development of the tunable photonic crystals will not only enrich the types of photonic crystal but also provide novel ideas and new methods for the development of optical devices. In the paper, the latest research progress on methods and technologies for tunable photonic crystal was reviewed. The relevant tuning principles were explained. The tunable photonic crystal was reviewed from five aspects including magnetic tuning, mechanical tuning, thermal tuning, electrical tuning and biochemical tuning. The representative examples were listed in detail, and advantages and applications of different tunable means were discussed. In the end, its development trend was predicted. The contents of this paper could provide assistance for the study and research of tunable photonic crystal, especially for the optical devices.

Optically induced dielectrophoretic (ODEP) chip is to combine their own advantages of optical tweezers and electrodynamics manipulation technologies, which can trap single particles in high resolution as well as enrich much of micro-/nanoparticles in high throughput. The paper analyzed the structure of optoelectronic tweezers (OET) chip, moreover, the frequency response of multi-membrane eukaryotic cells about 103–109 Hz. The Clausius–Mositti (CM) frequency factor in terms of cell membrane, cell cytoplasm, nuclear envelope thickness changes, and volume ratio was illustrated. In the end, the paper presented 3D numeric model of cells in OET chip. The dielectrophoresis force acting on the dipole of 11.8-μm cells subjected to a non-uniform electric field under 60-μm Gaussian-distributed beam spot could be simulated in the enrichment process. The separation of cells that were two different types of CM values was calculated. Furthermore, it was proved to be feasible to achieve the efficient separation of cells using ODEP technology in the biological numerical model. Comparing with the literature of experiment, the results in cell dielectric spectroscopy and numeric model findings were in general agreement. The simplified structure and numeric model of nucleated cell provide a theoretical basis for research of biosensor and complex life.

Optical-induced dielectrophoresis (ODEP) is a novel technology used in the field of micro-/nanoparticles manipulation. The finite element method was applied for ODEP to research the particles motion in this paper. The potential distribution in the optoelectronic chip, which was induced by the incident light spot, was attained through electric current module in the COMSOL 4.3a. The particles motion was studied by coupling the module of electric current and particle tracing for fluid flow. Compared with molecular dynamics, the method proposed in the paper could effectively simplify the tedious programming. The polystyrene sphere (PS) particles with the radius of 2, 5, 10, and 15 μm were, respectively, used as the objects. The kinetic energy of the PS created by the dielectrophoresis (DEP) forces, the Stokes drag forces, the gravity forces, and the Brownian motion forces was calculated during the whole manipulation process. The simulation results indicated that with the decreasing in the particle size, the time on enrichment of the smaller PS would become longer. It was because that for the smaller PS, the effect of DEP forces would play less important role in the system. The conclusions in this paper could be used as a theoretical guidance in the further research.

•A vibration sensor based on single-mode multimode single-mode fiber structure was demonstrated.•Preliminary experimental results verified the proposed idea.•Vibration frequency measurement error of ±0.5 Hz was obtained.•Measurement range was from 2 Hz to 80 Hz.A vibration-sensing system which is based on single-mode multimode single-mode (SMS) fiber structure was demonstrated in this paper. When the light is coupled from the lead-in single-mode fiber (SMF) into the multimode fiber (MMF), high-order Eigen modes of the MMF are excited and the interference between different modes occurs while the light propagates along the MMF. The interference results will be changed because the vibration signals are acted on the sensing MMF. Theoretical analysis was carried out, then preliminary experimental results verified the proposed idea. Vibration frequency measurement error of 2% as obtained within the range from 2 Hz to 80 Hz by using a lock-in amplifier as the signal detection method.

•An optical fiber Mach–Zehnder interferometer based on slow light in polymer-infiltrated PCW was proposed for an FBG displacement sensor.•By optimizing the structure of PI-PCW, slow light with high group index of 110 was realized.•Differential and orthogonal method was used to demodulate the interference fringes of MZI with high stability and good linearity.•The proposed method demonstrated a high sensitivity of 1.035 rad/mm with good linearity and wide measurement range of 55.6 mm.An optical fiber Mach–Zehnder interferometer (MZI) based on slow light in polymer-infiltrated PCW (PI-PCW) was proposed to enhance the demodulation sensitivity of fiber Bragg grating (FBG) displacement sensor. By optimizing the structure of PI-PCW, slow light with high group index of 110 was realized, which is advantageous to develop high-sensitive MZI. And thanks to electro-optic effect of the infiltrated polymer, the working wavelength of flat band slow light, as well as the demodulation range of MZI, could be flexibility changed and enlarged by tuning external driving voltage. At last, differential and orthogonal method was used to demodulate the interference spectrum of MZI with high stability and good linearity. The FBG was pasted on an Omega-like beam, thus the displacement variation of the free end of the Omega-like beam could be measured by monitoring the output phase of MZI. Finally, the proposed displacement sensor demonstrated a high sensitivity of 1.035 rad/mm with good linearity and wide measurement range of 55.6 mm.

•A refractive index sensor based on a Single-mode Multimode Single-mode fiber structure was experimentally demonstrated.•The dip in the measured spectrum caused by the modal interference shifted obviously when the surrounding RI changed.•The approximate linear relationship between surrounding RI and spectrum dip wavelength shift was obtained.•The measurement sensitivity up to 286.2 nm/RIU was showed with the surrounding RI range from 1.33 to 1.3775.A high sensitive refractive index (RI) sensor based on a Single-mode Multimode Single-mode (SMS) fiber structure was used and experimentally demonstrated. The dip in the measured spectrum signal caused by the modal interference shifted obviously when the surrounding RI changed. The approximate linear relationship between surrounding RI and spectrum dip wavelength shift was obtained experimentally. The measurement sensitivity up to 286.2 nm/RIU was showed with the surrounding RI range from 1.33 to 1.3775, which means the measurement resolution about 3.5 × 10−6 could be implemented if the wavelength shift measurement resolution of the Optical Spectrum Analyzer is 1 pm.

The birefringence value of the birefringent photonic crystal fiber (PCF) is not sensitive to the change of temperature, while the refractive index of ethanol will be changed with the varied temperature. So if the air-holes of PCF are filled with ethanol, the birefringence value will surely change along with the varied temperature, which can be used as a sensing principle to measure temperature. In this paper, a method called End Face Reflection of the optical fiber was used to measure the refractive index against the change of temperature. The experimental results indicated that, on the condition of the incident wavelength of 1550 nm, the refractive index of ethanol changed from 1.3691 to 1.3602 within the temperature ranging from 24.5 °C to 44.5 °C. The birefringence value of the filled PCF changed from 1.8900e-04 to 2.1500e-04. That is to say, the temperature sensitivity coefficient could reach 10e-6 order of magnitude.

FLRDS technique is popular in recent years. It is an absorption spectroscopic and detection technique. It makes use of an optical cavity, which not only realizes a long effective path-length through a sample, but also eliminates the affection of fluctuations of the light intensity. In this paper, a kind of FLRD gas concentration sensor is presented. We analyze the structural parameters, including the couplers, pulse setting parameters, gas cell, and optimization methods of finding ring-down peak signal, etc. By the experimental data, this system can be reached that gas resolution is up to 1000 ppm, stability is 1.02%, and the repeatability error is 2.37%. Finally, we prove the superiority of the system.

A high sensitive multi-component gas sensing system was proposed, in which the slotted photonic crystal waveguide (SPCW) was used as the gas cell and the fiber Fabry–Perot (FFP) was adopted as the wavelength filter. By choosing the refractive index that infiltrated in the first and second rows of air holes adjacent to the slot respectively, the working wavelength of SPCW could be tuned to matches with one absorption line of the measured gas, thus, the system could detect different kinds of gases selectively. Meanwhile, a very high group index of 150 with flat bandwidth could be achieved and the electric field enhancement factor would reach to 12. Combined with the wavelength modulation techniques (WMS) for signal processing, the minimum detection limit of 1.56 ppm was achieved.

Magnetic fluid has many unique optical properties. It has numerous potential applications in developing optical devices because of its versatile optical properties. This paper summarizes the physical origins and control mechanisms of the MF transmission properties, and the related optical devices based on the transmission properties of magnetic fluid. In recent years, there are many applications in optical information communication and sensing technology, such as optical switches, tunable optical gratings, coarse wavelength-division multiplexing, magnetic-field sensors, current sensor. The qualitative and quantitative analysis about the physical configuration, the operating principle, and the characteristics of those optical devices are given. The valuable potential problems and the solutions that are related to optical properties and optical devices based on magnetic fluid are expounded in detail, and potential new types of MF-based optical devices are proposed. It can be concluded that the transmission properties of MF will be improved greatly, and the characteristics of present optical devices based on magnetic fluid will be made better continually and it will play an important role in the fields of optical information communication and sensing technology.Highlights► Magnetic fluid has many unique optical properties. ► These physical origins and control mechanisms of the MF transmission properties have been described in detail. ► The magnetic fluid based optical devices have been concluded and the related characteristics have been analyzed. ► The valuable potential problems and the solutions of MF in optical area are expounded in detail according to our research.

Based on the characteristic of magnetic-controlling refractive index, the magnetic fluid filled in hollow-core photonic crystal fiber (HC-PCF) can be used as the sensitive medium in the cavity of a fiber Fabry–Perot (F–P) magnetic field sensor. The structure and the sensor principle are introduced. The theoretical simulations of the mode distribution of the HC-PCF filled with the magnetic fluid and the sensor output spectra are discussed in detail. The sensor multiplexing capability is indicated as well. Magnetic field measurement sensitivity is about 33 pm/Oe based on the proposed sensor.Highlights► A magnetic sensor is proposed based on an HC-PCF filled with magnetic fluid (MF). ► The principle is the characteristic of magnetic-controlling refractive index of the MF. ► Magnetic field measurement sensitivity is about 33 pm/Oe.

Based on the two structures of the high-birefringent fiber loop mirror (Hi-Bi FLM) with reflection probe, the relevant principles are carried out and experiments are performed to prove the principles. Temperature properties of one structure, known as the FLM with the Hi-Bi reflection probe, are experimentally studied. The temperature sensitivity is 1.698 nm/°C with good linearity, thus the temperature resolution can reach the scale of 0.04 °C in theory if the resolution of the optical spectrum analyzer (OSA) is 0.02 nm. Besides, the temperature range can be tuned by different length or birefringence of the Hi-Bi fiber. Since the reflection probe is one output of the Hi-Bi FLM, the structure can be used for the high sensitivity remote temperature sensing.Highlights► A new fiber sensor structure was proposed based on the high-birefringent fiber loop mirror with reflection probe. ► The relevant principles are carried out and experiments are performed to prove the principles of the new structure. ► This kind structure is useful for the high sensitivity remote temperature sensing. ► The temperature sensitivity is 1.698 nm/°C with good linearity, thus the temperature resolution can reach the scale of 0.04 °C in theory if the resolution of the optical spectrum analyzer (OSA) is 0.02 nm. ► Besides, the temperature range can be tuned by different length or birefringence of the Hi-Bi fiber.

A double-arched-beam-based fiber Bragg grating (FBG) displacement sensor is presented. Unlike most interrogation methods by measuring the resonance wavelength shifts, this kind of FBG sensor is proposed to measure the displacement by measuring the FBG's reflective spectra bandwidth using a blazed fiber Bragg grating (blazed FBG) and a fiber optic array. According to the strain distribution on the novel double-arched beam surface, the positive and negative strain will act on only one FBG, which is stuck on the beam surface. Thus, the positive and negative strain will make the spectra broaden as the displacement increases. What is more, the problem of cross-sensitivity in the FBG sensor is solved because temperature only affects the wavelength shift, but not the spectra bandwidth. The reflective signal of FBG sensor will be lead into a blazed FBG and the radiation light from blazed FBG with a certain radiation angle will be received by a fiber optic array, and then recorded by the infrared linear detector (IRLD). Based on the Gauss distribution theory, the radiation light spot size related to the FBG's reflective spectra bandwidth can be determined. Simulation and preliminary experimental results indicate the feasibility of the proposed idea.

The last decade has been of great significance for the development of slow light technology. Electromagnetically induced transparency (EIT), coherent population oscillation (CPO), stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), soliton collision, and photonic crystal waveguides have been used to slow down the velocity of light. In this paper, some important theoretical and technical developments of slow light technology that occurred over the last decade are discussed. Novel technologies for slowing down the velocity of light and their primary applications are introduced in detail. In addition, the future developing trends of slow light and its potential applications, especially in optical fiber sensors, are also forecasted and proposed in this paper.

•An in-fiber modal interferometer is presented and experimentally demonstrated.•The proposed sensor could be used for curvature and temperature measurement.•Sensing part is fabricated by splicing the single mode fiber and hollow core fiber.•The highest curvature sensitivity of 5.05 dB/m−1 is achieved.An in-fiber modal interferometer is presented and experimentally demonstrated for simultaneous measurement of curvature and temperature. The sensing part is fabricated by splicing single mode fiber (SMF) and hollow core fiber (HCF) via two abrupt tapered joints. Light couples to the wall of HCF due to the collapse in abrupt taper region and then modal interference occurs among multiple modes. Not only intensity but also wavelength of the dip around 1556 nm linearly responses to the curvature and temperature change thus a sensitivity matrix could be built to demodulate both curvature and temperature simultaneously. In addition, the interference dip around 1540 nm performs a decrease trend and the highest curvature sensitivity of 5.05 dB/m−1 is achieved in a wide range from 0.765 m−1 to 3.423 m−1.