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A fiber optic ethanol gas sensor with ZnSnO 3 hollow cube/nano-TiO 2 composite coating based on ultraviolet (UV) radiation sensitization was proposed, which can realize high sensitivity, high selectivity, and fast ethanol gas sensing at room temperature. The probe adopted a simple single-mode fiber (SMF)-multi-mode fiber-SMF structure. The ZnSnO 3 /TiO 2 gas-sensitive film was coated on the sensing area by dip-coating method. Experiment results show that the probe sensitized by UV radiation has a linear sensitivity of 3.85 pm/ppm for ethanol gas at 0–250 ppm, which is about 6.6 times more sensitive than the probe without UV radiation (0.582 pm/ppm). In addition, the probe with UV radiation has good time measurement stability, a fast response time (response time is about 20 s, recovery time is about 60 s), and excellent ethanol gas selectivity. This study proves that UV radiation is an effective method to increase the sensitivity of gas sensing. This is also the first application of ZnSnO 3 /TiO 2 composite material in fiber optic sensors. Combined with the advantages of fiber optic sensing, it shows anti-electromagnetic interference, micro size, high sensitivity, high selectivity, fast response, and remote sensing. • A Fiber optic room temperature ethanol sensor was proposed and fabricated. • The TiO2/ZnSnO3 composite material was firstly applied in fiber optic sensors. • Under the UV radiation, the probe behaves good stability, fast response, excellent selectivity. [ABSTRACT FROM AUTHOR]

A reflective fiber optic sensor is proposed and experimentally validated, which enables simultaneous measurement of temperature and mercury ion (Hg 2+ ) concentration. The sensor consists of a fiber Bragg grating (FBG) and a functionalized no-core fiber (NCF). The chitosan (CS)/poly acrylic acid (PAA), as Hg 2+ sensitive film, is assembled on the surface of the NCF by layer-by-layer self-assembly method. Besides, the silver film is coated on the end of NCF to ensure the reflection of light. This reflective fiber structure forms a fiber modal interferometer. When the sensor is immersed in the test solution, the Hg 2+ adsorption of sensitive film leads to change in its refractive index, and then the interference spectrum of the fiber modal interferometer will shift accordingly, which is the measurement principle of Hg 2+ sensor. Meanwhile, when the surrounding temperature changes, the interference fringe of the fiber modal interferometer and the Bragg wavelength of FBG will shift simultaneously. Therefore, the Hg 2+ concentration and temperature can be simultaneously measured by using dual-wavelength matrix method, and then the cross sensitivity problem of temperature and Hg 2+ concentration can also be solved. The experimental results show that the detection sensitivity of Hg 2+ concentration is 0.0823 nm/μM in the range of 0 μM–100 μM and 0.0178 nm/μM in the range of 100 μM–500 μM, respectively. The temperature sensitivity is 0.0147 nm/°C in the range of 20 °C–50 °C. [ABSTRACT FROM AUTHOR]

A hydrogen sensor based on high-birefringence fiber loop mirror (HBFLM) with sol-gel Pd/WO 3 coating was demonstrated. The sensing structure was formed by inserting a section of polarization maintaining fiber (PMF) with Pd/WO 3 coating in fiber loop mirror. The Pd/WO 3 coating was prepared by sol-gel method and coated on PMF by dip-coating method, which is simple fabrication, low cost, and has good binding force with fiber. When the hydrogen concentration around Pd/WO 3 coating was changed, it would induce the strain change of polarization maintaining fiber, and then shift the interference spectrum of HBFLM. Therefore, the hydrogen concentration can be measured by monitoring the wavelength shift of the interference spectrum. Experimental results showed that the resonance wavelength had a blue shift with the increase of hydrogen concentration and the total shift of the resonance wavelength was ∼2.18 nm within the concentration range of 0–1%. The sensor had simple structure, low cost, high sensitivity, good repeatability, and well stability. [ABSTRACT FROM AUTHOR]

A review for optical fiber hydrogen sensors based on palladium (Pd) and tungsten oxide (WO 3 ) thin films is presented, with specific focus on the measurement methods, probe structures, and sensing properties of different sensors. Firstly, the theoretical models behind the optical fiber hydrogen sensors, as well as their practical limitations, are addressed. Secondly, four mainstream measurement methods, including intensity, fiber Bragg grating (FBG), interferometer, surface plasmon resonance (SPR), which have been proposed to sense the physicochemical properties variations of sensitive thin films when exposed to hydrogen, are reviewed. Then, the advantages and disadvantages of all the above measurement methods are also discussed and compared. Finally, the existing problems and future prospects of optical fiber hydrogen sensors are pointed out. [ABSTRACT FROM AUTHOR]

In this paper, a new optical fiber hydrogen(H 2) sensor based on a self-assembled microbottle resonator was proposed. The self-assembled microbottle was simply obtained by coating the single-mode fiber (SMF) with an internal composite material layer of palladium-tungsten trioxide (Pd-WO 3) that was sensitive to H 2 and an outer organic polymer material layer of polydimethylsiloxane (PDMS) with a significant thermal response. The microbottle resonator was further fabricated by coupling a tapered fiber with the self-assembled microbottle to excite whispering gallery mode (WGM). When the WGM resonator is exposed to H 2 , H 2 molecules penetrate the PDMS and undergo redox reaction with the Pd-WO 3. After the PDMS absorbs the reaction heat, its volume and refractive index will change, along with the wavelength shift of the WGM resonator. Therefore, a simple and low-cost H 2 sensor is realized by observing the shift of resonance wavelength. Experimental results showed that the hydrogen sensor had a maximum sensitivity of − 3.091 nm/% at 25 ℃. And the resonance wavelength just shifted a small distance when the relative humidity increased from 22.87 % to 84.59 % or the temperature rose from 20.2 ℃ to 30 ℃, which indicated the sensor had preferable resistance to disturbances of external humidity and temperature. • A simple and low-cost optical fiber H2 sensor was proposed with high sensitivity of − 3.091 nm/%. • The self-assembled microbottle was simply fabricated by coating Pd-WO 3 and PDMS on a fiber in sequence. • The sensor has preferable resistance to disturbances of external humidity and temperature. [ABSTRACT FROM AUTHOR]

A new method for simultaneous measurement of liquid concentration and temperature is proposed by using a miniature photonic crystal sensor, where two cascaded cavities (H0 cavity and H1 cavity) are separately located adjacent to one waveguide. The standard liquid whose concentration is fixed and known will be infiltrated in defected holes of the H1 cavity, and the measured liquid whose concentration is unknown will be infiltrated in defected holes of the H0 cavity. Both the two independent resonant dips of cascaded cavities that can be simultaneously monitored at output spectrum of the waveguide will shift with the variation of liquid concentration or ambient temperature. By using finite difference time domain (FDTD) simulation, resonant properties of the two cavities are respectively optimized, and then the linear relationships between shifts of two resonant dips and liquid concentration/temperature are calculated. Finally, according to dual-wavelength matrix method, liquid concentration measurement with a resolution of 9.4322 ppm and temperature measurement with a resolution of 0.0136 K are simultaneously realized, which can not only solve the cross-sensitivity problem between liquid concentration and temperature, but also provide a solution for two-parameter measurement in a miniaturized system. [ABSTRACT FROM AUTHOR]

A miniaturized, high-sensitive , and high-precision gas sensor for the measurement of methane concentration was realized in a cryptophane E infiltrated photonic crystal (PC) microcavity, by combing selective adsorption property of cryptophane E to methane and excellent resonant property of PC microcavity. The concentration variation of methane would change the refractive index (RI) of cryptophane E that infiltrated in defected holes of PC microcavity, and then induce a shift in resonant wavelength , allowing precision measurement of methane concentration. Simulation results of PC microcavity showed that the RI sensitivity of 363.8 nm/RIU, quality factor of 12,923, and transmission of 0.9122 could be obtained by increasing the radii of defected holes, which does not require sophisticate geometrical adjustment and high-precision requirement for fabrication. However, its resonant property is comparable to other subtle-tuned PC microcavities . Besides, the designed methane sensor is only in response to methane, which can prevent the interrupt of other gases and enhance the measurement accuracy. Benefiting from the high RI sensitivity and high quality factor of the microcavity, a theoretical detection limit of 697.35 ppm for methane sensing can be achieved. [ABSTRACT FROM AUTHOR]

Abstract: 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 1.56ppm was achieved. [Copyright &y& Elsevier]

Abstract: A miniature and sensitivity gas sensor based on slotted photonic crystal waveguide (SPCW) was presented, in which the correlation spectroscopy method was applied for signal processing and the SPCW was used for the gas cell. To enhance the system sensitivity, the structure of the SPCW was optimized to possess extremely high group index and very large light confinement in the slot. Simulation results indicated that the measuring resolution of acetylene gas of 1ppm was achieved with good temperature stability. Additionally, the active region was only 1mm long, which could be of great advantage in the use of the limited space. [Copyright &y& Elsevier]

Abstract: This paper proposed a high sensitivity gas sensing system based on slow light in photonic crystal waveguide (PCW). The PCW was used as the gas chamber, and the harmonic detection method was adopted for signal processing. Combined the Maxwell equation with perturbation theory, the effective gas absorption factor was deduced which was greatly enhanced due to slow light in PCW. For the measurement of CO gas, a PCW was designed as the gas chamber and the detection sensitivity could be increased by 14.45 times compared with the traditional system that without PCW as the chamber. Thus the measuring detection limit of 21.5ppm is achieved with active region is only 1mm long. The proposed method provided a potential for expending the applications of slow light in the sensor fields, having powerful practicability and favorable application prospects. [Copyright &y& Elsevier]

Abstract: The discovery of photonic crystal (PhC) has realized human''s dreams of manipulating and controlling photons. More and more researchers show great interests in the properties and application technologies of PhC, and many novel optical sensors have been proposed based on the novel structure and unique optical properties of PhC. Not only does it provide a potential for high sensitivity and flexible sensor design, but also opens up a new way to design micro sensors and integrated optical circuits. In this paper, research advances of PhC gas and liquid sensors are presented. Because of the diverse structure types of PhC, many unique and important properties such as photonic band gap, photonic localization, slow light based on PhC are discussed. According to these sensing properties, the theory, structural parameters of PhC and the performances of each kind of application in sensors are introduced in detail. Finally, the key problems of PhC in sensing technology are concluded and the future research directions are put forward. [Copyright &y& Elsevier]

A new sensing method for simultaneous measurement of seawater temperature and salinity by C-type micro-structured fiber was proposed. The C-type fiber structure formed by removing the outer wall of one pore from a six holes micro-structured fiber, which would bring in birefringence and broke original pattern of degeneracy. By optimizing the parameters of this fiber structure, X polarization and Y polarization of fundamental mode were separate. Therefore, it could be used for double parameter measurement. Besides, gold film coated on the surface of structure to enhance sensing sensitivity by Surface Plasmon Resonance (SPR) principle. In this study, finite element analysis method was used to analyze the spectral transmission characteristics of C-type micro-structured optical fiber. The surface of wedge-shaped defect was contacted with seawater directly to feel salinity; meanwhile, thermo-optic material filled into pores to detect the temperature of seawater. Through model analysis, it was proved that the proposed filling structure could produce two SPR loss valleys, which had different responses to temperature and salinity. Under optimized structure, maximum salinity sensitivity of 1.402 nm/‰ was obtained for X-polarization and maximum temperature sensitivity of −7.609 nm/ °C was obtained for Y polarization, which demonstrated that the designed scheme could not only solve the cross sensitivity problem of two parameters but also achieve high sensitivity. In addition, C-type micro-structured fiber is non-cascade integrated, strong stability, and flexibility in design, which has great potential for sensing applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

An optical fiber ammonia (NH 3) sensor based on multi-mode interference (MF) and anti-resonance (AR) principle is proposed, which can detect the NH 3 and increase the function of temperature measurement to avoid the influence of ambient temperature. The sensor is made of single-mode fiber (SMF) - hollow core fiber (HCF) - SMF - coreless fiber (NCF) -SMF welding, wherein NCF deposited graphene oxide gas response coating by dip-coating method. HCF and NCF can excite AR valley and MF valley simultaneously in the same spectrum. Experimental results show that for NH 3 gas sensing, MF valley exhibits a NH 3 sensitivity of 3.28 pm/ppm with a response time of 60 s and a recovery time of 30 s, while AR valley is not sensitive. For temperature sensing, MF valley and AR valley are sensitive to temperatures of − 12.63 pm/℃ and 15.57 pm/℃, respectively. The simultaneous detection of NH 3 and temperature can be realized through spectral demodulation. The gas sensor offers several advantages, including straightforward preparation, cost-effectiveness, and low operating temperature. Most importantly, it solves the problem of temperature interference that is often ignored in fiber optic gas sensors, and the hybrid fiber structure is suitable for many multi-parameter measurement occasions and has important practical application significance. • A novel sensing technique based on anti-resonance and multi-mode interference superposition is proposed. • A micro-optical fiber optic sensor with two-parameter recognition capability is developed. • The simultaneous detection of ammonia and temperature is realized. • The problem of temperature cross sensitivity which is common in gas sensor is solved effectively. • The multi-parameter detection capability of this hybrid fiber structure makes it very potential for a variety of practical applications. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]