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High sensitivity volatile organic compounds (VOCs) sensors based on a tapered small core single mode fiber (TSCSMF) and a microfiber coupler (MFC) are reported. The TSCSMF had a waist diameter of ∼5.1 μm and the MFC had a waist diameter of ∼1.9 μm each and both were fabricated using a customized microheater brushing technique. Silica based materials containing immobilized Nile red prepared by sol-gel method with two different recipes (recipe I and recipe II) are investigated. Initially recipe I based coating materials were applied to the surfaces of the TSCSMF and MFC. The experimental results show that the sensor based on an MFC shows much better sensitivities of −0.130 nm/ppm and −0.036 nm/ppm to ethanol and methanol than those of the TSCSMF based sensor. The corresponding minimum detectable concentration change of the MFC based sensor are calculated to be ∼77 ppb and ∼281 ppb to ethanol and methanol respectively. Both sensors are demonstrated fast response times of less than 5 minutes, while the recovery times varied from 7 minutes to 12 minutes. In addition, another TSCSMF based sample (∼7.0 μm) coated with a mixed layer of sol silica and Nile red prepared by recipe II was fabricated to achieve simultaneous measurement of ethanol and methanol, employing a second-order matrix approach.

Whispering gallery modes (WGMs) excitation in a cylindrical microresonator formed by a section of silica optical fiber has been studied. Evanescent light coupling into the microresonator is realized using a tapered optical fiber, fabricated by a microheater brushing technique. Several types of silica fibers with different diameters are studied as microresonators, and the influence of the resonator's diameter on the excitation of WGMs is investigated. The excitation of WGMs in a cylindrical fiber resonator were studied with changes to the tilt angle between the microcylinder and the fiber taper in the range of angles from a perpendicular position (0 degrees) to large tilt angles (24 degrees). The evolution of the fiber taper transmission spectrum with the change of the tilt angle results in changes in the intensity, broadening of and a blue shift in the WGM resonance spectra. Overall losses in the taper transmission spectrum decrease with the increase of the taper tilt angle from its perpendicular position, followed by a complete disappearance of the WGM resonances at large tilt angles greater than 20 degrees.

A simple, cost effective high-temperature sensor (up to 1000 °C) based on a hollow core fiber (HCF) structure is reported. It is configured by fusion splicing a short section of HCF with a length of few millimeters between two standard single mode fibers (SMF-28). Due to multiple beam interference introduced by the cladding of the HCF, periodic transmission dips with high spectral extinction ratio and high-quality (Q) factor are excited. However, theoretical analysis shows that minor variations of the HCF cladding diameter may result in a significant decrease in the Q factor. Experimental results demonstrate that the position of periodic transmission dips are independent of the HCF length, but spectral Q factors and transmission power varies with different HCF lengths. A maximum Q factor of 3.3 × 10 4 has been demonstrated with large free spectral range of 23 nm and extinction ratio of 26 dB. Furthermore, the structure is proved to be an excellent high-temperature sensor with advantages of high sensitivity (up to 33.4 pm/ °C), wide working temperature range (from room temperature to 1000 °C), high resolution, good stability, repeatability, relatively low strain sensitivity (0.46 pm/μe), low cost, and a simple and flexible fabrication process that offers a great potential for practical applications. A thorough theoretic analysis of the HCF-based fiber structure has been proposed. The experimental results are demonstrated to be well matched with our simulation results.

A polarization-dependent all-fiber comb filter based on a combination effect of multimode interference and Mach-Zehnder interferometer was proposed and demonstrated. The comb filter was composed with a short section of multimode fiber (MMF) fusion spliced with a conventional single mode fiber on the one side and a short section of a different type of optical fiber on the other side. The second type of optical fiber is spliced to the MMF with a properly designed misalignment. Different types and lengths of fibers were used to investigate the influence of fiber types and lengths on the performance of the comb filter. Experimentally, several comb filters with free spectral range (FSR) values ranging from 0.236 to 1.524 nm were achieved. The extinction ratio of the comb filter can be adjusted from 6 to 11.1 dB by varying polarization states of the input light, while maintaining the FSR unchanged. The proposed comb filter has the potential to be used in optical dense wavelength division multiplexing communication systems.

A platinum-coated singlemode-multimode structure is investigated in this paper as an optical fibre sensor (OFS) to monitor the phase transition of a phase change material (PCM). Paraffin wax has been used as an example to demonstrate the sensor's performance and operation. Most materials have the same temperature but different thermal energy levels during the phase change process, therefore, sole dependency on temperature measurement may lead to an incorrect estimation of the stored energy in PCM. The output spectrum of the reflected light from the OFS is very sensitive to the bend introduced by the PCM where both liquid and solid states exist during the phase transition. The measurement of strain experienced by the OFS during the phase change of the PCM is utilized for identifying the phase transition of paraffin wax between the solid and liquid states. The experimental results presented in this paper show that the OFS can measure the phase change point of paraffin wax and the sensor with a multimode fibre length of 10 mm measured the phase transition temperature range from 37.8 °C to 57.7 °C.

We present a compact highly sensitive microfiber coupler-based twist sensor formed by fusing and tapering two optical fibers and then connecting two of the pigtails to form a Sagnac loop. The device is miniature, low cost, and easily fabricated. The sensor has high twist sensitivities of $\sim 0.9$ nm/° when measuring wavelength shift and $\sim 0.16$ dB/° when measuring transmission power, both of which confirm that the sensor has good potential for application in the structural health monitoring and other areas.

This paper reports a study of carbon monoxide sensing properties for a novel nanocomposite material obtained by intercalation of a mixture of liquid crystal with magnetite nanoparticles into a matrix of mesoporous alumina. The sensing principle is based on measurements of the shift in the selective light reflection peak of the proposed nanocomposite film due to interaction between CO molecules and magnetite nanoparticles dispersed in the liquid crystal. The influence of the alumina pores' size on the sensitivity of the structure to CO has been investigated. The highest sensitivity of the selective reflection peak to the CO concentration in air estimated as 0.85 nm/(mg/m 3 ) is observed for a nanocomposite film with 50 nm size pores in the region of CO concentrations less than 10 mg/m 3 .

A new method of signal analysis from a polarimetric optical fiber sensor is presented. The proposed method is based on the Hilbert transform and utilizes only one light detector to calculate a strain-induced phase shift, thereby overall sensor cost and complexity is minimized. The demodulation algorithm is optimized for the dynamic strain measurement used in composite materials monitoring. The method is implemented to measure the dynamic strain in a reflective sensor configuration. Vibration and mechanical impact measurements as well as Fourier analysis prove the reliability of the proposed method.

The well-known tradeoff between the propagation loss and mode confinement is a critical consideration for the plasmonic waveguide structures. Aiming to overcome this limitation, in this paper, we propose a compact plasmonic waveguide consisting of two identical dielectric wedge waveguides symmetrically placed on each side of a nanowedge-patterned thin metal film. The systematical analysis has demonstrated that the light can be confined to approximate 3000th of the diffraction spot size (ranging from lambda(2)/10 604 to lambda(2)/972) without sacrificing the propagation length (ranging from 1680 to 4724 mu m). Compared to the recent published structure which achieved the best tradeoff, to the best of our knowledge, the proposed waveguide could achieve a 9-fold enhanced mode confinement for the same propagation length and a 2.4-fold outspread propagation length for the same mode confinement.

Optical microfibre photonic components offer a variety of enabling properties, including large evanescent fields, flexibility, configurability, high confinement, robustness and compactness. These unique features have been exploited in a range of applications such as telecommunication, sensing, optical manipulation and high Q resonators. Optical microfibre biosensors, as a class of fibre optic biosensors which rely on small geometries to expose the evanescent field to interact with samples, have been widely investigated. Due to their unique properties, such as fast response, functionalization, strong confinement, configurability, flexibility, compact size, low cost, robustness, ease of miniaturization, large evanescent field and label-free operation, optical microfibres based biosensors seem a promising alternative to traditional immunological methods for biomolecule measurements. Unlabeled DNA and protein targets can be detected by monitoring the changes of various optical transduction mechanisms, such as refractive index, absorption and surface plasmon resonance, since a target molecule is capable of binding to an immobilized optical microfibre. In this review, we critically summarize accomplishments of past optical microfibre label-free biosensors, identify areas for future research and provide a detailed account of the studies conducted to date for biomolecules detection using optical microfibres.

This article comprises definitions of environmental services, environmental services market as well as the preconditions for creating a loyal environment for the development of environmental services market. We propose principles of improving the system of relationships environmental services subjects in the context of rational water resources use. The key moments of improving the system of rational water resources use while using market tools for environmental services have been described. We designate the principles of environmental services market forming in Europe. We analyze the issues of inheriting these principles in Ukraine and we propose the ways for their optimization.

Photonic crystal fibers (PCF) have been demonstrated for various novel sensing applications for which conventional optical fiber sensors either fail to provide a solution or have low sensitivity issues. A combination of liquid crystal (LC) materials with their tunable optical properties and PCFs with their air-hole microstructure provide the unique advantage to fabricate all-fiber tunable photonic devices. Liquid crystal infiltrated photonic crystal fibers (LCPCF) have been demonstrated for various tunable photonic devices such as filters, switches, polarizers, in-line fiber attenuators, polarization controllers etc. Fiber optic based electric field (e-field) and voltage sensors have been demonstrated using conventional fibers and fiber sensors such as fiber bragg gratings for applications in electric power industry. Due to the natural immunity of the fibers to electromagnetic fields, external transducers or actuators have to be used to induce e-field sensitivity resulting in sensors with low sensitivity. On LC infiltration PCF propagation properties become sensitive to external parameters such as electric fields. The inherent sensitivity of the LCPCF structure and the large e-field induced birefringence of the LC allows to fabricate all-fiber e-field sensor probes with high sensitivity and large measurable e-field range. The book chapter on "Liquid crystal photonic crystal fiber for sensing applications" focuses on the use of LCPCFs for electric field sensing. The book chapter will discuss miniature e-field probes with capability to detect and measure all the parameters of an unknown e-field such as amplitude, frequency and direction. The work presented in this book chapter includes authors' original contribution in this area. With a brief mention of the tunable properties of liquid crystals and the electric field sensing mechanism of LCPCFs, the book chapter will present details on fabrication and characterization of various LCPCF e-field probes. The capability of the e-field probe to simultaneously measure e-field intensity and frequency will be presented. The chapter will discuss a polarimetric e-field sensing scheme combined with LC infiltration length control to fabricate sensor probes customized for measurement in a particular e-field range. A true all-fiber directional e-electric field sensor using LCPCF will be discussed. The chapter will provide an insight into the future research and application areas of LCPCF such as an all-fiber sensor for both electric and magnetic fields. Fiber optic sensors with capability to simultaneously measure both electric current and voltage for practical applications in electric power industry and smart grids will also be discussed briefly.

An all-fiber magnetic field sensor based on whispering-gallery modes (WGM) in a fiber micro-resonator infiltrated with ferronematic liquid crystal is proposed and experimentally demonstrated. The cylindrical microresonator is formed by a 1 cm-long section of a photonic crystal fiber infiltrated with ferronematic materials. Both ferronematics suspensions are prepared based on the nematic liquid crystal 1-(trans-4-Hexylcyclohexyl)-4-isothiocyanatobenzene (6CHBT) doped with rod-like magnetic particles in the first case and with spherical magnetic particles in the second case. WGMs are excited in the fiber microresonator by evanescent light coupling using a tapered fiber with a micron-size diameter. The Q-factor of the microresonator determined from the experimentaly measured transmission spectrum of the tapered fiber was 1.975 × 103. Under the influence of an applied magnetic field the WGM resonances experience spectral shift towards shorter wavelengths. The experimentally demonstrated sensitivity of the proposed sensor was −39.6 pm/mT and −37.3 pm/mT for samples infiltrated with rod like and spherical like ferromagnetic suspensions respectively for a magnetic field range (0-47) mT. Reducing the diameter of the cylindrical micro-resonator by tapering leads to enhancement of the magnetic field sensitivity up to −61.86 pm/mT and −49.88 pm/mT for samples infiltrated with rod like and spherical like ferromagnetic suspensions respectively for the magnetic field range (0-44.7) mT.

In this paper, we investigate both theoretically and experimentally the influence of the agarose hydrogel layer thickness on the sensitivity of a proposed relative humidity (RH) sensor based on a silica microsphere resonator coated with agarose hydrogel. The operating principle of the sensor relies on excitation of whispering gallery modes (WGMs) in the coated silica microsphere using the evanescent field of a tapered fiber. A change in the ambient relative humidity is detected by measuring the wavelength shift of the WGMs in the transmission spectrum of the tapered fiber. Using perturbation theory, we analyze the influence of the agarose coating thickness on the sensitivity of the proposed sensor and compare the results of this analysis with experimental findings for different coating layer thicknesses. We demonstrate that an increase in the coating layer thickness initially leads to an increase in the sensitivity to RH and reaches saturation at higher values of the agarose layer thickness. The results of the study are useful for the design and optimization of microsphere sensor parameters to meet a performance specification.

A high-sensitivity temperature sensor based on a singlemode-multimode-polymer filled hollow core fibre-multimode-singlemode (SMHMS) fibre structure is proposed. This sensor was made from a short section of hollow core fibre filled with a high thermo-optic coefficient (TOC) polymer with a refractive index close to that of the fibre cladding, fusion spliced between two singlemode-multimode (SM) fibre structures. This sensor effectively improves the temperature sensitivity by over 200 times by comparison to a conventional singlemode-multimode-singlemode (SMS) fibre structure. In this report, we experimentally demonstrate that the proposed sensor provides a high temperature sensitivity of 2.16 nm/°C.

A simple volatile organic compound (VOC) sensor based on a tapered small core singlemode fiber (SCSMF) structure is reported. The tapered SCSMF fiber structure with a waist diameter of 7.0 μm is fabricated using a customized microheater brushing technique. Silica based material containing immobilized Nile red was prepared by a sol-gel method and was used as a coating applied to the surface of the tapered fiber structure. Different coating thicknesses created by a 2-pass and 4-pass coating process are investigated. The experiments demonstrate that both sensors show a linear response at different gas concentrations to all three tested VOCs (methanol, ethanol and acetone). The sensor with a thicker coating shows better sensitivities but longer response and recovery times. The best measurement resolutions for the 4-pass coating sensor are estimated to be 2.3 ppm, 1.5 ppm and 3.1 ppm for methanol, ethanol and acetone, respectively. The fastest response and recovery time of 1 min and 5 min are demonstrated by the sensor in the case of methanol.

In this paper, we propose a novel approach to measurements of low relative humidity (RH) levels based on the whispering gallery modes (WGMs) phenomenon in a silica microsphere resonator coated with Agarose. The spectral dips of the WGM resonances excited in the proposed sensor depend strongly on the changes in the refractive index (RI) of the coating material as well as the surrounding RI. A study of the humidity-induced RI changes in a thin Agarose layer, applied to the surface of a 162 μm-diameter silica microsphere was carried out by correlating the experimental results and numerical simulations performed using the perturbation theory. We experimentally demonstrate a linear sensing characteristic in a low-humidity range from 10% to 45% RH. The estimated quality factor of the micro-resonator is 2.82×106 and detection limit for the sensor is 0.057% RH, corresponding to the RI resolution of 8.4× 10−7 RIU.

In this paper we propose and investigate a novel magnetic field sensor based of a ring erbium-doped fiber laser combined with a fiber Bragg grating and a Sagnac loop containing a microfiber coupler and magnetic fluid. In addition to the magnetic field sensing capability, the proposed structure can simultaneously provide temperature information. Thanks to the dual-ring structure of the MFC-Sagnac loop and the FBG-assisted resonant cavity, the output has two distinct laser peaks. Experimentally demonstrated magnetic field sensitivity of one of the laser peaks is 15 pm/mT in the magnetic field range from 0 to 100 mT. The spectral position of the second laser peak is independent on the magnetic field but shifts towards long wavelengths with a sensitivity of 13 pm/°C.

Optical microfiber couplers (MFCs) with certain geometries are sensitive to local refractive changes. By incorporating a humidity-sensitive polyethylene oxide coating, a tapered optical MFC can work as a humidity sensor. We fabricated such a humidity sensor and demonstrated its sensing ability by monitoring spectral shifts in a wavelength domain. © 2015 Wiley Periodicals, Inc. Microwave Opt Technol Lett 57:457–460, 2015

A bend singlemode–multimode–singlemode (SMS) fibre structure based vibration sensor is proposed and developed. This sensor configuration is very simple and employs a bend SMS fibre structure and a broadband optical source. The vibration applied to the bend SMS fibre structure will change the bend radius and hence the intensity of the transmitted optical power will also vary. Experimental results show that the sensor can detect both vibration frequencies and amplitudes over a broad range with good sensitivity, from a hertz to 12 kHz.

The characteristics of polymer fiber Bragg gratings (FBGs) embedded in composite materials are studied in this paper and are compared with characteristics of their silica counterparts. A polymer FBG of 10 mm length which exhibits a peak reflected wavelength circa 1530 nm is fabricated and characterized for this purpose. A silica FBG with a peak reflected wavelength circa 1553 nm is also embedded in the composite material for a comparison study. The fabricated composite material sample with embedded sensors is subjected to temperature and strain changes and the corresponding effects on the embedded polymer and silica FBGs are studied. The measured temperature sensitivity of the embedded polymer FBG was close to that of the same polymer FBG in free space, while the silica FBG shows elevated temperature sensitivity after embedding. With an increase in temperature, spectral broadening was observed for the embedded polymer FBG due to the stress induced by the thermal expansion of the composite material. From the observed wavelength shift and spectral bandwidth change of the polymer FBG, temperature and thermal expansion effects in the composite material can be measured simultaneously.

In this letter, an ultra-compact plasmonic polarization beam splitter (PBS) is proposed and investigated by numerical simulations using the finite element method. The PBS is based on a three-core plasmonic directional coupler, which uses a long range surface plasmon polaritons waveguide as the middle waveguide to achieve polarization selective coupling. The calculations show that with proper structural parameters, the PBS with low insertion losses of 0.17 and 0.25 dB for TE and TM polarizations, respectively, and high extinction ratios of 20.17 and 19.83 dB for TE and TM polarizations, respectively, can be realized at a telecommunication wavelength of 1550 nm. Furthermore, an insertion loss and an extinction ratio > 14 dB can be realized across the entire C-band for both TE and TM polarizations.

In this article, we propose a miniaturized flexible interrogator for polarimetric and fiber Bragg grating (FBG) sensors based hybrid sensing scheme embedded in composite materials. The flexible interrogation module comprises of an arrayed waveguide (AWG) for FBG demodulation and an electro optic mach–zehnder interferometric (EO-MZI) intensity modulator for polarimetric sensor demodulation. A customized design of AWG and EO-MZI are presented, and the designed performance resulted in a wide channel spacing of 5 nm and low cross talk of −34 dB between adjacent channels for the AWG, and a low Vπ voltage of ±1.5 V for the MZI-EO. The method for fabrication of the flexible interrogator device is also presented, based on post processing of platinum coated polyimide substrate. The film type interrogation module is compact, with a size of 3.4 × 0.1 × 0.01 cm3. The surface attachable flexible hybrid sensor interrogator is proposed to integrate with photo detector arrays and wireless communication technology to enhance the competency of hybrid sensing scheme in smart sensing composite parts in motion for applications in aircraft, wind rotor blades and so forth. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:1167–1174, 2014

This paper proposes a novel design for a surface plasmon resonance (SPR) fiber sensor with an axisymmetric sub-wavelength metal grating layer. The relationship between the sensor performance (the sensitivity S and the quality factor Q of the SPR dip) and the characteristic parameters are investigated. Numerical simulation results show that the proposed sensor can achieve a maximum sensitivity of 13,000 nm/RIU (refractive index unit) for a refractive index range from 1.3 to 1.4.

We experimentally investigate white light optical trapping by generating a supercontinuum in a lead silicate fibre pumped by femtosecond pulses from a Ti:Sapphire laser near the zero-dispersion wavelength of 1030 nm, before confining the light using a microfibre half taper with a final tip diameter of 75 nm. Due to the high intensity gradient at the output, robust optical trapping is possible, as demonstrated for individual yeast cells using an average pumping power of 100 mW.

A simple integrated ratiometric wavelength-monitoring device based on a single directional coupler (DC) is proposed and designed. To meet the desired spectral response, a computationally fast method is proposed to optimize the separation distance between two waveguides and the interaction length of the DC based on the local supermodes solution. The wavelength discrimination of the designed structure is demonstrated numerically.

A single-mode-no-core-single-mode (SNS) fiber optical sensor for the detection of solid-liquid and liquid-solid phase changes in C18H38 n-alkane (n-octadecane) is proposed and demonstrated. The transmission-type sensor probe consists of a short section of no-core fiber sandwiched between two sections of a single-mode fiber. Phase changes in n-octadecane are accompanied by large step-like variations of its refractive index (RI). Such a large discontinuous change of the n-octadecane’s RI during its phase transition leads to the corresponding step-like change in the transmitted optical power that can reliably indicate the phase change of the sample in the vicinity of the sensor. The proposed sensor probe is simple, accurate and is capable of detecting the material’s phase based on a single measurement. The results of this work suggest that the proposed sensor is potentially capable of detecting liquid-solid phase changes in other materials whose thermo-optic properties are similar to those of n-octadecane.

Liquid level measurement in lab on a chip (LOC) devices is a challenging task due to the demand for a sensor with ultra-high resolution but miniature in nature. In this Letter, we report a simple, compact in size, yet highly sensitive liquid level sensor based on a hollow core fiber (HCF) structure. The sensor is fabricated by fusion splicing a short section of HCF between two singlemode fibers (SMFs). Sensor samples with different lengths of HCF have been studied; it is found that the sensor with a HCF length of ∼4.73 mm shows the best sensitivity of ∼0.014 dB/μm, corresponding to a liquid level resolution of ∼0.7 μm, which is over five times higher than that of the previous reported fiber optic sensors to date. In addition, experimental results have demonstrated that the proposed sensor shows good repeatability of measurement and a very low cross sensitivity to changes in the refractive index of the surrounding medium.

A novel fiber optic hybrid device for simultaneously measuring temperature and relative humidity (RH) is proposed and demonstrated in this paper. The device is composed of a fiber Bragg grating (FBG) and a reflection type photonic crystal fiber interferometer (PCFI) infiltrated with humidity sensitive material Agarose. The Agarose infiltrated PCFI is used to monitor the RH and an in-line FBG is used to monitor the temperature. The temperature is measured by detecting the shift in the Bragg wavelength and the humidity is measured by monitoring the change in the optical power of the reflected spectrum. The sensor shows a temperature sensitivity of 9.8 pm/°C and an optical power variation of >7 dB for an RH change of 75%, with less cross sensitivity.

This article presents performance evaluation of the use of one or two edge filter(s) based on singlemode-multimode-singlemode (SMS) fiber structure on an all-fiber ratiometric wavelength monitor system.Several aspects of noise, polarization-dependent loss (PDL), and temperature-dependent loss (TDL) that can influence the system's performance were investigated. It was demonstrated that the use of two SMS edge filters can improve the system's resolution and wavelength error when compared with the use of one edge filter. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1645–1649, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27619

In this paper, a novel metal-dielectric waveguide structure is proposed to support hybrid long range surface plasmon polaritons (LRSPPs) with a highly confined mode field. The simulation results showed that our proposed structure has better mode confinement and propagation length compared to that of conventional dielectric-loaded surface plasmon polaritons (DLSPPs) waveguides. This structure offers greater flexibility for the design of surface plasmon polaritons (SPPs) waveguides by altering the trade-off between mode confinement and propagation length. The proposed structure has significant potential for application in highly integrated photonic circuits.

The Cherenkov radiations (CRs) at visible wavelengths are efficiently generated based on the soliton self-frequency shift (SSFS) by coupling the 120 fs pulses into the fundamental mode propagated in the multi-knots of a hollow-core photonic crystal fiber cladding (HC-PCFC). When the pump operates at 830 nm with the average power of 300 mW at the knots 1–4 , the central wavelengths of generated CRs λCR are 476 nm, 454 nm, 554 nm, and 580 nm, the conversion efficiencies ηCR are 33%, 35%, 30%, and 29%, and the corresponding bandwidths BCR are 57 nm, 74 nm, 25 nm, and 50 nm, respectively. To the best of our knowledge, ηCR of 35% and BCR of 74 nm are the best experimental results reported with >100 fs pump pulse. The efficient and broadband CRs can be wavelength-tunable over the whole visible wavelength by adjusting the wavelengths of pump pulses coupled into different knots. The result can be used as the multi-channel ultrashort pulse source for many important applications in the multiphoton microscope and ultrafast photonics.

The humidity and temperature responses of a microfiber coupler (MFC) coated with silica gel are investigated. Two MFC structures with different waist diameters of 2.5 and 3.5 μm were fabricated by fusing and tapering two single-mode fibers using a microheater brushing technique. The influences of the coating thickness and tapered waist diameter on the sensing performance are analyzed. For the proposed sensor with a waist diameter of 2.5 μm and 8-layers thick coating, the change in the relative humidity (RH) results in an exponential blueshift with a maximum sensitivity of 1.6 nm/% RH in the range from 70 to 86% RH. In response to the temperature change, the sensor's transmission spectrum redshifts in a linear fashion with an average sensitivity of 0.55 nm/°C in the range from 20 to 40 °C. The study is important for the development of the proposed fiber structure as a humidity or temperature sensor.

A novel fiber sensor for simultaneous measurement of ammonia gas concentration and temperature is proposed. The sensor is fabricated from two sections of single-mode fiber which are cleaved and then a fusion splicer and which is then used to fabricate spherically shaped structures at the end facets. The fusion arc is used to soften the glass which naturally assumes a spherical shape due to surface tension. A short section of multimode fiber is then fusion spliced with the two spherical-shaped ends of the single mode fibers so both the core modes and the cladding modes of the multimode fiber are excited to create two kinds of interference dips: One is created by core modes only which is not sensitive to ammonia gas since the core is isolated by the cladding so the effective refractive index of the core does not change when the refractive index of the environment changes, The other dip is created by the coupling of the core mode and cladding mod, which with a suitable coating is sensitive to ammonia gas. Silica sol-gel was prepared and coated on the fiber surface as a sensing layer for detecting ammonia gas concentration. The experimental results show that the two dips have linear wavelength shift responses but with different sensitivities to ammonia gas concentration (5.03×10-4nm/ppm for dip1 and -2.5×10- 5nm/ppm for dip2) and temperature (0.0067 nm/oC for dip1 and 0.0149 nm/oC for dip2. By constructing a wavelength shifts matrix for the two dips vs. ammonia gas concentration and temperature, both the ammonia gas concentration and temperature can be measured simultaneously.

A high sensitivity ammonia sensor based on a tapered small core singlemode fiber (SCSMF) structure for measurement of ammonia concentration in water is reported. Two tapered SCSMF fiber structures with different waist diameters of 23 µm and 13.5 µm are fabricated by using a customized microheater brushing technique. The silica based material prepared by the sol-gel method is used as a coating applied to the surface of the tapered fiber structures. To investigate the influence of the coating thickness on the sensitivity to ammonia in water, silica coatings with different thicknesses (2-pass and 8-pass coatings) are deposited on the surface of the fiber sensor with a waist diameter of 23 µm. Experiments demonstrate that the sensor with a thicker (8-pass) silica coating shows better sensitivity of 0.131 nm/ppm to ammonia compared to that of 0.069 nm/ppm for the thinner silica coating (2-pass). To further improve the sensor sensitivity, the taper waist diameter is reduced. For an 8-pass coating (249nm at the taper waist section) applied to a tapered SCSMF structure based fiber sensor with a reduced waist diameter of 13.5 µm. Experimental results show that the sensitivity to ammonia is significantly improved to 2.47nm/ppm. The best measurement resolution for ammonia concentration in water is estimated to be 4 ppb while the response and recovery times are less than 2 and 5 minutes respectively. The proposed sensor also offers good performance in terms of repeatability and good selectivity for sensing ammonia compared to that of other common ions and organic molecules in water.

We propose a novel optical microfiber-loaded plasmonic TE-pass polarizer consisting of an optical microfiber placed on top of a silver substrate and demonstrate its performance both numerically by using the finite element method (FEM) and experimentally. The simulation results show that the loss in the fundamental TE mode is relatively low while at the same time the fundamental TM mode suffers from a large metal dissipation loss induced by excitation of the microfiber-loaded surface plasmonic mode. The microfiber was fabricated using the standard microheater brushing–tapering technique. The measured extinction ratio over the range of the C-band wavelengths is greater than 20 dB for the polarizer with a microfiber diameter of 4 µm, which agrees well with the simulation results.

A high sensitivity ammonia gas sensor based on a tapered small core single-mode fiber (SCSMF) structure for measurement of ammonia gas concentration is reported. The proposed tapered SCSMF fiber structure has a waist diameter of 13.5 µm and it is fabricated using a customized microheater brushing technique. The silica based material prepared by sol-gel method is used as a coating applied to the surface of tapered fiber structures. Experimental results show that sensor is very sensitive to ammonia. The best measurement resolution of the ammonia concentration is estimated to be 16 ppb for gas concentrations of 0.5 ppm to 2.5 ppm while the response and recovery times are less than 5 and 10 minutes respectively.

This paper provides an overview of the different types of fiber optic sensors (FOS) that can be used with composite materials and also their compatibility with and suitability for embedding inside a composite material. An overview of the different types of FOS used for strain/temperature sensing in composite materials is presented. Recent trends, and future challenges for FOS technology for condition monitoring in smart composite materials are also discussed. This comprehensive review provides essential information for the smart materials industry in selecting of appropriate types of FOS in accordance with end-user requirements.

This paper examines a technique that utilizes a Sagnac loop with a microfiber coupler (MFC) as a coupler which allows the MFC to operate effectively as a sensor but with larger than normal tapered fiber diameters. The proposed structure is found to be suitable for temperature and refractive index (RI) sensing. It is shown that a variation in the surrounding of the MFC RI results in a shift of the output spectrum, while a temperature variation leads to changes in the intensity of the interference dips. A decrease in the waist diameter of the MFC results in an increase in the sensitivity to temperature. For MFC structures based on a 5.6 μm and a 3 μm fiber waist diameter, the minimum transmission power level of a selected spectral dip decreases by 1.7 dB and 5.03 dB respectively, as the temperature changes from 18 °C to 44 °C. A change in the surrounding RI from 1.334 to 1.395 results in the spectral redshift of 8 nm using a 5.6 μm fiber waist diameter. By functionalizing the surface of the MFC with various materials, the structure could potentially be used for sensing of other parameters.

We report a detailed study of a miniature optical relative humidity (RH) sensor based on a polymer-infiltrated photonic crystal fiber interferometer. The sensor shows a high sensitivity to RH variations with a change in its reflected power of about 12 dB for a humidity change of 84% RH. The sensor has the advantages of a very compact length of 1 mm, and an end-type probe configuration makes it suitable for monitoring humidity in microenvironments. The repeatability, long-term stability, measurement accuracy, and temperature dependence of the sensor are studied in this paper. The observed low thermal sensitivity of the sensor suggests that temperature compensation may not be needed if it is used in normal environments. The response time of the sensor is found to be 400 ms for a change in RH of ~ 30% RH. The fast response time suggests that the sensor can potentially be used as a human breath rate monitor in a clinical situation.

This paper reports on a novel design for a tunable filter and plasmonic sensor based on the metal–insulator–metal waveguide with a nanocavity resonator. Simulation results show that as a one-channel filter, the resonance wavelengths show a linear red-shift with an increase in nanocavity length with a slope of 1742 nm/μm and a nonlinear blue-shift with an increase in nanocavity width, respectively. A two-channel filter can be realized using two nanocavities and the arrangement of the two nanocavities with respect to the waveguide and the value of the distance between the nanocavities has only a marginal effect on the filter notch wavelength. Finally, both in-slit and out-slit refractive index plasmonic sensors are investigated with a sensitivity of 710 nm/RIU and 250 nm/RIU, respectively.

A feasibility study of smart composite materials with embedded photonic crystal fiber (PCF) sensors for measurements of local and average strain and temperature is presented in this paper. For this purpose a composite material sample with embedded fiber sensors is fabricated and characterized. For temperature independent local strain measurements a short length PCF modal interferometer is utilized, while for an average strain measurement a polarization maintaining PCF Sagnac interferometric sensor is used. The strain and temperature sensitivities of a buffer coated and a buffer stripped PM-PCF Sagnac fiber loop mirror sensors are measured in free space and compared to those for similar sensors embedded in the composite material. It is found that the Sagnac interferometer with an acrylate coated PCF embedded in the composite material shows the same response as the one in free space while the unbuffered PCF Sagnac interferometric sensor which is temperature insensitive in free space shows significant temperature sensitivity when embedded in the composite material sample. A fiber Bragg grating array is also embedded in the composite material for comparison of the strain and temperature measured by the PCF modal interferometers. The results obtained in this study demonstrate that PCF sensors are a feasible technology that can be adapted for composite material strain measurement.

Low frequency vibration measurements in composite materials using embedded optical fiber sensors are presented in this paper. The two types of fiber sensors embedded are a polarization maintaining photonic crystal fiber (PM-PCF) polarimetric fiber sensor and a fiber Bragg grating (FBG) sensor. A glass fibre reinforced polymer composite material sample is fabricated with both fiber optic sensors embedded in it. A comparison between the performance of both the PM-PCF polarimetric fiber sensor and the FBG sensor embedded in the composite material is also presented.

The frequency dependence of a nematic liquid crystal infiltrated photonic crystal fiber (LCPCF) subjected to a time varying electric field is studied experimentally in the frequency range from 50 Hz-1 kHz. Due to the millisecond order response times of nematic liquid crystals the transmittance response of the infiltrated fiber is found to be highly dependent on the frequency of the input electric field. The time varying transmission response of the infiltrated fiber is found to vary sinusoidally with a frequency that is twice the frequency of the input electric field. It is demonstrated that the time varying transmission response of the LCPCF can be approximated using an appropriate sinusoidal function. It is shown that a fitted function can be used to extract the parameters of the electric field such as frequency and amplitude. The potential application of these structures as all-fiber sensors serving as the core of an electrical signal frequency and amplitude monitoring device is demonstrated and discussed.

An edge filter-based ratiometric wavelength measurement system is modeled and analyzed in this paper. The results confirm that the noise of input signal and photodetectors limits the resolution of the wavelength measurement system. The achievable resolution is calculated for a given noise level of the input signal and photodetectors' resolution. An improved ratiometric wavelength measurement system consisting of two fiber comb filters is presented both theoretically and experimentally, which performs coarse and fine wavelength measurements simultaneously. The resolution of the system is significantly improved to better than 5 pm while maintaining the potential for high measurement speed and wide measurable wavelength range.

A fast relative humidity sensor is presented which can operate over a wide humidity range, based on a bent single mode optical fiber. We discuss the selection of a suitable hygroscopic coating and also the influence of bend radius on sensor performance. A sensor fabricated using an agarose coating with a bend radius of 16.75 mm shows a linear response in the range 25–90% RH. The measured response time of the sensor is about 50 ms, which is significantly faster than other optical humidity sensors. The humidity sensitivity of the sensor is wavelength dependent and high sensitivity is observed at higher wavelengths. This sensor offers the advantages of simple structure and low cost and its response is reversible in nature.

The measurement of interaction forces in minimally invasive surgical devices, sensorized with photonic crystal fiber (PCF) sensors, is presented in this paper. Two types of PCF sensors are used: a tapered PCF interferometer and a microhole-collapsed PCF interferometer for the detection of interaction forces generated in surgical devices without the influence of ambient temperature variation. The demonstration devices used for force characterization are a laparoscopic scissor and a standard surgical scissor blade. The force sensitivity of each sensorized blade is examined and compared with fiber Bragg grating (FBG)-sensorized blades. Results show that the PCF-sensorized surgical blades outperform the blades fitted with the FBG sensors during static load measurement.

A hybrid sensor that operates in the intensity domain by converting the polarization and wavelength information from the photonic crystal fiber sensor and fiber Bragg grating (FBG) sensor, respectively, into intensity variation is presented in this paper. The hybrid fiber-optic sensor system involves a combination of a polarimetric sensor based on a photonic crystal fiber and a FBG sensor and is used for simultaneous strain and temperature measurement. The strain sensitivity of the polarization maintaining photonic crystal fiber at different lengths and the corresponding slope required for the edge filter which converts the FBG wavelength information into intensity are studied and presented in this paper. The proposed sensor configuration has a wide range of applications in smart fiber-optic sensing.