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A fiber optic strain sensor, based on the Vernier effect, was constructed using two parallel MZIs. The strain sensitivity of one MZI was 0.84 pm/µε, using this configuration the sensitivity increased up to 13.00 pm/µε.

Graphene-based materials are capable of enhancing the refractometric response of prism- and optical fiber-based surface plasmon resonance (SPR) sensors; however, complicated multistep and time-consuming attaching processes could limit their practical applications. Herein, for the first time, we demonstrate the immobilization of graphene oxide (GO) submicrometric sheets onto the surface of a gold-coated single-mode fiber using a coating of fungal self-assembling proteins, the hydrophobins (HFBs), as an adhesive nanolayer. Hetero-core fiber tip SPR structures used in this study, consisting of a mirrored multimode–single-mode fiber structure coated with different thin layers (a chromium layer of 3 nm and a gold layer of 30 nm on top) exhibited a refractive index sensitivity (SRI) of 1842 nm RIU−1 (RIU: refractive index unit) at a refractive index (RI) of 1.36. Self-assembly of GO over the SPR fiber tip via HFB, offered an enhancement of up to 20% in the SRI. Moreover, this HFB-GO coating prevented degradation of the Al thin film mirror caused by corrosive salt-water solutions. The process is very simple, harmless, rapid (around 15 min) and scalable, as it is mostly based on one plasma treatment, which can be performed in large chambers and two dip coating steps, in liquid baths. All these features make the use of self-assembled bio/non-bio hybrid coating a green industrial method to improve the performance of SPR fiber biosensors, if compared with traditional chemical methods. Materials applied in this technology, fungal proteins and derivatives of graphite, are sustainable and largely available.

We propose and demonstrate an extrinsic Fabry-Perot interferometer build with a strongly coupled multicore fiber. The interferometer can be used for measuring distances or any other parameter that induces changes in the cavity length.

In the schlieren method, the deflection of light by the presence of an inhomogeneous medium is proportional to the gradient of its refractive index. Such deflection, in a schlieren system, is represented by light intensity variations on the observation plane. Then, for a digital camera, the intensity level registered by each pixel depends mainly on the variation of the medium refractive index and the status of the digital camera settings. Therefore, in this study, we regulate the intensity value of each pixel by controlling the camera settings such as exposure time, gamma and gain values in order to calibrate the image obtained to the actual temperature values of a particular medium. In our approach, we use a color digital camera. The images obtained with a color digital camera can be separated on three different color-channels. Each channel corresponds to red, green, and blue color, moreover, each one has its own sensitivity. The differences in sensitivity allow us to obtain a range of temperature values for each color channel. Thus, high, medium and low sensitivity correspond to green, blue, and red color channel respectively. Therefore, by adding up the temperature contribution of each color channel we obtain a wide range of temperature values. Hence, the basic idea in our approach to measure temperature, using a schlieren system, is to relate the intensity level of each pixel in a schlieren image to the corresponding knife-edge position measured at the exit focal plane of the system. Our approach was applied to the measurement of instantaneous temperature fields of the air convection caused by a heated rectangular metal plate and a candle flame. We found that for the metal plate temperature measurements only the green and blue color-channels were required to sense the entire phenomena. On the other hand, for the candle case, the three color-channels were needed to obtain a complete measurement of temperature. In our study, the candle temperature was took as reference and it was found that the maximum temperature value obtained for green, blue and red color-channel was ∼275.6, ∼412.9, and ∼501.3 °C, respectively.

A surface plasmon resonance-based fiber-optic sensor for simultaneous measurement of refractive index and temperature of liquid samples is proposed and experimentally demonstrated. The sensor consists of a gold-coated MM-SM-MM optical fiber structure, whose sensitive section was partially covered with polydimethylsiloxane (PDMS) to generate two independent SPR resonance dips in the fiber transmission spectrum. One of the dips is generated by the bare gold-coated fiber section whose wavelength resonance is tuned by the refractive index and temperature of the surrounding medium. The other dip that is exclusively used to monitor the temperature variations of the liquid sample, whose central wavelength at 900 nm corresponds to PDMS refractive index at 20 °C, is produced by the polymerized gold-coated fiber section. The high refractive index and temperature sensitivity achieved, 2323.4 nm/RIU and −2.850 nm/°C respectively, the small size, the ease fabrication process, and the bio-compatibility of the proposed device are appealing characteristics that makes it ideal for practical bio-sensing applications.

A compact, simple-to-fabricate, low-cost, and highly sensitive optical fiber temperature sensor based on surface plasmon resonance (SPR) is reported. The sensor consists of a core mismatch fiber structure fabricated by splicing a small piece of single-mode fiber (SMF) between two multimode fibers (MMF). SPR is generated when evanescent field interacts with the gold layer deposited over the SMF cladding. Then, the sensor was embedded in polydimethylsiloxane (PDMS), which acts as a temperature to refractive index transducer. Due to PDMS high thermooptic coefficient, the SPR dip underwent a noticeable wavelength shift when a variation of temperature occurred. The device was tested in the 20–60 °C range showing a linear response and a sensitivity of 2.60 nm/°C. This sensor is appealing for temperature monitoring in microfluidic devices made of PDMS due to its high performance and simply fabrication process.

An optical fiber tip interferometer for the measurement of relative humidity (RH) and temperature is proposed. The optical fiber structure used, a dual-cavity optical fiber Fabry-Perot interferometer (DFFPI), is simply-to-fabricate, compact, and robust. The reflectance (RDFFPI) of the interferometer is sensitive to the refractive index (RI) and temperature of the external medium. Consequently, when the cross-section of the fiber tip was coated with a SnO 2 thin film, whose RI changes according to the humidity of the surrounding ambient, the measurement of the RH was possible. An increment of the RH produced a decrement of RI of the SnO 2 thin film, then the reflectance of the fiber tip end-face diminished, and this produced a decrement of the visibility of the interference fringes. The analysis of the RDFFPI was carried out in the Fourier domain, using a novel processing method it was possible to establish that the amplitude of two peaks of Fourier spectrum changed at a ratio of 39.49 × 10 -3 %RH -1 in the range of 40 to 90 RH%. On the other hand, the temperature of the humidity chamber was monitored, from 25 to 60 °C at a fixed RH%, by analyzing the phase shift of the interference pattern produced by the changes in the optical path length of the cavities. The good sensitivity, stability, reproducibility, and compactness of the fiber tip RH sensor make this proposal very appealing in a wide range of applications. This work was supported in part by the Consejo Nacional de Ciencia y Tecnología (CONACyT) from Mexico through the CIENCIA BÁSICA under Project 257046 and Project LN-293442, and in part by the Spanish Agencia Estatal de Investigación (AEI) and Fondo Europeo de Desarrollo Regional (FEDER) under Grant TEC2016-79367-C2-2-R. The work of Carmen E. Domínguez-Flores and Osvaldo Rodríguez-Quiroz were supported by the CONACyT for Ph.D. Scholarship.

The output signal of Mach-Zehnder curvature sensor was Fourier transformed to increase its curvature range to high value of 1.69 m-1, relative to the traditional fringe contrast measurement 1.09 m-1.

A multimode interference (MMI) sensor was designed and experimentally demonstrated for simultaneous measurement of curvature and temperature. A typical fiber structure of single-mode fiber – multimode fiber – single-mode fiber (SMS), mounted in a long and thin carbon steel sheet, and then coated with polydimethylsiloxane (PDMS) was manufactured and tested in curvature and temperature. Bending laboratory results showed that the proffered sensor has a curvature sensitivity of -0.9835 dB/m-1 over a range from 0 m-1 to 1.3652 m-1 , measurements were taken by keeping a constant temperature of 30°C. The laboratory temperature response was -119 pm/°C at a temperature range from 30°C to 60°C, showing an improvement in temperature response, temperature measurements were taken by keeping a constant bending of 0 m-1 . The results show that PDMS coatings are a good way to improve multimode interferometer sensitivity during temperature measurement while keeping a good curvature measuring response, moreover the device shows a linear response within the curvature and temperature ranges. Another advantage of the PDMS coating is that it makes the sensor insensitive to refractive index changes, it gives the sensor robustness and protection against dust.

In this paper, biconical tapered fibers are used to carry out sensing in a simple and effective way by using an incoherent source and a power meter. This approach reduces the cost and complexity of traditional schemes based on tunable lasers while providing similar performance. Refractive index and strain sensing measurements are presented and their performance discussed.

An easy to construct curvature sensor is proposed and experimentally demonstrated. The device was fabricated splicing a segment of hollow core fiber (HCF) between two segments of coreless fiber (CLF) forming a Mach-Zehnder interferometer (MZI).

We proposed and demonstrated a compact, simple-to-construct and highly sensitive optical fiber sensor capable of simultaneously measuring refractive index (RI) and temperature (T) based on surface plasmon resonance (SPR) effect. The device structure consists of a single mode fiber (SMF) spliced between two multimode fibers where whole SMF was coated with chromium and gold. The thicknesses of the layers of chromium and the gold were 5 nm and 30 nm, respectively. By covering half of the sensing region of the device with a polymer with high thermo-optic coefficient (TOC), it is feasible to detect RI and T changes with sensitives of 2664.540 nm/RIU and -2.852 nm/°C, respectively.

Localized surface plasmon resonance (LSPR) has recently emerged as an efficient and powerful tool for bio-photonic applications due to its high sensitivity to refractive index changes. One technique to excite LSP is by the interaction of the evanescent wave of the light guided by an optical fiber with metallic nanoparticles deposited over the surface of the fiber. This paper proposes a novel, simple, and fast method to attach gold nanoparticles to the optical fiber surface, which can be used to construct highly sensitive refractive index sensors based on localized surface plasmon resonance. A hetero-core structured fiber, composed by a small section of single-mode fiber inserted in a multimode fiber, was coated with nanoparticles using the method proposed here. A sensor sensitivity and resolution of 765 nm/RIU and ∼1 × 10−4 RIU, respectively, were estimated over a refractive index range of 1.333–1.365. This coating method is appealing to construct optical fiber refractive index sensors since it is very simple and low cost.

A simple-to-fabricate and highly sensitive optical fiber temperature sensor is presented. The device consists of an optical microfiber mode interferometer embedded in a polymer with high thermo-optic coefficient which is also used as a packaging material. Temperature modifies the refractive index of the polymer, hence the phase difference between the interfering modes, which results in a prominent wavelength shift of the interference pattern. The sensor was tested in the 20 °C–48 °C range. The temperature sensitivity was found to be 3101.8 pm/°C. The simplicity and the performance of our device make it attractive for several applications.

Nonlinear effects are observed in a non-adiabatically tapered optical fibre. The designed structure allows for the introduction of self-phase modulation, which is observed through pulse breaking and spectral broadening, in approximately a centimetre of propagation using a commercial telecom laser. These devices are simple to fabricate and suitable to generate and control a variety of nonlinear effects in practical applications because they do not experience short-term degradation as previously reported approaches. Experimental and theoretical results are obtained, showing a good agreement.

A Fabry-Perot interferometer built with a properly selected tapered optical fiber tip and a flat reflecting target is proposed for long range displacement sensing. By scaling down the diameter of the core and cladding of the optical fiber that forms the interferometer, the divergence of the output beam is reduced. The tapered fiber tip also couples the reflected light from the target more efficiently. As a result, well-defined interference patterns are observed even for long cavities which allows to sense displacements as long as 80 mm. The fabrication of the fiber tips is simple and reproducible and the interrogation of the interference patterns is fast as a conventional fiber Bragg grating interrogator can be used. The possibility of multiaxis displacement sensing is discussed.

This paper provides the design, fabrication, and experimental demonstration of a novel, compact, and simple-to-construct optical fiber-tip sensor for the simultaneous measurement of the refractive index (RI), temperature, and level of liquid samples by means of surface plasmon resonance (SPR) phenomenon. The proposed fiber-tip SPR sensor consists of a multimode fiber spliced to a sensing element, it comprises a gold-coated single-mode fiber tip, whose end face is coated with a metallic mirror. By covering part of the sensing element with a thermo-responsive material, whose thermo-optic coefficient is high, it is possible to measure temperature; whereas the metal-bare section is used to measure the changes in the RI and level of a liquid sample. As the RI and temperature of the liquid sample increase, the position of the resonance wavelengths ( ${\boldsymbol {\lambda }_{\boldsymbol {bare}}}$ and ${\boldsymbol {\lambda }_{\boldsymbol {Covered}}}$ ) shifts toward larger and shorter wavelengths, respectively. On the other hand, as the level of the sensed sample increases, the deepness of ${\boldsymbol {\lambda }_{\boldsymbol {bare}}}$ increases too, but the position of ${\boldsymbol {\lambda }_{\boldsymbol {bare}}}$ and ${\boldsymbol {\lambda }_{\boldsymbol {Covered}}}$ remains constant for a particular liquid and temperature. We expect that this novel sensor becoming a promising candidate for biomedical, food, and/or industrial applications.

A fiber optic Fabry-Perot interferometer (FPI) was proposed and demonstrated. The sensor was constructed by splicing a single mode fiber (SMF) and a hollow core fiber (HCF), cleaving the HCF to a length of 15 ± 1 μm and filling it with polydimethylsiloxane (PDMS). The sensitivity of the sensor tip was 0.61 nm/°C and was caused by a combined effect of the thermo-optical and expansion coefficients.

The main motives driving the trend toward increased implementation of structural monitoring systems are the need for structural health monitoring of existing and ageing structures and the desire for a better understanding of increasingly complex designs through performance monitoring of new structures. This drive is sustained by rapid progress in research and technology development on sensors and communications. This chapter presents three case studies focusing on the monitoring of tunnels using distributed fiber-optic sensing. The first case study presents the results of strain development in the sprayed concrete lining (SCL) of a new Crossrail station tunnel during excavation of a cross-passage. The second case study focuses on the short term monitoring of a century old cast iron tunnel during the proximity construction of a large platform tunnel for the Crossrail project. The final case study describes the early stage implementation of a long-term structural health monitoring program to assess the integrity of tunnels at CERN. For all these projects Brillouin-based time domain techniques were used. A brief description of the basic principles of the method and the fiber-optic systems used is presented before the case studies. The variety of optical fibers available today combined with the number of fibre devices, with improved characteristics, recently developed provides a countless potential sensor configurations for environmental monitoring. In this chapter, a background of the use of fiber optic sensor networks in environmental applications are discussed focus in the optical detection for gas leakage sensing, water contamination sensing, soil contamination sensing and mapping with distributed fiber-optic sensors for environment.

We compare the behavior of three temperature sensors based on fiber optic Fabry-Perot interferometers (FPI). The length of the cavities were 30 ± 2 µm and they were filled with polydimethylsiloxane (PDMS), poly(methyl methacrylate) (PMMA), and the optical adhesive NOA61. The sensitivities for each sensor were 2.20 nm/°C, 0.42 nm/°C, and 0.57 nm/°C, respectively. The refractive index (RI) sensitivity showed by these devices was caused by an integrated consequence of the thermo-optical and expansion coefficients.

An in-fiber Mach–Zehnder Interferometer (MZI), based on hollow core fiber (HCF), for measuring curvature is fabricated and experimentally demonstrated. The sensing part was fabricated by splicing a section of HCF between two sections of multimode fiber (MMF), and different HCF lengths were investigated in order to achieve the highest curvature sensitivity. These devices were tested in a transmission configuration using lead-in and lead-out single mode fibers (SMF). The sensor was attached to a steel sheet by using the polymer Polydimethylsiloxane (PDMS) for accurate control of the applied curvature. The modal analysis was carried out using a commercial software based on finite element method (FEM) and by incorporating some of the experimental data, it was feasible to determine the two dominant modes that interfere in this sensor. The devices were characterized by measuring the fringe contrast variations due to the curvature changes. The interferometer fabricated with a HCF 2.5 mm in length HCF showed the highest curvature sensitivity, −17.28 ± 2.30 dB/m−1, in a range from 1.84 m−1 to 2.94 m−1. Moreover, the sensor that exhibited a better performance was fabricated with a HCF length of 1 mm, combining the most extensive curvature range (from 0.95 m−1 to 2.68 m−1) and an adequate sensitivity (−11.80 ±1.30 dB/m−1). The analysis of the interferometric signal of this device in Fourier domain, allows us to establish a one to one relationship between the contrast and the curvature in a broader range (from 0 m−1 to 2.94 m−1). Moreover, the fringe contrast showed a very low dependency on temperature (from 30 °C to 90 °C), depicting that this device was not affected by temperate fluctuation.

Multiwavelength Erbium-doped fiber ring lasers using two fiber filters inserted into the laser cavity is reported. The first fiber comb filter is a Mach–Zender interferometer consisting of two concatenated fiber tapers whose transmission bands determine the lasing wavelengths. The second filter is a tapered fiber which acts as a broadband fiber filter that reshapes the Erbium-doped fiber gain spectrum, and can be tuned by S-bending. Both filters are fabricated on the same fiber thus providing a cheap and very simple method to obtain tunable multiwavelength laser generation.

A simple technique for sensing the acoustic vibration of a cantilever beam, using a single-fiber Mach–Zehnder interferometer, is presented. The interferometer consists of two concatenated low-loss fused fiber tapers, with a waist diameter of 60 μm, separated by an un-tapered fiber section of 10 mm length. The interferometer transmitted signal is modulated when the device is bent under the presence of an external acoustic signal. The optical fiber device glued directly on a metallic cantilever beam is capable of measuring frequency of the resonant modes. The interrogation set-up is simple consisting of a single tunable diode laser and a photodetector. The measured frequencies of the resonating modes agree with the numerical results obtained by the Finite Element Method.

We report an easy-to-build, compact, and low-cost optical fiber refractive index sensor. It consists of a single fiber loop whose transmission spectra exhibit a series of notches produced by the resonant coupling between the fundamental mode and the cladding modes in a uniformly bent fiber. The wavelength of the notches, distributed in a wavelength span from 1,400 to 1,700 nm, can be tuned by adjusting the diameter of the fiber loop and are sensitive to refractive index changes of the external medium. Sensitivities of 170 and 800 nm per refractive index unit for water solutions and for the refractive index interval 1.40-1.442, respectively, are demonstrated. We estimate a long range resolution of 3 × 10(-4) and a short range resolution of 2 × 10(-5) for water solutions.

An all-fiber comb-filter using a Mach-Zehnder interferometer based on concatenated tapers is presented. The central wavelength, the amplitude, the period, and the spectral width of the transmission interference pattern can be tailored by varying the diameter and length of the waist, the length of the up- and down-transition of the fiber tapers, and the separation between the two tapers. Depending on the geometrical parameters of the tapers, the central position and span of the spectral interference pattern were tailored in the range of 1200-1650 and 50-300 nm, respectively. By varying the interferometer length, the fringe period and bandwidth were tailored between 3-36 and 2.5-11 nm, respectively. The physical length of the devices fabricated ranges from 15 to 30 mm. The proposed device is simple and suitable for applications in optical communications systems, fiber lasers, and optical fiber sensors for the monitoring of strain and displacement.

We report on a contactless optical fiber refractive index (RI) sensor that can be used to measure the RI of solid or liquid samples. The sensor is simple to construct and consists of a Fabry-Perot interferometer (FPI) with long cavity. The cavity of our FPI consists of a tapered optical fiber tip and an external flat mirror. The output beam of the fiber tip is partially reflected from different interfaces of the sample present in the interferometer cavity. Each of such reflections interferes with the beam internally reflected by the fiber tip. Thus, a multiple-beam FPI is formed whose reflection spectrum is composed by the superposition of several two-beam interferences. The analysis of the multiple interference spectra was carried out in the Fourier domain. Several glass samples, water-sucrose and water-glycerol solutions were prepared and tested. Since the fiber tip is not in direct contact with the sample under test the measurement is simple and immediate. To our-knowledge, this is the first time that a fiber optic sensor can be used to measure the RI of solid and liquid samples without any modification.

The Raman response function and Raman nonlinear index of phosphosilicate fibers are calculated for several P2O5 concentrations. The nonlinear index of phosphosilicate fiber is estimated by using the Boling formula, and it is used to calculate the Raman fraction of the nonlinear index. On the other hand, the Raman response function of the phosphosilicate fibers is fitted to a superposition of six phase-shifted under-damped functions in order to use it in the numerical simulation of ultra-short pulse propagation. We show through numerical simulations that phosphosilicate fibers are a suitable medium to observe a scaling up of the Raman self-frequency shift when compared to silica fibers.

A micro-displacement sensor consisting of a fiber-loop made with a tapered fiber is reported. The sensor operation is based on the interaction between the fundamental cladding mode propagating through the taper waist and higher order cladding modes excited when the taper is deformed to form a loop. As a result, a transmission spectrum with several notches is observed, where the notch wavelength resonances shift as a function of the loop diameter. The loop diameter is varied by the spatial displacement of one end of the fiber-loop attached to a linear translation stage. In a displacement range of 3.125 mm the maximum wavelength shift is 360.93 nm, with 0.116 nm/μm sensitivity. By using a 1,280 nm broadband low-power LED source and a single Ge-photodetector in a power transmission sensor setup, a sensitivity in the order of 2.7 nW/μm is obtained in ≈ 1 mm range. The proposed sensor is easy to implement and has a plenty of room to improve its performance.

The spectral response of mechanically induced long-period fiber gratings (MLPFG) to the ambient temperature variation was experimentally study. In the MLPFG setting, a pressure rig with aluminum grooved plates on standard telecommunication fiber was used. We found that a relatively low change in temperature can produce a severe impact on the characteristic parameters of the attenuation bands, such as a critical decrease in the contrast and fast shift saturation. When the temperature increases from −10 to 70 °C, all the bands are shifted toward longer wavelengths with a mean sensitivity of approximately 180 pm/°C from 0 to 40 °C in the near-linear region, then they present a flattened zone beyond this temperature. Meanwhile, the contrast of the attenuation bands rapidly decreases from the maximum value to 0 dB with a quasi-cosinoidal behavior. These results are important and have to be considered when MLPFG are applied in a medium with ambient temperature variation. Furthermore, we show that MLPFG can be used as low cost ambient temperature sensors through intensity based measurements.

We report the use of the prism-based surface plasmon resonance (SPR) technique to differentiate between three types of tequilas white or silver, aged, and extra-aged. We used the angular interrogation method in which the structure is based on prism fabricated with BK7 glass coated with a gold layer as the SPR active layer. Our study was centered in the analysis of the resonant angle of the SPR generated by the three types of tequilas produced by the three major tequila-producing firms. We observed that each tequila sample produced a well-differentiated SPR curve. We found that resonant angle of the SPR curve produced by silver tequilas is larger than that produced by the aged and extra-aged tequilas of the same producer firm. We found that the position of the SPR curve is not exclusively determined by the alcohol contents; we believe that there are other parameters derived from the aging process that should be considered. The refractive index of the tequilas used in this study was estimated using the measured resonant angle.

Here we propose two methods using conventional evaporation process to produce tailored Pd/Au composite thin films for hydrogen sensing. In the first method Pd and Au are evaporated simultaneously using different evaporation rate. In the other, alternated nano-layer of Pd and Au are evaporated. Some characteristics of the thin films and their response to hydrogen when they are deposited over optical fibers are experimentally analyzed. Optical fiber sensors for detection of hydrogen concentration lower than 4% and response time of 10 s approximately are demonstrated. The possibility to precisely control the proportion of the constituents of Pd-alloys thin films to produce optical fiber hydrogen sensors with fast response makes these methods very attractive.

We present a study of spiral structures generated by superposition of a truncated Bessel beam and a plane wave. One of the main characteristics of spiral structures is that the shape of spiral beams remains unchanged upon propagation. The results of experimental observations using a Mach–Zehnder interferometer are compared with the numerically calculated intensity distribution. We show that when spiral beams are disturbed by a nontransparent obstacle, the amplitude of the transmitted field is distorted at nearby distances behind that obstacle, but during free-propagation, it regenerates to its initial form.

Optical microfibers decorated with PdAu nanoparticles are proposed for fast hydrogen sensing. The optical microfibers were fabricated by simply tapering conventional telecommunications fiber down to dimensions comparable to the wavelength of the guided light. The optical microfibers were then coated over a few millimeters with a PdAu layer in island form by depositing the layer at low deposition rate (0.1 A/s). Then the islands were grown with a thermal annealing process until composite nanoparticles were formed. The PdAu nanoparticles deposited on the optical microfibers experience optical and physical changes when they are exposed to hydrogen. This gives rise to reversible transmission changes with an unusual pulsed like behavior which is attributed to scattering of the guided light. The devices are promising for detecting low concentrations of hydrogen (up to 8%) at room temperature with response and recovery times on the order of seconds, i.e., faster than many other hydrogen sensors that exploit phenomena at the nanoscale. The sensing of other gases is also feasible if the optical microfibers are decorated with suitable gas-permeable nanoparticles.

We have investigated the effects of cladding etching in arc-induced long-period fiber gratings. Comparing the position of the resonant wavelengths at each diameter with the corresponding theoretical dispersion curves of effective refractive index, it is found that some of the resonances show a better match with anti-symmetrical cladding modes, while others with symmetrical cladding modes. Etching the cladding diameter to ∼39 μm, results in coupling to a high-order cladding mode that is very sensitive to changes in the ambient refractive index. The resonant peak shifts remarkably (∼230 nm) toward shorter wavelengths as the external refractive index increases. This sensitivity is higher than any previously reported in arc-induced long-period fiber gratings.

The characteristics of a mechanically induced long-period fiber grating (MLPFG) made by pressing a pair of grooved plates over single-mode fiber tapers are analyzed. Fiber tapers with a waist length of 80 mm and diameter ranging from 90 to 125 μm, fabricated using the heating and puling method, were used. We observed that the resonance wavelengths shift toward shorter wavelengths as the fiber taper waist diameter is reduced. A maximum shift of 254 nm in the position of the resonance peaks was observed when the fiber diameter was reduced to 90 μm. This technique is particularly suitable for tuning the resonance wavelengths to shorter wavelengths below the limit imposed by the grooved plate period.

The characteristics of leaky modes in microstructured fibers with finite-thickness absorbing coatings are studied theoretically and experimentally. It is shown that, if the thickness of the coating is constant, the fiber has a quasi-periodic transmission spectrum. The period and depth of modulation of the spectrum are determined by resonance coupling between the modes of the fiber and coating. Since the constants of mode propagation in the coating are very sensitive to environmental parameters, the discovered effect seems to be promising for sensor applications.

We present an experimental study performed to determine the hydrogen sensing characteristics of a stack of Pd and Au nano-layers. Hetero-core optical fibers were coated with a stack of 1, 2, 3, 4 and 5 double layers of 1.4 nm of Pd and 0.6 nm of Au, alternately deposited by thermal evaporation. The presence of hydrogen produces a decrease in the palladium refractive index, which causes attenuation changes in the optical fiber evanescent wave. Several samples, hetero-core optical fibers coated with different thin film thicknesses, were fabricated and tested in a very simple optical transmission measurement setup. A mixture of hydrogen and nitrogen (carrier gas) was used to test the devices. We observed a response time of 4.5 s for hydrogen concentration of 4%. When hydrogen flow was turned off, the multilayer thin film desorbs the gas in 13 s, indicating a fast recovery response.

A temperature sensor consisting of two three-core-fiber segments is demonstrated. Ratiometric measurements of the signals provide a sensitivity of 0.036/oC which is two orders of magnitude higher than their individual signals.

Thin alloy film of Pd and Au, formed by simultaneous electron-beam and thermal evaporation techniques, respectively, is used in the design of an optical fiber hydrogen sensor. The sensor consists of a multimode fiber (MMF) in which a short section of single mode fiber (SMF), coated with the Pd–Au thin film, is inserted. Due to core diameter mismatch, the SMF cladding guides light, allowing the interaction between the sensing layer and the guided light. When the sensor is exposed to hydrogen, the Pd–Au layer refractive index diminishes and causes attenuation changes on the transmitted light. Several samples with different layer thickness uniformity were fabricated and tested in a very simple experimental set-up. We have observed that the sensor signal change is dependant on layer thickness uniformity, since the effective interaction length between the evanescent field and the sensing layer is increased. By contrast, such uniformity practically has no influence on the time response of the sensor. The resulting Pd–Au film can detect 4% hydrogen with a response time of 15 s.

We report on optical fiber hydrogen sensors based on absorption changes of evanescent fields caused by a Pd or Pd/Au alloy thin film. The sensor consists of a small piece of standard single-mode fiber (SMF) coated with a palladium or Pd/Au thin film sandwiched between two multimode fibers (MMFs). Due to core diameter mismatch the SMF cladding guides light. When the device is exposed to hydrogen the layer refractive index diminishes and causes attenuation changes of the evanescent fields. Such attenuation changes can be easily detected with a simple transmission measuring setup. Robust sensors with time responses of 15 s and good sensitivity suitable for the detection of hydrogen concentration below the critical 4% are demonstrated.

For an expert nose, the aroma of a beverage is a fingerprint that can be used to certify its authenticity, distinguish between distillation processes, or even identify the raw material used to fabricate it. In this work, we propose a simple, automatic, and repeatable optical method, which can be used as a first and quick test to authenticate tequila samples. This method is based on the measurement of beam intensity changes, using the surface plasmon resonance technique, operating at a fixed angle. We observed that each tequila, depending on the alcohol content and aging process, produces a specific change in measured intensity level.

The theoretical modeling of large-mode-area silica holey fiber tapers with collapsed air holes that are fabricated with a nonadiabatic process is reported. The transmission spectra of such tapers exhibit multiple interference peaks due to the beating between two modes of the taper waist. The devices investigated here can be useful for a variety of sensor applications since the interference peaks are sensitive to the external environment. Our theoretical model confirms the experimental results

When a thin metal layer is deposited on a single-mode tapered optical fiber the layer takes a semi-cylindrical shape. The fundamental fiber mode can excite the different hybrid surface plasmon modes supported by the semi-cylindrical metallic shell. As a consequence, the transmission spectrum of the device exhibits multiple resonance peaks with either TE- or TM-polarized light. The positions of those peaks are remarkably shifted when the refractive index of the medium adjacent to the metal layer changes. Experimentally, we have found that by monitoring the first three resonance peaks a refractive index resolution of about 7 × 10 −7 can be achieved. Our devices can be used for high-resolution biosensing.