Showing papers on "Step-index profile published in 2014"

Enhanced refractive index sensing characteristics of optical fibre long period grating coated with titanium dioxide thin films

Abstract: A new type of fibre-optic refractive index sensor based on a long period fibre grating (LPFG) coated with a titanium dioxide (TiO2) thin film was demonstrated. The wavelength shift of the attenuation bands of this LPFG sensor to changes in the refractive index of the external media from 1.30 to 1.64 RIU was investigated. In order to optimize the sensor the TiO2 thin film thickness deposited around the LPFGs was varied from 10 to 80 nm. It was found that the TiO2 thin film increases the wavelength sensitivity of the LPFG to changes in the surrounding refractive index for values lower and higher than the cladding refractive index. As opposed to the bare LPFG it was shown the possibility to monitor refractive indices lower and higher than cladding refractive index tailoring the TiO2 thickness. An average wavelength sensitivity of 5250 nm/RIU was achieved in the range 1.444 to 1.456 RIU for a TiO2 thickness of 50 nm. In the region between 1.46 and 1.48 RIU the average sensitivity of about 825 nm/RIU was measured for a 40 nm thick film.

Refractive index dispersion of chalcogenide glasses for ultra-high numerical-aperture fiber for mid-infrared supercontinuum generation

Abstract: We select a chalcogenide core glass, AsSe, and cladding glass, GeAsSe, for their disparate refractive indices yet sufficient thermal-compatibility for fabricating step index fiber (SIF) for mid-infrared supercontinuum generation (MIR-SCG). The refractive index dispersion of both bulk glasses is measured over the 0.4 µm–33 µm wavelength-range, probing the electronic and vibrational behavior of these glasses. We verify that a two-term Sellmeier model is unique and sufficient to describe the refractive index dispersion over the wavelength range for which the experimentally determined extinction coefficient is insignificant. A SIF composed of the glasses is fabricated and calculated to exhibit an ultra-high numerical aperture >0.97 over the entire wavelength range 0.4-33 µm suggesting that the SIF glass pair is a promising candidate for MIR-SCG. Material dispersion characteristics and the zero dispersion wavelength, both critical design parameters for SIF for MIR-SCG, are derived.

Temperature-insensitive refractive index sensor based on in-fiber Michelson interferometer

Abstract: A novel intensity-modulated refractive index sensor based on an in-fiber Michelson interferometer is demonstrated by splicing a section of thin core fiber between two standard single mode fibers. Such a refractive index sensor exhibits an ultrahigh sensitivity of −208.24 and 125.44 dB/RIU at the refractive index of 1.440 and 1.500, respectively. The refractive index sensor is insensitive to temperature and thus solves the cross-sensitivity problem between temperature and surrounding refractive index. Moreover, the promising RI sensor has the advantages of short size (less than 2 mm) and easy fabrication.

Design of Few-Mode Fibers With M-modes and Low Differential Mode Delay

Abstract: In this paper, we investigate the design of few-mode fibers (FMFs) guiding 2 to 12 linearly polarized (LP) modes with low differential mode delay (DMD) over the C-band, suitable for long-haul transmission. Two different types of refractive index profile have been considered: a graded-core with a cladding trench (GCCT) profile and a multi-step-index (MSI) profile. The profiles parameters are optimized in order to achieve: the lowest possible DMD and macro-bend losses (MBL) lower than the ITU-T standard recommendation. The optimization results show that the MSI profiles present lower DMD than the minimum achieved with a GCCT profile. Moreover, it is shown that the optimum DMD and the MBL scale with the number of modes for both profiles. The optimum DMD obtained for 12 LP modes is lower than 3 ps/km using a GCCT profile and lower than 2.5 ps/km using a MSI profile. The optimization results reveal that the most preponderant parameter of the GCCT profile is the refractive index relative difference at the core center, Δnco. Reducing Δnco, the DMD is reduced at the expense of increasing the MBL. Regarding the MSI profiles, it is shown that 64 steps are required to obtain a DMD improvement considering 12 LP modes. Finally, the impact of the fabrication margins on the optimum DMD is analyzed. The probability of having a manufactured FMF with 12 LP modes and DMD lower than 12 ps/km is approximately 68% using a GCCT profile and 16% using a MSI profile.

Reflectivity enhanced refractive index sensor based on a fiber-integrated Fabry-Perot microresonator.

Abstract: We discuss a fiber-integrated refractive index sensor with strongly improved detection performance. The resonator has been implemented by means of focused-ion beam milling of a step index fiber and shows a sensitivity of about 1.15µm/RIU. Coating the resonator walls led to a strongly improved mirror reflectivity by a factor of about 26. Design rules for device optimization and a detailed mathematical analysis are discussed, revealing that the sensor operates as an optimized Fabry-Perot resonator. We also show that the performance of such kind of Fabry-Perot sensors is, in general, limited by the detection limit function – a quantity depending on the cavitiy’s finesse and on the measurement capabilities used.

Theoretical and experimental study of refractive index sensors based on etched fiber Bragg gratings

Abstract: In this work, a theoretical and experimental study of the effect of the etching process on the properties of fiber Bragg gratings (FBGs) used as refractive index sensors is presented. The theoretical study addresses the dependence of the effective refractive index on the cladding thickness of the etched FBG. The results of this study show that as the cladding thickness is reduced, the effective refractive index decreases exponentially. Based on this result, a simple analytic expression between the effective refractive index and the radius of the etched FBG was developed. Thus, the radius of the FBG after the etching process can be determined directly from the shift of the Bragg wavelength, without the need of extensive numerical simulations. Also, the sensitivity of the FBG sensor in characteristic environments of practical interest has been calculated as a function of the fiber radius, and found to be described by simple analytic functions. The time evolution of the Bragg wavelength during the etching process has also been investigated, both experimentally and theoretically. By means of detailed theoretical analysis of the experimental results, the heating and the etching rate of the FBG was calculated. Finally, the etched FBGs have been used as liquid level sensors for water and oil, having refractive indices lower and higher than the fiber core respectively. The analysis of the experimental results was based on the shift of the diffracted wavelength in the case of water and on the reduction of the reflected power in the case of oil.

UV-Curable Polymer Microhemisphere-Based Fiber-Optic Fabry-Perot Interferometer for Simultaneous Measurement of Refractive Index and Temperature

Abstract: A fiber-optic Fabry-Perot interferometer based on UV-curable polymer micro- hemisphere is proposed and demonstrated. The polymer microhemisphere is formed by adhering and solidifying a liquid microdroplet of UV-curable adhesive to the end face of a cleaved single-mode fiber. The height of polymer microhemisphere could be flexibly con- trolled by adjusting the diameter of a single-mode fiber. The theoretical and experimental results demonstrate that the refractive index (RI) and the temperature of external environ- ment can be simultaneously measured by the fringe contrast variation and the wave- length shift of reflection spectra separately, alleviating the cross sensitivity effectively. The obtained temperature and RI sensitivities are about 0.19 nm= � C and 260 dB/RIU in the RI range of 1.38-1.42.

Refractive Index Sensing of SMS Fiber Structure Based Mach-Zehnder Interferometer

Abstract: A Mach-Zehnder interferometer based on single-mode-thin-core-multimode-single-mode (STMS) fiber structure for refractive index (RI) measurement is proposed and experimentally demonstrated. It works on the basis of interference between the core mode and cladding mode. Using the multimode fiber core, the transmission spectrum of STMS structure is very sensitive to RI variations of the surrounding medium. The experimental results show that the sensor possesses a high sensitivity of 148.27 nm/RIU in the RI range of 1.333-1.403 and has a good linear response to the SRI.

Refractometric sensors based on multimode interference in a thin-film coated single-mode–multimode–single-mode structure with reflection configuration

Abstract: Thin-film coated single-mode-multimode-single-mode (SMS) structures have been analyzed both theoretically and experimentally with the aim of detecting different refractive indices. By adequate selection of the thickness of the thin film and of the diameter of the multimode segment in the SMS structure, a seven-fold improvement can be obtained in the sensitivity of the device to the surrounding medium refractive index, achieving a maximum sensitivity of 1199.18 nm/refractive index unit for the range of refractive indices from 1.321 to 1.382. Using layer-by-layer self-assembly for deposition, both on the cladding and on the tip of the multimode segment, allows the reflected power to increase, which avoids the application of a mirror on the tip of the multimode segment.

Thickness-dependent optical properties and nonlinear refractive index of a-Ge–Se–In thin films

Abstract: The present work reports the study of linear optical properties of a-Ge20Se80−xInx (x = 0, 5, 10, 15, 20) system as a function of film thickness and at different Se/In ratio using transmission spectra. Optical gap increases with the increase in film thickness, whereas no significant change is found in the linear refractive index for all the compositions. To evaluate their potential applications for all-optical ultrafast switching devices, we report the effect of In alloying on the nonlinear refractive index and third-order susceptibility (χ(3)), evaluated from the changes of index of refraction using Wang approximations. Comparative higher values of susceptibility of the order of 10−11 (in esu) are found and show a maximum at In = 10 at.%. The largest value of susceptibility can be expected for materials with higher values. Different formulations were used to predict the nonlinear behavior of Ge–Se–In system. The addition of In to Ge20Se80−xInx glass leads to an increase in the nonlinear refractive index,...

Multimode Interference Refractive Index Sensor Based on Coreless Fiber

Abstract: A multimode interference refractive index (RI) sensor based on the coreless fiber was numerically and experimentally demonstrated Two identical single mode fibers (SMF) are spliced at both ends of a section of the coreless fiber which can be considered as the equivalent weakly guiding multimode fiber (MMF) with a step-index profile when the surrounding refractive index (SRI) is lower than that of the coreless fiber Thus, it becomes the conventional single-mode multimode single-mode (SMS) fiber structure but with a larger core size The output spectra will shift along with the changes in the SRI owing to the direct exposure of the coreless fiber The output spectra under different SRIs were numerically studied, as well as the sensitivities with different lengths and diameters of the coreless fiber The predication and calculation showed the good agreement with the experimental results The proposed RI sensor proved to be feasible by verification experiments, and the relative error was merely 01% which occupied preferable sensing performance and practicability

Fiber structure based on a depressed inner cladding fiber for bend, refractive index and temperature sensing

Abstract: A fiber-optic structure based on a section of a double-clad fiber with depressed inner cladding is investigated for bend, refractive index and temperature sensing. The structure is formed by splicing a section of SM630 fiber between two standard fibers SMF-28. The operation principle relies on the sensitivity of cladding modes that are induced at a splice of fibers having different refractive index profiles. The mode structure of the double cladding fiber and the mechanism of formation of dips in the transmission spectra are discussed. The transmission spectra of the structure are measured for different curvatures of the inserted fiber section. The shift of dips to long wavelengths with increasing curvature of the fiber is observed and its dependence on the fiber section length and the direction of bending is investigated. The sensitivities of the spectral dips to the external refractive index and temperature are also measured.

Reflection-based fibre-optic refractive index sensor using surface plasmon resonance

Abstract: A reflection-based fibre-optic refractive index sensor using surface plasmon resonance (SPR) in a thin metal film sputtered on a bare core of a multimode optical fibre is presented. The sensing element of the SPR fibre-optic sensor is the core of a step-index optical fibre made of fused silica with a gold film double-sided sputtered on the whole core surface, including the core end face. Consequently, a terminated reflection-based sensing scheme to measure the refractive indices of liquids is realized. The sensing scheme uses a wavelength interrogation method and the refractive index of a liquid is sensed by measuring the position of the dip in the reflected spectral intensity distribution. As an example, the aqueous solutions of ethanol with refractive indices in a range from 1.333 to 1.363 are measured. In addition, the increase in the sensitivity of the SPR fibre-optic refractive index sensor with the decrease of the fibre sensing length is demonstrated.

Two-core transversally chirped microstructured optical fiber refractive index sensor

Abstract: We present a sensing architecture consisting of a two-core chirped microstructured optical fiber (MOF) for refractive index sensing of fluids. We show that by introducing a chirp in the hole size, the MOF can be a structure with decoupled cores, forming a Mach–Zehnder interferometer in which the analyte directly modulates the device transmittance by its differential influence on the effective refractive index of each core mode. We show that by filling all fiber holes with analyte, the sensing structure achieves high sensitivity (transmittance changes of 300 per RIU at 1.42) and has the potential for use over a wide range of analyte refractive index.

Scratch-resistant articles with a gradient layer

Abstract: Embodiments of this disclosure pertain to articles that exhibit scratch-resistance and improved optical properties. In some examples, the article exhibits a color shift of about 2 or less, when viewed at an incident illumination angle in the range from about 0 degrees to about 60 degrees from normal under an illuminant. In one or more embodiments, the articles include a substrate, and an optical film disposed on the substrate. The optical film includes a scratch-resistant layer and a refractive index gradient. In one or more embodiments, the refractive index includes a refractive index that increases from a first surface at the interface between the substrate and the optical film to a second surface. The refractive index gradient may be formed from a compositional gradient and/or a porosity gradient.

Tunable fiber laser based on the refractive index characteristic of MMI effects

Abstract: A tunable erbium-doped all-fiber laser has been demonstrated. This tunable laser is based on a tunable fiber filter using the refractive index characteristics of multimode interference effects. A thinner no-core fiber with a diameter of 104 μm is used to fabricate the tunable fiber filter. The joint point of the thinner no-core fiber with SMF is a taper, which improves its sensitivity for refractive index changes. The filter exhibits a very sensitive response to the change of the environmental refractive index, which is about 1000 nm/RIU in the RI range from 1.418 to 1.427. The tunable fiber laser based on the filter achieved a tunability of 32 nm, with the wavelength tuned from 1532 nm to 1564 nm covering the full C-band. The 3 dB bandwidth of the tunable laser is less than 0.02 nm with the signal-to-noise ratio of about 40 dB.

System and method for optical fiber

Abstract: In one embodiment, an optical fiber includes a first layer having a first refractive index and a second layer surrounding the first layer, where the second layer has a second refractive index, an inner radius, and an outer radius. The optical fiber also includes a third layer surrounding the second layer, where the third layer has a third refractive index, where the first refractive index is less than the second refractive index, where the third refractive index is less than the second refractive index, and where a ratio of the outer radius to the inner radius is less than 1.5.

Refractive Index Sensing Characteristics of Single-Mode Fiber-Based Modal Interferometers

Abstract: We present a theoretical and experimental investigation on refractive index (RI) sensing characteristics of single mode fiber (SMF) based modal interferometers. Theoretical analysis reveals that interference between different modes in an SMF has a quite different response to the RI variation of the external medium. The interference between the core and lower order cladding modes has negative RI sensitivity whereas that between the core and higher order modes, or between two different order cladding modes have positive sensitivity. A single-mode-multimode-single-mode (SMS) fiber Michelson interferometer with a large-core step-index multimode fiber (MMF) is employed for experimental verification. In the SMS-based Michelson interferometer, the MMF acts as a mode coupler to excite cladding modes in the SMF. The RI response of the SMS-based structures with two different lengths of MMF are respectively tested in sodium-chloride water solutions. Experimental results show excellent agreements with the theoretical analysis.

Reflection based Extraordinary Optical Transmission Fiber Optic Probe for Refractive Index Sensing.

Abstract: Fiber optic probes for chemical sensing based on the extraordinary optical transmission (EOT) phenomenon are designed and fabricated by perforating subwavelength hole arrays on the gold film coated optical fiber endface. The device exhibits a red shift in response to the surrounding refractive index increases with high sensitivity, enabling a reflection-based refractive index sensor with a compact and simple configuration. By choosing the period of hole arrays, the sensor can be designed to operate in the near infrared telecommunication wavelength range, where the abundant sources and detectors are available for easy instrumentation. The new sensor probe is demonstrated for refractive index measurement using refractive index matching fluids. The sensitivity reaches 573 nm/RIU in the 1.333–1.430 refractive index range.

Fiber Loop Ring-Down Refractive Index Sensor Based on High- $Q$ Photonic Crystal Cavity

Abstract: This paper proposed a novel ultracompact refractive index sensor formed by a high-quality (Q) photonic crystal (PhC) cavity-based fiber loop ring-down. In our proposed device configuration, we insert one pair of two air holes into the linear waveguide as two reflectors, and a point defect was defined between the two reflectors to form a nanocavity. Since the output resonant spectrum was extremely sensitive to a small refractive index variance attributed to the medium infiltrated in the holes, the ring-down time as well as the radiation intensity would be a function of the relative refractive index for input light at certain frequency. To improve the system sensitivity, the Q -factor of nanocavity was optimized by adjusting the radii of all air holes and the spacing between the two reflectors. The properties of the refractive index sensor were analyzed and simulated using the finite-difference time-domain method. Results showed that a quasi-linear measurement of refractive index was achieved with sensitivity of 20.34 ms per refractive index unit and Q -factor of 605. In addition, the sensing range could be broadened to 1.330-1.400 by using multiple operating frequencies.

Analysis of Straight Periodic Segmented Waveguide Using the 2-D Finite Element Method

Abstract: A numerical analysis of periodic segmented waveguides (PSWs) using the 2-D finite element method (2D-FEM) in the frequency domain is presented. This method has significantly lower computational cost when compared with 3-D methods that have been used to model PSWs, and can also model back reflected signals. Unlike photonic crystal waveguides, light confinement in a PSW is due to total internal reflection as in a continuous waveguide (CWG). We show that the dispersion relation of the guided modes in PSW is strongly influenced by the dielectric periodicity along the waveguide. We calculate the mode profile of a PSW in a region far away from the bandgap and we showed that it is comparable to the mode profile of the equivalent CWG even for relatively high values of averaged refractive index contrast.

Graded refractive index bending-resistant multimode optical fiber

Abstract: A graded refractive index bending-resistant multimode optical fiber includes a core layer and claddings. The core layer has a radius in a range of 22.5-27.5 μm; refractive indexes being a gradient-graded refractive index distribution with a distribution exponent α in a range of 1.99-2.06; and a maximum relative refractive index difference (RRID) Δ1% max in a range of 0.9%-1.3%. The claddings has an inner cladding surrounding the core layer, an intermediate cladding surrounding the inner cladding and an outer cladding surrounding the inner cladding. The inner cladding has a radius in a range of 25.5-34.5 μm, and an RRID Δ2% in a range of −0.02%-0.02%. The intermediate cladding is a pure quartz glass layer, and has a radius in a range of 30.5-49.5 μm, and an RRID Δ3% in a range of −0.01%-0.01%. The outer cladding has a radius in a range of 61.5-63.5 μm, and an RRID Δ4% is in a range of −0.20%-0.30%.

Temperature dependence of the refractive index of optical fibers

Abstract: Many experimental investigations on the temperature dependence of the refractive index of optical fibers have been reported previously, however a satisfying theoretical explanation for it is still absent. In this paper, a theoretical model about the temperature dependence of the refractive index of optical fibers is presented and it is in agreement with the previous experimental results. This work is a significant reference for the research and development of temperature sensors based on optical fiber delay lines.

Photonic Crystal Fiber-Based Modal Interferometer for Refractive Index Sensing

Abstract: A core-cladding-mode interferometer is fabricated by splicing a single-mode fiber to an endlessly single-mode photonic crystal fiber (ESM-PCF). The optimum collapsed length of ESM-PCF is investigated to ensure a higher order cladding mode of ESM-PCF to be excited, serving as a sensing beam. At the end of ESM-PCF, a spherical end-facet with big radius is proposed to ensure from where the higher order cladding mode is to be reflected and then it interferes with the core mode serving as a reference beam. Such a device has an enhanced refractive index sensitivity of ~ 199 nm/RIU, offering potentials in biological and chemical applications.

Method for refractive index measurement of nanoparticles

Abstract: A new method of measuring the refractive indices of nanoparticles has been proposed based on refractive index matching between nanoparticles and surrounding organic solvents. By finding the most transparent point of the transmittance spectrum, the refractive index of the nanoparticles is equal to that of the solvents at the corresponding wavelength. Utilizing a Rayleigh scattering model, the effects of refractive index mismatching (Δn) on the transmittance are investigated under different conditions. Some criteria for getting highly transparent nanoparticle dispersion and accurate refractive index measurements are suggested, which can support practical applications for nanomaterials in optical and optoelectronic applications.

Optical fiber with large mode field diameter and low microbending losses

Abstract: Optical fibers having a mode field diameter at 1310 nm of at least 8.8 μm, wire mesh covered drum microbending losses at 1550 nm less than 0.03 dB/km, and a 2 m cutoff wavelength less than 1320 nm. The fibers may include a central core region, an inner cladding region, an outer cladding region, a primary coating with an in situ modulus less than 0.20 MPa and glass transition temperature less than −35° C., and a secondary coating with an in situ modulus greater than 1500 MPa. The fibers may further include a depressed index cladding region. The relative refractive index of the central core region may be greater than the relative refractive index of the outer cladding region may be greater than the relative refractive index of the inner cladding region. The fibers may be produced at draw speeds of 30 m/s or greater.

Refractive index sensor based on the leaky radiation of a microfiber.

Abstract: In this work we present a refractive index sensor based on the leaky radiation of a microfiber. The 5.3um diameter microfiber is fabricated by drawing a commercial optical fiber. When the microfiber is immersed into a liquid with larger refractive index than the effective index of fiber mode, the light will leak out through the leaky radiation process. The variation of refractive index of liquid can be monitored by measuring radiation angle of light. The refractive index sensitivity can be over 400 degree/RIU in theory. In the experiment, the variation value 0.001 of refractive index of liquid around this microfiber can be detected through this technique. This work provides a simple and sensitive method for refractive index sensing application.