Fiber Refractive Index Research Articles

Low-cost wavelength-selective evanescent fiber optic temperature and refractive index sensor

We report here a low-cost evanescent field wave fiber optic temperature sensor. The amount of rays guided along the fiber probe is the result of the absorption of evanescent waves produced at the core-cladding interface. We systematically investigate the wavelength dependence of this sensor during the measurement of the temperature. The degree of variation with respect to the temperature is also determined by alteration of measuring liquid. We observe an enhanced responsivity of the fiber sensor towards a higher wavelength side. The sensor set-up offers simultaneous detection of refractive indices when correlated with temperature variation. All the results reported here bear a good repeatability.

Are optical fibers really uniform? Measurement of refractive index on a centimeter scale.

Many applications of optical fiber, such as specialized fiber Bragg gratings (FBGs), require high uniformity of a fiber's refractive index (RI) along its length. We show here that the mode effective index of most fibers is not constant, even on a short length scale. To help improve fiber manufacturing and selection, we demonstrate a technique for characterizing a meter length single-mode optical fiber's effective RI over a centimeter scale with a precision of 3×10-6 RI units (RIUs) and an absolute accuracy of 2×10-4 RIU. By writing several weak probe FBGs as frequency references and then measuring the frequency deviation of these probe FBGs along the length of the fiber with an optical frequency domain reflectometer, the RI distribution of the effective mode index may be found. We validate our measurements on reference and fibers under test with theoretical simulations.

HCPCF-based in-line fiber Fabry-Perot refractometer and high sensitivity signal processing method

An in-line fiber Fabry-Perot interferometer (FPI) based on the hollow-core photonic crystal fiber (HCPCF) for refractive index (RI) measurement is proposed in this paper. The FPI is formed by splicing both ends of a short section of the HCPCF to single mode fibers (SMFs) and cleaving the SMF pigtail to a proper length. The RI response of the sensor is analyzed theoretically and demonstrated experimentally. The results show that the FPI sensor has linear response to external RI and good repeatability. The sensitivity calculated from the maximum fringe contrast is –136 dB/RIU. A new spectrum differential integration (SDI) method for signal processing is also presented in this study. In this method, the RI is obtained from the integrated intensity of the absolute difference between the interference spectrum and its smoothed spectrum. The results show that the sensitivity obtained from the integrated intensity is about –1.34×105 dB/RIU. Compared with the maximum fringe contrast method, the new SDI method can provide the higher sensitivity, better linearity, improved reliability, and accuracy, and it’s also convenient for automatic and fast signal processing in real-time monitoring of RI.

Fiber-Optic Sensing Interrogation System for Simultaneous Measurement of Temperature and Transversal Loading Based on a Single-Passband RF Filter

In this paper, a fiber-optic sensing interrogation system for simultaneous measurement of temperature and transversal loading based on a single-passband RF filter has been proposed and experimentally demonstrated. This sensing interrogation system consists of a fiber Mach–Zehnder interferometer (FMZI) as the sensing device and a dispersive medium. The FMZI also acts as the spectrum slicer to induce continuous sampling, which results in the single-passband frequency response together with the dispersive medium. Two arms of the FMZI are used to measure the temperature and transversal loading respectively. The temperature change on one arm introduces the variation of fiber refractive index, which will change the central frequency of the RF filter’s passband, while the transversal loading induces the change of polarization state of the light, and thus, the amplitude of the RF filter’s passband will be changed accordingly. By tracking the central frequency and amplitude changes of the passband, temperature, and transversal loading can be monitored simultaneously. With the proposed fiber-optic sensing interrogation system, the temperature and transversal loading variation-induced optical spectra changes can be converted to the variation of frequency and amplitude of the RF filter’s passband in electronic domain, which greatly facilitates the measurement.

Multipoint refractive index and temperature fiber optic sensor based on cascaded no core fiber-fiber Bragg grating structures

A multipoint fiber optic sensor based on two cascaded multimode interferometer (MMI) and fiber Bragg grating (FBG) structures is proposed and demonstrated for simultaneous measurement of refractive index (RI) and temperature. The MMI is fabricated by splicing a section of no-core fiber (NCF) with two single-mode fibers. The suitable NCF lengths of 19.1 and 38.8 mm are selected by simulations to achieve wavelength division multiplexing. The two MMIs are sensitive to RI and temperature with the maximal RI sensitivities of 429.42228 and 399.20718 nm/RIU in the range of 1.333 to 1.419 and the temperature sensitivities of 10.05 and 10.22 pm/°C in the range of 26.4°C to 100°C, respectively. However, the FBGs are only sensitive to the latter with the sensitivities of 10.4 and 10.73 pm/°C. Therefore, dual-parameter measurement is obtained and cross-sensitivity issue can be solved. The distance between the two sensing heads is up to 12 km, which demonstrates the feasibility of long-distance measurement. During measurement, there is no mutual interference to each sensing head. The experimental results show that the average errors of RI are 7.61×10−4 RIU and 6.81×10−4 RIU and the average errors of temperature are 0.017°C and 0.012°C, respectively. This sensor exhibits the advantages of high RI sensitivity, dual-parameter and long-distance measurement, low cost, and easy and repeatable fabrication.

Dual-mode large-mode-area multi-core fiber with circularly arranged airhole cores

Multi-core fiber has aroused considerable interest as one of potential candidates for space division multiplexing that provides an additional freedom degree to increase optical fiber capacity to overcome the transmission bottleneck of current single-mode fiber optical networks. Few-mode fiber is also under intense study as a means to achieve space division multiplexing. We propose a novel dual-mode large-mode-area multi-core fiber (DMLMAMCF), which uses multi-core structure to realize few-mode condition when pursuing large mode-area. The proposed fiber consists of 5 conventional silica-based cores in the center region and 14 air hole cores surrounding the center cores. The outer circle with 12 air hole cores, which function similarly to the fluorine doping region in the bend-insensitive fiber, can mitigate the bending loss when keeping large mode area. The symmetrically distributed two cores on both sides of the center core in central region can reduce the half second-order LP11 mode consisting of two degenerate HE11 modes, TE01 mode, two degenerate HE21 modes and TM01 mode, thus leading to the remaining four vector modes, i.e. two degenerate HE11 modes and two degenerate HE21 modes. That is the reason why we call it strict dual-mode. We focus on large-mode-area properties and bending characteristics of the dual-mode. The influence of structural parameters that include corepitch Λ, refractive index difference between core and cladding Δn, and fiber core radius a, on mode characteristics and mode area of HE11 mode and HE21 mode is investigated in detail. The results reveal that it is helpful to increase the effective area of fundamental mode when we increase the value of corepitch, reduce the refractive index and fiber core radius. The effective mode area of HE11 is about 285.10 μm2 under the strict dual-mode condition. In addition, the relationship between bending loss and bending radius, and the relationship between effective mode area and bending radius of two modes are both investigated. For the HE11 mode, the least bending loss is about 5×10-5 dB/m while the least effective mode area with bending radius larger than 0.6 m is about 285.10 μm2. The HE21 mode is more sensitive to bend effect. The least bending loss is about 0.028 dB/m and the effective mode area is larger than 280.00 μm2 except for resonant coupling points. Large effective areas of both modes with low bending loss can be realized. Larger effective mode area with larger corepitch, appropriate refractive index difference and fiber core radius can be achieved. This fiber may find its usage in high power fiber lasers and amplifiers.

Temperature-insensitive refractive index sensor based on Mach–Zehnder interferometer with two microcavities

We propose a temperature-insensitive refractive index (RI) fiber sensor based on a Mach–Zehnder interferometer. The sensor with high sensitivity and a robust structure is fabricated by splicing a short photonic crystal fiber (PCF) between two single-mode fibers, where two microcavities are formed at both junctions because of the collapse of the PCF air holes. The microcavity with a larger equatorial dimension can excite higher-order cladding modes, so the sensor presents a high RI sensitivity, which can reach 244.16 nm/RIU in the RI range of 1.333–1.3778. Meanwhile it has a low temperature sensitivity of 0.005 nm/°C in the range of 33°C–360°C.

SPR optimization using metamaterials in a D-type PCF refractive index sensor

Abstract Using the finite element method (FEM), this paper presents a numerical investigation of the performance analysis of a D-type photonic crystal fiber (D-type PCF) for refractive index sensing, based on surface plasmon resonance (SPR) with a planar structure made out of a metamaterial. COMSOL Multiphysics was used to evaluate the design of the referred refractive index optical fiber sensor, with higher accuracy and considerable economy of time and resources. A study of different metamaterials concentrations conformed by aluminum oxide (Al 2 O 3 ) and silver (Ag) is carried out. Another structural parameters, which influences the refractive index sensor performance, the thickness of the metamaterial, is also investigated. The results indicate that the use of metamaterials provides a way of improving the performance of SPR sensors on optical fibers and allows to tailor the working parameters of the sensor.

Refractive Index Fiber Laser Sensor by Using Tunable Filter Based on No-Core Fiber

A novel refractive index fiber laser sensor based on a fiber Bragg grating (FBG) integrated with a section of no-core fiber (NCF) is proposed and experimentally demonstrated. The oscillating wavelength of the fiber laser is only decided by the central wavelength of the FBG that is insensitive to the surrounding refractive index (SRI). However, the output power of the fiber laser varies with the SRI because the laser output is filtered by the edge of the bandpass filter (BPF) formed by the multimode interference (MMI) effect in the NCF, which is sensitive to the SRI. By measuring the variation of the output power, a cost-effective power detection RI sensor could be realized. The fiber laser sensor has a linear relationship with SRI and a sensitivity of 113.73 dB/RIU in the RI range of 1.333–1.4076.

Adaptive simultaneous algebraic reconstruction technique for retrieving refractive index profiles of optical fiber

An efficient adaptive simultaneous algebraic reconstruction technique (ASART) to calculate optical fiber refractive index profiles is proposed based on phase difference curves obtained by digital holography technique. We develop adaptive relaxation parameter (ARP) on simultaneous algebraic reconstruction technique (SART) to increase the convergence speed and improve the reconstruction accuracy. A formula of ARP is derived mathematically and multilevel scheme (MLS) is used to increase convergence speed in the first iteration. Experimental results show the proposed ASART convergences over 40% faster than SART and achieve significantly higher reconstruction accuracy. Experimental verification shows that ASART is more efficient than SART and filtered back projection in image reconstruction, especially with few projection views. The running time of ASART is much shorter than that of SART, and ASART needs fewer iteration numbers to obtain the same reconstruction effects. In addition, it can be used to measure optical fibers with various diameters that cannot be measured with S14 refractive index profiler (S14).

Fast response Fabry-Perot interferometer microfluidic refractive index fiber sensor based on concave-core photonic crystal fiber.

We report a fast response microfluidic Fabry-Perot (FP) interferometer refractive index (RI) fiber sensor based on a concave-core photonic crystal fiber (CPCF), which is formed by directly splicing a section CPCF with a section of single mode fiber. The CPCF is made by cleaving a section of multimode photonic crystal fiber with an axial tension. The shallow concave-core of CPCF naturally forms the FP cavity with a very short cavity length. The inherent large air holes in the cladding of CPCF are used as the open channels to let liquid sample come in and out of FP cavity. In order to shorten the liquid channel length and eliminate the harmful reflection from the outside end face of the CPCF, the CPCF is cleaved with a tilted tensile force. Due to the very small cavity capacity, the short length and the large sectional area of the microfluidic channels, the proposed sensor provides an easy-in and easy-out structure for liquids, leading to great decrement of the measuring time. The proposed sensor exhibits fast measuring speed, the measuring time is less than 359 and 23 ms for distilled water and pure ethanol, respectively. We also experimentally study and demonstrate the superior performances of the sensor in terms of high RI sensitivity, good linear response, low temperature cross-sensitivity and easy fabrication.

Extra-broad Photonic Crystal Fiber Refractive Index Sensor Based on Surface Plasmon Resonance

We propose and investigate a photonic crystal fiber (PCF) refractive index sensor with triangular lattice and four large-size channels based on surface plasmon resonance. In such sensor, two gold wires obtained by chemical reaction are filled in two air holes between the upper and lower channels. Numerical results show that the refractive indexes of the analytes can be detected from 1.30 to 1.79. It’s a very broad detection range that is rarely seen in the field of photonic crystal fiber sensor based on surface plasmon resonance. The average sensitivity of this photonic crystal fiber could be 100 nm/refractive index unit (RIU) in the dynamic index range from 1.30 to 1.63. In the range from 1.63 to 1.79, the highest sensitivity could reach to 3233 nm/RIU. The ability to integrate four large-size microfluidic channels for efficient analyte flow in is attractive for the development of integrated highly sensitive PCF-SPR sensors.

Localized topological states in Bragg multihelicoidal fibers with twist defects

We have studied the influence of a twist defect in multihelicoidal Bragg fibers on the emerging of localized defect modes. We have shown that if such a fiber is excited with a Gaussian beam this leads to the appearance of a defect-localized topological state, whose topological charge coincides with the order of rotational symmetry of the fiber's refractive index. We have shown that this effect has a pronounced crossover behavior. We have also formulated a principle of creating the systems that can nestle defect-localized topologically charged modes. According to this principle, such systems have to possess topological activity, that is, the ability to change the topological charge of the incoming field, and operate in the Bragg regime.

High stability Michelson refractometer based on an in-fiber interferometer followed with a Faraday rotator mirror

We demonstrated an optical fiber refractive index (RI) sensor based on an in-fiber Michelson interferometer (MI), which is fabricated by splicing a Faraday rotator mirror (FRM) to a singlemode-thin core-singlemode (STS) optical fiber interferometer. From our experiments, it can be found that thus a FRM as reflector can eliminate the polarization random fluctuation of interference spectrum, which makes the sensor exhibit a high stability. Core to core splicing of thin core to standard single mode optical fibers and high reflectivity mirror of FRM can reduce the optical power loss and improve the fringe contrast of sensor spectrum. Both high stability and low power loss are very important in such kinds of reflection interference spectrum modulation and high resolution sensors. This sensor wavelength and intensity RI sensitivity is −48.858nm/RIU and −6.548dB/RIU at the liquid RI from 1.38 to 1.435, respectively. The temperature sensing performance of this sensor is also studied, the sensor wavelength versus temperature sensitivity is 66.18pm/ °C and intensity hardly change with temperature, which means that the temperature cross sensitivity can be eliminated when intensity modulated, and this sensor also has compact size, simple structure, easy fabrication and good repeatability.

Mode Coupling Effects in Ring-Core Fibers for Space-Division Multiplexing Systems

An optical fiber with weak mode coupling is desirable for future ultrahigh capacity space-division multiplexing (SDM) systems because mode coupling in an optical fiber results in extrinsic loss of the fiber and crosstalk between guided optical modes. To study the feasibility of a ring-core fiber (RCF) for SDM systems, in this paper, we investigate the mode coupling in the RCF supporting five or seven guided mode groups (MGs) at a wavelength of 1550 nm. For this purpose, the coupled mode/power theory with identified spatial power spectrum of random perturbations of fiber axis is used to estimate the bend loss/crosstalk of the RCF due to microbending. It is shown that based on the identified parameters for the spatial power spectrum in the 5/7-MG RCF, the estimated bend loss/crosstalk of the RCF agrees well with experimental results. In addition, the impact of the gradient parameter α and refractive index contrast Δ of the fiber refractive index profile on bend loss and crosstalk of the RCF is explored. Simulation results indicate that the Δ instead of the α significantly affects bend loss and crosstalk of the RCF. The magnitude improvement in bend loss by increasing the Δ is dependent on the spatial power spectrum.

Measurement of the focusing constant of gradient-index fiber lens and its application in developing GRIN fiber probes

The curve-fitting algorithm is adopted to measure the focusing constant of a gradient-index (GRIN) fiber lens, which is used to further design ultra-small GRIN fiber probes. Firstly, the refractive index profile of a GRIN fiber lens is acquired by using the Optical Fiber Refractive Index Profile tester. And the focusing constant of the GRIN fiber lens is obtained by means of the curve-fitting algorithm of quadraticpolynomial. Secondly, the measured focusing constant is used to design ultra-small GRIN fiber probes with different focusing properties. Thirdly, according to the designed optical probe models, GRIN fiber probe samples have been fabricated and tested. Finally, comparative analysis is conducted between the theoretical focusing performance parameters and the experimentally measured values. The results show that, in the given condition, the fitted value of the focusing constant of GRIN fiber lens and the theoretical focusing performance parameters of GRIN fiber probes, respectively agree well with the nominal value provided by the manufacturer and the experimentally measured data. Therefore, the proposed methods of measuring the focusing constant of GRIN fiber lens and its application in designing GRIN fiber probes are validated and feasible to further developing ultra-small GRIN fiber probes requiring specific optical focusing performance.

A proposal of T-structure fiber-optic refractive index sensor based on surface plasmon resonance

We present a compact and novel “T” structure optical fiber refractive index sensor proposal based on surface plasmon resonance. “T” structure sensing head consists of a single mode fiber (SMF) with a plasmonic facet and a cladding partly removed single mode fiber (CPR-SMF) with a gap. The gold film is deposited on the end of SMF instead of the side of the CPR-SMF. The simulation results show that the SPR based on the “T” structure can be excited effectively. The SPR transmission spectrum shifts towards longer wavelength with the sensing sample refractive index increasing largely. When we divide the refractive index range of the sensing sample to two parts, the linear relationships between the SPR wavelength and the refractive index can be used. The resolutions can be highly up to 7.115×10−6RIU and 3.525×10−6RIU for the refractive index ranges of 1.3333–1.36 and 1.37–1.4, respectively. The proposed “T” structure sensor works well for achieving the refractive index measurement with high sensitivity and wide range for samples with a tiny amount.

Single-Mode Multicore Fibers With Integrated Bragg Filters

Fiber Bragg gratings (FBGs) in single-mode fibers, used as narrowband filters, have reached a high maturity level, especially for applications in telecommunications and sensing. In spectroscopic applications, e.g., in Raman spectroscopy, FBG filters can be used to separate laser probing light from generated sensor light which simplifies Raman probes. For this purpose, light from a sample has to be collected with high efficiency into a fiber. The light collection efficiency in single-mode fibers is, however, strongly limited due to the small fiber core area. Therefore, we have investigated the possibility to use single-mode multicore fibers to increase the light collection efficiency while maintaining Bragg filter performance. An optical density filtering effect of 1.5, which is equivalent to a transmission of -15 dB, has been achieved with a FBG in a 61-core fiber. Simulations have been performed in order to clarify the influence of the fiber geometry and refractive index distribution on grating performance. The application of chirped gratings in the multicore fibers improves filter performance in comparison to homogeneous gratings.

Signals of an intermodal fiber interferometer induced by laser frequency modulation

Signals of an intermodal fiber interferometer induced by laser optical frequency modulation are studied. Dependences of signal amplitudes and spectra on the laser frequency deviation are examined theoretically and experimentally for various optical fibers. It is established that the sensitivity of the intermodal fiber interferometer to the laser frequency variation essentially depends on the fiber refractive index profile. The minimal sensitivity corresponds to graded-index (α ≈ 2) multimode optical fibers. Step-index optical fibers (α = ∞) are more sensitive to the laser frequency variation by more than a factor of 100.

Design of an Optical Fibre-based Receiver System for Atmospheric Sensing Lidar

In this paper we report on the design of an optical fiber-based light receiver which is meant for replacing the conventional beam steering secondary mirror of the Newtonian telescope of a biaxial Raman lidar. The influence of using optical fibre, acting as an aperture stop of constant diameter on the geometrical compression form factor; the transitional altitude and the resulting blur disk diameter have been determined. The viability of using a commercially available optical fibre as a receiver and the associated trade-offs have been discussed. A hot-fused fibre bundle-based receiver is proposed as a potential alternative for low altitude profiling to be enabled. These theoretical investigations have formed the basis for our current experimental work aimed at the development of all fibre-based reconfigurable beam delivery systems for mobile lidars. Full Text: PDF References A.M. Melesse et al., "Remote Sensing Sensors and Applications in Environmental Resources Mapping and Modelling", Sensors 51, 3209 (2007). CrossRef P. Wang et al., "A macrobending singlemode fiber refractive index sensor for low refractive index liquids", Phot. Lett Poland 2, 67 (2010). CrossRef J.R. Jenness et al., "Design of a lidar receiver with fiber-optic output", Appl. Optics 36, 4278 (1997). CrossRef J.V. Hey et al., "Determination of overlap in lidar systems", Appl. Optics 50, 5791 (2011). CrossRef K. Stelmaszczyk et al., "Analytical function for lidar geometrical compression form-factor calculations", Appl. Optics 44, 1323 (2005). CrossRef A.A.P. Boechat et al., "Bend loss in large core multimode optical fiber beam delivery systems", Appl. Optics 30, 321 (1991). CrossRef http://www.schott.com/china/chinese/download/284345_fo_light_it_right_e_without_tecspec.pdf DirectLink https://www.thorlabs.com/thorproduct.cfm?partnumber=APCH1500 DirectLink