Optical Fiber Refractive Index Sensor Research Articles

Enhanced Spectrum Resolution of Optical Fiber Refractive Index Sensors Based on Single-Arm Fiber-Optic Tapered Probe

Enhanced Spectrum Resolution of Optical Fiber Refractive Index Sensors Based on Single-Arm Fiber-Optic Tapered Probe

Fiber optic refractive index sensing using an inline dual semi-distributed interferometer

In this work, we present a fiber optic refractive index (RI) sensor based on a dual inline semi-distributed interferometer (SDI) device. The SDI is formed within a telecom-grade optical fiber by simply splicing a short length of a high-scattering fiber; the presence of a high density of scattering centers induces a distributed cavity effect, forming a low-finesse interferometer. Here, we propose an approach based on two serial SDIs, with few millimeters spacing between the two cavities; the outer SDI encodes the changes of surrounding RI, while the inner probe is RI-insensitive and can be used as a reference. Experimental analysis shows that the dual-SDI device has RI sensitivity of 111.2–466.2 dB/RIU with RI-sensitive mode, and 0.4–3.7 dB/RIU with RI-insensitive mode, with a ratio up to 1165.5 between sensitive and insensitive modes. In addition, the Vernier effect can be interrogated by detecting the wavelength shift of the narrow spectral lines, with sensitivity of 0.303 nm/RIU.The dual-SDI structure maintains the same fabrication ease of a single SDI, since all process is made through splicing and cleaving fibers with a telecom-grade fiber. The key advantage is the availability of modes having both high and near-zero RI sensitivity, which therefore can be used for a referenced RI detection as an in-situ probe.

High sensitivity refractive index sensor based on double peanut-shaped and etched multimode fiber structures

A fiber optic refractive index (RI) sensor based on an etched multimode fiber (MMF) with a double peanut-shaped structure is proposed and experimentally demonstrated. The sensor consists of two peanut-shaped and a section of etched MMF tapered fiber structure. The excitation of the fundamental mode to higher-order modes is facilitated by using the beam splitting/coupling effect of the double peanut-shaped and etched taper structures, and the higher-order modes can be excited into an evanescent field. In the sensing medium, the stronger the evanescent field, the stronger the energy shock between the fiber and the sensing environment. Experimental results showed that the sensitivity was 326.52 nm/RUI and 823.91 nm/RUI when the etched waist taper diameter was 51.92 μm and the glycerol solution index ranged from 1.3395 to 1.3945 and 1.3945 to 1.4200, respectively. Compared to the MMF sensor structure without etching, the RI sensitivity is improved by about 2 times. In addition, the temperature characteristics of the sensor were investigated over a range of 30 °C–100 °C, and the results showed a maximum temperature sensitivity of only 30.24 pm °m−1. The sensor structure has a low-temperature sensitivity and the temperature effect on the RI measurement results is negligible within the allowable error range. The sensor has the advantages of simple fabrication, wide measurement range, good stability, low cost, and compact structure, which has potential application value in the field of RI detection.

Multi-tapered polarization-maintaining fiber-optic sensor for refractive index and temperature measurements

In this paper, a fiber-optic refractive index and temperature sensor based on Mach-Zehnder interferometer (MZI) is designed and fabricated. The sensor structure consists of a section of polarization-maintaining fiber (PMF) with multi-tapers and sandwiched by two sections of multi-mode fiber (MMF). Due to the changes of ambient refractive index and temperature, the effective refractive index of the core and cladding modes of the multi-tapered PMF will be changed, which will cause the interference spectral shift of core mode and cladding mode. The series experiments show that the structure has better repeatability, the maximum refractive index sensitivity is 250.861 nm/RIU, the maximum temperature sensitivity is −78 pm/°C. As the sensitivity matrix is used to realize the two-parameter distinguishing measurement, the resolutions of 1.2 × 10−5 RIU and 0.38 °C for refractive index and temperature are achieved. The structure has superior mechanical properties, and has broad application prospects in petrochemical industry, underwater exploration and other fields.

Semi-distributed interferometers fiber-optic sensors for high-sensitivity refractive index detection: Design and sensitivity analysis

In this work, we present the design and analysis of fiber-optic refractive index (RI) sensors based on a simple semi-distributed interferometer (SDI). The SDI is a cavity formed between a cleaved fiber tip and the reflective interface between a single-mode fiber and a high-scattering fiber with core doped by Mg-silicate nanoparticles. The fabrication is rapid (∼1 min) and straightforward, as only a splice and cleave of the fibers are required; the resulting spectral fingerprint is an interferometer-shaped envelope, with a stochastic distribution due to the random scattering centers of the in-cavity fiber. We fabricated 18 sensors with 6 different enhanced-scattering fibers, having different drawing temperature and speed; by analyzing the RI sensitivity of each unit, we obtain sensitivity values up to 787.0 dB/RIU (refractive index units), whereas the optimal fiber allows achieving average sensitivity of 457.9 dB/RIU.

Distributed Refractive Index Sensing Based on Etched Ge-Doped SMF in Optical Frequency Domain Reflectometry.

A distributed optical fiber refractive index sensor based on etched Ge-doped SMF in optical frequency domain reflection (OFDR) was proposed and demonstrated. The etched Ge-doped SMF was obtained by only using wet-etching, i.e., hydrofluoric acid solution. The distributed refractive index sensing is achieved by measuring the spectral shift of the local RBS spectra using OFDR. The sensing length of 10 cm and the spatial resolution of 5.25 mm are achieved in the experiment. The refractive index sensing range is as wide as 1.33-1.44 refractive index units (RIU), where the average sensitivity was about 757 GHz/RIU. Moreover, the maximum sensitivity of 2396.9 GHZ/RIU is obtained between 1.43 and 1.44 RIU.

Highly Efficient Refractive Index Sensor Based on a Dual-Side Polished SMS Fiber Enabled by Femtosecond Laser Writing.

Fiber-optic refractive index (RI) sensors based on wavelength-shift-based interrogation continue to present a challenge in achieving high sensitivity for a wide detection range. In this paper, we propose a sensor for determining the RI of liquids based on femtosecond laser (fs-laser) writing of a dual-side polished singlemode-multimode-singlemode (SMS) fiber. The proposed sensor can determine the RI value of a surrounding liquid by detecting the dip wavelength in the transmission spectrum of the light propagating through the sensing area. The high RI sensitivity is attributed to the increased interaction area established by the fs-laser, which creates hydrophilic surfaces and maintains the wide detection range of the SMS structure. The results of the wavelength-shift-based interrogation reveal that the fabricated device exhibited a high sensitivity of 161.40 nm per refractive index unit (RIU) over a wide RI detection range of 0.062 RIU. The proposed device has high processing accuracy and a simple manufacturing process. Hence, it has the potential to be used as a lab-on-fiber sensing platform in chemical and biotechnological applications.

Review of Seawater Fiber Optic Salinity Sensors Based on the Refractive Index Detection Principle.

This paper presents a systematic review of the research available on salinity optic fiber sensors (OFSs) for seawater based on the refractive index (RI) measurement principle for the actual measurement demand of seawater salinity in marine environmental monitoring, the definition of seawater salinity and the correspondence between the seawater RI and salinity. To further investigate the progress of in situ measurements of absolute salinity by OFSs, the sensing mechanisms, research progress and measurement performance indices of various existing fiber optic salinity sensors are summarized. According to the Thermodynamic Equation of Seawater-2010 (TEOS-10), absolute salinity is recommended for sensor calibration and measurement. Comprehensive domestic and international research progress shows that fiber-optic RI sensors are ideal for real-time, in situ measurement of the absolute salinity of seawater and have excellent potential for application in long-term in situ measurements in the deep ocean. Finally, based on marine environmental monitoring applications, a development plan and the technical requirements of salinity OFSs are proposed to provide references for researchers engaged in related industries.

Artificial neural network based optimization for Ag grated D-shaped optical fiber surface plasmon resonance refractive index sensor

This study reports the optimization of fiber optic SPR refractive index sensor parameters with the simulation of finite element method (FEM) and artificial neural network (ANN) model. To demonstrate the applicability of the algorithm, we examined an Ag-grated D-shaped fiber optic sensor configuration with 4 basic input parameters with the aim of reaching the highest sensitivity. Through the conventional optimization, the best parameter set appeared to be a 10 nm air gap distance between the gratings (a), 20 gratings (N), 50 nm residual cladding thickness (d), and 70 nm silver layer thickness (Ag_th) at the indices of 1.35 and 1.39 yielding a sensitivity of 3775 nm/RIU. A close match is found between the actual and predicted sensitivity. 199 input data obtained from FEM are used for training by Leave One Out Cross-Validation (LOOCV) approach with R-squared value of 0.98, and the trained model with R-squared value of 0.97 is implemented in the Genetic Algorithm. We achieved the sensitivity of 3890 nm/RIU at the predicted a, N, d, and Ag_th of 10 nm, 20, 50 nm, and 75 nm, respectively. Future studies may further improve these results by integrating other algorithms. This method may apply to different and more complex structures to observe the correlation between the parameters, cover an entire range of parameters, and get more accurate results, especially with a high number of inputs requiring less time and computing effort. The proposed method carries great potential to improve the sensing ability and bring a new perspective to the literature.

Portable Multihole Plastic Optical Fiber Sensor for Liquid-Level and Refractive Index Monitoring

In this article, we report a low-cost method to fabricate plastic optical fiber (POF) liquid-level and refractive index (RI) sensors integrated with smartphones by molds and drilling microholes in side-growing POFs (SGPOFs) with high-temperature metal wire. These sensors can be used for real-time liquid-level and RI monitoring, especially under situations requiring portability and illumination. When the liquid level changes, the change of the RI around the microholes leads to a change in the light intensity transmitted in the fiber, enabling us to measure the liquid level and RI. Utilizing one sensor with six holes (0.45 mm radius and 15 mm spacing) in 2.0-mm SGPOF, the results of level and RI measuring are in good agreement with the theory. The liquid-level measurement of the sensor is carried out for water with an RI of 1.333, the level sensitivity is 0.29%/cm ± 0.02%/cm. In the RI measurement, the RI sensitivity of the sensor is −22.8%/RIU ± 0.6%/RIU in the measurement range from 1.333 to 1.475.

Refractive index sensor based on a gradually hot-pressed flatted plastic optical fiber

This paper proposes a gradually hot-pressure plastic optical fiber (POF) refractive index (RI) sensor with a flatted structure to detect the glucose concentration, and experimentally investigates the performance of RI sensor with straight and micro-bending POFs. The sensor is fabricated by a temperature-controllable metal rod to form the hot-pressed end face of POF which can increase signal-to-noise ratio (SNR) of the sensor. The experimental results show that the proposed sensor, utilizing a flatted structure POF with thickness of 0.5 mm and diameter of 0.4 cm, achieves maximum intensity sensitivity of up to 1748.91 %/RIU and resolution of up to 5.97 × 10−4 RIU in the RI range of 1.33 to 1.382. Furthermore, its simple design and manufacturing process shows potential for application, especially in biochemistry and environmental safety.

Nanophotonics and optical fibers: New avenues for sensing and active devices

Rapid development in electronic and optical devices has unequivocally eased human life to an appreciable extent, which, however, involved lots of hard work in the miniaturization of technology. The continual fabrication and characterization of devices in the field of spectroscopy, security/imaging, information technology, nanophotonics, opticalfiber-based devices, photodetectors, etc. is going on in full swing. By citing exemplary instances, this article comprehensively shows various applications based on nanophotonics and optical fiber technologies. This article focuses on the recent advancements in the field of nanophotonics and optical fiber to use them in sensing and active devices. Initiating with an overview of the recent areas of research, design, and implementation of nanophotonics and optical fiber, the paper discusses the instrumentation used for fabricating and characterization of surface plasmon resonance (SPR) based fiber optic refractive index sensor and visible photodetector furnished by cadmium sulfide (CdS) nanostructures. The fabricated CdS nanostructures were characterized using XRD, Raman, SEM, UV–vis and photoluminescence spectroscopic techniques so as to obtain the required/favourable properties for the realization of fiber-optic refractive index sensing device employing SPR technique and for visible photodetection applications. The performance parameters of both the fabricated devices were also evaluated and explained in detail. Lastly, the paper outlines the limitations, challenges as well as available opportunities in this avenue.

D-shaped optical fiber refractive index sensor for application in fuel oil mixtures and leakage detection

D-shaped optical fiber refractive index sensor for application in fuel oil mixtures and leakage detection

Enhanced Mach-Zehnder interferometer multimode–single-mode–multimode fiber optic refractive index sensor based on surface plasmon resonance

In this paper, an all-fiber Mach-Zehnder interferometer (MZI) sensor for refractive index (RI) measuring is presented, which is based on Multimode–Single-mode–Multimode (MSM) fiber. The effects of both reducing the radius of the sensing part and the surface plasmon resonance (SPR) on its efficiency are investigated. Increasing the interaction of high-order modes with external media, caused by etching the cladding layer of the single-mode fiber part, significantly improves the sensitivity. Both wavelength and intensity interrogation approaches are employed to study the Multimode–etched Single-mode–Multimode (MESM) fiber sensor. The intensity and the wavelength sensitivities for the RI measurement in the range of 1.428–1.458 are obtained as −2306.754 %/RIU and 1315.74 nm/RIU, respectively. Finally, the MESM-SPR sensor is proposed and characterized. Results exhibit high performance in the RI range of 1.333 to 1.357, in which the sensitivity of 1437.056 nm/RIU is achieved. The advantages like low cost, high sensitivity, and simple fabrication methods make these sensors promising devices for chemical, food industry, and biosensing applications.

Effect of temperature change on refractive index of an egg white and yolk: a preliminary study

In this article, the refractive index of an egg white and yolk depending on temperature in range 30 - 47 °C over 1550 nm was determined. The measurement head was constructed as fiber optic Fabry-Perot interferometer with interference between polished fiber end-face and aluminum weighing dish. The measurement setup has been made of an optical spectrum analyzer, a superluminescent diode with a central wevelength of 1550 nm, 2:1 fiber coupler and heat plate. Full Text: PDF ReferencesP. Magdelaine, "Egg and egg product production and consumption in Europe and the rest of the world, Improving the Safety and Quality of Eggs and Egg Products", Egg Chemistry, Production and Consumption, 3 (2011). CrossRef H. Kuang, F. Yang, Y. Zhang, T. Wang, and G. Chen, "The Impact of Egg Nutrient Composition and Its Consumption on Cholesterol Homeostasis", Cholesterol (2018). CrossRef J. Gienger, K. Smuda, R. Müller, M. Bär, and J. Neukammer, "Refractive index of human red blood cells between 290 nm and 1100 nm determined by optical extinction measurements", Sci. Rep. 9, 1 (2019). CrossRef P. Listewnik, M. Hirsch, P. Struk, M. Weber, M. Bechelany, and M. Jędrzejewska-Szczerska, "Preparation and Characterization of Microsphere ZnO ALD Coating Dedicated for the Fiber-Optic Refractive Index Sensor", Nanomaterials 9, 2 (2019) CrossRef Y. Wu, Y. Zhang, J. Wu, and P. Yuan, "Fiber-Optic Hybrid Structured Fabry-Perot Interferometer Based On Large Lateral Offset Splicing for Simultaneous Measurement of Strain and Temperature", J. Lightwave Technol., 35, 19 (2017). CrossRef M. Islam, M. Mahmood, M Lai, K. Lim, and H. Ahmad, "Chronology of Fabry-Perot Interferometer Fiber-Optic Sensors and Their Applications: A Review", Sensors 14, 4 (2014). CrossRef K. Karpienko, M. Wróbel, and M. Jędrzejewska-Szczerska, "Determination of refractive index dispersion using fiber-optic low coherence Fabry-Perot interferometer: implementation and validation", Opt. Eng. 53, 7 (2014). CrossRef M. Kosowska, D. Majchrowicz, K. Sankaran, M. Ficek, K. Haenen, and M. Szczerska, "Doped Nanocrystalline Diamond Films as Reflective Layers for Fiber-Optic Sensors of Refractive Index of Liquids", Materials 12, 13 (2019). CrossRef G. Xiao, A. Adnet, Z. Zhang, F. Sun, and C. Grover, "Monitoring changes in the refractive index of gases by means of a fiber optic Fabry-Perot interferometer sensor", Sensors and Actuators 118, 2 (2005). CrossRef

Step-etched U-bent silica fiber optic probes – Design and optimum geometry for refractive index sensing

This study presents a ray-optics based model for the multimode U-bent silica fiber optic refractive index (RI) sensor probes, followed by a detailed experimental investigation to identify the optimum fiber core (d) and bend diameter (D), and the optimal bend ratio (D/d). U-bent probes with different geometries (d = 200, 400 and 600 µm; D = 1.0, 1.5, 2.0, and 2.5 mm) were fabricated and evaluated for their RI sensitivity in terms of refractive losses. Among the bend ratios between 1.7 and 12.5, the U-bent probes with a bend ratio between 3 and 5 exhibit a higher RI sensitivity for a given core diameter. Among the twelve different geometries tested, U-bent probes with 600 µm core and ~2.5 mm bend diameter demonstrate the highest RI sensitivity of ~6 ∆A/RIU, measured as a change in absorbance (∆A) per refractive index unit (RIU). Further investigations to improve light-medium interactions by wet chemical step-etching of the silica fiber core show a gradual improvement in the RI sensitivity until a etched waist diameter of 200 µm, followed by an abrupt drop. At least a two-fold improvement in the RI sensitivity was observed. A compact LED-photodetector based dip-type handheld refractometer was realized as a proof-of-the-concept to demonstrate the feasibility of using these etched probes in reusable or disposable form for RI based analytical applications.

Ultra-Long Range Refractive Index Fiber Sensor

The dynamic range of optical fiber refractive index sensors is mainly determined by the refractive index (RI) of the sensor surface in contact with the sample under test. In case of the refractive index sensor based on the hybrid fiber Fabry–Perot interferometer (HFFPI), the largest measurable refractive index value is equal to that of the fiber core. In this work, we demonstrate that it is possible to extend the refractive index dynamic range of a HFFPI by simply adjusting the optical path length (OPL) of the air and solid cavity to be equal or differ by just a small amount. Two isometric versions of the HFFPI (i-HFFPI) with a total length of 100 and 172 μm, where the OPL of the air and solid cavity are very similar, were fabricated and tested. The interferometers were immersed in different samples with a refractive index ranging from 1.000 to 1.733. The response of the interferometers was analyzed in the Fourier domain, and it was possible to establish a one-to-one relationship between the refractive index of the liquid sample and the amplitude of one of the peaks in the Fourier spectra. The amplitude of this peak experienced a linear increment when the RI of the surrounding medium was increased. Tracking the amplitude changes of a Fourier spectrum peak is straightforward which simplifies the online monitoring of the sensor. These features make this compact refractive index fiber sensor very appealing for biosensing applications.

A study of highly sensitive D-shaped optical fiber surface plasmon resonance based refractive index sensor using grating structures of Ag-TiO2 and Ag-SnO2

In this study, we propose a design and simulation of a highly sensitive surface plasmon resonance (SPR) based single-mode D-shaped optical fiber refractive index sensor in the near-infrared (NIR) region. It consists grating structure of silver coated with titanium dioxide (TiO2) and Tin dioxide (SnO2) layer on the core-cladding interface. With the help of the finite element method (FEM), the proposed optical fiber sensors are optimized with various geometrical parameters. Proposed optimized structures are investigated and various performance parameters are calculated. It is found that performance parameters enhance with TiO2/SnO2 coated on Ag grating structure at a particular thickness of TiO2/SnO2. Due to this more generations of surface plasmons take place on metal-metal oxide structures. We find the maximum sensitivity of 15.31 μm / RIU and 9.81 μm / RIU for Ag-TiO2 and Ag-SnO2 grating structure respectively which shows a better result in comparison to the other reported work. So proposed structure may have great potential to enhance the sensing ability of optical fiber-based sensors as compared to the other conventional SPR based optical fiber sensors.

Optical fiber refractive index sensor based on the double cladding fiber

An optical fiber sensing structure based on double-cladding fiber (DCF) is described. The core and outer cladding of the DCF both supports more than one guided mode and the cladding mode resonance as well as modal interference in the fiber. Due to the W-type refractive index (RI) distribution, the outer cladding generates a stronger evanescent field to obtain higher sensitivity. In the transmitted spectrum, all three dips shift to the longer wavelength with the external RI changing from 1.33 to 1.43, and the high sensitivities are 595, 741, and 686 nm/RIU, respectively.

Differential Refractive Index Sensor Based on Coupled Plasmon Waveguide Resonance in the C-Band.

We proposed a differential fiber-optic refractive index sensor based on coupled plasmon waveguide resonance (CPWR) in the C-band. The sensor head is a BK7 prism coated with ITO/Au/ITO/TiO2 film. CPWR is excited on the film by the S-polarized components of an incident light. The narrow absorption peak of CPWR makes it possible to realize dual-wavelength differential intensity (DI) interrogation by using only one incident point. To implement DI interrogation, we used a DWDM component to sample the lights with central wavelengths of 1529.55 and 1561.42 nm from the lights reflected back by the sensor head. The intensities of the dual-wavelength lights varied oppositely within the measurement range of refractive index, thus, a steep slope was produced as the refractive index of the sample increased. The experimental results show that the sensitivity is 32.15/RIUs within the measurement range from 1.3584 to 1.3689 and the resolution reaches 9.3 × 10−6 RIUs. Benefiting from the single incident point scheme, the proposed sensor would be easier to calibrate in bio-chemical sensing applications. Moreover, this sensing method is expected to be applied to retro-reflecting SPR sensors with tapered fiber tip to achieve better resolution than wavelength interrogation.