Influence of Refractive Index Distribution on Brillouin Gain Spectrum in GeO_(2)-Doped Optical Fibers

GeO_(2) is commonly used as dopant to adjust the refractive index profile(RIP)and the acoustic velocity profile(AVP)in the fiber,thereby forming different Brillouin gain spectrum(BGS)characteristics such as Brillouin gain,acoustic mode number and peak intensity difference.When an optical fiber is used in optical fiber sensing or communication system,its BGS characteristics may play an important role in determining the performance of the system.In this paper,finite element analysis(FEA)method is used to study the influence of refractive index distribution and its corresponding AVP on the BGS in step-index,graded-index,and complex-index optical fibers.A new method has also been proposed to efficiently discriminate acoustic mode solution and obtain the new and full images of total Brillouin gain and acoustic modes number of the fiber as a function of the refractive index distribution,considering the influence of changing the refractive index difference and the geometric size simultaneously.For each type of optical fiber,the recommended parameter range is provided for optical fiber sensing and optical fiber communication.Moreover,the suitable optical fiber with close peak intensity in its multi-peak BGS is explored and achieved,which can be used in Brillouin beat spectrum detection systems to improve sensing accuracy.

Temperature dependence of the refractive index of optical fibers

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.

Nonlinear Refractive Index Measurement Utilizing Bistable Behavior of Double Coupling Optical Fiber Ring Resonator

A novel approach for measuring the nonlinear refractive index of an optical fiber utlizing the bistable behavior of the double coupling optical fiber ring resonator was proposed and investigated. The switch-off or switch-on power decreases with an increase in the nonlinear refractive index n2 （m2/W）, and the dependence of swith-off or switch-on power on the nonlinear refractive index was analyzed numerically. Simulation results showed that the switch-off power and switch-on power （in dBW） decreased linearly with loglo （n2） in a 100-m-length fiber ring resonator, when n2 changed from 3.2 ×10^-20 m2/W to 2.5 × 10^-17 m2/W or nearly n2 = 3.2 × 10^-20 m2/W. These mean that high accuracy as well as large-scale nonlinear refractive index measurement can be achieved by the proposed approach.

Long Period Grating Imprinted on a Flat-Shaped Plastic Optical Fiber for Refractive Index Sensing

A corrugated surface long period grating(LPG)was fabricated on a flat-shaped plastic optical fiber(POF)as a refractive index(RI)sensor by a simple pressing with the heat pressure and mechanical die press print method.The light propagation characteristics of an LPG imprinted on a multi-mode POF were analyzed by the method of geometrical optics.Theoretical and experimental results showed that the structural parameters of the sensor affected the RI sensing performance,and the sensor with a thinner flat thickness,a deeper groove depth of the corrugated surface LPG,and a longer LPG exhibited better RI sensing performance.When the POF with a diameter of 1 mm was pressed with the heat pressure to a flat shape with a thickness of 600μm,an LPG with a period of 300μm,a groove depth of 200μm,and a length of 6 cm was fabricated on it,and the RI sensitivity of 1447%/RIU was obtained with a resolution of 5.494×10^(−6) RIU.In addition,the influences of the POF cladding,tilting of LPG,and bending of the sensing structure were investigated.The results demonstrated that after removing the cladding and tilting or bending the LPG,the RI sensing performance was improved.When the LPG imprinted on the flat-shaped POF was bent with a curvature radius of 6/πcm,the highest sensitivity of 6563%/RIU was achieved with a resolution of 2.487×10^(−9) RIU in the RI range of 1.3330–1.4230.The proposed sensor is a low-cost solution for RI measurement with the features of easy fabrication,high sensitivity,and intensity modulation at the visible wavelengths.

Intensity modulated silver coated glass optical fiber refractive index sensor

Miniature optical fiber sensors with thin films as sensitive elements could open new fields for optical fiber sensor applications. Thin films work as sensitive elements and a transducer to get response and feedback from environments, in which optical fibers act as a signal carrier. A novel Ag coated intensity modulated optical fiber sensor based on refractive index changes using IR and UV-Vis （UV-visible） light sources is proposed. The sensor with an IR light source has higher sensitivity compared to a UV-Vis source. When the refractive index is en- hanced to 1.38, the normalized intensity of IR and UV-Vis light diminishes to 0.2 and 0.8. respectively.

Numerical Investigation of a Refractive Index SPR D-Type Optical Fiber Sensor Using COMSOL Multiphysics

Recently, many programs have been developed for simulation or analysis of the different parameters of light propagation in optical fibers, either for sensing or for communication purposes. In this paper, it is shown the COMSOL Multiphysics as a fairly robust and simple program, due to the existence of a graphical environment, to perform simulations with good accuracy. Results are compared with other simulation analysis, focusing on the surface plasmon resonance （SPR） phenomena for refractive index sensing in a D-type optical fiber, where the characteristics of the material layers, in terms of the type and thickness, and the residual fiber cladding thickness are optimized.

Cascaded tilted fiber Bragg grating for enhanced refractive index sensing

We proposed and experimentally demonstrated a cascaded tilted fiber Bragg grating(TFBG)for enhanced refractive index sensing.The TFBG is UV-inscribed in series in ordinary single-mode fiber(SMF)and reduced-diameter SMF with the same tilt angle,and then excites two sets of superposed spectral combs of cladding modes.The cascaded TFBG with total length of 18 mm has a much wider wavelength range over 100 nm and narrower wavelength separation than that of a TFBG only in the SMF,enabling an enlarged range and a higher accuracy of refractive index measurement.The fabricated TFBG with the merits of enhanced sensing capability and temperature self-calibration presents great potentials in the biochemical sensing applications.

Characterization of refractive index distribution of polymer optical fiber

A focusing method is developed to characterize the refractive index profile of polymer optical fiber (POF). Based on the refractive index profile the theoretical bandwidth and the core index exponent a (a 】 0) of POF are calculated. The results show that the value of theoretical bandwidth agrees well with the experimental data.

An HC-PCF Fluorescence Spectrocopy for Detection of Microsphere Samples Based on Refractive Index Scaling Law

This paper illustrates an efficient fluorescence detection of micro particles using hollow-core photonic crystal fibers (HC-PCFs) by applying the refractive index (RI) scaling law. The variations in the central wavelength for different filling material indices are illustrated for most commonly available HC-PCFs that have cladding made of pure fused silica with array of air holes running along the entire length of the fiber. The proposed concept is verified by immobilizing fluorescent microsphere samples inside two HC-PCFs of different central wavelengths and the quantification of fluorescence inside the fibers is performed through spectroscopic analysis. The sensitivity has been compared for similar fiber with different dispersed media and different fibers with same dispersed medium.

Refractive index and temperature sensing characteristics of an optical fiber sensor based on a tapered single mode fiber/polarization maintaining fiber

An evanescent field optical fiber sensor based on a short section of polarization maintaining fiber spliced with a tapered single mode fiber is proposed and experimentally investigated. We mainly focus on the refractive index （RI） and temperature sensing characteristics of this compact device. The transmission spectrum of the resonance wavelength, induced by the interference between the excited low order cladding modes and core modes, shows the quadratic function relationships with RI and linear relationships with temperature. Thus, the proposal of this simple-to-fabricate, compact, and low cost sensor shows its possible potential in the sensitive detection field.

Random Raman Fiber Laser as a Liquid Refractive Index Sensor

In this paper,a new concept of forward-pumped random Raman fiber laser(RRFL)-based liquid refractive index sensing is proposed for the first time.For liquid refractive index sensing,the flat fiber end immersed in the liquid can act as the point reflector for generating random fiber lasing and also as the sensing head.Due to the high sensitivity of the output power of the RRFL to the reflectivity provided by the point reflector in the ultralow reflectivity regime,the proposed RRFL is capable of achieving liquid refractive index sensing by measuring the random lasing output power.We theoretically investigate the effects of the operating pump power and fiber length on the refractive index sensitivity for the proposed RRFL.As a proof-of-concept demonstration,we experimentally realize high-sensitivity half-open short-cavity RRFL-based liquid refractive index sensing with the maximum sensitivity and the sensing resolution of-39.88W/RIU and 2.5075×10^(-5) RIU,respectively.We also experimentally verify that the refractive index sensitivity can be enhanced with the shorter fiber length of the RRFL.This work extends the application of the random fiber laser as a new platform for highly-sensitive refractive index sensing in chemical,biomedical,and environmental monitoring applications,etc.

Modeling of Gold Circular Sub-Wavelength Apertures on a Fiber Endface for Refractive Index Sensing

A finite-difference time-domain approach was used to investigate the excitation of surface plasmons of the circular sub-wavelength apertures on an optical fiber endface. This phenomenon provided the basis of a sensitive liquid refractive index sensor. The proposed sensor is compact and has the potential to be used in biomedical applications, having a sensitivity of （373 ± 16）nm per refractive index unit （RIU） as found through the variation of a reflection minimum with the wavelength.

Modeling of Refractive Index Sensing Using Au Aperture Arrays on a Bragg Fiber Facet

A finite-difference-time-domain(FDTD)approach is undertaken to investigate the extraordinary optical transmission(EOT)phenomenon of Au circular aperture arrays deposited on a Bragg fiber facet for refractive index(RI)sensing.Investigation shows that the choice of effective indices and modal loss of the Bragg fiber core modes will affect the sensitivity enhancement by using a mode analysis approach.The critical parameters of Bragg fiber including the middle dielectric RI,as well as its gap between dielectric layers,which affect the EOT and RI sensitivity for the sensor,are discussed and optimized.It is demonstrated that a better sensitivity of 156±5 nm per refractive index unit(RIU)and an averaged figure of merit exceeding 3.5 RIU‒1 are achieved when RI is 1.5 and gap is 0.02μm in this structure.

Modeling Refractive Index Change in Writing Long-Period Fiber Gratings Using Mid-Infrared Laser Radiation

The research on the use of fiber sensors based on long-period fiber gratings inscribed by CO2 laser mid-infrared radiation has increased in the last years. In this paper, a set of analytical expressions are used to model the interaction between laser radiation and an optical fiber and to determine the resulting refractive index change. Thermal and residual stress analysis is exemplified for a standard single mode fiber, demonstrating the capability of these models to point out the necessary parameters to achieve proper optical fiber devices based on long period fiber gratings. Experimental results are also presented.

Measurement of refrative index based on fiber Mach-Zehnder interference

In order to detect the refractive index of liquid with high precision,based on modular interference,Mach-Zehnder optical fiber refractive rate sensor was studied.Sensor structure is composed of ordinary single-mode fiber and multimode fiber,according to the singlemode multimode singlemode sequence to fuse together,and the fused optical fiber is used to process the taper.As a result,the diameter of the sensing head is about 10μm.Experimental results show that,as liquid refractive index increases range from 1.33 to 1.35,the loss peak of the transmission spectrum will shift to long wave direction.

A Novel High Sensitivity Refractive Index Sensor Based on Multi-Core Micro/Nano Fiber

A refractive index sensor based on a multi-core micro/nano fiber is proposed for low refractive index solutions. At first, the mode field distribution of the tapered multi-core fiber is analyzed with the finite element model (FEM). After that, the relationship between the refractive index sensitivity and the diameter of the multi-core micro/nano fiber is calculated. At last, four sensors with different sizes are explored, and when the taper length is 16.20 mm, the refractive index sensitivity of the sensor can reach 5815.50nm/RIU, which agrees with the theoretical analysis. The refractive index measurement error is less than 0.5 %o, which has a high practical application value. The longer the taper length is, the smaller the fiber diameter is. According to the theoretical analysis, when the fiber diameter is less than 4.864 gm, the structure sensor\ refractive index sensitivity is higher than 10000 nm/RIU. At the same time, when the sensor's taper length is 15.99 mm, its temperature sensitivity is -0.1084nm/°C. Compared with single-mode fiber, the sensor proposed here has the advantages of stability, compact structure, and high sensitivity, which has a potential in the field of seawater salinity measurement.

Novel optical fiber current sensor based on magnetic fluid

A novel fiber optic Fabry-Perot（F-P） current sensor is developed based on magnetic fluid as the medium in F-P interference cavity.A signal demodulation method based on slanted fiber Bragg grating（FBG） wavelength measurement system is proposed.Theory and principle of electromagnetic-controlled refractive index of the magnetic fluid is described,as well as the structure of the sensor system.Preliminary experiments are carried out,and the results indicate that there is a fairly good linearity of the measurement characteristic.The thickness of magnetic fluid film and initial concentration will affect the measurement results.

Nonadiabatic Tapered Optical Fiber for Biosensor Applications

A brief review on biconical tapered fiber sensors for biosensing applications is presented. A variety of configurations and formats of this sensor have been devised for label free biosensing based on measuring small refractive index changes. The biconical nonadiabatic tapered optical fiber offers a number of favorable properties for optical sensing, which have been exploited in several biosensing applications, including cell, protein, and DNA sensors. The types of these sensors present a low-cost fiber biosensor featuring a miniature sensing probe, label-free direct detection, and high sensitivity.

Optical fiber hydrogen sensor based on light reflection and a palladium-sliver thin film

Thin alloy films of palladium(Pd) and silver(Ag) are deposited onto glass substrates via the direct current(DC) magnetron technique.The hydrogen sensor probe consists of optical fiber bundle and Pd/Ag optical thin film.when the sensor is exposed to hydrogen,the refractive index of Pd/Ag optical thin layer will diminish and cause attenuation changes of the reflective light.It is observed that the thickness of Pd/Ag alloy layer can affect the hydrogen sensor signal.Under different substrate temperatures,several Pd/Ag samples are coated with different thicknesses of Pd/Ag alloy,and the results of a hydrogen sensor based on reflective light from the Pd/Ag alloy thin film are discussed.

Effect of Overlaid Material on Optical Transmission of Side-Polished Fiber Made by Wheel Side Polishing

The performance of optical power transmission through a side-polished fiber on which materials of different refractive indices were overlaid is investigated. The experiments show that the transmitted optical power through the side-polished fiber varies with the refractive index of the overlaid material. The result of our experiments fits well the theoretical calculation. Side-polished fiber manufactured by wheel polishing method can be used not only to control optical power transmission through the fiber core but also as a refractive index sensor.