Higher Order Cladding Modes Research Articles

Balloon-Shaped SMF Blood Glucose Concentration and Temperature Sensor Based on Core-Offset Structure

A blood glucose concentration and temperature sensor with a balloon-shaped single-mode fiber (SMF) based on a core-offset structure is proposed and experimentally demonstrated. The balloon-shaped SMF is created by offset-fusing a straight-line SMF between two other SMFs, thereby forming a Mach–Zehnder interferometer (MZI). The core-offset structure can effectively excite higher-order cladding modes. The experimental results showed that the maximum sensitivity of blood glucose concentration was 0.331 nm/(mmol/l) and the maximum sensitivity of temperature was 0.216 nm/°C when the offset distance was 10 μm. Dual-parameter measurement was achieved through a dual-parameter matrix. In addition, the sensor has characteristics such as simple structure, low cost, good stability, and electromagnetic interference resistance, making it potentially valuable for diagnosing high blood glucose and related conditions.

An asymmetric STNS-MZI structure and its applications in temperature and Cd2+ monitoring

In this paper, an asymmetric fiber Mach-Zehnder interferometer (MZI) is presented and investigated. The proposed asymmetric MZI structure is mainly constructed with thin core fiber (TCF) and no core fiber (NCF), sandwiched between single mode fibers (SMFs). Note that the TCF is spliced with a slight offset such that higher order cladding modes could be effectively exited. The SMF-TCF-NCF-SMF (STNS) structure is adjusted by a finite-difference beam propagation method simulation to achieve an optimal interference spectrum. Temperature monitoring performance is addressed and the calculated sensing resolution is about 0.28℃ with high precision of ± 0.3 °C. Moreover, as for the Cd2+ monitoring application, the TCF is further etched and then coated with 1-allyl-2-thiourea (ATU) forming cross-linked “-S-Cd-S-” structure. The results show that the resolution of Cd2+ could reach 2.37 × 10−11mol /L, which shows a four order of magnitude improvement compared with our previous work. Therefore, the proposed asymmetric STNS-MZI interference structure has great potential in future applications.

High-sensitive glucose sensor based on tilted fiber Bragg gratings

In this paper, we proposed a high-sensitive glucose sensor using the tilted fiber Bragg gratings (TFBGs). The TFBGs with different tilt angle has been fabricated by phase mask scanning method with an argon ion double-frequency 244 nm laser. As tilt angle increases, the bandwidth of the cladding mode excited by TFBGs gradually expands and cladding mode region shifts towards shorter wavelengths. The higher-order cladding modes appeared at the shorter wavelength side demonstrate a larger separation in the spectrum. TFBGs with different tilt angles for glucose concentration measurement have been investigated experimentally. When the concentration of glucose solution increases, the surrounding refractive index changing make the cut-off mode resonance shift to a longer wavelength. In addition, the tilt angle of TFBGs does affect the glucose sensitivity, that the larger tilt angle is, the higher sensitivity could achieve. In this work, a maximum glucose sensitivity of 80.91 pm/(mg/ml) could be obtained for TFBGs with tilt angle of 10°. The proposed TFBGs with superiorities of high glucose sensitivity and wider detection range, makes it potential for glucose detection in the field of chemical and biomedical sensing.

Trace detection of cadmium (II) ions based on an excessively tilted fiber grating

Cadmium (Cd2+) ion is one of the most crucial industrial pollutants that cause serious harm to the human body. We proposed and experimentally demonstrated a highly sensitive Cd2+ sensor based on hydrogel coated excessively tilted fiber grating. The hydrogel with the functional monomer of the allyl thiourea can specifically bind to Cd2+, and hence forming a complex. The grating excites high order cladding modes, and ensures a sufficient interaction between the light and hydrogel binding to Cd2+, providing highly sensitive monitoring. The results show that the sensor can detect 0-160 pM Cd2+ in aqueous solution. The maximum sensitivity is 10600 nm/µM, and the minimum detection concentration is 20 pM (about 0.004 ppb), which is much less than that of the international standard (3 ppb). The proposed sensor exhibits high sensitivity, ultra-low detection limit, specificity, and a compact structure, offering potential as a tool for Cd2+ detection in aqueous solution.

A highly sensitive optical fiber sensor enables rapid triglycerides-specific detection and measurement at different temperatures using convolutional neural networks

For specific recognition and sensitive detection of triglycerides (TGs), an optical fiber sensor (OFS) based on an enhanced core diameter mismatch was proposed. The sensitivity of the sensor is significantly increased due to the repetitive excitation of the higher-order cladding modes. A technique for immobilizing lipase using covalent binding technology was presented and demonstrated by Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy. The interference dip of the sensor was shifted due to TGs being hydrolyzed in the presence of lipase. The sensor shows an optimal response within 3 min and exhibits a high sensitivity of 0.9933 nm/(mg/ml) and a limit of detection of 0.0822 mg/ml in the concentration range 0–8 mg/ml at a temperature of 37 °C and a pH of 7.4. The response of the sensor to TGs concentration at different temperatures and pH was investigated. The reproducibility, reusability, and stability of the proposed sensor were tested and verified experimentally. The biosensor is highly specific for TGs and unaffected by many other interfering substances. Further, the measurement of TGs concentration at different temperatures was realized. This method provides a new way to detect TGs rapidly and reliably and has potential applications in medical research and clinical diagnosis.

A highly sensitive strain fiber sensor based on waved core structure

Strain and temperature, as two important parameters to evaluate structural health, is increasingly paid numerous attentions for years. In this article, we developed and experimentally tested a highly sensitive strain sensor based on SMS structure with waved core multi-mode fiber (MMF), which has ability to monitor temperature simultaneously. By introducing a section of bent-core fiber into the portion of MMF, the higher-order cladding mode is excited to enhance the sensitivity. Furthermore, the portion of bent-core displays significant influence on effective refractive index (ERI) of the MMF section as strain applied, which brings about higher strain sensitivity. The experimental results display that the highest sensitivity of the sensor reaches 125.5 pm/με within 0–200 με and the measuring range achieves 0–1200 με. By demodulating two dips, simultaneous measurement for strain and temperature is achieved by solving the equation matrix. This simple method displays minor influence on mechanical property and enhances the strain sensitivity considerably. It is strongly believed the proposed sensor brings about a competitive choice in engineering fields.

Optical Fiber Sensor with Novel Structure and Its Applications in Oil and Gas Exploration

Compared to electronic sensors, Optical Fiber Sensors (OFSs) have received increasing attention due to their advantages, such as small size, light weight, anti-electromagnetic interference, easy reuse, and remote sensing. This paper proposes a novel all-fiber mode interferometer. This interferometer excites higher-order cladding modes through mode mismatch between standard single-mode fibers and thin cores. There is interference between higher-order cladding mode and core layer mode, resulting in interference fringes used as sensing signals. Sensitivity testing is conducted on the designed OFS to demonstrate whether it meets the application requirements for petroleum exploration. Transmission-type Thin-Core Fiber Modal Interferometers (TCFMIs) with different lengths of Thin-Core Fibers (TCFs) (20 mm, 40 mm, and 60 mm) are produced in the experiment. Among them, TCFMIs at 60 mm TCF length can obtain ideal sensing signals. The TCFMI (with a length of 20 mm TCF) is encapsulated in a self-made aluminum groove in rosin to test its Refractive Index (RI). The results show that as the RI increases, the central wavelength shifts towards the long wavelength direction. Its sensitivity reaches 146 nm/R.I.U. When the temperature is increased, the central wavelength shifts towards the long wavelength direction, resulting in lower temperature sensitivity. In the sensitivity test, the phase change obtained by designing OFSs is proportional to the vibration acceleration. It is fixed on the vibration table to keep the acceleration of the vibration table constant and adjust the vibration frequency of the vibration table. The results show that the vibration spectral line of the sensor is relatively flat within 100 Hz, and resonance occurs within the range of 200 Hz to 350 Hz. Through phase demodulation, sensors loaded with different oscillator masses increase linearly in the low-frequency range. When the vibration frequency approaches the resonance frequency of the sensor, the phase sensitivity of the sensor increases nonlinearly.

Temperature insensitive salinity sensor with U-shaped structure based on few-mode fiber

A novel U-shaped few-mode fiber (FMF) salinity sensor is proposed, which is insensitive to temperature. It consists of a section of FMF fused between two sections of single-mode fiber (SMF). The flame heating method is used to form the FMF into a U-shaped optical fiber sensor. Because the bending makes the light energy of the core leak into the cladding, higher-order cladding modes are excited and the sensor becomes more sensitive to the external environment. In the experiments, the sensors with different bending diameters are fabricated and studied for salinity and temperature. The results demonstrate that the sensitivity reaches up to 1.847 nm/% in the salinity range of 0 % to 10 %. In the temperature range of 5 °C to 30 °C, it shows temperature insensitive characteristics. Compared with the traditional optical fiber sensor, the sensor can avoid the effect of temperature on measurement results, and can be easily fabricated. It has a good application prospect in the field of seawater temperature and salinity measurement.

Highly sensitive curvature fiber sensor based on an enhanced core diameter mismatch.

In this study, a curvature fiber sensor based on an enhanced core diameter mismatch is experimentally proposed and theoretically investigated. The structure is fabricated by splicing two types of step multimode and coreless fibers to excite the high-order cladding modes to improve the curvature sensitivity. Experimental results show that the highest curvature sensitivities of the structure reach -114.74 nm/m-1 in the Dip 1272 nm, -91.08 nm/m-1 in Dip 1408 nm, and -61.10 nm/m-1 in Dip 1644 nm in the measuring range of 0-0.49778 m-1. Meanwhile, the sensor's temperature and strain responses were also tested, which shows little influence on the curvature measurement. Additionally, the proposed fiber sensor exhibits features of easy fabrication, simple structure, and high mechanical strength. This study proposes a device for curvature measurement with potential use in material mechanics and optical fiber sensor design.

Spin-entangled orbital angular momentum filter of photons based on a chiral long-period fiber grating.

The orbital angular momentum (OAM) of light is particularly promising for multi-channel data transmission in optical fiber communications. In the implementation, one of the impediments is the lack of an effective all-fiber method to demultiplex and filter OAM modes. To solve the problem, by utilizing the inherent spiral characteristics of a chiral long-period fiber grating (CLPG), we propose and demonstrate experimentally a CLPG-based scheme for filtering spin-entangled OAM of photons. We unveil theoretically and verify experimentally that the co-handed OAM with the same chirality of helical phase wavefront as CLPG couples to higher-order cladding modes and suffers loss, while the cross-handed OAM with the opposite chirality freely passes in CLPG. Meanwhile, by combining its grating characteristics, CLPG can realize the filtering and detection of a spin-entangled OAM with arbitrary order and chirality without exerting extra loss for other OAM. Our work has great potential in analyzing and manipulating spin-entangled OAM, paving the way for the development of all-fiber OAM applications.

S-type fiber surface plasmon resonance strain sensor.

The fiber surface plasmon resonance (SPR) sensor is widely used in high-sensitivity refractive index detection, and there are few reports on SPR sensors used for strain measurement. In this paper, we propose and demonstrate an S-type fiber strain sensor based on SPR. The simulation and testing results indicate that the smaller the vertical axis offset of S-type fiber is, the larger the incidence angle of SPR is, and the closer the working band of SPR is to the shorter wavelength direction. By electrofusion, we fabricated an S-type structure on the single-mode fiber, and by the S-type structure, the high-order cladding mode was excited. A 50 nm gold film was coated on the surface of the fiber cladding behind the S-type structure. The evanescent field of the cladding mode contacted the gold film to produce SPR, and the strain can change the vertical axis offset of the S-type fiber and further change the incidence angle of SPR; hence an S-type fiber strain sensor based on SPR was realized. When the refractive index of the ambient medium is 1.345 RIU, the vertical axis offset and length of the S-shaped structure are 87 and 501 µm, respectively, the resonance wavelength of the fiber SPR strain sensor changes from 648.06 to 631.77 nm with a strain detection range of 0-1200µε, and its sensitivity is -14.38pm/µε. The proposed sensor provides a new solution for the strain measurement of the fiber SPR sensor, which is expected to be used in the fields of engineering, health monitoring, and early warning.

A High-FOM surface plasmon resonance sensor based on MMF-TUMMF-MMF structure of optical fiber

The figure of merit (FOM) of conventional surface plasmon resonance (SPR) sensors is low due to the low sensitivity and wide full width at half maxima (FWHM) caused by many low-intensity high-order cladding modes. To address this problem, this paper proposes a SPR sensor based on U-shaped tapered arm cascade of multimode fiber (TUMMF). The U-shaped macro-bend increases the intensity of the light field leaking into the cladding, and the tapered area makes the number of higher-order modes smaller to achieve high sensitivity and narrow FWMH sensing performance. Through simulation and experimental verification, the sensitivity of the sensor is proportional to the fiber curvature and taper ratio, and the FWMH is inversely proportional. Therefore, the parameters of the sensor are optimized by integrating the FOM and the mechanical strength of the sensor. The optimized sensor parameters are 3 mm bending diameter, 0.85 taper ratio, and 152 nm gold film thickness. The sensor was tested in the refractive index (RI) sensing range of 1.3324–1.3731 and showed a sensitivity of 4816.092 nm/RIU and a FWMH of 97.54 nm. The FOM of the proposed sensor is 49.37 /RIU, which is 3.18 times higher than that of the U-shaped multimode fiber SPR sensor. The probe has the advantages of simple fabrication, high environmental stability and easy modification, and has a wide range of applications in bioenvironmental and other fields.

Fiber cladding SPR sensor based on V-groove structure

The difficulty with fiber cladding Surface Plasmon Resonance (SPR) sensors lies in coupling the core mode to the cladding mode without removing the cladding. This paper hereby endeavors to demonstrate a fiber cladding SPR sensor based on V-grooves structure by using a CO2 laser. V-grooves are engraved on single-mode fiber, step-index multimode fiber, and graded-index multimode fiber. When the transmitted light reaches the V-grooves structure, the higher-order cladding mode is excited, and a metal film is coated on the 2 cm cladding area after the V-grooves to form fiber cladding SPR sensors. This paper investigates the refractive index and bending sensing of V-groove cladding SPR sensors of single-mode, graded-index multimode, and step-index multimode fibers. The experimental results show that: the refractive index sensitivity of three kinds of sensors is 2811.34 nm/RIU, 1862.33 nm/RIU, and 1418.85 nm/RIU, respectively, and the curvature sensitivity of three types of sensors is 0.42 nm/m−1, 1.03 nm/m−1, and −6.59 nm/m−1, respectively. The proposed sensors in this paper are easy to manufacture, demonstrate quality of high sensitivity, and can be applied extensively.

Design and simulation of a hybrid coated LPG-TFBG-FBG three-parameter sensor for an ocean environment

Based on the demand for monitoring conductivity, temperature, and depth (CTD) in the ocean, a hybrid coated long-period fiber grating, tilted fiber Bragg grating, and fiber Bragg grating (LPG-TFBG-FBG) structure is proposed, which could eliminate the mutual interference of CTD in measurements. First, according to coupled mode theory and phase matching conditions, the coupling equation of each segment of the grating is deduced. Next, through the difference iterative method, the total transfer matrix of coated LPG-TFBG-FBG is obtained. Then, to improve the sensitivity of the structure and make the demodulation more accurate and convenient, the structure of the grating is optimized. On this basis, the transmission spectrum of the structure is simulated, and three peaks formed by coupling the forward core mode with the forward cladding mode in LPG ( λ L c l ), the backward high-order cladding mode in TFBG ( λ TB c l ), and the backward cladding mode in FBG ( λ B c l ) are investigated. Furthermore, the sensing characteristics of the surrounding refractive index (SRI), temperature, and water pressure of the peaks are analyzed, and the three peaks show different sensitivities to environmental parameters. Therefore, a three-parameter matrix equation is established, and the three parameters can be measured simultaneously without interference. The simulation results of the response of the three transmission peaks to the three parameters show good performance in both bandwidth (less than 5 nm) and linearity (all larger than 0.99). The maximum sensitivities of the three transmission peaks to SRI, temperature, and water pressure are − 343 n m / R I U , − 1318.7 p m / ∘ C , and − 9.5 n m / M p a , respectively. Finally, the condition number of the parametric demodulation matrix is 581.5822, indicating that the inversion of the resonant wavelength offset has high precision. To our knowledge, this is the first optical fiber CTD sensor based on fiber gratings that can compensate for each other regarding CDT. The proposed coated LPG-TFBG-FBG structure has good linearity and high sensitivity, which give this structure great application potential in the fields of multi-parameter sensing and segmented sensing.

Highly sensitive humidity sensor based on tapered dual side-hole fiber

We proposed and demonstrated a highly sensitive humidity sensor based on a fiber Mach-Zehnder Interferometer (MZI). This sensor is composed of a short section of tapered dual side-hole fiber (DSHF) sandwiched between two single mode fibers. At the tapered section, both the fundamental and higher order cladding modes are stimulated and interfere with each other, forming an inter-modal MZI. The existence of air holes in the DSHF makes the cladding mode more easily excited and has higher refractive index sensitivity. In addition, by coating graphene oxide (GO) film with large specific surface area on the tapered DSHF, the humidity sensitivity of the sensor has been further improved. Experimental results show that the humidity sensitivity of the sensor can reach − 0.142 nm/%RH, which increased 7.9 times than that without GO coating. In addition, the rising and falling time of respiration were 0.23 s and 2.19 s, respectively. The excellent performance demonstrates it is a good candidate for humidity measurement.

TILTED FIBER BRAGG GRATING SENSORS FOR REFRACTIVE INDEX MEASUREMENTS OF LIQUID SOLUTIONS

This publication presents the basic assumptions about the light guiding mechanisms in tilted fiber Bragg gratings, transmission spectra of the fiber with such structures and their sensor properties related to the occurrence of the so-called cladding modes. The light in the form of modes coupled to the optical fiber cladding causes their spectral properties to depend on the difference in the refractive indices of the surrounding medium and the fiber cladding. With the introduction of a small inclination angle of the diffraction fringes forming the Bragg grating, the cladding modes show sensitivity to changes in the refractive index of the environment of aqueous solutions, which makes the spectrum of their applications broad. This publication presents changes in the spectra of selected modes measured for solutions with a specific refractive index, shift sensitivity, and changes in the mode transmission coefficient and processing characteristics at a selected concentration range of solutions. Experimental results show that high order cladding modes respond both by shifting the central wavelength as well as by changing the transmission minimum. In selected ranges, these parameters show a linear characteristic as a function of changes in the concentration of the cane sugar aqueous solution. In the case of TFBG with a tilt angle of 8 °, the sensitivity of wavelength changes is 0.012nm / RIU for solutions with concentrations ranging from 0% to 10% by weight.

Sensitivity-Enhanced Strain Sensor With a Wide Dynamic Range Based on a Novel Long-Period Fiber Grating

A long period fiber grating prepared by multimode fibers (MMF-LPFG) is proposed and experimentally demonstrated as a sensitivity-enhanced strain sensor. The core diameter mismatch between the MMFs excites the high order cladding mode to obtain a compact sensing structure (3.6 mm). The strain sensitivity of the MMF-LPFG structure is enhanced to −15.06 pm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \varepsilon $ </tex-math></inline-formula> when the grating period is closer to the dispersion turning point (DTP). Meanwhile, the novel fiber structure also has an excellent mechanical property to achieve a large dynamic range ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{0-4508} ~\mu \varepsilon $ </tex-math></inline-formula> ) in strain measurement.

Bend Sensor Based on Mach-Zehnder Interferometer Using Single-Mode Fiber With Helical Structure

An all-fiber bend sensor based on Mach-Zehnder interferometer (MZI) is proposed and experimentally demonstrated. The fiber interferometer consists of two helical structures in single mode fiber (SMF) and is fabricated by continuous arc discharge technique. The first helical structure forms a twisted long-period fiber grating (LPFG) to excite the higher-order cladding modes and acts as a beam splitter. The LPFG arrangement is adopted to improve the bending sensitivity. The second helical structure only contains two turns and can recouple the light in the cladding back into the core. The bending characteristics of the MZI are discussed. The experimental results show that high curvature sensitivities of 24.06 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-1}$ </tex-math></inline-formula> and −7.524 dB/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\text{m}^{-1}$ </tex-math></inline-formula> are obtained in the curvature range of 0-0.422m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> . Due to its low cost, small size, simple preparation, less time consuming, it could be a competitive candidate in many bending sensing applications.

High Sensitivity In-Situ Copper (∏) Detection of Chitosan Based on the Knotted-Shaped Fiber

A novel fiber sensor which can detect the concentration of copper ions indirectly was proposed and demonstrated by coating the knotted-shaped fiber with chitosan. The diameter of the knotted-shaped fiber was determined to be 10 mm. The Mach-Zehnder Interference generated due to bending deformation that excited some core-modes into higher order cladding-modes and two modes interfered. Chitosan was selected as the identification material for coating the fiber. In order to enhance the chelation between chitosan and copper ions, and improve the sensitivity of copper ions, glutaraldehyde was used to chemically modify chitosan. The original fiber sensor was annealed to obtain a stable structure, and then it was immersed in the modified chitosan solution and deionized water to form an available chitosan coating on the surface of the sensor. In the detection experiment with copper ions concentration of 0-330 ppm, the results show that interference spectra uniformly red-shift with the increase of the copper ions concentration and the sensitivity of the copper ion was 24.6 pm/10ppm. At the same time, ideal spectral fluctuations were demonstrated in the stability inquiry experiments.

A MMF-TSMF-MMF Structure Coated Magnetic Fluid for Magnetic Field Measurement

An optical fiber Mach-Zehnder interferometer (MZI) coated by magnetic fluid (MF) is fabricated for magnetic field measurement. The interferometer is composed of multimode fiber-taper single mode fiber-multimode fiber (MTSM). The two MMFs play the role of a beam splitter and a coupler. The TSMF induces the generation of high-order cladding mode and enhances the re-coupling ability of the interferometer. The transmission spectrum and cladding modes with different taper waist diameter are theoretically analyzed. Interference dips are sensitive to the alteration of magnetic field intensity when the taper waist diameter of interferometer is 34 μm. The highest sensitivity is up to -0.81 nm/mT at the magnetic field range of 8-17 mT.