Fiber Cladding Research Articles

Advances in Multicore Fiber Interferometric Sensors.

In this paper, a review of multicore fiber interferometric sensors is given. Due to the specificity of fiber structure, i.e., multiple cores integrated into only one fiber cladding, multicore fiber (MCF) interferometric sensors exhibit many desirable characteristics compared with traditional fiber interferometric sensors based on single-core fibers, such as structural and functional diversity, high integration, space-division multiplexing capacity, etc. Thanks to the unique advantages, e.g., simple fabrication, compact size, and good robustness, MCF interferometric sensors have been developed to measure various physical and chemical parameters such as temperature, strain, curvature, refractive index, vibration, flow, torsion, etc., among which the extraordinary vector-bending sensing has also been extensively studied by making use of the differential responses between different cores of MCFs. In this paper, different types of MCF interferometric sensors and recent developments are comprehensively reviewed. The basic configurations and operating principles are introduced for each interferometric structure, and, eventually, the performances of various MCF interferometric sensors for different applications are compared, including curvature sensing, vibration sensing, temperature sensing, and refractive index sensing.

PANI-ZnO clad modified multimode optical fiber pH sensor based on EWA

In the present investigation, an intrinsic intensity modulated optical fiber pH sensor is developed and experimentally tested at room temperature. To fabricate the sensor probe, original cladding of multimode fiber was replaced by a pH-sensitive polyaniline-zinc oxide (PANI-ZnO) nanocomposite matrix whose absorbance property and refractive index (RI) are pH dependent. The synthesized PANI, ZnO, and incorporation of ZnO into PANI was confirmed and analyzed by some characterization techniques. It was observed that the ZnO composition into PANI not only enhances the materials's optical activity, porosity and hence sensitivity of the sensor but also boosts the mechanical strength of the coating layer over silica surface. Moreover, high index ZnO material is used to enhance the evanescent field towards the sensing region, which can help in better interaction of light from the outer medium. The pH sensing principle depends upon evanescent wave absorption (EWA) due to swelling/shrinkage of PANI and hence modification in volume and RI of the layer leads to change in transmitted optical power. The average sensitivity of proposed sensor was found to be 2.46 μW/pH for a wide pH range of 2–10 unit. The sensor also possesses excellent stability, fast response time, low hysteresis, good repeatability and durability characteristics. Results suggest that this sensor can be productively utilized in biomedical and chemical sensing applications.

Investigation of Fiber Optic-Based-Refractometer for Biogas Sensing

In this study, a fiber-based refractometer (FOR) applied for biogas sensing has been investigated. Two types of fiber, single-mode (SMF) and multimode fiber (MMF) have been proposed as sensing elements. The research aims to investigate the spot and power attenuation of both fiber types in 4 main conditions; fiber cladding, de-cladding, compound coating, and biogas feeding. The experimental results showed that the spot diameters from both fiber types are constantly at 4 and 26 mm in any conditions. This causes the difference in core diameters and also the dispersion of light characteristics within the fibers. Moreover, when the sensing element has been modified by the following conditions, the results indicated that the output intensity has proportionally changed, according to the fiber modification and the concentration of biogas absorbed into the sensing element. Besides, the power attenuation from MMF is larger than SMF. This causes the length of fiber de-cladding and dispersion of light within the MMF can easily be induced by biogas feeding. Therefore, it can be concluded that the MMF is more suitable than SMF for employment as a sensing element of the fiber refractometer.

The Blue Beret

The Blue Beret

Immunosensing for Early Detection of Rheumatoid Arthritis Biomarkers: Anti-Cyclic Citrullinated Peptide Antibodies Based on Tilted-Fiber Bragg Grating Biosensor

Rheumatoid arthritis (RA) is regarded as a chronic, immune-mediated disease that leads to the damage of various types of immune cells and signal networks, followed by inappropriate tissue repair and organ damage. RA is primarily manifested in the joints, but also manifests in the lungs and the vascular system. This study developed a method for the in vitro detection of RA through cyclic citrullinated peptide (CCP) antibodies and antigens. The diameter of a tilted-fiber Bragg grating (TFBG) biosensor was etched to 50 μm and then bonded with CCP antigens and antibodies. The small variations in the external refractive index and the optical fiber cladding were measured. The results indicated that the self-assembled layer of the TFBG biosensor was capable of detecting pre- and post-immune CCP antigen and CCP peptide concentrations within four minutes. A minimum CCP concentration of 1 ng/mL was detected with this method. This method is characterized by the sensor's specificity, ability to detect CCP reactions, user-friendliness, and lack of requirement for professional analytical skills, as the detections are carried out by simply loading and releasing the test samples onto the platform. This study provides a novel approach to medical immunosensing analysis and detection. Although the results for the detection of different concentrations of CCP antigen are not yet clear, it was possible to prove the concept that the biosensor is feasible even if the measurement is not easy and accurate at this stage. Further study and improvement are required.

A Low-Flow Fiber-Optic Flowmeter Based on Bending Measuring Using a Cladding Fiber Bragg Grating

A low-flow flowmeter based on the bending measurement of a cladding fiber Bragg grating (CLFBG) has been proposed and demonstrated. The CLFBG is fixed in a cantilever beam structure composed of a spring and a circular target. A femtosecond laser side-illumination technique was utilized to ensure that the grating inscription remains close to the core–cladding interface of the multi-cladding fiber (MCLF). The cladding resonance presents fiber bending dependence because of the asymmetrical distribution of the CLFBG along the fiber cross section. The force exerted on the circular target by the slow flow rate of the fluid on the order of mm/s bends the fiber, which changes the peak intensity of the cladding mode. The experimental results show that the sensor responds in the range of 0–87 mm/s and has extremely low-temperature cross-sensitivity. The proposed sensor has potential application prospects in high-precision, low-start flow measurement and can be applied to oil pipelines, downhole, and other fields.

Theoretical analysis of mode evolution in a tapered double clad fiber based on the coupled local mode theory

Mode characteristics of a tapered double clad fiber should be analyzed based on three-layer waveguide model, so it is more complicated than that of a single clad fiber. In this paper, a method based on coupled local mode theory is introduced to calculate the mode evolution process in a tapered double clad fiber. Mode field characteristics of double clad fiber are calculated analytically with weak guidance approximation, and the expression of mode coupling coefficient is derived. Based on this, the local mode characteristics and mode coupling characteristics in a tapered double clad fiber are analyzed. The mode evolution characteristics of tapered double clad fibers with different parameters are simulated. As an example, the structure optimization process of a tapered double clad fiber is introduced. High efficiency coupling with a six-mode single clad fiber is realized, and the theoretical transmission loss is less than 0.3 dB.

Orthogonal single-mode helical Bragg gratings created in fiber cladding for vector bending measurement.

We demonstrate a novel, to the best of our knowledge, two-dimensional vector bending sensor based on orthogonal helical Bragg gratings inscribed in the cladding of a conventional single-mode fiber (SMF). The helical cladding fiber Bragg gratings (HCFBGs) are created by using a femtosecond laser direct writing technology and a quarter-pitch graded index fiber (GIF) is used in front of the HCFBGs to diverge the core mode into fiber cladding. In contrast to the multimode resonance observed in conventional cladding Bragg gratings inscribed by using a femtosecond laser point-by-point (PbP) or line-by-line (LbL) technology, the proposed HCFBGs exhibit stable narrowband single-mode Bragg resonance. An HCFBG with a low peak reflectivity of -50.77 dB and a narrow bandwidth of 0.66 nm was successfully fabricated by using a lateral offset of 45 µm between the HCFBG and the fiber core axis. Moreover, two orthogonal HCFBGs were fabricated in the SMF cladding and used for vector bending sensing. Strong orientation dependence could be seen in omnidirectional bending measurement, exhibiting a maximum bending sensitivity of up to 50.0 pm/m-1, which is comparable to that in a multicore FBG. In addition, both the orientation and amplitude of bending vector could be reconstructed by using the measured Bragg wavelength shifts in two orthogonal HCFBGs. As such, the proposed HCFBGs could be used in many applications, such as structural health monitoring, robotic arms, and medical instruments.

Computation of dimensionless propagation parameters of LP11 mode of double clad fibers

A scalar variational approximation is proposed to calculate the propagation constant and the normalised dispersion parameter of LP11 mode of double clad fibers. Single and two parameter variational approximations have been proposed in this work. Also, a numerical method has been developed for the computation of these parameters. It has been demonstrated that the two parameter variational approximation calculates the propagation parameters more accurately than single parameter variational approximation over a wide range of fiber parameters. These values of the parameters, computed using variational approximations, have been verified with the exact results obtained numerically.

Optical fiber modes SM450 and SMF-28

The sensitivity of fiber-inserted structures can be greatly increased by exciting cladding modes and coating the fiber cladding with a material with a refractive index higher than that of the cladding. The article reviews the mode characteristics of two types of coated and uncoated optical fibers.

γ-MnO2-deposited photonic crystal fiber: Novel photonic devices for multi-state solitons generation

Transition metal oxides, especially MnO2 nanoparticles, are often used as cathode of dry batteries and catalysts. They are also promising materials for ultrafast laser applications due to their strong absorption in visible light and near-infrared wavelength range. In this work, high-concentration MnO2 nanoparticles dispersions embedded in the hole cladding of dual-hole photonic crystal fiber is fabricated as a saturable absorber with a modulation depth of 4 % and a saturation intensity of 25 MW/cm2. Strong MnO2-light interaction occurs due to enhanced evanescent-field strength (over 10 cm) of the SA could enable stable mode locking operation at 1.55 μm region. By inserting a Sagnac fiber filter in the cavity, multi-state solitons are experimentally demonstrated with identical layout, respectively, which greatly improves the versatility of this laser. This study proves that MnO2 nanoparticles possess excellent nonlinear optical properties in the near-infrared band. The simple in-line structure of the proposed nanoparticles-deposited device could pave a way for high power and all-fiber applications of photonics.

DETECTION OF ADULTERANT TRACES IN EDIBLE OIL BASED ON FIBER OPTIC SENSOR TECHNOLOGY

Low-cost multimode fiber (MMF) optic sensor for detection and determination of adulterant traces of palm oil in coconut oil is experimentally demonstrated. The fundamental principle of detection is the sensitive dependence of the wavelength shift on the refractive index changes of the surrounding medium of multimode fiber cladding. The transmission spectral changes due to variation of adulteration levels have been investigated. Building up of this low-cost multimode fiber optic sensor is helpful in prevention of adulteration. This technique has resulted in 20% adulteration for coconut-palm oil binary mixture

Temperature characteristics analysis of a Tm3+-doped heterogeneous helical cladding fiber amplifier

Temperature characteristics analysis of a Tm3+-doped heterogeneous helical cladding fiber amplifier

Optical fiber with homogeneous material byside-array cladding.

Optical fibers are the core elements for various fiber-optic applications in communication, lasers, sensors, tweezers, quantum optics, and bio-photonics. Current optical fibers are based on a core-cladding structure with different refractive indices and are mainly fabricated using the stack-draw method. However, such a traditional fabrication method limits the realization of fibers with various advanced optical materials, thereby restricting the utilization of excellent optical properties offered by these materials. In this study, a novel structure for side-array cladding by laser drilling on the side of the fiber with homogeneous material is proposed. Accordingly, the confinement loss, mode characteristics, birefringence, and dispersion of the side-array cladding fiber are investigated based on the numerical simulation performed via the finite element method. Subsequently, an optimal fiber structure is obtained by taking the crystal material as an example. Essentially, our proposed side-array cladding fiber can eliminate the mismatch problem of core-cladding materials in the current stack-draw fabrication method. Potentially, the proposed approach can serve as a standard design and fabrication method of optical fibers with homogeneous material, by utilizing the rapid development of laser processing. In other words, a large number of advanced optical materials can be fabricated into optical fibers with the proposed technique, thus maximizing their technical advantages for different applications.

Efficient Suppression of Backward Light Scattering for Optical Fiber Cladding Light Stripper

This study investigates a method for the effective suppression of backscattering within optical fiber cladding light strippers (CLSs) with a periodically grooved structure. First, the relationship between the backscattering of cladding light and the surface roughness of the cladding for a periodically grooved CLS is theoretically investigated using the ray-tracing method. It is found that the backscattering of cladding light depends on the surface roughness of the grooves and that undesired backscattering significantly affects the stripping capability of a CLS. Based on the theoretical results, we propose a periodically grooved CLS structure with multiple unetched sections of a suitable length and verify its effectiveness by comparing its lightwave stripping performance with that of a conventional CLS design in both theoretical and experimental ways. Unlike the conventional structure, our proposed structure demonstrates almost uniform lightwave stripping without localized overheating.

A bent fiber optic sensor based on gold nanoparticle-decorated silver film for integration of cytosensor and plasmonic photothermal therapy: A simulation study

The fiber optic surface plasmonic resonance (SPR), with the unique characteristic of miniaturized devices, high refractive index sensitivity, and intense plasmonic photothermal effect, is a promising technique for advanced biosensors in clinical medicine. Studies on the synergistic influence factors of the dual performance of refractive index sensitivity and surface heat field are benefitted for integration of cytosensor and phototherapy. Herein, a gold nanoparticle-decorated silver film (AuNPs-AgFM) was simulated as the coating to construct a bent fiber optic surface plasmonic resonance (SPR) through a Finite Element Method (FEM). Bending leads core mode to shift away from the center of curvature into the fiber cladding, which further reacted with the AgFM and AuNPs to generate the short wavelength peaks for cytosensor and long wavelength peaks for plasmonic photothermal therapy, respectively. Furthermore, the parameters of bent radius, AuNPs’ size, and thickness of AgFM were optimized considering both refractive index sensitivity and surface temperature. Under the optimal condition, the designed SPR biosensor exhibit high amplification sensitivity (AS) of − 2301.6 (1/RIU), wavelength sensitivity (WS) of 3120 (nm/RIU), and surface temperature of 54.9 ℃ under irradiation of near-infrared ray (NIR) laser. The proposed AuNPs-AgFM-based fiber optic SPR biosensor provides an alternative scheme to integrate cytosensor and phototherapy, further pushing fiber optics to clinical application.

Consecutive 1015-1105-nm wavelength tunable "figure-of-9" mode-locked Yb:fiber oscillator.

A widely wavelength tunable mode-locked Yb-doped fiber oscillator based on nonlinear amplifier loop mirror (NALM) is reported, in which only a piece of short (∼0.5 m) single-mode polarization-maintaining (PM) Yb-doped fiber is employed, instead of the frequently used long (a few meters) double cladding (DC) fiber in previous papers. Experimentally, the center wavelength can be consecutively tuned from 1015 to 1105 nm by tilting the silver mirror, corresponding to a tuning range of 90 nm. To the best of our knowledge, this is the broadest consecutive tuning range in Yb:fiber mode-locked fiber oscillator. In addition, the mechanism of wavelength tuning is tentatively analyzed and attributed to the combined action of the spatial dispersion induced by a tilting silver mirror and the limited aperture in the system. Specific to the wavelength of 1045 nm, the output pulses with 13-nm spectral bandwidth can be compressed to 154 fs.

Twisted Fiber Optic SPR Sensor for GDF11 Concentration Detection.

There are few methods and insufficient accuracy for growth differentiation factor 11 (GDF11) concentration detection. In this paper, we designed a twisted fiber cladding surface plasmon resonance (SPR) sensor, which can achieve a high precision detection of GDF11 concentration. The new structure of the fiber cladding SPR sensor was realized by coupling the light in the fiber core to the cladding through fiber thermal fusion twisting micromachining technology; a series of functionalized modifications were made to the sensor surface to obtain a fiber sensor capable of GDF11 specific recognition. The experimental results showed when GDF11 antigen concentration was 1 pg/mL-10 ng/mL, the sensor had a detection sensitivity of 2.518 nm/lgC, a detection limit of 0.34 pg/mL, and a good log-linear relationship. The sensor is expected to play a role in the rapid and accurate concentration detection of pathological study for growth differentiation factors.

Improvement of Temperature Performance of Singlemode-Multimode-Singlemode Fiber Structure.

A theoretical model for studying the temperature properties of singlemode-multimode-singlemode (SMS) fiber structure fabricated by absorptive multimode fiber (MMF) cladding is established. Moreover, an SMS-based temperature sensor is fabricated and experimentally demonstrated. Experimental results show that the dip wavelength of the transmission spectrum changes linearly with temperature, which is in good agreement with the simulated results obtained by using the model. Further, a comprehensive study of temperature characteristics affected by the thermo-optic effect, thermal expansion effect, and thermal effect of absorption characteristics is performed for SMS fiber optic structures with different refractive indexes, thermo-optic coefficients, and absorption properties of MMF cladding, MMF core diameters, and thermal expansion coefficients of packaging shell. According to the obtained rules, investigations are carried out into the thermal response of an SMS fiber structure resulting from combined thermal effects for temperature performance optimization. Excellent temperature stability with a temperature sensitivity of 0 pm/°C or good temperature sensitivity of -441.58 pm/°C is achieved accordingly.

Liquid-Core Hydrogel Optical Fiber Fluorescence Probes.

This paper first reports a liquid-core hydrogel optical fiber fluorescence probe. It is composed of a liquid core, a high-refractive-index hydrogel fiber core, and a low-refractive-index hydrogel fiber cladding, which is completely different from many existing optical fiber fluorescence probes. The sensing solution with sensitive materials is sealed as a liquid core, and it can sufficiently react with small-molecule targets penetrating through the hydrogel fiber cladding and core, thus inducing variations in the fluorescence signals. These fluorescence signals can be localized and transmitted within the probe and finally collected and quantified for target detection. This proposed probe can be simply and rapidly fabricated and reused, and it was proven to have high sensitivity, accuracy, and selectivity in practical applications. Therefore, this liquid-core hydrogel optical fiber fluorescence probe will enable a novel sensing platform for small-molecule analyte detection that faces on-site detection challenges.