Splicing Single-mode Fibers Research Articles

‘Lab around fiber’ humidity-enhanced ammonia sensor: Multimode interference functionalized by graphene oxide

In the paper, a fiber-optic ammonia sensor based on a no-core fiber (NCF)-type multimode interferometer is proposed. This sensor can utilize high humidity to enhance the sensitivity for ammonia detection. It is easily constructed by fusion splicing single-mode fiber (SMF)-NCF-SMF structure. The NCF is tapered to improve sensitivity for surrounding refractive index. To impart ammonia sensing capability to the structure, the interferometer is coated alternately with graphene oxide (GO)/ polyethyleneimine (PEI) composite films using the layer-by-layer method (LbL). The surface morphology and composition of the GO/PEI composite films are characterized using scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The ammonia sensing performance is tested, and the experimental results indicate that the central wavelength has obvious response of ammonia concentrations changing. The ammonia concentrations sensitivity in a high humidity environment is higher than low humidity environment, and the maximum sensitivity is 22.8 pm/ppm. The fiber-optic ammonia sensor designed and fabricated in this study utilizes the enhancement effect of water molecules on ammonia adsorption to improve the sensitivity of the sensor. This approach addresses the issue of poor moisture resistance in traditional ammonia sensors, thereby further expanding the application scenarios of ammonia sensors.

Enhancing the sensitivity of a humidity sensor by using PVA and GQDs hybrid diaphragms and the harmonic Vernier effect.

What we believe to be a novel fiber optic Fabry-Perot interferometer (FPI) humidity sensor based on polyvinyl alcohol (PVA) doped graphene quantum dots (GQDs) was proposed and experimentally studied. This sensor consisted of a sensing FPI and a reference FPI in parallel. The two sensing cavities FPI were composed of humidity sensitive materials PVA and PVA-GQDs, respectively. Experimental comparative studies had found that doping GQDs in PVA increases humidity sensitivity by 2.1 times. Four reference cavity FPIs were prepared by splicing single-mode fiber and quartz capillary, and then they were combined with two sensing cavity FPIs to form two Vernier effect (VE) sensors and two harmonic Vernier effect (HVE) sensors. Experimental research had found that the sensitivities of PVA as a sensing material for the VE sensor and HVE sensor were -1.0804 nm/%RH and-1.6566 nm/%RH, respectively. The sensitivities of PVA-GQDs as sensing materials for the VE sensor and HVE sensor were-3.1527 nm/%RH and 7.3343 nm/%RH, respectively. Moreover, both HVE sensors had minimal temperature crosstalk. PVA-GQDs were excellent humidity sensitive materials that significantly improve the sensitivity of humidity sensors, making it promising candidate for humidity sensing in various applications.

Ultra-sensitive strain sensor based on Sagnac interferometer with different length panda fiber

A high-sensitivity harmonic Vernier effect (HVE) strain sensor is fabricated by cascading Sagnac interferometer (SI) and Fabry-Perot interferometer (FPI). SI is fabricated by the panda fiber (PF). PF is a typical polarization maintaining fiber (PMF) that has two pressure zones, making it particularly sensitive to strain. SI1, SI2, and SI3 are constructed using PF with lengths of 20 cm, 50 cm, and 80 cm, respectively. They have interference spectra similar to sine waves in the wavelength range of 1250 nm to 1650 nm. Research has found that the sensitivity of SI is directly proportional to the effective PF length under strain action. In addition, if the strain acts on the entire length of the PF, the strain sensitivities of SI1, SI2, and SI3 are 30.4 pm/με, 28.4 pm/με, and 30.1 pm/με, respectively, which are relatively close. Then, FPI is fabricated by splicing single mode fiber (SMF) and quartz capillary, and it is an “SMF-Capillary-SMF” sandwich structure, and the free spectral range (FSR) of FPI is approximately half of that of SI2. FPI and SI2 are cascaded to produce a first-order HVE sensor, further amplifying the strain sensitivity of SI2. The intersection point of the internal envelope based on the HVE sensor spectrum can provide accurate spectral wavelength position and drift, and the strain sensitivity of the HVE sensor reaches 293.0 pm/με, improving the strain sensitivity of SI2 by 10.3 times. The detection limit of the HVE sensor is 0.5 µε as the strain varies from 0 to 700 µε. This HVE strain sensor is relatively easy to manufacture, low-cost, linear and reliable, and can achieve high sensitivity strain measurement. It has certain application value in the field of strain measurement.

High-temperature measurement based on highly-sensitive miniature cascaded FPIs and the harmonic Vernier effect

A highly-sensitive miniature sensor based on cascaded FPIs and the harmonic Vernier effect is proposed for high-temperature measurement. The package-free sensor is fabricated by only cleaving and splicing single-mode fiber (SMF), suspended core fiber (SCF), and hollow core fiber (HCF). By carefully adjusting the lengths of SCF and HCF, the Vernier effect and harmonic Vernier effect are generated on two sensors built by the same method, respectively. The reflected spectra exhibit distinct characteristics depending on the direction of incident light. Leveraging on the harmonic Vernier effect, the sensor of this type could achieve the highest sensitivity of 109.72 pm/℃ and maintain a maximum temperature fluctuation of only 4.12 ℃ under 800 ℃ without any temperature-sensitive material. In addition, the repeatability has been verified and the hysteresis errors have been analyzed. This miniature sensor exhibits excellent repeatability and stability, making it well-suited for high-temperature measurements in harsh environments.

Polished hollow core Bragg fiber sensor for simultaneous measurement of cortisol concentration and temperature.

Disturbance of surrounding temperature inevitably affects the accuracy of fiber biosensors. To that end, we propose a compact label-free optofluidic sensor based on a polished hollow core Bragg fiber (HCBF) that can simultaneously measure the cortisol concentration and surrounding temperature in real-time. The sensor is comprised of fusion splicing single mode fiber (SMF), multimode fiber (MMF) and HCBF. HCBF is side polished to remove part of the cladding that the suspended inner surface of the fiber can contact the external environment. After the incident light passes through the MMF from the SMF, it enters the hollow area, high refractive index (RI) layers, respectively, where the anti-resonant reflecting optical waveguide (ARROW) guiding mechanism and Mach-Zehnder interferometer (MZI) are simultaneously excited. Taking advantage of the high RI layers of HCBF, compared to the fiber with uniform cladding, the light can be more confined in the cladding and more sensitive to inner surface medium. The inner surface of sensor is immobilized with cortisol aptamer for the sake of achieving high sensitivity and specific sensing of cortisol with the limit of detection (LOD) to be 4.303 pM. The proposed sensor has a compact structure, enables temperature compensation, and can be fabricated at low cost making it highly suitable for in-situ monitoring and high-precision sensing of cortisol and other biological analytes.

High sensitivity detection of SARS-CoV-2 by an optofluidic hollow eccentric core fiber.

Since the outbreak of coronavirus disease 2019 (COVID-19), efficient real-time monitoring has become one of the challenges faced in SARS-CoV-2 virus detection. A compact all-fiber Mach-Zehnder interferometer optofluidic sensor based on a hollow eccentric core fiber (HECF) for the detection and real-time monitoring of SARS-CoV-2 spike glycoprotein (SARS-CoV-2 S2) is proposed, analyzed and demonstrated. The sensor is comprised of fusion splicing single mode fiber (SMF), hollow core fiber (HCF) and HECF. After the incident light passes through the HCF from the SMF, it uniformly enters the air hole and the suspended micrometer-scale fiber core of the HECF to form a compact all-fiber Mach-Zehnder interferometer (MZI). HECF is side polished to remove part of the cladding that the suspended fiber core can contact the external environment. Subsequently, the mouse anti SARS-CoV-2 S2 antibody is fixed on the surface of the suspended-core for the sake of achieving high sensitivity and specific sensing of SARS-CoV-2 S2. The limit of detection (LOD) of the sensor is 26.8 pM. The proposed sensor has high sensitivity, satisfactory selectivity, and can be fabricated at low cost making it highly suitable for point-of-care testing and high-throughput detection of early stage of COVID-19 infection.

Fiber Bending Sensor With Turning Point in a Multimode Fiber Peanut-Like Structure

In this paper, we experimentally demonstrated a bending sensor based on Michelson interferometer (MI). The sensor was fabricated by fusion splicing single-mode fiber (SMF) and multimode fiber (MMF) spheres. The improved end-face packaging scheme was adopted to realize the measurement of bending. The advantage of MMF numerical aperture increases the mode field diameter and the number of modes. Fiber bending induces transition between modes, resulting in the generation and fade of fiber modes, achieving the emergence of bending turning points (BTPs). The BTP is defined as the point where the direction of interference dip shift changes due to fiber bending. By observing the direction of interference dip shift during bending, the degree of bending can be intuitively estimated. The simulation results verify the generation and fade of excited modes in the bending MMF peanut-like structure, which are in good agreement with the experimental results. When the sensor is installed in parallel and titled ways, due to the uneven force of the plate in the bending state, it shows the differences in sensitivity, BTP and initial bending response. Further, the BTP can be roughly adjusted by the MMF core diameter, and the deviation of the sensitivity is reflected in the length of the interference arm. Besides, compact size, low cost and low hysteresis make the sensor more competitive in harsh environment applications.

Hydroxyethyl cellulose sensitized SMDMS structure with optical fiber relative humidity and temperature simultaneous measurement sensor.

We have manufactured an intensity modulated optical fiber SMDMS sensor with hydroxyethyl cellulose (HEC) hydrogel coating for simultaneous measurement of RH and temperature. The SMDMS sensor was manufactured by splicing single-mode fiber (SMF), multi-mode fiber (MMF), dispersion compensation fiber (DCF), MMF, and SMF in sequence to form a structure of SMF + MMF + DCF + MMF + SMF (SMDMS). The cladding of MMFs and DCF were corroded by hydrofluoric acid (HF) and coated with HEC hydrogel to excite a strong evanescent field and increase the sensitivity of the SMDMS sensor. The adsorption of water molecules by HEC will cause a change in the effective refractive index of cladding mode, which will eventually change the intensity of the transmission spectrum. The experimental results indicate that the sensitivities are 0.507 dB/%RH and 0.345 dB/°C in the RH range of 30%-80% and temperature range of 10°C-50°C, respectively. At last, a dual-parameter measurement matrix is constructed based on the experimental results to achieve the simultaneous measurement of RH and temperature. The SMDMS sensor has the advantages of high sensitivity and good robustness, and has potential application prospects in daily life and other fields.

A Simple Combination Method of MZI and MI for Salinity Measurement

Generally, a sensing structure with high sensitivity has a small dynamic measurement range and vice versa. A simple combination method of Mach-Zehnder-interferometer (MZI) and a modal interference (MI) for salinity measurement is studied. It achieves the performance with high sensitivity and large measurement range. Here, the sensitivity of the MZI is higher than that of the MI by about two orders in magnitude. Then the MZI composed of large offset splicing single-mode fiber structure is adopted to work for high-sensitivity measurement of salinity. While the MI composed of a photonic crystal fiber (PCF) spliced with two single-mode fiber is adopted to extend the measurement range of the MZI structure. After proper data processing algorithm, it can meet the requirement of salinity measurement in seawater. According to experiments and data processing results, the sensitivity of proposed method could be up to -16602.8 nm/refractive index unit (RIU) in the range of 1.333-1.341, which is corresponding to 3.32056 nm/‰ in the salinity range of 0-40‰ at 25°C. The proposed method could effectively expand the measurement range of high-sensitivity MZI according to the requirement, and also has the potential for multi-parameter measurement.

Plug-in label-free optical fiber DNA hybridization sensor based on C-type fiber Vernier effect

A reflective Fabry-Perot interference (FPI) label-free biosensor based on Vernier effect is designed and implemented for in situ real-time DNA hybridization detection. The sensor is made by splicing single-mode fiber (SMF), a segment of C-type fiber and SMF. The Vernier effect occurs between the solid cavity FPI and the air cavity FPI. In our experiment, a high sensitivity of 10791.12 nm/RIU is obtained in the refractive index (RI) range of 1.3311–1.3450. On this basis, we immobilized probe DNA (pDNA) on the surface of SMF to detect the complementary DNA (cDNA), demonstrating use for label-free sensing of DNA hybridization. The experimental results show that the sensor can detect 1 µM cDNA with high specificity. The paper demonstrates the first research of optical fiber DNA biosensor based on the open microcavity FPI Vernier effect. In addition, the sensor has the advantages of high sensitivity, strong stability, simple structure and simple fabrication. It can also be applied to other biomolecule detection based on probe-target biological binding events, which has great application potential in medical diagnostics, food science, genetic engineering and other biological fields.

3D-Printed Tilt Sensor Based on an Embedded Two-Mode Fiber Interferometer

A 3D-printed tilt fiber sensor using a two-mode fiber interferometer (TMFI) as the sensing mechanism has been demonstrated. The TMFI was constructed by splicing single-mode fibers (SMF-28s) at both ends of a two-mode fiber (TMF) to generate a comb-like interference spectrum that is sensitive to bending. A 3D-printed cantilever with a weight attached to one end was used to induce bending as the structure was tilted and by embedding the TMFI onto the 3D printed cantilever, different tilt angles can be measured, as a result of the bending of the fiber. The TMFI exhibited a linear response towards the tilt angles, $\theta $ , with a responsivity of $1\times 10^{-2}$ nm/deg at negative $\theta$ (0 to −90°) and $5 \times 10^{-3}$ nm/deg at positive $\theta$ (0 to 90°), respectively. The sensor was able to detect small angle changes in increments as small as 1° and it performs better than embedded FBG sensors. The tilt sensor proposed has a small formfactor and simple design, as well as being cost-effective and light-weight, thus showing significant potential for a variety of civil engineering applications.

Temperature and Refractive Index-Independent Mode Converter Based on Tapered Hole-Assisted Dual-Core Fiber

A compact mode converter (MC) has been demonstrated based on tapered hole-assisted dual-core fiber (HADCF). The MC is fabricated by splicing single mode fiber (SMF) and a piece of HADCF that is composed of a central single mode core (SMC), an eccentric few mode core (FMC), and a large eccentric air hole. The HADCF is tapered in a controllable manner and the fundamental mode LP01 of the central core is coupled to the high-order LP11 mode of the eccentric core as two modes satisfy the phase matching condition. The high-order LP11 mode can be converted effectively across the range from O-band to C-band by only changing the tapered length. With the decreasing taper ratio, the mode conversion wavelength has a blue shift. The measured purity of the LP11 modes in the range from 1310 to 1550 nm are always higher than 95%. In addition, the tapered HADCF-MC is insensitive to the change of the temperature and external refractive index.

Highly Sensitive Reflective Fabry–Perot Magnetic Field Sensor Using Magnetic Fluid Based on Vernier Effect

A highly sensitive reflective Fabry-Perot (FP) magnetic field sensor using magnetic fluid (MF) based on the Vernier effect is presented and experimentally verified. Capillary action acts as an injection of MF into an FP cavity, which is composed of a single-mode fiber (SMF), capillary glass tube, and a short section of SMF coated with gold film. Vernier effect is formed by the MF cavity and air cavity in the FP magnetic field sensor, where the surface of MF is concave. The refractive index (RI) of MF depends on the magnetic field, and therefore, the wavelength of the reflection spectrum of the fabricated sensor can be sensitive to the magnetic field. The magnetic field sensor's sensitivity is up to 1.02602 nm/Gs from 118.768 to 166.261 Gs, and the resolution is 0.0078 Gs. The sensor shows good linearity within the range of 71.317-231.751 Gs, and the linearity is larger than 0.999. In this article, a new type FP cavity fabricated by splicing SMF inside the capillary glass tube is proposed, and MF can be injected into the FP cavity, which can also be applied to other liquids to generate Vernier effect. The concave surface of MF is creatively applied to the sensors, and the experimental verification is carried out in a capillary glass tube, which provides a basis on the capillary action of other fluids in fiber sensing.

A novel fiber in-line Michelson interferometer based on end face packaging for temperature and refractive index measurement

A novel fiber in-line Michelson interferometer based on end face packaging fabricated by fusion splicing single mode fiber (SMF) and hollow core silica tube (HCST) is proposed and experimentally demonstrated for measurements of refractive index (RI) and temperature. This proposed fiber end face packaging scheme enables in-line Michelson interferometer to sense the liquid refractive index and protect fiber ends from influences of external liquid. Temperature and refractive index (RI) can be demodulated by utilizing the phase-shift tracking scheme in the Fast Fourier Transform (FFT) spectra with advantages of low noise and high resolution. Experimental results show that the refractive index and temperature sensitivities are 8.1498rad/RIU (the RI range of 1.331–1.387) and −0.05rad/°C (the temperature range of 20 °C–90 °C), respectively. In addition, the proposed sensor is attractive due to its simple production process, compact size and low cost.

Temperature-Independent Micro-Refractometer Based on Cascaded In-Fiber Air Cavities With Strain-Error Correction

We proposed a new fabrication technology of in-fiber air-cavity by splicing single-mode fiber and hollow core fiber (HCF) filled with the glycerol solution in this paper. A fiber-optic interferometer composed of two cascaded ellipsoidal in-fiber air cavities was fabricated by the above-mentioned method. The two air cavities are the mode splitter and the mode combiner of the transmission-type multimode interferometer (MMI), respectively. The collapsed HCF with the length of $973~\mu \text{m}$ between them can be used as the refractive index (RI) sensing area of the MMI, and the temperature sensitivity of the MMI can be ignored compared with the RI sensitivity. Owing to the RI insensitivity and temperature insensitivity, the air cavity with the minimum wall thickness of $6.2~\mu \text{m}$ is used as the reflection-type Fabry–Perot interferometer to compensate the strain of the MMI. Research shows that the sensor can eliminate the effects of temperature and strain in the measurement of RI.

Switchable Dual-Wavelength Cylindrical Vector Beam Generation From a Passively Mode-Locked Fiber Laser Based on Carbon Nanotubes

Cylindrical vector beams (CVBs) with axial symmetry in both polarization and field intensity have attracted much attention because of their unique optical properties. Conventional methods to obtain CVBs including direct modulation of light beams in free space and high-order mode excitation by offset splicing single-mode fiber with few-mode fiber usually works at single wavelength with rather narrow bandwidth. Here, for the first time to the best of our knowledge, we demonstrate switchable dual-wavelength CVB generation from a passively mode-locked fiber laser using carbon nanotubes as saturable absorber for mode-locking and a home-made mode-selective coupler as both mode converter and birefringence filter. In experiments, the mode-locked fiber laser delivers CVB pulses of dual-wavelength (1532.5 nm and 1555.5 nm) and corresponding single wavelength with duration of hundreds of femtosecond, respectively. Moreover, the output polarization status is switchable between radially and azimuthally polarized states. The mode-locked CVBs with wavelength and polarization flexibility may have potential applications in mode-division multiplexing optical fiber communication, nanoparticle manipulation, material processing, nonlinear optics, and so on.

Design and fabrication of a heterostructured cladding solid-core photonic bandgap fiber for construction of Mach-Zehnder interferometer and high sensitive curvature sensor.

A heterostructured cladding solid-core photonic bandgap fiber (HCSC-PBGF) is designed and fabricated which supports strong core mode and cladding mode transmission in a wide bandgap. An in-line Mach-Zehnder interferometer (MZI) curvature sensor is constructed by splicing single mode fibers at both ends of a HCSC-PBGF. Theoretical analysis of this heterostructured cladding design has been implemented, and the simulation results are consistent with experiment results. Benefiting from the heterostructured cladding design, an enhanced curvature sensing sensitivity of 24.3 nm/m-1 in the range of 0-1.75 m-1 and a high quality interference spectrum with 20 dB fringe visibility are achieved. In order to eliminate the interference of longitudinal strain and transverse torsion on the result of the curvature sensing experiment, we measure the longitudinal strain and transverse torsion sensing properties of HCSC-PBGF, and the results show that the impact is negligible. It is obvious that this high-sensitivity and cost-effective all fiber sensor with a compact structure will have a promising application in fiber sensing.

Multipoint fiber-optic laser-ultrasonic actuator based on fiber core-opened tapers.

In this study, a novel fiber-optic, multipoint, laser-ultrasonic actuator based on fiber core-opened tapers (COTs) is proposed and demonstrated. The COTs were fabricated by splicing single-mode fibers using a standard fiber splicer. A COT can effectively couple part of a core mode into cladding modes, and the coupling ratio can be controlled by adjusting the taper length. Such characteristics are used to obtain a multipoint, laser-ultrasonic actuator with balanced signal strength by reasonably controlling the taper lengths of the COTs. As a prototype, we constructed an actuator that generated ultrasound at four points with a balanced ultrasonic strength by connecting four COTs with coupling ratios of 24.5%, 33.01%, 49.51%, and 87.8% in a fiber link. This simple-to-fabricate, multipoint, laser-ultrasonic actuator with balanced ultrasound signal strength has potential applications in fiber-optic ultrasound testing technology.

Temperature-insensitive fiber optic Fabry-Perot interferometer based on special air cavity for transverse load and strain measurements.

We experimentally demonstrate transverse load and strain sensing based on a fiber optic Fabry-Perot interferometer (FPI) with special air cavity, which was created by fusion splicing single mode fiber (SMF), hollow core fiber (HCF) and several electrical arc discharges. The cavity height of this structure is higher than the cladding diameter of SMF so that it can sense transverse load with high sensitivity. The transverse load sensitivity of this air cavity FPI sensor is 1.31 nm∕N and about 5 times more sensitive compared to the current fiber tip interferometer (0.2526 nm∕N). Meanwhile, this sensor also can measure strain and the strain sensitivity of 3.29 pm∕με is achieved. In addition, the low temperature sensitivity (1.08 pm/°C) of the sensor can reduce the temperature-induced measurement error. This novel air cavity FPI can be developed and used as high-sensitivity transverse load and strain sensor with temperature-insensitive.

In-fiber modal interferometer for simultaneous measurement of curvature and temperature based on hollow core fiber

An in-fiber modal interferometer is presented and experimentally demonstrated for simultaneous measurement of curvature and temperature. The sensing part is fabricated by splicing single mode fiber (SMF) and hollow core fiber (HCF) via two abrupt tapered joints. Light couples to the wall of HCF due to the collapse in abrupt taper region and then modal interference occurs among multiple modes. Not only intensity but also wavelength of the dip around 1556nm linearly responses to the curvature and temperature change thus a sensitivity matrix could be built to demodulate both curvature and temperature simultaneously. In addition, the interference dip around 1540nm performs a decrease trend and the highest curvature sensitivity of 5.05dB/m−1 is achieved in a wide range from 0.765m−1 to 3.423m−1.