Side-hole Fiber Research Articles

Fatigue and residual strain monitoring for thermoplastic composite using embedded FBG inscribed in highly‑birefringent side-hole elliptical core optical fiber

Monitoring of thermoplastic carbon reinforced composites is critical for the safe operation and optimization of composite structures. Strain measurement during their manufacturing and operation is often achieved with embedded Fiber Bragg Gratings (FBG). However, conventional FBGs can only measure axial strain. To measure multiaxial strain in thermoplastic composites, FBGs inscribed in highly-birefringent side-hole elliptical core optical fiber (SH2 FBG) are proposed. SH2 FBG were used to monitor composite produced by automated tape laying process, first − to measure residual in-plane and out-of-plane strain during the manufacturing stage, second − in plane transverse strain during cyclic loading. It was demonstrated that the sensors can be used to measure multiaxial residual strain, observe changes in material stiffness, and calculate the area of the mechanical hysteresis loop. As a result − it was shown that embedding SH2 FBG can be an effective and robust method for monitoring the carbon fiber reinforced thermoplastic composite.

Cascaded vernier effect optic fiber temperature sensor with DSHF-Based MZI and FPI

This paper introduces a optical fiber temperature sensor based on vernier effect. The sensor's design consists of two integrated interferometric structures: the Mach-Zehnder interferometer (MZI) and the Fabry-Perot interferometer (FPI). Both interferometric structures exhibit symmetry, but they employ different types of optical fibers. Specifically, the MZI employs dual side hole fiber (DSHF), while the FPI is constructed using hollow-core optical fibers.Given the MZI's superior sensitivity to temperature changes compared to the FPI, it is chosen as the sensing arm. The paper conducts theoretical research and experimental validation of the optical fiber sensor's sensing principles. The findings reveal a sensor sensitivity of −305.42 pm/°C when measuring temperatures within the 32–50 °C range. Stability testing, repeatability testing, and response time testing are carried out using a constant temperature chamber and a water bath, affirming the sensor's excellent repeatability and stability, along with a response time faster than that of conventional thermometers.

Compact Hybrid Interferometer Based on Twin- Core Side Hole Fiber and Harmonic Vernier Effect for Gas Pressure Measurement

Compact Hybrid Interferometer Based on Twin- Core Side Hole Fiber and Harmonic Vernier Effect for Gas Pressure Measurement

Highly birefringent side-hole fiber Bragg grating for high-temperature pressure sensing.

We demonstrate a novel, to the best of our knowledge, high-temperature pressure sensor based on a highly birefringent fiber Bragg grating (Hi-Bi FBG) fabricated in a dual side-hole fiber (DSHF). The Hi-Bi FBG is generated by a femtosecond laser directly written sawtooth structure in the DSHF cladding along the fiber core through the slow axis (i.e., the direction perpendicular to the dual-hole axis). The sawtooth structure serves as an in-fiber stressor and also generates Bragg resonance due to its periodicity. The DSHF was etched by hydrofluoric acid to increase its pressure sensitivity, and the diameter of two air holes was enlarged from 38.2 to 49.6 µm. A Hi-Bi FBG with a birefringence of up to 1.8 × 10-3 was successfully created in the etched DSHF. Two distinct reflection peaks could be observed by using a commercial FBG interrogator. Moreover, pressure measurement from 0 to 3 MPa at a high temperature of 700°C was conducted by monitoring the birefringence-induced peak splits and achieved a high-pressure sensitivity of -21.2 pm/MPa. The discrimination of the temperature and pressure could be realized by simultaneously measuring the Bragg wavelength shifts and peak splits. Furthermore, a wavelength-division-multiplexed (WDM) Hi-Bi FBG array was also constructed in the DSHF and was used for quasi-distributed high-pressure sensing up to 3 MPa. As such, the proposed femtosecond laser-inscribed Hi-Bi FBG is a promising tool for high-temperature pressure sensing in harsh environments, such as aerospace vehicles, nuclear reactors, and petrochemical industries.

Distributed and polarimetric pressure sensitivity in spun highly birefringent optical fibers.

We examine experimentally the influence of the fiber inelastic twist on polarimetric sensitivity to hydrostatic pressure and pressure sensitivity in a Rayleigh-scattering-based optical frequency-domain reflectometer (OFDR) for highly birefringent side-hole fibers. The fibers were drawn from the same preform with different spin pitches varying from 5 mm to 200 mm. We also demonstrate that the sensitivities of spun fiber can be estimated analytically based on its spin pitch, measured birefringence, and the sensitivity of the corresponding non-twisted fiber, showing good agreement with the experimental results. We proved that polarimetric sensitivity to pressure decreases with the shorter spin pitches, while distributed pressure sensitivity decreases for the one polarization eigenmode and increases for the second polarization eigenmode. Therefore, the spun-fibers can operate well as the polarimetric sensors of other physical parameters inducing circular birefringence even under varying pressure. Moreover, they can be used to obtain the desired difference between sensitivities for both polarization eigenmodes.

An Ultrasensitive Gas Pressure Sensor Based on Single-Core Side-Hole Fiber With Optical Vernier Effect

An ultrasensitive optical fiber gas pressure sensor with the Optical Vernier effect (OVE) is proposed based on fusion splicing a piece of the Single-core and Side hole (SCSHF) fiber between the No-core fiber (NCF) and the hollow core fiber (HCF). The fabrication process only refers to the operation of fusion splicing, which is suitable for mass production. The theoretical sensing principles of the proposed sensor are explicitly elucidated in this paper. The sensor can be used as a probe in a narrow place because it works based on the reflected mode. The experimental results demonstrated that the sensor could detect the gas pressure change with a sensitivity of ∼183 nm/MPa in the range of 0∼100 kPa. Repeated experiments are also conducted to verify the sensor's excellent sensing repeatability and long-term stability. In addition, the sensor possesses a low-temperature cross-sensitivity of ∼2.4 kPa/℃. Thus, the proposed sensor is expected to leverage its practical functionalities in sensing areas requiring high sensitivity, a simple fabrication process, and long-term stability.

Fiber Optic Temperature Sensor System Using Air-Filled Fabry-Pérot Cavity with Variable Pressure.

We report a high-resolution fiber optic temperature sensor system based on an air-filled Fabry-Pérot (FP) cavity, whose spectral fringes shift due to a precise pressure variation in the cavity. The absolute temperature can be deduced from the spectral shift and the pressure variation. For fabrication, a fused-silica tube is spliced with a single-mode fiber at one end and a side-hole fiber at the other to form the FP cavity. The pressure in the cavity can be changed by passing air through the side-hole fiber, causing the spectral shift. We analyzed the effect of sensor wavelength resolution and pressure fluctuation on the temperature measurement resolution. A computer-controlled pressure system and sensor interrogation system were developed with miniaturized instruments for the system operation. Experimental results show that the sensor had a high wavelength resolution (<0.2 pm) with minimal pressure fluctuation (~0.015 kPa), resulting in high-resolution (±0.32 ℃) temperature measurement. It shows good stability from the thermal cycle testing with the maximum testing temperature reaching 800 ℃.

Deep Learning Assisted Pressure Sensing Based on Sagnac Interferometry Realized by Side-Hole Fiber

In this paper, a novel deep learning characterization approach is proposed and demonstrated experimentally using a pressure sensor based on a Sagnac interferometer realized by a side-hole fiber. To enlarge the measurement range limited by the free spectral range, a long short-term memory (LSTM) neural network was proposed. The original spectra were recorded by a low-cost spectrometer, and the intensity of scaled spectra was used to construct one-dimension (1D) spectral data. The results show that the coefficient of determination ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R²</i> ) for the pressure prediction can reach 0.9996148 with the root mean square error (RMSE) equal to 2.559 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−2</sup> MPa. Moreover, two-dimension (2D) data were obtained with the ascension algorithm of the Gramian angle field (GAF). A better <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R²</i> of 0.9999908 and a lower RMSE of 4.365×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−3</sup> MPa can be obtained since the ascension algorithm could retrieve deeper features among the spectral data. The proposed approach can be adopted for a similar sensor structure, showing great potential in sensing applications requiring low cost and robustness.

Salinity and Temperature Dual-Parameter Sensor Based on Fiber Ring Laser with Tapered Side-Hole Fiber Embedded in Sagnac Interferometer.

This paper presented a new kind of salinity and temperature dual-parameter sensor based on a fiber ring laser (FRL) with tapered side-hole fiber (SHF) embedded in a Sagnac interferometer. The sensing structure is majorly composed of tapered SHF located in the middle of SHF inside the Sagnac interferometer loop structure. The influences of the SHF's diameters of different tapered in the Sagnac interferometer loop on the FRL sensing system are studied. The presence of air holes in the SHF makes the cladding mode easier to excite, and the interaction between the cladding mode with its surroundings is enhanced, thus having higher salinity sensitivity. Besides, the unique advantages of high resolution, narrower linewidth, and high signal-to-noise ratio (SNR) of fiber laser make the measurement results more accurate. In this experiment, the SHF with different taper diameters was made, and it was found that reducing the diameter of the taper waist diameter could further improve the salinity sensitivity. When the waist diameter was 9.70 μm, the maximum salinity sensitivity of 0.2867 nm/‱ was achieved. Temperature sensing experiments were also carried out. The maximum temperature sensitivity of the FRL sensing system was -0.3041 nm/°C at the temperature range from 20 to 30 °C. The sensor has the characteristics of easy manufacture, good selectivity, and high sensitivity, proving the feasibility of simultaneous measurement of seawater salinity and temperature.

Strain sensing with parallel air-cavity Fabry-Perot interferometers based on Vernier Effect

We proposed and investigated Vernier Effect (VE) based strain sensor, which comprises two parallel air cavity Fabry-Perot Interferometers (FPI). The sensor involves simple fabrication techniques, such as cleaving and fusion splicing. A small section of Double Side Hole Fiber (DSHF) is cleaved and spliced between two pieces of Single Mode Fiber (SMF). The core of the SMF-1 and the air core of the DSHF are aligned by a minor lateral offset to make an air cavity FPI. During the strain measurement, the sensor exhibits the strain sensitivity of 36.7 pm/με. The sensor improves the sensitivity with the highest amplification factor of 10. In addition, the proposed strain sensor shows the temperature sensitivity of only 1 pm/°C in the temperature range of 30 °C–100 °C, which has temperature cross-sensitivity of 2.72 × 10−2 με/°C, and appear to be insensitive to temperature. The proposed device has high strain sensitivity, low temperature cross-sensitivity, simple fabrication, and low cost. Therefore, it has many applications in different engineering fields, for example, structural health monitoring and civil engineering.

Gel Point Determination in Resin Transfer Molding Process with Fiber Bragg Grating Inscribed in Side-Hole Elliptical Core Optical Fiber.

Material as well as process variations in the composites industry are reasons to develop methods for in-line monitoring, which would increase reproducibility of the manufacturing process and the final composite products. Fiber Bragg Gratings (FBGs) have shown to be useful for monitoring liquid-composite molding processes, e.g., in terms of online gel point detection. Existing works however, focus on in-plane strain measurements while out-of-plane residual strain prevails. In order to measure out-of-plane strain, FBG inscribed in highly birefringent fiber (HB FBG) can be used. The purpose of this research is the cure stage detection with (a) FBG inscribed in single mode and (b) FBG inscribed in highly-birefringent side-hole fiber in comparison to the reference gel point detected with an in-mold DC sensor. Results reveal that the curing process is better traceable with HB FBG than with regular FBG. Thus, the use of HB FBG can be a good method for the gel point estimation in the RTM process.

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.

All-fiber biological detection microfluidic chip based on space division and wavelength division multiplexing technologies.

To further reduce the size of a microfluidic detection chip and the sample consumption and to shorten the chip manufacturing cycle, an all-fiber SPR detection multichannel microfluidic chip was proposed and demonstrated in this paper. The microfluidic channel of the proposed chip was provided by the air channel of a double side-hole fiber, the detection unit was fabricated using a dumbbell fiber with a fiber core exposed to air, and the sensing probe was composed and packaged by fiber micro-processing technology. The internal double channels of the fiber constructed from double side-hole and dumbbell fibers can realize dual channel detection based on space division multiplexing. 30 nm silver and 50 nm gold films were respectively coated on the left and right sides of the dumbbell fiber, which can realize the dual channel simultaneous detection based on wavelength division multiplexing. We employed the proposed microfluidic chip to detect immunoglobulin G and dopamine molecules, where the average sensitivity is 0.252 nm (mg mL-1)-1 and 0.061 nm (μg mL-1)-1, and the LOD is 0.397 mg mL-1 and 1.639 μg mL-1, respectively. The microfluidic channel and detection unit of all-fiber multi-channel SPR detection microfluidic chip are provided by a soft and flexible fiber, which is compact in structure, flexible in fabrication and short in manufacturing cycle, making it possible for the microfluidic chip to enter the human body for detection and enabling a new approach for the fabrication of wearable detection microfluidic devices. This provides a new idea for the development of microfluidic chips.

Fabrication and Refractive Index Sensing Characteristics of Exposed-Core Side-Hole Fiber Bragg Grating

Fabrication and Refractive Index Sensing Characteristics of Exposed-Core Side-Hole Fiber Bragg Grating

Ultrahigh-sensitivity temperature sensor based on resonance coupling in liquid-infiltrated side-hole microstructured optical fibers

A compact fiber-optic temperature sensor based on refractive index liquid (RIL) functionalized side-hole microstructured optical fibers (SHMOFs) is proposed and experimentally demonstrated. Transmission spectral characteristics of the proposed temperature sensor have been investigated from experimental as well as theoretical perspectives. Due to the resonance coupling between the fundamental core mode and RIL rod modes, multiple resonance dips are present in the transmission spectrum, which could be tuned by adjusting environmental temperature. Experimental results indicate that the highest temperature sensitivity reaches − 13.1 nm/°C for a temperature range of 80–100 °C. Hence, the SHMOF-based temperature sensor has several desirable merits such as ultrahigh sensitivity, wide measurement range and ease of fabrication, which make it a promising candidate for various applications in environmental monitoring, industrial and agricultural production.

In-Plane Strain Measurement in Composite Structures with Fiber Bragg Grating Written in Side-Hole Elliptical Core Optical Fiber.

In this paper, the application of a fiber Bragg grating written in a highly birefringent side-hole elliptical core optical fiber for two-axial strain measurement is presented. Hybrid optical fiber structures achieved by combining large side-holes and elliptical core result in a very high birefringence of 1 × 10−3 and thus high initial Bragg peak spectral separation of 1.16 nm, as well as a very high transverse force sensitivity, of up to 650 pm/(N/mm) or even −1150 pm/(N/mm), depending on the fiber orientation with respect to the applied force. Due to the ~22 %m/m GeO2 concentration in the core the fiber being highly photosensitive, which significantly simplifies FBG fabrication by UV illumination without the need for prior hydrogen loading, which worsens thermal stability. Finally, the developed FBGs written in the highly birefringent side-hole elliptical core optical fiber were embedded in the square composite plates and applied for strain measurements. Tests of two-directional four-point bending have shown usability of such FBG for two-axial in-plane strain measurement with a single FBG in iso-thermal conditions.

Temperature and Strain Sensing With Hybrid Interferometer

A hybrid interferometer for simultaneous measurement of strain and temperature is proposed and investigated experimentally. This hybrid design is composed of Fabry-Perot interferometer (FPI) and Michelson Interferometer (MI) cascaded with each other. It is developed by fusion splicing a single mode fiber (SMF), a multimode fiber (MMF), a dual side hole fiber (DSHF), a hollow core fiber (HCF) and a tapered-SMF. The tapered SMF was inserted into the HCF to form a reflection mirror for the FPI. The maximum temperature and strain sensitivity of the hybrid interferometer achieved through experiments is 11.6 pm/°C and 6.8 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> , respectively. The different sensitivities of FPI and MI to temperature and strain enable us to achieve simultaneous measurement. The proposed hybrid interferometer sensor has many attractive features such as, novel design, low cost, easy fabrication, compact size, and good sensitivity. Therefore, the proposed hybrid sensor could be widely deployed in plenty of applications, for instance, structural health monitoring, civil engineering, food manufacturing, chemical and medical fields.

Highly sensitive salinity sensor based on Mach-Zehnder interferometer with double-C fiber

This paper proposes a highly sensitive, compact, and low-cost optical fiber salinity sensor based on the Mach-Zehnder interferometer. The sensor is constructed using a single mode fiber (SMF) - no-core fiber - double-C fiber (DCF) - NCF-SMF structure, with the DCF prepared by etching the dual side-hole fiber with HF acid. The DCF's large-size exposed microfluidic channels solve the previous microstructured optical fiber's challenging liquid filling and replacement problems. Theoretical simulations and experiments demonstrate that the sensor is suitable for high-sensitivity salinity measurement. The sensor exhibits a high salinity sensitivity of -2.26 nm/‰ in the salinity range of 10‰-50‰, as demonstrated by the experimental results. Additionally, the sensor exhibits some fascinating characteristics, including high repeatability, hysteresis, reversibility, and stability.

Simultaneous Measurement of Temperature and Magnetic Field Based on Ionic-Liquid-Infiltrated Side-Hole Fibers

A dual-parameter fiber-optic sensor based on magnetic ionic liquids (MILs) functionalized side-hole fibers (SHFs) has been proposed and experimentally demonstrated for simultaneous measurement of temperature and magnetic field intensity. Multiple transmission dips are present in the transmission spectrum due to resonance coupling between the fundamental core mode and high-order modes in the MIL inclusion. The wavelength shifts of these resonance dips are highly sensitive to the environmental temperature and applied magnetic field intensity, providing an ideal sensing platform for simultaneous measurement of the above two physical parameters. Experimental results indicate that our proposed sensor exhibits temperature and magnetic field sensitivities up to −3.466 nm/°C and −0.0303 nm/Oe for the measurement ranges of 25 °C to 100 °C and 20 Oe to 200 Oe, respectively. The MILs-infiltrated SHFs sensor paves new way toward developing photonic devices for temperature/magnetic field dual-parameter sensing and high-precision wavelength-tuning based on functional materials.

Bovine serum albumin detection using side-hole fiber sensors

This paper proposes a biosensor based on side-hole fibers (SHF) formed by the structure of single-mode fiber (SMF)-coreless fiber (CLF)- SHF-CLF-SMF for the detection of bovine serum albumin (BSA). Among the fibers, the SHF’s misalignment splicing makes one hole exposed, which is very sensitive to the change of the external environment. Due to the unique structure of the SHF, two Mach-Zehnder interferences (MZI) are generated in this fiber sensor, which further improves the sensitivity of the sensor. Experiments show that the refractive index (RI) sensitivity of this sensor reaches 13421.638 nm/RIU, and for the BSA concentration detection, the sensitivity reaches 2.413 nm/(mg/ml). The proposed biosensor is simple, easy to manufacture, cost-effective, and can be used for many chemical and biological analyses.