Dual Side-hole Fiber Research Articles

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.

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.

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.

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 temperature and bending sensor based on Fabry-Perot interferometer with Vernier effect

We proposed and verified a sensor based on Fabry-Perot interferometer with Vernier effect for the simultaneous measurement of temperature and bending. The sensor mentioned is composed of eccentric-core fiber (ECF) and dual-side-hole fiber (DSHF). The ECF and DSHF are sequentially connected to form two cascaded Fabry-Perot interferometers (FPIs). The free spectral range (FSR) of the two interferometers are approximately equal, the Vernier effect can be occurs. Experimental results show that the temperature sensitivities of the high-frequency spectrum and the envelope spectrum are 6.76 pm/℃ and 57.2 pm/℃, which are amplified by about 8.46 times. In addition, the sensor of the structure has no obvious effect on the amplification of the bending sensitivity with the influence of the Vernier effect. Therefore, the amplification effect of the sensor temperature and the bending sensitivity can be simultaneously achieved.

Temperature-insensitive directional transverse load sensor based on dual side-hole fiber Bragg grating.

We propose and demonstrate a temperature-insensitive directional transverse load sensor based on a fiber Bragg grating (FBG) inscribed in a section of dual side-hole fiber (DSHF). The application of transverse load results in an effective change in the refractive index and, consequently, changes in the DSHF birefringence. The directional transverse load response of the fabricated DSH-FBG was studied by monitoring the wavelength separations with transverse load applied in different direction with 15° increments. The load sensitivity exhibited two maxima and two minima in a polar coordinate system, achieving a maximum value of 699 pm/(N/mm) for transverse load applied along the slow axis and a minimum value of 285 pm/(N/mm) for transverse load applied along the fast axis. Subsequently, a finite element analysis (FEA) was conducted to simulate the resulting strain distribution of the DSHF with applied directional transverse load. The temperature response of the DSH-FBG transverse load sensor was also tested, yielding a low sensitivity of 1.5 × 10-2 pm/°C. Hence, the compact size, directional transverse load sensitivity, and temperature insensitivity of this device make it suitable for intelligent transverse load monitoring.

A Pre-Twisted Taper in Dual-Side Hole Fiber for Torsion Measurement With High Sensitivity

We propose and demonstrate a novel torsion sensor based on a fiber Mach-Zehnder interferometer (MZI) in this paper. The MZI consists of a short section of dualside hole fiber (DSHF) sandwiched between two single-mode fibers, in which the DSHF is twisted and tapered by a hydrogen oxygen flame to form a spiral structure in the MZI. The spiral structure makes MZI not only sensitive to torsion, but also has the ability to distinguish the direction of torsion. Experimental results show that the torsion sensitivity of the interferometer is as high as -1.67 nm/(rad·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) and 1.65 nm/(rad·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> ) in the clockwise (CW) and counterclockwise (CCW) rotations, respectively. Furthermore, the temperature sensitivity of the structure is 7.66 pm/°C, corresponding to a low temperature cross-sensitivity of 0.008 rad·m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> /°C. The proposed structure is particularly suitable for the situations where high measurement sensitivity is required and the torsion direction can be identified.

Highly sensitive torsion senor based on dual-side-hole fiber Mach-Zehnder interferometer.

We experimentally demonstrate a highly sensitive directional torsion sensor based on a helical dual side-hole fiber (DSHF). The sensor is constructed by splicing a fraction of DSHF in between two short lengths of multi-mode fibers (MMFs), which forms a Mach-Zehnder interferometer (MZI) based on the interference between the core and cladding modes in the DSHF. The middle part of the DSHF is heated and twisted to get a spiral cladding, so that the cladding mode in the DSHF travels along a helical path. The introduction of the helical structure can greatly improve the torsion sensitivity of the senor and enables it to recognize the twist direction. Experimental results show that the torsion sensitivity can reach as high as 1.666 nm/(rad/m) and -1.413 nm/(rad/m) in the range of 0 ± 5.517 rad/m. Besides, strain sensitivity of the sensor is about -4.5 pm/µɛ.

Bending sensor with parallel fiber Michelson interferometers based on Vernier-like effect

A highly sensitive bending sensor based on two parallel fiber Michelson interferometers (MIs) with a Vernier-like effect is proposed and demonstrated. The interferometric structure consists of a dual-core fiber (DCF) spliced with a short section of dual-side-hole fiber (DSHF). The centric core of the DCF is aligned with one air-hole of the DSHF to form a reflective interface. The centric core of the DCF is connected to the core of the DSHF with a little lateral offset to make the light in the centric core couple to both the core and cladding of the DSHF. The light beams reflected by the end of the DSHF and the DCF interfere with each other, forming two parallel MIs. The free spectral range (FSR) difference between the two MIs is very small due to the tiny difference in refractive index (RI) between the core and cladding of the DSHF, which results in a Vernier-like effect. The most novel feature of the mentioned structure is that both MIs are sensitive to bending with the same sensitive direction. Experimental results indicate that the bending sensitivity of the sensor can reach 38.53 nm/m−1. This kind of sensor has far-reaching significance in the research of structural safety monitoring.

A high sensitivity gas pressure sensor based on femtosecond laser micro/nano processing

A compact and novel Mach-Zehnder interferometer used for gas pressure sensing is proposed and demonstrated experimentally. The sensor is constructed by fusion splicing a short piece of dual side-hole fiber (DSHF) in between two sections of multimode fibers (MMF). Interference occurs as the optical path length difference (OPLD) between light beams along air holes and silica of the DSHF. With the opening drilled on one air hole by femtosecond laser micro/nano processing, the filled gas directly affects light beam along this air hole, which results in the interference spectrum shift. Experimental results show that the sensor exhibits a high sensitivity of −8.58 nm/MPa with the increasing gas pressure. The sensor has potential application in gas pressure sensing for its advantages of high sensitivity, low temperature crosstalk and easy fabrication.

Highly Sensitive Vector Curvature Sensor Based on Two Juxtaposed Fiber Michelson Interferometers With Vernier-Like Effect

In this paper, a highly sensitive vector curvature sensor based on two juxtaposed fiber Michelson interferometers (MI) with Vernier-like effect has been proposed and verified. Different from the structures of the conventional Vernier effect which are composed of the cascaded elements, the proposed configuration consists of two juxtaposed MIs. A triple-core fiber (TCF) is connected with a quite short piece of dual-side-hole fiber (DSHF), and the core of the DSHF and the two eccentric cores of the TCF are respectively constituted two juxtaposed MIs. The Vernier-like effect is caused by the very small refractive index difference between the two eccentric cores of the TCF. The most outstanding feature of the proposed structure is that the two MIs are both sensitive to curvature with opposite sensitivities, which makes the curvature sensitivity magnification factor twice that of the conventional Vernier effect structure. Experimental results show that the curvature sensitivity of the structure can reach $-60.84~{\mathrm {nm/m}}^{-1}$ .

High sensitivity liquid level sensor based on dual side-hole fiber Mach–Zehnder interferometer

A novel highly sensitive liquid-level sensor based on an asymmetric dual side-hole fiber (DSHF) using the core mode as the sensing mode and the cladding mode as the reference mode is described. The side-hole located close to the fiber core is opened by polishing and used to access the core mode as the sensing mode, the side hole located furthest from fiber core being used to tightly confine the cladding mode as the reference mode. The small core size of the DSHF further enhances the sensitivity. Pure-water-level sensitivities of 1.496 nm/mm, 2.026 nm/mm and 4.019 nm/mm are demonstrated experimentally when the distance between the DSHF core and its proximal (opened) side hole are 1.75μm, 1.6μm and 1.1μm, respectively. The sensor also exhibits low-temperature cross-sensitivity of ∼0.013 mm/°C, ∼0.011 mm/°C and ∼0.006 mm/°C. The fabrication process in this case is not sensitive to the polishing process and its uniformity is determined mainly by the quality of the DSHF itself, which makes it possible to ensure repeatable fabrication. Such reliability of fabrication lends itself well to good reproducibility for potential multiple sensor unit manufacture.

Ultra-highly sensitive gas pressure sensor based on dual side-hole fiber interferometers with Vernier effect.

We have presented and demonstrated a fiber optic gas pressure sensor with ultra-high sensitivity based on Vernier effect. The sensor is composed of two integrated parallel Mach-Zehnder interferometers (MZIs) which are fabricated by fusion splicing a short section of dual side-hole fiber (DSHF) in between two short pieces of multimode fibers (MMFs). Femtosecond laser is applied for cutting off part of the MMF and drilling openings on one air hole of the DSHF to achieve magnified gas pressure measurement by Vernier effect. Experimental results show that the gas pressure sensitivity can be enhanced to about -60 nm/MPa in the range of 0-0.8 MPa. In addition, the structure possesses a low temperature cross-sensitivity of about 0.55 KPa/°C. This presented sensor has practically value in gas pressure detection, environmental monitoring and other industrial applications.

A Mach-Zehnder interferometer based on tapered dual side hole fiber for refractive index sensing

Abstract An in-line fiber Mach-Zehnder interferometer (MZI) based on tapered dual side hole fiber (DSHF) is proposed and fabricated for refractive index measurement. The MZI is composed of a short piece of tapered DSHF which is sandwiched between two single mode fibers (SMFs). The DSHF is heated and tapered by using an oxy-hydrogen flame. At the taper zone, the core mode of the lead-in SMF is coupled to several cladding modes of the DSHF, and interference happened between the core and cladding modes of the DSHF. Due to the existence of the air hole at the tapered DSHF zone, the cladding modes can be much more easily stimulated than general SMF. The experiments indicated that the proposed sensor has a high refractive index sensitivity of 816.1 nm/RIU in the refractive index range of 1.333–1.379 with the tapered DSHF diameter of 33 μm. In addition, the sensor had a low temperature response of 19.9 pm/°C in the range from 30 to 110 °C. The proposed sensor is attractive for its high refractive index sensitivity, and easy fabrication, making it of potential value for refractive index measurements for chemical and biological sensing.

Directional Bending Sensor Based on a Dual Side-Hole Fiber Mach–Zehnder Interferometer

A new directional bending sensor based on a dual side-hole fiber (DSHF) that can detect both the direction and magnitude of bending in a self-temperature compensated manner is described and experimentally demonstrated. The sensor is based on an in-fiber Mach-Zehnder interferometer (MZI), where a DSHF segment was spliced between two standard single-mode fibers (SMFs) as input and output. The sensor has two orthogonal axes which are formed by both the asymmetry in the side-hole location in DSHF and an offset in the fusion splice between the SMF and DSHF. The visibility in the MZI output intensity and the spectral shifts of MZI fringes show separate responses to the bending curvature and direction, which is a key feature of the directional curvature sensor. An inscribed fiber Bragg grating in the DSHF measure allows the temperature to be measured independently. The sensor can provide salient advantages in its unique capability to precisely quantify the direction and magnitude of bending along with its reproducibility, compactness, and suitability for mass production, which makes it suitable for many practical bending sensing applications.

Highly Sensitive Two-Axis Bending Sensor Based on Arc-Induced Long Period Fiber Grating in Dual Side-Hole Fiber

A highly sensitive two-axis bending sensor based on long period fiber grating (LPFG) in a dual side-hole fiber (DSHF) is presented and experimentally investigated. The LPFG is fabricated by periodically collapsing a piece of DSHF with automatic arc discharge technology. The existence of the two air holes in the DSHF makes the cladding modes, regardless of the polarization state, concentrate in the area perpendicular to the connection of the two holes. Such a feature leads to quite different bending responses at the directions perpendicular and parallel to the connection of the air holes, which makes the DSHF-based LPFG suitable for two-axis bending measurement. The bending sensitivity of the LPFG are 21.03 nm/m -1 and 15.77 dB/m -1 at the orthogonal directions, respectively. Besides, compared with the general arc-induced LPFGs in solid fibers, the size of the DSHF-based LPFG is effectively reduced because the periodic air hole collapse in the DSHF causes a large geometric deformation of the fiber core and increases coupling coefficient between the core and cladding modes. The sensing characteristics of strain, polarization, surrounding refractive index, and temperature are also investigated in our experiment.

An In-Fiber Dual Air-Cavity Fabry–Perot Interferometer for Simultaneous Measurement of Strain and Directional Bend

A simultaneous strain and directional bend sensor based on an in-fiber dual air-cavity Fabry–Perot interferometer (FPI) is presented and demonstrated. The hybrid structure is composed of a fraction of dual side-hole fiber fusion spliced in between a seven-core fiber and a multimode fiber. The bending sensitivities of the two air-cavity FPIs at two opposite most sensitive directions are 120.5 pm/ $\text{m}^{-1}$ and −122 pm/ $\text{m}^{-1}$ , 110.4 pm/ $\text{m}^{-1}$ , and −121.1 pm/ $\text{m}^{-1}$ , respectively. The strain sensitivities of the two FPIs are 1.59 pm/ $\mu \varepsilon $ and 1.52 pm/ $\mu \varepsilon $ , respectively. By using the method of difference between the two mutually independent air-cavity FPIs, the highest bending sensitivities can be improved to be 242.5 pm/ $\text{m}^{-1}$ and −231.5 pm/ $\text{m}^{-1}$ , respectively, and the axial strain effect can be eliminated simultaneously.