In-fiber directional coupler for high-sensitivity curvature measurement

Highly Coupled Seven-Core Fiber for Ratiometric Anti-Phase Sensing

A ratiometric fiber optic temperature sensor based on a highly coupled seven-core fiber (SCF) is proposed and experimentally demonstrated. A theoretical analysis of the SCF's sinusoidal spectral response in transmission configuration is presented. The proposed sensor comprises two SCF devices exhibiting anti-phase transmission spectra. Simple fabrication of the devices is shown by just splicing a segment of a 2 cm long SCF between two single-mode fibers (SMFs). The sensor proved to be robust against light source fluctuations, as a standard deviation of 0.2% was registered in the ratiometric measurements when the light source varied by 12%. Its low-cost detection system (two photodetectors) and the range of temperature detection (25 °C to 400 °C) make it a very attractive and promising device for real industrial applications.

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.

High-sensitivity vector bend sensor based on a fiber directional coupler inscribed by a femtosecond laser

In this Letter, we demonstrate a high-sensitivity vector bend sensor based on a fiber directional coupler. The fiber directional coupler is composed of two parallel waveguides inscribed within a no-core fiber (NCF) by a femtosecond laser. Since the two written waveguides have closely matched refractive indices and geometries, the transmission spectrum of the fiber directional coupler possesses periodic resonant dips. Such a fiber directional coupler exhibits a good bending-dependent spectral shift response due to its asymmetric structure. Experimental results show that bending sensitivities of -97.11 nm/m^-1 and 58.22 nm/m^-1 are achieved for the 0° and 180° orientations in the curvature range of 0-0.62 m^-1, respectively. In addition, the proposed fiber directional coupler is shown to be insensitive to external humidity changes, thus improving its suitability in high-accuracy bending measurements.

Optical curvature sensor based on polarization characteristics of optical fiber

Curvature measurement plays an important role in various applications. An optical curvature sensor based on polarization characteristics of optical fiber is proposed and verified by experiments. The direct bending of the fiber causes a change in birefringence, which results in a change of Stokes parameters of the transmitted light. The large curvature measurement range of tens to more than 100 m^-1 has been realized in the experiment. For micro bending, a cantilever beam structure is used to achieve a sensitivity of up to 12.26/ m^-1 and a linearity of 99.49% in the measurement range of 0 to 0.15 m^-1, with a resolution of up to 10^-6m^-1 order of magnitude, which reaches the level of the latest report. The method with the advantages of simple fabrication, low cost and good real-time performance gives a new development direction to the curvature sensor.

A Review of Sensitivity Enhancement in Interferometer-Based Fiber Sensors

Optical fiber sensors based on an interferometer structure play a significant role in monitoring physical, chemical, and biological parameters in natural environments. However, sensors with high-sensitivity measurement still present their own challenges. This paper deduces and summarizes the methods of sensitivity enhancement in interferometer based fiber optical sensors, including the derivation of the sensing principles, key characteristics, and recently-reported applications.The modal coupling interferometer is taken as an example to derive the five terms related to the sensitivity: (1) the wavelength-dependent difference of phase between two modes/arms ∂ϕd/∂λ, (2) the sensor length Lw,A, (3) refractive index difference between two modes/arms Δneff,A, (4) sensing parameter dependent length change α, and (5) sensing parameter dependent refractive index change γ. The research papers in the literature that modulate these terms to enhance the sensing sensitivity are reviewed in the paper.

Coupled-core fiber Bragg gratings for low-cost sensing

Sensors based on Bragg gratings inscribed in conventional single mode fibers are expensive due to the need of a sophisticated, but low-speed, interrogation system. As an alternative to overcome this issue, in this work, it is proposed and demonstrated the use of coupled-core optical fiber Bragg gratings. It was found that the relative reflectivity from such gratings changed when the coupled-core fiber was subjected to point or periodic bending. This feature makes the interrogation of such gratings simple, fast, and cost-effective. The reflectivity changes of the gratings are attributed to the properties of the supermodes supported by the coupled-core fiber. As potential applications of the referred gratings, intensity-modulated vector bending and vibration sensing are demonstrated. We believe that the results reported here can pave the way to the development of many inexpensive sensors. Besides, coupled-core fiber Bragg gratings may expand the use of grating technology in other areas.

Highly sensitive vector bending sensor based on an embedded multimode D-shaped LPFG

In this letter, a highly sensitive bending sensor based on an embedded multimode D-shaped long period fiber grating (EMD-LPFG) is proposed. The novel sensor is applied to carry out vector bending measurement. The proposed LPFG is fabricated by polishing on the prepared structure which is formed by periodically splicing between single mode fiber (SMF) and multimode fiber (MMF). Since the cross section of the embedded MMF is D-shaped, we named it EMD-LPFG. Due to the asymmetric modulation of the refractive index on the fiber by the CO₂ laser, the sensor has the ability to distinguish the bending directions, and the MMFs provide higher bending response. The experimental transmission spectrum can match the simulation results well. The experimental results show that the average bending sensitivities in three orthogonal directions are 70.21 nm/m^-1 (0°), 9.75 nm/m^-1 (90°), -12.04 nm/m^-1 (180°) and 9.98 nm/m^-1 (270°), respectively. Meanwhile, the temperature sensitivity is 30 pm/°C in the range of 25 °C to 75 °C. According to the ultra-compact structure with the total length of 2.5 mm, high bending sensitivity and ability to distinguish the bending direction, the novel sensor has potential in bending measurement.

Highly sensitive optofluidic refractive index sensor based on a seven-liquid-core Teflon-cladding fiber

We propose and theoretically demonstrate a highly sensitive optofluidic refractive index (RI) sensor based on a spectral filter formed by a segment of liquid-filled seven-hole Teflon-cladding fiber sandwiched by two standard single mode fibers (SMFs). When liquid flows through the air hole channels of the seven-hole Teflon-cladding fiber, it forms a seven-liquid-core fiber (SLCF) and the lightwaves are well guided by the liquid cores owing to total inner reflection. When the input SMF is aligned to the central core of the SLCF, the light excited in the central core will couple to outer cores periodically along the length of the SCLF. At the detection port, the output SMF is also aligned to the central core of the SLCF. Since the coupling coefficient depends on wavelength, the coupling efficiency is also wavelength dependent, leading to a filter spectrum for a given length of the SLCF. The spectral response of the filter to the change in RI of the liquid cores is numerically simulated based on the coupled-mode theory through finite-element method. The dependence of the RI sensitivity on the diameter and pitch of air holes of the SLCF are studied, respectively. Finally, a very high sensitivity of 25,300 nm/RIU for RI around 1.333 is achieved.

Stress homogenization effect in multicore fiber optic bending sensors

In this work we study the particular case of an optical fiber subjected to compression-bending load, the most common loading configuration for testing fiber optic bending sensors. Our analysis is based on the foundations of column theory and reveals a progressive stress homogenization across the optical fiber with increasing bending. This effect is general to any optical fiber subjected to this load configuration and it is of particular interest for structures with multiple cores since the state of stress experienced by each core can significantly differ even for a condition of constant load. The approach outlined here captures relevant features observed in experiments with multicore fiber optic bending sensors. Also, this approach can be incorporated into coupled-mode theory for assessing the performance of spectrally operated fiber sensors based on multicore coupled structures under realistic conditions commonly encountered in the experiments and without the need of performing computationally expensive simulations. The progressive stress homogenization, as well as the regime of homogeneous stress dominated by the bending contribution, is experimentally demonstrated using a multicore optical fiber with three coupled cores. Our observations are similar to those reported in recent experiments using other multicore fibers with different number of cores.

Few-mode fiber based distributed curvature sensor through quasi-single-mode Brillouin frequency shift

We proposed and demonstrated a few-mode fiber (FMF) based optical-fiber sensor for distributed curvature measurement through quasi-single-mode Brillouin frequency shift (BFS). By central-alignment splicing FMF and single-mode fiber (SMF) with a fusion taper, a SMF-components-compatible distributed curvature sensor based on FMF is realized using the conventional Brillouin optical time-domain analysis system. The distributed BFS change induced by bending in FMF has been theoretically and experimentally investigated. The precise BFS response to the curvature along the fiber link has been calibrated. A proof-of-concept experiment is implemented to validate its effectiveness in distributed curvature measurement.

Temperature- and strain-insensitive curvature sensor based on ring-core modes in dual-concentric-core fiber

We report on a high-performance curvature sensor based on a long-period grating (LPG) in a dual-concentric-core fiber (DCCF). The LPG is inscribed to couple light from the fundamental mode of the central core to the ring-core modes, resulting in the generation of a series of resonant dips. Two adjacent dips shift toward each other when the LPG is bent. By monitoring the variation of the wavelength interval between these two dips, this LPG can be applied in curvature measurement with a sensitivity as high as -9.046  nm/m(-1). More importantly, such a wavelength interval is almost immune to the cross impacts of temperature and axial strain, since the sensitivities to temperature and axial strain are only 2.6 pm/°C and 0.083 pm/με, respectively.

A Compact Fiber Inclinometer Using a Thin-Core Fiber with Incorporated an Air-Gap Microcavity Fiber Interferometer

A compact fiber-optic inclinometer is proposed and experimentally demonstrated based on a Fabry-Perot interference (FFPI). The sensing head consists of a short segment of thin-core fiber (TCF) following with a piece of hollow-core fiber (HCF). High-order cladding modes have been excited because of core diameter mismatch. A clear interference spectrum has been obtained as the consequence of interference among the reflected core modes and cladding modes. Fringe contrast of the interference spectrum is highly sensitive to fiber bending with direction independence, and good linearity has been observed during the bending range from 1° to 3° with a sensitivity of 2.71 dB/deg.

Bending sensor combining multicore fiber with a mode-selective photonic lantern

A bending sensor is demonstrated using the combination of a mode-selective photonic lantern (PL) and a multicore fiber. A short section of three-core fiber with strongly coupled cores is used as the bend sensitive element. The supermodes of this fiber are highly sensitive to the refractive index profiles of the cores. Small bend-induced changes result in drastic changes of the supermodes, their excitation, and interference. The multicore fiber is spliced to a few-mode fiber and excites bend dependent amounts of each of the six linearly polarized (LP) modes guided in the few-mode fiber. A mode selective PL is then used to demultiplex the modes of the few-mode fiber. Relative power measurements at the single-mode PL output ports reveal a high sensitivity to bending curvature and differential power distributions according to bending direction, without the need for spectral measurements. High direction sensitivity is demonstrated experimentally as well as in numerical simulations. Relative power shifts of up to 80% have been measured at radii of approximately 20 cm, and good sensitivity was observed with radii as large as 10 m, making this sensing system useful for applications requiring both large and small curvature measurements.

Asymmetrical in-fiber Mach-Zehnder interferometer for curvature measurement

We demonstrated a compact and highly-sensitive curvature sensor based on a Mach-Zehnder interferometer created in a photonic crystal fiber. Such a Mach-Zehnder interferometer consisted of a peanut-like section and an abrupt taper achieved by use of an optimized electrical arc discharge technique, where only one dominating cladding mode was excited and interfered with the fundamental mode. The unique structure exhibited a high curvature sensitivity of 50.5 nm/m^-1 within a range from 0 to 2.8 m^-1, which made it suitable for high-sensitivity curvature sensing in harsh environments. Moreover, it also exhibited a temperature sensitivity of 11.7 pm/°C.

Optimization of multicore fiber for high-temperature sensing

We demonstrate a novel high-temperature sensor using multicore fiber (MCF) spliced between two single-mode fibers. Launching light into such fiber chains creates a supermode interference pattern in the MCF that translates into a periodic modulation in the transmission spectrum. The spectrum shifts with changes in temperature and can be easily monitored in real time. This device is simple to fabricate and has been experimentally shown to operate at temperatures up to 1000°C in a very stable manner. Through simulation, we have optimized the multicore fiber design for sharp spectral features and high overall transmission in the optical communications window. Comparison between the experiment and the simulation has also allowed determination of the thermo-optic coefficient of the MCF as a function of temperature.