Microfiber Bragg Grating Research Articles

Flexible Arc-Shaped Micro-Fiber Bragg Grating Array Three-Dimensional Tactile Sensor for Fingertip Signals Detection and Human Pulse Monitoring.

A flexible arc-shaped micro-Fiber Bragg Grating (mFBG) array three-dimensional tactile sensor for fingertip signal detection and human pulse monitoring is presented. It is based on a three mFBGs array which is embedded in an arc-shaped poly (dimethylsiloxane) (PDMS) elastomer, which can effectively discriminate the normal force, left force, and right force by monitoring the reflected intensity variation of the three mFBGs. Different from the traditional FBG sensors, this sensor measures force by detecting changes in light intensity, effectively avoiding the wavelength cross-sensitivity impact of temperature variations on the sensor performance. This design strategy simplifies the sensor structure, reduces the system complexity and signal interrogation cost, and enhances reliability and practicality. Through systematic experiments, we successfully validated the sensor's superior performance, achieving a minimum detection force of 0.01 N and providing robust data support for practical applications. In addition, the sensor has been used to monitor human pulse accurately. The successful fabrication and experimental validation of this sensor lay a foundation for its widespread application in fields such as robot perception and human vital signal detection.

Temperature-compensated high-sensitivity refractive index, liquid level and PH sensing realization enabled by a small-period microfiber long period grating cascaded with microfiber Bragg grating

Conventional reported optical fiber sensors could not have both good temperature-compensated characteristic and high PH sensing sensitivity. In this study, we proposed and fabricated a small-period microfiber long period grating (SP-mFLPG) cascaded with a line-by-line microfiber Bragg grating (mFBG) by using femtosecond (fs) laser directly writing technique for high-sensitivity temperature-compensated sensing refractive index(RI), liquid level(LL), PH value. To be the different from traditional LPG devices, the temperature sensitivities of SP-mFLPG was measured as −8.61 pm/℃ owing to the strong coupling occurred between the fundamental mode of TE11 mode and the higher-order mode of TE111, which could be compensated with the mFBG device with a temperature sensitivity of 7.85 pm/℃. On the basis of this temperature-insensitive sensing strategy, our proposed SP-mFLPG sensor can sense RI, LL and pH with high sensitivities of 866.343 nm/RIU, 7.224 nm/mm, −5.70 nm/PH, respectively. Our obtained PH sensitivity is much higher than numerous reported state-of-the-art optical fiber PH sensors. Thus, our proposed optical microfiber device can be found potential applications in real-time measuring some important biochemical reaction procedures.

Multi-node wearable optical sensor based on microfiber Bragg gratings.

Flexibly wearable sensors are widely applied in health monitoring and personalized therapy. Multiple-node sensing is essential for mastering the health condition holistically. In this work, we report a multi-node wearable optical sensor (MNWOS) based on the cascade of microfiber Bragg gratings (µFBG), which features the reflective operation mode and ultra-compact size, facilitating the functional integration in a flexible substrate pad. The MNWOS can realize multipoint monitoring on physical variables, such as temperature and pressure, in both static and dynamic modes. Furthermore, the eccentric package configuration endows the MNWOS with the discernibility of bending direction in addition to the bending angle sensing. The multi-parameter sensing is realized by solving the sensing matrix that represents different sensitivity regarding the bending and temperature between FBGs. The MNWOS offers great prospect for the development of human-machine interfaces and medical and health detection.

Cantilevered Microfiber Bragg Grating Sensor With High Ultrasonic Sensitivity and Designable Resonant Frequency

Cantilevered Microfiber Bragg Grating Sensor With High Ultrasonic Sensitivity and Designable Resonant Frequency

Near-infrared microfiber Bragg grating for sensitive measurement of tension and bending.

Fiber-optic devices working in the visible and near-infrared windows are attracting attention due to the rapid development of biomedicine that involves optics. In this work, we have successfully realized the fabrication of near-infrared microfiber Bragg grating (NIR-µFBG), which was operated at the wavelength of 785 nm, by harnessing the fourth harmonic order of Bragg resonance. The NIR-µFBG provided the maximum sensitivity of axial tension and bending to 211 nm/N and 0.18 nm/deg, respectively. By conferring the considerably lower cross-sensitivity, such as response to temperature or ambient refractive index, the NIR-µFBG can be potentially implemented as the highly sensitive tensile force and curve sensor.

Flexible Wearable Optical Sensor Based on Optical Microfiber Bragg Grating

Wearable sensing devices, which can find tremendous applications in healthcare and automation, are attracting great attention in recent years. Herein, we report a flexible wearable optical sensor (FWOS) which consists of the microfiber Bragg grating ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\mu }$</tex-math></inline-formula> FBG) as the core sensing node and a ductile PDMS film as the sensor encapsulation. Due to the strain amplification effect of the microfiber, the sensitivity of FWOS to pressure, bending angle and temperature is significantly enhanced to 0.03 nm/kPa, 0.19 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> and 0.04 nm/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> C, respectively. Furthermore, thanks to the better flexibility of the microfiber structure, the FWOS device could be achieved for real-time monitoring of wrist pulses, finger knuckle bending, and human body temperature. The proposed FWOS creates new ideas for the development of future wearable optical devices, which is critical for the advancement of next-generation robotics, and healthcare monitoring.

Temperature-Insensitive Ferrofluid-Clad Microfiber Bragg Grating for Magnetic Field Sensing

In this paper, a temperature-insensitive ferrofluid (FF)-clad microfiber Bragg Grating (MF-BG) magnetic field sensor is proposed. Through optimizing the diameter of MF-BG, we can effectively suppress its thermal property. The experimental research results show that when the diameter of MF-BG is ~2.94 μm, its reflection spectrum shift owing to ambient temperature change can be substantially small within the range of 20–80 °C. The thermal stable sensor has a magnetic field sensitivity of 0.667 pm/Gs with a linearity of more than 0.985 at 20 °C.

Microfiber Bragg Grating Bonded Using Tapered Cantilever for High-Sensitivity Ultrasonic Detection

A microfiber Bragg grating (MFBG) bonded using tapered cantilever was designed to improve ultrasonic sensitivity, and was investigated using experimental, theoretical, and numerical methods. The ultrasonic signal was amplified by the tapered structure while the influences of the static strain and evanescent field were eliminated by the cantilever. Compared with the cantilevered fiber Bragg grating, the sensitivity of the sensor presents an upward trend with the descent of the diameter as there are 331% and 108% sensitivity increment by MFBGs with 20 and 40 μm diameters, and the improvement is apparent when the frequency of the ultrasonic signal is high.

A Fully-Encircled Polymerized Microfiber Bragg Grating by 3D Femtosecond Laser Nanofabrication.

Through the merits of the arbitrary three-dimensional (3D) fabrication ability and nanoscale resolution of two-photon polymerization, we demonstrated a fully encircled polymerized microfiber Bragg grating using 3D femtosecond laser nanofabrication. In order to generate strong enough polymer Bragg grating units around the microfiber surface, and to possess a possible smaller unit pitch and structure size, the composition of photoresist and grating dimensions were both experimentally optimized. A fast-curing, high-adhesion, great-heat-resistant acrylate monomer EQ4PETA was chosen as the cross-linking element, and a high-efficiency photoinitiator DETC was used. Along the tapered microfiber with a diameter of 2 microns, dozens of grating units of 300 nm thickness were successively fabricated. The resonance wavelength was approximately 1420 nm, with a unit pitch of 1 μm, slightly different with varying unit pitches. The refractive index sensitivity reached up to ~440 nm/RIU, which is much higher than other microfiber grating sensors. We also measured the temperature and strain sensitivity of this fully encircled microfiber Bragg grating, and this was estimated at 88 pm/°C and 6.3 pm/µε. It is foreseeable that with the continuous progress of fabrication technology, more highly integrated functional optical devices will emerge in the future.

Plasmonic Gold Nanostar Mediated Self-Photothermal Modulation of Microfiber Bragg Grating

Photothermal actuation that leverages optical fiber and plasmonic materials would not only add a practical function but also spur many important applications for the fiber-optic sensors, for example, the self-temperature governing and fiber-optic photothermal medicine probe. In this paper, we propose and demonstrate a microfiber Bragg grating device that integrates the functionality of self photothermal modulation mediated by the plasmonic gold nanostar (AuNSt). As an excellent plasmnoic nanomaterial, AuNSt stands out due to its tailorable surface plasmon resonance(SPR) spectrum and pronouced “lightning rod” effect. After exploring the optimized AuNSt colodial solution for immobilization on the fiber surface, the AuNSt-functionalized microfiber Bragg grating presented a high pump-heat slope <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\sim }$</tex-math></inline-formula> 0.06 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> C/mw and the maximum temperature elevation of 18.5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^{\circ }$</tex-math></inline-formula> C in the liquid environment. Furthermore, the enhanced electromagnetic field of AuNSt effectively improves the sensitivity of the microfiber Bragg grating. The functional grating probe also performs good stability and fast on-off switch of the photoheating. The demonstration heralds the potential of the plasmonic microfiber grating for steering the interfacial temperature through the photothermal effect, which will be beneficial for the bio-chemical sensing and medical therapeutics.

Rare Earth-Doped Microfiber Bragg Grating Refractive Index Sensor With Self-Photothermal Manipulation

We demonstrate a microfiber Bragg grating as the reflective refractometer with the function of self-photoheating. The rare earth-doped fiber provides not only the photosensitivity to the UV modulation but also the effective pump laser absorption and photoheating conversion. The rare earth-doped microfiber Bragg grating, which is cladding-removed by the hydrofluoric acid, can detect the tiny refractive index change with a resolution of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> RIU scale. The self-photoheating could be realized by 980 nm pump absorption and non-radiative transition process of the ytterbium and erbium ions, which are well-preserved by the core structure after etching. Monitored by the reflective grating signal, the photoheating can manipulate 30 °C increase of the device in the air, 8 °C in the ethanol, and 4 °C in the water. The second time-scale heating/cooling response allows the sensor to act as ad optical switch for engineering temperature. The work may open a new route for accelerative biosensing and pollutant determination and treatment.

Preparation of Adiabatic Microfiber Bragg Grating by Chemical Etching

Micro fiber Bragg grating (MFBG) is sensitive to temperature and refractive index at the same time, and has excellent sensing performance. Scholars all over the world have carried out a lot of research on it. At present, for the preparation of adiabatic MFBGs, micro fibers meeting adiabatic conditions are prepared by heating stretching method, and then fabricated by mask method or etching method. In order to simplify the preparation process, based on chemical etching method, a simple and easy technique for preparing adiabatic MFBG is proposed. In this technique, the FBG immersed in corrosion solution is gradually lifted by stepping motor, which can form a transition region with decreasing diameter, so that the cone angle of the transition region meets the adiabatic conditions, and then an adiabatic MFBG is formed. Several adiabatic MFBGs with different diameters are actually fabricated. The results show that adiabatic MFBG has better spectral and wavelength stability than ordinary MFBG. Finally, the refractive index and temperature sensing of the MFBGs are realized, the results showed that for MFBG with a diameter of about 9 μm, its sensing sensitivity to refractive index and temperature are 5200 pm/RIU and 10.06 pm/°C, for MFBG with a diameter of about 12 μm, the sensing sensitivity are 1125 pm/RIU and 10.33 pm/°C.

Mid-infrared microfiber Bragg gratings.

We report mid-infrared (mid-IR) Bragg gratings fabricated on sub-wavelength-diameter chalcogenide glass (ChG) microfibers. ChG microfibers with diameters around 3 µm are tapered drawn from As2S3 glass fibers, and the mid-IR microfiber Bragg gratings (mFBGs) are inscribed on microfibers using interference patterns of near bandgap light at a 532 nm wavelength. At a wavelength of about 4.5 µm, the mFBG has an extinction ratio of 15 dB and a positive photo-induced refractive index change of 2×10-2. The dependence of the grating formation on accumulated influence of exposure power density and time is investigated. The mid-IR mFBGs demonstrated here may be used as building blocks for micro-photonic circuits or devices in the mid-IR spectral range.

Study of Temporal Thermal Response of Microfiber Bragg Grating

Fiber Bragg grating has been successfully fabricated in the silica microfiber by the use of femtosecond laser point-by-point inscription. Temporal thermal response of the fabricated silica microfiber Bragg grating has been measured by the use of the CO2 laser thermal excitation method, and the result shows that the time constant of the microfiber Bragg grating is reduced by an order of magnitude compared with the traditional single-mode fiber Bragg grating and the measured time constant is ~ 21ms.

Superstructure microfiber grating characterized by temperature, strain, and refractive index sensing.

Microfiber gratings with diameters in the subwavelength scale have recently attracted much attention for developments of sensitive sensors; however, a specific structure is usually chosen for sensing one parameter according to the optical response. In this work, a superstructure microfiber grating combined with microfiber Bragg grating and long-period microfiber grating is reported for the first time. The proposed superstructure is formed by ultraviolet laser inscription and femtosecond laser scratching techniques, which simultaneously endows the unique properties of the two individual gratings. The reflection and transmission spectral characteristics differing to conventional counterparts are demonstrated. The responsivities of the two gratings to temperature, strain and refractive index are investigated, providing a possibility for simultaneous multi-parameter sensing.

Directional acoustic signal measurement based on the asymmetrical temperature distribution of the parallel microfiber array.

A parallel microfiber array for the measurement of directional acoustic signals is proposed and experimentally demonstrated. Two microfiber Bragg gratings (micro-FBGs) in single-mode fibers were placed on two sides of a Co2+-doped microfiber, forming an array of three parallel microfibers. The micro-FBGs can measure the temperature difference between the two sides of the Co2+-doped microfiber through interrogation of the matched FBGs. Due to the asymmetrical temperature distribution of the Co2+-doped microfiber under the applied acoustic signal, sound source localization can be realized through the acoustic particle velocity. The experimental results show that an acoustic particle velocity sensitivity of 44.2 V/(m/s) and a direction sensitivity of 0.83mV/deg can be achieved at a frequency of 1000 Hz, and the sound source localization has been realized through the orthogonal direction responses of two crossed Co2+-doped microfibers. The results demonstrate that the parallel microfiber array has the ability to recognize orientation, offering potential for directional acoustic signal detection with miniature size.

Temperature monitorable refractometer of microfiber Bragg grating using a duet of harmonic resonances.

To overcome the temperature cross-sensitivity of the microfiber Bragg grating (m-FBG) refractometer, we propose a novel refractive-index-temperature dual-sensing paradigm involving the third harmonic Bragg resonance that presents distinctive sensing characteristics. Strong resonances are obtained in both 1060nm and 1550nm wavebands under the modulation of the UV Talbot pattern. Moreover, higher-order transverse mode coupled resonance is also observed at the third harmonic waveband, supplementing an independent signal for enabling a sensing trio potentially. It is believed that the proposed dual-sensing paradigm would contribute to the m-FBT-based chemoprobes/bioprobes.

Helical Microfiber Bragg Grating Printed by Femtosecond Laser for Refractive Index Sensing

A microfiber helical Bragg grating is demonstrated for use in refractive index measurements. The proposed sensor was fabricated using a femtosecond laser-induced multiphoton polymerization technique. The grating was cured along with the microfiber, and its reflection spectra, transmission spectra, refractive index, and temperature cross sensitivity were investigated. The refractive index sensitivities of ~229 and ~120 nm/RIU were achieved, corresponding to high order and fundamental modes, respectively. This helical microfiber Bragg grating exhibits high refractive index sensitivity and good mechanical strength and could potentially be used for optical functionality integration.

High-speed refractive index sensing system based on Fourier domain mode locked laser.

A high-speed refractive index sensing system based on the Fourier domain mode locked laser (FDML) and a microfiber Bragg grating (mFBG) is theoretically studied and experimentally demonstrated. Unlike traditional physical parameter sensing systems, which directly use the FDML as the wavelength scanning source and the optical sensor as the spectra shaping component, we inserted an mFBG into the FDML cavity in order to generate time domain pulse signals used for sensing. The wavelength shift in optical frequency domain is converted into time domain pulse drift. The sensitivity of the proposed refractive index (RI) sensing system is improved by two orders of magnitude, compared with the wavelength monitoring method. The scanning speed is as high as 43 kHz. Moreover, the sensitivity curve can be adjusted by tuning the direct current voltage. The nonlinear sensitivity and linear sensitivity with RI can be achieved.

Simultaneous measurement of temperature and refractive index based on microfiber Bragg Grating in Sagnac loop

Abstract In this paper, a simultaneous measurement system of temperature and refractive index (RI) is proposed and demonstrated. The dips in the output spectrum of the proposed sensor are formed by the microfiber Bragg Grating (mFBG) and Sagnac loop. Due to different interference mechanisms, the dual-parameter measurement is realized by mFBG and Sagnac loop with different sensitivities to temperature and RI. Compared with the traditional Sagnac loop sensors, the proposed Sagnac loop with mFBG is much sensitive to surrounding RI. The results show that the temperature sensitivity is 0.033 nm/°C and −0.015 dB/°C. And the RI sensitivity is 39.30 nm/RIU and 41.51 dB/RIU. The proposed sensor has a simple structure and great potential in biological and chemical fields.