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A new type of human immunoglobulin G (IgG) sensors based on the surface plasmon resonance (SPR) in the low refractive index (RI) plastic optical fiber (POF) and an antibody immobilization method is presented. A 50-nm-thick gold film was formed on the polished D-shaped fiber surface by magnetron sputtering. The RI response of the POF sensor is 30 049.61 nm/RIU, which is 26.5 times higher than that of single mode fiber (SMF) SPR sensors. The proposed SPR biosensor can be developed by simple and rapid modification of the gold film with 11-mercapto undecanoic acid (MUA). Upon immobilization of the goat anti-human IgG antibody, the resonance wavelength shifts by 11.2 nm. The sensor can be used to specifically detect and quantify the human IgG at concentrations down to 245.4 ng/mL with the sensitivity of 1.327 7 nm per µg/mL, which offers an enhancement of 12.5-fold compared to that of the conventional SMF based SPR sensors. The proposed device may find the potential applications in the case of use at the point of care. [ABSTRACT FROM AUTHOR]

Mechanical metamaterials can adjust mechanical properties of structures flexibly through a mechanical structural design based on the premise that the materials remain unchanged. Here, a cantilever probe microstructure is designed using mechanical metamaterials for an optical fiber microforce sensor tip that can be prepared by femtosecond laser-induced two-photon polymerization. The elastic constant k of the fabricated fiber-optic microforce sensor has been adjusted by two orders of magnitude from 0.165 to 46 N/m, and the geometric configuration of the cantilever beam can be tailored to match the mechanical properties of biological specimens. This fiber microforce sensor shows an ultra-high force sensitivity of 154 nm/µN and a force resolution of up to 130 pN. The optical fiber microforce sensor that shows the lowest force resolution in a direct-contact mode has high potential for biosensing applications, and the results reveal a potential design strategy for special scanning tunneling microscope probes with unique physical properties. [ABSTRACT FROM AUTHOR]

We experimentally studied the inscription of fiber Bragg gratings by using femtosecond (fs) laser point-by-point (PbP) technology. The effects of the focusing geometry, grating order, laser energy and grating length on the spectral characteristics of the PbP FBG were investigated. After optimizing these parameters, a high-quality first-order PbP FBG with a reflectivity > 99.9% (i.e., Bragg resonance attenuation of 37.7 dB) and insertion loss (IL) of 0.03 dB was successfully created. Moreover, taking advantage of the excellent flexibility of the fs laser PbP technology, high-quality FBGs with various Bragg wavelengths ranging from 856 to 1902.6 nm were inscribed. Furthermore, wavelength-division-multiplexed (WDM) FBG arrays consisting of 10 FBGs were rapidly constructed. Additionally, a Fabry-Perot cavity was realized by using two high-quality FBGs, and its birefringence could be reduced from 3.04 × 10−5 to 1.77 × 10−6 by using a slit beam shaping-assisted femtosecond laser PbP technology. Therefore, such high-quality FBGs are promising to improve the performance of optical fiber sensors, lasers and communication devices. [ABSTRACT FROM AUTHOR]

An all-fiber glucose sensor is proposed and demonstrated based on a helical intermediate-period fiber grating (HIPFG) produced by using a hydrogen/oxygen flame heating method. The HIPFG, with a grating length of 1.7 cm and a period of 35 μm, presents four sets of double dips with low insertion losses and strong coupling strengths in the transmission spectrum. The HIPFG possesses an averaged refractive index (RI) sensitivity of 213.6 nm/RIU nm/RIU in the RI range of 1.33–1.36 and a highest RI sensitivity of 472 nm/RIU at RI of 1.395. In addition, the HIPFG is demonstrated with a low-temperature sensitivity of 3.67 pm/°C, which promises a self-temperature compensation in glucose detection. In the glucose-sensing test, the HIPFG sensor manifests a detection sensitivity of 0.026 nm/(mg/mL) and a limit of detection (LOD) of 1 mg/mL. Moreover, the HIPFG sensor exhibits good stability in 2 h, indicating its capacity for long-time detection. The properties of easy fabrication, high flexibility, insensitivity to temperature, and good stability of the proposed HIPFG endow it with a promising potential for long-term and compact biosensors. [ABSTRACT FROM AUTHOR]

High-accuracy temperature measurement plays a vital role in biomedical, oceanographic, and photovoltaic industries. Here, a highly sensitive temperature sensor is proposed and demonstrated based on cascaded polymer-infiltrated Mach–Zehnder interferometers (MZIs), operating near the dispersion turning point. The MZI was constructed by splicing a half-pitch graded index fiber (GIF) and two sections of single-mode fiber and creating an inner air cavity based on femtosecond laser micromachining. The UV-curable polymer-infiltrated air cavity functioned as one of the interference arms of MZI, and the residual GIF core functioned as the other. Two MZIs with different cavity lengths and infiltrated with the UV-curable polymers, having the refractive indexes on the different sides of the turning point, were created. Moreover, the effects of the length and the bending way of transmission SMF between the first and the second MZI were studied. As a result, the cascaded MZI temperature sensor exhibits a greatly enhanced temperature sensitivity of −24.86 nm/°C based on wavelength differential detection. The aforementioned result makes it promising for high-accuracy temperature measurements in biomedical, oceanographic, and photovoltaic applications. [ABSTRACT FROM AUTHOR]

With the current trend of device miniaturization, the measurement and control of interfacial adhesion forces are increasingly important in fields such as biomechanics and cell biology. However, conventional fiber optic force sensors with high Young's modulus (>70 GPa) are usually unable to measure adhesion forces on the micro- or nano-Newton level on the surface of micro/nanoscale structures. Here, we demonstrate a method for interfacial adhesion force measurement in micro/nanoscale structures using a fiber-tip microforce sensor (FTMS). The FTMS, with microforce sensitivity of 1.05 nm/μN and force resolution of up to 19 nN, is fabricated using femtosecond laser two-photon polymerization nanolithography to program a clamped-beam probe on the end face of a single-mode fiber. As a typical verification test, the micronewton-level contact and noncontact adhesion forces on the surfaces of hydrogels were measured by FTMS. In addition, the noncontact adhesion of human hair was successfully measured with the sensor. [ABSTRACT FROM AUTHOR]

Blood glucose concentration is important for metabolic homeostasis in humans and animals. Many diabetic patients need to detect blood glucose daily which burdens community hospitals and family healthcare. Optical fiber sensors are widely used in biomedical detection because of their compact structure, fast response, high sensitivity, low cost, and ease of operation. In this work, we constructed a Fabry-Perot (FP) cavity biosensor for the fast detection of glucose concentration in serum. The femtosecond laser micromachining was applied to fabricate the FP cavity by printing the fiber-tip fixed-supported bridge at the end face of the optical fiber. An additional hemisphere was printed at the center of the outer surface of the bridge to avoid multi-beam interference. The results demonstrated that the proposed biosensor had high refractive index (RI) detection sensitivity, roughly 1039 nm/RIU at a wavelength of 1590 nm, and the detection sensitivity for glucose was around 0.185 nm/ (mg/mL) at a wavelength of 1590 nm. Due to its high sensitivity, compact structure, and fast response, the FP cavity biosensor has great potential to be applied in family healthcare for glucose concentration detection of diabetic patients. [ABSTRACT FROM AUTHOR]

Fiber Bragg grating (FBG) is the most widely used optical fiber sensor due to its compact size, high sensitivity, and easiness for multiplexing. Conventional FBGs fabricated by using an ultraviolet (UV) laser phase-mask method require the sensitization of the optical fiber and could not be used at high temperatures. Recently, the fabrication of FBGs by using a femtosecond laser has attracted extensive interests due to its excellent flexibility in creating FBGs array or special FBGs with complex spectra. The femtosecond laser could also be used for inscribing various FBGs on almost all fiber types, even fibers without any photosensitivity. Such femtosecond-laser-induced FBGs exhibit excellent thermal stability, which is suitable for sensing in harsh environment. In this review, we present the historical developments and recent advances in the fabrication technologies and sensing applications of femtosecond-laser-inscribed FBGs. Firstly, the mechanism of femtosecond-laser-induced material modification is introduced. And then, three different fabrication technologies, i.e., femtosecond laser phase mask technology, femtosecond laser holographic interferometry, and femtosecond laser direct writing technology, are discussed. Finally, the advances in high-temperature sensing applications and vector bending sensing applications of various femtosecond-laser-inscribed FBGs are summarized. Such femtosecond-laser-inscribed FBGs are promising in many industrial areas, such as aerospace vehicles, nuclear plants, oil and gas explorations, and advanced robotics in harsh environments. [ABSTRACT FROM AUTHOR]

Hydrogen is widely used in industrial production and clinical medicine, and as fuel. Hydrogen becomes explosive when the hydrogen–air mixture ranges from 4 to 76 vol%; thus, a rapid hydrogen concentration measurement is particularly important in practical applications. We present a novel fiber optic hydrogen sensor with fast response fabricated from a graphene–Au–Pd sandwich nanofilm and an ultrashort fiber Bragg grating. The response time is only 4.3 s at a 3.5 vol% hydrogen concentration. When the measured hydrogen concentration was increased from 0 to 4.5 vol%, the optical resonance dip in the sensor near 1550 nm shifted by 290 pm. In addition, the sensor has an insertion loss of only −2.22 dB, a spectral contrast of 10.8 dB, and a spectral finesse of 5. Such a flexible, fast-response sensor is expected to be used in the development of hydrogen sensors with low power consumption. [ABSTRACT FROM AUTHOR]

Single-mode-no-core-single-mode (SNS) optical fiber structures have valuable potential to encapsulate as high-precision temperature sensors, due to their great sensitivity. This paper is to improve the temperature sensitivity of SNS fibers by studying the parameters of fiber length, diameter, and refractive index of the seal liquid. Transmission spectra of the fibers are given in both aspects of experiment and numerical simulation. We find that using a slim no-core fiber section sealed with liquid of high refractive index can effectively improve the sensitivity, while the choice of fiber length mainly affects the width of transmission peak. The results may contribute to the development of high-sensitivity optic temperature sensor. [ABSTRACT FROM AUTHOR]

We demonstrated a refractive index (RI) sensor based on optical fiber Michelson interferometer (MI), which was fabricated by splicing a section of thin core fiber (TCF) to a standard single mode fiber with a core offset. Experimentally, such a MI-based RI sensor with a core offset of 8 μm and a TCF length of 3 mm exhibits a high resolution of 4.9 × 10 −6 RIU and sensitivity of −202.46 dB/RIU, which is two or three times higher than that of intensity-modulated RI sensors reported previously. In contrast, our MI-based RI sensor is insensitive to temperature, thus overcoming the cross-sensitivity problem between surrounding RI and temperature. Moreover, intensity modulation, rather than wavelength modulation, was used in the proposed MI-based RI sensor, and the sensor also has the advantages of compact size (8 mm), simple structure, easy fabrication, and good repeatability. [ABSTRACT FROM AUTHOR]

A nondestructive measurement method based on an Optical frequency domain reflectometry (OFDR) was demonstrated to achieve Young's modulus of an optical fiber. Such a method can be used to measure, not only the averaged Young's modulus within the measured fiber length, but also Young's modulus distribution along the optical fiber axis. Moreover, the standard deviation of the measured Young's modulus is calculated to analyze the measurement error. Young's modulus distribution of the coated and uncoated single mode fiber (SMF) samples was successfully measured along the optical fiber axis. The average Young's modulus of the coated and uncoated SMF samples was 13.75 ± 0.14, and 71.63 ± 0.43 Gpa, respectively, within the measured fiber length of 500 mm. The measured Young's modulus distribution along the optical fiber axis could be used to analyze the damage degree of the fiber, which is very useful to nondestructively estimate the service life of optical fiber sensors immersed into smart engineer structures. [ABSTRACT FROM AUTHOR]

Different from the basic sensing theory of the fiber Bragg grating (FBG) sensors based on spectral analysis, a new method exploiting the polarization properties of FBG for diametric load sensing is proposed. In this paper, the evolution of the differential group delay (DGD) response of FBG with respect to the diametric load is completely studied. The centroid information of the DGD evolution of FBG is subsequently proposed for diametric load sensing. Experiments were carried out to verify the reasonability and feasibility of the sensing method. Good agreements between experimental results and numerical simulations had been obtained. [ABSTRACT FROM AUTHOR]

Abstract: Different from the basic sensing theory of the fiber Bragg grating (FBG) sensors based on direct spectral analysis, a new method exploiting the polarization properties of FBG for temperature-insensitive transverse load sensing is proposed. In this paper, the birefringence effects on FBG under transverse load are analyzed. The evolution of the polarization dependent loss (PDL) of FBG with wavelength for the transmitted signals with respect to the transverse load is also studied theoretically and experimentally. We proposed utilizing the PDL evolution of FBG to precisely recover the wavelength spacing between the two peaks of x-mode and y-mode for accurate transverse load measurement. Good agreements between experimental results and numerical simulations had been obtained. [Copyright &y& Elsevier]

Abstract: Long-period fiber gratings (LPFGs) are written by use of a focused CO2 laser beam to notch periodically on the surface of optical fibers. Temperature characteristics of the CO2-laser-notched LPFG are investigated in detailed to develop its sensing applications. It is shown that, for such LPFG, temperature sensitivity of resonant wavelength can be increased by creating deeper notches. The coupling from the fundamental core mode to a cladding mode in the LPFG can be enhanced by increasing the ambient temperature. The polarization-dependent loss (PDL) of the LPFG depends strongly on the ambient temperature. Such unique temperature characteristics can be used to solve the cross-sensitivity problem between the temperature and the tensile strain. So, the LPFG is a promising temperature sensor based on the wavelength modulation, on the intensity modulation and/or on the PDL modulation. [Copyright &y& Elsevier]

The conventional methods for monitoring IgG levels suffer from some apparent problems such as long assay time, multistep processing, and high overall cost. An effective and suitable optical platform for label-free biosensing was investigated by the implementation of antibody/antigen immunoassays. The ultrasensitive detection of IgG levels could be achieved by exploiting the dispersion turning point (DTP) existing in the tapered two-mode fibers (TTMFs) because the sensitivity will reach ±∞ on either side of the DTP. Tracking the resonant wavelength shift, it was found that the fabricated TTMF device exhibited limits of detection (LOD) down to concentrations of 10 fg/mL of IgG in PBS solution. Such immunosensors based on DTP have great significance on trace detection of IgG due to simple detection scheme, quick response time, and miniaturization. [ABSTRACT FROM AUTHOR]

This paper presents a review of our work concerning the recent progress in fabrications and applications of heating-induced long period fiber gratings (LPFGs). Firstly, three kinds of heating fabrication techniques based on CO2 laser, hydrogen–oxygen flame and arc discharge are demonstrated to fabricate LPFGs, i.e., standard LPFGs (SLPFGs) and helical LPFGs (HLPFGs), in different types of optical fibers such as conventional fibers, photonic crystal fibers, and photonic bandgap fibers. Secondly, the all-fiber orbital angular momentum (OAM) mode converters based on heating-induced SLPFGs and HLPFGs in different types of fibers are studied to increase the transmission capacity. Finally, the heating-induced SLPFGs and HLPFGs are investigated to develop various LPFG-based strain, pressure, torsion and biochemical sensors. [ABSTRACT FROM AUTHOR]

A highly sensitive temperature sensor based on surface plasmon resonance (SPR) of a side-polished single mode fiber is demonstrated. The sensor consists of a gold film coated side-polished fiber covered by a layer of UV-curable adhesive. Before introducing the UV-curable adhesive, the gold-coated fiber exhibits refractive index (RI) sensitivity of 1691.6 nm/RIU to 8800 nm/RIU in the range of 1.32 to 1.43. The resonant wavelength of the SPR sensor shifts to 650 nm when the adhesive is coated on the gold film, and is fixed at about 725 nm when the adhesive is cured. Due to the high thermo-optic and thermal expansion coefficient of the adhesive, the sensor structure achieves a temperature sensitivity of −0.978 nm/°C between 25 °C and 100 °C. The proposed optical fiber SPR sensor is simple, highly sensitive and cost effective, which may find potential applications for temperature measurements in the biomedical and environmental industries. [ABSTRACT FROM AUTHOR]

A simple vibration sensor is proposed and demonstrated based on an optical fiber Fabry-Perot interferometer (FPI) with an in-fiber collimator. The device was fabricated by splicing a quarter-pitch graded index fiber (GIF) with a section of a hollow-core fiber (HCF) interposed between single mode fibers (SMFs). The static displacement sensitivity of the FPI with an in-fiber collimator was 5.17 × 10−4 μm−1, whereas the maximum static displacement sensitivity of the device without collimator was 1.73 × 10−4 μm−1. Moreover, the vibration sensitivity of the FPI with the collimator was 60.22 mV/g at 100 Hz, which was significantly higher than the sensitivity of the FPI without collimator (11.09 mV/g at 100 Hz). The proposed FPI with an in-fiber collimator also exhibited a vibration sensitivity nearly one order of magnitude higher than the device without the collimator at frequencies ranging from 40 to 200 Hz. This low-cost FPI sensor is highly-sensitive, robust and easy to fabricate. It could potentially be used for vibration monitoring in remote and harsh environments. [ABSTRACT FROM AUTHOR]

• A novel fiber optic temperature sensor based on multimode interference is proposed. • The sensitivity is improved by splicing NCFs with different diameters and tapered fusion. • An ultra-high temperature sensitivity of −14.448 nm/K is achieved. • The reproductivity and repeatability of the proposed sensor are remarkable. For the advancement of temperature monitoring devices, research into high sensitivity fiber optic temperature sensors is valuable. This research suggests a fiber optic temperature sensor with great sensitivity. Specifically, the NCF (no core fiber) spliced with various diameters is employed within the multimode interference region of the typical SNS (single mode fiber-no core fiber-single mode fiber) sensor. The morphology of the splice sites connecting two NCFs is also regulated by fusing. The mixture of ethanol and glycerol is used to coat the entire structure. According to the experimental results, the proposed sensor can achieve a temperature sensitivity of −14.448 nm/°C. Additionally, the sensor has good performance in repeatability and temperature cycling thanks to its simple, dependable structure. It is anticipated that the relevant results will be useful for the development and applications of precise temperature measurement. [ABSTRACT FROM AUTHOR]

This work proposes an optical fiber acoustic sensor using a Fabry-Perot interferometer (FPI), which consists of an optical fiber end face and a gold diaphragm with an effective diameter of 75 µm and a thickness of 20 nm. The sensor has contrast of up to 34 dB and achieves audible acoustic signal sensing over a broad frequency response range from 70 Hz to 20 kHz with a linear response to sound pressure. The minimum detectable sound pressure is 815.5 μPa/Hz1/2 at 70 Hz. The sensor made of a pure gold diaphragm has stable performance and can work consistently over a long period, promising to be applied as an optical microphone for real-time communication in some extreme environments. [Display omitted] • Proposed an optical fiber acoustic sensor using a FPI with a 75 μm diameter and 20 nm thickness gold film. • Achieves audible acoustic signal sensing over a broad frequency response range from 70 Hz to 20 kHz. • Linear response to sound pressure with minimum detectable sound pressure of 815.5 μPa/Hz1/2 at 70 Hz. • Sensor made of pure Au film has stable performance and can work consistently over a long period. [ABSTRACT FROM AUTHOR]

We demonstrate a surface plasmon resonance refractive sensor based on the side-polished single mode optical fiber and investigated the sensor performance decay induced by silver coating degradation. The sensor showed fast response to the refractive index liquid in the range from 1.32 to 1.40 RIU. The highest sensitivity up to 4365.5 nm/RIU and a FOM of 51.61 RIU −1 were achieved, which were comparable to the reported polished-fiber-based SPR sensors. Characteristics of the silver-coated sensor after exposure to the air as well as to the liquid environment were investigated. The sensor always stored in dry air could respond stably two weeks later with sensitivity comparable with the fresh one and lower resolution due to the broadened resonant dip. But the sensor immersed in the refractive liquid for 30 min firstly and then exposed to ambient air for hours decayed dramatically due to silver degradation. The presented silver-coated SPR sensor could be used as a simple, cost-effective, disposable device in bio-chemical, environment, and food safety detection. [ABSTRACT FROM AUTHOR]

Tiny and soft breath sensors offer unique capabilities in a continuous and long-term recording of vital physiological parameters for human health monitoring. Here, we propose an all fiber-optic breath sensor based on helical intermediate-period fiber grating (E-HIPFG) written in an elliptic core polarization-maintaining fiber. This E-HIPFG-based breath sensor shows ultrafast response time and recovery time, which are 34.46 ms and 41.34 ms, respectively. The response time and recovery time are three orders faster than the corresponding values of conventional optical fiber-based breath sensors with humidity-sensitive material. In addition, this sensor does not combine with any additional coating materials or fiber architectures, increasing the stability in a continuous and long-term detection for breath monitoring. The E-HIPFG sensor is insensitive to external influences, such as temperature, torsion and strain, endowing the possibility of stable detection in the moving body. Particularly, the temperature sensitivity of the sensor is 2.75 pm/°C, which is two orders lower than the corresponding value of previous reports. Due to its considerable stability, quick response, and insensitivity to external influences, the E-HIPFG-based breath sensor offers significant potential for long-term and compact breathing monitoring devices. • A breath sensor is designed with elliptical core helical intermediate-period fiber grating (E-HIPFG). • The E-HIPFG is an all-fiber transducer without sensitive material and restructure. • The E-HIPFG sensor gains an ultrafast response time and recovery time. • The E-HIPFG-based breath sensor shows excellent resistance to environmental disturbances. [ABSTRACT FROM AUTHOR]

There is a growing need to measure respiratory rate (RR) in a variety of applications, including in clinical and occupational settings, as well as during physical exercises. Fiber optic sensors (FOSs) is an attractive solution for wearable RR monitoring because of their small size, multiplexing capability, chemical inertness, and immunity to electromagnetic fields. Here, we review recent advances in FOSs for breath monitoring. We presented the sensing mechanisms of state-of-the-art sensors and analyze their advantages and disadvantages. We classified recently reported FOSs based on sensing principles and then critically analyze the strengths and weaknesses of representative recent works. Finally, we summarized the challenges and future prospects of breath FOSs, with the aim of applying them to real applications. [ABSTRACT FROM AUTHOR]