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An ultra-compact integrated sensor comprised of an in-fiber core-offset Fabry–Perot interferometer (FPI) and a fiber Bragg grating (FBG) is demonstrated for simultaneous measurement of humidity and temperature. The FPI is proposed by splicing three single-mode fiber segments together to realize multi-mode interference, along with UV-glue immersed in the core-offset region to enhance the sensitivity. Thus, a relative humidity sensitivity of 11.2 pm/%RH, and a temperature sensitivity of −74 pm/°C are obtained based on FPI. We also demonstrate a temperature measurement with sensitivity of 9.2 pm/°C based on the FBG. Since FBG is not response on humidity variation, it could be used as a compensating element to decouple humidity information from the total spectrum shift in the FPI-based relative humidity measurement, which finally realize simultaneous measurement of relative humidity and temperature. Considering ultra-compact size, easy fabrication, and dual-parameter measurement, our proposed fiber-optic sensor provides a great potential for practical applications. [ABSTRACT FROM AUTHOR]

We propose and investigate a fiber-optic sensor of As2Se3 chalcogenide (ChG) microfiber laterally adhered on the hollow-core fiber (HCF) end face. HCF is fused with a single-mode fiber to form Fabry–Perot interferometer (FPI) between single-mode fiber (SMF) end face and ChG microfiber. ChG microfiber is ultra-sensitive to the small perturbations in the surroundings, because of its high refractive index and small Young's modulus. Furthermore, relative to traditional fiber-optic FPI comprised by thin film, our proposed FPI with thin-diameter ChG microfiber could respond to mechanical vibrations over a broad range of frequencies. Based on finite-element simulation, ChG microfiber has ultra-broadband dynamic frequency response, up to several hundreds of megahertz accompanied by a very small response strength decay, in comparison to the frequency response of ChG film which is typically less than 30 MHz. ChG microfiber-based FPI is also used in the static refractometric sensing with sensitivity of 1563 nm/RIU. Such a novel fiber-optic sensor offers excellent potential for the nondestructive detection and advanced biomedical sensing. [ABSTRACT FROM AUTHOR]

In this paper, a novel nano sensor structure is proposed, which consists of a metal-insulator-metal waveguide (MIM) with rectangular baffles and a water droplet cavity with rectangular stubs (WDCRS). The WDCRS structure optimizes the sensitivity of a single water droplet cavity and makes the transmission curve clearer and smoother. The transmission characteristics of WDCRS structure were simulated using finite element method (FEM). The transmission characteristics of the exported structure were analyzed in detail. In addition, the influence of structural geometric parameters on sensing performance was also studied, and it was found that the size of the water droplet cavity is a key factor in improving sensitivity. When applied to a refractive index sensor, the structure achieves a sensitivity of up to 2,300 nm/ RIU with a corresponding figure of merit (FOM) of 60.5. These works provide some ideas for the design of high-performance nanostructures and multiple Fano resonance excitation structures. [ABSTRACT FROM AUTHOR]

Three-dimensional Au network films with flexibility and transferability were fabricated based on sputtering deposition onto electrospun nanofibers as a template. The films are constructed using long Au nanotubes that are cross-linked with each other and that have dense nanoparticles on the tube wall surface. The surface plasmon resonance (SPR) peaks for the films are tunable in a wide range, from visible light to the near-infrared region, by tuning the inner diameter and/or wall thickness of the nanotubes. Such structured film exhibits significant surface-enhanced Raman scattering (SERS) activity with good signal uniformity and stability, and possesses great potential in the in situ detection of trace organic pollutants on a solid surface by simple transferring. This study provides a Au film with a unique structure and widely tunable SPR for in situ SERS sensing and other needs. [ABSTRACT FROM AUTHOR]

Strain sensor with low temperature crosstalk over large strain range is essential for field monitoring of structural safety. We develop a tension-compression strain sensor with high strain sensitivity, large strain range, low temperature, and polarization crosstalk due to many high-order modes interferences from asymmetric off-core microsphere-waveguide. It is based on BaTiO3 (BTO) microsphere embedded on the surface of the silica core-offset fiber via negative thermal coefficient UV glue. Multi-beam reflection and whispering-gallery modes (WGMs) in the microsphere enhance the multi-mode interference, and then improve the strain sensitivity to 17.29 pm/μϵ, which is three times higher than that without microsphere, along with the temperature sensitivity of −0.87 pm/°C, and the strain accuracy of 0.5 μϵ. Thanks to the reduced effective Young's modulus by microsphere and UV glue on an asymmetric core-offset structure, strain range of tension and compression is increased to 153.2 mϵ. [ABSTRACT FROM AUTHOR]

A compact hybrid structure of silica taper-assisted dual micro-bubbles is proposed and fabricated for broadband ultrasound sensing. UV-cured adhesive is dripped on silica taper to form dual elliptic microbubbles at preselected position. Scattering and reflected light in each microbubble converges at the two focal points of the ellipse to form two new point light sources, enhancing the emergence of high-order modes with strong multi-mode interference. The second microbubble with multi-mode interference further enhances the modulated light propagating from the first microbubble, realizing high contrast transmission spectrum. By exciting high-order harmonics of piezoelectric transducer (PZT) center at 3.7 MHz as ultrasound source, broad bandwidth acoustic signal from 3 kHz to 23.5 MHz is detected, owing to this hybrid structure with strong multi-mode interference along with polymer microbubbles with small Young’s modulus and small sizes. This unique dual-microbubble device makes attractive features for biomedical application and nondestructive test inspection. [ABSTRACT FROM AUTHOR]

Fiber-optic sensors fabricated by core-offset splicing method with 2 to 12 unequal-length single-mode fiber (Corning SMF-28) segments are demonstrated for the ultrasound detection. High-order modes are created in the air and silica cladding to enhance multi-mode interference, leading to large spectrum range comparing to that case with the same number of equal fiber segments. This leads to larger wavelength tuning range which overcomes the free-spectrum-range limitation in Fabry-Perot interferometer with multiple equal fiber segments. Furthermore, wavelength shift associated with power change, and characterized as high spectrum slope defined as 1st derivative of the reflection spectrum, is realized based on these unequal-length and asymmetric waveguides, which enhances the sensitivity of ultrasound detection. Using piezoelectric transducer as ultrasound generator at 260 kHz with 10 V peak-to-peak driving voltage, we detect ultrasound response based on the fiber devices with 2 to 12 segments as sensing unit or as feedback/sensor in a fiber laser. In the former sensor case, the ultrasound response via photo-detector could be increased from 15.5 mV for the core-offset fiber with 2 unequal-length segments to 115.3 mV for that with 12 segments, corresponding to a factor of 7.4 times improvement due to narrow linewidth of reflection spectrum in the fiber-optic sensor with more unequal-length fiber segments. The fiber laser sensor for the ultrasound detection brings improvement from 31.6 mV to 207.1 mV with the enhancement of 6.6 times, thanks to higher quality factor (Q) due to the optical filter formed by unequal-length core-offset fiber. [ABSTRACT FROM AUTHOR]

Fiber-optic ultrasound transmitters can be used in structural health monitoring, material characterization, and biomedical imaging. However, the bandwidth of current fiber-optic ultrasound transmitters is tens of MHz which prevents the ultrasound image on cells with resolution of $\mu \text{m}$ level. Here, ultra-compact in-fiber ultrasound transmitter is proposed by using the multi-mode interference in the curved UV-cured adhesive waveguide, which is fabricated between two fiber endfaces over a sandwiched core-offset spliced fiber segment. The curved adhesive waveguide can improve the contrast of device reflection spectrum from 2.7 dB to 21.7 dB. The ultrasound is generated by optical pulse excitation via photoabsorption and thermal expansion of UV-cured adhesive, and then detected via the multi-mode interference of the same waveguide with hundreds of Hz to 306 MHz broad transverse acoustic waves. This ultra-compact and cost-effective device offers new opportunities to the advanced biomedical and ultrasound-based applications. [ABSTRACT FROM AUTHOR]

We propose and numerically analyze a fiber-optic sensor based on a silica taper-assisted multiple polymer microspheres to realize high-sensitivity refractometric sensing due to the excitation of multi-beam interference and whispering-gallery modes (WGMs) in each microsphere. Up to 5 UV-cured adhesive microspheres are dripped in sequence on the 2 µm-diameter silica taper with the certain distances in between. Scattering and reflection of light in each microsphere enhance the multi-beam interference because of the emergence of high-order modes. Moreover, WGMs with certain resonant wavelengths are excited in the microspheres, further enhancing the transmission spectrum with high contrast and quality factor. Furthermore, irregular transmission spectrum corresponds to the large wavelength tuning range. Propagating light between the microspheres could be strongly affected by the environmental factor with the large transmission spectrum shift, therefore realizing the refractometric sensing with high sensitivity of 846 nm/RIU based on the taper-assisted 2-spheres sensor. Furthermore, another two microspheres are placed on both sides of 2-spheres sensor to form 4-spheres-loop sensor, re-collecting those large-angle scattering light with the enhancement of multi-beam interference and WGMs. Therefore, the sensitivity of 4-shperes-loop sensor is further improved to the maximum of 1296.3 nm/RIU. [ABSTRACT FROM AUTHOR]