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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]

Large strain with submicro resolution is essential for steel structural monitoring; however, the fiber base sensors are limited by the glass extension to be less than 1%. Here, we propose a U-shape core-offset fiber sensor including four fiber segments to realize a large strain sensor. Four fiber segments with slight length differences in between are core-offset fused together to achieve U-shape spring-like microstructure fiber for large transverse bending radius. The reflected high-order modes at three silica/air interfaces interfere to give a broad spectrum due to unequal segment length, which enables continuous strain detection over 35 mɛ. The air and glass hybrid structure of the device enables the large bending, and hence compression and tension measurement can be achieved simultaneously. The strain sensitivity is up to 20.75 pm/µɛ with the strain accuracy of 0.5 µɛ. This novel, to the best of our knowledge, core-offset fiber has high strain sensitivity and large strain range for compression and tension strain measurement. Furthermore, the proposed strain sensor can be fabricated easily for practical applications where large strain with high strain accuracy is needed.

An ultracompact twisted silica taper with an asymmetric structure is fabricated by fire-drawing two twisted single-mode fibers for broadband ultrasound sensing. A piezoelectric transducer (PZT), peaked at 3.7 MHz, is used as an ultrasound generator. A steel plate with a silica taper attached is adhered to the PZT and is used as the ultrasound propagation medium. The transmission spectrum of the silica taper has high contrast owing to multimode interference in this twisted silica structure. Specially, the taper waist length and waist diameter are optimized for the highest optical sensitivity with high contrast at high slope in the transmission spectrum. The ultrasound sensitivity is characterized by a different thickness of the steel plate from 0 to 2.36 mm to achieve the highest ultrasound frequency response. With the taper waist length of 5 mm, waist diameter of 5 µm, and steel thickness of 0 mm, a broadband ultrasound frequency of 20 kHz to 94.4 MHz can be detected at high harmonics of the PZT, verifying the high sensitivity of the compact twisted silica taper.

Combined compression-tension strain sensors with a range of 1 micro to a maximum of 20 milli-strain based on non-uniform multiple-core-offset fibers have been realized. A large strain range with high resolution is ideal for monitoring deformation of steel structures where a large compressive and tensile strain co-exists. Thanks to core-offset splicing of non-uniform fiber segments, unique asymmetric waveguides reduce the degeneracy of each section, realizing a reflection spectrum with a large range and irregular shape. Furthermore, enhanced multi-mode interference induced from high-order modes in silica cladding and air results in the large strain range with high resolution in both compression and tension regions. The sensitivity of 7.93 pm/µε with a strain step of 1.7 µε is achieved for micro-strain measurement. For milli-strain measurement, a strain coefficient of 1.298 nm/mε over a tensile strain of 13.2 mε is realized; in the compressive strain case, a coefficient of -1.251 n m / m ε over compression of 20.1 mε is observed.

Whispering-gallery-mode optical microresonators have found impactful applications in various areas due to their remarkable properties such as ultra-high quality factor (Q-factor), small mode volume, and strong evanescent field. Among these applications, controllable tuning of the optical Q-factor is vital for on-chip optical modulation and various opto-electronic devices. Here, we report an experimental demonstration with a hybrid structure formed by an ultra-high-Q microtoroid cavity and a graphene monolayer. Thanks to the strong interaction of the evanescent wave with the graphene, the structure allows the Q-factor to be controllably varied in the range of 3.9 × 10 5 ∼ 6.2 × 10 7 by engineering optical absorption via changing the gap distance in between. At the same time, a resonant wavelength shift of 32 pm was also observed. Besides, the scheme enables us to approach the critical coupling with a coupling depth of 99.6%. As potential applications in integrated opto-electronic devices, we further use the system to realize a tunable optical filter with tunable bandwidth from 116.5 MHz to 2.2 GHz as well as an optical switch with a maximal extinction ratio of 31 dB and response time of 21 ms.

An ultra-compact ultrasound sensor with a chalcogenide (ChG) microfiber and a silica microfiber is fabricated. It can detect the ultrasound wave generated by a piezoelectric transducer peaked at 3.7 MHz. The sensor shows the broadband ultrasound frequency response with a high signal-to-noise ratio (SNR), owing to the high refractive index and small Young's modulus of ChG material. A ChG (${{\rm As}_2}{{\rm Se}_3}$As 2 Se 3 ) microfiber of 2 µm diameter is adhered to the surface of a silica microfiber with a diameter of 5 µm via Van der Waals force; the transmission spectrum has high contrast due to multi-mode interference in this hybrid structure. The SNR response could be up to 70 dB, especially at low frequency due to the soft ChG microfiber as a sensing unit to magnify the ultrasound signal, along with an SNR over 12 dB at ultrasound of 31.2 MHz. As a comparison, a silica taper with the same size as the ChG microfiber is also placed on the silica microfiber with a diameter of 5 µm to detect an ultrasound signal from 18 kHz to 9.4 MHz with an SNR lower than that based on the ChG microfiber up to 38.8 dB, showing the high sensitivity of the ChG microfiber.

Highly sensitive and broadband ultrasound (tens of megahertz) detectors can extend the capabilities of photo-acoustic imaging in biomedical application and nondestructive test inspection. Here ultra-compact fiber-based multi-mode dual-cavity Fabry-Perot interferometer (DC-FPI) ultrasound sensors are proposed by splicing three sections of cleaved standard single-mode fibers with the fiber off-core cross section in the middle. The contrasts, reflectivity, and linewidths of the DC-FPIs have been comprehensively characterized. The broadband frequency responses, ranging from 5 kHz to 45.4 MHz, are demonstrated by using the high harmonics of a piezoelectric transducer centered at 3.7 MHz. In addition, the influences of different resonant modes in the DC-FPI on the ultrasound frequency response have been demonstrated. The high-frequency and broadband response of such a simple and cost-effective ultrasound device offers new opportunities to the industrial ultrasound-based and advanced biomedical applications.

We experimentally demonstrate a hybrid structure microlaser on chip with a single CdSe nanowire attached to a high-Q silica microdisk cavity at room temperature. When pumped by a 532 nm pulse laser, both single-longitudinal mode and multi-longitudinal mode lasers with linewidth of 0.18 nm are obtained from the hybrid structure with a 58-µm-diameter microdisk and a 250-nm diameter nanowire. The measured lasing threshold of the microlaser is as low as 100 μJ/cm².

We experimentally demonstrate an all-optical analog to electromagnetically induced transparency (EIT) on chip using coupled high-Q silica microtoroid cavities with Q-factors above 10(6). The transmission spectrum of the all-optical analog to EIT is precisely controlled by tuning the distance between the two microtoroids, as well as the detunings of the resonance frequencies of the two cavities.