Whispering gallery modes in a single silica microparticle attached to an optical microfiber and their application for highly sensitive displacement sensing

Multimode sensing based on optical microcavities

Optical microcavities have the ability to confine photons in small mode volumes for long periods of time, greatly enhancing light-matter interactions, and have become one of the research hotspots in international academia. In recent years, sensing applications in complex environments have inspired the development of multimode optical microcavity sensors. These multimode sensors can be used not only for multi-parameter detection but also to improve measurement precision. In this review, we introduce multimode sensing methods based on optical microcavities and present an overview of the multimode single/multi-parameter optical microcavities sensors. Expected further research activities are also put forward.

Rapid and high-precision displacement sensing based on the multiple mode dip areas in a SNAP microresonator

Whispering gallery mode (WGM) microresonators offer significant potential for precise displacement measurement owing to their compact size, ultrahigh sensitivity, and rapid response. However, conventional WGM displacement sensors are prone to noise interference, resulting in accuracy loss, while the demodulation process for displacement often exhibits prolonged duration. To address these limitations, this study proposes a rapid and high-precision displacement sensing method based on the dip areas of multiple resonant modes in a surface nanoscale axial photonics microresonator. By employing a neural network to fit the nonlinear relationship between displacement and the areas of multiple resonant dips, we achieve displacement prediction with an accuracy better than 0.03 µm over a range of 200 µm. In comparison to alternative sensing approaches, this method exhibits resilience to temperature variations, and its sensing performance remains comparable to that in a noise-free environment as long as the signal-to-noise ratio is greater than 25 dB. Furthermore, the extraction of the dip area enables significantly enhanced speed in displacement measurement, providing an effective solution for achieving rapid and highly accurate displacement sensing.

A Micron-Range Displacement Sensor Based on Thermo-Optically Tuned Whispering Gallery Modes in a Microcapillary Resonator

A novel micron-range displacement sensor based on a whispering-gallery mode (WGM) microcapillary resonator filled with a nematic liquid crystal (LC) and a magnetic nanoparticle- coated fiber half-taper is proposed and experimentally demonstrated. In the proposed device, the tip of a fiber half-taper coated with a thin layer of magnetic nanoparticles (MNPs) moves inside the LC-filled microcapillary resonator along its axis. The input end of the fiber half-taper is connected to a pump laser source and due to the thermo-optic effect within the MNPs, the fiber tip acts as point heat source increasing the temperature of the LC material in its vicinity. An increase in the LC temperature leads to a decrease in its effective refractive index, which in turn causes spectral shift of the WGM resonances monitored in the transmission spectrum of the coupling fiber. The spectral shift of the WGMs is proportional to the displacement of the MNP-coated tip with respect to the microcapillary's light coupling point. The sensor's operation is simulated considering heat transfer in the microcapillary filled with a LC material having a negative thermo-optic coefficient. The simulations are in a good agreement with the WGMs spectral shift observed experimentally. A sensitivity to displacement of 15.44 pm/µm and a response time of 260 ms were demonstrated for the proposed sensor. The device also shows good reversibility and repeatability of response. The proposed micro-displacement sensor has potential applications in micro-manufacturing, precision measurement and medical instruments.

Displacement sensing in a multimode SNAP microcavity by an artificial neural network

Benefiting from the coupling between the Surface Nanoscale Axial Photonics (SNAP) microcavity and the waveguide, i.e., influenced by their abrupt field overlap, multiple axial modes in the transmission spectrum form a functional relationship with the coupling position, thus enabling displacement sensing. However, this functional relationship is complex and nonlinear, which is difficult to be fitted using analytical methods. We introduce a back-propagation neural network (BPNN) to model this functional relationship. The numerical results show that the multimode sensing scheme has great potential for practical large-range, high-precision displacement sensing platforms compared with the single-mode sensing based on the whispering gallery mode (WGM) resonators.

Simulation and Optimization of SNAP-Taper Coupling System in Displacement Sensing

Sensing applications based on whispering gallery mode (WGM) microcavities have attracted extensive attention recently, especially in displacement sensing applications. However, the traditional displacement sensing scheme based on shift in a single resonance wavelength, has a lot of drawbacks. Herein, a novel displacement sensing scheme based on the surface nanoscale axial photonics (SNAP) is proposed to achieve a wide range and high-resolution displacement sensor through analyzing the transmittance of multiple axial modes. By analyzing the surface plot of the resonance spectrum with different coupling positions, the ideal coupling parameters and ERV for displacement sensing are obtained. In the following, displacement sensing with high sensitivity and a wide range is theoretically realized through adjusting the sensitivity threshold and the number of modes. Finally, we present our views on the current challenges and the future development of the displacement sensing based on an SNAP resonator. We believe that a comprehensive understanding on this sensing scheme would significantly contribute to the advancement of the SNAP resonator for a broad range of applications.

Off-axis microsphere photolithography patterned nanohole array and other structures on an optical fiber tip for glucose sensing

Off-axis microsphere photolithography (MPL) was used as a method to create a plasmonic fiber-based sensor for glucose sensing. Sensitivity of 906 nm per RIU has been achieved. And multiple nanostructures have been successfully created on a fiber tip.

Tunable polarization beam splitter and broadband optical power sensor using hybrid microsphere resonators

Based on silica microsphere resonators embedded with iron oxide nanoparticles, we proposed and fabricated an all-optical and continuously tunable polarization beam splitter (PBS), and a broadband optical power sensor (OPS) with high sensitivity. The PBS is realized since the effective refractive indexes of the transverse-electric and transverse-magnetic polarization modes in the microsphere resonator are different. Due to the excellent photothermal effect of iron oxide nanoparticles, we realized the all-optical and continuously tunable PBS based on the hybrid microsphere resonator. A maximum polarization splitting ratio of 20 dB and a tuning range of 5 nm are achieved. Based on this mechanism, the hybrid microsphere resonator can also be used as a broadband OPS. The sensitivity of the OPS is 0.487 nm/mW, 0.477 nm/mW, and 0.398 nm/mW when the probe wavelength is 690 nm, 980 nm, and 1550 nm, respectively. With such good performances, the tunable PBS and the broadband OPS have great potential in applications such as optical routers, switches and filters.

An Optical Microfiber Biosensor for CEACAM5 Detection in Serum: Sensitization by a Nanosphere Interface

The detection of carcinoembryonic antigen (CEA)-related cell adhesion molecules 5 (CEACAM5) is significant in cancer prewarning. Early diagnosis can effectively alleviate the danger of cancer. Point-of-care testing (POCT) has become a competitive technology for early detection. Fiber optic biosensors have great potential as POCT tools. However, their limits of detection (LODs) are not sufficient to afford ultralow concentration detection at the early stage. Herein, this work presents an optical microfiber sensor functionalized by a polystyrene@gold nanosphere (PS@Au nanosphere) interface for a synergistic sensitization effect to detect the ultralow CEACAM5 concentrations in serum at the early stage. The sensor's LOD achieves 3.54 × 10^-17 M in pure solution and 5.27 × 10^-16 M in serum, with the sensitization effect coupled with surface area enlargement and electromagnetic enhancement of interface. This LOD is about 6 orders of magnitude lower than that of current methods. It can be employed to detect the biomarkers at ultralow concentrations present in serum in the early stages of cancer. As the interfacial synergistic sensitization strategy is suitable for refractive index (RI)-based optical transducers, this work provides new opportunities to employ fiber optic biosensors as effective POCT tools.

Magnetic-field tuning whispering gallery mode based on hollow microbubble resonator with Terfenol-D-fixed

We propose a method of magnetic-field tuning whispering gallery modes (WGMs) based on a hollow microbubble resonator (HMBR) with Terfenol-D-fixed. WGMs are excited by the evanescent field from a tapered fiber coupling with an HMBR. Both ends of the HMBR are fixed with Terfenol-D and vary with different lengths of the Terfenol-D. The length of the Terfenol-D varies with the external magnetic field for the high magnetostriction coefficient of Terfenol-D. The magnetic field sensitivity of 0.081 pm/mT in the magnetic field range of 0.14 mT-21.8 mT is achieved. The $Q$Q-factor of the HMBR can be regulated up to ${2.07} \times {{10}^4}$2.07×10⁴ with physical stretching HMBR. This work provides a novel tuning whispering gallery mode scheme and a broad application prospect in the fields of optical measurement and precise optical clocks in the future.

Femtosecond laser enabled selective micro-holes drilling on the multicore-fiber facet for displacement sensor application

We experimentally demonstrate a femtosecond laser enabled selective micro-holes drilling technique on the multicore-fiber facet. The precise position of individual cores at the seven-core fiber facet is initially locked by the image processing algorithm, and then six micro-holes are successfully fabricated after the pulse energy of femtosecond laser is optimized. Meanwhile, the use of fabricated seven-core fiber for the application of reflective intensity-modulated fiber optics displacement sensor (RIM-FODS) is comprehensively investigated. By using the beam propagation method (BPM), we theoretically investigate the effect of micro-hole depth on the RIM-FODS performance, in terms of both dead zone and measurement range. We identify that, with the increase of micro-hole depth, the dead zone range can be substantially reduced at the expense of measurement range reduction. However, multiple micro-holes with a successive depth difference can overcome such problem. When the micro-holes with depths of 5, 10, 15, 20, 25, 30 μm are fabricated on the seven-core fiber facet, and the dead zone range can be substantially reduced from 150 μm to 20 μm, together with an extension of measurement range from 250 μm to 400 μm.

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.

Micro/Nanofibre Optical Sensors: Challenges and Prospects

Micro/nanofibres (MNFs) are optical fibres with diameters close to or below the vacuum wavelength of visible or near-infrared light. Due to its wavelength- or sub-wavelength scale diameter and relatively large index contrast between the core and cladding, an MNF can offer engineerable waveguiding properties including optical confinement, fractional evanescent fields and surface intensity, which is very attractive to optical sensing on the micro and nanometer scale. In particular, the waveguided low-loss tightly confined large fractional evanescent fields, enabled by atomic level surface roughness and extraordinary geometric and material uniformity in a glass MNF, is one of its most prominent merits in realizing optical sensing with high sensitivity and great versatility. Meanwhile, the mesoporous matrix and small diameter of a polymer MNF, make it an excellent host fibre for functional materials for fast-response optical sensing. In this tutorial, we first introduce the basics of MNF optics and MNF optical sensors, and review the progress and current status of this field. Then, we discuss challenges and prospects of MNF sensors to some extent, with several clues for future studies. Finally, we conclude with a brief outlook for MNF optical sensors.