Magnetic field sensing using whispering-gallery modes in a cylindrical microresonator infiltrated with ferronematic liquid crystal

Whispering-gallery mode sensor based on coupling of tapered two-mode fiber and glass capillary

A novel whispering-gallery mode (WGM) sensor is fabricated by coupling a tapered two-mode fiber and a glass capillary. By utilizing the relatively large orifice of glass capillaries, polydimethylsiloxane (PDMS) and magnetic fluid are directly injected into two WGM structured glass capillaries, respectively, allowing these materials to substantially interact with the light field of the WGM, thereby achieving temperature, pressure, and magnetic field measurements. λ1 and λ2 are the two resonant peak wavelengths of the WGM after injecting PDMS into a glass capillary. λ3 is the resonant peak wavelength of the WGM after injecting the magnetic fluid into the glass capillary. The experiments found that λ1 and λ2 exhibit high sensitivity to temperature and air pressure, and λ3 is sensitive to magnetic field. The temperature and air pressure sensitivities of λ1 are -624 pm/°C and -7.04 nm/MPa, respectively. The temperature and air pressure sensitivities of λ2 are -964 pm/°C and -15.08 nm/MPa, respectively. The magnetic field sensitivity of λ3 is 107 pm/mT in the range of 9.45-62.91 mT. The proposed sensors have the advantages of low cost, simple fabrication, and high sensitivity, and they can be applied to temperature, gas pressure, and magnetic field measurements.

AC field modulated DC magnetic field sensor based on optical whispering gallery mode microcapillary resonator

A sensitive DC magnetic field sensor is constructed by measuring the signal-to-noise ratio of an AC-modulated magnetic field at a particular frequency from an optical whispering gallery mode microcapillary resonator. The sensing element consists of an optical whispering gallery mode microcapillary resonator bonded to a magnetostrictive material that enables it to respond to external magnetic fields. A DC magnetic field sensitivity of 0.1703dB/Oe and a linear detection range from 4.8Oe to 65.7Oe are realized under an AC modulation field of 168.1kHz in the unshielded environment at room temperature. To our best knowledge, this sensitivity is about 2.3 times of the maximum sensitivity of other DC magnetic field sensors based on magnetic fluid or magnetostrictive material integrated fiber systems that use the dissipative sensing scheme. Furthermore, the sensor can operate at a stable temperature in the range of [-11∼45]°C, as long as the modulation frequency of the AC-modulation field is adjusted according to the ambient temperature. This sensor provides us with a novel DC magnetic field sensing scheme, which may play a role in industrial fields related to current and position detection in the future.

Packaged optofluidic microbottle resonator for high-sensitivity bidirectional magnetic field sensing

We demonstrate a high-sensitivity bidirectional magnetic field sensor based on a packaged optofluidic microbottle resonator (OFMBR) filled with magnetic fluid (MF). The relationship between sensitivity and different wall thicknesses and radial modes of OFMBR is theoretically analyzed. Then the thin-wall OFMBR is fabricated by etching a capillary with the fusion discharge process. The OFMBR and tapered fiber is packaged with a portable and robust coupling configuration. By applying perpendicular or parallel magnetic field directions to the OFMBR, opposite refractive index responses of the MF can be obtained, with resonant wavelengths redshifted or blueshifted as the magnetic field intensity is increased. A magnetic field sensitivity of 98.23 pm/mT can be obtained by using the second-order radial mode when the magnetic field is perpendicular to the packaged OFMBR. When the magnetic field is parallel to the packaged OFMBR, the sensitivity is -304.80 pm/mT by using the third-order radial mode and the detection limit reaches 0.0656 mT. The proposed sensor has the advantages of easy fabrication, high sensitivity, and reliability, showing a great potential in bidirectional magnetic field application.

Magnetic Field Sensing Based on Whispering Gallery Mode with Nanostructured Magnetic Fluid-Infiltrated Photonic Crystal Fiber

A kind of novel and compact magnetic field sensor has been proposed and investigated experimentally. The proposed sensor consists of a tapered single mode fiber coupled with a nanostructured magnetic fluid-infiltrated photonic crystal fiber, which is easy to be fabricated. The response of magnetic fluid to magnetic field is used to measure the intensity of magnetic field via whispering gallery mode. The magnetic field-dependent shift in resonance wavelength is observed. The maximum magnetic field intensity sensitivity is 53 pm/mT. The sensor sensitivity is inversely proportional to the thickness of the photonic crystal fiber cladding.

Label-Free MicroRNA Optical Biosensors

MicroRNAs (miRNAs) play crucial roles in regulating gene expression. Many studies show that miRNAs have been linked to almost all kinds of disease. In addition, miRNAs are well preserved in a variety of specimens, thereby making them ideal biomarkers for biosensing applications when compared to traditional protein biomarkers. Conventional biosensors for miRNA require fluorescent labeling, which is complicated, time-consuming, laborious, costly, and exhibits low sensitivity. The detection of miRNA remains a big challenge due to their intrinsic properties such as small sizes, low abundance, and high sequence similarity. A label-free biosensor can simplify the assay and enable the direct detection of miRNA. The optical approach for a label-free miRNA sensor is very promising and many assays have demonstrated ultra-sensitivity (aM) with a fast response time. Here, we review the most relevant label-free microRNA optical biosensors and the nanomaterials used to enhance the performance of the optical biosensors.

Tunable Whispering Gallery Mode Photonic Device Based on Microstructured Optical Fiber with Internal Electrodes

We propose and experimentally demonstrate the first tunable whispering gallery mode (WGM) photonic device based on side-hole microstructured optical fiber (SH-MOF) with internal electrodes, in which the WGM quality factors do not decrease significantly during the tuning process. The resonant modes are redshifted simply by increasing the temperature. A description of the thermal tuning properties of the WGMs in SH-MOF with internal electrodes is performed by using a two-stage computational methodology, where the effects of metal filling process are considered. SH-MOF devices with internal electrodes are tested and the experimental results show excellent agreement with the theory. A linear relationship between the shift rate of the WGM modes and temperature is observed. The tunable SH-MOF microresonator with internal electrodes is anticipated to find potential applications in optical filtering, optical switching, and highly integrated tunable photonic devices.

Infiltrated Photonic Crystal Fibers for Sensing Applications

Photonic crystal fibers (PCFs) are a special class of optical fibers with a periodic arrangement of microstructured holes located in the fiber's cladding. Light confinement is achieved by means of either index-guiding, or the photonic bandgap effect in a low-index core. Ever since PCFs were first demonstrated in 1995, their special characteristics, such as potentially high birefringence, very small or high nonlinearity, low propagation losses, and controllable dispersion parameters, have rendered them unique for many applications, such as sensors, high-power pulse transmission, and biomedical studies. When the holes of PCFs are filled with solids, liquids or gases, unprecedented opportunities for applications emerge. These include, but are not limited in, supercontinuum generation, propulsion of atoms through a hollow fiber core, fiber-loaded Bose⁻Einstein condensates, as well as enhanced sensing and measurement devices. For this reason, infiltrated PCF have been the focus of intensive research in recent years. In this review, the fundamentals and fabrication of PCF infiltrated with different materials are discussed. In addition, potential applications of infiltrated PCF sensors are reviewed, identifying the challenges and limitations to scale up and commercialize this novel technology.

Broadband laser-tuned whispering gallery mode in a micro-structured fiber embedded with iron oxide nanoparticles

A grapefruit microstructured fiber-based resonator embedded with iron oxide nanoparticles is demonstrated in this paper. Due to efficient photon-to-heat conversion and transfer of the magnetic nanoparticles, such a device possesses broadband all-optical wavelength tuning with high sensitivity. Experimental results show that the tuning range and sensitivity can be up to ∼5.32  nm and 0.106 nm/mW, respectively, when pump laser with a wavelength of 1550 nm is injected into the resonator. Moreover, it exhibits other excellent features such as ease of fabrication and excellent repeatability, making it a good candidate for potential applications in the area of optical filtering.