Optical Whispering Gallery Mode Cylindrical Micro-Resonator Devices for Sensing Applications

Whispering gallery mode (WGM) micro-resonators are devices attractive for many practical applications including optical sensing, micro-lasers, optical switches, tunable filters and many others. Their popularity is due to the high Q-factors and the exceptional sensitivity of their optical properties to the resonator’s size, refractive index as well the properties of the surrounding medium. The main focus of this thesis is on the cylindrical WGM micro-resonators due to the simplicity of their fabrication and light coupling that they offer in comparison with other WGM devices. At first, an in-depth experimental investigation of the WGM effect in different types of cylindrical micro-resonators was carried out in order to establish the influence of the resonator’s geometry and coupling conditions on the resulting WGM spectrum. As one of the outcomes of this study, a novel method for geometrical profiling of asymmetries in thin microfiber tapers with submicron accuracy has been proposed and demonstrated. The submicron accuracy of the proposed method has been verified by SEM studies. The method can be applied as a quality control tool in fabrication of microfiber based devices and sensors or for fine-tuning of microfiber fabrication setups. The study also resulted in better understanding of the optimum conditions for excitation of WGMs in cylindrical fiber resonators, the influence of the tilt angle between the micro-cylinder and the coupling fiber taper. A novel strain sensor formed by a polymer-wire cylindrical micro-resonator has been developed. Accurate and repeatable measurements of strain have been demonstrated experimentally with the proposed sensor for the upper range of limit of detection up to 3250 με. Practical packaging method for the proposed strain sensor on a glass microscope slide has also been realized making the sensor portable and easy to use. A study of thermo-optic tuning of the WGMs in a nematic liquid crystal-filled hollow cylindrical microresonator has been carried out. A simple and robust packaging has been realized with the proposed tunable device to ensure its stable and repeatable operation. The demonstrated thermo-optic method for the WGMs tuning is potentially useful for many tunable photonic devices. Two novel all-fiber magnetic-field sensors have been designed based on photonic crystal fibers infiltrated with a magnetic fluid and a ferronematic liquid crystal utilizing the magnetic field tunability of WGM resonances. The highest experimentally demonstrated magnetic field sensitivity was 110 pm/mT in the range of magnetic fields from 0 to 40 mT. Finally, a packaged inline cascaded optical micro-resonators (ICOMRs) design is proposed for coupling multiple micro-resonators to a single fiber and simultaneous sensing of multiple parameters (strain, temperature, humidity, or refractive index) at multiple points in space has been demonstrated. The proposed design principle can find applications in quasi-distributed sensing, optical coding, optical logic gates and wavelength division multiplexed optical communications systems

Evaluation of functional outcome of patients with fracture neck of femur treated with Austin Moore prosthesis at government medical college, Thrissur-a prospective study during October 2019 to October 2020

Background: Femoral neck fractures commonly affects the elderly population. 90% of femoral neck fractures in the elderly result from simple fall. Elderly age group suffers from innumerable illnesses such as diabetes, hypertension, coronary artery disease, chronic obstructive pulmonary disease, all of which can aggravate following a femoral neck fracture because patient can become completely bedridden. The aim of treating these patients is to make them mobilize as early as possible to achieve his or her pre-fracture functional status. The incidence of femoral neck fractures is increasing dramatically as the mean age of the population increases. Prosthetic replacement allows immediate weight bearing to elderly patients and thus helps to avoid complications of recumbency and inactivity. To study the functional outcome in patients with femoral neck fractures treated with Hemiarthroplasty using Austin Moore prosthesis.Methods: Prospective study was done among 42 consented patients who sustained neck of femur fractures and were admitted in department of orthopaedics, government medical college, Thrissur, between 01/10/19 to 01/10/20, who were aged above 60 years and presenting within 3 weeks of injury. Patients were followed up at an interval of 6 weeks, 3 months and 6 months functional outcome was analysed by modified Harris hip scoring system.Results:  Among the 42 subjects, 52.4 % were females and the rest were males. Mean age of the study population was 71.57±7.047 years. Harris hip score at end of 6 months ranged from 35 to 94. 33.3% of hemiarthroplasties had hip score of 91-100 (excellent), 38.1% of the hemiarthroplasties had scores of 81-90 (good), 19% had satisfactory/fair results whereas only 9.5% of hemiarthroplasties fell in poor category with scores below 70.Conclusions: Hemiarthroplasty with Austin Moore prosthesis is still a good option in elderly patients with femoral neck fractures

Agarose Coated Spherical Micro Resonator for Humidity Measurements

A new type of fiber optic relative humidity (RH) sensor based on an agarose coated silica microsphere resonator is proposed and experimentally demonstrated. Whispering gallery modes (WGMs) in the micro resonator are excited by evanescent coupling using a tapered fiber with ~3.3 μm waist diameter. A change in the relative humidity of the surrounding the resonator air induces changes in the refractive index (RI) and thickness of the Agarose coating layer. These changes in turn lead to a spectral shift of the WGM resonances, which can be related to the RH value after a suitable calibration. Studies of the repeatability, long-term stability, measurement accuracy and temperature dependence of the proposed sensor are carried out. The RH sensitivity of the proposed sensor depends on the concentration of the agarose gel which determines the initial thickness of the deposited coating layer. Studies of the micro- resonators with coating layers fabricated from gels with three different Agarose concentrations of 0.5%, 1.125% and 2.25 wt./vol.% showed that an increase in the initial thickness of the coating material results in an increase in sensitivity but also leads to a decrease of quality factor (Q) of the micro resonator. The highest sensitivity achieved in our experiments was 518 pm/%RH in the RH range from 30% to 70%. The proposed sensor offers the advantages of a very compact form factor, low hysteresis, good repeatability, and low cross sensitivity to temperature

Compact Humidity Sensor Based on a Multi-layer Agarose Hydrogel Coated Silica Microsphere Resonator

In this paper we report on a novel approach to implementing a compact humidity sensor that utilizes whispering gallery mode (WGM) phenomena in a silica microsphere coated with Agarose hydrogel. The spectral positions of the WGM resonances for such a sensor depend strongly on the refractive index and thickness of the coating. The WGM’s spectral shift occurs due to adsorption/desorption of the water vapor in response to changes in ambient humidity and also due to the corresponding changes of the coating thickness. We experimentally investigated the WGMs spectral shift for a 100 μm diameter silica microsphere coated with Agarose hydrogel over a wide range of relative humidity (RH) values from 30%RH to 70%RH at a constant temperature. Six dip coating cycles of 2.25% wt. /vol. Agarose hydrogel were carried out in sequence with a characterization of the sensor performed for each coating thickness. A resonance shift of 16 nm is achieved in our experiment for the six-layer Agarose hydrogel coating senso

Sensing of multiple parameters with whispering gallery mode optical fiber micro-resonators

Monitoring of multiple physical parameters, such as humidity, temperature, strain, concentrations of certain chemicals or gases in various environments is of great importance in many industrial applications both for minimizing adverse effects on human health as well as for maintaining production levels and quality of products. In this paper we demonstrate two different approaches to the design of multi-parametric sensors using coupled whispering gallery mode (WGM) optical fiber micro-resonators. In the first approach, a small array of micro-resonators is coupled to a single fiber taper, while in the second approach each of the micro-resonators within an array is coupled to a different tapered fiber section fabricated along the same fiber length. Simultaneous measurement of relative humidity and ammonia concentration in air is demonstrated with an array of two microspheres with different functional coatings coupled to a single fiber taper. Sensitivity to ammonia of 19.07 pm/ppm ammonia molecules and sensitivity to relative humidity of 1.07 pm/% RH have been demonstrated experimentally. In the second approach, an inline cascade of two cylindrical micro-resonators fabricated by coupling to multiple tapered sections along a single optical fiber is demonstrated for measurement of strain and temperature simultaneously. A strain sensitivity of 1.4 pm/με and temperature sensitivity of 330 pm/ºC have been demonstrated experimentally. Both the proposed sensing systems have the potential for increase of the number of microresonators within an array for sensing of a larger number of parameters allowing for reduction of the overall cost of sensing system

Thermo-optic Tuning of a Packaged Whispering Gallery Mode Resonator Filled with Nematic Liquid Crystal

Thermo-optic tuning of whispering gallery modes (WGMs) in a nematic liquid crystal-filled thin-walled capillary tube resonator is reported. WGMs were excited by the evanescent field from a tapered optical fiber. Tapered optical fiber fabrication and reduction of wall thickness of the capillary tube was carried out by a ceramic micro-heater brushing technique. A simple and robust packaging technique is demonstrated to ensure stable and repeatable operation of the device. Tunability of WGMs with temperature was demonstrated with a sensitivity of 267.5 ± 2.5 pm/°C. The demonstrated thermo-optic method for WGMs tuning is potentially useful for many tunable photonic devices and sensors

A Packaged Whispering Gallery Mode Strain Sensor Based on a Polymer-Wire Cylindrical Micro Resonator

We propose a whispering gallery mode (WGM) strain sensor formed by a polymer-wire cylindrical micro resonator for strain measurement applications. WGMs are generated by evanescently coupling light into the polymer-wire resonator from a silica fiber taper fabricated by the micro heater brushing technique. Accurate and repeatable measurements of strains up to one free spectral range shift of the WGMs (corresponding to 0.33% of the polymer-wire elongation, 3250 με) are demonstrated experimentally with the proposed sensor. Practical packaging method for the proposed strain sensor on a glass microscope slide has also been realized making the sensor portable and easy to handle. The robustness of the packaged coupling system is confirmed by vibration tests. The performance of the packaged strain sensor is evaluated and compared with that for an unpackaged sensor

A Comprehensive Experimental Study of Whispering Gallery Modes in a Cylindrical Micro-Resonator Excited by a Tilted Fiber Taper

Whispering gallery modes (WGMs) excitation in a cylindrical microresonator formed by a section of silica optical fiber has been studied. Evanescent light coupling into the microresonator is realized using a tapered optical fiber, fabricated by a microheater brushing technique. Several types of silica fibers with different diameters are studied as microresonators, and the influence of the resonator\u27s diameter on the excitation of WGMs is investigated. The excitation of WGMs in a cylindrical fiber resonator were studied with changes to the tilt angle between the microcylinder and the fiber taper in the range of angles from a perpendicular position (0 degrees) to large tilt angles (24 degrees). The evolution of the fiber taper transmission spectrum with the change of the tilt angle results in changes in the intensity, broadening of and a blue shift in the WGM resonance spectra. Overall losses in the taper transmission spectrum decrease with the increase of the taper tilt angle from its perpendicular position, followed by a complete disappearance of the WGM resonances at large tilt angles greater than 20 degrees

Thermo-optic tuning of a packaged whispering gallery mode resonator filled with nematic liquid crystal

Thermo-optic tuning of whispering gallery modes (WGMs) in a nematic liquid crystal-filled thin-walled capillary tube resonator is reported. WGMs were excited by the evanescent field from a tapered optical fiber. Tapered optical fiber fabrication and reduction of wall thickness of the capillary tube was carried out by a ceramic micro-heater brushing technique. A simple and robust packaging technique is demonstrated to ensure stable and repeatable operation of the device. Tunability of WGMs with temperature was demonstrated with a sensitivity of 267.5 ± 2.5 pm/°C. The demonstrated thermo-optic method for WGMs tuning is potentially useful for many tunable photonic devices and sensors

Whispering Gallery Modes in a Ferronematic-Infiltrated Fibre Micro-Resonator for Magnetic Field Sensing

Introduction Whispering gallery mode (WGM) micro-resonators are remarkable optical devices whose small mode volumes and high Q-factors make them attractive for a wide range of applications such as filters, lasers, modulators and sensors [1]. WGMs can be excited in dielectric structures with circular symmetry, such as microspheres, micro-disks, and optical fibres, and can be imagined as closed-trajectory rays confined within the cavity by almost total reflections from the curved surface of the resonator. Spectral positions of the WGM resonances are highly sensitive to the geometrical dimensions of the resonator and refractive indices of both the resonator itself and its surrounding medium. Any refractive index perturbation of the interrelated media or a change in diameter results in variations of the WGM spectrum, offering the possibility to design various optical devices [2, 3]. Photonic crystal fibres (PCF) containing micron-sized air holes can be used as cylindrical WGM resonators whose refractive index can be altered by infiltrating them with various materials. In this work we propose a novel magnetic field sensors based on a PCF resonator infiltrated with a ferronematic liquid crystal. Effective refractive index of such a WGM resonator varies in the presence of magnetic field, and thus the value of the field can be related to the spectral shift of the WGMs