Whispering Gallery Mode Shifts Research Articles

Improving the performance of resonator-based optical sensors using time-lagged correlation and Fourier phase shift analysis

The performance of time-lagged correlation and Fourier phase shift analysis in resonator-based optical sensors was evaluated and compared to centroiding algorithms. 100 emission spectra of an Er3+ oxyfluoride microsphere were recorded and the average of all data was used as a noiseless spectrum. The Whispering Gallery Mode (WGM) signal, superimposed on the Er3+ emission band, was subtracted and shifted to simulate sensor measurements. Additive random Gaussian noise with different values of the variance was added to the original and WGM-shifted spectra, and the WGM shift was estimated using time-lagged correlation, Fourier phase shift analysis and centroiding algorithms. The procedure was repeated 10000 times to evaluate the accuracy and precision of all numerical methods, and different values of the noise level and WGM shift were considered. These pseudo-experimental studies show that the use of time-lagged correlation and Fourier phase shift analysis can improve the accuracy and precision of the sensor when compared to traditional centroiding algorithms. Additionally, signal analysis can be automated without the need for peak identification and matching.

Photonic Seismometer Based on Whispering Gallery Modes

Abstract We present the concept of an all-optical seismometer based on the principle of optical whispering gallery modes (WGMs). The proposed sensor is compact, rugged, low power, and resistant to electromagnetic interference. A cantilever configuration of a fiber-pigtailed photonic integrated circuit with a ring resonator is employed as the sensing element. The measurement approach is based on the optical excitation of the WGMs of a ring resonator using a 1313 nm tunable diode laser. A digital signal processing system analyzes the recorded WGM scans. The base acceleration is calculated from the WGM shifts caused by the deformation of the optical ring resonator. A prototype seismometer is developed, calibrated, and tested. The frequency response of the seismometer is assessed by observing the free vibration of the sensor. The preliminary results are encouraging and suggest that a WGM-based optical seismometer is feasible.

Temperature Sensor Based on Whispering Gallery Modes of Metal-Filled Side-Hole Photonic Crystal Fiber Resonators

In this study, we propose and analyze a temperature sensor based on whispering gallery modes (WGMs) in a cylindrical microresonator formed by metal-filled side-hole photonic crystal fiber (mSH-PCF). WGMs in the mSH-PCF microresonator are excited by evanescent light coupling using a tapered single-mode silica fiber. The application of temperature to the mSH-PCF microresonator causes a change in both the geometric shape of the microresonator and the refractive index produced by the thermal expansion of the filler metal, resulting in a linear shift of the WGM resonances in which the WGM quality factors do not decrease significantly during temperature changes. We also demonstrate that the temperature sensitivity of this platform is governed by the interaction between the microresonator and the fiber taper. Bismuth and indium were used to examine the effect of metal composition on temperature sensitivity. The measurements show that a maximum temperature sensitivity of 18.72 pm/°C can be achieved with the indium-filled PCF. The experimental results are in good agreement with the numerical simulations. WGM shifts were found to depend mainly on cavity deformation.

Controllable gap in microsphere resonator integrated with a deformable ferrofluid droplet for magnetic field sensing

Abstract A magnetic field sensor based on silica microsphere resonator and ferrofluid is proposed and experimentally demonstrated in this paper. A silica fiber taper with a waist diameter of 2 μm is employed to couple the evanescent light into the microsphere with a diameter of 31.21 μm. Whispering gallery modes (WGMs) are excited inside the microsphere resonator and circulate inside the equatorial plane. The essence of the sensing concept is to control the coupling gap between fiber taper and microsphere using the magnetically induced deformation of ferrofluid droplet. The shape-varying ferrofluid droplet is operated under different magnetic fields to observe the resonance shift and Q-factors of WGMs. As the magnetic field rises gradually, a longer resonant wavelength shifts with a decreasing Q-factor and a high maximum sensitivity of 120.9 pm/mT in the vertical magnetic field. On the contrary, resonance under the parallel magnetic field shifts to shorter wavelength with an increasing Q-factor and a low maximum sensitivity of 16.4 pm/mT. All experimental results are in good agreement with the theoretical model, which provides a disparate approach to integrate photonics with ferrofluid for signal processing and sensing applications.

Micro-optical vibrometer/accelerometer using dielectric microspheres.

In this paper, an optical vibrometer/accelerometer is designed using micro-optical dielectric sensors based on whispering gallery modes (WGM) phenomena. The proposed design is created as a sphere made from polydimethylsiloxane (PDMS). The dielectric sensor would be laid between a piezo stack and a proof mass of 10-4 kg (implemented as body force to the sensor) in the vertical position. Once the piezo stack starts to impose a vibration in the vertical direction, the proof mass starts to vibrate, leading to a change in the morphology of the sensor. In turn, the WGM shifts would be seen on the transmission spectrum and would be related to the vibration/acceleration of the piezo stack. An analysis based on the force transmissibility from the piezo stack to the optical sensor and calibrations are carried out along with preliminary designs and experiments. Results showed that the proposed sensor could reach a very high resolution up to 8nano-g with sensitivity dλ/da≈2.33 pm/μg.

Acoustic sensor based on fiber-optic polymeric resonator

Acoustic measurements are the obvious target of acoustic investigations in this paper, using an opto-mechanical frequency analyzer. This proposed sensor is designed using micro-optical dielectric sensors based on whispering gallery modes (WGMs). This phenomenon, commonly referred to as WGM, is excited by evanescently coupling light from a tunable laser diode using a tapered single-mode optical fiber. A measurement arrangement can typically be separated into source and receiver components. The receiver component consists of a sound level meter or sound analyzer that we call an opto-mechanical frequency analyzer, which displays the total sound level in frequency-dependent data. It can also produce results such as spectra or impulse responses. Furthermore, measurements are based on special measurement environments that allow several reference sound fields to be created. The proposed design is composed of a tuner that consists of slots with different geometries with dielectric beams placed on each one. These beams are made from polydimethylsiloxane. The spherical optical polymeric resonators are mechanically coupled to the dielectric beams placed at each slot. The tuner has a different resonant frequency at each slot, which depends on the effective length of the beam. Sound waves deform the polymeric cavity, causing a shift in its transmission spectrum. A technique based on signal cross-correlation is used to calculate that shift, which is known as WGM shift. An analysis and calibration are carried out along with preliminary designs and experiments. Results prove that the proposed technique can be used as a very high-resolution frequency analyzer due to the high quality factor (Q-factor) of the resonators, which is 106 compared to the typical electrical frequency analyzer that has a Q-factor up to 102. Similar to an optical prism, this device can be used to split sound into its constituent frequencies.

Experimental Demonstration of Temperature Sensing with Packaged Glass Bottle Microresonators.

Whispering gallery mode (WGM) glass bottle microresonators are potential highly sensitive structures for a variety of physical and bio-chemical sensing applications. In this paper, we experimentally demonstrate the practical use of glass bottle resonators as temperature sensors. The basic parameters, such as WGM resonance wavelengths, free spectral ranges, and Q factors, have been investigated by coupling light from a tapered fiber to the bottle structure. We show the spectral characteristics of the WGMs by choosing different bottle dimensions and taper diameters. For practical measurements, a robust 3D-printed package that includes the bottle resonator and the tapered fiber has been proposed. The packaged bottle has a central diameter Dc = 207 µm and a length L = 300 µm. Temperature sensing experiments were also performed. A linear response of the WGM shifts as a function of the temperature is confirmed. The fitted experimental data indicate a temperature sensitivity of 10.5 pm/K at λ ~ 1550 nm, resulting in a limit of detection of 0.06 K. These values can be compared with values reported for other WGM resonators. Additionally, bottle resonators are made with simple splicing methods and their assembly method can be easily defined due to large coupling tolerances.

Probing Stress-Induced Optical Birefringence of Glassy Polymers by Whispering Gallery Modes Light Localization.

An optical resonance method for the determination of the strain- and stress-optical coefficients of optically transparent polymers is presented and exemplified for monodisperse and bidisperse molecular weight polystyrene (PS). This method employs whispering gallery modes (WGMs) resonation inside a spheroid polymeric cavity, suspended on an optical fiber taper waist, which, in turn, is used for subjecting the polymeric resonator to controlled strain conditions. The wavelength shifts of equal order transverse electric and transverse magnetic polarization WGMs are measured, as well as their relative birefringence versus applied strain. For monodisperse PS microspheroids (2 and 50 kDa) the stress-optical coefficient is negative, contrary to the results for bulk PS in the glassy state indicating different phenyl group orientation of the PS monomer with respect to the strain direction. In the bidisperse (2 and 50 kDa) spheroid with a symmetric monomer composition, local structural irregularities are probably responsible for the observed coupling between WGMs. The method possesses metrological capabilities for probing the molecular orientation of polymer-based resonators.

Ultra-Sensitive Optical Resonator for Organic Solvents Detection Based on Whispering Gallery Modes

In this paper, a novel technique using an ultra-sensitive optical resonator based on whispering gallery modes (WGM) is proposed to detect the diffusion of organic solvents. The sensor configuration is a micro-cavity made of polymeric material. When the solvent starts to diffuse, the polymer of the cavity starts to swallow that solvent. A swollen elastomer is in fact a solution, except that its mechanical response is now elastic rather than viscous. As solvents fill the network, chains are extended. In turn, that leads to the change of the morphology and mechanical properties of the sensing element. These changes could be measured by tracking the WGM shifts. Several experiments were carried out to measure that swelling force. Ethanol and methanol are used in this paper as candidates to study their driving force of diffusion (concentration gradient) on the cavity. Additionally, this sensing design can be used for biological sensing application. Breath diagnosis can use this configuration in diabetes diagnosis since a solvent like acetone concentration in human breath leads to a quick, convenient, accurate, and painless breath diagnosis of diabetes. The optical resonator results are verified through two different analyses: theoretical and experimental modeling. These micro-optical cavities have been examined using preliminary experiments to fully investigate their response and to verify the numerical analysis. Results show that the proposed sensor yields sensitivity for the driving force of diffusion (concentration gradient) (9.405 × 1013 pm/N) with a measurement precision of ~3.6 fN.

Mathematical Model for Electric Field Sensor Based on Whispering Gallery Modes Using Navier’s Equation for Linear Elasticity

This paper presents and verifies the mathematical model of an electric field senor based on the whispering gallery mode (WGM). The sensing element is a dielectric microsphere, where the light is used to tune the optical modes of the microsphere. The light undergoes total internal reflection along the circumference of the sphere; then it experiences optical resonance. The WGM are monitored as sharp dips on the transmission spectrum. These modes are very sensitive to morphology changes of the sphere, such that, for every minute change in the sphere’s morphology, a shift in the transmission spectrum will happen and that is known as WGM shifts. Due to the electrostriction effect, the applied electric field will induce forces acting on the surface of the dielectric sphere. In turn, these forces will deform the sphere causing shifts in its WGM spectrum. The applied electric field can be obtained by calculating these shifts. Navier’s equation for linear elasticity is used to model the deformation of the sphere to find the WGM shift. The finite element numerical studies are performed to verify the introduced model and to study the behavior of the sensor at different values of microspheres’ Young’s modulus and dielectric constant. Furthermore, the sensitivity and resolution of the developed WGM electric filed sensor model will be presented in this paper.

Integrating Whispering Gallery Mode Refractive Index Sensing with Capillary Electrophoresis Separations Using Phase Sensitive Detection.

Whispering gallery mode (WGM) resonators are small, radially symmetric dielectrics that recirculate light through continuous total internal reflection. High-Q resonances are observed that shift in response to changes in surrounding refractive index, leading to many applications in label-free sensing. Surface binding measurements with WGM resonators have demonstrated competitive analytical detection metrics compared to other sensing schemes. Similar figures of merit for detecting bulk refractive index changes, however, have proven more challenging. This has limited their use in applications such as capillary electrophoresis (CE), where their compact footprint and refractive index sensitivity offers advantages in nondestructive, universal detection. Here we couple WGM detection with CE by introducing a modulation scheme to improve detection limits. Phase sensitive WGM (PS-WGM) detection is developed to monitor real-time shifts in the WGM spectrum due to changes in surrounding refractive index. We directly compare phase sensitive detection with spectral measurements normally used to track WGM shifts. We report an improvement in detection limits by almost 300-fold using the PS-WGM method. The integrated CE with PS-WGM approach is demonstrated by detecting the separation of a three-component mixture of cations (Na(+), Li(+), and K(+)).

High-Q whispering gallery modes in a polymer microresonator with broad strain tuning

We have reported the high- Q whispering gallery modes (WGMs) in a polydimethylsiloxane (PDMS) optical microresonators with broad tuning range. The PDMS microresonators are fabricated at the center of two collimating fiber tips, which can be controlled by the piezoelectric stage. Through stretching the fiber stem, the tuning range of WGMs are demonstrated more than 50 nm. Further investigations demonstrated that the WGM shift has a high force sensitivity (~ 19.7 pm/μN) of the gravitation when the microcavity is stretched by a weight. The theoretical analysis reveals that the high force sensitivity of polymer microresonator can be used for the weak force or height measurement.

Effect of wall pressure and shear stress on embedded cylindrical microlasers.

In this paper, we carried out numerical experiments to study the effect of the shear stress and the wall pressure on the optical mode shift of two embedded cylindrical microlasers. The optical cavities (laser) are encapsulated in a slab that is clamped at the bottom surface while the other sides of the slab are free-stress boundaries. When a uniform shear stress and pressure is applied on the top surfaces of the slab, the morphology of the optical resonators are perturbed. This leads to a shift in the optical modes [commonly referred to as the whispering gallery mode (WGM)] of the resonators. The effect of the geometry (size and position of the optical cavities) and materials properties on the optical mode shift are studied. The results show a linear dependency of the WGM shift on the applied external pressure. In addition, the optical mode shift is slightly dependent on the geometry and the material properties. The effect of the shear stress on the WGM shift shows a quadratic dependency and this nonlinearity is strongly dependent on the position of the resonators within the slab. The studies also show that the proposed configuration could be used as a sensor for simultaneous measurements of wall pressure and shear stress.

Carrier-induced refractive index change observed by a whispering gallery mode shift in GaN microrods

Vertical oriented GaN microrods were grown by metal-organic vapor phase epitaxy with four different n-type carrier concentration sections above 1019 cm−3 along the c-axis. In cathodoluminescence investigations carried out on each section of the microrod, whispering gallery modes can be observed due to the hexagonal symmetry. Comparisons of the spectral positions of the modes from each section show the presence of an energy dependent mode shift, which suggest a carrier-induced refractive index change. The shift of the high energy edge of the near band edge emission points out that the band gap parameter in the analytical expression of the refractive index has to be modified. A proper adjustment of the band gap parameter explains the observed whispering gallery mode shift.

Temperature response of the whispering gallery mode resonances from the green upconversion emission of an Er3+–Yb3+ co-doped microsphere

Whispering gallery mode resonances lying in the green emission from an Er3+–Yb3+ co-doped glass microsphere have been analyzed under different temperatures in the range 290–380 K. Exciting the microsphere at 997 nm results in an upconversion emission with broad bands centered at 530 and 550 nm and overlapped resonances due to the spherical cavity. Both the band shape and the position of the resonances are temperature dependent which allow the correlation of the positions with the temperature inferred by the fluorescence intensity ratio technique. A resonances shift of 4.7 pm K−1 is achieved by laser heating in the power range 50–300 mW. The minimum temperature resolution has been found to be 0.7 K in the fluorescence intensity ratio technique and 100 times smaller for the whispering gallery mode shifts.

Photonic detection and characterization of DNA using sapphire microspheres.

A microcavity-based deoxyribonucleic acid (DNA) optical biosensor is demonstrated for the first time using synthetic sapphire for the optical cavity. Transmitted and elastic scattering intensity at 1510 nm are analyzed from a sapphire microsphere (radius 500 µm, refractive index 1.77) on an optical fiber half coupler. The 0.43 nm angular mode spacing of the resonances correlates well with the optical size of the sapphire sphere. Probe DNA consisting of a 36-mer fragment was covalently immobilized on a sapphire microsphere and hybridized with a 29-mer target DNA. Whispering gallery modes (WGMs) were monitored before the sapphire was functionalized with DNA and after it was functionalized with single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA). The shift in WGMs from the surface modification with DNA was measured and correlated well with the estimated thickness of the add-on DNA layer. It is shown that ssDNA is more uniformly oriented on the sapphire surface than dsDNA. In addition, it is shown that functionalization of the sapphire spherical surface with DNA does not affect the quality factor (Q . ≈ 04) of the sapphire microspheres. The use of sapphire is especially interesting because this material is chemically resilient, biocompatible, and widely used for medical implants.

Photonic electric field sensor based on polymeric microspheres

ABSTRACTThe detection of electric field by monitoring the optical whispering gallery mode shifts of polymeric microspheres is demonstrated. Two types of spheres are considered; (i) a polydimethylsiloxane (PDMS) sphere with 60 parts base silicon elastomer‐to‐1 part polymer curing agent by volume and; (ii) a silica sphere coated with a PDMS (uncured) base. The optical mode shifts are caused by perturbations to the resonator morphology induced by electrostriction effect in the presence of an external electric field. Preliminary experiments show that the latter microsphere yields higher sensitivity (0.027 pm/V m−1) with a measurement precision of ∼1.8 V/m. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 276–279

Effects of uniaxial pressure on polar whispering gallery modes in microspheres

We investigate the detuning of whispering gallery modes (WGMs) in solid polystyrene microspheres (PM) as a function of axisymmetric stress applied to two antipodal points of the microsphere we call poles. We specifically investigate WGMs passing close to these poles, so-called polar WGMs. The applied uniaxial pressure reduces the geometrical circumference of the PM but also increases locally the refractive index at the flattened poles. Our experiments show that the WGMs shift to higher frequencies with increasing pressure and that the magnitude of the strain-induced shift depends on the radial mode number n. Furthermore an energy splitting between azimuthal modes linearly increasing with the pressure is observed. A theoretical model based on a classical ray optics approach is presented which reproduces the main results of our experimental observations.

Terahertz tuning of whispering gallery modes in a PDMS stand-alone, stretchable microsphere

We report on tuning the optical whispering gallery modes (WGMs) in a poly dimethyl siloxane-based (PDMS) microsphere resonator by more than 1 THz. The PDMS microsphere system consists of a solid spherical resonator directly formed with double stems on either side. The stems act like tie-rods for simple mechanical stretching of the microresonator, resulting in tuning of the WGMs by one free spectral range. Further investigations demonstrate that the WGM shift has a higher sensitivity (0.13 nm/μN) to an applied force when the resonator is in its maximally stretched state compared to its relaxed state.

A full free spectral range tuning of p-i-n doped gallium nitride microdisk cavity

Effective, permanent tuning of the whispering gallery modes (WGMs) of p-i-n doped GaN microdisk cavity with embedded InGaN quantum dots over one free spectral range is demonstrated by irradiating the microdisks with a ultraviolet laser (380 nm) in DI water. For incident laser powers between 150 and 960 nW, the tuning rate varies linearly. Etching of the top surface of the cavity is proposed as the driving force for the observed shift in WGMs and is supported by experiments. The tuning for GaN/InGaN microdisk cavities is an important step for deterministically realizing nanophotonic devices for studying cavity quantum electrodynamics.