High Sensitivity Fiber Refractive Index Sensors Based on Asymmetric Supermodes Interference in Tapered Four Core Fiber

We demonstrate high sensitivity fiber refractive index (RI) sensor based on asymmetric supermode interferences in tapered four core fiber (TFCF). To make TFCF-based RI sensors, the whitelight was launched into any one of the cores to define the excitation orientation and is called a vertex-core excitation scheme. When the four-core fiber (FCF) was gradually tapered, the four cores gathered closer and closer. Originally, the power coupling occurred between its two neighboring cores first and these three cores are grouped to produce supermodes. Subsequently, the fourth diagonal core enters the evanescent field overlapping region to excite asymmetric supermodes interferences. The output spectral responses of the two cores next to the excitation core are mutually in phase whereas the spectral responses of the diagonal core are in phase and out of phase to that of the excitation core at the shorter and longer wavelengths, respectively. Due to the lowest limitation of the available refractive index of liquids, the best sensitivity can be achieved when the tapered diameter is 10 μm and the best RI sensitivity S is 3249 nm/RIU over the indices ranging from 1.41–1.42. This is several times higher than that at other RI ranges due to the asymmetric supermodes.

Theoretical study on the sensitivity of dynamic acoustic force measurement through monomodal and bimodal excitations of rectangular micro-cantilever

We present a detailed analysis on measurement sensitivity of dynamic acoustic forces via numerical simulation of the micro-cantilever responses. The rectangular micro-cantilever is regarded as a point mass in the dynamic model of forced and damped harmonic oscillator. We use single- and bimodal-frequency excitation schemes for actuation of the micro-cantilever in the presence of dynamic acoustic forces. In bimodal-frequency excitation scheme, the micro-cantilever is excited at its first two eigenmode frequencies simultaneously as opposed to single-frequency excitation. First, we numerically obtain micro-cantilever deflections by solving the Equations of Motions (EOMs) constructed for the first two eigenmodes. Then, we determine oscillation amplitude and phase shift as a function of acoustic force strength within different frequency regions. Moreover, we relate amplitude and phase shift to virial and energy dissipation in order to explore the interaction between flexural modes in multifrequency excitation. The simulation results point out that bimodal-frequency excitation improves the measurement sensitivity of dynamic acoustic forces at particular frequencies. Herein, simultaneous application of driving forces enables higher sensitivities of observables and energy quantities as acoustic force frequencies become around the eigenmode frequencies. For our case, we obtain the highest phase shift (approximately 178 degrees) for the acoustic force strength of 100 pN at the frequency of around 307.2 kHz. Therefore, this method can be easily adapted to improve measurement sensitivity of dynamic acoustic forces in a wider frequency window.

Virial and Energy Dissipation in Measurement of Dynamic Acoustic Forces Using Bimodal-frequency Excitation of Micro-cantilever Array

Virial and energy dissipation, related to oscillation observable responses, possess complementary information regarding acoustic force measurements. In this paper, we introduce a mathematical framework describing the analytic relationship between oscillation observables and energy quantities at the second eigenmode in the measurement of dynamic acoustic forces. We utilize a bimodal-frequency excitation scheme for actuation of the micro-cantilever array to obtain high-sensitivity frequency bands. Herein, we analyze the virials of acoustic force interaction and the energy dissipation levels on the domain of acoustic force frequency. For our case, we obtain the high-frequency bands of around 200-270 kHz and 440-570 kHz for the force strengths in the range of 4.0-36.0 pN. In addition, results of virials and dissipated power with respect to acoustic force strengths are introduced for low- and high-sensitivity frequency regions. Therefore, the energy quantities can be robustly utilized to determine high-sensitivity frequency windows in the measurement of dynamic acoustic forces.

High-Harmonic Injection-Based Brushless Wound Field Synchronous Machine Topology

This paper discusses the design and analysis of a high-harmonic injection-based field excitation scheme for the brushless operation of wound field synchronous machines (WFSMs) in order to achieve a higher efficiency. The proposed scheme involves two inverters. One of these inverters provides the three-phase fundamental-harmonic current to the armature winding, whereas the second inverter injects the single-phase high-harmonic i.e., 6th harmonic current in this case, to the neutral-point of the Y-connected armature winding. The injection of the high-harmonic current in the armature winding develops the high-harmonic magnetomotive force (MMF) in the air gap of the machine beside the fundamental. The high-harmonic MMF induces the harmonic current in the excitation winding of the rotor, whereas the fundamental MMF develops the main armature field. The harmonic current is rectified to inject the direct current (DC) into the main rotor field winding. The main armature and rotor fields, when interacting with each other, produce torque. Finite element analysis (FEA) is carried out in order to develop a 4-pole 24-slot machine and investigate it using a 6th harmonic current injection for the rotor field excitation to both attain a brushless operation and analyze its electromagnetic performance. Later on, the performance of the proposed topology is compared with the typical brushless WFSM topology employing the 3rd harmonic current injection-based field excitation scheme.

Influence of Pumping Regime on Temperature Resolution in Nanothermometry

In recent years, optical nanothermometers have seen huge improvements in terms of precision as well as versatility, and several research efforts have been directed at adapting novel active materials or further optimizing the temperature sensitivity. The signal-to-noise ratio of the emission lines is commonly seen as the only limitation regarding high precision measurements. The role of re-absorption caused by a population of lower energy levels, however, has so far been neglected as a potential bottleneck for both high resolution and material selection. In this work, we conduct a study of the time dependent evolution of population densities in different luminescence nanothermometer classes under the commonly used pulsed excitation scheme. It is shown that the population of lower energy levels varies when the pump source fluctuates in terms of power and pulse duration. This leads to a significant degradation in temperature resolution, with limiting values of 0.5 K for common systems. Our study on the error margin indicates that either short pulsed or continuous excitation should be preferred for high precision measurements. Additionally, we derive conversion factors, enabling the re-calibration of currently available intensity ratio measurements to the steady state regime, thus facilitating the transition from pulse regimes to continuous excitation.

Investigation of a new method for sound generation – Advanced Digital Sound Reconstruction

The current loudspeaker market has a high demand for portable audio devices. Hence, the miniaturization of loudspeakers (microspeakers) is of great importance for manufacturers. Traditional loudspeakers – for example the electrodynamic loudspeaker – are the forerunners, but so-called MEMS loudspeakers (Micro-Electro-Mechanical-System) have emerged recently. MEMS devices have already been used for sensors (i.e., microphones) to a great extend due to their advantages regarding form factor and production efficiency. Albeit additional challenges for actuators like moving enough air with a microstructure – as it is the case for a loudspeaker – the usage of MEMS technology for loudspeakers is very attractive.Since especially low-frequency audio signals often pose problems for microspeakers, this article focuses on a new sound generation technique called Advanced Digital Sound Reconstruction (ADSR) which is especially well-suited for low-frequency audio signals since ADSR can generate more volume displacement relative to its size. Based on a general description of the principle, an outlook of the possibilities regarding achievable sound pressure compared to the classical excitation scheme is derived. Furthermore, measurements are presented, which aim to prove the concept of ADSR based on already existing actuators.

Simultaneous Two-Photon Fluorescence Microscopy of NADH and FAD Using Pixel-to-Pixel Wavelength-Switching

Two-photon fluorescence (TPF) microscopy of intrinsic fluorophores provides physiological and pathological information from biological tissues. Reduced nicotinamide adenine dinucleotide (NADH) and flavin adenine dinucleotide (FAD) are two endogenous fluorescent coenzymes existing on the intracellular scale. Autofluorescence images of NADH and FAD have been applied to noninvasively record changes during metabolism, according to their distributions and concentrations. However, the widely used sequential (non-simultaneous) excitation scheme results in artifacts caused by sample motion or laser power fluctuation. The single-wavelength illumination scheme suffers from low excitation efficiency and spectral bleed-through. In this paper, we demonstrate a new imaging system simultaneously capturing autofluorescence images from NADH and FAD, with high excitation efficiency and negligible spectral bleed-through. Two temporally multiplexed and spatially overlapped excitation beams were achieved with fast-switching light paths based on an electro-optic modulator. The switching beams were centered at 750 and 860 nm, enabling independent excitations of NADH and FAD. Autofluorescence images of NADH and FAD were acquired at the wavelength ranges of 415–455 nm and 500–550 nm, respectively. The electro-optic modulator was synchronized with the pixel clock from the microscope, achieving pixel-to-pixel wavelength-switching. The capability of the system was demonstrated by performing TPF imaging of freshly excised mouse colon tissues. The microenvironment of the colon wall was depicted by the distributions of colonocytes, goblet cells, and crypts of Lieberkühn, and the relative concentrations of NADH and FAD were estimated. The experimental results show that the system can effectively perform simultaneous imaging of NADH and FAD, and is considered a promising tool for investigations into metabolism-associated processes and diseases.

Multicolor three-photon fluorescence imaging with single-wavelength excitation deep in mouse brain

Multiphoton fluorescence microscopy is a powerful technique for deep-tissue observation of living cells. In particular, three-photon microscopy is highly beneficial for deep-tissue imaging because of the long excitation wavelength and the high nonlinear confinement in living tissues. Because of the large spectral separation of fluorophores of different color, multicolor three-photon imaging typically requires multiple excitation wavelengths. Here, we report a new three-photon excitation scheme: excitation to a higher-energy electronic excited state instead of the conventional excitation to the lowest-energy excited state, enabling multicolor three-photon fluorescence imaging with deep-tissue penetration in the living mouse brain using single-wavelength excitation. We further demonstrate that our excitation method results in ≥10-fold signal enhancement for some of the common red fluorescent molecules. The multicolor imaging capability and the possibility of enhanced three-photon excitation cross section will open new opportunities for life science applications of three-photon microscopy.

Focused X-ray Luminescence Computed Tomography using a Continuous Scanning Scheme

X-ray luminescence computed tomography (XLCT) imaging is a hybrid molecular imaging modality combining the merits of both conventional x-ray imaging (high spatial resolution) and optical imaging (high measurement sensitivity). The narrow x-ray beam based XLCT imaging has been shown to be promising. However due to the selective excitation scheme, the imaging speed is slow thus limiting its practical applications for in vivo imaging. In this work, we have introduced a continuous scanning scheme to acquire data for each angular projection in one motion, eliminating the previous stepping scheme and reducing the data acquisition time, which makes it feasible for multiple transverse scans for three-dimensional (3D) imaging. We have introduced a high accuracy vertical stage to our focused x-ray beam based XLCT imaging system to perform high-resolution and 3D XLCT imaging. We have also included a scintillator crystal coupled to a PMT to act as a single-pixel detector for boundary detection purposes to replace our previous flat panel x-ray detector. We have verified the feasibility of our proposed scanning scheme and imaging system by performing phantom experimental studies. A phantom was embedded with a set of cylindrical targets with 200 µm edge-to-edge distance and was scanned in our imaging system with the proposed method. To test the feasibility for 3D scanning, we took measurements from 4 transverse slices with a vertical step size of 1 mm. The results of the experiments verified the feasibility of our proposed method to perform 3D XLCT imaging using a narrow x-ray beam in a reasonable time.

Hyperpolarized 13C Spectroscopy with Simple Slice-and-Frequency-Selective Excitation

Hyperpolarized 13C nuclear magnetic resonance spectroscopy can characterize in vivo tissue metabolism, including preclinical models of cancer and inflammatory disease. Broad bandwidth radiofrequency excitation is often paired with free induction decay readout for spectral separation, but quantification of low-signal downstream metabolites using this method can be impeded by spectral peak overlap or when frequency separation of the detected peaks exceeds the excitation bandwidth. In this work, alternating frequency narrow bandwidth (250 Hz) slice-selective excitation was used for 13C spectroscopy at 7 T in a subcutaneous xenograft rat model of human pancreatic cancer (PSN1) to improve quantification while measuring the dynamics of injected hyperpolarized [1-13C]lactate and its metabolite [1-13C]pyruvate. This method does not require sophisticated pulse sequences or specialized radiofrequency and gradient pulses, but rather uses nominally spatially offset slices to produce alternating frequency excitation with simpler slice-selective radiofrequency pulses. Additionally, point-resolved spectroscopy was used to calibrate the 13C frequency from the thermal proton signal in the target region. This excitation scheme isolates the small [1-13C]pyruvate peak from the similar-magnitude tail of the much larger injected [1-13C]lactate peak, facilitates quantification of the [1-13C]pyruvate signal, simplifies data processing, and could be employed for other substrates and preclinical models.