Change In Cavity Length Research Articles

Computational comparison of passive control for cavitation suppression on cambered hydrofoils in sheet, cloud, and supercavitation regimes

Cavitation is a transient, highly complex phenomenon found in numerous applications and can have a significant impact on the characteristics as well as the performance of the hydrofoils. This study compares the evolution of transient cavitating flow over a NACA4412(base) (NACA stands for National Advisory Committee for Aeronautics) cambered hydrofoil and over the same hydrofoil modified with a pimple and a finite (circular) trailing edge. The assessment covers sheet, cloud, and supercavitation regimes at an 8° angle of attack and the Reynolds number of 1×106, with cavitation numbers ranging from 0.9 to 0.2. The study aims to comprehensively understand the role of the rectangular pimple in controlling cavitation and its impact on hydrodynamic performance across these regimes. Numerical simulations were performed using a realizable model and the Zwart–Gerber–Belamri (ZGB) cavitation model to resolve turbulence and cavitation effects. The accuracy of the present numerical predictions has been verified both quantitatively and qualitatively with available experimental results. The present analysis includes the time evolution of cavities, temporal variation in total cavity volume, time-averaged total cavity volume, distributions of vapor volume fractions along the chord length, and their hydrodynamic performance parameters. Results demonstrate that rectangular pimples have significant impacts in the different cavitation regimes. In the sheet cavitation regime (σ=0.9), the NACA4412(pimpled) hydrofoil exhibits minimal cavity length and transient volume changes as compared to the NACA4412(base) hydrofoil. In the cloud cavitation regimes (σ=0.5), cavity initiation occurs differently, starting from the pimpled location for the NACA4412(pimpled) hydrofoil, unlike the initiation just downstream of the nose in the case of base hydrofoil. In the supercavitation regimes (σ=0.2), the cavity length remains comparable, but the NACA4412(pimpled) hydrofoil exhibits larger cavity volume evolution in both cloud and supercavitation regimes (σ=0.5 and σ=0.2) after initial fluctuations. Furthermore, hydrodynamic performance for the NACA4412(pimpled) hydrofoil shows 41%, 36%, and 17% lower lift coefficients, and 46%, 27%, and 9% lower drag coefficients in sheet, cloud, and supercavitation, respectively.

All-solid highly sensitive fiber-tip magnetic field sensor based on a Fabry-Perot interferometer with a breakpoint structure.

An all-solid fiber-tip Fabry-Perot interferometer (FPI) coated with a nickel film is proposed and experimentally verified for magnetic field sensing with high sensitivity. It is fabricated by splicing a segment of a thin-wall capillary tube to a standard single-mode fiber (SMF), then inserting a tiny segment of fiber with a smaller diameter into the capillary tube, and creating an ultra-narrow air-gap at the SMF end to form an FPI. When the device is exposed to magnetic field, the capillary tube is strained due to the magnetostrictive effect of the nickel film coated on its outer surface. In addition, owing to the unique breakpoint sensitivity-enhancement structure of the air-gap FPI, the elongation of the capillary tube whose length is over 100 times longer than the air-gap width is entirely transferred to the cavity length change of the FPI, and the sensor is extremely sensitive to the magnetic field as proved by our experiments, achieving a high sensitivity of up to 2.236 nm/mT for a linear magnetic field range from 40 to 60 mT, as well as a low-temperature cross-sensitivity of 56µT/°C. The all-solid stable structure, compact size (total length of ∼3.0 mm), and reflective working mode with high magnetic field sensitivity indicate that this sensor has good application prospects.

Free spectral range measurement based on a multi-longitudinal mode laser self-mixing ellipse fitting degree detection technique.

A method by detecting the ellipse fitting degree of the trajectory equation formed by two self-mixing (SM) signals in the multi-longitudinal mode laser SM system with a Wollaston prism is presented to test the free spectral range (FSR) of the laser. By utilizing the orthogonal vector and phase-shift characteristics between adjacent longitudinal modes, the fluctuations in multi-mode SM effects caused by changes in the external cavity length are transformed into alterations in the trajectory composed of two orthogonal SM signals. The FSR is calculated by detecting the difference in external cavity lengths between the two positions, where the trajectory of the two SM signals best fits an ellipse. To achieve an automatic FSR measurement, the ellipse fitting degree is proposed as the criterion for positioning the external cavity mode. Experimental results indicate that the FSR of the laser diode is measured to be 85.23 GHz with a resolution of 0.48 GHz, while the corresponding external cavity resolution is 10 µm, and the resolution of the ellipse fitting degree is less than 1. The compact and straightforward design, coupled with high sensitivity, automated measurements, and immunity to optical feedback, holds significant promise as a robust tool for measuring FSR and assessing laser performance.

Experimental study on vertical water entry of the projectile with canard-wing

Flow control techniques play an important role during water entry. In this paper, the idea of water entry of the projectile with single canard-wing is proposed and applied to the water entry problem. The cavity evolution and motion characteristics of projectile with canard-wing were investigated through experiments, and the cavity length, trajectory, and attitude changes of projectile with canard-wing during water entry were quantified. The results show that, different from the water entry process of projectile without wing, the projectile with canard-wing has the typical characteristics of forming the attached cavity on the wing. Due to the influence of canard-wing, the trajectory deflection is always toward the side without the wing, and the initial moment of trajectory deflection is advanced with the increase in the impact velocity. The length of the fore-end cavity and the attached cavity on the wing increases as the impact velocity increases and the pinch-off depth of the fore-end cavity also increases. Moreover, the deviation of the trajectory and the attitude angle of the projectile with canard-wing increases as the impact velocity increases during water entry. The results can provide important support for the passive flow control during the water entry of the projectile and the development of the trans-media aircraft.

A Grating Interferometric Acoustic Sensor Based on a Flexible Polymer Diaphragm.

This study presents a grating interferometric acoustic sensor based on a flexible polymer diaphragm. A flexible-diaphragm acoustic sensor based on grating interferometry (GI) is proposed through design, fabrication and experimental demonstration. A gold-coated polyethylene terephthalate diaphragm was used for the sensor prototype. The vibration of the diaphragm induces a change in GI cavity length, which is converted into an electrical signal by the photodetector. The experimental results show that the sensor prototype has a flat frequency response in the voice frequency band and the minimum detectable sound pressure can reach 164.8 µPa/√Hz. The sensor prototype has potential applications in speech acquisition and the measurement of water content in oil. This study provides a reference for the design of optical interferometric acoustic sensor with high performance.

Modeling photothermal effects in high power optical resonators used for coherent levitation

Radiation pressure can be used to enable optomechanical control and manipulation of the quantum state of a mechanical oscillator. Optomechanical interaction can also be mediated by photothermal effects which, although frequently overlooked, may compete with radiation pressure interaction. Understanding of how these phenomena affect the coherent exchange of information between optical and mechanical degrees of freedom is often underdeveloped, particularly in mesoscale high-power systems where photothermal effects can fully dominate the interaction. Here we report an effective theoretical model to predict and successfully reconstruct the dynamics of a unique optomechanical system: a cavity-enhanced setup for macroscopic optical levitation, where a free-standing mirror acts as the optomechanical oscillator. We decompose the photothermal interaction into two opposing light-induced effects, photothermal expansion, and thermo-optic effects. We then reconstruct a heuristic model that links the intracavity field to four types of cavity length changes caused by acoustic (), centre of mass (), photothermal () and thermo-optic () displacements. This offers refined predictions with a higher degree of agreement with experimental results. Our work provides a means to precisely model the photothermal effect of high power optomechanical systems, as well as for developing more precise photothermal modeling of photonics systems for precision sensing and quantum measurements.

Three-wavelength phase demodulation technique for extrinsic Fabry-Perot interferometric sensors

A three-wavelength quadrature phase compensation method for recovering dynamic signals in extrinsic Fabry–Perot interferometers (EFPIs) is proposed and experimentally demonstrated. The influence of the direct current component is eliminated by using three symmetrical interferometric signals, and two orthogonal signals are obtained by the phase compensation algorithm to extract the phase change. A sinewave with a frequency of 1000 Hz was applied on an EFPI sensor. The cavity length of the EFPI sensor is 643.0299 μm. The demodulated signal is a sinewave with a frequency of 1000 Hz. The demodulated frequency shows good agreement with the frequency of the applied signal. This technique is suitable for measuring EFPI sensor cavity length changes under dynamic signals.

Design Method of Three-Component Optic Fiber Balance Based on Fabry-Perot Displacement Sensor.

This article proposes a new type of three-component optic fiber balance based on Fabry-Perot displacement measurement technology based on the structure of the pulse wind tunnel balance. This paper systematically introduces the force measurement principle and design process of a three-component optic fiber balance and conducts relevant simulation analysis and experimental verification. The simulation results show that the Fabry-Perot sensor can achieve significant sensitivity to cavity length changes, and when used in existing balance structures, sensitivity gains can be achieved by changing the probe height without the need to modify the original structure of the balance. Finally, the feasibility of the design method was verified through calibration experiments: the optic fiber balance has high sensitivity and good linearity compared to simulation sensitivity, the error is less than 6%, and the calibration accuracy of each component is better than 0.13%, which is better than the existing traditional strain balance (0.37%). The pulse wind tunnel force measurement test has a short test time and a large model mass, and the balance needs to have a large stiffness to meet the short-term force measurement requirements. The introduction of more sensitive optic fiber balance force measurement technology is expected to solve the contradiction between the stiffness and sensitivity of force measurement systems.

Four-Wavelength Passive Demodulation Technique for the Measurement of Static/Dynamic Composite Signals

A four-wavelength passive demodulation technique for the measurement of static/dynamic composite signals is proposed and experimentally demonstrated. Those previously reported passive demodulation methods for extrinsic Fabry–Perot intererometrics (EFPIs) will collapse if the cavity length changes significantly during signal demodulation. However, the proposed technique can demodulate the measurand even if the cavity length changes significantly. The phase difference between different interferometric signals is firstly calculated by using four interferometric signals. The phase compensation of these interferometric signals is then implemented to eliminate the influence of cavity length variations on the demodulation. The change of the cavity length can then be recovered. Dynamic signals, static signals, and static/dynamic composite signals were interrogated by the demodulation technique with relative errors of ~0.1%. An EFPI with a cavity length ranging from 32.53 μm to 1219.29 μm is detected by the same demodulator. The fast measurement of a cavity length change with an amplitude of ~80 μm is achieved. The technique is applicable to different types of signals applied on EFPIs with an unsteady cavity length.

High‐Resolution Investigation of a Grating‐Stabilized Laser with a Fabry–Pérot Interferometer

A diode‐based external cavity diode laser (ECDL) is built in Littrow configuration and its modal behavior as well as its current and temperature‐induced wavelength shift is investigated. The ECDL is based on a laser diode (LD) in the blue spectral range with a straight ridge and low‐reflectivity front facet. A high‐resolution grating spectrometer (SPEX 1404 0.85 m double spectrometer) as well as a scanning Fabry–Pérot Interferometer (FPI, Burleigh RC‐110 with RG‐93 ramp generator) are used for spectral characterization. With the high resolution of the FPI, an upper bound of the ECDL linewidth of about 50 fm is found, which is 75 MHz in terms of frequency. The spectral shifts with temperature and with driving current are determined. The shift of 1.15 pm mA−1of the individual mode of the external cavity is caused by a change of the group refractive index and corresponding change of the effective cavity length. The shift of 2.7 pm mA−1of the spectral center of gravity can be explained by the faster shift of the internal modes. The single‐mode shift with temperature of 16.71 pm K−1is consistently measured both by the spectrometer and the FPI.

Design and Demodulation of a Fiber-Optic Fabry-Perot Sensor Applied in a High- Frequency Pneumatic System

This study proposes a high-frequency pneumatic system and demodulation method based on polydimethylsiloxane (PDMS) film-embedded fiber-optic Fabry-Perot sensor to meet the conflicting requirements of small sizes and wide frequency-response range not achieved by conventional pneumatic probes. The high modulus of elasticity of PDMS films provides the potential for an upper limit of a pneumatic frequency response up to 20 kHz, and the new demodulation algorithm, based on the ability of the F-P cavity to return to its initial length from the maximum cavity length change, is utilized to analyze its variation. The process of recovering F-P cavity corresponds with the sparse part of the signal, which takes advantage of the unique characteristics of interference fringes in this part. Moreover, the relationship between the relative change of the F-P cavity length and pneumatic pressure, as well as the relationship between the duration of the relative change of the F-P cavity length and the frequency are analyzed comprehensively. The rationality of the proposed demodulation scheme is verified from the angle-error analyses. These analyses could be helpful for integrated and high-frequency response pneumatic detection.

Miniature tri-axis accelerometer based on fiber optic Fabry-Pérot interferometer

A fiber optic accelerometer with a high sensitivity, low noise, and compact size is proposed for low-frequency acceleration sensing. The sensor is composed of a 20 mm diameter spherical outer frame and a three-dimensional spring-mass structure as the inertial sensing element. Three Fabry-Pérot interferometers (FPI) are formed between flat fiber facets and cubic mass surfaces to measure the FPI cavity length change caused by acceleration. The dynamic signal sensing of the designed accelerometer is performed, which shows a high acceleration sensitivity of 42.6 dB re rad/g with a working band of 1-80 Hz. An average minimum detectable acceleration of 4.5 µg/Hz1/2 can be obtained. The sensor features simple assembling, small size, light weight, and good consistency. Its transverse sensitivity is measured to be less than 3% (-30 dB) of the sensitive axis. The experimental result indicates that the proposed accelerometer has application potential in areas such as seismic wave detection and structural health monitoring.

Spider dragline silk-based humidity alarm sensor with ultra-high sensitivity

The sensitivity of Fabry–Perot (FP) sensor is determined by the change of cavity length. To achieve high sensitivity, we employ supercontraction property of spider dragline silk (SDS) to fabricate a FP cavity, and the length of the cavity can change up to 99.9% of the initial cavity length. The cavity consists of two reflecting surfaces which are the fiber end and the glass plane coated with gold. These two reflecting surfaces are connected by 1 cm long SDS, and the initial length of the FP cavity is 2 mm. Supercontraction property enables SDS shrink axially in length under high relative humidity (RH) environment, shrinking SDS pulls two reflecting surfaces closer to each other, so that the cavity length decreases and the free spectral range (FSR) of interference spectrum increases when environmental RH is more than 68%. Thus the proposed FP cavity length is adjustable with the change of RH, and it can change dramatically in the high humidity. FSR of the obtained spectrum can change from 0.600 nm to 466.538 nm when the RH increases from 68%RH to 95%RH. The experimental results indicate that the average sensitivity of the proposed SDS-based sensor is 27.355 nm/%RH in the range of 78%RH–95%RH, and the maximum sensitivity is 266.825 nm/%RH when RH increases from 94%RH to 95%RH. Supercontraction property makes SDS have the potential for RH sensing in high humidity range with high sensitivity. The proposed humidity sensor works only when the environmental RH increases to more than 68%RH, and its sensitivity increases with the increase of RH, thus it can work as a humidity alarm to protect storage from wetting.

Fiber-Tip Fabry–Perot Cavity Pressure Sensor With UV-Curable Polymer Film Based on Suspension Curing Method

We investigate a novel and compact fiber-tip pressure sensor based on Fabry-Perot interferometer (FPI). The proposed device is fabricated by welding a Section of hollow silica tube (HST) on the end of single mode fiber (SMF) and then coating with ultraviolet (UV) curable polymer film at the tip of HST. A simple suspension curing method is introduced to control the formation of the UV curable polymer layer. Therefore, different thicknesses of the polymer film can be easily controlled. The pressure sensing mechanism is based on the cavity length change induced by ambient pressure. The sensor characteristics are studied theoretically and experimentally. The proposed fiber-tip pressure sensor with polymer film thickness of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$5.7\,\, \mu \text{m}$ </tex-math></inline-formula> exhibits a pressure sensitivity of 396 pm/kPa in the range of 0–30 kPa based on wavelength shift of the interference dip. Moreover, temperature crosstalk is measured, and the thermal-induced deformation of the polymer film is simulated using the Finite Element Analysis method. The proposed fiber-tip Fabry-Perot cavity pressure sensor has advantages of high sensitivity, fast response, excellent repeatability, compact size, cost effectiveness and easy fabrication, which makes it suitable for high sensitivity pressure detection in a limited space.

Slow light frequency reference cavities—proof of concept for reducing the frequency sensitivity due to length fluctuations

Length changes due to thermo-mechanical noise originating from, for example, Brownian motion are a key limiting factor of present day state-of-the-art laser frequency stabilization using Fabry–Pérot cavities. We present a laser-frequency stabilization concept using an optical cavity with a strong slow-light effect to reduce the impact of cavity length changes on the frequency stability. The resulting noise-reduction factor is proportional to the ratio between the light phase and group velocities in the highly dispersive cavity spacer. We experimentally demonstrate a proof-of-principle implementation of this laser-frequency stabilization technique using a rare-earth doped crystalline cavity spacer in conjunction with semi-permanent spectral tailoring to achieve precise control of the dispersive properties of the cavity. Compared to the same setup in the absence of the slow-light effect a reduction in frequency sensitivity of four orders of magnitude was achieved.

THE DEMODULATION ALGORITHM OF OPTICAL FIBER FABRY–PEROT SENSOR USING THE NONLINEAR DYNAMIC CHARACTERISTICS OF THE OPTICAL FIBER BROADBAND LIGHT SOURCE

The purpose is to optimize the optical fiber Fabry–Perot (F-P) sensor’s performances to provide a realistic basis for the optical fiber’s development. First, the classification of the optical fiber F-P sensors is summarized. The optical fiber F-P sensors’ fundamental principles are analyzed, focusing on the phase demodulation method that converts the optical fiber F-P sensing system’s input light source into a broadband light source. The broadband light source’s lightwave changes are analyzed, and the Fourier transform method and the fringe counting method are proposed to optimize the phase demodulation method’s function and analyze the light source’s nonlinear dynamic characteristics. According to the proposed demodulation method, the optical fiber F-P sensor with a cavity length change of [Formula: see text][Formula: see text][Formula: see text]m is simulated, and the cavity length selected for the simulation spectrum is the change range’s center value, 185[Formula: see text]um. The wavelength division multiplexer (WDM) decomposes the two optical signals, and an equation directly calculates the wavelength value orthogonal to its phase. The results show that the light intensity’s initial value of the actual spectrum is 0.04, and that of the standard function is 0.06. The light intensities for the maximum peak values are the same, 0.12, with the wavelength in [Formula: see text]–[Formula: see text][Formula: see text]m. When the wavelength exceeds [Formula: see text][Formula: see text]m before the next peak, the light intensity difference between them is large. The two orthogonal optical signals’ peaks change between the cavity length 0.18[Formula: see text]nm and 0.19[Formula: see text]nm after the wavelength selected, and the two signals’ peak values are orthogonal with the cavity length changes. The two signals’ initial and end peak values are different. The curve obtained by the two signals division calculation shows a tangent trend with the highest peak value of 87 and the lowest of [Formula: see text]. The peak curve changes obtained by the two signals’ arctangent calculation are in an orthogonal state, with the maximum peak value of 1.3 and the minimum of [Formula: see text]. The reflected light peak value changes of the optical fiber F-P sensor are in an orthogonal state, with the highest value of [Formula: see text] and the lowest [Formula: see text], and the adjacent peak changes in the wavelengths of 1500–1580 are the same. The curve’s fitting linearity is 1, and the linearity in the dynamic demodulation is 0.9776. The sampling frequency in the phase demodulation is [Formula: see text][Formula: see text]Hz, and the sampling frequency in the dynamic demodulation is [Formula: see text][Formula: see text]Hz, showing that upgrading the interface can increase the dynamic sampling frequency continuously to improve the device performance.

Optical Interferometric Force Sensor Based on a Buckled Beam

This paper reports a novel extrinsic Fabry-Perot interferometer (EFPI)-based fiber optic sensor for force measurement. The prototype force sensor consists of two EFPIs mounted on a stainless steel rectangular frame. The primary sensing element, i.e., the first EFPI, is formed between the endface of a horizontally-placed optical fiber and a stainless steel buckled beam. The second EFPI, fashioned between a longitudinally-placed optical fiber and a silver-coated glass beam, is arranged to demonstrate the amplification mechanism of the buckled beam structure. When the sensor is subjected to a tension force, the pre-buckled beam will deflect backward, resulting in a longitudinal/axial displacement of the pre-buckled beam. The axial displacement is further transferred and amplified to a horizontal/vertical deflection at the middle of the buckled beam, leading to a relatively-significant change in the Fabry-Perot cavity length. A force sensitivity of 796 nm/N (change in cavity length/Newton) is achieved with a low-temperature dependence of 0.005 N/°C. The stability of the sensor is also investigated with a standard deviation of ± 5 nm, corresponding to a measurement resolution of ±0.0064 N. A simulation is conducted to study the axial displacement and stress distribution of the sensor when it is subjected to a tension load of 250 N. It is demonstrated that the maximum stress of the sensor is tremendously reduced attributed to the buckled design, enabling a long service life cycle of the force sensor. The robust and simple-to-manufacture force sensor has great potential in structural health monitoring, robotics control, and oil/ gas refining systems.

Development of a Compact and Robust Mid-Infrared Spectrometer by Using a Silicon/Air Hyperspectral Filter

The molecular absorption spectrum in the mid-infrared (mid-IR) region is typically measured by Fourier-transform infrared spectroscopy (FTIR). Such spectrometers require complex and delicately aligned interferometers, which increases their size and cost. Here, we present an alternative compact mid-IR spectrometer, which uses a variable mid-IR filter with a gradual change of cavity length between two silicon/air dielectric mirrors. When combined with a modern uncooled thermal imaging camera, hyperspectral filtering can provide a powerful solution for the miniaturization of mid-IR spectrometers. By using the hyperspectral filter, light from a broadband light source can be dispersed and assigned to individual pixels of a microbolometer array of the thermal imaging camera. This technology offers an inexpensive and compact mid-IR spectrometer design with no moving parts and rapid acquisition time.

Spider dragline silk-based FP humidity sensor with ultra-high sensitivity

We utilize the supercontraction property of spider dragline silk (SDS) to fabricate a Fabry-Perot (FP) humidity sensor whose FP cavity length can change up to 61.87% of the initial cavity length. Therefore, the sensor has ultra-high humidity sensitivity (up to 23.14 nm/%RH), because the change of cavity length determines the sensitivity of FP sensor. The FP interferometer consists of two reflective surfaces: the end of a single-mode fiber (SMF) and the glass plane coated with gold. The SMF is inserted into and fixed with a round quartz tube which is connected to two SDS-based humidity driven-muscles and two elastic blocks. Supercontraction property enables the SDS-based muscles to shrink axially in length in the environment with high relative humidity (RH). Under the contraction force of muscles, the fiber end will be pulled away from the gold-coated reflective plane when humidity increases. When the humidity decreases, the optical fiber driven by elastic force will be moved close to the gold-coated reflective plan, and the humidity-driven muscle will restore its original length. Therefore, the sensor can reversibly detect humidity. The average sensitivity of the sensor is 11.89 nm/%RH when humidity ranges from 32%RH to 95%RH, and the maximum sensitivity reaches 23.14 nm/%RH when RH varies from 32%RH to 55%RH. The sensor shows reversibility, good repeatability, little temperature crosstalk, and the potential to be applied in physiological detection such as respiratory monitoring.

Analyzing random lasing spectra of the zinc oxide bulk and the multiple quantum wells by empirical mode decomposition and fast Fourier transform

Empirical mode decomposition (EMD) was used to efficiently distinguish weak random-cavity emission from large broad spontaneous emission of the ZnO bulk and the multiple quantum wells (MQWs) structures. By fast Fourier transforming (FFT) the EMD results, we obtained the optical cavity lengths of random lasing and their corresponding emission intensity. With increasing pumping power, the EMD-FFT method confirms that the nonlinear trend of the random lasing emission in ZnO bulk and the change in optical cavity length is a result of change in refractive index dispersion due to the bandgap renormalization with high excited carrier density in nonpolar a-plane ZnO MQWs.