Air-gap Cavity Research Articles

Transformer oil temperature sensing utilizing bundle plastic optical fiber sensor

A bundle plastic optical fiber (POF) that works based on an intensity modulation technique is experimentally demonstrated to sense the temperature of transformer oil. The sensor was developed using a bundle POF that is located perpendicular to an aluminum reflective film with an airgap cavity between these two elements. The simplicity of the architecture allows the development of an economical optical sensor system. To avoid interference effects by other substances in the oil, the sensor head is encapsulated with a metal protecting tube. The temperature measurement was realized in this study by monitoring the output light intensity in the visible light spectrum. For linearity range from 40 °C to 75 °C, the tested sensor exhibits a sensitivity of 0.0064 °C−1, a linearity coefficient of 0.95 and a resolution of 1.56 °C. These results demonstrate the suitability of the developed sensor for temperature oil monitoring in an electrical power transformer system.

A compact linearly polarized RLSA antenna integrated with circular ring resonator (CRR) for 5G mm-wave applications

This article introduces a compact linearly polarized (LP) radial line slot array antenna (RSLA) that incorporates a metamaterial of circular ring resonator (CRR) reflector for improving the bandwidth. Hence, this enables the antenna to operate at n257 (26.5–29.5), n258(24.25–27.5), and n261(27.5–28.35) frequency bands which are standardized by Federal Communications Commission (FCC) with a minor reduction of the directivity. Due to the short wavelength of the millimeter wave bands, the technique of the RLSA design has been advantageous for achieving high gain. The prototype of the RLSA antenna and the metamaterial CRR reflector are designed and fabricated at 28 GHz resonance frequency utilizing Rogers 5880 substrate and fed using an SSMA coaxial connector with an air gap cavity equivalent to half guide wavelength (λg/2) between substrate and ground plane. The method of integrating the CRR reflector with the LP-RLSA antenna delivered a passband transmission operating at (22–29.5) GHz. The proposed antenna has a similar agreement between simulated and measured for directivity and bandwidth. it achieved a peak directivity of 20 dB with a squinted main lobe at 12° and a fractional bandwidth of 29.13%. The fabricated RLSA antenna dimensions are 130 mm × 130 mm × 10.65 mm which is compact compared to conventional RLSA antennas.

Low-noise microwave generation with an air-gap optical reference cavity

We demonstrate a high finesse, microfabricated mirror-based, air-gap cavity with volume less than 1 ml, constructed in an array, that can support low-noise microwave generation through optical frequency division. We use the air-gap cavity in conjunction with a 10 nm bandwidth mode-locked laser to generate low phase noise 10 GHz microwaves, exhibiting a phase noise of −95 and −142 dBc/Hz at 100 Hz and 10 kHz offset frequencies, respectively. This is accomplished using the 2-point lock optical frequency division method, where we exploit 40 dB common-mode rejection of two lasers separated by 1.29 THz and locked to the same air-gap cavity. If used with an octave spanning comb, the air-gap cavity is capable of supporting 10 GHz phase noise below −160 dBc/Hz at 10 kHz offset, a level significantly lower than electronic synthesizers. These results show how extremely small optical reference cavities, operated without the benefit of vacuum enclosures or thermal insulation, can, nonetheless, support state-of-the-art microwave phase noise in compact and portable systems.

Multi-Point Optical Fiber Fabry-Perot Curvature Sensor Based on Microwave Photonics

This article reports a multi-point curvature sensor system based on multiplexed optical fiber Fabry-Perot interferometric (FPI) sensor devices and a microwave photonics interrogation technique. The FPI sensor is fabricated with the assistance of a capillary tube, where a short section of the capillary is sandwiched between two single-mode fibers, forming the air-gap Fabry-Perot cavity. Bending of the FPI device leads to changes in the fringe contrast of its reflection spectrum. Based on the microwave photonics filtering technique, variations of the fringe contrast are encoded into the changes in the peak magnitude of the passband in the frequency response of the FPI device. By multiplexing such FPI devices with different cavity lengths, multi-point measurements of curvature can be realized by tracking changes in corresponding passbands in the frequency response of the system. The FPI curvature sensor is easy-to-manufacture and cost-effective, and the microwave photonics-based system provides an alternative and robust solution to interrogating the multiplexed FPI sensors for multi-point curvature sensing that could be desired in structural health monitoring, human-machine interface sensing, and other related fields.

Effects of Building Materials on Building Thermal Load in Malaysian Institutional Library

Green building, net-zero energy building, low energy building, and sustainable building are the common terms being used in the building industry nowadays, with the primary aim to produce buildings with low energy and low carbon dioxide (CO2) emissions. Different types of building materials can have significant implications for energy efficiency, especially in the warming climate. The energy crisis, future global warming, and climate change can be mitigated by choosing the right building materials, which will also lower the building's energy consumption and carbon dioxide emissions. This paper presents the effects of different building materials on the cooling capacity and cooling energy consumption of a building for four (4) different climate change weather data, namely, the present time scenario, the 2020s, 2050s, and 2080s time scenario using TRNSYS simulation software. The main library of a university in Petaling Jaya, Malaysia, was chosen as the case study. Three (3) types of building materials were studied, namely, Type 1 (brick walls with single-glazed windows), Type 2 (foam insulation cavity walls with double-glazed windows), and Type 3 (air gap cavity walls with double-glazed windows). The simulation results showed that the Type 2 building materials could reduce the yearly cooling energy consumption by 14.5%, compared with the current building materials (brick walls with single-glazed windows). The Type 3 building materials (air gap cavity walls with double-glazed windows) with the lower installation were wound to reduce the yearly cooling energy consumption by 9% compared with the Type 1 building materials. For architects, designers, politicians, and library administrators, the research's conclusions have immediate practical implications that will help them make decisions and implement energy-saving plans. In the end, this research helps libraries in Malaysia remain sustainable and resilient in the face of global warming and the energy crisis.

Air-gap Fabry-Pérot cavity filtered 30 nm broadband electro-optic frequency combs for high-order coherent communications.

Broadband electro-optic (EO) frequency combs, which have flexible and high repetition frequencies, are prospective light sources for dense-wavelength-division-multiplexed coherent optical communications. In most cases, nonlinear spectral broadening and amplification procedures are needed to achieve broadband and high-power EO frequency combs. This leads to a low optical carrier-to-noise ratio (OCNR) for comb lines, limiting the transmission capacity. Here, we propose to use an air-gap Fabry-Pérot (FP) cavity to improve the OCNR for all the comb lines covering a 30 nm broadband spectrum. A 12 dB OCNR (0.1 nm bandwidth) improvement is obtained experimentally via using an FP cavity with ∼790 MHz bandwidth. We apply a 150-channel filtered EO comb with 25 GHz channel spacing and load 20 GBaud signals on each comb line to demonstrate the effect of OCNR improvement. The 137/150 channels have a bit error rate below the threshold of soft-decision forward error correction when using the 128 quadrature amplitude modulation (QAM) format. However, none of these channels can support this modulation format without cavity filtering. We also investigate dispersion tolerance and the long-term stability when using an air-gap FP cavity, highlighting its advantages. Our results show a practical solution to boost the transmission capacity when applying broadband EO combs in optical communications.

Measurement, modeling, and optimization of sound absorption performance of Kenaf fibers for building applications

The use of natural fibers in the buildings and construction industries as a sustainable and biodegradable product with the aim of noise pollution control has attracted the attention of many researchers. This work aims to study the acoustic behavior of porous absorbers made of natural Kenaf fibers. To this end, samples of sound absorber were fabricated with thicknesses of 10–40 mm at two different bulk densities of 150 and 200 kg/m3, and their sound absorption coefficient (SAC) was determined by standing wave sound impedance tube at different air gap cavities. A hybrid numerical-mathematical model was also proposed to investigate the acoustic behavior of the samples. To this end, a code was developed to simulate the 3D virtual structure of samples, and flow resistivity was calculated by numerically solving the flow of air in the structures. Tortuosity and two characteristic lengths were obtained using an inverse method programmed in MATLAB®. These parameters were then imported into the Johnson-Champoux-Allard (JCA) model to predict SACs at different frequencies. Afterward, considering the cost and sound absorption average (SAA), samples were optimized using factorial design. Consequently, the acoustic behavior of the optimized acoustic panels was investigated in the reverberation room in terms of reverberation time and random absorption coefficient. Moreover, in order to provide aesthetically and artistically pleasing appearance, the samples were covered with spacer fabrics, and their sound absorption behavior was also studied.The results revealed the promising sound absorption performance of Kenaf fibers. It was found that the SAC at low, mid, and high frequencies increases significantly with increasing the bulk density. The average of SACs for frequencies above 1250 Hz for samples of 40 mm thickness was found to be 0.95, while these values for samples of 30 and 20 mm thickness were respectively 0.85 and 0.7. The introduction of the air gap was found to improve the SAC at low-frequency bands and shift the peak of absorption toward low frequencies. Very good consistency was observed between the predicted and experimental data. The results of the statistical analysis suggested a thickness of 33 mm and a bulk density of 150 kg/m3 for the optimized panels. The results showed that the mean of SAC increased from 0.68 to 0.72 after covering the optimized panels with spacer fabrics.

Broadening sound absorption coefficient with hybrid resonances

In the last years, a great research effort has been focused on the noise mitigation at low frequencies. Membrane-type acoustic metamaterials (AMM) are one of the most promising solutions to meet the growing demand for low frequency sound absorbers. Typically, acoustic membrane absorbers require large back cavities to achieve low frequency sound absorption, which is usually categorised by a single narrow absorption peak. This paper presents an acoustic resonator unit cell, comprising of a thin elastic silicone plate with an air gap cavity with broadband absorption in a frequency range between 250 and 400 Hz. The broadband and multiple peak sound absorption showed by the proposed resonator is due to hybrid resonances which occur in the frequency range due to coupling of the structural dynamic response of the plate with the acoustic response of the air cavity. A numerical model based on acoustic-structural interaction, validated for experimental data, has been used to explain how the broadening gain in the sound absorption level is strictly related to the hybrid resonances of the unit cell resonator. We demonstrated that hybrid resonances are a function of the geometrical parameters and the ratio between the Young’s modulus and the density of the material plate, thus the proposed resonators absorption frequency range is tuneable at low frequencies allowing a wider broadband not achievable with acoustic membrane absorbers.

Measurement of milli-Newton axial force and temperature using a hybrid microsilica sphere Fabry-Perot sensor.

An optical fiber sensor based on the Hybrid Fabry-Perot for simultaneous measurement of milli-Newton axial force and temperature is proposed. This structure is composed of a single-mode optical fiber (SMF) integrated to a silica capillary tube (SCT) with polydimethylsiloxane (PDMS). A microsilica sphere cavity (MSSC) is fabricated at the tip of a capillary tube. Consequently, an air gap followed by a microsilica sphere forms two cascade cavities. To explain the transferring load from the SMF to the SCT through PDMS, a shearing mechanism is employed. The experimental results show that axial force and temperature sensitivities of the air gap cavity in the range of 0-3.43mN and 30-65°C are 170pm/mN and 24pm/°C, respectively, while the MSSC did not show any force sensitivity due to its rigidity. However, its temperature sensitivity is 34pm/°C. The different sensitivities enable us to implement simultaneous sensing of force and temperature.

Design of a Vertical Cavity Optical Micro-Electromechanical Filter

Based on Micro-Opto-Electro-Mechanical System (MOEMS) technology, vertical cavity devices with an air-gap enable very efficient ultra-wide continuous tuning with only a single control parameter. The vertical cavity filters and Lasers consist of an air-gap cavity embedded between two highly reflective Bragg mirrors. Vertical Cavity Surface Emitting Lasers (VCSELs) have an active layer additionally integrated in the cavity. The ultra-wide continuous tuning is achieved by micro-electromechanical actuation of the mirror membranes. Our research activities concentrated on Bragg mirror material system with InP/air-gap mirror structures. The resonator quality depends on the refractive index contrast between the Bragg mirror layer materials and on the number of periods. The advantage of the semiconductor/air-gap system is the very high refractive index contrast ensuring a high reflectivity of more than 99.8% with only three layer periods. The semiconductor/air-gap structures forming the membranes of one Bragg mirror are n-doped while the other mirror membranes are p-doped forming a diode structure. Due to device miniaturization very efficient tuning is achieved by electrostatic actuation. The investigations presented focus on an InP/air-gap Fabry-Perot-filter for air =1.55μm with an air-gap cavity thickness equal to a multiple of ½ air embedded between two InP/air Bragg mirrors. The InP membranes have a thickness of of ¾ InP, the air-gaps in between are ¼air thick. The tuning behaviour is determined by the mechanical flexibility of the membranes and by the optical properties of the vertical cavity. The electrostatic deflection due to a reverse voltage is calculated with FEM simulations incorporating the effect of stress in the membranes. The results of the model calculations compared very well with measured values. This study describes the optical micro-electromechanical design of the novel low cost device structure applicable in optical filters, vertical cavity surface emitting lasers (VCSELs) and a host of other optical applications. We also present a solution to one of the most challenging problems in the design of micro-electromechanical devices consisting of thin-walled structures - that is residual stress. Residual stress alters device characteristics if not taken into consideration during design and characterisation.

MEMS-based air-gap cavity for entangled photon generation via polariton-polariton scattering

A structure for entangled photon generation via polariton-polariton scattering is proposed. We have designed an air-gap planar cavity with embedded quantum wells, in which the photonic modes are in strong coupling regime with excitons of quantum wells. An electrical tuning mechanism is designed by introducing an air-gap layer and an integrated electrical actuator to the distributed Bragg reflector (DBR) mirror structure. Our structure consists of 40 (17) bottom (top) DBRs with 12 GaAs/AlAs (7/3 nm) quantum wells at the antinodes of the electromagnetic field working at 767-nm wavelength. Initial air-gap thickness is 1500 nm. The Q factor and Rabi splitting of the structure are 1742 and 14 meV, respectively. Reflectivity of cavity/air interface is 0.32, and suspended DBR reflectivity is 0.97. Average indices of suspended mirror and cavity are 3.2062 and 3.1901, respectively. According to our simulation results, by changing air-gap thickness from 1000 to 1350 nm, the wavelength tuning range is extended to 30 nm via applying a maximum voltage of 12 V.

Temperature-independent fiber salinity sensor based on Fabry-Perot interference

We present a novel fiber Fabry-Perot (FP) interference salinity sensor based on polyimide (PI) diaphragm. With an increase in water salinity, the PI diaphragm shrinks, and the PI diaphragm constriction causes the increase of the width of the air-gap, which causes the red shift of the interference fringes. We fabricated salinity sensor prototypes with different air-gap lengths and 20μm PI diaphragm. When salinity increases from 0mol/L to 5.47mol/L, the maximum sensitivity is 0.45nm/(mol/L). We verify that the sensitivity can be enhanced by reducing air-gap cavity length. We also choose appropriate air cavity length and PI diaphragm length to solve the cross-sensitivity between temperature and salinity. As a robust and ultra-compact salinity sensor, which is easy to be fabricated and need no alignment, this fiber interferometer can be applied for real-time salinity sensing applications.

Design and Optimization of Microbolometer Multilayer Optical Cavity

Microbolometers are the most widely used detectors in long-wave infrared uncooled thermal imagers. An optical cavity is required within a microbolometer structure to increase its optical absorption. In this work we present a detailed study on the design and optimization of a microbolometer optical cavity using Essential-Macleod package. In the simulations, the cavity is considered as thin film multi-layers that form cascaded Fabry-Perot optical cavities. In the design phase, the layers structures are selected including materials and initial thickness. The absorbing layers are chosen to be vanadium-pentoxide (V2O5) and titanium (Ti). In the optimization phase, the designed layer thicknesses are varied to maximize optical absorption within the absorbing layers. The simulations show that Ti layer absorption dominates over V2O5 layer. Also, the optimization proves that the air-gap cavity thickness is not simply quarter-wavelength because of the complex cascaded Fabry-Perot structure. The optimized air-gap thi...

Hybrid optical fiber Fabry-Perot interferometer for simultaneous measurement of gas refractive index and temperature.

We present a hybrid miniature optical fiber Fabry-Perot interferometer for simultaneous measurement of gas refractive index and temperature. The interferometer is fabricated by cascading two short sections of capillary tubes with different inner diameters. One extrinsic interferometer is based on the air gap cavity formed by the capillary tube with large diameter. Another section of capillary tube with small inner diameter performs as an intrinsic interferometer and also provides a channel enabling gas to enter and leave the extrinsic cavity freely. The experiment shows that the different dips or peaks in fringe exhibit different responses to the changes in gas refractive index and temperature. Owing to this feature, simultaneous measurement of the gas refractive index and temperature can be realized.

High Detectivity Uncooled Thermal Detectors With Resonant Cavity Coupled Absorption in the Long-Wave Infrared

We describe uncooled thermal detectors with a peak detectivity of at least 3 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> cm √{Hz}/W with spectrally selective absorption in the long-wave infrared. The spectral selectivity in absorption is achieved through resonant cavity coupling of a thin metal film with a low-order air-gap optical cavity. The electrical readout uses thermoelectric thin films with a Johnson noise limited performance. The detectors are of multiple sizes but those with 100- μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area have time constants of 58 ms and thermal conductances of 2.3 ×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-7</sup> W/K.

Negative optical pressure between the two coupled gold slabs under an external magnetic field

Strong light-induced negative optical pressure between the two coupled gold slabs under an external magnetic field is investigated theoretically. The negative optical pressure indicates attractive interaction between the two coupled gold slabs with the air-gap cavity local resonance. Plasmon resonances lead to extremely large localized field, thereby resulting in large mutual coupling forces between the two coupled gold slabs. The light coupling into metallic microcavity can stimulate collective electron oscillations of plasmatic resonance, and cause the contraction of the negative pressure in metallic cavity wall. The light-induced negative optical pressure between the two coupled gold slabs can also be changed with the length of slab, air-gap distance, the external magnetic field, and the angle of incidence.

The negative electromagnetic attractive forces arising from kinetic energy of conduction electrons in double-layer metallic nanowire arrays

The coupled-nanowire plasmatic resonances and very strong negative electromagnetic force between the double-layer metallic nanowire arrays is investigated theoretically. The negative electromagnetic force indicates attractive interaction between the metallic nanowires with the air-gap cavity local resonance effect. Plasmon resonances lead to extremely large localized field, thereby resulting in large mutual coupling forces between the double-layer metallic nanowire arrays. The light coupling into metallic microcavity can stimulate collective electron oscillations of plasmatic resonance, and cause the contraction of the negative pressure in metallic cavity wall. The electromagnetic field of the plasmatic resonance mode is mainly localized inside the air-gap region between the two wires.

Optofluidic membrane interferometer: An imaging method for measuring microfluidic pressure and flow rate simultaneously on a chip

We present a novel image-based method to measure the on-chip microfluidic pressure and flow rate simultaneously by using the integrated optofluidic membrane interferometers (OMIs). The device was constructed with two layers of structured polydimethylsiloxane (PDMS) on a glass substrate by multilayer soft lithography. The OMI consists of a flexible air-gap optical cavity which upon illumination by monochromatic light generates interference patterns that depends on the pressure. These interference patterns were captured with a microscope and analyzed by computer based on a pattern recognition algorithm. Compared with the previous techniques for pressure sensing, this method offers several advantages including low cost, simple fabrication, large dynamic range, and high sensitivity. For pressure sensing, we demonstrate a dynamic range of 0-10 psi with an accuracy of ±2% of full scale. Since multiple OMIs can be integrated into a single chip for detecting pressures at multiple locations simultaneously, we also demonstrated a microfluidic flow sensing by measuring the differential pressure along a channel. Thanks to the simple fabrication that is compatible with normal microfluidics, such OMIs can be easily integrated into other microfluidic systems for in situ fluid monitoring.

성덕대왕신종의 명동과 간극의 공명조건

한국종은 지면으로부터 일정한 간극을 두고 매달리며, 종 아래에 명동이라고 불리는 독특한 음향학적 요소를 갖고 있다. 간극과 명동의 적절한 설계는 공명효과에 의하여 종소리를 극적으로 증폭시킨다. 종체의 내부공동과 간극, 그리고 명동으로 구성된 음향공동계는 공명주파수와 모드를 갖는데, 공명주파수가 종체의 고유진동수에 일치하게 되면, 그 주파수성분이 크게 증폭된다. 이 연구는 성덕대왕신종을 대상으로 간극효과를 고려한 명동의 공명조건을 처음으로 제시한다. 공명조건을 구하기 위하여 신종의 실제 치수를 사용하여 SYSNOISE를 이용한 경계요소해석을 수행하였다. 또한, 계절에 따른 기온의 차이가 공명조건에 어떠한 영향을 미치는가를 규명하고, 가변형 명동의 개념을 제시함으로써, 현장에서 온도차를 보정하여 정확한 공명조건을 구현시킬 수 있는 방안을 제공한다. Korean bell is hung with some air gap between the bell bottom and the ground. In addition, it has a peculiar acoustic element, so called resonance cavity below the bell. A proper design of the air gap and cavity size dramatically amplifies the bell sound by resonance effect. Bell interior cavity, air gap and resonance cavity consist of an acoustic cavity system. When the acoustic cavity frequency coincides with the natural frequency of the bell body, the frequency component is significantly amplified. On the Sacred Bell of the Great King Seongdeok, this study proposes a resonance condition of the cavity system considering air gap effect for the first time. With the exact dimension of the bell, boundary element analysis is performed using SYSNOISE. Finally, this study reveals how the temperature in season influences the resonance condition and proposes a concept of variable type resonance cavity. By using the variable type resonance cavity, the cavity size is controlled on site and exact resonance is available regardless of temperature difference in season.

Proposal of gain-matched VCSELs with a thermally actuated MEMS structure for wide temperature operations

We propose a novel gain-matched VCSEL with a thermally actuated cantilever structure, which enables the lasing wavelength to be matched with a gain-peak wavelength even under wide temperature changes. The proposed structure includes an AlxOy layer in an air-gap cavity, which results in the increase of the temperature dependence of lasing wavelengths. The modeling result shows a potential of wide temperature operations of over 200K.