Interference Filters Research Articles

Thin-film interference filters illuminated by tilted apertures.

Thin-film interference filters can be illuminated by a circular aperture at different angles. Each situation produces a different transmittance spectrum. We present an analytical model that, for small tilt angles, predicts the change in transmittance for an arbitrary position of the filter in three-dimensional space. The model is extended to take into account higher-order harmonics. We also derive a formula to predict the change in central wavelength, and we validate our results by comparison with thin-film transfer-matrix calculations. A key property of our approach is that the model can be combined with empirical data to predict the transmittance without knowing the filter design.

Optical Gas Imaging With Uncooled Thermal Imaging Camera - Impact of Warm Filters and Elevated Background Temperature

The goal of the presented article is to investigate the impact of warm filters and elevated background temperature on the operation of an uncooled wideband camera used for optical gas imaging (OGI). The noise equivalent temperature difference (NETD) parameter is measured for a VOx wideband OGI camera built for the purpose of the experiments, with and without an interference filter. The results demonstrate that warm filters have a clear effect on the performance of the OGI camera. Based on the standard NETD measurement procedures, a camera-scene noise equivalent temperature difference (CSNETD) parameter is proposed to quantify both the camera and scene noise. The CSNETD measurements are used to evaluate the impact of elevated background temperature on the performance of the OGI camera. Finally, the camera is used to image methane leaks in the mid-wave infrared (MWIR) and long-wave infrared (LWIR) bands. The results are consistent with the presented NETD/CSNETD measurements and confirm the impact of warm filters and elevated background temperature on the operation of the uncooled thermal imaging camera.

Flexible 2.5D Metamaterial with High Mechanical Bearing Capacity for Electromagnetic Interference Filters at Microwave Frequency

Herein, the conception of 2.5D metamaterial is proposed. The basic idea of the 2.5D metamaterial is to build “3D” metamaterial by stacking 2D metasurfaces layer‐by‐layer. In this design, the trapezoidal prism units (3D) is built up by stacking a series of rectangle units (2D) together. This means, in the conception of 2.5D metamaterial, any kind of 3D structure with complex geometrical shape can be easily built up by controlling the 2D shape in each layer, which provide a new way for the fabrication of 3D complex units. Furthermore, paper is chosen as the substrate of 2D layer, because the interconnected lignocellulosic microfibers in paper will not only bring in flexibility to the 2.5D metamaterial but also strong mechanical strength. The results show that the 2.5D metamaterial still maintain a good filtering performance at 11 GHz when the pressure loading rise up to 7 MPa, whereas the classic 3D metamaterial (built by copper‐clad plate) undergoes a fracture failure. The conception and demonstration of 2.5D metamaterial with high mechanical property not only overcome the lacking of the basic structural carrying capacity in 3D electromagnetic metamaterial but also give new idea to the design of multifunctional metamaterial.

Investigate and Reduce Capacitive Couplings in a Flyback Adapter With a DC-Bus Filter to Reduce EMI

In consumer electronics, high power density power adapters are designed to minimize the adapter size. As a result, the components are getting very close and the near-field coupling issue tends to be severe. This compromises the performance of electromagnetic interference filters, especially at high frequencies. This article investigates near-field capacitive couplings and the reduction techniques in a high power-density power adapter with a dc-bus filter. The parasitic capacitive coupling theory is developed and parasitic coupling capacitances are experimentally extracted. The common-mode (CM) noise model with parasitic capacitive couplings is developed and the techniques to reduce the CM noise due to parasitic capacitive couplings are explored. Simulation and experiments were conducted to verify the analysis and the proposed techniques.

Development of an interference-filter-type external-cavity diode laser for resonance ionization spectroscopy of strontium.

A frequency tunable external-cavity diode laser (ECDL) using a narrow bandwidth (∼0.3 nm) interference filter has been developed for resonance ionization spectroscopy of strontium (Sr) with high isotopic selectivity. Improved wavelength and single mode stabilities of this interference-filter-type ECDL (IF-ECDL) over a commonly used (also home-made) Littrow-type ECDL were theoretically expected and experimentally confirmed by both a wavelength meter and a home-made Fabry-Perot interferometer. The measured spectral profile of the dominant isotope 88Sr using our IF-ECDL in the 689.4 nm intercombination transition shows that the Lorentzian component (∼1.3 MHz) of the spectrum width is consistent with the obtained fringe width of the interferometer. High 90Sr isotopic selectivity of ∼104 with respect to 88Sr is expected in this transition, which indicates that even if the manufacturing accuracy is not comparable to commercial Littrow-type ECDLs, our compact IF-ECDL having sufficient wavelength stability is a promising laser source for background-free analysis of radioactive 90Sr in marine samples.

Frequency tracking and mitigation method based on CPHD filter and adaptive multiple linear Kalman notch filter for multiple GNSS interference

In this paper, a frequency tracking method based on a cardinalized probability hypothesis density (CPHD) filter with cardinality compensation and fuzzy-c means (FCM) clustering is proposed to precisely estimate multiple interference frequencies in the received Global Navigation Satellite System (GNSS) signal. In addition, an adaptive multiple linear Kalman notch filter is applied in order to mitigate multiple GNSS interference signals by using tracking results from the proposed CPHD filter. The cardinality refers to the number of interference frequencies, and estimation accuracy on the cardinality has an effect on the tracking and mitigation performance of the filter. For that reason, the cardinality compensation process and FCM clustering are added in the CPHD filter. The proposed tracking and mitigation method is evaluated by theoretical analysis including simulations. It is confirmed that the proposed algorithm has better tracking and mitigation performance compared with the conventional algorithms.

A Survey of Active EMI Filters for Conducted EMI Noise Reduction in Power Electronic Converters

Wide bandgap devices enable high power density power converters. Despite the advantages of increased switching frequency, the passive components are still a major bottleneck toward enabling high power density. Among the passive components in the converter, the passive electromagnetic interference (EMI) filters are unavoidable to ensure compliance with conducted EMI standards. Active EMI filters (AEFs) help reduce the volume of the passive components and have been around for three decades now. The design and implementation of the AEFs depend upon the type of noise (common-mode or differential-mode) and power converter (ac–dc, dc–dc or inverter). This article presents a comprehensive survey of different AEFs and their implementations for different power converters presented in the literature. A comprehensive survey of noise-sensing, noise-processing, and noise-cancellation circuits is presented. Also, a comparison of attenuation provided by the AEFs for common-mode and differential-mode noises for different converters is carried out. Furthermore, other facets of AEFs such as the auxiliary power supply, power loss, and protection methods for the AEF are also summarized. This article is intended to be a useful reference for power converter designers in both industry and academia.

MANTIS: A real-time quantitative multispectral imaging system for fusion plasmas.

This work presents a novel, real-time capable, 10-channel Multispectral Advanced Narrowband Tokamak Imaging System installed on the TCV tokamak, MANTIS. Software and hardware requirements are presented together with the complete system architecture. The image quality of the system is assessed with emphasis on effects resulting from the narrowband interference filters. Some filters are found to create internal reflection images that are correlated with the filters' reflection coefficient. This was measured for selected filters where significant absorption (up to 65% within ∼70 nm of the filter center) was measured. The majority of this was attributed to the filter's design, and several filters' performance is compared. Tailored real-time algorithms exploiting the system's capabilities are presented together with benchmarks comparing polling and event based synchronization. The real-time performance is demonstrated with a density ramp discharge performed on TCV. The behavior of spectral lines' emission from different plasma species and their interpretation are qualitatively described.

A Proposed Terminal-Ground EMI Filter for Reduction of Conducted Emissions Considering Cable Radiation and Safety

This article proposes an optimal design of a terminal-ground electromagnetic interference filter (TGEF) for reduction of common-mode (CM) conducted emissions (CEs) with considering cable radiations and safety. The TGEF is free from risks of magnetic saturation and heat problem. In experimental tests with a 3.3-kW air conditioner system, a prototype TGEF well reduced low-frequency CM CE noises, but increased disturbance power (DP) due to cable radiations. To investigate the principle of cable radiations, the impedance and parameters in the DP measurement setup are extracted by measurements using a vector network analyzer and validated with full-wave simulations. The effects of the TGEF on total CM DP currents at power cables are also experimentally demonstrated and modeled in simulations. Based on the analysis of DP measurement environments, a quantitative guideline for a TGEF design was proposed without increasing cable radiations while maintaining CE noise attenuation performances. The performances of the optimized TGEF are then validated by CE and DP measurements in the real product system. Moreover, safety of the optimized TGEF in a ground-fault condition was experimentally confirmed.

Preliminary exploration of atmospheric water vapor, liquid water and ice water by ultraviolet Raman lidar.

Water is the only atmospheric component with three phases. In this work, ultraviolet Raman lidar is developed for synchronous measurements of water vapor, liquid water and ice water in Xi'an (34.233°N, 108.911°E), China. Different interference filters are designed to construct individual water Raman channels, and the corresponding central wavelength and bandwidth are determined by 399.0 nm (3.1 nm), 403.0 nm (5.0 nm) and 407.6 nm (0.6 nm) in ice water, liquid water and water vapor Raman channels, respectively. The mutual interference effect originating from the overlapping characteristics of water Raman spectra is further analyzed, and an accurate retrieval method based on linear simultaneous equations and mutual interference degrees is proposed for synchronous three-phase water mixing ratio profiles. Preliminary measurements are carried out in the Centre for lidar remote sensing research of Xi'an University of Technology, and representative measurement examples are obtained and validated for the performance of the Raman lidar system. Synchronous mixing ratio profiles in water vapor, liquid water and ice water are retrieved, and the corresponding extinction coefficient and relative humidity profiles are also combined to reveal the variation characteristics in three-phase waters. The possible aerosol fluorescence are analyzed as well, and it is inferred that the aerosol fluorescence might affect (possibly overestimate) the derived mixing ratio values of the liquid water and ice water. The effective detection can reach up to a height of 5 km under cloudy weather, and synchronized growth in water vapor and liquid water content is obtained in cloud layers. Continuous observations are also made under hazy weather conditions, and the temporal and spatial evolution trends of three-phase waters in clouds are successfully realized. Preliminary exploration and results validate the feasibility of ultraviolet Raman lidar for synchronous measurements of atmospheric water vapor, liquid water and ice water.

Demonstration of daytime wind measurement by using mobile Rayleigh Doppler Lidar incorporating cascaded Fabry-Perot etalons.

A well-designed filter assembly is incorporated to an earlier mobile Rayleigh Doppler Lidar developed at University of Science and Technology of China (USTC) for wind measurement round the clock. The filter assembly consists of two cascaded Fabry-Perot Etalons (FPEs) and a narrow-band interference filter (IF), which are optimized to filter out strong solar background radiation during daytime. The high resolution FPE is mainly used to compress the whole bandwidth of the filter assembly, whereas the low resolution FPE with relatively large free spectral range (FSR) is primarily used to block the unwanted periodic transmission peaks of high resolution FPE arising within the narrow-band IF passband. Some test experiments are carried out and demonstrate that the filter assembly have an overall peak transmission of 33.32% with a bandwidth of 2.41 pm at 355 nm. When applying it to the USTC mobile Rayleigh Doppler Lidar, the daytime background is only 3% or less than before. Consequently, the detectable altitude during daytime increases to ∼51 km with wind velocity accuracy of ±7.6 m/s.

Fluorescent Label-Free Aptasensor Integrated in a Lab-on-Chip System for the Detection of Ochratoxin A in Beer and Wheat.

This paper reports on the development of a fluorescent label-free aptamer assay integrated in a lab-on-chip (LoC) system for the detection of Ochratoxin A (OTA). The detection system relies on the integration, on a single glass substrate, of an array of amorphous silicon photosensors and a long pass interferential filter. The aptamer assay, integrated into the microfluidic network, is an aptasensor having affinity versus OTA, selected as a case study. The fluorescent molecule is a "light switch" complex [Ru(phen)2(dppz)]2+. The aptamer is directly anchored into a layer of poly(2-hydroxyethyl methacrylate) polymer brushes grown inside the channels. The fluorophore is intercalated between the base pairs of the aptamer. Upon the interaction of OTA with the aptasensor, a change of the aptamer conformation causes the release of the fluorophore, yielding a decrease of the fluorescent signal detected by the array of the amorphous silicon photosensors positioned underneath the microfluidic network. The developed LoC is a portable system capable of performing the analysis with a small volume of sample (about 10 μL) in a short time (5 min) with a limit of detection for OTA equal to 1.3 ng/mL. The LoC has been applied for the detection of OTA (5-200 ng/mL)in beer and wheat samples.

Detection of OH radicals in atmospheric-pressure plasma jet by evanescent-wave laser-induced fluorescence spectroscopy

This paper reports the signal and noise in the detection of OH radicals by evanescent-wave laser-induced fluorescence (EW-LIF) spectroscopy. We adopted this method to detect OH radicals, which were produced within an atmospheric-pressure argon plasma jet, in the vicinity of a quartz surface. The dominant noise was the stray laser light which could not be eliminated by using a monochromator and an interference filter. This was because the detection optics looked at the total reflection point of the laser beam in the EW-LIF spectroscopy. It was impossible to detect the LIF signal in the duration of the laser pulse because of the intense stray light. On the other hand, thanks to the slow decay of the LIF intensity, we succeeded in detecting the LIF signal after the laser pulse. The intensity ratio between the LIF signal originated from the bulk plasma and the one induced by EW near the surface was 5000–10000, which can be explained by the difference in the observation volumes. The proportionality between the LIF intensities from the bulk plasma and the surface vicinity, when changing the discharge conditions, suggests that the surface loss probability of OH on the quartz surface was not affected by these changes.

Active EMI Filter Design With a Modified LCL-LC Filter for Single-Phase Grid-Connected Inverter in Vehicle-to-Grid Application

More and more attention has been gained in the vehicle-to-grid (V2G) system due to its attractive features, such as transferring bidirectional power between electric vehicles and grid, regulating grid voltage/frequency. This paper develops the active electromagnetic interference (EMI) filter (AEF) with significant system volume and weight reduction compared to the traditional passive EMI filter (PEF). While the high-frequency parasitic parameters of the AEF and the restricted loop gain result in limited AEF EMI noise attenuation in the high-frequency range. Thus, an additional passive EMI filter is commonly needed to suppress the EMI noise. For avoiding external passive filter components, the active EMI filter is designed and integrated with a modified LCL-LC filter as a hybrid filter to guarantee EMI noise attenuation. The modified LCL-LC filter can maintain satisfactory harmonic suppression with a higher EMI reduction ability compared to other types of harmonic filters. Based on a single-phase grid-connected inverter, the total harmonic distortion (THD) and the conducted EMI measurement results are compared with the results of traditional LCL, LLCL, LCL-LC filters. The effectiveness of the proposed circuit is verified by simulation and experimental results. The proposed circuit successfully improves the filter design of the inverter in the suppression of harmonics and EMI noise.

Quantum interference effects in 1D parallel high-Tc SQUID arrays with finite inductance

The quantum interference effects of one-dimensional (1D) parallel arrays of high-temperature superconducting (HTS) SQUIDs were investigated experimentally and theoretically via the voltage–magnetic field responses for 4–81 Josephson junctions. The sensitivity of the arrays generally decreased as the number of junctions (and SQUIDs) in parallel increased, contrary to the predictions of models in the low (zero) inductance limit. A full theoretical description was developed to describe 1D parallel HTS SQUID arrays with finite inductances in an applied magnetic field, by extending the model for a single DC SQUID to multiple loops in parallel and including the flux generated by currents circulating through all loops in the array. Calculations were extended from SQUID arrays with equal loop areas to arrays with a distribution of loop areas, otherwise known as superconducting quantum interference filters. The model uses parameters relevant to HTS arrays, including typical variations (up to 30%) in HTS Josephson junction parameters, such as critical current and normal resistance. The effect of the location of the current biasing leads was also explored through the calculations. This model shows good agreement with experimentally measured 1D arrays of different lengths and highlights the importance of the geometry of the current biasing leads to the arrays when optimizing the array response.

Monolithic integrated cyclic 64-channel AWG with MZI filters and arrayed vertical reflecting mirrors for WDM-PON application.

The paper introduces a silica-on-silicon monolithic integrated cyclic arrayed waveguide grating (AWG) with Mach-Zehnder interference (MZI) filters and arrayed vertical reflecting mirrors in silicon to realize effective and stable optical transmission between waveguides and photodiodes. The cyclic AWG acts as both multiplexer over the L-band for upstream traffic and demultiplexer over the C-band for downstream traffic. The integrated chip, including AWG, MZI filters, and arrayed reflecting mirrors, has been made successfully with a 6.0 dB insertion loss, which is less than the discrete devices. At the same time, the arrayed reflecting mirrors are more stable than separate reflectors.

Novel Measurement Procedure for Switched-Mode Power Supply Modal Impedances

The measurement of modal impedances of switching-mode power supplies (SMPSs) is of paramount importance in electromagnetic compatibility applications, especially when designing electromagnetic interference filters to suppress the conducted and radiated electromagnetic emissions is the target. A novel measurement technique that makes use of an impedance analyzer and a coupling transformer and allows extracting the complex value of common- and differential-mode noise source impedance of an SMPS is proposed. The measurement procedure is first studied from the theoretical point of view, then experimentally validated with passive components, and eventually implemented in a CISPR-25 conducted emission test setup to measure the magnitude and phase of modal impedances of an automotive SMPS under different operation conditions.

Vibration-proof ECDL with an Intracavity Interference Filter

A new external cavity diode laser (ECDL) structure with an interference filter (IF) as a selective element is proposed. Key units of the developed ECDL, providing high mechanical stability, and hence, reproducible device characteristics, are the units of selective element rotation and radiation output from the laser cavity. Such ECDL is proposed to be used in onboard electro-optical systems.

Identifying Electromagnetic Noise-Source Impedance Using Hybrid of Measurement and Calculation Method

Identifying the noise impedance is a key step to improve the performance of an electromagnetic interference (EMI) filter. This paper proposes an impedance perturbation method to measure and calculate unknowns under a wide frequency range. Not only the differential mode/common mode (DM/CM) impedances but also the magnitude and the phase of impedances will be obtained at the same time. Moreover, the extraneous solution introduced by computation is checked. In addition, the profile of the perturbation infrastructure almost keeps the same under different cases, and it will limit the error of measurement under a very low level, which is also discussed in detail. Finally, the experimental results validate the methods very well.

Thomson scattering measurements in the divertor region of the TCV Tokamak plasmas

In order to better study conventional and alternative divertor configurations, the TCV Tokamak was recently upgraded. In-vessel graphite baffles were installed in the TCV chamber to delimit a separate divertor chamber from the core that required enhanced diagnostic capabilities. To date, TCV's Thomson scattering diagnostic provided electron temperature and density profiles of the hot core and the pedestal of the plasma with an array of 96 polychromators. To study the new divertor region, an upgrade added 20 new polychromators specifically to diagnose the plasma in the relatively cold divertor region. New 4-channel polychromators were designed to provide Te measurements down to ∼1 eV with ne∼1.5× 1019 m−3. Interference filters of only 1.9 nm FWHM were required to achieve this goal. Te and ne measurements from the TCV divertor region have been obtained for the first time. Specific diverted plasma discharges where the outer divertor leg is scanned in and out of the TS laser beam's path are used to demonstrate the diagnostic's new capabilities and potential with measurements of Te as low as 1.4 eV obtained for electron densities of 2× 1018 m−3. The system's 16 mm integration length along the laser line is providing, for the first time, Te and ne profiles with sufficient spatial resolution across the divertor legs to resolve the divertor plasma dynamics.