Transmission Filters Research Articles

Ultra-Narrow Bandwidth Fabry-Perot Optical Filter Illuminated by a Gaussian Beam

We implement an ultra-narrow bandwidth (UNB) Fabry-Perot (FP) optical filter that has two flat mirrors illuminated by a normally incident Gaussian beam from an optical fiber. Owing to the finite waist radius of the Gaussian beam, the UNB FP optical filter’s transmittance curve deviates significantly from that of the plane wave incidence case. Applying an amplified spontaneous emission to the UNB FP optical filter, we find the waist radius of the Gaussian beam from the beat noise spectrum of the UNB FP optical filter output. Then, the transmittance of the UNB FP optical filter is evaluated by expanding the Gaussian beam as infinitely many plane waves. With this measurement method, we implement the UNB FP optical filter having its optical bandwidth down to 270 MHz.

Temperature tunable Bragg transmission multichannel filters made of two quarter-wave stacks separated by defect layers

We report on simulated temperature-tunable single-channel/multichannel transmission filters with 0.37 nm/K shift in the peak wavelength is observed in the infrared region (1300 nanometers −1650 nanometers) using a one-dimensional photonic crystal structure. A single channel can be selected in the photonic bandgap region based on the thickness of the quarter wave stacks and temperature. The transmission coefficient of the transmitted defect modes is approximately the same as that required for telecommunication. For 20000 defect layers, 4000 channels were created with full width at half maximum of 0.7 picometers at the center wavelength of ∼1550 nm and channel separation of ∼0.18 nanometer between 1500 nanometers-1600 nanometers.

Tunneling from general Smith–Volterra–Cantor potential

We study the tunneling problem from the general Smith–Volterra–Cantor (SVC) potential of finite length L characterized by the scaling parameter ρ and stage G. We show that the SVC(ρ) potential of stage G is a special case of the super periodic potential (SPP) of order G. By using the SPP formalism developed by us earlier, we provide the closed form expression of the tunneling probability TG(k) with the help of the q-Pochhammer symbol. The profile of TG(k) with wave vector k is found to saturate with increasing stage G. Very sharp transmission resonances are found to occur in this system, which may find applications in the design of sharp transmission filters.

Efficient low-density grating setup for monochromatization of XUV ultrafast light sources.

Ultrafast light sources have become an indispensable tool to access and understand transient phenomenon in material science. However, a simple and easy-to-implement method for harmonic selection, with high transmission efficiency and pulse duration conservation, is still a challenge. Here we showcase and compare two approaches for selecting the desired harmonic from a high harmonic generation source while achieving the above goals. The first approach is the combination of extreme ultraviolet spherical mirrors with transmission filters and the second approach uses a normal-incidence spherical grating. Both solutions target time- and angle-resolved photoemission spectroscopy with photon energies in the 10-20 eV range but are relevant for other experimental techniques as well. The two approaches for harmonic selection are characterized in terms of focusing quality, photon flux, and temporal broadening. It is demonstrated that a focusing grating is able to provide much higher transmission as compared to the mirror+filter approach (3.3 times higher for 10.8 eV and 12.9 times higher for 18.1 eV), with only a slight temporal broadening (6.8% increase) and a somewhat larger spot size (∼30% increase). Overall, our study establishes an experimental perspective on the trade-off between a single grating normal incidence monochromator design and the use of filters. As such, it provides a basis for selecting the most appropriate approach in various fields where an easy-to-implement harmonic selection from high harmonic generation is needed.

Multifunctional terahertz device based on a VO2-assisted metamaterial for broadband absorption, polarization conversion, and filtering

This paper proposes a multifunctional terahertz device based on VO2 with a simple structure that needs only one step lithography. The designed device can realize broadband absorption, reflective broadband cross-polarization conversion, reflective linear-to-circular polarization conversion, transmissive narrowband cross-polarization conversion, and filtering under different working conditions. When VO2 is in a metallic state, the device can perform two different functions depending on the direction of the incident wave. When the electromagnetic wave is a forward incident, the device is a broadband absorber in the frequency range from 3.53 THz to 9.68 THz with a corresponding absorption efficiency above 90% and a relative bandwidth of 93%. When the electromagnetic wave is a backward incident, the device can work as a reflective broadband cross-polarization converter in the frequency range from 0.77 THz to 1.79 THz with a polarization conversion rate greater than 97% and a relative bandwidth of 80% and a reflective linear-to-circular polarization converter at 0.66 THz and 1.86 THz. While for VO2in the insulating state, the device exhibits the transmissive narrowband cross-polarization converter with polarization conversion rate greater than 90% at 1.32 THz and the function of the transmissive narrowband filter with transmittance more than 60% at 1.84 THz for both forward and backward incident waves. This multifunctional device may have great potential in miniaturized terahertz systems.

A Survey on Microstrip Single/Multiband Bandpass Filter for 5G Applications

For a long time, extensive research on microstrip bandpass filters has been documented to meet the standards of modern multiservice wireless communication systems. The multiband bandpass filters (BPF) are required for the receiver front end to function as a single device that caters to many bands at the same time. Planar bandpass filters are particularly promising because of their simplicity of manufacture using printed circuit technology, compactness, and low integration cost. Due to the need for high integration, low cost, and high-speed data transmission, the design and implementation of filters for fifth-generation (5G) mobile communication systems is difficult. This paper presents and discusses a detailed survey of existing research on microstrip singlemultiband bandpass filter designs for fifth-generation applications, with a focus on the latest advancements in this research and the difficulties that researchers face. Various designs and techniques were given. A detailed comparison of several design techniques is presented and discussed to offer researchers the benefits and drawbacks of each technique and design that may be of interest to a certain application. Recent microstrip single/multiband BPFs are included in the surveys, which use different design techniques and achieve varying performance for current and future fifth-generation applications.

Sensitivity enhanced all-fiber displacement sensor compatible with passive and intracavity system based on Lyot filter

An all-fiber high-sensitivity displacement sensor based on 45°-spliced PM Lyot filter is proposed and its sensing performance is investigated experimentally. The transfer function of PM Lyot filter transmission spectrum is derived by the Jones matrix. It is indicated by the simulation results that the birefringence index and the splicing angle of the PM fiber have obvious influences on the displacement sensitivity and the spectral modulation depth, respectively. According to the relationship between the dips and the displacements, the Lyot-filter based sensing head has a good linearity. The sensitivity of 122.81 pm/μm is finally obtained in the range of 200μm displacement variation and its R square is larger than 0.997. Moreover, the sensing head can be compatible to the intracavity spectroscopy to narrow linewidth and enhance the high signal-to-noise ratio (SNR) so as to realize high-resolution sensing. It is demonstrated experimentally that the sensitivity of the intracavity displacement sensor can be 60 pm/μm with a linewidth narrower than 0.05 nm and a SNR higher than 55 dB. The excellent performance of the proposed sensor compatible with passive and intracavity system not only provides a new approach for optical fiber displacement sensor, but also further expands the application of Lyot filter in the sensing field.

Single- and Multiple-Material Additively Manufactured Electronics: A Further Step From the Microwave-to-Terahertz Regimes

Additively manufactured electronics (AME) is an emerging technology that is advancing the design capability and shaping the functions of today’s microwave components. Many complex components, which are difficult to implement using conventional methods, can be prototyped and realized by AME fabrication technology. As one of the fundamental manufacturing methods in Industry 4.0, AME is becoming a driving force for the revolutionary manufacturing of microwave components with high-performance, high-accuracy, low cost, and short production cycles. The materials utilized in AME have also developed from single-material (dielectric or conductive) to multimaterial manufacturing (both dielectric and conductive materials), extending the possibility of AME-inspired designs from the microwave to the terahertz regime. This article presents an overview of single-material-to-multimaterial AME fabricated circuits and devices and their applications in microwave, millimeter-wave (mm-wave), and terahertz designs, including transmission lines, lenses, frequency-selective surfaces (FSSs), filters, waveguides, and antennas.

An end-to-end in-flight calibration of Mini-EUSO detector

Mini-EUSO is a wide Field-of-View (FoV, 44°) telescope currently in operation from a nadir-facing UV-transparent window in the Russian Zvezda module on the International Space Station (ISS). It is the first detector of the JEM-EUSO program deployed on the ISS, launched in August 2019. The main goal of Mini-EUSO is to measure the UV emissions from the ground and atmosphere, using an orbital platform. Mini-EUSO is mainly sensitive in the 290–430 nm bandwidth. Light is focused by a system of two Fresnel lenses of 25 cm diameter each on the Photo-Detector-Module (PDM), which consists of an array of 36 Multi-Anode Photomultiplier Tubes (MAPMTs), arranged in blocks of 2 × 2 called Elementary Cells (ECs), for a total of 2304 pixels working in photon counting mode, in three different time resolutions of 2.5 µs (defined as 1 Gate Time Unit, GTU), 320 µs and 40.96 ms operating in parallel. In the longest time scale, the data is continuously acquired to monitor the UV emission of the Earth. It is best suited for the observation of ground sources and therefore has been used for the observational campaigns of the ground-based UV flasher in order to perform an end-to-end calibration of Mini-EUSO. In this contribution, the assembled UV flasher, the operation of the field campaign and the analysis of the obtained data are presented. The result is compared with the overall effi ciency computed from the expectations which takes into account the atmospheric attenuation and the parametrisation of different effects such as the optics effi ciency, the MAPMT detection effi ciency, BG3 filter transmittance and the transparency of the ISS window.

Nonreciprocal transmission of electromagnetic waves with nonlinear active plasmonic metasurfaces

Nonreciprocity is important for optical information processing, full-duplex communications, and protection of sensitive laser equipment from unwanted reflections. However, it is very challenging to obtain strong nonreciprocal response in optical frequencies, especially when nanoscale configurations are considered. In this work, we solve this problem by demonstrating a compact bifacial plasmonic metasurface that acts as an ultrathin nonreciprocal transmission filter at near-infrared frequencies. The proposed nanostructure is simple to be practically implemented, since it is composed of two silver nanostripes with different dimensions placed on both sides of an ultrathin active dielectric spacing layer. The introduced gain leads to an exceptional point formation in the linear operation regime, where unidirectional perfect transmission due to loss compensation is achieved from the presented non-Hermitian parity-time symmetric nanoscale system. The demonstrated plasmonic metasurface breaks the Lorentz reciprocity law at the nanoscale when its nonlinear response is considered, mainly due to its spatially asymmetric geometry combined with enhanced nonlinearity. Strong and broadband nonreciprocity with unity transmission contrast from opposite directions is realized under relatively low-input laser intensity values. Furthermore, asymmetric nonlinear third-harmonic generation with a pronounced contrast is also illustrated by the same nonreciprocal metasurface design. The findings of this work can lead to the realization of various self-induced nonreciprocal nanophotonic configurations, such as integrated ultrafast switches, ultrathin protective coatings, and asymmetric directional imaging devices.

Strong resonance response with ultrahigh quality factor in grating-multilayer systems based on quasi-bound states in the continuum

Optical bound states in the continuum (BICs) exist in many photonic crystals and periodic structures with a strong resonance and ultrahigh Q factor. Such phenomena can be used in the designs of narrowband transmission filters, lasers, and sensors. In this paper, we consider the energy bands of a complex structure consisting of a grating and a multilayer substructure to obtain the position of the BIC in the structure. Hence, the higher Q factor can be obtained in the grating-multilayer structure than can be realized in the simple grating geometry. We analyze the wave propagation process in the complex structure and the change in the Q value via the use of transmission matrix theory. In addition, the reflectance spectrum is found to exhibit a series of asymmetric line-shapes with different values of the asymmetry parameter, δ, due to the interference between the two channels. One of these channels is the broadband channel, induced by the Fabry–Perot resonance, and the other channel is the narrowband channel, induced by guided mode resonance. Quasi-BICs are seen to transform into BICs as the value of δ is decreased gradually to zero. Therefore, a large Goos–Hänchen shift can be achieved as a result of the high Q factor and quasi-BIC. This work designs a complex structure with ultrahigh Q factor and strong resonance properties, which has significant implications for exploring the phenomenon of BICs.

A Multi-Spectral Thermal Gas Detection Imager Using Uncooled Infrared Camera

Gas remote detection is useful for early warning of gas leakage and toxic chemicals. Optical gas imaging (OGI) built with an uncooled infrared camera is superior to cooled detectors in terms of cost. Current mainstream OGI technologies fall short in their detection of gases at ambient temperature and their ability to classify multiple gases. A multi-spectral uncooled imager is developed to try to solve these problems, which is constructed from a commercial uncooled thermal camera and wide band filters. To solve filter self-radiation and unevenness, a correction method is devised, with an ambient temperature blackbody placed in front and subtracted from the measured image. Based on waveband cutoffs, filters are classified into target-sensitive filters and background filters. Multi-spectra are simulated according to wide band filter transmittance, which can be used in gas classification. A sulfur hexafluoride (SF6) experiment is conducted outdoors at a distance of 10 m. An SVM model is trained to classify gas release in real time. Detection with a cold sky background is improved with the aid of data cube differences in a time sequence. The SF6 outdoor experiment concluded with preliminary effective results of ambient temperature gas remote detection.

Design and analysis of tunable multichannel transmission filters with a binary photonic crystal of silver/silicon

Design and analysis of tunable multichannel transmission filters with a binary photonic crystal of silver/silicon

Random Access Identifier-Linked Receiver Beamforming with Transmitter Filtering in TDD-Based Random Access

Random Access Identifier-Linked Receiver Beamforming with Transmitter Filtering in TDD-Based Random Access

3D Printed Fabry–Pérot Filters for Terahertz Spectral Range

3D printing is evolving into a standard tool for prototyping various optical components suitable for application in the terahertz range of the electromagnetic spectrum. This work takes the next step in this evolution by demonstrating the fabrication and subsequent evaluation of Fabry–Pérot interferometers (FPIs). Large optical area (centimetre scale) Fabry–Pérot transmission filters have been 3D printed with polylactic acid (PLA) using a commonly used low-cost 3D printer. The advantages of the proposed approach include low cost, rapid prototyping and repeatability. Terahertz transmission measurements for two demonstrated filter designs realised to target optimisation of either signal transmission or spectral filter performance have been performed using terahertz time-domain spectroscopy (THz-TDS) and demonstrate good agreement with the simulated response in the operating spectral band of 0.30–0.75 THz (wavelengths from 1000 down to 400 μm). The critical spectral characteristics assessed were the filter peak transmission magnitude, central wavelength and full width at half-maximum (FWHM) of the transmission peak, as well as the free spectral range (FSR). The signal transmission levels were observed to reach beyond 90% for the first series of filters that targeted optimisation of this aspect; however, this was accompanied by diminished out-of-band rejection and broader transmission peaks in comparison to the second series of filters which targeted the overall performance. For the latter filter series, the resolution in terms of FWHM values of the transmission peaks was reduced to 40–50 GHz, with the out-of-band rejection approaching a ratio of 10:1. This level of spectral performance, along with the achieved signal peak transmission characteristics of 65–75%, provides adequate performance for many applications harnessing the terahertz spectral range.

Ultra-wideband transmission filter based on guided-mode resonances in two terahertz metasurfaces.

This paper reports on a broadband transmission filter that employs the guided mode resonances pertaining to a terahertz metasurface composed of metallic gold disks with a quartz slab. Unlike structures involving conventional metasurfaces, two identical metasurfaces are placed on the upper and lower sides of a thick quartz slab. This structure can excite both even and odd guided mode resonances. The interaction of the two resonances at similar frequencies produces a broadband transmission peak. The sharp spectral feature of each resonance leads to the abrupt degradation of the transmission at the spectral edge, which can enable the development of the filter application. The proposed scheme can facilitate practical applications such as those of broadband filters at a terahertz frequency.

Development of a filter-based near-surface remote sensing system to retrieve far-red sun-induced chlorophyll fluorescence

Observations of sun-induced chlorophyll fluorescence (SIF) by remote sensing have improved our understanding of the structural and physiological dynamics of vegetation. Substantial efforts have been made to measure SIF with ground-based sensing systems, but field observation data for various plant functional types are still sparse. This is partly due to the limited availability of commercial SIF measurement systems, the relatively high cost of hyperspectral spectroradiometers, and the difficulties of sensor calibration and maintenance in the field. We developed a filter-based smart near-surface remote sensing system for SIF (4S-SIF) to overcome the technical challenges of monitoring SIF in the field, which also decreased the sensor cost, thus enabling more comprehensive spatial sampling. To retrieve SIF, we combined ultra-narrow bandpass filters (full width half maximum <1.3 nm) and photodiode detectors to observe electromagnetic radiation at specific wavelengths (757, 761, and 770 nm). We confirmed that the spectral and radiometric performance of the bandpass filters was satisfactory to retrieve SIF by comparing them to a high-spectral-resolution spectroradiometer that served as a reference. In particular, we confirmed that the digital numbers (DNs) from 4S-SIF exhibited linear relationships with the DN from the reference spectroradiometer in each band (R2 > 0.99). In addition, we developed equations to correct for temperature-induced changes in filter transmittance, such that SIF can be reliably extracted in outdoor environments without the need to actively stabilize the temperature. Furthermore, we confirmed that the SIF signal from 4S-SIF had a strong linear relationship with the reference spectroradiometer-based SIF. Importantly, this relationship held even when the physiological mechanisms of the plant were altered by a herbicide treatment that induced substantial changes in the SIF signal (R2 = 0.85, relative RMSE = 0.22), which indicated that 4S-SIF could be used to retrieve SIF. We believe that 4S-SIF will be a useful tool for collecting in-situ SIF data across multiple spatial and temporal scales.

Tunable directional filter for mid-infrared optical transmission switching.

Controlling the spectral and angular response of infrared (IR) radiation is a challenging task of paramount importance to various emerging photonic applications. Here, we overcome these problems by proposing and analyzing a new design of a tunable narrowband directional optical transmission filter. The presented thermally controlled multilayer filter leverages the temperature dependent phase change properties of vanadium dioxide (VO2) to enable efficient and reversible fast optical switching by using a pump-probe laser excitation setup. More specifically, transmission is blocked for high intensity probe lasers due to the VO2 metallic properties induced at elevated temperatures while at low probe laser intensities high transmission through the filter occurs only for a narrowband IR range confined to near normal incident angles. The proposed multilayer composite dielectric filter is expected to have applications in optical communications, where it can act as dual functional infrared filter and optical switch.

Balance of the 6 TH and 10 TH spatial harmonics amplitudes of a quadrupole mass filter with round rods

The purpose of this work is to find the electrode structure of the analyser with round rods, in which the amplitudes of the 6th and 10th harmonics are equal. For these conditions, we determine the shape of the mass peak and the modified stability diagram. The method is the representation of the electric field in the form of a complex potential which allows to write the equations of ion motion in analytic form. Numerical integration of the motion equations for an ensemble of ions with a given distribution of initial conditions is used to obtain the mass filter transmission contour. It is shown that radial displacement of two opposite rods can be used to balance the amplitudes of the 6th and 10th spatial harmonics and obtain an increase in the resolving power.

Ultra-narrow linewidth transmission filters based on the cladding mode assisted Fabry-Perot effect in a planar waveguide.

We propose and analyze a counterpropagating cladding mode assisted tunable frequency Fabry-Perot interferometer formed by a Bragg grating (BG) cavity in a liquid crystal coated planar optical waveguide. A full vector modal analysis has been used to obtain the transmission spectra of the individual Bragg reflectors, and the cavity effects have been incorporated by employing a suitable phase matrix. We show that the cavity resonances that appear from two fiber BGs forming a resonator can be efficiently explained by incorporating appropriate phase shifts in one BG grating period. We further show that utilizing the cladding mode evanescent field, a liquid crystal overlay can be used to tune the cavity resonance over the entire free-spectral range of the cavity transmission spectra. Our study should find application in designing highly tunable integrated optical Fabry-Perot interferometers.