Tunable Bandstop Filter Research Articles

Numerical Verification of a Polarization-Insensitive Electrically Tunable Far Infrared Band-Stop Meta-Surface Filter

Tunable filters have many potential applications in diverse fields, including high-capacity communications, dynamic beam shaping and spectral imaging. Although providing a high-performance solution for actively tunable devices, metasurface combined with tunable materials faces the great challenges of limited tuning range and modulation depth. Here, we propose a far-infrared tunable band-stop filter based on Fabry-Perot (FP) resonators and graphene surface plasmons. By switching the wavelength of the critical coupling condition of the filter via the gate voltage applied on graphene, achieving the dynamically tunable band-stop filtering at the central wavelengths from 12.4 μm to 14.1 μm with a modulation depth of more than 99%. Due to the symmetry of the proposed meta-atoms, the filter is insensitive to the polarization direction of the incident light. And it can realize more than 85% filtering efficiency within 60° angle of incidence around the vertical direction. By adjusting the geometry of the meta-atoms structure, it is feasible to move the operational range from the near-infrared to terahertz bands.

Tunable band-stop fiber filter based on laser-induced graphene metamaterial in THz frequency

As an important device in the application of terahertz (THz) technology, a THz filter has broad application prospects in the fields of THz communication, imaging, and sensing. In this paper, a THz filter based on grating structure laser-induced graphene (LIG)/ side polishing terahertz fiber composite structure is proposed. In the experiment, we achieved the maximum Q factor of 23.83 at the central resonant frequency of 0.715 THz. By modifying the grating structure, a tunable operational span of 269 GHz was achieved, along with a tunable range of 21 GHz through laser stimulation. In testing, we found that LIG materials prepared with circular filling are more sensitive to relatively high-power pump lasers, while LIG samples prepared with line filling exhibit better linear response to laser power. Furthermore, the compact and highly integrated nature of the device suggests its broad potential utility in the realm of THz frequency selection.

Temporally-topological defect modes in photonic time crystals.

In this paper, we investigate the properties of temporally-topological defect modes (TTDMs) (or temporally-topological interface states) in the topological photonic time crystal (PTC) systems. The PTC systems are constructed by the cascade of multiple sub-PTCs that possess temporal inversion symmetries and different topologies. The cases of two-, three-, and multiple-sub-PTC for the topological PTC system are studied. By transfer matrix method, we find that the TTDMs appear when the topological signs of the corresponding gaps in the sub-PTCs are different. The positions of TTDMs can be adjusted by changing the modulation strength of the refractive index, the time duration, and the period of the sub-PTCs. Moreover, the number of TTDMs is one less than the number of sub-PTCs. In addition, the robustness of the systems is also studied. We find that the topological PTC systems have good robustness, especially on the random configuration of the refractive index and time duration for the temporal slabs in the systems. Such research may provide a new degree of freedom for PTC applications, such as novel PTC lasers, tunable band-stop or band-suppression PTC filters, and many others, in the field of integrated photonic circuits for optical communications.

Dielectric coated conductive rod resonantly coupled with a cut transmission line as a tunable microwave bandstop filter and sensor

The resonant interaction of a dielectric-coated conductive rod with the X-band microwave field is investigated. The magnetic field distribution of the Goubau standing radial surface waves was experimentally visualized by using a thermo-elastic optical indicator microscope, and the corresponding electric field distribution was determined via numerical simulations. These field distributions are characterized by a certain pattern of antinodes distinctive for standing waves. An analysis of these field distributions allows one to couple a coated rod with a cut Goubau line. A rod placed in the gap region perpendicular to the Goubau line results in a sharp rejection band in the transmission spectrum which is extremely sensitive to the changes in the surrounding media. The shifting rate of the resonance as a function of the dielectric shell thickness is approximately 1.4 GHz/mm. The Q-factor of copper rods depends on their size and dielectric shell thickness. Longer rods with more energy localization areas have higher Q-factors, typically 1.7 times higher (12.7 vs. 7.5). Moreover, incorporating a dielectric shell enhances energy confinement and can elevate the Q-factor by as much as 22 %. When a 25 mm Cu rod is situated inside a cut Goubau line system, the Q-factor values are significantly higher, with a ratio of 275 to 13. With the addition of a dielectric shell, the Q-factor can be elevated by 58 %. The versatility of the proposed controllable system makes it possible to tune the operating spectrum towards higher GHz and THz frequencies.

Terahertz tunable band-stop filter using topological valley photonic crystals.

In recent years, there has been a growing interest in the wideband propagation and control of terahertz (THz) radiation due to its potential for a variety of applications, such as 6G communication, sensing, and imaging. One promising approach in this area is the use of valley photonic crystals (VPCs), which exhibit properties like wider band gaps and robust propagation. In this paper, a two-dimensional dielectric silicon-air VPC is studied, which is constructed from a method of inversion symmetry breaking providing a band gap of 109.4GHz at a mid-gap frequency of 0.376THz. We employ an optimized bearded-stack interface to construct the VPC waveguide for wideband THz propagation along straight and Z-shaped paths. We demonstrate that a band-stop response can be achieved in a VPC by introducing periodic defects along the domain wall. Furthermore, the stop range can be tuned by varying the refractive index of the defects through incorporating liquid crystal along the domain wall of VPC. Our proposed structure and the techniques employed could be promising for the development of a band-stop filter (BSF) and other photonic components having potential applications in 6G communication and beyond.

Hybrid multi-channel electrically tunable bandstop filter based on DAST electro-optical material

A voltage tunable hybrid multi-channel bandstop filter based on a metal–insulator–metal (MIM) waveguide is presented in this work, which can realize three narrowband and one broadband filtering functions simultaneously. The filter comprises two asymmetric composite cavities, which are filled with organic electro-optical material of 4-dimethylamino-N-methyl-4-toluenesulfonate (DAST). The composite cavity is composed of a rectangular cavity and an annular cavity, and the annular cavity is formed by two rectangular cavities connected with two semi-elliptical annular cavities. The transmission spectrum and magnetic field distribution of the filter are studied and analyzed by the finite element method (FEM), and the effects of the structure parameters on the transmission spectrum are discussed. Our analysis indicates that the bandstop filter has minimum transmittances of 0.02%, 0.29%, and 0.1%, minimum bandwidths of 5 nm, 9 nm, and 25 nm, and maximum quality factors (Q) of 123.7, 87.1, and 44.2, respectively, in three narrowband modes. The stopband bandwidth at the broadband mode is 70 nm, and the adjustable range is 1695–2065 nm. Additionally, the filter characteristics can be adjusted by imposing a control voltage, providing a high degree of tunability and maintaining stable filter performance. Finally, the basic structure is optimized yielding an increased bandwidth of 238 nm for the broadband mode, which does retain great electrical tuning characteristics. Consequently, the proposed structure can be applied with huge potential in high-density integrated circuits and nano-optics.

Tunable ultra-wideband graphene-based filter with a staggered structure.

We present a tunable ultra-wideband band-stop filter utilizing graphene with a straightforward staggered structure. The transmission spectrum has been meticulously analyzed using the effective-index-based transfer matrix method (EIB-TMM). The results demonstrate that the filtering properties can be precisely tailored by manipulating the Fermi energy level of graphene. Importantly, we have successfully achieved a remarkable ultra-wideband stopband by optimizing the staggered parameters. Our exploration of redefining the staggered structure through adjustments to three critical parameters has revealed a crucial role in expanding bandwidth. This investigation deepens our understanding of how nonperiodic structures can effectively broaden bandwidth and holds great promise for the prospective design of ultra-wideband band-stop devices.

Highly compact tunable hourglass-shaped graphene band-stop filter at terahertz frequencies

In this research paper, we introduce a band-stop filter based on an hourglass-shaped graphene nanoribbon topology. Employing the three-dimensional finite difference time domain (3D-FDTD) method, we conduct comprehensive numerical simulations to investigate transmission spectra and electromagnetic field distributions. Our primary objectives are to achieve high transmission efficiency, increase tunability, and ensure compact dimensions. The hourglass-shaped band-stop filter exhibits a remarkable bandwidth of 1.2 THz, a 90 % transmission efficiency in the passband, an impressive attenuation of −52.5 dB at the resonance frequency of 32.3 THz, and a compact footprint of 400×300 nm2. By manipulating the chemical potential (or bias voltage) to control the conductivity and dielectric constant of the graphene surface, we set the center frequency of the filter in the range of 28 to 36 terahertz. The remarkable combination of high transmission efficiency, high tunability, and compact design makes our proposed filter an ideal candidate for integration.

Graphene based filter design using triangular patch resonator for THz applications

In this paper, graphene based tunable bandpass and bandstop filters are designed in terahertz frequency regime using a triangular microstrip patch resonator. Initially, a bandpass filter is designed at an operating frequency of 1.65 THz with a bandwidth of 65 GHz. Further a dual-band bandstop filter is designed with resonance frequencies of 1.25 THz and 2.14 THz. In both designs, the triangular patch is coupled to the transmission line to achieve bandpass and bandstop characteristics. A graphene layer is deposited between the dielectric and the conductor layer to enhance the propagation of plasmonic waves. The simulation results reveal that the designed filters are capable of achieving the desired frequency response. By varying the graphene’s chemical potential, a shift in the transmission response’s resonance frequency is observed. The proposed filters have the potential to be used as key components for future terahertz band communications systems.

A new tunable bandstop filter square-ring resonator using varactor diodes.

This work presents the development of a bandstop filter with a tunable response. Varactor diodes are used as control elements. Studies and investigations demonstrate the influence of the variable capacitance on the input admittance and on the S-parameters frequency responses of the proposed square-ring resonator geometry. The design of the square-ring resonator is based on mathematical modeling of ideal transmission lines, considering parameters of characteristic admittance and electrical length for odd and even excitation modes. Based on S-parameters in ports, an equivalent circuit model of the resonator geometry is presented. The corresponding results are compared with numerical simulations. Comparative analyses are presented in order to guide the process of optimizing the physical dimensions of the layout. A prototype with dimension 0.0272 [Formula: see text] was designed, fabricated, and tested. As a measured result, a filter with two rejection bands was obtained, the first at 0.6-1.15 GHz and the second at 1.71-2.28 GHz, with 63.0 and 29.0% tuning range, respectively. In comparison with bandstop filters from the literature, the proposed reconfigurable filter presents a larger tuning range for the first band, sufficient inband rejection levels for several applications, and reduced physical dimensions. The proposed configuration is an attractive reconfigurable filtering device for use in modern communication systems operating below 3.0 GHz.

Design and analysis of tunable optical filter based on silicon-on-insulator photonic crystal for visible light communication

In this paper, tunable narrow bandwidth band-stop and band-pass optical filters for visible light communication (VLC) are designed and numerically investigated using a photonic crystal (PC) structure. Designing a tunable filter for visible light communication is crucial for enhancing spectral utilization and maximizing communication efficiency. A rectangular defect with optimized sides length utilized in the center of the PC structure based on the desired filtering characteristics. To obtain a favorable narrow bandwidth filter for visible light wavelengths, this defect is filled with a specific liquid crystal. In addition, an electric field is applied to the proposed structure in order to change the refractive index of the liquid crystal and achieve tunability in the desired range. The functional wavelength ranges of band-pass and band-stop filters are 550–570 nm and 565–585 nm, respectively. Applying an electric field causes a shift in the central wavelengths of the filters. It is shown that a 1.5 nm central wavelength shift for a band-stop filter and a 0.9 nm central wavelength shift for a band-pass filter can be achieved by changing the voltage source up to 1.5 V. In addition, the impact that the variation in the length of the rectangular defects’ sides has on the filtering characteristics of the proposed device is studied.

Tunable bandstop filtering specialities in superconducting Thue–Morse photonic multilayers

We theoretically investigate wide and tunable bandstop filters based on one-dimensional superconducting photonic multilayers. These multilayers are composed of superconductors (Nb) and semiconductors (GaAs), arranged in accordance with the Thue–Morse sequence. The wide transmission bandgap of the multilayers can be utilized as a bandstop filter, with greater accuracy achieved by larger sequence number and thinner semiconductor. Our results demonstrate that both the bandwidth and center frequency decrease as the ambient temperature and semiconductor thickness increase. Especially, the stopband disappears when the temperature exceeds the critical temperature (Tc) of Nb. On the other hand, increasing hydrostatic pressure and superconductor thickness leads to monotonous increases in both the bandwidth and center frequency. Moreover, for thin superconductors or high temperatures close to Tc, the quality factor is high and tunable in a wide range. Our work presents a flexible solution for optical bandstop filters, which may also find potential applications in optical switching and modulating.

Tunable bandstop filter using spoof surface plasmon polaritons for terahertz applications

This paper presents the design and analysis of a spoof surface plasmon polaritons (SSPP)-based bandwidth tunable bandstop filter (BSF) operating at 0.26 THz frequency. The proposed filter is comprised of SSPP T-shape resonators which are coupled to the spoof transmission line. Odd and even mode analysis is carried out for the proposed design, and using Co-EM simulation, it is shown that the designed band stop filter is capable of offering tunable bandwidth at THz frequency using a variable capacitor. In order to experimentally validate the proposed approach, design studies are carried out at microwave frequencies and the BSF is developed on a Rogers R4003C substrate of thickness 1.524 mm. The proposed filter designs have a 3 dB bandwidth tunability of 3 GHz (0.038–0.041 THz) and 50 MHz (0.55–0.60 GHz) in the THz and microwave frequency regimes, respectively. The proposed terahertz filter has the potential to be used as an ultra-compact BSF in future applications of photonic integrated circuits in terahertz imaging.

Multioctave Interference Detectors With Sub-Microsecond Response

High-power interferers are one of the main hurdles in wideband communication channels. To that end, this article presents a wideband interferer detection method. The presented technique operates by sampling the incoming signal as an input, and produces the frequency and the power readings of the detected interferer. The detection method relies on driving an open circuit stub, where the voltage is proportional to the power of the interferer, and the standing wave pattern is an indicator of its frequency. This approach is feasible over multioctave bandwidth with a wide power dynamic range. The concept is analyzed for design and optimization, and a prototype is built for a proof-of-concept. The measured results demonstrate the ability to detect an interferer within the 1–16 GHz frequency range, with a power dynamic range between <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$-$</tex-math> </inline-formula> 20 and 20 dBm. The detection concept is also fit with different types of tunable bandstop filters (BSFs) for automatic detection and suppression of the interferer if its power exceeds a programmable threshold. With a measured response time of 500 ns, the presented method is a technology enabler for wideband receivers.

Proposal of optically tunable and reconfigurable multi-channel bandstop filter using sum-frequency generation in a PPLN waveguide

A multi-wavelength bandstop filter is proposed and numerically demonstrated using the sum-frequency generation (SFG) process in a waveguide of periodically poled lithium niobate (PPLN). This proposed device achieves channels number reconfigurable, central filtering wavelength of each filtering channel independently tunable and extinction ratios (ERs) equalized via all-optical methods.

Electronically Tunable UWB Band Stop Filter Using Varactor Diode

An electronically tunable Ultra-Wideband (UWB) Band Stop Filter (BSF) using the varactor diode is investigated. The open stubs and Step Impedance Resonator (SIR) topology are utilized to obtain UWB bandstop response. The concept of variable capacitive loading to get tuning, which is achieved by a varactor diode (SMV2019-079LF) and a biasing network is clearly elucidated. The validation of the |S| parameter of the proposed tunable BSF filter is carried out using the filter's transmission line model (TLM), consequently, even and odd mode impedances are calculated analytically, and a lumped equivalent circuit model of the proposed BSF filter is extracted. Moreover, the filter is fabricated, and corresponding |S| parameters are measured which offers a UWB response (2.44 GHz–9.65 GHz) with five Transmission Zeros (TZ) and has tuning of 1 GHz frequency range (8.65 GHz–9.65 GHz) at the upper edge. The proposed miniaturized BSF has a size of 0.23λg × 0.32λg. The simulation results of the electromagnetic (EM)/distributed model, and the measured results of the fabricated prototype are in good agreement. Additionally, a very high skirt factor (SF) of 380 dB/GHz is achieved.

Low-Power, Reconfigurable Brillouin RF Photonic Bandstop Filters

Stimulated Brillouin Scattering (SBS) has been widely used to realize narrowband as well as wideband microwave photonic bandstop filters. However, most Brillouin based wide-band bandstop filters require large powers and have high complexity. In this letter, we propose a novel approach to realize a wideband, reconfigurable and tunable Brillouin-based microwave photonic bandstop filter using RF phase engineering. We demonstrate bandstop filters with suppression of >15 dB for moderate bandwidths and maximum bandwidth reconfigurability up to 1 GHz by using two pumps with powers of 8.2 dBm and 9.3 dBm. Further, we also demonstrate switching from a bandpass to a bandstop filter with simple phase manipulations. This letter presents numerical simulations, analysis, and experimental results for the proposed filter. Compared to conventional Brillouin systems, the proposed filter is extremely power-efficient.

Tunable and reconfigurable bandstop filters enabled by optically controlled switching elements

The authors report a novel approach for designing tunable and reconfigurable bandstop filters by employing bias-free optically-controlled photoconductive radio frequency (RF) switching elements. To verify the effectiveness of the approach, a bandstop filter with two stopbands centred at 4.05 and 4.75 GHz was designed using a split-ring-coupled microstrip transmission line on RO3010 substrates. To enable reconfigurability, a micromachined Si chip with a thickness of ∼73 μm was embedded in the gap of each resonator. The tuning and reconfiguring of the filter are accomplished by selectively illuminating the Si chips using fibre-coupled laser diodes with a wavelength of 808 nm. By turning on and off each laser diode, the filter stop bands can be dynamically reconfigured. In addition, the suppression of each stop band can be continuously and independently tuned by changing the light intensity from 0 to 20 W/cm2. With geometric scaling, this approach is promising for realizing a novel class of compact and high-performance tunable and/or reconfigurable circuits from the microwave to mmW-THz region.

Plasma-based GHz tunable bandstop filter

Tunability is an important feature for the filter. As a special electromagnetic medium, the plasma has its permittivity being altered in a wideband range. In this work, based on the surface plasmon polaritons of plasma–dielectric–plasma waveguides, we propose a double-stubs structure submerged in a gaseous discharge plasma medium to realize tunable filtering properties in the giga-hertz (GHz) regime. The finite element method is applied to numerically compute filtering properties. The coupled mode theory and orthogonal design method are introduced to verify simulation results and estimate the effect of simulation parameters on the filtering properties. It is shown that the height of two stubs has the most important influence on filtering performance. Although once the filter is fabricated, its size cannot be modified, one can, nevertheless, vary the plasma frequency to effectively adjust the plasma frequency for the best filtering. Thus, such a plasma-modified filter provides a feasible scheme to dynamically adjust the filtering frequency.

Tunable plasmonic band stop filter based in graphene nanoribbon

In this work we suggest and analyze a simple and compact structure that can work as a graphene-based plasmonic band stop filter for the THz region. The device is formed by graphene nanoribbons, where the propagation and blocking of surface polariton plasmon waves occurs, which fully return to the device’s input port. The graphene nanoribbons are deposited on a two-layer dielectric substrate formed by SiO2 (silica) and Si (silicon). The band stop filter can be dynamically controlled by applying an electrostatic field that changes the chemical potential of graphene, changing the filter’s operating range. Numerical simulations show that the insertion losses reach levels around -55 dB with a bandwidth of 7.3 THz, showing that the device has good performance for the frequency range used in our work and can be used for future applications.