Tunable Filter Research Articles

A tunable dual-narrowband HTS filter using coding superconducting metamaterials aided by grey wolf algorithm

Abstract The design of tunable dual-narrowband high-temperature superconducting (HTS) filter based on coding superconducting metamaterials aided by grey wolf algorithm (GWA) is proposed in this paper. By virtue of the characteristics of right/left-handed bands and scale of deep sub-wavelength, the tunable metamaterials unit loaded with varactor is designed, with coupling types of coding pairs further studied. A tunable sixth-order HTS filter circuit is then constructed, whose layout is effectively optimized by newly developed GWA method. Simulated frequency responses demonstrate that the center frequencies of dual-band respectively locate at 170.1-177.2 MHz (Band I) and 185.2-194.1 MHz (Band II), with 3-dB fractional bandwidth (FBWs) of 0.65%-0.61% and 0.62%-0.57%. The deeply miniaturized circuit size is 0.037×0.046 λ2 g, and the close-to-band rejection level reaches 80-dB. Experimental results of fabricated prototype measured at 70 K validate the feasibility of described design method for high order tunable HTS filter.

An edge-coupled magnetostatic bandpass filter

The further development of 5G and 6G communication systems introduced new frequency allocations beyond 6 GHz, necessitating the development of compact bandpass filters that can operate over wide gigahertz frequency ranges. Herein, we report on the design, fabrication, and characterization of an edge-coupled magnetostatic forward volume wave bandpass filter (MSFVW). Using micromachining techniques, we fabricate both 2-pole and 4-pole filters from a yttrium iron garnet (YIG) film grown on a gadolinium gallium garnet (GGG) substrate with inductive transducers. By adjusting an out-of-plane magnetic field, we demonstrate linear center frequency tuning for a 4th-order filter from 4.5 GHz to 10.1 GHz while retaining a fractional bandwidth of 0.3%, an insertion loss of 6.94 dB, and a − 35 dB rejection level. We characterize the filter nonlinearity in the passband and stopband with IIP3 measurements of − 4.85 dBm and 25.84 dBm, respectively. In this work, we demonstrate a compact octave tunable narrowband channel-select filter with a significant degree of design flexibility and performance comparable to the state-of-the-art.

A dual mode circular waveguide cavity resonator based frequency tunable band pass filter using a single tuning element

AbstractIn this letter, we present a tunable band pass filter (BPF) using a dual‐mode circular waveguide cavity resonator using a single tuning element. For the proof of the concept, a 2nd order tunable BPF has been designed and developed over a tuning range from 4.92 to 6.1 GHz (21.4%). The measured bandwidth is 85 ± 10 MHz over the entire tuning range. The peak insertion loss is within 1.3 dB and return loss is better than 15 dB. The tunable filter is proposed to be used in high Q reconfigurable radio systems.

Acousto-optic femtosecond laser beam shaping for bottle-beam generation

Abstract This study explores the properties of a laser beam shaping system comprising an acousto-optic dispersive delay line (AODDL) and an acousto-optic tunable filter (AOTF) for ultrashort laser pulse shaping. The system demonstrates the AODDL’s ability to generate signals with arbitrarily positioned transmission windows and investigates the impact of spectral width on field distributions. The AOTF’s frequency scanning and simultaneous control of two and three spectral components are presented. The experimental setup utilizes femtosecond laser radiation, and the results highlight the system’s versatility for programmable spectral and spatial filtering, offering potential applications in advanced optical manipulation.

Temperature impact on acoustic wave reflection in quasi-collinear acousto-optic devices.

Quasi-collinear geometry is a special configuration of acousto-optic (AO) diffraction that applies the acoustic wave reflection from the AO cell input optical face and provides an extremely large interaction length for achieving abnormally high spectral resolution of AO tunable filters. As a result, it becomes possible to implement the multifrequency diffraction which has found important applications for laser pulse shaping. The operation of quasi-collinear AO devices in the multifrequency diffraction regimen is accompanied by the appearance of the longitudinal and transverse temperature gradients in the crystal, mainly due to the acoustic power absorption. Temperature changes the AO cell material stiffness moduli, affecting the characteristics of the incident and reflected acoustic waves (propagation velocities and walk-off angles), and the reflection condition in general. On the example of paratellurite crystal is shown that the AO cell heating near the reflecting facet leads to a deviation of the reflected acoustic beam propagation direction from that specified during the AO cell manufacturing. The deviation magnitude depends on the reflection geometry choice and, in the paratellurite, may exceed several degrees, which adversely affects the AO diffraction characteristics, reducing the AO interaction efficiency and distorting the transmission function shape. The reflected beam deviation may be compensated by means of choosing the angle between the AO cell reflecting face and the piezoelectric transducer face, taking into account the operating AO device thermal regimen.

Tunable terahertz meta-resonator with switchable single- and dual-resonance characteristic

A tunable terahertz meta-resonator (TTM) is proposed to be capable of switching single- and dual-resonance characteristics in this article. The presented TTM device consists of two symmetrical E-shaped electric split-ring resonators (eSRRs) on the silicon (Si) substrate. Through the change of the gap between two eSRRs, it allows the resonances to be tuned about 78 GHz, spanning from 0.780 THz to 0.702 THz in transverse magnetic (TM) mode. When one eSRR is anchored on the Si substrate and the other suspended eSRR is heightened along the z-axis direction, the TTM device exhibits resonance-insensitive behavior in transverse electric (TE) mode, along with a convertible feature that enables it to transition between single-resonance and dual-resonance responses in TM mode. Besides, the practicality of TTM device is further enhanced by its capability to operate effectively in ambient surroundings with varying refractive indexes. In TE and TM modes, the high linearity relationship between resonances and refractive indexes is achieved with correlation coefficients both exceeding 0.99. The TTM device provides potential possibilities in tunable filters, switchable logic gates, environmental sensors, and other dynamically controllable THz-wave optoelectronics devices.

Wavelength-tunable mode-locked fiber laser based on a bending strain-controlled filter

Wavelength-tunable mode-locked fiber lasers are highly versatile and find applications in pump–probe measurement, optical communications, optical parametric oscillation, among others. Herein, we introduce a novel approach for tuning carbon nanotube-based passively mode-locked fiber laser using a bending-based mechanism. Our method involves a bending strain controlled multimode interference all-fiber filter. This filter consists of a hybrid structure built from a cascaded seven-core fiber (SCF)-twin-hole twin-core fiber (THDCF)-SCF. It achieves a continuous wavelength tuning range of 25 nm from 1558 nm to 1583 nm with a tuning efficiency of −3.38 nm/m−1. By increasing the pump power, the dual-wavelength locked modes are also achieved at specific curvature. Owing to advantages of simple structure, low cost, and high bending sensitivity, our tunable filter shows promise for efficient wavelength tuning in ultrafast fiber laser.

Design of high selectivity tunable bandpass filter with switchable single‐/dual‐band responses

AbstractThis article proposes a switchable filter with four operating states, namely dual‐band mode, lower band mode, upper band mode, and all‐stop mode. The transition between these states is achieved by using positive intrinsic negative diodes, while the center frequency tuning is done by using varactors. In order to verify the design concept, a prototype has been fabricated on an F4BME substrate with a dielectric constant of 2.25 and a thickness of 1 mm, and the measured results were in agreement with the simulations. Notably, the dual‐band mode yields four transmission zeros (TZs), while the single‐band mode produces two TZs. The final circuit size of the designed filter is 0.18 × 0.14 λg, making it ideally suited for reconfigurable multi‐band communication systems.

Design of near-infrared solid-state tunable Fabry-Perot filters based on VO2/P4VP films

The significant refractive index change of VO2 during phase transition makes it attractive for the design of optical switches and filters, but high absorption of VO2 limits its development in the field of filter. In this paper, we firstly design near-infrared solid-state tunable/switchable single-/dual-band bandpass Fabry-Perot (F-P) filters based on VO2/P4VP composite films using COMSOL software. The simulation results show that the single-passband tunable filter can achieve a continuous thermal tuning range of 19 nm at 1064 nm, the full width at half maxima (FWHM) is 28.8 nm, and the peak transmittance is >30%. On this basis, we simulated a tunable dual-band bandpass filter, the left peak can be heated to achieve a tuning range of 15 nm (945 nm), the right peak is 24 nm (1168 nm), the peak transmittances of both the two peaks are >35%. When boosting the VO2 fraction, the continuous tunability of the above-mentioned single-/dual-band filters will be switchable due to the high absorption of VO2, and then switchable single-/dual-band bandpass filters are designed. This work has guiding significance for the design of novel filter structures and optical tunable devices based on phase change materials such as VO2 in the future.

Tunable narrow band-rejection and band-pass filter based on long-period waveguide grating on a thin-film lithium niobate rib waveguide

We propose and demonstrate experimentally an electro-optic (EO) and thermo-optic (TO) tunable wavelength filter with band-rejection and band-pass dual-function. Our proposed filter is based on a long-period waveguide grating (LPWG) formed on a lithium niobate on insulator (LNOI) rib waveguide with a channel-shaped polymer cladding waveguide. The LPWG formed on the surface of the LNOI core enables efficient mode coupling between the two fundamental modes of the LNOI waveguide and the polymer cladding waveguide and hence dual-function filtering. For the z-/x-polarized input light, our fabricated best filter shows an 18.1-dB/8.2-dB band-rejection contrast and a 15.7-dB/17.4-dB band-pass contrast at 1565.26/1594.24 nm wavelength, with a relatively narrow 3-dB bandwidth of 2.5/2.3 nm, respectively. In addition, the fabricated filter also shows an EO tuning efficiency of ∼32 pm/V and a TO tuning efficiency of 0.465 nm/°C, respectively. Our proposed filter could find applications in wavelength division multiplexing, temperature sensing, and other fields of optical communication and optical information processing.

Bandwidth modulation and pulse characterization of passively Q-switched erbium-doped fiber laser

We demonstrate the modulation of laser bandwidth by utilizing an ultranarrow tunable bandpass filter (UNTBF) in a passively Q-switched erbium-doped fiber laser. The passive Q-switch mechanism is enabled by using carbon nanotubes as saturable absorber at a Q-switched threshold of 35.5 mW. Based on spectral filtering effect introduced by the UNTBF, the 3 dB laser bandwidth can be tuned from 0.016 nm to 0.478 nm at a fixed pump power of 75.9 mW. The corresponding pulse behavior for each different bandwidth is characterized, and the results reveals that the pulse width can be as well tuned from 7.8 to 2.6 μs against the laser bandwidth, which agrees with the rule of time-bandwidth product. Correspondingly, the pulse repetition rate and the pulse energy vary from 16.23 kHz to 26.16 kHz and from 0.67 to 1.03 μJ respectively across the laser bandwidth. Further investigation of the pulse performance is performed against the pump power increment up to 107.2 mW. To the best of our knowledge, this is the first demonstration of spectrum bandwidth modulation in a passively Q-switched fiber laser, which can be useful for fully exploiting the possibilities of Q-switched pulse applications.

Microelectromechanical System-Based Reconfigurable Terahertz Metamaterial for Polarization Filter, Switch, and Logic Modulator Applications.

The terahertz (THz) metamaterials integrated with microelectromechanical systems (MEMS) have led to the realization of dynamic control in amplitude, phase, polarization, and spin angular momentum of the THz wave. In this study, we demonstrate an MEMS-based reconfigurable THz metamaterial (RTM) composed of a split ring resonator (SRR) for real-time modulation of THz wave. By gradually increasing the polarization angle of the incident THz wave, the resonant frequency of SRR switches from 0.74 to 1.16 THz, and the maximum modulation depth is more than 70%. When the MEMS-based RTM is actuated by different DC bias voltages, the polarization-dependent transmission intensity and resonant frequency of the device can be actively tuned. MEMS-based RTM shows logical function characteristics that can be used for logic modulators by performing the driving voltages and polarization states as 2-bit input signals and quantizing the transmission response as "on" and "off" states. The logic gates of "NAND" are at 0.439 THz and "AND" is at 0.732 THz. These results offer potential applications for the proposed MEMS-based RTM in tunable and reconfigurable polarization filters, optical switches, programmable logic modulators, and so on.

Microwave magnetic excitations in U-type hexaferrite Sr4CoZnFe36O60 ceramics

Microwave (MW) transmission, absorption, and reflection loss spectra of the ferrimagnetic U-type hexaferrite Sr4CoZnFe36O60 ceramics were studied from 100 MHz to 35 GHz at temperatures between 10 and 390 K. Nine MW magnetic excitations with anomalous behavior near ferrimagnetic phase transitions were revealed. They also change under the application of the weak bias magnetic field (0–700 Oe) at room temperature. Six pure magnetic modes are assigned to dynamics of the magnetic domain walls and inhomogeneous magnetic structure of the ceramics, to the natural ferromagnetic resonance (FMR), and to the higher-frequency magnons. Three modes are considered the magnetodielectric ones with the dominating influence of the magnetic properties on their temperature and field dependences. The presence of the natural FMR in all ferrimagnetic phases proves the existence of the non-zero internal magnetization and magnetocrystalline anisotropy. Splitting of the FMR into two components without magnetic bias was observed in the collinear phase and is attributed to a change in the magnetocrystalline anisotropy during phase transition. The high-frequency FMR component critically slows down to phase transition. At room temperature, FMR splitting and essential suppression of the higher-frequency modes were revealed under the weak bias field (300–700 Oe). The highly nonlinear MW response and FMR splitting are caused by the gradual evolution of the polydomain magnetic structure to a monodomain one. The high number of magnetic excitations observed in the MW region confirms the suitability of using hexaferrite Sr4CoZnFe36O60 ceramics as MW absorbers, shielding materials and highly tunable filters.

Evaluation of the Tellurium Dioxide Crystal Shear Acoustic Wave Attenuation at 40-140 MHz Frequency.

The attenuation of slow shear acoustic waves in the (110) plane of tellurium dioxide (TeO2) crystals was investigated. The strong acoustic anisotropy of TeO2 crystals results in a non-uniform acoustic power distribution, which can introduce errors in conventional acousto-optic testing methods. In this study, we propose a general method to measure the acoustic power distribution along the propagation direction of acoustic waves in non-collinear acousto-optic tunable filters (AOTFs). Additionally, we analyze the errors introduced by the non-uniform acoustic field resulting from strong acoustic anisotropy in acousto-optic testing methods. The measurements were carried out for a crystal cutoff angle of 6.5° from the [110] axis, for the ultrasound frequency range from 40 to 140 MHz. The attenuation coefficients were determined and their quadratic dependence on ultrasound frequency was confirmed.

Fully tunable microwave photonic narrow bandpass filter using an on-chip dual-drive microring resonator

We propose and experimentally demonstrate a fully tunable microwave photonic narrow bandpass filter based on phase modulation to intensity modulation (PM-IM) conversion. In the filter implementation, an on-chip dual-drive microring resonator (MRR) is a key component. This resonator leverages a multimode waveguide to enable a high Q-factor. A metallic micro-heater and a lateral PN junction are simultaneously created for resonance wavelength tuning. When one driving signal is applied to the micro-heater, a large tuning range of the resonance wavelength is resulted; when another driving signal is applied to the PN junction, a fast tuning speed of the resonance wavelength is caused. By jointly using two different tuning mechanisms, the realized microwave photonic filter features a large frequency tuning range as well as a fast tuning speed. In addition, the filter bandwidth can also be tuned. A silicon-based dual-drive high-Q racetrack MRR chip is designed, fabricated, and evaluated. By incorporating the chip in a microwave photonic filter system, a bandpass filter with a narrow bandwidth of 1.27 GHz is achieved. An ultra-wide frequency tuning range from 3 to 51 GHz, an ultra-fast tuning speed less than 0.54 ns, and a tunable bandwidth from 1.27 to 4.47 GHz is experimentally demonstrated. This fully tunable filter offers significant potential in future radar and next-generation wireless communication applications.

A simple and cost-effective strategy for electrochemiluminescence spectral determination

BackgroundElectrochemiluminescence (ECL), as a unique and powerful analytical technique, has been widely used in various fields. The determination of ECL spectra plays a crucial role in understanding ECL reaction mechanisms and conducting spectra-resolved ECL analysis. ECL intensity is typically detected using a photomultiplier tube, which offers high sensitivity for detecting extremely weak light signals but does not allow for spectral identification. Due to the time-dependent variation of ECL intensity caused by the applied potential and electrochemical reaction processes, it is challenging to perform ECL spectral detection using conventional wavelength-scanning spectrometers. ResultsIn this study, we present a straightforward and cost-effective ECL spectral detection strategy by incorporating an automatically controlled tunable optical filter device between a commonly used PMT detector and a specially designed ECL reaction cell. The effectiveness of this approach was confirmed through initial validation, where the spectrum of a green LED spotlight was measured and compared with a commercial spectrometer. In a dynamic system with stable ECL signals, the ECL spectrum of the typical Ru(bpy)32+/TPA system was rapidly acquired by adjusting the bandpass filters. To account for time-varying ECL signals in practical measurements, time-based correction algorithms were implemented to rectify variations in ECL intensity. By integrating time-based correction algorithms and an automatically controlled tunable optical filter device into a commonly utilized PMT detector, the rapid and sensitive ECL spectra determination was achieved. Experimental results demonstrated the reliability of the proposed strategy. SignificanceThis strategy is based on the widely used high-sensitivity PMT detection component, enabling the rapid and sensitive measurement of ECL spectra without altering the ECL detection hardware. It is simple, fast, efficient, and cost-effective, with the potential to be widely used for rapid ECL spectral detection and spectra-resolved ECL analysis.

Theoretical study of a tunable optical device on the base of magnetoactive cholesteric liquid crystals

We investigated the light reflection from half-space of cholesteric liquid crystal (CLC) being in external static magnetic field directed along the helix axis at normal light incidence. In this case there appear three photonic band gaps (PBGs) and these bands can be tuned in large intervals. We found the eigenpolarizations (EPs) at reflection of light from half-space and investigated their peculiarities. We have shown that in the absence both of absorption and of external magnetic field and at minimal influence of dielectric boundary (when the dielectric permittivity of isotropic half-space which borders with CLC half-space is equal to the CLC's mean dielectric permittivity) these polarizations are orthogonal. While, in the presence of external magnetic field they become quasi-orthogonal everywhere outside the main PBG (which exist also at the absence of external magnetic field), and they are significantly non-orthogonal within the main PBG. At some values of helix pitch they become significantly non-orthogonal practically everywhere. Moreover, in this case there exist some special points where the EPs degenerate. This system can apply as tunable narrow-band or broad-band filter and mirror, a highly tunable broad/narrow-band optical diode, and other devices.

A Multi-Format, Multi-Wavelength Erbium-Doped Fiber Ring Laser Using a Tunable Delay Line Interferometer

This work demonstrates the use of an erbium-doped fiber amplifier (EDFA), a tunable bandpass filter (TBF), and a tunable delay line interferometer (TDLI) to form a ring laser that produces multi-format, multi-wavelength laser beams. The TDLI serves as the core of the proposed laser generation system. TDLI harnesses the weak Fabry–Pérot (FP) interferences generated by its built-in 50/50 beamsplitter (BS) with unalterable filtering characteristics and the interferences with free spectral range (FSR) adjustable from each of its two outputs with nearly complementary phases to superpose and generate a variable interference standing wave. The interferometric standing wave and weak FP interferences are used to form a spatial-hole burning to promote the excitation of multi-format and multi-wavelength lasers. The proposed system enables dual-wavelength spacing ranging from 0.3 nm to 3.35 nm, with a switchable wavelength position at approximately 1527 nm to 1535 nm, providing flexible tunability.

Enhancement of Brillouin nonlinearities with a coupled resonator optical waveguide.

Stimulated Brillouin scattering (SBS) is a nonlinear optical phenomenon mediated from the coupling of photons and phonons. It has found applications in various realms, yet the acousto-optic interaction strength remains relatively weak. Enhancing the SBS with resonant structures could be a promising solution, but this method faces strict constraints in operational bandwidth. Here, we present the first demonstration to our knowledge of the broadband enhancement of Brillouin nonlinearities by a suspended coupled resonator optical waveguide (CROW) on an SOI platform. By comprehensively balancing the Brillouin gain and operational bandwidth, a 3-fold enhancement for the Brillouin gain coefficient (GB) and a broad operational bandwidth of over 80 GHz have been achieved. Furthermore, this 1.1 mm device shows a forward Brillouin gain coefficient of 2422 m-1W-1 and a high mechanical quality factor (Qm) of 1060. This approach marks a pivotal advancement toward wide bandwidth, low energy consumption, and compact integrated nonlinear photonic devices, with potential applications in tunable microwave photonic filters and phonon-based non-reciprocal devices.

Tunable terahertz filter based on graphene photonic crystals with defective layers

In this paper, we design a high-precision tunable terahertz filter by using transfer matrix method. The filter structure mainly consists of graphene embedded photonic crystals (GPCs). The front part of the GPCs contains artificial synthetic material and air layer, the back part of the GPCs is composed by and periodic stack of isotropic dielectric slabs (MgF2) embedded with graphene sheets, where air defect layer is located in the middle of the GPC as a central layer. Our simulation reveals that graphene layer and air defective layer strongly affect the filter performance. And we can get a relatively pure transmission peak in a wide frequency region. Additionally, the influence of incidence angle of terahertz wave, thickness of air layer, the unit number of front periodic structure and chemical potential of the graphene sheets can also modulate the function of the filter. And the filter has strong stability when the temperature changes from 150 K to 350 K.The results indicate that single channel, dual and multiple channels filter in a narrow frequency can be obtained by optimizing the structure parameter.