Tunable Filter Research Articles

Tuning stability enhancement of all-fiber acousto-optic tunable filter based on multi-piezoelectric transducer

All-fiber acousto-optic tunable filters (AOTFs) have advantages of wide wavelength tuning range, fast tuning speed and low insertion loss. The resonance wavelength of the all-fiber AOTF can be tuned by adjusting the driving frequency of the piezoelectric transducer (PZT). In general, the driving voltage needs to be tuned in wide range in the wavelength tuning process, due to the existence of resonance frequency of the PZT. In this work, we propose and demonstrate a stability-enhanced all-fiber AOTF based on multi-PZT, which can reduce the dependence on the resonant frequency of a single PZT. In the experiment, the acousto-optic coupling efficiency can maintain larger than 80%, when the resonance wavelength of the AOTF is tuned from 1400 nm to 1600 nm, under the condition of a fixed driving voltage. The high tuning stability of the proposed multi-piezoelectric driving configuration can facilitate the practical applications of the all-fiber AOTF, such as the laser wavelength tuning, wavelength-division-multiplexing optical communication, fast optical spectroscopy, and so on.

Kalman Filter Tuning for State Estimation of Lithium‐Ion Batteries by Multi‐Objective Optimization via Hyperspace Exploration

For the estimation of the state of charge of lithium‐ion batteries Kalman filters are the state of the art. To ensure precise and reliable estimations these filters use covariance matrices, which need to be tuned correctly by the developer. This process is time‐consuming and depends largely on the experience and skill of the developer. Hence, filter tuning is not reproducible and not optimal with regard to goals as accuracy and convergence speed. Herein a multiobjective optimization framework called hyperspace exploration is used for the first time to automate the filter tuning procedure for an extended Kalman filter and two versions of adaptive extended Kalman filters. Four key performance indicators, including the maximum error in the estimation of the state of charge and the according root mean square error, are used to describe, validate, and compare the filter performance. This automated process enables optimal usage of the degrees of freedom in filter tuning and no longer requires manual tuning while the whole hyperspace, including different use cases and validation scenarios, is considered in the optimization. Furthermore, the proposed approach yields a novel method for the evaluation of filter parameters and their influence on the estimation behavior.

Study transmission characteristics of graded photonic crystal as low pass filter

Here, we investigate propagation characteristics of graded thickness one-dimensional photonic crystal structure made of silicon and air. The optical transmission is obtained by transfer matrix method (TMM) and studied impact of graded index and refractive index variation on the transmission spectra. The optical transmission of suggested structure drops as frequency increases and falls to zero more rapidly as graded index increases. The graded index variable can be used to tune the suggested structure’s cut off frequency, which corresponds to half of the transmission value. So, such device has useful application as tunable optical low-pass filter.

A Novel Frequency Tuning Scheme for a C-Type Filter Based on the Best Filtering Frequency

The C-type filter is widely applied in harmonic suppression. As an approximate formula of tuning frequency is used in C-type filter design, the calculated tuning frequency has a deviation. Therefore, an accurate C-type filter tuning frequency formula is derived. In this paper, we find that the tuning frequency of the C-type filter may not exist and reveal the existence conditions. Different from single-tuned filters, the best filtering frequency of C-type filters is not the tuning frequency. However, because the best filtering frequency can hardly be obtained by solving the derivative zeros of the equivalent impedance equation, the tuning frequency is still regarded as the best filtering frequency. The C-type filters do not have the best filtering performance for the above reasons. Therefore, by introducing the critial best filtering frequency and the limit best filtering frequency, an equation between the best filtering frequency and capacitor ratio parameter is established, which can avoid solving the derivative zeros. Finally, two C-type filter design schemes based on the best filtering frequency are proposed, and the superiority of the schemes is verified based on the IEEE-14 bus test system.

Constant absolute bandwidth six-pole quasi-elliptic tunable bandpass filter using synchronously tuned dual-mode resonators

This paper presents the design of a 6th-order tunable bandpass filter (BPF) that achieves constant absolute bandwidth (ABW) and transmission zeros (TZs) across a wide frequency tuning range. The proposed tunable BPF employs three synchronously tuned dual-mode resonators, along with a frequency-dependent coupling structure realized using a T-network consisting of series and shunt capacitors. To achieve constant ABW, the tunable external quality factors are implemented using series capacitor and shunt varactor. As an illustrative example, a 6th-order BPF is fabricated and measured, results revealing that the center frequency of BPF can be tuned from 1.39 GHz to 1.81 GHz (26.25 %) with two TZs on the lower and upper side of the passband, 1-dB constant ABW of 62 ± 1.5 MHz and return loss > 12 dB in every tuning state.

Ultra-Wideband Tunable Microwave Photonic Filter Based on Thin Film Lithium Niobate

In signal processing of the growing semaphore, a microwave photonic filter (MPF) is capable of dealing with high-frequency signals, offering the advantages of high bandwidth, easy tuning, and more. This paper presents an efficient tunable microwave photonic filter that features a wideband tuning capability and narrow-band filtering effect based on the lithium niobate on insulator (LNOI) material platform. A multi-mode waveguide race-track type microring resonator has been designed, along with a thermal electrode that utilizes the thermo-optical effect of lithium niobate to adjust the microring resonator. The packaged device has been tested, with a wideband tunable range of 4.7~38.2 GHz achieved. This allows for cross-band continuous tuning across the C-band to Ka-band range. When supplied with 29.1 mW of electric power, the thermal tuning efficiency reaches 9.2 pm/mW, enabling high-frequency tuning of up to 38.2 GHz. The filter possesses a high resolution, exhibiting a 3 dB bandwidth of 662 MHz.

Exceptional points and lines and Dirac points and lines in magnetoactive cholesteric liquid crystals

We investigated the properties of cholesteric liquid crystals (CLCs) being in external static magnetic field directed along the helix axis. We have shown that in the case of the wavelength dependence of magneto-optic activity parameter, and in the presence of absorption new features appear in the optics of CLCs. We have shown that in this case new photonic band gaps (PBGs) appear. This new PBG is sensitive to the polarization of the incident light. But if the chirality sign of the polarization of the incident diffracting light for the basic PBG (which exist also at the absence of external magnetic field) is determined only by the chirality sign of the CLC helix, then for the second one it is determined by the external magnetic field direction (i.e., on whether the directions of the external magnetic field and the incident light are parallel, or they are antiparallel). We have shown that in this case besides Dirac points there appear also Dirac lines as well as exceptional points and exceptional lines. And moreover, near some of these points and lines the narrow bands or wide bands of magnetically induced transparency, and magnetically induced absorption appear, and near the others a wide coherent perfect absorption band appears that is insensitive to incident light polarization. And finally at some wavelength the same reflection, transmission and absorption takes place for any polarization of incident light, although it is not Dirac point nor exceptional point. This system can apply as tunable narrow-band or broad-band filters and mirrors, a highly tunable broad/narrow-band coherent perfect absorber, transmitter, ideal optical diode, and other devices.

Designing cladding mode interference device – a host for sensors and all-fiber inline gain-flattening component and broad-band filter

This article describes how to devise interference and self-imaging of core and cladding modes based devices using dissimilar core concatenated all-fibers. The core diameter mismatch in the structure excites the cladding modes with the core mode that leads to a progressive interference imaging between the cladding modes and the core mode yielding wavelength and refractive index dependent loss. Through a systematic study, we tune the device parameters to gauge temperature and concentration with sensitivity of 0.0713 dB/°C and 0.0372 dB/(% mL/mL) respectively as a high-performance fiber sensor. Next, optimizing the wavelength sensitive parameters of the host configuration, we investigate the transmission characteristics as gain-flattening filters (GFF) of Erbium-doped fiber amplifiers (EDFAs) with maximum gain variations of 0.91 and 0.97 dB, for two different EDFAs respectively over a wavelength band of 33 and 25 nm. Finally, we demonstrate a tunable and two-flattop band rejection filters with rejection band of 14, 26 and 46 nm, respectively.

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.

Design of dual-layer serpentine-shaped metamaterial with tunable resonant infrared wavelength and spectra intensity

We present a design of dual-layer serpentine-shaped metamaterials (SSM) in the infrared (IR) wavelength range. It is composed of one serpentine-shaped gold (Au) nanostructure array on silicon (Si) substrate and another serpentine-shaped Au nanostructure array suspended atop. SSM device shows multi-resonance in the wavelength range from 1 μm to 3 μm in transverse electric (TE) and transverse magnetic (TM) modes. By changing the gap of top SSM layer, the resonance at the wavelength of 1.15 μm can be modulated from 23% to 47% and that at the wavelength of 1.78 μm can be modulated from 82% to 2% in TE mode. In TM mode, the tuning ranges of two resonances are over 80%. Moreover, the proposed SSM design is sensitive to the polarization angle of incident wave, which shows switching characteristics at resonant wavelengths under the conditions of different polarization angles. To increase the flexibility and applicability of SSM device, it is exposed on the ambient environment with different refractive indexes. The relationships of resonances and refractive indexes are quite linear. Hence, the SSM device exhibits the potential application to be used to realize logic devices, switchable optic devices, environmental detectors, tunable filters and so on.

Generation of dual and quad-optical frequency combs in the injected radiation free mode-locked frequency-shifted feedback laser

The results of an optoelectronic system—frequency-shifted feedback (FSF) laser experimental examination are presented. The considered FSF laser is seeded only with optical amplifier spontaneous emission (ASE) and operates in the mode-locked regime, whereby the output radiation is sequence of short pulses with a repetition rate determined by the delay time in its optical feedback circuit. In the frequency domain, the spectrum of such a pulse sequence is an optical frequency comb (OFC). These OFCs we call initial. We consider the possibility of tunable acousto-optic (AO) dual and quad-comb frequency spacing downconversion in the FSF laser seeded with ASE and operating in the mode-locked regime. The examined system applies a single frequency shifting loop with single AO tunable filter as the frequency shifter that is fed with several radio frequency signals simultaneously. The initial OFCs with frequency spacing of about 6.5 MHz may be obtained in the wide spectral range and their width, envelope shape and position in the optical spectrum may be tuned. The dual-combs are obtained with a pair of initial OFCs aroused by two various ultrasound waves in the acousto-optic tunable filter (AOTF). The dual-combs frequency spacing is determined by the frequency difference of the signals applied to the AOTF piezoelectric transducer and can be tuned simply. The quad-combs are obtained with three initial OFCs, forming a pair of dual-combs, appearing when three ultrasound frequencies feed the AOTF transducer. The quad-combs frequency spacing is defined by the difference between the frequency spacing of dual-combs. Quad-combs with more than 5000 spectral lines and tunable frequency spacing are observed. The successive frequency downconversion gives the possibility to reduce the OFC frequency spacing form several MHz for initial OFC to tens of kHz for quad-combs.Graphical abstract

Power-efficient polarization-insensitive tunable microring filter on a multi-layer Si3N4-on-SOI platform.

We develop and demonstrate a non-duplicate polarization-diversity tunable bandpass optical filter by leveraging the bi-directional transmission of add-drop dual-coupled microring resonators (MRRs) on a multi-layer Si3N4-on-silicon-on-insulator (SOI) platform. By using Euler-bends, we implement compact and low-loss Si3N4 MRRs with an equivalent bending radius of ∼38 µm. Fiber-to-fiber (on-chip) insertion loss of 4.3 dB (1.7 dB) with a low polarization-dependent loss of <0.5 dB and low differential group delay of <2.5 ps is achieved. The extinction ratio is more than 30 dB. The thermo-optic tuning efficiency of the Si3N4 MRRs is improved with a suspended micro-heater design. As a result, a wavelength tuning range of ∼2 nm and a 3-dB bandwidth tuning range of 20 GHz are experimentally demonstrated. The tuning efficiency is 33 pm/mW, which is ∼7.5 times higher than the previous design. This reconfigurable polarization-insensitive filter, with low loss, low cross talk, and high power efficiency, is highly promising for practical applications in optical communication and signal processing.

Electroluminescence hyperspectral imaging of light-emitting diodes using a liquid crystal tunable filter

We demonstrate the use of a low-cost liquid-crystal-based wavelength-tunable filter and CMOS video camera to add hyperspectral imaging capabilities to a probe station equipped with a simple optical microscope. The resultant setup is used to rapidly resolve the spectral and spatial variations in electroluminescence typically observed for InxGa1−xN/GaN light-emitting diodes. Applying standard statistical analyses of variation within the multivariate datasets, such as moments and principal components, we observe inhomogeneities on a spectral scale significantly smaller than the bandwidth of the tunable filter. The resultant tool offers an alternative to scanning beam luminescence techniques for high-throughput hyperspectral analysis of optoelectronic devices.

Electrically Tunable Topological Notch Filter for THz Integrated Photonics

AbstractElectromagnetic filtering is essential for emerging integrated photonic technologies and widely adaptable processing of high‐bandwidth signals. The conventional electro‐optic modulator‐based photonic approach for signal filtering at microwave frequencies cannot be implemented at terahertz (THz) frequencies due to the unavailability of the THz signal‐driven optical modulator. Here, an electrically tunable on‐chip THz photonic notch filter based on the topologically protected valley hall waveguide‐cavity platform is demonstrated. The device shows a significantly large notch suppression depth of more than 20 dB with a return loss of 13 dB in the entire tuning range of notch frequency. The feedback control circuit enables precise control of the notch frequency shift with a minimum step size of 7 MHz. This work extends the application of topological photonic crystals in developing THz‐integrated photonic devices for transformative technologies, including sixth‐generation (6G) communication and high‐resolution spectral sensing.

Spin filtering in Dresselhaus spin-orbit-coupled, layered-semiconductor microstructure

Based on Dresselhaus spin-orbit-coupling effect in a layered semiconductor microstructure, an electron-spin filter is proposed. It is found that an obvious spin polarization effect appears and is related to electron energy and in-plane wave vector. It is also found that both magnitude and sign of spin polarization can be controlled by strain engineering or layer's thickness. This layered-semiconductor microstructure can act as a tunable spin filter.

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.

Quasi-Elliptic Lossy Filter With Reconfigurable Bandwidth

This letter proposed a quasi-elliptic lossy filter with a wide tunable bandwidth and enhanced passband flatness. For adapting the limited circuit sizes, a new type of lossy technique named center-loaded resistive-cross-couplings (CLRCCs) is proposed. By loading resistor coupling at the centers of half-wavelength resonators, additional mid-passband losses can be introduced to flatten the passband. A bandwidth reconfigurable five-pole filter with a hybrid structure is developed for the examination. The measured promising results showed the superiority of the proposed novel lossy method for wideband tunable filters.

Electro-optic tunable optical filter based on long-period waveguide grating in lithium niobate on insulator with absorption ribbons.

We propose an electro-optic tunable optical filter based on sidewall long period waveguide grating (LPWG) in lithium niobate on insolator (LNOI). The operation of our proposed filter is based on the mode coupling, filtering, and absorption achieved, respectively, with two corrugated sidewall LPWGs, a tapered waveguide, and two metal ribbons. Our typical fabricated devices achieved a 16.32-dB rejection band and an EO tuning efficiency of ∼0.344 nm/V. Our proposed LPWG and filter are compact and could be integrated with other LNOI waveguide devices to realize more sophisticated functions for on-chip optical signal processing.

Spectral Variability in Fixed Windows using Fractional Fourier Transform: Application in Power Spectral Density Estimation

In statistical signal processing, power spectral density estimation is a frequency domain analysis in which power contents of a signal are measured with respect to frequency components of that signal. The power estimation of a signal can be carried out more precisely by using a window with a narrower 3-dB bandwidth and higher side-lobe attenuation. Theoretically, these two spectral parameters show trade-off in variable windows and remain constant in fixed windows. In this work, spectral behavior of fixed windows has been elaborated using Fractional Fourier Transform (FRFT) keeping their inherent time domain behavior intact. The FRFT is an extension of conventional Fourier transform with an additional variable parameter, known as rotation angle, which makes it more flexible and useful in various signal processing applications viz. power estimation and designing of tunable transition band FIR filters. In this article, variability in 3-dB bandwidth and sidelobe attenuation of fixed windows has been achieved by exploiting the available flexibility in FRFT and obtained variability has been applied in the estimation of signal power. Simulation results demonstrate that both of these two spectral parameters are improved and hence, trade-off problem between resolution and spectral leakage in the power spectral density estimation is overcome upto an extent.

A single-mode tunable plasmonic sensor based on an 8-shaped resonator for cancer cell detection

In this paper, a novel 8-shaped resonator coupled to metal–insulator–metal waveguides is used for designing plasmonic filters and sensors. The resonator supports two resonance modes, which result in peaks in the transmission spectrum of the structure. A Q-factor of 247.4 which can reach up to 270 at the wavelength of 1187.5 nm is observed. By placing vertical and horizontal metal blades in the resonator, two tunable single-mode plasmonic filters are obtained at the first and second resonance modes, respectively. The effect of structural parameters on the transmission spectrum is investigated using the finite-difference time-domain (FDTD) method. Based on the obtained results, the proposed plasmonic structure can be used for biosensing applications such as the detection of basal cancer cells with a sensitivity of 1200 nm/RIU. It is of great significance that both the sensitivity and Q-factor values for the proposed structure are higher than most recent sensors reported in the literature. Therefore, the proposed structure is a potentially promising candidate for filtering and sensing applications.