Bandwidth Tunable Filter Research Articles

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.

Bandwidth-Reconfigurable Coaxial Bandpass Filter With Multioctave Tuning Using a Single Element

A bandwidth (BW) tunable coaxial-loaded waveguide filter is presented in this letter based on a novel concept using a single tuning element. The operation principle relies on the rotation of coaxial resonators with respect to fixed irises in order to change the inter-resonator (IR) coupling strengths that control the BW. This mechanism features multioctave tuning while maintaining a fixed operating frequency and a stable unloaded quality factor with only one rotary post. Furthermore, the filter can be switched off by using the same tuning mechanism. A prototype is manufactured and measured to verify the proposed filter. The measured results demonstrate multioctave BW tunability from 12 to 83 MHz (tuning ratio <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&gt;$</tex-math> </inline-formula> 6.9:1), promoting the technique for multistandard communication systems and applications.

Tunable Coaxial Bandpass Filters Based on Inset Resonators

A new class of compact, high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> , tunable coaxial filter is presented in this article based on a novel inset resonator concept. The tuning concept is based on the displacement of movable resonators inside a properly modified metallic housing which features wide tuning capabilities and stable high <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> -factor performance with minimum variation throughout the tuning window. Various prototypes are designed and implemented to demonstrate and validate the proposed concept. A single tunable inset resonator is first designed and measured showing distinctive results of a 43% tuning range, stable high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> of 4100% ± 4%, spurious-free band up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.8\times f_{0}$ </tex-math></inline-formula> , and volume-saving up to 50% when compared with the conventional combline and half-wavelength structures. The design procedure for constant absolute bandwidth (CABW) tunable filters is then presented, and two different tunable inset filters are designed and implemented. First, a manually tunable four-pole filter is demonstrated with the merits of a wide 39.3% tuning range, while maintaining a constant bandwidth of 116 MHz ± 6% and a stable high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> of 1820% ± 6%. Next, an automatically tunable third-order inset filter is designed and measured using high-accuracy piezomotors. Similarly, the measured results exhibit a wide 1.3-GHz tuning range from 2.65 to 3.95 GHz with a stable insertion loss that is less than 0.35 dB, a return loss that is better than 15 dB, and a good spurious performance up to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2.8\times f_{0}$ </tex-math></inline-formula> . To our own knowledge, the proposed tuning technique and tunable components represent state-of-the-art tuning range and stable high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$Q$ </tex-math></inline-formula> with minimal variation when compared with similar loaded-waveguide designs.

Widely-tunable harmonic mode-locked fiber laser by the combination of spectral filtering and gain management

Controlled flexible ultrafast pulses are of great importance in many application fields, such as metrology. Here, we report a compact pulse width tunable passively harmonic mode-locked fiber laser by exploiting the combination effect of spectral filtering and gain management. The spectral filtering is achieved by adopting a bandwidth tunable filter, resulting in pulse width tunable and changing the harmonic order (i.e., repetition rate). On the other hand, gain management can tune the harmonic order, while the emission pulse width typically remains the same. Thus, a combination of these two effects can make the pulse width tunable in a broad range at a fixed repetition rate. At the 6th-order harmonic mode-locking (76.8 MHz), the pulse width can be tuned from ∼ 0.8 ps to 4.3 ps by adjusting the bandwidth and pump strength. The development of pulse width tunable laser sources at a high-repetition rate would find potential applications in fields such as optical sensing and biomedical imaging.

33.2: A Tunable Filter with Single Layer Twist Structure Liquid Crystals

A bandwidth tunable filter and a multi‐pitch filter based on twist structure liquid crystals (LCs) were proposed. The LC filters can be fabricated by multiple templating and refilling process, which shows large bandwidth and good extensibility. The bandwidth of the twist structure LC filter can be tuned from 111 nm to 244 nm with central wavelength shift of 16 nm. Furthermore, the bandwidth can be broadened to a great extent theoretically by the simple fabrication process. Three‐pitch twist structure LC filter was obtained, and the pitch number can be increased by utilizing multiple templating technique. The fabricated tunable twist structure LC filters show a potential application in optical communication device and spectral systems.

P‐105: Cholesteric Liquid Crystal Filters with Single Layer template

The template effects on bandwidth tunable and reflectivity improvement of cholesteric liquid crystal (CLC) filters were investigated. A single layer bandwidth tunable CLC filter was fabricated by multiple templating process. In addition, high reflectivity CLC filters with single layer templating process was proposed which may reflect both right‐ and left‐ circularly polarized light. High‐reflectivity of 65%, 69%, and 72% in the red, green, and blue color regions was achieved. The templated CLC filters show great potential applications in optical community, display, smart windows, and lasing.

Parity-Time Symmetry Enabled Band-Pass Filter Featuring High Bandwidth-Tunable Contrast Ratio

Integrated optical filters based on microring resonators play a critical role in many applications, ranging from wavelength division multiplexing and switching to channel routing. Bandwidth tunable filters are capable of meeting the on-demand flexible operations in complex situations, due to their advantages of scalability, multi-functionality, and being energy-saving. Recent studies have investigated how parity-time (PT) symmetry coupled-resonant systems can be applied to the bandwidth-tunable filters. However, due to the trade-off between the bandwidth-tunable contrast ratio and insertion loss of the system, the bandwidth-tunable contrast ratio of this method is severely limited. Here, the bandwidth-tunable contrast ratio is defined as the maximum bandwidth divided by the minimum bandwidth. In this work, we show that a high bandwidth-tunable contrast ratio and low insertion loss of the system can be achieved simultaneously by increasing the coupling strength between the input port and the resonant. Theoretical analysis under different coupling states reveals that the low insertion loss can be obtained when the system initially operates at the over-coupling condition. A high bandwidth-tunable contrast ratio PT-symmetry band-pass filter with moderate insertion loss is shown on the Silicon platform. Our scheme provides an effective method to reduce the insertion loss of on-chip tunable filters, which is also applicable to the high-order cascaded microring systems.

Wavelength and bandwidth tunable filter and its application in a dissipative soliton fiber laser.

A wavelength and bandwidth tunable filter and its application in a dissipative soliton (DS) Yb-doped fiber laser are demonstrated. The spectral filter consisting of six cascaded temperature-sensitive long-period fiber gratings is designed and fabricated for the first time, to the best of our knowledge. The corresponding spectral characteristics of the filter are also characterized with temperature. Its 3-dB bandwidth can be adjusted from 4.84 to 11.02 nm, and the center wavelength is continuously adjustable from 1036 to 1045 nm. The sensitivity of the variation of the center wavelength and the linearity of the center wavelength variation are 32 pm/°C and 99.53%, respectively. This home-made spectral filter has two notable features: i) with regard to the tunable spectrum, the 3-dB bandwidth of the spectrum filter can be unchanged; ii) with regard to the spectral tunability, the 3-dB bandwidth of the spectral filter can also be quantitatively changed as needed by changing the heating mode. The home-made spectral filter is used in the DS fiber laser to further realize the continuous adjustment of DS with a tuning accuracy of 0.03 nm by a step size of 1°C. Such a wavelength-tunable DS fiber laser greatly enhances the design flexibility of the coherent anti-Stokes Raman scattering source.

Bandwidth Tunable Optical Bandpass Filter Based on Parity-Time Symmetry.

A chip-scale tunable optical filter is indispensable to meeting the demand for reconfigurability in wavelength division multiplexing systems, channel routing, and switching, etc. Here, we propose a new scheme of bandwidth tunable band-pass filters based on a parity-time (PT) symmetric coupled microresonator system. Large bandwidth tunability is realized on the basis of the tuning of the relative resonant frequency between coupled rings and by making use of the concept of the exception point (EP) in the PT symmetric systems. Theoretical investigations show that the bandwidth tuning range depends on the intrinsic loss of the microresonators, as well as on the loss contrast between the two cavities. Our proof-of-concept device confirms the tunability and shows a bandwidth tuning range from 21 GHz to 49 GHz, with an extinction ratio larger than 15 dB. The discrepancy between theory and experiment is due to the non-optimized design of the coupling coefficients, as well as to fabrication errors. Our design based on PT symmetry shows a distinct route towards the realization of tunable band-pass filters, providing new ways to explore non-Hermitian light manipulation in conventional integrated devices.

Bandwidth Tunable Filter Based on Ideal Quasi-Critical Coupling State in WGM Cavity

A tunable bandwidth add-drop filter in ideal quasi-critical coupling (QCC) state was analyzed in theory in detail and demonstrated in experiment, with two vector modes HE $^{\rm X}_{11}$ and HE $^{\rm Y}_{11}$ coupled single whispering gallery mode (WGM) as its basic configuration. In the experiment, the bandwidth tuning range at the Drop port was from 54.0 MHz to 775.8 MHz with the conversion efficiency maintained above 95%, and bandwidth tuning range at the Through port was from 6.6 MHz to 720.6 MHz with the insertion loss fluctuation less than 5%. The demonstrated filter gains advantages of stable efficiency, narrow band, and broad bandwidth tuning range, which will be helpful for its application in microwave photonics, narrow linewidth lasers and nonlinear optics.

Tunable bandpass filters with constant absolute bandwidth cascaded trisection topology

A novel cascade trisection (CT) coupling structure, based on half-wavelength resonators, is proposed for designing constant absolute bandwidth (CABW) tunable bandpass filters. The tunable resonators have both electric and magnetic dominant coupling regions with predefined coupling coefficients. The third-order and fifth-order tunable bandpass filters (BPFs), which can realize CABW and a high side cross-coupling transmission zero (Tz), are designed based on the proposed CT structure. Measurement results show that the central frequency tuning of the third-order filter is from 1.47 to 1.83 GHz with −3dB bandwidth of 108.75 ± 2.05 MHz, and the high side 3 to 25 dB attenuation slope is 327.32–623.57 dB/GHz. The tuning range of the fifth-order filter is from 1.45 to 1.92 GHz with −3dB bandwidth of 101.9 ± 1.3 MHz, and high side 3 to 30 dB attenuation slope is 442.75–843.59 dB/GHz.

Thermally chirped contra-directional couplers for residueless, bandwidth-tunable Bragg filters with fabrication error compensation.

Bandwidth-tunable filters are essential in elastic optical networks for dynamic bandwidth allocation. Existing solutions in silicon photonics face challenges to meet requirements in real-world applications due to design trade-offs and fabrication errors. In this Letter, we propose and experimentally demonstrate a silicon photonic tunable add-drop filter in a single-stage, hyperbolic-tangent-apodized contra-directional coupler with a segmented microheater. It allows to create an arbitrary temperature profile along the device for bandwidth tuning in both through and drop responses. We show that the algorithmic operation of the device can effectively compensate local fabrication nonuniformity and improve the out-of-band suppression ratio by 69%. Applying proper temperature offsets and slopes allows to continuously tune the filter's center wavelength over 8 nm and its drop-port 3 dB bandwidth between 14.0 and 22.4 nm.

Widely wavelength-tunable mode locked Er-doped fiber oscillator from 1532 nm to 1594 nm with high signal-to-noise ratio

A widely wavelength tunable mode locked Er-doped fiber laser oscillator based on nonlinear polarization rotation (NPR) effect was demonstrated. By intentionally introducing a position-and-width adjustable slit and a pair of reflective gratings into the cavity, which worked as a wavelength and bandwidth tunable spectral filter, continuous wavelength tunable mode locked operation from 1532 nm to 1594 nm was achieved, corresponding to a tuning range of 62 nm, which is a new record for mode locked continuously wavelength-tunable single mode Er-doped fiber oscillators based on nonlinear polarization rotation, to the best of our knowledge. During the wavelength tuning, the pulse durations for different wavelengths changed slightly and closed to the Fourier-transform limit. The signal-to-noise ratio (SNR) of the radio frequency (RF) spectrum of the output pulses is higher than 89 dB over the entire wavelength tuning range, superior to the typical SNR of 50–70 dB reported in most papers. In addition, the pulse duration could be tuned from 904 fs to 1339 fs by adjusting the width of the slit, for which the corresponding spectral bandwidth changed from 3.4 nm to 1.7 nm.

Ultracompact bandwidth-tunable filter based on subwavelength grating-assisted contra-directional couplers.

An ultracompact, bandwidth-tunable filter has been demonstrated using a silicon-on-insulator (SOI) wafer. The device is based on cascaded grating-assisted contra-directional couplers (GACDCs). It also involves the use of a subwavelength grating (SWG) structure. By heating one of the heaters on GACDCs, a bandwidth tunability of ∼6 nm is achieved. Owing to the benefit of having a large coupling coefficient between SWG and strip waveguides, the length of the coupling region is only 100 µm. Moreover, the combination of the curved SWG and the tapered strip waveguides effectively suppresses the sidelobes. The filter possesses features of simultaneous wavelength tuning with no free spectral range (FSR) limitation. A maximum bandwidth of 10 nm was experimentally measured with a high out-of-band contrast of 25 dB. Similarly, the minimum bandwidth recorded is 4 nm with an out-of-band contrast of 15 dB.

All-normal dispersion fiber laser with a bandwidth tunable fiber-based spectral filter.

We investigate numerically and experimentally an all-fiber, bandwidth tunable spectral filter comprising birefringent fibers. The spectral bandwidth tunability of the filter is based on the compensation of birefringence in polarization maintaining fibers. This unique filter allows mode-locked operation of a fiber oscillator with the ability to generate distinct laser modes with different output spectral shapes and pulse evolutions.

Effects of spectral filtering on pulse dynamics in a mode-locked fiber laser with a bandwidth tunable filter

We investigate numerically the impact of spectral filtering on pulse characteristics in anomalous dispersion erbium-doped fiber laser mode-locked with carbon nanotube-based saturable absorber. The laser can operate only in a single-pulse state without a filter, while it will transit to bound soliton states once the saturation energy of the gain fiber and the bandwidth of the filter are carefully chosen, respectively. Further analyzation shows that the operation of a single-pulse lasing from dissipative soliton to conventional soliton can be selectively targeted by changing the bandwidth of the filter through the loss and frequency narrowing effects. Laser performances, such as spectrum structure, pulse duration, pulse energy and peak power in both the frequency and the time domains, will all be affected. This paper provides a guidance for the mode-locked fiber laser and proposes a feasible way to control the pulse dynamics.

A switchable absolute bandwidth tunable filter with etched slot

In this paper, a reconfigurable filter with tunable center-frequency and switchable bandwidths is proposed. Frequency flexibility is achieved by varactors in the inner resonant parts. The switchable bandwidth states are implemented by pin diode connected with the etched slot on the ground. The coupling coefficient could be adjusted by the physical size of grounded slot, which would determine filter’s bandwidth. To demonstrate the scheme, a proof-of-concept prototype filter is designed at 800 MHz. The measurement results show the center frequency of the designed filter could be tuned from 720 to 820 MHz; meanwhile, the two states switches the passband bandwidth from 45 to 36 MHz via turning on/off the grounded pin diode. The proposed filter has been fabricated, and measured, and exhibits excellent performance.

Tunable narrow bandwidth of a bandpass filter combined microstrip and suspended substrate stripline

AbstractThis paper introduces a narrow bandwidth tunable filter. The filter has been tuned electronically by using varactor diodes. A set of mechanically tuning screws have been utilized to overcome the manufacturing tolerance of the filter's resonators. In contrast to most the combline filters, which depend on the cross‐coupling approach to generate the transmission poles, the proposed filter has a pair of transmission zeros located on both sides of the passband, by employing two uncoupled microstrip resonators that have been electronically tuned independently. The proposed filter bandwidth tuning range is 59–88 MHz with 3 dB fractional bandwidth 1.94–2.76 respectively. A detailed demonstration of the suggested tunable filter is introduced based on a mathematical approach.

Sharp tunable and additional noise-free optical filter based on Brillouin losses

In this paper, we propose an additional noise-free, independent center frequency and bandwidth tunable optical filter based on stimulated Brillouin scattering (SBS) losses. By suppressing the out-of-band signal with two broadened symmetric SBS losses, tunable pass bandwidths from 500 MHz to 9.5 GHz and the independent center frequency tunability are demonstrated. Considering the limited SBS interaction in the center frequency range, a flat-top response with minimum 0.3 dB ripple is achieved. Assisted by the extra suppression from polarization pulling, a maximum selectivity of 20 dB and an ultrahigh 250 dB/GHz roll-off are reached. A gain-based SBS filter adds noise to the filtered signal. However, for our proposed filter setup, no additional noise is detected due to the transparency in the passband. Considering the wide independent bandwidth and center frequency tunability, flat-top response, and low-noise characteristic, our proposed filter can be perfectly used as a supplement of most commercialized conventional tunable optical single bandpass filters, whose minimum bandwidth is limited by 10 GHz.

5/60 GHz $0.18~\mu m$ CMOS Dual-Mode Dual- Conversion Receiver Using a Tunable Active Filter for 5-GHz Channel Selection

A dual-mode dual-conversion receiver with two far-apart carrier frequencies, 5 and 60 GHz, and two very different channel bandwidths, tens-MHz and GHz, is demonstrated in a standard $0.18~\mu \text {m}$ CMOS technology. The dual conversion architecture receives 60-GHz mode using Schottky diodes at the first conversion stage and merges 5-GHz mode at the second conversion stage. The 5-GHz RF channel selection is achieved by inserting a 20-MHz bandwidth tunable active filter after a 5-GHz low-noise amplifier for the noise concern and before the second down-conversion mixer to relax the linearity requirement of subsequent stages. A tunable active filter with improved linearity using a transformer-coupled scheme is employed. As a result, the narrow 20 MHz channel bandwidth selection at 5-GHz mode shows a 500 MHz tuning range with noise figure of 9 dB and conversion gain of 28 dB. For 60-GHz operation, the 3-dB IF bandwidth is 1 GHz with noise figure of about 20 dB and conversion gain of −3 dB.