Large Tuning Range Research Articles

Tunable metasurface realizing dynamic chiroptical responses in infrared band

The chiral metasurfaces composed of chiral element structures have attracted much attention because of their interesting optical response. However, with the rapid development of chiral metasurface research, people gradually realize that the traditional chiral metasurfaces lacks tunability in practical application. Therefore, it is the latest research direction to design the tunable / reconfigurable chiral metasurfaces. In this work, a double split-ring metasurface consist of metal/phase change material (PCM) Ge2Sb2Se4Te1(GSS4T1)/metal/dielectric multilayer stack is designed, asymmetric transmission (AT) and circular dichroism (CD) dynamic control are achieved in the infrared band with a large tuning range of more than 50 THz by switching between the amorphous and crystalline states of the Ge2Sb2Se4Te1. The design in this work is of great significance for realizing the GSST-enabled tunable polarization control in the infrared band.

SIW-based enhanced gain self-diplexing antenna using higher order modes

A self-diplexing antenna using slot-loaded cavity-backed Substrate Integrated Waveguide (CB-SIW) is presented with the enhanced gain. Higher-order modes (TE130 and TE310) in a square-shaped SIW cavity are utilized to emit linear and orthogonally polarized radiation with gain enhancement. Two concentric square ring slots are etched on the top conducting layer of the cavity to achieve broadside radiation. The asymmetric width of the outer square ring slot helps in achieving two different resonant frequencies at TE130 and TE310 modes. The proposed antenna is verified by observing a good match between the simulated and measured results. An excellent self-diplexing characteristic is achieved by getting isolation better than 28.0 dB between the two ports for a large tuning range. The proposed antenna radiates unidirectionally at two close frequencies 11.4 GHz and 12.2 GHz exhibiting gain of 9.8 dBi and 9.15dBi, respectively. In addition, the proposed design is single-layered, easy to fabricate as compared to traditional diplexing antennas.

A Capacitively Coupled Noise Circulating VCO

A capacitively coupled noise circulating voltage-controlled oscillator (VCO) is proposed. Through capacitive coupling from the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">LC</i> tank to the gates of the active devices, a simple inductor-based resonator can realize the same noise circulating effect as the conventional transformer-based topology, leading to a more compact chip area, as well as larger tuning range. The proposed VCO is fabricated with a 28-nm CMOS process using only thick-oxide devices. The core chip area is 0.09 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The measured oscillation frequency ranges from 6.17 to 10.45 GHz, and the phase noise at 1-MHz offset is from −121.6 to −115 dBc/Hz, with a power dissipation from 32 to 41 mW under a 1.8-V supply voltage across the frequency tuning range. The achieved figure-of-merit with tuning range (FoM <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$_{T}$ </tex-math></inline-formula> ) is from 193.1 to 196.4 dBc/Hz.

Ultra-Compact Band-Pass and Band-Stop Tunable Filters Based on Loop-Cascaded Nanobeam Structure

We propose an ultra-compact band-pass and band-stop filter with bandwidth tunability based on the loop-cascaded nanobeam structure in simulation, with a small coupling region of 41×1 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . It features low insertion loss, large bandwidth tunable range and high sidelobe suppression ratio. By two-stage cascading, the bandwidth of the band-pass filter can be adjusted in the range of 6 nm-24 nm. We also experimentally demonstrate the loop-cascaded nanobeam band-pass and band-stop filter with a coupling region of only 17.5×1 μm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Reinforced design method for moiré metalens with large spacing.

Although metalens has attracted many research interests for its advantages of light weight, ultrathin size, and high design freedom in realizing achromatic and aberration-free optical devices, it still lacks adjustability in zoomable optical systems. Moiré metalens, which consists of two cascaded metasurface layers, can realize large focus tuning range by the mutual rotation of the two layers, and becomes a possible solution to realize real application of reconfigurable metalenses. However, due to the spacing between the two metasurface layers, it suffers from aberration caused by diffraction, leading to a dramatically decreased efficiency with the spacing. In this paper, we propose a reinforced design method for moiré metalenses with large spacing based on diffraction optics. Simulation results demonstrate that at the wavelength of 810 nm, when the spacing of the two metasurfaces is 10λ, the focusing efficiency of the reinforced moiré metalens is 3.4 times larger than the traditional moiré metalens. Furthermore, in order to consider the situation that the spacing between the two metasurfaces cannot be controlled precisely, we also propose a reinforced design method for multiplex spacings, which can make the device maintain a high focusing efficiency (3 times larger than the traditional moiré metalens) for the spacing in a range of 6λ∼10λ. The new design method is anticipated to be applied in realizing tunable metalenses in integrated continuously zoomable optical systems.

A Study on the Variable Inductor Design by Switching the Main Paths and the Coupling Coils

In a radio frequency (RF) system, it is possible to use variable inductors for providing tunable or selective frequency range. Variable inductors can be implemented by the microelectromechanical system (MEMS) process or by using transistors as switches to change the routing of coils or coupling quantities. In this paper, we investigated the design method of a variable inductor by using MOS transistors to switch the main coil paths and the secondary coupled coils. We observed the effects of different metal layers, turn numbers, and layout arrangements for secondary-coupled coils and compared their characteristics on the inductances and quality factors. We implemented two chips in the 0.18 μm CMOS process technology for each kind of arrangement for verification. One inductor can achieve inductance values from about 300 pH to 550 pH, and the other is between 300 pH and 575 pH, corresponding to 59.3% and 62.5%, respectively, inductance variation range at 4 GHz frequency. Additionally, their fine step sizes of the switched inductances are from 0.5% to 6% for one design, and 1% to 12.5% for the other. We found that both designs achieved a large inductance tuning range and moderate inductance step sizes with a slight difference behavior on the inductance variation versus frequency.

Power-Optimized Digitally Controlled Oscillator in 28-nm CMOS for Low-Power FMCW Radars

This work presents the design of a 24-GHz digitally controlled oscillator (DCO) in an advanced 28-nm bulk CMOS technology for short-range frequency-modulated continuous-wave radar system-on-chip for mobile and Internet-of-Things devices. The power minimization is therefore the primary focus. The oscillator consumes a record low power of 1.2 mW at a 0.65-V supply voltage. It achieves a very large frequency tuning range (TR) of 5.8 GHz (27%) and a 150 kHz resolution without significantly degrading the phase noise (PN). The proposed design methodology results in a state-of-the-art -193 dBc/Hz FoM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> .

Wideband tunable optoelectronic oscillator based on stimulated Brillouin scattering and a Mach-Zehnder interferometer.

We propose a novel optoelectronic oscillator (OEO) based on stimulated Brillouin scattering (SBS) and a Mach-Zehnder interferometer (MZI). The SBS and MZI form a microwave photonic filter (MPF), which completes the frequency selection of the OEO. The width of the MPF passband can be changed by adjusting the DC voltage of the MZI. When the gap between the passbands is aligned with the side mode, the side mode can be suppressed; in this way, a single-mode optoelectronic oscillator is achieved. Compared with the previous structure, the proposed structure is more stable and easier to integrate. A stable frequency signal at 9.4 GHz is obtained with a phase noise of -84.05dBc/Hz at 10 kHz, the side-mode-suppression ratio is 42 dB, and the frequency drift is below 50 kHz within 420 s. By adjusting the wavelength of the tunable laser source, a 280 MHz tuning range from 9.25 to 9.53 GHz is obtained. If an independent pump is introduced, a large tuning range is achieved from 9.4 to 24.9 GHz.

Large-tuning-range frequency stabilization of an ultraviolet laser by an open-loop piezoelectric ceramic controlled Fabry-Pérot cavity.

We demonstrate a laser frequency stabilization method with large tuning range to stabilize a UV laser by installing piezoelectric ceramic actuators into a Fabry-Pérot cavity with an ultra-low expansion spacer. To suppress piezoelectric drift, a two-layer symmetrical structure is adopted for the piezoelectric actuator, and a 14.7 GHz tuning range is achieved. The short-term drift of the piezoelectric ceramics caused by temperature and creep is eliminated, and the long-term drift is 0.268 MHz/h when the Fabry-Pérot cavity is sealed in a chamber without a vacuum environment. The long-term frequency drift is mainly caused by stress release and is eliminated by compensating the cavity voltage with an open loop. Without the need for an external reference or a vacuum environment, the laser frequency stabilization system is greatly simplified, and it can be extended to wavelengths ranging from ultraviolet to infrared. Owing to its simplicity, stability, and large tuning range, it is applicable in cold atom and trapped ion experiments.

Graphene-enabled active terahertz focusing with wide tuning range

Tunable terahertz (THz) metalenses using graphene have the advantages of good tunability, fast tuning speed, and ultra-small thickness for compact THz photonic systems. However, it is still a great challenge to realize such an active metalens with a large tuning range of focal length. Here, we propose a reflective metamirror with a graphene-coated metallic antenna array. Using an algorithm to find the C-shaped antennas with large phase changes when the graphene is gated, the metamirror with a wide tuning range is designed. The simulation results indicate that the focal length is able to be continuously changed by 96λ when the graphene chemical potential increases from 0.04 eV to 0.52 eV at 1 THz. The full width at half maximum of the focus is close to the diffraction limit. This method is anticipated to open up a new way for designing graphene-based widely tunable THz metasurfaces.

Ultra-broadband tunable and switchable multi-wavelength mode-locking Tm-doped all-fiber laser

Through regulating the in-band absorption of used gain fiber, we demonstrate an ultra-broadband tunable and switchable multi-wavelength mode-locking Tm-doped all-fiber laser. The tuning range can be enlarged as much as 153 nm (from 1828.45 to 1981.35 nm, the largest tuning range so far) by employing different lengths of used gain fiber with different in-band absorption and birefringent filtering mechanism. In addition, benefiting from four in-line polarization controllers and broadband gain profile of Tm-doped fiber, switchable quad-wavelength mode locking with part binary control can be observed clearly, even penta-wavelength. Such kind of ultra-broadband tunable and switchable mode-locking laser provide simpler and elastic way for practical application, such as optical signal processing, spectroscopy, optical communications, and so on.

Widely tunable silicon Raman laser

Stimulated Raman scattering is an effective means of wavelength conversion and can largely extend the operating spectral range of an optical source. We demonstrate a high-performance tunable Raman laser on a sub-micrometer-thick silicon on insulator wafer using a standard foundry process. The key feature to this laser is the use of a tunable coupling mechanism to adjust both pump and signal coupling coefficients in the ring cavity, allowing demonstration of laser emission over a large wavelength tuning range of 83 nm. This Raman laser demonstrates efficient (slope of up to 26% and a maximum pump-to-signal power conversion efficiency of 10%) on-chip nonlinear wavelength conversion. Our results indicate great promise for substantially increasing the optical spectral resources available on a silicon chip.

Tunable liquid crystal frequency selective surface with the compact unit cell, large tuning range, and the passband of flat-top and sharp roll-off

In this paper, a design method for tunable liquid crystal (LC)-based frequency selective surface (FSS) with the compact unit cell, large tuning range, and the passband of flat-top and sharp roll-off is proposed. A very compact LC-based bandpass FSS unit cell with the dimension of 0.077 λ × 0.077 λ is designed based on a pair of planar vortex-shaped metallic structure with two large parallel patches, between which the LC is sandwiched. Results show that the central frequency of the FSS is tuned from 11.56 to 12.58 GHz, and the tuning range reaches up to 8.5%. In order to obtain the transmission characteristics of flat-top and sharp roll-off, a double-layer structure is proposed and combined with the LC-based FSS design. Theoretical analyses are conducted to declare the working principle of the double-layer LC-based FSS. A prototype sample for the double-layer LC-based FSS is designed and fabricated. Results show that the double-layer LC-based FSS structure achieves frequency selective characteristics with a flat-top passband of about 450 MHz and sharp roll-off for arbitrary LC states with −15 dB rectangle coefficient of 2.44 for arbitrary LC states. Moreover, stable frequency responses of the double-layer LC-based FSS under various incident angles from 0° to 45° are obtained for arbitrary LC states.

Synthesis of High-Order Continuously Tunable Low-Pass Active-R Filters

This work discusses the synthesis and design methodology of high-order and frequency-tunable low-pass active-R filter architectures for multi-standard wireless applications. Active-R filters use the inherent integrator-like behavior of amplifiers to realize their frequency response. The main advantages of this type of filter are high-frequency performance and a low integrated area. Active-R filters only need the Miller capacitor used in internally compensated amplifiers. In this work, amplifiers with configurable unity-gain frequencies enable the continuous tuning of active-R filters. Three different filter architectures realize a fifth-order Butterworth prototype tunable in the 1-50 MHz frequency range. These filters are designed, fabricated, and tested using the TSMC 0.18 μm process. Each fifth-order filter consumes power in the range 7.45 mW to 9.38 mW from a 1.8 V supply and has an integrated area ≤ 0.33 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Compared with state-of-the-art active filters, the filters presented in this work have the largest tuning range without linearity degradation and the smallest area per tuning range ratio. Also, the measured filters show a competitive FoM 153.3 dB/J when operating at high frequencies.

Excitation of Multi-Beam Interference and Whispering-Gallery Mode in Silica Taper-Assisted Polymer Microspheres for Refractometric Sensing

We propose and numerically analyze a fiber-optic sensor based on a silica taper-assisted multiple polymer microspheres to realize high-sensitivity refractometric sensing due to the excitation of multi-beam interference and whispering-gallery modes (WGMs) in each microsphere. Up to 5 UV-cured adhesive microspheres are dripped in sequence on the 2 µm-diameter silica taper with the certain distances in between. Scattering and reflection of light in each microsphere enhance the multi-beam interference because of the emergence of high-order modes. Moreover, WGMs with certain resonant wavelengths are excited in the microspheres, further enhancing the transmission spectrum with high contrast and quality factor. Furthermore, irregular transmission spectrum corresponds to the large wavelength tuning range. Propagating light between the microspheres could be strongly affected by the environmental factor with the large transmission spectrum shift, therefore realizing the refractometric sensing with high sensitivity of 846 nm/RIU based on the taper-assisted 2-spheres sensor. Furthermore, another two microspheres are placed on both sides of 2-spheres sensor to form 4-spheres-loop sensor, re-collecting those large-angle scattering light with the enhancement of multi-beam interference and WGMs. Therefore, the sensitivity of 4-shperes-loop sensor is further improved to the maximum of 1296.3 nm/RIU.

Π-type reconfigurable coupler based on a complementary tunable method

A complementary tunable method is proposed based on the Hybrid coupler. The odd-even mode analysis shows that the method can achieve continuously tunable power division ratio by changing two transmission lines in the circuit, and it can achieve larger tunable range by improving the complementary tunable method based on the cascaded Hybrid coupler. Using the improved method, a novel π-type reconfigurable coupler, which works at 2.45 GHz, is designed by replacing transmission lines with π-type circuit. The power division ratio of the fabricated reconfigurable coupler is changed in the range of −20.5–21.3 dB, with the bias voltage being 0∼6 V; meanwhile, the isolation and return loss are above 20 dB.

Orthogonally polarized RF optical single sideband generation with integrated ring resonators

We review recent work on narrowband orthogonally polarized optical RF single sideband generators as well as dual-channel equalization, both based on high-Q integrated ring resonators. The devices operate in the optical telecommunications C-band and enable RF operation over a range of either fixed or thermally tuneable frequencies. They operate via TE/TM mode birefringence in the resonator. We achieve a very large dynamic tuning range of over 55 dB for both the optical carrier-to-sideband ratio and the dual-channel RF equalization for both the fixed and tunable devices.

An Ultra-Compact Multi-Band VCO Achieving − 196 dB FoMA with Single-Ended Tunable Active Inductor

This paper presents a wide tuning-range LC-tank voltage controlled oscillator (VCO). To perform large frequency tuning range, the proposed circuit topology uses a tunable active inductor (TAI) for the coarse frequency tuning, while the fine-tuning is controlled only by a MOS varactor. The TAI design is based on toggling varactor arrays by which four channels are able to be selected for multi-band applications. The proposed VCO is designed in 90-nm CMOS process. Simulation results show that the VCO can operate from 760 MHz to 4.370 GHz representing an FTR of 140.74%, with a tuning gain Kvco from 654 to 1882 MHz/V. The achieved phase noise is lower than − 105 dBc/Hz at an offset frequency of 1 MHz from the carrier tuned to 2.52 GHz. The VCO core circuit draws 3.3 mA from a 1V voltage supply and occupies a small active area of (45 × 37) μm2. The fully implemented VCO provides excellent performances such as attractive FOMA value of − 196 dBc/Hz.

Near-field radiation assisted smart skin for spacecraft thermal control

Thermal control is of critical importance for normal operation of spacecraft. Given thermal radiation is the only means of heat dissipation in space, an efficient thermal control approach for spacecraft is to coat the radiator with a tunable-emittance “skin” that can tune its heat dissipation according to various thermal conditions. The existing schemes solely relying on far-field thermal radiation, which are based on mechanical, electrochromic or thermochromic working principles, are difficult to combine the advantages of all-solid-state structure, actively and accurate tuning, and large tuning range of heat flux. In this work, we propose a near-field radiation assisted (NFRA) smart skin for thermal control which can tune the heat rejection accurately and in a large range. It contains a metal-insulator-semiconductor (MIS) structure, where the carrier distribution in the semiconductor layer can be electrically altered. In this way, the near-field heat flux, and ultimately the skin emission power expressed using effective emittance, can be controlled as a function of the applied voltage. The variation range of the effective emittance can exceed 0.7 when adjusting the applied voltage from −10 V to 100 V with our preliminary design. This work opens a new way of smart skin design for active spacecraft thermal control.

Dynamically reversible and strong circular dichroism based on Babinet-invertible chiral metasurfaces.

We propose a Babinet-invertible chiral metasurface for achieving dynamically reversible and strong circular dichroism (CD). The proposed metasurface is composed of a VO2-metal hybrid structure, and when VO2 transits between the dielectric state and the metallic state, the metasurface unit cell switches between complementary structures that are designed according to Babinet's principle. This leads to a large and reversible CD tuning range between ±0.5 at 0.97 THz, which is larger than the one found in the literature. We attribute the CD effect to extrinsic chirality of the proposed metasurface. We envision that the Babinet-invertible chiral metasurface proposed here will advance the engineering of active and tunable chiro-optical devices and promote their applications.