Continuous Tuning Research Articles

Tunable phononic crystal waveguides based on the double tuning method

To realize the frequency tuning of phononic crystals (PCs) and the functional design of tunable PCs, acoustic components with more flexible working frequencies are manufactured to meet the various requirements of engineering applications. We proposed a combined tuning method that combines the change of the Young’s modulus of the shape memory alloy and the rotation of the scatterer. The tunable band structure and transmission spectra of the method were calculated using the finite element method. We analyzed the effect of fill rate and viscosity of matrix on the band structure and studied the regulation law of the dual regulation mode. The numerical results show that the double tuning method makes up for the shortcomings of the single tuning method and has the characteristics of widely tuning range, continuous adjustment, and more tuning modes. In addition, a PC waveguide is constructed by using this combined tuning method, which realizes the flexible construction of waveguide channels and the continuous tuning of wide frequency range. It is an important guideline for the research of tunable waveguides, the design of acoustic components, and the application of practical engineering.

Tunable noise-like pulse and Q-switched erbium-doped fiber laser.

A switchable, widely wavelength-tunable noise-like pulse (NLP) and Q-switched Er-doped fiber (EDF) laser with a linear cavity structure is proposed and experimentally demonstrated in this work. The net-normal-dispersion mode-locked NLP operation based on a semiconductor saturable mirror (SESAM) is realized in a 57 nm continuous tuning range from 1528 to 1585 nm by using a tunable filter (TF). When the pump power is 500 mW, the NLPs produce a maximum average output power of about 16 mW with a 3-dB spectral bandwidth of about 17 nm at the central wavelength of 1555 nm, while the average peak power is about 58.8 W. The measured characteristics of the output NLPs at 1555 nm are consistent with the numerical results under the condition of Δβ2, net = 0.095 ps2, and Esat = 0.77 nJ. In addition, stable Q-switched pulses with a 67 nm wavelength tuning range from 1518 to 1585 nm are obtained by adjusting the central wavelength of the filter. The maximum pulse energy reaches 231.4 nJ at the center wavelength of 1555 nm, corresponding to a peak power of about 278.8 mW. The proposed wavelength-tunable fiber laser is simple and versatile, demonstrating significant potential for numerous practical applications.

Frequency-tunable optical fiber passband-squeezed birefringence filter based on quarter-wave polarization conversion

A polarization-diversified loop structure (PDLS) formed by a four-port polarizing beam splitter has been employed to establish the input polarization independence of optical devices. Here we report a PDLS-based optical fiber birefringence filter, which can provide continuous frequency tunability in passband-squeezed comb spectra by harnessing quarter-wave polarization conversion. The birefringence filter is composed of a four-port PBS, two high birefringence fiber (HBF) segments whose lengths are equal, the first polarization controller (PC) comprised of a quarter-wave retarder (QWR) and a half-wave retarder (HWR), which is located in front of the first HBF segment, and a set of two QWRs as the second PC that lies before the second HBF segment. The dual QWRs, chosen for the second PC, are the simplest form of a PC that can convert arbitrary input polarization into desired polarization. The slow axis of the second HBF segment is oriented at 22.5° with respect to the horizontal axis of the PBS. Through the Jones matrix formulation, we derived the sinusoidal passband-squeezed transmittance function of the filter, whose absolute phase (ϕ) should be continuously varied for the contiguous wavelength tuning of filter spectra. Then, using this filter transmittance, we found the possible loci of the orientation angles (OAs) of the four wave retarders as the functions of ϕ. By utilizing the OA loci of the wave retarders, the theoretical passband-squeezed transmission spectra of the filter were calculated for eight equally spaced values of ϕ from 0° to 315° (with an increment of 45°). This theoretical calculation was also verified by measuring the wavelength-tuned passband-squeezed spectra of the fabricated filter. We theoretically and experimentally confirmed that our filter could provide the wavelength tunability of the polarization-independent passband-squeezed comb spectrum by appropriately controlling the OAs of the wave retarders.

Laser‐Induced Tuning and Spatial Control of the Emissivity of Phase‐Changing Ge2Sb2Te5 Emitter for Thermal Camouflage

AbstractThe tuning of a material's emissivity in the atmospheric window range of 8–13 µm is crucial for infrared (IR) adaptive applications such as thermal camouflage, IR stealth, and radiative cooling. A resonator structure comprising a phase‐changing Ge2Sb2Te5 (GST) film on a metal mirror is promising as a tunable thermal emitter because its long‐wavelength IR emissivity can be varied by adjusting the degree of crystallization of the GST film through annealing. However, the space‐selective tuning of the emissivity required for practical use remains a significant challenge. Herein, a hybrid method combining laser irradiation and heat treatment is presented for space‐selective and continuous tuning of the emissivity of a GST‐based resonator. The current study shows that a laser‐induced crystalline layer formed in the near‐surface region of a 400 nm thick amorphous GST film can act as heterogeneous nuclei for crystallization when the film is subsequently annealed, thereby increasing the crystallization rate of the GST film. This results in a large difference in emissivity between the irradiated and nonirradiated areas, and the emissivity difference can be tuned by changing the annealing temperature. Thermal camouflage is experimentally demonstrated by making high‐and low‐temperature regions with low and high emissivity, respectively.

Tunable dual-band metamaterial absorber in the infrared range based on split-ring-groove array.

In this paper, we present a tunable dual-band perfect metamaterial absorber working in the infrared band by integrating a metallic split-ring-groove resonator array with a liquid crystal (LC) layer atop a metal substrate. By varying the height of the central nanodisks, the absorptivity of the dual-band absorption peaks can be simultaneously adjusted. The dual-band resonance frequencies of the proposed absorber exhibit continuous tunability by adjusting the refractive index of the LC, which can be controlled by applying external voltage. The mechanism of the perfect absorption is attributed to the gap plasmonic resonance coupling regime. The presented absorber exhibits good tolerance to incidence angles up to 60° and shows polarization dependent performance, which may offer promising applications in sensing, modulator, and optical absorption switching in the infrared regime.

Design of an Optical Parametric Oscillator using a BBO partial cylinder for a continuous tunability between 0.4 μm and 0.9 μm

This work describes the different steps of the design of a cylindric Optical Parametric Oscillator. It is based on a BBO nonlinear crystal shaped as a partial cylinder to be pumped by a commercial micro-laser at 0.355 μm for an energetic and sub-nanosecond emission continuously tunable between 0.4 μm and 0.9 μm.

An Active Metamaterial Absorber With Ultrawideband Continuous Tunability

In this paper, an active metamaterial (MM) absorber loaded with PIN diodes is proposed. By adjusting the bias currents of PIN diodes to change the value of the introduced equivalent resistance, the Q factors of multiple resonances of the whole structure are altered. Therefore, multiple broadband absorption states with moderate absorption performance at different frequencies are obtained. These states cover continuously tunable absorption frequencies in an ultrabroad frequency band. Bias circuits for PIN diodes are also specially designed to obtain polarization-insensitive absorption performance. Optimizing works were carried out by analyzing the operating mechanism based on the simulated field distribution and power loss distribution. The presented absorber covers an ultrawideband absorption bandwidth with a reflection coefficient below -10 dB from 0.78 GHz to 4.62 GHz (142.2% in relative bandwidth) by adjusting the PIN diodes. It is worth noting that the absorption bandwidth of the low-resistance state reaches 21.0% in the P band, which greatly improves performance in the low-frequency regime. The absorber shows good angular stability under oblique incident angles from 0° to 30°. In addition, the total thickness of the absorber is only 1/16 of the wavelength at the lowest operating frequency with an areal density of 2.06 kg/m2.

Photonic meta-switch based on phase change and catenary-enabled continuous phase regulation

Aiming at the characteristics of passive and discrete phase regulations inherent in current metasurfaces, we combine optimized isowidth catenary with non-volatile phase change dielectrics and explore a type of bistable phase-change-based wavefront meta-switch of continuous phase tuning and active switching. First, the switchable wavefront deflector is demonstrated in the mid-IR range between 9 µm and 10 µm. Upon phase transition between crystalline state and amorphous state, the incident wave can be switched into anomalous reflection and regular reflection, i.e. the “on” state and “off ” state of wave deflection. Further, a type of dynamically tunable Bessel beam switch is demonstrated. In the amorphous state, the polarization conversion efficiency approaches to 100% with an incident wave of 9.6 µm in wavelength. Therefore, the normal geometrical phase and the second-order Bessel focus are switched “on”. However, the cross-polarization and geometrical phase are switched “off ” upon phase changing into crystallized state. Intrinsically, non-dispersive spin-orbit interaction ensures that this kind of device possesses the broadband characteristics. Such a devise has great potential applications in active optoelectronic integration, optical communications, etc.

A CMOS 24–30-GHz Low-Phase-Variation Variable Gain Amplifier Design for 5G New Radio

This letter presents a 24–30-GHz variable gain amplifier (VGA) achieving low-phase variation in a standard RF 65-nm CMOS process. The proposed VGA employs two stages. Phase compensation during the gain variation is realized with the dynamic complementary phase variation at the first stage. The second stage is utilized to increase the gain and keep linearity. Measurement results prove that the proposed VGA achieves a continuous gain tuning range of 8.3 dB. Under this condition, low-phase variation is realized with the root-mean-square (RMS) phase error less than 3° from 24 to 30 GHz. Moreover, the RMS phase error is less than 2° at the desired 5G frequency band from 27.5 to 29.5 GHz. The peak gain is measured with 18.7 dB at 29 GHz. With the max gain setting, the measured OP_1dB and OIP₃ are 5.1 and 9.3 dBm, respectively, and the measured noise figure is from 4.6 to 6.8 dB at the frequency range from 24 to 30 GHz.

High repetition rate actively mode-locked Er:fiber laser with tunable pulse duration

An actively mode-locked fiber laser with controllable pulse repetition rate and tunable pulse duration is presented, in which an optical delay line (ODL) is used to adjust the cavity length precisely for regulating the repetition rate, and a semiconductor optical amplifier (SOA) is introduced for enabling the pulse duration control. Experimentally, continuous tuning of the repetition rate from 2 GHz to 6 GHz is realized, which is limited by the availability of an even higher repetition rate radio-frequency (RF) source. Specifically, when the repetition rate is fixed at 2.5 GHz, the pulse duration can be tuned from 4 ps to 30 ps, which is, to the best of our knowledge, the widest tuning range of pulse duration ever achieved in a gigahertz (GHz) repetition rate actively mode-locked 1.5 µm fiber laser oscillator.

Wide-Bandwidth Electronically Programmable CMOS Instrumentation Amplifier for Bioimpedance Spectroscopy

An instrumentation amplifier (IA) with continuous tuning of the voltage gain, suitable for operation over a wide frequency range, and aimed to electrical bioimpedance spectroscopy, is proposed. The operation principle of the IA is based on indirect current feedback (ICF), which leads to an almost-constant bandwidth regardless of the value of the programmed voltage gain. The use of improved voltage followers in the transconductors required in the ICF technique allows achieving a compact implementation with a bandwidth compatible with bioimpedance spectroscopy applications. The tuning strategy relies on a continuously programmable current mirror that can be electronically adjusted by means of a control current. The IA has been designed and fabricated in 180 nm CMOS technology to operate with a 1.8-V single supply. The experimental characterization of the silicon prototypes showed a gain programmability range higher than 45 dB, between –4.6 dB and 41.2 dB, a BW around 3 MHz, and a maximum CMRR at DC higher than 86 dB, all this with a minimum current consumption of 144.8 μA and an area occupation of 0.0196 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Nearly Optimal Tunable MPC Strategies on Embedded Platforms

We present an embeddable optimization-free application of a near-optimal MPC implementation with continuous tuning capabilities. We propose a strategy combining the advantages of explicit model predictive control with tunable properties that is implementable on embedded platforms with limited memory and computational resources. We consider a neural network (NN) learning procedure to mimic the control actions of an MPC strategy. While acknowledging limited guarantees on the constraints satisfaction with just the NN-based controller, we introduce an optimization-based corrector of the mimicked control action. Such a corrector then steers the control authority of the mimicked controller such that constraints on manipulated and process variables are enforced. To demonstrate the efficacy of the proposed control strategy, a case study implemented on an embedded platform is shown.

Widely tunable single-mode slot waveguide quantum cascade laser array.

We report designs and experimental demonstrations of a widely tunable single-mode quantum cascade laser array based on slot waveguide structures in the mid-infrared region. The laser array device realized a continuous tuning range of 71 cm-1 from 9.66 μm to 10.37 μm at 300K only using the current tuning without any external heatsink temperature adjustments, in good agreement with the design. Stable single-mode operations free of undesired mode-hops have been obtained over the whole tuning range. Another slot waveguide QCL array with a 41 cm-1 continuous tuning range around 7.3 μm has also been realized with the same design principle, demonstrating the universal applicability of the array design. The broadly continuous tuning with simple processing makes the array device a suitable candidate for mid-infrared sensing and spectroscopy application.

Continuous Fermi level tuning of Nb-doped WSe2 under an external electric field

The possibility of continuous Fermi level tuning of Nb-doped WSe2 under an external electric field is investigated, using the density functional theory combined with the effective screening medium method. The Fermi level monotonically increases and decreases as the carrier concentration increases and decreases, respectively, by controlling the external electric field. The electronic structure of Nb-doped WSe2 is insensitive to the Nb concentration and arrangement. Furthermore, it was demonstrated that the electric field simply shifts the Fermi level of Nb-doped WSe2, resulting in the constant quantum capacitance through the gate voltage, irrespective of the Nb concentration and arrangement.

Robust and low cost in-fiber acousto-optic Mach-Zehnder interferometer and its application in a dual-wavelength laser.

A robust in-fiber tunable acousto-optic Mach-Zehnder interferometer with a taper-shaped sandwich-like fiber structure is proposed and characterized experimentally, based on which tunable dual-wavelength lasers are demonstrated. The fiber structure was prepared by two-step etching methods, which could be used to fabricate either a symmetric structure for a continuous tuning dual-wavelength laser or an asymmetric structure for a switchable one. The proposed structure has advantages of low cost, low driving power, and robustness. The method for preparing the fiber structure is agile, which paves the way for its applications.

Monolithic, Optically Coupled, Multi-Section Mid-IR Quantum Cascade Lasers

Mid-infrared (mid-IR λ ≈ 3–12 μm), single-mode-emission Quantum Cascade Lasers (QCLs) are of significant interest for a wide range of applications, especially as the laser sources are chosen for laser absorption spectroscopy. In this work, we present the design, fabrication and characterization of multi-section, coupled-cavity, mid-IR quantum cascade lasers. The purpose of this work is to propose a design modification for a coupled-cavity device, yielding a single-mode emission with a longer range of continuous tuning during the pulse, in contrast to a 2-section device. This effect was obtained and demonstrated in the work. The proposed design of a 3-section coupled-cavity QCL allows for a single-mode emission with 35 dB side-mode suppression ratio. Additionally, the time-resolved spectra of the wavelength shift during pulse operation, show a continuous tuning of ~3 cm−1 during the 2 μs pulse. The devices were fabricated in a slightly modified, standard laser process using dry etching.

Frequency-Tunable Pulsed Microwave Waveform Generation Based on Unbalanced Single-Arm Interferometer Excited by Near-Infrared Femtosecond Laser

Femtosecond laser-excited generation of frequency-tunable microwave pulses, based on an unbalanced single-arm interferometer with frequency-to-time mapping, has been proposed and demonstrated with easy-to-obtain commercial devices. The optical wave-to-microwave frequency conversion, which involves continuous tuning in the range from 2.0 GHz to 19.7 GHz, was achieved based on simple spatial–optical group delay adjustment. Additionally, the pulse duration of the microwave waveform was measured to be 24 ns as the length of the linear dispersion optical fiber was fixed at 20 km. In addition, owing to the designs of the single-arm optical path and polarization-independent interference, the generated microwave pulse train had better stability in terms of frequency and electrical amplitude. Furthermore, a near-triangular-shaped microwave pulse at 4.5 GHz was experimentally obtained by the superposition of two generated sinusoidal signals, which verified the potential of this system to synthesize special microwave waveform pulses.

Development of a Doppler-broadened NICE-OHMS system for trace gas detection based on a single sideband phase modulator

To expand the applicability of noise-immune cavity-enhanced optical heterodyne molecular spectrometer (NICE-OHMS), a universal system incorporating a fiber-coupled single-sideband modulator (f-SSM) for control of the laser frequency has been developed. A homemade PID servo mainly composed of two integrators has been designed, resulting in a locking bandwidth of 170 kHz and a continuous tuning range of 2.2 GHz. The system exhibits a noise-equivalent Doppler-broadened absorption limit of 8.0×10−14 cm−1 for an integration time of 64 s. Since the f-SSM is the sole external frequency actuator, this opens up for NICE-OHMS based on a multitude of laser systems.

An Extended Interaction Oscillator Capable of Continuous Multimode Operation

A nine-gap extended interaction oscillator (EIO) circuit capable of continuous multimode operation is proposed to overcome the continuous electronic tuning range limitation of the conventional single-mode operation EIO circuit. Circuit analysis shows that it has the capability of stable continuous mode switching between multiple operating modes with the sweeping operating voltage. Particle-in-cell simulation shows that a continuous mode switching between the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> /8 mode and the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$2\pi $ </tex-math></inline-formula> mode is achieved with a continuous electronic tuning range of 95.50–96.23 GHz. Working mechanism of the continuous multimode operation is preliminary studied by the “cold cavity” and “hot cavity” analysis.

Enhanced Tuning Performance of In-Series REC-DFB Laser Array

In this study, a tunable in-series multi-wavelength distributed feedback laser array (DFB-MLA) based on reconstruction-equivalent-chirp (REC) technique with simple wavelength tuning scheme is presented and experimentally demonstrated. Unlike traditional DFB laser tuned by thermal-electric cooler (TEC), the proposed laser tuned by injection current can realize faster wavelength-tuning and reduce TEC power consumption. By comparing the thermal influence of different structures of heatsink blocks, several optimized block structures are selected for appropriate thermal conductivity and controllable heat dissipation, resulting in a broad and continuous wavelength tuning range. The proposed in-series DFB-MLA on optimized heatsink achieves better tuning performance with a wavelength tuning range of ~2.4 nm per channel from 40 to 120 mA and stable output power of over 20 mW compared to the laser mounted on the standard heatsink block with the tuning range of 1.4 nm per channel. Furthermore, the tunable laser keeps good single-mode stability, with a side mode suppressing ratio (SMSR) of over 40 dB. Thus, this study provides a new perspective for designing a low-cost continuous tunable semiconductor laser with a simple tuning mechanism in the future.