Frequency-tunable Microwave Research Articles

Continuously and Precisely Tunable Microwave Frequency Comb Generation Based on Actively Mode-Locked OEO

Continuously and Precisely Tunable Microwave Frequency Comb Generation Based on Actively Mode-Locked OEO

Tunable microwave and optical frequency comb generator with the aid of cascaded MZM structure

Tunable microwave and optical frequency comb generator with the aid of cascaded MZM structure

Frequency tunable non-invasive microwave blood glucose sensor based on negative impedance circuit

Abstract This article proposes a frequency adjustable non-invasive blood glucose microwave sensor based on negative impedance circuit. Firstly, this article establishes a skin-blood-skin human tissue model for the detection of non-invasive microwave sensors. Compared to traditional passive microwave blood glucose sensors, this design uses a negative impedance compensation circuit (NIC), greatly increasing microwave penetration and improving its detection sensitivity. In addition, this design achieves the goal of adjustable operating frequency by adding an additional dielectric layer at the detection end. This function is of great significance for microwave non-invasive blood glucose sensors. On the one hand, it can adjust to the optimal operating frequency band according to the characteristics of blood glucose changes; On the other hand, it is possible to measure changes in blood glucose concentration at multiple frequency points, thereby increasing the accuracy of detection. The relevant results show that adjusting the frequency band can reach 1–4 GHz, and the specific method of frequency adjustment is provided in the article; Meanwhile, four initial frequency points were selected in the article, at which the results show that the sensitivity reaches 1.356 × 10−3, 3.142 × 10−3, 2.071 × 10−3, 1.903 × 10−3. The above results demonstrate the superiority of our design, and blood glucose sensors based on this principle are expected to break through the limitations of current blood glucose sensors.

Wideband finely tunable, ultralow-phase noise microwave generation in a Brillouin cavity.

A novel, to the best of our knowledge, approach to generate frequency-tunable microwave sources with low-phase-noise based on a Brillouin laser frequency comb is proposed and experimentally demonstrated. The Brillouin laser frequency comb is generated by combining stimulated Brillouin scattering, frequency shifting optical injection locking, modulation sideband optical injection locking (MSOIL), and four-wave mixing effects. By beating the generated comb lines, the microwave is generated with an extremely low-level phase noise of -120 dBc/Hz at a 10-kHz offset. The frequency of the microwave signal can be finely tuned in steps of a Brillouin cavity mode spacing (i.e., 2 MHz) and coarsely adjusted to integer times the applied RF signal frequency in the MSOIL unit. Remarkably, the phase noise of the microwave source can be kept at almost the same low level during the whole tuning process over the frequency range of 30-75 GHz. The proposed tunable low-phase-noise microwave generation approach has great potential applications in communications, radars, and metrology.

Research on Optical Mutual Injection to Generate Tunable Microwave Frequency Combs

In this study, a scheme for generating tunable microwave frequency combs (MFCs) based on optical mutual injection is proposed and experimentally investigated. The scheme is based on the optical injection of lasers to generate MFCs, and constitutes a feedback loop by using dual-laser mutual injection to obtain MFCs with a large continuous bandwidth and tunable comb spacing. The experimental setup analyzes the effects of injected optical power, modulation frequency and amplitude, and wavelength detuning on the generated MFC signals. The experimental results indicate that when the single-frequency electrical signal is set to 2 GHz, flat MFCs with amplitude variations within 10 dB can be obtained by optimizing the injected power and the frequency detuning between the two semiconductor lasers. Furthermore, the comb spacing of the MFCs can be made tunable by varying the modulation frequency and selecting the matched operating parameters to adapt to different application scenarios.

Design of 5CB liquid crystal antenna for tunable microwave frequency applications

ABSTRACT Cyanobiphenyls (CBs) are a class of molecules with unique properties that make them highly versatile in various fields. Their anisotropic nature, low viscosity, and sensitivity to external stimuli make them ideal for applications in display technology, photonic devices, sensors, and in tunable antennas. In this article, we present the design and simulation of a 5CB LC antenna for tunable microwave frequency applications due to its fast response time and low dielectric loss than other CB materials. The proposed antenna contains two glass substrates with an air gap of 0.004 mm between them. The lower glass contains a size of 20 × 15 mm2, while the upper glass contains a size of 13 × 15 mm2. The proposed antenna produces two bands when there is an air gap between the glass plates; three bands when diodes are used for the radiating patch; and four bands when 5CB LC material is introduced between the glass plates as a dielectric medium. The same structure produces various frequency bands, which is a notable advantage compared to the existing state of the art. All the results shown in this manuscript were simulated and measured with the body of data available from the literature.

Tunable microwave frequency comb generation via periodic relaxation oscillation in directly modulated laser

In this paper, a novel and simple generation scheme based on a directly modulated laser (DML) is proposed for tunable microwave frequency combs (MFCs). When the input modulated RF power is intentionally controlled to be sufficiently high to drive the DML into its nonlinear operating region periodically, relaxation oscillations will be excited in the same period. The modulated light from the DML is subsequently detected by a photodetector, and a special kind of microwave signal source can be optically produced and shows up in the form of MFC in the frequency domain. In the proof-of-concept experiment, tunable MFCs are demonstrated with the bandwidth of more than 25 GHz, the comb spacing of 0.5 ∼ 3 GHz, and the flatness of less than ± 2 dB with the frequency from 3 GHz to 18 GHz. The bandwidth of the generated MFC is in accordance with the relaxation oscillation frequency of the DML. The comb spacing is strictly equal to the frequency of the modulated RF signal and thus flexibly adjustable. The carrier-to-noise ratio can be up to 48 dB, and the single-sideband phase noise is measured to be lower than − 90 dBc/Hz@1 kHz for MFCs with comb spacings of 0.7 ∼ 3 GHz.

Optical Tunable Frequency-Doubling OEO Using a Chirped FBG Based on Orthogonally Polarized Double Sideband Modulation

We propose and experimentally demonstrate a tunable frequency-doubling optoelectronic oscillator (FD-OEO) based on a single-bandpass dispersion-induced microwave photonic filter (MPF) consisting of a Mach–Zehnder modulator (MZM), a linearly chirped fiber Bragg grating and polarization-multiplexed dual-loop. Thanks to the polarization dependence of the MZM, a special double sideband modulation is implemented where the optical carrier (OC) and subcarriers are orthogonally polarized. By simply tuning the PC in the OEO loop, the phase difference between the orthogonal polarization carrier and two sidebands can be controlled, and thus the center frequency of the fundamental OEO can be tuned. Furthermore, a PC and a polarizer are placed outside the OEO to achieve optical carrier suppression (OCS) modulation, which ensures that a frequency-tunable microwave signal at the second-harmonic frequency is generated. In the experiment, a fundamental frequency signal with tunable frequency from 3.6 to 6.85 GHz and FD-OEO with a tunable frequency range from 7.2 to 13.7 GHz are generated.

Frequency-tunable microwave quantum light source based on superconducting quantum circuits

A non-classical light source is essential for implementing a wide range of quantum information processing protocols, including quantum computing, networking, communication and metrology. In the microwave regime, propagating photonic qubits, which transfer quantum information between multiple superconducting quantum chips, serve as building blocks for large-scale quantum computers. In this context, spectral control of propagating single photons is crucial for interfacing different quantum nodes with varied frequencies and bandwidths. Here a deterministic microwave quantum light source was demonstrated based on superconducting quantum circuits that can generate propagating single photons, time-bin encoded photonic qubits and qudits. In particular, the frequency of the emitted photons can be tuned in situ as large as 200 MHz. Even though the internal quantum efficiency of the light source is sensitive to the working frequency, it is shown that the fidelity of the propagating photonic qubit can be well preserved with the time-bin encoding scheme. This work thus demonstrates a versatile approach to realizing a practical quantum light source for future distributed quantum computing.

Tunable High-Purity Microwave Frequency Combs Generation Based on Active Mode-Locking OEO

A novel approach to generate tunable high-purity microwave frequency combs (MFCs) based on active mode-locking optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. In the proposed method, one cavity mode of the OEO is injection-locked by a local oscillator (LO) signal resulting in single-mode oscillation. Under the assistance of an additional radio frequency signal, the MFCs generation is realized by loss modulation the single-mode oscillation. A key significance of our scheme is that no tunable filter is needed in OEO cavity. By altering the frequency of the LO signal, the MFCs center frequency can be flexibly precisely tuned over a bandwidth of 1.6 GHz. Furthermore, the high suppression of the supermode noise is obtained due to the injection locking of the OEO decreasing the required loop gain. The experimental results show that the generated MFCs maintain a large supermode noise suppression ratio over the entire tuning range.

Entangled Frequency-Tunable Microwave Photons in a Superconducting Circuit

We propose a frequency-tunable source to emit entangled microwave photons on the platform of a superconducting circuit, in which two superconducting transmission-line resonators are coupled via a capacitor and one resonator is inserted with a superconducting quantum interference device (SQUID) in the center. By pumping the circuit appropriately with an external coherent microwave signal through the SQUID, microwave photons are emitted in pairs out of the circuit. The entanglement between the two modes is demonstrated by numerically calculating the second-order coherence function and the logarithmic negativity of the output microwave signals. Due to the tunability of SQUID’s equivalent inductance, the frequencies of the entangled microwave photons can be tuned by an external flux bias in situ. Our proposal paves a new way for obtaining entangled frequency-tunable two-mode microwave photons.

Frequency-tunable microwave generation with parity-time symmetry period-one laser dynamics.

A novel frequency-tunable microwave signal generation method is proposed by incorporating parity-time (PT) symmetry in period-one (P1) laser dynamics in an optically injected semiconductor laser. In this method, P1 oscillation enables a large frequency tuning range and PT symmetry leads to excellent side-mode suppression and low phase noise. In an experimental demonstration, the side-mode suppression ratio reaches 58.4 dB and the phase noise is -126.2 dBc/Hz at 10 kHz offset when generating a 6.98 GHz signal, which are improved by 44.5 dB and 13.5 dB, respectively, compared with the previously reported optoelectronic oscillator-based P1 oscillation. By simply adjusting the optical injection strength, the frequency of the microwave signal generated by PT symmetry P1 dynamics is tuned from 5.07 GHz to 15.22 GHz, in which the phase noise is kept below 120 dBc/Hz at 10 kHz offset. The proposed method is expected to find applications in high-performance wireless communication and radar systems.

Tunable Microwave Frequency Comb Generation Based on Actively Mode-Locked OEO

A novel photonics-assisted method for tunable microwave frequency comb (MFC) generation based on an actively mode-locked optoelectronic oscillator (AML-OEO) is proposed and experimentally demonstrated in this letter. Two key factors for the proposed system, periodic gain modulation and widely tunable center frequency, are implemented by an externally introduced intensity-modulated optical signal and a microwave photonic filter (MPF) based on the phase-shifted fiber Bragg grating (PS-FBG) respectively. Unlike a conventional OEO with only one dominant oscillation mode, an AML-OEO operates in a stable multimode oscillation state and thus can generate MFC signal. Through simply adjusting the center frequency of the PS-FBG-based MPF, the center frequency of the generated MFC changes accordingly. In the proof-of-principle experiment, two sets of MFC signals with center frequency tuning from 3.5 GHz to 6.5 GHz and frequency intervals of 1.02 MHz and 340.46 kHz are generated.

Design of U-Shaped Frequency Tunable Microwave Filters in MEMS Technology

U-shaped microwave resonators implemented by RF MEMS switches can be considered the result of a novel design approach for obtaining small-footprint tunable resonators, owing to the bent shape of the resonator and the microsystem solution for changing the frequency of resonance. In this paper, we discuss the design approach for potential configurations of U-shaped structures combined with ohmic RF MEMS switches. Owing to their prospective application in RADAR and satellite systems, the devices were assessed for K-Band operation, specifically for 15 GHz, 20 GHz, and 26 GHz. The ON-OFF states determined by an electrostatic actuation of metal beams composing the RF MEMS ohmic switches allow for selecting different path lengths corresponding to different frequencies. In this contribution, initial configurations were designed and manufactured as a proof-of-concept. The advantages and critical aspects of the designs are discussed in detail.

Ultra-Wide and Tunable Microwave Frequency Down-Conversion Based on Re-Circulating Opposite Frequency Shifting

A photonic-assisted scheme is proposed and demonstrated for microwave frequency down-conversion based on re-circulating opposite frequency shifting. The proposed method consists of a microwave signal receiver and a re-circulating opposite frequency shifter. The microwave signal under down-conversion is received by electro-optical modulation (EOM). The microwave frequency is down-converted with a ring-assisted re-circulating EOM, in which the 0th and 1st optical sidebands of the microwave signal are frequency shifted in the opposite direction with the frequency step of the local oscillator (LO) signal in optical frequency domain, respectively. Two optical frequency combs (OFCs) with the same frequency spacing and different center frequency are generated. After photodetection, the frequency of the down-converted IF signal is the frequency difference between the generated two OFCs. The re-circulating opposite frequency shifter enables accurately tunable and ultra-wide frequency operation of down-conversion by simply adjusting the low-frequency electrical LO. As compared with the conventional EOM-based methods, our method eliminates the bandwidth limitation of devices for the wideband LO signals EOM. Prior to the conventional OFC-based methods, ours enables flexible frequency down-conversion by tuning the frequency of LO. In the demonstrations, 6∼40 GHz radio-frequency (RF) signals are experimentally down-converted to IF band below 3 GHz with a low-frequency electrical LO in the range of 6∼6.5 GHz.

Optically Transparent Frequency-Tunable Microwave Absorber Based on Patterned Graphene-ITO Structure

In this work, the authors report the design, fabrication, and measurement of an optically transparent microwave absorber with tunable absorption frequency based on a patterned graphene-indium tin oxide (ITO) hybrid structure. This transparent tunable absorber consists of periodically patterned graphene sandwich structure (GSS), patterned ITO layer, polyvinyl chloride (PVC) substrate, and ITO ground plane from top to bottom. The frequency of absorption peak can be dynamically controlled from 13.1 to 10.9 GHz by regulating the bias voltage between the graphene sheets from 0 to 30 V, in which the peak absorption rates remain above 99.7%. An inhomogeneous doping model for graphene patterns is introduced to interpret the usage of relatively large tuning voltage. The engineered absorber also exhibits polarization-insensitive characteristics attributed to the symmetrical design of the hybrid structure. This tunable device with optical transparency achieved by integrating different transparent materials has broad prospects in the application of multispectral and smart stealth technology.

Polarization multiplexed active mode-locking optoelectronic oscillator for frequency tunable dual-band microwave pulse signals generation.

We propose and experimentally demonstrate a polarization multiplexed active mode-locking optoelectronic oscillator (AML-OEO) based on a single dual-polarization binary phase-shift keying (DP-BPSK) modulator for frequency tunable dual-band microwave pulse signal generation. In order to realize mode-locking, two single-tone signals whose frequency are integer multiple of the free spectrum range (FSR) of AML-OEO are applied as active modulation signals (AMSs) at the bias ports of the DP-BPSK modulator. By dividing the AML-OEO into two loops with polarization demultiplexing, both the carrier frequency and pulse repetition frequency (PRF) of the dual-band microwave pulses are independently adjustable. In the experiment, microwave pulses with different PRFs of 162.4 kHz, 324.8 kHz and 812 kHz are generated based on fundamental, second-order harmonic and fifth-order harmonic mode-locking, respectively. In addition, the carrier frequency tunability within 4∼10 GHz is verified by inserting a frequency tunable electrical filter. The phase noise of the generated pulse signal at 10 kHz offset is better than -125 dBc/Hz.

Tunable microwave frequency comb generation using a Si3N4-MDR based actively mode-locked OEO.

Microwave frequency combs (MFCs) have important applications in communication and sensing owing to their characteristics of large number of comb lines, wide frequency range, and high precision of comb spacing. In many applications, MFCs are required to emit signals with tunable center frequency and variable comb spacing to accommodate different operating frequency bands and accuracies. Here, we demonstrate a tunable MFC by injecting a low-frequency electrical signal into a tunable optoelectronic oscillator (OEO). Tuning of MFC's center frequency and comb spacing are realized, allowing a frequency tuning range from 1 to 22 GHz and 50 comb lines within a 5 MHz bandwidth obtained in the MFC generator. In addition, the introduction of the silicon nitride micro-disk resonator (Si3N4-MDR) in the system paves the way for the integration of MFC generator.

Micromagnetic prediction strain and current co-mediated spindynamics in skyrmion-based spin-torque nano-oscillator

In this article, we theoretically investigate the influence of strain on the oscillation frequency characteristics of a skyrmion-based spin-torque nano-oscillator (STNO) using micromagnetic simulations. The system consists of an ultrathin nanopillar magnetic film with interfacial Dzyaloshinskii–Moriya interactions and a ring-shaped region to offer a tunable spatial strain source. The applied strain gives rise to an extra effective field in the magnetic film through magnetoelastic coupling and forms a strong confined potential for skyrmion dynamics. Our simulation results show that the operation frequency of the STNO can be effectively modulated by the strain. Furthermore, we introduce a simplified analytical expression of the skyrmion gyration dynamics to explain the micromagnetic simulation results. Our results will be useful in the development of wide-range frequency-tunable microwave emitters and frequency-shift keying for wireless communication.

Phase Tunable Microwave Source With Chromatic-Dispersion-Induced Power Fading Suppression

A frequency and phase tunable microwave source based on a single sideband modulated optoelectronic oscillator (OEO) is proposed and demonstrated. A dual-drive Mach-Zehnder modulator (DDMZM) is utilized to achieve single sideband modulation in the OEO loop. Phase reconfigurability is realized by phase shift element, which is composed of a phase modulator (PM) and a fiber Bragg grating (FBG). The phase of the generated signal can be tuned by adjusting the bias voltage of the PM. The single sideband modulated optical signal can overcome the chromatic-dispersion-induced power fading which can be used for long-distance transmission. In the experiment, the generated signal has a frequency range of 2–14 GHz and a phase range of 0–360°. The phase noise of the generated signal at an offset frequency of 10 kHz is −105 dBc/Hz.