Microwave Generation Research Articles

Ultrahigh-Frequency Hop Dryer with Tiered Resonators

Introduction. From the analysis of hop dryers of different systems and designs follows the prospect of hop drying by complex influence of the energy of the electromagnetic field of ultrahigh frequency and convective heat. Aim of the Article. The article aims at developing a small-sized microwave convective hop dryer with justified parameters for intensive drying technology of freshly harvested hops. Materials and Methods. Taking into account the justified criteria for the design of a hop dryer and the analysis of existing resonators, there was proposed a methodology for the development of a hop dryer with an energy supply in an electromagnetic field, including requirements for structural design, operational and economic indicators, and technology. The electrodynamic parameters of the resonator were investigated according to the CST Studio 2017 program. Results. The dielectric parameters of hops are theoretically investigated and functional dependences on humidity at a frequency of 2,450 MHz are obtained. The dynamics of hop heating is investigated when its dielectric loss factor changes during the action of an ultrahigh frequency electromagnetic field. There has been developed a design and technological scheme of a radio-hermetic microwave convective hop dryer of continuous flow action with tiered resonators for low-power agricultural enterprises. Resonators are arranged in tiers in the screening cylindrical housing: the first and third resonators are hemispherical, and the middle one is made in the form of an ellipsoid to ensure a high electric field strength. The transportation of raw materials takes place by rotating the discs in a gentle mode. Discussion and Conclusion. The expected specific energy costs of a hop dryer with a capacity of 12–13 kg/h at the microwave generator power of 4.0 kW for drying freshly harvested hops with a humidity of 76–82% to a humidity of 11–14% are 0.30–0.33 kWh/kg. The required electric field voltage of 2 kV/cm in all three resonators is provided, therefore, disinfection of raw materials occurs at a temperature of 65–70°C for 5–6 minutes of stay in three resonators. The intensity of moisture release from hop cones during endogenous convective heating increases 5–6 times compared to the convective drying method. The introduction of microwave drying technology using a convective method of evaporation and removal of moisture from the drying chamber reduces the duration of the process, saves valuable components of cones for brewing.

Search for the optimal shape of the envelope of a microwave pulse of a powerful generator in the maximum permissible radiation mode

In the article, the problem of choosing the shape of the envelope of the microwave pulse of a powerful generator, which provides the maximum radiated surface antenna energy in the pre-breakdown mode, is formulated in the form of an isoperimetric problem of the calculus of variations, and its solution is obtained. Optimization was carried out at a fixed effective duration of the envelope. It has been established that the optimal shape depends on the effective pulse duration and is a truncated oval. As the pulse duration increases, the optimal shape tends to be rectangular. Analytical expressions are obtained that relate the effective duration and maximum allowable energy to the breakdown field for pulses with optimal, rectangular, Gaussian, and triangular envelopes. The dependences of the breakdown field and the maximum allowable energy of pulses with the indicated envelope shapes on the effective pulse duration are calculated in the range of realizable durations of powerful relativistic microwave generators. A comparative analysis of the obtained results is given.

Compact, portable, thermal-noise-limited optical cavity with low acceleration sensitivity.

We develop and demonstrate a compact (less than 6 mL) portable Fabry-Pérot optical reference cavity. A laser locked to the cavity is thermal noise limited at 2 × 10-14 fractional frequency stability. Broadband feedback control with an electro-optic modulator enables near thermal-noise-limited phase noise performance from 1 Hz to 10 kHz offset frequencies. The additional low vibration, temperature, and holding force sensitivity of our design makes it well suited for out-of-the-lab applications such as optically derived low noise microwave generation, compact and mobile optical atomic clocks, and environmental sensing through deployed fiber networks.

Broad Tunable and High-Purity Photonic Microwave Generation Based on an Optically Pumped QD Spin-VCSEL with Optical Feedback

Spin-polarized vertical-cavity surface-emitting lasers (spin-VCSELs) with birefringence-induced polarization oscillations have been proposed to generate desired photonic microwave signals. Here, we numerically investigate the generation of photonic microwave signals in an optically pumped quantum dot (QD) spin-VCSEL. First, the influence of intrinsic key parameters on period-one (P1) oscillations and microwave properties is discussed. Second, the difference between microwave generation based on the quantum well (QW) and QD spin-VCSELs is analyzed by controlling the carrier capture rate that is described in the spin-flip model. The QD spin-VCSEL shows superior microwave quality in the low-frequency range (e.g., 10 GHz~20 GHz) compared with the QW spin-VCSEL. Finally, to boost the performance of the generated photonic microwave signal, optical feedback is introduced. The results show that dual-loop feedback can simultaneously narrow the microwave linewidth and suppress the side modes that are derived from the external cavity mode.

The assessment of the atmospheric air breakdown voltage generated by the interaction between microwaves and metallic wires

The aim of this research is to understand the plasma initiation process generated by metallic wires when interacting with high energy density microwaves. Lead (Pb) and molybdenum (Mo) wires of 0.5 mm diameter were investigated in this experiment. The tip of the metallic wire was placed into the nodal point of a waveguide cavity attached to a microwave generator, where it was exposed to the high energy density of the microwave field. Following the interaction between microwaves and the metallic wire, a plasma was initiated having as effect the wire vaporization. The experiments were conducted in atmospheric air at ∼1 bar pressure. From optical emission spectroscopy investigations it was observed that electronic excitation of the plasma has high values and it is in a local thermal equilibrium. The theoretical calculation of the voltages induced in the metallic wires when exposed to high energy density microwave field are similar to those measured in air breakdown experiments. The scanning electron microscopy analysis of the tips of the metallic wires showed that the field emission process is responsible for the ignition of the metallic wires and plasma generation.

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.

Numerical investigation of photonic microwave generation in an optically pumped spin-VCSEL subject to optical feedback.

Photonic microwave generation based on period-one (P1) dynamics of an optically pumped spin-polarized vertical-cavity surface-emitting laser (spin-VCSEL) is investigated numerically. Here, the frequency tunability of the photonic microwave generated from a free-running spin-VCSEL is demonstrated. The results show that the frequency of the photonic microwave signals can be widely tuned (from several gigahertz to hundreds of gigahertz) by changing the birefringence. Furthermore, the frequency of the photonic microwave can be modestly adjusted by introducing an axial magnetic field, although it degrades the microwave linewidth in the edge of Hopf bifurcation. To improve the quality of the photonic microwave, an optical feedback technique is employed in a spin-VCSEL. Under the scenario of single-loop feedback, the microwave linewidth is decreased by enhancing the feedback strength and/or delay time, whereas the phase noise oscillation increases with the increase of the feedback delay time. By adding the dual-loop feedback, the Vernier effect can effectively suppress the side peaks around the central frequency of P1, and simultaneously supports P1 linewidth narrowing and phase noise minimization at long times.

Innovations in designing microwave electro-technological units with hybrid chambers

Aim of study: Microwave (MW) electro-technological units based on electromagnetic radiation of ultrahigh-frequency can involve thermal MW modification of dielectrics and non-thermal MW modification of polymers. Area of study: Russian Federation. Material and methods: The paper considers a method for making a unit with a hybrid chamber, where thermal and non-thermal MW modifications were carried out simultaneously, and the remaining energy after non-thermal MW modification of polymers was used for heating the dielectric. Main results: A microwave electro-technological unit with a hybrid chamber replaced two separate devices that implemented these MW modifications. It was cheaper and required one MW generator. The unit took up less space than two separate apparatuses, and upgraded the existing microwave dryer to perform thermal MW modification of a lumber pile and non-thermal MW modification of polymer materials. The existing microwave dryer was redeveloped by solving the boundary value problem in electrodynamics and heat and mass transfer. Research highlights: The research presents a microwave electro-technological unit with a hybrid chamber, combining thermal and non-thermal MW modifications of dielectric and polymer materials. As a result of upgrading the existing microwave dryer, it was possible to carry out both thermal and non-thermal MW modifications, namely, microwave drying of timber and microwave drying of up to seven different polymer objects.

Optical and Microwave Metrology at the 10−18 Level with an Er/Yb:glass Frequency Comb

AbstractOptical frequency combs are essential tools for precision metrology experiments ranging in application from remote spectroscopic sensing of trace gases to the characterization and comparison of optical atomic clocks for precision time‐keeping and searches for physics beyond the standard model. Presented here is a description of the architecture and a full characterization of a telecom‐band, self‐modelocking frequency comb based on a free‐space laser with an Er/Yb co‐doped glass gain medium. The laser provides a robust and cost‐effective alternative to Er:fibre laser based frequency combs, while offering stability and noise performance similar to Ti:sapphire laser systems. Finally, the utility of the Er/Yb:glass frequency comb is demonstrated in high‐stability frequency synthesis using two ultra‐stable optical references at 1157 nm and 1070 nm and in low‐noise photonic microwave generation by dividing these references to the microwave domain.

Electro‐Optically Tunable Low Phase‐Noise Microwave Synthesizer in an Active Lithium Niobate Microdisk

AbstractPhotonic‐based low‐phase‐noise microwave generation with real‐time frequency tuning is crucial for a broad spectrum of subjects, including next‐generation wireless communications, radar, metrology, and modern instrumentation. Here, for the first time to the best of the authors’ knowledge, narrow‐bandwidth dual‐wavelength microlasers are generated from nearly‐degenerate polygon modes in a high‐Q active lithium niobate microdisk. The record‐high‐Q (≈107) nearly‐degenerate polygon modes formation with independently controllable resonant wavelengths and free spectral ranges is enabled by the weak perturbation of the microdisks using a tapered fiber. Moreover, because a high spatial overlap factor between the pump and the dual‐wavelength laser modes is achieved, the gain competition between the two lasing modes spatially separated with a π‐phase difference is suppressed, leading to stable dual‐wavelength laser generation with low threshold, and in turn, the low noise microwave source. The stable beating signal confirms the low phase‐noise achieved in the tunable laser. Without the need of external phase stabilizers, the measured microwave signal shows a phase noise of −123 dBc Hz−1 and an electro‐optic tuning efficiency of −1.66 MHz V−1. The linewidth of the microwave signal is measured as 6.87 kHz, which is more than three orders of magnitude narrower than current records based on integrated dual‐lasers.

Test of KW Class Photonic Microwave Generation Using Vanadium-Compensated 6H-SiC PCSS and Burst-Mode-Operation Pulse Laser

Photoconductive semiconductors operating in linear mode can be used for adaptive high-power microwave (HPM) generators by modulating incident light. This paper presents the design scheme and preliminary test results of a narrowband kW class adaptive photonic microwave generator by employing a wide-bandgap semi-insulating 6H-SiC photoconductive semiconductor and a burst-mode laser. The experimental scheme of the generator is described along with the circuit simulation of the radio frequency generator. The laser operated at a wavelength of 532 nm with a pulse width of 100 ns and a repetition rate of 100 Hz. The laser modulates the frequencies of the generated microwave, and preliminary tests are conducted in the frequency range of 0.8–1.2 GHz. The maximum output microwave power of the designed scheme reaches up to 2.6 kW when the bias voltage is 10 kV. The results confirm that this method can be used for high-power frequency-adjustable microwave signal generation. The microwave source system presented in this paper has continuously adjustable frequency and flexible waveform control.

Focus on test, measurement, quantum metrology, and analytical equipment

Focus on test, measurement, quantum metrology, and analytical equipment

Effect of the Plasma Density on the Generation Efficiency and Changes in the Spectrum of a Plasma Relativistic Microwave Generator

Effect of the Plasma Density on the Generation Efficiency and Changes in the Spectrum of a Plasma Relativistic Microwave Generator

A Compact Pulse Magnetron Microwave Generator Based on a Solid-State Switch

A Compact Pulse Magnetron Microwave Generator Based on a Solid-State Switch

Advances in multi-wavelength Brillouin fiber lasers: An outlook across different spectral regions

In recent years, Brillouin fiber lasers have become a promising optical source technology for academic and industrial purposes. Among the promising laser solutions that have emerged with low threshold, high efficiency, small linewidth, and low resonator loss, multi-wavelength architecture has attracted the most attention recently. However, the development of configurations with an increased number of Stokes channels that present high flatness and reasonable optical-to-signal ratio (OSNR) is a challenge that this fiber laser technology still faces, and the development of solutions for this is an open problem. From this perspective, this paper presents a review focused on recent progress in multi-wavelength Brillouin fiber laser (MBFL) devices. Specifically, microwave generation and temperature sensing applications are discussed. Key results in the spectral regions of 1.0μm, 1.3μm, 1.5μm, 1.6μm, and 2.0μm are reported, as well as future perspectives are addressed in this review for the advancements in multi-wavelength fiber laser sources.

Time-frequency characteristics of microwaves generated by hypervelocity impact

When space debris impinges upon a spacecraft shield structure at an ultra-high speed, part of the debris’ kinetic energy is converted into electromagnetic energy, which radiates outward in the form of electromagnetic waves and can interfere with the spacecraft's normal communication. To investigate the radiation characteristics of the emitted electromagnetic waves, a series of experiments were conducted using a two-stage light gas gun, in which a spherical aluminum alloy (diameter:6.4 mm) projectile impinges on a 23-mm-thick target of the same material at velocities of 3.3–6.3 km/s. A real-time spectrum analyzer and super-heterodyne receiver were used to measure the microwaves in the frequency and time domains, respectively. Then, the time–frequency spectrum of the microwaves was obtained by Hilbert-Huang Transform. Further, two mechanisms of microwave generation—plasma expansion and material destruction—in an aluminum–aluminum impact are proposed, and the radiation powers of the two mechanisms are estimated based on the theoretical analysis and experimental results. Finally, the effect of the plasma cutoff frequency on the microwave propagation is explained, and the relationship between the radiation mechanism and the impact velocity is obtained. When the impact velocity is less than 5.2 km/s, material destruction is the main mechanism of microwave radiation, whereas at impact velocities > 5.2 km/s, plasma expansion dominates.

A Monolithically Integrated Two-Section Laser for Wideband and Frequency-Tunable Photonic Microwave Generation

A monolithically integrated two-section laser is presented for wideband and frequency-tunable photonic microwave generation. The laser consists of two back-to-back DFB sections forming a mutually coupled structure. By properly adjusting the bias currents of two sections, the laser can stably work at the state of period-one oscillation over a wide range of frequency detuning. Based on this, continuous and linear tuning of photonic microwave signals can be achieved. Experimental results confirm that a large tunable range from 12.45 to 80.30 GHz can be realized using this laser.

Comparative analysis of microwave granulators implementing electrophysical methods for processing secondary agricultural raw materials

Introduction. A promising direction for granulation of combined secondary raw materials of animal and vegetable origin is high-temperature molding of granules under the influence of an electromagnetic field of super highfrequency (SHF-EMF), which improves the microbiological parameters of protein feed.Results. Microwave granulators of multicomponent raw materials containing different design versions of resonators have been developed— for example, toroidal, cylindrical and ellipsoidal resonators. The electrodynamic parameters of these resonators are determined and the technical characteristics of the granulators are described. The required specific heat for the processing of secondary fat-containing raw materials has been calculated. With a granulator capacity of 200 kg/h and a specific power of the microwave generator of 1.4–2 W/g, the duration of exposure to the raw material weighing 0.8–1 kg in the resonator chamber should be 10–12 s. This ensures the minimum specific energy costs of 0.06 kW·h/kg.

Frequency-selectable microwave generation based on on-chip switchable spectral shaping and wavelength-to-time mapping.

We propose and experimentally demonstrate a scheme for the photonic generation of pulsed microwave signals with selectable frequency based on spectral shaping and wavelength-to-time mapping (WTTM) technique. The frequency selectivity is realized by channel switching on an integrated silicon-on-insulator (SOI) spectral shaping chip. The incident signal is spectrally shaped by the asymmetric Mach-Zehnder interferometer (MZI) in the selected channel, and an optical spectrum with uniform free spectral range (FSR) can be generated in a broad bandwidth up to dozens of nanometers, implying large microwave signal duration after WTTM if a pulse light source with matched bandwidth is available. Microwave pulses of frequency from 3.6 GHz to 28.4 GHz with a fixed interval are experimentally generated respectively. The realization of eight microwave frequencies selectable with only one shared dispersive element (DE) required indicates high expansibility in the frequency cover range of our scheme by tuning the dispersion value in WTTM.

Advantages and problems of electric microwave processing plants with hybrid working chambers

This article presents information about the design and purpose of microwave processing plants of thermal, non-thermal microwave modification and microwave processing plants with hybrid working chambers that have not yet been widely used. This article also presents and justifies advantages of electric microwave processing plants with hybrid chambers over electric microwave processing plants used for thermal treatment of dielectric materials and non-thermal treatment of polymer materials. These advantages give reason to talk about the expediency of widespread use of electric microwave processing plants with hybrid working chambers in industry and agriculture. This article formulates and justifies the problems of electric microwave processing plants with hybrid working chambers. Thus, high prices for microwave generators reduce the profitability of the plant. The research is being carried out within the framework of a grant as subsidies in science from the budget of the Republic of Bashkortostan for the state support of young scientists – candidates of sciences (the competition code is REC–Federal and State Management – 2022).