Microwave Oscillators Research Articles

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AbstractA regime of quasi-stationary microwave generation in moderately relativistic microwave oscillator of the twistron type with 50 ± 20% efficiency of electron beam power conversion into electromagnetic radiation has been obtained in experiment through optimization of the electron–wave interaction. For the selected electron beam parameters (diode voltage, 210 kV; beam current. 1.36 kA) the output microwave power at 10.63 GHz frequency was 140 ± 40 MW in a guiding magnetic field of about 1.9 T. The duration of microwave pulses was about 16 ns.

Stable and Low-Spurs Optoelectronic Oscillators: A Review

An optoelectronic oscillator (OEO) is an optoelectronic hybrid oscillator which utilizes ultra-low-loss fiber as an electro-magnetic energy storage element, overcoming the limits of traditional microwave oscillators in phase noise performance. Due to their ability to generate ultra-low phase noise microwave signal, optoelectronic oscillators have attracted considerable attentions and are becoming one of the most promising and powerful microwave signal sources. In this paper, we briefly introduce the operation principle and discuss current research on frequency stability and spurious suppression of optoelectronic oscillators.

Sub-hertz fundamental linewidth photonic integrated Brillouin laser

The demand for high-performance chip-scale lasers has driven rapid growth in integrated photonics. The creation of such low-noise laser sources is critical for emerging on-chip applications, ranging from coherent optical communications, photonic microwave oscillators remote sensing and optical rotational sensors. While Brillouin lasers are a promising solution to these challenges, new strategies are needed to create robust, compact, low power and low cost Brillouin laser technologies through wafer-scale integration. To date, chip-scale Brillouin lasers have remained elusive due to the difficulties in realization of these lasers on a commercial integration platform. In this paper, we demonstrate, for the first time, monolithically integrated Brillouin lasers using a wafer-scale process based on an ultra-low loss Si3N4/SiO2 waveguide platform. Cascading of stimulated Brillouin lasing to 10 Stokes orders was observed in an integrated bus-coupled resonator with a loaded Q factor exceeding 28 million. We experimentally quantify the laser performance, including threshold, slope efficiency and cascading dynamics, and compare the results with theory. The large mode volume integrated resonator and gain medium supports a TE-only resonance and unique 2.72 GHz free spectral range, essential for high performance integrated Brillouin lasing. The laser is based on a non-acoustic guiding design that supplies a broad Brillouin gain bandwidth. Characteristics for high performance lasing are demonstrated due to large intra-cavity optical power and low lasing threshold power. Consistent laser performance is reported for multiple chips across multiple wafers. This design lends itself to wafer-scale integration of practical high-yield, highly coherent Brillouin lasers on a chip.

A phase-stabilized superradiant Ka-band oscillator driven by nanosecond voltage pulses with amplitude variations and reduced rise rates

This paper presents the results of experiments and a numerical simulation of the conditions for phase stabilization (standard deviation < 0.5 rad) of a subgigawatt relativistic superradiant Ka-band backward wave oscillator with the accelerating voltage amplitude varying within 15%. It is shown that phase stabilization can be achieved for the voltage rise rate (dU/dt)max ∼ 2 MV/ns by using our recently developed approach. Particular emphasis is given to the cases of substantially lower voltage rise rates at which stabilization of the microwave oscillation phase is also possible. The mode with (dU/dt)max ≪ 1 MV/ns is considered for specific voltage waveforms that ensure efficient kinematic peaking of the beam current. In this case, stabilization of the microwave oscillation phase can be achieved by initiating a voltage prepulse, provided that the accelerating pulse leading edge has a segment of stable rise rate and stable shape.

Active ring resonance cavity assisted single-mode all-optical microwave oscillator

Abstract A pure optical microwave oscillator with single mode oscillation is proposed and demonstrated. This system has no any electrical microwave devices as well as electro-optic (E/O) and opto-electronic (O/E) conversion. The oscillation frequency is chosen by Brillouin-selective sideband amplification, and the optical envelope detection and feedback modulation are implemented by a semiconductor optical amplifier (SOA). As a high-Q tunable optical comb filter, an active ring resonance cavity (ARRC) is employed to select one oscillation mode within the Brillouin gain region, and consequently a single mode oscillation signal can be generated. In the experiment, a single mode photonic microwave signal with frequency of 10.83 GHz and side-mode suppression ratio of 48 dB is obtained. The measured phase noise is −88 dBc/Hz@10 kHz. In addition, the tunability is also demonstrated by adjusting the wavelength of the light source.

Advances in the accuracy, stability, and reliability of the PTB primary fountain clocks

Improvements of the systematic uncertainty, frequency instability, and long-term reliability of the two caesium fountain primary frequency standards CSF1 and CSF2 at PTB (Physikalisch-Technische Bundesanstalt) are described. We have further investigated many of the systematic effects and made a number of modifications of the fountains. With an optically stabilized microwave oscillator, the quantum projection noise limited frequency instabilities are improved to for CSF1 and for CSF2 at high atom density. The systematic uncertainties of CSF1 and CSF2 are reduced to and , respectively. Both fountain clocks regularly calibrate the scale unit of International Atomic Time (TAI) and the local realization of Coordinated Universal Time, UTC(PTB), and serve as references to measure the frequencies of local and remote optical frequency standards.

Microwave oscillator using piezoelectric thin-film resonator aiming for ultraminiaturization of atomic clock.

We developed a microwave oscillator and a micro electromechanical systems-based rubidium cell for the miniaturization of atomic clocks. A thin-film bulk acoustic resonator (FBAR) having a resonant frequency of the fundamental mode in the 3.5 GHz band was employed instead of a crystal resonator. It delivers a clock transition frequency of Rb atoms of 3.417 GHz without the need for a complicated frequency multiplication using a phase-locked loop. This topology considerably reduces the system scale and power consumption. For downsizing the atomic clock system toward the chip level as well as mass production, a microfabricated gas cell containing Rb and N2 gases was also developed. These microcomponents were incorporated into an atomic clock test bench, resulting in a clock operation with a short-term frequency instability of 2.1 × 10-11 at 1 s. To the best of our knowledge, this is the first report of a coherent population trapping clock operation using an FBAR-based microwave oscillator as well as a microfabricated gas cell.

Experimental investigation of an S-band triple cavity oscillator

The triple cavity oscillator (Tritron) is an improved high power microwave (HPM) oscillator from the dual cavity oscillator (Bitron), in which an intermediate cavity is introduced to enhance the beam modulation depth and improve the conversion efficiency by optimizing the phase between the modulated beam and the field in the intermediate cavity. There are only three cavities in the device, which makes it fit for developing compact HPM source. In this paper, an S-band Tritron is proposed and investigated experimentally. By means of adjusting the axial position of the support posts, the self-oscillation phenomenon observed in the experiment is efficiently suppressed without affecting the characteristics of the working mode. An output power of 1.8GW with the pulse width of 46ns and the frequency of 2.46GHz is obtained in the experiment under the condition of the 0.7T guiding magnetic field, and the axial length of the interaction region is only 11cm.

A real-time humidity sensor based on a microwave oscillator with conducting polymer PEDOT:PSS film

In this paper, we propose a humidity sensor based on a negative resistance oscillator using conducting polymer (CP) PEDOT:PSS film at room temperature. The proposed humidity sensor basically consists of a microstrip line and a negative resistance circuit, which creates a negative resistance for high-frequency oscillation. The microstrip line has a circuit structure that is connected through a via hole between the ground plane and the signal line with a CP thin film. From our experimental results, we find that the oscillation frequency of the sensor gradually decreases as the relative humidity (RH) increases. Owing to the conductivity variation of the CP film with the RH value, the oscillation frequency of the sensor sensitively changes in real time. Therefore, we suggest that our microwave-oscillator-based sensor scheme is a good candidate for the design of a robust and stable humidity sensor.

Low-Noise X-Band Tunable Microwave Generator Based on a Semiconductor Laser With Feedback

The stabilization of a relatively simple microwave oscillator tunable across the full X-band based on a laser subjected to optical feedback is achieved. Specifically, a resonance effect based on locking the two inherent dynamic frequencies of the system, as well as, optoelectronic feedback are utilized to achieve a sub-ps jitter and typical phase noise in the range of −107 dBc/Hz at 10 kHz offset from an oscillation frequency that can be tuned from 5.5 to 12.1 GHz. Further, the microwave signal is extracted from the laser-diode injection terminals and eliminates the need for multiple lasers, radio-frequency filters, and external RF sources. This architecture, therefore, realizes a compact and low-cost tunable microwave photonic oscillator.

Possible gyrotron operation in the “no start current” zone caused by the axial dependence of the phase of the resonator field

It is known that gyrotrons (as well as other electron beam driven microwave and millimeter-wave oscillators) can operate in the regime of either soft or hard self-excitation. In the regime of soft self-excitation, the beam current exceeds its starting value; thus, the oscillations can start to grow from the noise produced by electrons. In the regime of hard self-excitation, the beam current is less than its starting value. Therefore, for exciting the oscillations, a certain start-up scenario is required, which may include the variation of the mod-anode and/or beam voltage or the guiding magnetic field. It was found recently [O. Dumbrajs and G. S. Nusinovich, Phys. Plasmas 25, 013121 (2018)] that some gyrotrons can also operate in the region of magnetic fields where there is no start current at all. In the present paper, it is shown that this sort of operation can be attributed to the presence of the axial dependence of the phase of the resonator field.

Coupled-mode analysis for two-dimensional coaxial Bragg structures with helical ripples

Making use of field expansion and the equivalent boundary, an analytical model is set up to describe the multi-mode intercoupling in two-dimensional coaxial Bragg structures with either one or both of the conductors corrugated with helical ripples. The coupled-mode equations and the explicit formulae of the coupling coefficients for all possible mode combinations are derived, and the general mode coupling rules are discussed. Based on the analytical model, the dependence of the coupling coefficients on structural parameters of the two-dimensional coaxial Bragg structures with overmoded operation is investigated by numerical calculation. The results show that the additional conductor provides more flexibility for adjustment of coupling coefficients compared with other types of two-dimensional Bragg structures with a single conductor. Taking advantage of this feature, a two-dimensional coaxial Bragg structure can provide strong reflection with high mode purity under overmoded operation by suppressing the parasitic coupling using an optimized combination of corrugated outer and inner conductors. In this way, the structure has potential applications in the design of high-Q improved Bragg resonators for high-power microwave oscillators operating in the millimeter wave or terahertz range.

A Phase-Noise Reduced Microwave Oscillator Sensor With Enhanced Limit of Detection Using Active Filter

A microwave planar negative resistance oscillator with higher frequency stability and lower phase noise (PN) is introduced for sensing applications. Split-ring resonator is used as sensing element at 2.4 GHz and is used for microfluidic concentration sensing of high lossy solution of water in ethanol. Stabilizing the frequency response with the use of active filter leads to 14-times enhancement in the limit of detection, from 7% (passive filter) to 0.5% (active filter), while the linearity of the sensor is preserved; the sensing range is then increased. Nearly 13-dBc/Hz improvement in the measured PN, in great agreement with theory, also verifies the frequency domain stability of the oscillator.

A 45-GHz/20-kW Gyrotron-Based Microwave Setup for the Fourth-Generation ECR Ion Sources

A 45-GHz gyrotron-based microwave oscillator setup has been purposefully developed for powering of a superconducting electron cyclotron resonance (ECR) ion source. The setup provides a smoothly regulated output power in the 0.1–20-kW range (up to 26 kW in manual mode) with an efficiency of up to 50% and collector depression in both continuous wave (CW) and pulse modes. Pulse mode specifications: duration 5–200 ms, leading edge less than ${2}~\mu \text{s}$ , trailing edge less than ${1}~\mu \text{s}$ , repetition rate 1–10 Hz, and triggered operation using an external reference oscillator. The set of high-voltage power supplies (HVPSs) is equipped with a fast electronic protective system to ensure trouble-free operation of the gyrotron. The functional structure of the setup corresponds to the general design method for feedback-controlled microwave sources. The feedback loop consists of a power meter (or a detector unit, to the user’s choice), a control system, and an anode HVPS, the regulated voltage of which controls the output power of the gyrotron. The feedback characteristic time is less than 1 s. Feedback control is provided for both CW and pulse modes. The setup includes a unique quasi-optical line that transmits the microwave beam through a 2-m air gap, as well as a synthesized mode converter from the Gaussian beam to the TE01 or TE11 mode (to the user’s choice) installed on a 300-kV platform. During the tests, the developed setup proved to be a reliable and user-friendly tool applicable for long-term powering of the fourth-generation ECR ion sources.

Хаотизация колебаний СВЧ диапазона в микрополосковых антеннах-генераторах при воздействии низкочастотного шумового сигнала.

Хаотизация колебаний СВЧ диапазона в микрополосковых антеннах-генераторах при воздействии низкочастотного шумового сигнала.

Microstrip Antenna–Oscillators Integrated with a Waveguide Built in a Dielectric Substrate

The design of a microwave oscillator based on a microstrip log-periodic antenna integrated with a field effect transistor and a waveguide built in a dielectric substrate has been developed and analyzed. The waveguide geometry provides the possibility of propagation and radiation at both the fundamental frequency of the log-periodic antenna and harmonics. Computer simulation of the oscillator design in a frequency range near resonance frequencies of the log-periodic antenna is conducted. The possibility of summation of powers of several antenna–oscillators arranged ias a linear phased array is investigated.

Simple-structured, subfemtosecond-resolution optical-microwave phase detector.

We demonstrate a simple all-fiber photonic phase detector that can measure the phase (timing) difference between an optical pulse train and a microwave signal with subfemtosecond resolution and -60 dB-level amplitude-to-phase conversion coefficient. It is based on passive phase biasing of a Sagnac loop by the intrinsic phase shift of a symmetric 3×3 fiber coupler. By eliminating the necessity of magneto-optic components or complex radio frequency (RF) electronics for phase biasing of the Sagnac loop, this phase detector has potential to be implemented as an integrated photonic device as well. When using this device for synchronization between a 250MHz mode-locked Er-fiber laser and an 8GHz microwave oscillator, the minimum residual phase noise floor reaches <-154 dBc/Hz (at 8GHz carrier) with integrated root mean square (rms) timing jitter of 0.97fs [1Hz-1MHz]. The long-term rms timing drift and frequency instability are 0.92fs (over 5000s) and 4×10-19 (at 10,000s averaging time), respectively.

Observation of parity-time symmetry in microwave photonics

Symmetry plays a crucial role in explorations of the laws of nature. Parity-time (PT) symmetry phenomena can lead to entirely real spectra in non-Hermitian systems, which attracts considerable attention in the fields of optics and electronics because these phenomena provide a new tool for the manipulation of oscillation modes and non-reciprocal signal transmission. A potential new field of application is microwave photonics, an interdisciplinary field in which the interaction between microwaves and optical signals is exploited. In this article, we report the experimental use of PT symmetry in an optoelectronic oscillator (OEO), a key microwave photonics system that can generate single-frequency sinusoidal signals with high spectral purity. PT symmetry is theoretically analyzed and experimentally observed in an OEO with two mutually coupled active oscillation cavities via a precise manipulation of the interplay between gain and loss in the two oscillation cavities. Stable single-frequency microwave oscillation is achieved without using any optical/electrical filters for oscillation mode selection, which is an indispensable requirement in traditional OEOs. This observation opens new avenues for signal generation and processing based on the PT symmetry principle in microwave photonics.

A single chip electron spin resonance detector based on a single high electron mobility transistor

Single-chip microwave oscillators are promising devices for inductive electron spin resonance spectroscopy (ESR) experiments on nanoliter and subnanoliter samples. Two major problems of the previously reported designs were the large minimum microwave magnetic field (0.1–0.7 mT) and large power consumption (0.5–200 mW), severely limiting their use for the investigation of samples having long relaxation times and for operation at low temperatures. Here we report on the design and characterization of a single-chip ESR detector operating with a microwave magnetic field and a power consumption orders of magnitude lower compared with previous designs. These significant improvements are mainly due to the use of a high electron mobility transistor (HEMT) technology instead of a complementary metal-oxide semiconductor (CMOS) technology. The realized single-chip ESR detector, which operates at 11.2 GHz, consists of an LC Colpitts oscillator realized with a single high-electron mobility transistor and a co-integrated single turn planar coil having a diameter of 440 μm. The realized detector operates from 300 K down to 1.4 K, at least. Its minimum microwave magnetic field is 0.4 μT at 300 K and 0.06 μT at 1.4 K, whereas its power consumption is 90 μW at 300 K and 4 μW at 1.4 K, respectively. The experimental spin sensitivity on a sensitive volume of about 30 nL, as measured with a single crystal of α,γ-bisdiphenylene-β-phenylallyl (BDPA)/benzene complex, is of 8 × 1010 spins/Hz1/2 at 300 K and 2 × 109 spins/Hz1/2 at 10 K, respectively. In a volume of about 100 pL, located in proximity to the coil wire, the spin sensitivity improves by two orders of magnitude.

Numerical and experimental investigation of 4 mm wavelength microwave oscillator based on high-current compact accelerator

The concept of two-wave relativistic Cherenkov oscillator for the generation of nanosecond microwave pulses at ∼72 GHz central frequency has been applied. A moderately oversized sectioned slow wave structure (average diameter ≈2.5 wavelength) provides the interaction of electrons simultaneously with a slow (or surface) TM01 wave and a fast (or volume) TM02 wave, which govern the microwave energy output. The generation of 85 MW, 1.3-ns long microwave pulses has been demonstrated in a single pulse operation with a pulsed guiding magnetic field of 3.8 T on the base of the desktop high-current accelerator RADAN-303.