Several Tens Of Gigahertz Research Articles

Mechanically Actuated Kerr Soliton Microcombs

AbstractMode‐locked ultrashort pulse sources with a repetition rate of up to several tens of gigahertz greatly facilitate versatile photonic applications such as frequency synthesis, metrology, radar, and optical communications. Dissipative Kerr soliton microcombs provide an attractive solution as a broadband, high‐repetition‐rate compact laser system in this context. However, its operation usually requires sophisticated pump laser control to initiate and stabilize the soliton microcombs, particularly in millimeter‐sized ultrahigh‐Q whispering‐gallery resonators. Here, a mechanically actuated soliton microcomb oscillator is realized with a microwave repetition rate of 15 GHz. This enables direct initiation, subsequent long‐term stabilization, and fine tuning of soliton combs without pump laser tunability that is generally required for soliton microcomb operation. The prospects for using this method with a wide range of applications are revealed that will benefit from mechanical soliton actuation such as optical clocks, spectral extension, and dual‐comb spectroscopy.

High-frequency resolution diamond nitrogen-vacancy center wide-spectrum imaging technology

High-resolution wide-spectrum measurement techniques have important applications in fields such as astronomy, wireless communication, and medical imaging. Nitrogen-vacancy (NV) center in diamond is well known for its high stability, high sensitivity, real-time monitoring, single-point detection, and suitability for long-term measurement, and has an outstanding choice for spectrum analyzers. Currently, spectrum analyzers based on NV centers as detectors can perform real-time spectrum analysis in the range of several tens of gigahertz, but their frequency resolution is limited to a MHz level. In this study, we construct a quantum diamond microwave spectrum imaging system by combining continuous wave-mixing techniques. According to the spin-related properties of the NV center in diamond, we implement optical pumping by 532 nm green laser light illuminating the diamond NV center. A spherical magnet is used to produce a magnetic field gradient along the direction of the diamond crystal. By adjusting the size and direction of the magnetic field gradient, spatial encoding of the resonance frequency of the NV center is achieved. The magnetic field gradient induces the Zeeman effect on the diamond surface at different positions, generating corresponding ODMR signals. Through accurate programming, we coordinate the frequency scanning step size of the microwave source with the camera exposure and image storage time, and synchronize them circularly according to the order of image acquisition. Ultimately, after algorithmic processing, we successfully obtain comprehensive spectrum data in a range from 900 MHz to 6.0 GHz. Within the measurable spectrum range, the system employs continuous wave-mixing, simultaneously applying resonant microwaves and slightly detuning auxiliary microwaves to effectively excite the NV center. This method triggers off microwave interference effects, disrupting the balance between laser-induced polarization and microwave-induced spontaneous relaxation. Specifically, microwave interference causes the phase and amplitude of the fluorescence signal to change, leading to the generation of alternating current fluorescence signals. This further enhances the response of the NV magnetometer to weak microwave signals. The method enables the system to achieve a frequency resolution of 1 Hz in the measurable spectrum range, and it can separately measure the frequency resolution of multiple frequency points with a frequency step size of 1 MHz. The research results indicate that the wide-spectrum measurement based on NV centers can achieve sub-hertz frequency resolution, providing robust technical support for future spectrum analysis and applications.

High-speed metasurface modulator using perfectly absorptive bimodal plasmonic resonance

Free-space electro-optic (EO) modulators operating at gigahertz and beyond are attractive for a wide range of emerging applications, including high-speed imaging, free-space optical communication, microwave photonics, and diffractive computing. Here, we experimentally demonstrate a high-speed plasmonic metasurface EO modulator operating in a near-infrared wavelength range with a gigahertz modulation bandwidth. To achieve efficient intensity modulation of reflected light from an ultrathin metasurface layer, we utilize the bimodal plasmonic resonance inside a subwavelength metal–insulator–metal grating, which is precisely tuned to satisfy the critical coupling condition. As a result, perfect absorption of −27 dB (99.8%) and a high quality (Q) factor of 113 are obtained at a resonant wavelength of 1650 nm. By incorporating an EO polymer inside the grating, we achieve a modulation depth of up to 9.5 dB under an applied voltage of ±30 V. The 3-dB modulation bandwidth is confirmed to be 1.25 GHz, which is primarily limited by the undesired contact resistance and the output impedance of the driver. Owing to the high electrical conductivity of metallic gratings and a compact device structure with a minimal parasitic capacitance, the demonstrated device can potentially operate at several tens of gigahertz, which opens up exciting opportunities for ultrahigh-speed active metasurface devices in various applications.

Xilinx RFSoC를 사용한 레이다 펄스 신호 실시간 스펙트럼 탐색기 개발

In this paper, we present a design and implementation method for a high-speed spectrum analyzer that can detect the spectrum of a received radar pulse signal in real time. To respond to an enemy’s target search or tracking in an electronic warfare environment, it is first necessary to quickly detect the enemy radar signal. Signal search is mainly performed through spectrum analysis, and radar signals exist over a wide band of several tens of gigahertz (GHz). Therefore, signal detection is performed by dividing the entire band into several subbands and searching sequentially or in parallel. Because the search time can be reduced as the subband becomes wider, a searcher capable of processing the wideband in real time is required. In this paper, we design a digital searcher capable of detecting radar pulse signals within a 1 GHz bandwidth in real time and present a hardware implementation method. The requirements for real-time spectrum search for the hardware used are also presented. In the proposed method, the received signal is converted to digital through a high-speed analog-to-digital converter, and the spectrum of the signal can be searched in real time by performing an FFT that reflects the characteristics of the pulse signal. The spectrum analyzer was implemented on a Xilinx RFSoC board and its performance was examined.

МЕТОД ГЕНЕРАЦИИ СВЕРХШИРОКОПОЛОСНЫХ СВЧ-СИГНАЛОВ С ЛИНЕЙНО-ЧАСТОТНОЙ МОДУЛЯЦИЕЙ НА ОСНОВЕ САМОГЕТЕРОДИНИРОВАНИЯ ИЗЛУЧЕНИЯ ЛАЗЕРНОГО ДИОДА

In this paper, microwave photonic technique of ultrawideband linearly frequency modulated signals generation is proposed. The proposed method based on direct modulation of the laser diode current, which provides a periodic change in the emission frequency. The Michelson interferometer summarizes laser signal and its replica with time delay exceeding 10 ms, which, in turn, is defined by time difference between arms. Optoelectronic conversion is performed in high-speed high-power photodiode. The proposed approach makes it possible to generate microwave signals with linear frequency modulation in the frequency range up to several tens of gigahertz by means of laser pump current modulation in the frequency range less than 100 MHz. The method of ultra-wideband microwave signals generation is implemented using a Michelson fiber-optic interferometer with Faraday mirrors. Such interferometer topology provides a doubling of the path length, and reduced gyroscopic effects due to a counter-propagating double pass. Changing the parameters of the generated microwave ultrawideband signals with frequency modulation is achieved by pump current shape and amplitude control. It is also proposed to use real-time oscilloscope without need of changing of the internal structure of such generator to pre-tune it. Time-frequency analysis is used to extract frequency versus time dependence. By so doing, direct impulse response estimation is performed. This method allows to characterize laser diode frequency response for arbitrary microwave waveform generation by means of pump current waveform calculation from experimental data.

Radio absorption in high-mass gamma-ray binaries

High-mass γ-ray binaries consist of a presumptive pulsar in orbit with a massive star. The intense outflows from the star can absorb radio emission from the pulsar, making the detection of pulsation difficult. In this work, we present the basic geometry and formulae that describe the absorption process of a pulsar in binary with an O/B star and apply our model to two typical and well-studied binaries: PSR B1259−63/LS 2883 and LS 5039. We investigate the influences of the equatorial disc of LS 2883 with different orientations on the dispersion measure and free-free absorption of the radio pulsation from PSR B1259−63. The observed data are consistent with the disc inserted on the orbital plane with a relatively large inclination angle. For LS 5039, due to its tight orbit, it was believed that the strong wind absorption makes detecting radio emissions from the putative pulsar unlikely. However, considering the wind interaction and orbital motion, a bow shock cavity and a Coriolis shock would be formed, thereby allowing the pulsations to partially avoid stellar outflow absorption. We investigate the dependence of the radio optical depth on the observing frequencies, the orbital inclination angle, and the wind parameters. We suppose that the presumptive pulsar in LS 5039 is similar to PSR B1259−63 with pulsed emission extending to several tens of gigahertz. In that case, there could be a transparent window for radio pulsations when the pulsar is moving around the inferior conjunction. The following deep monitoring of LS 5039 and other systems by radio telescopes at high radio frequencies might reveal the nature of compact objects in the future. Alternatively, even a null detection could still provide further constraints on the properties of the putative pulsar and stellar outflows.

Determination of the Contribution of the Ground-Shield Losses to the Microwave Performance of On-Chip Coplanar Waveguides

In this article, we characterize and model two parasitic effects that become apparent in the performance of coplanar waveguide interconnects in CMOS. One is the transverse resistance introduced by a patterned ground shield in coplanar waveguide interconnects, which significantly contributes to the shunt losses. The other one is the parasitic coupling between the input and output ports through the ground shield. The latter effect is particularly accentuated in relatively short lines and complicates the determination of the propagation constant using line–line algorithms at several tens of gigahertz. We demonstrate that using the proposed methodology, excellent model–experiment correlation can be achieved in the modeling of these types of interconnects up to at least 60 GHz.

Far Off-Resonance Laser Frequency Stabilization Technology

In atomic physics experiments, a frequency-stabilized or ‘locked’ laser source is commonly required. Many established techniques are available for locking close to an atomic resonance. However, in many instances, such as atomic magnetometer and magic wavelength optical lattices in ultra-cold atoms, it is desirable to lock the frequency of the laser far away from the resonance. This review presents several far off-resonance laser frequency stabilization methods, by which the frequency of the probe beam can be locked on the detuning as far as several tens of gigahertz (GHz) away from atomic resonance line, and discusses existing challenges and possible future directions in this field.

Picosecond-Scale Terahertz Pulse Characterization With Field-Effect Transistors

Precise real-time detection of terahertz (THz) pulses is a key requirement for characterization of pulsed THz sources and nondestructive testing applications. We experimentally evaluate the speed limits of THz rectification in field-effect transistors (FETs) using the example of pulses from a free-electron laser (FEL). We develop an improved model for the description of these THz pulses and demonstrate its validity experimentally by comparison to spectroscopic data as well as to expectation values calculated from FEL physics. The model in conjunction with the high speed of the detectors permits the detection of an exponential rise time of the pulses as short as 5 ps despite a Gaussian postdetection time constant of 11 and 14 ps for a large area and an antenna-coupled detector, respectively. This proves that FETs are excellent compact, room-temperature THz detectors for applications that require an intermediate frequency bandwidth of several tens of gigahertz.

Tunable all-optical actively mode-locked fiber laser at 2 µm based on tellurite photonic crystal fiber

Although thulium-doped fiber lasers operating at 2 µm have been intensively investigated, lasers with high repetition rate (up to several tens of gigahertz) and tunable output characteristics are difficult to achieve at this band. In this paper, an all-optical actively mode-locked fiber laser is proposed for this purpose. By employing a section of specially-designed highly nonlinear group-velocity-matched tellurite photonic crystal fiber as the mode-locker, the proposed thulium-doped fiber laser can easily produce stable pulses with tunable pulse width and repetition rate up to 40 GHz. Moreover, through investigating the pulse dynamics in the cavity, a transfer-limited soliton pulse is successfully achieved. The proposed laser can find application in future optical fiber communication systems at 2 µm.

Modeling and Parameter Extraction for the Metal Surface Roughness Loss Effect on Substrate Integrated Waveguides From S-Parameters

This paper presents a new simple model for representing and characterizing the loss effect introduced by the roughness of the metal-to-dielectric interface on the conductor attenuation of substrate integrated waveguides. The proposal allows for the parameter determination using simple linear regressions which can be implemented directly from S-parameter data and avoids the use of data corresponding to structures with perfectly smooth conductors. In fact, no previous knowledge of the metal surface profile is required to obtain the model parameters. Excellent agreement between full-wave simulations and experimental data at several tens of gigahertz is achieved.

Optimization of SFQ Logic Gate Considering Dependence of its Signal Propagation Delay on the Bias Voltage

Superconductive single flux quantum (SFQ) digital circuits can operate at a clock frequency of several tens of gigahertz. The operating margin of an SFQ logic circuit generally decreases with an increase in the operating frequency because of the timing error in the low-bias region caused by the difference in bias dependence of the signal-propagation delay between the data and clock lines. We proposed an improvement in the operating margin by controlling the dependence of the signal-propagation time on the bias voltage. In the present study, we investigated a new optimization method for an SFQ logic gate. We developed a new circuit parameter optimizer, which takes into account not only the bias margin but also the dependence of the signal-propagation delay on the bias voltage. We defined an appropriate evaluation function for the optimization. From the result of the optimization using the defined evaluation function, we designed and demonstrated an SFQ AND gate optimized using the new optimizer.

High-power high-repetition-rate subpicosecond monolithic Yb:KGW laser with self-mode locking

We report on a high-power subpicosecond monolithic self-mode-locked Yb:KGW laser with the pulse repetition rate up to several tens of gigahertz. Experimental results reveal that not only the repetition rate but also the pulse width depend on the length of the laser crystal. Using a coated Yb:KGW crystal with a length of 3.36 mm, mode-locked pulses with pulse duration of 850 fs at the repetition rate of 22.4 GHz have been achieved. With an incident pump power of 10.5 W, an average output power of 3.6 W was achieved which corresponds to the optical conversion efficiency of 34.3%.

Monolithically Integrated DFB Lasers for Tunable and Narrow Linewidth Millimeter-Wave Generation

A monolithic optoelectronic device for the generation of tunable and narrow linewidth millimeter-wave signals is presented. The device consists of three mutually injected distributed feedback lasers whose emission frequencies are stabilized via a four-wave mixing process. Their beating on a high-speed photodetector generates a narrow linewidth electrical signal that is continuously tunable over several tens of gigahertz.

Improvement of Operating Margin of SFQ Circuits by Controlling Dependence of Signal Propagation Time on Bias Voltage

Superconductive single flux quantum (SFQ) digital circuits can operate at a clock frequency of several tens of gigahertz. However, the operating margin of these circuits decreases with an increase in the operating frequency because a timing error occurs in the low bias region. In this study, a novel design method that enables a wide operating margin at a high operating frequency has been investigated. The proposed circuits incorporate an additional bias feeding line in addition to the conventional bias feeding lines of the conventional Josephson transmission lines (JTLs) and can control the dependence of signal propagation time on the bias voltage. We have shown experimentally that in our proposed JTLs, the signal propagation time becomes more sensitive to the bias voltage. Timing errors can be avoided by inserting proposed JTL cells in the critical data path of the SFQ digital circuits. Circuit simulation results indicate that the operating margin of a bit-serial SFQ full adder, designed assuming the 2.5 kA/cm2 Nb process, can be improved by 15% compared with the conventional design at a frequency of 20 GHz by employing our novel design method.

Chaos Synchronization and Communication in Mutually Coupled Semiconductor Lasers Driven by a Third Laser

We numerically investigate the chaos synchronization and message transmission between two mutual coupling semiconductor lasers (MCSLs) subject to identical unidirectional injections (UIs) from an external cavity semiconductor laser (ECSL). The synchronization between the MCSLs is realized through injection locking in conjunction with symmetric operation. The simulation results show that stable isochronal synchronization between the MCSLs can be achieved under proper driving injections. This type of synchronization is robust to parameter mismatch up to tens of percentage and frequency detuning of several tens of gigahertz, which is much better than those of the MCSLs systems with self-feedback. Moreover, the investigations on the mutual chaos pass filtering effects and the message transmission indicate that the isochronal synchronization allows mutual message exchange with a bit rate higher than 10 Gb/s, when the chaos modulation technology is adopted. In addition, the MCSLs can synchronize with the ECSL for proper UI and MC, which provides an opportunity for array chaos synchronization and chaos communication networks. Finally, leader/laggard synchronization can also be achieved in the proposed system.

Terahertz and infrared photodetection using p-i-n multiple-graphene-layer structures

We propose to utilize multiple-graphene-layer structures with lateral p-i-n junctions for terahertz and infrared (IR) photodetection and substantiate the operation of photodetectors based on these structures. Using the developed device model, we calculate the detector dc responsivity and detectivity as functions of the number of graphene layers and geometrical parameters and show that the dc responsivity and detectivity can be fairly large, particularly, at the lower end of the terahertz range at room temperatures. Due to relatively high quantum efficiency and low thermogeneration rate, the photodetectors under consideration can substantially surpass other terahertz and IR detectors. Calculations of the detector responsivity as a function of modulation frequency of THz and IR radiation demonstrate that the proposed photodetectors are very fast and can operate at the modulation frequency of several tens of gigahertz.

Low-Voltage High-Speed (18 GHz/1 V) Evanescent-Coupled Thin-Film-Ge Lateral PIN Photodetectors Integrated on Si Waveguide

This letter presents the device performance of scaled thin-film-Ge lateral PIN photodetectors integrated on a Si waveguide. The photodetectors are with closely spaced p+/n+ regions (0.8 mum) on a Ge region with short length (5-20 mum) and narrow width (2.4 mum). Though with a thin Ge layer (~220 nm including bottom SiGe buffer), light is evanescent-coupled from the Si waveguide effectively to the overlying Ge detector. The device exhibits f3 dB bandwidth of 18 GHz with external responsivity of 0.13 A/W for 1550 nm at -1 V. Considering the coupling loss and waveguide loss, the internal responsivity is as high as 0.65 A/W. It is shown that with increasing detector length, the devices' internal quantum efficiency can be improved to ~90% and by suppressing parasitic effects, speed can be boosted further towards several tens of gigahertz.

Radiation admittance of an open‐ended parallel plate metallic waveguide with exterior planar dielectric coating

A methodology is presented for the calculation of the radiation admittance of an open‐ended, homogeneously filled, parallel plate metallic waveguide with exterior planar dielectric coating, operated at the fundamental (transverse electromagnetic wave) mode. The methodology relies on the definition of an auxiliary electromagnetic boundary value problem that is simple to solve using an integral equation approach. In order to provide for broadband calculation of the radiation admittance of the open‐ended waveguide from DC to several tens of gigahertz, a loop‐tree decomposition scheme is used in the method of moments solution of the resulting integral equation. A rational function‐fitting process with passivity enforcement is used to generate a broadband, passive macromodel for the calculated radiation impedance. Such a macromodel is suitable for use as a local truncation boundary condition in the finite difference/finite element modeling of electromagnetic interactions in multilayered signal and power distribution networks encountered in printed circuit boards and packages.

Theory and illustration of the three-dimensional elasto-optic interaction in picosecond ultrasonics

The three-dimensional (3D) photoelastic interaction involved in the detection mechanism of picosecond ultrasonics is investigated in micrometric metallic films. In pump-probe experiments, the laser source beam is focused to a spot size less than 1 µm. A 3D diffracted acoustic field is generated at high frequencies of several tens of gigahertz, containing longitudinal and shear waves altogether. Their propagation changes the dielectric permittivity tensor and the material becomes optically heterogeneous. Consequently, the detection process is modeled through the propagation of the laser probe beam in a material with dielectric properties varying in all directions. Thus, the solving of Maxwell equations leads to a differential system, the source term of which is proportional to the acoustic field itself. The scattered electromagnetic field is described using the matricant and the ensuing analytical solution allows then analyzing the 3D photoelastic interaction. Good agreement with experiments performed in a 1 µm thick aluminum film is found.