Microwave frequency transfer over a 112-km urban fiber link based on electronic phase compensation

We demonstrate the transmission of a microwave frequency signal at 10 GHz over a 112-km urban fiber link based on a novel simple-architecture electronic phase compensation system.The key element of the system is the low noise frequency divider by 4 to differentiate the frequency of the forward signal from that of the backward one,thus suppressing the effect of Brillouin backscattering and parasitic reflection along the link.In terms of overlapping Allan deviation,the frequency transfer instability of 4.2×10-15 at 1-s integration time and 1.6×10-18 at one-day integration time was achieved.In addition,its sensitivity to the polarization mode dispersion in fiber is analyzed by comparing the results with and without laser polarization scrambling.Generally,with simplicity and robustness,the system can offer great potentials in constructing cascaded frequency transfer system and facilitate the building of fiber-based microwave transfer network.

Optimal Microwave Radiation Field Parameters for Mercury Ion Microwave Frequency Standards

We propose a method to determine the optimal power of the microwave resonance transition that simultaneously improves the signal-to-noise ratio and reduces line width based on saturation broadening theory and experiment. Saturation broadening spectra of the ground state hyperfine transition of trapped 199Hg＋ ions are measured and analyzed. The value of the optimal microwave power is obtained by using the proposed method and is verified. Rabi oscillations decay spectra of trapped 199Hg＋ ions are observed and the optimal microwave irradiation time for the maximum transition signal intensity is determined. This work will help to improve the short-term frequency stability of the mercury ion microwave frequency standard.

Tunable and broadband microwave frequency combs based on a semiconductor laser with incoherent optical feedback

Based on a semiconductor laser （SL） with incoherent optical feedback, a novel all-optical scheme for generating tunable and broadband microwave frequency combs （MFCs） is proposed and investigated numerically. The results show that, under suitable operation parameters, the SL with incoherent optical feedback can be driven to operate at a regular pulsing state, and the generated MFCs have bandwidths broader than 40 GHz within a 10 dB amplitude variation. For a fixed bias current, the line spacing （or repetition frequency） of the MFCs can be easily tuned by varying the feedback delay time and the feedback strength, and the tuning range of the line spacing increases with the increase in the bias current. The linewidth of the MFCs is sensitive to the variation of the feedback delay time and the feedback strength, and a linewidth of tens of KHz can be achieved through finely adjusting the feedback delay time and the feedback strength. In addition, mappings of amplitude variation, repetition frequency, and linewidth of MFCs in the parameter space of the feedback delay time and the feedback strength are presented.

Effect of microwave frequency on plasma formation in air breakdown at atmospheric pressure

Microwave breakdown at atmospheric pressure causes the formation of a discrete plasma structure. The onedimensional fluid model coupling Maxwell equations with plasma fluid equations is used to study the effect of the microwave frequency on the formation of air plasma. Simulation results show that, the filamentary plasma array propagating toward the microwave source is formed at different microwave frequencies. As the microwave frequency decreases, the ratio of the distance between two adjacent plasma filaments to the corresponding wavelength remains almost unchanged（on the order of 1/4）, while the plasma front propagates more slowly due to the increase in the formation time of the new plasma filament.

Controllable microwave frequency comb generation in a tunable superconducting coplanar-waveguide resonator

Frequency combs are useful in a wide range of applications,such as optical metrology and high-precision spectroscopy.We experimentally study a controllable frequency comb generated in a tunable superconducting coplanarwaveguide resonator in the microwave regime.A two-tone drive is applied on one of the resonance modes of the resonator and comb generation is observed around the resonance frequency of the resonator.Both central frequency and teeth density of the comb are precisely controllable,and the teeth spacing can be adjusted from Hz to MHz.Moreover,we show that a few hundreds of sidebands can be generated using a sufficiently strong drive power and the weakest drive power needed to generate the comb can be reduced to approach the quantum limit.These experimental results can be qualitatively explained via theoretical analysis.

Experimental investigation of evaporative cooling mixture of bosonic ^(87)Rb and fermionic ^(40)K atoms with microwave and radio frequency radiation

We investigate sympathetic cooling fermions 40K by evaporatively cooling bosonic 87Rb atoms in a magnetic trap with microwave and radio frequency induced evaporations in detail. The mixture of bosonic and fermionic atoms is prepared in their polarized spin states IF = 9/2, mF = 9/2） for 40K and IF = 2, mF=2〉 for 87Rb, which is trapped in Quadrupole-Ioffe-Configuration trap. Comparing microwave with radio frequency evaporatively cooling bosonic STRb atoms with sympathetically cooling Fermi gas 40K, we find that the presence of rubidium atoms in the [2, 1} Zeeman states, which are generated in the evaporative process, gives rise to a significant loss of 40K due to inelastic collisions. Thus, the rubidium atoms populated in the [2, 1} Zeeman states should be removed in order to effectively perform sympathetically cooling 40K with the evaporatively cooled STRb atoms.

Dick Effect in a Microwave Frequency Standard Based on Laser-Cooled 113Cd+ Ions

The Dick effect is one of the main limits to the frequency stability of a passive frequency standard, especially for the fountain clock and ion dock operated in pulsed mode which require unavoidable dead time during interrogation. Here we measure the phase noise of the interrogation oscillator applied in the microwave frequency standard based on laser-cooled 113^Cd＋ ions, and analyze the Allan deviation limited by the Dick effect. The results indicate that the Dick effect is one of the key issues for the cadmium ion dock to reach expected frequency stability. This problem can be resolved by interrogating the local oscillator continuously with two ion traps.

Dependence of Switching Current Distribution of a Current-Biased Josephson Junction on Microwave Frequency

We investigate the distribution of the switching current of a current-biased Josephson junction （CBJJ） and its dependence on the microwave frequency using two theoretical methods, one of which is the quantum trajectory method and the other is the master equation method. Both the methods show that the distribution of the switching current of CBJJ will exhibit double peaks in a certain range of microwave frequency if proper microwave power is given, and the gap between the two peaks will increase with the microwave frequency. The obtained results can be used to identify the energy difference of the ground and first excited states in a Josephson junction for any bias current.

Tunable off-resonant Rydberg microwave frequency comb spectroscopy based on metawaveguide coupled Rydberg atoms

Studying Rydberg microwave frequency comb(MFC)spectroscopy helps increase the working bandwidth of the Rydberg receiver.This Letter demonstrates off-resonant Rydberg MFC spectroscopy in a meta-waveguide-coupled Rydberg atomic system.An off-resonant MFC field couples with the Rydberg atoms through a meta-waveguide.The system can receive the microwave field in the working band from 0.5 GHz to 13.5 GHz,and the MFC spectroscopy covers a span of 36 MHz at three different arbitrarily-chosen frequencies of 2 GHz,3 GHz,and 5.8 GHz.The MFC spectrum that covers a wide range of 125 MHz is also verified.This work is significant for tunable wide-band instant microwave signal detection in the Rydberg atomic system,which is useful in microwave frequency metrology,communication,and radar.

Magnetic sensitivity technology based on microwave modulation of solid state spin system NV center

In view of the low resolution and accuracy of traditional magnetometer,a method of microwave frequency modulation technology based on nitrogen-vacancy(NV)center in diamond for magnetic detection was proposed.The magnetometer studied can reduce the frequency noise of system and improve the magnetic sensitivity by microwave frequency modulation.Firstly,ESR spectra by sweeping the microwave frequency was obtained.Further,the microwave frequency modulated was gained through the mixed high-frequency sinusoidal modulation signal generated by signal generator.In addition,the frequency through the lock-in amplifier was locked,and the signal which was proportional to the first derivative of the spectrum was obtained.The experimental results show that the sensitivity of magnetic field detection can reach 17.628 nT/Hz based on microwave frequency modulation technology.The method realizes high resolution and sensitivity for magnetic field detection.

Investigation on latch-up susceptibility induced by high-power microwave in complementary metal–oxide–semiconductor inverter

The latch-up effect induced by high-power microwave（HPM） in complementary metal–oxide–semiconductor（CMOS） inverter is investigated in simulation and theory in this paper. The physical mechanisms of excess carrier injection and HPM-induced latch-up are proposed. Analysis on upset characteristic under pulsed wave reveals increasing susceptibility under shorter-width pulsed wave which satisfies experimental data, and the dependence of upset threshold on pulse repetitive frequency（PRF） is believed to be due to the accumulation of excess carriers. Moreover, the trend that HPMinduced latch-up is more likely to happen in shallow-well device is proposed.Finally, the process of self-recovery which is ever-reported in experiment with its correlation with supply voltage and power level is elaborated, and the conclusions are consistent with reported experimental results.

Permittivity and its temperature dependence in hexagonal structure BN dominated by the local electric field

This paper presents an analysis of the local electric field in hexagonal boron nitride （h-BN） by introducing a modified parameter. Based on the determination of the modified parameter of h-BN, the revised Lorenz equation is developed. Then the permittivity at high temperature and in the microwave frequency is investigated. In addition, this equation is derived for evaluating the temperature coefficient of the permittivity of h-BN. The analyses show that the permittivity increases with increasing temperature, which is mainly attributed to the positive temperature coefficient of the ionic polarizability.

Microwave Reflectometry on HL-2A Tokamak

One of the most important electron density diagnostics, microwave reflectometry, has been developed on many large and medium nuclear fusion devices in recent years . Not only the electron density profiles with high temporal and spatial resolutions, but also the profiles of plasma rotation and turbulence can be obtained with this diagnostic system.

Photonic-assisted approach for instantaneous microwave frequency measurement with tunable range by using Mach-Zehnder interferometers

A novel photonic-assisted approach to microwave frequency measurement is proposed and experimentally demonstrated. The proposed scheme is based on the frequency-to-power mapping with different transmis- sion responses. A polarizer is used in one output branch of a phase modulator to simultaneously implement phase modulation and intensity modulation. Owing to the complementary nature of the transmission re- sponses and the Mach-Zehnder interferometers （MZIs）, this scheme theoretically provides high resolution and tunable measurement range. The measurement errors in the experimental results can be kept within 0.2 GHz over a freauencv ranee from 0.1 to 5.3 GHz.

Ultra-flat broadband micro wave frequency comb generation based on optical frequency comb with a multiple-quantumwell electro-absorption modulator in critical state

In this paper,we proposed a novel ultra・flat broadband microwave frequency comb(MFC)generation based on optical frequency comb(OFC)with a multiplequantum-well electroabsorption modulator(MQW-EAM)in critical state.The scheme is simple and easy to adjust.The performance of the MFC generation scheme is investigated using software Optisystem.The results show that the comb spacing of MFC can be adjusted from 5 to 20 GHz by changing RF signafs frequency and the MFC is almost independent on the linewidth of the tunable laser diode.The performance of the MFC can be improved by reasonably increasing the voltage of the RF,the smallsignal gain of the Erbium-doped fiber amplifier(EDFA)and the responsivity of the photodetector.The MFC generated by this scheme has 300 GHz effective bandwidth with 15 comb lines,whose power variation is 0.02 dB,when the components9 parameters in the Optisystem are set as follows:the power of tunable laser diode(TLD)is 0 dBm,the wavelength is 1552.52 nm,and linewidth is 1 MHz;RF signaPs frequency is 20 GHz and the voltage is 10 V;the reverse bias voltage of MQW-EAM is 6.92 V;the small-signal gain of the EDFA is 40 dB;the responsivity of the photodetector(PD)is 1 A/W.

Microwave frequency upconversion employing a coupling-modulated ring resonator

We present a method to generate a frequency-doubled microwave signal by employing a coupling-modulated ring resonator. Critical coupling is achieved when the resonator intrinsic loss is perfectly balanced by the external coupling enabled by a Mach–Zehnder interferometer coupler. The high suppression of the carrier leads to a clean two-tone optical signal with the frequency interval two times larger than that of the input microwave frequency.The beating of the two-tone signal at a photodiode generates the frequency upconverted microwave signal.A theoretical model is established to analyze the modulation process and the microwave signal generation.Experimental results show that the electrical harmonic suppression ratio is around ~20 dB(29 dB) for an input microwave signal with 5 dBm(10 dBm) power.

Novel photonic approach to microwave frequency measurement using tunable group delay line

A photonic approach for measuring microwave frequency over a wide bandwidth is proposed. An optic group delay line composed of several magneto-optical switches and a 1.6-km single-mode fiber is used as a tunable dispersive medium in the measurement setup. A minimum frequency accuracy of 80 MHz in the range of 1-20 GHz is achieved experimentally.

Instantaneous microwave frequency measurement based on hybrid microwave photonic filter

A novel photonic technique for instantaneous microwave frequency measurement based on hybrid microwave photonic filter(HMPF) is proposed and experimentally demonstrated. The HMPF is composed of an all-pass filter and a band-pass filter with negative coefficients. By properly controlling the power relationship between the all-pass filter and the band-pass filter, the HMPF can realize a monotonic frequency response, and then a unique relationship between the output power and the input frequency is established. A high measurement resolution can be achieved for a given frequency range.

Self-mode-locking optoelectronic oscillator with ultrashort time delay

The optoelectronic oscillator(OEO)is a typical time-delay system with rich nonlinear dynamical characteristics.Most of the previous research on OEOs has been focused on analyzing the properties of OEOs with a long time delay,which makes it difficult to realize mode locking without additional phaselocking mechanisms.We have achieved,for the first time to our knowledge,a self-mode-locking OEO and generated stable microwave frequency combs by analyzing the characteristics of OEOs with an ultrashort time scale.In the experiment,the self-mode-locking OEOs with fundamental mode,second-order harmonic,and sixth-order harmonic were realized by adjusting the system parameters,all of which produced uniform square wave signals with tunable duty cycles,steep rising and falling edges,and periods of less than 20 ns.The self-fundamental-mode-locking OEOs with different time delays were also implemented and experimentally realized.Furthermore,the experiment revealed the self-hybrid mode-locking OEO,which is the coexistence and synchronization of the three measured self-locking modes in one OEO cavity,demonstrating the complex nonlinear dynamical behaviors of the OEO system and enabling the generation of periodic nonuniform hybrid square wave signals.The realization of the self-mode-locking OEO and the generation of flexible and stable square wave signals at ultrashort time scales enrich the study of OEO nonlinear dynamics in the realm of complex microwave waveform generation,offering promising applications in areas such as atomic clocks,radars,communications,and optoelectronic neural networks.

PTX-symmetric metasurfaces for sensing applications

In this paper,we introduce an ultra-sensitive optical sensing platform based on the parity-time-reciprocal scaling(PT^-symmetric non-Hermitian metasurfaces,which leverage exotic singularities,such as the exceptional point(EP)and the coherent perfect absorber-laser(CPAL)point,to significantly enhance the sensitivity and detectability of photonic sensors.We theoretically studied scattering properties and physical limitations of the PTX-symmetric metasurface sensing systems with an asymmetric,unbalanced gain-loss profile.The PTLY-symmetric metasurfaces can exhibit similar scattering properties as their Pr-symmetric counterparts at singular points,while achieving a higher sensitivity and a larger modulation depth,possible with the reciprocal-scaling factor(i.e.,X transformation).Specifically,with the optimal reciprocalscaling factor or near-zero phase offset,the proposed PTX-symmetric metasurface sensors operating around the EP or CPAL point may achieve an over 100 dB modulation depth,thus paving a promising route toward the detection of small-scale perturbations caused by,for example,molecular,gaseous,and biochemical surface adsorbates.