Microwave Photonics for Modern Radar Systems

The emerging new concepts and technologies based on microwave photonics have led to an ever-increasing interest in developing innovative radar systems with a net gain in functionality,bandwidth /resolution,size,mass,complexity and cost when compared with the traditional implementations. This paper describes the techniques developed in the last few years in microwave photonics that might revolutionize the way to design multifunction radar systems,with an emphasis on the recent advances in optoelectronic oscillators( OEOs),arbitrary waveform generation,photonic mixing,phase coding,filtering,beamforming,analog-to-digital conversion,and stable radio-frequency signal transfer. Challenges in implementation of these components and subsystems for meeting the technique requirements of the multifunction radar applications are discussed.

Advance in Microwave Photonics

Microwave photonics is a combination of microwave and photonics in concepts,devices and systems.Its typical research involves optical generation,processing and conversion of microwave signals,as well as distribution and transmission of microwave signals on optical links.Research achievements of microwave photonics have promoted the development of some new technologies,including Radio over Fiber (RoF) communication,subcarrier multiplexing and fiber transmission in Cable Television (CATV) system,optical controlled beam forming network with phased array radar and measurement technologies in microwave frequency domain.

Temperature-insensitive fiber-optic refractive index sensor based on cascaded in-line interferometer and microwave photonics interrogation system

A compact and high-resolution fiber-optic refractive index(RI)sensor based on a microwave photonic filter(MPF)is proposed and experimentally validated.The sensing head utilizes a cascaded in-line interferometer fabricated by an input single-mode fiber(SMF)tapered fusion with no-core fiber-thin-core fiber(TCF)-SMF.The surrounding RI(SRI)can be demodulated by tracing the passband’s central frequency of the MPF,which is constructed by the cascaded in-line interferometer,electro-optic modulator,and a section of dispersion compensation fiber.The sensitivity of the sensor is tailorable through the use of different lengths of TCF.Experimental results reveal that with a 30 mm length of TCF,the sensor achieves a maximum theoretical sensitivity and resolution of-1.403 GHz∕refractive index unit eRIUT and 1.425×10^(-7) RIU,respectively,which is at least 6.3 times higher than what has been reported previously.Furthermore,the sensor exhibits temperature-insensitive characteristics within the range of 25℃-75℃,with a temperatureinduced frequency change of only±1.5 MHz.This value is significantly lower than the frequency change induced by changes in the SRI.The proposed MPF-based cascaded in-line interferometer RI sensor possesses benefits such as easy manufacture,low cost,high resolution,and temperature insensitivity.

Recent advances in microwave photonics

Microwave photonics （MWP） is an interdisci- plinary field that combines two different areas of microwave engineering and photonics. It has several key features by transferring signals between the optical domain and microwave domain, which leads to the advantages of broad operation bandwidth for generation, processing and distribution of microwave signals and high resolution for optical spectrum measurement. In this paper, we comprehensively review past and current status of MWP in China by introducing the representative works from most of the active MWP research groups. Future prospective is also discussed fi＇om the national strategy to key enabling technology that we have developed.

Microwave photonics connected with microresonator frequency combs

Microresonator frequency combs （microcombs） are very promising as ultra-compact broadband sources for microwave photonic applications. Conversely, microwave photonic techniques are also employed inten- sely in the study of microcombs to reveal and control the comb formation dynamics. In this paper, we reviewed the microwave photonic techniques and applications that are connected with microcombs. The future research directions of microcomb-based microwave photonics were also discussed.

Broadband photonic ADC for microwave photonics-based radar receiver

A broadband photonic analog-to-digital converter（ADC） for X-band radar applications is proposed and experimentally demonstrated. An X-band signal with arbitrary waveform and a bandwidth up to 2 GHz can be synchronously sampled and processed due to the optical sampling structure. In the experiment, the chirp signal centered at 9 GHz with a bandwidth of 1.6 GHz is sampled and down-converted with a signal-to-noise ratio of 7.20 d B and an improved noise figure. Adopting the photonic ADC in the radar receiver and the above signal as the transmitted radar signal, an X-band inverse synthetic aperture radar system is set up, and the range and cross-range resolutions of 9.4 and 8.3 cm are obtained, respectively.

Multifunctional mixed analog/digital signal processor based on integrated photonics

Photonic signal processing offers a versatile and promising toolkit for contemporary scenarios ranging from digital optical communication to analog microwave operation.Compared to its electronic counterpart,it eliminates inherent bandwidth limitations and meanwhile exhibits the potential to provide unparalleled scalability and flexibility,particularly through integrated photonics.However,by far the on-chip solutions for optical signal processing are often tailored to specific tasks,which lacks versatility across diverse applications.Here,we propose a streamlined chip-level signal processing architecture that integrates different active and passive building blocks in silicon-on-insulator(SOI)platform with a compact and efficient manner.Comprehensive and in-depth analyses for the architecture are conducted at levels of device,system,and application.Accompanied by appropriate configuring schemes,the photonic circuitry supports loading and processing both analog and digital signals simultaneously.Three distinct tasks are facilitated with one single chip across several mainstream fields,spanning optical computing,microwave photonics,and optical communications.Notably,it has demonstrated competitive performance in functions like image processing,spectrum filtering,and electro-optical bandwidth equalization.Boasting high universality and a compact form factor,the proposed architecture is poised to be instrumental for next-generation functional fusion systems.

Single-photon microwave photonics

With the rapid development of microwave photonics technology, high-speed processing and ultra-weak signal detection capability have become the main bottlenecks in many applications. Thanks to the ultraweak signal detection capability and the extremely low timing jitter properties of single-photon detectors, the combination of single-photon detection and classical microwave photonics technology may provide a solution to break the above bottlenecks. In this paper, we first report a novel concept of singlephoton microwave photonics(SP-MWP), a SP-MWP signal processing system with phase shifting and frequency filtering functionalities is demonstrated based on a superconducting nanowire single photon detector(SNSPD) and a successive time-correlated single photon counting(TCSPC) module.Experimental results show that an ultrahigh optical sensitivity down to-100 d Bm has been achieved,and the signal processing bandwidth is only limited by the timing jitter of single-photon detectors. In the meantime, the proposed system demonstrates an ultrahigh anti-interference capability, only the signal which is phase locked by the trigger signal in TCSPC can be extracted from the detected signals combining with noise and strong interference. The proposed SP-MWP concept paves a way to a novel interdisciplinary field of microwave photonics and quantum mechanism, named by quantum microwave photonics.

High-power InAlAs/InGaAs Schottky barrier photodiodes for analog microwave signal transmission

The design,manufacturing and DC and microwave characterization of high-power Schottky barrier In Al As/In Ga As back-illuminated mesa structure photodiodes are presented.The photodiodes with 10 and 15μm mesa diameters operate at≥40 and 28 GHz,respectively,have the output RF power as high as 58 m W at a frequency of 20 GHz,the DC responsivity of up to 1.08 A/W depending on the absorbing layer thickness,and a photodiode dark current as low as 0.04 n A.We show that these photodiodes provide an advantage in the amplitude-to-phase conversion factor which makes them suitable for use in highspeed analog transmission lines with stringent requirements for phase noise.

Eight-channel microwave photonics transceiver photonic integrated circuit

Microwave photonics(MWP)studies the interaction between microwaves and light waves,including the generation,transmission,and processing of microwave signals.Integrated MWP using photonic integrated circuits(PICs)can achieve compact,reliable,and green implementation.However,most PICs have recently been developed that only contain one or a few devices.Here,we propose a multi-channel PIC that covers almost all devices in MWP.Our PIC integrates lasers,modulators,amplifiers,and detectors in the module,successfully manufacturing an eight-channel array transceiver module.We conducted performance tests on the encapsulated transceiver module and found that the cascaded bandwidth of the eightchannel transceiver module was greater than 40 GHz,and the spurious-free dynamic range(SFDR)of the broadband array receiver module was greater than 94 dBm·Hz2/3.The noise figure(NF)is less than-35 dB and the link gain is greater than-26 dB.The success of multi-channel PIC marks a crucial step forward in the implementation of large-scale MWP.

Switchable down-,up-and dual-chirped microwave waveform generation with improved time–bandwidth product based on polarization modulation and phase encoding

A switchable down-,up-and dual-chirped microwave waveform generation technique with improved time–bandwidth product(TBWP)is proposed and demonstrated based on a dual-polarization dual-parallel Mach–Zehnder modulator(DPDPMZM)cascaded with a polarization modulator(Pol M).By properly controlling the phase shifts of the radio frequency signals applied to the DP-DPMZM,switchable down-,up-and dual-chirped waveforms with simultaneous frequency and bandwidth doubling can be generated.To enlarge the TBWP further,splitting parabolic signal and phase-encoding splitting parabolic signal are used to drive the Pol M for the enhancement of bandwidth and time duration.Numerical results demonstrate the generation of down-,up-and dual-chirped microwave waveform with TBWP of 8,160 and 10240.The proposed method may find applications in future multifunction radar systems due to the high performance and flexibility.

Modulation of energy spectrum and control of coherent microwave transmission at single-photon level by longitudinal field in a superconducting quantum circuit

We study the effect of longitudinally applied field modulation on a two-level system using superconducting quantum circuits. The presence of the modulation results in additional transitions and changes the magnitude of the resonance peak in the energy spectrum of the qubit. In particular, when the amplitude ,λz and the frequency COl of the modulation field meet certain conditions, the resonance peak of the qubit disappears. Using this effect, we further demonstrate that the longitudinal field modulation of the Xmon qubit coupled to a one-dimensional transmission line could be used to dynamically control the transmission of single-photon level coherent resonance microwave.

Photonic generation of RF and microwave signal with relative frequency instability of 10^(-15)

We demonstrate the ultra-stable frequency sources aiming to improve the short-time instability of primary frequency standards.These sources are realized by using photonic generation approach,and composed of ultra-stable lasers,optical-frequency-combs,optical signal detecting parts,and synthesizers.Preliminary evaluation shows that the sources produce fixed-frequency at 9.54(/9.63)GHz,10 MHz,and tunable-frequency around 9.192 GHz with relative frequency instability of 10^(-15) for short terms.

Numerical investigation on photonic microwave generation by a sole excited-state emitting quantum dot laser with optical injection and optical feedback

Based on three-level exciton model,the enhanced photonic microwave signal generation by using a sole excited-state(ES)emitting quantum dot(QD)laser under both optical injection and optical feedback is numerically studied.Within the range of period-one(P1)dynamics caused by the optical injection,the variations of microwave frequency and microwave intensity with the parameters of frequency detuning and injection strength are demonstrated.It is found that the microwave frequency can be continuously tuned by adjusting the injection parameters,and the microwave intensity can be enhanced by changing the injection strength.Moreover,considering that the generated microwave has a wide linewidth,an optical feedback loop is further employed to compress the linewidth,and the effect of feedback parameters on the linewidth is investigated.It is found that with the increase of feedback strength or delay time,the linewidth is evidently decreased due to the locking effect.However,for the relatively large feedback strength or delay time,the linewidth compression effect becomes worse due to the gradually destroyed P1 dynamics.Besides,through optimizing the feedback parameters,the linewidth can be reduced by up to more than one order of magnitude for different microwave frequencies.

Microwave photonic filter with a continuously tunable central frequency using an SOI high-Q microdisk resonator

Utilizing a high-Q microdisk resonator （MDR） on a single silicon-on-insulator （SOI） chip, a compact microwave photonic filter （MPF） with a continuously tunable central frequency is proposed and experimentally demonstrated. Assisted by the optical single side-band （OSSB） modulation, the optical frequency response of the MDR is mapped to the microwave frequency response to form an MPF with a continuously tunable central frequency and a narrow 3-dB bandwidth. In the experiment, using an MDR with a compact size of 20×20 μm^2 and a high Q factor of 1.07×10^5, we obtain a compact MPF with a high rejection ratio of about 40 dB, a 3-dB bandwidth of about 2 GHz, and a frequency tuning range larger than 12 GHz. Our approach may allow the implementation of very compact, low-cost, low-consumption, and integrated notch MPF in a silicon chip.

Tunable Single-Passband Microwave Photonic Filter Based on Sagnac Loop and Fabry-Perot Laser Diode

A tunable single-passband microwave photonic filter is proposed and demonstrated, based on a laser diode (LD) array with multiple optical carriers and a Fabry-Perot (F-P) laser diode. Multiple optical carriers in conjunction with the F-P LD will realize a filter with multiple passbands. By adjusting the wavelengths of the multiple optical carriers, multiple passbands are merged into a single passband with a broadened bandwidth. By varying the number of the optical carrier, the bandwidth can be adjusted. The central frequency can be tuned by adjusting the wavelength of the multiple optical carriers simultaneously. A single-passband filter implemented by two optical carriers is experimentally demonstrated.

Wireless Communication Based on Microwave Photon-Level Detection with Superconducting Devices:Achievable Rate Prediction

Future wireless communication systemembraces physical-layer signal detection with highsensitivity, especially in the microwave photon level.Currently, the receiver primarily adopts the signal detection based on semi-conductor devices for signal detection, while this paper introduces high-sensitivityphoton-level microwave detection based on superconducting structure. We first overview existing works onthe photon-level communication in the optical spectrum as well as the microwave photon-level sensingbased on superconducting structure in both theoreticaland experimental perspectives, including microwavedetection circuit model based on Josephson junction,microwave photon counter based on Josephson junction, and two reconstruction approaches under background noise. In addition, we characterize channelmodeling based on two different microwave photondetection approaches, including the absorption barrierand the dual-path Handury Brown-Twiss (HBT) experiments, and predict the corresponding achievablerates. According to the performance prediction, it isseen that the microwave photon-level signal detectioncan increase the receiver sensitivity compared withthe state-of-the-art standardized communication system with waveform signal reception, with gain over 10dB.

Versatile quantum microwave photonic signal processing platform based on coincidence window selection technique

Quantum microwave photonics(QMWP)is an innovative approach that combines energy-time entangled biphoton sources as the optical carrier with time-correlated single-photon detection for highspeed radio frequency(RF)signal recovery.This groundbreaking method offers unique advantages,such as nonlocal RF signal encoding and robust resistance to dispersion-induced frequency fading.We explore the versatility of processing the quantum microwave photonic signal by utilizing coincidence window selection on the biphoton coincidence distribution.The demonstration includes finely tunable RF phase shifting,flexible multitap transversal filtering(with up to 14 taps),and photonically implemented RF mixing,leveraging the nonlocal RF mapping characteristic of QMWP.These accomplishments significantly enhance the capability of microwave photonic systems in processing ultraweak signals,opening up new possibilities for various applications.

High-frequency characterization of high-speed modulators and photodetectors in a link with low-speed photonic sampling

We propose a low-speed photonic sampling for independent high-frequency characterization of a Mach–Zehnder modulator(MZM)and a photodetector(PD)in an optical link.A low-speed mode-locked laser diode(MLLD)provides an ultrawideband optical stimulus with scalable frequency range,working as the photonic sampling source of the link.The uneven spectrum lines of the MLLD are firstly characterized with symmetric modulation within the interesting frequency range.Then,the electro-optic modulated signals are down-converted to the first Nyquist frequency range,yielding the self-referenced extraction of modulation depth and half-wave voltage of the MZM without correcting the responsivity fluctuation of the PD in the link.Finally,the frequency responsivity of the PD is self-referenced measured under null modulation of the MZM.As frequency responses of the MZM and the PD can be independently obtained,our method allows self-referenced high-frequency measurement for a high-speed optical link.In the proof-of-concept experiment,a 96.9 MS/s MLLD is used for measuring a MZM and a PD within the frequency range up to 50 GHz.The consistency between our method and the conventional method verifies that the ultra-wideband and self-referenced high-frequency characterization of high-speed MZMs and PDs.

Instantaneous frequency measurement using two parallel I/Q modulators based on optical power monitoring

A scheme for instantaneous frequency measurement(IFM)using two parallel I/Q modulators based on optical power monitoring is proposed.The amplitude comparison function(ACF)can be constructed to establish the relationship between the frequency of radio frequency(RF)signal and the power ratio of two optical signals output by two I/Q modulators.The frequency of RF signal can be derived by measuring the optical power of the optical signals output by two I/Q modulators.The measurement range and measurement error can be adjusted by controlling the delay amount of the electrical delay line.The feasibility of the scheme is verified,and the corresponding measurement range and measurement error of the system under different delay amounts of the electrical delay line are given.Compared with previous IFM schemes,the structure of this scheme is simple.Polarization devices,a photodetector and an electrical power meter are not used,which reduces the impact of the environmental disturbance on the system and the cost of the system.In simulation,the measurement range can reach 0 GHz-24.5 GHz by adjusting the delay amount of the electrical delay lineτ=20 ps.The measurement error of the scheme is better at low frequency,and the measurement error of low frequency 0 GHz-9.6 GHz can reach-0.1 GHz to+0.05 GHz.