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.

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.

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.

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.

Microwave photonic sideband selector based on thin-film lithium niobate platform

We propose and demonstrate an integrated microwave photonic sideband selector based on the thin-film lithium niobate(TFLN)platform by integrating an electro-optic Mach-Zehnder modulator(MZM)and a thermo-optic tunable flat-top microring filter.The sideband selector has two functions:electro-optic modulation of wideband RF signal and sideband selection.The microwave photonic sideband selector supports processing RF signals up to 40 GHz,with undesired sidebands effectively suppressed by more than 25 d B.The demonstrated device shows great potential for TFLN integrated technology in microwave photonic applications,such as mixing and frequency measurement.

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.

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‑resolution silicon photonic sensor based on a narrowband microwave photonic flter

Microwave photonic sensors are promising for improving sensing resolution and speed of optical sensors.In this paper,a high-sensitivity,high-resolution temperature sensor based on microwave photonic flter(MPF)is proposed and demonstrated.A micro-ring resonator(MRR)based on silicon-on-insulator is used as the sensing probe to convert the wavelength shift caused by temperature change to microwave frequency variation via the MPF system.By analyzing the frequency shift with high-speed and high-resolution monitors,the temperature change can be detected.The MRR is designed with multi-mode ridge waveguides to reduce propagation loss and achieves an ultra-high Q factor of 1.01×10^(6).The proposed MPF has a single passband with a narrow bandwidth of 192 MHz.With clear peak-frequency shift,the sensitivity of the MPF-based temperature sensor is measured to be 10.22 GHz/℃.Due to higher sensitivity and ultra-narrow bandwidth of the MPF,the sensing resolution of the proposed temperature sensor is as high as 0.019℃.

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.

A broadband integrated microwave photonic mixer based on balanced photodetection

An integrated microwave photonic mixer based on silicon photonic platforms is proposed,which consist of a dual-drive Mach–Zehnder modulator and a balanced photodetector.The modulated optical signals from microwave photonic links can be directly demodulated and down-converted to intermediate frequency(IF)signals by the photonic mixer.The converted signal is obtained by conducting of-chip subtraction of the outputs from the balanced photodetector,and subsequent fltering of the high frequency items by an electrical low-pass flter.Benefting from balanced detection,the conversion gain of the IF signal is improved by 6 dB,and radio frequency leakage and common-mode noise are suppressed signifcantly.System-level simulations show that the frequency mixing system has a spurious-free dynamic range of 89 dB·Hz^(2/3),even with deteriorated linearity caused by the two cascaded modulators.The spur suppression ratio of the photonic mixer remains higher than 40 dB when the IF varies from 0.5 to 4 GHz.The electrical-electrical 3 dB bandwidth of frequency conversion is 11 GHz.The integrated frequency mixing approach is quite simple,requiring no extra optical flters or electrical 90°hybrid coupler,which makes the system more stable and with broader bandwidth so that it can meet the potential demand in practical applications.

Adaptive microwave photonic angle-of-arrival estimation based on BiGRU-CNN [Invited]

An adaptive microwave photonic angle-of-arrival(AOA) estimation approach based on a convolutional neural network with a bidirectional gated recurrent unit(BiGRU-CNN) is proposed and demonstrated.Compared with the previously reported AOA estimation methods based on phase-to-power mapping,the proposed method is unnecessary to know the frequency of the signal under test(SUT) in advance.The envelope voltage correlation matrix is obtained from dual-drive Mach–Zehnder modulator(N-DDMZM,N > 2) optical interferometer arrays first,and then AOA estimations are performed on different frequency signals with the aid of BiGRU-CNN.A three-DDMZM-based experiment is carried out to assess the estimation performance of microwave signals at three different frequencies,and the mean absolute error is only 0.1545°.

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.

Switchable microwave photonic filter using a phase modulator and a silicon-on-insulator micro-ring resonator

A switchable microwave photonic filter(MPF) using a phase modulator(PM) and a silicon-on-insulator microring resonator(MRR) is proposed and demonstrated. By adjusting the polarization controller between the PM and the MRR, the filtering function of the MPF can be switched between a band-stop filter and a band-pass filter. In a proof-of-concept experiment, an MPF with a rejection ratio of 30 dB(or 15 dB) for the band-stop(or band-pass) response and a frequency tuning range from 9.6 to 20.5 GHz is achieved.

Few-mode fibre-optic microwave photonic links

The fibre-optic microwave photonic link has become one of the basic building blocks for microwave photonics.Increasing the optical power at the receiver is the best way to improve all link performance metrics including gain,noise figure and dynamic range.Even though lasers can produce and photodetectors can receive optical powers on the order of a Watt or more,the power-handling capability of optical fibres is orders-of-magnitude lower.In this paper,we propose and demonstrate the use of few-mode fibres to bridge this power-handling gap,exploiting their unique features of small acousto-optic effective area,large effective areas of optical modes,as well as orthogonality and walk-off among spatial modes.Using specially designed few-mode fibres,we demonstrate order-of-magnitude improvements in link performances for single-channel and multiplexed transmission.This work represents the first step in few-mode microwave photonics.The spatial degrees of freedom can also offer other functionalities such as large,tunable delays based on modal dispersion and wavelength-independent lossless signal combining,which are indispensable in microwave photonics.

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.

Ultra-high resolution microwave photonic radar with post-bandwidth synthesis

We demonstrate microwave photonic radar with post-bandwidth synthesis, which can realize target detection with ultra-high range resolution using relatively small-bandwidth radio frequency(RF) frontends. In the proposed radar, two temporal-overlapped linear frequency-modulated(LFM) signals with the same chirp rate and different center frequencies are transmitted. By post-processing the de-chirped echoes in the receiver, a signal equivalent to that de-chirped from an LFM signal with the combined bandwidth is achieved. In a proof-ofconcept experiment, two LFM signals with bandwidths of 8.4 GHz are exploited to achieve radar detection with an equivalent bandwidth of 16 GHz, and a range resolution of 1 cm is obtained.

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.

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.

Application of SOl microring coupling modulation in microwave photonic phase shifters

Phase shifter is one of the key devices in microwave photonics. We report a silicon microring resonator with coupling modulation to realize microwave phase shift. With coupling tuning of the Mach-Zehnder interferometer （MZI） coupler to change the resonator from under-coupling to over-coupling, the device can realize a π phase shift on the incoming microwave signal with a frequency up to 25 GHz. The device can also realize 2.5π continuous phase tuning by manipulating the three DC bias voltages applied on the MZI coupler.