Microwave photonic I/Q mixer for wideband frequency downconversion with serial electro-optical modulations.

In this paper, we propose a serial electro-optical (EO)-modulation-based microwave photonic in-phase and quadrature (I/Q) mixer and investigate its performance for wideband frequency downconversion. The proposed I/Q mixer uses two EO modulators and a programmable optical processor in a serially cascaded structure, which ensures good phase stability and flexibility to achieve high-performance broadband frequency downconversion. A proof-of-concept experiment is carried out in which the frequency downconversion of the RF signals in the range from 10 to 40 GHz is demonstrated with an average image rejection ratio of 38.66 dB. The spurious-free dynamic range (SFDR) measured at around 15 GHz is 86 dBc·Hz2/3. Based on this microwave photonic I/Q mixer, a broadband radar receiver is established and de-chirping of linearly frequency modulated (LFM) radar echoes with an instantaneous bandwidth of 8 GHz (10-18 GHz) is achieved. The results verify its feasibility for high-performance I/Q mixing in practical applications.

Broadband stepped-frequency radar waveform generation by Fourier domain mode-locking period-one laser dynamics.

A stepped-frequency (SF) radar waveform generation method based on Fourier domain mode-locking (FDML) period-one laser dynamics is proposed and demonstrated. By fast controlling the optical injection strength of a semiconductor laser through electro-optical modulation, a broadband SF signal is generated. By further introducing an optoelectronic feedback loop with its round trip time delay matched with the temporal period of the SF signal, FDML is enabled through which the frequency stability, accuracy, and in-band signal-to-noise ratio are greatly improved. In the experiment, SF signals with a bandwidth of 6 GHz (12-18 GHz) and a frequency step of 150 MHz are generated. By comparing the qualities of signals generated with and without FDML, advantages of the proposed SF signal generation method are verified. Based on the proposed SF signal generation method, high-resolution inverse synthetic aperture radar (ISAR) imaging is also demonstrated, in which the 2D imaging resolution reaches 2.6 cm × 3.82 cm.

Frequency-tunable microwave generation with parity-time symmetry period-one laser dynamics.

A novel frequency-tunable microwave signal generation method is proposed by incorporating parity-time (PT) symmetry in period-one (P1) laser dynamics in an optically injected semiconductor laser. In this method, P1 oscillation enables a large frequency tuning range and PT symmetry leads to excellent side-mode suppression and low phase noise. In an experimental demonstration, the side-mode suppression ratio reaches 58.4 dB and the phase noise is -126.2 dBc/Hz at 10 kHz offset when generating a 6.98 GHz signal, which are improved by 44.5 dB and 13.5 dB, respectively, compared with the previously reported optoelectronic oscillator-based P1 oscillation. By simply adjusting the optical injection strength, the frequency of the microwave signal generated by PT symmetry P1 dynamics is tuned from 5.07 GHz to 15.22 GHz, in which the phase noise is kept below 120 dBc/Hz at 10 kHz offset. The proposed method is expected to find applications in high-performance wireless communication and radar systems.

Deep-learning-based time-frequency domain signal recovery for fiber-connected radar networks.

A deep-learning-based time-frequency domain signal recovery method is proposed to deal with the signal distortion in fiber-connected radar networks. In this method, the deteriorated signal is converted to the time-frequency domain, and a two-dimensional convolutional neural network is used to conduct signal recovery before inverse conversion to the time domain. This method can achieve high-accuracy signal recovery by learning the complete features in both time and frequency domains. In the experiment, distorted linear frequency modulated radar signals with a bandwidth of 2 GHz after 8-km fiber transmission are recovered with the noise effectively suppressed. The proposed signal recovery method works well under different input signal-to-noise ratios. Specially, the average peak to floor ratio after radar pulse compression is improved by 25.5 dB. In addition, the method is proved to be able to recover radar signals of multiple targets.

Frequency-modulated microwave signal generation by dual-wavelength-injection period-one laser dynamics.

In this Letter, dual-wavelength-injection period-one (P1) laser dynamics is proposed for the first time, to the best of our knowledge, to generate frequency-modulated microwave signals. By injecting light with two different wavelengths into a slave laser to excite P1 dynamics, the P1 oscillation frequency can be modulated without external control of the optical injection strength. The system is compact and stable. The frequency and bandwidth of the generated microwave signals can be easily adjusted by tuning the injection parameters. Through both simulations and experiments, the properties of the proposed dual-wavelength injection P1 oscillation are revealed, and the feasibility of the frequency-modulated microwave signal generation is verified. We believe that the proposed dual-wavelength injection P1 oscillation is an extension of laser dynamics theory, and the signal generation method is a promising solution for generating broadband frequency-modulated signals with good tunability.

Towards small target recognition with photonics-based high resolution radar range profiles.

Photonics-based radar expands the bandwidth of traditional radars and enhances the radar range resolution. This makes it possible to recognize small-size targets using the high resolution range profiles (HRRPs) acquired by a photonics-based broadband radar. In this paper, we investigate the performance of small target recognition using HRRPs of a photonics-based radar with a bandwidth of 8 GHz (28-36 GHz), which is built based on photonic frequency multiplication and frequency mixing. A convolutional neural network (CNN) is used to extract features of the HRRPs and classify the targets. In the experiment, recognition of four types of small-size targets is demonstrated with an accuracy of 97.16%, which is higher than target recognition using a 77-GHz electronic radar by 31.57% (2-GHz bandwidth) and 8.37% (4 GHz-bandwidth), respectively. Besides the accuracy, target recognition with photonics-based radar HRRPs is proved to have good generalization capability and stable performance. Therefore, photonics-based radar provides an efficient solution to small target recognition with one-dimension HRRPs, which is expected to find import applications in air defense, security check, and intelligent transportation.

Photonics-based 3D radar imaging with CNN-assisted fast and noise-resistant image construction.

Photonics-based high-resolution 3D radar imaging is demonstrated in which a convolutional neural network (CNN)-assisted back projection (BP) imaging method is applied to implement fast and noise-resistant image construction. The proposed system uses a 2D radar array with each element being a broadband radar transceiver realized by microwave photonic frequency multiplication and mixing. The CNN-assisted BP image construction is achieved by mapping low-resolution images to high-resolution images with a pre-trained 3D CNN, which greatly reduces the computational complexity and enhances the imaging speed compared with basic BP image construction. Besides, using noise-free or low-noise ground truth images for training the CNN, the CNN-assisted BP imaging method can suppress the noises, which helps to generate high-quality images. In the experiment, 3D radar imaging with a K-band photonics-based radar having a bandwidth of 8 GHz is performed, in which the imaging speed is enhanced by a factor of ∼55.3 using the CNN-assisted BP imaging method. By comparing the peak signal to noise ratios (PSNR) of the generated images, the noise-resistant capability of the CNN-assisted BP method is soundly verified.

Millimeter-level resolution through-the-wall radar imaging enabled by an optically injected semiconductor laser.

Millimeter-level resolution through-the-wall radar (TWR) imaging is demonstrated using a broadband nonlinear frequency-modulated (NLFM) signal that is generated by an optically injected semiconductor laser. The proposed system uses period-one dynamics of a semiconductor laser, together with an optical frequency downconversion technique to generate NLFM signals, which addresses the problem of traditional period-one oscillation not being able to generate broadband signals in the low-frequency region. In the experiment, an NLFM signal having a broad bandwidth of 18.5 GHz (1.5-20 GHz) is generated with a corresponding radar range resolution of 8.1 mm. Using this signal, TWR imaging is demonstrated, in which the use of the NLFM signal achieves good side-lobe suppression during pulse compression, and a modified back projection imaging algorithm with sub-aperture weighting is proposed to improve the imaging quality.

Soil water-stable aggregates and microbial community under long-term tillage in black soil of Northern China.

Long-term frequent tillage would cause black soil degradation and serious soil erosion as soil microbial communities and soil structure are extremely sensitive to tillage process. However, there is no unified conclusion on the relationship between the distribution of soil water-stable aggregates (WSAs), and microbial community construction and diversity under long-term tillage in black soil during different seasons. In this study, we used wet-sieving method to evaluate the composition and stability of soil WSAs and employed Illumina MiSeq high-throughput sequencing technology to study the diversity, taxonomic composition and co-occurrence network properties of microbial community, comparing outcomes between uncultivated soil and long-term cultivated soil for 60 years in Keshan farm of Heilongjiang Province. The results showed that after long-term tillage, the proportion of larger than 1 mm WSAs reduced by 34.17-51.37%, and the stability of WSAs, soil pH, organic matter (OM), total nitrogen (TN) contents decreased significantly in all seasons (P < 0.05), while soil available phosphorus (AP) and available potassium (AK) contents increased remarkably (P < 0.05). The diversity of bacteria increased, while that of fungi decreased. Soil fungal communities were more susceptible to long-term tillage than bacterial and archaeal communities. Actinobacteria mainly exist in large WSAs (˃1 mm), and when their relative abundance is high, it is beneficial to improve the water-stability of black soil; while Proteobacteria and Gemmatimonadetes may exist in small WSAs (˂1 mm), whose high relative abundance will weaken the water-stability of black soil. The experimental results provide a scientific theoretical basis for sustainable utilization of black soil.

Photonic scanning receiver for wide-range microwave frequency measurement by photonic frequency octupling and in-phase and quadrature mixing.

A photonic scanning receiver with optical frequency scanning and electrical intermediate frequency envelope detection is proposed to implement wide-range microwave frequency measurement. This system applies photonic in-phase and quadrature frequency mixing to distinguish and measure the signals in two frequency bands that mirror each other. Combined with the photonic frequency octupling technique, the proposed system has a frequency measurement range that is 16 times that of the sweeping range of the electrical signal source. Besides, optical frequency sweeping with up and down chirps is used to relax the requirement for precise synchronization between the sweeping source and the analog-to-digital converter. In the experiment, using an electrical sweeping local oscillator having a bandwidth of 1.75 GHz, the system achieves a frequency measurement range as large as 28 GHz. The measurement errors are kept within 24 MHz with an average error of 9.31 MHz.

Deep neural network-assisted high-accuracy microwave instantaneous frequency measurement with a photonic scanning receiver.

A microwave instantaneous frequency measurement system with a photonic scanning receiver is proposed in which deep neural network (DNN)-assisted frequency estimation is used to deal with the system defects and improve the accuracy. The system performs frequency-to-time mapping by optical-domain frequency scanning and electrical-domain intermediate frequency envelop detection. Thanks to the optical frequency multiplication, the system can measure high frequency signals in a large spectral range. The DNN establishes an accurate mapping between the digital samples and real frequencies, based on which high-accuracy measurement is achieved. The measurement of signals from 43 to 52 GHz is experimentally demonstrated. Compared with the direct measurements, the DNN-assisted method achieves obviously reduced average errors of about 3.2 MHz.

Intelligent Image De-Blurring for Imaging Flow Cytometry.

By virtue of the combined merits of optical microscopy and flow cytometry, imaging flow cytometry is a powerful tool for rapid, high-content analysis of single cells in large heterogeneous populations. However, its efficiency (defined by the ratio of the number of clearly imaged cells to the total cell population) is not high (typically 50-80%), due to out-of-focus image blurring caused by imperfect fluidic focusing of cells, a common drawback that not only reduces the number of cell images useable for high-content analysis but also increases the probability of false events and missed rare cells. To address this challenge and expand the efficacy of imaging flow cytometry, here, we propose and demonstrate intelligent deblurring of out-of-focus cell images in imaging flow cytometry. Specifically, by using our machine learning algorithms, we show an 11% increase in variance and a 95% increase in first-order gradient summation of cell images taken with an optofluidic time-stretch microscope. Without strict hardware requirements, our intelligent de-blurring method provides a promising solution to the out-of-focus blurring problem of imaging flow cytometers and holds promise for significantly improving their performance. © 2019 International Society for Advancement of Cytometry.

High-resolution phased array radar imaging by photonics-based broadband digital beamforming.

A photonics-based broadband phased array radar is demonstrated to realize high-resolution imaging based on digital beamforming. This photonics-based phased array radar can achieve a high range resolution enabled by a large operation bandwidth, and can realize squint-free beam steering by digital true time delay (TTD) compensation. In addition, the photonic dechirp processing applied in the receiver can alleviate the hardware requirements for data sampling and storage, and hence remarkably enhance the real-time signal processing capability. In a proof-of-concept experiment, target imaging by a photonics-based 1 × 4 phased array radar that has a bandwidth of 4 GHz (22-26 GHz) is demonstrated, of which the range and azimuth resolution is measured to be 3.85 cm and 2.68°, respectively. The proposed scheme provides good solution to overcoming the bandwidth limitation and implementing high-resolution imaging in a phased array radar.

Photonics-based dual-functional system for simultaneous high-resolution radar imaging and fast frequency measurement.

A photonics-based dual-functional system is proposed that can simultaneously implement high-resolution radar imaging and fast frequency measurement. In this system, the radar is realized based on photonic frequency doubling and de-chirp receiving, and the frequency measurement is achieved by a novel frequency-to-time mapping method. In the experimental demonstration, the radar works in Ku band with a bandwidth of 6 GHz (12-18 GHz), through which inverse synthetic aperture radar imaging with a resolution as high as ∼2.6 cm×∼2.8 cm is achieved. The frequency measurement module operates in Ka band, which can achieve a measurement frequency range from 28 GHz to 37 GHz, with a measurement resolution of 40 MHz and a refresh rate of 100 kHz.

Photonics-based MIMO radar with high-resolution and fast detection capability.

A photonics-based multiple-input-multiple-output (MIMO) radar is proposed and demonstrated based on wavelength-division-multiplexed broadband microwave photonic signal generation and processing. The proposed radar has a large operation bandwidth, which helps to achieve an ultra-high range resolution. Compared with a monostatic radar, improved radar performance and extended radar applications originated from the MIMO architecture can be achieved. In addition, low-speed electronics with real-time signal processing capability is feasible. A photonics-based 2 × 2 MIMO radar is established with a 4-GHz bandwidth in each transmitter and a sampling rate of 100 MSa/s in the receiver. Performance of the photonics-based multi-channel signal generation and processing is evaluated, and an experiment for direction of arrival (DOA) estimation and target positioning is demonstrated, through which the feasibility of the proposed radar system can be verified.

Fiber-distributed ultra-wideband radar network based on wavelength reusing transceivers.

A fiber-distributed ultra-wideband (UWB) radar network based on wavelength-reusing transceivers is proposed and demonstrated. In the proposed system, wavelength-division multiplexing technology is applied to connect a central unit (CU) and several spatially separated transceivers. The optically generated UWB pulses in different transceivers are designed to have different polarities or shapes, so the CU can easily identify which transceiver the echo UWB pulse is emitted from. By applying the wavelength reusing, the wavelength of the uplink UWB pulse can also be used by the CU to identify which transceiver it is received by. Therefore, with very simple cooperative signal processing in the CU, the information of the target in the radar detection area can be extracted. In addition, because the fiber lengths in the network are known, clock synchronization in the transceivers is not required, which simplifies further the entire system. In an experiment, 2-D localization with localization accuracy of about one centimeter is achieved using the proposed radar network with two distributed transceivers.

Performance enhancement of an optically-injected-semiconductor-laser-based optoelectronic oscillator by subharmonic microwave modulation.

An approach to enhancing the performance of an optically-injected-semiconductor-laser-based optoelectronic oscillator (OEO) is proposed by subharmonic microwave modulation. A free-running OEO is first established based on period-one dynamics of an optically injected semiconductor laser. The oscillation frequency can be tuned in the range of 8.87 to 18.41 GHz by controlling the injection strength, but the output signal suffers from strong side modes and poor frequency stability. To address these problems, subharmonic microwave modulation is applied to the injected semiconductor laser. In the experiment, microwave modulation with 1/2, 1/4, and 1/6 subharmonics is demonstrated. The side-mode suppression ratio is improved by over 40 dB, while the phase noise at a 1 kHz offset is reduced by about 18 dB. Furthermore, the frequency drift over a period of 20 min, which characterizes the long-term stability, is reduced from 8.7 kHz to less than 1 Hz, indicating a significant reduction of over three orders.

Self-calibrating and high-sensitivity microwave phase noise analyzer applying an optical frequency comb generator and an optical-hybrid-based I/Q detector.

Phase noise analyzers (PNAs) are indispensable for evaluating the short-term stability of microwave signals. In this Letter, a high-sensitivity PNA with self-calibration capability is proposed based on an optical frequency comb generator and an optical-hybrid-based I/Q detector. The negative factors that result in inaccurate measurement, including the direct component interference, amplitude noises of the microwave signal under test and the laser, and phase noise of the laser, are all eliminated through digital signal processing. A proof-of-concept experiment is performed. The established PNA can achieve accurate phase noise measurement with a high sensitivity of -146.1 dBc/Hz at 10 kHz, and the self-calibrating property of the PNA is also verified.

Fast and wide-range optical beam steering with ultralow side lobes by applying an optimized multi-circular optical phased array.

A multi-circular optical phased array (OPA) is proposed and investigated through simulation to realize fast and wide-range optical beam steering with ultralow side lobes. The proposed multi-circular OPA has a circularly symmetrical distribution, and its radical element spacings are optimized by a modified genetic algorithm to achieve the best side lobe suppression. Specifically, the peak side lobe level (PSLL) of the far-field pattern in 0° beam direction reaches as low as 0.0715 for a 73-element optimized multi-circular OPA, which is much better than a multi-circular OPA with uniform radical element spacing (the PSLL is 0.3686). The prominent feature of the proposed OPA is that, once the OPA is optimized towards a specific elevation direction by the modified genetic algorithm, a wide-angle optical beam steering with nearly the same side lobe suppression can be achieved without updating the OPA distribution, which makes it possible for fast optical beam steering over a wide scanning range. In the simulation, ultralow side lobe beam steering with an elevation angle from 0° to 30° and an azimuth angle from 0° to 360° is achieved with the PSLL variation less than 0.001. The relationship between the optimized PSLL and the elevation angle used for OPA optimization is also investigated, which is helpful in achieving the best side lobe suppression for different scanning ranges. The proposed OPA is expected to find applications such as laser radar, high-resolution display, and free space optical communications.

Wideband microwave phase noise measurement based on photonic-assisted I/Q mixing and digital phase demodulation.

A wideband microwave phase noise measurement system is proposed based on quadrature phase demodulation of the mixing components of a signal under test (SUT) and its delayed replica. The time delay is introduced by a low-loss optical fiber, which can be sufficiently large to achieve a high phase noise measurement sensitivity, and the quadrature phase demodulation is achieved by photonic-assisted in-phase and quadrate (I/Q) mixing together with digital signal processing. Thanks to the optoelectronic hybrid quadrature phase demodulation, the use of feedback loops, which are usually required in conventional photonic-delay-line-based phase noise measurement systems, is avoided, and the measurable frequency range is expanded. An experiment is implemented. Accurate phase noise measurement of SUTs in a frequency range of 5-35 GHz is demonstrated. With a 2-km single-mode fiber serving as the photonic delay line, the phase noise floor is as low as -131 dBc/Hz at the offset frequency of 10 kHz. The proposed scheme can be applied for evaluating the performance of microwave systems using low-phase-noise and wideband tunable microwave sources.