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.

Coherent dual-frequency signal generation in an optoelectronic oscillator.

Coherent dual-frequency microwave signal generation using an optoelectronic oscillator (OEO) is presented and demonstrated. In the proposed OEO, a dual-band bandpass filter (DB-BPF) is utilized to select two oscillation modes. An external signal is injected into the OEO loop with its frequency equaling the frequency interval of the two oscillation modes. Owing to the modulation nonlinearity of the Mach-Zehnder modulator, the two oscillation frequencies interact with the injection frequency. When the phase and gain conditions are satisfied within the loop, injection locking between the two oscillation signals will be established, and their phases will be synchronized. The effect of gain competition in the OEO loop, which leads to single-frequency oscillation, is suppressed. An experiment is carried out, and two frequencies, of 16.083 GHz and 9.998 GHz, are generated at the same time. The phase noise values are -140.1 and -141.0 dBc/Hz @ 10 kHz, respectively. The coherence between the generated signals and sidemode suppression performance are evaluated.

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.

Coherent stepped-frequency waveform generation based on recirculating microwave photonic frequency conversion.

We propose a novel method for generating coherent and wideband stepped-frequency waveforms using recirculating microwave photonic frequency conversion (MWP-FC). By injecting a narrowband signal into an MWP-FC loop utilizing a dual-parallel Mach-Zehnder modulator (DPMZM), the signal frequency is continuously converted to produce a stepped-frequency waveform with a wide bandwidth. Within the MWP-FC loop, photo-electric conversion is achieved based on self-mixing detection, where the optical phase noise can be suppressed, guaranteeing stability and coherence of the generated signal. In a proof-of-concept experiment, a stepped-frequency signal with a frequency interval of 2 GHz and a bandwidth of about 16 GHz and a stepped-frequency chirp signal with a frequency interval of 3 GHz and a bandwidth of about 15 GHz are generated. In addition, coherence of the generated signals is verified by coherent integration and de-chirping.

Optical pulse interharmonic extraction and repetition rate division based on a microwave photonic phase detector.

Microwave photonic phase detectors (MPPDs) can extract ultrastable microwaves from a mode-locked laser (MLL), but their frequencies are often limited by the pulse repetition rate. Few works studied methods to break the frequency limitation. Here, a setup based on an MPPD and an optical switch is proposed to synchronize an RF signal from a voltage-controlled oscillator (VCO) to an interharmonic of an MLL and to realize the pulse repetition rate division. The optical switch is employed to realize pulse repetition rate division, and the MPPD is followed to detect the phase difference between the frequency-divided optical pulse and the microwave signal from the VCO, which is then fed back to the VCO via a proportional-integral (PI) controller. Both the optical switch and the MPPD are driven by the signal from the VCO. When the system reaches its steady state, the synchronization and repetition rate division are achieved simultaneously. An experiment is conducted to verify the feasibility. The 80½th, 80⅓rd, and 80⅔rd interharmonics are extracted, and pulse repetition rate division factors of two and three are realized. The phase noises at offset frequency of 10 kHz are improved by more than 20 dB.

Selective nitridation-corrosion process to recover vanadium, titanium, chromium, and iron from vanadium slag.

Vanadium slag (V-slag) is an important secondary V source, but other valuable elements are discarded in the tailings in industry. Herein, a green nitridation-corrosion process is proposed for the comprehensive recovery of valuable elements (V, Ti, Cr, Fe) from V-slag without producing hazardous waste. Thermodynamic results indicate that ammonia gas (NH3) can selectively reduce Fe and nitride V, Cr, and Ti. The main phase composition of the nitrided V-slag included metallic Fe, nitrides, and diopside under optimal conditions, and their relative contents were 42.5, 26.2, and 31.3%, respectively, after roasting at 1000 °C for 6 h. The effects of the main parameters on corrosion test were investigated, and the highest weight-gain ratio attained was 19.6%. FeOOH crystallizes on the surface of the nitrided V-slag due to the oxidization of metallic Fe. The phase evolution during the entire process is spinel/olivine/diopside → Fe/nitrides/diopside → FeOOH/nitrides/diopside. Owing to finer particle sizes, most FeOOH is separated by wet sieving (<1400 mesh). The purity of the enriched nitrides attained was 43% after pickling to remove excess Fe. The total recovery rates of Fe, V, Ti, Cr were 87.76%, 95.92%, 92.92%, 92.11%, respectively. This paper provides a sustainable strategy for the comprehensive utilization of V-slag, and guides the cleaner treatment of other similar minerals.

Third-harmonic-assisted four-wave mixing in a chip-based microresonator frequency comb generation.

Microcombs generated in photonic integrated circuits can provide broadband and coherent optical frequency combs with a high repetition rate from microwave to terahertz. Coherent microcombs formed in normal group velocity dispersion microresonators usually have a flat-top temporal profile, called platicon. Here, we propose a novel scheme to generate platicon in Si3N4 microresonator with the assistance of third-harmonic generation. The nonlinear coupling between the fundamental and the third-harmonic waves that draws support from third-order sum/difference frequency generation provides a new mechanism to achieve the phase matching of four-wave mixing in normal dispersion microresonators. We show that single or multiple platicons can be obtained by changing the third-harmonic nonlinear coupling strength and phase matching condition for third-order sum/difference frequency generation. Our work provides a promising solution to facilitate coherent and visible microcomb generation in a pure χ(3) microresonator, which is potential for self-referencing combs and optical clock stabilization.

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.

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.

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.

Frequency-modulated continuous-wave laser ranging using low-duty-cycle signals for the applications of real-time super-resolved ranging.

A frequency-modulated continuous-wave laser ranging method using low-duty-cycle linear-frequency-modulated (LFM) signals is proposed. A spectrum consisting of a dense Kronecker comb is obtained so that the frequency of the beat signal can be measured with finer resolution. Since the dense comb is provided, super-resolved laser ranging can be achieved using a single-parametric frequency estimation method. Therefore, the run times of the estimation are reduced which promises real-time applications. A proof-of-concept experiment is carried out, in which an LFM signal with a bandwidth of 5 GHz and a duration of 1 µs is used. The duty-cycle of the LFM signal is 10%. The time delay of a scanning variable optical delay line is obtained in real time from the frequency of the highest comb tooth, of which the measurement resolution is 20 ps. Moreover, a single-parametric nonlinear least squares method is used to fit the envelope so that the time delay can be estimated with super-resolution. The standard deviation of the estimation displacements is 2.3 ps, which is 87 times finer than the bandwidth-limited resolution (200 ps). Therefore, the variation of the time delay can be precisely monitored. The proposed method may be used to achieve real-time high-resolution laser ranging with low-speed electronic devices.

Optical vector analyzer with time-domain analysis capability.

Time-domain analysis (TDA) is useful for measuring optical devices along with a link and for diagnosing a long device. In this Letter, an optical vector analyzer with TDA capability is proposed and experimentally demonstrated. The key to realizing TDA is a low-coherence optical carrier, which is achieved by modulating an electrical broadband signal on a continuous-wave light via acousto-optic modulation. Then, optical single-sideband modulation and vector balanced detection are used to measure the total frequency response of multiple devices under test (DUTs). Through an inverse Fourier transform, the obtained DUT impulses are distinguished in the time domain. Finally, time-domain gating and Fourier transform are applied to extract the frequency response of each DUT. An experiment is performed in which a fiber link comprising three DUTs and an H 13 C 14 N gas cell with a breakpoint inserted is characterized. The frequency setting resolution is 5 MHz, and a time-domain resolution of 30.84 ns is proved, which can reach 14.881 ns in theory.

Comprehensive vector analysis for electro-optical, opto-electronic, and optical devices.

High-performance electro-optical (E-O), opto-electronic (O-E), and optical (O-O) devices are widely used in optical communications, microwave photonics, fiber sensors, and so on. Measurement of the amplitude and phase responses are essential for the development and fabrication of these devices. However, the previous methods can hardly characterize the E-O, O-E, and O-O devices with arbitrary responses. Here we propose a comprehensive vector analyzer based on optical asymmetrical double-sideband (ADSB) modulation to overcome this difficulty. The ADSB solves the problem of frequency aliasing and can extract information from both the +1st- and -1st-order sidebands. Thus, most devices in photonic applications, including phase modulators, can be characterized. In the experiment, a commercial photodetector, a phase modulator, and a sampled FBG are used as the O-E, E-O, and O-O devices under test, respectively. A frequency resolution of 2 MHz, an electrical sweeping range of 40 GHz, and an optical sweeping range of 80 GHz are achieved.

Photonic approach for simultaneous measurement of microwave DFS and AOA.

A photonic scheme that can simultaneously estimate the microwave Doppler-frequency shift (DFS) and angle-of-arrive (AOA) is demonstrated. In the proposed system, the transmitted signal is independently mixed with two echo signals by a dual-channel microwave photonic mixer. By measuring the frequency of the intermediate frequency (IF) signal output from the two channels and the phase difference between them, the DFS (with direction identification) and AOA parameters can be obtained. In a proof-of-concept experiment, the errors are less than ${{\pm}}\;{0.08}\;{\rm{Hz}}$ for the DFS measurement within a range of ${\rm{\pm 100}}\;{\rm{kHz}}$ and less than ${\rm{\pm 1}.{3}}\deg$ for the AOA measurement ranging from 0° to 90°, respectively.

Dual-output filter-free microwave photonic single sideband up-converter with high mixing spur suppression.

A dual-output filter-free microwave photonic single sideband (SSB) up-converter with the mixing spurs highly suppressed is proposed and experimentally demonstrated. By introducing the balanced Hartley structure using a 90° optical hybrid, the lower sideband (LSB) and upper sideband (USB) up-converted RF signals can be generated simultaneously and output separately, with no need of either optical or electrical filtering. The structure avoids the special requirement with the optical modulation format of the local oscillator (LO) signal. The intermediate frequency signal is modulated with the optical carrier suppressed -SSB modulation format. The undesired optical components are highly suppressed. In this way, the high sideband and LO leakage suppression ratios of the SSB up-converter are guaranteed. The dual-output SSB up-conversion is experimentally achieved within the working frequency range of 10-30 GHz. The undesired sideband and LO leakage suppression ratios are larger than 67 dB for the whole frequency range. The spurious-free dynamic range of larger than 95.6dBc⋅Hz2/3 has also been achieved experimentally for both the LSB and USB up-conversion conditions.

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.

Photonic generation of tunable dual-chirp microwave waveforms using a dual-beam optically injected semiconductor laser.

An approach to generating dual-chirp microwave waveforms (DCMWs) is proposed and experimentally demonstrated. The proposed scheme consists of a typical semiconductor slave laser (SL), which is subject to a dual-beam optical injection from two master lasers with one being positively detuned (${\rm ML}_1$ML1) and the other negatively detuned (${\rm ML}_2$ML2) from the SL. Under proper injection conditions, the SL operates in the so-called Scenario B of dual-beam injection. After optical-to-electrical conversion, a dual-frequency microwave signal can be generated with one of its two frequencies increasing linearly and the other decreasing linearly as the ${\rm ML}_1$ML1 injection strength is increased. By incorporating a fast injection strength controller (formed by an intensity modulator and an electrical control signal), a DCMW with a large time-bandwidth product can be generated. In the experimental demonstration, a DCMW with a temporal period of 1 µs has been obtained. This simultaneously offers an up-chirp (13.4-20.2 GHz) and a down-chirp (27.3-20.5 GHz), and its frequency tunability has been achieved by simply adjusting the injection parameters. Furthermore, the auto-ambiguity function of the generated DCMW has also been investigated, which proves that the proposed scheme has the ability to improve the range-velocity resolution and, thus, could be promising for use in radar systems.