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Quantification of regional soil carbon changes in boreal forests in China is difficult for high spatial heterogeneity, especially considering soil CH4 fluxes in permafrost regions. This study attempted to quantify the variation of soil CO2 emission and its relationship with other soil properties along a permafrost hillslope in Larix gmelinii Kuzen. forest. Using a closed-chamber method, the soil CO2 emission was measured at four slope positions in the Greater Xing’an Range of China in two growing seasons. The results showed that soil CO2 changes have high spatial variability in Larix gmelinii forest along the slope, and average soil CO2 emission at the upper part of the slope was 64% higher than at the bottom. Soil CO2 fluxes showed high positive correlation with soil temperature at 10 cm depth and fungi numbers and negative correlation with soil CH4 change. This study showed the complexity of CO2 emission and could provide data support for forest carbon measurement caused by hillslope in the boreal forest of China. Study Implications: The forest area of Larix gmelinii Kuzen. in the Greater Xing’an Range accounts for 13.2% of the total forest area China. Therefore, the accurate calculation of carbon sequestration of Larix gmelinii forest is significant to the forest carbon measurement of China. However, due to the topographical complexity of the Greater Xing’an Range, the measurement of soil carbon has always been a problem. This study explored the soil carbon dioxide emissions at different slope positions along a hillslope and provided some methods and data support to solve measurement problems caused by hillslope in boreal forest in China.

Photonic digital-to-analog converters (PDACs) have a broad application prospect due to the ability to overcome the non-idealities in electronic circuits. PDACs are usually implemented by quantizing and summing the optical intensities of multiple lasers. The relative intensity noise of laser sources plays a critical role in determining the signal-to-noise ratio (SNR) and effective number of bits (ENOB). We present a detailed noise analysis for PDACs. Both the traditional binary-weighted structure and the recently proposed segmented-weighted structure are investigated. The results show that laser noise imposes a fundamental limit to the maximum SNR and ENOB that can be achieved in binary-weighted PDACs, while segmented PDACs can break this limitation and have a continuously increasing SNR with the quantization bit number (QBN). A novel configuration based on laser multiplexing and balanced detection, to the best of our knowledge, is also proposed and analyzed to increase the number of bits when the number of lasers is limited. Numerical simulations are performed to evaluate the SNR evolution with the QBN in different types of PDACs. The results are in good agreement with the theoretical analysis. Our analysis provides useful insights and can be important guidance for implementing high-performance PDACs.

Solitons are self-reinforcing localized wave packets arising from a balance of linear and nonlinear effects. This definition encompasses the interplay of nonlinear gain and loss, leading to the concept of dissipative solitons that has been instrumental in understanding the wide variety of mode locking phenomena in ultrafast optics. To date, most studies have involved the group velocity dispersion as a key ingredient for soliton generation. Here, we report on a novel kind of soliton, both theoretically and experimentally, which builds up in spectrally confined cavities when dispersion is practically absent. Precisely, the interplay between the Kerr nonlinearity and spectral filtering results in an infinite hierarchy of eigenfunctions which, combined with optical gain, allow for the generation of stable dispersion-less dissipative solitons in a previously uncharted regime. When the filter order tends to be infinite, we find an unexpected link between dissipative and conservative solitons, in the form of Nyquist-pulse-like solitons endowed with an ultra-flat spectrum. In contrast to the dispersion-enabled solitons, these dispersion-less Nyquist solitons build on a fully confined spectrum and their energy scaling is not constrained by the pulse duration. This study broadens the fundamental scope of dissipative soliton physics and opens new avenues for engineering optical solitons endowed with superior temporal and spectral features., Comment: 25 pages, 16 figures

In this article, we firstly analyze the specific effect of time and phase synchronization errors on the coherent gain of microwave photonic wideband distributed coherent aperture radar (DCAR), and further propose a robust design based on bandwidth segmentation. In our design, the intermediate-frequency narrowband linear frequency modulated waveform (LFMW) sub-pulses are mixed with the stepped-frequency local oscillator signals at the transmitter, generating the stepped-frequency LFMW. To ensure the waveform purity, a photonics-based single-sideband up-converter is proposed as the mixer. At the receiver side, the echo wave is processed by a photonics-based down-converter combined with the matched-filtering processing. The relatively low-resolution range profiles with low coherent gain loss can be achieved by the narrowband sub-pulses, and the high-resolution range profile with low coherent gain loss can be synthesized from these low-resolution range profiles. The coherent gain of the proposed system is less sensitive to time synchronization error comparing to the system where the wideband LFMW is transmitted. Higher level of robustness also enables easier implementation of such system. To show the effectiveness of our design, we conduct simulations and further establish an X-band (8–12 GHz) microwave photonic system with two transmitters and one receiver. The coherent gain under different time synchronization errors are measured. Both the simulation and experimental results confirm that the proposed system achieves a higher robustness to time synchronization error. For instance, to keep the coherent gain loss less than 0.3 dB, the required time synchronization accuracy can be relaxed from 35 ps to 127 ps.

Temporal solitons can arise in spectrally confined cavities with no chromatic dispersion. When the filter order tends to infinity, the soliton tends to a Nyquist pulse with a fully confined ultra-flat spectrum and a transform-limited waveform.

We propose an ultra-wideband photonic compressive receiver based on random codes shifting with image-frequency distinction. By shifting the center frequencies of two random codes in large frequency range, the receiving bandwidth is flexibly expanded. Simultaneously, the center frequencies of two random codes are slightly different. This difference is used to distinguish the “fixed” true RF signal from the differently located image-frequency signal. Based on this idea, our system solves the problem of limited receiving bandwidth of existing photonic compressive receivers. In the experiments, with two channels of only 780-MHz outputs, the sensing capability in the range of 11–41 GHz has been demonstrated. A multi-tone spectrum and a sparse radar-communication spectrum, composed of a linear frequency modulated (LFM) signal, a quadrature phase-shift keying (QPSK) signal and a single-tone signal, are both recovered.

A novel photonic-enabled high frequency broadband RF canceller for in-band full-duplex wireless communication system or frequency modulated continuous-wave (FMCW) radars is proposed. The phase and amplitude between reference and interference signal is matched by optical spectrum processing, and the delay time is aligned by using the dispersive element and finely tuning the wavelength of the tunable lasers. The experiment is performed. RF selfinterference cancellation with the bandwidth up to 6GHz is experimentally demonstrated with the cancellation depth of more than 26.9dB in K band.

We demonstrated a photonics-based X-band radar system, in which the transmitter generates a linear frequency modulated signal centered at 10GHz with 2GHz bandwidth based on photonic arbitrary waveform generator (PAWG), and the receiver is based on photonic de-chirping. We conducted a field experiment on this radar system and achieved inverse synthetic aperture radar (ISAR) imaging of non-cooperative targets (airplane), verified the possibility of its application in future radar applications.

In recent years, attacks from pseudo base stations and other kinds of illegal wireless access have emerged one after another, and more and more telecom frauds are causing numerous property losses. There is an urgent need for real-time monitoring of the utilization of wireless spectrum, so as to identify illegal signals and take appropriate protective measures promptly. Traditional wireless spectrum monitoring solutions used to be narrow-band, and due to the insufficient capabilities of data acquisition equipment, the amount of equipment needed is huge, which will lead to the increase of cost, energy consumption and maintenance complexity. We need to transmit signals from various areas to a specific place, where there is a global and full-spectrum perception device that could process and analyze wide-band signals. Therefore, a wireless channel monitoring architecture is proposed and experimentally demonstrated. Based on microwave photonics, this architecture is consisted of an optical-wireless converged network and an all-optical channelized receiver. The frequency error of the channelized receiver is no more than 50 kHz, meeting the requirement for signal analysis in 5G communications. The experiment results show that the proposed architecture could effectively monitor collected signals of different channels.

In this paper, we propose and demonstrate a stable millimeter-wave (mm-wave) local oscillator (LO) phase dissemination trunk network utilizing homogeneous multicore fibers (MCFs). The repetition frequency of an optical frequency comb acts as a probe signal to sense the delay variation by round trip transmission over the different cores of the MCF. The probe signal is then photomixed with its second harmonic to generate a phase-conjugated signal at the local site. At the remote site and an arbitrary access site, the forward phase-conjugated signal is photomixed with the tapped probe signal to obtain a photonic mm-wave signal, of which the phase is stabilized against the environmental perturbations. Simultaneously, the two-stage microwave photomixing implements optical frequency quadrupling of the microwave reference to enable the network to have flexible frequency dissemination capabilities. Herein, the backscattering or optical reflections in the bidirectional transmission can be mitigated through the parallel propagation paths offered by MCFs. A 36-GHz mm-wave LO signal is steadily disseminated through a 1.2-km seven-core fiber trunk network to multiple access sites. The relative frequency stability of around 3 × 10−17 level at 20 000 s averaging time can be realized in this stable phase dissemination network. The crosstalk and skew fluctuation issues of MCFs are also discussed. This scheme presents the application potential of MCFs in stable frequency transfer over fiber.

High-precision imaging of dim-small target is demonstrated using a photonics-assisted broadband distributed coherent aperture radar (DCAR). The photonics-based methods lead to a high resolution, making it possible to discriminate small targets. In addition, the cooperative work of multiple radars enhances the signal-to-noise ratio (SNR) and helps to improve the imaging precision. Experimentally, the inverse synthetic aperture radar (ISAR) coherent imaging for a small unmanned aerial vehicle is conducted by a two-unit X-band DCAR (8-12 GHz), and the range and cross-range resolution are ~3.4 cm and ~3.6 cm, respectively. About 8.5 dB image SNR gain can be achieved in full-coherence mode compared to that in monostatic mode. Taking advantage of the SNR gain, the scatters that are below the noise floor when in monostatic mode can be clearly seen when full-coherence is realized, which reveals that the proposed photonics-assisted DCAR improves the imaging precision and makes it easier to identify dim-small targets.

We propose and demonstrate a photonics-based coherent dual-band radar for super-resolution range profile in this paper. This radar cannot only generate coherent dual-band linear frequency modulated signals with large bandwidths, but also realize a paralleled coherent de-chirping processing to the dual-band echoes simultaneously based on a single compact photonics-assisted transceiver. The sharing of a single transceiver for signals with different bands ensures perfect stability and coherence among the waveforms, which makes it possible to realize a super-resolution range profile by fusion of dual-band signals without heavy computation on phase compensation. In the experiment, the radar operates in S -band with a bandwidth of 1.5 GHz and X -band with a bandwidth of 3 GHz, and results show its range resolution is up to 1.6 cm by coherent fusion processing, which is nearly corresponding to the resolution gotten by a linear frequency modulated wave with a bandwidth of 9.5 GHz ranging from 2 GHz to 11.5 GHz, much higher than the scenario when the radar is operated in the single-band mode. This technology would improve the accuracy of classification and recognition of the targets.

In this paper, we proposed a tunable K/W-band OFDM integrated radar and communication system based on Optoelectronic Oscillator (OEO) for intelligent transportation. All-optical signal processing including amplitude asymmetric filtering and quadratic phase manipulating is applied in OEO to achieve a high-frequency and tunable self-excited oscillation, which supports the K/W-band OFDM signal generation. Its product of maximum detection range and communication capacity is cB/4Δf (m·Gbaud), where c is light speed and Δf is subcarrier spacing of OFDM. A proof-of-concept experiment is carried out in K-band with bandwidth B = 2 GHz and W-band with bandwidth B = 10 GHz. The range resolution ΔR, detection range Rmax and communication capacity C of 0.075 m, 75 m, 12.8 Gbps, and 0.015 m, 300 m, 32 Gbps are experimentally demonstrated in K/W-band respectively.

A system consisted of spectrum acquisition and transmission network and radio frequency channelization receiver is proposed to achieve global and full-spectrum perception, which could help the communication system make better use of the spectrum resources.

This paper reports a photonics-assisted joint radar and communication system for intelligent transportation based on an optoelectronic oscillator (OEO). By manipulating the optical multi-dimensional processing module inserted in the OEO loop, two phase-orthogonal integrated signals are generated with low phase noise and high frequency, as the communication data loaded on the overall polarity of radar pulses. At the receiver, single-channel matched filtering and two-channel IQ data fusion are utilized to retrieve the communication data and the range profile, without any performance deterioration of either. In this way, the contradiction between the performance of two functions existing in the previous scheme is solved, and the integrated performance can be further optimized as bandwidth increases. A proof-of-concept experiment with 2 GHz bandwidth at 24 GHz, which is the operating frequency of short-range automotive radar, is carried out to verify that the proposed system can meet the requirement of the intelligent vehicles in the short-range scene. A communication capacity of 335.6 Mbps, a range profile with a resolution of 0.075 m, and a peak-to-sidelobe ratio (PSLR) of 20 dB is demonstrated under the experimental condition. The error vector magnitude (EVM) curve and constellation diagrams versus received power are measured, where the EVM is −8 and −14.5 dB corresponding to a power of −14 and 6 dBm, respectively.

The previously reported photonics-based radar working with a large bandwidth has the advantages of realizing high-resolution imaging of targets with low velocity. However, the high velocity of a target will introduce Doppler dispersion to the echo signals, which severely deteriorates the imaging resolution. This problem becomes more noticeable as the bandwidth increases. In this paper, we propose a radar receiver based on a reconfigurable photonic fractional Fourier transformer (PFrFTer). The order of the PFrFTer can be reconstructed flexibly by changing the optical transform kernel. When the transform order matches the velocity of the target, the chirp echo signals behave as narrow impulses in the fractional Fourier domain, showing the range information with a high resolution. In the experiment, a PFrFTer is established and applied to process the echo signals with a bandwidth of 12 GHz. A lossless range resolution of 1.4 cm is obtained in range profiles and inverse synthetic aperture radar imaging for high-speed targets. This range resolution is much higher than that in the classical optical de-chirping receiver. These results demonstrate the PFrFTer is immune to the Doppler dispersion effect and is excellent for high-resolution imaging of high-speed target. The introduced technique would be of practical interest in the detection and recognition of targets.

Existing photonic compressive receivers have the problem of resolution deterioration when applied in wide-range radar detection. In this study, we propose a photonic-assisted space-frequency two-dimensional (2D) compressive radar receiver capable of achieving high-resolution detection in wide-range scenarios. For the space dimension, the compression process is realized by employing a spatially adaptive photonic projection basis, which guarantees complete mapping of arbitrarily delayed echoes—the key to high-resolution wide-range detection. For the frequency dimension, photonic compressive sensing is employed to further compress the bandwidth of the projected sparse signal. Therefore, the proposed system can achieve wide-range radar detection without resolution deterioration with compressed output. Herein, with two channels of 630 MHz outputs, high-resolution distance detection within a range of 21 km with a resolution of up to 2.3 cm is achieved. Moreover, inverse synthetic aperture radar (ISAR) imaging of two sets of four-point turntables distributed within the range of 21 km with a resolution of 2.3 cm × 5.7 cm is realized. The proposed photonic-assisted 2D compressive radar receiver is a viable solution to overcome the tradeoff between detection resolution and range of existing photonic compressive receivers, which indicates a path for the further development of high-resolution wide-range radar detection.

In this paper, a broadband photonic beam processor is presented for the all-optical multifunction integrated receiver. By implementing echo signals with optical beam multi-domain processing based on space-to-time mapping and time-to-frequency mapping, the non-mechanical control of expected beam pointing is enabled while the target within the beam can be imaged simultaneously. A proof-of-concept experiment with a 4-element phased array is performed in Ka band. The beam pointing is set to be 0° and 12.5°, where two-dimensional images of moving targets inside the beam region are obtained, respectively. The suppression ratio to the beam region outside is measured to be 26.8 dB. And the range and cross-range imaging resolution is 0.042 m × 0.051 m. A comparison with a cascade mode of single-function microwave photonic modules shows that the multifunction integrated photonic beam processor has reduced the system loss by 32.4 dB. The proposed beam processor enables multi-element broadband phased arrays with less complexity and power consumption.

We propose a microwave photonic 2D time-frequency filter based on a photonic time-frequency prism. A time-varying frequency response is realized by deviating the passband of a 1D ordinary frequency filter in the 2D time-frequency plane. The proposed time-frequency filter features highly reconfigurable frequency-sweeping speed and bandwidth, thanks to the software-defined photonic time-frequency prism. With the proposed technique, separation of multiple linear and nonlinear chirp signals with overlapped spectra is experimentally demonstrated.

Three-dimensional (3D) imaging radar is an advanced sensor applied in space surveillance and target recognition for supplying 3D geometric features and supporting visualization. However, high 3D resolution requires both broadband operation and a large 2D aperture, which are difficult and complex for conventional radars. This paper presents a photonics-enabled distributed multiple-input and multiple-output (MIMO) radar with a centralized architecture. By use of photonic multi-dimensional multiplexing, multi-channel signal generation and reception are implemented on a shared reference signal in a central office, enabling a highly coherent network with a simple structure. Additionally, a sparse array and a synthetic aperture are combined to efficiently reduce the required transceivers, further weakening the dilemma between system complexity and angular resolution. A 4 × 4 MIMO radar is established and evaluated in field tests. A high-resolution 3D image of a non-cooperative aircraft is obtained, in which rich details are displayed. From a comparison with electronics-based radar, significant resolution improvement is observed. The results verify the superior imaging capability and practicability of the proposed radar and its great potential to outperform conventional technologies in target classification and recognition applications.

In this paper, we propose and demonstrate a novel spectrum stitching method for broadband linear frequency-modulated waveform (LFMW) generation. An optical frequency comb (OFC) is modulated by a narrowband LFMW whose bandwidth matches the free spectral range of the OFC. Optical injection locking is employed in extracting one broadband frequency sweeping component from the modulated OFC. In this way, seamless spectrum stitching is realized and a broadband LFMW with a multi-fold time-bandwidth product (TBWP) is obtained. Our scheme has a simple structure, which requires only a single OFC, a modulation module and a baseband waveform generator. An LFMW as broad as 20 GHz is generated from a baseband LFMW with 2GHz bandwidth experimentally. The TBWP is 100 times as large as that of the baseband LFMW. Moreover, the power fluctuation and the phase jumps are both eliminated, ensuring an excellent pulse compression performance. Benefiting from the injection locking technique, the linearity reaches 2.0 × 10−6. The central frequency tuning ability of our scheme is also demonstrated.

In this paper, we present a distributed aperture coherent microwave photonic radar (DCMPR) system by means of a high-precision fiber-optic time-frequency synchronization network (OTFSN). The microwave photonic radar units distributed at different geographic locations are connected with the fiber network. Meanwhile, the time and frequency reference of the central controlling station are stably transferred over the fiber network to each radar unit, of which transmit and receive times are synchronized by the reference signal to cohere the multiple radar apertures. Experimentally, we demonstrate a two-unit DCMPR system with a 12-km OTFSN, where both radar units are operated in X-band and with a bandwidth of 4 GHz. Through the OTFSN, the time difference of the transmitted waveforms at the two radar units can be maintained within about 26 ps. When full coherence on transmit and receive is achieved, the signal-to-noise ratio (SNR) can be increased by about 8.1 dB and 7.9 dB respectively for two unit radars. Moreover, three radar reflectors are clearly imaged and probed by utilizing the mutually coherent operation, yet they are not be detectable by the single radar case.

This paper reports a photonic chirp rates estimator for the piecewise linear frequency modulated waveforms (PLFMWs). The estimator is based on the photonic self-fractional Fourier transform (FrFT) by utilizing the input PLFMW as the transform kernel. In this way, the self-FrFT operation can be finished with short latency time and the chirp rates of all LFM sub-pulses can be retrieved according to their fractional frequencies. In experiment, the chirp rates estimation of a four-stage PLFMW is performed. The measurement range from −964.47 GHz/µs to 50.76 GHz/µs with resolution of 0.0518 GHz/µs is demonstrated for the experimental system. And the absolute measurement error within ±0.06 GHz/µs and the relative measurement error less than 2% is also obtained.

For the first time, to the best of our knowledge, we propose a photonic fractional Fourier transformer (PFrFTer), which is used in chirp radar for detecting multiple non-cooperative targets. Based on photonic rotation of the time-frequency plane, the optimal fractional Fourier domain is formed, and the received broadband chirp signals are projected on it, where they behave as impulses. Moreover, through manipulating the fractional Fourier transform spectrum, the PFrFTer contributes to the cancellation of two ghost target sources, so that the ghost targets in multiple-target circumstances are removed. The simulation and experimental results show that the proposed PFrFTer can adapt to multiple non-cooperative targets environments and is immune to ghost targets at optimal working conditions, which agrees well with the theoretical analysis.

Chromatic dispersion (CD) has been considered as a significant issue that needs to be addressed over the past decades. Nowadays, as the bandwidth of radio frequency signals has expanded to multi-gigahertz, CD becomes increasingly troublesome for some wideband signals such as linear frequency modulated waveform (LFMW), which requires a flat amplitude response and a linear phase response. In this paper, we first investigate the CD-induced amplitude distortion and phase distortion that a wideband signal will suffer in traditional double-sideband (DSB) modulated and single-sideband (SSB) modulated radio-over-fiber (RoF) links with an emphasis on the LFMW. The detrimental impacts of the CD-induced distortions are subsequently discussed based on the pulse compression performance. Then, a two-tap microwave photonics equalizer (MPE) model is proposed and its operating principle for overcoming the CD-induced distortions is demonstrated. Based on the model, we design the MPE, which is mainly composed of an integrated dual-polarization dual-parallel Mach–Zehnder modulator and a differential group delay module. In the proof-of-concept experiment, the RoF link utilizing the proposed MPE is built, and a 1–10 GHz LFMW is transmitted over the 66.3-km single-mode fiber. Thanks to the MPE, both CD-induced amplitude distortion and phase distortion are successfully compensated. Pulse compression results show that the pulse, compared with the signal transmitted in the SSB-based link.