Photonic architecture for remote multi-parameter measurement and transmission of microwave signals.

A photonic architecture for remote multi-parameter measurement and transmission of microwave signals is proposed and demonstrated, which utilizes a dual-parallel dual-drive Mach-Zehnder modulator (DP-DDMZM) in the antenna unit and a dual-drive Mach-Zehnder modulator (DDMZM) in the processing unit. Doppler frequency shift (DFS) and angle of arrival (AOA) can be determined by analyzing the down-converted intermediate frequency signals. Introducing a reference signal in the processing unit ensures DFS measurement without directional ambiguity. The proposed architecture can also be applied for instantaneous frequency measurement based on down-conversion. Due to the use of optical single sideband modulation, long-distance transmission of radio frequency (RF) signals without dispersion-induced power fading can be achieved. Experiments for accurate and stable DFS and AOA measurement as well as long-distance RF signal transmission with dispersion-induced power fading are presented. The approach avoids the use of optical filters and polarization-related devices, facilitating wideband and stable operation, which is highly desirable. The proposed architecture is a potential solution for microwave photonic antenna remoting, offering support for both remote transmission and multi-parameter measurement.

Harmonic active mode-locking optoelectronic oscillator with suppressed supermode noise based on pulse intensity feedback.

A harmonic active mode-locking optoelectronic oscillator (HAML-OEO) with pulse intensity feedback is proposed and experimentally demonstrated. It is capable of generating microwave pulses characterized by suppressed supermode noise, uniform intensity, and tunable repetition rates. Unlike traditional HAML-OEOs, active mode-locking and pulse intensity feedback are simultaneously achieved through the use of a dual-drive Mach-Zehnder modulator (DDMZM). By synchronously feeding back the generated microwave pulses to the DDMZM, each pulse undergoes a loss proportional to its intensity, facilitating pulse intensity equalization and supermode noise suppression. In the experiment, intensity-equalized microwave pulse trains with repetition rates of 499 kHz and 998 kHz are generated by the 5th- and 10th-order HAML-OEOs, respectively, with the measured supermode noise suppression ratios exceeding 40 dB.

Instantaneous frequency measurement based on photonic compressive sensing with sub-Nyquist pseudo-random binary sequences.

We propose a novel, to the best of our knowledge, approach to realizing instantaneous frequency measurement with ultrahigh measurement bandwidth, which utilizes three-channel photonic compressive sensing (CS) with sub-Nyquist pseudo-random binary sequences (PRBSs). In each CS channel, an alias frequency is recovered due to the sub-Nyquist property of the applied PRBS. A frequency identification algorithm is employed to determine the frequency of the signal under measurement according to the three alias frequencies. The proposed approach significantly reduces the bit rate of the applied PRBSs and the sampling rate required by the digitizers in CS. A proof-of-concept experiment for measuring frequency in the Ku band is demonstrated using PRBSs at 1 Gb/s and digitizers with a sampling rate of 250 MS/s.

Photonic generation and transmission for an X-band dual-chirp waveform with frequency multiplication and power-fading compensation.

The generation of an X-band dual-chirp waveform, which is capable of pulse compression, plays an important role in radar, electric warfare, and satellite communication systems. With the development of applications such as multi-static radar, transmission over long distances has attracted considerable attention. In this paper, a photonic system for X-band dual-chirp waveform generation and transmission based on frequency multiplication and power-fading compensation is put forward and experimentally carried out. Based on a compact dual-parallel Mach-Zehnder modulator (DPMZM), the dual-chirp waveforms of 8.6-9.6 GHz and 9.6-10.6 GHz are generated by an RF carrier of 4.8 GHz and transmitted through a 40 km single-mode fiber (SMF) spool. The dispersion-induced power fading of the chirp waveform is compensated for by about 13 dB. The full width at half maximum (FWHM) and the peak-to-sidelobe ratio (PSR) of the compressed pulses are 1 ns and 11.5 dB, respectively. Moreover, the compensation of power fading in the entire X-band is verified to demonstrate the applicability of our system. By flexibly adjusting the bias voltage of the built-in phase shifter, the system can be applied in more scenarios.

Association Between Hypoperfusion Intensity Ratio and Postthrombectomy Malignant Brain Edema for Acute Ischemic Stroke.

BACKGROUND: Malignant brain edema (MBE) is a life-threatening complication that can occur after mechanical thrombectomy (MT) for acute ischemic stroke. The hypoperfusion intensity ratio (HIR) reflects the tissue-level perfusion status within the ischemic territory. This study investigated the association between HIR and MBE occurrence after MT in patients with anterior circulation large artery occlusion. METHODS: We conducted a retrospective cohort study of patients who received MT at a comprehensive stroke center from February 2020 to June 2022. Using computed tomography perfusion, the HIR was derived from the ratio of tissue volume with a time to maximum (Tmax) > 10 s to that with a Tmax > 6 s. We dichotomized patients based on the occurrence of MBE following MT. The primary outcome, assessed using a multivariable logistic regression model, was the MBE occurrence post MT. The secondary outcome focused on favorable outcomes, defined as achieving a modified Rankin Scale score of 0-2 at 90 days. RESULTS: Of the 603 included patients, 90 (14.9%) developed MBE after MT. The median HIR exhibited a significantly higher value in the MBE group compared with the non-MBE group (0.5 vs. 0.3; P < 0.001). Multivariable logistic regression analysis indicated that a higher HIR (adjusted odds ratio [aOR] 8.98; 95% confidence interval [CI] 2.85-28.25; P < 0.001), baseline large infarction (Alberta Stroke Program Early Computed Tomography Score < 6; aOR 1.77; 95% CI 1.04-3.01; P = 0.035), internal carotid artery occlusion (aOR 1.80; 95% CI 1.07-3.01; P = 0.028), and unsuccessful recanalization (aOR 8.45; 95% CI 4.75-15.03; P < 0.001) were independently associated with MBE post MT. Among those with successful recanalization, a higher HIR (P = 0.017) and baseline large infarction (P = 0.032) remained as predictors of MBE occurrence. Furthermore, a higher HIR (P = 0.001) and the occurrence of MBE (P < 0.001) both correlated with reduced odds of achieving favorable outcomes. CONCLUSIONS: The presence of a higher HIR on pretreatment perfusion imaging serves as a robust predictor for MBE occurrence after MT, irrespective of successful recanalization.

Photonic compressive sampling of wideband sparse radio frequency signals with 1-bit quantization.

Photonic compressive sampling (PCS) is an effective method to recover wideband sparse radio frequency (RF) signals. However, the noisy and high-loss photonic link leads to signal-to-noise ratio (SNR) degradation of the RF signal to be tested, which limits the recovery performance of the PCS system. In this paper, a random demodulator-based PCS system with 1-bit quantization is proposed. The system consists of a photonic mixer, a low-pass filter, a 1-bit analog-to-digital converter (ADC), and a digital signal processor (DSP). The 1-bit quantized result is used to recover the spectra of the wideband sparse RF signal with the binary iterative hard thresholding (BIHT) algorithm, which can alleviate the negative impact of the SNR degradation caused by the photonic link. A full theoretical framework of the PCS system with 1-bit quantization is given. Simulation results show that the PCS system with 1-bit quantization can provide better recovery performance than the traditional PCS system under low SNR and stringent bit budget.

Photonic distributed compressive sampling of multi-node wideband sparse radio frequency signals.

A photonic distributed compressive sampling (PDCS) approach for identifying the spectra of multi-node wideband sparse signals is proposed. The scheme utilizes wavelength division multiplexing (WDM) technology to transmit multi-node signals to a central station, where distributed compressive sampling (DCS) based on the random demodulator (RD) model is employed to simultaneously identify the signal spectrum. By exploiting signal correlations among nodes, DCS achieves a higher compression ratio of the sampling rate than single-node compressive sampling (CS). In a semi-physical simulation experiment, we demonstrate the feasibility of the approach by recovering the spectra of two wideband sparse signals from nodes located 20 km and 10 km away. The spectra of two signals with a mixed support-set sparsity of 2 and 4 are recovered with a compression ratio of 8 and 4, respectively. We further investigate the impact of common parts and the number of nodes on PDCS performance through numerical simulation. The proposed system takes advantage of the ultra-high bandwidth of photonic technology and the low loss of optical fiber transmission, making it suitable for long-distance, multi-node, and large-coverage electromagnetic spectrum identification.

Photonics-assisted compressed sensing of radio frequencies with a rate-doubled bipolar random sequence.

A novel, to the best of our knowledge, photonics-assisted scheme for compressed sensing (CS) of sparse RF signals is proposed. An architecture with time-delayed dual-channel modulation of a pseudo-random binary sequence in combination with differential detection enables the generation of a rate-doubled bipolar random sequence, which largely increases the bandwidth of the CS system. In addition, the bipolarity of the random sequence ensures a zero-mean measurement matrix, which helps improve the signal recovery performance. Experimental results are presented to verify the performance improvement of the approach in comparison with the conventional single-ended photonic CS system.

Effect of leukoaraiosis on collateral circulation in acute ischemic stroke treated with endovascular therapy: a meta-analysis.

BACKGROUND AND OBJECTIVE: The recruitment of collateral circulation correlates with a balance of the microvasculature. Uncertainty remains to be made about the association of leukoaraiosis with leptomeningeal collaterals. To explore the effect of leukoaraiosis on leptomeningeal collaterals in patients treated with endovascular therapy. METHODS: Observational studies exploring the correlation between leukoaraiosis and leptomeningeal collaterals in large vessel occlusion treated with endovascular therapy were searched from PubMed, EMBASE, and Cochrane Libraries databases. Two independent reviewers retrieved eligible literature, extracted purpose-related data, and utilized the Newcastle-Ottawa Scale to evaluate the risk of bias. A Mantel-Haenszel method was used to calculate the odds ratio (OR). Meta-regression and subgroup analyses were conducted to clarify heterogeneity. RESULTS: Data from 10 studies with 1606 patients were extracted for pooled analysis. Compared to non-severe leukoaraiosis, patients with severe leukoaraiosis showed significant relevance to poor leptomeningeal collaterals (OR, 2.13; 95% confidence interval [1.27-3.57]; P = 0.004). Meta-regression indicated that sample size (coefficient = -0.007299, P = 0.035) and the number of female patients (coefficient = -0.0174709, P = 0.020) were sources of heterogeneity. Furthermore, all of the countries (USA versus France versus China, Q = 3.67, P = 0.159), various assessment scales of leukoaraiosis (the Fazekas scale versus Non-Fazekas scales, Q = 0.77, P = 0.379), and different imaging methods of leukoaraiosis (computed tomography versus magnetic resonance imaging, Q = 2.12, P = 0.146) and leptomeningeal collaterals (computed tomography angiography versus digital subtraction angiography, Q = 1.21, P = 0.271) showed no contribution to the effect size. CONCLUSION: Severe leukoaraiosis is associated with poor leptomeningeal collaterals in patients treated with endovascular therapy. Further studies may focus on whether the finding applies to different stroke subtypes.

Broadband and linearized photonic time-stretch analog-to-digital converter based on a compact dual-polarization modulator.

A broadband linearization scheme for a photonic time-stretch (PTS) analog-to-digital converter (ADC) is proposed based on a dual-polarization binary phase-shift keying (DP-BPSK) modulator with complementary parallel single-sideband (SSB) modulation. In this system, two stretched radio frequency signals, corresponding to the two orthogonal polarizations of the DP-BPSK modulator, are complementary, which can realize differential operation to eliminate the second-order harmonic distortion and pulse envelope distortion. Meanwhile, SSB modulation is used to avoid dispersion-induced power fading. Theoretical analysis and simulation results show that the proposed scheme can effectively improve the linear performance of PTS ADC, even considering the polarization-dependent delay deviation and amplitude offset.

Photonic generation of dual-band dual-chirp waveforms with anti-dispersion transmission.

A photonic approach for generating dual-band dual-chirp waveforms with the capability of anti-dispersion transmission is proposed. In this approach, an integrated dual-drive dual-parallel Mach-Zehnder modulator (DD-DPMZM) is adopted to realize single-sideband modulation of a RF input and double-sideband modulation of baseband signal-chirped RF signals. By properly presetting the central frequencies of the RF input and the bias voltages of DD-DPMZM, dual-band dual-chirp waveforms with anti-dispersion transmission can be achieved after photoelectronic conversion. A complete theoretical analysis of the operation principle is presented. Full experimental verification of the generation and anti-dispersion transmission of dual-chirp waveforms centered at 2.5 and 7.5 GHz as well as 2 and 6 GHz over two dispersion compensating modules with dispersion values equivalent to 120 km or 100 km standard single-mode fiber is successfully carried out. The proposed system features a simple architecture, excellent reconfigurability, and immunity to dispersion-induced power fading, which are highly desired in distributed multi-band radar networks with optical-fiber-based transmission.

Photonic generation of dual-mode multi-format chirp microwave signals.

In this paper, we present a novel, to the best of our knowledge, photonic scheme for the generation of dual-mode multi-format chirp microwave signals, utilizing a dual-drive dual-parallel Mach-Zehnder modulator (DD-DPMZM). By inputting a single-chirp signal and controlling the input binary sequences, the proposed method can generate up-, down-, dual-, or triangular-chirp signals in both pulse and continuous-wave modes. Moreover, the duty cycle of the generated chirp signals in the pulse mode can be easily adjusted by manipulating the injected binary sequences. The compact structure of the proposed scheme eliminates the need for polarization control in signal switching and avoids the use of any optical filter. Experimental verification confirms the feasibility of our approach, while also pointing towards its promising applications in multi-functional radar systems.

Impact of Cerebral Microbleeds on Gait, Balance, and Upper Extremities Function in Cerebral Small Vessel Disease.

BACKGROUND AND PURPOSE: White matter hyperintensites (WMHs) , lacunes and brain atrophy have been demonstrated to be positively related to gait disorder. However, cerebral microbleeds (CMBs) as a manifestation of cerebral small vessel disease (CSVD) is still under-investigated. Therefore, correlations between CMBs and upper extremity, gait and balance performance were investigated in this study. METHODS: A cross-sectional study of middle-aged to older adults was conducted. CSVD burden was measured with magnetic resonance imaging (MRI) and the location and number of CMBs were analysed. Gait and balance functions were evaluated using a four meter walkway, Tinetti, Timed-Up-and-Go (TUG) and Short Physical Performance Battery (SPPB) tests. Upper extremity function was measured by 10 repeated pronation-supination time, 10 repeated finger tapping time, and 10 repeated opening and closings of the hands. RESULTS: A total of 224 participants were included in this study, with a mean age of 60.6 ± 10.5 years. The prevalence of CMB was 34.8% and most was lobar. Multiple linear regression analysis showed that CMB was associated with lower gait velocity, wider stride width, longer TUG test time, and poor performance on Tinetti and SPPB tests independently of other coexisting CSVD markers and risk factors. These relationships appeared to be explained by CMBs in the frontal, temporal, basal ganglia and infratentorial regions. The motor function of upper extremity also had independent correlations with CMBs especially in frontal, parietal, and temporal areas, and in the basal ganglia. CONCLUSIONS: CMBs were found to be associated with both gait, balance and upper extremity disturbances. The presence of CMB seems to be another major driving force for CSVD on lower and upper extremity impairment in healthy elderly subjects.

Polarization multiplexed active mode-locking optoelectronic oscillator for frequency tunable dual-band microwave pulse signals generation.

We propose and experimentally demonstrate a polarization multiplexed active mode-locking optoelectronic oscillator (AML-OEO) based on a single dual-polarization binary phase-shift keying (DP-BPSK) modulator for frequency tunable dual-band microwave pulse signal generation. In order to realize mode-locking, two single-tone signals whose frequency are integer multiple of the free spectrum range (FSR) of AML-OEO are applied as active modulation signals (AMSs) at the bias ports of the DP-BPSK modulator. By dividing the AML-OEO into two loops with polarization demultiplexing, both the carrier frequency and pulse repetition frequency (PRF) of the dual-band microwave pulses are independently adjustable. In the experiment, microwave pulses with different PRFs of 162.4 kHz, 324.8 kHz and 812 kHz are generated based on fundamental, second-order harmonic and fifth-order harmonic mode-locking, respectively. In addition, the carrier frequency tunability within 4∼10 GHz is verified by inserting a frequency tunable electrical filter. The phase noise of the generated pulse signal at 10 kHz offset is better than -125 dBc/Hz.

Wideband sparse signal acquisition with ultrahigh sampling compression ratio based on continuous-time photonic time stretch and photonic compressive sampling.

In this paper, an approach to realizing wideband sparse signal acquisition with an ultrahigh sampling compression ratio based on continuous-time photonic time stretch (CT-PTS) and photonic compressive sampling (PCS) is proposed and experimentally demonstrated. In the system, a wideband sparse signal is slowed down in the time domain by a CT-PTS module and then down-sampled and reconstructed by a random-demodulator-based PCS scheme in which random mixing is realized with a pseudo-random binary sequence. Virtual time gating based on wavelength-to-time mapping and wavelength division multiplexing is used to realize CT-PTS to increase the length of the sampling window and finally improve the performance of PCS. In addition, single sideband modulation is employed to solve the problem of dispersion-induced power fading in PTS and therefore increase the bandwidth of the system. Due to the techniques of CT-PTS and PCS, wideband sparse signals can be acquired with sampling rates far below the Nyquist rate of the original signal. In the experiment, a sparse signal within 2-40 GHz bandwidth is successfully recovered with a sampling rate of 800 MS/s, which means a sampling compression ratio as high as 100.

Compressive sensing based on optical mixing using a spectral shaper with bipolar coding.

Photonic compressive sensing (CS) has attracted great research interest for its potentials in the acquisition of wideband sparse signals with relatively low sampling rate. The photonic CS scheme based on optical mixing using a spectral shaper can realize the mixing of a sparse signal with a high-speed pseudo-random bit sequence (PRBS), but avoids the use of high-speed electronics. In this approach, by utilizing the frequency-to-time mapping (FTTM) of chirped pulses, the spectral information on the spatial light modulator (SLM) within a spectral shaper can be projected into the time-domain waveform. However, the generated PRBS in the time domain is a unipolar sequence that alternates between 0 and 1, which leads to a nonzero-mean measurement matrix. This would result in a poorer performance of signal reconstruction compared to that with a zero-mean measurement matrix. Moreover, the length of PRBS that can be recorded in the SLM is also limited by the far-field condition. In this paper, we propose an optical mixer for photonic CS, which utilizes an SLM-based spectral shaper with complementary outputs as well as a balanced photodetector in order to generate bipolar PRBS. The performance of signal reconstruction can be significantly improved owing to the zero-mean measurement matrix induced by bipolar PRBS. In addition, the constraint on the length of PRBS can be greatly alleviated, since the obtained PRBS can still be kept zero-mean even if the PRBS is longer than that the far-field condition demands. Experimental and simulation results are presented to demonstrate the feasibility and advantage of the given approach.

Photonic arbitrary waveform generation based on the temporal Talbot effect.

In this paper, we propose a novel photonic approach for generating arbitrary waveform. The approach is based on the property of real-time Fourier transform in the temporal Talbot effect, where the spectrum of the modulating analog signal is converted into the output time-domain waveform in each period. We present a concise and strict theoretical framework to reveal the relationship of real-time Fourier transform between the optical signals before and after the dispersion. A proof-of-concept experiment is implemented to validate the presented theoretical model. We propose to generate symmetrical or asymmetrical arbitrary waveforms by using double-sideband or single-sideband modulation, respectively, which is verified by simulation results. It is shown that the given approach can be used to generate a repetition-rate multiplied optical pulse train with arbitrary waveform by simply using a multi-tone RF signal with appropriate frequencies and powers.

Photonic approach for generating bandwidth-doubled and switchable multi-format chirp waveforms.

A novel photonic approach for generating switchable multi-format chirp RF waveforms is proposed and experimentally demonstrated. The key component in the scheme is a dual-parallel Mach-Zehnder modulator (DPMZM). By properly setting the bias voltage on the DPMZM, different chirp RF waveforms with a doubled bandwidth, including the dual-, up-, and down-chirp waveforms, can be generated, when an unmodulated RF signal and a chirp RF signal are injected. The experiments are implemented to demonstrate the generation of switchable chirp waveforms with bandwidths of 2 and 0.8 GHz. The proposed scheme features its compact structure, easiness in switching control, as well as bandwidth doubling, which has potential applications in multifunctional radars.

Distortion compensation in continuous-time photonic time-stretched ADC based on redundancy detection.

In continuous-time photonic time-stretched analog-to-digital conversion (PTS-ADC), a wavelength-division multiplexer (WDM) is widely used to separate the overlapped stretched signal segments in both the wavelength and time domains. However, the spectrum shape of the WDM is not an ideal rectangle, and signal distortion occurs when we reconstruct the stretched signal by connecting the segments carried by the rising or falling edges of the spectrum. A signal distortion compensation scheme based on redundancy detection is proposed in this paper. Two WDMs with complementary spectrum division are employed; if we properly set the central wavelength of each channel of both WDMs, the signal segments that might be distorted by inter-channel mismatch can be redundantly detected and the distortion can be completely avoided. Moreover, by adjusting the dispersion amount of the first dispersive medium, the optical carrier obtained by connecting the stretched chirped pulses is partly overlapped, which guarantees the redundant modulation of signal segments that might be distorted by inter-pulse mismatch. Accordingly, the signal distortion induced by inter-pulse mismatch can be perfectly removed. In addition, the effective bandwidth of each chirped optical pulse is given. The maximum system stretch factor is also derived in this paper, and experiments based on the proposed signal distortion compensation scheme are carried out. The results obtained provide instructive guidelines for the design of a continuous-time PTS-ADC with the desired performance.

Photonic time-stretch based on phase modulation for sub-octave applications.

Photonic time-stretch (PTS) has been extensively studied due to its great potential in analog-to-digital converters. Here, we propose and demonstrate a PTS system based on phase modulation for sub-octave applications. Different from the PTS system using a Mach-Zehnder modulator (MZM), the PTS system, which uses a phase modulator (PM), has an operation bandwidth within an octave and is more suitable for preprocessing of sub-octave signals. Within the sub-octave band, the system is free of all second-order spurious signals. Because there is no direct current bias in a PM, the problem of bias drift, as well as the nonlinear distortion caused by it, can be thoroughly avoided. In addition, based on phase modulation and direct detection, the proposed PTS system has higher stability and a more simplified structure than that based on coherent detection. An exact analytical model has been established, and some compact expressions have been derived to fully characterize all frequency components of the PM-based PTS system. System properties, including the power transfer function, 3-dB bandwidth, and nonlinear distortion have been discussed, and numerical and experimental results on the performance of the PM-based PTS have been presented. In addition, a dual-channel PTS that employs a PM and a push-pull MZM has been proposed to extend the operation bandwidth to multi-octave.