We report on the observation of a non-uniform dark count rate in Superconducting Nanowire Single Photon Detectors (SNSPDs), specifically focusing on an afterpulsing effect present when the SNSPD is operated at a high bias current regime. The afterpulsing exists for real detection events (triggered by input photons) as well as for dark counts (no laser input). In our standard set-up, the afterpulsing is most likely to occur at around 180 ns following a detection event, for both real counts and dark counts. We characterize the afterpulsing behavior and speculate that it is not due to the SNSPD itself but rather the amplifiers used to boost the electrical output signal from the SNSPD. We show that the afterpulsing indeed disappears when we use a different amplifier with a better low frequency response. We also examine the short-lived enhancement of detection efficiency during the recovery of the SNSPD due to temporary perturbation of the bias and grounding conditions.

Due to short coherent length of the laser, traditional single frequency lidar (SFL) uses echoes of adjacent samples to retrieve Doppler wind velocity, which inevitably brings about a dilemma of high range resolution and velocity resolution. Dual-frequency lidar (DFL), however, with a frequency difference locating at the radar band, can process the laser echoes with a radar processing framework while maintaining a good wind sensing ability in clear air. In this paper, performances of SFL and DFL are analyzed in terms of different conditions like pulse duration, number of sampling points in processing, wind distribution, signal to noise ratio (SNR), etc. It is shown that, for a complex wind field, DFL has the potential to retrieve the Doppler velocity with high velocity and range resolution, even in a relatively low SNR condition.

We report on the switchable generation of versatile patterns of multiple noise-like pulse (NLP) and dissipative soliton resonance (DSR) pulses in a dual-pump figure-eight mode-locked fiber laser. Benefiting from the change of saturation power influenced by pump power intercoupling, harmonic DSR pulses, dual-pulse DSR, dual-pulse and tri-pulse NLP are formed depending on different pump power combinations and intra-cavity polarization states. We find that the NLP pulse profile can turn from h-shape to triangular-shape after splitting. The dual-wavelength spectrum of the pulse, consisting of an obvious stokes line, can tune its center from 1075 to 1097 nm. This type of fiber laser may be helpful for further in-depth study on the underlying dynamics of multiple NLP and DSR pulses.

It is quite challenging for conventional estimators to extract the radial wind velocities from the raw spectrum data of coherent Doppler wind Lidar (CDWL) at very low signal-to-noise ratios (SNR). This work proposes a new wind velocity estimation method based on the constraints from neighboring wind profiles, which can effectively and robustly extend the reliable detection range of CDWL. Considering the spatial continuity of wind field, priori information from the velocity probability distributions in previous range bins is utilized to reshape the contaminated spectrum of the current range bin. The influences of different preceding range bins are weighted by their overlapping ratios to the contaminated range bin. Iterations are carried out when computing the variance of the Gaussian-shaped probability, which can preserve the wind field details while removing the parameter dependence in practical applications. The method is verified theoretically in simulated atmosphere echoes as well as experimentally in our self-developed dual-frequency pulsed CDWL system. First results show that, reasonable estimates can be obtained in places far beyond the previous detection boundary provided by conventional estimators.

We numerically present the generation of four types of pulses using nonlinear Schrödinger equations in an all-normal-dispersion dual-pump mode-locked fiber laser based on a nonlinear amplifying loop mirror (NALM). Transition state (TS) between dissipative soliton (DS) and dissipative soliton resonance (DSR), typical DSR, DS, noise-like pulse (NLP), and nonmode-locked state are investigated in a two-dimensional pump power space. Different from previous research, we first find that the pulse peak power and width vary dependently and nonmonotonously under asymmetric coupling ratios in the NALM. Under different coupler ratios and amplifier powers, the shift and distortion of simulated NALM transmittivity curve which are caused by the pump power intercoupling and the amplifier saturation are demonstrated. The ratio of incident power and the saturable power of NALM (when the ratio is >2, 1 ∼2, ∼1, <1) directly determines the pulse states (NLP, DSR, unstable, DS, or TS) in the cavity. Overall, the utilization of NALM could effectively enlarge the saturable power as well as the transmittivity, which provide a road to increase the DSR peak power.

The generalized Radon–Fourier transform (GRFT) is a well-established coherent accumulation technique for high-speed and high-mobility target detection. However, this method tends to suffer from the difficulty of identifying the main lobe from multiple blind speed sidelobes (BSSLs) and the computational complexity is generally high. To address these challenges, we propose a new method, namely the BSSL Traction Particle Swarm Optimization (BTPSO), to robustly and accurately extract the main lobe. In the method, the relationship between the main lobe and the BSSLs is used to attract particles to potential positions of the main lobe in the group when trapped in local optimal, and a new termination criterion in which multiple particles should converge to the same optimal value is proposed to avoid local convergence. Simulation examples show that the proposed method can improve the probability of converging to the main lobe peak while reducing cost time, and its good adaptability to low signal-to-noise ratio (SNR) cases is well verified.