Nonlinear adaptive switching control for a class of non-affine nonlinear systems

An improved nonlinear adaptive switching control method is presented to relax the assumption on the higher order nonlinear terms of a class of discrete-time non-affine nonlinear systems. The proposed control strategy is composed of a linear adaptive controller, a neural network(NN) based nonlinear adaptive controller and a switching mechanism. An incremental model is derived to represent the considered system and an improved robust adaptive law is chosen to update the parameters of the linear adaptive controller. A new performance criterion of the switching mechanism is designed to select the proper controller. Using this control scheme, all the signals in the system are proved to be bounded. Numerical examples verify the effectiveness of the proposed algorithm.

High-resolution mid-infrared single-photon upconversion ranging

Single-photon laser ranging has widespread applications in remote sensing and target recognition.However,highly sensitive light detection and ranging(lidar)has long been restricted in the visible or near-infrared bands.An appealing quest is to extend the operation wavelength into the mid-infrared(MIR)region,which calls for an infrared photon-counting system at high detection sensitivity and precise temporal resolution.Here,we devise and demonstrate an MIR upconversion lidar based on nonlinear asynchronous optical sampling.Specifically,the infrared probe is interrogated in a nonlinear crystal by a train of pump pulses at a slightly different repetition rate,which favors temporal optical scanning at a picosecond timing resolution and a kilohertz refreshing rate over-50ns.Moreover,the cross-correlation upconversion trace is temporally stretched by a factor of 2×10^(4),which can thus be recorded by a low-bandwidth silicon detector.In combination with the time-correlated photon-counting technique,the achieved effective resolution is about two orders of magnitude better than the timing jitter of the detector itself,which facilitates a ranging precision of 4μm under a low detected flux of 8×10^(−5) photons per pulse.The presented MIR time-of-flight range finder is featured with single-photon sensitivity and high positioning resolution,which would be particularly useful in infrared sensing and imaging in photon-starved scenarios.

Mid-infrared photon counting and resolving via efficient frequency upconversion

Optical detectors with single-photon sensitivity and large dynamic range would facilitate a variety of applications.Specifically,the capability of extending operation wavelengths into the mid-infrared region is highly attractive.Here we implement a mid-infrared frequency upconversion detector for counting and resolving photons at 3μm.Thanks to the spectrotemporal engineering of the involved optical fields,the mid-infrared photons could be spectrally translated into the visible band with a conversion efficiency of 80%.In combination with a silicon avalanche photodiode,we obtained unprecedented performance with a high overall detection efficiency of 37%and a low noise equivalent power of 1.8×10^(-17) W∕Hz1∕2.Furthermore,photon-number-resolving detection at mid-infrared wavelengths was demonstrated,for the first time to our knowledge,with a multipixel photon counter.The implemented upconversion detector exhibited a maximal resolving photon number up to 9 with a noise probability per pulse of 0.14%at the peak detection efficiency.The achieved photon counting and resolving performance might open up new possibilities in trace molecule spectroscopy,sensitive biochemical sensing,and free-space communications,among others.

Environmentally stable Er-fiber mode-locked pulse generation and amplification by spectrally filtered and phase-biased nonlinear amplifying long-loop mirror

We report on environmentally stable long-cavity ultrashort erbium-doped fiber lasers,which self-start mode-locking at quite low thresholds by using spectrally filtered and phase-biased nonlinear amplifying long-loop mirrors.By employing 100-m polarization-maintaining fiber(PMF)in the nonlinear loop,the fundamental repetition rate reaches 1.84 MHz and no practical limitation is found to further decrease the repetition rate.The filter used in the long loop not only suppresses Kelly sidebands of the solitons,but also eliminates the amplified spontaneous emission which exists widely in lowrepetition-rate ultrafast fiber lasers.The bandwidth of the filter is optimized by using a numerical model.The laser emits approximately 3-ps pulses with an energy of 17.4 p J,which is further boosted to 1.5μJ by using a fiber amplifier.

Scalar optical hopfions

Hopfions are three-dimensional(3D)topological states discovered in field theory,magnetics,and hydrodynamics that resemble particle-like objects in physical space.Hopfions inherit the topological features of the Hopf fibration,a homotopic mapping from unit sphere in 4D space to unit sphere in 3D space.Here we design and demonstrate dynamic scalar optical hopfions in the shape of a toroidal vortex and expressed as an approximate solution to Maxwell’s equations.Equiphase lines correspond to disjoint and interlinked loops forming complete ring tori in 3D space.The Hopf invariant,product of two winding numbers,is determined by the topological charge of the poloidal spatiotemporal vortices and toroidal spatial vortices in toroidal coordinates.Optical hopfions provide a photonic testbed for studying topological states and may be utilized as high-dimensional information carriers.

Study on the key technology of spectral reflectivity reconstruction based on sparse prior by a single-pixel detector

By studying the traditional spectral reflectance reconstruction method, spectral reflectance and the relative spectral power distribution of a lighting source are sparsely decomposed, and the orthogonal property of the principal component orthogonal basis is used to eliminate basis; then spectral reflectance data are obtained by solving a sparse coefficient. After theoretical analysis, the spectral reflectance reconstruction based on sparse prior knowledge of the principal component orthogonal basis by a single-pixel detector is carried out by software simulation and experiment. It can reduce the complexity and cost of the system, and has certain significance for the improvement of multispectral image acquisition technology.

Non-approximate method for designing annular field of two-mirror concentric system

Annular field aberrations of a three-reflection concentric system,which are composed of two spherical mirrors,are analyzed.An annular field with a high level of aberration correction exists near the position where the principal ray is perpendicular to the object-image plane.Aberrations are determined by the object height and aperture angle.In this letter,the general expression of the system aberration is derived using the geometric method,and the non-approximate design method is proposed to calculate the radii of the annular fields that have minimum aberrations under different aperture angles.The closer to 0.5（the ratio of the radius of convex mirror to the radius of concave mirror）is,the smaller the system aberration is.The examples analyzed by LABVIEW indicate that the annular field designed by the proposed method has the smallest aberration in a given system.

60 nJ, 95 fs environmentally stable Er-doped fiber laser system

We demonstrate here an environmentally stable and extremely compactable Er-doped fiber laser system capable of delivering sub-100-fs temporal duration and tens of nanojoules at a repetition rate of 10 MHz.This laser source employs a semiconductor saturable absorber mirror mode-locked soliton laser to generate seed pulses.A singlemode-fiber amplifier and a double-cladding-fiber amplifier(both with double-pass configuration)are bridged by a divider and used to manage the dispersion map and boost the soliton pulses.By using 64 replicas,pulses with as high as 60 n J energy within 95 fs duration are obtained at 10 MHz,corresponding to 600 kW peak power.

Cubic polynomial curve-guided method for isochromatic determination in three-fringe photoelasticity

A method for isochromatic determination in three-fringe photoelasticity is presented. It combines the phaseshifting method with cubic polynomial curve-fitting technology to eliminate the errors caused by color repetition.We perform a demonstration of the method on a circular disc subjected to compressive loading and an injectionmolded cover with residual stresses. The test results compare well with the theoretical results.