We report a five-step processing algorithm for photon counting depth imaging under strong background noise environment, and experimentally verified that this method can realize single photon counting 3D imaging under signalto-noise ratio (SNR) less than 1. In order to accurately locate the target position when the ambient flux is high, a computational pile-up correction is performed to recover the underlying signal photons, then performing an adaptively full-pixel target position locating. After the target position is determined, the signal and noise photons are separated pixel-wisely using a cluster method. Pixels which have no arrival photons are filled by using the information from neighborhood. At last, by using total variation spatial regularization, the depth images are reconstructed accurately. To validate the proposed method, a single photon counting 3D imaging system is established and experiments at different noise levels are carried out. Experimental results show that accurate depth imaging can be reconstructed with the SNR as low as 0.41. This approach is suitable for depth imaging under high background noise and also very suitable for the noncooperative target imaging with no prior knowledge of the target distance for its adaptive range gating.
We presented three-dimensional image including reflectivity and depth image of a target with two traditional optical imaging systems based on time-correlated single photon counting technique (TCSPC), when it was illuminated by a MHz repetition rate pulsed laser source. The first one is bi-static system of which transmitted and received beams path are separated. Another one called mono-static system of which transmit and receive channels are coaxial, so it was also named by transceiver system. Experimental results produced by both systems showed that the mono-static system had more advantages of less noise from ambient light and no limitation about field area of view. While in practical applications, the target was far away leading to there were few photons return which was prejudicial to build 3D images with traditional imaging system. Thus an advanced one named first photon system was presented. This one was also a mono-static system on hardware system structure, but the control system structure was different with traditional transceiver system described in this paper. The difference was that the first return photon per pixel was recorded across system with first photon system, instead of overall return photons per pixel. That’s to say only one detected return photon is needed for per pixel of this system to rebuild 3D images of target with less energy and time.
Based on previous researches, we construct a pseudo-thermal light ghost imaging system suited for remote imaging applications. By using pulsed pseudo-thermal light, the transmitted power is improved to ghost imaging long distant targets. By using imaging lens system, the path lengths of reference and signal light need not keep equal, as in lensless ghost imaging system, thus the transmitter, receiver, and correlator circuit can be integrated and keep compact. Furthermore, the revolution is improved by reducing the sizes of speckles. And the number of imaging frames is decreased (thus reduced the image-reconstruct time) and the signal-noise-ratio of ghost image is improved by compressed sensing. Based on the constructed experimental system, we implemented ghost imaging of a target at about 30m range.
We demonstrate the methods for increasing the observed squeezing level in the generation of squeezed states of light at
1064 nm with periodically poled KTiOPO4 (PPKTP) crystal. We analyze the technical limits to the reduction of noise in
the squeezed quadrature theoretically, including the intra-cavity loss of the optical parametric amplifier (OPA) cavity, the
normalized amplitude, the detection frequency, and the mode matching on the balanced homodyning stage. To observe a
high degree of squeezing, we improve the quality of mode matching into the OPA cavity and the mode cleaning (MC)
cavity experimentally. By optimizing mode matching of the light into cavities, the finesse of the cavities would be higher
in practice and the desirable spatial mode can be realized. As the intensity noise of the laser light reach the shot noise
limit above the frequency of 1 MHz, which is sufficient for our experiment, so the MC cavity is incorporated to filter
higher-order transverse modes of the local oscillator (LO) beam for the balanced homodyne detection. The experimental
and theoretical results show that it is essential to optimize the mode matching efficiency to generate and detect high
degree of squeezing otherwise an inefficiency mode matching will throw away the squeezing and transform the squeezed
field into a vacuum field.
We analyze the balanced homodyne detection technique in the detection of squeezed light, which is controlled by dither
locking scheme. We discuss how the balanced homodyne detection efficiency influences the detected degree of
squeezing. Also, fluctuation in the relative phase between the local beam and the squeezed light is discussed, since a little
phase fluctuation would decrease the detected degree of squeezing greatly. Then, the dither locking technique is studied
in detail, which is used to lock the relative phase between the local beam and the squeezed light. The simulation
experiments and theoretically results show that the balanced homodyne detection technique and the dither locking
scheme are efficient methods to get more accurate degree of squeezing in the preparation of the squeezed states of light.
Ghost imaging (correlated imaging) has been extensively investigated in recent years, both theoretically and
experimentally. By using the second-order or high-order coherence properties of light field and the correlation
measurement, ghost imaging was realized with quantum entangled light, pseudo-thermal light and even true thermal light.
In this work, basing on the theory of statistical optics, we model the dynamic process of thermal variation, and obtain the
ghost interference and ghost imaging by means of simulated calculation. In the later experiment, a pseudo-thermal source
is firstly prepared by using a laser beam to pass through a rotating ground glass plate, and the parameters of the
pseudo-thermal source are obtained via Hanbury-Brown-Twiss (HBT) experiment. With the pseudo-thermal light, we
perform ghost interference. The experimental results demonstrate the accordance of numerical prediction. And our
conclusion shows that the quality of ghost interference is influenced by the size of the pinhole in the reference path, the
little pinhole due to a higher quality of ghost interference.
Photoexcitation of biased semiconductor photoconductive antennas by femtosecond pulses is the most common and
convenient technique for generating strong terahertz (THz) pulses. In this paper, we use the three-dimensional (3D)
finite-difference-time-domain (FDTD) to analyze electric field distribution of THz pulses in the near-field from a
photoconductive antenna. The simulation is based on solving Maxwell’s equations and the carrier rate equations
simultaneously on realistic dipole antenna structures. The 3D FDTD simulation gives detailed features of THz electric field distribution in and out of the antenna. It is found that the difference of near-field distribution between the substrate and free space is considerably large. The fields of the alternating-current dipole exhibit an unsymmetrical distribution and a large deviation from those calculated using the simple Hertzian dipole theory. The magnitude of THz field in and out of the substrate attenuates rapidly while it holds the line in the gap center. The high-frequency components of THz radiation emission come only from the dipole antenna, while the low-frequency components are from both the center electrodes and coplanar stripline waveguide. This work can be used to optimize the design of antenna geometry and raise the radiation field power.
An analytical solution of the optical susceptibility of quantum wells driven by a terahertz (THz) field is achieved
based on the density-matrix formalism. A simplified three-level model is adopted for the quantum well structures,
with the levels coupled by a near-infrared (NIR) and a THz field. The equation of motion for the polarization is
derived from Liouville's equation for the density matrix. Using Floquet's theorem, and under the rotating-wave
approximation with respect to the NIR probe field, analytical expression for the optical susceptibility of the
driven quantum wells is obtained. Different features rising induced by the driving THz field in the absorption
spectrum of the NIR probe are discussed for the THz field near resonance and out of resonance of the intraexcitonic
transition. This analytical investigation of the susceptibility of the THz-field-driven quantum wells is
much useful for clearly identifying the physical principles obscured in the full numerical calculations.
Excitonic optical absorption in quantum wells (QWs) under intense terahertz (THz) waves polarized along the
grown-direction is investigated. The characteristics induced by the strong THz field in the optical absorption
spectrum in the QWs near the band gap edge is analyzed by coherent wave approach. The calculated results
with and without the presence of THz field are presented. It shows that the excitonic absorption peaks may split
or broaden when an intense THz field is applied. The presence of an intense THz field results in the formation
of replicas in the absorption spectrum. The replicas are not symmetric and presence on both sides of the main
excitonic peaks due to the Coulomb interaction between electrons and holes.
The optical absorption properties of semiconductors and their nanostructures under intense terahertz (THz)
radiation are investigated theoretically. We derived the extended Semiconductor Bloch Equations (SBEs), which
include the effects of the Coulomb interaction among the photoexcited carriers and the effects of the applied
external classical (static and/or THz) electric and magnetic fields. We presented two formulations of the SBEs in
wavevector space and real space, respectively, which are appropriate to analyzing respective semiconductor structures.
Different semiconductor nanostructures (such as quantum wells, quantum wires, quantum rings, quantumdot-
superlattice nanowires, and quantum-dot-superlattice nanorings) in different configurations of the applied
external fields (with the THz electric field applied along the heterostructure interface or the growth-direction)
are considered. We showed that the driving of the THz field can give rise to many intriguing phenomena, such
as THz dynamical Franz-Keldysh effect, ac Stark effect, THz-sideband, replica of dark exciton states. We also
suggested some potential application of these new phenomena in developing novel semiconductor optoelectronic
devices.
Microstrip line pulse generators based on gallium arsenide (GaAs) photoconductive semiconductor switches have been
fabricated and tested experimentally. The GaAs photoconductive semiconductor switches were optically triggered with a
70 W, 85 ns duration (FWHM) high power laser diode. Short electrical pulses have been obtained and reported with
practical applications emphasized. The risetimes and the durations of the output electric pulses can be less than 2 ns and
6 ns, respectively, which are in remarkable contrast to the 20 ns risetimes and the 85 ns durations of the activating optical
pulses.
We investigate the optical properties of a one-dimensional semiconductor quantum wire in the presence of a static electric field applied along the axis. Optical absorption spectra near the band gap are nonperturbatively calculated by solving the low-density semiconductor Bloch equations in real space. The influence of the Franz-Keldysh effect and the exciton ionization on the continuum of the absorption spectrum in semiconductor quantum wire is investigated. We found substantial and tunable absorption oscillations appear above the band gap for reasonable electric strengths. This shows semiconductor quantum wire has great promise of the potential applications in electric-optical devices.
The optical absorption in semiconductor nanorings under a lateral DC field and a perpendicular magnetic field is numerically simulated by coherent wave approach. The exciton dominated optical absorption is compared with the free-carrier interband absorption to demonstrate the key role of Coulomb interaction between electron and hole. The influence of the lateral DC field and the perpendicular magnetic field on the optical absorption are discussed in detail. It shows that the lateral DC field can significantly enhance the Aharonov-Bohm effect of the neutral excitons in semiconductor nanorings.
This research has focused on modeling of optically triggered, high gain nonlinear GaAs switches. A complete model with dynamics of deep level trap, carries, direct band-gap recombination radiation and heat involved has been constructed. The various generation and recombination mechanism have been discussed and presented. Photo-ionization, thermal emission of deep level traps, intrinsic impact ionization, standard Shockley-Read-Hall recombination, direct band-gap recombination and Auger recombination have been considered.
This research has focused on optically triggered, high gain nonlinear GaAs switches for high speed, high power microwave generation. A microstrip and a parallel-plate pusle generator have been constructed and tested. The experimental results are reported. Further studies on optically triggered, high gain, nonlinear PCSS are proposed.
An experimental and theoretical investigation of PCSS's behaviors has been performed with emphasis on 'lock-on' is intimately related to bias voltage, triggering optical energy and the concentration of deep energy level trap in PCSS's by 2D simulation with MEDICI. This effect is observed only when the three factors all satisfy corresponding threshold values that vary with operational conditions. The reason is that under such condition, the distributions of the electric field, potential, carriers and current densities in PCSS's can be notably changed, the accumulation of carriers will be formed and electric field will be gradually enhanced in some region. It ultimately reaches the critical electric field for avalanche ionization, thus avalanche multiplication of carriers occurs.
The reliable use of piezoelectric ceramics as actuators in smart structures hinges on a fundamental understanding of the fracture process in these materials. However, despite the success of fracture mechanics theories in explaining the cracking behavior of a wide range of engineering materials, the extension of these accepted criteria to piezoelectrics fails to predict even qualitatively their response to combined electrical and mechanical loads. A new fracture criterion is presented here, in which a multiscale point of view is adopted in order to account for a zone of combined mechanical brittleness and electrical ductility near the crack tip. As a starting point for the investigations, we assume that the region of electrical nonlinearity is confined to aline segment ahead of the crack, analogous to the Dugdatle zone of plasticity in metals. This mathematical simplification represents the physical situation in which a distribution of excess electric dipoles is aligned on a finite segment in an otherwise linear piezoelectric solid. By applying this model to both insulated and conducting cracks subjected to far-field loading, we obtain local-scale energy release rates whose dependence on applied tractions and electric fields agrees with the trends observed experimentally. One important feature of the analytical expressions for crack driving force is that they are independent of the strength and size of the nonlinear zone.
The isothermal full Gibbs function which is negative definite and the adiabatic internal energy which is positive definite are used to develop a linear fracture theory for isothermal and adiabatic cracking in a piezoelectric material, respectively. Both the full Gibbs function release rate and the internal energy release rate for crack propagation are evaluated and revealed to be positive functions of crack lengths and remote loads, even under pure electric loads only. When the electric permittivity inside the crack is treated as zero, each of the full Gibbs function release rate and the internal energy release rate has a compact form in terms of the mode II, I, III stress and electric displacement intensity factors.
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