High Dynamic Range Imaging Based on Attenuation Microarray Mask has broad application prospects due to its good real-time performance and small size. But at the current level of craftsmanship, it is hard to fabricate a microattenuation array mask whose attenuation rate is adjustable. This leads to the fact that the imaging dynamic range cannot adapt to changes in scene brightness in most cases. To this end, this paper proposes a novel imaging system whose dynamic range can adaptively change according to the brightness of the scene. The core components are the micro polarization array mask mounted on the CMOS surface and the on-sensor rotatable linear polarizer in front of the lens. By controlling the rotation angle of the polarizer placed before the lens, the CMOS pixel exposure can be precisely controlled. Therefore, the imaging system dynamic range can be adjusted adaptively according to the scene brightness. Through horizontal comparison with Sony’s multi-quadrant polarization chip, we determined the optimal parameters of the multi-quadrant micro-polarization array mask for extending the dynamic range. The experimental results show that the imaging performance remains consistently good even when the dynamic range of the photographed scene is large, and the dynamic range of the device can be adapted to the dynamic range of the shooting scene, so that the image after shooting and processing can always show sufficient details. By rotating the polarizer in front of the lens to a specific angle, the high dynamic imaging of the scene can be significantly improved.
Modeling and simulation platform of optical teaching experiments SeeLight was proposed. Its modeling range covers a variety of simple or complex optical systems including geometrical optics, wave optics, adaptive optics, information optics, polarization analysis, and so on. It has seven categories of models and more than 50 simulation application examples, so that many optical teaching experiments can be done on this platform. The platform is based on the component-based software architecture. And the operating efficiency is greatly improved based on high-performance computing framework. And it’s not only a stand-alone version, but also web version, which provides convenience for the user to build optical simulation systems. So that the complex optical experiment can be easily implemented on the platform, and the parameters of the experimental system can be adjusted flexibly, which greatly reduce the experimental cost and improve the efficiency of the experimental analysis. And in the process of simulation, not only the results of all optical components which have different spatial locations can be observed, but also users can observe the results over time. And the high-performance computing framework greatly improves the efficiency of the platform. Multiple optical simulation systems of comparative experiments can be simultaneously simulated. The functional structure, typical features and key simulation difficulties of this platform were discussed in detail. Finally, several simulation examples were given.
In this paper, the physical properties of multimode beam are analysed by using the theory of partially coherent light. Based on the spatial coherence measurement results of a multimode fiber laser, we provide a theoretical basis for aberration correction for multimode beams. To improve the beam quality of multimode lasers, phase correction of multimode laser based on a dual-phase-only liquid-crystal spatial light modulator is presented which is used as aberration correction device. The phase distribution was optimized by the stochastic parallel gradient descent algorithm. In this paper the power in the bucket of the far field was used as the evaluation function and the multimode beam included multiple higher order Laguerre-Gaussian beam modes. The real-time aberration correction of Multimode beam by stochastic parallel gradient descent Algorithm is simulated studied and experimental analysed respectively. According to the results the parameters of stochastic parallel gradient descent algorithm can be adjusted and the efficiency and practicability of the algorithm are determined.
Bessel beam has the advantages of reducing scattering artefacts and increasing the quality of the image and penetration.
This paper proposed to generate a guide star by Bessel beam with vortex phase, and to use the beacon with special spot
structure to measure the atmosphere turbulence aberrations. With the matching algorithm of measured characteristic spot
in each subaperture, the detection accuracy of Hartmann wavefront sensor can be improved. Based on wave optics
theory, the modeling of Bessel beam guide star and wavefront sensing system was built. The laser guide star beacon
generated by Bessel beam with vortex phase and beacon echo wave measured by Hartmann sensor were both simulated.
Compared with the results measured by echo wave from Gauss beam generated guide star beacon, this novel method can
reduce the error of wavefront detection and increase the detection accuracy of Hartmann sensor.
Light-spots centroid positions detection is one of the major error sources in the application of Hartmann-Shack (H-S) wavefront sensors. The double images (ghosting) in part of sub-aperture, caused by multiple reflections from optical elements or paraxial stray light from operating environment, result in centroid detection errors and that the precisions of wavefront aberration reconstruction decline. The conventional threshold method often leads to the loss of available information of light-spots, though, by which ghosting can been brought under effective control. In this paper, an improved method and procedure, aiming at ghosting detection and local removal automatically, is proposed by combining several algorithms which include nonlinear processing, autocorrelation, convolution, and local filtering, and so on. Then, the corresponding research has been carried out by a numerical simulation platform established by ourselves, and the results can show that this method and procedure is effective.
Full-aperture noninterferometric phase retrieval system, namely one single shot, can overcome the impact of low
Signal-to-Noise ratio in the condition of weak illumination by extended beacon. Contributing to its robustness and
practicability, the technology has been widely applied in industrial inspections. However, the technology is limited by
the operational speed and the accuracy of the phase retrieval algorithm in most situations. Based on phase space optics,
an analytical relationship can be set up between the phase of the quasi-coherent light field from the extended beacon of
small field of view and 3 adjacent intensity distributions, which may be resolved fast. That is, the unknown phase is
equal to the convolution of the partial differential of the difference value of the three intensities with respect to the
rotation angle of the phase space and the sign function. This paper introduces a design and realization which
accomplishes this goal using a specially designed chromatic aberration lens and a 3CCD camera. By this way, three high
resolution images of the beacon can be captured within a single shot. The numerical simulation results show that the
method can accurately recover aberrations of more than 10 orders.
Atmospheric turbulence induces laser guide star (LGS) spot wandering in the sodium layer, which introduces trouble to adaptive optics systems. Experimental study of LGS spot wandering usually needs on-sky test. However, the on-sky test of LGS spot wandering is expensive and complicated. Since spatial light modulators (SLMs) are able to simulate atmospheric turbulence, we have designed and set up a SLMs based LGS simulator to study LGS spot wandering. This LGS simulator is prominent to build a bridge between theoretical study and on-sky test. Its performance is tested for vortex beam generated annular LGS which was proposed to reduce the LGS spot wandering in our former paper.
We present an annular laser guide star (LGS) concept for large ground-based telescopes in this paper. The more stable annular LGS is generated by turbulence-resisted vortex beam. In the uplink, a vortex beam tends to wander more slightly than a Gaussian beam does in atmospheric turbulence. This may enable an annular LGS to wander more slightly than a traditional Gaussian beam generated LGS does, which would ease the burden of uplink tip-tilt mirror and benefit a dynamical closed-loop adaptive optics system. We conducted numerical simulation to validate the feasibility of this concept. And we have gotten 31% reduced variance of spot wandering of annular LGS. Besides, we set up a spatial light modulator based laser guide star simulator for beam propagation in turbulent atmosphere to experimentally test the annular LGS concept. Preliminary experimental results are given. To the best of our knowledge, it is the first time this concept is formulated.
In this paper, a novel criterion for evaluating the beam quality of high energy laser is proposed, which is called “power outside the large bucket”, POLB for short. The novel criterion does not demonstrate the divergence of the beam but focuses on the high spatial frequency wavefront aberration of the beam. The POLB values of the laser beams with various aberrations are calculated. It shows that the more high spatial frequency components in the aberration the larger POLB value is. Moreover, it is theoretically analyzed that the laser beams with various aberrations are corrected by ideal adaptive optics (AO) systems with different deformation mirror (DM) actuator numbers. It is shown that the residual error of the corrected wavefront aberration with many high spatial frequency components is quite large. Finally, the dependence of the residual wavefront error on the POLB value is investigated. Only if the POLB is smaller than 3.8/3.4/2.5/1.7 the residual error of the wavefront may be smaller than λ/10, which is corrected by an ideal AO system with 127/61/37/19 actuators. It is necessary to employ a complicated AO system for improving the laser beam of which the POLB value is large. The novel evaluating criterion POLB is able to demonstrate the amount of high spatial frequency aberration and the residual wavefront error corrected by AO system. It is an accessible and useful criterion for evaluating the beam quality of high energy lasers.
Propagation simulation method and choosing mesh grid are both very important to get the correct propagation results in wave optics simulation. A new angular spectrum propagation method with alterable mesh grid based on the traditional angular spectrum method and the direct FFT method is introduced. With this method, the sampling space after propagation is not limited to propagation methods no more, but freely alterable. However, choosing mesh grid on target board influences the validity of simulation results directly. So an adaptive mesh choosing method based on wave characteristics is proposed with the introduced propagation method. We can calculate appropriate mesh grids on target board to get satisfying results. And for complex initial wave field or propagation through inhomogeneous media, we can also calculate and set the mesh grid rationally according to above method. Finally, though comparing with theoretical results, it’s shown that the simulation result with the proposed method coinciding with theory. And by comparing with the traditional angular spectrum method and the direct FFT method, it’s known that the proposed method is able to adapt to a wider range of Fresnel number conditions. That is to say, the method can simulate propagation results efficiently and correctly with propagation distance of almost zero to infinity. So it can provide better support for more wave propagation applications such as atmospheric optics, laser propagation and so on.
When using numerical simulation method study laser system, modeling and simulation energy distribution of the target echo on the detector is studied in order to achieve closed-loop optical path. From the perspective of Fresnel formula, using bidirectional reflectance distribution function (BRDF) model to calculate the intensity distribution of the target reflection; calculation of light vector angle expression reflects the phase change between reflected light and incident light when light travelling in a single medium surface. Setting position parameters and attitude parameters of different components in the laser simulation system, through the calculation of geometric relationship, the energy distribution under the view of the detector is achieved. Target surface shape was respectively set for planar, spherical and cylindrical. Analyzed the influence of targets surface roughness root mean square (RMS), zenith angle and azimuth angle of the incident light to targets reflection characteristics respectively. Results show that this method can accurately achieve the detection simulation of simple geometric shape surface target in laser system.
KEYWORDS: Adaptive optics, Wavefront sensors, Sensors, Distortion, Mirrors, Data fusion, High power lasers, Detection and tracking algorithms, Beam splitters, Wavefronts
In a high-power laser system, a beam splitter refers to the mirror which locates at the cross point of the path of highpower beam and the weak light section. Because of the thermo-optic effect and elasto-optic effect, a beam splitter deforms under intense laser radiation. This deformation adds extra phase on the incident waves and deliveries inaccurate information to the wavefront sensor. Consequently, the output laser focuses at finite distance and gets divergent when arrives at the target. To settle the above problem, this paper presents a new method for real-time correction of the thermal distortion of beam splitter, based on algorithm of the data fusion of two Shack-Hartmann wavefront sensors (SH-WFS). Different from the traditional AO system, which contains a wavefront sensor, a corrector and a servo controller, two extra Shack-Hartmann wavefront detectors are adopted in our AO system, to detect the transmitted and reflected aberrations of beam splitter mirror. And these aberrations are real-timely delivered to the wavefront sensor. Based on coordinate conversion and data fusion algorithm, it makes the wavefront sensor of AO can “see” the aberrations of splitter mirror by itself. Thus, the servo system controls the corrector to compensate these aberrations correctly. In this paper, the theoretical model of data fusion algorithm is carried out. A closed-loop AO system, which consists of a typical AO system and two extra Shack-Hartmann wavefront detectors, is set up to validate the data fusion algorithm. Experimental results show that, the distortion of a CaF2 beam splitter can be real-time corrected when the AO closedloop control is on. The beam quality factor of output laser decreases from 4 to 1.7 times of diffraction limit.
High-power laser propagation through the inner optical path will produce a significant thermal effect on the beam-splitter mirror which will cause phase aberrations. Based on the three-dimensional transient heat conduction equation and the thermal elastic stress-strain equation, a simulation model of reflector mirror was built with three-dimensional finite element method (FEM). The temperature increase and thermal displacements of two kinds of mirror substrates (Al2O3 crystal and spinel) were especially investigated with different laser intensity, output duration and absorption coefficient. The effects of mirror thermal distortion on laser beam phase aberrations were also evaluated on both reflection and transmission directions. The experiments of high–power laser propagation through two kind materials of beam-splitter mirrors samples with diameters of 50mm and thicknesses of 5mm were carried out to measure the thermal effects induced by the absorbed laser energy. Both two kinds mirror samples were deposited the same film layer of same reflectance. A high power semiconductor laser was expanded to a beam of 35mm diameter, and double Shack-Hartmann wavefront sensors were used to detect both reflection and transmission thermal distortions of the mirror samples. The measurements showed that reflection aberrations of spinel mirror sample were larger than those of Al2O3 crystal mirror sample while its transmission aberrations were slightly less than Al2O3 crystal mirror sample. The results of experiments and simulations showed a very good consistency.
When laser propagation through atmospheric turbulence, the effect of anisoplanatic error will affect the compensation results of adaptive optics system. Based on the wave optics propagation model, laser propagation through horizontal path atmospheric turbulence with AO compensating simulation system was established to study the anisoplanatic error in turbulence with diffraction effect. Propagation experiments through a 4-km horizontal path outside were conducted. The AO compensating effects in different isoplanatic angles were tested at different turbulence strength. Both simulation results and experimental data show that during the isoplanatic angle with influence of piston and tilt terms removing the AO system possess effective compensation results.
An 4km optical link was established to examine the effect of beam wander on the scintillation characterization for
multiple focus beams. Multiple beams that sample different portions of the atmosphere reduce the scintillation if the
atmospheric turbulence each beam traverses is independent. How much the beam wander affects scintillation is
determined by the number of the beams and the size of the beam wander variance. The number of mutually incoherent
beams was 1, 3 or 6. Individual laser was launched from a separate region of the transmitter and focused on the flat
screen with the horizontal distance of 4km by 350mm telescope. We recorded the illuminated flat screen with a CCD
camera operating at 1kHz. We calculated the scintillation index of the illuminated area with the size of 8x8cm on the
condition of untracked and tracked beams. A tracked beam generally refers to the situation in which the beam is tilt-corrected.
However, There was no tilt-corrected in the experiment. Instead, the rms beam centroid of the area was
aligned with the optical axis at each frame. Experimental results show that the scintillation index in tracked beam was
larger than in the untracked beam case when the size of the Rytov variance σ2R is the same for one beam case, but for
three and six beams, the scintillation values are almost. It is the reason that the root mean square variance of wander
angle decreases as the number of beams increases. So the effect of beam wander is not evident for three and six beams.
And increasing the number of beams on the condition of untracked and tracked cases does reduce the scintillation index
as predicted.
Adaptive optics system can work with beacon produced by illumination the dim target with laser beam. But atmospheric
turbulence distributed over the entire propagation path produces intensity scintillation of the illumination laser beam. The
multi-beam illumination approach can improve the intensity uniformity by adding different intensity distributions
produced by separate illumination beams passing through independent atmospheric paths. A 4 km horizontal propagation
path was established to examine the illumination effect with multi-beam on the target. Individual laser beams were
launched from a separate region of the transmitter telescope. The far-field irradiance profile was received by target-plane
and recorded with a CCD camera operating in 1k Hz placed in front of the target-plane. The uniformity of target area
was determined by the number of the illuminating beams and the turbulence strength. Data were recorded with 1, 3 or 6
incoherence lasers illumination and for a range of R0 parameter. We calculated the irradiance of a 8x8 cm2 area in the
target-plane to characterize uniformity of illumination. Results show a good reduction in intensity variance with number
of illumination beams increasing and that the uniformity of illumination on target turns to worse when the atmosphere
turbulence became worse. The back backscatter from the target-plane was received by another telescope nearby the
transmitter side and imaged by a CCD camera. The images of the rough reflector showed that the image resolution and the
illumination effects improved with the number of illuminating beams increasing.
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