Coherent Beam Combining (CBC) can scale the overall output power while maintaining high beam quality. In recent years, this technique has also been employed to generate the high-power Optical Vortex Beam (OVB). In this work, we designed a coherent phased array, and an Optical Vortex Beam Array (OVBA) was generated via the CBC technique. Numerical simulation was studied, and an experimental setup was set up. The results showed that the OVBA could be generated in the far field. Meanwhile, the phase detection results indicated that the OVBA included several phase singularities. This work can present a new idea for manipulating the high power structured optical fields.
We explore the influence of cooling temperature on mode instability (MI) effect based a 2kW oscillator that can work stably at low temperature. The corresponding MI threshold of the oscillator is carefully measured with different operating temperatures of the Ytterbium-doped fiber (YDF) and laser diodes (LDs). It is found that whether decreasing the cooling temperature of the LDs or YDF, the MI threshold would rise, but decreasing the cooling temperature of the LDs has better effects. In our experiments, the MI threshold increases by 21.6%,from 1752W to 2130W when the operating temperature of the LDs changes from 25°C to 5°C due to the central wavelength of the LDs shifts from 976nm to 970nm, corresponding to a lower thermal load. In the process that only the cooling temperature of the YDF drops, although the increase is small, we have observed a rising trend of the laser MI threshold. This work can clarify the influence of cooling temperature on the laser thermal effect, which is conducive to perfecting the theoretical model of the MI effect of the fiber lasers.
This paper reports a wavelength selective switch (WSS) employing a thermally expanded core (TEC) fiber array (FA) as the input/outputs, which enables a compact WSS with convenience for alignment and assembly. Simulation of the WSS module based on ZEMAX shows that conical diffraction of the grating results in horizontal offset of the output beam spots. An excess insertion loss will be caused due to the offset if a linearly aligned FA is employed as the input/outputs. This paper designs a parabolically aligned TEC FA that the fibers in the array can be positioned by V-grooves with different depth. With the parabolically aligned TEC FA as the input/outputs, the WSS module is expected to be compact with low loss.
The plenoptic camera is a new camera structure which can record the intensity, color and the direction of the light by adding a microlens array in front of the image sensor. Based on some basic concepts of the focused plenoptic camera, we first analyze the imaging characteristics. Then, the design method was given. Furthermore, we establish the depth resolution model after detailed derivation. The imaging characteristics of the focused plenoptic camera with a novel structure of four types focal lengths microlens array was analyzed. The simulation results show that our designed camera has the advantages of large depth of field and high depth resolution. The method proposed in this paper can provide reference for designing plenoptic cameras for specific application scenarios.
In order to find out the wavefront distribution characteristics of supercontinuum light source, adding wavefront distortion description into the evaluation of supercontinuum light source beam quality, a comprehensive and accurate measurement about the wavefront distribution characteristics of a 400-1700nm supercontinuum light source developed by National University of Defense Technology (NUDT) is carried out in this paper. According to the experimental results, wavefront distribution characteristics in different wavelength bands of this 400nm-1700nm supercontinuum source are basically the same, mainly composing of defocus and astigmatism, however, values of distortions are different in specific wavelength bands.
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.
In order to measure the absorption coefficient and performance degradation characteristics of optical components which
used in high power laser system, an intra-cavity device was established based on a discharge-drived CW chemical laser.
Two pieces of 45 degree reflecting mirrors were tested. Each mirror was tested for more than twenty times, and high
power laser irradiation on the testing mirrors lasted 100 seconds continuously in each test. The dependence of absorption
coefficients on irradiation times was acquired. The testing results of both reflecting mirrors showed that, the differences
between the experimental absorption coefficients and their fitting curve were up to 30.7% and 21.6% respectively, and
the differences were independent of irradiation condition, such as irradiation energy, irradiation power and beam crosssectional
area. The uncertainty of absorption coefficient was composed of two parts. For one thing, the uncertainty of the
direct measurement results, such as the temperature rise of optical components, can cause the uncertainty of absorption
coefficients. This part of uncertainty was about 11.3%. For another, the resonant cavity need to be adjusted again when
other optical components were replaced, which lead to the change of the incident angle of the optical components to be
measured. A typical film system of 24 layers (12 pairs) was calculated by Thin Film Design Software called TFCalc,
which showed the absorption coefficients increased with the increase of incident angle. When the angle of incidence was
0.5 degree from the design value, there would be -60~71ppm difference of absorption coefficient from the original one,
and the uncertainty was 14.5%. When there was a deviation of 1 degree, the difference of absorption coefficient and the
uncertainty were -112~155ppm and 31.7% respectively. This results showed that, the deviation of incident angle was
between 0.5~1 degree in the test. In order to reduce the testing uncertainty of absorption coefficients, the deviation
between the incident angle of optical components and the design value should be reduced as much as possible. This
provides guidance for measuring the absorption coefficients of optical components with an intra-cavity device.
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.
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.
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.
Polarization imaging plays an important role in various fields, especially for skylight navigation and target identification, whose imaging system is always required to be designed with high resolution, broad band, and single-lens structure. This paper describe such a imaging system based on light field 2.0 camera structure, which can calculate the polarization state and depth distance from reference plane for every objet point within a single shot. This structure, including a modified main lens, a multi-quadrants Polaroid, a honeycomb-liked micro lens array, and a high resolution CCD, is equal to an “eyes array”, with 3 or more polarization imaging “glasses” in front of each “eye”. Therefore, depth can be calculated by matching the relative offset of corresponding patch on neighboring “eyes”, while polarization state by its relative intensity difference, and their resolution will be approximately equal to each other. An application on navigation under clear sky shows that this method has a high accuracy and strong robustness.
Kinds of wavefront aberrations exist among optical systems because of atmosphere disturbance, device displacement and a variety of thermal effects, which disturb the information of transmitting beam and restrain its energy. Deformable mirror(DM) is designed to adjust these wavefront aberrations. Bimorph DM becomes more popular and more applicable among adaptive optical(AO) systems with advantages in simple structure, low cost and flexible design compared to traditional discrete driving DM. The defocus aberration accounted for a large proportion of all wavefront aberrations, with a simpler surface and larger amplitude than others, so it is very useful to correct the defocus aberration effectively for beam controlling and aberration adjusting of AO system. In this study, we desired on correcting the 3rd and 10th Zernike modes, analyze the characteristic of the 3rd and 10th defocus aberration surface distribution, design 3-element actuators unimorph DM model,study on its structure and deformation principle theoretically, design finite element models of different electrode configuration with different ring diameters, analyze and compare effects of different electrode configuration and different fixing mode to DM deformation capacity through COMSOL finite element software, compare fitting efficiency of DM models to the 3rd and 10th Zernike modes. We choose the inhomogeneous electrode distribution model with better result, get the influence function of every electrode and the voltage-PV relationship of the model. This unimorph DM is suitable for the AO system with a mainly defocus aberration.
The Performance of wavefront aberration detection and recovery, is limited by the spatial resolution in subaperture, especially for the high-order aberration. To improve the accuracy of wavefront reconstruction, our paper focuses more on the phase retrieval using the stochastic parallel gradient descent (SPGD) algorithm with lower subaperture. In this paper, the theoretical that Shack-Hartmann wavefront sensor can get a single subaperture high spatial frequency components, which are four-dimensional spatial position and spatial frequency information of the two-dimensional light field is researched.Numerical simulations show that compared with conventional methods, the SPGD algorithm can effectively improve the phase retrieval precision of higher-order aberrations, and decrease the influence of the spatial resolution in subaperture. At the same time, by selecting the appropriate algorithm iterative initial value can effectively enhance the speed of wave front reconstruction.
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.
The non-uniform response of the sensors are increasingly prominent with infrared focal plane arrays (IRFPAs) being more and more used in infrared measuring systems, which gives uncorrected images and measurement error. The merits and faults of several non-uniformity correction algorithms attracting broad attention in recent years, including two-point correction, multipoint correction, temporal high pass filter correction (THPFC), artificial neural network correction (ANNC), were analyzed and discussed in this paper. In addition, the algorithm validation has been respectively carried out on these methods, using existing infrared images sequences and the possibility of application of scene-based non-uniformity correction techniques for infrared measuring systems, is discussed by preliminary research and exploration.
Traditional stereo imaging technology is not working for dynamical translucent media, because there are no obvious characteristic patterns on it and it’s not allowed using multi-cameras in most cases, while phase space optics can solve the problem, extracting depth information directly from “space-spatial frequency” distribution of the target obtained by plenoptic sensor with single lens. This paper discussed the presentation of depth information in phase space data, and calculating algorithms with different transparency. A 3D imaging example of waterfall was given at last.
The phase distribution of light field at a certain location can be calculated from two close intensities in first-order optical system along the propagation direction, and can be considered as the wavelet transform coefficient of the pupil light field. According to such theory, a new phase retrieval algorithm based on several intensities of different layers is presented in this paper, which can quickly calculate the phase with low frequency, and then gradually increase the resolution by adding more intensity to calculate.
The extended beacon belongs to partially coherent light field, and Phase space optics is an effective
tool for analyzing such light field, especially in calculating the phase of multi-angle and multi-layer.
Due to the lack of directly detection means, the research of this theory is crippled a long time, in this
paper an optical structure to obtain the spectrogram which is a kind of phase space distribution has
been presented, while the resolution problem and an improved method for possible has been
discussed too, and two new methods to get high quality astronomical Image are appearing with such
algorithm.
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.
The Gerschberg-Saxton (GS) algorithm is a classic algorithm for phase retrieval. It is usually based on FFT (fast Fourier
transform) and IFFT (inverse fast Fourier transform). We improve the GS algorithm based on the Wigner distribution.
Instead of FFT and IFFT, Wigner distribution is not only used in the propagation but also filtered to optimize the signal
during the iteration. The simulation results illustrate that the new method could effectively improve the efficiency and
accuracy of phase retrieval.
Laser beam shaping is required in many applications for improving the efficiency of the laser
systems. In this paper, the near field beam shaping based on the combination of simulated annealing
algorithm and Zernike polynomials is demonstrated. Considering phase distribution can be represented
by the expansion of Zernike polynomials, the problem of searching appropriate phase distribution can
be changed into a problem of optimizing a vector made up of Zernike coefficients. The feasibility of
this method is validated theoretically by translating the Gaussian beam into square quasi-flattop beam
in the near field. Finally, the closed control loop system constituted by phase only liquid crystal spatial
light modulator and simulated annealing algorithm is used to prove the validity of the technique. The
experiment results show that the system can generate laser beam with desired intensity distributions.
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