Wavefront Distortion Research Articles

A New Method for Analyzing Aero-Optical Effects with Transient Simulation

Aero-optical effects reduce the accuracy of optical sensors on high-speed aircraft. Current research usually focuses on light refraction caused by large-scale density structures in turbulence. A method for analyzing photon energy scattering caused by micro-scale structures is proposed in this paper, which can explain the macro image distortion caused by moving molecules in inhomogeneous airflow. Quantitative analysis of the propagation equation indicates that micro-scale structures may contribute more to the wavefront distortion than the widely considered large-scale structures. To analyze the micro mechanism of aero-optical effects, a transient simulator is designed based on the scaling model of transient distorted wavefronts and the artificial vortex structure. The simulation results demonstrate that correct aero-optical phenomena can be obtained from the micro mechanism of photon energy scattering. Examples of using the transient simulator to optimize the parameters of the star sensor on a hypersonic vehicle are provided. The proposed analysis method for micro-scale structures provides a new idea for studying the aero-optical effects.

Study of Optical Modulation based on Binary Masks with Finite Pixels

Binary holography has been applied with fast switching digital masks, e.g., digital micromirror device (DMD), to modulate light in recent years for a wide range of optical and scientific applications, such as trapping of cold atoms and 3D random-access two-photon fluorescence microscopy. However, the binarized modulation errors associated with the effect of discrete sampling, finite pixels, and intrinsic noises of algorithms have yet to be systematically investigated. In this paper, we present the development of an analytical model for a general binary mask with finite pixels. The model has established a deterministic link between the quality of the reconstructed wavefront to important parameters of the binary mask, including pixel size, pixel geometry, fill factor, and aperture. Based on the model, three case studies are presented, including (1) beam shaping, (2) 3D random-access scanning, and (3) multi-focus generation based on binary masks. The results show the model can precisely predict the wavefront distortion, power efficiency, position accuracy, and intensity uniformity for multi-focus generation as a function of binary mask parameters. As digital systems are highly repeatable, the predicted errors, e.g., position errors, can be compensated by adding appropriate phase to the designed holograms. The model and the simulation results may provide useful guidance to select appropriate binary devices and optimization algorithms for specific optical engineering applications, e.g., nanofabrication or nonlinear microscopy.

Low-loss microscope optics with an axicon-based beam shaper.

We present low-loss microscope optics using an axicon-based beam shaper, which can convert a Gaussian beam to a ring beam to minimize the optical loss from blocking by the back aperture of the objective lens while maintaining spatial resolution. To design the beam shaper, we characterize the position-dependent transmittance of high-transmittance objective lenses and numerically calculate the beam propagation in the beam shaper. We also clarify the effect of misalignments of the beam shaper and wavefront distortion of the input beam. Furthermore, we experimentally demonstrate a low-loss microscope optical system with a high transmittance of 86.6% and high spatial resolution using the full numerical aperture of the objective lenses.

Study of the Optical Atmospheric Distortions using Wavefront Sensor Data

The paper studies the optical turbulence structure at the Large Solar Vacuum Telescope of the Baikal Astrophysical Observatory. Some results on the study of the wavefront distortions are given to expand the data archive. The possibilities of detecting layers with enhanced optical turbulence intensity in the atmosphere are discussed. The altitudes of the atmospheric turbulent layers within the boundary layer are estimated.

Adaptive beam clean-up of high power slab lasers using least-squares wavefront reconstruction algorithm with performance-based filtering

The edge effects of high-power slab lasers cause severe wavefront distortions with large amplitude and gradients, which is a great challenge for the adaptive optics systems for improving the beam qualities. The standard least-squares wavefront reconstruction method would inherently assign higher priorities to correcting the edge aberrations which contribute little to the laser power in the near diffraction-limit areas in the far-field, while the other aberrations are not well corrected. To solve the problems, we propose a least-squares reconstruction algorithm with performance-based filtering, in which the correction capability of the deformable mirror is analyzed in the spatial frequency domain, and then the input phase distortions are filtered accordingly. The uncorrectable components of input distortions are ignored so that the residuals of the correctable components could be further minimized. Experiments with a 20 kW Nd:YAG slab laser indicate that the proposed method is effective to improve the beam quality of high-power slab laser with edge effect.

Phase Compensation of the Non-Uniformity of the Liquid Crystal on Silicon Spatial Light Modulator at Pixel Level.

Phase compensation is a critical step for the optical measuring system using spatial light modulator (SLM). The wavefront distortion from SLM is mainly caused by the phase modulation non-linearity and non-uniformity of SLM’s physical structure and environmental conditions. A phase modulation characteristic calibration and compensation method for liquid crystal on silicon spatial light modulator (LCoS-SLM) with a Twyman-Green interferometer is illustrated in this study. A method using two sequences of phase maps is proposed to calibrate the non-uniformity character over the whole aperture of LCoS-SLM at pixel level. A phase compensation matrix is calculated to correct the actual phase modulation of the LCoS-SLM and ensure that the designed wavefront could be achieved. Compared with previously known compensation methods, the proposed method could obtain the phase modulation characteristic curve of each pixel on the LCoS-SLM, rather than a mono look-up table (LUT) curve or multi-LUT curves corresponding to an array of blocks over the whole aperture of the LCoS-SLM. The experiment results show that the phase compensation precision could reach a peak-valley value of 0.061λ in wavefront and this method can be applied in generating freeform wave front for precise optical performance.

Improved 3D imaging of phase shifting digital holographic microscope by compensation for wavefront distortion

This paper is focused on improving the performance of quantitative phase imaging via phase-shifting digital holographic microscope. The development of the interferometric techniques is important for technology and biomedicine, since the surface and structure of samples can be monitored in real-time by non-destructive investigations. The novelty of the optical arrangement is that in the reference beam of the digital holographic microscope a liquid crystal variable retarder is introduced. Thus, it became possible to actively control the polarization state of light for realizing the necessary phase-shifts. In addition, a spatial light modulator is integrated in the optical setup to produce computer-controlled compensation of the spatial distortion. An ad hoc hologram processing technique was developed to execute the numerical correction of the physical phase-shifting errors. Topographical investigations of phase masks recorded on carbazol-based azopolymers provided the experimental testing of the achieved accuracy in phase reconstruction.

Structural design of the optical bench and enclosure for MAORY, adaptive optics module for the ELT

This paper outlines an overview of the mechanical design of the optical bench and the enclosure for MAORY (Multi-conjugate Adaptive Optics RelaY) for the Extremely Large Telescope. MAORY will enable high-angular resolution observations in the near infrared by employing real-time compensation of the wave-front distortions due to atmospheric turbulence and other disturbances on the telescope. The main purpose of the optical bench is to provide support to the opto-mechanical mountings and subsystems that will be integrated on it. The design philosophy behind the proposed architecture is a truss spatial structure with the aim of optimizing the mass of the Main Structure. The enclosure has to protect the optomechanical elements and to achieve a uniform temperature distribution in its internal environment. The mechanical design of the bench and the enclosure was supported by a set of structural FE analyses, to verify the design compliance with ESO (European Southern Observatory) requirements.

10 J operation of a conductive-cooled Yb:YAG active-mirror amplifier and prospects for 100 Hz operation.

We report, to the best of our knowledge, the highest power conductive-cooled active-mirror amplifier (CcAMA) using Yb:YAG with a pulse energy of 10 J. By using four liquid-nitrogen circulating cooled laser heads, we achieved a repetition rate, pulse energy, and average power of 33.3 Hz, 9.3 J, and 310 W, respectively. The problem of wavefront distortion, which is difficult to solve with a large-aperture active-mirror laser, is suppressed by using reinforcing materials with the same thermal expansion coefficient. We have confirmed that the wavefront distortion is small (0.15λ P-V per head) at 100 Hz operation, which paves the way for 100 Hz operation with the CcAMA concept.

Modified Method to Detect the Turbulent Layers in the Atmospheric Boundary Layer for the Large Solar Vacuum Telescope

A method to detect the atmospheric turbulent layers using a single Shack–Hartmann wavefront sensor is discussed. In order to determine the height distribution of the atmospheric turbulence above a telescope, we register the wavefront distortions at different regions of the aperture from a single light solar object moving in time. Changes of the spatial position of the solar object on the sky give us the possibility to estimate the angular shift of an object. Cross-correlation analysis of the low-frequency component of wavefront slopes spaced on the telescope aperture at different times allows us to estimate characteristics for different atmospheric layers. Knowledge of the height profiles of atmospheric turbulence as well as the Fried parameter is critical for wide-field adaptive optics (AO).

Robust 17 W single-pass second-harmonic-generation at 532 nm and relative-intensity-noise investigation.

We demonstrate a 17 W single-frequency, low-intensity-noise green source at 532 nm, by single-pass second-harmonic generation of a 50 W continuous-wave fiber laser in a 30 mm MgO-doped periodically-poled stoichiometric lithium tantalate crystal. The maximum conversion efficiency is about 37%. A nearly Gaussian beam (M2<1.15 at 15 W) and low wavefront distortion are obtained. The system shows stable behavior over 100 h of uninterrupted operation. The evolution of the relative-intensity-noise transfer from the fundamental to the second harmonic is theoretically and experimentally investigated with high resolution.

Wavefront distortion due to the shock wave and boundary layer in the supersonic flow over a compression ramp

Optical devices equipped in supersonic flight vehicles are subjected to distortions of optical paths due to aero-optical effects, potentially resulting in the degradation of sensing and imaging performance. In this study, the aero-optical effects of the supersonic flow over a two-dimensional compression ramp are investigated through experimental and numerical methods. The experiment is conducted by measuring the wavefront of the laser beam propagated through the test section of a supersonic wind tunnel using a two-dimensional Shack-Hartmann sensor. The individual contribution of the shock wave and boundary layer to the wavefront distortion is identified by conducting a ray-tracing computation through the density of the flow field obtained from a two-dimensional numerical simulation that solves the Reynolds-averaged Navier-Stokes equations. The numerically-obtained wavefront is compared with the time-averaged wavefront measured using the Shack-Hartmann sensor. The deflection angle of the ray at the center of the laser beam is analyzed to assess the aero-optical effects caused by the shock wave and boundary layer. The individual contribution of the shock wave and boundary layer to the wavefront distortion was analyzed for various incidence angles of the laser beam. For the present experimental setup, as boundary layers at the top and bottom walls have the opposite direction of the density gradient, the resultant laser beam deflections are mostly governed by the density gradient across the shock wave.

Optimized dynamic interference smoothing via asynchronous phasemodulation of SSD

In dynamic interference smoothing (DIS) scheme, the speckles of focal spots are swept in multi-directions within several picoseconds, leading to the improvement of irradiation uniformity. In this work, the theoretical limitation of the smoothing performance of DIS scheme is revealed and then an optimized dynamic interference smoothing (ODIS) scheme is proposed. The ODIS scheme is implemented by applying asynchronous sinusoidal phase modulations induced by electro-optic modulators with time-delayed voltage signals to the beamlets in a laser quad. Results indicate that the irradiation uniformity of the ODIS scheme is further improved than that of DIS scheme owing to the asynchronous spatiotemporal phases. In addition, the polarization of the ODIS scheme also rotates on picosecond timescale, which may be potential to mitigate the parametric backscattering. Moreover, though the ODIS scheme has a relatively high requirement for temporal synchronization of beamlets, it shows a wide tolerance for the spatial wavefront distortion.

Temperature characteristics of high repetition rate water-cooled Nd:YAG active mirror amplifier

In order to deal with the thermal management problem of high-energy high-repetition rate laser amplifiers, the efficient heat removal in water-cooled Nd:YAG active mirror amplifiers is investigated in detail through numerical modeling and experimental analysis. According to the low Reynolds number &lt;i&gt;k-ε&lt;/i&gt; turbulence model, a full fluid-solid conjugate heat transfer model is established to give a comprehensive model of flow and thermal characteristics in three dimensions. The thermal distributions obtained from the model are then used to calculate all mechanical stresses in the laser medium and thermally-induced wavefront distortions. In comparison with the standard &lt;i&gt;k-ε&lt;/i&gt; turbulence model, the influences of the near-wall treatments of the above model on the process of fluid flow, convection diffusion and heat conduction, and temperature distributions are analyzed. Meanwhile, the effects of coolant flow rate and pump parameter on the flow field characteristics, temperature and wavefront distributions of the YAG disk are also studied. Numerical simulation results reveal that the temperature distribution of the laser medium is closely related to the viscous effect in the solid-liquid boundary layer. Although the heat deposition distribution of the laser medium is symmetrical, the temperature profile is asymmetrical as a result of the increasing water temperature along the water flow. The maximum temperature rise of the disk is at the outlet end, and the position remains almost unchanged. The front-surface temperature distributions and wavefront profiles of Nd:YAG vary nonlinearly with the coolant flow rates, but linearly with the pump parameter. Model predictions show that when the laser amplifier operates at a repetition rate of 50 Hz, the thermal diffusion of the coolant mainly occurs in a range of 100 μm, and the maximum temperature difference of the coolant reaches up to 10.85 ℃. Correspondingly, the maximum temperature variation over the front-surface active region is less than 4 ℃, with an average temperature of 49.62 ℃, which leads to a total peak-to-valley wave front distortion of 7.27&lt;i&gt;λ&lt;/i&gt;. The experimentally measured temperature distributions are in reasonable agreement with numerical simulations. The research results are beneficial to designing and optimizing the high-energy, high-repetition rate water-cooled Nd:YAG active mirror amplifiers.

Application progress of the stimulated Brillouin scattering phase conjugate mirror in high power nanosecond lasers

Stimulated Brillouin Scattering (SBS) is a third-order nonlinear optical effect, the reflected beam of which has the property of phase conjugation. The reflected beam is time inversed on amplitude, phase and polarization with the incident beam and maintains the same wavefront as the incident light. In the Master Oscillator Power Amplifier (MOPA) nanosecond laser system, the thermal effect in the laser amplifiers and a large number of optical elements in the optical path cause serious wavefront distortion of the transmitted beam, which not only deteriorates the beam quality, but also limits the possibility of further power improvement. Stimulated Brillouin Scattering Phase Conjugate Mirror (SBS-PCM) is utilized widely in this kind of laser system for the fact that using SBS-PCM by making the beam transmit along a round trip in amplifiers which bring in serious wavefront distortion compensates the distortion in real time, therefore is capable of optimizing the beam quality and improving the output power, so as to promote the development of nanosecond laser system in the direction of both high power and high beam quality. In this paper, the basic principle and phase conjugation characteristics of SBS-PCM were briefly introduced theoretically. Then the characteristics and application scope of different SBS-PCM media were compared. The research progress by domestic and foreign institutions on SBS-PCM and the typical application and development process of SBS-PCM in high power nanosecond lasers were summarized. Finally, the development trend of SBS-PCM was prospected.

Method of inverting wavefront phase from far-field spot based on deep learning

In the adaptive optics system, the accuracy and robustness of wavefront sensor greatly affect the ability of aberration detection and closed-loop correction. In the condition of the nonuniformity of amplitude distributions or insufficient of beacon light energy, it will cause accuracy decrease of Hartmann wavefront sensing due to the lack of sub-aperture light. Meanwhile, the real-time performance of the wavefront sensing-free adaptive system based on far-field spot inversion cannot meet the practical requirements. The method of the wavefront inversion based on deep learning is to directly obtain aberrations by inputting the far-field light intensity image, which can be used as an effective supplement to the adaptive optical system. Through numerical simulation, this paper proved that the deep residual neural network could directly predict the Zernike coefficient of the wavefront phase through the far-field spot. And experimental demonstrated the corrected residual RMS between input and reconstructed wavefront phase was 0.08 waves, the average computation time was less than 2 ms by GPU acceleration. This method can predict the Zernike coefficient of incident wavefront distortion more accurately, and has a good aberration correction capability, suitable for measuring and correcting the main components of wavefront distortion in traditional adaptive optics method, or providing a good initial wavefront estimation for optimized adaptive optics.

Compensation for aberration distortions of a laser beam wavefront by aero-optical effects on aircraft - satellite paths based on backscatter signals

Compensation for aberration distortions of a laser beam wavefront by aero-optical effects on aircraft - satellite paths based on backscatter signals

基于改进型SPGD算法的涡旋光波前畸变校正

基于改进型SPGD算法的涡旋光波前畸变校正

Inside Cover

AbstractWavefront shaping technology can correct for wavefront distortion in turbid media and restore the diffraction‐limited imaging. However, the extent of each correction's validity, or the isoplanatic patch, in bone is unknown. In this paper, we present 1) direct measurement and 2) an in‐situ modeling method of the isoplanatic patch in the murine skull. (The authors thank Ms. Rachel Carlyle for her help with the design of the cover illustration.)Further details can be found in the article by Kayvan Forouhesh Tehrani, Nektarios Koukourakis, Jürgen Czarske, and Luke J. Mortensen (e202000160). image

1178 J, 527 nm near diffraction limited laser based on a complete closed-loop adaptive optics controlled off-axis multi-pass amplification laser system

AbstractA 1178 J near diffraction limited 527 nm laser is realized in a complete closed-loop adaptive optics (AO) controlled off-axis multi-pass amplification laser system. Generated from a fiber laser and amplified by the pre-amplifier and the main amplifier, a 1053 nm laser beam with the energy of 1900 J is obtained and converted into a 527 nm laser beam by a KDP crystal with 62% conversion efficiency, 1178 J and beam quality of 7.93 times the diffraction limit (DL). By using a complete closed-loop AO configuration, the static and dynamic wavefront distortions of the laser system are measured and compensated. After correction, the diameter of the circle enclosing 80% energy is improved remarkably from 7.93DL to 1.29DL. The focal spot is highly concentrated and the 1178 J, 527 nm near diffraction limited laser is achieved.