Laser beam formation by adaptive optics

Laser beam propagation in the atmosphere is inevitably affected by turbulence and extinction, the far-field intensity distribution of the spot changes greatly relative to the laser exit. When the beam quality parameters are measured by the traditional method, the atmospheric effect is difficult to remove and the analysis mechanism is complex. Since the Hartmann-Shack wavefront sensor can detect the wavefront phase of the laser beam. The far-field situation of laser beam can be obtained by processing the phase information of near-field, which can be applied to the diagnosis of laser beam quality. However, for the wavefront distribution of large aperture spot, especially the wavefront distribution of high power laser, the traditional Hartmann-Shack wavefront detection method is no longer applicable due to the limitation of aperture size and the anti-damage detection threshold of detection components. In this paper, a wavefront direct measurement system based on Hartmann detection principle is proposed, which can measure large aperture and high power laser beam. The simulation results show that the wavefront recovery error of our method is low in a large detection range. The performance of this method is also evaluated on a more complex wavefront, and the wavefront recovery accuracy is lower than 0.20λ. The system proposed in this paper is suitable for large aperture laser beam measurement. It is of great importance to improve the measurement accuracy of beam quality.

Performances of a 20-element bimorph deformable mirror are measured and a closed-loop adaptive optics system, which consists of the bimorph mirror and a Shack-Hartmann wave-front sensor with 13×13 sub-aperture array, is established to test the mirror’s capability of correcting static aberrations. Experimental results show that, the mean displacement response to voltage of discrete actuators is 0.57 μm/100 V and that of defocus electrode is 1.09 μm/100 V. The hysteresis of actuator is limited to ten percents of the maximal displacement. With the ambient temperature increasing from 13℃ to 25℃, the P-V of defocus of the mirror surface is enlarged by 1.37 μm and the P-V of other distortions, such as astigmatism, coma and spherical aberration, is enlarged by 0.15 μm. The frequency response of the bimorph mirror is about 4 kHz. When the closed-loop control is on, intensity distributions at the image plane of distorted wave-fronts are effectively improved. The Strehl ratios increase from 0.02 to 0.95, and 0.03 to 0.54， respectively.

The HELEN laser is a three-beam, large aperture Nd:glass laser, used for plasma physics studies at the Atomic Weapons Establishment in the UK. Two of the beams nominally deliver 500 J each in 1 ns at the second harmonic (527 nm). The third beam, the “backlighter”, has recently been upgraded to operate as a chirped pulse amplification system and it now routinely delivers 70 J to target in 500 fs. Optimal focal spot performance is achieved using a closed-loop adaptive optics system, which ensures good wavefront characteristics, irrespective of whether previous firing of the amplifiers has induced refractive index variations in the laser glass. The system uses a 32 element bimorph mirror with 98 mm aperture, roughly half way through the laser chain. A Shack-Hartman wavefront sensor, positioned at the output of the laser is the diagnostic used to provide feedback to the deformable mirror. Correction of the static and slowly varying aberrations on the beam has been demonstrated. The fast aberrations induced during the flashlamp discharge have been evaluated. The improved focal spot characteristics result in an intensity on target of significantly greater than 1019 Wcm-2.

High average power operation of the Mercury Laser induces dynamic aberrations to the laser beam wavefront. Analysis of recent data indicates that up to 4 waves of low order aberration (mainly focus error or power, with spatial resolution < 0.5 cm{sup -1}) could be expected at each pass. Because of the magnitude of the wavefront error, the logical position is to place a deformable mirror (DM) at the M11 position, where the DM will correct the beam between passes 1 & 2 and 3 & 4. Currently, there are only two established commercial vendors offering complete adaptive optic (AO) systems that can accommodate the Mercury beam size (45 x 75 mm) which are compatible with high damage threshold coatings. Xinetics (MA, USA) offers a complete AO system along with a Shack-Hartmann wavefront sensor. The Xinetics DM is based on lead magnesium niobate (PMN) technology. A number of US aerospace firms as well as NIF use Xinetics PMN technology for their DMs. Phasics (Paris, France) offers a complete AO solution with its proprietary SID-4, a four-way shearing interferometric wavefront sensor capable of high resolution (over 100 x 100 sampling points). The Phasics system includes a bimorph deformable mirror from Night-n-Opt (Moscow, Russia) that uses lead zirconate titanate (PZT) technology. Various high power laser laboratories around the world such as LULI (France), HELEN (UK), and GEKKO (Japan) are using the PZT-based bimorph DM in their system. While both DM technologies are equivalent and have been deployed in high-energy laser systems, the PZT based bimorph DM offers two distinct features that makes it more attractive for high average power laser systems. The bimorph DM uses two layers of PZT actuators with the outer layer acting as power correctors, capable of correcting up to 20 waves of power. The Xinetics DM offers a maximum stroke of 4 waves. In addition, Night-N-Opt has also designed a water-cooled DM with a silicon based substrate (as opposed to a glass substrate) specifically for high average power laser systems--an option that is currently not available for PMN based DMs.

Information is given about the Projects # 1929 and # 2631 supported by ISTC and concerned with improving laser beam quality and interesting for adaptive optics community. One of them, Project # 1929 has been recently finished. It has been devoted to development of an SBS phase conjugation mirror of superhigh conjugation quality employing the kinoform optics for high-power lasers with nanosecond scale pulse duration. With the purpose of reaching ideal PC fidelity, the SBS mirror includes the raster of small lenses that has been traditionally used as the lenslet in Shack-Hartmann wavefront sensor in adaptive optics. The second of them, Project # 2631, is concerned with the development of an adaptive optical system for phase correction of laser beams with wavefront vortex. The principles of operation of modern adaptive systems are based on the assumption that the phase is a smooth continuous function in space. Therefore the solution of the Project tasks will assume a new step in adaptive optics.

The plenoptic wavefront sensor (PlWFS) has been proposed as being suitable for operating on extended objects in adaptive optics (AO) systems. We propose a new self-referencing method that uses the cell image of PlWFS as a correlation reference and enables the simultaneous measurement of high-order aberrations and tilt. Simulations have been performed for different operating conditions to verify the feasibility of the proposed method. These show that using the cell image results in better AO performance compared to existing reference generation techniques and allows stable wavefront tilt control with a small residual variance.

The pyramid wavefront sensor is known for its high sensitivity and dynamic range that can be tuned by mechanically altering its modulation amplitude. Here, a novel modulating digital scheme employing a reflecting phase only spatial light modulator is demonstrated. The use of the modulator allows an easy reconfigurable pyramid with digital control of the apex angle and modulation geometry without the need of any mechanically moving parts. Aberrations introduced by a 140-actuator deformable mirror were simultaneously sensed with the help of a commercial Hartmann-Shack wavefront sensor. The wavefronts reconstructed using the digital pyramid wavefront sensor matched very closely with those sensed by the Hartmann-Shack. It is noted that a tunable modulation is necessary to operate the wavefront sensor in the linear regime and to accurately sense aberrations. Through simulations, it is shown that the wavefront sensor can be extended to astronomical applications as well. This novel digital pyramid wavefront sensor has the potential to become an attractive option in both open and closed loop adaptive optics systems.

Measurements of the wavefront of light reflected from the retina of the human eye can be used to determine optical aberrations of the human eye for large pupils. An instrument based on the Hartmann-Shack principle was developed. The wavefront is refracted by a microlens array and detected by a CCD camera. In first clinical studies human volunteer eyes and preoperative and postoperative refractive surgical patient eyes have been examined. An adaptive optical closed loop system has been devised for preoperative simulation of refractive outcomes of aberration free refractive surgical procedures.

An innovative experimental method for studying the influence of supersonic flow on laser beam transmitting has been proposed. The wavefront phase properties and the dynamic wavefront change of laser beam traversing supersonic flow can be detected by Hartmann-Shack (H-S) wavefront sensor. Beam aberrations are constructed by adopting Zernike mode-reconstruction theory. The centroid change of laser beam and the far-field properties can be obtained by further analyzing the experimental data. In the paper, the experimental principle is presented, and wavefront aberrations with several different flow parameters are given. Some beam-quality criteria such as Zernike aberration coefficient, PV and RMS value, circle energy, Strehl ratio, and centroid change of beam, are also calculated. Experimental results show that the stage of flow establishing, stabilizing and ending can be detected by using H-S wavefront sensor. By comparing the result of no flow with that of a given-parameter flow, defocus and astigmatism aberration increase obviously. At the same time, the Strehl ratio is reduced and the beam quality is degraded. The same conclusion can be obtained by comparing no test-model with the given-structural test-mode in same flow. Moreover, the wavefront aberration of laser beam in the condition of different flow parameters can also be measured and compared accurately by using the test method.

The Shack-Hartmann wavefront sensor is the most common wavefront sensor used today in adaptive optics. While the most common way to employ a Shack-Hartmann wavefront sensor is with a centroiding-based algorithm, recent studies have shown how its accuracy can be increased by using a correlation-based algorithm. This paper compares the accuracy of a correlation-based Shack-Hartmann wavefront sensor to a centroiding-based Shack-Hartmann wavefront sensor by measuring the distortions of a point source viewed through strong turbulence where scintillation, variations in the amplitude of the field, plays an important role. In particular, this paper demonstrates that a correlation-based algorithm generally does outperform the centroiding-based algorithm, but by using a simple threshold a centroiding-based Shack-Hartmann can produce similar results as a correlation-based Shack-Hartmann. Also, in very low signal levels, such as when the average signal level is below the detector read noise, a regular centroiding-based Shack-Hartmann outperforms all other methods tested.

Light-induced material organization extends over a broad area of research, from photon momentum transfer to atoms, molecules and particles, serving the basis for optical trapping, and expands into the laser-induced changes of material properties through photopolymerization, photodarkening, and materials ablation. Relevant phenomena are observed over many orders of magnitude of light intensity, from a few kW cm-2 for the optical trapping of living cells to 1014 W cm-2 encountered in femtosecond laser micromachining and micro-explosion. Relevant interactions reveal a rich palette of novel phenomena in the solid state, from subtle excitations and material organization to phase transformations, non-equilibrium and transient states. The laser-induced material modifications relate to changes in the crystal structure and the molecular bonding, phase transitions in liquid state, ablation and plasma production associated with extreme pressure and temperature conditions towards entirely new states of matter.

The recent 2020 Decadal Survey of Astronomy and Astrophysics listed habitable exoplanet imaging with future extreme adaptive optics (AO) on 30m-class telescopes as a key priority in the coming decade. However, there is a current 100x contrast gap between the best systems today and what is needed to enable this goal. Astronomical AO is a required approach to enable ground-based diffraction-limited imaging of exoplanets on future extremely large telescopes. Time lag between the end of an exposure and the application of deformable mirror commands is a major contributor to the error budget in many AO systems, and detector read time is often a large component of this lag. We present two designs for a modified Shack Hartmann wavefront sensor (SHWFS), named Focal plane Actualized Shifted Technique Realized for a SHWFS (fastrSHWFS), to reduce the time lag component. This design steers the spot pattern at the focal plane into a rectangular or linear array with a custom aspect ratio, reducing readout time. The mask with focus yields aberrated results while the mask with tip/tilt only yields some defined spots. This essay outlines the current SHWFS concept, our fastrSHWFS theoretical solution to addressing time lag, reflection and quality analysis of printed mask designs, and results from testing both masks on the High Contrast Testbed at Lawrence Livermore National Lab. This work follows the test of a previous fastrSHWFS design.

Adaptive optics (AO) systems greatly improve the resolution of retinal imaging instruments by actively correcting ocular aberrations. In this Letter, closed-loop correction as well as ocular aberration compensation of a 62-element silicon unimorph deformable mirror (DM) driven by only positive voltage is performed. The experimental results show that the root-mean square (RMS) wavefront of the initial mirror surface is reduced to 0.011 μm in a closed-loop AO system. The DM reproduces Zernike shapes from the third to 35th mode accurately. The simulated compensation of 200 ocular wavefronts shows that the average RMS value after correction is reduced to 0.017 μm.

Several telescopes include large Deformable Mirrors (DM) located directly inside the telescope. These adaptive telescopes trigger new constraints for the calibration of the Adaptive Optics (AO) systems as they usually offer no access to an artificial calibration source for the interaction matrix measurement. Moreover, the optical propagation between the DM and the Wave-Front Sensor (WFS) may evolve during the operation, resulting in mis-registrations that highly affect the AO performance and thus the scientific observation. They have to be measured and compensated, for instance by updating the calibration. A new strategy consists of estimating the mis-registrations and injecting them into synthetic models to generate noise-free interaction matrices. This pseudo-synthetic approach is the baseline for the Adaptive Optics Facility working with a Shack-Hartmann WFS and seems particularly suited for the future Extremely Large Telescope as the calibration will have to be regularly updated, for a large numbers of actuators. In this paper, the feasibility of a pseudo synthetic calibration with Pyramid WFS at the Large Binocular Telescope (LBT) is investigated. A synthetic model of the LBT AO systems is developed, and the procedure to adjust the mis-registrations parameters is introduced, extracting them from an experimental interaction matrix. We successfully tested an interaction matrix generated from the model on the real system in high-order AO mode. We recorded a slightly better performance with respect to the experimental one. This work demonstrates that a high accuracy calibration can be obtained using the pseudo synthetic approach with pyramid WFS.

This paper presents the design of an Adaptive Optics (AO) system for retinal imaging applications. The development of retinal imaging systems allows for early diagnosis of eye diseases. Such systems can increase the quality of life of patients as well as curtail increasing health care costs through early eye disease detection and treatment. Until recently, AO systems have been prohibitively expensive and cumbersome. This has been mainly due to the size and cost of flexible membrane mirrors normally used as the aberration correction device. Recent developments in the technology of Microelectromechanical System (MEMS) based actuators allow the implementation of AO systems which would have been difficult to implement a few years ago due to exorbitant costs. The aim of this paper is to present the design of a compact and flexible low cost AO system using off the shelf components to measure and compensate for the aberrations of the eye. The design is based around the system’s main components which include a 52 channel magnetically actuated deformable membrane mirror, a Shack Hartmann wavefront sensor and a control system which runs on a single processor personal computer. All the components are commercially available. The use of the MEMS-based magnetically actuated mirror allows for increased resolution and force compared to conventional membrane mirrors designed mainly for use in astronomical applications. The performance of the closed-loop system is evaluated through experiments. Although designed as a diagnostic tool for eye diseases, such a system will find a number of applications in basic research in the visual sciences, including the study of microscopic structures in the living retina that could not be seen before. Optometrists, retinal surgeons, and ophthalmologists will also benefit from using such a system, through potential improvements on commonly used instruments such as phoropters and fundus cameras.