Adaptive system for wavefront correction of the PEARL laser facility

Supercontinuum laser, as a new type of laser source, compared with monochromatic or quasi-monochromatic light, has incandescent wide spectra and the brightness of the laser. However,the beam quality in high power is usually bad and greatly affected by atmospheric turbulence in the transmission process. Therefore, how to improve the beam quality to ensure the energy transmission and image quality has become a popular topic. According to the measurement results of the near field wavefront characteristics of the supercontinuum laser in different wavelength, we know that the wavefront phase of the light source is relatively stable and change slowly over time. The types of wavefront distortion at each wavelength are basically the same, mainly made up of defocus and astigmatism. In order to conform to the wavefront characteristics of supercontinuum laser, the input of simulation is low-order static distortion wavefront generated by using Zernike polynomia with defocus and astigmatism. Stochastic Parallel Gradient Descent (SPGD) algorithm with the thorlabs DMP40 deformable mirror and CCD camera as the imaging sensor, is used to construct a simulation model for wavefront correction of low-order static distortion aberrations. Since B-SPGD algorithm is superior to U-SPGD in terms of convergence speed and correction effect, as well as adaptability to the near field wavefront distortion [1], we use B-SPGD algorithm to construct the simulation program for wavefront correction simulation research. The ability and convergence of SPGD algorithm to correct wavefront distortion are studied, mainly about the random disturbance amplitude and influence coefficient of the core parameters. In order to get close to the evaluation standard of laser in practical application, Strehl Ratio(SR) is selected as the performance evaluation function. According to the effective diameter of deformable mirror and the distance between electrodes, the influence function of deformable mirror is normalized to the unit circle to meet the requirements of simulation calculation. In order to improve the stability of SPGD algorithm and reduce the possibility of SPGD algorithm falling into local extreme values, we use Hessian matrix to optimize SPGD algorithm. After simulation and analysis of distortion correction of Supercontinuum laser wavefront, deformable mirror controlled by SPGD algorithm has good correction capability for low-order static aberration.

When the Daniel K. Inouye Solar Telescope (DKIST) achieves first light in 2019, it will deliver the highest spatial resolution images of the solar atmosphere ever recorded. Additionally, the DKIST will observe the Sun with unprecedented polarimetric sensitivity and spectral resolution, spurring a leap forward in our understanding of the physical processes occurring on the Sun. The DKIST wavefront correction system will provide active alignment control and jitter compensation for all six of the DKIST science instruments. Five of the instruments will also be fed by a conventional adaptive optics (AO) system, which corrects for high frequency jitter and atmospheric wavefront disturbances. The AO system is built around an extended-source correlating Shack-Hartmann wavefront sensor, a Physik Instrumente fast tip-tilt mirror (FTTM) and a Xinetics 1600-actuator deformable mirror (DM), which are controlled by an FPGA-based real-time system running at 1975 Hz. It is designed to achieve on-axis Strehl of 0.3 at 500 nm in median seeing (r₀ = 7 cm) and Strehl of 0.6 at 630 nm in excellent seeing (r₀ = 20 cm). The DKIST wavefront correction team has completed the design phase and is well into the fabrication phase. The FTTM and DM have both been delivered to the DKIST laboratory in Boulder, CO. The real-time controller has been completed and is able to read out the camera and deliver commands to the DM with a total latency of approximately 750 μs. All optics and optomechanics, including many high-precision custom optics, mounts, and stages, are completed or nearing the end of the fabrication process and will soon undergo rigorous acceptance testing. Before installing the wavefront correction system at the telescope, it will be assembled as a testbed in the laboratory. In the lab, performance tests beginning with component-level testing and continuing to full system testing will ensure that the wavefront correction system meets all performance requirements. Further work in the lab will focus on fine-tuning our alignment and calibration procedures so that installation and alignment on the summit will proceed as efficiently as possible.

Two combined systems of synchrotron radiation and laser have been constructed for solid-state research. One is a laser-induced fluorescence system to observe synchrotron radiation-induced desorption of alkali atoms from ionic crystals, which consists of a laser diode with a high repetition rate and synchrotron radiation under a single-bunch operation. The other is a system of two-photon spectroscopy, which is based on the combination of synchrotron radiation pulses with a low intensity and high repetition rate and Nd:YAG laser pulses with a high intensity and low repetition rate. The experimental systems and the preliminary results are presented in this report.

Earth observation satellite with a large mirror telescope has been studied in Japan Aerospace Exploration Agency to offer a fine ground sampling distance from geostationary orbit. Our latest optical design has a 3.6-m primary mirror and thus aims to obtain a ground sampling distance of sub-10 m from an altitude of 36,000 km at visible wavelengths. To achieve diffraction-limited performance in such optics on orbit, the telescope equips with not only actuators to control primary and secondary mirrors but also a deformable mirror (DM) at the exit pupil plane for the fine phasing. For on-orbit wavefront correction with a deformable mirror, a wavefront sensorless image-based aberration correction scheme is advantageous from the viewpoint of severely limited hardware resources in satellites. Phase diversity (PD) and stochastic parallel gradient descent (SPGD) optimization are known for promising image-based approaches. The former is model-based and thus the estimation accuracy of wavefront aberration significantly depends on the model accuracy, while the other requires many measurements to compensate for large aberration. To alleviate these issues, we propose sequential use of PD and SPGD optimization to efficiently reduce wavefront correction. We first developed an optical testbed with an incoherent light source, a MEMS DM, and extended image targets, and then a wavefront correction experiment was carried out. As a result, the proposed method successfully achieved diffraction-limited imaging performance with a small number of measurements. We will also discuss the image dependence of the wavefront correction performance.

In wavefront correction the pixels position error will affect wavefront corrector precision. To solve this problem a circulation feedback control method based on image boundary definition was proposed and an experiment is done on wavefront correcting based on liquid crystal spatial light modulator (LC SLM). The measured wavefront aberration is transformed a gray image to search edge pixels and centroid coordinates by boundary detection method of the digital image processing. On this basis the wavefront aberration is corrected by circulation feedback control. The experimental results show that the PV value of the static wavefront aberration is 0.66λ, the RMS is 0.13λ and the Strehl ratio is 0.54 before correcting and the PV is 0.17λ, the RMS is 0.02λ and the Strehl ratio is 0.98 after correcting. The PV value is less than λ/4 and the Strehl ratio is near diffraction limit, which proves that the above method is effective.

: The end-to-end performance achieved by an adaptive optical (AO) imaging system is determined by a combination of the residual time-varying phase distortions associated with atmospheric turbulence, and the quasi-static unsensed and uncorrectable aberrations in the optical system itself. Although the effects of these two errors on the time averaged Strehl ratio and the time averaged optical transfer function (OTF) of the AO system are not formally separable, such an approximation is found to be accurate to within a few per cent for a range of representative residual wave-front errors. In these calculations, we have combined static optical system aberrations and time-varying residual phase distortion characteristic of deformable mirror (DM) fitting error, wave-front sensor (WFS) noise, and anisoplanatism. The static aberrations consist of focus errors of varying magnitudes as well as a combination of unsensed and uncorrectable mirror figure errors derived from modeling by the Gemini beta-meter Telescopes project. The overall Strehl ratios and OTF's due to the combined effect of these error sources are well approximated as products of separate factors for the static and time-varying aberrations, as long as the overall Strehl ratio due to both errors is greater than about 0.1. For lower Strehl ratios the products provide lower bounds on the actual values of the Strehl ratio and OTF. The speckle transfer function (STF) is also well approximated by a product of two functions, but only where AO compensation is sufficiently good that speckle imaging techniques are usually not required.

A shape of a wave-front that is transformed by an optical element (or system) is changed depending on physical and geometric properties of such a system. This fact can be used in many areas of science and technology. For example, various applications can be found in adaptive optics where active optical elements are used for wave-front correction. Our work presents a detailed analysis of aberration properties of a simple deformable planar mirror from both the aspect of modeling the shape of reflecting surface of the mirror and the mechanisms that enable to reach this desired shape for correction of spherical aberration. Finally, a simple experimental device for practical realization of the optical system for partial wave-front correction is proposed.

The next generation of extremely large telescopes (ELTs) that will be built on a scale of 30–50 meters in diameter necessarily require an increased ability to correct wavefront errors. Currently, wavefront correction is achieved through the use of a deformable mirror (DM). The degree and amount of correction depends on amechanical displacement of segments of themirror (stroke), and the number of actuators in the mirror surface (order). An effective ELTwill require a high-stroke (10microns) and high-order (100X100 actuators) mirror, a costly prospect using current technology.1 Lowering the cost while improving the performance of DMs is possible using microelectromechanical systems (MEMS) technology. However, current surface machining processes used to fabricate MEMS deformable mirrors, such as the Sandia ultra-planar multilevel MEMS technology (SUMMiT) and MEMSCAP’s PolyMUMPS process, limit a DM’s stroke due to the use of thin-film sacrificial layers (which are dissolved away after the etching process to free up micromechanical components in the actuators).2, 3 Commercially available surface micromachined MEMS mirrors are limited in stroke to approximately 5.5 microns, about half of the stroke required by ELTs. A high aspect ratio fabrication process capable of depositing thick layers will allow the DM’s to provide both high-stroke and highorder corrections, thus bypassing the need for a complex dual woofer (high stroke, low order)/tweeter (high order, low stroke) DM configuration.4 In our work, we tested different actuator designs with a bonded faceplate constructed using the LIGA (German acronym for lithography, electroplating, and molding) process, enabling multilayer fabrication of MEMS devices.5 Various types of high-stroke gold actuators consisting of folded springs with Figure 1. Diagram showing the final release of the monolithic fabricated structure created using our technique. The gap for the Xbeam electrostatic actuators is 20 microns, enabling a mirror stroke of 10 microns. WMS-15: specific glass-ceramic substrate. Not to scale.

The wave front corrector is one of the three key elements in adaptive optics, along with the wave front sensor and the control computer. Low cost, compact deformable mirrors are increasingly available. We have tested the AOptix bimorph deformable mirror, originally developed for ultra-high bandwidth laser communication systems, to determine its suitability for vision science applications, where cornea and lens introduce optical aberrations. Measurements of the dynamic response of the mirror to a step input were obtained using a commercial Laser Doppler Vibrometer (LDV). A computer-controlled Twyman-Green interferometer was constructed to allow the surface height of the deformable mirror to be measured using Phase-Shifting Interferometry as a function of various control voltages. A simple open-loop control method was used to compute the control voltages required to generate aberration mode shapes described by the Zernike polynomials. Using this method, the ability of the deformable mirror to generate each mode shape was characterized by measuring the maximum amplitude and RMS error of each Zernike mode shape up to the fifth radial order. The maximum deformation amplitude was found to diminish with the square of the radial order of the Zernike mode, with a measured deformation of 8 microns and 1.5 microns achieved at the second-order and fifth-order Zernike modes, respectively. This deformation amplitude appears to be sufficient to allow the mirror to correct for aberrations up to the fifth order in the human eye.

The impact and thermal action of laser sparks on the reflector of a laser engine in which the propulsion is produced by repetitively pulsed radiation is estimated. It is shown that for a low pulse repetition rate, the thermal contact of a plasma with the reflector and strong dynamic resonance loads are inevitable. These difficulties can be surmounted by using the method based on the merging of shock waves at a high pulse repetition rate.

The capabilities of a novel deformable mirror to correct ocular aberrations are analyzed. The deformable mirror, (MIRAO52 Imagine Eyes) is incorporated within a complex retinal imaging system able to produce simultaneous en- face optical coherence tomography (OCT) and scanning laser ophthalmoscopy (SLO) images of the retina. The performances of the deformable mirror have been examined by evaluating the amount of aberrations which the deformable mirror is able to correct for, the improvement in the signal-to-noise ratio and transversal resolution in both OCT and SLO images obtained from an artificial eye. Pairs of C-scan images were obtained at a frame rate of 2 Hz, with and without wave-front correction. The correction of aberrations increased the signal-to-noise ratio in images obtained from artificial eyes by up to 6 dB in the OCT channel and up to 31 dB in the SLO channel. Also, we demonstrated the high capability of the MIRAO52 deformable mirror to correct for high amounts of wave-front aberrations.

The Photo-controlled deformable mirror (PCDM) presents an interesting alternative to the conventional deformable mirror technologies as it addresses their two major bottlenecks. It offers enormous spatial resolution while keeping its complexity low due to its elegant control of the deformable surface by light projection. The PCDM prototype was a membrane type deformable mirror driven by a modified commercial mini-projector. Unique properties of PCDM allowed experimental study of actuator array disposition influence on the wavefront shaping performance of the deformable mirror. While keeping all other aspects and parameters of the experiment constant, the four actuator-array configurations were compared and resulting wavefront shaping performance assessed. Moreover, the Photo-controlled deformable mirror was numerically modelled and the experimental results were reproduced with high degree of fidelity. By using the numerical model of PCDM, a novel method to control high spatial resolution wavefront corrector was studied with promising results.

The required surface shape of the intra-cavity deformable mirror (DM) is not conjugate with the aberration in an unstable standing-wave resonator, which makes the adaptive correction inside the resonator complicated. In this letter, an unstable travelling-wave resonator structure with an intra-cavity adaptive optics (AO) system is proposed to solve this problem. More specifically, the Shark-Hartmann wavefront sensor (SHWFS) and a 980 nm probe beam are employed to measure the aberration of the Nd:YAG slab during lasing operation. A tip/tilt mirror (TTM) is used to correct the intra-cavity tilt, and a DM is employed to compensate for all the remaining intra-cavity aberration. By placing both the wavefront correctors (DM and TTM) and the Nd:YAG slab in the collimated part of the travelling-wave resonator, the required surface shape of the DM is well conjugate with the measured aberration. Experimental results show the proposed method can effectively improve the performance of solid-state slab lasers. After correction, the output power increased from 1312 W to 1587 W, and the corresponding optical-to-optical efficiency increased from 19.5% to 23.6%. Meanwhile, the beam quality <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\beta $ </tex-math></inline-formula> was improved from 9.7 to 1.8.

Dental lasers represent a promising therapeutic tool in the treatment of periodontal and peri-implant diseases. However, their clinical application remains still limited. Here, we investigated the potential biostimulatory effect of low pulse energy neodymium:yttrium-aluminum-garnet (Nd:YAG) laser irradiation on different cells representative of the oral microenvironment and elucidated the underlying molecular mechanisms. Saos-2 osteoblasts, H-end endothelial cells, and NIH/3T3 fibroblasts pre-treated or not with photosensitizing dye methylene blue (MB), were irradiated with low pulse energy (20 mJ) and high repetition rate (50-70 Hz) Nd:YAG laser, and evaluated for cell viability and proliferation as well as for the expression of specific differentiation markers by confocal immunofluorescence and real-time RT-PCR. Changes in intracellular Ca(2+) levels after laser exposure were also evaluated in living osteoblasts. Nd:YAG laser irradiation did not affect cell viability in all the tested cell types, even when combined with pre-treatment with MB, and efficiently stimulated cell growth in the non-sensitized osteoblasts. Moreover, a significant induction in the expression of osteopontin, ALP, and Runx2 in osteoblasts, type I collagen in fibroblasts, and vinculin in endothelial cells could be observed in the irradiated cells. Pre-treatment with MB negatively affected cell differentiation in the unstimulated and laser-stimulated cells. Notably, laser irradiation also caused an increase in the intracellular Ca(2+) in osteoblasts through the activation of TRPC1 ion channels. Moreover, the pharmacologic or genetic inhibition of these channels strongly attenuated laser-induced osteopontin expression, suggesting a role for the laser-mediated Ca(2+) influx in regulating osteoblast differentiation. Low pulse energy and high repetition rate Nd:YAG laser irradiation may exert a biostimulative effect on different cells representative of the oral microenvironment, particularly osteoblasts. Pre-treatment with MB prior to irradiation hampers this effect and limits the potential clinical application of photosensitizing dyes in dental practice.

Studies have been taken into the breakdown voltage jitter of corona-stabilized switch at high repetition rate up to several kHz. It is thought that the gas recovery affect the jitter of breakdown voltage directly at high repetition rate, because the gas switch will breakdown at a low voltage if the gas cannot fully recover. The breakdown voltage jitter is called voltage jitter in the following paper. However, based on the experimental results, the voltage jitter still exists at low repetition rate, even at single-shot mode. This is not explained by the previous studies. In this paper, voltage jitter below 100 Hz of a corona-stabilized switch is investigated experimentally. A corona-stabilized switch is introduced, using a cylindrical electrode as the cathode and a plane electrode as the anode. The gas used between the electrodes is SF6. A negative pulse with a maximum width of $30~\mu \text{s}$ is used to charge this switch. The tested factors include gas pressure, gap space, repetition rate, and switch geometry. Experimental results show that voltage jitter is mainly affected by the gas pressure and switch geometry. It is seen that voltage jitter decreases as gas pressure increases and a suitable cathode geometry presents a low voltage jitter. Critical volume is a region close to the highly stressed electrode, where the presence of an electron will lead to an electron avalanche which attains a critical size and hence leads to a stabilizing corona. Critical volume is introduced to explain the experimental results. It is found that less voltage jitter can be achieved with a smaller and more concentrated critical volume, as well as efficient corona stabilization. The results indicate that voltage jitter may be reduced by optimize the critical volume at low repetition rate.