Wavefront correction with photo-controlled deformable mirror

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.

Nearly every new large-scale laser system application at LLNL has requirements for beam control which exceed the current level of available technology. For applications such as inertial confinement fusion, laser isotope separation, laser machining, and laser the ability to transport significant power to a target while maintaining good beam quality is critical. There are many ways that laser wavefront quality can be degraded. Thermal effects due to the interaction of high-power laser or pump light with the internal optical components or with the ambient gas are common causes of wavefront degradation. For many years, adaptive optics based on thing deformable glass mirrors with piezoelectric or electrostrictive actuators have be used to remove the low-order wavefront errors from high-power laser systems. These adaptive optics systems have successfully improved laser beam quality, but have also generally revealed additional high-spatial-frequency errors, both because the low-order errors have been reduced and because deformable mirrors have often introduced some high-spatial-frequency components due to manufacturing errors. Many current and emerging laser applications fall into the high-resolution category where there is an increased need for the correction of high spatial frequency aberrations which requires correctors with thousands of degrees of freedom. The largest Deformable Mirrors currently availablemore » have less than one thousand degrees of freedom at a cost of approximately $1M. A deformable mirror capable of meeting these high spatial resolution requirements would be cost prohibitive. Therefore a new approach using a different wavefront control technology is needed. One new wavefront control approach is the use of liquid-crystal (LC) spatial light modulator (SLM) technology for the controlling the phase of linearly polarized light. Current LC SLM technology provides high-spatial-resolution wavefront control, with hundreds of thousands of degrees of freedom, more than two orders of magnitude greater than the best Deformable Mirrors currently made. Even with the increased spatial resolution, the cost of these devices is nearly two orders of magnitude less than the cost of the largest deformable mirror.« less

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.

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 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.

For satisfying the boarder application requirement of adaptive optics (AO) and solving the problem of large volume and high cost of conventional deformable mirrors (DM), micro DM based on micro-electro-mechanical system (MEMS) technology is developed and measured. The developed DM has 140 hexagonal parallel plate capacitor electrostatic actuators. The actuators are arranged as a square array and the pitch is 400 μm. A DM prototype is fabricated by MEMS surface micromachining process and packaged by a ceramic pin grid array (CPGA). A miniaturization multi-channel high voltage driver for the DM is developed too. The measurement results show that the prototype has a surface PV value of 411 nm, RMS value of 78 nm, reflectivity of about 80% in 600 nm to 900 nm wavelength, stroke of 1.8 μm, actuator coupling of 15%, working bandwidth of 13 kHz and step response time of 23 μs. Thus the DM has the advantages of small volume, low cost and fast response. Besides the measurement of single element, the whole DM is controlled open loop to fit Zernike aberration and its fitting capability is demonstrated. Above results indicate that the DM prototype can satisfy initially the requirement of AO system.

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 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.

ACMAS program, in partnership with CNES and DGA, intends to develop an innovative concept of deformable mirror (DM) for next generation space telescopes, to manufacture an engineering model and to test it on optical and mechanical points of view. <br/> The design of this space compatible DM is based on ISP System’s technology, and is adapted to space constraints (ECSS) particularly in terms of vibrations and reliability. These electro-mechanical DMs offer some innovative characteristics for space telescopes with low speed wavefront variations. <br/> A specific version of the patented AME actuator is developed for space applications, and tested to TRL6. The deformable mirror’s actuators pattern is optimized in terms of quantity and position with respect to the wavefront corrections expected to apply during mission. The DM design is optimized thanks to finite element modeling in order to withstand the vibrations and shocks due to launching, and improve thermo-mechanical stability, and an innovative redundancy method is developed. <br/> The performance of an engineering model is measured using an optical test bench with high resolution wavefront sensor. The correction thermal stability is also measured and the performance with one and two disabled actuators is evaluated. Vibrations are then injected into the DM. All these measures are compared to simulation values and show an impressive correlation..

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 &mu;s. 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.

We report on the development of a new class of electrostatic MEMS deformable mirror (DM) fabricated through a combination of bulk micromachining, wafer bonding, and surface micromachining. The combination of these fabrication technologies introduces four major improvements over previous MEMS DMs, which are fabricated using surface micromachining alone. First, the MEMS DM structural components (mirror surface and actuator array) are made entirely of single crystalline silicon by use of the device layer of a whole 4-inch silicon-on-insulator (SOI) wafer bonded together via anodic bonding. Unlike current MEMS DMs fabricated entirely using surface micromachining, bulk micromachining steps in this fabrication process require no etch access holes, print through is inexistent, and no polishing steps are required. This leads to reduced diffraction of light from the mirror surface, improved mirror surface optical quality, and elimination of manufacturing processing steps. Second, through-wafer interconnects are used to connect the densely-packed electrostatic actuator array to driver electronics. This eliminates the need for wirebonding at the periphery of the DM, increasing the surface area available for actuators and removes the need for bulky wire bundles to connect the device to its driver. Third, by using the full area of a silicon wafer for each mirror, these MEMS DMs offer a larger optical aperture than any previously-reported MEMS DM. The larger aperture will achieve higher angular resolution, providing larger wavefront correction. Finally, the mirror and actuator thicknesses are not limited to several micrometers, unlike in surface micromachining. The thickness limits using this fabrication process is prescribed by the device layer thickness in SOI wafers, which vary between several micrometers to several hundred of microns.

High-power laser plays an important role in many fields, such as directed energy weapon, optoelectronic contermeasures, inertial confinement fusion, industrial processing and scientific research. The uniform nearfield and wavefront are the important part of the beam quality for high power lasers, which is conducive to maintaining the high spatial beam quality in propagation. We demonstrate experimentally that the spatial intensity and wavefront distribution at the output is well compensated simultaneously in the complex high-power solid-state laser system by using the small-aperture spatial light modulator (SLM) and deformable mirror (DM) in the front stage. The experimental setup is a hundred-Joule-level Nd:glass laser system operating at three wavelengths at 1053 nm (1ω), 527 nm (2ω) and 351 nm (3ω) with 3 ns pulse duration with the final output beam aperture of 60 mm. While the clear arperture of the electrically addressable SLM is less than 20 mm and the effective diameter of the 52-actuators DM is about 15 mm. In the beam shaping system, the key point is that the two front-stage beam shaping devices needs to precompensate the gain nonuniform and wavefront distortion of the laser system. The details of the iterative algorithm for improving the beam quality and the strategy of achiving high beam quality on spatial intensity and wavefront simultaneously are presented. Experimental results show that the output wavefront RMS value is 0.06, and simultaneously the output near-field modulation is 1.38:1 and the fluence contrast is 10.5% at 3 ns at 1053nm with 40-Joule-level energy.

In this paper we develop new results on control design for spatially distributed linear systems using an n- D systems approach. As a possible application example, we consider a deformable mirror used as a wavefront corrector in adaptive optical system. The first new results consist of stability analysis plus stability tests. This is followed by control law design in a polynomial setting where a Youla-Kucera parametrization of all stabilizing controllers is specialized to the objective of reducing vibrations of the mirror based on minimization of the H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> system norm.