Adaptive Optics Configuration Research Articles

Deep tissue multi-photon imaging using adaptive optics with direct focus sensing and shaping.

High-resolution optical imaging deep in tissues is challenging because of optical aberrations and scattering of light caused by the complex structure of living matter. Here we present an adaptive optics three-photon microscope based on analog lock-in phase detection for focus sensing and shaping (ALPHA-FSS). ALPHA-FSS accurately measures and effectively compensates for both aberrations and scattering induced by specimens and recovers subcellular resolution at depth. A conjugate adaptive optics configuration with remote focusing enables in vivo imaging of fine neuronal structures in the mouse cortex through the intact skull up to a depth of 750 µm below the pia, enabling near-non-invasive high-resolution microscopy in cortex. Functional calcium imaging with high sensitivity and high-precision laser-mediated microsurgery through the intact skull were also demonstrated. Moreover, we achieved in vivo high-resolution imaging of the deep cortex and subcortical hippocampus up to 1.1 mm below the pia within the intact brain.

Wavefront sensor-less adaptive optics using deep reinforcement learning.

Image degradation due to wavefront aberrations can be corrected with adaptive optics (AO). In a typical AO configuration, the aberrations are measured directly using a Shack-Hartmann wavefront sensor and corrected with a deformable mirror in order to attain diffraction limited performance for the main imaging system. Wavefront sensor-less adaptive optics (SAO) uses the image information directly to determine the aberrations and provide guidance for shaping the deformable mirror, often iteratively. In this report, we present a Deep Reinforcement Learning (DRL) approach for SAO correction using a custom-built fluorescence confocal scanning laser microscope. The experimental results demonstrate the improved performance of the DRL approach relative to a Zernike Mode Hill Climbing algorithm for SAO.

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.

Cascading optofluidic phase modulators for performance enhancement in refractive adaptive optics

We discuss the implementation and performance of an adaptive optics (AO) system that uses two cascaded deformable phase plates (DPPs), which are transparent optofluidic phase modulators, mimicking the common woofer/tweeter-type astronomical AO systems. One of the DPPs has 25 electrodes forming a keystone pattern best suited for the correction of low-order and radially symmetric modes; the second device has 37 hexagonally packed electrodes better suited for high-order correction. We also present simulation results and experimental validation for a new open-loop control strategy enabling simultaneous control of both DPPs, which ensures optimum correction for both large-amplitude low-order, and complex combinations of low- and high-order aberrations. The resulting system can reproduce Zernike modes up to the sixth radial order with stroke and fidelity up to twice better than what is attainable with either of the DPPs individually. The performance of the new AO configuration is also verified in a custom-developed fluorescence microscope with sensorless aberration correction.

Theoretical research on the novel adaptive optics configuration based on the tubular deformable mirror for the aberration correction of the annular laser beam.

The annular laser beam (ALB) has been widely used in many fields for its unique intensity distribution. Especially, in the materials processing, the power and the beam quality of the large-aperture thin-wall ALB are of vital. However, limited by the aperture, the actuators' spacing or the damage threshold, the existing deformable mirrors (DMs) are not suitable for the correction of the ALB. Considering the stretching effect of the oblique incidence, in this paper, by using the tubular DM (TDM), a novel adaptive optics (AO) configuration is promoted to increase the number of the effective actuators covered by the input ALB. The coordinate transformation equations and correction principle of the novel AO configuration are derived based on the ray tracing. A typical TDM prototype is designed based on the coordinate transformation equations. The influence function characteristics of the TDM is analyzed using the finite element method, and the correction ability of the novel AO configuration based on the TDM is verified. Simulation results show that the TDM could perfectly compensate the wavefront distortions described by the 2th to 15th order Zernike annular aberrations.

Optical characterization and adaptive optics correction of polymer adaptive lens aberrations.

Adaptive lenses based on fluid-filled polymer membranes allow for great simplification of optical systems providing large focal length variation and reduction of size, weight, and power consumption. However, aberrations can reduce their optical quality and, for some demanding applications, their correction by means of adaptive optics implies increased complexity, especially if reflective wavefront correctors are used. In this work, we characterize two adaptive lenses in terms of optical power and aberrations. We then correct the gravity-induced aberrations by means of a multiactuator adaptive lens in a closed-loop adaptive optics configuration, with a minimal increase in optical setup complexity. The improvements in the performance of an imaging system are shown.

Linear controller error budget assessment for classical adaptive optics systems.

Understanding limitations of adaptive optics (AO) systems is crucial when designing new systems. In particular, analyzing the potential of different controllers is of great interest for the upcoming AO systems of the very large telescopes (VLTs) and extremely large telescopes (ELTs). This paper thus details a complete error budget assessment formalism, based on analytic formulas involving the disturbance temporal power spectral density (PSD) and the controller transfer function, and is applicable to any linear controller. This formalism is presented here for the special case of classical AO systems, but can be extended to any closed- or open-loop, single- or multi-conjugated AO configuration. Special attention is paid to the "control-dependent" errors, the importance of which is directly related to the type of control used in the AO system. The proposed method is applied to a NAOS/VLT-type single conjugated AO system, using disturbance PSD derived from a simulated turbulence trajectory or estimated from wavefront sensor measurements, enabling the construction of detailed error budgets for an integrator and different linear quadratic Gaussian controllers. Application to ELT-sized systems is discussed in the conclusion.

Conjugate adaptive optics in widefield microscopy with an extended-source wavefront sensor

Subsurface microscopy is often limited by poor image quality due to sample-induced aberrations. Adaptive optics (AO) can counter such aberrations, though generally over limited fields of view. In most applications, AO is either slow or requires a “guide star” in the sample to serve as a localized reference target. We describe a fast closed-loop feedback implementation of AO that requires no guide stars, where the sample itself serves as the reference. Several features of our implementation are new. First, it is based on a high-resolution, single-shot wavefront sensor that is compatible with extended samples. Second, it is applied to widefield (i.e., nonscanning) microscopy in a conjugate AO configuration that increases field of view. Third, it makes use of a fast algorithm to identify sample-induced aberrations using illumination from an arbitrarily shaped source. We present the principle of our technique and proof-of-concept experimental demonstrations.

Fast reconstruction and prediction of frozen flow turbulence based on structured Kalman filtering

Efficient and optimal prediction of frozen flow turbulence using the complete observation history of the wavefront sensor is an important issue in adaptive optics for large ground-based telescopes. At least for the sake of error budgeting and algorithm performance, the evaluation of an accurate estimate of the optimal performance of a particular adaptive optics configuration is important. However, due to the large number of grid points, high sampling rates, and the non-rationality of the turbulence power spectral density, the computational complexity of the optimal predictor is huge. This paper shows how a structure in the frozen flow propagation can be exploited to obtain a state-space innovation model with a particular sparsity structure. This sparsity structure enables one to efficiently compute a structured Kalman filter. By simulation it is shown that the performance can be improved and the computational complexity can be reduced in comparison with auto-regressive predictors of low order.

Compensation of Multimode Fiber Dispersion by Optimization of Launched Amplitude, Phase, and Polarization

In previous work, a technique was proposed for compensation of modal dispersion in multimode fiber (MMF) systems by using adaptive optics. A spatial light modulator (SLM), with amplitude and phase adjustable over discrete blocks, was used to control the field launched into a MMF. The system impulse response was found as a function of the SLM settings and the field patterns of the MMF principal modes, which were assumed to be known. Using principles of convex optimization, an optimal solution for the SLM settings was obtained, which minimizes intersymbol interference (ISI) subject to physical constraints. Here, we extend the previous method to include control over launched polarization. We propose five adaptive optics configurations, some permitting control only of overall polarization, and others permitting block-wise polarization control. For each configuration, we find optimal adaptive optics settings, which minimize ISI. We show that the best performance is obtained by two configurations that permit independent block-wise control of amplitude, phase and polarization.

Performance comparison of wavefront reconstruction and control algorithms for Extremely Large Telescopes

Extremely Large Telescopes (ELTs) are very challenging with respect to their adaptive optics (AO) requirements. Their diameters and the specifications required by the astronomical science for which they are being designed imply a huge increment in the number of degrees of freedom in the deformable mirrors. Faster algorithms are needed to implement the real-time reconstruction and control in AO at the required speed. We present the results of a study of the AO correction performance of three different algorithms applied to the case of a 42-m ELT: one considered as a reference, the matrix-vector multiply (MVM) algorithm; and two considered fast, the fractal iterative method (FrIM) and the Fourier transform reconstructor (FTR). The MVM and the FrIM both provide a maximum a posteriori estimation, while the FTR provides a least-squares one. The algorithms are tested on the European Southern Observatory (ESO) end-to-end simulator, OCTOPUS. The performance is compared using a natural guide star single-conjugate adaptive optics configuration. The results demonstrate that the methods have similar performance in a large variety of simulated conditions. However, with respect to system misregistrations, the fast algorithms demonstrate an interesting robustness.

Multiple parallel FPGA implementations of a Kolmogorov phase screen generator

Modelling the effects of wavefront distortions over a finite aperture is an essential component in the simulation of adaptive optics configurations, prediction of performance of laser designators and atmospheric imaging simulations like generation of infrared (IR) scenes in the presence of atmospheric turbulence. In all of these applications many thousands of phase screens need to be generated. The computation time required for a large iterations of algorithms that model this effect is important an issue and for this reason there have been many previous attempts to improve the computation speed such algorithms. In this paper, the computation performance of the best previous algorithm that models this phenomenon is substantially improved using high performance reconfigurable computing through acceleration of the key computationally intensive steps of the algorithm on a field programmable gate array (FPGA). Our best hardware implementation can provide a speedup of more than 60 times the original algorithm.

An ultra fast Kolmogorov phase screen generator suitable for parallel implementation

Modelling phase fluctuations due to Kolmogorov turbulence is important in many areas of applied optics such as simulating adaptive optics configurations, prediction of the performance of laser designators and simulation of infrared (IR) scenes in the presence of atmospheric turbulence. The computational performance of algorithms implementing this model is an important issue because in many situations a large number of phase screens is required. For example, in IR scene simulation a different phase screen is required for each pixel in the scene, and in other situations there exists a need for many thousands of phase screens to be calculated to obtain a statistical average. Whilst there have been previous attempts to increase the computational speed of these algorithms, the computation time required for a large number of phase screens still remains an issue. In this paper, we apply linear and statistical properties to improve the performance of the previous best published algorithm by 60 times when implemented on a sequential processor in software. Because the new algorithm is now trivially parallelizable, a further 20 times speedup can easily be achieved through a parallel software or hardware implementation.