Fourier transform-based continuous phase-plate design technique: a high-pass phase-plate design as an application for OMEGA and the National Ignition Facility

Predicting hot electron generation in inertial confinement fusion with particle-in-cell simulations

A series of two-dimensional particle-in-cell simulations with speckled laser drivers was carried out to study hot electron generation in direct-drive inertial confinement fusion on OMEGA. Scaling laws were obtained for hot electron fraction and temperature as functions of laser/plasma conditions in the quarter-critical region. Using these scalings and conditions from hydro simulations, the temporal history of hot electron generation can be predicted. The scalings can be further improved to predict hard x-rays for a collection of OMEGA warm target implosions within experimental error bars. These scalings can be readily implemented into inertial confinement fusion design codes.

Full-aperture random polarization smoothing for a low-coherence laser facility

Two new random polarization smoothing methods using full-aperture elements are proposed on low-coherence lasers, one using birefringent wedge and one using flat birefringent plate. By designing the crystal axis direction and wedge angle of the birefringent plates, the methods can selectively introduce time delay and spatial displacement, so as to obtain fast random evolution of transient polarization by utilizing low spatiotemporal coherence of the laser focal field. Both methods avoid the near field discontinuity and can be used under high fluence. The method using birefringent wedge can slightly improve focal spot uniformity, and the method using flat birefringent plate can obtain non-polarization with DOP lower than 2%. Theoretical studies show that the resulting focal polarization evolves rapidly on sub-picosecond timescales and rapidly covers the entire Poincaré sphere. The method using birefringent wedge is achieved in experiment. The results show that the degree of polarization of the focal spot is reduced from 1 to 0.27, which proves the effectiveness of the full-aperture random polarization smoothing. The full-aperture random polarization smoothing can generate a focal field very close to unpolarized thermal light, which is expected to suppress the laser plasmas instability.

Deformable mirror resolution-matching-based two-stage wavefront sensorless adaptive optics method

In high-power laser facilities, the application of a traditional wavefront control method is limited under the influence of a continuous phase plate (CPP). In order to obtain a satisfactory far-field intensity distribution at the target of the beamline with the CPP, a novel deformable mirror (DM) resolution-matching-based two-stage wavefront sensorless adaptive optics method is proposed and demonstrated. The principles of the DM resolution-matching method and two-stage wavefront sensorless adaptive optics method are introduced, respectively. Based on the numerical model, the matching relationship between the actuator space of the DM and the spatial period of the CPP is investigated. By using the resolution-matched DM, the feasibility of the two-stage wavefront sensorless adaptive optics method is numerically and experimentally verified. Both the numerical and the experimental results show that the presented DM resolution-matching-based two-stage wavefront sensorless adaptive optics method could achieve the target focal spot control under the influence of the CPP, and the profile and the intensity uniformity of the corrected focal spot are optimized close to the designed ideal focal spot.

Experiment and theory of beam smoothing using induced spatial incoherence with a lens array

The smoothing effect of induced spatial incoherence combined with a lens array on a large-bandwidth and short-coherence-time laser is reported. A theoretical model based on statistical optics is developed to describe the spatial and temporal characteristics of the focal spot. Theoretical simulation is consistent with the experimental results. A method was proposed to remove or reduce the residual interference fringes of the experimental focal spot, and both the simulation and analysis show that this method does not affect the smoothing speed of the focal spot.

High-energy parametric amplification of spectrally incoherent broadband pulses

We study and demonstrate the efficient parametric amplification of spectrally incoherent broadband nanosecond pulses to high energies. Signals composed of mutually incoherent monochromatic lines or amplified spontaneous emission are amplified in a sequence of optical parametric amplifiers pumped at 526.5 nm, with the last amplifier set in a collinear geometry. This configuration results in 70% conversion efficiency from the pump to the combined signal and idler, with a combined energy reaching 400 mJ and an optical spectrum extending over 60 nm around 1053 nm. The spatial, spectral, and temporal properties of the amplified waves are investigated. The demonstrated high conversion efficiency, spectral incoherence, and large bandwidth open the way to a new generation of high-energy, solid-state laser drivers that mitigate laser-plasma instabilities and laser-beam imprint via enhanced spectral bandwidth.

Phase retrieval with complexity guidance

Iterative phase retrieval methods based on the Gerchberg-Saxton (GS) or Fienup algorithm typically show stagnation artifacts even after a large number of iterations. We introduce a complexity parameter ζ that can be computed directly from the Fourier magnitude data and provides a measure of fluctuations in the desired phase retrieval solution. It is observed that when initiated with a constant or a uniformly random phase map, the complexity of the Fienup solution containing stagnation artifacts stabilizes at a numerical value that is higher than ζ. We propose a modified Fienup algorithm that uses a controlled sparsity-enhancing step such that in every iteration the complexity of the resulting guess solution is explicitly made close to ζ. This approach, which we refer to as complexity-guided phase retrieval, is seen to provide an artifact-free phase retrieval solution within a few hundred iterations. Numerical illustrations are provided for both amplitude as well as phase objects with and without Poisson noise introduced in the Fourier intensity data. The complexity-guidance concept may potentially be combined with a variety of phase retrieval algorithms and can enable several practical applications.

Performance of target irradiation in a high-power laser with a continuous phase plate and spectral dispersion

We report on the performance of target irradiation at the SG-II high-power laser facility with a continuous phase plate (CPP) and the technique of smoothing by spectral dispersion (SSD). Simulative and experimental results are presented, where the irradiation uniformity and energy concentration of the target spots are analyzed. The results show that the designed CPP can focus the spot energy into the desired region and shape a profile with steep edge and flat top, but the actual performance of the fabricated CPP needs some improvements. It is also proved that the CPP is insensitive to the long-scale wavefront distortion in the incident beam. The one-dimensional SSD configuration evidently works in smoothing the fine-scale intensity modulation inside the target spot.

A novel design method for continuous-phase plate

A continuous-phase plate (CPP) is a key element for beam smoothing in a high-power laser system. For the beam-smoothing effect, the surface shape of a CPP is one of the most important facts. In this paper, the change law of the transmission direction of light rays has been analyzed according to the geometrical optical principle. It is discovered that the 2-dimensional histogram of a surface gradient can be used to show the far-field distribution of a CPP. Drawing on the experience of histogram modification technology in digital image processing, a novel method is proposed to design a CPP. The design steps of a 1-dimensional CPP are introduced in detail. The far-field distribution and spatial frequency spectrum of this CPP are calculated. The results show that this method is efficient and can reflect the relationship between the surface figure and the far-field distribution of a CPP directly.

Phase retrieval algorithms: a personal tour [Invited]

This paper gives the reader a personal tour through the field of phase retrieval and related works that lead up to or cited the paper "Phase Retrieval Algorithms: a Comparison," [Appl. Opt.21, 2758 (1982)].

Fast and precise registration of quasi-planar free-form wavefronts

Quasi-planar free-form wavefronts are featured with vertical fluctuations of small amplitude and high frequency. They can be considered a plane superposed with waviness at the scale of a wavelength, which is the key to enabling control of the optical performance. This paper presents a method for fast and precise registration of quasi-planar free-form wavefronts to obtain the true difference between the test wavefront and the nominal one. We first adapt the planar subaperture stitching algorithm to the registration of two wavefronts with almost 100% overlap. This will usually fail because the registration result is very sensitive to lateral misalignment. Hence a fast and precise method is proposed to directly register two point sets in three-dimensional (3D) space. It is based on a novel procedure to calculate the closest distances of 3D points to another 3D point set. Then registration of a 320 mmx320 mm wavefront with the method is simulated and compared with the subaperture stitching method. Finally a corrective machining case is given to demonstrate the validity of the method to achieve high precision.