Continuous Phase Plate Research Articles

Key technology of vector removal decoupling in a slope-based figuring model and application in continuous phase plate fabrication.

For the high-precision fabrication of a continuous phase plate (CPP), a combined decoupling algorithm of single-step decoupling based on the Clairaut-Schwarz theorem and global decoupling by stagewise iteration is proposed. It attempts to address the problem of the low accuracy and limitation of the existing slope-based figuring (SF) model in two-dimensional applications caused by the vector removal coupling between the tool slope influence function and the material removal slope due to the inherent convolution effect in the SF model. The shortcomings of CPP interferometry and the application bottleneck of the Hartmann test in traditional height-based figuring model are studied. The generation mechanism of vector removal coupling is analyzed and compensated. A CPP of 85m m×85m m was successfully machined by the decoupled slope-based figuring model, and the root mean square (RMS) of the surface height error accounted for 6.01% of the RMS of the design value. The research results can effectively improve the convergence and certainty of CPP fabrication using the slope-based figuring model.

Influence of Focal Spot Distribution on Characteristics of Continuous Phase Plates

Influence of Focal Spot Distribution on Characteristics of Continuous Phase Plates

The influence of chemical etching on the surface quality of phase components

Large-aperture fused silica phase optical components such as continuous phase plate (CPP) are widely used in large-scale laser devices to achieve beam homogenization and improve beam quality. However, under the action of high-energy lasers, their lower damage threshold seriously restricts their service life and increases cost of using. Compared with other fused silica components, chemical processing technology with hydrofluoric acid solution (HF) is lacking in the processing of phase components because the residual root mean square value (RMS) of phase elements is very high, and it can not guarantee this. Therefore, it is necessary to carry out research on the influence of chemical treatment on phase components. In this paper, we improved the chemical treatment process to achieve the change of residual RMS value not more than 3 nm, and improved the ability of resisting laser damage that the damage threshold of 29J/cm2 was obtained under the test conditions 351nm@3ns. Finally, we successfully mastered the chemical control process of phase components and applied it to the CPPs as other fused quartz materials engineering production process.

Fully continuous spiral phase plate for ultraintense optical vortices.

Ultraintense optical vortices carrying orbital angular momentum have attracted much attention in strong-field laser physics due to their spiral phase and hollow intensity. This Letter introduces a fully continuous spiral phase plate (FC-SPP) that enables the generation of an ultraintense Laguerre-Gaussian beam. An optimization design method based on the spatial filter technique and chirp-z transform is proposed to match the polishing processing and the tightly focusing performance. To enable its use in high-power laser systems, a large-aperture (200 × 200 mm2) FC-SPP has been fabricated on a fused silica substrate through magnetorheological finishing without the use of mask techniques. The far-field phase pattern and intensity distribution based on vector diffraction calculation were compared with those of ideal spiral phase plate and fabricated FC-SPP, which confirmed the high quality of the output vortex beams and their feasibility for producing high-intensity vortices.

Investigation of an innovative stitching-embedded characterization method for imprinting large-aperture continuous phase plates

In high-power laser systems, the large-aperture continuous phase plates (CPPs) are designed and fabricated with continuous varying structured topography to serve as the class of phase-modulating beam-conditioning optics. To achieve better optical performance, its fabrication highly depends on the reiterative figuring to eliminate the form error relative to the designed CPP surface. However, the characteristics of large aperture and complex structured topography make the quality inspection of CPPs still challenging. A typical inspection process includes measuring the CPPs with the sub-aperture stitching technique, followed by the characterization procedure with respect to the CPP design description. This paper proposes an innovative stitching-embedded characterization method for imprinting the large-aperture CPPs, which combines the sub-aperture stitching and characterization procedure together. First, the concept and mathematical model of the stitching-embedded characterization method is presented. Then, both numerical simulations and experiments were undertaken to verify the proposed method, and the results demonstrate its feasibility and reasonability to provide the more reliable form error evaluation while simplify the analysis procedure compared with the conventional inspection process.

In-situ measurement and compensation of complex spatio-temporal couplings in ultra-intense lasers

Diffraction wavefront errors of large-scale Treacy compressors may induce complex spatio-temporal couplings (CSTCs) in chirped pulse amplification and degrade the quality of the pulsed beam in both near and far fields. This problem is particularly pronounced in petawatt laser facilities that require tiled compressor gratings. Here, to the best of our knowledge, we present the first in-situ measurement method named pseudo lateral shearing interferometry (PLSI) for the CSTC wavefront error and manufacture a continuous phase plate (CPP) according to the measurement to reduce the distortion. A Shark-Hartmann wavefront sensor is used to characterize wavefronts of the entire system at two different wavelengths, and the CSTC wavefront error can be reconstructed similarly to lateral shearing interferometry (LSI). Our approach strongly reduces systematic errors and the impact of other optics. The root-mean-square (RMS) value of the resulting error is 40 nm in a single measurement, and the day-to-day repeatability is 55 nm. The employed CPP reduces the peak-to-valley (PV) value of the CSTC wavefront error from 675 to 312 nm, and the focused peak intensity is improved by 51%.

Figuring Method of High Convergence Ratio for Pulsed Ion Beams Based on Frequency-Domain Parameter Control.

The continuous phase plate (CPP) provides excellent beam smoothing and shaping impacts in the inertial confinement fusion application. However, due to the features of its dispersion, its surface gradient is frequently too large (>2 μm/cm) to process. When machining a large gradient surface with continuous ion beam figuring (IBF), the acceleration of the machine motion axis cannot fulfill the appropriate requirements, and the machining efficiency is further influenced by the unavoidable extra removal layer. The pulsed ion beam (PIB) discretizes the ion beam by incorporating frequency-domain parameters, resulting in a pulsed beam with a controlled pulse width and frequency and avoiding the extra removal layer. This research evaluates the processing convergence ability of IBF and PIB for the large gradient surface using simulation and experiment. The findings reveal that PIB offers obvious advantages under the same beam diameter. Compared with the convergence ratio (γ = 2.02) and residuals (RMS = 184.36 nm) of IBF, the residuals (RMS = 27.48 nm) of PIB are smaller, and the convergence ratio (γ = 8.47) is higher. This work demonstrates that PIB has better residual convergence in large gradient surface processing. It is expected to realize ion beam machining with a higher convergence ratio.

Continuous cubic phase plate with long depth of focus fabricated by femtosecond laser and its optical performance

Continuous cubic phase plate with long depth of focus fabricated by femtosecond laser and its optical performance

Characteristics of the beam smoothing using the combination of induced spatial incoherence and continuous phase plate

Beam smoothing effect of induced spatial incoherence combined with continuous phase plate is reported. General theoretical models that can be used to describe the spatiotemporal characteristics of the focal region in beam smoothing is proposed. The focal region evolution using induced spatial incoherence is studied and compared with that of using smoothing by spectral dispersion. The result shows that, when using induced spatial incoherence combined with continuous phase plate for beam smoothing, the focal region spatiotemporal evolution is chaotic and random, and the size of speckle on spacetime is determined by the coherence time of the laser. The comparison shows that the smoothing speed of induced spatial incoherence is higher. Focal spot time-resolved spectrum and focal spot intensity distribution using induced spatial incoherence are measured experimentally. The results show that the beam smoothing method has an excellent ability to obtain the desired low coherence in focal region, and can completely suppress the middle-high frequency nonuniformity of the focal spot.

Characteristics of the speckle field under sharper focusing conditions.

In inertial confinement fusion (ICF) systems, a continuous phase plate is often used to generate a speckle pattern in order to reduce the growth of parametric instabilities. An analytic model based on the vector diffraction theory is proposed in order to analyze the characteristics of the speckle field generated under sharper focusing conditions. The statistical and spatial autocorrelation function of the speckle field are deduced and calculated. Results show that more accurate results can be achieved by vector theory than by scalar theory. Furthermore, since light-field components in different directions are involved in the vector diffraction integral, more comprehensive information on statistical characteristics of the speckle field can be discovered. Additionally, the impact of the speckle field on three waves plasma parametric instabilities are discussed. Results show that speckle field generated under sharp focusing conditions can efficiently increase the phase mismatch among the three waves and then limit the growth of the instability.

High-Accuracy Surface Topography Manufacturing for Continuous Phase Plates Using an Atmospheric Pressure Plasma Jet.

The continuous phase plate (CPP) is the vital diffractive optical element involved in laser beam shaping and smoothing in high-power laser systems. The high gradients, small spatial periods, and complex features make it difficult to achieve high accuracy when manufacturing such systems. A high-accuracy and high-efficiency surface topography manufacturing method for CPP is presented in this paper. The atmospheric pressure plasma jet (APPJ) system is presented and the removal characteristics are studied to obtain the optimal processing parameters. An optimized iterative algorithm based on the dwell point matrix and a fast Fourier transform (FFT) is proposed to improve the accuracy and efficiency in the dwell time calculation process. A 120 mm × 120 mm CPP surface topography with a 1326.2 nm peak-to-valley (PV) value is fabricated with four iteration steps after approximately 1.6 h of plasma processing. The residual figure error between the prescribed surface topography and plasma-processed surface topography is 28.08 nm root mean square (RMS). The far-field distribution characteristic of the plasma-fabricated surface is analyzed, for which the energy radius deviation is 11 μm at 90% encircled energy. The experimental results demonstrates the potential of the APPJ approach for the manufacturing of complex surface topographies.

Analysis of the misalignment effect and the characterization method for imprinting continuous phase plates.

Continuous phase plates (CPPs) are increasingly being used to realize beam shaping and smoothing in high-power laser systems. With computer controlled optical surfacing (CCOS) technology, CPPs can be imprinted with high accuracy by a series of processing iterations, in which the characterization of the imprinted CPP surface plays a key role. However, the form accuracy evaluation is sensitive to the misalignment caused by the difference between the designed and measured coordinates. In this paper, the matching problem, which is the critical part of characterization, is first summarized as a least squares problem in accordance with the processing principle of CPPs. Then, the misalignment effect on the form error evaluation is analyzed. Necessary attention is paid to the CPP features and the sensibility analysis for different misalignments is conducted. To improve the efficiency and accuracy, an automatic characterization method based on image registration and nonlinear optimization is presented. Considering the smoothness of the CPP surface, the height difference tracing method is proposed to evaluate the matching performance and embedded into the characterization method. Finally, a series of simulations and experiments were undertaken to verify the performance of the proposed characterization method. The results demonstrated the feasibility of the proposed method, indicating that it can provide the reliable form error evaluation with sub-nanometer accuracy for imprinted CPPs.

Optimization of continuous phase plate for reducing near-field diffraction modulation of back-stimulated Brillouin scattered light

A continuous phase plate (CPP) is often employed to shape and smooth laser beams to improve focal spot quality in inertial confinement fusion facilities; however, near-field modulation resulting from back-stimulated Brillouin scattering (SBS) and the CPP may damage optical devices. In this study, we first explored the relationship between the back-near-field diffraction intensity and the CPP phase structure based on SBS and CPP diffraction models. Subsequent theoretical analysis and simulations showed that strong modulation of the back-near-field diffraction occurs near the extremum of the CPP phase Laplacian. Then, the probability distribution of the CPP phase Laplacian was calculated to determine optimize threshold in real-time, and CPP phase local spatial filtering was performed to reduce phase fluctuation to constrain the near-field modulation in the iteratively designed CPP. The improved CPP can effectively reduce the back-near-field modulation, including the local maximum intensity and overall contrast over a long diffraction distance, while the uniformity of the far-field focal spot is slightly reduced within an acceptable range of 2% to satisfy the high requirements for the focal spot uniformity. The optimization of the CPP is of great significance for reducing the damage of high-power laser devices and practical application of CPP.

Frequency division combined machining method to improve polishing efficiency of continuous phase plate by bonnet polishing.

Continuous phase plate (CPP), as a key diffractive optical element, is difficult to manufacture owing to its random and small features. In this paper, a novel frequency division combined machining (FDCM) method was proposed to improve polishing efficiency of CPP by optimizing the tool influence functions (TIFs) over targeted frequency bands. In addition, the convergence rate of power spectral density (CR-PSD) was proposed to evaluate the correct ability of TIF in different frequency bands, and to determine the division frequency for the combined processing. Through simulation verification, the combined processing with optimized TIFs by FDCM enabled high precision in less total time than that with single TIF processing. The experimental results verified that the method could imprint a 300 × 300 mm CPP with residual root-mean-square 24.7 nm after approximately 6-h bonnet polishing. Comparing the focal spots of designed and fabricated CPPs, the deviation of their energy concentration within 500 microns is only 0.22%. Hence, bonnet polishing using the FDCM is a new technical option for the production of large-aperture CPPs. Furthermore, the FDCM method shows a significant increase in efficiency, and it could be a generic method for CPP processing through other technologies, including magnetorheological and ion beam finishing.

Observation of perturbations in underdense plasmas induced by ablative Rayleigh–Taylor instability

Plasma homogeneity is of great importance for inertial confinement fusion because of its effects on laser absorption and subsequent implosion. Perturbations occurring near the ablation front and induced by hydrodynamic instabilities may influence the homogeneity of the underdense corona as the plasma expands. In this paper, perturbations in underdense plasmas produced from plastic targets at laser irradiances of ∼ 10 15 W/cm2 are observed, which were effectively suppressed by the application of a continuous phase plate. Taking account of the directions of the perturbations, the laser, and the target, the perturbations may be interpreted as the result of ablative Rayleigh–Taylor instability. Calculations of their linear growth rates are performed based either on radiation hydrodynamics simulations or on a fixed cutoff wavenumber corresponding to the measured spectra of the perturbations, and better agreement with the experimental results is found for the second approach. The difference between the two approaches is interpreted in terms of different ranges of the Froude number, which in the experiments was greater than unity, beyond the usual range at relatively low laser irradiance.

A simple method for ultrafast smoothing and polarization rotation by interfering beamlets with spatial phase difference

The coherent superposition of beamlets with proper frequency shift, spatial phase difference and polarization control can smooth the irradiation uniformity and rotate the polarization of focal spot on picosecond timescale. In our previous work, the introduction of spatial phase difference requires insertion of additional phase plates that might increase the laser cost and complexity. In this paper, we propose a simple method to introduce the required spatial phase difference without inserting any additional optical elements in the beamline, but good-working in improving irradiation uniformity and rotating polarization of the focal spot. This novel method takes full advantages of continuous phase plate to explore its potential in introducing the spatial phase difference among beamlets. Two representative approaches have been presented, i.e., the first is designing individual CPP for each beamlet which can generate focal spot with same intensity envelope but different speckles individually, and the second is mounting the identical CPP for each beamlet in different orientations. Results indicate that, this simple method can effectively improve the irradiation uniformity within the sub-picosecond timescale and bring about polarization rotation to suppress the parametric backscattering.

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.

Measuring the orbital angular momentum of high-power laser pulses

In this article, we showcase the experimental results of methods to produce and characterize orbital angular momentum (OAM) carrying high-power lasers. The OAM pulses were produced on the ASTRA laser of the Central Laser Facility using a continuous spiral phase plate. Three different characterization methods were then used to measure the OAM content of the beam. The methods that were used were a cylindrical lens diagnostic, an interferometric diagnostic, and a projective diagnostic. We further discuss the relative advantages and disadvantages of each method in the context of high-power laser experiments.

Effects of continuous phase plate on plasma corona homogeneity and laser-plasma instabilities in experiments with three target materials

Corona homogeneity and spectra of scattered light and hot electrons produced by laser-plasma instabilities inside laser-produced plastic, aluminum, and gold plasmas were investigated with and without the use of continuous phase plate (CPP) to the laser beam. Improvement of the corona homogeneity was observed for all three materials after applying CPP, while the inhibition of the intensity of backward-scattered light and the amount of emitted hot electrons was not always synchronous for different materials, which is interpreted as a result of the changes in thresholds of the stimulated Raman scattering (SRS) and the two-plasmon decay (TPD) instability before and after the application of CPP. By comparing the changes of SRS scattered light intensity with the amount of hot electrons in different kinetic energy ranges for all three target materials in our experiments, we conclude that SRS is more responsible for the diagnosed hot electrons between 50–150 keV, and those above 150 keV should be generated by TPD, which could be explained by the difference in phase velocity of electron plasma waves between SRS and TPD calculated from measured and simulated parameters of corona and laser-plasma instabilities.

Conformal smoothing of mid-spatial frequency surface error for nano-accuracy Continuous Phase Plates (CPP)

This paper presented a conformal smoothing theory, and smoothing capability evaluation was established on the proposed theory. According to pressure distribution model, processing parameters have been optimized and the CPP sample with a size of 340 × 340 mm was applied in conformal smoothing. The middle spatial frequency was effectively corrected with the total polishing time of 750 min, and energy was constringed 32.2 times (improved from 57.68 nm2·mm to 1.79 nm2·mm). Meanwhile, surface roughness RMS (root mean square) maintained at the same scale (changed from 265.4 nm to 265.2 nm). Parametric conformal smoothing was proven to be an effective method to control the middle spatial frequency error of CPPs.