Large-aperture Laser Beam Research Articles

Large-aperture laser beam scanning system based on liquid crystal polarization grating

Large-aperture laser beam scanning system based on liquid crystal polarization grating

Simulation Analysis of a Wavefront Reconstruction of a Large Aperture Laser Beam

In order to solve the problem of atmospheric influence on the far-field measurement of the quality of a laser beam, we proposed a direct wavefront measurement system based on the Hartmann detection principle, which can measure large apertures and high-power laser beams. The measuring system was composed of a lens array and a detector. The wavefront detection of a large aperture laser beam could be realized by controlling the distance between the lenses and the size of the lens. The influence of different duty cycle factors on the accuracy of the wavefront reconstruction under the same arrangement and different arrangement conditions was simulated and analyzed. The simulation results showed that when the sub-lenses of the system were not in close contact, the reconstruction accuracy of the duty factor of 0.8 was close to that of the case of the duty factor of 1. Within a certain detection range, the hexagonal arrangement of 19 lenses and the arrangement of 8 × 8 lens arrays had a high wavefront restoration accuracy; both were lower than 0.10 λ. The system proposed in this paper was suitable for measuring a large aperture laser beam, providing a new idea for measuring and analyzing the quality of large aperture laser beams. It also has an important significance for improving the measurement accuracy of the beam quality.

The Multiparameter Measurement Technique in a Large-Aperture Rectangular Laser Beam Aberration Correction System

Adaptive optics (AO) can effectively improve the beam quality of solid-state slab lasers. However, the aperture of the output beam increases as the output power of the laser increases, resulting in a larger measurement system. Ultimately, a more complex AO system needs to be designed. To meet the requirements of conjugate imaging in an AO system, it is of research significance to coordinate and optimize the system's structural dimensional parameters while enabling the detection of multiple parameters, such as the wave-front and beam quality. In this paper, a multiparameter measurement technique in a large-aperture rectangular laser beam aberration-correction system is proposed. The system layout is optimized with total dimensions of 400 mm × 150 mm × 246 mm (L × W × H). The AO system conforms to the requirements for conjugate detection and can perform wave-front detection, far-field evaluation, and near-field detection of a 160 mm × 120 mm rectangular beam produced from a solid-state slab laser. The findings reveal that the measured PV values of wave-fronts of the measurement system are less than 0.288 μm, the RMS is no more than 0.079 μm, the average far-field beam quality factor is 1.248 times the diffraction limit, and the average near-field beam uniformity is 0.533 at a temperature of 20 °C ± 10 °C; these results satisfy the technical parameters.

Compact internal sensing phase locking system for coherent combining of fiber laser array

A compact internal sensing phase locking system for coherent combining of large aperture fiber laser beams is demonstrated in this paper. Herein, the phase noises in the laser channels are detected and compensated before the lasers form a large cross-section laser array. Therefore, the large aperture beam splitters and lens are no longer needed for phase locking, and the phase-locking system can be more compact, especially for the coherent laser array with large cross-section. We built a 7-channel fiber laser array to experimentally verify the feasibility of this technique. The phase deviation was less than λ/31 RMS, and the visibility of far-field interference pattern of the combined beam was ∼96.3%. This technique can be used in high-power coherent beam combining system with the large aperture laser beams.

Advantage of a disposable focusing lens in petawatt laser for inertial confinement fusion

The inertial confinement fusion laser facility usually has several hundred large-aperture laser beams. A focusing off-axis parabolic (OAP) mirror combined with a debris shield to focus the petawatt lasers must be placed outside the target chamber for target space limitation. Further increasing the focusing power density of ps-petawatt lasers for large F-number focusing is difficult. However, a disposable lens with a smaller F-number that is used to simultaneously replace the parabolic mirror and debris shield can be placed inside the chamber by an extension pipe. The lens only needs a small target space and can increase the focusing power density of the petawatt laser over 35 times more than an OAP mirror under a 20° solid angle. The nonlinear effect of self-phase modulation may decrease the quality of the spot distribution, but under the correction of a deformable mirror with νcutoff = 0.0625 mm−1, a better focusing spot distribution can be obtained.

Range of multiple filamentation of a terawatt-power large-aperture KrF laser beam in atmospheric air

The range of multiple low-ionized filaments, either stochastically distributed or regularly arranged, produced in atmospheric air by terawatt-power picosecond pulses of a KrF laser was investigated for a nearly collimated beam of 60 mm in diameter. Filamentation patterns, energy attenuation, and spectra evolution were monitored along a 100 m pass in the laboratory air. Resonance-enhanced multiphoton (2+1) ionization of water vapor naturally contained in the atmosphere and stimulated rotational Raman scattering in nitrogen are identified as the main features of the filamentation mechanism, while a self-induced loss of laser radiation coherence is the main factor limiting a multiple-filamentation range.

Effect of non-linear amplification of phase and amplitude modulations on laser-induced damage of thick fused silica optics with large beams at 351 nm

Nonlinear propagation of high power, large aperture laser beams leads to the amplification of the spatial pre-existing small scale modulations, and therefore locally increases the laser energy density as the beam propagates through the optical component. The measurement of the energy density distribution on the exit face of the tested component is therefore more challenging, as the impact of the propagation must be taken into account. It is for this reason that the damage densities of thick fused silica components have never been precisely measured. To this end, a dedicated imaging setup has been developed to accurately measure the energy density distribution on the exit face of 34-mm thick fused silica optical components during laser damage experiments with a centimeter-scale laser beam. The impact of nonlinear propagation on the beam spatial profile has been studied experimentally and confronted to numerical simulations that solve the nonlinear Schrödinger equation for the experimental spatial and phase profiles. The results clearly indicate that the imaging setup can accurately measure the amplification of the beam spatial modulations. The beam energy density on the exit face is therefore correctly measured and, for the first time, the damage density of thick optics can be accurately determined.

Theoretical and experimental research of tiling error compensation method based on a small-size mirror for large-aperture tiled-grating compressors

Tiled-grating techniques have been adopted in many multi-kilojoule petawatt laser facilities to meet the size requirements for compression gratings. To realize coherent tiling, we propose a new simple method of tiling error compensation for large-aperture tiled-grating compressors. In this method, errors are controlled by adjusting a small-size mirror. We theoretically analyze that this method improves errors tolerances many times higher than that of traditional compensation method and approaches the limit of adjustment accuracy. An experiment also has been carried out to demonstrate the feasibility. The method significantly reduces the difficulty of tiling, increases the stability of large-aperture tiled-grating compressor system and could be applied to coherent combination of large-aperture short laser beams.

Mechanisms governing the interaction of metallic particles with nanosecond laser pulses.

The interaction of nanosecond laser pulses at 1064- and 355-nm with micro-scale, nominally spherical metallic particles is investigated in order to elucidate the governing interaction mechanisms as a function of material and laser parameters. The experimental model used involves the irradiation of metal particles located on the surface of transparent plates combined with time-resolved imaging capable of capturing the dynamics of particle ejection, plume formation and expansion along with the kinetics of the dispersed material from the liquefied layer of the particle. The mechanisms investigated in this work are informative and relevant across a multitude of materials and irradiation geometries suitable for the description of a wide range of specific applications. The experimental results were interpreted using physical models incorporating specific processes to assess their contribution to the overall observed behaviors. Analysis of the experimental results suggests that the induced kinetic properties of the particle can be adequately described using the concept of momentum coupling introduced to explain the interaction of plane metal targets to large-aperture laser beams. The results also suggest that laser energy deposition on the formed plasma affects the energy partitioning and the material modifications to the substrate.

The effect of laser beam size on laser-induced damage performance

The influence of laser beam size on laser-induced damage performance, especially damage probability and the laser-induced damage threshold (LIDT), is investigated. It is found that damage probability is dependent on beam size when various damage precursors with different potential behaviors are involved. This causes the damage probability and the LIDT to be different between cases under a large-aperture beam and a small-aperture beam. Moreover, the fluence fluctuation of the large-aperture laser beam brings out hot spots, which move randomly across the beam from shot to shot. Thus this leads the most probable maximum fluence after many shots at any location on the optical component to be several times the average beam fluence. These two effects result in the difference in the damage performance of the optical component between the cases under a large-aperture and small-aperture laser.

Statistical analysis of noise-induced multiple filamentation

The propagation of high-power large-aperture laser beams in a Kerr medium is considered. A statistical approach is developed for the growth of filaments from small-amplitude small-scale initial modulations. Closed-form expressions are derived for the intensity distribution, contrast, and maximal beam intensity, which are valid up to the blowup of the most intense filament. Numerical experiments are found to be in good agreement with theoretical predictions.

Highly sensitive wave-front sensor with a non-zero-order phase plate

We propose a novel highly sensitive wave front detection method for a quick check of a flat wave front by taking advantage of a non-zero-order pi phase plate that yields a non-zero-order diffraction pattern. When a light beam with a flat wave front illuminates a phase plate, the zero-order intensity is zero. When there is a slight distortion of the wave front, the zero-order intensity increases. The ratio of first-order intensity to that of zero-order intensity is used as the criterion with which to judge whether the wave front under test is flat, eliminating the influence of background light. Experimental results demonstrate that this method is efficient, robust, and cost-effective and should be highly interesting for a quick check of a flat wave front of a large-aperture laser beam and adaptive optical systems.

Phased-array electro-optic steering of large aperture laser beams using ferroelectrics

We present a device concept for scanning large (>1cm) laser beams using a domain microengineered ferroelectric device. In our design, the large input beam is divided into smaller beamlets, which are then individually deflected through an angle, and then recombined in the far-field to reconstruct the large beam. As a demonstration of this concept, a five-stage cascaded rectangular domain micropatterned scanner device with 13 beamlet channels was fabricated in LiTaO3 and was demonstrated to deflect a 1.064μm infrared laser beam by a total of 10.3° at 5.39kV∕mm.

Generation of arbitrary waveforms with electro-optic pulse-shapers for high energy—multimode lasers

Precisely controlled pulse-shaping operation with large-aperture high-energy laser beams may imply numerous limitations when some minimal flexibility together with high optical performance are required. This paper presents an original electro-optical concept for the production of very high dynamic range arbitrary-waveforms with high temporal resolution, using laser–diode coupled photo-conductive bars coupled onto large coplanar micro-strip lines. This concept simply involves a short-impulse high-voltage generator at the input and a standard Pockels cell at the output. A number of prototypes have been tested on large-aperture fast Pockels cells for the validation of both electrical and optical performance. Such pulse-shapers may find specific applications in the field of optical smoothing related to fusion lasers and other areas, i.e. whenever one needs to provide computer-controlled, low- or high-voltage, pulses with any pre-defined arbitrary waveform. Apart from the coupling application to a Pockels cell, this concept is fully scalable for higher electrical energies and higher voltages.

Harmonic conversion of large-aperture 105-μm laser beams for inertial-confinement fusion research

To provide high-energy, high-power beams at short wavelengths for inertial-confinement fusion experiments, we routinely convert the 1.05-microm output of the Nova, Nd:phosphate-glass, laser system to its second- or third-harmonic wavelength. We describe the design and performance of the 3 x 3 arrays of potassium dihydrogen phosphate crystal plates used for type-II-type-II phase-matched harmonic conversion of the Nova 0.74-m diameter beams. We also describe an alternate type-I-type-II phasematching configuration that improves third-harmonic conversion efficiency. These arrays provide conversion of a Nova beam of up to 75% to the second harmonic and of up to 70% to the third harmonic.

Fast-response system for stabilization of the axis of the angular distribution of laser radiation

A fast-acting system for stabilization of the axis of the angular distribution of radiation from a continuous-flow CO2 laser is considered. The results of a simulation experiment are reported: they show that it is possible to suppress, by 24–28 dB, fluctuations of the position of the axis in the spectral range 0–20 Hz. This makes the proposed system a promising method for large-aperture laser beams.

Laser focusing limitations from nonlinear beam instabilities

Compression of fusion targets by irradiation with Nd : glass lasers requires precise focusing of large-aperture high-power laser beams. Nonlinear propagation instabilities cause growth of small-scale amplitude perturbations which reduce the focusing properties of such beams. We present temporal measurements of the focus of a high-power glass laser and we show that the integrated nonlinear phase retardation of the beam (or B value) can be used to determine the maximum focusable energy available from an unspatially filtered amplifier chain.