Laser Challenges for Fast Ignition

Improved ion heating in fast ignition by pulse shaping

The fast ignition paradigm for inertial fusion offers increased gain and tolerance of asymmetry by compressing fuel at low entropy and then quickly igniting a small region. Because this hot spot rapidly disassembles, the ions must be heated to ignition temperature as quickly as possible, but most ignitor designs directly heat electrons. A constant-power ignitor pulse, which is generally assumed, is suboptimal for coupling energy from electrons to ions. Using a simple model of a hot spot in isochoric plasma, a pulse shape to maximize ion heating is presented in analytical form. Bounds are derived on the maximum ion temperature attainable by electron heating only. Moreover, arranging for faster ion heating allows a smaller hot spot, improving fusion gain. Under representative conditions, the optimized pulse can reduce ignition energy by over 20%.

Large area ion beam sputtered dielectric ultrafast mirrors for petawatt laser beamlines

The latest advances in petawatt laser technology within the ELI Beamlines project have stimulated the development of large surface area dielectrically coated mirrors meeting all demanding requirements for guiding the compressed 30 J, 25 fs HAPLS laser beam at 10 Hz repetition rate and a center wavelength of 810 nm entirely in vacuum. We describe the production and evaluation of Ta₂O₅/HfO₂/SiO₂ ion beam sputtered coated (440 × 290 × 75) mm³ beam transport mirrors. No crazing was observed after thirty vacuum-air cycles. A laser induced damage threshold of 0.76 J/cm² (fluence on mirror surface) was achieved and maintained at high shot rates.

Dynamic chromatic aberration pre-compensation scheme for ultrashort petawatt laser systems

A novel chromatic aberration pre-compensation scheme for ultrashort petawatt laser systems was proposed. The pre-compensation scheme consists of a convex lens, group of concave lenses, and a spherical reflector combined with a conventional vacuum chamber. It provides a versatile method to accurately compensate the chromatic aberration of an entire laser system via controlling the amount of propagation time delay (PTD) induced by the compensator without changing the input and output beam size. A compensator, tailored based on the proposed scheme, was designed and experimentally evaluated for the Shen-Guang-II 5PW (SG-II 5PW) laser system at Shanghai Institute of Optics and Fine Mechanics (SIOM). The experimental results verified that chromatic aberration in the laser system was almost fully compensated: the size of laser beam focused by an f/2.42 off-axis parabolic mirror (OAP) was reduced tremendously from 32×18μm²to about 4×4μm²at full width at half maximum (FWHM). The proposed scheme provides the flexibility to accurately correct chromatic aberration in high-power laser systems within a wide dynamic range.

Influence of absorption-edge properties on subpicosecond intrinsic laser-damage threshold at 1053 nm in hafnia and silica monolayers

Owing to their relatively high resistance to laser-induced damage, hafnia and silica are commonly used in multilayered optical coatings in high-power laser facilities as high- and low-refractive-index materials, respectively. Here, we quantify the laser-induced-damage threshold (LIDT) at 1053 nm in the short-pulse regime of hafnia and silica monolayers deposited by different fabrication methods, including electron-beam evaporation, plasma ion-assisted deposition and ion-assisted deposition. The results demonstrate that nominally identical coatings fabricated by different deposition techniques and/or vendors can exhibit significantly different damage thresholds. A correlation of the LIDT performance of each material with its corresponding absorption edge is investigated. Our analysis indicates a weak correlation between intrinsic LIDT and the optical gap of each material (Tauc gap) but a much better correlation when considering the spectral characteristics in the Urbach tail spectral range. Spectrophotometry and photothermal absorption were used to provide evidence of the correlation between the strength of the red-shifted absorption tail and reduced LIDT at 1053 nm.

Chirped-pulse-amplification seed source through direct phase modulation

This work presents integration of a directly chirped laser source (DCLS) into a high-energy optical parametric chirped-pulse-amplification (OPCPA) system. DCLS is an all-fiber, chirped laser source that produces nanosecond, linearly chirped laser pulses at 1053 nm for seeding high-energy chirped-pulse-amplification systems. DCLS produces a frequency chirp on an optical pulse through direct temporal phase modulation. A 1-ns, linearly chirped pulse with a 3-nm bandwidth is produced by applying an ~1000-rad (300π) quadratic temporal phase. The chirped pulse is amplified to 76 mJ in an OPCPA system and compressed to close to its Fourier transform limit, producing an intensity autocorrelation trace with a 1.5-ps width.

Robust optimization of the laser induced damage threshold of dielectric mirrors for high power lasers

We report on a numerical optimization of the laser induced damage threshold of multi-dielectric high reflection mirrors in the sub-picosecond regime. We highlight the interplay between the electric field distribution, refractive index and intrinsic laser induced damage threshold of the materials on the overall laser induced damage threshold (LIDT) of the multilayer. We describe an optimization method of the multilayer that minimizes the field enhancement in high refractive index materials while preserving a near perfect reflectivity. This method yields a significant improvement of the damage resistance since a maximum increase of 40% can be achieved on the overall LIDT of the multilayer.

Achieving unlimited recording length in interference lithography via broad-beam scanning exposure with self-referencing alignment

Large-area holographic gratings are of great importance in diverse fields including long-range interference metrology, high-resolution astronomical telescopes, and chirped-pulse-amplification systems. However, in conventional interference lithography, the recording length is limited by the aperture of the collimating lenses. Here we propose broad-beam scanning exposure which employs the latent grating generated continuously during scanning for real-time dynamic fringe locking and thus achieves unlimited recording length. This method is experimentally proved to make high-quality gratings, and is expected to be a new type of interference lithography.

Single-shot high-resolution characterization of optical pulses by spectral phase diversity

The concept of spectral phase diversity is proposed and applied to the temporal characterization of optical pulses. The experimental trace is composed of the measured power of a plurality of ancillary optical pulses derived from the pulse under test by adding known amounts of chromatic dispersion. The spectral phase of the pulse under test is retrieved by minimizing the error between the experimental trace and a trace calculated using the known optical spectrum and diagnostic parameters. An assembly composed of splitters and dispersive delay fibers has been used to generate 64 ancillary pulses whose instantaneous power can be detected in a single shot with a high-bandwidth photodiode and oscilloscope. The diagnostic is experimentally shown to accurately characterize pulses from a chirped-pulse-amplification system when its stretcher is detuned from the position for optimal recompression. Pulse-shape reconstruction for pulses shorter than the photodetection impulse response has been demonstrated. Various investigations of the performance with respect to the number of ancillary pulses and the range of chromatic dispersion generated in the diagnostic are presented.

Spatio-temporal modeling and optimization of a deformable-grating compressor for short high-energy laser pulses

Monolithic large-scale diffraction gratings are desired to improve the performance of high-energy laser systems and scale them to higher energy, but the surface deformation of these diffraction gratings induce spatio-temporal coupling that is detrimental to the focusability and compressibility of the output pulse. A new deformable-grating-based pulse compressor architecture with optimized actuator positions has been designed to correct the spatial and temporal aberrations induced by grating wavefront errors. An integrated optical model has been built to analyze the effect of grating wavefront errors on the spatio-temporal performance of a compressor based on four deformable gratings. A 1.5-meter deformable grating has been optimized using an integrated finite-element-analysis and genetic-optimization model, leading to spatio-temporal performance similar to the baseline design with ideal gratings.

Laser damage density measurement of optical components in the sub-picosecond regime

A rasterscan procedure adapted to the sub-picosecond regime is set to determine laser-induced damage densities as function of fluences. Density measurement is carried out on dielectric high-reflective coatings operating at 1053 nm. Whereas laser-induced damage is usually considered deterministic in this regime, damage events occur on these structures for fluences significantly lower than their intrinsic damage threshold. Scanning electron microscope observations of these "under-threshold" damage sites evidence ejections of defects, embedded in the dielectric stack. This method brings a new viewpoint for the qualification of optical components and their optimization for a high resistance in the sub-picosecond regime.

Analysis and minimization of spacing error of holographic gratings recorded with spherical collimation lenses

This research proposes a feedback method to adjust the dual-beam exposure system with spherical collimation lenses to achieve gratings with low spacing error. Through theoretical analysis and numerical calculation, it is proved that the interference aberration can be analyzed with the Zernike polynomials and the adjustment errors can be estimated according to the linear relationship between the errors and the polynomial coefficients. Moreover moving the substrate along its normal is proposed to decrease the spacing error but keep the grating's period unchanged. In the experiments, the wavefront measurement results of the ± 1st orders are used to deduce the spacing error. Based on the feedback adjustment method, the grating with a spacing error of 0.03 λ within 70 mm × 70 mm is fabricated with the collimation lenses of 0.6 λ spherical aberration.

Phase control requirements of high intensity laser beam combining

Aiming at getting the general requirements of the beam combine for ignition scale laser facilities, the analytical expressions including the factors affecting the combine results are derived. The physical meanings of every part are illustrated. Based on these expressions, the effects of the factors, including the beam configuration, piston error, and tip/tilt error, are studied analytically and numerically. The results show that the beam configuration cannot affect the Strehl ratio (SR) of the combined beam, but it influences the FWHM of the main peak and the ratio of the main peak and the side peak. The beam separation should be no more than 1.24 times the individual beam width for the multibeam combine, and be close to the individual beam width for the two-beam combine as much as possible. The piston error can change the characteristics of the combine beam focus, including the peak intensity, the focal spot morphology, the fractional energy contained within a certain area, and the center of mass. For the two-beam combine, a piston error less than 2π/5 rad is suitable, and for the multibeam combine, the standard deviation of the piston error should be no more than 2π/10 rad. The tip/tilt error has a great influence on the combined results. It affects the superposition degree of the focal spots of the combined elements directly. A requirement of 0.5~1 μrad for the standard deviation of the tip/tilt error is adequate.

Amplifying nanosecond optical pulses at 1053 nm with an all-fiber regenerative amplifier

Nanosecond, 1053 nm optical pulses are amplified from 15 pJ to 240 nJ by a Yb-doped all-fiber regenerative amplifier (AFRA), achieving an overall gain of 42 dB. To the best of our knowledge, this is the highest output-pulse energy from an AFRA ever reported. Techniques for suppressing amplified spontaneous emission are employed to favor the signal gain 23 nm off the gain peak of the Yb-doped fiber. Numerical simulations show that increasing the number of round trips and operating the AFRA at saturation will increase the output level and improve the output stability. This is currently limited by the onset of bifurcation instability, which can be avoided at low repetition rates.

High-gain fiber, optical-parametric, chirped-pulse amplification of femtosecond pulses at 1 μm

Fiber-based optical-parametric chirped-pulse amplification is reported at 1μm in a microstructured fiber in the femtosecond regime. The signal has been highly stretched by an Öffner triplet and then amplified with an all-fiber, pulsed-pump, fiber optical-parametric amplifier. More than 30dB gain has been achieved over 8.3nm, and the amplified signal has been recompressed.

Experimental demonstration of optical parametric chirped pulse amplification in optical fiber

We experimentally demonstrate optical parametric chirped pulse amplification for the first time (to our knowledge) in a completely integrated all-fiber optical system. A single chirped fiber Bragg grating, achieving both the stretching and compression stages, is combined with a cw-pumped fiber optical parametric amplifier. As a proof of principle, we demonstrate the amplification of picosecond Fourier-transform-limited pulses at 1550nm.

Direct spectral phase measurement with spectral interferometry resolved in time extra dimensional

The complete spectral characterization of ultrashort pulses is demonstrated with a new diagnostic called Spectral Interferometry Resolved in Time Extra Dimensional. This method, based on spectral shearing interferometry, is self-referenced and self-calibrated. It yields directly to an interferogram pattern displaying an intuitive representation of the derivative of the spectral phase. No iterative algorithm is needed for phase measurement making this method suitable for real time and easy characterization. This technique is highlighted by the spectral phase characterization of pulses out of a folded nondispersive line and the pulse shape is compared with a trace recorded with an intensity autocorrelator.

Fabrication of optical mosaic gratings with phase and attitude adjustments employing latent fringes and a red-wavelength dual-beam interferometer

We present a method to make optical mosaic gratings that uses the exposure beams and the latent grating created by the previous exposure to adjust the lateral position and readjust the attitude of the substrate for the current exposure. As thus, it is a direct method without using any auxiliary reference grating(s) and it avoids the asynchronous drifts between otherwise independent exposure and alignment optical sub-systems. In addition, the method uses a red laser wavelength in the plane-mirror interferometers for the multi-dimensional attitude adjustment, so the adjustment can be done at leisure. The mosaic procedure is described step by step, and the principles to minimize substrate alignment errors are explained in detail. Experimentally we made several mosaics of (50 + 30) x 50 mm(2) final grating area. The typical peak-valley and root-mean-square values of the measured -1st-order diffraction wavefront errors are 0.036 lambda and 0.006 lambda, respectively.

Two-beam SPIDER for dual-pulse single-shot characterization

We describe an innovative design of the spectral-phase-interferometry-for-direct-electric-field-reconstruction (SPIDER) technique applicable to the simultaneous measurement of the electric field of two ultrashort pulses, e.g., a reference and probe pulse. We demonstrate the utility of the device by measuring material dispersion in a single shot by subtracting the reconstructed phase of a dispersed pulse and a reference pulse and by quantifying phase differences arising from nonlinear effects. Our two-beam technique is particularly suitable for use in experiments performed on high-energy, ultrafast laser systems operating in a single-shot mode with significant pulse-to-pulse variations.

Generation of tunable picosecond pulses by pulse stacking in an Yb-fiber gain-assisted pulse stacker

We report on the generation of tunable picosecond pulses of high repetition rate by pulse stacking in an Yb-fiber stacker that benefited from high optical gain, properly time delay and laser synchronization. The gain-assisted pulse stacker could be controlled for pulse shaping to produce tunable pulse duration from ps to sub-ns range by managing the intracavity dispersion and adjusting the time delay. The energy loss during the pulse shaping process was compensated by the gain of a 60-cm-long Yb-fiber pumped by a diode laser. The temporal profiles of the output pulses were measured by using a special cross-correlation technique. The duration of the stacked pulses could be tuned from 5 to 200 ps with a controllable time interval.

A dual-channel, curved-crystal spectrograph for petawatt laser, x-ray backlighter source studies

A dual-channel, curved-crystal spectrograph was designed to measure time-integrated x-ray spectra in the approximately 1.5 to 2 keV range (6.2-8.2 A wavelength) from small-mass, thin-foil targets irradiated by the VULCAN petawatt laser focused up to 4x10(20) W/cm(2). The spectrograph consists of two cylindrically curved potassium-acid-phthalate crystals bent in the meridional plane to increase the spectral range by a factor of approximately 10 compared to a flat crystal. The device acquires single-shot x-ray spectra with good signal-to-background ratios in the hard x-ray background environment of petawatt laser-plasma interactions. The peak spectral energies of the aluminum He(alpha) and Ly(alpha) resonance lines were approximately 1.8 and approximately 1.0 mJ/eV sr (approximately 0.4 and 0.25 J/A sr), respectively, for 220 J, 10 ps laser irradiation.

Spot-shadowing optimization to mitigate damage growth in a high-energy-laser amplifier chain

A spot-shadowing technique to mitigate damage growth in a high-energy laser is studied. Its goal is to minimize the energy loss and undesirable hot spots in intermediate planes of the laser. A nonlinear optimization algorithm solves for the complex fields required to mitigate damage growth in the National Ignition Facility amplifier chain. The method is generally applicable to any large fusion laser.

High-dynamic-range single-shot cross-correlator based on an optical pulse replicator

The operation of a single-shot cross-correlator based on a pulse replicator is described. The correlator uses a discrete sequence of sampling pulses that are nonlinearly mixed with the pulse under test. The combination of a high reflector and partial reflector replicates an optical pulse by multiple internal reflections and generates a sequence of spatially displaced and temporally delayed sampling pulses. This principle is used in a cross-correlator characterizing optical pulses at 1053 nm. A dynamic range higher than 60 dB is obtained over a temporal range larger than 200 ps.

Split-aperture laser pulse compressor design tolerant to alignment and line-density differences

We introduce a four-pass laser pulse compressor design based on two grating apertures with two gratings per aperture that is tolerant to some alignment errors and, importantly, to grating-to-grating period variations. Each half-beam samples each grating in a diamond-shaped compressor that is symmetric about a central bisecting plane. For any given grating, the two half-beams impinge on opposite sides of its surface normal. It is shown that the two split beams have no pointing difference from paired gratings with different periods. Furthermore, no phase shift between half-beams is incurred as long as the planes containing a grating line and the surface normal for each grating of the pair are parallel. For grating pairs satisfying this condition, gratings surfaces need not be on the same plane, as changes in the gap between the two can compensate to bring the beams back in phase.

Grating mosaic based on image processing of far-field diffraction intensity patterns in two wavelengths

A practical grating mosaic method is proposed based on quantitative image processing of three far-field diffraction intensity patterns in two wavelengths. This method aims at making a perfect mosaic of two planar gratings that can substitute for a single and larger grating without introducing wavefront aberration at any wavelength. The zeroth-order and first-order far-field patterns of one wavelength are analyzed for separating and eliminating the angular mosaic errors. The first-order far-field patterns of two wavelengths are applied for separation of the lateral and longitudinal phase errors. Then the three patterns are considered together to enlarge the target range of coarse adjustment required for further fine adjustment in longitudinal position. Experimentally, angular and positional detection sensitivities of less than 6 microrad and 14 nm were achieved, respectively, and the periodicity in positional adjustment was checked, which departed less than 1.8% from the theoretical period. The performance of the perfect mosaic grating was diagnosed with the far-field diffraction intensity pattern in a third wavelength, and the necessity for a perfect mosaic was verified.

Chromatism compensation of the PETAL multipetawatt high-energy laser

High-energy petawatt lasers use series of spatial filters in their amplification section. The refractive lenses employed introduce longitudinal chromatism that can spatially and temporally distort the ultrafast laser beam after focusing. To ensure optimum performances of petawatt laser facilities, these distortions need to be corrected. Several solutions using reflective, refractive, or diffractive optical components can be addressed. We give herein a review of these various possibilities with their application to the PETAL (Petawatt Aquitaine Laser at the Laser Integration Line facility) laser beamline and show that diffractive-based corrections appear to be the most promising.

Piston measurement by quadriwave lateral shearing interferometry

We present what is to our knowledge a new method for measuring the relative piston between two independent beams separated by a physical gap, typical of petawatt facilities. The feasibility of this measurement, based on quadriwave lateral shearing interferometry, has been demonstrated experimentally: piston has been measured with accuracy and sensitivity better than 50 nm.

Synthetic aperture compression scheme for a multipetawatt high-energy laser

High-energy petawatt lasers using the chirped-pulse amplification technique require meter-sized gratings to limit the beam fluence on the surface of the grating. An alternative, studied by many groups, is a mosaic grating consisting of smaller, coherently added gratings. We propose what we believe to be a new compression scheme consisting of beam phasing instead of grating mosaic phasing. This synthetic aperture compression scheme allows us to control the beam thanks to a unique segmented mirror equipped with three degrees of freedom. With this configuration, the beam is divided into small subapertures adapted to the classical grating size. After compression, these subapertures are coherently added before the focusing stage. Therefore the alignment processes are simplified.