Degradation of temporal contrast from post-pedestal interference with a chirped pulse in an optical parametric amplifier.

Pre-pedestal generation is observed in a 0.35-PW laser front end coming from a post-pedestal via instantaneous gain and pump depletion in an optical parametric amplifier during chirped-pulse amplification. Generalized simulations show how this effect arises from gain nonlinearity and applies to all optical parametric chirped-pulse-amplification systems with a post-pedestal. An experiment minimizing the effect of B-integral is used to isolate and study the newly observed conversion of a continuous post-pedestal into a continuous pre-pedestal. Matching numerical simulations confirm experimental results and additionally reveal how third-order dispersion largely controls the slope of the generated pre-pedestal.

Final amplifier of an ultra-intense all-OPCPA system with 13-J output signal energy and 41% pump-to-signal conversion efficiency.

Optical parametric chirped-pulse amplification (OPCPA) using high-energy Nd:glass lasers has the potential to produce ultra-intense pulses (>1023 W/cm2). We report on the performance of the final high-efficiency amplifier in an OPCPA system based on large-aperture (63 × 63-mm2) partially deuterated potassium dihydrogen phosphate (DKDP) crystals. The seed beam (180-nm bandwidth, 110 mJ) was provided by the preceding OPCPA stages. A maximum pump-to-signal conversion efficiency of 41% and signal energy up to 13 J were achieved with a 52-mm-long DKDP crystal due to the flattop super-Gaussian pump beam profile and flat-in-time pulse shape.

Achieving 100 GW idler pulses from an existing petawatt optical parametric chirped pulse amplifier.

Optical parametric chirped-pulse-amplification produces two broadband pulses, a signal and an idler, that can both provide peak powers >100 GW. In most cases the signal is used, but compressing the longer-wavelength idler opens up opportunities for experiments where the driving laser wavelength is a key parameter. This paper will describe several subsystems that were added to a petawatt class, Multi-Terawatt optical parametric amplifier line (MTW-OPAL) at the Laboratory for Laser Energetics to address two long-standing issues introduced by the use of the idler, angular dispersion, and spectral phase reversal. To the best of our knowledge, this is the first time that compensation of angular dispersion and phase reversal has been achieved in a single system and results in a 100 GW, 120-fs duration, pulse at 1170 nm.

Design and characterization of "flow-cell" integrated-flow active cooling for high-average-power ceramic optics.

We used COMSOL Multiphysics to design a prototype actively cooled "flow-cell" substrate targeted at high-average-power applications, fabricated the prototype from cordierite ceramic, and tested the substrate under load in our thermal loading test stand. Sub-aperture testing revealed average-power handling up to 3.88-W/cm2 absorbed power density, in excellent agreement with model predictions. Gratings fabricated on 2-in.-diam cordierite coupons were subjected to laser-damage testing and showed a damage threshold of 250 mJ/cm2.

Impact of the optical parametric amplification phase on laser pulse compression.

Optical parametric chirped-pulse amplification (OPCPA) is an effective way to generate ultrashort pulses that has been used extensively for a variety of applications requiring high peak intensities. Precise control and measurement of a system's spectral and spatial phases are required for Fourier-transform-limited pulse compression and diffraction-limited focusing. Phase accumulated during optical parametric amplification (OPA) can degrade the compressibility and focusability of the pulse, reducing peak intensity. We used analytic and numerical analysis of OPA to study the influence of crystal parameters, the wavefront of the pump and signal, and their relative optical alignment on the accumulated phase. We show that the accumulated signal phase is largely independent of amplifier saturation and, with significant local wavefront gradients in the signal or pump beam, the quality of the compressed pulses can be degraded. We use first-order expressions for the pump- and signal-angle sensitivity to evaluate an OPCPA system consisting of a highly deuterated potassium dihydrogen phosphate amplifier designed to support bandwidth for 15 fs pulses centered at 920 nm.

Effect of the pump beam profile and wavefront on the amplified signal wavefront in optical parametric amplifiers.

We present a theoretical and experimental analysis of the signal phase introduced by the pump-beam wavefront and spatial profile during optical parametric amplification (OPA) process. The theory predicts the appearance of an additional wavefront in the amplified signal beam that is proportional to the spatial derivative of the pump-beam wavefront. The effect of the pump-beam profile on the signal-beam wavefront is also investigated. Our experiments tested these theoretical predictions by comparing the wavefront of the signal beam before and after amplification in a multi-joule broadband OPA. The measured signal wavefront was shown to have the expected dependence on the pump-beam profile and wavefront. These results can be considered when designing petawatt-scale ultrabroadband optical parametric chirped-pulse-amplification systems.

Simultaneous contrast improvement and temporal compression using divided-pulse nonlinear compression.

We experimentally demonstrate how divided-pulse nonlinear compression can be used to improve the temporal contrast of a laser pulse train while simultaneously temporally compressing the pulses. We measure a contrast improvement of almost four orders of magnitude on a nanosecond time scale and temporally compress the pulses from 1.2 ps to 187 fs. The efficiency of our method is also competitive with other contrast improvement methods, with 72% efficiency measured for the main pulse. We expect the method will be useful in the continuing development of high-power, Yb regenerative amplifiers, which suffer from both significant prepulses and relatively long pulse durations.

Energy scaling beyond the gas ionization threshold with divided-pulse nonlinear compression.

We demonstrate how pulse energy in hollow-core fiber can be scaled beyond gas-ionization limitations using divided-pulse nonlinear compression. With one pulse, ionization limits our fiber's output pulse energy to 2.7 mJ at an input of 4 mJ. By dividing the pulse to four low-energy pulses before the fiber, we eliminated the ionization and scaled the pulse energy 2.5× to 6.6 mJ at an input energy of 10 mJ. Larger energy scaling is possible, as our maximum pulse energy has not reached the new gas ionization threshold. Our results motivate applying the method to state-of-the-art systems for large pulse energy scaling without prohibitive system size increases.

Analysis of pump-to-signal noise transfer in two-stage ultra-broadband optical parametric chirped-pulse amplification.

In optical parametric chirped-pulse amplification (OPCPA), pump temporal intensity modulation is transferred to the chirped-signal spectrum via instantaneous parametric gain and results in contrast degradation of the recompressed signal. We investigate, for the first time to our knowledge, the pump-to-signal noise transfer in a two-stage ultra-broadband OPCPA pumped by a single laser and show the dependence of pump-induced signal noise, characterized both before and after pulse compression, on the difference in pump-seed delay in the two stages. We demonstrate an up-to-15-dB reduction of the pump-induced contrast degradation via pump-seed delay optimization. Experiments and simulations show that, even when parametric amplifiers are operated in saturation, the pump-seed delay can be used to minimize the pump-induced contrast degradation that is attributed largely to the noises from the unsaturated edges of the pulse and that of the beam.

High-efficiency, fifth-harmonic generation of a joule-level neodymium laser in a large-aperture ammonium dihydrogen phosphate crystal.

High-energy deep ultraviolet (UV) sources are required for high-density plasma diagnostics. The fifth-harmonic generation of large-aperture neodymium lasers in ammonium dihydrogen phosphate (ADP) can significantly increase UV energies due to the availability of large ADP crystals. Noncritical phase matching in ADP for (ω + 4ω) was achieved by cooling a 65 × 65-mm crystal in a two-chamber cryostat to 200 K. The crystal chamber used helium as the thermally conductive medium between the crystal and the crystal chamber, which was surrounded by a high-vacuum chamber with a liquid nitrogen reservoir. A temperature variation of 0.2 K across the crystal aperture was obtained. The total conversion efficiency from the fundamental to the fifth harmonic at 211 nm was 26%.

Advanced laser development and plasma-physics studies on the multiterawatt laser.

The multiterawatt (MTW) laser, built initially as the prototype front end for a petawatt laser system, is a 1053 nm hybrid system with gain from optical parametric chirped-pulse amplification (OPCPA) and Nd:glass. Compressors and target chambers were added, making MTW a complete laser facility (output energy up to 120 J, pulse duration from 20 fs to 2.8 ns) for studying high-energy-density physics and developing short-pulse laser technologies and target diagnostics. Further extensions of the laser support ultrahigh-intensity laser development of an all-OPCPA system and a Raman plasma amplifier. A short summary of the variety of scientific experiments conducted on MTW is also presented.

Overcoming gas ionization limitations with divided-pulse nonlinear compression.

We simulate Kerr and plasma nonlinearities in a hollow-core fiber to show how plasma effects degrade the output pulse. Our simulations predict the plasma effects can be avoided entirely by implementing divided-pulse nonlinear compression. In divided-pulse nonlinear compression, a high-energy pulse is divided into multiple low-energy pulses, which are spectrally broadened in the hollow-core fiber and then recombined into a high-energy, spectrally broadened pulse. With the plasma effects overcome, spectral broadening can be scaled to larger broadening factors and higher pulse energies. We anticipate this method will also be useful to scale spectral broadening in gas-filled multipass cells.

Design and alignment of an all-spherical unobscured four-mirror image relay for an ultra-broadband sub-petawatt laser.

We describe the design and alignment of an unobscured reflective image relay for use in an ultra-broadband half-petawatt laser system in development at the University of Rochester's Laboratory for Laser Energetics. The design consists of four spherical mirrors in an unobscured configuration. We show the theoretical basis for such a four-mirror design using first-order optical matrix methods and nodal aberration theory. We also describe the process of aligning this design in a test bed and show the results of this alignment method. We were able to achieve a diffraction-limited nominal design. Upon aligning the design in our test bed, we were able to successfully align the design to achieve our target wavefront error of less than 0.3 waves peak to valley and 0.07 waves rms at the central laser-operating wavelength of 910 nm.

The Dynamic Compression Sector laser: A 100-J UV laser for dynamic compression research.

The Dynamic Compression Sector (DCS) laser is a 100-J ultraviolet Nd:glass system designed and built by the Laboratory for Laser Energetics for experimental research at the DCS located at the Advanced Photon Source (Argonne National Laboratory). Its purpose is to serve as a shock driver to study materials under extreme dynamic pressures. It was designed to deposit energy within a uniformly illuminated 500-µm spot on target, with additional optics provided to implement spot sizes of 250 and 1000 µm. Designed after larger-scale glass lasers such as OMEGA and the National Ignition Facility, the laser consists of a fiber front end with interferometer-based pulse shaping, a Nd:glass regenerative amplifier, a four-pass rod amplifier, and a 15-cm glass disk amplifier, through which six passes are made in a bowtie geometry. The output is frequency tripled from 1053 to 351 nm by using a pair of type-II phase-matched KDP crystals, with a third to increase conversion bandwidth. The super-Gaussian spot in the far field is achieved with a distributed phase plate and a 1-m aspherical focusing lens. Beam smoothing is achieved by smoothing by spectral dispersion and polarization smoothing, resulting in a root-mean-square variation in intensity on target of ±8.7%.

Measurements of heat flow from surface defects in lithium triborate.

We present what is, to our knowledge, the first measurement of temperature distributions in a nonlinear optic resulting from absorption in a localized surface defect. These measurements were performed on principal cut samples of lithium triborate with damage spots centered on their front surfaces, pumped by a kW-scale continuous-wave laser. The changes in optical-path length associated with this heating were measured with a Mach-Zehnder interferometer, from which the temperature distribution could be inferred. These distributions have sharper features with larger magnitudes than would be expected with bulk-absorption heating. Comparison with both numerical and analytical models is used to qualify the measurements and to estimate the total power absorbed at a given site using this bulk material response. While sensitivity is dependent on the properties of the material of study, we demonstrate measurements of absorption levels of one part in 105.

Simulation of grating compressor misalignment tolerances and mitigation strategies for chirped-pulse-amplification systems of varying bandwidths and beam sizes.

The effects of pulse compressor grating misalignment on pulse duration and focusability are simulated for chirped-pulse-amplification systems of varying bandwidths, beam sizes, groove densities, and incident angles. Tilt-alignment tolerances are specified based on a 2 drop in focused intensity, illustrating how tolerances scale with bandwidth and compressor beam size, which scales with energy when transformed via known grating damage thresholds. Grating-alignment tolerance scaling with grating groove density and incident/diffracted angles is investigated and applied to compressor design. A correlation between grating tip and in-plane rotation error sensitivity is defined and used to compensate residual out-of-plane angular dispersion, even for ultra-broadband pulses. Simulation of dispersion compensation methods after grating misalignment is shown to mitigate pulse lengthening, limited by temporal contrast degradation and higher-order effects for ultrabroad bandwidths.

Record fifth-harmonic-generation efficiency producing 211 nm, joule-level pulses using cesium lithium borate.

The fifth harmonic of a pulsed Nd:YLF laser has been realized in a cascade of nonlinear crystals with a record efficiency of 30%. Cesium lithium borate is used in a Type-I configuration for sum-frequency mixing of 1053 and 266 nm, producing 211 nm pulses. Flat-topped beam profiles and pulse shapes optimize efficiency. The energies of the fifth harmonic up to 335 mJ in 2.4 ns pulses were demonstrated.

Ionization Waves of Arbitrary Velocity.

Flying focus is a technique that uses a chirped laser beam focused by a highly chromatic lens to produce an extended focal region within which the peak laser intensity can propagate at any velocity. When that intensity is high enough to ionize a background gas, an ionization wave will track the intensity isosurface corresponding to the ionization threshold. We report on the demonstration of such ionization waves of arbitrary velocity. Subluminal and superluminal ionization fronts were produced that propagated both forward and backward relative to the ionizing laser. All backward and all superluminal cases mitigated the issue of ionization-induced refraction that typically inhibits the formation of long, contiguous plasma channels.

Spectrally tunable, temporally shaped parametric front end to seed high-energy Nd:glass laser systems.

We describe a parametric-amplification-based front end for seeding high-energy Nd:glass laser systems. The front end delivers up to 200 mJ by parametric amplification in 2.5-ns flat-in-time pulses tunable over more than 15 nm. Spectral tunability over a range larger than what is typically achieved by laser media at similar energy levels is implemented to investigate cross-beam energy transfer in multibeam target experiments. The front-end operation is simulated to explain the amplified signal's sensitivity to the input pump and signal. A large variety of amplified waveforms are generated by closed-loop pulse shaping. Various properties and limitations of this front end are discussed.

Thermal effects in an ultrafast BiB3O6 optical parametric oscillator at high average powers.

An ultrafast, high-average-power, extended-cavity, femtosecond BiB3O6 optical parametric oscillator was constructed as a test bed for investigating the scalability of infrared parametric devices. Despite the high pulse energies achieved by this system, the reduction in slope efficiency near the maximum-available pump power prompted the investigation of thermal effects in the crystal during operation. The local heating effects in the crystal were used to determine the impact on both phase matching and thermal lensing to understand limitations that must be overcome to achieve microjoule-level pulse energies at high repetition rates.