The Mercury Project: A High Average Power, Gas-Cooled Laser for Inertial Fusion Energy Development

Demonstration of a 1 TW peak power, joule-level ultrashort Tm:YLF laser

We report on the demonstration of a diode-pumped, Tm:YLF-based, chirped pulse amplification laser system operating at λ ≈ 1.9 µm that produces amplified pulse energies exceeding 1.5 J using a single 8-pass power amplifier. The amplified pulses are subsequently compressed to sub-300 fs durations by a diffraction grating pair, producing record >1 TW peak power pulses. To the best of our knowledge, this is the highest peak power demonstrated for any solid-state, near-2 µm laser architecture and illustrates the potential of Tm:YLF for the next generation of high-power, diode-pumped ultrashort lasers.

Out-of-plane multilayer-dielectric-grating compressor for ultrafast Ti:sapphire pulses

Extreme heat loads on optics, in particular the final pulse compression gratings, are a major hurdle to overcome in the ongoing push towards high average power (kW) and high repetition rate (kHz) operation of terawatt-class Ti:sapphire lasers. Multilayer dielectric (MLD) diffraction gratings have been suggested as a potential alternative to traditionally gold-coated compressor gratings, which are plagued by high energy absorption in the top gold layer. However, to support the required bandwidth (and ultimately the desired pulse duration) with MLD gratings, the gratings have to be operated in an out-of-plane geometry near the Littrow angle. Here, we report on the design of an MLD-based out-of-plane test compressor and a matching custom stretcher. We present a full characterization of the MLD compressor, focusing on its spectral transmission and the significance of laser pulse polarization in the out-of-plane geometry. To demonstrate compression of 40 μJ pulses centered at 800 nm wavelength to 26 fs pulse duration, we use the compressor with an MLD and gold grating configuration, and fully characterize the compressed pulses. Extrapolating our results indicates that MLD-grating-based out-of-plane compressors can support near-transform-limited pulses with sub-30 fs duration and good quality, demonstrating the viability of this concept for kW-level ultrafast Ti:sapphire laser systems.

1 GW peak power and 100 J pulsed operation of a diode-pumped Tm:YLF laser

We report on the generation of high energy, high power pulses in a tabletop diode-pumped Tm:YLF-based laser system, which delivers amplified pulse energies up to 108 J, as well as GW peak power performance when seeded with nanosecond duration pulses. Furthermore, the high power and efficiency capabilities of operating Tm:YLF in the multi-pulse extraction (MPE) regime were explored by seeding the experimental setup with a multi-kHz burst of pulses exhibiting a low individual pulse fluence, resulting in a 3.6 kW average power train of multi-joule-level pulses with an optical-to-optical efficiency of 19%.

Index-leveling for forced-flow turbulent face-cooling of laser amplifiers

Direct laser slab face-cooling by a fluid crossing the main and pump laser beams is an important method to reach high average laser powers. However, the flow regime is usually maintained at low Reynolds numbers to prevent the onset of turbulence features in the flow that would degrade the wavefront quality. We show here how bringing the fluid temperature to the thermo-optical null point, close to the water/ice transition in the case of water, allows one to mitigate the optical consequences of hydrodynamic instabilities, by bleaching optically the temperature inhomogeneities within the flow. This optical process, dubbed index-leveling, opens the door to a highly efficient forced-flow, weakly turbulent face-cooling regime that should be instrumental to boost the kilowatt capabilities of next-generation high-power lasers.

Demonstration of a compact, multi-joule, diode-pumped Tm:YLF laser

We report the demonstration of a diode-pumped Tm:YLF laser operating at 1.88 µm that produces pulse energies up to 3.88 J in 20 ns. The compact system consists of a Q-switched cavity-dumped oscillator generating 18 mJ pulses, which are then amplified in a four-pass power amplifier. Energies up to 38.1 J were obtained with long-pulse amplifier operation. These results illustrate the high energy storage and extraction capabilities of diode-pumped Tm:YLF, opening the path to high peak and average power mid-infrared solid-state lasers.

Modeling and Measurement of Thermal Effect in a Flashlamp-Pumped Direct-Liquid-Cooled Split-Disk Nd:LuAG Ceramic Laser Amplifier

In this paper, a model to predict the thermal effects in a flashlamp-pumped direct-liquid-cooled split-disk Nd:LuAG ceramic laser amplifier has been presented. In addition to pumping distribution, the model calculates thermal-induced wavefront aberration as a function of temperature, thermal stress and thermal deformation in the gain medium. Experimental measurements are carried out to assess the accuracy of the model. We expect that this study will assist in the design and optimization of high-energy lasers operated at repetition rate.

Large-aperture coatings for fusion-class laser systems

Optical coatings for fusion-class laser systems pose unique challenges, given the large substrate sizes, the high intensities incident on the coatings, and the system-focusing requirements, necessitating a well-controlled optical wavefront. Significant advancements have taken place in the past 30 years to achieve the coating capabilities necessary to build laser systems such as the National Ignition Facility, Laser Mégajoule, OMEGA EP, and OMEGA. This work summarizes the coating efforts and advancements to support such system construction and maintenance.

Bidirectional ring amplifier with twin pulses for high-power lasers

A novel bidirectional ring amplifier with twin pulses for high-power lasers is proposed, and the performances on output energy capability and extraction efficiency are comprehended with detailed simulation. The results show that an extraction efficiency of 62.3% and the output energy of 13.4 kJ per pulse at the B integral limit can be obtained at low average fluence of 10.3 J/cm² and the low injection energy of 3.9 mJ in the bidirectional ring amplifier. Compared with the multi-pass amplifier, the bidirectional ring amplifier is more compact and the extraction efficiency is much higher at low injection energy and low laser fluence operation, which is beneficial to simplify the preamplifier system and reduce the effects of nonlinear phase shift.

12 J, 10 Hz diode-pumped Nd:YAG distributed active mirror amplifier chain with ASE suppression

Experimental amplification of 10-ns pulses to an energy of 12.2 J at the repetition rate of 1-10 Hz is reported from a diode-pumped room-temperature distributed active mirror amplifier chain (DAMAC) based on Nd:YAG slabs. Efficient power scaling at the optical-optical efficiency of 20.6% was achieved by suppressing the transverse parasitic oscillation with ASE absorbers. To the best of our knowledge, this is the first demonstration of a diode-pumped Nd:YAG active-mirror laser with nanosecond pulse energy beyond 10 joules. The verified DAMAC concept holds the promise of scaling the energy to a 50 J level and higher by adding 10-12 more pieces of active mirror in the chain.

Aperture scalable, high-average power capable, hybrid-electrode Pockels cell

We demonstrate a hybrid-electrode Pockels cell (HEPC) using a thin z-cut deuterated potassium dihydrogen phosphate (DKDP) crystal. The device is a reflective, longitudinally driven, longitudinally cooled Pockels cell, constructed with a plasma chamber providing the incident-side electrode and a silicon-substrate mirror serving the three purposes of a mirror, a back-side electrode, and a heat sink. The mirror and the DKDP are thermally coupled through a sub-millimeter, inert gas-filled gap. A time-multiplex pass-by driving method is proposed to favor low-voltage and fast-response operation. The experimental results support that the HEPC would be a competent device for high-energy and high-repetition-rate lasers.

Jones calculus modeling and analysis of the thermal distortion in a Ti:sapphire laser amplifier

The mathematical modeling of an anisotropic Ti:sapphire crystal with a significant thermal load is performed. The model is expressed by the differential Jones matrix. A thermally induced distortion in the chirped-pulse amplification process is shown by the solution of the differential Jones matrix. Using this model, the thermally distorted spatio-temporal laser beam shape is calculated for a high-power and high-repetition-rate Ti:sapphire amplifier.

Low temperature diode pumped active mirror Yb3+:YAG disk laser amplifier studies

An experimental study of a static helium gas gap heat switch concept for laser amplification is presented. High single pass gains with large co-sintered ceramic Yb:YAG disks are recorded in the 80-200K temperature range on a diode pumped active mirror amplifier.

100 J-level nanosecond pulsed diode pumped solid state laser

We report on the successful demonstration of a 100 J-level, diode pumped solid state laser based on cryogenic gas cooled, multi-slab ceramic Yb:YAG amplifier technology. When operated at 175 K, the system delivered a pulse energy of 107 J at a 1 Hz repetition rate and 10 ns pulse duration, pumped by 506 J of diode energy at 940 nm, corresponding to an optical-to-optical efficiency of 21%. To the best of our knowledge, this represents the highest energy obtained from a nanosecond pulsed diode pumped solid state laser. This demonstration confirms the energy scalability of the diode pumped optical laser for experiments laser architecture.

DiPOLE: a 10 J, 10 Hz cryogenic gas cooled multi-slab nanosecond Yb:YAG laser

The Diode Pumped Optical Laser for Experiments (DiPOLE) project at the Central Laser Facility aims to develop a scalable, efficient high pulse energy diode pumped laser amplifier system based on cryogenic gas cooled, multi-slab ceramic Yb:YAG technology. We present recent results obtained from a scaled down prototype laser system designed for operation at 10 Hz pulse repetition rate. At 140 K, the system generated 10.8 J of energy in a 10 ns pulse at 1029.5 nm when pumped by 48 J of diode energy at 940 nm, corresponding to an optical to optical conversion efficiency of 22.5%. To our knowledge, this represents the highest pulse energy obtained from a cryo cooled Yb laser to date and the highest efficiency achieved by a multi-Joule diode pumped solid state laser system. Additionally, we demonstrated shot-to-shot energy stability of 0.85% rms for the system operated at 7 J, 10 Hz during several runs lasting up to 6 hours, with more than 50 hours in total. We also demonstrated pulse shaping capability and report on beam, wavefront and focal spot quality.

High-energy diode-pumped D2O-cooled multislab Yb:YAG and Yb:QX-glass lasers

We investigated the lasing performance of a multislab Yb:QX and Yb:YAG laser amplifiers using a facet-cooled design. Di-deuterium oxide (D2O) was used as the coolant flowing between the active slabs with the pump and laser light passing through the very low absorbing heavy-water films. A square pump profile at a maximum intensity of 40  kW/cm2 drove the amplifier with a peak fluence of 5.5  J/cm2 and a pulse duration of 6 ns. We demonstrated a maximum pulse energy of 1 J for each gain medium as well as a repetition rate of 10 Hz for Yb:YAG and 1 Hz for Yb:QX glass, thus showing the feasibility and scalability of directly water-cooled, diode-pumped, high-energy short-pulse lasers.

16.6 J chirped femtosecond laser pulses from a diode-pumped Yb:CaF2 amplifier

We report the amplification of laser pulses at a center wavelength of 1034 nm to an energy of 16.6 J from a fully diode-pumped amplifier using Yb:CaF2 as the active medium. Pumped by a total optical power of 300 kW from high-power laser diodes, a gain factor of g=6.1 was achieved in a nine-pass amplifier configuration agreeing with numerical simulations. A measured spectral bandwidth of 10 nm full width at half-maximum promises a bandwidth-limited compression of the pulses down to a duration of 150 fs. These are, to our knowledge, the most energetic laser pulses achieved from a diode-pumped chirped-pulse amplifier so far.

High-efficiency 10 J diode pumped cryogenic gas cooled Yb:YAG multislab amplifier

We report on the first demonstration of a diode-pumped, gas cooled, cryogenic multislab Yb:YAG amplifier. The performance was characterized over a temperature range from 88 to 175 K. A maximum small-signal single-pass longitudinal gain of 11.0 was measured at 88 K. When amplifying nanosecond pulses, recorded output energies were 10.1 J at 1 Hz in a four-pass extraction geometry and 6.4 J at 10 Hz in a three-pass setup, corresponding to optical to optical conversion efficiencies of 21% and 16%, respectively. To our knowledge, this represents the highest pulse energy so far obtained from a cryo-cooled Yb-laser and the highest efficiency from a multijoule diode pumped solid-state laser system.

High-gain magnetized inertial fusion

Magnetized inertial fusion (MIF) could substantially ease the difficulty of reaching plasma conditions required for significant fusion yields, but it has been widely accepted that the gain is not sufficient for fusion energy. Numerical simulations are presented showing that high-gain MIF is possible in cylindrical liner implosions based on the MagLIF concept [S. A. Slutz et al Phys. Plasmas 17, 056303 (2010)] with the addition of a cryogenic layer of deuterium-tritium (DT). These simulations show that a burn wave propagates radially from the magnetized hot spot into the surrounding much denser cold DT given sufficient hot-spot areal density. For a drive current of 60 MA the simulated gain exceeds 100, which is more than adequate for fusion energy applications. The simulated gain exceeds 1000 for a drive current of 70 MA.

Optimising the efficiency of pulsed diode pumped Yb:YAG laser amplifiers for ns pulse generation

We present a numerical model of a pulsed, diode-pumped Yb:YAG laser amplifier for the generation of high energy ns-pulses. This model is used to explore how optical-to-optical efficiency depends on factors such as pump duration, pump spectrum, pump intensity, doping concentration, and operating temperature. We put special emphasis on finding ways to achieve high efficiency within the practical limitations imposed by real-world laser systems, such as limited pump brightness and limited damage fluence. We show that a particularly advantageous way of improving efficiency within those constraints is operation at cryogenic temperature. Based on the numerical findings we present a concept for a scalable amplifier based on an end-pumped, cryogenic, gas-cooled multi-slab architecture.

Influence of ASE on the gain distribution in large size, high gain Yb3+:YAG slabs

Amplified spontaneous emission (ASE) in large size, high gain Yb(3+):YAG slabs severely impacts the gain/energy storage capability. We will discuss numerical simulations and experimental results obtained on large size Yb(3+):YAG slabs. The spatial distribution and temporal evolution is shown under different conditions.

Terawatt diode-pumped Yb:CaF2 laser

We present what we believe to be the first terawatt diode-pumped laser employing single-crystalline Yb:CaF(2) as the amplifying medium. A maximum pulse energy of 420 mJ at a repetition rate of 1 Hz was achieved by seeding with a stretched femtosecond pulse 2 ns in duration, preamplified to 40 mJ. After recompression, a pulse energy of 197 mJ and a duration of 192 fs were obtained, corresponding to a peak power of 1 TW. Furthermore, nanosecond pulses containing an energy of up to 905 mJ were generated without optical damage.