Diode-pumped solid-state lasers are a promising choice for inertial fusion energy beamlines of the future.
Since first achieving fusion ignition in December 2022, Lawrence Livermore’s National Ignition Facility (NIF), the world’s highest-energy laser system, has kept the Laboratory at the forefront of advancing the field. Building on decades of expertise and research efforts, Livermore is looking ahead to new laser system developments, both for future high-yield facilities for the stockpile modernization mission and inertial fusion energy (IFE) facilities to power a new energy future.
Achieving ignition was an outstanding accomplishment, but NIF’s laser architecture was designed in the 1990s to cost-effectively deliver multiple experiments in a day. NIF is not an appropriate choice for IFE due to its large facility size, low shot rate, and wall plug efficiency. Because NIF uses a flashlamp-pumped design, the facility requires 100 times as much energy from the electrical grid than the laser energy it delivers to the target. An IFE laser system will require higher energy, higher efficiency, and a shot rate nearly 1,000,000 times higher, all in a cost-effective package. Simply tweaking the current NIF blueprints will be insufficient.
To achieve next-generation power production, more advanced laser technology is a priority, and a promising choice to advance the state of the art is diode-pumped solid-state lasers (DPSSLs). Moreover, advancing DPSSLs and the associated laser technologies will enable capabilities for national security and other important applications. DPSSLs use diodes to deliver optical energy into a solid-state gain material, which becomes excited and emits laser light at wavelengths suitable for a variety of applications, including laser cutting, medical procedures, semiconductor chip manufacturing, laser-particle acceleration, and fusion experiments. While alternative approaches, such as gas-based excimer lasers, are also being explored by the IFE community as potential driver technologies, DPSSLs are favored candidates due to their potential for high average power and efficiency.
DPSSLs, though, like all other systems, have limitations. To determine an optimal approach for DPSSL implementation, a Livermore team has leveraged previous laser research through a mid-scale Laboratory Directed Research and Development (LDRD) project. “Internally, we saw fusion ignition as the spark of many great new research projects, as so much has happened since the LIFE (Laser Inertial Fusion Energy) project nearly 14 years ago, which was the last time the Laboratory focused on nurturing fusion energy as an application,” says Issa Tamer, laser physicist in the Advanced Photon Technologies (APT) program at Livermore. The IFE Drivers LDRD team, led by Tamer as the principal investigator, has tapped into expertise from around the Laboratory and provided Livermore with several options to pursue across multiple disciplines to improve DPSSLs—enough to spur the transformation of the mid-scale project into a large-scale effort that provides additional funding and extends it over the next few years.
A LIFEtime of Research
Between 2009 and 2013, Livermore conducted a detailed assessment of IFE plant designs with the start of the LIFE project. (See S&TR, July/August 2011, Igniting our Energy Future.) The Laboratory was uniquely equipped to dive into IFE research due to its stockpile stewardship activities, large laser project expertise, supercomputing resources, optical materials and target design capabilities, and materials science and engineering background.
Building off Livermore’s laser capabilities, the LIFE project developed initial concepts for a future laser-driven fusion energy power plant using principles from NIF’s approach to ignition: indirect drive using a target capsule and hohlraum and the use of solid-state lasers to drive the implosion. “LIFE took a very broad look at the wide range of issues around an IFE plant, including chamber materials, target engagement, target fabrication at acceptable costs, and the laser driver itself,” says Robert Deri, distinguished member of technical staff of Livermore’s Directed Energy Technologies Programs under the NIF and Photon Science Principal Directorate. “Target engagement is the ability to reliably and repeatedly hit targets with the laser beams as they fly through the target chamber at a repetition rate of 10 hertz (Hz) or more, or approximately one target every 100 milliseconds.”
LIFE’s laser research and development occurred under the CELL (compact efficient low-cost lasers) large-scale LDRD—the CELL being the building block of LIFE. The CELL project, led by Deri, was completed in 2012 and defined a novel architecture for a DPSSL subsystem that met many of the needs for an IFE plant more effectively than prior architectures.
The technologies developed during the CELL project would be foundational to future IFE research. Through the invention of a new spatial filter and demonstration of compact current drivers for the diode lasers that pump the solid-state laser, the CELL team designed a compact, transportable laser beamline that enabled factory construction and refurbishment and minimized the size and significant costs of building an IFE power plant. In addition, their subsystem technology consisting of the pulser and diodes, called the High- Power Intelligent Laser Diode System (HILADS), resulted in multiple patents and received an R&D 100 Award. (See S&TR, January/February 2016, Powerful Laser System Improves Experimental Capabilities.) The CELL project also addressed the need for efficiency in IFE plants, with its design exhibiting approximately three times more efficiency than prior IFE laser designs. The LIFE laser design further formed the basis for the High-Repetition-Rate Advanced Petawatt Laser System (HAPLS) (see S&TR, January/February 2014, Lighting a New Era of Scientific Discovery), a scientific exploration laser that became fully integrated and operational at the Extreme Light Infrastructure (ELI) Beamlines Research Center in the Czech Republic in 2018 and is still in operation today.
Around the time of the December 2022 ignition achievement, the U.S. Department of Energy (DOE) had been convening laser scientists to define requirements for a future fusion energy plant, including core component technology and target needs. The achievement of ignition during the process generated significant excitement, leading to the launch of Tamer’s LDRD project, which builds on the LIFE effort to pursue improved IFE designs. “With the immense success of fusion ignition and the renewed global excitement for a laser-driven IFE power plant, our LDRD aims to continue the development of high-energy, high-power laser systems spearheaded by Livermore,” says Tamer. “The Laboratory is not looking to build an IFE power plant but, rather, to develop laser architectures broadly applicable to a number of different IFE approaches and potentially useful to multiple private companies.” An additional outcome of the IFE effort is the global increase of fusion companies post-ignition, accelerating timelines and increasing the urgency of the Laboratory’s efforts to advance IFE-relevant technologies. Building on this momentum, the Laboratory established the Livermore Institute for Fusion Technology (LIFT) (see S&TR, July/August 2025, From Ignition to Energy) in 2025 to centralize its fusion energy expertise and establish public–private partnerships that advance fundamental science and technology to make fusion energy a reality.
To improve IFE laser beamline designs, Tamer and team have been exploring how research findings following the LIFE project can solve ongoing roadblocks. “We are looking at new innovations Livermore has developed that can solve problems we now know would occur if we were to try to build the original LIFE laser system,” says Tamer.
In the mid-scale LDRD project, Tamer and team first selected the most promising and efficient laser optic materials for diode-pumping applications, then identified key issues in DPSSL development at IFE-relevant performance. These roadblocks include aperture scaling, optical damage, costs, beamline footprint, and thermal handling.
Leveraging Livermore expertise in DPSSLs, the team developed novel technologies to enable further laser system scaling in pulse energy and repetition rate, including a stacked mosaic amplifier design to address aperture scaling and efficiency, optical metasurfaces to improve the robustness of optics to laser-induced damage, and a scheme to reduce light lost to depolarization at high powers, among other advancements. Thus far, these advances show immense promise in tearing down roadblocks to DPSSLs and IFE systems in pursuit of higher power, and the results lay the groundwork for the follow-on large-scale LDRD project.
Gain Material Mosaics
High-energy, high-repetition-rate solid-state lasers for IFE rely on optical amplification, and advancing these systems requires overcoming fundamental limits in conventional amplifier designs. In such lasers, powerful pump sources—typically semiconductor laser diodes—are used to excite laser gain materials. The diodes emit light at wavelengths that match the absorption bands of the gain medium, allowing optical energy to be stored efficiently. When a low-energy seed pulse passes through this pumped material, the pulse stimulates the emission of additional photons that are identical in phase and direction to the seed. This process amplifies the seed pulse and converts the stored energy into a high-energy, coherent output pulse.
However, the seed pulse does not extract all the stored energy, creating serious challenges for controlling energy flow in the system. Some of the stored energy is released as spontaneous emission, with photons emitted in random directions and phases. As these photons propagate through the gain material, they can undergo a process known as amplified spontaneous emission (ASE). Different from the desired amplification along the main beam direction, ASE can grow along many directions, including sideways across the gain medium. In large-aperture amplifiers, the long transverse path length can produce very high transverse gain, often orders of magnitude larger than the gain in the main beam direction. This can lead to parasitic lasing that drains the stored energy before the seed pulse arrives. The result is substantial energy loss, increased heating of the gain material, and a significant reduction in efficiency with which the laser can extract useful energy. Modeling indicates that in high-gain amplifiers, for large beam apertures, the transverse ASE can reduce the usable energy output by more than 70‑percent, severely limiting overall system performance.
In addition to ASE, the choice of gain material itself presents major practical and economic constraints. Many materials that appear promising from a physics standpoint are either too expensive to manufacture at the large scales required, or they cannot yet be produced with the large apertures needed for IFE class lasers. As Tamer notes, “The gain material is the heart of every beamline, so to determine the most efficient laser architectures, we needed to evaluate essentially all known laser gain materials that could support high-energy pulsed operation. By leveraging the insights from multiple previous design efforts at the Laboratory—including LIFE and the mid-scale Kilowatt LDRD—and now equipped with much more powerful laser modeling and optimization capabilities supported in part by the LDRD program, we were able to conduct more in-depth evaluations of gain material candidates with a heavy focus on overall system efficiency.” The team carried out a systematic down-selection process, weighing factors such as maximum achievable aperture size, intrinsic laser properties, thermal behavior under intense pumping, manufacturability, and cost. Several candidate materials emerged as highly promising, but they are not currently available in the required sizes, creating a significant barrier to their deployment in next-generation IFE laser architectures.
Saumyabrata Banerjee, a laser physicist in the APT group, has spearheaded the LDRD team’s charge toward a solution to these issues. Instead of increasing the aperture of a single laser beam to a size large enough for the multi-kilojoule (kJ) outputs needed for IFE—as large as 50 by 50 centimeters—the team designed a conceptual mosaic of smaller beams adjacent to one another with restricted apertures, so each of them would experience a much smaller transverse ASE while still combining to achieve the necessary longitudinal gain and output power.
Beyond ASE suppression, a mosaic architecture offers additional advantages. Rather than splitting into many independent beamlines, the mosaic architecture focuses on compactness and maintains a single integrated beamline comprising tightly packed sub-apertures, which greatly reduces the number of amplifier heads, pumping and cooling hardware, and optics in some configurations. By reusing expensive amplifier technology and packing more laser energy into a smaller footprint, the IFE facility size, capital cost, and beamline servicing time can all be substantially reduced compared with multiple separate beamlines. Additionally, the modular design improves thermal management by increasing the surface area for gas and liquid cooling, which is essential for high-repetition-rate operations. Smaller, modular gain elements are easier to fabricate and can be produced to higher optical quality standards. The architecture also supports gradient doping for uniform gain and thermal distribution, the mixing of different laser gain materials, and simplified maintenance by allowing individual replacement of damaged elements. Moreover, the mosaic format enables advanced spectral engineering and pulse shaping, supporting broad bandwidths and dynamic beam profiles required for advanced IFE schemes.
The enabling technology behind the mosaic amplifier is the novel geometry-matching diode pump delivery system. In this setup, the output of megawatt-class laser diode arrays has a configuration matching the mosaic layout, and advanced optical homogenization ensures uniform pumping of each sub-aperture. This approach maximizes pump efficiency, maintains spatially uniform gain, and allows for selective control of individual beamlets.
In a theoretical system under study, which used a gain medium of ytterbium-doped yttrium aluminum garnet (Yb:YAG) as an example case, the team first designed a mosaic-based amplifier, comprising multiple stacks of mosaic slabs. The slabs have different doping concentrations ensuring that they absorb the pumped energy uniformly throughout the amplifier. Crucially, uniform pumping leads to uniform gain and heat load, enabling effective cooling by the laser system to reach the high repetition rates necessary for IFE. “For some gain media such as Yb:YAG at cryogenic temperatures, we found through initial modeling of this concept that we can reduce ASE by several orders of magnitude,” says Banerjee. “For example, a full-aperture large beam aiming for 10 kJ of output may lose 8 kJ by the end of pumping owing to ASE losses, but if we use a mosaic of four gain media capable of 2.5 kJ of energy instead of one large one, they may only lose 0.2 kJ of energy each as ASE losses are significantly reduced for smaller beam sizes, leading to a total output of 9.2 kJ.”
After the project’s excellent start, the team is planning follow-on efforts to implement the stacked mosaic structures in experimental settings and validate the efficacy of the concept in practice. “The prospect of the stacked mosaic amplifier can carve the future IFE landscape by allowing the necessary energies out of such systems,” says Banerjee.
Metasurfaces Are the New Coatings
Optics are another limiting factor in taking laser systems to the extreme powers needed for IFE and similar applications. The delicate interfaces associated with traditional coating technologies that make up each optical component see burn damage when power is too high. Systems generate too much heat, but coatings’ effects are still necessary to minimize optical transmitted loss. This problem is further exacerbated for systems operating at high repetition rates, where initiated laser damage can continue to grow under subsequent laser shots, jeopardizing both the optic itself as well as downstream optics. “Optical coatings burn at this combination of high laser fluences and high thermal loads, so we needed to move away from traditional coatings in this project,” says Tamer. “Instead of putting a coating on the optic, what if we pulled from previous Livermore technology and structured the surface of the optic to behave like a coating?”
Metasurfaces offer an avenue to create coating-like surfaces on a variety of optics and have been heavily researched at Livermore. (See S&TR, April/May 2024, A New Dimension of Glass.) At the Laboratory, researchers fabricate metasurfaces using etching masks that contain patterns with details finer than the wavelength of the laser light itself, which they engrave onto the optic surface. The result is a textured surface with no new materials or material interfaces added—just the etch mask pattern engraved into the bulk optical material itself—removing the limitations of multiple material interfaces while providing control over the optical, physical, and mechanical properties of the optic surface. “Metasurfaces enable us to dial down the reflectivity of the surface, even incorporating broadband anti-reflectivity when needed, without being limited by the refractive index of the optical material. Advantageously, their capabilities extend beyond that,” says Nathan Ray, staff scientist in Livermore’s Materials Science Division. “We can control the mechanical properties and physical properties of the surface, induce birefringence (a state in which the refractive index is different for light polarized along different directions) in non-birefringent materials, influence how the surface interacts with water, and more.”
One of the biggest selling points for incorporating optics with metasurfaces into DPSSLs is the increased laser damage threshold they provide. As opposed to coatings of different materials, each with their own damage thresholds, metasurfaces for DPSSL applications converge upon laser damage thresholds close to those of the optical material itself. Uniform, high-damage threshold optics are crucial for the high fluences required by IFE plants. While a strong idea in theory, the LDRD team explored whether metasurfaces would be a good solution to meet this need in practice.
Ray and team fabricated an anti-reflective metasurface on fused silica—a key material employed as lenses and windows in DPSSL lasers—and tested it for laser damage, hitting it with about 40,000 laser shots. The optic did not damage below a fluence of 46 joules per square centimeter (J/cm2)—the goal for DPSSLs is approximately 10 J/cm2 operating fluence for efficient laser extraction fluence. “The fact that the metasurface technology could withstand 40,000 shots at over four times our desired fluence validated what we expected, and we were happy to see it bear fruit in the lab,” says Ray. “Now we have an optical surface that is a viable technology for high-energy pulsed lasers operating at this repetition rate.”
Part of the transition from the mid-scale LDRD project to the large-scale project will upgrade the aperture of the experimental optic and develop the same process for a selection of key DPSSL gain materials. “We consider our recent result a slam dunk, and we’re incredibly excited,” says Tamer. “Now, we’re going to take these results to the next level.” Adds Ray, “Metasurfaces provide solutions to several problems that DPSSLs will face, and I’m looking forward to realizing that potential with the team over the next couple of years.”
Minimization of Depolarization
Light is a transverse electromagnetic wave with magnetic and electric fields oscillating perpendicularly to its direction of travel. Laser light is typically polarized, meaning these oscillations have a well-defined orientation, most often linear. However, IFE applications require DPSSLs with higher power and repetition rates, and the extreme heat generated in these laser systems can introduce stress-induced birefringence and other effects that degrade polarization purity, which results in lower laser efficiency. Birefringence causes a linearly polarized beam to change its polarization, often becoming elliptical or rotated.
Elliptical polarization poses problems for generating the energy needed to ignite an IFE target. When light’s polarization rotates, substantial portions of the laser energy are lost from the beamline after interacting with polarization-sensitive components and do not hit the desired target. “Lasers can lose much more than 10 percent of their light through depolarization, and these percentages are important at the 100 kilowatt-scale average powers for IFE beamlines,” says František Batysta, a laser physicist in the APT group.
Batysta and the team developed a scheme to tackle high-power DPSSLs’ depolarization effects, in which the laser passes through a quartz rotator crystal and relay-imaged, multipass amplifier geometry that undoes stress-induced depolarization rotations. Livermore developed the concept during the Matter in Extreme Conditions Upgrade project at SLAC National Accelerator Laboratory and improved it during this LDRD project, culminating in the testing of the scheme in a surrogate laser system that mimicked the beam path and deposition sources of a larger-scale DPSSL.
the scheme decreased the amount of distorted, depolarized light by 47 times. The team also assessed its effectiveness in more realistic scenarios beyond a surrogate in the laboratory. “Reducing the depolarization factor by this much means that we aren’t losing several percent of the light, only less than 1 percent, which has a significant impact to a laser’s final output,” says Batysta. “We have shown the principles of how our setup works and how to scale it into real high-power laser systems, so I am very optimistic.”
The Future Is Brighter
Looking ahead to a potentially fusion-powered world, Livermore’s advancements in optic metasurfaces, depolarization mitigation, and amplifiers for DPSSLs have laid the groundwork for the evolution of the mid-scale project into a large-scale, three-year LDRD project, titled “STELLAR: Scalable Technologies for Efficient, Low-Cost Laser ARchitectures,” that will continue efforts to surmount the roadblocks to IFE. During the STELLAR project, the team will explore new architecture designs and further their efforts in beamline technology development and validation, including advanced laser diode and compact spatial filter concepts.
In addition to next steps in research, Livermore is reengaging in the global DPSSL effort, notably through hosting the High-Energy-Class Diode-Pumped Solid- State Laser (HEC–DPSSL) Workshop in March 2025. Last hosted by Livermore in 2012, this workshop convened approximately 75 leading experts in lasers and diodes from the U.S., the United Kingdom, Germany, France, the Czech Republic, Lithuania, and Japan and served as needed representation for the U.S. after several stagnant years, uniting the global front around DPSSL innovation into the future. Representing the Laboratory, Tamer and colleague Salma Helwa were the event’s main organizers, with technical program and organizational support from Livermore’s Tammy Ma, Jeff Bude, Robert Deri, James McCarrick, Thomas Spinka, Jeff Wisoff, and others.
The combination of NIF’s 10 ignition shots, the enabling technical innovations from the mid-scale IFE LDRD project, and the recent demonstration of HEC-DPSSL leadership has now enabled Livermore to engage in a dedicated public–private partnerships to further develop DPSSL laser designs and technologies in collaboration with industry. This cooperative research is distinct but synergistic with the thrusts of the new large-scale, three-year LDRD project. IFE developments through this work fall under the umbrella of the recently established LIFT, which is looking to foster public–private partnerships and help share technologies to support the broader ecosystem.
The creation of a DPSSL beamline fully compatible with IFE requirements will be no small feat, but Livermore’s existing expertise, diligent research efforts, and continued global outreach position the Laboratory at the forefront of the challenge. “DPSSL-driven IFE is the most near-term and exciting option moving forward,” says Tamer. “Laser-driven fusion is the method that has been proven, so I’m motivated and feel incredibly fortunate to be working on this. The return on investment can be enormous for this type of LDRD work, and we’re hopeful this next phase of the project, the STELLAR project, will be even more successful.”
—Lilly Ackerman
For further information contact Issa Tamer (925) 422-2697 (user1 [at] llnl.gov (tamer1[at]llnl[dot]gov)).
