Lasers used for sophisticated scientific experiments and advanced manufacturing do much more than turn on and off: They are often patterned in time to control the laser–plasma interaction and enhance their effect. The capability to meticulously craft the laser light’s power and wavelength versus time can be used to optimize the desired effect of the laser light or frustrate unwanted interactions. Defining the pulse’s parameters (for example, how much power is delivered to the target and the wavelength over different durations) is essential, especially in the case of high-energy-density (HED) experiments, which happen millions of times quicker than the blink of an eye. Scientists require equipment that can pattern laser pulses just as fast as light interacts with matter and do so over relatively long periods of time. To meet these requirements, Livermore researchers have created a novel optics device, STILETTO (Space–Time Induced Linearly Encoded Transcription for Temporal Optimization), which can fine-tune the laser pulse profiles relevant to the experimental regimes of inertial confinement fusion (ICF) and inertial fusion energy (IFE), filling a long-enduring gap in researchers’ toolboxes.
Seeking Stability
In a common experimental setup at the National Ignition Facility (NIF), which achieved fusion ignition via inertial confinement in December 2022, laser pulses strike the inner surface of a metal hohlraum, producing x-rays that inundate and compress from all sides a small capsule containing deuterium–tritium fuel. This approach, which involves amplifying an initial “seed” pulse at the laser system’s front end before channeling it into NIF’s 192 laser beamlines, requires exquisitely timed and tuned pulse profiles to properly control the target implosion as it unfolds over the course of mere nanoseconds. Individual features of the laser pulses driving these experiments must be controlled with even finer temporal resolution.
“In NIF’s early days, when experiments were just starting to achieve basic implosions, we quickly realized that we weren’t achieving the desired reactions,” says John Heebner, group leader in the Materials Engineering Division (MED) and lead scientist for the front end of the NIF laser. Instead of maintaining a symmetric, spherical implosion, the target would often deform at the equator or at the poles, creating shapes respectively dubbed “sausages” or “pancakes.” Such distortions are the end results of compounding instabilities between the laser and the generated plasma, which develop over tens of picoseconds—hundreds of times faster than the nanoseconds-long implosion event. “Some control over this has been achieved using different wavelengths for the four cone angles of NIF beams, but more tailored approaches to mitigating these effects have been challenging to develop and deploy,” says Heebner.
One of the biggest experimental challenges when using highly energetic laser systems is that once the lasers begin to create plasma from the target, the plasma begins to interact with the laser light still being delivered to the target, potentially compromising energy delivery. “When manipulating high-energy matter, like we do at NIF, material shifts around relatively slowly—that is, on nanosecond timescales. The interaction of light with materials is a much faster phenomenon,” says MED researcher, Daniel Mittelberger. Energy from light can react with materials in several ways, including by reflecting away or transforming into sound waves, which detracts from energy being delivered on-target. Therefore, ensuring the beam stays concentrated on the target despite these reactions is imperative. Once plasma forms, the material moves more freely and exhibits confounding behaviors. Mittelberger says, “We need a way to modulate the properties of the applied laser light to get around these instabilities to better control the implosion process for longer.”
A Tilt in Time
Laser pulse shaping entails modifying a laser’s amplitude (power) and sometimes its wavelength (color). Scientists can choose from a repertoire of techniques to control energy delivery and mitigate plasma instabilities. For instance, they could fire brief pulselets in rapid succession or quickly vary the wavelength of light arriving at the target. These modulations in laser power and wavelength are called pulse features. “The idea that bandwidth is important in shaping pulses on the picosecond (ps) timescale is widely recognized in the ICF and IFE research communities, but no consensus yet exists on what the optimal power or wavelength profile should be. Many opinions have been shared, but not many experiments have taken place,” says Heebner.
This debate continues largely because researchers lack a technology that can shape pulses at the desired subpicosecond resolution and do so for the entirety of nanoseconds-long pulses inherent to ICF and IFE experiments. When dealing with such fine features, performing pulse shaping over longer durations becomes increasingly difficult to achieve. For example, leading electro-optic pulse shapers, which vary the amplitude and wavelength of light by adjusting an applied voltage, can operate for any duration. However, they are limited to a coarse feature resolution of approximately 20 to 100 ps. Conversely, pulse shaping done by manipulating the spectrum using optical circuitry (via an ensemble of short-pulse lasers, lenses, filters, and other optical elements) can define features with subpicosecond resolution, yet record length is limited to tens of picoseconds—too short for the experiments in question.
To address this temporal gap, STILETTO provides subpicosecond pulse-shaping capabilities at record lengths exceeding one nanosecond (1,000 ps). STILETTO is inspired by the principles of an optical device called a monochromator (see figure above). “The role of a monochromator is to take a short, broadband pulse, which contains many different wavelengths of light, and transform it into a long, single-wavelength pulse,” says MED researcher, Ryan Muir. “However, defocusing the monochromator generates a long pulse with a rainbow separation of colors in time. By further manipulating the light as it travels through the monochromator, we can draw nearly any type of feature on a long pulse.”
STILETTO is similar to a monochromator, but with the crucial addition of a spatial light modulator (SLM). Muir explains, “SLMs are commonplace technologies, such as the liquid crystal screens in computer monitors and televisions. Whereas monitor screens provide a spatially programmable polarization rotation, the SLM in our setup provides a spatially programmable optical deflection. At every point in our SLM, we can manipulate the light by deflecting it to a new, arbitrary angle.”
STILETTO operates by first reflecting an ultrafast, subpicosecond broadband pulse off an angled diffraction grating. Due to the angle of attack, light reaches one part of the grating before it reaches the other. “This angle makes the pulse tilted in space and therefore tilted in time,” says Muir. The light then passes onto the SLM, which can deflect portions of the tilted pulse in plane to change its wavelength or out of the optical circuit to attenuate with high spatial precision. After focusing the remaining light into an optical fiber, these spatially defined features translate into temporal features, resulting in a long-record pulse with fine feature resolution. The result is a device that can generate a long optical pulse with any combination of amplitude and wavelength at every finely resolved point in time, providing a valuable, long-sought-after capability for HED experiments.
The Other Half of the Equation
While in principle, STILETTO can function alone to shape pulses, unlocking its full potential requires the ability to iteratively refine its output to match the user’s desired pulse profile. Prior to STILETTO, pulse shapers capable of picosecond resolution and nanosecond-record length did not exist, nor did optical recording devices that could adequately analyze the faint, intricate pulses that STILETTO could create at the necessary timescales. As with pulse shapers, optical recorders contend with a trade-off between resolution and record time. They can either resolve fine changes in light’s amplitude and wavelength, or they can operate for long stretches, but not both. “STILETTO represents a new frontier in pulse shaping, so we needed a diagnostic that could work alongside it,” says Muir. “The world of optical recorders is rich, but none worked for STILETTO’s combination of resolution and record length. Creating a diagnostic for this purpose was a tall order and something researchers have been trying to achieve for decades.”
The team devised three-phase spectral interferometry (3PSI), a novel adaptation of the nearly 50-year-old spectral interferometry technique. Spectral interferometry is a powerful technique that can measure both the amplitude and wavelength of dim pulses at every point in time for long records and with high resolution. The process works by taking a well-characterized, ultrafast pulse and placing it at the same place and time as the signal pulse in question. Using spectrometers to measure the interfering pulses, researchers can mathematically deduce the unknown signal spectrum by comparing it to the known reference spectrum. The interference pattern results in an optical amplification of the signal, making the technique extremely sensitive. However, this sensitivity is a double-edged sword. Any small error in measurement or calibration results in large errors when retrieving the signal. Though an appealing capability, spectral interferometry has never been worked into a desirable diagnostic despite decades of effort from optics researchers.
The team’s new technique, 3PSI, combines the signal and reference in a three-way optical splitter, which yields three uniquely phased copies of the interfering pulses. The three outputs are then measured with high-resolution spectrometers. According to Muir, the Livermore team was the first to discover that a polarization-maintaining splitter was essential to providing interferometric stability. Data from the spectrometers then must be interpreted. The 3PSI device is relatively simple to build, but understanding how to retrieve the signal from the three spectrometer measurements proved challenging. “Combining these three measurements is difficult, and we wanted to avoid using tricks to simplify the problem that predecessors to this study had relied on. The mathematical expressions for the signal retrieval were so complicated that it took our computer nearly a day to derive,” says Mittelberger.
The 3PSI device does more than allow the team to analyze the pulses that STILETTO produces with high accuracy. It also helps them adjust shot parameters so that the observed pulse more closely matches the desired pulse—a process known as closed-loop shaping. As with any complicated experimental setup, nonidealities separate expectation from reality. In the realm of laser pulses boasting critical features on the order of picoseconds, deviations in the signal’s timing, amplitude, and wavelength make the observed pulse an imperfect match to the ideal, resulting in an “as-drawn” pulse profile. Using 3PSI’s readings, the team applies an algorithm that analyzes differences between the observed signal and the requested signal and makes iterative adjustments to zero in on the user-requested shape. “In a laboratory setting, nothing ever works perfectly, especially the first time. We can tell the computer to turn whatever knobs are needed to correct nonidealities and iteratively close in on the desired pulse,” says Mittelberger.
The STILETTO and 3PSI duo is installed at the Jupiter Laser Facility (JLF), Livermore’s intermediate-scale institutional user facility for HED research. At JLF, STILETTO is qualified for a record time of roughly 700 ps with subpicosecond resolution; and as a standalone device, 3PSI is qualified for record times greater than 2 nanoseconds with finer than 500-femtosecond (one-half trillionth of a second) resolution. “If we were to entertain adding this setup to NIF one day, the most serious challenge would be extending the record time to approximately 30 nanoseconds,” says Heebner.
JLF serves as a proving ground for these new technologies where the team can optimize the devices while enabling researchers to perform their own experiments. While STILETTO is primarily used in HED regimes, 3PSI is being eyed as a diagnostic with diverse applications and is related to another established technology, optical coherence tomography, commonly used in medicine to generate grayscale volumetric images of tissue. The 3PSI device could be used to add color information to these images, offering a substantial upgrade to this useful medical diagnostic tool.
—Elliot Jaffe
For further information contact Ryan Muir (925) 423-3842 (muir3 [at] llnl.gov (muir3[at]llnl[dot]gov)).
