Beam Smoothing for Picket-Pulse Fusion Experiments - Laboratory for Laser Energetics

Streaked spectrometer image showing the ripple-like pattern produced by SSD wavelength modulation (wavelength versus time, with color indicating intensity). The timing of this modulation is adjusted so that the initial short “picketˮ pulse occurs at the zero crossing, where there is no wavelength shift and the beam points in the correct direction.

How Spectral Dispersion and Timing Improve Irradiation Uniformity on OMEGA

The OMEGA Laser System is designed to deliver laser energy as uniformly as possible to a spherical target using 60 beams arranged symmetrically around the target chamber. This uniformity depends not only on beam arrangement but also on the smoothness of each individual beam. The goal is to compress the spherical target evenly from all sides. If the laser light is uneven—too intense in some places and too weak in others—the target can distort as it implodes, reducing performance and making experiments harder to interpret. OMEGA uses three techniques that work together to smooth each laser beam:

Distributed phase plates (DPPs), which split each beam into many small beamlets that overlap on the target
Distributed polarization rotators (DPRs), which create overlapping speckle patterns with different polarizations that average together
Smoothing by spectral dispersion (SSD), which rapidly moves the speckle pattern during the laser pulse

While all three smoothing techniques are important, SSD plays a unique role because it actively moves the laser speckle pattern during the pulse. Together, these methods ensure that the target receives a very smooth time-averaged intensity, even though the beam is not perfectly uniform at any instant. This smooth illumination is essential for symmetric inertial confinement fusion (ICF) implosions.

How SSD Works

SSD works by modulating the laser wavelength at high speed. Like light passing through a prism, each spectral component travels through the laser system at a slightly different angle. As the wavelength shifts back and forth, the beam traces a repeating pattern on the target known as a Lissajous pattern. At any one moment, the beam still has a speckle pattern, but because the pattern is moving rapidly, the target responds to a much smoother average intensity.

One way to understand SSD is to think about a long-exposure photograph. If the subject moves while the camera shutter is open, the camera does not record the instantaneous position of the subject; it records the average light over the entire exposure, resulting in a blurred image that represents the average motion over time. SSD on OMEGA works in a similar way. This motion-based smoothing helps ensure that the target is driven evenly during ICF implosions on OMEGA.

The Short-Pulse Challenge

Many ICF experiments use carefully shaped laser pulses that include a very short picket at the beginning of the pulse. These pickets help set up the implosion before the main drive pulse arrives.

However, picket pulses can be extremely short, typically around 100 ps. In that short time, the SSD motion does not complete a full cycle. As a result, the average beam pointing during the picket can be slightly offset from the pointing during the main pulse. Even a small pointing offset during the picket can introduce large-scale asymmetries in the implosion, affecting performance and making results more difficult to interpret.

SSD Sync: Optimizing SSD for Short Pulses

To solve this problem, engineers developed a technique called SSD Sync. SSD has been used on OMEGA for many years, but SSD Sync was developed to support modern pulse shapes that include very short picket pulses.

Figure 1. (a) Streaked spectrometer image showing the time-resolved laser spectrum. One axis represents wavelength, and the other represents time, while color indicates intensity. The oscillating pattern reveals the SSD wavelength modulation used to set the RF phase for SSD sync. (b) With SSD off, the wavelength remains constant in time, producing a narrow, stationary trace. With SSD on, the wavelength oscillates in time, producing the ripple pattern.

This system carefully times the picket pulse relative to the radio-frequency (rf) signal that controls the SSD wavelength modulation as measured with a streaked spectrometer. There is a specific moment in the rf cycle, called the zero crossing, when there is no wavelength shift. With no wavelength shift, there is no angular shift in the beam, so it points in the same direction as the time-averaged beam.

Using a streaked spectrometer, engineers measure how the laser spectrum changes in time (Fig. 1) and adjust the phase of the rf signal so that the peak of the picket pulse occurs exactly at this zero-crossing point. This time-averaged beam pointing ensures that the pointing during the picket matches the pointing during the rest of the pulse.

Improving Consistency from Shot to Shot

SSD also introduces a small amount of unavoidable amplitude modulation as the beam moves through apertures in the laser system. By locking the rf phase, SSD Sync ensures that this modulation is the same from shot to shot. This consistency removes a variable from the experiment, making it easier for scientists to analyze implosion performance and compare results across experiments.

Why This Matters

The OMEGA laser was designed to deliver extremely uniform irradiation to fusion targets, and beam smoothing is critical to achieving that goal. Phase smoothing (DPPs), polarization smoothing (DPRs), and SSD work together to reduce laser nonuniformity and improve implosion symmetry.

SSD Sync is a refinement of this system that becomes especially important for modern pulse shapes that include very short picket pulses. By ensuring accurate beam pointing and consistent laser performance, SSD Sync helps improve experimental repeatability and the overall quality of the scientific data collected on OMEGA.

In ICF research, success depends on precision, symmetry, and repeatability. Technologies like SSD and SSD Sync operate behind the scenes, but they play a critical role in the success of ICF experiments on OMEGA.

Streaked Spectrometer

Photo of a Streak Spectrometer at Laboratory for Laser Energetics.

To ensure that SSD Sync is working correctly, engineers need a way to observe how the laser’s wavelength changes over time. This is done using a diagnostic called a streaked spectrometer, which combines two instruments into one.

A spectrometer spreads light into its component wavelengths (colors), mapping each to a different position along a line, much like a prism creating a rainbow. On its own, it provides a time-integrated measurement, showing which colors are present but not how they change during the pulse.

A streak camera, on the other hand, records how light intensity changes over time. It converts incoming light into electrons at a photocathode, then rapidly sweeps those electrons across a detector so that one dimension of the image represents time.

By combining these two instruments, the streaked spectrometer records both color and time simultaneously. The spectrometer first spreads the light into a line of wavelengths, and that line is then imaged onto the streak camera. As the signal is swept across a 2D detector, one axis represents wavelength, and the other represents time.

Each vertical slice of the image shows the instantaneous spectrum of the laser at a given moment, while each horizontal slice shows how the intensity of a particular wavelength changes over time. This allows engineers to observe the sinusoidal wavelength modulation produced by SSD and to align the timing of the picket pulse with the rf zero crossing.

In practice, the streaked spectrometer produces images like the one shown in Fig. 3, where the oscillating pattern reveals how the laser wavelength shifts during the pulse. By analyzing this pattern, the SSD phase can be adjusted to ensure proper synchronization and maintain accurate beam pointing during short picket pulses.

A version of this article appears in Issue 9 of LLE In Focus, the magazine of the University of Rochester’s Laboratory for Laser Energetics.