Coherence is the essential and fundamental property of waves. And for light, it’s a front row seat for Maxwell’s Equations.
Coherence is valuable because it encodes information in retrievable ways and lets us explore regimes of time and distance that would otherwise be beyond our reach.
For a few decades, since 3rd generation storage rings led to smaller electron beams, and periodic magnetic insertion devices gave us extremely bright x-ray sources, the coherence properties have been there, drowned out in the phase space of partial coherence and imperfect beamline optics.
You can, and we do, extract a coherent fraction of the light by spatial and wavelength filtering, but it comes at the expense of flux, throwing light away that doesn’t fit through the pinhole.
But now we find ourselves at the convergence of two developments that add constructively. First, diffraction-limited storage rings* (DLSR) promise significantly smaller source sizes that increase the coherent fraction into the soft x-ray and tender x-ray range. Second, beamline optical systems are no longer lagging behind. With superior optical quality, new adaptive elements, and advancing wavefront sensors all becoming available, we are steps away from delivering reliable, beamlines that are worthy of the source improvements that drive them.
This is our vision for the Advanced Light Source Upgrade (ALS-U), and this is what we are committed to make possible.
From 2016 to mid-2019, I led the group working to develop ALS-U Beamlines and Optical Systems. In addition to two new and two upgraded beamlines that will fully utilize the new source’s coherence properties, every beamline within the ALS will be affected by the project. Our job is to deliver new tools, and position the facility’s experimental systems to be ready for the next 25 years.
Our team has brought the project through the US Department of Energy’s Critical Decision 1 (CD-1) milestone, emerging from the Conceptual Design phase, we are well into Preliminary Design now. Within the project, the Beamlines and Optical Systems Group is comprised of beamline veterans, engineers, technicians, and subject matter experts in a variety of fields directly related to achieving and maintaining beamline optical systems with diffraction-limited performance (that is, performance approaching the limits of what is possible, for efficiency and optical quality).
In the near future, as advances in accelerator technology allow, “DLSR” Diffraction-Limited Storage Rings will come online at facilities around the world. A new generation of beamlines will be created that are worthy of the new source properties. Today, the quality we seek is at the edge of our grasp. By the work we are engaged in, soon it will be commonplace, yet no less extraordinary.
*Diffraction-limited has related meanings in the context of light sources, such as synchrotrons, and optical systems, such as lenses, mirrors, or whole beamlines.
For a light source to be “diffraction-limited” means that the beam of light emanates from a point so small, or so well collimated, that the light is highly coherent, and can produce interference like a laser beam. Larger light sources are either incoherent or they are partially coherent, and the interference gets washed out to varying degrees. Whether a light source is diffraction-limited depends on the source size, the wavelength, and the cone angle of the light that emanates from the source.
For an optical system to be called “diffraction-limited” we require that the optical aberrations that arise from imperfections, misalignments, etc., are small enough that it performs as though it is nearly perfect. There are various standards that dictate how close to perfect they need to be, but the essential idea is that the lenses, mirrors, gratings, and other optical elements, work together in a nearly perfect way, giving close to the highest possible performance.