This post originally appeared on CLEO 2011 by Frank Kuo and is reproduced with permission from its author.

With so many different kinds of lasers play essential roles in modern researches and daily life, it is tempting to find out what are the extremes among them. Thanks to this conference, this question intrigues me once again during a talk (QMF3) where a gigantic free electron laser (FEL) was mentioned and used to probe the atomic structures. Searching with the conference program brochure and within my memory, here is what I can find.

The winners of “the biggest” prize go to the FELs. Taking the one in the U.S. soil as an example, a FEL powered by a two-mile-long linear accelerator (linac) in Stanford Linear Acceleration Center (SLAC) has a grand name associated with it  – Linac Coherent Light Source (LCLS). Technically speaking, it is a laser of more than two miles in length and many many tons in weight (I don’t think people actually weight this monster, figure 1). Basically, after SLAC’s linac accelerates very short pulses of electrons to 99.9999999 percent of the speed of light; the LCLS takes them through a 100-meter stretch of alternating magnets that force the electrons to undulate back and forth. This motion causes the electrons to emit X-rays. Since the electron motion is in phase with the field of the light already emitted, the fields add together coherently.  As many as 10 trillion X-ray photons can be produced and squeezed into a bunch that’s a mere 100 femtoseconds long. This giant laser has a sibling across the Atlantic. In Europe, an x-ray free electron laser (European XFEL) shared by 14 countries is powered by a 2.1 km long superconducting linear accelerator.

Figure 1. The aerial view of the monster FEL laser in SLAC.

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