Lights, Camera, Nanosecond Action!

Nebeker sets up shop one afternoon at the labs of Spectra-Physics in
Mountain View, California, where he can use a laser capable of generating
7-nanosecond pulses of brilliant green laser light. The camera he brought shoots only two frames; Nebeker wants a pair of sequential pictures—an ultrashort movie, essentially—that will mark the progress of the light over a short period of time.



The laser light will travel through a series of spaces as it bounces through angled mirrors that face each other in pairs along a 9-foot-long table. A 7-foot-long pulse of light will travel through the mirror course in 90 nanoseconds.



With a camera capable of shooting 10-nanosecond frames, the trick is figuring out which 10 nanoseconds to capture, there being 90 million such periods in one second alone. There is little time for reaction lag, and even though the laser’s hardware generates a pulse to trigger Nebeker’s camera at the same time that it triggers the laser, the circuitry and the 20-foot cable attaching laser to camera add nanoseconds of delay here and there.



In order to capture the image, Nebeker has to measure and synchronize the delay of the laser with the delay of the camera. Employing a trial-and-error process, he ultimately gets the camera to switch on precisely while the pulse is passing through the mirrors. As predicted, a 100-nanosecond exposure produces an image showing, in the first frame, the entire 90-foot course lit up; the second frame is dark. He then cuts exposure time down to 50 nanoseconds, which produces a first-frame image showing half the course illuminated. The second frame, however, remains dark. Tweaking the delay between frames, he produces a second frame that shows the tail end of the pulse disappearing into the terminating “beam dump” box at the far end of the mirror course.



Nebeker adjusts the exposure time to 10 nanoseconds, the camera’s limit, and the attending Spectra-Physics engineers begin to get excited about what they’re seeing. As expected, the computer screen shows a first frame in which only one leg of the mirror course has laser light in it. The rest is dark. In the second frame, also 10 nanoseconds long and exposed after 10 nanoseconds’ delay, a leg of the course two over from the first is illuminated. Nebeker has captured two discrete portraits of a laser light beam traveling within the 90-foot course.



The grainy black-and-white digital images would not pass muster with John Woo. Nonetheless, they show light the way science fiction has long depicted it—moving as a discrete packet across a short space (think of Kirk’s phaser, set to stun, in the old Star Trek). The small crowd of engineers is delighted. “I think we need one of those!” one engineer calls out as he looks longingly at the Cordin Model 220 camera.