Optics 

Dr. Thompson’s primary areas of research are the physics of pulsed lasers and the interaction of these pulses with glass optical fiber. Much of this work requires the precise control of various properties of the laser pulses. As a result, we work with lasers that we build rather than off-the-shelf laser systems. The picture below shows a close-up of part of a laser system used in our experiments.

The laser emits light in the near-infrared part of the spectrum, so no scattered light from the laser beam can be seen in the picture. With great care one can obtain short intense pulses from the laser with one central color and a highly stable temporal shape. “Pulse” is the technical name for a short burst or flash of light.

In this case the flashes last for about 30 billionths of a second and have peak powers of around a 1,000 watts. However, even these highly stable pulses fluctuate from one pulse to the next. These fluctuations manifest themselves as small erratic changes in the peak brightness of each pulse and affect everything for which the laser is used.

When such laser pulses are coupled into optical fiber, the interaction between the light and the glass changes various properties of the light. One thing that can change is the color, or, in technical terms, the frequency or wavelength.

The graph on the right shows a spectrum of the laser light after it has interacted with the glass. Before the interaction there was only one central color, but afterward an additional peak appears. This additional peak is a signal that a process known as "Brillouin scattering" has occurred, shifting the frequency of part of the laser light down by about 14 GHz (a frequency change of about 1 part in 20,000, as it turns out).

One of the things about these interactions of particular interest is how the scattered light fluctuates in brightness (in other words how stable or unstable it is). The scattered light fluctuates for two distinct reasons: the laser pulses that drive the scattering fluctuate in brightness, and the interaction between the light and the glass introduces additional fluctuations that are intrinsic to the interaction.

Knowing about this is important because these effects that shift the color of laser light are used to make intense light sources that aren’t always available in currently existing lasers. Most applications require that these sources of light be very stable. From a fundamental perspective, nature is replete with events or processes that depend in some way on random fluctuations; consequently, what we learn from these experiments may help us think more clearly about natural processes involving randomness.