Simpler Approach Produces Highly Chirped Laser Pulses

Researchers at the University of Rochester’s Institute of Optics have produced ultrashort, extremely high-energy (chirped) laser pulses using a spectral filter in a Kerr resonator. Kerr resonators, which operate without amplification, are of interest to researchers globally due to their ability to support behaviors that include broadband bursts of light. The pulses created in the work remained stable in low quality-factor resonators, despite large dissipation.

The demonstration used relatively low-quality and inexpensive equipment, the researchers reported. By adding a spectral filter in a Kerr resonator, a type of optical cavity, they were able to manipulate a laser pulse in the resonator to widen its wavefront by separating the beam’s color.

The work comes more than two years after researchers demonstrated a technique to create ultrashort, extremely high-energy laser pulses. The 2018 Nobel Prize in physics was shared by the the University of Rochester researchers who pioneered that work.

An illustration of the optical fiber Kerr resonator, which Rochester researchers used with a spectral filter to create highly chirped laser pulses. The rainbow pattern in the foreground shows how the colors of a chirped laser pulse are separated in time. Courtesy of the University of Rochester, Michael Osadciw.

In the current method, as the laser pulse widens, the pulse peak is reduced, said William Renninger, assistant professor of optics and a co-author of the study. The effect is advantageous: More overall energy can be put into the pulse before it reaches a high peak power.

The researchers’ work takes advantage of the way that light is dispersed as it passes through optical cavities. Most prior cavities require rare “anomalous” dispersion, which means that blue light travels faster than red light through the cavity. The chirped pulses, in the current demonstration, live in what is known as “normal” dispersion cavities, in which red light travels faster than blue light. The dispersion is called “normal” because it is a more common type of dispersion, and it will increase the number of cavities that can generate pulses.

Prior cavities are also designed to have less than 1% loss, whereas the chirped pulses can survive in the cavity despite very high energy loss.

“We are showing chirped pulses that remain stable even with more than 90% energy loss, which really challenges the conventional wisdom,” Renninger said. “With a simple spectral filter, we are now using loss to generate pulses in lossy and normal dispersion systems. In addition to improved energy performance, it really opens up what kinds of systems can be used.”

Ultrashort pulse and frequency comb sources that are simpler and more effective for spectroscopy, communications, and metrology are among the technologies and applications the work supports, according to the study. More specific applications include improved telecom systems, improved astrophysical calibrations (such as those used to find exoplanets), atomic clocks with increased accuracy, and precise devices to measure chemical contaminants, including those in the atmosphere.

The University of Rochester and the National Institute of Biomedical Imaging and Bioengineerng at the National Institutes of Health supported the project.

The work was published in Optica (www.doi.org/10.1364/OPTICA.419771).