50th Anniversary of Supercontinuum Laser Marked at SPIE’s BiOS/Photonics West

Light is one of the most vital and versatile phenomena in nature with salient properties. Without it, no one can see. And because of it, short pulses of light can reveal what’s hidden in the molecular world.

That belief drove one of the seminal discoveries of the 20th century: supercontinuum light.

Five decades ago, Robert Alfano, Ph.D., and Stanley L. Shapiro, Ph.D., both members of GTE Lab in Bayside, N.Y., observed a type of laser light that spanned a large part of the visible spectrum over 10,000 cm⁻¹ under excitation of 532 nm green picosecond pulses. The observations were chronicled in three separate papers appearing in Physical Review Letters in 1970.

“It was essentially a light bulb. I called it the supercontinuum,” recalled Alfano in opening remarks that kicked off a Feb. 2 special session dedicated to the discovery of the supercontinuum at SPIE Photonics West 2020.

“Our idea was to probe the fundamental process of nature,” Alfano said. “And in order to do that you need to use the salient properties of light; you need to know wavelength, time polarization, and coherence.”

Alfano recalled his early work when he and Shapiro sent green laser pulses through solids, observing the continuum in rare gas and liquids (argon and krypton) and in solid krypton. What they observed was four wave rings, and at the center was the supercontinuum, he said. A pattern of 5-picosecond green pulses passed through a transparent piece of glass — a breakthrough that would set in motion later discoveries, including the use of fiber laser for the first tabletop SC laser developed in later years.

After the discovery and application of the supercontinuum phenomenon came a series of technological advances in the ’70s, ’80s, ’90s, and 2000s.

On Sunday, Imperial College’s James R. Taylor, Ph.D., enumerated key developments during these years that led to extensive spectral versatility and allowed the temporal format of the excitation pumps to range from the continuous wave to the femtosecond regime. Such advances, including the demonstration of temporal optical solitons in fiber, led to tabletop, pulsed, solid-state lasers and compact, all-fiber, integrated configurations.

“We’ve seen scientific and commercial success between 300 nm to 4.5 µm,” Taylor said, also noting supercontinuum laser sources emitting in the femtosecond, picosecond, and nanosecond ranges.

Spectral ranges today span 110 nm to 13 µm, with an average power greater than 100 W, he said.

Defining tumor margins

Stephen Boppart, Ph.D., M.D., of the Beckman Institute for Advanced Science and Technology, credited advances in supercontinuum light sources with breakthroughs in fiber-based simultaneous label-free autofluorescence multiharmonic SLAM microscopy and powering optical coherence tomography (OCT), notably finding tumor margins in vivo in real time.

“Through in vivo, real-time, label-free imaging, we’ve gained a new understanding of cellular dynamics,” Boppart said during Sunday’s session.

This has led to the discovery of a wealth of new biomarkers for assessing the tumor microenvironment and diagnosing disease.

“By developing widely coherent supercontinuum from photonic crystal fibers, new excitation wavelengths can be generated to tailor the light stimulus in new ways,” Boppart said. As a result, SLAM microscopy enables clinicians to visualize the intrinsic molecular, metabolic, and structural information in cells and tissues, he said. Results suggest broad potential of this stain-free, slide-free imaging approach for real-time point-of-procedure applications.

For more on the supercontinuum, see “Evolution of the Supercontinuum Light Source,” which appeared in the January 2018 issue of Photonics Spectra.