Harnessing Randomness to Improve Lasers

Randomness could be key in improving the performance of lasers in sensing and imaging applications, a new study shows.

Randomly arranged, or irregular, surfaces typically have poor optical properties, as the roughness (randomness) can obscure the view of an object. But now, a team from the A*STAR Singapore Institute of Manufacturing Technology (SIMTech) and Nanyang Technological University has discovered a way to make efficient use of randomness. In its research, the team demonstrated the first electrically pumped mid-infrared random laser, which is as bright as conventional diode lasers but produces less-speckled images.

Lightwaves from a conventional laser oscillate in perfect synchronicity across both time and space. Perfect alignment of the lightwaves at different times and locations across the beam profile is known as temporal and spatial coherence, respectively.

Designed at A*STAR, the first electrically pumped mid-infrared random laser will enhance sensing and imaging applications.

When a laser illuminates a surface, a speckled pattern typically is visible, which indicates spatial coherence. The speckles are the result of the laser beam’s reflectance from different parts of the surface. Because the waves are in sync, they create spatial interference effects in the eye of an observer.

This distortion is undesirable, particularly in biomedical imaging applications conducted in the IR region of the spectrum. Random lasers are the solution to this type of distortion, said Ying Zhang of SIMTech.

“Random lasers show the same high temporal coherence as that of other lasers, but have a lower spatial coherence,” he said. “High temporal coherence gives the desirable brightness, but it is the low spatial coherence that removes the speckles caused by interferences.”

To realize a random laser in the mid-IR spectrum, researchers used a semiconductor quantum cascade laser into which they had drilled a random pattern of nanoholes. At a sufficiently high density, these holes prevented the formation of a regular laser pattern within the semiconductor. Instead, the pattern of a random laser forms, with low spatial coherence.

More work is needed to bring random lasers to the current market, however.

“In the long term, we plan to extend random lasers from the infrared to even longer wavelengths … for quality control of printed electronics, biomedical imaging, among other applications,” said Hou Kun Liang of SIMTech.

For more information, visit www.simtech.a-star.edu.