MIT's phase-locked laser array yields better beam

June 14, 2016 // By Julien Happich
In the Nature Photonics paper "Phase-locked laser arrays through global antenna mutual coupling", researchers from the MIT and Sandia National Laboratories have demonstrated a natural phase-locking mechanism of closely integrated nanolasers, showing that the typically unwanted lateral emissions could contribute to a long-range global coupling of all the lasers in the 2D array.

"Phase locking of an array of lasers is a highly effective method in beam shaping because it increases the output power and reduces the lasing threshold", the researchers wrote, having built a demonstrator array of 37 subwavelength short-cavity surface-emitting lasers across a die area less than 1mm square.

The laser elements, emitting at circa 3THz, were distributed in different array configurations, only spaced apart by roughly the equivalent of their fundamental wavelength (100µm). Being phase-locked to each other through "far-field radiations with definite phase relations", they jointly delivered up to 6.5mW (in a pulsed operation) single-mode operation, with a maximum 450mW A –1 slope efficiency and a near-diffraction-limited beam divergence, the researchers reported.

By designing the nanolasers into well-defined grid patterns, the researchers leveraged the nanolasers' divergent beam patterns (typically lost energy) to obtain a strong coupling among the other lasers in the array, and shape a better focused and stronger beam than what they would have achieved with the sum of the individual lasers taken independently.

They took their inspiration from the field of microwaves, where the use of phased arrays of antennas is common place to achieve narrow beam-forming.

According to the researchers, the larger the array, the more laterally emitted radiation can be recaptured to stimulate lasing of nearby lasers, lowering the overall power threshold at which the array can produce laser light. This research could bring industrial applications such as terahertz spectroscopy within reach of terahertz quantum cascade lasers.

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