Adaptive pumping for spectral control of random lasers

TL;DR

This paper demonstrates a novel active pumping technique that enables spectral and mode control in random lasers, overcoming inherent randomness and extending their tunability and directionality.

Contribution

The authors experimentally implement a spatial pump shaping method to control emission wavelength and modes in an optofluidic random laser with overlapping modes, a previously unexploited approach.

Findings

Spectral selectivity down to 0.06 nm achieved.

More than 10 dB side-lobe rejection demonstrated.

Control over random laser emission without prior mode knowledge.

Abstract

A laser is not necessarily a sophisticated device: Pumping energy into an amplifying medium randomly filled with scatterers, a powder for instance, makes a perfect "random laser." In such a laser, the absence of mirrors greatly simplifies laser design, but control over emission directionality or frequency tunability is lost, seriously hindering prospects for this otherwise simple laser. Lately, we proposed a novel approach to harness random lasers, inspired by spatial shaping methods recently employed for coherent light control in complex media. Here, we experimentally implement this method in an optofluidic random laser where scattering is weak and modes extend spatially and strongly overlap, making individual selection a priori impossible. We show that control over laser emission can indeed be regained even in this extreme case by actively shaping the spatial profile of the optical pump. This unique degree of freedom, which has never been exploited, allows selection of any desired wavelength and shaping of lasing modes, without prior knowledge of their spatial distribution. Mode selection is achieved with spectral selectivity down to 0.06nm and more than 10dB side-lobe rejection. This experimental method paves the way towards fully tunable and controlled random lasers and can be transferred to other class of lasers.