Ultra-high spectral purity laser derived from weak external distributed perturbation

Ultra-high spectral purity lasers are of considerable research interests in numerous fields such as coherent optical com-munication, microwave photonics, distributed optical fiber sensing, gravitational wave detection, optical clock, and so on. Herein, to deeply purify laser spectrum with compact size under normal condition, we propose a novel and practical idea to effectively suppress the spontaneous radiation of the laser cavity through weak external distributed perturbation. Sub-sequently, a laser configuration consisting of a main lasing cavity and an external distributed feedback cavity is pro-posed. The feedback signal with continuous spatio-temporal phase transition controlled by a distributed feedback struc-ture is injected into the main cavity, which can deeply suppress the coupling rate from the spontaneous radiation to the stimulated emission and extremely purify the laser spectrum. Eventually, an ultra-narrow linewidth on-chip laser system with a side mode suppression ratio greater than 80 dB, an output linewidth of 10 Hz, and a relative intensity noise less than-150 dB/Hz is successfully obtained under normal conditions. The proposed concept in this work provides a new perspective for extreme regulation of laser parameters by using weak external distributed perturbation, which can be val-id for various gain-type lasers with wide wavelength bands.

Automated summary

This study proposes a new method to deeply purify laser spectrum with compact size under normal conditions, by effectively suppressing the spontaneous radiation of the laser cavity through weak external distributed perturbation. By injecting a feedback signal with continuous spatio-temporal phase transition controlled by a distributed feedback structure into the main cavity, the coupling rate from the spontaneous radiation to the stimulated emission can be deeply suppressed, resulting in an extremely purified laser spectrum. Eventually, an ultra-narrow linewidth on-chip laser system with excellent performance is successfully obtained under normal conditions.