Title : External cavity self-adaptive lasers assisted by distributed feedbacks
With excellent properties of the high purity spectrum and long coherent length, narrow-linewidth lasers can be applied in many fields, such as optical quantum information, low-noise microwave generation, high-resolution spectroscopy, precise sensing, high-speed optical coherent communication and so on. Herein, commencing from the in-depth analysis for the effective suppression of spontaneous emission under the excitation of a cavity signal, we propose a compression scheme to extremely compress laser linewidth assisted by a distributed feedback. Subsequently, we develop a novel laser configuration that comprises the main laser cavity and an external cavity with a distributed feedback characteristic. To provide an excitation signal required for linewidth compression, we inject into the main cavity through the distributed feedback structure a cavity mode signal matched with the output wavelength. Moreover, we conduct an experimental investigation based on an on-chip laser system with a distributed feedback. Eventually, an ultra-narrow linewidth laser with a side mode suppression ratio (SMSR) greater than 80 dB, an output linewidth of 10 Hz, and a relative intensity noise (RIN) less than -150 dB/Hz is successfully obtained under normal conditions. The proposed concept is valid in any other gain-types lasers with various wavelengths, providing a new perspective for extreme modulation of other laser parameters. We further explore the linewidth compression in tunable lasers. With all-optical tunable narrow-band fiber Bragg gratings and RBS linewidth compression, we obtain tunable fiber lasers with a precise tuning step of less than 0.02 nm and linewidth of about 200 Hz. With all-fiber acousto-optical tunable filters, we obtain fast tunable narrow-linewidth fiber lasers with a tuning band of 20 nm and switching time of less than 1 ms. Finally, we propose parameter measurement methods of ultra-narrow-linewidth lasers. For frequency-stabilized lasers, we propose an ultra-narrow linewidth measurement method based on the short delayed fiber self-heterodyne interferometer (SDSHI). For wavelength-swept lasers, we propose the measurement method of transient frequency noise and linewidth.