Single frequency laser sources with low frequency noise are now at the heart of precision high-end scientific applications, from the most precise optical atomic clocks to the gravitational-wave detection, thanks to the rapid development of laser frequency stabilization techniques based on optical or electrical feedback from an external reference cavity. Despite the tremendous progress, these laser systems are relatively high in terms of complexity and cost, essentially suitable for the laboratory environment.

Nevertheless, more and more commercial applications also demand laser sources with low noise to upgrade their performance, such as fiber optic sensing and LiDAR, which require reduced complexity and good robustness to environmental perturbations.

A research team from the Shanghai Institute of Optics and Fine Mechanics of the Chinese Academy of Sciences (CAS) has made research progress in the frequency noise reduction of single frequency fiber laser. Based on the frequency noise self-feedback mechanism, the frequency noise of fiber laser has reached the thermal noise limit at low Fourier frequency successfully. The results were published in Opt. Express.

In this study, researchers have developed an ultralow noise DFB fiber laser with self-feedback mechanics by utilizing the inherent photothermal effect through regulation of the thermal expansion coefficient of laser cavity. A systematic theoretical model is proposed and experimental study shows that a frequency noise self-feedback mechanism is established via the photothermal effect.

By utilizing the inherent photothermal effect through regulation of the thermal expansion coefficient of laser cavity, the pump induced noise and environmental noise is reduced completely and over 20 dB frequency noise reduction is achieved. The frequency noise of fiber laser has reached to the thermal noise limit at low Fourier frequency successfully, which is better than the one of NKT Koheras X15 fiber laser.

The findings would make contribution to applying the free-running DFB fiber laser into the commercial application areas such as fiber optic sensing, LiDAR, etc.

This work is supported by the National Natural Science Foundation of China, the Civil Aerospace "13th Five-Year" Preliminary Research Project, the Strategic Priority Research Program of CAS, the Shanghai Sailing Program and the Natural Science Foundation of Shanghai.

The different level of laser frequency noise in different conditions. (Image by SIOM)