Research On Tunable Narrow-linewidth Fiber Lasers Based On Random Distributed Feedback

Currently,ultra-narrow linewidth lasers are mainly generated by semiconductor lasers,solid-state lasers and fiber lasers,providing low-threshold,controlled output laser sources for scientific research and industrial applications.Stable fiber lasers with narrow linewidth and relatively high output power have a wide range of promising applications in optical communication,metrology and sensing due to their high coherence and low noise characteristics,which are important requirements for improving the detection range and accuracy of these systems.At the same time,wavelength-tunable lasers are widely used in applications such as lidar,coherent optical communication,spectroscopy and precision measurement.Wavelength-tunable spectrally coherent light sources offer good system portability and help to improve the signal-to-noise ratio and accuracy of communication and measurement systems.Therefore,the realization of wavelengthtunable and stable narrow linewidth laser output has been an important research topic in the field of laser science and technology.In response to the above needs,this thesis proposes a low-cost tunable narrow linewidth fiber laser with laser linewidth compression by means of combined filtering of a random feedback grating array in an active fiber system.The main research elements and findings are as follows:Firstly,the spectral patterns(including transmission and reflection spectra)of a random distributed fiber grating array are theoretically simulated,while the grating array is fabricated by femtosecond machining,and the simulated and measured spectral patterns are analyzed.To achieve a narrow linewidth fiber laser with a smaller linewidth and a single excitation mode,this thesis proposes the use of two random distributed feedback disordered fiber grating arrays to construct a toroidal cavity for a narrow linewidth fiber laser.In an active fiber system,the combination of transmission and reflection spectra of the random grating array can provide a selective resonant tunneling effect to select and compress the laser modes without excessive feedback points,allowing for ultra-narrow laser excitation with good stability and high edge-mode rejection ratio.The optimized allfiber laser architecture allows for a narrow linewidth and stable single longitudinal mode spectrum at 570 Hz with side-mode suppression ratio of 63 d B.The maximum output power achievable from this laser is approximately 6 m W at the maximum available pump power,with no observed saturation of the output power,and can be further scaled up to over 25 m W.The results show the effectiveness of random distributed weak feedback for narrow linewidth single longitudinal mode selection,and that it increases with the accumulation of random feedback,providing an effective method of compressing single longitudinal mode linewidth using edge-mode suppression.The output characteristics of this narrow linewidth laser are then further measured and improved.The beat frequency signal of the output narrow linewidth laser is calculated by interferometry and the output wavelength drift is measured to be about 0.8 pm,which is a good wavelength stability.To tune the output wavelength,the grating array is temperature-controlled from 20 °C to 120 °C to achieve a partially tunable output wavelength in the 445 pm range.The frequency noise and phase noise of the laser are further analyzed,and noise suppression methods are explored to achieve a narrow linewidth fiber laser with excellent output characteristics and low noise.Subsequent work can be carried out to extend the output wavelength tuning range to provide a lowresolution light source for later super-resolution imaging implementations.