Research On The Tuning Technology Of Ultra-narrow Linewidth Lasers Based On Controllable Fiber Gratings

Wavelength-tunable lasers are widely applied in lidar,microwave photonics,coherent optical communication,spectroscopy and precise measurements.Light sources with wavelength tunability and high spectral coherence have a good system portability,which is conducive to improve the signal to noise ratio and accuracy of communication and measurement systems.At the same time of laser linewidth narrowing,the realization of precise,fast,large-range and linear wavelength tunablity is one of the key technologies to promote developments of coherent optical communication and precise measurement fields.At present,the tunability of narrow-linewidth lasers depends on the wavelength selection elements in cavities,and mechanical,stress,temperature-and electronic-control mechanisms are still the main methods for wavelength tunability.Due to the limit of control characteristics of tuning components and devices,their tuning speed,linearity,precision and range are still facing challenges at the industrial application level.Therefore,this paper,utilizing the fiber as the carrier,focuses on the research of laser linewidth narrowing technologies and explores the wavelength-tunable devices that are compatible with the fiber laser systems,and it systematically studies key technologies of ultra-narrow linewidth tunable lasers based on the tunable devices.The contents of this paper are shown as follow:(1)Summary the research status of tunable lasers in depth,the research route from tunable devices to tunable narrow linewidth lasers is established.Principles of optical-controlled wavelength tuning based on graphene and the acousto-cotrolled tuning based on the polarization conversion are studied.The narrow-band gain of Brillouin laser,the filtering characteristic of saturable-absorption self-built grating and the wavelength independence of Rayleigh-scattering-based laser linewidth compression are analyzed.The principles of controllable grating and linewidth compression of lasers provide theoretical supports for the fabrication of controllable gratings and the wavelength tuning of narrow linewidth lasers.(2)Two kinds of controllable fiber grating devices for laser tuning are developed.Firstly,the precisely optical-controlled fiber grating based on graphene are fabricated.The experiment shows that the device has a linear wavelength-tuning performance.The optical-controlling response time reaches 10ms,and the tuning precision reaches100MHz order.The tuning speed of traditional precise temperature control is improved based on the sensitive and fast thermal conductivity of graphene.In order to achieve a larger-range and fast tunability,the band-pass acoustically-induced fiber grating based on the polarization conversion is prepared.The experiment proves that the linear fitting R~2 of the grating in the wavelength range of?35nm reaches 0.99421,and the response time reaches 100?s order.These two kinds of controllable fiber gratings provide the device foundation for precise,fast,large-range and linear tunability of laser systems.(3)The tuning dynamics of the controllable grating is revealed with the perspective of transient spectra for the first time.Based on the dissipative-soliton-Fourier-transform spectrum measurement system,the transient spectral evolution process of the acousto-optically induced grating in the fast tuning process is characterized,and the tuning speed can reach 13000nm/s in the wavelength range of?4nm.This research proves the availability of grating for maintaining the spectral bandwidth in fast and linear tuning process,which provides guidance for the construction and optimization of tunable laser systems.(4)An optical-controlled tunable ultra-narrow linewidth Brillouin fiber laser is proposed and constructed.A single-longitudinal-mode laser with a linewidth of?750Hz is obtained by the side longitudinal mode suppression of Brillouin narrow-band gain.The precision lasing wavelength tuning with a sensitivity of 13.2pm/m W and linear fitting R~2 of 0.99897 in the tuning range of 3.67nm is achieved by the optical-contolled fiber grating,and the good linearity is maintained when the tuning step size(28pm)approaches the spectrometer's resolution limitation.(5)An optical-controlled tunable ultra-narrow linewidth fiber laser based on the Rayleigh scattering linewidth compression is proposed and constructed.On the basis of single-longitudinal-mode laser operation,the linewidth of the optical-controlled laser is further compressed to?200Hz,and linewidth compression characteristics of Rayleigh scattering at different lasing wavelengths are discussed.(6)An acoustical-controlled large-range and fast tunable ultra-narrow linewidth fiber laser is proposed and built.The acousto-optical frequency shift is effectively suppressed by the polarization conversion,and backgrounds of laser frequency noise and relative intensity noise are as low as 10Hz~2/Hz and-135d Bc/Hz,respectively.The tuning range of?20 nm is realized by the acoustically-induced fiber grating,whose linear fitting R~2 reaches 0.99781,and the laser maintains the ultra-narrow linewidth characteristic of?2k Hz under different tuning channels.Due to the laser relaxation oscillation during dynamic tuning,the tuning response time is 800?s.Based on the acousto-optical tuning dynamic characteristics,a scheme for optimizing the tuning performance utilizing the semiconductor gain is further proposed,which extends the tuning range to 36nm(mainly limited by the working bandwidth of the cavity devices),shortens the response time to?200?s and effectively suppresses the relaxation oscillation during the laser tuning process.In this paper,from tunable optical fiber devices to tunable laser systems,the statical and dynamical tunning response characteristics are studied in depth.Precisely optical-controlled tunable ultra-narrow linewidth lasers are proposed,which deeply narrows the laser linewidth using Rayleigh scattering and improves the response speed of the lasing wavelength tunnability.Optical-controlled tunability is very important to the remote and non-contact control of lasers.The acousto-controlled tunable ultra-narrow laser is proposed,which provides a method for lasers with fast,highly linear tunability,linewidth narrowing and large-range wavelength extension.