| Linear cavity single-frequency fiber laser has been gradually paid more and more attention by many researchers due to their advantages of simple and compact structure,low noise,easy integration,difficult to jump mode and high stability and so on.In this paper,the effective cavity-length of distributed Bragg reflector(DBR)single-frequency fiber laser is significantly changed by precise temperature control of fiber Bragg grating(FBG)in the cavity to realize narrow laser output line width.A flexible and controllable distributed feedback(DFB)fiber laser with tunable wavelength is proposed and designed by the theoretical and experimental study of the output spectral characteristics of linearly chirped fiber grating with local point heating.Details are as follows:1.The output spectral characteristics of linearly chirped fiber gratings under localized point heating are studied theoretically and experimentally.The results show that the central wavelength and transmittance of narrow-band transmission peaks of the output spectrum changed periodically under the localized point heating with different heating temperatures,heating widths.When the heating positions are changed,the central wavelength of narrowband transmission peaks will shift linearly and the transmittance will not change basically.Based on these results,a flexible,controllable and wavelength tunable DFB fiber laser is proposed and designed,and the output spectral characteristics and the distribution of optical power intensity in the cavity are analyzed theoretically in the preliminary stage.2.The operation characteristics of DBR single-frequency fiber laser with tunable cavity-length are studied.Based on the group delay phenomenon of FBG,the effective cavity-length of DBR single-frequency fiber laser is significantly changed by changing the temperature interval ΔT of the two FBGs in the cavity to make their spectrum partially overlapped,and the experimental measurement of the effective cavity-length is carried out.The results show that when the temperature interval ΔT of two FBGs is greater than13 ℃ and the effective cavity-length increases obviously,the output linewidth of the fiber laser is certainly narrowed to the extent,the output wavelength is not sensitive to the cavitylength,and the output power increases in fluctuation with pump power.3.The intensity noise of DBR fiber laser with tunable cavity-length is studied experimentally.When the temperature interval ΔT of two FBGs is less than 13 ℃(the effective cavity-length of resonant cavity is basically unchanged),the relaxation oscillation frequency in the relative intensity noise spectrum increases with pump power.When ΔT is greater than 13 ℃,the relaxation oscillation frequency increases in fluctuation with pump power.When the pump power is constant and the temperature interval ΔT between two FBG is less than 13 ℃,the relaxation oscillation frequency of DBR fiber laser is basically unchanged.However,when ΔT is greater than 13 ℃,the relaxation oscillation frequency decreases rapidly.When the DBR fiber laser operates in dual-longitudinal mode,low frequency relaxation oscillation peaks appear in the relative intensity noise spectrum due to the multi-longitudinal mode anti-phase dynamics.The high frequency and low frequency components of the relaxation oscillation peaks will change regularly when the strength of the two longitudinal modes is different. |
