Multiwavelength Fiber Laser Based On Random Feedback

Randomly distributed feedback fiber laser(RDFFL)is a new kind of fiber laser developed in recent years.Different from the conventional laser structure,RDFFLs do not have a fixed reflective cavity mirror.It can achieve stable,spatially incoherent,and continuous laser output.This new fiber laser is considered as an important new light source to be used for nonlinear optics,optical communications sensing and other fields.This work studied and implemented several random distributed feedback fiber lasers based on Erbium-doped fibers:1.A random fiber laser with stimulated Brillouin scattering(SBS)and Erbium-doped fiber(EDF)as hybrid gains is proposed and experimentally demonstrated.Two spools of 20 km single-mode fibers are used to form a fully opened cavity and provide random optical feedback by Rayleigh scattering.For fixed Brillouin pump wavelength and pump power of 1550.00 nm and 2.19 mW,the proposed laser can achieve random lasing of two wavelengths with frequency downshift of about 0.088 nm and 0.174 nm relative to the Brillouin pump wavelength when EDF pump power increases.EDF pump powers corresponding to the 1st and 2nd SBS are 190 mW and 370 mW,respectively.Maximal output powers from the two ends are 1.60 mW and 1.68 mW when EDF pump power is 433 mW.When EDF pump power is obviously higher than the threshold power,the output laser has a 3 dB bandwidth of about 0.022 nm and keeps stable.The peak intensity and wavelength have no observable fluctuations over time.2.Our group has experimentally demonstrated a multiwavelength Brillouin-Erbium random fiber laser with a linear half-open cavity using the optical feedback from both Rayleigh scattering and the reflection of a chirped fiber Bragg grating.Multiwavelength random fiber laser is obtained with flattened output spectra,which is verified by both about 4.3 dB peak power difference between the first and fourth channel and inconspicuous peak power discrepancy between the odd and even Stokes lines.However,the lasing wavelength range is limited by the limited grating bandwidth.To overcome the limitation,a tunable multi-wavelength random fiber laser,with single-mode fiber and ring structure forming half-open cavity,has been proposed.Tunable Brillouin pump laser is used to achieve tunable random lasing.The results show that the proposed laser can generate up to four wavelengths.The number of wavelength can be accurately controlled by the output power of Erbium-doped fiber amplifier.The threshold powers for the first Stokes line to the fourth Stokes line are 12,31.6,73 and 610 mW,respectively.The slope efficiency of the first stokes wave is 12.5%.When the power of the Erbium-doped fiber amplifier is fixed at 631 mW,the output power of the proposed laser decreases exponentially with the lengths of the optical fibers used as random distributed feedback media.The output wavelength of the random fiber laser can be tuned between 1528 nm and 1580 nm.Furthermore,the half-open cavity design can effectively eliminate the peak power difference between the odd-order and even-order Stokes lines.