| Narrow linewidth fiber lasers are widely used in fields such as coherent detection,beam combination,and nonlinear frequency conversion due to their advantages such as high coherence,high beam quality,and convenient thermal management.High-power fiber lasers based on narrow-linewidth fiber Bragg gratings(FBG)pair have been extensively studied in recent years due to their unique advantages of simple structure.However,the temporal instability caused by the self-pulsing effect will induce the stimulated Raman scattering effect(SRS)during the power amplification process,and spectral broadening is also serious.Thus related research about the optimization of the cavity structure were carried on,which aims to improve the performance of fiber lasers around the three goals of power boosting,stabilizing laser timing sequences and narrowing the laser linewidth.Higher-performance seed laser would be provided for the master oscillator power amplifier structure(MOPA)structure.Firstly,the temperature control was studied for FBG-based laser cavity.FBG is sensitive to temperature due to the thermo-optic effect,and its reflected spectrum will move to the long-wave direction as the temperature rises.It was verified that temperature control of FBGs is beneficial to narrow the laser linewidth.And based on this,through cross-splicing a pair of polarization-maintaining(PM)FBGs,a narrow-linewidth fiber laser with power of 190.7 W,-3 d B bandwidth of 0.16 nm,and polarization extinction ratio(PER)of 13 d B was achieved in a single-transverse-mode fiber oscillator.During the experiment,it was unexpectedly discovered that the timing of laser cavity became extremely unstable,and high-peak-power self-pulses appeared,which lead to strong nonlinear effects and realized supercontinuum laser output.Secondly,aiming at the problem of common linear cavity lasers that are easily restricted by the SRS due to unstable timing sequences,a composite cavity laser was built by introducing an additional grating into the linear cavity.And the ability of SRS suppression was verified through a long passive fiber.The results show that the composite cavity structure can indeed stabilize the timing and suppress the SRS,and optimizing the cavity length of the composite cavity will further enhance its Raman suppression ability.In the end,the laser was achieved with power of 77.3 W,-3 d B bandwidth of 0.2 nm and the temporal properties are significantly better than that of a common line cavity structure.Thirdly,the laser in the common linear cavity and the composite cavity is a standing wave field,while the laser in a ring cavity structure is a traveling wave field.Due to this feature,a variety of spectral filter components can be inserted into the ring cavity,which has a large operating space and is widely used in the single-frequency lasers.However,the current power of ring cavity lasers is low,in the order of hundreds of milliwatts.In this paper,a narrow-linewidth ring cavity laser with a power of 61.1 W and-3 d B linewidth of 0.1 nm is built,which provides a new idea for the MOPA structure’s seed laser. |
