Research On Brightness Enhancement Technology Of High Power Raman Fiber Laser

Raman fiber laser has the advantages of flexible wavelength selection and cascade operation.With appropriate pump laser,Raman fiber laser at any wavelength within the transparent window of optical fibers can be generated.Besides,it has no effects of photon darkening,gain saturation and amplified spontaneous emission,which is easier to obtain high-power laser.Up to now,Raman fiber lasers are the only proven technology to generate high-power wavelength-agile fiber sources,and the output power of it has reached to several kilowatts level.However,further power scaling is limited by the high-order Raman light and mode instability.Some universal nonlinear suppression schemes need to be proposed.Therefore,the following studies are carried out.Based on graded index fiber,a fiberized Raman amplifier with a maximum power of 400 W was built.Based on the stochastic parallel gradient descent algorithm,the output beams were decomposed into individual modes,and the beam purification process is characterized from the modal level.In addition,the effects of pump temporal stabilities on the high-order Raman threshold and output characteristics of the laser are experimentally investigated.The feasibility of suppressing high-order Raman light and improving the output beam quality by optimizing the pump timing was verified.Raman laser with output of 943 W is realized with the M~2 of 1.6 and BE of 27.Experimental studies on graded index fiber based Raman lasers including oscillators and random distributed feedback lasers are carried out to explore the potential of mode selection in cavity combined with graded index fiber on beam purification.All fiber Raman oscillator and random distributed feedback Raman laser were built based on different number of gratings.We obtained Raman oscillator laser with 334 W and 443 W output power and their M~2 is 2.8 and 3.5 corresponding the BE of 5.6 and 4.2.And 306W and 722 w random Raman laser with M~2 of 2.35 and 4.65 were realized corresponding the BE of 6.1 and 8.9.A random distributed feedback Raman laser with all fiber structure was built based on self-designed triple clad fiber.By designing the core cladding area ratio of the fiber,the high-order Raman light was effectively suppressed.Cladding pumped by high-power fiber laser,random Raman fiber laser with output power of 1005 w was obtained,verifing the feasibility of using cladding pumping scheme to enhance the power and brightness of random Raman laser.Based on the stochastic parallel gradient descent algorithm,the output beams at different power were decomposed and the distortion dynamics was characterized and preliminarily explored from the modal perspective.