Tunable Passively-mode-locked All-fiber Lasers Based On Multimode Interference Effect

Tunable passively mode locked fiber lasers have significant applications in fields such as manufacturing,medicine and scientific research.As the key element to affect the tunability of fiber systems,the characteristics of spectral filter is of great importance.Spectral filters based on multimode interference(MMI)effect in fiber are widely used in various fiber systems owing to their all-fiber-based structure,easy fabrication,multiple tuning methods and designable filtering spectrum.By employing MMI spectral filters in passively mode locked fiber lasers,one can obtain all-fiber-based configuration and tune the output spectrum of mode locked fiber lasers.The utility of MMI spectral filters can also expand the configurations of tunable passively mode locked fiber lasers and inspire more designs of tunable laser systems as well.In this thesis,we analyzed the multimode interference effect in fiber and performed research on MMI based tunable passively mode locked fiber lasers.The main work is demonstrated as follow.Firstly,we summarized mode theory in fiber and analyzed the characteristic and power distribution of transverse modes in fiber.Mode capacity of multimode fibers was also calculated in our analysis.Then,a theoretical study of MMI effect and self-imaging effect in fiber is performed based on fiber mode theory and wave equation.The multimode interference process when a fundamental mode of single mode fiber was coupled into a multimode fiber was simulated in our study.The evolution of selfimaging quality factor along multimode fiber and the transmission spectrum of MMI filter with single-mode-multimode-single-mode(SMS)structure were also simulated in this work.Furthermore,we introduced the basic principles of beam propagation method and finite difference beam propagation method,and then summarized the numerical simulation of MMI effect in fibers based on these methods.Secondly,the propagation properties of single-wavelength and wide-bandwidth optical signal in MMI filters were simulated,respectively.Moreover,the influences of signal wavelength,length,core size,refractive index distribution and radial stress of multimode fiber on MMI effects in fiber were studied,and their influences on transmission spectrum of MMI filters were also analyzed.We also fabricated some samples of MMI filters with SMS structure and tested their wavelength tunability.What's more,the spectral filtering effect of passively mode locked fiber lasers based on nonlinear polarization rotation(NPR)was studied,and the tunability of NPR laser cavity with MMI filters was studied as well.Thirdly,a MMI-based tunable passively mode locked fiber laser system and a tunable fiber amplifier were designed and fabricated in this research.Owing to the tunability of MMI filter,the NPR mode locked fiber laser obtained mode locked pulses with tunable center-wavelength and tunable spectral bandwidth.What's more,in the tunable fiber amplifier,we used an amplification stage with large mode area Yb-doped phosphate fiber to amplify the output of a MMI-based tunable mode locked fiber laser and obtained mode locked pulses with an average power of 2.11 W and a tuning range within 1031.9-1062.8 nm.