Fiber Laser Devices Based On Specialty Long Period Gratings

Optical fiber lasers have been explosive growth in the past decades due to their poten-tial for realizing robust,alignment free,high power,compact and rugged laser sources.They have been the mainstay in many fields such as cutting,drilling and welding where large mode area fibers are required for high intensity tolerance,high speed telecommu-nications where a single coherent mode is needed for long-haul transmission and mod-ulation,microscopy imaging where special wavelengths are desired to match various targets,and so on.And those applications have led to many interesting laser compo-nents such as band-rejection filters,dispersion compressors and stretchers,and mode converters etc.Unfortunately,Part of these components are constructed with free space optics which may require extra techniques to make them highly compatible with fiber laser system,like bulky glasses,diffraction gratings and binary phase plates and so on.In most recent decades,in-fiber laser devices have taken root in many industries given that they can be easy to assemble via simple splicing routine,leading to well-compatible to fiber laser cavities and very-compact size of prototype for commercial uses.The category of ll-fiber laser devices include coupler,wavelength division mul-tiplexer?WDM?,circulator,isolator,and modulator and so on.In this paper,we us long period fiber gratings?LPG?technique to realize functions such as band-rejecting,dispersion-compensating,and mode-converting.LPG written by ultraviolet light into the core of an optical fiber has been a critical fiber devices due to its low insertion loss,high extinction,and potentially low cost.LPG has similar principle with fiber Bragg gratings?FBG?except that LPG is a transmissive device,leading to lower insertion loss when used in ring or transmissive laser cavities than FBG.However,FBGs are much more maturely developed than long period fiber gratings?LPGs?,which in contrary is operating in transmissive high order modes?HOMs?instead of reflective fundamental mode in FBGs.Part of this disparity stems from the assumption that high order modes?HOMs?cannot stably propagate in multi-moded step-index structure for long lengths without freely coupling to other waveguide modes.However,the stably propagation property of HOMs have been demonstrated in multi-moded fibers?MMFs?with dis-tinct effective index splitting of modes(?10^-4)to avoid mode coupling caused by fiber defection or outer cladding?OD?fluctuation.More particularly,an uncoated?coating stripped?SMF can be treated as a highly multi-moded fiber with excellent geometry as well when we consider the cladding mode.Thus this has motivated the development on HOM-based long period fiber gratings?LPGs?,and also has given rise to the poten-tial for various HOM-based techniques in fibers including terabit-scale high bandwidth communications,atomic acceleration and trapping,and perhaps most relevant to this work:mode conversion and dispersion compensation.In this thesis,we investigate interesting in-fiber laser devices using HOM-based specialty long period fiber grating techniques,including band-rejection and long-pass filter,ultra-fast fiber laser dispersion compensator,all-fiber orbital angular momentum?OAM?mode generator,for various fiber laser sources.Numerical models based on a transfer matrix approach and a direct integration ap-proach are built to simulate and analyze the properties and performance of specialty LPGs in terms of transmission spectra,group delay,and dispersion.We introduce an advanced computer-controlled UV-inscription setup for scalar-mode LPGs fabrication-s,and we discuss the design and characteristics of them including Gaussian-apodized LPGs,Raised-Gaussian-apodized LPGs,chirped LPGs,Raised-Gaussian-apodized Chi-rped LPGs.These LPGs are demonstrated with high conversion efficiencies??99%?with negligible insertion loss.A dual-frequency counter-pumped Master Oscillator Power-Amplifier?MOPA?continuous-wave?CW?fiber laser is designed and built.We firstly realize the power amplifying of the optical-carried microwave signal in a MOPA system,achieving 10W power of amplified microwave signal.The frequency can be tuned from 125MHz to165MHz with high signal-to-noise ratio.A wavelength tunable bi-pumped MOPA pulse fiber laser are designed and built.While it encounter the issue that the backward pump light or parasitic resonating light may damage upstream optical devices.Therefore we firstly propose and demonstrate a band-rejection long-pass filter using apodized LPG with the LP_0,13in a SMF for reject-ing the backward lights,and band-passing the forward signal light.We achieve?99.9%extinction of backward light at arbitrary wavelength such as 976nm and 993nm,and?0.5dB insertion loss of forward signal light over the wavelength range from 1020nm to 1080nm.In addition,we use this band-rejection filter to build a 3-stage fiber MOPA system,achieving 46kW peak power with the pulse duration of 0.65ns and the repetition of 20kHz at 1030nm.A second set of experiments demonstrates that,chirped LPG is a promising mean of developing a compact,low loss,transmissive,and power-tolerant dispersion com-pensation module,comparing to the current techniques including chirped fiber Bragg gratings?FBGs?,photonics crystal fibers?PCFs?,and LP_0,2-based compensators.We demonstrate that ultra-short femtosecond?fs?optical pulse stably propagating through a fabricated CLPG and also conduct experiment on pulse compressing via CLPG,but the CLPG is operating in HOM output.We use concatenation of CLPGs to realize fundamental mode input and output;and we investigate an sub-100fs pulse dispersion compensator using a concatenated chirped LPG with the LP_0,13,which is character-ized by a cross-correlation?C²?dispersion measurement setup and is demonstrated to successfully compensate?1m SMF fiber laser cavity.Furthermore,a new technique using raised-super-Gaussian apodization is proposed for breaking through the bottle-neck caused by the tradeoff of bandwidth and dispersion,and for designing dispersion compensator which is capable of supporting sub-50fs and balancing?10m SMF fiber laser cavity.This gives a very wide prospect in ultra-compact mode-locked fiber laser design.In a separate experiment,we demonstrate orbital angular momentum?OAM?mode generation at visible wavelengths with low loss?0.7dB?efficient??90%?using acousto-optic?AO?LPGs on a vortex fiber designed to propagate OAM modes stably,with tunability of 210nm-to the best of our knowledge,the highest tuning range achieved from such devices compared to the state of the art.This covers almost 85%of the fluorescent dyes used in microscopy techniques such as stimulated emission depletion?STED?microscopy and leads to more potentials in other applications.Based on these results,the specialty LPG are a very promising technique for de-signing and developing interesting in-fiber laser devices such as band-rejection long-pass filters,ultra-fast optics diserpsion compensator,and OAM converter discussed in following chapters.Especially,concatenated CLPG technology offers a solution for ultra-low loss,ultra-compact and compatible dispersion compensators.In addition,demonstration of such a high-dispersion compensator using the raised-apodized-super-Gaussian chirped LPG technique would be revolutionary,as it could supplant current dispersion compensators based on FBGs and PCFs,etc..Furthermore,exploration and discovery of new techniques based on specialty LPGs would be a very attractive topic and would have very good industry and market prospect.