Research On Microstructured Fiber Technology For High Power Fiber Lasers

The output power of high power fiber laser systems has been promoted sharply in recent years,which is largely dependent on the advances in specialty fiber design and fabrication allowing impressive mode area scaling.The microstructured fiber technology for high power fiber laser is investigated in detail in this dissertation.Firstly,a general overview on specialty fibers and microstructured fibers are given.Then,a systematicly research have been foused on the hollow core fibers,multicore fibers and all-solid large mode area fibers.The design,fabrication,characteristics and application of those fibers are introduced in the following main contents:(1)A simplified hollow-core microstructured fiber with core diameter 31?m was fabricated and investigated.The thickness of the bridges t is about 280 nm.A Gaussian-shaped beam profile with a visible pattern guided by the large core was observed in the far field.This large core broadband transmission fiber offers a good medium for applications in high energy delivery,novel gas-cell based laser and sensing devices.Moreover,A kind of all-solid Bragg fiber with large core diameter up to 28?m was fabricated by PCVD process,which is useful for the delivery of high power laser energy.(2)We numerically investigate the seven-core photonic crystal fiber(PCF)with the zero dispersion wavelength designed in the range of 1000-1080 nm,particularly suitable for the ytterbium-doped fiber laser pumping.Then,a new type of seven-core photonic crystal fiber was designed and fabricated for high power supercontinuum(SC)generation,which has smaller six holes in the first layer than the other holes.Such design has the merits of high efficient coupling,and especially high beam quality of M2-factor measured to be-1.05.In addition,we drew a 500 m-long PCF taper directly on the industry drawing tower.The fiber taper has a uniform cross-section structure with galss diameter from 170 ?m to 70 ?m,and demonstrates very good beam quality.The PCF taper was pumped with a picosecond laser source at wavelength of 1064 nm,and generated 200mW output power of SC covering from-450 nm to 1600 nm.Lastly,the high average power PCF-based SC are presented.104.2 W all-fiber-integrated SC is generated by pumping a piece of seven-core PCF with a self-made picosecond fiber laser.(3)We have demonstrated a new approach for developing very large mode area silica-based microstructured Ytterbium(Yb)-doped fibers.The microstructured region acting as pump cladding around the core is composed by periodically arranged low-index Fluorine-doped silica inclusions with an extremely low filling ratio of 0.088.To the best of our knowledge,we achieved the most accurate controlling on cladding index by 1 × 10-5 via our passively doped cladding(PDC)method.Two fibers with 127?m and 50?m core diameter respectively were fabricated from the same final preform designed by this approach.It is verified that our 50?m core diameter fiber can maintain robust single mode behavior at 1064nm wavelength,while 127?m core diameter fiber could conditionally operate in single mode behavior in active configuration.The advantage of all-solid structure along with much simpler fabrication process makes our approach very suitable for realizing very large mode area fibers for high power fiber laser application.