Study On Miniaturized Wavelength-tunable Mid-infrared All-fiber Pulsed Laser

Laser sources operating at the mid-infrared region(2-20 ?m)of the electromagnetic spectrum have potential applications in industrial manufacturing,laser surgery,molecular detection,atmospheric sensing and many other fields.In the past 40 years,the field of mid-infrared photonics has made great progress,various mid-infrared laser sources have been reported,including solid-state lasers,quantum cascade lasers,optical parametric amplifiers,and fiber lasers using rare earth ions doped soft glass fibers as gain media.Among them,mid-infrared fiber lasers possess advantages such as high conversion efficiency,good beam quality and low cost compared with other types of lasers,thus having attracted extensive attention and investigations from researchers all over the world.Despite their rapid development,mid-infrared fiber lasers still face some challenges,as following:(1)Mid-infrared fiber laser cavities are usually composed of fluoride fiber,which is difficult to be spliced with commonly-used silica fiber.Therefore,the traditional mid-infrared fiber lasers often use lens with bulk structure as laser coupling device for laser input and output,which is not beneficial for the development of a stable all-fiber laser system.The advent of fluoride fiber gratings provides a solution,however,direct inscription of fiber Brugg gratings in a fluoride fiber has certain difficulties in operations and may also increase the overall cost of the system.Therefore,it is of great practical significance to propose a new method for constructing an all-fiber mid-infrared laser cavity.(2)At present,most tunable mid-infrared fiber lasers use bulk diffraction gratings as wavelength-tuning devices.The adoption of such devices also destroys the all-fiber structure of the laser cavity and makes the laser system more complicated,undermining its stability.Thus,it is necessary to explore a new wavelength-tuning technique suitable for laser cavity with all-fiber structures.To address the above challenges,this thesis aims at the research topic of miniaturized all-fiber tunable mid-infrared pulsed fiber lasers.The relatedexperimental research is carried out around Ho3+-doped fluoride fiber lasers operating at 3 ?m waveband.The main contents are as following:(1)Using fiber end-facet mirrors instead of conventional bulk lens as laser coupling device,a stable miniaturized all-fiber 3?m mid-infrared laser cavity was successfully constructed.In order to optimize the performance of 3?m Ho3+-doped fluoride fiber laser,comparative experiments were executed using gain fibers of different lengths and laser pump sources of different wavelengths.In the same cavity structure,home-made Raman fiber lasers with wavelengths of 1.15 ?m and 1.19 ?m were used to pump Ho3+:ZBLAN gain fibers with lengths of 40 cm,55 cm and 95 cm,respectively,and the basic parameters such as 3 ?m laser threshold,laser wavelength,output power and slope efficiency were compared.The experimental results will provide important instructions for the design and optimization of 3?m Ho3+-doped fluoride fiber lasers.(2)A novel wavelength-tuning method for all-fiber lasers was proposed.This method requires a pair of eccentric ceramic ferrule fiber connectors to form a loss adjustment device,and continuous tuning of the mid-infrared laser wavelength can be realized by adjusting the intra-cavity loss.Applying this method to a 3 ?m Ho3+-doped fluoride fiber laser,a miniaturized tunable 3 ?m all-fiber laser was achived.In addition,based on this compact and stable all-fiber structure,an effective scheme for 3 ?m laser pulse generation was also proposed,and self-Q-switching operation was achieved for the first time in a 3 ?m fiber laser.3 ?m laser pulses were generated while maintaining the laser itself a compact and all-fiber structure.(3)The miniaturized all-fiber pulsed laser was further investigated using the gain-switching technique.The laser pulses at 3?m waveband were successfully obtained by pumping the Ho3+-doped fluoride fiber with a home-made 1.15 ?m pulsed Raman fiber laser.The dynamic evolution of 3 ?m laser pulses under different pump powers and different pump pulse repetition rates is studied.