Research On Few-mode Fiber Grating And Orbital Angular Momentum Fiber Laser Based On Femtosecond Laser Fabrication

Recently,fiber lasers have broad application prospects in the industry,medicine,and scientific community because of their compact structure,stable output,good heat dissipation,and low cost.With the development of laser light field manipulation technology,all-fiber Orbital angular momentum(OAM)lasers have attracted considerable attention.OAM is a special light beam with a spiral phase distribution whose center intensity is zero.Due to its unique properties,the OAM has great potential in optical communication,highresolution imaging,optical tweezers,and material processing.Most all-fiber OAM lasers use the extra-cavity mode converter,turning fundamental mode into OAM mode.The direct OAM output in the intra-cavity reduces the insertion loss,which is the essential method to level up the conversion efficiency and stability in the future.In this paper,the femtosecond laser direct-writing technology was utilized to fabricate fiber gratings.The OAM laser based on few-mode fiber Bragg gratings(FBG)was realized.A scheme that oscillated in high-order mode directly was proposed.Furthermore,we designed and realized a novel wavelength-switchable all-fiber OAM laser.The paper is organized as follow:(1)The femtosecond laser direct-writing method was studied.An experimental platform for fabricating fiber gratings by the femtosecond laser was built.Based on the point-by-point and line-by-line method,various fiber gratings were fabricated,including uniform FBG and phase-shifted FBG.The fabrication techniques of different gratings were explored.Their spectrum characteristics and the insertion loss were measured.(2)The mode interference principle of the few-mode fiber Fabry-Perot interferometer(FPI)was analyzed,and its spectrums and mode characteristics were studied.The FPI was fabricated in a few-mode fiber using the femtosecond laser direct-writing technique.Then,we built a scheme for mode field measuring.And the OAM mode manipulation of the fewmode fiber FPI was measured.By changing the polarization state of input light or the direction of polarizer transmitted by the reflected light beam,the ±1 OAM modes can be achieved.(3)The fiber laser that can realize multi-wavelength output and transverse modes switching was demonstrated.Due to the polarization dependence and multi-resonant wavelengths of the few-mode FBG,single-wavelength,dual-wavelength and triplewavelength output can be realized by changing the polarization inside the cavity.Meanwhile,the specific laser mode was selected.The ±1 OAM modes can be achieved at the wavelength of 1547.4 nm.(4)The all-fiber OAM laser with a switchable wavelength that consists of a few-mode FBG and a FPI was constructed.The OAM laser can selectively output 1547.48 nm and1547.61 nm wavelengths by using the comb filtering property of FPI.At each wavelength,different OAM modes of +1 and-1 orders can be output via polarization manipulation.