Long Period Fiber Gratings Based Multi-pulse Dissipative Soliton All-normal-dispersion Yb-doped Fiber Laser And Refractive Index Sensing

As important passive optical components, optical fiber gratings have received great research interests because of their wide applications in optical fiber sensors, optical fiber communications and optical fiber lasers. Long-period fiber grating（LPFG） as a new type of fiber gratings has exhibited great potentials due to its advantages such as low insertion-loss, broad-band, sensitive to the changes of environments, easy fabrication and low-cost. This thesis presents an investigation on the applications of LPFGs in fiber lasers and fiber sensing. LPFGs based multi-pulse dissipative soliton all-normal-dispersion（ANDi） Yb-doped fiber laser and refractive index sensing are experimentally studied. The main contents and results are as follows:（1） Experimental fabrication of LPFGs using high-frequency CO₂ laser line-by-line inscription technology are investigated. A fabricated cascaded long-period fiber grating（C-LPFG） is then applied to an all-normal-dispersion multi-pulse dissipative soliton Yb-doped fiber laser as a spectral filter. It is experimentally found that different number of multi-pulse dissipative solitons could be obtained with different cavity lengths while the time intervals between the solitons are independent of the length of the cavity, in which 1) Two types of twin-pulse dissipative solitons with a time interval of 21.1ns and 33.6ns, respectively, are observed with a 220 m cavity length; 2) triple-pulse solitons with equal time intervals of 33.6ns and non-equal time triple-pulse and four-pulse solitons（with time intervals of 21.1ns and 12.5ns, respectively） are observed with a 350 m cavity length; and 3) when the cavity length is increased to 1140 m, a maximum number of five-pulse dissipative solitons with equal time intervals of 33.6ns or non-equal time five-pulse or non-equal time eight-pulse（with time intervals of 21.1ns, 12.5ns and 9ns, respectively） are also observed. Possible physical mechanism of the generation of the multi-pulse dissipative solitons with different time intervals are analyzed.（2） Soliton bunches in an all-normal dispersion Yb-doped fiber laser are observed and discussed. Based on mode-locking mechanism of nonlinear polarization evolution（NPE） in the laser, and a C-LPFG as the bandpass filter, as much as 6 solitons can be achieved in a soliton bunch and the number the solitons in a soliton bunch can be increased or decreased by either rotating the polarization waveplates or changing the pump power. The formation of the soliton bunches are discussed experimentally.（3） Fabrication method and sensing characteristics of micro LPFGs are experimentally studied. LPFGs are fabricated on a microfiber using a CO₂ laser inscription system in which the microfiber is obtained by etching a conventional SMF with HF solution. The fabricated micro LPFG is then applied to a liquid refractive index sensing. It is experimentally shown that a resonant wavelength response of the micro LPFG to external refractive index was found to be ¹98.1nm/RIU with a diameter of³2μm fiber and a 45-period micro LPFG in the fiber, which is ⁴ times larger than that of a normal LPFG with the same period number of 45.