Research On Bending Resistant Large Mode Area Few-Mode Fiber And Few-Mode Fiber Laser

With the increasing output power of fiber laser,the nonlinear effects have become the most important factor limiting the power improvement.The mode instability also damages the beam quality of the laser output.Large mode area fibers have the advantages of low nonlinearity,low loss and bending resistance,which can effectively improve the power and beam quality of lasers.In this paper,many kinds of large mode area fibers with high bending resistance and single mode operation are proposed to reduce the nonlinearity of optical fibers.The theoretical research and structure design of large mode area few-mode fiber is further studied.The main work and innovative achievements of this paper are as follows:1.Based on the high extinction ratio of single core offset mode interferometric filters,a multi-wavelength erbium-doped fiber laser is experimentally fabricated,and stable switchable triple-wavelength oscillation,quadruple-wavelength oscillation and quintuple-wavelength oscillation is obtained at room temperature.The theoretical and experimental results of mode interference comb filter based on large mode area fibers show that the extinction ratio of the filter increases with the core radius of few-mode fiber.Core offset fusion can farther improve extinction ratio of filters,but at the same time it will lead to the increase of device loss.2.A Multi-Trench Fiber(MTF)structure with leaky channels is proposed.It is found that bending leads to the separation of LP11v mode and LP11h mode in MTF,and increase the loss of LP11v mode can effectively improve the single mode property.By drilling holes in low refractive index grooves,the leakage loss of higher-order modes(HOMs)in optical fibers is increased,and the loss ratio between lowest-HOMs and fundermantal mode(FM)is increased by 170%.When the bending radius is 20 cm,the mode area of FM can reach 869?m2,and the lowest-HOMs to FM loss ratio is 2600.The single mode operation can be maintained within the bending orientation ranges between ±10 degrees.This design has the characteristic of smaller bending radius and stronger single-mode operation.3.An eight-segment Multi-Trench Fiber is proposed.When the bending radius is 20 cm,the FM mode area of traditional MTF can reach 790?m2 maximum.By divided the outermost low refractive index trench of traditional MTF into eight segments,the mode area of the optical fiber is significantly expanded by the combined effect of resonance and leakage.When the bending radius is 20cm,the mode area of FM is increased from 790?m2 to 920?m2,and the performance of this design is independent of the bending direction.4.The bending characteristics of large mode area segmented cladding fiber(SCF)are investigated.It is proved that SCF can still achieve large mode area and maintain single-mode operation under bending conditions.A novel kind of SCF is proposed,which has excellent HOMs suppression.When the bending radius is 15 cm,the mode area of FM can reach 715?m2.Moreover,two kinds of single trench SCFs are proposed.The single trench contact segmented cladding fiber(STC-SCF)has 76%improvement of HOMs suppression than traditional SCF in straight.It has mode area of 797?m2 at bending radius of 15 cm.Meanwhile,the loss ratio between lowest-HOMs and FM is larger than 100,and the performance of this design is stable with bending orientation.5.A segmented cladding fiber with a parabolic-profile core is proposed.The fiber combines the parabolic-profile refractive-index core and segmented cladding,which improve the bending characteristics of segmented cladding fiber and effectively increase the leakage loss of HOMs.It can achieve good single-mode operation with large mode area.When the bending radius is 15cm,the mode area of FM can reach 980?m2 and loss ratio between lowest-HOMs and FM is larger than 10.Meanwhile,the mode area of FM is insensitive with the bending radius and bending orientation.