Investigation On High-order Mode Fiber Lasers And Modes Control

Fiber lasers have important applications in fundamental science,optical communication,and industrial laser processing due to their compact structure,strong anti-interference ability,and efficient heat dissipation.Earlier studies mainly focused on single-mode fiber lasers with fundamental mode output.In recent years,high-order mode(HOM)lasers have attracted wide attention.Due to unique intensity distribution,polarization distribution,and phase distribution,HOM fiber lasers show specific applications in particle manipulation,surface Raman spectral enhancement,laser material processing,super-resolution imaging,optical communication,and high-power fiber laser system.In addition,compared with the fundamental mode in optical fiber,HOM has a larger effective mode area,which can effectively suppress the nonlinear effect in optical fiber and improve the damage threshold of optical fiber devices.Meanwhile,the ultra-short pulse HOM output can be obtained by combining modelocking technology,which is conducive to the further development of high-power and high-energy pulsed fiber lasers.However,mostly mode-locked fiber lasers are composed of single-mode fiber and essentially oscillate in fundamental mode,which needs to use mode converters to obtain HOM output.Consequently,the mode purity and output efficiency of the HOM fiber laser are limited by the performance of the mode converter and the lack of flexibility of the mode-switchable.A series of studies are carried out based on the all-few modes fiber(FMF)structure in this paper,We proposed high-efficiency fiber lasers with HOM direct oscillation,and the output mode can be flexibly manipulated by modulating mode gain in the cavity,which can meet the requirements of various practical applications at the same time.The main research results are as follows:1.We proposed and demonstrated an all-FMF laser with HOM direct oscillation by using a few-mode fiber grating and band-pass filter to suppress the fundamental mode.In a continuous wave regime,the fiber laser directly oscillates in HOM with a slope efficiency of 46%and mode purity of 98.3%without mode converter and polarization sensitivity.Moreover,a self-started HOM pulsed fiber laser is achieved by using the SESAM.2.We demonstrated a transverse mode switchable mode-locked fiber laser in an allFMF cavity.The mode gain profile can be controlled by using a specially designed filter and ring gain fiber.Pulsed HOM output with a slope efficiency of 12%,mode purity of 98.2%,and pulse width of 31.5ps is realized.Besides,by adjusting the mode competition in the cavity,the oscillating mode can be flexibly switched between the fundamental mode and HOM.This high-efficiency mode-locked fiber laser with a controllable oscillating mode has important applications in laser material processing、mode division multiplexing systems.3.The all-fiber saturable absorber is made based on multimode fiber,and a cylindrical vector beam(CVB)mode-locked fiber laser is built.The stable pulsed HOM output is achieved by using broadband long-period fiber grating that served as a mode converter.By simply adjusting the pump power,the operation state of the HOM fiber laser can be flexibly switched among continuous-wave,Q-switched mode locking,and mode-locked regimes.When the pump power is 380 mW,mode-locked pulse CVB with a repetition frequency of 3.592 MHz,pulse width of 4.62 ns,and mode purity of 97.6%is obtained.The output power of 142 mW is obtained with a slope efficiency of 20.2%,and the maximum pulse envelope energy can reach 510 nJ,which has guiding significance for the realization of a high-energy CVB fiber laser system and its spatiotemporal manipulation research.4.We proposed and demonstrated an FMF random laser with HOM direct oscillation based on mode injection technology to promote the competition of HOM.The laser threshold is as low as 88 mW,and the CVB output with mode purity of 97.2%and a slope efficiency of 7.2%is obtained.By controlling the power of the signal light,the oscillating mode of the random fiber laser can be flexibly switched among the fundamental mode,the mixed mode,and HOM.5.The multi-dimensional control of all-FMF random fiber laser is proposed and realized.A specially designed Lyot filter is used to control the gain of the modes in the cavity.The cavity mode experiences different gains and losses by temperature tuning.Thus,the high-efficiency random fiber laser with HOM direct oscillation is realized.Under the quantitative temperature control,the random fiber laser can realize the controllable output of single-wavelength,double-wavelength,and threewavelength,and the operating transverse mode can be flexibly switched between the fundamental mode and HOM.In addition,speckle contrast measurement experiments are carried out to verify the coherence control of random fiber lasers,which further confirms the value of the laser in speckle suppression.The innovation of this paper is as follows:1.Self-starting HOM directly oscillating fiber laser is proposed in an all FMF cavity.HOM output is obtained without a mode conversion device and polarization sensitivity when the laser operates in a continuous wave regime.Self-starting HOM pulse output is also realized by combining mode-locked pulse technology,which provides an important reference value for realizing the stable output of any highorder mode fiber laser.2.An all-FMF mode-locked pulsed fiber laser with a switchable oscillating mode is proposed.A specially designed filter and annular gain fiber are used to control the mode gain in the cavity without using mode converters.HOM direct oscillation with high slope efficiency is achieved,and the oscillating transverse mode can be switched flexibly between the fundamental mode and HOM,which has important application value in mode division multiplexing and laser material processing.3.The nonlinear multimode interference structure is used as an all-fiber saturable absorber,which solves the shortcomings of the material saturable absorber,such as complex production and low damage threshold.Pulsed CVB output with high energy and high output power is realized.In addition,the operation state of the HOM fiber laser is flexibly controllable among continuous wave,Q-switched mode-locked,and mode-locked regimes.Meanwhile,the output of azimuthally polarized and radially polarized beams can also be switched.This controllable HOM fiber laser with controllable temporal-spatial properties has important applications in time/mode division systems and laser processing.4.Based on the mode injection locking technique,an all-few mode fiber random laser with HOM direct oscillation is realized for the first time.By controlling the power of signal light,the oscillating mode controllable output of random fiber laser can also be realized.5.The spectrum and spatial mode of the random fiber laser can be manipulated quantitatively.The mode gain profile can be changed through temperature tuning,which solves the instability and uncontrollability of the output spectrum and spatial mode caused by strong mode competition in the multimode/few-mode random fiber laser,and HOM direct oscillation output with a slope efficiency as high as 17.9%is realized.In addition,the unique potential of speckle-free imaging and secure encrypted communication are also pointed out.