Study On Erbium-doped Fiber Lasers With WS₂ As Saturable Absorbers

Passively mode-locked fiber lasers can produce pulses with very narrow pulse width, high-peak power and a wide spectrum. Thanks to the advantages of low-cost, high gain efficiency, compact size and without alignment, passively mode-locked fiber lasers prosper in fast-diagnosis, femtosecond processing, bio-medicine, laser radar and so on. With the development of nanotechnology and new materials, the new two-dimensional material for passively mode-locked fiber lasers attract most of the researcher's eyes. Since the single-walled carbon nanotubes, graphene and topological insulators were deeply studied, another kind of material called transition metal dichalcogenide?TMDC? draws lots of attention. As a member of TMDC, WS₂ was brought to the forefront naturally. The excellent electrical and optical properties, such as ultra-fast relaxation time and ultra-wideband saturable absorption, indicate pervasive applications in ultrafast laser. In this thesis, based on passively mode-locked erbium-doped fiber laser, novel two-dimensional material WS₂ was explored to achieve fiber laser mode-locking.The main contents of this thesis are as follows:1. The research progress of passively mode-locked fiber laser is introduced. Several ways of passive mode-locking are detailed, such as NPR, "8" cavity mode-locking, carbon nanotubes and graphene, the new two-dimensional nano-materials.2. The band structure of WS₂ is analyzed, also the principles of the saturable absorption. Pulses forming mechanism in mode-locked laser is theoretical analyzed, several key factors of mode-locked laser are outlined.3. By using the Pulsed Laser Deposition?PLD? method, WS₂ saturable absorber?SA? is prepared with low insertion loss?IL? and good power handling capability. The saturation absorption exhibits thickness correlation. When the pump power arrived 54 mW, fundamental mode locking is achieved in the erbium-doped fiber laser with output pulses duration of 675 fs, signal to noise ratio of 65 dB. Increasing pump power to the maximum of 395 mW, pulses with duration reduced to 452 fs and SNR of 48 dB are observed, and the repetition rate of mode-locked fiber laser is 1.04 GHz corresponding to 53rd-order harmonic.4. By filling WS₂ nanosheets alcohol solution into the single-mode photonic crystal fiber?SMPCF?, a super-fine saturable absorber is prepared. The modulation depth, saturation intensity and non-saturation loss of SA are measured to be 3.53%, 159 MW / cm2 and 23.2% respectively. By applying the fine SA into the EDFL, the mode-locking is observed with pulse width of 524 fs and repetition frequency of 19.57 MHz. Also the SNR of output pulses is up to 60.5 dB.5. The specific structure and the preparation method of the fiber integrated WS₂-SAM are both presented. By a linear erbium-doped fiber laser, the mode-locking characteristics of prepared WS₂-SAM are measured. Experimental results show that the erbium-doped fiber laser can be achieved mode-locking. The mode-locking pulses are Q-switched, and the SNR is measured to be 40 dB. In this experiments, pulses are stable which verifying that the WS₂-SAM is really effective, and optimization methods also mentioned.