The Study Of New Saturable Absorbers Based Passively Q-switched And Passively Mode-locked Fiber Lasers

Attributed to the excellent beam quality, high efficiency, compact performance and convenient heat management, fiber lasers have lead the revolution of laser technology. Pulse fiber lasers can generate large pulse energy and high peak intensity, so they are widely applied in industrial production, science research, fiber communication and so on. Date to now, there are two mainly methods to produce short pulses:Q-switched technique and mode-locked technique. In both of these two techniques, saturable absorber plays a very important role. Saturable absorber based passively Q-switched and passively mode-locked lasers have the advantages of compact configuration, small size, low cost and high environment stability. So, we can say, the development of pulse lasers are mainly dependent on the development of the saturable absorber materials.Traditional saturable absorbers, like dye and semiconductor saturable absorber mirror (SESAM), have many distinct disadvantages, and have greatly limit the development of pulse lasers. With the progress of materials science, some new nanomaterials came out, such as carbon nanotube (CNT), graphene, transition metal dichalcogenides (TMDs) and so on. Based on the large improvement of their properties and the price reduction, pulse lasers have greatly developed. In this dissertation, the saturable absorption properties of these new nanomaterials and their applications in pulse fiber lasers are mainly investigated, including passively Q-switched fiber laser, passively mode-locked fiber laser and passively Q-switched and mode-locked all-fiber lasers with cylindrical-vector beam emission. Considering the applications in the occasion of demanding smooth flat-top beam, in the last of this dissertation, we have also investigated the smoothing technique aiming at the output light field of fiber lasers.The major works in this dissertation:K By using z-scan and twin-arms detection, we experimentally investigated the ultra-wide saturable absorptions of the newly TMD-tungsten disulphide(WS2) at 1064 nm and 1550 nm. We have also demonstrated its polarization-independence and ultra-high damage threshold. So we can conclude that WS2 is a promising saturable absorber material.2、 By fabricating the composite film of WS2 and polyvinyl alcohol (PVA), we have respectively demonstrated a WS2-based erbium-doped and ytterbium-doped passively Q-switched fiber laser. In the Er-doped Q-switched fiber laser, with the increasing of the pump power from 279 mW to 609 mW, the repetition rate increases from 88.67 kHz to 157.29 kHz, and the corresponding pulse duration decreases from 1.24 μs to 917 ns. The available maximum pulse energy is up to 89.23 nJ. In the Yb-doped Q-switched fiber laser, by adding a tunable filter into the cavity, the operating wavelength can be continuously tuned from 1027 nm to 1065 nm at a fixed pump power with the stable Q-switched state existing all the time. When at a fixed wavelength of 1048.1 nm, the repetition rate increases from 60.2 kHz to 97.0 kHz by varying the pump power and the shortest pulse duration of 1.58μs is achieved.3、 We demonstrate a WS2-based Yb-doped passively mode-locked fiber laser through fabricating a WS2-PVA film with gradient thickness and concentration by vertical evaporation. By incorporating the WS2-PVA film into a Yb-doped fiber laser cavity, stable dissipative solitons are obtained. The pulse train has a duration of 206 ps and fundamental repetition rate of 20.66 MHz. The time-bandwidth product (TBP) is calculated of 30, indicates the mode-locking pulses are highly chirped, which can be further compressed outside the cavity.4、We propose a scheme to produce an all-fiber passively Q-switched and passively mode-locked CVB laser. We choose a few-mode fiber Bragg Grating (FMFBG) as a transverse mode selector to filter out the fundamental mode and get pure TM01 (radially polarized beam) and TE01 (azimuthally polarized beam) mode output. At the same time, we respectively adopt WS2-PVA and CNT-PVA film as a saturable absorber. Benefitting from their saturable absorption and polarization-independence, the CVB can be easily generated with Q-switched or mode-locked state. Compared with other schemes, our scheme is very compactness and the Q-switched or mode-locked state are not easily lost during the adjusting CVB.5、 In the inertial confinement fusion (ICF), laser display and laser projection, the uniform on-target irradiation is one of the key requirements. So we develop a scheme with multiple lasers staggered overlaying and Lens Array (LA) to smooth and shape the on-target laser patterns. Based on the theoretical analysis and related numerical simulation, we have verified the feasibility of the scheme and study the effects of several parameters. At last we have also demonstrated the scheme in experiment and eventually got a flat-top beam with nonuniformity below 4.6%.Highlights of the dissertation are as follows:1、 To the best of our knowledge, it’s the first time to expand the application of WS2 to the wavelength of 1064 nm. We have demonstrated a WS2-based passively Q-switched fiber laser with a wide tunable range from 1027 nm to 1065 nm while Q-switched state exists all the time. Moreover, we have successfully achieved a WS2-based Yb-doped passively mode-locked fiber laser and obtained stable dissipative solitons with repletion rate of 20.66 MHz and duration of 206 ps.2、 By choosing a FMFBG as a mode selector and WS2 as a saturable absorber, we demonstrate an all-fiber passively Q-switched laser with CVB emission. The repetition rate can be tuned from 80 kHz to 120kHz with available minimum duration of 958 ns. By tuning the polarization controllers in the cavity, both radially polarized beam and azimuthally polarized beam can be easily generated under Q-switched state.3、 By choosing a FMFBG as a mode selector and CNT as a saturable absorber, we demonstrate an all-fiber passively mode-locked laser with CVB emission. Eventually, we get a stable mode-locked operation with repetition rate of 10.61 MHz and duration of 22.73 ps. Meanwhile, a radially polarized beam with mode purity about 98.03% is obtained.4、 We propose a scheme with multiple lasers staggered overlaying and Lens Array (LA) to smooth and shape the on-target laser patterns. Based on the theoretical analysis and related numerical simulation, we have verified the feasibility of the scheme. At last we have demonstrated the scheme in experiment. Our configuration provides a compact and highly tolerant method for uniform irradiation.