【作者】 林健；

【导师】 许祖彦； 许立新；

【作者基本信息】 中国科学技术大学 ， 光学， 2016， 博士

【摘要】 光纤激光器具有光束质量好、效率高、结构紧凑、易于热管理等诸多优点,掀起了激光技术产业的革命。而脉冲光纤激光器因脉冲能量大、峰值功率高,在工业生产、科学研究、激光通信等领域拥有极其广泛的应用。目前,产生脉冲的主要方式是采用调Q技术和锁模技术,在这两种技术中,可饱和吸收体(SA)都起着至关重要的作用,由它构造的被动调Q和被动锁模激光器具有结构简单、体积小、价格低廉、环境稳定性高等优点。所以,我们可以说,脉冲激光器的发展在很大程度上决定于可饱和吸收材料的发展。传统的可饱和吸收体,如染料、半导体可饱和吸收镜等,面临着诸多缺陷,大大限制了脉冲激光器的发展。随着材料科学的进步,相继涌现出像碳纳米管(CNT)、石墨烯(Graphene)、过渡金属硫化物(TMDs)等各种新型纳米材料,它们的性能得到质的提升,价格也不断下降,脉冲激光器借此得到蓬勃发展。本论文主要针对新型纳米材料的可饱和吸收特性以及在脉冲光纤激光器中的应用展开研究,包括基于它们的被动调Q和被动锁模光纤激光器以及全光纤的被动调Q和被动锁模轴对称偏振光束(CVB)激光器。论文的最后,我们还对光纤激光器输出光场的匀滑技术进行了研究,以便应用在各种需要光滑平顶光束的场合。本论文的主要研究工作有：1、在实验上分别采用z扫描和双臂探测法研究了一种新型过渡金属硫化物—二硫化钨纳米片在1064nm和1550hm波段的超宽带可饱和吸收特性,并且验证了它的偏振无关特性和高损伤阈值,指出这是一种非常理想的可饱和吸收材料。2、利用二硫化钨州S2)和聚乙烯醇(PVA)制得的复合薄膜,我们分别搭建了基于二硫化钨的掺铒被动调Q光纤激光器和掺镱被动调Q光纤激光器。对于掺铒调Q激光器,当泵浦功率从279mW增加到609mW时,调Q脉冲重复频率从88.67kHz增加至157.29kHz,对应的脉宽从1.24岬减小到917ns,单脉冲能量最大达89.23nJ。在掺镱调Q激光器中,我们通过加入可调滤波器,实现了工作波长在1027-1065nm间连续可调,而调Q脉冲始终稳定存在。在固定工作波长1048.1nm的情况下,通过增大泵浦功率,调Q脉冲重复频率可以从60.2kHz增加到97.0kHz,最短脉冲宽度1.65μs。3、利用垂直蒸发法制备的厚度和浓度渐变的WS2-PVA薄膜,我们搭建了基于WS2的掺镱被动锁模光纤激光器。通过合理设计激光器结构和选择合适参数的SA,最终获得了0.55nm光谱宽度、20.66MHz重复频率、206ps脉宽的稳定耗散孤子脉冲。脉冲时间带宽积为30,带有非常大的啁啾,脉宽可在腔外得到进一步压缩。4、提出了一种全光纤被动调Q和被动锁模CVB激光器的方案,采用少模光纤光栅(FMFBG)作为横模选择元件,滤除基模,获得纯净的TM01(径向偏振光)和TE01(角向偏振光)模输出；同时分别采用WS2-PVA和CNT-PVA作为可饱和吸收体,利用它们的可饱和吸收特性和偏振无关特性,最终在保证调Q和锁模脉冲输出的同时,获得了高纯度的轴对称偏振光。相较于其它方案,该方案结构简单,轴对称偏振光方便获得,调Q状态和锁模状态不容易丢失。5、针对激光惯性约束核聚变和激光投影、激光显示中对靶面辐照均匀性的要求,我们设计了一种多束激光错位叠加和阵列透镜结合的束匀滑整形方案。通过理论分析和相关的数值模拟探讨了方案的可行性,并且详细研究了各个参数对靶面匀滑效果的影响。最后,我们还在实验上对方案进行了验证,最终得到不均匀度RMS低于4.6%的平顶光束。本论文的创新点：1、首次将WS2的应用扩展到1064nm波段,实现了波长和重复频率同时可调的掺镱调Q光纤激光器,保证波长在1027-1065nm间连续可调的同时,稳定调Q状态始终存在：另外,我们也实现了基于WS2的掺镱被动锁模光纤激光器,获得了20.66MHz重频、206ps脉宽的稳定耗散孤子脉冲。2、采用FMFBG作为横模选择元件,WS2作为调Q元件,搭建了全光纤被动调Q轴对称偏振光束激光器,重频在80KHz到120KHz间可调,最短脉宽958ns。通过调节腔内的偏振控制器,在调Q状态下获得了高模式纯度的径向偏振光和角向偏振光。3、采用FMFBG作为横模选择元件,CNT作为锁模元件,实现了全光纤被动锁模CVB激光器,最终在10.61MHz重频,22.73ps脉宽稳定锁模的同时,获得了模式纯度高达98.03%的径向偏振光。4、提出了一种多束激光错位叠加和阵列透镜结合的束匀滑整形方案。通过理论分析和相关的数值模拟探讨了方案的可行性,并在实验上进行了验证,该方案简单易行、适用性广,冗余度高。更多还原

【Abstract】 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.更多还原