【作者】 张远耕；

【导师】 张行愚； Valentin Petrov；

【作者基本信息】 山东大学 ， 光学工程， 2014， 博士

【摘要】 在激光器诞生之后的50多年中,激光技术得到了迅猛的发展。发光功率、峰值功率、光束质量、脉冲宽度、体积及成本等各项指标不断得到改善。其中,全固态激光器具有效率高、性能稳定、体积小等特点,已经成为了激光技术研究中的主流,在空间通讯、器件加工、医疗、光学成像等诸多领域有了广泛的应用。人们一般把脉冲宽度在皮秒到飞秒量级的脉冲,称为超短脉冲。超短脉冲具有脉宽窄、峰值功率高、光谱宽等特点。如今,超短脉冲也已经广泛应用于人们日常生活和工业生产中。锁模技术是获得超短脉冲的重要手段。锁模技术主要可以分为主动锁模和被动锁模,后者使用可饱和吸收体作为锁模元件,可以获得更短的脉冲宽度。在过去的20年中,随着半导体激光器的发展和新型光学可饱和吸收体出现及成熟,被动锁模技术有了很大的发展,获得了极短脉冲宽度(小于10fs)、极高的峰值功率(经放大可达PW)和超高的脉冲重复率(超过100GHz)。目前,在被动锁模激光器中主要使用的可饱和吸收体包括：半导体可饱和吸收镜(SESAM)、单层碳纳米管(SWCNT)和石墨烯(Graphene)。相比于半导体可饱和吸收镜,单层碳纳米管和石墨烯材料具有制备过程简单、工作光谱范围广等优点。双钨酸盐晶体(KRE(WO4)2, RE=Gd, Lu和Y)拥有优良的物理、化学性质,是一种优良的激光基质材料。通过在晶体中掺杂Nd、Yb、Tm等稀土离子,已经实现了高效激光运转。同时,此类晶体属于单斜晶系,由于低对称性导致其吸收、发射光谱较宽,有利于获得超短脉冲。无序晶体在一定程度上综合了光学玻璃和有序晶体的特性。相对于光学玻璃,无序晶体的热导率高,受激发射界面大。同时,由于无序晶体中的离子在一定范围内的无序分布,使其拥有了类似玻璃的无序结构,可以实现宽发射光谱。钙锂铌镓石榴石(CLNGG)是一种典型的无序晶体。本论文的研究工作主要集中新型被动锁模激光器的研究上。实验中采用SESAM和SWCNT作为可饱和吸收体,实现了多个波段的被动锁模激光器。在1μm波段,我们分别使用Nd:KLu(WO4)2和Yb:CLNGG作为激光晶体；在1.5μm波段,使用Cr:YAG作为激光晶体；在2μm波段,使用Tm,Ho:KLu(WO4)2作为激光晶体。采用不同参数的SESAM和SWCNT作为可饱和吸收体,分析了不同的可饱和吸收体的参数对锁模效果的影响。实验研究了被动锁模激光器的特性并用现有理论加以解释。同时,研究了被动调Q拉曼激光器中的自锁模现象,分析了自锁模现象和可饱和吸收体的小信号透过率、泵浦功率以及谐振腔长度之间的关系。本论文的主要研究内容如下：1.研究了LD泵浦的Nd:KLu(WO4)2连续激光特性和被动锁模激光特性。当吸收泵浦功率为4.4W时,输出1070.2nm连续光功率为3.3W,对应的转换效率为75%。使用SESAM作为可饱和吸收体,采用Z型腔,实现了连续锁模运转。最大输出功率为1.4W,脉冲重复率为102MHz,对应的单脉冲能量为6.8nJ。脉冲宽度为11.5ps,对应的峰值功率为0.6kW。在实验过程中,观察到在高泵浦功率的条件下,连续锁模脉冲出现脉冲分裂现象。2.研究了Yb:CLNGG被动锁模激光器的特性。使用SWCNT作为可饱和吸收体。当输出镜的透过率为0.4%时,获得最短脉冲宽度为90.2fs；当输出镜的透过率为3.0%时,获得最大平均输出功率为90.5mW,脉冲宽度为137.3fs。在高泵浦功率的条件下,锁模脉冲的光谱中出现了连续分量,成为限制输出功率增长的因素。使用SESAM作为可饱和吸收体。当输出镜的透过率为1.0%时,获得脉冲宽度为55.6fs的超短脉冲,平均功率为60.1mW。当输出镜的透过率为3.0%时,获得了最大的平均功率为150.5mW,脉冲宽度为115.6fs。3.研究了Cr:YAG被动锁模激光器的特性。使用SWCNT作为可饱和吸收体。获得了49.6fs的超短脉冲,中心波长为1509.4nm,光谱宽度50.8nm。系统研究了锁模激光器的特性,找到了最佳泵浦功率。使用频率分辨光学开关法(FROG)测量了谐振腔中的色散,为下一步优化谐振腔设计做好了准备。4.研究了Tm,Ho:KLu(WO4)2被动锁模激光器的特性。使用SWCNT作为可饱和吸收体,获得了2.4ps的超短脉冲,中心波长为2058.9nm,光谱宽度为1.7nm,对应的时间带宽积为0.3。使用透过率为3.0%的输出镜,获得了最大平均输出功率110.7mW。使用SESAM作为可饱和吸收体,获得了7.2ps的超短脉冲。实验中尝试了多种参数不同的SWCNT和SESAM,并通过改变棱镜对的间距改变谐振腔中的色散补偿,但始终无法获得飞秒脉冲。5.研究了全固体态拉曼激光器中的自锁模现象。以Nd:YAG晶体作为激光增益介质,以SrWO4晶体作为拉曼工作介质,以Cr:YAG作为被动调Q元件,实现了内腔式被动调Q拉曼激光器。并研究了自锁模现象和谐振腔长度,泵浦功率,Cr:YAG的小信号透过率之间的关系。实验发现,当谐振腔的长度大于50cm时,自锁模现象明显且稳定。自锁模现象和泵浦功率、小信号透过率之间没有明显关系。在全泵浦范围内都可以观察到调制深度为100%的自锁模现象。本研究论文的主要创新点如下：1.研究了LD泵浦Nd:KLu(WO4)2被动锁模激光特性。使用SESAM作为可饱和吸收体,获得了11.4ps的超短脉冲,平均输出功率为1.4W。这是首次有关LD泵浦Nd:KLu(WO4)2被动锁模激光器的报道。证明了Nd掺杂的双钨酸盐晶体可以用来获得高功率皮秒脉冲。2.首次研究了以Yb:CLNGG作为增益介质,以SWCNT为可饱和吸收体的被动锁模激光特性。当输出镜的透过率为0.4%时,获得最短脉冲宽度为90.2fs,这是目前使用透射式SWCNT在1μm波段所获得最短脉冲。3.首次研究了以Yb:CLNGG作为增益介质,以SESAM为可饱和吸收体的被动锁模激光特性。获得了55.6fs的超短脉冲,平均输出功率为60.1mW。这是目前为止,在Yb掺杂的各向同性的晶体中,获得的最短的超短脉冲。4.首次研究了Tm,Ho:KLu(WO4)2被动锁模激光器的特性。使用SWCNT作为可饱和吸收体,获得了2.4ps的超短脉冲。使用SESAM作为可饱和吸收体,获得了7.2ps的超短脉冲。这是首次有关Tm,Ho:KLu(WO4)2晶体被动锁模的报道。5.研究了被动调Q拉曼激光器中的自锁模现象。实验证明,被动调Q拉曼激光器中的自锁模现象和泵浦功率、小信号透过率没有明显的关系。更多还原

【Abstract】 Laser technology had experienced a great development since the first successful laser was reported. The parameters of laser have perfected constantly, such as average output power, beam quality, pulse duration, and cost. Compared to other laser systems, all-solid-state lasers have the advantages of high efficiency, high stability, compact structure, and long lifetime. Due to these advantages, these kinds of lasers have been widely used in the fields of communication, material processing, medical, light display and so on.Generally, pulses with pulse durations of picoseconds and femtoseconds can be called as ultrashort pulses. The ultrashort pulses have the advantages of ultrashotr pulse duartion, high peak power, and broad spectrum and so on. Mode-locking technology is an efficient way to generate ultrashort pulses. Mode-locking technology can be divided into active mode-locking and passive mode-locking and the latter one can genetate much shorter pulse using saturable absorbers. In the recent20years, semiconductor lasers for optical pumping and optical saturable absorbers have dramatically improved these mode-locking lasers which have extremely short pulse durations (shorter than10fs), extremely high peak power (with amplifer, PW) and extremely high pulse repetition rates (greater than100GHz). Today, saturable absorbers used in passively mode-locked laser include Semiconductor saturable absorber mirror (SESAM), single-walled carbon nanometer tube (SWCNT), and Graphene. Compared to SESAM, SWCNT and Graphene have the advantages of broadband absorption, short recovery time, and easy fabrication.Double tungastates (KRE(WO4)2,RE=Gd, Lu and Y) crystals have been proposed to be ideal laser crystals with excellent properties of physics and chemistry. The high efficient laser operations have been obtained using rare earth ions doped KRE(WO4)2crystals. At the same time, KRE(WO4)2crystals belong to monoclinic system, space group C2/c. Due to their low symmetry, this kind of crystals have broader emission spectra, which are beneficial for mode-lcoking. Disordered crystals attract more and more attention as hosts for rare-earth ions in mode-locked lasers, because inhomogeneous broadening provides broader spectral emission than common crystals while compared to the even more broadband laser glasses, they exhibit higher thermal conductivity. Calcium-lithium-niobium-gallium garnet (CLNGG) is one of the typical disordered crystals.In this thesis, the work is focusd on novel passively mode-locked lasers. During experiments, we achieved some passively mode-locked lasers operating at different wavnlengths, using SESAM and SWCNT as saturable absorbers. Around1μm, Nd:K.Lu(WO4)2crystal and Yb:CLNGG crystal were used as the laser gaim medium; around1.5μm, a Cr:YAG crystal was used as the laser gain medium; around2μm, a Tm,Ho:(WO4)2crystal was used as the laser gain medium. Different SWCNTs and SESAMs with different paramaters were used to obtain mode-locking, and analyze the influence of the parameters of the saturable absorbers on mode-locking performance. We studied the characteristics of the passively mode-locked lasers and explained theses phenomena. We also studied the self mode-locking phenomenon in passively Q-swtiched Raman lasers.The main contents of this thesis are as follows:1. The characteristics of diode-pumped CW and passively mode-locked Nd:KLu(WO4)2lasers were studied.3.3W of1070.2nm CW laser was obtained with an absorbed pump power of4.4W. A Z-type cavity was used to archieve the passively mode-locked laser using SESAM as the saturable absorber. The maximum output power was1.4W, corresponding to a single pulse energy of6.8nJ. The pulse duration was11.5ps, corresponding to a peak power of0.6kW. During experiment, we obtained the phenomenon of pulse splitting at high pump power.2. The characteristics of a diode-pumped passively mode-locked Yb:CLNGG laser were studied. When the SWCNT was used as the saturable absorber, the minmum pulse duration of90.2fs was obtained, using an output coupler with a transmission of0.4%. When the transmission of the output coupler was3.0%, a maximum output power of90.5mW was obtained with a pulse duration of137.3fs. Appearance of CW component in the spectrum was the limit to obtain higher output power. When a SESAM was used as the saturable absorber, the shortest pulse duration of55.6fs was obtained, using an output coupler with a transmission of1.0%. When the transmission of the output coupler was3.0%, the maximum output power of150.5mW was obtained with a pulse duration of115.6fs. 3. The characteristics of a passively mode-locked Cr:YAG, using SWCNT as the saturable absorber, were studied. A pulse duration as short as49.6fs was obtained at1509.4nm. We studied the different performances of passively mode-locked lasers at different pump powers and found the optimum pump power for mode-lokcing. We used Frequency-Resolved Optical Gating (FROG) technology to measure the dispersion in the cavity.4. The characteristics of a Ti:sapphire laser pumped passively mode-locked Tm,Ho:KLu(WO4)2were studied. When SWCNT was used as the saturable absorber, a pulse duration as short as2.4ps was obtained. The spectrum centred at2058.9nm with a width of1.7nm, corresponding to a time-bandwidth product of0.3. When the transmission of the output couple was3.0%, the maximum output power of110.7mW was obtained. A pulse duration of7.2ps was obtained using SESAM as the satuarble absorber. During the experiment, we tested different saturable absorbers with different parameters and changed the dispersion in order to get fs pulses, unfortunately, we cound not get fs pulses.5. The self mode-locking phenomenon in all-solid-state Raman lasers was demonstrated. A intracavity passively Q-switched Nd:YAG/SrWO4/Cr:YAG Raman laser was used to study the phenomenon of self mode-locking. The self mode-locking phenomena with different cavity lengths, different small signal transmissions and different pump powers were studied. When the cavity length was longer than50cm, stable self mode-locking of Raman laser can be obtained. Self mode-locking phenomenon with100%modulation depth can be obtained with different pump powers and different small signal transmissions.The main innovations of this thesis are as follows:1. The characteristics of diode-pumped CW and passively mode-locked Nd:KLu(WO4)2lasers were studied. A pulse duration as short as11.4ps was obtained with a SESAM as the saturable absorber. This is the first report about passively mode-locked Nd:KLu(WO4)2laser. Our experiment demonstrated this kind of crystal is a candidate to get high power mode-locked laser with ps pulse duration.2. Passively mode-locked Yb:CLNGG laser, using SWCNT as the saturable absorber, was demonstrated for the first time. A pulse duration as short as90.2fs was obtained with a0.4%output coupler. This is the shortest pulses to our knowledge for a diode-pumped1-μm laser using transmissive SWCNT.3. A passively mode-locked Yb:CLNGG laser, using SESAM as the saturable absorber, was demonstrated for the first time. A pulse duration as short as55.6fs was obtaind with output power of60mW. This is the shortest pulses to our knowledge for Yb doped isotropic crystals.4. The characteristics of a Thsapphire laser pumped passively mode-lcocked Tm,Ho:KLu(WO4)2laser were demonstrated for the first time. Pulse durations as short as2.4ps and7.2ps were obtained, using SWCNT and SESAM as saturable absorbers respectively.5. The mode-locking phenomenon in passively Q-swtitched Raman laser was studied for the first time. Our experiment demonstrated that there was no obvious influence of small signal transmission and pump power on self mode-locking phenomenon.更多还原