Enhanced performance of thin-disk lasers by pumping into the zero-phonon line

Pumping Yb:YAG or Yb:LuAG into the zero-phonon line at 969?nm instead of using the common pump wavelength of 940?nm reduces the heat generation by 32%. In addition to the 3% increase of the Stokes efficiency, this significantly reduces the diffraction losses caused by the thermally induced phase distortions leading to a remarkable increase of the overall efficiency especially of fundamental-mode thin-disk lasers. Using this pumping scheme in an Yb:LuAG thin-disk laser, we achieved 742?W of nearly diffraction limited (M²≈1.5) output power at an unprecedented high optical efficiency of 58.5%. For multimode operation (M²≈15) the maximum optical efficiency of an Yb:YAG thin-disk laser was increased to 72%.     

大口径谐振腔式固体激光中的动力学不稳定性研究

大口径谐振腔式固体激光定标到足够高功率后,由于非稳定腔内激光不对称振荡导致介质上激光强度横向不均匀,激光提取与介质产热耦合会引起激光输出功率和光束质量在时域上的不稳定性。针对三种激光器构型:Nd:YAG薄片、Yb:YAG薄片和浸入式液冷叠片激光器,通过理论分析与数值模拟揭示了其各自不同的光热耦合机制及其影响因素,给出了激光输出的瞬态演化规律。结果表明,前两种构型中基于激光动力学的光热耦合具有饱和效应,其动力学不稳定性只出现在特定参数区间,可以通过恰当的设计加以避免;第三种构型中动力学不稳定性表现出明显的阈值特征,在强光状态下只能通过减少冷却液吸收系数来抑制。     

Multilayer polarizing grating mirror used for the generation of radial polarization in Yb:YAG thin-disk lasers

The design, fabrication and characterization of a multilayer polarizing grating mirror developed for an Yb:YAG thin-disk laser resonator are reported. The potential of the proposed solution is discussed together with the first demonstration of a radially polarized Yb:YAG thin-disk laser.     

Bulk crystal growth and efficient diode-pumped laser performance of Yb3+:Sc2SiO5

A bulk crystal of Yb:Sc₂SiO₅ (Yb:SSO) with favorable thermal properties was successfully obtained by the Czochralski method. The energy level diagrams for Yb:SSO crystal were determined by optical spectroscopic analysis and semi-empirical crystal-field calculations using the simple overlap model. The full width at half maximum of the absorption band centering at 976 nm was calculated to be 24 nm with a peak absorption cross-section of 9.2×10^-21 cm². The largest ground-state splitting of Yb³⁺ ions is up to 1027 cm^-1 in a SSO crystal host. Efficient diode-pumped laser performance of Yb:SSO was primarily demonstrated with a slope efficiency of 45% and output power of 3.55 W.     

Passively mode-locked Yb:KLu(WO4)2 thin-disk oscillator operated in the positive and negative dispersion regime

We demonstrate a passively mode-locked femtosecond Yb:KLu(WO(4))(2) thin-disk laser oscillator. Chirped-pulse operation in the positive dispersion regime as well as solitary operation have been realized, and the laser performance of both configurations are compared. In the solitary mode-locking regime the output power exceeds 25 W in a diffraction-limited beam, and pulse durations as short as 440 fs at a 34.7 MHz repetition rate have been generated. For the first time we present a chirped-pulse operation of a thin-disk oscillator that yields a maximum average output power of 9.5 W with a Fourier limit of 450 fs.     

Multi-wavelength mode-locked Er/Yb Co-doped fiber laser with square nano-second pulse output

We report a multi-wavelength mode-locked Er/Yb codoped fiber laser with square nano-second pulse output, which is constructed with a “figure-of-eight” cavity structure. A section of polarization-maintaining Er/Yb co-doped double clad fiber was used as the gain medium and pumped by a fiber pigtailed multimode laser diode working at 975 nm via a multimode fiber combiner. Several megahertz repetition rate output pulses with peak power up to 9.2 W and pulse duration around several nanoseconds were obtained.     

Observation of spectral modulation coupled with broadband transverse-mode-locking in an Yb:CALGO frequency-degenerate cavity

A frequency-degenerate cavity(FDC) is the resonator that the ratio of transverse and longitudinal mode frequency spacings is a simple rational number. When an optical resonator is close to the FDC, transverse-mode-locking(TML) takes place with drastic changes of laser mode. We report for the first time, to the best of our knowledge,the multi-frequency emission and spectral modulation effects coupled with TML in FDC. The Yb:CaGdAlO_4(Yb:CALGO) crystal with large gain bandwidth was used as a gain medium in an off-axis-pumped hemispherical FDC for realizing broadband emission. Interestingly, the spectrum can transform from a single smooth packet shape to a multi-peak structure; meanwhile, the transverse pattern accordingly transforms into some exotic wave-packet profiles through controlling off-axis displacement in a special degenerate state.     

Generation of 15 W femtosecond laser pulse from a Kerr-lens mode-locked Yb:YAG thin-disk oscillator

We demonstrated a robust power-scalable Kerr-lens mode-locked(KLM) operation based on a Yb:YAG thin-disk oscillator.15-W,272-fs pulses were realized at a repetition rate of 86.7 MHz with an additional Kerr medium and a 2.5 mm hard aperture in the cavity.247-fs pulses with an average power of 11 W could also be obtained by using a 2.4 mm hard aperture.Based on this shorter pulse,high efficient second-harmonic generation(SHG) was performed with a 1.7-mm-long Li B3O5(LBO) crystal.The SHG laser power was up to 5 W with the power fluctuation RMS of 1% measured over one hour.     

High-power 200 fs Kerr-lens mode-locked Yb:YAG thin-disk oscillator

We demonstrate a power-scalable Kerr-lens mode-locked Yb:YAG thin-disk oscillator. It delivers 200 fs pulses at an average power of 17 W and a repetition rate of 40 MHz. At an increased (180 W) pump power level, the laser produces 270 fs 1.1 μJ pulses at an average power of 45 W (optical-to-optical efficiency of 25%). Semiconductor-saturable-absorber-mirror-assisted Kerr-lens mode locking (KLM) and pure KLM with a hard aperture show similar performance. To our knowledge, these are the shortest pulses achieved from a mode-locked Yb:YAG disk oscillator and this is the first demonstration of a Kerr-lens mode-locked thin-disk laser.     

Yb:CaGdAlO4 thin-disk laser

We present the first demonstration of a Yb:CALGO thin-disk laser. In a slightly multimode configuration, we obtained up to 30?W of average power at a slope efficiency of 40% and an optical-to-optical efficiency of 32%. With a single-mode cavity, an average power of 25?W was achieved. A tuning range from 1018 to 1052?nm could be demonstrated by inserting a prism into the cavity. In the Q-switched regime, we obtained 1?mJ of pulse energy at a repetition rate of 100?Hz.     

Study of the optical bistability in the laser oscillation of Yb:GdVO4 crystal

We have studied the effect of optical bistability exhibited in the laser oscillation of Yb:GdVO₄ crystal, revealing the complexities arising from the coexistence and switching of σ and π polarization states characteristic of this crystal. In terms of absorbed pump power, the range for bistable operation can be in excess of 1 W, while the output power generated at the up-threshold can reach as high as 0.71 W. The studies also show significant influence of the Yb concentration, the crystal thickness, the output coupling, and the resonator configuration on the bistable laser operation.     

两种国产新型掺Yb氟化物激光晶体的光谱特性研究

掺Yb氟化物激光材料是继掺Yb氧化物激光材料之后的另一类重要的掺Yb激光材料,已经成为可调谐激光和超快激光领域中研究的热点之一。针对两种国产新型掺Yb的氟化物激光材料：混晶材料Yb∶CaF₂-SrF₂和共掺离子型的单晶材料Yb, Y∶CaF₂,进行了详细的光谱特性比对实验研究。通过荧光比对实验,发现这两类激光材料的荧光光谱完全不同,并分析了不同荧光产生的物理机制。通过吸收率比对实验,讨论了两类材料中的激活离子Yb或共掺离子Y的掺杂浓度对晶体吸收特性的影响,得到了最佳掺Yb离子或共掺Y离子的浓度。利用激光二极管作为泵浦源,实现了这两类新型材料在折叠腔型下的连续激光输出运转,其中对于共掺离子型Yb, Y∶CaF₂晶体是首次实现连续激光运转。通过激光对比实验,获得了两类激光材料(四种样品)的激光输入—输出关系曲线,测量了各自的斜效率和激光光谱特征。通过系统地比较两类激光晶体的吸收率、荧光光谱特性、激光光谱特性以及连续激光运转的阈值功率和斜效率等参数得出以下结论：在四种实验样品中,共掺离子型单晶材料中的3at%Yb, 6at% Y∶CaF₂晶体具有最好的光谱和激光特性,具有良好的应用前景。这些实验结果为进一步提升此类激光材料的性能提供了有益的参考。     

增益介质位置对腔外倍频效率的影响

采用MgO掺杂的周期性极化铌酸锂晶体作为非线性晶体,搭建了线性平-凹腔结构的外腔式倍频系统,分析了增益介质和输入耦合镜之间的距离对二次谐波转换效率的影响。实验中,使用光纤耦合二极管泵浦Nd: YVO₄激光器作为基频光源,通过改变增益介质和输入耦合镜的间距,测量了激光器的纵模数量、锁模脉冲稳定性、基频光线宽以及光-光转换效率。实验结果显示,增加增益介质和输入耦合镜之间的距离,可以在一定程度上提升自锁模脉冲的稳定性并有效减小基频光线宽。当泵浦功率较高时,适当增加该间距可以有效提升二次谐波转换效率。     

LDA侧面Zigzag泵浦多边形薄片激光放大器模拟与实验研究

介绍了一种激光二极管叠阵（LDA）侧面Zigzag泵浦多边形薄片激光放大器构型，采用三维光线追迹方法进行了详细的模拟仿真，优化设计了此放大器系统的泵浦耦合结构，主要研究了多边形增益介质的掺杂离子浓度与侧面切角对介质内部泵浦光分布的影响。在晶体厚度1.5 mm、端面口径16 mm的条件下，侧面切角在35°～65°，Nd3+掺杂浓度为0.20 at.%～0.30 at.%时，模拟仿真中Nd:YAG多边形薄片对泵浦光的吸收分布较均匀，泵浦光分布均匀性均优于0.1，同时在实验中得到了平顶的荧光分布和增益分布。介质内储能的均匀平顶分布有利于实现高功率高光束质量的激光输出，为侧面Zigzag泵浦多边形薄片激光器系统的设计与进一步实验提供了重要参考。     

CW laser at 484 nm based on frequency-doubled diode-pumped Yb:YAG crystal

We report for the first time, to the best of our knowledge, a Yb:YAG laser operating in a CW on the three-level laser at 968 nm, based on the ² F _5/2-² F _7/2 transition, generally used for a 1030 nm emission. The use of a pump module with 16 passes through the crystal allowed the realization of a Yb:YAG thin-disk laser with 507 mW of continuous wave (CW) output power at 968 nm. Moreover, intracavity second-harmonic generation (SHG) has also been achieved with a power of 58 mW at 484 nm by using a BiB₃O₆ (BIBO) non-linear crystal.     

Review of collisional excitation neon-like and nickel-like soft X-ray lasers pumped by multiple infrared laser pulses at the Institute of Laser Engineering,Osaka University

We report on the soft X-ray laser developments performed at the Institute of Laser Engineering, Osaka University especially for the collisional excitation X-ray lasers at wavelengths between 25 nm and 4 nm. The performance of neon-like and nickel-like ion X-ray lasing pumped by a train of short laser pulses has been investigated. We have also demonstrated double pass amplification using a soft X-ray multi-layer mirror at wavelengths of 19.6 nm and 7.9 nm. Based on these results and experimental technique, two targets were placed in series to double the gain length. Two opposing laser beams irradiated the double targets with a suitable time difference for quasi-traveling wave pumping. The double target amplification was successfully demonstrated with two beam irradiation for Ag, Nd, Yb, Hf and Ta lasing. The estimated absolute photon flux from the saturated amplification Ag X-ray laser was ∼300 μJ. Based on the experimental results, we also discuss the applications of an X-ray laser as a super-high brightness soft X-ray source.     

A model describing the single and multiple line spectra of tunable microcrystal lasers

A theoretical model is derived by which the occurrence of the single and multiple emission line spectra of tunable microcrystal lasers is described. These spectra in general exhibit several emission lines, arising from the Stark-split atomic levels of the lasing ions in the crystal field. The model is based on the shift of the resonator frequencies due to the thermally induced shift of the optical resonator length and demonstrates that the coincidence of the resonator frequencies with the laser gain lines leads to the emission of single or multiple line spectra of microcrystal lasers. These spectra can be described by the model. The model is given as a set of criteria. In this way not only can predictions of the single emission-line tuning range be made but also resonator lengths can be optimized in order to obtain a maximum tuning range. Furthermore, Q-switched operation can be achieved for specific parameters by periodically shifting the resonator frequencies. The linewidth of the gain used in this model depends on the laser threshold and is folded with the thermal shift of the atomic transitions. Therefore the centre wavelength of the gain is assumed to be constant. The advantage is that this experimentally relevant linewidth can be measured easily with microcrystal lasers themselves, whereas spectroscopic data do not take laser threshold behaviour into account. It is shown that the results of the model are in good agreement with experimental data measured for two different Nd: YAG crystals. Simply by inserting other material and laser parameters, the model can easily be applied to other laser crystals and other wavelengths.     

High-efficiency 1.9 µm Tm3+:LiLuF4 thin-disk laser

We present the first demonstration of a 792 nm diode-pumped Tm3+:LiLuF4 thin-disk laser operation around 1.9 μm. In multimode configuration, up to 21 W of output power and a maximal slope efficiency of 49% with an optical-to-optical efficiency of 40% was demonstrated. A tuning range from 1899 nm to 1927 nm could be achieved by inserting an etalon into the cavity.     

Ultrafast laser performance of Yb3 +: Sc2SiO5 crystal with a single-walled carbon nanotube absorber

Single-walled carbon nanotube (SWCNT) absorber fabricated by vertical evaporation is used in passively mode-locked Yb^3 +:Sc₂SiO₅ (Yb:SSO) ultrafast laser for the first time. The performance of Yb:SSO ultrafast laser with pulse width as short as 880 fs is studied and the average output power is 712 mW. To our knowledge, this is the highest output power of femtosecond lasers with SWCNT-SAs reported. In addition, we firstly demonstrate a passively mode-locked picosecond Yb:SSO laser without inserting any dispersion compensation device. The pulses width is as short as 5.4 ps and the output power is 940 mW.     

Erbium-ytterbium co-doped multi-wavelength double-clad fiber laser around 1612 nm

We report an erbium-ytterbium (Er/Yb) co-doped multi-wavelength laser operation around 1612 nm. The fiber laser was constructed in a figure-of-eight configuration and using a piece of polarization-maintaining (PM) Er/Yb double clad fiber as the gain medium. The oscillation laser lines around 1612 nm could be controlled by carefully adjusting the polarization controllers in the cavity, which might result in single line, dual line or triple line fiber laser operation. The line space was measured about 3.3 nm (±0.2 nm) which was believed to be strongly dependent on the length of the PM Er/Yb co-doped double-clad fiber.