准分子激光振荡一放大系统中光束发散角分析

从光学谐振腔中近衍射极限模式的建立角度出发,分析了准分子激光振荡──放大系统的输出光束发散角,并讨论了与之相关的激光腔工作参数．     

氢气中的超宽带拉曼脉冲压缩

对氢气中的超宽带拉曼脉冲压缩进行了实验研究，利用工作在阈值附近的拉曼振荡器，当准分子激光抽运源的脉冲宽度为２３ ｎｓ时，得到了６ ｎｓ的拉曼种子光，并采用反向拉曼放大得到了放大倍数近３ 倍的放大拉曼光。并就实验中的气压和气体掺杂对实验结果的影响进行了讨论。     

425mJ高光束质量特殊取向Nd:YAG激光放大器

基于主振荡功率放大结构,采用特殊取向Nd:YAG激光放大器,获得高脉冲能量、高光束质量的激光输出。激光放大结构包含种子源、预放大级和主放大级三部分。在主放大级中,采用串联放置的激光二极管侧面抽运Nd:YAG棒状放大模块对种子光进行放大。为了获得高光束质量的输出光束,对不同切割方向Nd:YAG晶体棒的热退偏损耗进行了模拟。根据模拟结果,放大模块选择[100]切割方向的Nd:YAG晶体棒作为增益介质。在重复频率为200Hz、脉宽为25ns、脉冲能量为40μJ、光束质量接近衍射极限的种子光注入条件下,获得了425mJ脉冲能量输出,输出光光束质量因子为1.37,功率稳定度为0.81%。     

双光栅非相干组束系统单光束耦合效率分析

为了保证组束激光的光束质量,在非相干光纤激光组束系统中再引入一个与之平行的衍射光栅.这种双光栅非相干组束系统的单光束耦合效率是单程耦合腔效率和光栅衍射效率的合成,通过理论分析和数值计算,结果表明在设定的组束参数下双光栅非相干组束系统的单光束耦合效率可以达到75.9%左右.     

光纤激光器外腔型光谱组束研究

实验研究了两路光纤激光器外腔反馈型光谱组束(SBC),采用透射式衍射光栅作为组束元件,实现了1060nm波长激光与1080nm波长激光的光谱组束输出。输出光束组束方向光束质量为1.328,非组束方向光束质量为1.257,组束输出功率为57.3 W,光-光效率为91.7%。采用该组束方案可以降低单路激光器线宽的要求,同时验证了多路激光进行组束的可行性,通过增加组束光束个数可以进一步提高组束输出功率。     

高能飞秒激光光束锯齿光阑截趾整形

针对高能飞秒固体激光器光强分布光滑均匀的要求，研究了飞秒光束通过锯齿光阑和空间滤波器的传输特性。和单色激光的衍射不同，宽带的飞秒光束衍射调制较小，直接利用锯齿光阑，便能获得近场光强分布相对均匀的光束。     

高重复率高峰值功率高光束质量Nd∶YAG倍频激光器

研究设计了高效均匀抽运、均匀冷却的聚光腔;采用双YAG棒中间加90°旋光器,输出镜采用特制的变反射率介质膜与全反镜构成非稳定谐振腔,使振荡级在较高输出能量时获得近衍射极限的激光输出;放大级采用光学性能一致的YAG棒("姊妹棒")中间加90°旋光器构成减少热退偏的放大单元,经过两个放大单元及相应的级间隔离光学扩束、匹配等减小光束畸变,获得了高重复频率、高光束质量、高峰值功率激光输出.其性能指标为: 1) 输出峰值功率:λ1=1064 nm,P1=388 MW/p;λ2=532 nm,P2=158 MW/p; 2) 脉冲宽度:τ1=8 ns (λ1=1064 nm),τ2=6.8 ns(λ2=532 nm); 3) 光束质量:M2=1.62(1.6倍衍射极限); 4) 重复频率:f=61 Hz; 5) 连续工作时间:t=20 min.(OC28)     

相位共轭镜用作光隔离器

在Nd:YAG高增益激光放大系统中,采用相位共轭镜兼作光隔离器,对1uJ、22ns的单频脉冲激光种子,经六次放大,最终获得了大于100mJ、近衍射极限的单频激光输出。     

近衍射极限半导体激光束波面检测

在星间半导体激光通信系统中，如何检测发射光束波面的质量是个较难处理的问题，为了较好地解决这一问题，在简单介绍白光横向双剪切干涉仪的基础上，报道了用此干涉仪对近衍射极限半导体激光光束波面的检测，在此基础上推导出计算远场发散度的公式。实验测得近场光束的波高差为0．2A，通过夫朗和费衍射求得光束的发散度仅为64．8μrad，这表明光束接近光学衍射极限。同时，表明双剪切干涉仪灵敏度高、实用性好。     

新产品

蓝激光光源　　ＢｌｕｅＳｋｙＲｅｓｅａｒｃｈ公司将已获专利的微透镜技术与蓝激光二极管结合 ,实现了能够产生高功率、低发散、衍射极限光束的蓝激光光源。将微透镜直接安装到二极管发光边缘的正面 ,可获得近1 0 0 %的二极管能量。仅用一个步骤即可使光束呈环状 ,并能纠正任意偏差。这种技术无须使用多元光学件、棱镜对或强光束孔径。这种蓝激光光源可应用于复印、光学数据存储和分析仪器中。(Ｎｏ .34)在线偏振器　　ＧｅｎｅｒａｌＰｈｏｔｏｎｉｃｓ公司设计的在线偏振器可用于偏振模色散补偿和测量、偏振稳定化、分复用、偏振分析和…     

Shallow angle beam combining using a broad-band XeF laser

Experiments in which two xenon fluoride laser pump beams (353 nm) were crossed at an angle of 2.5 mrad in a 565-cm-long Raman amplifier containing high-pressure (10-atm) hydrogen are described. Bisecting the angle made by these two pump beams was a seed beam at the vibrational Stokes shifted wavelength of 414 nm. This seed beam was generated from a portion of the pump beam that was split off and brought to focus in a separate Raman seed generator. All three beam paths were adjusted so they arrived in the main Raman amplifier at the same time. For the 1.4-Å (FWHM) xenon fluoride pump radiation this required path offset precision to within 0.2 mm over many meters of optical path. The beam quality of the resulting amplified Stokes beam was determined through shearing interferometry techniques to be near the diffraction limit, whereas the pump beams had significant optical distortion     

Raman beam cleanup of a severely aberrated pump laser

Distortion-free amplification of a diffraction-limited (D.L.) Stokes beam in a hydrogen Raman amplifier pumped by a severely aberrated XeCl laser (120 × D.L.) has been observed with an attendant power conversion efficiency of the order of 30 percent. The corresponding increase in available far field intensity over that from the aberrated pump beam is 5000. An optical integrator was used to focus the poor quality pump beam into the amplifier and to remove all near-axial components in the pump field. Numerical study of this process using a two-dimensional propagation code shows that the presence of near-axial pump components can cause phase matched four-wave mixing interactions with the Stokes, leading to increased angular divergence of the amplified Stokes beam and the development of secondary sidebands in the far field. When a moderately aberrated pump beam (20 × D.L.) was used, spatial sidebands of the Stokes beam were generated due to increased coherence length for the mixing process, significantly reducing the far field Stokes intensity.     

Steady-state coherent Raman beam combining with multiaxial mode lasers

Forward-stimulated Raman scattering can be used to combine multiple broad-band pump beams into a single coherent Stokes wave. In this paper, the equations of motion for the Stokes electric field are derived in the presence of multiple pump waves propagating at slightly different angles with respect to the Stokes propagation direction. The solutions with pump depletion are presented for fully coherent pump waves. It is shown that the Raman gain is lower for incoherent pump waves than for coherent pump waves. Stokes field solutions are used to evaluate the far-field intensity of the Stokes output from a saturated Raman beam combiner. It is shown that the condition for coherent combination depends on the beam diameter, pump coherence length, and the angle between the pump and the Stokes. It is also shown that multiaxial mode Raman shifting can generate Stokes waves with higher beam quality than the pump waves, both with phase perturbed or highly distorted pump beams.     

Phase front reproduction in Raman conversion

The Raman gain coefficient for nonplanar pump wave is calculated. The gain dependence for wavefront reproducing Stokes conversion and for an injected plane Stokes wave on the pump beam quality is derived. The amount of plane Stokes wave injection required to generate a plane Stokes wave output is discussed. A comparison is given for the homogeneity requirement of the pump laser medium as reflected in the pump beam quality for the cases with and without Raman conversion for near diffraction limited output. The results indicate that with the appropriate plane Stokes wave injection, the medium homogeneity requirement of the laser can be relaxed.     

Closed-form solution for parametric second Stokes generation inRaman amplifiers

The efficiency of a first Stokes Raman amplifier can be seriously reduced by the generation of a second Stokes beam. The dominating process for this generation is often a Raman four-wave mixing interaction which couples the pump and second Stokes beams. A one-dimensional model for this effect has a closed-form solution, simply described in terms of an equivalent input, under conditions where high first Stokes conversion efficiency is possible. Excellent agreement with numerical integration results has been obtained. Use of the model for maximizing conversion efficiency of pump beams with large temporal or spatial intensity nonuniformity is described, with an example worked out for a triangular pulse shape     

The stimulated Raman scattering threshold for anondiffraction-limited pump beam

The threshold power of stimulated Raman scattering (SRS) has been calculated for various nondiffraction-limited pump beams using a numerical model. These calculations were performed for the following pump beam profiles: Gaussian-Hermite, Gaussian-Laguerre, Gaussian-Schell-model (GSM), and several superpositions of Gaussian-Hermite modes. It is shown that the threshold power, for a tightly focused beam, depends on the M² factor, which is a dimensionless factor that compares the divergence of a given beam with the divergence of a (diffraction-limited) Gaussian beam, related to the pump beam quality. The influence of the exact pump beam profile on the threshold is small. It is shown that the measured SRS thresholds in methane for two pump beams, one with an M² of 1.12 and the other with M²=8.7, correspond with the calculated thresholds for two GSM beams with the same M²     

Transfer of pump spatial variations to Stokes phase in Raman amplifiers

Many applications of forward Raman amplifiers require that transverse spatial amplitude and phase aberrations in the pump beam not be transferred to the phase of the amplified Stokes signal. Detailed one-dimensional depleted-pump calculation are given for two mechanisms that transfer pump aberrations to the Stokes phase. The first is multilongitudinal mode pump fluctuations that vary faster than the coherence time of the Raman transition. The second is saturated dispersion combined with a detuning either due to Raman operation off of line-center or due to the dynamic Stark effect.     

Studies of the spatial distribution and beam quality of Stokesoutput from a Raman generator pumped by a broad-band KrF laser

One-dimensional stimulated Raman scattering (SRS) theory is extended to three dimensions by the concept of transverse modes associated with the Fresnel number. Two simplified models are introduced to evaluate the beam parameters and waist position under low gain. The dependence of waist size and position of the Stokes beam on the focus position and Rayleigh length of the focused pump laser is analyzed and verified by experiment. The Fresnel number approach is utilized to evaluate the energy density distribution and the mode structure of the pump laser in a Raman cell under high gain. The different configurations are classified and relevant mode formulation and spatial distribution are discussed. The experimental result shows that the beam quality of the Stokes output from a Raman generator depends essentially on the pump configuration and the geometric parameters of the pump laser and is insensitive to the practical pump power, which agrees with the theoretical analysis. The results obtained are adopted to improve the beam quality of the first-order Stokes output from the Raman generator with different configurations     

Stimulated Raman forward scattering with a divergent or convergent pump beam

Stimulated Raman forward scattering is investigated for a divergent or convergent pump beam. The rate equations for this case are derived and analytically solved, including pump depletion. The solutions are compared to an experiment where the generation of a Stokes pulse from noise is investigated with a pump beam of variable divergence. Theoretical and experimental results are in good agreement. Stimulated Raman forward scattering thus may be controlled by the beam geometry, e.g., in Raman pulse compressors. An example is discussed in the Appendix.