光参量振荡器阈值的研究

为获得光参量振荡器（OPO）的高效率运转,尤其是在小功率泵浦的情况下,降低它的阈值成为研究的重要内容。通过分析影响OPO阈值的因素,得到了通过以下手段来降低阈值的方法：选择有效非线性系数大的晶体,选择满足走离条件下较长的晶体,增大泵浦光与信号光模式匹配因子,双程泵浦,短的OPO的腔长和泵浦光脉冲,降低信号光在腔内往返损耗,选择合适的输出镜强度反射率等。并重点对影响阈值因素中的泵浦光与信号光模式匹配因子进行了研究,得到了双凹腔结构腔内任意位置的模式匹配计算的一般方法,当腔镜曲率半径趋于无穷时,该结果可以过渡到平平腔或平凹腔的特殊结构,并在实验上实现了小功率腔内OPO的运转。     

The tandem optical parametric oscillator

We report the theoretical analysis of a tandem optical parametric oscillator (TOPO) in which the principal OPO generates a signal which pumps a secondary OPO. The signal produced by the principal OPO is resonated in a high-Q cavity containing both nonlinear crystals. This cavity also has low Q for the signal of the secondary OPO. The two OPO's are singly resonant, since neither idler is resonated. We show that the useful output, consisting of the two idlers and the signal of the secondary OPO, can be produced with high efficiency over a large dynamic range in the pump intensity. As an example, we discuss modeling in three space dimensions of a Nd:YAG-pumped monolithic TOPO based on periodically poled LiNbO₃. This TOPO could generate three infrared wavelengths in high-transmission bands of atmospheric water vapor     

角锥棱镜腔Cr4+:YAG被动调Q内腔式光参量振荡器

通过对角锥棱镜腔实现KTPⅡ类非临界相位匹配光参量振荡的理论分析,发现腔内存在特殊偏振方位角的本征模式,其满足Ⅱ类非临界相位匹配条件。实验上不但实现了该腔的Cr4 ∶YAG被动调Q内腔式光参量振荡器(OPO)运转,而且得到了优于平-平腔的转换效率,改善了信号光的稳定性。分析了Cr4 ∶YAG初始透过率和腔内功率密度的关系,实验获得不同Cr4 ∶YAG初始透过率、抽运能量和信号光能量的关系曲线。通过腔内功率密度的优化,电光效率高达0.25%,设计出结构紧凑、小型高效的工程实用光参量振荡器器件,抽运能量阈值为4.7 J,输出能量为11.8 mJ,脉宽为6.9 ns,发散角为7 mrad,能量稳定性为5%。     

声光调Q DPL脉冲宽度空间分布的研究

研究了在声光调Q二极管泵浦固体激光器（DPL）中,光斑不同部位的脉冲宽度的空间分布.由多模振荡引起脉冲宽度在空间分布的不均匀,导致脉冲宽度的展宽.以声光调Q DPL为例进行分析,通过数值计算,得出了在一定的泵浦速率和重复率下,TEM00模与TEM01模两种模式的脉冲建立时间及脉冲宽度的空间分布与上能级反转粒子数的关系.由计算结果得出：若调节二极管泵浦固体激光器中抽运光的空间分布能够使TEM00模和TEM01模在脉宽接近时同时起振,对低阶模和高阶模进行模式合成,从而减小由于多模振荡引起的脉冲宽度的展宽.该结果有助于调Q DPL的脉宽压缩和模式合成.     

2μmTm,Ho∶YLF激光抽运ZnGeP_2光参量振荡技术研究

ZnGeP2晶体具有宽的透明范围(0.7～12μm),较大的非线性系数(d36=75pm/V),最高损伤阈值能量密 度为10J/cm2,较高的热导率(0.18W/(m·K)),因而非常适合作为高功率中红外光参量振荡器(OPO)晶体。理 论上分析了ZnGeP2光参量振荡器相位匹配特性,实现3～5μm连续调谐范围输出的Ⅰ类相位匹配角在52.5～ 55.2°之间。实验上,以15W光纤耦合激光二极管(LD)抽运的2.05μm高重复频率声光调QTm,Ho∶YLF激光 器作为抽运源,其最大平均功率4W,脉冲宽度小于40ns,脉冲重复频率100Hz～10kHz可调。为降低准三能级 系统激光器阈值,提高激光脉冲能量抽取效率,Tm,Ho∶YLF晶体采用液氮制冷方式,工作在77K温度条件下。 非线性频率转换晶体ZnGeP2长15mm,55.7°切割,光参量振荡器谐振腔为平平腔,腔长约20mm。在3.6W的抽 运功率下,脉冲重复频率10kHz,实现了4.1μm附近中红外激光输出,参量光脉冲宽度为20ns,平均输出功率为 0.7W,光 光转换效率为20%,抽运光阈值功率为0.65W。     

基于高品质PPMgLN器件的中红外光参量振荡器

介绍了一种基于高品质周期畴极化反转掺镁铌酸锂晶体的高效可调谐中红外固体激光器——光参量振荡器的研制情况。该晶体长40 mm、厚1 mm,采用高压电脉冲触发技术制作,其中氧化镁掺杂浓度为6 mol％。光参量振荡器采用双向单共振结构,泵浦源为工作波长在1.064 μm、线偏振的声光调Q掺钕钒酸钇激光器。当输入泵浦功率为10.6 W时,光参量振荡器输出功率为4.8 W,信号光的调谐范围为1.4~1.8 μm,闲散光的调谐范围为2.7 ~4.4 μm,光光转换效率大于44％。由于该类激光器的极高转换效率和方便灵活的调谐能力,非常适合于作为中红外波段的激光光源,用于各类光电对抗、激光探空雷达、大气激光雷达等系统。     

基于内腔的调Q锁模光参量振荡器的研究

为了实现内腔型光参量振荡器（OPO）的调Q锁模脉冲输出,通过设计并匹配OPO谐振腔和基频激光腔的腔长,满足了同步泵浦条件,最终在实验上得到了信号光的调Q锁模输出;在实验中,采用氙灯泵浦Nd:YAG作为基频激光,以KTP晶体为非线性转换介质,采用电光开关作为调Q手段,测量了OPO的近红外信号光的输出波形、输出能量、光谱构成等输出特性。在泵浦能量12.8 J、调制频率20 kHz时,得到了锁模深度为100%的信号光输出,并发现信号光锁模脉冲重复率依赖于基频激光;得到了调Q锁模信号光输出能量随泵浦能量、电光调制频率的变化关系。     

KTP光学参量振荡器输出激光的空间模式和光束质量

理论上通过二维傅里叶变换求解耦合波方程 ,分析了KTP光学参量振荡器 (OPO)信号光的空间分布 ;实验上利用Nd∶YAG倍频激光 ( 5 3 2nm)抽运非临界 (θ =90°,φ=0°) 及临界相位匹配KTP (θ =62 7°,φ=0°) OPO ,测量了参量光的空间分布、远场发散角及M2 因子等参数 ,讨论了抽运功率、谐振腔长、残余光后向二次抽运对OPO参量光的发散角和光束质量因子M2 的影响。     

A simultaneously phase-matched tandem optical parametric oscillator

We report the theoretical analysis of a simultaneously phase-matched tandem optical parametric oscillator (STOPO) in which the principal OPO generates a signal which pumps a secondary OPO. Both OPO interactions are assumed to be phase-matched in a single crystal. The cavity has high Q for the signal produced by the principal OPO and has lower Q for the signal of the secondary OPO. The pump and the two idler frequencies are not resonated. The five waves undergoing nonlinear mixing are assumed to be nondegenerate in either frequency or polarization. We show that the useful output, consisting of the two idlers and the signal of the secondary OPO, can be produced with high efficiency over a large dynamic range in the pump intensity. We discuss the tuning with temperature and grating period of a STOPO based on periodically poled LiNbO₃ pumped at 1.064 μm. We present results based on both plane-wave modeling and modeling in three spatial dimensions     

掺镁铌酸锂微结构多周期调谐光学参量振荡器

从傅里叶展开三波耦合波方程出发,对准相位匹配光学参量振荡进行了初步的理论分析,同时对准相位匹配周期极化掺镁铌酸锂微结构光学参量振荡进行了实验研究。通过改变微结构周期,实现了信号光从1.45~1.72 μm的输出,最小阈值为30 μJ。在温度30 ℃,抽运功率为300 mW,最大信号光输出功率为56 mW,斜率效率达18.7%。由于掺镁铌酸锂微结构抗光损伤性能显著提高,无需在高温下进行运转,使得掺镁铌酸锂微结构光学参量振荡器在常温条件下实现连续运转成为可能。与同成份铌酸锂微结构参量振荡器相比,结构更加紧凑,易于实现小型化。     

振幅调制连续光产生高功率超短脉冲串的研究

高功率超短脉冲串需求广泛,但是有限的平均功率和能量的低利用率限制了超短脉冲串的应用。基于不同于锁模机制的非线性孤子效应,利用振幅调制连续光抽运,数值模拟了光纤中高功率超短脉冲串的稳定产生。分别研究各输入参数如抽运功率、调制深度、调制频率以及光纤参数包括非线性系数、群速度色散系数等对高功率超短脉冲串输出特性的影响规律。为了综合考量各参数对抽运效率的影响,以利于光纤参数和调制参数的选择,引入了归一化调制频率。研究结果表明:选择具有小非线性及较大群速度色散的光纤,适当增加调制频率和调制深度,都可以在保证高平均功率输出的同时获得相对高的抽运效率,而较小抽运功率则有利于能量提取。     

紧凑型1.57μmOPO激光器

设计了紧凑型1.57μmOPO激光器,以满足工程化的需要。分别采用两种非稳腔腔型的1.57μmOPO激光器,同样为Cr4+:YAG被动调Q,在7J的电能量输入下,得到输出能量6mJ、发散角3.0mrad和输出能量4mJ、发散角为3.0mrad的两种高光束质量的1.57μm人眼安全激光输出。以极其简单的结构克服了OPO激光器光束质量不好的困难。     

Modeling of longitudinally pumped CW Ti:sapphire laser oscillators

Longitudinal pumping of CW Ti:sapphire oscillators is considered to predict the power output of such oscillators using folded, astigmatically compensated cavities. The model predicts how the oscillator performance is affected by the selection of Ti³⁺ concentration, material figure of merit, rod length, and pump and cavity mode waists. Experimental results for broadband CW oscillators pumped by CW argon ion lasers are reported, and these are shown to be in excellent agreement with theory. Nearly 30% slope efficiency is achieved with a four-mirror folded cavity and even higher efficiencies are predicted for optimized pumping     

Analytic theory of a tapered gyrotron resonator

An analytic theory has been derived for determining the eigenfrequencies, RF-field distribution and Q of the TE_mpq modes of a gyrotron resonator consisting of a circular cylinder joined to a slowly tapered section. Explicit results are obtained for a linear taper. The cavity modes are found to have an RF-field distribution which is useful for prebunching the electron beam and enhancing efficiency. For high Q cavities, the cavity Q depends on axial mode number q as q^?2, which is important for mode discrimination. Proper selection of taper length is found to reduce the Q of high q modes, also aiding in mode discrimination. The present approach may be applied to other forms of weakly irregular cavities, such as cavities with nonlinear tapers.     

Impact of modulation frequency on cross-phase modulation effect inintensity modulation-direct detection WDM systems

The impact of modulation frequency on the crossphase modulation (XPM) effect in intensity modulation (IM)-direct detection wavelength-division multiplexing systems is investigated theoretically and numerically. A simple expression for IM is derived, verified by simulation and its validity is presented. The dependence of XPM-induced IM on the fiber length, fiber dispersion, channel separation and pump modulation frequency is assessed. It is shown that at very low frequency the walkoff effect has almost no influence on the XPM-induced IM efficiency which increases with the square of the frequency; at higher frequency the IM efficiency can be reduced significantly by the walkoff and scales linearly with modulation frequency     

Nd：YAG激光器最佳泵浦条件的实验研究

本文报导不同泵浦腔的比较及几何传输效率的数值计算．研究泵浦腔几何尺寸，灯的类型，反射膜层与泵浦效率的关系．并讨论泵浦均匀性及灯放电回路对泵浦效率的影响．     

An excimer-pumped β-BaB2O4 opticalparametric oscillator tunable from 354 nm to 2.370 μm

A pulsed singly resonant optical parametric oscillator (OPO) has been constructed which can provide tunable coherent radiation over the entire wavelength range extending from 354 nm in the near ultraviolet, throughout the visible, to 2.370 μm in the near infrared using a single β-BaB₂O₄ crystal. The oscillator is pumped at 308 nm by a pulsed narrow band injection-seeded XeCl excimer laser in a near-field pumping configuration, and energy conversion efficiencies (optical-to-optical) in excess of 10% have been obtained in a 12-nm-long crystal across the OPO tuning range. The requirements placed upon the spectral and spatial coherence of the pump laser for optimum OPO operation are discussed. Experimental results on several oscillator parameters, including tuning range, oscillation threshold, energy conversion efficiency, and spatial and temporal characteristics, are presented     

Nonlinear propagation of pulses in multimode fiber with strong linear coupling

Multimode fiber has a richer spatial dimension than single-mode fiber, and is an ideal platform for studying many novel nonlinear effects. We established a strong linear coupling and short-range fiber model to understand the interactive effects of linear coupling and nonlinear effects. We find that strong linear coupling can compensate for the group delay between eigenmodes and cause energy fluctuation between modes which weakens the nonlinear effects. In high energy pulses, the interaction of linear coupling and nonlinear effects can help producing weak dispersion waves when the spectrum is broadened. Since linear coupling in a mode group is common and unavoidable, these results may provide a certain theoretical explanation for multi-mode nonlinear phenomena.     

Characterization of an Nd:YAG-pumped β-BaB2O4 optical parametric oscillator in collinear and noncollinearphase-matched configurations

We present the characterization of type I (e→o+o) phase matching in beta-barium borate (BBO) optical parametric oscillators pumped by the third harmonic of a Q-switched Nd:YAG laser in a collinear phase-matched geometry and three separate noncollinear configurations. Using a 12 mm long crystal, we show that the optical conversion efficiency increases from 10.7% in the collinear case to 40% with a noncollinearity of 90.8 mrad, and a corresponding fall in the operational threshold from 274 to 188 mJ·cm^-2. We discuss the constraints imposed upon the spectral and spatial coherence of the pump beam, and show that the spatial restriction is eased in the noncollinear regime. Included in the paper is a theoretical model of noncollinear phase matching which predicts the tuning responses of each OPO accurately. Several of the oscillators were capable of tuning ranges from 450 nm to 1.68 μm using a single mirror set. We discuss in some detail the observed increases in conversion efficiency and linewidth, and the fall in operational threshold with noncollinearity, with particular attention to the pump beam walkoff