Efficiency of short-pulse type-I second-harmonic generation with simultaneous spatial walk-off, temporal walk-off, and pump depletion

We have developed a theory which for the first time predicts the efficiency of type-I second-harmonic generation (SHG) with simultaneous spatial and temporal walk-offs and with pump depletion. In order to validate our theory, we have compared its predictions with a numerical solver. We also have measured the conversion efficiency for type-I SHG in experiments with /spl sim/120--fs laser pulses in BBO and LBO crystals long enough that both temporal and spatial walk-off are important. Comparison supports our theoretical analysis. Our theory provides a useful tool for estimating SHG efficiency with pulsed sources in both nondepletion and depletion regions.     

Variations of optical M-waveguides for direct phase matching inAlGaAs

Two variations of an optical indexwise M-shaped waveguide structure in AlGaAs are presented as results of numerical simulations. The principal differences of the waveguiding aspects are discussed. The impact of the optical layer thicknesses is investigated with respect to phase matching and conversion efficiency for second-harmonic generation (SHG). Despite large differences in the interacting mode distributions, both waveguide configurations yield comparable nonlinear normalized conversion efficiencies. The technologically simple waveguide designs represent an attractive alternative for phase matched SHG     

High-power long-pulse second harmonic generation and optical damage with free-running Nd:YAG laser

Frequency doubling with a free-running long-pulse Nd:YAG laser and LBO or KTP nonlinear crystals yields conversion efficiency of up to 17.5% and 162 W peak power in the second harmonic. This efficiency is obtained for a TEM/sub 00/ beam with rectangular temporal pulse shape of 50 to 400 /spl mu/s. To our knowledge, this is the highest second-harmonic generation (SHG) efficiency reported for the long-pulse free-running configuration. The efficiency is limited by optical damage with much lower threshold than in the Q-switch domain. The damage is preceded by a saturation effect of the SHG efficiency. Both wavelengths (fundamental and second-harmonic) are necessary for the creation of the catastrophic damage. We present first evidence for a mechanism that involves creation of transient absorption centers by the second-harmonic radiation due to multiphoton absorption. Absorption of the fundamental wave at these centers leads to local heating and ultimately catastrophic damage.     

Optimization of line-focusing geometry for efficient nonlinearfrequency conversion from copper-vapor lasers

Detailed parametric investigations of line foci for efficient nonlinear frequency conversion of copper-vapor lasers are presented. For a single medium-scale copper-vapor laser (nominally 20 W), the optimum focal geometry for efficient second-harmonic and sum-frequency generation in BBO is to focus a 3.75-mm-diameter beam into the crystal with an f=60 mm cylindrical lens, Using such a focal geometry, UV powers of up to 1.75 W (SHG of the green), 1.2 W (SHG yellow), and 1.5 W (SFG) have been produced with peak instantaneous conversion efficiency of up to 48%. Conversion efficiencies are most sensitive to the F-number of the cylindrical focusing, with the optimum F-number being approximately 16, and are relatively insensitive to the width of the focused beam     

40-GHz picosecond-pulse second-harmonic generation in an MgO-doped PPLN waveguide

Second-harmonic generation (SHG) of 40-GHz picosecond optical pulses with different pulsewidths, pulse energies, and central wavelengths in a MgO-doped periodically poled lithium niobate (PPLN) waveguide is studied experimentally and theoretically. In the experiments, the picosecond pulses are generated from a 40-GHz mode-locked fiber laser and two tunable filters, with which the lasing wavelength can be tuned from 1530 to 1570 nm, and the pulsewidth can be tuned from 2 to 7 ps. The second-harmonic (SH) pulses are generated when the picosecond pump pulses pass through the MgO-doped PPLN waveguide. Dependences of SHG on pump pulsewidth, average pump power, and pump central wavelength are then investigated systematically. Meanwhile, dynamic behaviors of both pump and SH pulses in propagation are simulated numerically. Based on the temporal and spectral characteristics of SHG, a quantitative and systematic analysis on SHG efficiencies in terms of both pulse energy and spectral peak is presented. The simulation results are in good agreement with the measured data.     

New organic materials with large optical nonlinearities

We have investigated a large number of new nonlinear organic materials by second-harmonic generation (SHG) in powders, and found three compounds with SHG efficiencies an order of magnitude larger than urea. The results are interpreted in terms of bond conjugation, charge transfer, and crystal structure. These compounds are also found to have very high optical damage thresholds.     

Efficient Frequency Doubling of a Low-Power Femtosecond Er-Fiber Laser in BiB$_{3}$ O$_{6}$

BiB₃O₆ (BIBO) crystal has been used for efficient second-harmonic generation (SHG) of a low-power femtosecond Er-fiber laser-amplifier system operating at 56 MHz. At the maximum input power of 65 mW, an internal conversion efficiency of 23% was achieved for SHG at 782 nm, with a pulse duration of 64 fs. A comparison with beta-BaB₂O₄ reveals superior properties of BIBO for such ultrashort-pulse ultra-broadband SHG.     

Induced second-harmonic generation in planar waveguides by anexternally applied periodic DC electric field: efficiency as a functionof field structure

The efficiency of second-harmonic generation (SHG) induced by an externally applied periodic DC electric field was investigated. A finite element method was used to find the static electric field due to different interdigitated electrode structures. The resulting fields were used to calculate the overlap integrals which determine the SHG efficiency in an optical waveguide. The results show that there are reasonable differences in SHG efficiencies for the structures investigated     

Theoretical analysis of Cerenkov-type optical second-harmonicgeneration in slab waveguides

Optical second-harmonic generation in the Cerenkov-radiation scheme in slab waveguides has been theoretically investigated for all the possible type I configurations. By solving the nonlinear wave equations analytically, a simple general expression for the second-harmonic power has been obtained. Characteristics of the Cerenkov-type second-harmonic generation and optimum conditions for attaining high conversion efficiencies are discussed on the basis of the theoretical analysis. It is shown that, in order to attain high conversion efficiencies, off-diagonal elements of the nonlinear optical coefficient tensor should be utilized taking advantage of birefringence     

Observation of both second-harmonic and multiphoton-absorption-induced luminescence in ZnO

The confusion over second-harmonic generation (SHG) and multiphoton-absorption (MPA)-induced luminescence in ZnO is observed under excitation with femtosecond laser between 800 and 1000 nm at room temperature. Excitation power dependence measurements show SHG is a two-photon process, and MPA is a three- or four-photon process. These are confirmed with interferometric pulse autocorrelation experiments. Excitation power density is a main regulatory factor for this confusion.     

Intersubband optical second-harmonic generation in multiple quantumwells: radiative coupling effects

An analysis of intersubband optical second-harmonic generation (SHG) in multiple quantum wells (QW's) is presented, in which special attention has been paid to radiative coupling effects. Starting from the Maxwell-Lorentz equations, the field at the second-harmonic frequency in the QW system is determined. Following the same framework, the field at the fundamental frequency is also obtained. In a reflection geometry, the SH conversion efficiency is derived for a p-polarized incident field. Detailed numerical calculations for different numbers of QW's, angles of incidence, as well as barrier thicknesses show that the SHG conversion efficiency can be significantly modified due to radiative coupling among wells     

脉冲形状对三次谐波转换的影响分析

本文研究了基频光脉冲波形对三倍频转换效率的影响,对高斯脉冲和平顶脉冲的不同谐波转换特点作了详细分析,研究结果表明,当基频光脉冲形状偏离理想的平顶分布时,适当地降低二倍频转换效率,可提高三倍频转换效率。当基频光为高斯脉冲时,在3GW/cm2的输入条件下最佳的二倍频转换效率为56.8％,低于理论预计的66.7％。进一步的,计算了不同功率密度下,最佳二倍频转换效率与超高斯脉冲阶数的关系。     

脉冲形状对3次谐波转换的影响分析

为了使谐波转换系统保持高效稳定的3次谐波转换,采用数值模拟的方法,研究了基频光脉冲波形对3倍频转换效率的影响,并对高斯脉冲和平顶脉冲的不同谐波转换特点作了详细分析.当基频光为高斯脉冲时,在3GW/cm2的输入条件下最佳的2倍频转换效率为56.8%,低于理论预计的66.7%.进一步计算了不同功率密度下,最佳2倍频转换效率与超高斯脉冲阶数的关系.当考虑空间走离效应,基频光时间和空间均为平顶分布时,最佳的2倍频转换效率为62%;若基频光时间为平顶分布、空间分布为高斯分布时,最佳2倍频转换效率为51%,进一步地偏离66.7%.结果表明,当基频光脉冲形状偏离理想的平顶分布时,适当地降低2倍频转换效率,可提高3倍频转换效率.     

Optical frequency conversions in nonlinear medium with periodicallymodulated linear and nonlinear optical parameters

A new method considering both linear and nonlinear optical parameter modulation and propagation loss is proposed to treat second-order nonlinear optical interactions in nonlinear media having periodic structures. Mathematical expressions are derived for difference frequency generation (DFG) and compared with those for second-harmonic generation (SHG). Wavelength conversions around 1.55 μm, which are very interesting in optical communication systems, are studied for semiconductor DFG devices with periodic structures. Semiconductor (e.g., AlGaAs) DFG devices of standard device lengths (several mm) and pump light intensities (10⁵ kW/cm²) are shown capable of practical level conversion efficiencies (~10%) and extremely wide bandwidths (⩾100 nm). Effects of propagation loss in DFG devices are also examined     

Wavelength conversions in quasi-phase matched LiNbO/sub 3/ waveguide based on double-pass cascaded /spl chi//sup (2)/ SFG+DFG interactions

Based on the cascaded nonlinear interactions (/spl chi//sup (2)/:/spl chi//sup (2)/) of sum- and difference-frequency generation (SFG+DFG), a novel all-optical wavelength conversion scheme is proposed for the first time in periodically poled LiNbO/sub 3/ (PPLN) waveguide, in which a double-pass configuration is introduced. The performance of this scheme is thus different from the previous single-pass SFG+DFG scheme. The concept of the "balance condition" is presented to optimize the power and frequencies of the two pump sources. Under this condition, the energy is transferred irreversibly from the pump waves to the SF wave during the forward propagation. The equations describing the SFG can be solved analytically under this condition. Subsequently, the DFG equations are solved under the assumption that the SF wave would be constant during the backward propagation. Theoretical expressions are derived and are found to be consistent with numerical calculations. Compared with the conventional converter based on the cascaded /spl chi//sup (2)/:/spl chi//sup (2)/ interactions of second-harmonic generation and difference frequency generation SHG+DFG, the same conversion efficiency can be achieved in our scheme by employing two pump sources with lower power, or conversely higher conversion efficiency can be reached using two pump sources similar to that used in SFG+DFG scheme. The profile of the conversion efficiency can be further improved by adjusting the wavelengths of the two pump sources. In addition, compared with the single-pass SFG+DFG scheme, the main advantage of this new scheme rests on the fact that the conversion efficiency can be enhanced significantly. The advantages of the double-pass SHG+DFG scheme and the single-pass SFG+DFG scheme are combined in this new design to a great extent.     

Three-dimensional vector beam-propagation method for secondharmonic generation analysis

A full vectorial three-dimensional beam-propagation method (BPM) based on the finite-element method is described for the analysis of second harmonic generation (SHG). Quasi-phase-matched SHG devices with periodically domain-inverted GaAlAs- and LiTaO₃-based waveguides are analyzed. The influences of the shape of domain-inverted regions and of the inversion width on the conversion efficiencies are investigated in detail. The results of full-wave analysis are compared to those of approximate scalar analysis     

Generation of picosecond optical pulses with highly RF modulated AlGaAs DH laser

AlGaAs DH lasers with strong RF modulation superimposed on the relatively low dc bias below the oscillation threshold were demonstrated to be feasible for generation of a train of approximately 30 ps optical pulses at a repetitive frequency of sub-GHz range. The pulse envelope width was measured by three different methods: a fast response photodetector, the second-harmonic generation (SHG) correlation method, and an ultrafast streak camera. The results of the SHG correlation and the streak camera agreed fairly well. In order to explain the generation mechanism and the characteristics of these ultrashort optical pulses in a highly RF modulated semiconductor diode laser, the rate equation analysis was performed and the results were generally in good agreement with the experiment. Furthermore, from the computer simulation for the analysis of the SHG correlation traces, it was inferred that an individual ultrashort optical pulse has internal substructures made of fluctuating fields whose spike widths were of the order of subpicoseconds, due to the randomness of the phases among lasing modes.     

TEA CO2激光在AgGaSe2晶体中的倍频实验研究

实验上实现了TEACO2 激光在AgGaSe2 非线性光学晶体中的倍频光产生。着重研究了TEACO2 激光的能量、脉冲重复频率以及基波的焦点位置对于倍频光输出的影响。实验中得到了倍频光的角度调谐曲线 ,将入射光聚焦在晶体的中心区域 ,获得最大倍频光输出能量为 1 32mJ。脉冲能量转换效率为 2 4 % ,主脉冲的最大转换效率为 4 %。还分析了脉冲重复频率的变化 ( <10Hz)对倍频转换效率的影响。实验结果发现抑制倍频转换效率的主要因素为晶体的光学质量、激光脉冲低功率密度的拖尾以及激光脉冲的不稳定。特别由于脉冲附带长的拖尾加剧了热透镜效应 ,使得脉冲重复频率的增加引起转换效率的降低。     

Analysis of Ultrafast All-Optical OTDM Demultiplexing Based on Cascaded Wavelength Conversion in PPLN Waveguides

Ultrafast all-optical optical time-division multiplexing demultiplexing based on the cascaded second-harmonic generation and difference-frequency generation wavelength conversion in quasi-phase-matched periodically poled lithium niobate waveguides are studied theoretically. For a typical 160- to 10-Gb/s demultiplexing process, conversion efficiency, pulse shape, time delay, phase variation, and crosstalk are investigated in two different arrangements of the 160-Gb/s signals, and both the wavelength conversion and pulse reshaping can be realized