A novel quasi-phase-matching frequency doubling technique

A new quasi-phase-matching technique for efficient second-harmonic generation is reported. It is based on the spatial periodic modulation of the light intensity along the propagation direction, rather than the conventional spatial periodic modulation of the nonlinear optical coefficients. It can be realized by using a novel dual-channel waveguide frequency doubler structure for the desired light intensity distribution. This dual-channel waveguide device has major advantages including very small beam size, high light intensity within long nonlinear-waveguide interaction length, highly efficient second-harmonic generation, ease in fabrication of the nonlinear channel waveguides without any spatially periodic poling, and low waveguide propagation losses. The new quasi-phase-matching technique can also be applied to third-harmonic generation and other nonlinear optics processes.     

Spatiotemporal collapse in a nonlinear waveguide with a randomly fluctuating refractive index

Analytical results, based on the virial theorem and the Furutsu-Novikov theorem, of the spatiotemporal evolution of a pulse in a nonlinear waveguide with a randomly fluctuating refractive index are presented. For initial conditions in which total collapse occurs in a homogeneous waveguide, random fluctuations postpone the collapse. Sufficiently large-amplitude and short-wavelength fluctuations can cause an initially localized pulse to spread instead of contracting. We show that the disorder can be applied to induce a high degree of controllability of the spatiotemporal extent of the pulses in the nonlinear waveguide.     

Blue light generation in a ridge waveguide MgO:LiNbO3 crystal pumped by a fiber Bragg grating stabilized laser diode

We report cw blue light generation by using a periodically poled MgO:LiNbO(3) crystal with a ridge waveguide pumped by a fiber Bragg grating stabilized laser diode with 0.25 nm spectrum width in the coherent collapse regime. Blue light generation of 73 mW with 27% conversion efficiency and a wide temperature tolerance of 5.7 degrees C were attained.     

Thermally induced dephasing of high power second harmonic generation in MgO:LiNbO3 waveguides

High power second harmonic generation(SHG) in MgO-doped LiNbO3 waveguides is investigated using a three-dimensional(3D) coupled thermo-optical model.Simulations performed for a 1111.6-nm fundamental laser show the in?uence of the absorptions and the thermally induced dephasing on the conversion efficiencies of the different waveguides.The onset of the thermally induced dephasing effect for each waveguide is also indicated.As a result of high light intensity in the waveguide,nonlinear absorptions are identified as the possible main factors in efficiency losses in specific cases.     

Spatial optical (2+1)‐dimensional scalar‐ and vector‐solitons in saturable nonlinear media

(2+1)‐dimensional optical spatial solitons have become a major field of research in nonlinear physics throughout the last decade due to their potential in adaptive optical communication technologies. With the help of photorefractive crystals that supply the required type of nonlinearity for soliton generation, we are able to demonstrate experimentally the formation, the dynamic properties, and especially the interaction of solitary waves, which were so far only known from general soliton theory. Among the complex interaction scenarios of scalar solitons, we reveal a distinct behavior denoted as anomalous interaction, which is unique in soliton‐supporting systems. Further on, we realize highly parallel, light‐induced waveguide configurations based on photorefractive screening solitons that give rise to technical applications towards waveguide couplers and dividers as well as all‐optical information processing devices where light is controlled by light itself. Finally, we demonstrate the generation, stability and propagation dynamics of multi‐component or vector solitons, multipole transverse optical structures bearing a complex geometry. In analogy to the particle‐light dualism of scalar solitons, various types of vector solitons can ‐ in a broader sense ‐ be interpreted as molecules of light.     

Spatial weak-light solitons in an electromagnetically induced nonlinear waveguide

We show that a weak probe light beam can form spatial solitons in an electromagnetically induced transparency (EIT) medium composed of four-level atoms and a coupling light field. We find that the coupling light beam can induce a highly controllable nonlinear waveguide and exert very strong effects on the dynamical behavior of the solitons. Hence, in the EIT medium, it is not only possible to produce spatial solitons at very low light intensities but also simultaneously control these solitons by using the coupling-light-induced nonlinear waveguide.     

Third-harmonic generation in slow-light chalcogenide glass photonic crystal waveguides

We demonstrate third-harmonic generation (THG) in a dispersion-engineered slow-light photonic crystal waveguide fabricated in AMTIR-1 chalcogenide glass. Owing to the relatively low loss and low dispersion in the slow-light (c/30) regime, combined with the high nonlinear figure of merit of the material (～2), we obtain a relatively large conversion efficiency (1.4×10(-8)/W(2)), which is 30× higher than in comparable silicon waveguides, and observe a uniform visible light pattern along the waveguide. These results widen the number of applications underpinned by THG in slow-light platforms, such as the direct observation of the spatial evolution of the propagating mode.     

Second-harmonic generation in three-well and bound-to-continuum GaAs-based quantum-cascade lasers

We present efficient second-harmonic generation in state of the art GaAs-based quantum-cascade lasers with nonlinear output powers up to 100 μW. The nonlinear output was significantly improved by applying an AlGaAs waveguide structure, which guides both, the fundamental and nonlinear light. We further show the influence of the doping in the active region on the nonlinear light generation by comparing two similar structures with different doping levels. PACS 42.55.Px; 42.65.Ky; 81.05.Ea     

飞秒脉冲在硅波导中产生超连续谱的研究

利用非线性薛定谔方程对飞秒脉冲在硅波导中的传输以及超连续谱的产生进行了研究,计算和分析了色散参量及非线性损耗在超连续谱的产生过程中的影响.结果表明,孤子分裂是飞秒脉冲在硅波导中产生超连续谱的主要机理.飞秒脉冲中心波长与波导零色散点的相对位置对超连续谱的产生有极大影响.当中心波长位于近零色散点的反常色散区时,孤子分裂现象最明显,谱宽远大于在零色散波长及正常色散处入射时的情况,并且达到稳定展宽所需波导长度最短.其次,高阶色散的大小也会影响光谱展宽,三阶色散绝对值较小时,能够获得较大的展宽.另外,由于双光子吸收效应带来大量损耗,限制了谱宽,并且随着初始脉冲功率的逐渐增大,展宽出现饱和现象. 关键词： 超连续谱 孤子分裂 色散 硅波导     

Four-wave mixing of picosecond pulses in hollow fibers: Phase matching and the influence of high-order waveguide modes

The processes of third-harmonic and difference-frequency generation through the four-wave mixing of picosecond pulses in gas-filled hollow fibers are experimentally studied. Due to the improvement of phase-matching conditions with an appropriate choice of the gas pressure and optimal parameters of the hollow fiber, we were able to use hollow fibers with a large length (up to 30 cm) for difference-frequency generation, which resulted in a considerable increase in the power of the difference-frequency signal at the output of the fiber. Our experimental data reveal a considerable influence of high-order waveguide modes on four-wave mixing processes in a hollow fiber. It is shown that the waveguide regime of nonlinear optical interactions implemented in hollow fibers removes the limitations on the efficiency of third-harmonic and sum-frequency generation, which are characteristic of the tight-focusing regime in media with normal dispersion and which are due to the geometric phase shift arising in tightly focused light beams.     

基于飞秒激光抽运的石墨烯包裹微光纤波导结构的级联四波混频研究

基于石墨烯的光学非线性特性和器件研究正在成为新一代微纳光子器件的一个重要方向. 采用峰值功率为kW量级的飞秒脉冲抽运和P型掺杂石墨烯薄膜包裹的微光纤所构成的复合波导结构, 在1550 nm波段成功激发并观察到级联四波混频现象. 实验 结果表明, 这种P型掺杂石墨烯包裹的微光纤复合波导具有非线性系数高、结构紧凑, 可承受高功率和超快响应的特点, 对基于该结构的级联四波混频特性的研究在基于超快光学的多波长光源、光参量放大以及全光再生等领域具有参考价值和应用意义     

466 mW green light generation using annealed proton-exchanged periodically poled MgO: LiNbO3 ridge waveguides

We report high-power efficient green light generation by frequency doubling from a periodically poled MgO doped LiNbO(3) ridge waveguide. The ridge waveguide is fabricated by the annealed proton-exchanging and precise diamond blade dicing techniques. The ridge structure exhibits a surface roughness of only 3.7 nm, and near-90° vertical sidewall. The total insertion loss of an 8.5 μm wide and 1.4 cm long uncoated waveguide is 3.0 dB under direct fiber coupling. 466 mW of continuous-wave green light with an optical-to-optical conversion efficiency of 69.7% is obtained. To the best of our knowledge, this is the highest green light output power reached to date using a ridge-type LiNbO(3) waveguide device. Phase-matching temperature shift, tuning curve distortion, and waveguide loss increase are observed under high power operation. Our analysis shows that the photorefractive effect and the green induced infrared absorption are responsible for the observed phenomena, which becomes prominent under several megawatt per square centimeter power density.     

基于六角格子光子晶体波导的高效全光二极管设计

提出了一种基于六角格子光子晶体波导微腔和Fabry-Perot(FP)腔非对称耦合的全光二极管结构, 它由一个包含非线性Kerr介质的高Q值微腔与一个光子晶体波导中的FP腔组成. 通过有限时域差分方法对其传输特性进行了仿真, 发现通过两腔的非对称耦合可以实现在特定光强度下的正向传输、反向截止的功能. 在靠近微腔方向光入射时, 特定强度的光可以激发非线性微腔的Kerr效应, 改变了Fano腔的共振频率, 从而变成透射状态. 而远离微腔方向光入射, 由于这个不对称的结构造成场局域的分布不对称, 激发微腔Kerr效应的光强还不够, 所以光不能透射. 所设计的全光二极管结构具有良好的性能参数: 最大透射率高和高透射比、光强阈值低和易于集成等.     

Second harmonic generation in planar optical waveguides

We study optical second harmonic generation (SHG) in planar waveguide structures composed of several layers with different dielectric constants. We develop a general formalism for the calculation of mode generation by a planar antenna embedded in the waveguide. As an application we consider a monolayer of high second-order susceptibility adsorbed at the interface between two layers of the waveguide structure. Periodic modulation of the nonlinear susceptibility allows phase matching leading to dramatically enhanced second harmonic intensities. We investigate the SHG-efficiency of various experimentally realizable geometries.     

基于光子晶体光纤和拉锥式单模光纤的超连续光谱产生的实验研究

本文使用重复频率为250 MHz、脉冲宽度为135 fs、最大功率为2.2 W的锁模掺镱光纤激光作为种子源,利用光子晶体光纤和自制的拉锥式单模光纤两种高非线性光纤研究了超连续光谱的产生特性,通过对比两种光纤的结构、色散等特性,分析了拉曼孤子、色散波及其他非线性效应对产生的超连续谱形状的影响,并均得到了大于一个倍频程的超连续光谱,特别是拉锥式单模光纤产生的超连续光谱,耦合效率达到60%,这为众多研究领域,尤其是光学频率梳的建立提供了实用的超连续光源.     

Nonlinear optical properties of a channel waveguide produced with crosslinkable ferroelectric liquid crystals

A binary mixture of ferroelectric liquid crystals (FLCs) was used for the design of a channel waveguide. The FLCs possess two important functionalities: a chromophore with a high hyperpolarizability and photoreactive groups. The smectic liquid crystal is aligned in layers parallel to the glass plates in a sandwich geometry. This alignment offers several advantages, such as that moderate electric fields are sufficient to achieve a high degree of polar order. The arrangement was then permanently fixed by photopolymerization which yielded a polar network possessing a high thermal and mechanical stability which did not show any sign of degradation within the monitored period of several months. The linear and nonlinear optical properties have been measured and all four independent components of the nonlinear susceptibility tensor have been determined. The off-resonant d-coefficients are remarkably high and comparable to those of the best known inorganic materials. The alignment led to an inherent channel waveguide for p-polarized light without additional preparation steps. The photopolymerization did not induce scattering sites in the waveguide and the normalized losses were less than 2 dB/cm. The material offers a great potential for the design of nonlinear optical devices such as frequency doublers of low-power laser diodes. Received 7 March 2000     

The ridge waveguide fabrication with periodically poled MgO-doped lithium niobate for green laser

Ridge waveguides were fabricated using an external field, a precision lapping machine and neutron loop discharge (NLD) in magnesium-oxide-doped lithium niobate. The measured quasi-phase-matching (QPM) wavelength of the second-harmonic generation (SHG) in a 30 mm long periodically poled magnesium-doped lithium niobate (PPMgLN) ridge waveguide which has a domain period of 6.8 μm is about 532 nm. A fabricated periodically poled magnesium-doped lithium niobate ridge waveguide was duty cycle of 51.9 ± 2.83% and demonstrated second-harmonic generation. By using this periodically poled magnesium-doped lithium niobate ridge waveguide, highly effective, low-cost optical devices with high power or short wavelength can be achieved.     

Highly efficient blue-light generation from a compact, diode-pumped femtosecond laser by use of a periodically poled KTP waveguide crystal

We present a simplified, potentially portable, and highly efficient blue-light source from a periodically poled KTP waveguide crystal with a compact femtosecond Cr:LiSAF laser. This light source generates 5.6 mW of blue average output power at 424 nm with 27 mW of incident fundamental in a single-pass extracavity arrangement at room temperature. The overall system efficiency of electrical power to blue light is 0.5%, and the internal second-harmonic generation conversion efficiency is as high as 37%. The slope efficiency of 5.5% pJ(-1) at low pulse energies is, to our knowledge, the highest slope efficiency yet reported for frequency conversion into the blue spectral region.     

Nonclassical light in symmetric nonlinear directional coupler

We examine the quantum-statistical properties of light beams in a directional symmetric nonlinear coupler composed of one linear waveguide and two nonlinear waveguides operating by the second-harmonic generation. Linear and nonlinear mismatches are taken into account. Non-classical behaviour of single and compound modes in such a device is investigated. This work was partially supported by the Grant 202/96/0421 of the Czech Grant Agency and by the Complex Grant VS96028 of Czech Ministry of Education.     

锥形空心银波导的聚焦特性

利用时域有限差分(FDTD)方法数值演示了一种能光学引导和聚焦激光的锥形空心银波导,数值模拟结果表明这种锥形空心银波导能把激光聚焦成一个极小的、直径只有1 μm、强度高度局域性的光斑.对锥形波导能光学引导和聚焦激光的物理机理作了分析和探索,并进一步探讨了激光倾斜入射、波导几何结构对锥形波导聚焦特性造成的影响,这些研究结果对于锥形光学的应用、快点火或产生高能带电粒子中锥靶结构的最佳设计具有重要的参考价值. 关键词： 锥形波导 聚焦特性 时域有限差分