Optical bistability in one-dimensional photonic band gap structures with coupled nonlinear defects

Using the transfer matrix method, we study the transmission property of one-dimensional photonic band gap structure with two coupled nonlinear defects separated by a linear middle layer and find that such composite structure exhibits a bistability that’s strongly dependent of the linear middle layer. The switching threshold values can be greatly reduced just by choosing the proper parameters of the linear middle layer.     

Optically S-polarized surface waves in symmetrical nonlinear sensors: Thermal effects

A surface wave in a planar nonlinear waveguide is a current problem that has several applications in modern electronics and optics such as optical sensors design. The effect of thermal stress on the optical performance of a nonlinear symmetrical sensor is studied. The mathematical forms of the dispersion equation and thermal-stress sensitivity are analytically derived and plotted numerically. It is found that the thermal sensitivity of the sensor can be controlled by tuning the core size, by changing the loading materials, and by carefully selecting the materials.     

Increasing absorption optical bistability operating in the front surface layer of high absorption samples

The increasing absorption optical bistability in a bulk ZnSe sample with quite high absorption has been observed. Our experiments show that the bistability operates only in the front surface layer of sample and that the back layer of sample can be regarded as a linear absorber and heat sink.     

Experimental observation of tunable wavelength down- and up-conversions of ultra-short pulses in a periodically poled LiNbO3 waveguide

All-optical tunable wavelength conversion of ps-pulses is proposed and experimentally demonstrated by use of cascaded sum- and difference frequency generation (cSFG/DFG) in a periodically poled LiNbO₃ (PPLN) waveguide. The amplified spontaneous emission (ASE) noise is effectively suppressed by employing two tunable filters. As a result, tunable wavelength down- and up-conversions are simultaneously observed. The temporal evolutions of the signal, pump, control and idler waves propagating along the PPLN waveguide are simulated. From the temporal waveforms and optical spectra in the simulations, it is interesting to find that continuous wave (CW) pump and control are evolved into optical pulses during the cSFG/DFG nonlinear interactions. Moreover, some new sidebands in the output pump spectrum are observed both in the simulations and experiments.     

On the distribution of 1550-nm photon pairs efficiently generated using a periodically poled lithium niobate waveguide

We report on efficient generation of 1550-nm photon pairs in a periodically poled lithium niobate waveguide using the spontaneous parametric down-conversion process. Such photon pairs are expected to find applications in fiber-based long-distance quantum communication. Pumping the waveguide with a pulsed semiconductor laser with a pulse rate of 800 kHz and a maximum average pump power of 50 μW, we obtain a coincidence rate of 600 s⁻¹. Despite only two single-photon detectors are used, we gain some information about the photon-number distribution. Our measurements are found to be in agreement with a Poissonian photon-pair distribution, but clearly differ from the expected outcomes for both conventional and two-mode squeezed states, the latter corresponding to a thermal photon-pair distribution. The Poissonian photon-pair distribution is also explained by comparing the coherence time of the pump light and of the detected photons. An average of 0.9 generated photon pairs per pulse can thus be inferred.     

Low threshold, actively Q-switched Nd:YVO4 self-Raman laser and frequency doubled 588 nm yellow laser

We reported an actively Q-switched, intracavity Nd³⁺:YVO₄ self-Raman laser at 1176 nm with low threshold and high efficiency. From the extracavity frequency doubling by use of LBO nonlinear crystal, over 3.5 mW, 588 nm yellow laser is achieved. The maximum Raman laser output at is 182 mW with 1.8 W incident pump power. The threshold is only 370 mW at a pulse repetition frequency of 5 kHz. The optical conversion efficiency from incident to the Raman laser is 10%, and 1.9% from Raman laser to the yellow.     

Bandwidth enhancement and response flattening of cascaded sum- and difference-frequency generation-based wavelength conversion

We investigate bandwidth enhancement and response flattening of wavelength conversion based on cascaded sum-frequency generation and difference-frequency generation (SFG/DFG) in quasi-phase-matched periodically poled lithium niobate waveguides by use of two pump lights. Analysis shows that the conversion properties deeply depend on the pump settings. The bandwidth is efficiently enhanced by augmenting the pump wavelength difference. The critical bandwidth corresponding to the response fluctuation of 1 dB in a 3-cm-long waveguide reaches 80-90 nm, which is enough to cover the entire conventional-band and long-wavelength-band. The cascaded SFG/DFG bandwidth is about 24% broader than that of the cascaded second-harmonic generation and difference-frequency generation-based configuration with the same waveguide length. However, the response fluctuation is also enhanced together with the bandwidth enhancement. The method of pump detuning is presented to flatten the response fluctuation. The fluctuation is reduced by more than 1 dB and it can be further reduced by increasing the pump detuning value with a little efficiency penalty.     

Determination of nonlinear coefficient and group-velocity-dispersion of bismuth-based high nonlinear optical fiber by four-wave-mixing

Nonlinear coefficient and group-velocity-dispersion of bismuth-based nonlinear fibers were determined by four-wave-mixing measurements. The wavelength dependence of refractive index of bulk bismuth-based glasses was also measured to estimate the material dispersion and waveguide dispersion. A newly developed bi-directional four-wave-mixing configuration enabled us to determine all fiber parameters simultaneously. The obtained fiber nonlinearity γ ∼ 1000 W⁻¹ km⁻¹ of bismuth-based nonlinear fiber is the highest one in the step-index fiber made of oxide glasses as expected from a high refractive index at 1550 nm. Dispersion analysis reveals that the both material dispersion and waveguide dispersion affect to the large group-velocity-dispersion of bismuth-based nonlinear fiber.     

Intra-cavity spectroscopy with diode lasers

A multi-mode diode laser with an external cavity is studied experimentally and theoretically for its application to intra-cavity spectroscopy. One facet of a typical Ga_0.89Al_0.11As laser diode was antireflection-coated by deposition of HfO₂ such that 10^–3 residual reflectivity was left over. This diode was placed in an external optical cavity. The emission spectrum of this diode laser is highly sensitive to any frequency-dependent loss in the cavity, and the detectivity of such a loss grows with the pump rate. Even close to threshold, the absorption at 780 nm of Rb atoms with a density of 5×10¹⁰ cm^–3 has been detected. An adequate model for diode lasers based on rate equations and including frequency-dependent gain saturation is developed and applied to the calculations of output spectra. The sensitivity of these spectra to intra-cavity absorption is determined by the overall cavity loss — which is rather high — and the fraction of spontaneous emission in the total emission, in contrast with dye lasers where it is limited by nonlinear mode coupling. Various criteria for the sensitivity are suggested. The smallest detectable absorption with a perfectly antireflection-coated laser is 10^–6 cm^–1. Improvement of the characteristics of the laser diode would increase the sensitivity.     

Enhanced third harmonic reflection and diffraction in Silicon on Insulator photonic waveguides

Enhanced third harmonic (TH) generation from Silicon-On-Insulator (SOI) planar waveguides as well as SOI photonic crystal (PhC) slabs is studied in different angular configurations, both in the visible and infrared energy ranges. In the SOI planar waveguide, the multilayer structure causes the optical properties such as TH reflection to be different from those of bulk silicon samples. This behavior is well reproduced by calculations of TH reflectance.Measurements of third-harmonic reflection and diffraction from one-dimensional PhC slabs etched in the SOI waveguide are also reported. The angular positions of TH peaks at various diffraction orders agree well with those calculated from a nonlinear grating equation. Both reflection and diffraction processes contribute to enhanced TH generation efficiency in the PhC slabs.TH reflectance measurements performed on PhC slabs in the near infrared show a resonant interaction between the incident beam and the photonic structure, dependent on the angle of incidence. This leads to a nonlinear conversion efficiency which is strongly enhanced with respect to that of the SOI waveguide, due to the excitation of strong local fields associated with the presence of photonic modes in the PhC slab.     

Generation of 100 μJ pulses at 82.8 nm by frequency tripling of sub-picosecond KrF laser radiation

. Investigations of the efficient generation of powerful coherent radiation at 82.8 nm by frequency tripling of short-pulse KrF laser radiation are presented. Argon gas is selected as nonlinear medium due to the resonantly enhanced 3rd-order susceptibility χ⁽³⁾(-3ω,ω,ω,ω). Pulse energies of 100 μJ at 82.8 nm have been measured for a pump pulse energy of 14 mJ. An upscaling to more than 500 μJ is expected with available more powerful pump lasers. Features of this XUV source and possible applications are discussed. Received: 26 July 2002 / Published online: 15 November 2002 RID="*" ID="*"Corresponding author. Fax: +49-511/7622211, E-mail: reinhardt@iqo.uni-hannover.de     

Optical bistability in fluorescein dyes

Optical bistability has been observed in highly concentrated fluorescein dye solutions and in thin (1 m) doped polymeric films. At concentrations larger than 10^–5 mole/l dye dimers are formed. For fluorescein dye, the dimer-monomer equilibrium constant is 10⁵ l/mole so that most of the dye species are in the dimer form. At 480 nm the dimer absorption cross section is 10^–18 cm²/molecule, while that for the dye monomer molecule is 7.6×10^–17 cm²/molecule. Upon laser excitation dimers dissociate to form monomers thus providing a highly nonlinear laser induced absorption. This high nonlinear absorption coefficient can be utilized for optically bistable response of the dye system.Optical bistability was observed by placing dye solutions or dye thin films inside a Fabry-Perot resonator and exciting it with 480 nm dye laser pulses of 10 ns duration. The effect is more pronounced in 10^–4 mole/l fluorescein than in 10^–6 mole/l fluorescein in which dimer formation is not that efficient.In disodium fluorescein no significant dimer formation is observed even at 10^–3 mole/l dye concentration. The observed bistability both in solution and in thin films can be explained in terms of recent models for optical bistability in nonlinearly absorbing molecular systems.     

Modelling the vertical grating-waveguide coupler for the description of its nonlinear symmetry-breaking dynamics

We propose a new scheme to observe the symmetry-breaking dynamics of two counter-propagating beams in a Kerr-type nonlinear waveguide coupler. Using a grating to couple light at normal incidence upon the waveguide allows for a single-beam geometry. The ability of the grating to couple light symmetrically is demonstrated on the basis of a linear coupled-mode approach especially adapted to vertical coupling. The nonlinear symmetry-breaking dynamics is studied analytically.     

Cascaded compression of the first and second Stokes pulses during forward transient stimulated Raman amplification

The results of numerical modelling of cascaded compression of the first and second Stokes pulses during regenerative regime of the forward transient stimulated Raman amplification are presented for the case when the walk-off length of the first Stokes pulse due to group velocity mismatch is shorter than the length of the nonlinear medium. The influence of the initial amplitudes of the seed first Stokes pulses, its durations and its time delay with respect to the pump pulse, the Kerr nonlinearity of the medium on the conversion efficiency, duration and propagation factor M² of the first and second Stokes pulse are studied. It is demonstrated that for the pump pulse duration of 1 ps the duration of the compressed second Stokes pulses in a KGW crystal near the beam axis may be approximately 14 times shorter than the pump pulse duration. It is shown that the propagation factor of the compressed pulses increases significantly because of complex spatial-temporal dynamics of compression and the influence of Kerr nonlinearity of Raman medium.     

非线性一维光子晶体波导光双稳

利用非线性折射率系数较大且非线性时间响应较快的CdS_xSe_1-x玻璃为材料,设计并制备了非线性一维光子晶体波导光双稳器件,该器件的折射率空间分布呈正弦形式。实验测得双稳开关的阈值功率密度为1.60×10⁵W/cm²,开关时间为63ps。采用时域有限差分方法讨论了光子晶体带隙随入射光强变化而移动的情况,随着入射光功率密度的增加,光子晶体的带隙中心向短波方向移动。同时计算了该器件的双稳特性,理论计算得到双稳开关的阈值功率密度为1.40×10⁵W/cm²,开关时间约为50ps。获得了理论与实验基本一致的结果。     

Numerical analysis of the spatial behaviour of nanosecond optical parametric oscillators of beta-barium borate

We report on a numerical analysis of the temporal and spatial beam properties of nanosecond optical parametric oscillators (OPOs). The analysis is performed for a 355-nm-pumped critically phase-matched OPO of beta-barium borate. The calculations provide detailed information on the dependence of the OPO beam quality (measured by the quality factor M ²) on pump energy. An important result is the strong increase of the M ² value for pump energies exceeding 1–2 times the energy at threshold. Furthermore, a temporal analysis of single OPO pulses indicates that the M ² value strongly increases during the first few nanoseconds of the OPO oscillation. This increase is understood by considering the temporal dynamics of the spatial profiles of the OPO signal beam and the depleted pump radiation. Received: 1 April 1999 / Revised version: 26 July 1999 / Published online: 20 October 1999     

掺Tm激光晶体的可见与红外发光转换及双稳多色开关

理论研究了648nm激光抽运掺Tm晶体的可见与红外发光转换和发光双稳特性.根据系统的耦合速率方程理论,推导了发光转换阈值的解析表达式,数值研究了Tm³⁺能级布居数,辐射光谱以及发光强度等随抽运光强的变化.数值结果表明,648nm激光抽运下可以实现452/469nm蓝光与1716 nm红外光谱的发光转换.在光子雪崩阈值附近,可以观察到掺Tm晶体的辐射光谱双稳,荧光色度双稳和发光强度双稳等现象.利用掺Tm晶体的可见与红外发光转换与双稳特性,可以实现抽运功率控制的全固态发光转换器和双稳波长 关键词： 光学双稳 多色开关 掺Tm晶体 光子雪崩     

Well-confined Yb:GdVO4 laser waveguide formed by MeV C ion implantation

The planar waveguide in x-cut Yb:GdVO₄ crystal has been fabricated by 6.0 MeV carbon ion implantation with the fluence of 1 × 10¹⁴ ions/cm² at room temperature. The modes of the waveguide were measured by the prism-coupling method with the wavelength of 633 nm and 1539 nm, respectively. An enhanced ordinary refractive index region was formed with a width of about 4.0 μm beneath the sample surface to act as a waveguide structure. By performing a modal analysis on the observed transverse magnetic polarized modes, it was found that all the transverse magnetic polarized modes can be well-confined inside the waveguide. Strong Yb-related photoluminescence in Yb:GdVO₄ waveguide has been observed at room temperature, which reveals that it exhibits possible applications for integrated active photonic devices.     

Modulational instability in one-dimensional saturable waveguide arrays: Comparison with Kerr nonlinearity

Discrete modulational instability within the first band of uniform one-dimensional waveguide arrays possessing a saturable self-defocusing nonlinearity is investigated in detail within the coupled mode approach. Explicit analytical results for both the threshold and the maximal gain of instability are compared with the corresponding data from waveguide arrays exhibiting Kerr nonlinearity. We find that saturation bounds the interval of existence of discrete modulational instability, stabilizes the frequency region of perturbations around ±π/2 and decreases both gain and critical spatial frequency of perturbations.     

基于双矩形腔边耦合波导的等离子体诱导透明效应

为了降低功耗、实现超快速响应,设计了一种基于双矩形腔边耦合等离子体波导系统,并研究了其等离子体诱导透明效应.采用光学Kerr效应超快调控石墨烯-Ag复合材料波导结构,实现1 ps量级的超快响应时间.动态调控等离子体波导的传输相移,当泵浦光强为5.83 MW/cm^2时,等离子体诱导透明系统能够实现透射光谱π相移,这是因为基于石墨烯-Ag复合材料结构等离子体波导具有大的等效光学Kerr非线性系数,表面等离子体激元局域光场和等离子体诱导透明效应慢光对光学Kerr效应产生了协同增强作用,大大降低了系统获得透射光谱π相移的泵浦光强.等离子体诱导透明效应透明窗口的可调谐带宽为40 nm,系统的群延时控制在0.15 ps到0.85 ps之间,并且光波通过间接耦合或者相位耦合机制实现了等离子体诱导透明效应相移倍增效应.耦合模式理论计算结果很好地吻合了时域有限差分法仿真模拟结果,研究结果对于低功耗、超快速非线性响应和紧凑型光子器件的设计和制作具有一定的参考意义.