The mode structure and spectral properties of supercontinuum emission from microstructure fibers

The mode structure and spectral properties of supercontinuum emission generated by femtosecond pulses of Ti: sapphire laser radiation in microstructure fibers are studied. The long-wavelength (720–900 nm) and visible (400–600 nm) parts of supercontinuum emission are shown to be spatially separated in microstructure-fiber modes, which can be isolated with an appropriate spectral filtering. The spatial modes thus isolated in spectrally sliced supercontinuum emission possess a spatial quality sufficient for further efficient frequency conversion. The possibility of achieving a high spectral quality of supercontinuum emission is also demonstrated. We explore the ways to control the spectrum of supercontinuum emission by matching parameters of the pump pulse with the parameters of a microstructure fiber and by tuning the initial chirp of the pump pulse. The results of our studies show that supercontinua produced in microstructure fibers offer new approaches to designing a new generation of optical parametric amplifiers and broadband radiation sources for spectroscopic, metrological, and biomedical applications.     

多孔微结构光纤中飞秒激光脉冲超连续谱的产生

报道了利用800nm飞秒激光脉冲在多孔微结构光纤中产生超连续谱展宽的现象，连续谱展宽范围为440—890nm.基于标量波近似理论对微结构光纤包层的有效折射率和基模的有效面积以及光纤的色散特性进行了计算，发现微结构光纤具有特殊的控制色散和波导特性的能力，对超连续谱展宽的机理进行了初步解释.本文的理论分析和实验结果有较好的一致性，认为即使包层由无序填充气线组成的多孔微结构光纤也可以出现超连续谱展宽效应. 关键词： 多孔微结构光纤 超连续谱 有效折射率 色散     

非均匀微结构光纤中双折射现象的研究

报道了利用飞秒脉冲激光与非均匀微结构光纤相互作用中产生超连续光谱后在非均匀微结构光纤传输中双折射拍频现象的研究.利用35?fs的飞秒激光脉冲在高双折射微结构光纤中的传输过程中直接观察到了拍频现象.并利用有限元方法对该光纤进行了模拟计算分析，计算得出在600?nm处拍频长度为毫米量级.所得结果与实验一致. 关键词： 双折射效应 微结构光纤 超连续光谱 有限元法     

石英光子晶体光纤中高功率中红外超连续谱的产生

非石英光纤在产生大功率超连续谱方面存在难以克服的局限性.本文首次报道了采用石英光纤产生大功率中红外超连续谱.精心设计光纤结构使色散有利于超连续谱向中红外波段展宽,同时保证相对较大的芯径以承受较高的泵浦功率.合理选择光纤长度,在保证光谱展宽到3.4 μm的情况下使光纤损耗的影响降低到最小限度.研究表明,在1.95 μm皮秒脉冲泵浦下,采用色散适宜的石英光子晶体光纤可以产生20 dB带宽覆盖1 550~3 420 nm的超连续谱.超连续谱的平均功率可达56.6 W.     

Observation of discrete supercontinuum spectra from a multi-pulse pumped distributed feedback dye laser

A new approach to the generation of uniformly sliced supercontinuum is reported. It is based on a multi-pulse pumped distributed feedback dye laser rather than on the conventional propagation of femtosecond pulses through air or photonic molecules. Recent developments in supercontinuum generation and laser filamentation are critically reviewed to relate our findings to published work. Novelty of the proposed technique, in terms of ease of generation and economy, makes the method an attractive alternative. This work was carried out by pumping dye solution with six beams of a Q. switched and mode-locked Nd:YAG laser. A total energy of 421 J was used to generate a 5.5 nm wide discrete supercontinuum consisting of nine distinct coherent lines. The principle of multiple line generation may be extended to any desired spectral range depending upon the available gain medium and number of pumping pulses. Threshold power intensity for sliced supercontinuum generation using a frequency doubled Nd:YAG laser for solution of R6G in ethanol is found to be less than 24×10⁸ W/cm². Successful design and operation of a coherent off-white discrete supercontinuum source is also presented. PACS 42.60.By; 42.65.Re; 42.25.Bs     

Simulation of femtosecond pulse propagation in sub-micron diameter tapered fibers

Ultrashort pulse propagation and supercontinuum generation in tapered and microstructured optical fibers is usually simulated using the corrected nonlinear Schrödinger equation. One of the underlying approximations is the use of a wavelength-independent effective area or, equivalently, of a constant nonlinear coefficient . In very thin waveguide structures with strong light confinement, including silica wires and sub-micron tapered fibers and some microstructured fibers, the validity of such an approximation comes into question. In this paper we present an improved model in which all modal properties are fully taken into account as functions of the wavelength. We use comparative numerical simulation to identify certain regimes in which an improved model is needed for quantitatively correct results. PACS 02.70.Hm; 02.60.Cb; 42.65.Wi; 42.65.Re; 42.81.Pp     

Supercontinuum generation in visible to mid-infrared region in square-lattice photonic crystal fiber made from highly nonlinear glasses

Square-lattice microstructure geometry of photonic crystal fibers made with high nonlinearity non-silica glass composition material as well as fused silica glass are studied here for modal propagation, dispersion analysis and supercontinuum generation. Our investigation of nonlinear propagation of low threshold power femtosecond pulse aims at generating supercontinuum in the near and mid infrared region using these highly nonlinear fibers. We performed a series of analyses through varying parameters of square-lattice geometry to map a desired broadband window. We also demonstrate a compact high power broadband source in visible region generated within few centimeter length of the fused silica square-lattice microstructure.     

Design of solid core photonic bandgap fibers for visible supercontinuum generation

We present a numerical study of two dimensional solid core photonic bandgap fiber design criteria for their particular application to blue/visible supercontinuum generation. By exploiting their strong frequency-dependent dispersion when compared to index guiding micro-structured fibers, we highlight the design of solid core photonic bandgap fibers to fulfill group index matching conditions between the first ejected optical soliton and the trapped dispersive wave generated in the visible wavelength range. We study how these matching conditions depend on the opto-geometrical parameters of the micro-structured cladding, and we use frequency-domain numerical simulations to determine the expected supercontinuum spectral characteristics for selected cases. We investigate design criteria to generate short wavelengths by pumping in such photonic bandgap fibers in different pulse duration regimes and we identify a novel class of short wavelength (blue/visible) supercontinuum generation in the 3rd bandgap of a typical structure by pumping into the 2nd bandgap through a high attenuation spectral region.     

UV generation in a pure-silica holey fiber

We report supercontinuum generation extending to 300 nm in the UV from a pure-silica holey fiber. The broad spectrum was obtained by launching ultra-short pulses (150 fs, 10 nJ at 820 nm) from an amplified Ti:sapphire laser. The extension of holey-fiber-based supercontinuum generation into the UV should prove to be of immediate application in spectroscopy. By slightly detuning the launch conditions we excited a higher order spatial mode, which produced a narrower supercontinuum, but with enhanced conversion efficiency at a series of blue/UV peaks around 360 nm. We present numerical simulations, which suggest that differences in the dispersion profiles between the modes are an important factor in explaining this enhancement. In a related experiment, using the same laser source and fiber, we demonstrate a visible supercontinuum from several subsidiary cores, with distinct colours in each core. The subsidiary cores were excited by an appropriate input coupling. Fabrication of a fiber with a range of core sizes (dispersion profiles) for tailored supercontinuum generation can therefore be envisaged for practical applications. PACS 42.72.Bj; 42.79.Nv; 42.81.Dp     

The sublimation growth of AlN fibers: transformations in?morphology & fiber direction

The growth of AlN fibers using sublimation method was investigated in the temperature range from 1600 °C to 2000 °C. Large-scale AlN fibers are obtained with diameters from 100 nm to 50 μm and lengths up to several millimeters. The fiber morphology and growth direction are characterized by X-ray diffraction (XRD), field emission scanning electron microscope (SEM), high-resolution transmission electron microscopy (HRTEM), and Raman scattering. The fibers change from wire-like to prism-like in morphology and increase in diameter as rising temperatures, accompanying a transformation in axial direction from [10 ] to [0001]. The transformation in the growth direction is discussed in terms of AlN structure and supersaturation of AlN gas species. These results provide useful information for controlling the growth of large-scale AlN fibers.     

Visible supercontinuum generation in tapered photonic crystal fiber pumped by picosecond pulse at 1064 nm

Short photonic crystal fibers (PCFs) with different tapered waist diameter are made to extend the continuum spectrum in the visible range. The diversification of output continuum spectrum with the diameter of the tapered waist is experimentally observed. An all fiber visible supercontinuum source with 1.88 W output is demonstrated in our experiments. To the best of our knowledge, it is the highest all fiber visible supercontinuum generation in tapered PCF, pumping by picosecond pulse at 1064 nm. The suitably designed short tapered PCF can extend the visible spectrum, while, how to preserve the tapered waist is crucial for the all fiber visible supercontinuum source in the practical applications.     

Supercontinuum generation in short tellurite microstructured fibers pumped by a quasi-cw laser

We investigate supercontinuum (SC) generation in highly nonlinear tellurite microstructured fibers pumped by a continuous wave (cw)/quasi-cw laser. We investigate two types of tellurite fibers. One type has the constant core diameter, and the other type has a longitudinally varying core diameter. For the fibers with a constant core diameter, when pumped in the anomalous dispersion region, the SC is symmetric in a fiber that has a zero dispersion wavelength close to the pump wavelength. For the fibers with a longitudinally varying diameter, the calculated phase-matching conditions show that they have a broad wavelength range of dispersive waves, and therefore the measured SC spectrum can be broader than one octave. In this work, the fiber lengths are as short as several tens of centimeters, and the pump power is in the watt level.     

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.     

Fluorescence-Labeled Pyrenesulfonamide Response for Characterizing Polymeric Interfaces in Composite Materials

E-glass fibers were silanized using a 1% (v/v) aqueous solution of -aminopropyltriethoxysilane (APES). Pyrene—sulfonamide conjugates were formed by reaction of 1-pyrenesulfonyl chloride (PSC) in acetonitrile (AcN), with the amine groups immobilized on the glass fiber surface. These pyrene—sulfonamide conjugates were used as fluorescence probes, being a relatively simple analytical method to study the coating microstructure of polyorganosiloxane layer on glass fibers. The first aim of this work was to estimate possible interactions of the polyaminosiloxane coating with surrounding molecules of different solvents (solvent accessibility to the chromophore). For this study, the fluorescence response of pyrene—sulfonamide dye (PSA) was correlated with solvent polarity parameters. It was concluded that all the studied solvents were accessible to the chromophore, and they can gather in two groups, depending on their ability to swell the poliorganosiloxane layer. The second objective was to estimate the rigidity of the coating polymer from the temperature dependence of PSA emission. At about 180 K, a sudden change in the behavior of different photophysical parameters of PSA were observed. This phenomenon was interpreted as a density change in the polyaminosiloxane attached to the glass fibers.     

基于低非线性系数光子晶体光纤的全光纤高效超连续谱光源

报道了一种基于低非线性系数光子晶体光纤的全光纤高效率超连续谱产生系统。将光纤锁模激光器输出的脉宽5 ps、重复频率20 MHz、平均功率50 mW的脉冲,输入到15 μm的大模场光纤中进行放大,通过与两级芯径较小的短光纤模场匹配缩小输出的模场直径后,输入到20 m低非线性系数的光子晶体光纤,获得的超连续谱波长覆盖范围宽于650～1 700 nm。输入光子晶体光纤的泵浦光功率为740 mW,输出超连续光功率为670 mW,转换效率大于90%。实验研究了超连续光谱展宽的过程,从理论上进行了分析解释。     

基于低非线性系数光子晶体光纤的全光纤高效超连续谱光源

 报道了一种基于低非线性系数光子晶体光纤的全光纤高效率超连续谱产生系统。将光纤锁模激光器输出的脉宽5 ps、重复频率20 MHz、平均功率50 mW的脉冲,输入到15 μm的大模场光纤中进行放大,通过与两级芯径较小的短光纤模场匹配缩小输出的模场直径后,输入到20 m低非线性系数的光子晶体光纤,获得的超连续谱波长覆盖范围宽于650～1 700 nm。输入光子晶体光纤的泵浦光功率为740 mW,输出超连续光功率为670 mW,转换效率大于90%。实验研究了超连续光谱展宽的过程,从理论上进行了分析解释。     

Fabrication method for ultra-long optical micro/nano-fibers

Nonlinear optical interactions can be enhanced dramatically by tight light-confinement and long interaction-length. Optical fiber tapers with micro/nano-thickness waists considerably increase light-matter interactions in or near their waists. Here, we propose and demonstrate a novel tapering method of fabricating uniform, low-loss, and ultra-long micro/nano fibers. The technique comprises three steps for conventional flame-brushing and pulling, recalibration, and one-directional pulling. Using the proposed method, two strands of tapered fibers are fabricated, having 0.82-μm/1.6-μm diameters, 220-mm/500-mm uniform lengths with <55-nm/66-nm diameter variances, and high transmittances of 90.2%/91.5%. A figure of merit indicating the nonlinear-optic efficiency is defined and used to compare the results obtained in this study with those for tapered fibers in references. The proposed tapering method will be very useful for the fabrication of tapered fiber devices exploiting nonlinear optic effects, including Brillouin scattering, Raman amplification, and other third-order nonlinearities for supercontinuum generation.     

Pyrene-Doped Polyorganosiloxane Layers over Commercial Glass Fibers

Commercial glass fibers have been subjected to different activation treatments under neutral and acidic conditions to achieve different coating degrees when silanized with -aminopropyltriethoxisi lane (APES). A fluorescent sulfonamide (PSA) was formed between the amine residue and a fluorescent probe, pyrenesulfonyl chloride (PSC). Reflectance UV–Vis spectra of the pyrene-doped fibres show that pyrene is present in the form of preassociated dimers when the coating degree is low. Emission and excitation fluorescence spectra reveal the existence of a charge transfer ground-state complex with exciplex emission at 460–515 nm and absorption red-shifted with respect to the S₀ S₁ transition. Lifetime measurements yield three lifetimes, which are assigned to dimer, exciplex, and monomer emission. From the photophysical data it is concluded that the fibers with the highest silane content have an open structure with the highest fraction of isolated fluorescent moieties.     

高激光损伤阈值Ge-As-S硫系玻璃光纤及中红外超连续谱产生

测量了Ge-As-S系列硫系玻璃在中红外波段的飞秒激光损伤阈值,研究了它与玻璃化学组成的关系.基于优化的玻璃组成,采用棒管法制备了芯径为15μm的阶跃折射率非线性光纤.采用飞秒脉冲抽运光纤,研究了光纤中超连续谱(supercontinuum,SC)的产生特性.在研究的Ge-As-S硫系玻璃中,具有化学计量配比的Ge0....     

Mode emission characteristics of semiconductor microdisc and microring lasers

We fabricate both microdisc and microring InGaAsP-InGaAs quantum well semiconductor lasers using an etching-and-bonding technique. Their emission properties are studied by optical pumping at 77 K. Single mode lasing is achieved for discs with a diameter of 5 m. Multimode emission is observed for rings with larger diameters. Both fundamental and higher order lasing modes are observed and are found to exhibit interesting emission characteristics.