Gain dependent pulse regimes transitions in a dissipative dispersion-managed fibre laser

For the first time, we demonstrate the possibility to switch between three distinct pulse regimes in a dissipative dispersion-managed (DM) fibre laser by solely controlling the gain saturation energy. Nonlinear Schrödinger equation based simulations show the transitions between hyper-Gaussian similaritons, parabolic similaritons, and dissipative solitons in the same laser cavity. It is also shown that such transitions exist in a wide dispersion range from all-normal to slightly net-normal dispersion. This work demonstrates that besides dispersion and filter managements gain saturation energy can be a new degree of freedom to manage pulse regimes in DM fibre lasers, which offers flexibility in designing ultrafast fibre lasers. Also, the result indicates that in contrast to conservative soliton lasers whose intensity profiles are unique, dissipative DM lasers show diversity in pulse shapes. The findings not only give a better understanding of pulse shaping mechanisms in mode-locked lasers, but also provide insight into dissipative systems.

Intensity dynamics and statistical properties of random distributed feedback fiber laser

We present first experimental investigation of fast-intensity dynamics of random distributed feedback (DFB) fiber lasers. We found that the laser dynamics are stochastic on a short time scale and exhibit pronounced fluctuations including generation of extreme events. We also experimentally characterize statistical properties of radiation of random DFB fiber lasers. We found that statistical properties deviate from Gaussian and depend on the pump power.

Fabrication of surface nanoscale axial photonics structures with a femtosecond laser

Surface nanoscale axial photonics (SNAP) structures are fabricated with a femtosecond laser for the first time, to the best of our knowledge. The inscriptions introduced by the laser pressurize the fiber and cause its nanoscale effective radius variation. We demonstrate the subangstrom precise fabrication of individual and coupled SNAP microresonators having the effective radius variation of several nanometers. Our results pave the way to a novel ultraprecise SNAP fabrication technology based on the femtosecond laser inscription.

Random distributed feedback fiber laser

Researchers conducted investigations to demonstrate the advantages of random distributed feedback fiber laser. Random lasers had advantages, such as simple technology that did not require a precise microcavity and low production cost. The properties of their output radiation were special in comparison to those of conventional lasers and they were characterized by complex features in the spatial, spectral, and time domains. The researchers demonstrated a new type of one-dimensional laser with random distributed feedback based on Rayleigh scattering (RS) that was presented in any transparent glass medium due to natural inhomogeneities of refractive index. The cylindrical fiber waveguide geometry provided transverse confinement, while the cavity was open in the longitudinal direction and did not include any regular point-action reflectors.

Isolator-free switchable uni- and bidirectional hybrid mode-locked erbium-doped fiber laser

An Erbium-doped fibre ring laser hybrid mode-locked with single-wall carbon nanotubes (SWNT) and nonlinear polarisation evolution (NPE) without an optical isolator has been investigated for various cavity conditions. Precise control of the state of polarisation (SOP) in the cavity ensures different losses for counter-propagating optical fields. As the result, the laser operates in quasi-unidirectional regime in both clockwise (CW) and counter-clockwise (CCW) directions with the emission strengths difference of the directions of 22 dB. Furthermore, by adjusting the net birefringence in the cavity, the laser can operate in a bidirectional generation. In this case, a laser pumped with 75 mW power at 980 nm generates almost identical 790 and 570 fs soliton pulses with an average power of 1.17 and 1.11 mW. The operation stability and pulse quality of the soliton pulses in both unidirectional regimes are highly competitive with those generated in conventional ring fibre lasers with isolator in the cavity. Demonstrated bidirectional laser operation can find vital applications in gyroscopes or precision rotation sensing technologies.

Broadly tunable dual-wavelength InAs/GaAs quantum-dot laser for THz generation

We demonstrate an ultra-compact, room-Temperature, continuous-wave, broadly-Tunable dual-wavelength InAs/GaAs quantum-dot external-cavity diode laser in the spectral region between 1150 nm and 1301 nm with maximum output power of 280 mW. This laser source generating two modes with tunable difference-frequency (300 GHz-30 THz) has a great potential to replace commonly used bulky lasers for THz generation in photomixer devices.

Rogue waves generation via nonlinear soliton collision in multiple-soliton state of a mode-locked fiber laser

We report for the first time, rogue waves generation in a mode-locked fiber laser that worked in multiple-soliton state in which hundreds of solitons occupied the whole laser cavity. Using real-time spatio-temporal intensity dynamics measurements, it is unveiled that nonlinear soliton collision accounts for the formation of rogue waves in this laser state. The nature of interactions between solitons are also discussed. Our observation may suggest similar formation mechanisms of rogue waves in other systems.

Statistical properties of radiation of multiwavelength random DFB fiber laser

In the presented paper, the temporal and statistical properties of a Lyot filter based multiwavelength random DFB fiber laser with a wide flat spectrum, consisting of individual lines, were investigated. It was shown that separate spectral lines forming the laser spectrum have mostly Gaussian statistics and so represent stochastic radiation, but at the same time the entire radiation is not fully stochastic. A simple model, taking into account phenomenological correlations of the lines' initial phases was established. Radiation structure in the experiment and simulation proved to be different, demanding interactions between different lines to be described via a NLSE-based model.

Influence of atomic densities on propagation property for ultrashort pulses in a two-level medium

The influence of atomic densities on the propagation property for ultrashort pulses in a two-level atom (TLA) medium is investigated. With higher atomic densities, the self-induced transparency (SIT) cannot be recovered even for 2π ultrashort pulses. New features such as pulse splitting, red-shift and blue-shift of the corresponding spectra arise, and the component of central frequency gradually disappears.

相干模表示的部分相干高斯谢尔光束的相位空间积

与传统的相干激光光束的评价不同, 对部分相干光束质量进行评价时, 不仅要反映其远场发散特性, 而且还要能体现光源本身的部分相干性。根据部分相干光的相干模表示法, 推导了由部分相干光源所产生光束的相位空间积Q。与前人在相干光源情形下得到的结论相比, 得出的相位空简积Q不仅包含各个全相干模基元线性组合的贡献, 而且还有来自不同的全相干模基元之间相互作用的贡献。

激光脉冲宽度对有质动力加速电子的影响

基于真空中单电子运动模型，编程计算得到了高斯激光脉冲与初始位于激光传播轴上电子的相互作用结果。不同激光参鼍条件下，得到了电子的能量增益与激光强度、焦斑大小和脉冲宽度关系。结果表明，高斯激光脉冲焦斑较大时，电子没有明显的能量增益，高斯激光脉冲焦斑太小时，电子也没有明显的能量增益。电子的能量增益有一个最佳焦斑大小。在相同激光强度下，电子能量增益的最佳焦斑大小随脉冲宽度的增大而增大，但最佳焦斑大小与脉冲宽度的比值基本上是不变的。

变栅距光栅实现啁啾脉冲光谱整形

为了充分利用放大介质的增益带宽，获得脉宽更短，功率更高的输出脉冲，需要将输入到主放大链的种子脉冲进行光谱整形来补偿放大过程中的增益窄化效应。提出了利用变栅距反射光栅实现中心波长1053nm，谱宽6nm啁啾脉冲的光谱整形。运用严格的光栅衍射耦合波理论分析光栅的衍射特性，发现该方案不会引入相位畸变。分别计算和分析了刻槽深度、入射角大小、光栅周期以及入射光波长的变化对衍射效率的影响，通过选取适当的光栅参量可获得0．5％～84％的光谱调制深度。

Montage et caractérisation d’un système de spectroscopie Raman accordable en longueur d’onde utilisant des réseaux de Bragg comme filtre : application aux nanotubes de carbone

La spectroscopie Raman est un outil non destructif fort utile lors de la caractérisation de matériau. Cette technique consiste essentiellement à faire l’analyse de la diffusion inélastique de lumière par un matériau. Les performances d’un système de spectroscopie Raman proviennent en majeure partie de deux filtres ; l’un pour purifier la raie incidente (habituellement un laser) et l’autre pour atténuer la raie élastique du faisceau de signal. En spectroscopie Raman résonante (SRR), l’énergie (la longueur d’onde) d’excitation est accordée de façon à être voisine d’une transition électronique permise dans le matériau à l’étude. La section efficace d’un processus Raman peut alors être augmentée d’un facteur allant jusqu’à 106. La technologie actuelle est limitée au niveau des filtres accordables en longueur d’onde. La SRR est donc une technique complexe et pour l’instant fastidieuse à mettre en œuvre. Ce mémoire présente la conception et la construction d’un système de spectroscopie Raman accordable en longueur d’onde basé sur des filtres à réseaux de Bragg en volume. Ce système vise une utilisation dans le proche infrarouge afin d’étudier les résonances de nanotubes de carbone. Les étapes menant à la mise en fonction du système sont décrites. Elles couvrent les aspects de conceptualisation, de fabrication, de caractérisation ainsi que de l’optimisation du système. Ce projet fut réalisé en étroite collaboration avec une petite entreprise d’ici, Photon etc. De cette coopération sont nés les filtres accordables permettant avec facilité de changer la longueur d’onde d’excitation. Ces filtres ont été combinés à un laser titane : saphir accordable de 700 à 1100 nm, à un microscope «maison» ainsi qu’à un système de détection utilisant une caméra CCD et un spectromètre à réseau. Sont d’abord présentés les aspects théoriques entourant la SRR. Par la suite, les nanotubes de carbone (NTC) sont décrits et utilisés pour montrer la pertinence d’une telle technique. Ensuite, le principe de fonctionnement des filtres est décrit pour être suivi de l’article où sont parus les principaux résultats de ce travail. On y trouvera entre autres la caractérisation optique des filtres. Les limites de basses fréquences du système sont démontrées en effectuant des mesures sur un échantillon de soufre dont la raie à 27 cm-1 est clairement résolue. La simplicité d’accordabilité est quant à elle démontrée par l’utilisation d’un échantillon de NTC en poudre. En variant la longueur d’onde (l’énergie d’excitation), différentes chiralités sont observées et par le fait même, différentes raies sont présentes dans les spectres. Finalement, des précisions sur l’alignement, l’optimisation et l’opération du système sont décrites. La faible acceptance angulaire est l’inconvénient majeur de l’utilisation de ce type de filtre. Elle se répercute en problème d’atténuation ce qui est critique plus particulièrement pour le filtre coupe-bande. Des améliorations possibles face à cette limitation sont étudiées.

Nonlinear refractive index of RECOB (RE = Gd and La) crystals

The Z-scan technique is employed to obtain the nonlinear refractive index (n (2)) of the Ca(4)REO(BO(3))(3) (RECOB, where RE = Gd and La) single crystals using 30 fs laser pulses centered at 780 nm for the two orthogonal orientations determined by the optical axes (X and Z) relative to the direction of propagation of the laser beam (k//Y// crystallographic b-axis). The large values of n (2) indicate that both GdCOB and LaCOB are potential hosts for Yb:RECOB lasers operating in the Kerr-lens mode locking (KLM) regime.

Cellular and tissue effects induced by photogemA (R) and red LED in photodynamic therapy

In order to consider the photodynamic therapy (PDT) as a clinical treatment for candidosis, it is necessary to know its cytotoxic effect on normal cells and tissues. Therefore, this study evaluated the toxicity of PDT with PhotogemA (R) associated with red light-emitting diode (LED) on L929 and MDPC-23 cell cultures and healthy rat palatal mucosa. In the in vitro experiment, the cells (30000 cells/cm(2)) were seeded in 24-well plates for 48 h, incubated with PhotogemA (R) (50, 100, or 150 mg/l) and either irradiated or not with a red LED source (630 +/- 3 nm; 75 or 100 J/cm(2); 22 mW/cm(2)). Cell metabolism was evaluated by the MTT assay (ANOVA and Dunnet`s post hoc tests; p < 0.05) and cell morphology was examined by scanning electron microscopy. In the in vivo evaluation, PhotogemA (R) (500 mg/l) was applied to the palatal mucosa of Wistar rats during 30 min and exposed to red LED (630 nm) during 20 min (306 J/cm(2)). The palatal mucosa was photographed for macroscopic analysis at 0, 1, 3, and 7 days posttreatment and subjected to histological analysis after sacrifice of the rats. For both cell lines, there was a statistically significant decrease of the mitochondrial activity (90-97%) for all PhotogemA (R) concentrations associated with red LED regardless of the energy density. However, in the in vivo evaluation, the PDT-treated groups presented intact mucosa with normal characteristics both macroscopically and histologically. From these results, it may be concluded that the association of PhotogemA (R) and red LED caused severe toxic effects on normal cell cultures, characterized by the reduction of mitochondrial activity and morphological alterations, but did not cause damage to the rat palatal mucosa in vivo.