Laser filaments generated and transmitted in highly turbulent air

The initiation and propagation of a filament generated by ultrashort laser pulses in turbulent air is investigated experimentally. A filament can be generated and propagated even after the beam has propagated through strongly turbulent regions, with structure parameters C(n)2 as many as 5 orders of magnitude larger than those encountered in the usual atmospheric conditions. Moreover, the filament's position within the beam is not affected by the interaction with a turbulent region. This remarkable stability is allowed by the strong Kerr refractive-index gradients generated within the filament, which exceed the turbulence-induced refractive-index gradients by 2 orders of magnitude.     

Self-assembled volume vortex grating induced by femtosecond laser pulses in glass

We presented a microfabrication process for optical volume vortex grating inside glass by femtosecond laser pulses. The self-trapped filament of femtosecond laser pulses can induce hundreds μm-long region refractive-index changes in glass. We realized the restructured optical vortex beams using a collimated He–Ne laser beam. The maximum first-order diffraction efficiency was about 19.6%. The volume vortex grating structure fabricated in glass is polarization dependent.     

Refractive index change mechanisms in femtosecond laser written ceramic Nd:YAG waveguides: micro-spectroscopy experiments and beam propagation calculations

The effect that femtosecond laser filamentation has on the refractive index of Nd:YAG ceramics, and which leads to the formation of waveguide lasers, has been studied by micro-spectroscopy imaging, beam propagation experiments and calculations. From the analysis of the Nd³⁺ luminescence and Raman images, two main types of laser induced modifications have been found to contribute to the refractive-index change: (i) a lattice defect contribution localized along the self-focusing volume of the laser pulses, in which lattice damage causes a refractive-index decrease, and (ii) a lattice strain-field contribution around and inside the filaments, in which the pressure-driven variation of the inter-atomic distances causes refractive-index variations. Scanning near-field optical-transmission and end-coupling experiments, in combination with beam propagation calculations, have been used to quantitatively determine the corresponding contribution of each effect to the refractive-index field of double-filament waveguides. Results indicate that the strain-field induced refractive-index increment is the main mechanism leading to waveguiding, whereas the damage-induced refractive-index reduction at filaments leads to a stronger mode confinement.     

Fabrication of high-efficiency diffraction gratings in glass

We investigated a microfabrication process for optical gratings with periods of micrometer order that use ultrafast laser pulses in semiconductor-doped glass. ZnS- or PbS-doped SiO2-A12O3-B2O3-CaO-ZnO-Na2O-K2O glass was prepared by a melting method. Glass transmission diffraction gratings with a high refractive-index difference were fabricated with femtosecond laser pulses. The first-order diffraction efficiencies of these gratings were approximately 80%, and the first-order diffraction angles of these gratings were 8 degrees at telecommunication wavelengths.     

Half-cycle pulses in the mid-infrared from a two-color laser-induced filament

Four-wave mixing (FWM) of femtosecond near-infrared laser pulses and its second harmonic in the filamentation regime is shown to give rise to ultrashort field waveforms in the mid-infrared with pulse widths as short as a half of the field cycle and produce ultrabroadband supercontinuum spectra stretching from the mid-IR to the terahertz region. Generation of 7-fs pulses centered at 4.35 μm is demonstrated by a two-color filamentation experiment, where the 25-fs, 800-nm fundamental-wavelength output of a Ti: Sapphire laser is mixed with its second harmonic. The spectral and temporal properties of the mid-IR waveforms, as well as their emission pattern, are consistent with the FWM scenario of frequency conversion generalized to include the Kerr effect and ionization-induced refractive-index modulation.     

Oscillation of the refractive index at the focal region of a femtosecond laser pulse inside a glass

The temporal evolution of refractive-index change produced by a tightly focused femtosecond (fs) laser pulse inside a soda-lime glass plate was investigated by use of a transient lens method with subpicosecond time resolution. An oscillating behavior of the light intensity in the central region of the probe beam was observed 0-1500 ps after irradiation of the plate. The oscillation was interpreted in terms of a rapid temperature increase and the ensuing propagation of the pressure wave. This study is to our knowledge the first real-time observation of refractive-index change inside a glass induced by a fs laser pulse.     

Infrared photosensitivity in silica glasses exposed to femtosecond laser pulses

We investigate the use of infrared femtosecond laser pulses to induce highly localized refractive-index changes in fused-silica glasses. We characterize the magnitude of the change as a function of exposure and measure index changes as large as 3x10(-3) and 5x10(-3) in pure fused silica and boron-doped silica, respectively. The potential of this technique for writing three-dimensional photonic structures in bulk glasses is demonstrated by the fabrication of a Y coupler within a sample of pure fused silica.     

Direct femtosecond laser writing of waveguides in As2S3 thin films

Single-channel waveguides and Y couplers were fabricated in chalcogenide thin films by use of femtosecond laser pulses from a 25-MHz repetition rate Ti:sapphire laser. Refractive-index differentials (delta n > 10(-2)) were measured through interferometric microscopy and are higher than the typical values reported for oxide glasses. The dependence of the index differential on the peak intensity reveals the nonlinear nature of the photosensitivity in arsenic trisulfide below its bandgap energy, and the refractive-index change is correlated to the photoinduced structural changes inferred by Raman spectroscopy data. A free-electron model to predict the parametric dependence of delta n is proposed.     

Backward supercontinuum emission from a filament generated by ultrashort laser pulses in air

Backward emission of the supercontinuum from a light filament induced by high-intensity femtosecond laser pulses propagating in air has been observed to be enhanced compared with linear Rayleigh-Mie scattering. This enhancement is interpreted as a nonlinear scattering process onto longitudinal refractive-index changes induced by the laser pulse itself. The spectral dependence of the supercontinuum angular distribution is also investigated.     

基于拉盖尔-高斯光束的单光子捕获概率研究

单光子源通常采用基于高斯光束的高度衰减激光脉冲,假设激光束具有初始高斯时域脉冲波形和TEM01模拉盖尔-高斯空域分布.基于折射率起伏的Rytov近似和修正von Karman谱模型,研究了大气湍流对星地量子通信单光子捕获概率的影响;建立了上行信道和下行信道的单光子捕获概率理论模型;针对低轨卫星-地面站间激光链路,对单光子捕获概率进行了分析.结果表明:上行信道的单光子捕获概率强烈依赖于地面折射率结构常数C2n(0),且随着C2n(0)的增加而减小;然而,下行信道的单光子捕获概率并不依赖于C2n(0),即大气湍流对其没有影响.     

Wavelength division with three-dimensional couplers fabricated by filamentation of femtosecond laser pulses

Refractive-index changes can be induced by filamentation of 800-nm, 1-kHz femtosecond laser pulses in silica glass. Two-dimensional translation of a 40-microm-long filament leads to the formation of a curved waveguide because of bending by the previously induced refractive-index change. The fabrication of 2-mm directional couplers to split the coupled beam into 1:1 at a wavelength of 632.8 nm is demonstrated. The realization of three-dimensional directional couplers and wavelength division in the output from the couplers is also demonstrated.     

Composition dependence of the photoinduced refractive-index change in lead silicate glasses

A large photoinduced refractive-index change (as great as Dn=0.21+/-0.04) is obtained in lead silicate glasses by irradiation with the frequency-quadrupled output of a Q -switched YAG laser (266 nm). An approximately exponential relationship is found between the photoinduced refractive-index change and the lead cation mole fraction over the composition range from 18.7% to 57%. The induced refractive-index change is permanent and shows no decay after heating to 360 degrees C during 1 h. Dispersion of the refractive-index change suggests that the photosensitivity is associated with changes in the intrinsic glass absorption edge.     

Generation of soliton-like wave packets and wave packets with linear phase modulation in gaining optical wave-guides with saturable nonlinearity

We have considered the dynamics of soliton-like pulses and stable frequency-modulated self-similar pulses in an active medium with saturable nonlinearity and a parabolic refractive-index profile. We show that, based on gaining gradient fibers with a parabolic distribution of the refractive index and saturable nonlinearity, it is possible to create complexes that ensure generation of laser pulses with a high (above 1 TW) peak power.     

Birefringent Fresnel zone plates in silica fabricated by femtosecond laser machining

We demonstrate maskless, single-step fabrication of strongly birefringent Fresnel zone plates by focusing of femtosecond laser pulses deep within silica substrates. The process allows us to produce alternate zone rings directly by inducing a local refractive-index modification of the order of n~10(-2) . The embedded zone plates shown in this Letter exhibit efficiencies that vary by as much as a factor of ~6 for orthogonal polarizations. Focal lengths of primary and secondary foci are shown to compare well with theory.     

Dynamics of a Wave Packet of Whispering-Gallery-Mode Type in an Optical Waveguide in the Presence of a Traveling Refractive-Index Wave

It is demonstrated that strong ultrafast modulation of a tunneling wave packet of the whispering-gallery-mode type can be implemented in a cylindrical optical waveguide exhibiting spatiotemporal inhomogeneity induced by a travelling refractive-index wave. The corresponding optical waveguides can be used for efficient generation of picosecond and subpicosecond laser pulses.     

In situ observation of dynamics of plasma self-channeling and bulk modification in silica glasses induced by a high-intensity femtosecond laser

The time-resolved dynamics of plasma self-channeling and refractive index bulk modification in silica glasses were first observed in situ using a high-intensity femtosecond (110 fs) Ti:sapphire laser (λ_p=790 nm). Plasma channeling is induced in silica glass at an irradiation higher than an input intensity of 1.5×10¹² W/cm² and photoinduces either the refractive-index modification or optical crack modification. In the domain of refractive-index modification, the lifetime of induced plasma self-channeling was 20 ps and the structural transition time for forming the refractive-index change was 10 ps. In the domain of optical cracks, however, the lifetime of induced plasma formation was 30 ps and the structural transition time for forming the optical cracks was 40 ps. According to electron spin resonance spectroscopic (ESP) measurement, it was found that the defect concentration of the SiE^′ center increased significantly in the refractive index modification region. A maximum value of the refractive-index change Δn was measured to be 1.6×10^-2. The intensity profile of the output beam transmitted through the refractive-index modification showed that the bulk modification produced a permanent optical waveguide. Received: 8 April 2002 / Accepted: 12 April 2002 / Published online: 22 November 2002 RID="*" ID="*"Corresponding author. Fax: +81-48/462-4682, E-mail: shcho@riken.go.jp     

Dynamics of plasma formation and permanent structural transformation in ZBLAN excited by tightly focused femtosecond laser pulses

Time-resolved dynamics of plasma formation and bulk refractive-index modification in fluoride glass (ZBLAN) excited by a tightly focused femtosecond (130 fs) Ti:sapphire laser (λ_p=790 nm) was observed in situ. The femtosecond time-resolved pump–probe measurement with perpendicularly linear polarized beams was used to study the dynamics of both plasma formation and induced permanent structural transformation with refractive-index change. In the refractive-index domain, the lifetime of induced plasma formation is ～35 ps and structural transition time for forming the refractive-index change is ～80 ps. In the optical damage domain, however, the lifetime of induced plasma formation is ～40 ps and structural transition time for forming the optical damage is ～140 ps. We found that the process of refractive-index bulk modification is significantly different from that of optical cracks. From the diffraction efficiency of Kogelnik's coupled mode theory, the maximum value of refractive-index change (Δn) was estimated to be 1.3×10^?2. By the scanning of fluoride glass on the optical X–Y–Z stages, the fabrication of internal grating with refractive-index modification was demonstrated in fluoride glass using tightly focused femtosecond laser.     

High photosensitivity and nanometer-scale phase separation in GeO(2)-SiO(2) glass thin films

Amorphous xGeO(2)-(1-x)SiO(2) thin films exhibit large negative index changes (4-8%) in the high GeO(2) region (x>~0.25) on irradiation with ArF laser pulses. The sign of the index change is opposite the low GeO(2) region X<0.25, and the magnitude of the index change is larger by an order of magnitude than that reported so far. Cross-sectional transmission electron microscope observation has revealed that nanometer-scale phase separation is induced in these highly photosensitive glasses by irradiation with ArF excimer laser light pulses or electron beams. This is a first finding of microphase separation in SiO(2)-GeO(2) glasses by irradiation and provides an essential constraint on the modeling of photonic effects induced by irradiation in these glasses.     

Writing Waveguide in LN With fs Laser

We investigated the waveguide formation in Lithium Niobate with Femtosecond laser pulse writing directly. The output optical field through waveguide has been observed and refractive-index change was characterized by using grating method.     

飞秒激光诱导有机薄膜自组装微结构

通过改变辐照激光脉冲数、激光的平均功率、显微镜物镜倍数/数值孔径等研究微皇冠结构形成与加工条件的关系,并借鉴激光熔池形成和液滴溅射的模型对其进行理论分析。当激光脉冲数达到一定数量后,烧蚀区域会出现微皇冠结构。烧蚀区域尺寸的改变是由于形成的液体区域中热毛细作用和化学毛细作用共同所致;微皇冠结构溅射个数的改变是由于形成的气体因多光子吸收体积迅速膨胀所致。