Glass Laser Pulses Research Articles

Influence of ambient air pressure on the energy conversion of laser-breakdown induced blast waves

Influence of ambient pressure on energy conversion efficiency from a Nd : glass laser pulse (λ = 1.053 µm) to a laser-induced blast wave was investigated at reduced pressure. Temporal incident and transmission power histories were measured using sets of energy meters and photodetectors. A half-shadowgraph half-self-emission method was applied to visualize laser absorption waves. Results show that the blast energy conversion efficiency ηbw decreased monotonically with the decrease in ambient pressure. The decrease was small, from 40% to 38%, for the pressure change from 101 kPa to 50 kPa, but the decrease was considerable, to 24%, when the pressure was reduced to 30 kPa. Compared with a TEA-CO2-laser-induced blast wave (λ = 10.6 µm), higher fraction absorption in the laser supported detonation regime ηLSD of 90% was observed, which is influenced slightly by the reduction of ambient pressure. The conversion fraction ηbw/ηLSD≈90% was achieved at pressure >50 kPa, which is significantly higher than that in a CO2 laser case.

Time resolved measurements of gratings photo-induced by femtosecond pulses in a lead doped glass

We report on the formation of gratings photo-induced by femtosecond laser pulses in SF59 glass. Depending on the number of pulses used to excite the sample and the pump power density, transient or permanent gratings are induced. We demonstrate that the grating formation is not instantaneous and is produced by laser-induced defects. This results in a change of both the real and imaginary part of the index of refraction. A simple set-up that records the temporal evolution of both parameters during the laser excitation is also presented. It makes it possible to evaluate the weight of both contributions to the grating diffraction efficiency.

Correlation of the refractive index change at the surface and inside phosphate glass upon femtosecond laser irradiation

A study of the surface modification induced by single femtosecond laser pulses in phosphate glass has revealed surface swelling at fluences just below the ablation threshold. This behavior is different from that observed in other dielectric materials. Optical micrographs obtained with monochromatic light show a pattern of Newton rings within the swollen region whose number scales inversely with the illumination wavelength, acting as a micro Fabry–Perot etalon. The swollen surface layer has lower refractive index than the bulk glass and can reach a maximum thickness of 820 nm. We relate these findings to results obtained during subsurface waveguide writing inside phosphate glass, which also show a refractive index decrease at energies near threshold for waveguide fabrication. We have identified low density free-electron plasma to be the trigger of the refractive index change.

Ultraviolet conical emission produced by high-power femtosecond laser pulse in transparent media

In this work, supercontinuum conical emission (SC CE) accompanying the filamentation of powerful ultrashort laser pulse in BK7 glass and fused silica is studied. The SC CE is controlled by the laser power density and the sample thickness, and the minimum SC CE cut-off wavelength is about 309 nm in the BK7 glass and 237 nm in the fused silica. The angular distributions of the SC CE in the wavelength range less than 510 nm are measured by using a new method, and it cannot be explained by the Cerenkov emission theory but the unabridged X-Waves solution theory. Meanwhile numerical simulations of the propagation of femtosecond laser pulse in sample are performed to provide theoretical support to our results.

Transmission properties of a new glass ceramic and doped with Co2+ as saturable absorber for 1.54 µm Er glass short pulse laser

The preparation and characteristics of a new transparent glass ceramic were described. Crystal phase particles with nanometer size were successfully precipitated in glass matrix, which was confirmed to be one of indium aluminum zinc oxide compounds (InxAlyZnzO). The presence of aluminum (Al) and indium (In) impurities in the zinc oxides (ZnO) crystal lattice leads to some changes of the carrier concentration in the material and then promote the sharply changes of transmission spectra in IR range wavelength. And subsequently, the IR cut-off edge blue shifted from 5.5 μm in base glass to 3 μm in transparent glass ceramic sample. Furthermore, passive Q switched 1.54 μm Er glass laser pulses with pulse energy of 10 mJ and pulse width of 800 ns were successfully obtained by using the cobalt doped transparent glass ceramic as a saturable absorber.

The theoretical study on abnormal experimental phenomenon of the elongated rod Nd:glass pulsed laser

According to the principle of laser and a large number of experimental data proved and also considering the influence of the ignored spontaneous emisson, we study the abnormal phenomenon that the Nd: glass rod which was longer than the optical pump cavity is optically amplified without pumping. The theoretical result is almost the same as experimental result.

Time-resolved axial-view of the dielectric breakdown under tight focusing in glass

We present time-resolved studies of the dielectric breakdown using tightly focused femtosecond (fs) laser pulses in glass. Axial evolution of the breakdown and material modifications have been retrieved over the time span from 0 to 1 ns with a 50 fs resolution and ∼ 1 μm spatial resolution using interferometric pump-probe technique. It is shown that even at pulse power slightly above critical Pcr ≃ 1 MW/pulse, the filamentation was limited at tight focusing and the central focal region with resolidified glass was localised axially within ∼ 10 μm; it can be used for the waveguide recording. Mechanisms of light-matter interaction at tight focusing and application potential are discussed. The electron-ion scattering time, τe–i ≃ 1.1 fs, for the glass at electron concentration ne ≃ (4–5)×1020 cm−3 was determined within Drude approximation.

Low threshold power density for the generation of frequency up-converted pulses in bismuth glass by two crossing chirped femtosecond pulses

We investigated the generation of frequency up-converted femtosecond laser pulses by nondegenerate cascaded four-wave mixing (CFWM) in a bismuth-oxide glass (BI glass). Broad-bandwidth light pulses with different propagation directions were simultaneously obtained by using two small-angle crossing femtosecond laser pulses in BI glass. Experimental results show that the threshold power density for the generation of broad-bandwidth femtosecond pulses in BI glass is one order of magnitude lower than that in fused silica.

Nanomorphing with Ultrafast Lasers and Biomedical Applications

ABSTRACTClassically, the limit for optical machining is on the order of the wavelength of the incident light. However, by taking advantage of precise, nonlinear damage mechanisms that occur for femtosecond laser pulses, damage can be achieved on a scale an order of magnitude lower, allowing precise removal of very small amounts of material to produce holes mere tens of nanometers wide. Femtosecond laser nanomachining can be carried out in a variety of dielectrics, and in transparent substrates machining can be sub-surface, in contrast to other nanomachining techniques such as using an electron beam or focused ion beam. We focus on the use of glass, as it is in many ways an ideal material for use in biological applications due to its chemical, optical, electrical and mechanical properties. By precisely placing laser pulses in glass, three dimensional nano and microfluidic channels and devices can be formed including nozzles, mixers, and separation columns. Recent advances in this technique allow the formation of high aspect ratio nanochannels from single pulses, thus helping address the fabrication speed limitations presented by serial processing. These nanochannels have a range of applications including the fabrication of nanoscale pores and nanowells that may serve as vias between fluidic channels, or from channels to a surface. These nanochannels have applications as a standalone technique for fabrication of nanopores and nanowells, but can also complement other fabrication techniques by allowing precisely placed jumpers that can connect channels that are out of plane. We discuss applications for diagnostic microfluidic devices, and basic cell biology research.

Numerical study on the efficient generation of 351 nm broadband pulses by frequency mixing of broadband and narrowband Nd: glass lasers

Efficient frequency-tripling of phase-modulated broadband Nd:glass laser pulses is of interest to inertial confinement fusion. We report and theoretically study an efficient frequency-tripling scheme for 351 nm broadband pulses generation by use of broadband and narrowband Nd:glass lasers. Based on the conventional two-crystal doubling and tripling baseline configuration, employing an additional narrowband laser can increase the bandwidth acceptance of Nd:glass up to 320 GHz, which is 3.5 times larger than that of using the conventional configuration. The proposed scheme may also dramatically reduce the conversion of amplitude modulation from the frequency modulation. The efficiency sensitivities on both the fundamental intensity and the doubler orientation are discussed for practical applications.

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.

Characteristics of filamentation in ZK7 glass by negatively chirped femtosecond laser pulses

The filamentation characteristics of femtosecond laser pulses in ZK7 glass are investigated experimentally as a function of initial negative chirps. It is found that the filament threshold power grows rapidly and the filament length extends over a long distance with increasing initial temporal chirps. The measurement of supercontinuum reveals that the plasma generation process within filamentation becomes weaker as the initial negative chirp increases, leading to a self-guiding long light channel dominated by Kerr nonlinearity. The interference of transverse rings in multifilamentation of the chirped laser pulses is observed as well. Analyses and discussions give an interpretation of this chirp-induced ionization-free filamentation. These results indicate that initial chirps will play a crucial role in the filament formation of ultrashort laser pulses in transparent media.

Surface oxidation of low carbon steel during laser treatment, its dependence on the initial microstructure and influence on the laser energy absorption

Laser treatment of steels in air is accompanied with formation of oxide layer on the surface. This leads to significant change of the absorption coefficient for the laser radiation and thus influences the temperature field in the material and the level of properties modification. By the methods of X-ray powder diffraction the parameters of the oxide films formed during surface laser treatment of sheet low carbon steels with Nd:glass pulsed laser have been investigated. The obtained experimental results show that the initial microstructure influences the parameters of the formed oxide films. The kinetics of the oxide film growth at the initial stages (within the laser pulse duration, 7 ms) has been simulated by the created computer codes based on the finite elements method. During the performed simulations the temperature dependencies of the thermal conductivity and specific heat capacity have been considered. The influence of the growing oxide film on the laser energy absorption has been analysed.

Acoustic monitoring of microplasma formation and filamentation of tightly focused femtosecond laser pulses in silica glass

Contact acoustic technique has been employed to perform spatially resolved in situ detection of microplasma formation and filamentation of tightly focused femtosecond laser pulses with supercritical pulse powers in bulk dielectrics, via corresponding acoustic emission. Investigation of acoustic generation mechanisms related to the plasma formation and filamentation effects reveals the critical character of the opaque microplasma and provides estimates of its gigapascal-level pressures and energy densities of a few kJ∕cm3. The acoustic measurement enables real-time in situ monitoring and revealing of basic mechanisms of ionization and filamentation in bulk dielectrics.

Concept study of a 100-PW femtosecond laser based on laser ceramics doped with chromium ions

A new concept of a superpowerful femtosecond laser based on CPA in polycrystalline chromium doped ceramics (Cr:YAG, Cr:YSGG, etc.) pumped by a Nd:glass laser is proposed. In contrast to amplifiers in available petawatt laser systems (neodymium glass, titanium-doped sapphire, and parametric amplifiers with a DKDP crystal), these ceramics combine three key favorable properties: a large gain bandwidth to amplify chirped pulses with less than 20-fs durations, a wide aperture to amplify chirped pulses to the multikilojoule level, and a high conversion efficiency of narrowband Nd:glass laser pulses into chirped pulses. These properties open up the opportunity to create a unique laser with a peak power of 100 PW at a 10-kJ pump power.

Hollow-fiber compression of visible, 200 fs laser pulses to 40 fs pulse duration

We demonstrate the use of a very simple, compact, and versatile method, based on the hollow-fiber compression technique, to shorten the temporal length of visible laser pulses of 100-300 fs to pulse durations shorter than approximately 50 fs. In particular, 200 fs, frequency-doubled, Nd:glass laser pulses (527 nm) were spectrally broadened to final bandwidths as large as 25 nm by nonlinear propagation through an Ar-filled hollow fiber. A compact, dispersive, prism-pair compressor was then used to produce as short as 40 fs, 150 microJ pulses. A very satisfactory agreement between numerical simulations and measurements is found.

Amplified luminescence and generation dynamics of the diode pumped erbium-ytterbium laser

The inuence of the cavity trapped amplied luminescence (CTAL) on the threshold, power and dynamic characteristics of Er, Yb : glass lasers with end and side diode pump congurations has been studied. The CTAL loss coefcient has been evaluated using the luminescence balance equation. The inverse proportion dependence of the CTAL loss coefcient on the cavity length has been revealed. It has been shown that amplication of luminescence in the laser cavity leads to a signicant increase of the threshold pump power (»3n5 times). The Er, Yb : glass laser generation dynamics has been modelled taking into account the luminescence evolution beginning with the onset of the diode pumping pulse. Depending on the diode pumping pulse amplitude, the increase or decrease of the Er, Yb : glass laser pulse generation delay can be observed.

Metallic nanocluster formation in neutral gas-confined laser produced plasma afterglow

Dense atom vapors are created as remnants of a three-dimensional (3D) laser-produced plasma (LPP) plume driven from a solid aluminum target by a gigawatt-class Nd:glass laser pulse. Formation of nanoclusters as the terminal state of aluminum vapors has been investigated. By plasma structure diagnosis, we have fully characterized the LPP plume in 3D as a function of time and from its evolution modeled its cooling to the critical point in more than 110 000 vapor cells. Clustering of atoms is calculated by numerical simulation of interparticle collisions. When the cluster size histogram is rescaled relative to the maximum population and the maximal radius, the rescaled histograms all collapse into a single functional form regardless of the time or the initial atom vapor density. The cluster size distribution for the entire plasma plume is obtained by summing over all plasma cells. Independently, the clusters are captured onto electron microscope grids and size analyzed by means of transmission electron microscopy. The computed result is in excellent agreement with the measured histograms of clusters by size.

Second-harmonic generation of weak femtosecond pulses under the condition of vanishing group-velocity mismatch

In this paper we theoretically study second-harmonic generation (SHG) of weak femtosecond pulses at the wavelength with vanished group-velocity mismatch (GVM) where group-velocity dispersion (GVD) plays a major role. While the principles behind the modelling are general, the simulations of SHG for Nd:glass laser pulses using the actual dispersion of a partially deuterated KDP are included. We demonstrate that both the proper pre-chirping of the pump pulse and optimization of the phase-matching condition may improve the conversion efficiency. The defined optimum dispersing parameter is calculated for several typical cases, and the results are compared with those obtained by Boyd and Kleinman in the space domain. The dependence of the SH spectrum on the phase-matching condition, GVDs and crystal length are evaluated respectively, and shows that optimum phase matching may enhance while strong GVDs and/or long crystal length narrow the SH bandwidth.

Quasi-soliton and multifocal propagation of high-intensity laser pulses in silica glass

The results of numerical simulation of propagation of high-intensity ultrashort pulses in silica glass are presented. It is shown that in the case of a dynamic balance between self-focusing and defocusing induced by ionisation, radiation propagating in the glass over the distance of the order of a diffraction length can be captured to form a quasi-soliton. For certain relations between the parameters of radiation and glass over lengths exceeding the diffraction length, the multifocal propagation regime is possible.