The transverse modes in microchip lasers (MCL) appear different from the classical mode theory, as the resonator consists of a set of plane mirrors (similar to semiconductor edge-emitting lasers and VCSELs). Then the parabolic potential is absent, and the electromagnetic radiation is confined in the transverse space of the resonator by other mechanisms, essentially by gain guiding. Thermal lensing also can play some role in confining the radiation. Laser engineers use several intuitive assumptions about the number of supported modes in the resonator, relating with the Fresnel number of the resonator. However, despite some attempts to calculate the modes in such resonators and investigate combined gain with index guiding, no clear and straightforward methods were provided to calculate the beam radiation profiles. We aim to fill this gap by providing analytical and numerical treatment of MCL modes in two dimensions. We analyze the transverse modes in microchip laser formed due to both gain guiding and thermal lensing. Analytical and numerical results are compared with the results of experimental measurements. Using the cylindrical pumping profile approach, we provide a simple 2D theory of the gain-guided and thermal-lens induced modes in such plane-mirror resonators. We estimate the mode beam quality factor dependency on pumping strength. The model is versatile and applicable to wider range of optical resonators consisting of plane mirrors with longitudinal pumping. Finally, we compare the experimental measurements of beam quality factor with our theoretical model.
Possible scenarios of high-intense vortex (and Gaussian) pulsed beam propagation in Kerr media and light bullet (LB) formation conditions are considered. The system of modified nonlinear Schroedinger equation for the complex envelope of the electric field and kinetic equation for the electron plasma density is exploited. Two-scale variational analysis is combined with direct numerical simulations based on finite-difference methods. Hamiltonian approach allows to reveal LB formation conditions. It is shown that the LB parameters correspond to minimum of potential energy when the whole balance of competing processes occurs. Numerical experiment confirms the results obtained on the base of variational analysis, demonstrating at the same time softer conditions for LB formation. It is emphasized that the linear and nonlinear dynamics of spatial and temporal radii obey the coupled oscillator theory.
We search for efficient schemes of second and terahertz harmonic generation in nanocomposites consisted of metal-oxide semiconductor quantum dots incorporated into a dielectric matrix, when the quantum dots are in resonance and the dielectric matrix is out of resonance with femtosecond light pulse. It’s established that large efficiency of frequency up-conversion is possible to attain, which may be for the optimal quantum dot concentration in above mentioned nanocomposites by 70% higher than in pure nonlinear dielectric matrix.
Spatial filtering techniques are important for improving the spatial quality of light beams. Photonic crystals (PhCs) with a selective spatial (angular) transmittance can also provide spatial filtering with the added benefit transversal symmetries, submillimeter dimensions and monolithic integration in other devices, such as micro-lasers or semiconductor lasers. Workable bandgap PhC configurations require a modulated refractive index with period lengths that are approximately less than the wavelength of radiation. This imposes technical limitations, whereby the available direct laser write (DLW) fabrication techniques are limited in resolution and refractive index depth. If, however, a deflection mechanism is chosen instead, a functional filter PhC can be produced that is operational in the visible wavelength regime. For deflection based PhCs glass is an attractive choice as it is highly stable medium. 2D and 3D PhC filter variations have already been produced on soda-lime glass. However, little is known about how to control the scattering of PhCs when approaching the smallest period values. Here we look into the internal structure of the initially symmetric geometry 2D PhCs and associating it with the resulting transmittance spectra. By varying the DLW fabrication beam parameters and scanning algorithms, we show that such PhCs contain layers that are comprised of semi-tilted structure voxels. We show the appearance of asymmetry can be compensated in order to circumvent some negative effects at the cost of potentially maximum scattering efficiency.
Glass drilling realized with the help of femtosecond lasers attract industrial attention, however, desired tasks may require
systems employing high numerical aperture (NA) focusing conditions, low repetition rate lasers and complex fast motion
translation stages. Due to the sensitivity of such systems, slight instabilities in parameter values can lead to crack
formations, severe fabrication rate decrement and poor quality overall results. A microfabrication system lacking the
stated disadvantages was constructed and demonstrated in this report. An f-theta lens was used in combination with a
galvanometric scanner, in addition, a water pumping system that enables formation of water films of variable thickness
in real time on the samples. Water acts as a medium for filament formation, which in turn decreases the focal spot
diameter and increases fluence and axial focal length. This article demonstrates the application of a femtosecond (280fs)
laser towards rapid cutting of different transparent materials. Filament formation in water gives rise to strong ablation at
the surface of the sample, moreover, the water, surrounding the ablated area, adds increased cooling and protection from
cracking. The constructed microfabrication system is capable of drilling holes in thick soda-lime, hardened glasses and
sapphire. The fabrication time varies depending on the diameter of the hole and spans from a few to several hundred
seconds. Moreover, complex-shape fabrication was demonstrated.
Glass drilling and welding applications realized with the help of femtosecond lasers attract industrial attention , however, desired tasks may require systems employing high numerical aperture (NA) focusing conditions, low repetition rate lasers and complex fast motion translation stages. Due to the sensitivity of such systems, slight instabilities in parameter values can lead to crack formations, severe fabrication rate decrement and poor quality overall results. A microfabrication system lacking the stated disadvantages was constructed and demonstrated in this report. An f-theta lens was used in combination with a galvanometric scanner, in addition, a water pumping system that enables formation of water films of variable thickness in real time on the samples. Water acts as a medium for filament formation, which in turn decreases the focal spot diameter and increases fluence and axial focal length . This article demonstrates the application of a femtosecond (280fs) laser towards two different micromachining techniques: rapid cutting and welding of transparent materials. Filament formation in water gives rise to strong ablation at the surface of the sample, moreover, the water, surrounding the ablated area, adds increased cooling and protection from cracking. The constructed microfabrication system is capable of drilling holes in thick soda-lime and hardened glasses. The fabrication time varies depending on the diameter of the hole and spans from a few to several hundred seconds. Moreover, complex-shape fabrication was demonstrated. Filament formation at the interface of two glass samples was also used for welding applications. By varying repetition rate, scanning speed and focal position optimal conditions for strong glass welding via filamentation were determined.
Generation of free-carrier plasma and filamentation of the ultra-short laser pulse were investigated and modeled.
Experimental results of filamentation are supported by numerical model which takes into account accumulation
of refractive index modifications due to multi-pulse exposure. A contact acoustic monitoring technique was
employed to perform spatially-resolved in situ detection of micro-plasma formation and filamentation of focused
femtosecond laser pulses with critical and sub-critical powers in glass. The recorded acoustic signals reveal freecarrier
generation mechanisms associated with the formation of plasma and filamentation of the propagating
laser pulses. Optical opacity of the plasma region, which sets in at the irradiance of a few kJ/cm3 (close to
the dielectric breakdown threshold) using pulse focusing optics with numerical aperture NA = 0.75, reveals its
critical character, and allows the estimation of acoustic pressure in the ~GPa range. The pressure depended on
the irradiance as P ~ I0.59. In the case of loose focusing (NA = 0.035) filamentation of fs-pulses occurred at
sub-critical plasma density with P ~ I. Detection and interpretation of these acoustic signatures thus enable
real-time in situ monitoring of optical ionization, pulse filamentation in bulk dielectrics under the irradiation by
femtosecond laser pulses.
Nonlinear losses experienced by the self-focusing femtosecond pulse is shown to have an important effect on the
refractive index modifications in fused silica. The region of the maximum induced change is found to coincide
with that of the maximum nonlinear losses of the pulse. It is found as well that material densification and the
formation of color centers both contribute to the index change in that zone. Experimental results are supported
by numerical simulations using model that takes into account accumulation of the permanent refractive index
changes and their influence back on the pulse. Both the color
center- and compaction-induced changes cause the
modification to develop into a waveguide and lead to the narrowing of supercontinuum spectra.
The theoretical and experimental studies of the cumulative effects on refractive index modification dynamics
during filamentation from femtosecond light pulses in transparent media are presented. Residual changes of the
refractive index were shown as developing both in forward and backward directions forming prolonged tracks of
modified index profiles (waveguides) during repetitious laser shots.
We report on the refractive index grating formation by filamentary propagation of femtosecond pulses in fused
silica. The relevant exposure and work cycles are considered both experimentally and through numerical study,
involving a model of light filaments supported by conical wave, capable to capture permanent glass refraction
index changes.
Self-focusing and filamentation of nearly-Gaussian femtosecond laser pulses propagating in photosensitive silicate glass was investigated. The filamentation was visualized by the precipitation of NaF nano-crystallites along the beam pass after post-exposure treatment of photosensitive glass, which provides a direct proof that ionization of silicate matrix takes place along the filament propagation lines. Theoretical model of the multi-filamentation based on the propagation of a Gaussian beam with elliptical transverse intensity profile modulated by a spatial noise in a medium with multi-photon absorption is proposed. Self-action of the femtosecond Gaussian-Bessel pulses in borosilicate glass was observed at high fluence. This model reproduces qualitatively the dotted damage lines observed after the beam propagation in borosilicate glass.
Dynamics of damage formation by focusing intense femtosecond pulses inside the fused silica glass is studied in wide energy range. Damage usually is initiated in the zone near geometrical focus, which is preceded by the zone where beam propagates in the form of multiple filaments. For high repetition rate pulses damage appears as an extended narrow track along the beam path, which forms due to the propagation of the initial damage zone toward the laser source. For low repetition rate pulses extended damage tracks don't form.
For high intensity lasers it is very important to choose appropriate optical elements. Since invention of high power lasers laser-induced damage of optical coatings was subject of extensive investigations. At high laser intensities the self-focusing in optical elements appears and intensity at rear optics surface can be much higher than at the front surface. Due to this damage of rear-surface can be reached much faster than damage of the front surface. We investigated the influence of self-focusing on damage threshold in fused-silica windows with anti-reflective coatings on both sides. In our experiments we used titanium-sapphire chirped pulse amplification system (130 fs, 2 mJ, 1 kHz repetition rate pulses at 800 nm). We have tested 1 mm, 3 mm and 6 mm thickness fused-silica windows with identical anti-reflective coatings. The front surface of the samples was placed in the waist of focused beam. The experiments were performed for effective spot diameters on the front 145 μm, 95 μm and 43 μm respectively. The experiments showed the self-focusing of beam inside the fused silica window and self-focusing dependence on initial beam diameter. The damage behavior was dependent on irradiation history. Also we found quite strong nonlinear absorption in fused silica.
We report experimental observation of modification and damage in bulk of fused silica induced by intense multiple femtosecond pulses. At power levels several times exceeding critical power for self-focusing the modification is in the form of some permanent changes inside the material and these effects are closely correlated with the beam filamentation process. Longer distances of propagation and much higher pulse energies produce bulk damage in the form of scattering zones. The formation of damage in anti-reflective-coated fused silica windows is also reported. Numerical simulations involving self-focusing, multiphoton absorption and permanent change of the refractive index of the bulk material were found to be in agreement with the experimental results.
The dynamics of multiple pulse laser-induced damage in the form of cracking or nonlinear coloration in bulk materials (fused silica and borosilicate K8 glass) was studied under the irradiation by femtosecond pulses at 800 nm wavelength. A Ti:sapphire chirped pulse amplification system with ~130-fs pulse duration and ~1-mJ pulse energy at 1-kHz repetition rate was used in the experiment. Self-guided propagation of femtosecond pulses over greater than 1-cm lengths accompanied by intensive supercontinuum generation was observed and studied in an interaction geometry where the laser beam was focused in the middle of the thick (~4 cm) sample. The pulse energy value at which self-guided propagation and supercontinuum generation in fused silica was observed was ~60 times lower than the laser-induced damage threshold. The nonlinear coloration in K8 glass was present at pulse energy values which exceeded the threshold for self-guided propagation. Numerical simulations involving self-focusing, temporal dispersion and multiphoton absorption were found to be in good agreement with the experimental results.
Quantum beats in transient absorption of PIC-J-aggregates, modelled by a modified 3-level- system are calculated. There is considered the influence of the temperature dependent phase relaxation on the temporal evolution of quantum beat signals.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.