Ultra-short pulses with high average power are required for a variety of technical and medical applications. Single, multi-pass, and regenerative amplifiers are used, in order to increase the power of ultra-short lasers. Typical laser crystals for such amplifiers include Ti:Sapphire or Yb:YAG laser crystals. Difficulties in the amplification of ultra-short pulses include gain narrowing effects and dispersion effects in the laser crystal. In particular, these complications arise, when a pulse stretcher is needed before amplification of the laser beam. We present a technique to model and simulate the amplification of ultra-short pulses. This technique allows to model both gain narrowing effects and decrease of beam quality caused by amplification of the laser beam. This requires a detailed 3-dimensional simulation of population inversion. Gain narrowing effects are taken into account by analyzing the gain of the spectrum of the laser beam. It is important to distinguish amplifiers with one or only two passes and a regenerative amplifier. These two different kind of amplifiers are modeled by different approaches. A regenerative amplifier is modeled by a set of time dependent rate equations. However, a single pass amplifier is modeled by a set of spatial dependent rate equations. In both cases, a system of rate equations arises from spectral discretization of the laser beam. Detailed simulation results are presented.
Modern simulation of solid state lasers and amplifiers is a multi-physics problem. It requires the simulation of light with different techniques, as well as coupling of optical effects with other physical effects like deformation and stress inside a laser crystal. We present and overview simulation techniques which are necessary for an accurate modelling of solid state laser. These include ray tracing for modeling of pump light, Gauss mode analysis for stability calculations of a laser cavity, and vectorial beam propagation method for the amplifier simulation. It is also necessary to perform mechanical or rate equations simulations. Structural mechanics simulation is required for the calculation of thermal lens and thermally induced birefringence effects inside a laser crystal. Coupling Gauss mode analysis and rate equations leads to the dynamic mode analysis (DMA), which is used to calculate output power and beam quality of a laser. Furthermore, we present a mode amplification method, which allows us to simulate spectral narrowing in laser amplifiers and power and beam quality calculation. The above mentioned simulation techniques have been integrated into laser simulation software package ASLD. We have used highly efficient algorithms and modern software techniques, together with a user friendly GUI. This enables users to efficiently simulate complex resonator and amplifier designs and save their development time and costs. Furthermore, ASLD can be used for educational purpose at universities and research institutes.
Thin film silicon solar cells are optimized to increase their efficiency. One technique to obtain higher efficiency
is to increase path length of light using textured surfaces. The impact of these layers on efficiency is usually
studied using experimental methods. This requires building of a solar cell and is time consuming and prone to
error. Simulation is used to predict light scattering effects in large domains with textured layers. We studied
these effects using a conformal finite integration technique (FIT) that efficiently simulates complex geometries
with surface roughness. The simulated external quantum efficiency EQE for a solar cell with a μc-Si:H and aSi:H
layers with surface roughness are presented.
Composite, core-doped, and ceramic crystals are used to reduce the thermal lensing effect in laser crystals. An accurate simulation of laser resonators is needed to nd optimal doping structures for a required laser. Also simulation helps apply these crystals efficiently. In this work, results of simulations performed on resonators containing core-doped Nd:YAG ceramic crystals with radial dependent doping concentration, and composite crystals with different layers of doping concentration are presented. The absorbed pump light in crystals is simulated using the ray tracing algorithm. The thermal lensing effect has been computed by the nite element analysis of the temperature and deformation. The dynamic mode analysis (DMA) was used to compute the laser beam quality and output power. The DMA solves rate equations separately for each Gauss mode. It also calculates population inversion by solving rate equations on nite volume grids. This approach yields a few thousands of rate equations for population inversion on the nite volume grid. The advantage of this method for both mentioned crystals is that different decay rates can be associated to regions with different doping concentration. Simulation results have shown that sophisticated laser crystals are able to reduce thermal lensing effect and increase beam quality.
Semiconductor saturable absorber mirrors (SESAMs) are used to produce passively Q-switched ultrashort pulsed
lasers. Numerical modeling of physical effects of SESAM is required to effectively design this type of lasers. For this purpose, simulations are performed to study the dynamic behavior of Gauss modes, gain of modes and saturation of the saturable absorber mirror. The laser beam quality has to be good enough in order to avoid chaotic laser behavior. We extended our dynamic mode analysis (DMA) algorithm to calculate laser beam quality. This simulation technique is based on rate equations for a set of Gauss modes and population inversions. Gain of each mode can be calculated separately by solving the corresponding set of rate equations. We have assumed that the reflectivity of the mirror is spatially invariant in the SESAMs model. An additional rate equation is required to include the saturation of SESAM. This equation considers parameters such as modulation depth, saturation fluence and relaxation time. Simulation results show that our model can predict pulse energy and non-chaotic behavior of the laser.
Passive Q-switched lasers are constructed using saturable absorbers (SA). One characteristic of these lasers is
that they are built with small dimensions. There are difficulties in designing lasers with a given pulse repetition
rate or pulse energy using saturable absorbers. Numerical simulation of Q-switches facilitates the design and
production of such lasers and helps to reduce development time and cost. This paper presents a new simulation
method which calculates beam quality, maximal output power, pulse-width and pulse energy.
We present a finite integration technique (FIT) simulator for modelling light diffraction from lithographic masks with
complex shapes. This method has high flexibility in geometrical modelling and treating curved boundaries. The inherent
feature of FIT allows more accurate rigorous electromagnetic field simulation in complex structures. This technique is
also suited for fast EMF simulations and large 3D problems because of its parallelisation potential.
We applied this method to investigate the effect of various mask shapes on lithographically printed images. The imaging
results were obtained using Dr.LiTHO's imaging simulator. We demonstrate results for attenuated phase-shift mask
(PSM) with different absorber deviations from ideal shapes such as footing and oblique sidewalls.
We present a finite integration technique (FIT) simulator for modelling light diffraction from lithographic masks with
complex shapes. This method has high flexibility in geometrical modelling and treating curved boundaries. The inherent
feature of FIT allows more accurate electromagnetic field simulation in complex structures. This technique is also suited
for fast EMF simulations and large 3D problems because of its parallelisation potential.
We applied this method to investigate the effect of complex mask shapes on the printed image. We demonstrate results
for a phase-shift mask (PSM) with footing extensions and surface roughness.
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