The three basic ideas behind LASCAD
LASCAD - The Optical Workbench on the PC
The easy-to-use and clearly organized user interface of LASCAD permits intuitive modeling and design of laser cavities. It helps the engineer to understand experimental results without wasting valuable time studying complicated manuals:
LASCAD - The Laser Cavity Analysis and Design Tool
FEA is used to compute temperature distribution, deformation, and stress or fracture mechanics in laser crystals. Results of thermal analysis are used for the simulation of thermal lensing effects of a variety of pump configurations and cooling systems in solid-state lasers (SSL) or diode-pumped solid-state lasers (DPSSL).
The FEA code of LASCAD has been specifically developed to meet the demands of laser simulation. It uses an automatic meshing algorithm to generate a semi-unstructured grid. This terminus means that the grid has regular and equidistant structure inside the crystal that is invaluable for use of the FEA results with optical codes
Predesigned FEA models with adjustable parameters, such as dimensions of crystal or material properties, are provided to assist the engineer with different laser cavity design concepts. These models also include laser crystals composed of different materials as well as of doped and undoped sections, including thin disk lasers, side pumped sandwiched slabs etc., which are subject of current research. Very flexible modeling of pump light distributions can be carried through by the use of super-gaussian functions. To allow numerical modeling of the absorbed pump power density interfaces to the reliable and well known ray tracing codes ZEMAX and TracePro are available.
To visualize pump light distribution, boundary conditions, and results of FEA sophisticated 2D and 3D graphical tools based on OpenGL are available.
The results of FEA can be used with an ABCD gaussian beam propagation code as well as with a wave optics code.
When using the FEA results with the ABCD gaussian mode propagation code, the temperature distribution, multiplied by the temperature dependence of the refractive index is fitted parabolically at right angles to the optical axis using the finite element grid subdivisions. In the same way, a fit of the deformed end faces of the crystal is carried through. The obtained parabolic coefficients are then used as input for the ABCD gaussian beam code.
Computations are carried through in two plains perpendicular to the resonator axis to allow modeling of astigmatism of laser cavity and crystal. Gaussian mode plots are shown simultaneously for both plains, also higher order gaussian resonator modes can be displayed.
For many configurations, end pumped rods for example, this approximation delivers reliable results for the laser mode.
In case of standing-wave resonators a stability diagram based on generalized g-parameters can be shown.
The obtained gaussian mode shape and the pump light distribution are used to compute the laser power output. Solution of the laser rate equations is computed by iterative integration over the crystal volume.
Overlap of laser modes and pump beam can be visualized at arbitrary positions along the crystal axis.
Parabolic approximation and ABCD gaussian beam propagation code are not always sufficient, however. In these cases FEA results can alternatively be used as input for a physical optics propagation code, which is based on the paraxial wave equations, and provides full 3-D simulation of the interaction of a propagating wavefront with the hot, thermally deformed crystal, without using parabolic approximation.
The physical optics code propagates a wavefront in small steps through the crystal, taking into account the distribution of the local refractive index, as well as the deformed end faces of the crystal, as obtained from FEA.
Based on the principle of Fox and Li, a series of roundtrips through the laser resonator is computed, which finally converges to the fundamental or to a superposition of higher order transversal laser modes.
Different from the ABCD gaussian beam code the physical optics propagation code also takes into account diffraction effects due to apertures, misalignment effects, and gain guiding.
The physical optics code therefore delivers realistic results for important features of a laser resonator like intensity and phase profile of the output beam.
This information may then be compared directly with physical wavefront measurements after the laser cavity is constructed.
In addition, the wave optics propagation code offers numerical analysis of the transverse eigenfrequencies of the laser resonator and also of the shape of the transverse eigenmodes of the resonator.
Propagation of the laser beam through an optical system outside the cavity can be carried through with the ABCD matrix as well as with the BPM code for instance coupling the laser beam into a fiber.
DMA is a new tool for the dynamic analysis of multimode and Q-switch operation of lasers. For this purpose, time dependent rate equations describing the individual photon numbers in a predefined set of gaussian transverse eigenmodes are solved by the use of a finite element solver. This approach provides a detailed description of mode competition, power output, beam quality, and pulse shape (see LASCAD Tutorial 4: Dynamic analysis of multimode competition and Q-Switch operation (PDF)).
Results turned out to be in good agreement with experimental measurements (see "Dynamic multimode analysis of Q-switched solid state laser cavities"(PDF)).
The new DMA code offers important new features:
LASCAD - The Educational Tool
Its easy-to-use and clearly organized user interface makes LASCAD ideally suited for educational purposes for students, and practicing scientists or engineers.
The principles of ABCD gaussian beam propagation can be studied interactively.
The behavior of complicated heterogeneous laser resonator design concepts or the combined influence of thermal lensing and gain guiding can be clearly demonstrated.
Verification of Results
Results obtained by the use of LASCAD for the temperature distribution in diode-end-pumped laser crystals have been verified by the Solid-State Lasers and Applications Team (ELSA) at LCFIO-Université Paris Sud. High resolution absolute temperature mappings measured by the use a "thermo-camera" have been in very good agreement with simulation results delivered by LASCAD.
Download the paper in the publications section.
The laser group of Prof. Richard Wallenstein at the University of Kaiserslautern, Germany uses the program already over a couple of years for analysis and optimization of composite crystals in diode-pumped high-power picosecond lasers and amplifiers. A detailed series of measurements was carried through, which have determined laser resonator parameters carefully and verified the results of simulation to a high degree.