The title of my paper refers to "very high power densities", and this implies here, of course, optical power, but first I should quantify what I mean by "very high". For the purpose of this paper, it shall mean power densities at and beyond which optical glasses are no longer transparent. This occurs basically due to the creation of free electrons by multi-photon absorption, which then cause stress and subsequent physical damage, such as cracks, in the glass. There is a threshold for this phenomenon, typically of the order of 1010 W/cm2 but strongly material dependent. For comparison: this is 7 orders of magnitude higher than what we can ever achieve by focussing light from the sun, our most powerful natural radiation source. It is, needless to say, readily achieved with current technology solid state pulsed lasers.
The design and implementation of a software system for supporting laser material processing work is presented. The system integrates a database, numerical simulation and various other computer techniques and provides a user friendly environment. The general system structure is described. Research results on some related topics for the system design are also presented in this paper. This includes a neural network analysis on laser surface hardening and cladding, and the implementation of simulation models for cutting and welding of steel. A PC version of the software, named CALMP (Computer Assisted Laser Material Processing), is briefly described.
KEYWORDS: Lenses, Temperature metrology, Absorption, High power lasers, Laser systems engineering, Thermography, Distortion, Ray tracing, Refraction, Interfaces
Focus shift of lenses used in high power laser machining systems is a serious problem. Unavoidable energy absorption in lenses combined with cooling at the outer rims causes radial thermal gradients and ensuing optical distortion. An existing ray tracing program was adjusted to compute the influence of stationary radial temperature profiles on the performance of lenses. Three effects were taken into account: a change of the lens thickness and curvatures caused by thermal expansion, an alteration of the angles of refraction at both air-lens- interfaces, and a curved light path through the lens. The last two effects are caused by the thermally induced radial refractive index gradient. Major consequence is a shortening of the focal length. Reasonable agreement was obtained between measurements of the focal lengths of ZnSe-lenses and theoretical predictions based on measured power absorption.
For best results and the highest speed of cutting sheet metal by high power lasers, the numerical aperture of the focussed beam must be properly matched to the material thickness. To alleviate the need for frequent changes of fixed-focal-length lenses a zoom lens system which allows fast and continuous adaptation is desirable. Requirements include near diffraction limited performance in a range of at least f/3 to f/8 and a back focus in excess of the focal length for lens protection against fumes and spatters. Four zoom lens designs for use with CO2-lasers in the 1 to 2 kW range are presented, which differ in technical complexity (use of an aspheric surface versus an additional lens element) and in the ranges of numerical apertures (f/2.8 to f/8 and f/2 to f/7). Only one of these has so far been built and measured, and cutting tests have shown slightly better performance at the short focal lengths over that of single (ZnSe) lenses of (optimized) meniscus shape.
The use of electronic computers in optical design and analysis is well
established. In fact, optical calculations were among the first
applications of the first computers that were built in the 40's, and
Donald Feder, starting in 1951 [1], proved that the use of these
machines went far beyond removing the tediousness of laborious
calculations and offered new dimensions in understanding the actual
design process [2]. By today's standards, of course, the equipment
which had such a remarkable impact at its time, was primitive and
slow. A modern inexpensive programmable calculator easily outperforms
the any computers in both speed and memory capacity, not to mention
accuracy and reliability. This also implies, that today, even
computers at the low end of the cost and performance scale can be
turned into remarkably powerful tools for optical design and analysis.
This has been demonstrated for the class of programmable calculators
[3] but applies, of course, even more convincingly to the present
generation of low-cost personal computers, which are typically based
on 16- or 32-bit processors, and where prices start well below
$ 1000.-. Any degree of higher performance is available at steadily
increased prices, so that there appears to be a fit for each
requirement.
The computer information system combines the features of a data base management
system (DBMS), some artificial intelligent (A.I.) techniques and the computer
language C into one integrated system capable of handling a variety of data and
knowledge types available in the area of laser materials processing (LMP).
Experimental data and general guideline data and knowledge are stored in data
bases using the DBMS. Models for parameter prediction and process simulation are
implemented in the computer language C. Some heuristic reasoning rules are
implemented to handle the searching of general knowledge and experimental data
from the data bases. A so-called "control center", which is implemented in
Prolog, functions as a shell, providing an interactive user environment and
guiding the user in accessing different parts of the system.
The problem of determining stresses, phase compositions and temperature distributions
during the transformation hardening of steel with a CO-laser beam is investigated.
To model the different phases in each material point a parallel fraction model is
used, in which each fraction represents one phase.
The description of phase transformations is obtained by an incremental formulation
of the Avrami-equation for isothermal transformation.
A model is developed to describe the superheating of ferrite and pearlite. The model
is demonstrated by a detailed computation of stresses, deformations and phase compositions
in the case of a stationary laser flash.
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