Improved Design of Optical Components for Lasers

ALGOR HELPS IMPROVE DESIGN OF OPTICAL COMPONENTS FOR LASERS

A von Mises stress contour of an optical component modeled and analyzed with the Algor system. Analyses like this help II-VI, Inc. determine how the optic will react to stress induced by high-power lasers.

A manufacturer of optical components for infrared (IR) devices is using the Algor Finite Element Analysis (FEA) system to analyze steady-state temperature and stress conditions incurred in applications based on high-power lasers. II-VI Inc., Saxonburg, Pennsylvania, uses Algor software to analyze and compare optics made from various substrate materials, such as ZnSe, GaAs, Si, and copper. (The company's name, II-VI, is derived from Group IIB and VIA of the Periodic Table of Elements. Elements in these groups are combined to create the semiconductor materials that make up optical components.)

"When an optical component is used in a high-power laser beam, some of the laser power is absorbed by the optic and converted into heat," explains Gary Herrit, II-VI Engineer. The thermal expansion and stress associated with the heating of the optic causes both physical and optical distortions. The distortions adversely effect the way the optic transmits or reflects a high-power laser beam.

"Users of high-power lasers usually pick an optical material by the trial-and-error method. With Algor, however, we are able to help our customers choose which optical materials are best suited for their particular application," says Herrit.

II-VI Engineer Gary Herrit, right, displays Algor analysis results with Herman Reedy, II-VI Vice President and General Manager of Quality and Engineering.

II-VI's largest markets are those using high-power industrial lasers and medical lasers. The lasers in these markets, in particular the CO₂ type, range in power from 10 watts to 10,000 watts. The high-power lasers are used in cutting, welding, and heat treating steels and other types of metals. Low-power to medium-power lasers (10 watts to several hundred watts) are used in medicine and for cutting soft materials, such as plastics and cloth.

"The optics we analyze are cylindrical in shape, with diameters of 1.0, 1.1, 1.5, and 2.0 inches, and thicknesses from three to nine millimeters," Herrit continues. "In general, IR optics have coatings applied to their surfaces to control the amount of laser light that is reflected or transmitted from these surfaces. The analyses done by II-VI include optics with various reflectives."

Designing Lasers

Designers of CO₂ lasers must choose optics according to a particular application. Though sometimes one type of optic stands out as a clear winner, other times two different optics perform almost identically, making the choice difficult.

In the past, designers could rely on one of two methods for determining which optic will perform best for their system. One method is a Figure of Merit (FOM) function that was developed in the late 1960's. This function ranks optics used in high-power laser systems. Originally, the function was developed for uncoated optics, however, it can be adapted for use with coated optics. The FOM function has some deficiencies when it is used to select optics, primarily when the optics have partially reflective coatings. "We do use an FOM function to help us in our comparisons, "Herrit states. "It is used in conjunction with the Algor FEA."

The second method for choosing optics is the costly and inefficient trial-and-error method. The user buys one of each of the optics that are possible candidates and tries them in the laser system being designed. Again, if the two different optics perform reasonably well in the laser system, a choice between the two will be difficult. The issue may also be clouded by the quality of the samples used for the trial-and-error test. If for some reason a defective or sub-standard optic is used in the trial, then the user will get the false impression that the material is not suitable for this application.

Designing with Algor

Originally II-VI purchased the PRIME version of the Algor system to do furnace design and analysis. As Herrit explains, "We thermally modeled CdTe ingot growth in a Modified Vertical Bridgman furnace as part of a military contract under the SBIR (Small Business Innovative Research ) Program. The objective of this work was to analyze the thermal gradients in the furnace and determine which furnace configuration produced the optimum gradients for crystal growth. Adapting the FEA program to our optic problems was a natural extension."

II-VI now runs Algor's Hyper-386 version of FEA software on an 80386 COMPAQ 386/25™. The Linear Static Stress Analysis Processor, Part 94-3, performs the mechanical analysis and the Steady-State Heat Transfer Processor; Part 101-3, provides the thermal analysis information. Plotted results show the thermal expansion of the optics.

"We are also incorporating Algor FEA into our future plans in optic analysis," states Herrit. "The FEA analysis we now perform on the optics produces data in a form that will be used by a lens design program we are planning to develop. Our goal is to predict the effect a distorted optic will have on a laser beam."

Heat transfer analysis of an optical component reveals areas affected by laser power absorbed by the optic and converted to heat.