This article provides an example of modeling laser pumping using different reflector shapes.
Authored By: Vasili Karneichyk
INTRODUCTION
The goals of modern laser pumping are to focus as much light as possible into the active medium of a laser and to achieve a uniform distribution of light at any given cross section of the active medium. The reflector design is the most significant influence on the efficiency of laser pumping.
LASER PUMPING EXAMPLE
The last page of this article contains the files that model the systems described.
A glass rod of 4 mm diameter is used as the active medium for laser pumping. The material used is a custom model of YAG defined in the "private.agf" glass catalog contained in the archive file. The model uses Nd:YAG (1% Nd) absorption data from Northrop Grumman (www.st.northropgrumman.com) and refractive index data from VLOC (www.vloc.com).
The light source is modeled using a source tube, which is appropriate for lamp modeling. The reflector is initally modeled as a reflective, diffuse ellipse.
The optical properties of the reflector coating used are similar to the Duraflex coating from Lapshpere (www.labsphere.com). The specific properties of the reflector are shown in the object properties dialog below.
A detector volume is placed to envelop the active medium of the laser. The results of the raytrace are shown as absorbed power for a cross section of the laser rod.
The peak absorbed flux is 1.12 microwatts.
Sometimes a more simple reflector shape is used as shown in the layout below.
The absorbed energy is shown for this type of reflector below.
The peak absorbed flux is 1.38 microwatts; slightly more than the elliptical reflector design.
By using a more sophisticated reflector, the total absorbed flux can be improved. The following shape was modeled using a custom DLL that defines a Bezier curve.
The parameters of the Bezier DLL are optimized using the following merit function.
The merit function targets maximum absorption of light in the laser rod (operands 4,5)and maximum uniformity in the laser rod (operands 6-18).
The raytrace results after running an optimization are shown below.
The peak absorbed flux is 1.78 microwatts; the largest value achieved among the reflector shapes presented.
SUMMARY
The laser pumping example in this article has illustrated the following:
- Zemax has the ability to show the effect of different reflector shapes, reflectivity and scattering on the efficiency of a laser cavity.
- A Bezier, or free form, reflector can be used to improve the efficiency of a laser pumping system. This example shows a 30-60% improvement in pumping efficiency by using a Bezier reflector.