Divergence of far-infrared laser beam and collimation for Galilean and Keplerian system designs

Local confinement of the electromagnetic fields in a waveguide is a boundary condition problem which is often discussed as an example of wave propagation. A wave can also propagate in the air in a region without boundaries. In this case, the beam of electromagnetic waves is merely diverging as no condition of total internal reflection exists in a uniform medium such as air. We develop equations predicting the proper separation distance between a pair of lenses to collimate a far-infrared laser beam for both a Galilean and Keplerian telescopic systems. For a typical CO2 laser Gaussian beam at λ=10.6 μm, it is found that the separation distance is much more than the simple sum of the focal lengths for both lenses used in a given telescopic system. This substantial increase in the separation distance between the pair of lenses is mostly due to the divergence of the emitting radiation of the CO2 laser in the air, which is large compared to that of other commercially available laser sources. Some experimental results are given for both the Galilean and Keplerian telescopes and the discrepancies are compared from the calculations. A simple methodology is also described to test the collimation of the invisible CO2 laser beam after it is transmitted through the pair of collimated ZnSe lenses. Good estimations for the separation distance between a pair of collimation lenses and beam magnifications are very useful when focusing a beam of CO2 laser to a very small spot in various designs, where high-power density is required. Computations for the separation distance and magnification are found to be very close to the experimental values reported with a discrepancy of less than 4%.