Studies of High Power CO₂ Gas Laser as a Heat Source (Report 1)

Characteristics of CO₂ Gas Laser beam focusing

Abstract

Systematic research has been made on a high power CW CO₂ gas laser to clarify its characteristics as a heat source, which include the characteristics of the laser beam focussing, interaction between laser beam and materials, distribution of temperature in laser heating and the characteristics of heat processing such as welding, cutting and drilling.
This paper discusses the results of the analysis on the characteristics of the beam focussing of the CO₂ gas laser. The size and shape of the focussed laser beam are measured expediently using film and the locations of the focal points and focal lines are determined. The method for measuring the actual energy distribution at focal point and focal line is developed.
Conclusions obtained are summarized as follows;
1) As an optical system to focus the high power CO₂ gas laser beam, a concave reflecting mirror is recommendable rather than a lens.
2) The astigmatism is inevitable in this optical system due to off axial incident beam. It becomes, however, negligible when incident angle θ<6°. When θ<6°, energy density distribution at focal point may be approximated by Gaussian curve and the diameter of the focussed laser beam at the e^-1 power point is about 0.5mm, which is approximately four times the diffraction limit. This provides an energy density of 5.6×10⁴ Watts/cm² at the center of the spot for 100 Watts incident beam.
3) The diameter of the laser beam measured agrees with the diameter calculated from measured beam divergence, 1.8 milliradians, and given focal length.
4) Focussed laser beam can be deformed easily by means of the astigmatism which increases gradually with an increasing θ when θ>6°.