Detection and sizing of small particles in an open cavity gas laser

The open cavity of a He-Ne gas laser oscillating at 632.8 microm was used to detect and size aerosol particles. This was done for two transverse modes of the laser, the TEM(oo) and TEM(o1) *. Plots of particle extinction loss vs laser power are shown to agree with the theory for an inhomogeneously broadened line under the condition of small saturated gain per pass. The TEM(ol)*, doughnut, mode is shown to be preferable for use in an operational instrument. Extrapolation of the measurement to particles smaller than the wavelength of the laser radiation is discussed.