Pulsed source thermal lens. Part 1.—Theoretical analysis

Abstract

The principle of operation of a pulsed source thermal lens is described and a practical system, based on a pulsed, CO₂, line-tunable laser, is outlined. The time variation of the thermal lens is probed by a continuous He/Ne laser monitored on a photomultiplier tube behind a pinhole mounted on the beam axis in far field. This photomultiplier signal is fully analysed theoretically as a function of time and the physical properties of the sample gas. It is shown that this analysis enables calculation of the magnitude of the temperature at any point in the cell as a function of time. The risetime of the signal is related to the vibrational–translational relaxation time of the sample and the lifetime of the signal decay is related quantitatively to the thermal conductivity coefficient.