Pulsed laser thermal lens spectrophotometry of liquid samples using an optical fiber beam guide with interference orthogonal signal processing

An apparatus for thermal lens spectrophotometry of liquid samples is conshucted using a pulsed nitrogen laser excitation source and a heliium-neon laser probe. Both laser beams are passed through a fused silica optlcal fiber prlor to being tightly focused Into the sample cell. The optical flber is used to reduce mode and pointng varlatlons in the laser outputs. A dlgltal correlation filter is developed and used for estlmatlon of the thermal lens signal. This dlgltal estimation procedure exhlblts a considerably enhanced slgnal-to-nolse ratio over that of other estimation methods. Moreover, It is relatively Immune to background Interference and rapid In estlmatlon time. Finally, the effectlve sample path length for focused beams In pulsed laser exclted thermal lens spectrophotometry is examlned from a theoretical basis. It is found that the slgnal will be very near maximum for cell path lengths on the order of 4 times the Raylelgh range.