Instrumentation for Coherent Raman Spectroscopy.

Abstract

Instrumentation is developed for two coherent Raman techniques as alternatives to spontaneous Raman spectroscopy in dilute aqueous solutions. The two coherent Raman techniques investigated are coherent anti-Stokes Raman spectroscopy (CARS) and stimulated inverse Raman spectroscopy (SIRS). Development of the CARS optical apparatus includes a symmetrical beam crossing angle device which is more easily adjusted and automated than other systems. A digital data acquisition system for CARS is also developed. The design is based on an 8-bit microprocessor, two modular gated integrators and a multiplexed analog-to-digital converter. In addition to ratioing and averaging data pulses, the miroprocessor controls and synchronizes the laser and data acquisition hardware. The design and construction of a pulsed dye laser for CARS experimentation as well as the fundamental principles involved in such a project are presented. The laser is a Hansch configuration for transverse nitrogen laser pumping. It includes an intracavity polarizer, beam exp and ing telescope and diffraction grating end reflector. Typical output powers are about 15 kW, with a spectral b and width of about 0.05 nm (FWHM). CARS peak height ratios, variable b and shapes, b and interference effects and polarized spectra are shown and their relationship to spontaneous Raman spectroscopy is discussed. The complex concentration dependence of CARS signal intensity is also demonstrated using aqueous KNO(,3) solutions. A concentration series from 2.6 M down to a detection limit of 0.05M is used to show a linear calibration curve at low concentrations and a square law dependence for more concentrated solutions. Stimulated inverse Raman spectroscopy (SIRS) is investigated using a tunable nitrogen pumped dye laser and a single frequency argon ion laser. Absorptions are measured out of the CW beam using a fast photodiode, AC-coupled electronics and a commercial boxcar integrator as a matched filter. Spectra are presented which have better signal-to-noise ratios than corresponding CARS spectra. SIRS spectra are also shown to have peak height ratios, b and shapes and depolarization ratios that are identical to spontaneous Raman data. SIRS signal intensities are found to scale linearly with aqueous KNO(,3) concentration from 2.5M down to 0.05M. Detection limits for this technique are found to be 0.005M, and are limited by power supply noise in the CW laser. Ultimate single pusle shot noise limited detection limits are calculated to be 0.001M with this experiment.