Abstract

We have controlled the relative phases of two successively generated coherent anti-Stokes Raman scattering (CARS) signals with dispersive material placed between the two CARS signals. A home-made phase shifting unit¹ made of BK7 glass was used as the dispersive medium. To control the relative phase between two CARS signals that changes with the wavelength of Stokes beam scanned, we adjusted the thickness of phase shifting unit adjustable to compensate for the change in a wide spectral range. With this nonlinear interferometric technique,^1,2 we could selectively suppress the undesired contribution from the 3rd order nonlinear susceptibility.^1,3 The experimental setup is shown in Fig. 1. We placed two gas cells serially and filled those with gas samples of interest. A phase shifting unit was placed between the sample cells. Parallel-polarized BOX-CARS phase matching was employed for the experiment. To show that the undesired background could be suppressed by applying nonlinear interferometry, first we obtained the CARS spectrum of 400 Torr pure propane used as the background in this experiment, which is shown in Fig. 2(a). Then, filling the first cell with the mixture of 15 Torr hydrogen chloride and 400 Torr propane, we measured the spectrum of the mixture shown in Fig. 2(b). To reject the contribution from propane, we filled the second cell with 400 Torr propane and adjusted the phase shifting unit to cause destructive interference between the two different propane signals from the first and the second cells. We finally observed the spectrum of hydrogen chloride shown in Fig. 2(c), in which the propane signal was suppressed.

© 1994 IEEE