OCR Text |
Show 14 N2 contribution appears as an addition to the peaked spectrum rather than as a modulation of the background as one would expect if the signals were created in the same region of space. A study of the origin of this background32 found that it is caused by particle induced laser breakdown. It is well known that moderately sized particles (>1 ym) can lower the laser breakdown threshold by one or two orders of magnitude.33"35 When breakdown occurs a plasma is generated that expands to a volume with a diameter of ^ 1 mm within a few nanoseconds. Because the ion and electron densities in a flame are reduced compared to those in STP breakdown, the discharge region remains transmissive to visible radiation. The enhanced linear susceptibility in the plasma increases the third order nonresonant susceptibility which, in turn, generates a nonresonant CARS signal large enough to compete with the N2 resonant signal. One way to eliminate this problem is to reduce the focal intensity so that it remains below the breakdown threshold when particles are present. Without an accompanying increase in the interaction length, this leads to an unacceptably large decrease in signal strength. Increasing the focal length of the crossing lens serves both these aims. The anti-Stokes signal strength P ^ I 2I £2A where I and I are the pump and Stokes a s p s a s p s r r focal intensities, i is the interaction length and A is the cross as sectional area of the signal generation volume. Noting I = d/sin (0/2) = 2d/0, 0 s D/f and d = 6f where d is the beam focal waist, 0 is the crossing angle, D is the pump beam separation at the lens, f is the focal length of the crossing lens and 6 is the beam divergence before entering the lens, we find 1. - p s . (D as 2 2 s assuming d ^ d and A *v* d2. Thus tripling the focal length decreases 0 s p as p ro & the focal intensity (and spatial resolution) nearly an order of magnitude without diminishing the signal strength. Replacing the crossing lens with a Galilean telescope is a useful method to increase the effective focal length without an unduly large increase in the lens-focal point distance. |