Optical damage considerations and the study of SBS. The threshold intensity for optical damage to fused silica is approximately 3 GW/cm2 and is of the same order of magnitude for most optical materials. (See, for example, Lowdermilk and Milam, 1981.) Use this fact and the value of the SBS gain factor at line center quoted in Table 9.3.1 to determine the minimum length of a cell utilizing fused-silica windows that can be used to excite SBS in acetone with a collimated laser beam. Assume that the laser intensity is restricted to 50% of the threshold intensity as a safety factor to avoid damage to the windows. If the laser pulse length is 20 nsec, what is the minimum value of the laser pulse energy per unit area that can be used to excite SBS? (SBS is often excited by tightly focused laser beams rather than by collimated beams to prevent optical damage to the windows of the cell