Hook method: Recovery of density information from interferograms distorted by large spatial gradients

The hook method is a well-established technique for measuring the spatial distribution of species’ densities in the gas phase, particularly in optically thick plasmas. However, in the presence of large density gradients (such as those occurring in a metal vapor laser plasma), the hook interferogram suffers severe distortion and the standard hook equation is invalid. By the use of a computer simulation of fringe formation, it is shown that this effect arises as a result of the strong wavelength-dependent lensing of probe rays in the test medium. On the basis of this lensing mechanism, a criterion has been derived for the maximum permissible density gradient, above which the standard hook analysis cannot be accurately applied. Finally, a new technique is presented that permits density data to be recovered from interferograms that are too distorted to analyze by the use of standard techniques. This technique is based on extracting density gradient values from distortion-free features of the fringe pattern. The new technique also permits density data to be obtained with an increased spatial resolution over that of the standard hook analysis.