Digital Interferometry Applied to Transient Dense Plasmas

The work presented in this paper proposes a simple interferometric technique of recording and digital processing, which allows the obtaining of three possible interferometric records of variations at the refraction index, both in large scale and in small scale. The experimental setup basically consists of the Mach-Zehnder interferometer and a digital camera (with CMOS technology) coupled to a PC. The interferometric arrangement originally forms a pattern of very thin parallel fringes (15-20 lines/mm on the CMOS device), which could eventually be used to obtain information of the refractivity of the phase object to submillimeter scales, looking directly to the original interferogram. The digital interferometric technique shown here is for triple exposure: the first exposure contains the plasma information and the next two are only referential records (reference interferometric patterns), similar to optical holographic interferometry, but instead of each exposure occurring on the same holographic plate, the record is done in consecutive captures of a digital acquisition system. One of the referential records is captured in the same condition as the one with plasma and the other one slightly changing the incidence angle of the reference beam. Thus, with this digital interferometry technique, it is possible to obtain (for the same plasma) a microinterferogram, which is an interferogram in fringes of infinite width and an interferogram in fringes of finite width. The technique is depicted in three different cases, namely, synthetic interferograms of a Gaussian plasma profile (simulated), laser-produced plasma, and a Z-pinch discharge at the SPEED2 generator. The first case describes the actual scope of the technique. For the other two pulsed plasmas, an Nd-YAG power laser (at the second harmonic, 532 nm) is used to produce the fringe pattern.