3384 IEEE TRANSACTIONS ON PLASMA SCIENCE, VOL. 40, NO. 12, DECEMBER 2012 Digital Interferometry Applied to Transient Dense Plasmas Cristian Pavez, José Pedreros, Carlos Curín, Gonzalo Muñoz C., and Leopoldo Soto Abstract—The work presented in this paper proposes a sim- ple interferometric technique of recording and digital process- ing, 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 arrange- ment 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 interfer- ogram. 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 interfer- ometric 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. Index Terms—Digital interferometry, plasma density measure- ment, refractive optical diagnostics. Manuscript received May 1, 2012; revised August 23, 2012 and September 27, 2012; accepted September 30, 2012. Date of publication November 16, 2012; date of current version December 7, 2012. This work was supported by Fondo Nacional de Desarrollo Científico y Tecnológico (FONDECYT) under Grant 11090377. C. Pavez is with the Comisión Chilena de Energía Nuclear, Santiago 650-0687, Chile, with the Center for Research and Applications in Plasma Physics and Pulsed Power (P 4 ), Santiago, Chile, and also with the Universidad Nacional Andrés Bello, Santiago 7600713, Chile (e-mail: cpavez@cchen.cl). J. Pedreros is with the Departamento de Ingeniería Eléctrica, Universidad de Santiago de Chile, Santiago 9170019, Chile. C. Curín is with the Universidad Nacional Andrés Bello, Santiago, Chile, and also with the Universidad de Santiago de Chile, Santiago 9170019, Chile. G. Muñoz C. is with the Pontificia Universidad Católica de Chile, Santiago 7820436, Chile. L. Soto is with the Comisión Chilena de Energía Nuclear, Santiago 650-0687, Chile, with the Center for Research and Applications in Plasma Physics and Pulsed Power (P 4 ), Santiago, Chile, with the Universidad Nacional Andrés Bello, Santiago 7600713, Chile, with the University of Concepción, Concepción 4070386, Chile, and also with the University of Talca, Talca 3340000, Chile. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPS.2012.2223237 I. I NTRODUCTION D IGITAL interferometry started when a digital acquisition system was incorporated into the interferometric diagno- sis. Thus, this technique can be plainly defined as the electronic record of an irradiance pattern of some interferometric scheme. Some of the interferometric diagnostics that fall into this defi- nition are those that record the interference pattern directly on a charge-coupled device (CCD), without an additional process. This is the most used in interferometry of Z -pinch plasmas. A hybrid technique, which combines the optical holography with the digital image acquisition, was developed in [1] and [2]. In this technique, the optical reconstruction of the hologram is handled and recorded on a CCD. The saved information is processed to determine the interferometric phase directly from a set of image irradiance measurements. On the other hand, from the digital interferometry techniques, in which digital processing is necessary for the formation of the interference pattern, the most important is the digital speckle pattern inter- ferometry (DSPI), which is sometimes called as “digital image plane holography” [3], [4], or simply “digital holography.” In this methodology, the interference pattern corresponds to a low- frequency modulation of the high-frequency speckle noise [5]. An interferometric technique based in a method of digital processing similar to the one developed with DSPI [5] is described in [6]. With this method, the fringe pattern appears as the result of a digital operation, without using the speckle pattern as an information carrier. The work presented in this paper proposes a simple pulsed interferometric technique of recording and digital processing. With only three exposures (one with a phase object and two reference records), this technique allows the obtaining of simul- taneous interferometric records of variations of the refractive index at large and small scale, similar to the optical method proposed in [7]. Each record is made with a parallel fringe interference pattern of high frequency (microinterferogram), of which spacing is between 15 and 20 lines/mm, or higher, depending on the CCD resolution. With digital processing (as in [6]) of the three saved images, we generate interference patterns with fringes of finite and infinite width, 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. It should be noted that a submillimetric inspection on the microinter- ferometric record (on an enlarged image) would allow one to measure changes in the refractive index to small scale inside a macroscopic phase object. On the other hand, the direct visualization of the plasma microinterferogram possesses the 0093-3813/$31.00 © 2012 IEEE