Analysis of phase measurements in combined interferometer and polarimeter systems

Abstract

Combined interferometer and polarimeter systems, using a single detecting element per line of sight, are susceptible to perturbation of the interferometric phase when modulation of the polarization vector is applied. This issue has been investigated extensively for the case of a rotating elliptically polarized probing beam, demonstrating that here a perturbation is inevitable. In this article the analogy between this analysis and earlier work is pointed out, and the underlying physics discussed. It will be demonstrated that schemes have been proposed in which the perturbation has been avoided or kept well within acceptable limits.

Polarimetric characterization of light and media

“…Different kinds of polarimeters are used in many different environments such as remote sensing [32]; scattering applications [33,8,34]; ITER&Tokamak [35]; Astronomy [36], etc. For some applications, spectral polarimetry and interferometry are combined into a whole device [35,[37][38][39][40].…”

High-frequency measurement of the interferometric phase, the Faraday rotation, and the Cotton-Mouton effect with a single detector in a far-infrared interferometer-polarimeter

“…On the other hand, for p > 1 the azimuth spans only a limited angular range, symmetrical with respect to zero, indicating that during a modulation cycle the E-field vector oscillates symmetrically around the x axis (alternating modulation). The possibility of producing an input beam with alternating modulation is important, as it allows to control and mitigate the perturbation of the interferometric phase [15,27]. Figure 5 shows a plot of the perturbation of the interferometric phase for the four values of p. These data have been calculated accordingly to the expression in [18], which corrects some misprint errors contained in [14].…”

“…However, in plasmas with a high transverse B-field, the polarization of the probing beam is also affected by the Cotton-Mouton effect, which may introduce a non-negligible and in principle unknown contribution to the Faraday rotation angle [11,12]. In addition, density measurements also suffer from a perturbation of the interferometric phase that, although in general very small, in some circumstances may introduce a considerable error [13][14][15].…”

“…is the interferometric phase, C = 2.82 × 10 −15 m −3 and L is the total length of the chord in the plasma [13]. ∆ contains the information on the line density Ĺ n e dl of the plasma chord and φ is an additional term known as the perturbation of the interferometric phase [14,15]. This term is always present in an interferometer-polarimeter and is related to the plasma birefringence.…”

Poloidal polarimeter for current density measurements in ITER

“…However, it is very difficult to do this as long as the technical implementation of the polarimeter has not been fixed. 13,14 Nevertheless, it seems to be a rather obvious way to calibrate the polarimeter between consecutive ITER discharges. In that case, for an ideal optical system the expected Faraday rotation and Cotton-Mouton effect should be zero.…”