Linear array focused-laser differential interferometry for single-shot multi-point flow disturbance measurements

Abstract

In this Letter, a modification of the well-known focused-laser differential interferometer (FLDI) is demonstrated, with the primary focus being increasing the number of probed locations efficiently. To generate multiple beams in the FLDI system, a diffractive optical element is used. This approach is significantly more cost-effective and easier to implement than the current approach of generating multiple FLDI beam pairs using a series of Wollaston prisms. The measurements shown here utilize a 1D linear array of points, and the ability to generate a 2D array is demonstrated using two linear diffractive optical elements in tandem. Therefore, this technique, referred to as linear array FLDI (LA-FLDI), is able to provide measurements of fluid disturbances at multiple discrete locations while allowing for high data acquisition rates ( > 1 M H z ). This technique provides a much simpler approach to multipoint FLDI measurements and can increase the throughput of FLDI measurements in impulse aerospace testing facilities.

Multi-depth focused laser differential interferometer based on chromatic aberration

“…The authors believe that the present setup can be improved by using RGB color photodetectors, which may eliminate the need to separate the channels using a dichroic mirror prior to the photodetectors. Several researchers have shown cameras can be used for FLDI detection [19,26,27], and a color camera could be used as described to eliminate the final dichroic mirror and digitally separate the two colors based on the sensor RGB wavelength sensitivity. Additionally, it may be possible to reduce the overlap of the sensitivity bands by having optics set up for tighter focusing.…”

“…This cylindrical lens approach was used as the basis of a one-dimensional FLDI measurement capability developed by Weisberger et al [17,18]. Gragston et al [19,20] also used diffractive optical elements within an FLDI setup to allow for the generation of arbitrary arrangements and numbers of FLDI probe points. Yet, in each of these modifications, no extension to measurement along the direction of the beam axis was developed, which could enable three-dimensional measurements and additional correlations that may be of interest in the analysis of three-dimensional flows.…”

Experimental investigation of boundary layer instabilities and transition on sharp and blunt cones in Oxford’s High-Density Tunnel

“…The multipoint FLDI is positioned with its focus approximately 40𝑚𝑚 below tunnel centreline and 65𝑚𝑚 downstream of the nozzle exit. The single-point FLDI employed in previous HDT experiments [23], which utilizes a Novanta Photonics Ventus solid state (𝜆 0 = 671𝑛𝑚) laser, is split into a 3x3 (9-point) grid with a diffractive optic element from HOLO/OR, as first suggested by Gragston et al [33]. The nine signals are recorded using a custom photodiode array inspired by, and very similar to the array described by Davenport et al [34].…”

A low-effort and inexpensive methodology to determine beam separation distance of multi-foci FLDI

“…Duplication of the basic FLDI into two or more closely spaced systems can be achieved in many different ways (Ceruzzi and Cadou 2019;Jewell et al 2019;Bathel et al 2020;Weisberger et al 2020;Gragston et al 2021a). In the present work, the D-FLDI is produced by means of a Wollaston prism of 2 • splitting angle together with a polarizer, highlighted in red in Fig.…”

“…Depending on its position with respect to the expanding and field lenses on the pitch-side and their focal lengths, multiple values of bundle separation x 2 at the D-FLDI center plane can be achieved with the same Wollaston prism, which can be advantageous in investigations aiming at, e.g., comparing spectral amplitudes between multiple points. A novel approach following this objective with a grid of FLDIs is presented in Gragston et al (2021a).…”