Linear and 2D arrays for Focused Laser Differential Interferometry using a high-speed camera

Abstract

Since the introduction of linear array focused laser differential interferometry (LA-FLDI), its use has rapidly proliferated as it extends the capabilities of FLDI to multiple measurement positions, greatly increasing the amount of data that can be simultaneously collected. To collect and store the signal data from LA-FLDI, arrays of photodiodes have primarily been utilized in recent literature, and, while photodiodes have the distinct advantage of reaching sampling rates on the order of 100＋ MHz, custom setups for photodiode arrays can be cumbersome to build and align as the number of points in the FLDI beam array increases and nonlinear array designs begin to be considered. As an alternative to photodiode arrays, high-speed cameras are promising, but their usage for LA-FLDI has been quite limited thus far. Compared to photodiodes, modern high-speed cameras are both expensive and possess lower sampling rates, but they are easy to align and provide expanded ability to measure larger arrays of FLDI points. The camera sensor needs only to be placed perpendicular to the array path, and, if appropriately sized, the entire array can be captured without individually aligning the beams. In this work, a high-speed camera is used to record measurements of spectral content, in a Mach 1 free jet flow obtained via imaging FLDI with 5 × 5, 3 × 3, 1 × 5, 1 × 10, and 1 × 12 arrays created using a combination of a 5 × 5 diffractive optical element and post-process sampling of individual pixels to form what we have named “virtual arrays”. For the sake of comparison and proof of similar applicability, the signal from the center point of the 5 × 5 grid is also measured using a high frequency bandwidth photodiode. The experiments show similar capability between the photodiode and camera results when measuring dominant frequencies in the spectral content, even when sampling individual pixels within FLDI points. Possibly due to the significantly smaller size of the pixels, the magnitudes of the individual pixel spectra are often less than that of the photodiode. However, the camera results imply the potential for arrays of arbitrary sizes and shapes, which are only limited by camera sensor size, to be imaged and yield informative estimates of spectral content. Furthermore, sampling individual camera pixels appears to provide significantly enhanced spatial resolution. For FLDI applications involving lower frequency ranges, say 0–2 MHz, current high-speed cameras may provide a suitable if not advantageous alternative to photodiode arrays.