Focused laser differential interferometry (FLDI) is an important diagnostic for measuring density fluctuations in high-speed flows. Currently, however, high dynamic range FLDI is limited to photodiode measurements. In order to spatially resolve multiple locations within complex flows, we present a novel, to the best of our knowledge, refractive-optic imaging FLDI concept that not only produces two-dimensional images without scanning but also reduces the measurement noise floor of those images. To demonstrate this concept, a 33 × 33 grid of FLDI points is first generated using a microlens array. Then, the beams are split and recombined using two polarized Mach–Zehnder interferometers to maximize flexibility in beam separation and optimize signal sensitivity. Next, the FLDI points are collected slightly out of focus on a high-speed camera in order to increase the number of pixels n per FLDI point, thereby reducing noise floor by √n$\mathrm {\mathbf {\sqrt {n}}}$. Finally, an under-expanded jet with a characteristic screech at 14.1 kHz is tested with the imaging FLDI setup, showing clear barrel and reflected shock features as well as spatially varying turbulence densities. Overall, this unique concept enables the creation of reduced-noise-floor, two-dimensional FLDI datasets for the study of supersonic and hypersonic flows.

中文翻译：

---

成像聚焦激光微分干涉测量

聚焦激光微分干涉仪 (FLDI) 是测量高速流动中密度波动的重要诊断方法。然而，目前高动态范围 FLDI 仅限于光电二极管测量。为了在空间上解析复杂流中的多个位置，据我们所知，我们提出了一种新颖的折射光学成像 FLDI 概念，该概念不仅无需扫描即可生成二维图像，而且还降低了这些图像的测量本底噪声。为了演示这个概念，首先使用微透镜阵列生成 33 × 33 的 FLDI 点网格。然后，使用两个偏振马赫曾德尔干涉仪对光束进行分离和重新组合，以最大限度地提高光束分离的灵活性并优化信号灵敏度。接下来，在高速相机上稍微失焦地收集 FLDI 点，以增加每个 FLDI 点的像素数n ，从而将本底噪声降低√名词$\mathrm {\mathbf {\sqrt {n}}}$​最后，使用成像 FLDI 设置测试具有 14.1 kHz 特征尖叫声的欠膨胀射流，显示出清晰的桶状和反射激波特征以及空间变化的湍流密度。总体而言，这一独特的概念能够创建低本底噪声的二维 FLDI 数据集，用于研究超音速和高超音速流。