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

Abstract

A new method is presented to measure the separation distance between probing volumes of closely spaced multi-foci Focused Laser Differential Interferometers (FLDI). The accuracy and precision of this distance measurement directly translate into the quality of convection velocity measurements performed by means of arrays of FLDI. The suggested method is based on the detection of a propagating weak blast wave, generated with a simple and inexpensive apparatus using an automotive spark plug. Demonstration is conducted using an FLDI with two foci (D-FLDI). The generated blast wave is probed at multiple distances from its source to verify its weakening into an acoustic pulse, which offers ideal conditions to the proposed methodology. D-FLDI separation distance measurement using the new approach is compared to measurements using beam profiler images and to the alternative currently established in the literature, based on the FLDI response to a moving weak lens. Tests are made on varying internal configurations of the D-FLDI, while the distance between the two systems is kept constant. Results show the present method to have improved accuracy and robustness in comparison to the moving lens approach, while requiring significantly less effort. Measured separation distances obtained from blast wave detections in a single location are within 0.5% of the reference value measured through the beam profiler. This procedure is therefore a practical and reliable alternative to the measurement using beam profiler imaging, with similar quality. Its advantages concern associated costs, flexibility when measuring in constrained spaces such as in proximity to walls, and applicability to systems in which beam imaging is not an option, such as multipoint line FLDI.