Progress on focused laser differential interferometry in measuring supersonic/hypersonic flow field

As a nonintrusive and high spatial/temporal resolution testing method, Focused Laser Differential Interferometry （FLDI） is very suitable for use in extreme experimental environment, such as hypersonic wind tunnel. Starting from the typical composition of the optical path, the principle of FLDI and the spatial filtering characteristics are introduced. Thereafter, a series of recent improvements based on typical FLDI is reviewed. Those improvements were implemented to meet different research needs. This is followed by applications and conclusions of FLDI in the field of hypersonic flow field measurement, including hypersonic freestream disturbance, hypersonic boundary layer transition, and supersonic jet noise. This review shows the potential of FLDI in supersonic and hypersonic flow field measurement, and it may provide reference for the follow-up improvement of FLDI testing technology and related precision measurement of hypersonic flow field.