Optical-Axis Spatial Sensitivity of a Simulated Focused Laser Differential Interferometer

A focused laser differential interferometer (FLDI) simulation was used to study the sensitivity of the FLDI technique to a variety of factors. The model takes in a series of density fields and computes the FLDI beam phase difference in response to those fields. For pure sinusoidal signals of finite width, the frequency was found to have no effect on the computed FLDI response below the photodetector threshold. The signal wavelength, however, has a direct correlation with FLDI amplitude; peak amplitude was achieved with a wavelength of double the FLDI beam separation when measured at the focal points. The ability of the FLDI to suppress signals away from the foci of the beams was observed. Simulated narrow-nozzle jet results using a large-eddy simulation were compared to an experimental test with good agreement. The jet simulation was then used to study the effects of varying the beam separation and profile diameter on the FLDI sensitivity falloff, with smaller beam spacing and larger profiles away from the foci corresponding to shorter optical-axis sensitivity lengths.