Accurate flow speed measurement through correlation ratios using a mode-selective photonic lantern in optical coherence tomography.

A novel method for measuring non-axial flow speed using optical techniques such as optical coherence tomography is introduced. The approach was based on the use of a modally-specific photonic lantern, which permits simultaneous probing of the sample with three distinct coherent spread functions. Transverse flow speed is measured from the ratio between the cross-correlation and autocorrelation of the signals. It achieved a 3 to 5 times higher accuracy than common autocorrelation approaches and measured flows as slow as 0.5 mm/s for an integration time of 1 second. Additionally, the method gives information on the flow's three-dimensional orientation, does not require information about the diffusion coefficient, and is more robust to bias errors such as a gradient in the axial flow velocity.