Laser Doppler velocimetry for three-dimensional distribution measurement of the velocity component by combining two-dimensional spatial encoding and non-mechanical scanning.

We present a differential laser Doppler velocimeter (LDV) for measuring the velocity distribution in a three-dimensional (3D) space. Our conventional research so far has proposed some methods for measuring two-dimensional (2D) distribution measurement. One of the proposed methods was spatial encoding of measurement points arranged on a 2D plane. Besides, we also have proposed laser Doppler cross-sectional velocity distribution measurement based on a non-mechanical scanning method. We propose a 3D method by combining 2D spatial encoding and non-mechanical scanning, in which the measurement points distributed on a 2D plane are spatially encoded with different bias frequencies, and these points are scanned non-mechanically in another direction by changing the wavelength. As a feasibility study of the proposed method, experiments were conducted using a 4×4 channel optical setup and were performed at five wavelengths over 1537-1553 nm. The experimental results indicate that the proposed method could successfully measure the 3D distribution of the velocity component. The spectral peaks of the beat signals for 68 measurement points for the rotational speed of the target of 2.0 and those for 67 measurement points for the rotational speed of -2.0 out of 80 measurement points were successfully observed within the measurement error of 2.8%-4.8%.