Optical fiber LDA offering high resolution and relatively long working distance

A differential LDA system is described, which employs twin front lens transmitting optics and provides a very small probe volume of 9 microns diameter combined with relatively large working distance of 135 mm. The system incorporates optical fibers and frequency shifting using either a Bragg cell or rotating diffraction grating. The advantages of the twin front lens approach for systems with a high ratio of working distance to size of probe volume are discussed and the positional tolerances required when using a rotating diffraction grating in conjunction with single mode fibers are analyzed. Finally, some experimental data are presented to confirm the optical properties and performance of the system.