Photonic Doppler Velocimetry Probe Designed with Stereo Imaging
The velocity distribution of an imploding curved surface is measured with a miniature fiber-optic probe. The probe uses fiber matrix arrays that are focused onto different regions of this curved surface. A pyramid reflector deflects the interrogation onto specific regions of this curved surface. The fiber arrays record velocity along several hundred lines of sight with sub-nanosecond resolution. Reflected light from each spot on the moving surface is Doppler shifted with a small portion of this light propagating backwards through the launching fiber. The reflected light is mixed with a reference laser in a technique called Photonic Doppler velocimetry, providing continuous time records. Within the probe, seven groups of miniature optical relay lenses are grouped into a close-packed array, allowing the interrogation of seven regions of interest. A six-faceted pyramid reflector with a conical hole drilled into its center is used to direct the light beams onto different regions. To minimize back reflections, the fiber matrix arrays are index matched onto the first relay lens made of fused silica. The optical lens design minimizes the laser beam diameters and also provides excellent imaging capabilities. Therefore, two of the fiber matrix arrays are replaced by coherent bundles for stereo imaging recorded by a high-speed framing camera. This requires additional cutouts of the pyramid reflector to allow imaging overlap. Six additional 1.5 mm diameter incoherent bundles are attached to the edges of the pyramid reflector and are used for illumination. Precision metrology on the direction cosines of the trajectories is measured to satisfy environmental requirements for vibration and temperature.