Autocollimation Optical Doppler Velocimeter: Velocity Measurement of Hard-to-Access Objects

All-fiber autocollimation schemes of the laser Doppler velocimeter (LDV) are presented that enable measuring velocities of hard-to-access objects in the range from several cm/s to several km/s. The autocollimation scheme covered in this work allows distant (several tens and hundreds of meters away) objects with the only working channel less than 5 mm in diameter to be investigated using a narrow laser beam. An acousto-optic modulator is used to modify Mach–Zehnder interferometer scheme to extend the range of measured velocities and allow distinguishing the motion direction. Air guns with the 4.5-mm-diameter bore have been used for test measurements. Velocities ranging from 0.1 to 330 m/s have been successfully measured.