Photonic Doppler velocimetry with 62.5 µm graded index multimode fiber at 1550 and 532 nm

Abstract

Photonic Doppler Velocimetry (PDV) systems measure velocities in shock physics experiments on novel materials. In many experiments, diffusive materials limit the amount of Doppler signal collected, which degrades the measurement. Increasing the laser power comes with a cost when many channels are supplied at the same time and could damage some sensitive surfaces. The use of a multimode fiber with a 62.5 µm core diameter in the collimator or focuser brings more signal into the interferometer but, on the other hand, the multimode behavior of the fiber reduces interference contrast. PDV systems with 62.5 µm multimode fiber were tested at 1550 and 532 nm on a test-bench at low velocities. The interference amplitudes are discussed according to tilt angle and for different surface qualities. The trends differed with wavelength. A multimode system was also tested under real conditions during a ramp compression experiment with velocities greater than 100 m/s. Practically, the use of multimode fibers greatly facilitates probe alignment, especially at 532 nm. Furthermore, 532-nm singlemode fiber are rapidly limited in terms of their optical power. For the two wavelengths tested, the high-speed measurements show clear interference fringes with at least the same amplitude level as with singlemode PDV systems. The benefits of multimode PDV systems appear clearer for highly diffusive surfaces and when probe alignment cannot be guaranteed. At 532 nm, better results are expected using fibers with a core diameter in the range of 20 µm if the optical components become available.