Method to dynamically measure optical return loss from shocked metal surfaces

Optical return loss from a surface is the fraction of light returned along a line-of-sight from the given surface. It is a useful quantity in determining whether interrogation via Photonic Doppler Velocimetry (PDV) or other optical diagnostics is possible. In this contribution, we present a novel method to make this measurement for shock metal surfaces with a time resolution of nanoseconds and a power resolution of about one in ten million. By comparing the power of the Broadband Laser Ranging (BLR) signal before and after shock breakout on a given surface, a relative change in reflection loss is observed at 40 MHz. When combined with optical backscatter reflectometer measurements which measure static reflection losses at a surface, a measure of the total dynamic reflection loss can be constructed. We present a series of small-scale, proof-of-principle xperiments performed at Los Alamos National Laboratory. An explosive drive consisting of an RP-1 detonator initiating a PBX-9501 pellet was used. Directly in contact with the high explosive was a stainless steel disk. Three surface finishes were investigated: 32 μin (0.8 μm) regularly machined grooves, a near mirror finish, and 32 μin surface that had been Scotchbrited to irregularly scratch the surface. Results show that surface treatment had minimal impact on the change in return loss after shock break-out. Statistically, changes in total reflection loss were centered around 0 dB with decreasing standard deviation as the probe angle was increased.