Luna Reflectometers - An Indispensable Tool for PDV System Diagnostics
Research in compression and shock physics includes the characterization of short duration, high-speed dynamic phenomena, which can provide important information about material properties and behavior. Photonic Doppler Velocimetry (PDV), a heterodyne laser interferometry technique capable of measuring km/s scale velocities, has become an increasingly important tool in this field. The availability of technologies developed for telecommunications, including stable, narrow linewidth lasers, low-loss single-mode optical fiber, fiber optic circulators, and high-speed detectors and oscilloscopes, enabled the construction of the first PDV systems at Lawrence Livermore National Labs (LLNL) in the early 2000s.
Being able to build it as a fiber-based system provided several advantages including the ability to use smaller, more densely placed probes and the ability to locate lasers and detectors farther from the experimental location and to easily reconfigure them. However, having the optical path in fiber necessitates verifying the absence of fiber damage, tight bends, or bad connections throughout the entire optical path.
Luna’s optical frequency domain reflectometry (OFDR) technology has facilitated PDV system development since the early days of PDV. The OBR 4600’s high spatial resolution reflectivity measurement and ability to characterize a complete optical path in one scan make it extremely useful as a diagnostic tool for PDV systems.
Probe Characterization and Verification
Probe selection and verification are a critical to PDV experiments. The OBR 4600’s ability to measure and distinguish reflections from different parts of the probe and from the target are useful not only during the design phase, to studying the properties of different probes[i], but also during experimental setup to verify probe integrity and distance to the surface under test.
Optical Path Verification
Fiber interferometer systems typically require precise control of fiber lengths, verification of the fiber path, and elimination of unwanted sources of reflectivity. For PDV in particular, optical path verification is critical. Since the test is usually destructive to the material under test and the probes, there is only one chance to perform each experiment. A compromised optical path can result in an unrecoverable loss of data.
Modern PDV systems contain increasing numbers of channels whose optical paths include long distances in fiber that can contain multiple fiber segments connected by fragile connectors. In addition, experiments are often conducted in inhospitable environments in the field, in which fiber or connector damage can occur at any time due to causes as varied as a person walking over a cable or a rodent chewing on cable insulation. For each channel, it is necessary to test the entire optical path, without disconnecting fibers, to ensure that there are no fiber breaks, damaged connectors, or tight fiber bends. Enabling the expansion of such systems requires faster measurement tools that can be integrated into automated test systems with multiport optical switches[ii].
Luna now offers multiple products for different system needs. For fiber networks under 500m, the new LWA 7601-C provides fast measurement (0.08-0.5s in continuous mode), high resolution at long lengths (20µm sample spacing up to 200m or 80µm sample spacing at 500m), simpler event identification, and easier automation via SCPI. The OBR 4600 covers the longest ranges (up to 2km in extended range mode) and offers the highest sampling resolution at short distances (10µm sample spacing at 20m). A portable OBR (6200 series) is also available for field measurements where a desktop/rack mounted instrument isn’t practical.
During assembly of a fiber optic system, it is often necessary to confirm that optical fiber segments match the prescribed lengths. The LWA 7601-C simplifies length measurement with an events table function and unprecedented delay measurement accuracy (±0.001% up to 200m).
[i] Shinas, M. and Briggs, M. “PDV probe efficiency study using the Luna Optical Backscattering Reflectometer”,
PDV Conference and Workshop, Sept. 3-4, 2008.
[ii] Burk,M., “Quality Assurance ‘LUNA’ Tool for Many Point Experiments”,
PDV Workshop, June 24-26, 2014.