Hybrid FOSS (HyFOSS)

Armstrong researchers are continuing their efforts to further develop FOSS technologies. A hybrid FOSS technique (HyFOSS) employs conventional continuous grating fibers and then overlays sections every 3-4 feet with “strong” gratings that can be sampled at higher rates. The new and stronger gratings can be sampled at rates up to 5,000 Hertz (Hz) while the continuous grating sections continue to be sampled at the lower 100 Hz rate. This technique enables higher spatial resolution at specific targets without sacrificing resolution in other areas. The ultimate goal is to achieve sampling rates up to 20 kHz.

Work to date: The team began investigating the technique in early 2013 after a request from NASA Kennedy Space Center. Many aerospace and space vehicles – fighter aircraft, UAVs, launch vehicles, and spacecraft – could benefit from FOSS integration. Some of the applications require higher sample rates to maintain high spatial resolution. NASA Kennedy’s Launch Services Group requested that the NASA Armstrong Advanced Structures and Measurements Group investigate the development effort that would be required to increase the sample rate from 100 hertz (Hz) to over 20 kHz. This increased sampling capability would allow structural features related to high frequency shock and/or vibration to be captured. To date, the OFDR technology does not have the capability to achieve these higher sample rates, though the possibility of fusing Wavelength Division Multiplexing (WDM) is feasible yet with limited spatial resolution. To achieve this with existing instrumentation would require the installation of two technologies, one utilizing OFDR and the other using WDM, and would increase the weight and installation requirement of fiber optic instrumentation.

Combining the best of OFDR and WDM technologies into new hardware that utilizes the same optical fiber would allow for high spatial resolution with lower sample rates in addition to the ability to obtain high sample rates at strategically spaced points along the fiber. With this goal, the hybrid FOSS (hyFOSS) system was developed.

A series of weak and strong FBGs are written onto the optical fiber. Weak FBGs allow for spatial resolution of 0.25 inches. The maximum number of stronger, unique-wavelength FBGs and the minimum separation interval between the sensors are determined by the length of the fiber and the wavelength spectrum of the light source. However, there is latitude to allow for the number and placement of the sensors to be tailored to the testing application.

Looking ahead: Researchers have asked the company that currently provides the fibers to supply a 40-ft strand embedded with the new technology. Next steps involving investigating the specimen in the laboratory environment. The Armstrong team is also investigating the possibilities of pushing the overall sample rate to 20kHz.

Benefits

• More measurements: Offers higher sampling rates (up to 5,000 Hz) for specific portions of the fiber
• Fast processing: Data can be collected at different resolutions enabling high resolution without sacrificing speed

Applications

• Aeronautics and launch vehicles