In this research, distributed sensing based on Rayleigh scattering is used to measure temperature and strain in a composite panel during a high energy laser strike. The ultimate goal is to rapidly detect a laser strike by sensing the localized, rapid temperature rise caused when directed energy is incident on the surface of a composite structure. A secondary goal is to determine if the thermal response can be detected even in the presence of applied strain. Initial results will be discussed for composite structures comprised of carbon fiber/epoxy of various thicknesses using embedded distributed optical fiber sensors (DOFS) to rapidly detect temperature changes greater than 1000° on the surface or between plies of the composite. Measurements of the temporal and spatial response are taken at rates greater than 20Hz with sub-millimeter resolution. An infrared camera is used to validate the temperature measurements obtained using DOFS. In addition, since DOFS respond to strain as well as to temperature, any strain in the composite as a result of mechanical loading is coupled into the embedded fiber and is also detected by the sensor. Initial measurements are taken to demonstrate the simultaneous response to both temperature and strain and to characterize the typical strain that results. A DOFS-based sensing architecture can then be designed to mitigate the mechanical response of the sensor, allowing for isolation and rapid detection of the thermal response when high energy radiation is incident on the composite surface.
This paper describes a unique technique that implements photoconductive sensors in a radio frequency (RF) switching network designed to locate in real-time the position and intensity of IR radiation incident on a composite structure. In the implementation described here, photoconductive sensors act as rapid response switches in a two-layer RF network embedded in an FR-4 laminate. To detect radiation, phosphorous-doped silicon photoconductive sensors are inserted in GHz range RF transmission lines. By permitting signal propagation only when a sensor is illuminated, the RF signals are selectively routed from lower layer transmission lines to upper layer lines, thereby pinpointing the location and strength of incident radiation. Simulations based on a high frequency three-dimensional planar electromagnetics model are presented and compared to the experimental results. The experimental results are described for GHz range RF signal control for 300- and 180-mW incident energy from 975- to 1060-nm wavelength lasers, respectively, where upon illumination, RF transmission line signal output power doubled when compared to nonilluminated results. The experimental results are also reported for 100-W incident energy from a 1060-nm laser. Test results illustrate real-time signal processing would permit a structure to be controlled in response to incident radiation.
Rapid sensing of near infrared (IR) energy on a composite structure would provide information that could mitigate
damage to composite structures. This paper describes a novel technique that implements photoconductive sensors in a
radio frequency (RF) switching network designed to locate in real time the position and intensity of IR radiation incident
on a composite structure. In the implementation described here, photoconductive sensors act as rapid response switches
in a two layer RF network embedded in an FR-4 laminate. To detect radiation, phosphorous doped silicon
photoconductive sensors are inserted in GHz range RF transmission lines. Photoconductive sensors use semiconductor
materials that are optically sensitive at material dependent wavelengths. Incident radiation at the appropriate wavelength
produces hole-electron pairs, so that the semiconductor becomes a conductor. By permitting signal propagation only
when a sensor is illuminated, the RF signals are selectively routed from the lower layer transmission lines to the upper
layer lines, thereby pinpointing the location and strength of incident radiation on a structure. Simulations based on a high
frequency 3D planar electromagnetics model are presented and compared to experimental results. Experimental results
are described for GHz range RF signal control for 300 mW and 180 mW incident energy from 975 nm and 1060 nm
wavelength lasers respectively, where upon illumination, RF transmission line signal output power doubled when
compared to non-illuminated results. Experimental results are reported for 100 W incident energy from a 1060 nm laser.
Test results illustrate that real-time signal processing would permit a structure or vehicle to be controlled in response to
In this research, fiber Bragg grating (FBG) temperature sensors are embedded in composites in order to detect highly localized temperature gradients in the composite structures. The primary goal is to perform structural health monitoring on a composite structure. A secondary goal is to use the sensors as a diagnostic tool to determine the optimal composite materials, architectures, or structures that are the least susceptible to thermal damage. Initial results will be discussed for two composite materials using a single sensor to measure temperature variations. The tests include measurements of the temporal and spatial thermal response of the composite resulting either from an applied heat source or to high energy radiation incident on the surface. Additional tests demonstrate the response using a 3x2 array of sensors to simultaneously measure the temperature at three varying depths in the composite, using three FBGs aligned with the heat source, and three FBGs located a short lateral distance (3cm) away from the heat source. In addition, since FBGs respond to strain as well as to temperature, any strain in the composite is coupled into the embedded fiber and is also detected by the FBG sensors. Initial measurements demonstrate the simultaneous response of FBG sensors to both temperature and strain. The various components of strain that are observed in the composite will be discussed, and possible methods to isolate these components and mitigate their response will be considered.
Long distance data transmission using solitons multiplexed on different wavelengths makes more efficient use of fiber bandwidth than transmission on a single wavelength channel. However, perturbations and nonlinear distortions limit the number of wavelengths which can be multiplexed and detected at the end of the fiber. Perturbations, such as loss, cause permanent frequency shifts if a collision occurs between solitons widely separated in frequency. Densely packing the solitons spectrally, though, results in distortions in spectral intensity which limit the use of standard wavelength demultiplexing techniques. We examine methods by which solitons, densely multiplexed in wavelength, may still be detected even during collisions. The theoretical feasibility of encoding the data on the eigenvalues of the linear evolution equations associated with soliton propagation by the inverse scattering transform is discussed, as are more practical techniques using only the spectral intensity of the waveform.