Portendo has in collaboration with FOI, the Swedish Defence Research Agency, developed a world-leading
technique of trace detection of explosives at standoff distance using Raman spectroscopy. The technology is
further developed in order to enhance the sensitivity of the method and to be able to extend the field of
applications. Raman scattering is a well-established technique able to detect substances down to single
micrograms at standoff distances, however, one of the obstacles limiting the detection possibilities is interfering
fluorescence, originating either from the substance itself or from the surrounding material. One main challenge
for this technology is thus to either omit the excitation of the fluorescent process altogether or to be able to
separate the two processes and only detect the Raman signal.
Due to the difference in the temporal behavior of the two processes - Raman scattering occurs in the order of
femtoseconds while fluorescence typically has a lifetime in the order of nanoseconds - one way to theoretically
separate them is to limit the measurement to as short time as possible, cutting off most of the emitted
fluorescence. The improvement depends on how much of the fluorescence can be omitted without decreasing
the Raman signal. Experimentally, we have verified this improvement in signal to noise ratio when using a laser
with picosecond pulses instead of nanosecond pulses, which has resulted in an improvement in SNR of up to 7
times for bulk ANFO. These results verify the predicted signal enhancement and suggest higher sensitivity for
standoff detection in future systems.
In this paper, we report the development of a new bonding agent and method for the surface mounting of optical fiber Bragg grating (FBG) strain and temperature sensors for use in high temperature environments--where there is a presence of water, moisture, dust, susceptibility to corrosion and/or elevated temperatures up to 800°C. To ensure a stable reflectivity response of FBGs and their survival at elevated temperatures, we are using chemical composition gratings (CCGs). The refractive index modulation in these gratings is caused by a chemical change, which results in a higher activation energy and stable behavior up to 1000°C. Samples of CCGs were successfully encapsulated and mounted onto metal shims. The packaged sensors were tested for strain (+/- 1000με) and temperature (to +400 °C) response. The encapsulated sensors display a linear response with an increase in the temperature sensitivity of the FBG, with a factor of ~ 28.34pm/°C, and a strain gauge factor of 1.7pm/με.