The principal source of acoustic noise in the infrasound frequency range is wind turbulence. In a typical prior infrasound sensor based on a pipe filter connected to a microbarograph, the sound is summed acoustically from multiple locations connected to the microbarograph via the pipe filter in order to obtain an averaging or smoothing effect that suppresses the relative contribution of noise. Unfortunately, the acoustic sum is not a true average and is affected by the frequency response of the pipe filter, which response is not flat across the entire infrasound frequency band and can be determined only with extreme difficulty. In contrast, the response of an OFIS depends on the optical response of the fiber but is substantially independent of the acoustic frequency.

Figure 2. The Directionality of an OFIS increases with length, as illustrated by four example polar plots of relative amplitude (represented by radius) versus angle of incidence (¸ represented by azimuthal angle).

A(θ) = sin[Nπcos(ı)]/Nπcos(θ).

As illustrated by a few examples in Figure 2, this response peaks at broadside incidence (θ = 90°) and becomes more sharply peaked as N increases.

This work was done by Mark A. Zumberge and Jonathan Berger of the University of California for the Defense Threat Reduction Agency. For more information, download the Technical Support Package (free white paper) at www.defensetechbriefs.com/tsp under the Photonics category. DTRA-0001

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