Optical-Fiber Infrasound Sensors
Features include directionality and relatively flat frequency response.
Optical-fiber infrasound sensors (OFISs) are being developed for detecting acoustic pressures in the frequency range from a few millihertz to a few hertz. As explained below, these sensors were conceived to overcome some of the limitations of prior infrasound sensors based on pipe filters connected to microbarographs.
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.
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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Optical-Fiber Infrasound Sensors
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Overview
The document discusses the relationship between integral computations and pressure signals, particularly focusing on the amplitude response of a system as a function of the arrival angle. It introduces a mathematical framework where the amplitude ( A(\theta) ) is expressed in terms of the cosine of the angle ( \theta ) and a parameter ( N ), which is defined as the ratio of the instrument length ( L ) to the wavelength ( \lambda ) (i.e., ( N = \frac{L}{\lambda} )).
The analysis begins with the computation of an integral that relates to the pressure signal at a specific point. By comparing this integral to the pressure signal, the document derives the expression for amplitude ( A(\theta) ), which is given by:
[ A(\theta) = N \cdot n \cdot \cos(\theta) ]
This relationship indicates that the amplitude is influenced by both the parameter ( N ) and the cosine of the angle of arrival, suggesting that the system's response varies with the angle at which the signal arrives.
The document includes graphical representations (referred to as Figures 11 and 12) that illustrate the angular response of the system for various values of ( N ). These graphs provide visual insights into how the amplitude changes with different angles, highlighting the dependence of the system's response on the ratio of instrument length to wavelength.
The findings emphasize the importance of understanding the angular response in applications where pressure signals are critical, such as in acoustics or fluid dynamics. By plotting ( A(\theta) ) for different values of ( N ), researchers can better predict how the system will behave under varying conditions, which is essential for designing instruments and interpreting data accurately.
Overall, the document serves as a concise exploration of the mathematical and graphical analysis of amplitude response in relation to pressure signals, offering valuable insights for researchers and practitioners in fields that rely on such measurements. The interplay between the parameters involved and the resulting angular response is crucial for advancing knowledge in this area.
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