Biomedical Engineering Reference
In-Depth Information
radar and other threat warning systems, such that multispectral multisen-
sor data clouds provide improved countermeasure implementation and
improved vehicle survivability.
4.19.1 System Requirements
The key requirement of an LWR is to provide high probability of intercept for
laser threats while maintaining an acceptably low false alarm rate. High prob-
ability of intercept implies a wide dynamic range and broadband response to
cover all likely threat wavelengths. The LWR must determine enough informa-
tion about the threat, in terms of bearing and other relevant parameters (e.g.,
wavelength, pulse width, and pulse repetition frequency) to identify it unam-
biguously or at least to characterize it sufficiently to enable the timely implemen-
tation of appropriate countermeasures. Various fielded systems include unique
fiber bundle collection apertures for low ambiguity wide field of view cover-
age of multiple laser threats, high-resolution optical wavelength analyzers with
holographic dispersion gratings, phase delay interpretation for angle of arrival
determination, and means of retrieval and identification of laser threat data.
4.19.2 Laser Threats
The most commonly encountered lasers used for range finding and target
designation are based on ruby, neodymium, and carbon dioxide systems.
CO
2
lasers in the infrared are better able to penetrate rain, haze, and smoke
than the older visible and very near infrared. They also have the advantage of
being eye safe at low output levels, as are those emitting at 1.54 and 2.06 μm.
Rangefinder pulse widths are typically 10 (Nd:YAG) or 50 ns (CO
2
), and
pulse repetition rates are between a few and a few tens per second. Output
pulse energies are typically ~20 mJ, and beamwidths are ~0.5 mrad. Thus, at a
likely engagement range of 1 km, the pulse energy density at the target could
be in the region of 10
−3
J m
−2
. Considerations of atmospheric transmission,
different source-target ranges, and the variety of threat sources lead to the
requirement for an LWR with a rather extended dynamic range. This is com-
pounded by the nonuniform illumination across the laser beam (which may
be modeled as Gaussian in profile, but where the speckle effect caused by
atmospheric turbulence must also be taken into account). Overall, a dynamic
range of around 10
6
in optical power will be needed.
4.19.3 Laser Detection
The LWR must detect radiation from pulsed sources operating at discrete
wavelengths within the visible to near-infrared spectral range, identify
these wavelengths, and accurately measure laser pulse angle of arrival
(see Figure 4.30 for a block diagram). Fiber input bundles with collection
apertures embedded in the outer surface of a military platform direct inci-
dent radiation to a concave holographic grating where the laser radiation is
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