AAWSAP DIRD, State of the Art and Evolution of High-Energy Lasers, March 2010

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Advanced airframe structures, pressure vessels and radar domes are frequently
fabricated using cut glass fibers or whole fiberglass cloth layers which are impregnated
with epoxy resin. These materials typically fail through a combination of thermal
ablation of the epoxy and delamination. Like metals, if the piece is under aerodynamic
load or is part of a pressure vessel, failure will typically occur before complete burn­
through although crack initiation and propagation is less likely due to the residual
strength of the fiberglass cloth after the epoxy is removed. Also, like metals the energy
required for penetration is thickness dependent.
Ceramic materials, similar in composition to common Corning Ware, are routinely used
in high temperature applications such as radomes for multi-mach air-to-air missiles or
engine components. Although designed to handle heat when absorbed slowly, it will
fracture and shatter if thermally shocked at moderate irradiance levels. Due to the
poor thermal conductivity of ceramic materials, a thick piece of ceramic will shatter at
almost the same irradiance and fluence levels as a much thinner piece.
A discussion of laser-material interaction requires background information on the
hazards of eye damage from laser radiation. There is a popular misconception that
there are "eye safe" laser wavelengths in the infrared. The reality is that any
wavelength in the UV, visible or infrared can damage the eye or skin. The damage
mechanisms and damage thresholds, however, are a function of the laser's wavelength,
pulse length and pulse energy (if pulsed) or average irradiance (if continuous). Laser
wavelengths in the UV, visible and near infrared out to about 1.2 µm are the most
dangerous. These wavelengths pass easily through the cornea and ocular fluid and are
then focused on the retina where individual rods and cones can easily be permanently
destroyed. This damage leaves the person with localized blind spots in the eye's field
of view. Infrared wavelengths from about 1 µm to about 1.5 µm pass through the
cornea but are absorbed by the eye's ocular fluid which results in localized heating.
The damage threshold in this wavelength range is considerably higher than for retinal
damage but in extreme cases the eye can be destroyed. Infrared radiation at
wavelengths longer than about 1.5 µm is absorbed by the cornea and results in scarring
similar to cataracts. Although it requires considerably more energy to damage the
cornea than the retina, eye and skin safety at these wavelengths cannot be ignored.
There are detailed ANSI standards which establish the Maximum Permissible Exposure
(MPE) as a function of wavelength and pulse/continuous beam format. Figure 18
provides an example of single pulse MPE values for a variety of wavelengths.
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