Thermally Activated Transmissive Optical Elements for Rejection of Laser Radiation
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ABSTRACT The sacrificial thermal shutter/reflector concept is suitable for optical power limiting in optical systems containing an intermediate focus and uses a thermally activated chemical process to form a mirror that reflects a large fraction of the incident laser light. Under normal illumination, the thermal reflector component is a transmissive optical element at or near a focal plane of an optical system. A pulse from an incident laser beam heats a film of precursor chemicals on the optical element and thereby converts it to a localized reflective metallic mirror. For visible laser pulses longer than 0.1 ms, the thermal shutter provides optical limiting at energies below the ANSI Maximum Permissible Exposure (MPE) standard for laser eye protection. For shorter pulses, near-MPE limiting is achieved. See-through during exposure is also possible with these elements. Various thermal shutter chemistries and efforts to form optically clear thin precursor films are described. The laser testing results highlight the importance of the substrate and absorber in these devices.
DESCRIPTION OF CONCEPT The sacrificial thermal shutter/reflector (TS) concept, represented schematically in Fig. 1, uses thermally activated chemistry to generate a metallic mirror that subsequently reflects a large fraction of the incident laser light. Under ambient illumination, the TS component is a
transmissive optical element at or near a focal plane in a direct view optical system. A pulse from an incident laser beam heats, by absorption, a thin film of precursor chemicals on a transparent optical substrate and thereby converts it to a reflective metallic mirror. Since these units will be part of a transmissive path, the optical quality constraints (e.g., flatness, spectral purity) are not as severe as they would be for reflective optics, and often these elements are easier to incorporate into existing direct-view optical systems to provide retrofitted laser eye protection. Since only a small spot is converted to a mirror, see-through during exposure is also possible with the TS concept, and a single element can withstand many laser pulses before its see-through properties are impaired such that the element must be replaced with a fresh one.
EXPERIMENTAL CONDITIONS A systematic study of TS precursor materials was undertaken to identify the optimum stoichiometries, the most effective initiator species, and the best substrate material. The key criteria in this investigation were the laser energy required to initiate a response, the optical density of the resulting material, and the resistance of the deposited material to subsequent irradiation. TS chemistries were initially carried out on glass substrates in an -250'C oven to assess mirror formation potential. The systems described below represent the most promising from this study. 319
Mat. Res. Soc. Symp. Proc. Vol. 374 01995 Materials Research Society
Ambient Light Exposure AM
Laser Pulse Exposure 1-------
Polycarbonate Substrate Coated With a Thin Film (5-10 gim) of Heat Sensitive
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