Optical Limiters: Spatial, Temporal, and Bio-Optical Effects
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1E+5
SPATIAL ISSUES
1E+4 CM1E+3 The efficiency with which a - 1E+2 system can f/5 diffraction-limited a 1E+1 is concentrate energy onto a focal plane C illustrated by the Airy pattern plotted in 1 E+0 log-log form in figure 1. The values 1E-1 shown relate to input energy 1 mJ. Inside ...... 1E-2 a real limiter, nonlinear processes would, 0 50 100 -100 -50 of course suppress the on-axis fluence far position (,m) below the 12000 J/cm2 illustrated in the figure; this section merely considers the amount of energy in the wings of the Figure 1: Airy disc pattern of f/5 focal spot spatial distribution where the fluence is below the 0.1 J/cm2 activation threshold of a good limiter. Even for this diffraction-limited system, the pattern contains more than 10 ptJ at positions where the limiter will not be activated, so no currently-available limiter can keep the transmitted energy below the familiar I fiJ injury threshold of the retina. Fortunately, this uncontrollable energy will not all be focussed into a single spot on the retina. The effectiveness of nonlinear eye protection therefore depends crucially on the distribution of laser energy across the retina. In a real optical system containing aberrations and imperfections, simple experiments in which a small "black spot" mask is placed at the focal plane readily reveal that a significant proportion of the input energy "misses" the focal spot. Even in a well-engineered f/5 system, as much as 2% of the input energy may escape past a mask of diameter 1.5 mm. The above arguments explain why all currently-available limiters must be expected to produce a broad retinal image when operating at high input levels. This statement is not confined to materials which exhibit nonlinear refraction and nonlinear scattering which would themselves broaden the retinal image; nonlinear absorption alone can produce the effect. The image is broadened because the low-intensity wings of the original diffraction pattern become important components when the centre is suppressed by strong nonlinear processes. Even for well-known nonlinear scatterers such as carbon suspensions, careful analysis of the transmitted light [1] can show that most of the non-focusable transmitted energy was already present in the off-axis components of the input beam. 0.6
TEMPORAL ISSUES
0.5
input
focusable Threshold arguments reveal that, at 0.4 output CD high input levels, the limiter must also output 0.3 change the temporal form of the transmitted a%0.2 pulse. The limiter must be driven from its 0.1 normal high-transmission state to a highly0 opaque state very early in the laser pulse. During this initial stage, the limiter transmits 10 6 8 4 2 0 a very brief pulse of highly-focusable light time (1s) as shown in figure 2. The non-focusable components are transmitted throughout the Figure 2: pulse-shortening by an optical limiter. Carbon suspension limiter driven by 3 ps laserpulse. remainder of the input pulse.
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The reduced duration of the hazardous focusable component must be taken into account in any assess
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