Optimization of Reverse Saturable Absorber Limiters: Material Requirements and Design Considerations
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ABSTRACT We present numerical beam-propagation simulations of optimized reverse-saturable absorption (RSA) based optical limiters where the depth of focus of the input beam is much smaller than the thickness of the nonlinear material. The optimization is achieved by allowing the molecular concentration to vary along the propagation path, allowing the entire length of the limiter to reach the maximum possible nonlinear absorption before eventual damage to the limiter. We review in detail the analytic model originally derived by Miles [1] to determine the design and performance of such limiters. This model requires the usual 5-level model used in the numerical solution to be approximated by a quasi-three-level system. We show that this effective 3-level excited-state cross section is both pulsewidth and fluence dependent. The numerical propagation output shows that there is considerable diffractive beam distortion, which cannot be accounted for in the analytic model. The end result is that while there is qualitative agreement with numerical results, the magnitude of the limited output can be an order-of-magnitude underestimated. We determine that the fluence level at all parts of the limiter must be at least ten times the saturation fluence to efficiently utilize the nonlinear absorption. We further describe how the optimized distribution of molecular density is the limit of the multi-element tandem limiter for an infinite number of elements. By carefully accounting for saturation over the entire length of each individual element, we show how a multi-element limiter may be designed to closely approach the performance of the optimized distribution for as few as four elements. With current materials technology the damage threshold of solid hosts needed to vary the molecular density is much lower than that of glass cuvettes used for liquid based limiters. Therefore, a multi-element liquid based tandem limiter can be used in full saturation so that better limiter performance should be obtained. Ultimately, however, the operation of all RSA-based limiters involves a strict trade-off between performance and linear transmittance. 1. INTRODUCTION: Reverse-saturable absorbing (RSA) molecules are promising candidates for use in passive optical limiters.[2-4] The excited-state absorption (ESA) cross section in such materials is larger than that of the ground state, causing the absorption to increase with input fluence, F (energy per unit area); hence the name "reverse-saturable absorber". While the strongest effect will be obtained by placing the nonlinear material at focus, from a practical viewpoint, the material is also most likely to undergo laser-induced damage at this position. For liquid-based limiters this may not be a problem and can actually help limiting, i.e., the resulting plasma may block incoming light.[5] However, solid-state hosts undergo irreversible damage as do the optical cells that hold liquid limiter materials.[5] This has prompted researchers to devise means of protecting the limiter itself from damage b
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