a Theoretical and Experimental Investigation of Enhanced Internal Reflection.
Abstract
Internal reflection from the interface between a transparent incident region and a region with gain is examined theoretically and experimentally. The gain, related to population inversion, is established by optical excitation. A Beer's Law absorption of the pump beam produces a gain which decreases exponentially with distance from the interface. Infinite series solutions to the wave equations are derived in the gain region. The solutions are composed of two parts: one represents a wave traveling toward the interface, the other a wave traveling away from the interface. Theoretical values of the reflectance and incident angle are presented. Reflectances less than unity occur for incident angles near normal incidence. A maximum greater than unity, is reached in the vicinity of critical angle. As the incident angle is increased toward grazing incidence, the reflectance decreases toward unity. Singularities may occur in the amplitude reflectance coefficient for certain combinations of incident angle and gain region thickness. The angular width of the reflectance curve as a singularity is approached is extremely narrow (less than .01(DEGREES)). These singularities are considered non-physical and a consequence of the assumption that the gain coefficient is independent of the amount of energy removed from the gain region by the transmitted wave. This assumption is violated in the rate instances where the reflectance becomes extremely large (greater than 200 for the experimental case described). An experiment designed to observe and measure the reflectance from an optically excited laser dye is described. Experimental parameters are discussed. The method by which the data were acquired is described and the data are interpreted. The theoretical reflectance curves are appropriately averaged to account for incident beam divergence and fluctuations of certain experimental parameters. Experimental data are shown to be in favorable agreement with theory, in particular the angular position of the maximum reflectance and the magnitude of reflectance on either side of the maximum. It is concluded that the exponentially decreasing gain model is a good description for the gain region of the experiment.
- Publication:
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Ph.D. Thesis
- Pub Date:
- 1982
- Bibcode:
- 1982PhDT........74C
- Keywords:
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- Physics: Optics