DESIGN OF TAPERED WAVE-GUIDES FOR IMPROVED OUTPUT COUPLING

Waveguides whose width increases linearly with distance are shown to b e an excellent means for decreasing the beam divergence of the guided mode and thus improving output coupling, provided the taper angle is c arefully chosen. Such tapered waveguides can provide nearly diffractio n-limited output while incurring virtually no energy penalty. Beam pro pagation calculations together with a dimensional analysis allow the f irst quantitative prediction of output beam divergence for a wide rang e of input parameters. Performance is shown to be optimal for taper an gles of about 0.4 times the input beam's full width at half maximum (F WHM) divergence angle, subsequently reducing the latter by a factor of five.