Variable numerical-aperture temporal-coherence measurement of resonant-cavity LEDs

The first interferometric measurements of temporal-coherence length variation with numerical aperture (NA) are described for 650 nm, resonant-cavity light-emitting diodes (LEDs) agreeing with spectrally derived results. The interferometrically measured coherence length (22 μm to 32 μm) reduced by 37% for a 0.42 increase in NA. For a larger range of NA (0-1), this would give coherence lengths (10 μm-40 μm) lying in the gap between that of conventional LEDs (∼5 μm) and superluminescent diodes (∼60 μm).