Fig. 2: Left: Temporal coherence of light-emitting diodes (LEDs), conventional single-mode lasers (e.g., distributed-feedback laser (DFB) lasers), and superluminescent diodes (SLDs).
From: Low-coherence semiconductor light sources: devices and applications
a The temporal coherence of an incoherent light source, like LEDs, exhibits noise-like intensity due to multiple frequencies and the random phase noise. b Light sources with high temporal coherence, like single-mode DFB lasers, exhibit low phase noise and narrow spectrum. c Light sources with low temporal coherence, like SLDs, exhibit moderately wide spectra with lower phase noise than LEDs and a lower number of frequencies, m, than that of LEDs, n. Right: Spatial coherence demonstration in Young’s double-slit interference experiment. d A spatially incoherent source yields no intensity fringes on the screen. e A high-spatial-coherence light source generates intensity fringes with a high contrast. f In a low-coherence light source, the contrast of fringes is in between those two extreme cases.
