Discussion
The following light source has the best directivity :
Answer: B
The source with the best directivity is the edge emitting LED.
A surface emitting LED, of which the Burrus LED is the classic type, emits from a broad area with a nearly Lambertian pattern, about 120° wide. An edge emitting LED confines the light in a thin active stripe sandwiched between heterojunction layers, like a waveguide, and emits from the edge in a much narrower beam, especially in the plane perpendicular to the junction (about 30°). Its higher radiance and directivity give better coupling into fibres.
The double heterojunction is a structure both types use for carrier and optical confinement, not a separate geometry with the best directivity.
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