A certain semiconductor p-n junction laser when biased as required has a gain coefficient g = 70 cm−1, a loss coefficient α = 5 cm−1, and a length between Fabry-Perot endfaces of L = 300 μm. It is...

A certain semiconductor p-n junction laser when biased as required has a

gain coefficient g = 70 cm−1, a loss coefficient α = 5 cm−1, and a length

between Fabry-Perot endfaces of L = 300 μm. It is desired to add antireflection coatings to the endfaces. What is the smallest endface reflectivity R that

can be used without going below the lasing threshold?

(Assume that both endfaces have the same reflectivity, and that the confinement factor d/D = 1.)