Consider a piece of semiconductor of index n = 3.5 and length d. It can be viewed as a Fabry-Perot cavity because the surfaces can be regarded as two mirrors by using reflection coatings with reflectivity |r|².

(a) Express the round-trip time trt symbolically in terms of n and d. Note that the effects of the refractive index should be included. 

(b) From the Fabry-Perot transmission spectrum shown above, please estimate d. 

(c) Find the photon lifetime by deducing from the spectrum.