3) The spontaneous emission from a GaAs semiconductor laser can be approximated by the graph shown below. The length of the wafer is 680 m, the index of refraction is 3.6, the facet reflectivity is 0.3, the residual absorption coefficient in the crystal is 10 cm, and the recombination lifetime is 1 ns. 1 um 230 m 680 m (a) What is the wavelength of peak gain? (b) What is the FWHM of the gain coefficient in Hz and cm-? 1.476 eV 43 meV 12 meV (c) What must be inverted carrier density to bring this wafer to threshold? (d) This carrier density must be sustained by some sort of pumping-carrier injection, photo pumping, or E-beam excitation. Estimate the minimum pumping power to maintain an inversion of 1016 cm3 through the wafer.