Calculate minimum mirror reflectivity R needed for optical gain in a semiconductor double- heterostructure (DH) laser, assuming both mirrors in the optical cavity are same (R = R1 = R2). Fabry-Perot cavity length is 20 , cavity loss (i) is 10 cm-1, optical conferment factor () is 1, threshold gain in the medium (gth) is 10^3 cm-1.