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+// Example no.3.7
+// To calculate (a) the photon lifetime, (b) the threshold current, and (c) the current required to generate a mean photon density of 8.5 × 10^21 m−3
+// Page no.130
+
+clc;
+clear;
+
+// Given data
+w=3*10^(-6);                                            // The active area width in meter
+d=0.3*10^(-6);                                          // The active area thickness in meter
+L=500*10^(-6);                                          // The length
+Te=1*10^(-9);                                           // Electron lifetime
+Neth=0.8*10^(24);                                       // Threshold electron density
+aint=46*10^2;                                           // Internal cavity loss in m^-1
+n=3.5;                                                  // Refrective index of the medium
+R1=0.65;                                                // The reflectivity of ligth wave which is reflected at A
+R2=0.65;                                                // The reflectivity of ligth wave which is reflected at B
+
+// (a)The photon lifetime
+amir=(1/(2*L))*log(1/(R1*R2));                          // The loss due to mirrors per m
+c=3*10^8;                                               // Speed of ligth in m/s
+v=c/n;                                                  // Speed of ligth in medium (m/s)
+Tp=1/(v*(aint+amir));                                  // The photon lifetime in sec
+
+// Displaying the result in command window
+printf('\n The photon lifetime = %0.2f ps',Tp*10^12);
+
+// (b)The threshold current
+V=w*d*L;                                               //The active volume in m^3
+q=1.602*10^(-19);                                      //The electron charge in C
+Te=10^-9;                                              //The electron lifetime in sec
+Ith=(Neth*q*V)/Te;                                    //The threshold current in mA  
+
+// Displaying the result in command window
+printf('\n The threshold current = %0.1f mA',Ith*10^3);
+
+// The answer calculated in book is wrong
+
+// (c)The current required to generate a mean photon density of 8.5 × 10^21 m−3
+Nph=8.5*10^21;                                         //Mean photon density
+Tph=Tp;                                               //The photon lifetime in sec
+I=(Ith+(Nph*q*V)/Tph);                                //The current required to generate a mean photon density of 8.5 × 10^21 m−3
+
+// Displaying the result in command window
+printf('\n The current required to generate a mean photon density of 8.5 × 10^21 m^-3 = %0.2f mA',I*10^3)
+
+// The answer calculated in book is wrong