calculate the maximum length of an optical fibre

Question

Calculate the maximum length of an optical fibre that exhibits 0.8dB/km attenuation if the optical output is 10mW and the power launched at the input is 150mW. [Hint: Loss, L=10 log (P_o/P_i)]

Ans:

      Given 

                P_o=10mW

                P_i=150mw

We have

           Loss, L=10 log (P_o/P_i)

       Substituting for P_o and P_i,

         L= -11.76dB

For 1km , the loss is 0.8dB.

Let the length of the cable be x km. Then, for x kms, the loss is -11.76dB.

Solving for x, we have

         x= (11.76 x 1)/0.8

           =15km

i.e the length of the cable is 15km approximately.