hi jyoti rana .. as all said your results are quite good… if you BER make it to zero your system is consider to be a under utilized performance coz BER zero is only possible for ideal system which is not possible in practical aspect.. if still to want to get .. you can try by reducing fiber length ..increasing gain after fiber .. change photodector APD insated of PIN .. or by reduing noise in photodector .. linewidth can also be reduced ..