  In the present work, we report experimental realization of an optical fiber
based COW protocol for QKD in the telecom wavelength (1550 nm) where the
attenuation in the optical fiber is minimum. A laser of 1550 nm wavelength,
attenuator and intensity modulator is used for the generation of pulses having
average photon number 0.5 and repetition rate of 500 MHz. The experiment is
performed over 40 km, 80 km and 120 km of optical fiber and several
experimental parameters like disclose rate, compression ratio, dead time and
excess bias voltage of the detector are varied for all the cases (i.e., for 40
km, 80 km and 120 km distances) to observe their impact on the final key rate.
Specifically, It is observed that there is a linear increase in the key rate as
we decrease compression ratio or disclose rate. The key rate obtains its
maximum value for least permitted values of disclose rate, compression ratio
and dead time. It seems to remain stable for various values of excess bias
voltage. While changing various parameters, we have maintained the quantum bit
error rate (QBER) below 6%. The key rate obtained is also found to remain
stable over time. Experimental results obtained here are also compared with the
earlier realizations of the COW QKD protocol. Further, to emulate key rate at
intermediate distances and at a distance larger than 120 km, an attenuator of 5
dB loss is used which can be treated as equivalent to 25 km of the optical
fiber used in the present implementation. This has made the present
implementation equivalent to the realization of COW QKD upto 145 km.
