This study has given the investigation for achievable net coding gain (NCG) of optical forward error correction (FEC) coding scheme with convolutional code to control the received sensitivity following a required signal-to-noise ratio in the communication channel. The achievable NCG is obtained from the difference between the upper bound NCG and the power penalty of the optical exclusive-OR (XOR) gate, which is based on a four-wave mixing in highly non-linear fibre. The upper bound NCG was analytically derived by calculating error probability of optimal convolutional code, and the power penalty was numerically derived by comparing bit-error rates between with and without the optical FEC coding. To confirm the feasibility for achieving the highest NCG, the optimal operating condition of the optical XOR gate is experimentally evaluated to minimise their power penalty. A 0.5 dB power penalty was obtained at BER = 10−9 under the optimised condition with 223−1 pseudo-random binary sequence differential phase-shift keying-modulated return to zero signal at 10 Gbps.