Security Improvement Using $\pm\pi/2$-Phase-Shifted SSFBG En/Decoder in Time-Spreading OCDMA

The existence of dips in the waveform encoded by 0/π-phase-shifted superstructured fiber Bragg grating (0/π -SSFBG) allows eavesdroppers to extract code sequence straightforwardly. In this letter, the code extraction technique for 0/π-SSFBG is summarized and demonstrated. To address this security issue, we propose using ±π/2-phase-shifted ±π/2-SSFBG as encoder/decoder to improve security in time-spreading optical code-division multiple-access. The security and coding performances for both ±π/2-SSFBG and 0/π-SSFBG are compared; ±π/2-SSFBG exhibits improved security and better coding performance. The influence of input pulse over the security performance for ±π/2 -SSFBG is discussed. Short input pulse can improve the security of the encoded signal.