Security Improvement Using ±π/2-Phase-Shifted SSFBG En/Decoder in Time-Spreading OCDMA

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