Fig. 1. Schematic illustration of optical frequency modulation span broadening based on the FCM technique by using the FWM process. Δω is the modulation span of the pump.

Fig. 2. Experimental setup of the proposed BOCDA system based on the FCM technique. BPF: bandpass filter; VOA: variable optical attenuator; ADC: analog-to-digital converter.

Fig. 3. Measured optical spectrum after the FWM process at the end of the HNLF.

Fig. 4. 3D plot of the distributed BGS around the end of the FUT when (a) the pump and (b) Idler-1 are used as the light source of the BOCDA system, respectively.

Fig. 5. (Color online) (a) Distribution map of the BFS measured with the pump and Idler-1; the measured BGS of the DSF when (b) the pump and (c) Idler-1 are used as light source, respectively.

Fig. 6. (a) 3D plot of the distributed BGS around the end of the FUT. (b) The measured BFS around the 10 cm DSF section with differential measurement techniques. The insets show the BGS of the SMF and DSF.