Interrogation of Cascaded FBGs With Narrow Spacing Using Double Wide Pulse Signals

An optical time-domain reflectometry (OTDR) based fiber Bragg grating (FBG) interrogation technique using wide pulse signals for the large-capacity cascaded FBGs with narrow spacing is proposed and demonstrated in this letter. The time-domain waveform subtraction at each scanned lasing wavelength obtained by injecting two separated pulse signals with different pulse-width to the FBG array is used to distinguish each FBG. The spatial resolution is determined by the effective pulse-width of the differential pulse signal of the two pulse signals rather than the pulse-width of either injecting pulse signal. The experimental results show that a spatial resolution of 0.16 m is realized using 11-ns and 10-ns wide pulses with a fall-time of 2.6 ns, and 1259 cascaded FBGs with 0.2-m grating spacing are interrogated with an error of 13.98 pm. The proposed technique can be used for sub-meter spatial resolution sensing using meter equivalent pulses, avoiding complex and costly system setup.