Research On Frequency Division Multiplexing Probe Based Coherent Optical Time Domain Reflectometry

Coherent optical time domain reflectometry (C-OTDR) is an important instrument for health monitoring of multi-repeater super long distance optical transmission line. As conventional C-OTDR is single frequency probe based, its measurement efficient is very low. As to this problem, this dissertation reports the progress that has been made on improving the dynamic range and reducing fading noise of detected trace by frequency division multiplexing probe based coherent optical time domain reflectometry (FDM-COTDR). Compared with conventional C-OTDR, the FDM-COTDR can simultaneously detect and extract multiple coherent intermediate frequencies, so it has much higher measurement efficiency. The FDM-COTDR is developed from conventional C-OTDR, which adopts a phase modulator to convert single frequency probe light to multi-frequency light and uses coherent detection method to extract the backscattered Rayleigh light power of each probe frequency propagating in fiber under test. Under different modulation depths of phase modulator, dual-frequency probe based C-OTDR, three multiplexing frequency probe based C-OTDR and four multiplexing frequency probe based C-OTDR are developed. Theoretical analysis and experimental results show that, compared with conventional C-OTDR, the above three FDM-COTDR types can enhance measurement dynamic range by3.0dB,2.4dB and3.8dB respectively, and the fading noise convergence speed has a twofold, threefold and fourfold improvement respectively.Based on the idea of frequency shift averaging to reduce fading noise of detected trace used in conventional C-OTDR, we have proposed and experimentally demonstrated a new scheme which employs both the frequency division multiplexing probe light and local oscillator with synchronous frequency sweeping. It adopts sequential radio frequencies to drive a phase modulator to achieve synchronous frequency sweeping for both probe light and local oscillator. Experimental results show that by this C-OTDR scheme the fading noise is reduced with fast convergence speed, whose performance is near that obtained by laser frequency hopping method.To further improve the performance of FDM-COTDR, time division multiplexing (TDM) technology is combined with dual frequency probe based C-OTDR, which employs an arbitrary waveform generator (AWG) to generate sequential radio frequency signals to drive PM, then the PM generates frequency precisely coded probe light. Both the AWG and AOM are controlled by synchronous trigger to obtain TDM probe light of multi-frequencies. For the purpose of direct IF synthesizing, in the light path of LO, an AOM which has the same frequency shift function with that in probe light path is used. Experiment results using20multiplexing frequencies demonstrate that compared with conventional C-OTDR it can bring8.0DR enhancement.Some nonlinear effects such as self-phase modulation (SPM), cross-phase modulation (XPM), four wave mixing (FWM), stimulated Brillouin scattering (SBS) and so on, which may exist and limit the performance of FDM-COTDR are theoretically analyzed and experimentally investigated, and some methods which can suppress these nonlinear effects are proposed.