Study On Optical Pulse Compression Refelctometry Based On Nonlinear Frequency Modulation

Optical time-domain reflectometry(OTDR) has been extensively used in optical-fiber cable installation and fault detection. It suffers the tradeoff relationship between spatial resolution and measuring range. In contrast, optical frequency-domain reflectometry(OFDR) succeeds in high spatial resolution but its measurement range is limited by the coherent length of laser source. Most recently, a new reflectometry, called optical pulse-compression reflectometry(OPCR), has been proposed and demonstrated. OPCR can overcome the tradeoff relationship of spatial resolution and measurement range. For instance, it provides the similar measurement range of OTDR and the comparable spatial range of OFDR. Previous study on the OPCR based on linear frequency modulation(LFM) shows that the full width at half magnitude of autocorrelation function is not determined by pulse width, but only by chirp width.This thesis studies a novel OPCR based on nonlinear frequency modulation(NLFM). Theoretical analysis, numerical simulation and experimental verification are carried out. It is shown that time-frequency traces, autocorrelation functions and OPCR reflection trances depend on the parameters of nonlinear frequency modulation. Experimental study demonstrates that the impact of nonlinear frequency modulation pulse signal lies on all devices of voltage controlled oscillator, low noise amplifier, and single side-band modulator. In consequence, the PSLR of NLFM-OPCR system trace is better than LFM-OPCR. The main contents of this thesis are divided into the following parts:(1) Theoretical analysisIn this part, we mainly introduce the working principle and theory of OPCR system. Firstly, we introduce the basis theory of pulse compression in radar system, including I/Q demodulation and matched filter. Secondly, we present the comparison of linear frequency modulation pulse signals and nonlinear frequency modulation pulse signals. After that, we introduce the parameters of autocorrelation function, such as, spatial resolution, peak side lobe ratio(PSLR), integrated side lobe ratio(ISLR). Finally, this thesis establishes a simple systematic model, and gives the analytical expression of OPCR trace.(2) Simulation analysisAccording to the OPCR theory above, the simulations of frequency modulation pulse signals and OPCR traces in different cases are implemented. Firstly, we simulate chirp pulse signals and autocorrelation functions. The PSLR of NLFM pulse signal is better than LFM pulse signal. PSLR of NLFM chirp pulse can reach 60 dB while PSLR of LFM pulse signal is only 13 d B. Secondly, this thesis compares the difference of LFM pulse, LFM windowing pulse, and the NLFM pulse. Finally, this thesis simulates and discusses the influence of NLFM’s alpha on improvement of the spatial resolution and measurement range. It proves the NLFM-OPCR system can improve the PSLR and dynamic range of OPCR traces than LFM-OPCR system.(3) Experimental resultThis part verifies the theoretical analysis and numerical simulation and optimizes the experimental parameter. To verify the effect of photoelectric devices, a “straight” optical pulse compression system was presented. We measure the performances of voltage controlled oscillator(VCO), low noise amplifier(LNA), and single side-band modulator(SSBM), respectively. We configure a “reflection” system of NLFM-OPCR. It can distinguish the two endpoints of 30 cm fiber after 5.4km fiber and its PSLR is better than LFM-OPCR. It also analyzes the influence of parameter alpha in improving the dynamic range of NLFM-OPCR system. At last, after optimizing the experimental parameter, the measuring range reaches 42 km and the nominal spatial resolution is about ten centimeters.