Ultra-high Resolution Optical Spectral Analysis Based On Brillouin Dynamic Gratings

High-resolution optical spectroscopy has found extensive applications in manyareas, including high-accuracy optical sensing, diagnostic and monitoring of signalsin optical communication systems, characterizing and evaluating optical frequencycomb, and etc. The best spectral resolution of the traditional grating-based opticalspectrum analyzer (OSA) can only reach a few picometers, which is limited by thegroove-line densities of the grating and maximum optical beam diameter. Thescanning Fabry-Perot interferometer can usually provide better spectral resolutionthan grating-based OSA, but with narrower range of wavelength coverage. Thecoherent detection-based OSA can provide the best resolution of40fm (5MHz).However, it cannot provide a higher-resolution with a wide band spectrummeasurement.In this thesis, we propose a novel scheme for an ultrahigh-resolutionspectroscopy based on the so-called Brillouin dynamic gratings (BDGs) andperforms the theoretical and experimental studies on the characteristics of BDGspectrum.Firstly, an mathematical model is proposed to describe the writing and readingprocess of BDG. Based on this model, we study the characteristics of the BDGspectrum in both steady state with a small-signal and transient state. The resultsshow that the bandwidth of the BDG decrease as increaseing the grating length, andthe pump depletion will induce the broadening of the BDG spectrum.Secondly, we study the impact of the fiber length and the optical power on thecharacteristics of the BDG spectrum. A Brillouin pump-probe scheme was used tocharacterize the distributed birefringence of the SMF segments. The result showsthat the nonuniform broadening of the BDG spectrum in an SMF is caused by thebirefringence of the SMF, which is the limition for us to obtain a narrower BDGspectrum.In the end, a BDG-based optical spectrometry is demonstrated, where thegrating pitch and the reflection central wavelength can be tuned by sweeping thefrequency of a tunable laser. The result shows that a best4fm (0.5MHz) spectralresolution is achieved by operating a BDG in a400m SMF segment.