Generation Of Orbital Angular Momentum Of Light And Its Application To Strain Sensing

Angular momentum is one of the most basic physical quantities in classical and quantum mechanics. The angular momentum of laser beam consists of SAM (Spin Angular Momentum) and OAM (Orbital Angular Momentum). The OAM of light corresponds to the rotation of phase structure, which reveals another degree of freedom that is totally different from SAM. Therefore, OAM has received significant attention.In recent years, obvious progress has been made in the applications based on the OAM of light, especially in the fields such as microscopy imaging, target detection, optical communications and RF communications. Even so, there are still a lot of problems to be settled, and the generation of OAM of light is a fundamental problem among them. CFG (Chiral fiber grating) has proven to be flexible and effective in generating the OAM, so relevant theoretical and experimental works are worthy of attention. Besides, as the light carrying OAM can generate a special interference pattern, it raises some thinking about its application in sensing, and the application potential has been shown preliminarily. Therefore, in this thesis, based on the CFG, we study the generating platform of OAM of light, and make a thorough analysis and design on the strain measurement system which is based on this platform.First, we start with coupled mode theory and archive the OAM conversion rules of CFG through derivation. Based on this, we study the generating scheme of OAM with topological charge one and two. The design method of CFG and the effect on the spectrum characteristics of parameter errors of CFG are discussed in this part. Then we begin to design the experimental platform which can generate and verify OAM simultaneously. The construction of the platform is carried out as well.Second, we analyze the strain sensing mechanism based on OAM of light, explain the feasibility of the method and work out a schematic diagram of the sensing system. Afterwards, for the purpose of earthquake precursor measurement, combined with digital image processing technique, we make a thorough discussion on the sensing characteristics of the sensor with topological charge one, and confirm its superior performance. Besides, we compare the sensing characteristics for the cases of utilizing different topological charges, and get the optimum solution.Finally, on the basis of above, we consider the specific design of the sensor. We use the generating scheme of OAM based on CFG to generate optical vortex. The range of fiber length of the sensing arm and the range of wavelength are both considered in order to ensure an efficient work system. We also improve the scheme of our sensor to overcome the cross-sensitivity of strain and temperature and the limitation of detection limit respectively.