Structure Design And Characteristic Study Of Helical-core Fiber

In recent years, with the widely use of high power lasers in civil and military fields, thedevelopment of laser is rapid. High-power fiber lasers have some obvious advantages inefficiency, size, cooling and beam quality. More and more domestic and foreign researchersare interested in the investigation of high-power fiber lasers. However, the optical damage offiber material which caused by thermal effect and nonlinear effect limit fiber lasers outputpower. One well known way to reduce thermal effect and nonlinear effect is to make fiberswith large mode area (LMA). In order to obtain the high-power output, the fiber core diameterat least40μm. However, when the core size is too large, the fiber becomes multimoded,leading to the decline in the quality of the output laser beam. Modal discrimination techniquesare then required to achieve single mode operation.Currently, people usually bend fiber to achieve the large core fiber single-mode operation.But for large core size, bending deforms the mode field distribution and reduces the modearea, which puts a limit on the benefit of using large core. In present thesis, we haveresearched on a kind of special optical fiber—helical-core fiber. The advantage of a helicalcore fiber is that the bending is built into the fiber. By replacing the bend radius with thecurvature radius of the helix, it can achieve small bending radius which bending can notachieve. The core diameter of the single mode helical-core fiber is well exceeding that of theconventional large-mode-area fiber. The helical-core fiber can provide effectivelylarge-mode-area single-mode operation without coiling fiber and selecting excitation modeand can be used for a high-power fiber laser.The thesis introduces the present research situation of the helical-core fiber andelaborates the transmission principle of helical-core fiber. Marcuse model and Beampropagation method (BPM) are used to analyze the impacts of the geometric parameters andphysical parameters on the properties of mode losses of the helical-core fiber. The design ofhelical core fiber geometric parameters to eliminate high order mode method is very effective.The propagation loss is0.32dB/m for the fundamental mode and propagation loss is20.95dB/m.The axis of the core is not parallel to the fiber axis. In practice, this means that aftercleaving or polishing in a standard way, the emitted beam will be asymmetric and distorted, thus reducing its beam quality. In order to overcome this shortcoming, and keep theadvantages of the helical-core fiber, we built a new model which has a helical core and astraight central core into the cylindrical cladding. BPM is used to analyze the impacts of thepith of helical core, edge-to-edge separation distance between the central and the side cores,the straight central core diameter and refractive index between core and cladding on theproperties of mode losses of the central core. By the introduction of helical-core structure, anduse high power, high brightness pump light source, we can obtain Large-mode-area andsingle-mode operation fiber laser. This thesis can provide a reference for the study of highpower laser and it has practical significance for the drawing of helical-core fiber.