利用即時側向繞射位置監控及紫外光接續寫入法製作布拉格光纖光柵

Abstract

在本論文中我們首度發展出在紫外光接續寫入布拉格光纖光柵過程中的新型即時光纖位置監控方法，為了即時對準待曝照的次光柵位置，我們使用側向繞射法來偵測另一個單一週期的光纖光柵之相位作為位置的參考點，如此可以較不受量測飄移的影響。 在本研究中我們也仔細校正了紫外光曝光量與光纖引致的折射率變化量之非線性關係圖，經由事先作紫外光處理，可以避免在紫外光量微弱時，因光纖的折射率變化量對紫外光強度呈非線性曲線而造成誤差。我們也發展出使用光強度不相同的新型雙光束干涉法來達到在單次寫入光纖過程中，從而可以保持光纖折射率變化與寫入的紫外光量成正比。 我們也提出一個簡單的方法來達到光纖光柵平均折射率維持定值而折射率調變形狀可加以控制的光纖光柵製造方法。此方法在每一個欲寫入次光柵的位置，將紫外光待曝光量分成前後兩次曝照，而折射率調變的形狀可藉由控制前後寫入兩紫外光的相對相位和強度來調整，如此沿著整個光纖光柵的平均折射率都保持固定值，而局部調整折射率調變的形狀卻可以任意控制。 整體而言，藉著我們發展出的這套光纖光柵製作技術可以建造一個精良的光纖光柵製作平台，從而可以針對不同應用製造出複雜的光纖光柵。

In this dissertation we develop a new real-time fiber position monitoring method for fiber Bragg grating (FBG) sequential UV-writing processes. To real-time accurately align the position of every exposed FBG, a single-period reference fiber grating is probed by applying an interferometric side-diffraction method to measure the grating phase as the position reference, so that the writing process can be less sensitive to the measurement drifts. During the research we have also calibrated carefully the nonlinear relationship between the UV flux and the induced index change. The pre UV treatment method can avoid the nonlinear index change response regime when the UV flux is low. An improved unequal-intensity UV two-beam interference scheme is proposed and demonstrated to achieve the index response linearization in a single scan. Finally, a simple method for attaining pure apodization of FBGs has also been proposed. The UV exposure at every exposed location is achieved in two exposure steps and the ac index modulation is adjusted by controlling the superimposed phase and amplitude of the two imprinted UV fringes. The average refractive index is kept constant, and the ac index modulation can be locally and independently changed. Based on the grating inscription skills, we have developed an excellent grating fabrication platform to produce complex FBGs for various applications.