藉由二氧化碳雷射加熱與張力拉伸製作長週期光纖光柵

Abstract

在本論文中，我們提出一種週期性熔錐形狀的長週期光纖光柵及製作方法。有別於以往二氧化碳雷射產生不對稱的長週期光纖光柵，我們產生的長週期光纖光柵是對稱的結構，而這個對稱的結構是藉由熔拉所產生。我們並藉由解三層結構光纖模態的數值模擬得到和實驗相吻合的結論。我們發現當週期性熔錐形狀明顯時，其對溫度的靈敏度升高。而將具週期性熔錐形狀光纖光柵置於較高折射率的介質中時，其對溫度的靈敏度亦升高。 具週期性熔錐形狀光纖光柵的製作方法如下：首先將高功率的二氧化碳雷射經過柱狀透鏡聚焦後加熱光纖，此光纖的一端固定而另一端則懸掛重物(重物即為光纖所受到的張力)，熔融的光纖經張力拉伸後產生熔拉的形狀。將上述光纖裝置於一個移動平台上，並用一個開關去控制二氧化碳雷射的加熱的時間。我們利用Labview進行自動控制，藉由控制開關打開的時間可以決定加熱的時間，藉由移動平台的移動的距離可以決定光柵的週期。此對稱光纖光柵對溫度的靈敏度(週期為700 μm且拉伸後的直徑100 μm的樣本)測得為0.238 nm/˚C，比傳統的0.059 nm/˚C改善很多。

In this thesis, we demonstrate a periodically tapered long period fiber grating and its fabrication method. Not as the same as the asymmetric long period fiber gratings produced before, we have produced devices with a symmetric structure tapered by a CO2 Laser. The produced LPFG with deep corrugated surface structure and immersed in higher index media results in high temperature sensitivity. The numerical results performed by solving waveguiding modes in three-layer optical fibers agree with the experimental observation. The procedure of making a periodically tapered long period fiber grating is as following: First, let the high power CO2 laser beam pass through the cylindrical lens and focused on the fiber, which is fixed at one end and tagged with a weight at the other end ( the stress of the fiber equals the weight ). Then, the melting fiber will be extruded and shaped like a tapered fiber with laser heating. The fiber is put on a translation stage and a shutter is used to control the heating time of the CO2 laser. We use the Labview software to control the translation stage and the shutter automatically so that we can set the parameters like the heating time and period easily. This kind of symmetric long period fiber gratings is more sensitive to temperature. We have measured the temperature sensitivity 0.238 nm/˚C for one of our LPFG samples, which is much better than the temperature sensitivity of traditional long period fiber gratings (0.059 nm/˚C).