Abstract
遠在十年以前,單模光纖已被用來做感測器,且廣泛的應用在工業,軍事和 科學研究方面.廣泛的使用光纖感測器有下列幾點理由:(一)小體積(二)免 於電磁的干擾(三)使用安全(四)具有遙測能力(五)高靈敏度.不像其它干 涉儀,馬克-詹德干涉儀(單光纖或雙光纖型式)在完成上較簡單.在雙光纖 型式,一光纖被用來感測另一光纖當作參考.單一光纖干涉儀(又稱為雙折 射感測器)較雙光纖干涉儀簡單,但靈敏度卻較差.每一種光纖感測方法,在 實際應用時,需能不受光纖延伸線的影響.在遙測系統中,對正確的量測而 言,光纖延伸線效應將是一嚴重的問題.所以,延伸線效應必須被消除.在單 模光纖感測器簡單應用中,如溫度感測器或聲波感測器,延伸線效應可借由 極化旋轉聯結器的概念加以消除.在此極化旋轉聯結器乃指兩極化保持光 纖的主軸旋轉四十五度加以熔接.當一線性極化光射入其中一極化保持光 纖時,在另一光纖內將激發兩同能量的極化模.在此光纖感測方法中,三段 極化保持光纖組成兩個極化旋轉聯結器.在此研究中,我們証實了建造一能 消除延伸線效應之單一光纖聲波感測器的可能性.為了穩定感測器,一反饋 訊號被加在相位調制器上,用來補償溫度變化所造成的相位雜訊.實驗結果 証明光纖延伸線效應成功地消除.本論文分為四章:第一章簡介,第二章聲 波感測器的理論,第三章實驗結果與討論,第四章結論. Single-mode optical fibers have been widely used as se- nsors since more than a decade ago, in a tremendous num- ber of applications for industrial, military, or scientific research purposes. Some of reasons for the spread use of fibers are (1)small size(2)immunity from electromagnetic interference(3) safety in use(4)capability of remote meas- uring(5)high sensitivity. Every fiber-optic sensing scheme, when applied for pr- actical use, should suffer from no effect of fiber lead. In a remote sensing system, the fiber lead effect can become a severe problem in accurately determining a measuring and is necessarily to be eliminated. In a simple application of single- mode fibers as sensors, the fiber lead effect can be eliminated by using the idea of polarization rotation coup- lers(PRC's). A PRC concerned here is simply a set of two polarization maintainin(PM) fibers with their principal axes rotated by 45 degrees and butt-jointed together. When a lin- early polarized light is launched into one of the two PM fi- bers, two polarization modes of equal power are excited wit- hin the other PM fiber. In the fiber-optic sensing scheme proposed here, two PRC's are formed with three sections of PM fibers. In this study, we demonstrate the possibility of constructing a one-fiber interferometric sensor with no fib- er lead effect for acoustic wave detection. This paper is organized as four chapters. Chapter 1, introduction. Chapter 2, the theory of the proposed acoustic sensor. Chapter 3, experimental results and discussion. Chap- ter 4, conclusion.