以激發少數模態光纖的基本模態重建其折射率分布

Refractive index profiling of a few-mode fiber from fundamental mode excitation

張哲愷(Che-Kai Chang)

指導教授 ： 蔡宛卲

國立暨南國際大學/科技學院/應用材料及光電工程學系/碩士(2014年)

摘要

在本研究中，我們利用絕熱性錐形熔接技術，結合一原本設計在1310/1550奈米波長單模傳導的小型纖核步階式折射率分布少數模態光纖，與設計在可見光波長633奈米的單模光纖，並且成功地在可見光波長633奈米選擇性激發少數模態光纖的基本傳播模態。
我們先介紹錐形熔接光纖的導光原理，先利用直接激發耦合系統激發單模光纖，光在單模光纖內是以纖核模態傳播並且在經過錐形區域時會絕熱性地逐漸轉變為纖殼模態，在轉換的過程中幾乎無能量損耗，而單模光纖的纖殼模態在經過熔接點的地方時會保持不變地傳送到少數模態光纖之中，因為兩條錐形光纖的腰部被均勻地控制住，之後光在進入到少數模態光纖的錐形區域後，纖殼模態又會逐漸地轉換回纖核之中，因此少數模態光纖的基本傳播纖核模態就被激發出來。
成功激發少數模態光纖的基本模態後，可以從電荷耦合元件攝影機觀察到。並且可以用改良式直接激發耦合系統量測到基本傳播模態分布的一階及二階空間微分場，再配合微分逆計算法能重建少數模態光纖的折射率分布，但是這套方式只適用於單一傳播模態。
重建後的折射率分布大致符合已知的光纖參數，如近似於步階式分布、折射率差大約為0.006、少數模態光纖的纖核直徑大小測量為8.2微米，其結果與製造商提供的參數相比，折射率差的誤差值只有0.36%。

關鍵字

絕熱性錐形熔接技術 ； 改良式直接激發耦合系統

並列摘要

The fundamental mode of a small-core step-index few-mode fiber (FMF), originally designed for single-mode operation at 1310/1550 nm, was selectively excited at visible wavelengths 633nm from a single-mode fiber (SMF) designed at 633nm by adiabatically tapering the fusion splice.
The SMF was first excited using end-fire coupling. Light propagated as the core mode through the SMF and was adiabatically transferred into cladding mode at the tapered region. The cladding mode remained unchanged at the splice point where the waists of two tapered fibers were uniformly controlled. After entering the tapered region of the FMF, the cladding mode remained and was gradually transferred into the fundamental guided core mode of the FMF.
The fundamental mode profile of the FMF was thus excited, and it can be observed with a charge-coupled-device (CCD) camera. First- and second-order spatial derivatives of the fundamental guided mode profiles were measured using the modified end-fire coupling method, and the refractive index profile (RIP) of the FMF can then be reconstructed by applying the differential inverse calculation algorithm, which applies only for single-mode guiding.
Good accordance can be seen from the reconstructed RIP and the known parameters of the fiber. A nearly step-index profile can be seen, with approximate index difference of 0.006. The core size of the FMF is estimated to be 8.2 ?m. Compared with the parameters provided by the manufacturer, with expected refractive index difference of 0.36%, these results show good agreement.

並列關鍵字

Adiabatically tapering the fusion splice ； Modified end-fire coupling method

參考文獻

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