光纖浸水感測器之設計及實現

Design and Realization of Optical Fiber Sensors for Water Immersion Detection

林宣宏(Hsuan-Hung Lin)

指導教授 ： 楊淳良

淡江大學/工學院/電機工程學系碩士班/碩士(2011年)

https://doi.org/10.6846/TKU.2011.00336

摘要

光纖為網路傳輸系統之最佳傳輸媒介，可提供高速率及長距離的光訊號傳輸，因此光
纖非常適合作為即時、遠端、及分佈偵測。因為許多特性使得光纖成為優異的感測器，可
作為一神經系統並且非常適合於工廠及民生的應用。目前常見的商品化產品有液體準位感
測器(Liquid Level Sensor)，其主要用於裝設在單一處所的感測器，其感測器的表面容易因
外在環境因素(如：塵土)而影響其功能，並且不適合架設於光纖偵測線路上。日本NTT研
發團隊研發出一種感測器可用在光纖線路上進行浸水之偵測，其感測原理是使用不織布吸
水膨脹導致光纖產生彎曲損耗， 並配合使用光時域反射儀(Optical Time-Domain
Reflectometer, OTDR)觀察。但此種感測器浸水時之彎曲損耗卻高達10 dB，因此無法在
OTDR上觀察到是否同時多個處所發生浸水狀況，以及容易因為某個偵測點斷裂而癱瘓後
面的偵測線路。
因此我們提出基於OTDR光纖浸水感測器，以利觀察同時多個處所浸水狀況。本偵測
線路拓樸採分支設計，因此不會因為偵測點的故障或分支光纖斷裂而影響其他偵測點及主
幹光纖偵測線路的運作。一旦分支偵測點受損或浸水，只須將感測器模組卸除再換新即可，如此便於施工及維護。
所提出的反射式光纖感測器主要是由一般8 度斜角APC 端面光纖連接器及一塊透水-漫射
反射材料，例如：美耐皿海綿（Melamine Sponge）。我們所設計的感測器可設置在任何
想偵測的區域中，其成本低、結構簡單且輕巧，工作人員容易維護及更換。光纖連接頭
APC 端面與美耐皿海綿構成一個感測介面，因為美耐皿海綿本身內部結構的關係，乾與濕
會讓APC 端面與美耐皿海綿的介面處呈現不同的狀態，乾的接面因未浸水，內部結構較
為乾燥也比較粗糙，產生任意角度漫射現象，因此讓反射光回到光纖的機率變大，OTDR
所量到的脈衝高度(Pulse-Height)也隨之變高。然而，濕的美耐皿海綿因浸水後變得較綿密
且呈現平滑光亮面，在APC 端面與濕美耐皿海綿的介面處則呈現鏡面反射，因此APC 端
面8 度斜角造成反射光不易返回到光纖內而產生較大損耗，OTDR 所量到的脈衝高度也就
下降了。在此美耐皿海綿扮演兩個重要角色：雜質濾水器及光波散光器(Lightwave
Diffuser)。
表

關鍵字

光纖感測器 ； 光時域反射儀 ； 浸水偵測

並列摘要

Optical fibers have become the best network transmission media, providing
high-speed and long-distance optical signal transmission, so they are also ideal for
real-time, remote, and distributed sensing. Since many features make optical fibers
excellent sensors, they can be used as a nerve system and are very suitable for plant and
civil applications. A commercialized optical liquid level sensor is made up of an infrared
LED and a phototransistor that need to be accurately positioned at the base of the sensor’s
tip, which is unsuitable for deploying in optical fiber networks and susceptible to external
factors such as dust depositing and improper mounting to affect its function. NTT R&D
group developed a sensing apparatus based on the fiber bending mechanism to be used in
optical fiber links for detecting water immersion of underground ducts. The water sensors
implemented in the optical fiber links can always be monitored by an optical time-domain
reflectometer (OTDR). However, such sensors present higher bending loss while they are
immersed in water, and therefore water immersion occurring at overmuch premise
couldn’t simultaneously be discovered.
In this paper, we propose novel OTDR-based optical fiber sensors, which are especially
suitable for simultaneous multipoint sensing. Belonging to the quasi-distributed branch
topology, it will be helpful to reach its sensing ranges and to avoid the failure of sensing
points introducing an interruption to the rear sensing networks. Once the branched sensing
points are damaged or immersed in water, their relative states can immediately be observed from an OTDR, and then those sensing points can easily be located and repaired
to facilitate the construction and maintenance.
The proposed sensors, which possess low-cost, compact and easily do-it-yourself (DIY)
features, normally present the diffuse reflection at the boundary between the two media of
melamine sponge and 8-degree APC end facet. After water immersion, they instantly
change into the specular reflection at this boundary. Owing to the diffuse reflection at the
dry boundary, the pulse-height measured by the OTDR becomes higher while the incident
light backscattered into the fiber becomes larger. However, under water immersion the wet
boundary results in specular reflection; and the 8-degree APC end facet can largely
suppress the reflected light back to the optical fiber, which leads to a large return loss to
the sensor and reduces the pulse-height shown on OTDR. Here, the melamine sponge
plays two key roles as a water impurity filter and a lightwave diffuser.

並列關鍵字

Optical fiber sensor ； optical time-domain reflectometer (OTDR) ； water immersion.

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