將光纖抽絲塔微小化以製作微奈米導光線與其應用

Abstract

微奈米導光線是將一般光纖以傳統光纖抽絲塔方法抽細而成，具有微小尺寸(0.5~10微米)、可撓曲、低傳輸損耗以及可大量製造的特點，可以用來替代各式光纖元件，使其可更為輕量化，或是根本上改變其效能。微奈米導光線的製作是將光纖垂直通過高溫爐，並接附於上下兩端之進出料系統，高溫爐(1400~1600℃)可將光纖變成熔融態，利用兩側平台與滾輪移動速度的差異，經由計算質量守恆定律可製作出不同直徑之微奈米導光線。光纖抽絲法之特點在於可以製作出較長的微奈米導光線(165公分)，遠超過其他製作方法(10公分)。另外，此法製作出之微奈米導光線於穩態時有幾乎沒有直徑變化，理論上可以得到更低的傳輸損耗。我們模擬了微奈米導光線的模態與場型，並量測其光學特性與品質隨時間變差的情形。目前製作出之微奈米導光線的傳輸損耗為0.002 dB/mm，雖然比不上傳統光纖，但仍有改善的空間，而且此值已可用來製作各種不同功能之光纖元件。 微奈米導光線為強導光性(strong guiding)，外界的汙染會更容易影響其效能，因此我們嘗試被覆聚二甲基矽氧烷(polydimethylsiloxane, PDMS)於其表面形成包覆層，測試經過18小時暴露於大氣中後也不會使微奈米導光線增加損耗。這些實驗結果代表著微奈米導光線具有潛力製作成類似於一般光纖的商品，具備大量生產與長久使用的能力。 當各式微奈米導光線元件製作完成後，需要一個技術將彼此連接起來，以達成更完整的功能。這時微奈米導光線融接技術便成為首選。我們將兩條微奈米導光線以凡得瓦力附著，利用電弧放電加熱耦合區域，於適當的距離、電流和時間下，可以將兩者融化並形成一根微奈米導光線。融接損耗可以小至0.16 dB，足以用於連接兩元件的需求。 應用至光纖元件方面，首先我們嘗試使用微奈米導光線於光互連技術(optical interconnect)中。微奈米導光線比起矽波導(0.1 dB/mm)具有更低的傳輸損耗，因此可用於光電基板中。以微影術定義位置，可精確控制製作出之元件的效能並具備重複性，又由於其放置方式，具有使用於3D結構光互連的潛力。接下來製作溝槽於PDMS上並以鎢針將微奈米導光線置入形成環形共振腔之結構，可成功形成共振條件，並達到了104的Q值。這個結果和以矽作為傳輸媒介的情形相似，表示微奈米導光線的確有取代矽波導的可行性。 我們測試的另一個光纖元件應用是鋸齒狀微光纖光柵感測器，光纖感測器具有電磁抗擾性(electromagnetic immunity)，可同時置放多個光纖感測元件而不會互相干擾，然而光纖本身的尺寸仍會限制其應用，無法放置於更狹小的空間中，因此需要製作微奈米導光線的感測器。我們將微奈米導光線以製程方式製作成表面鋸齒狀的週期性結構，因為強導光性而形成了長週期微光纖光柵，可用來量測溫度與折射率的變化。特別的是，因為微奈米導光線的模態分佈已經不同於一般光纖，於量測外界折射率時可以得到更高的靈敏度，可達2100nm/RIU。此外，本製程方法可同時製作多根微奈米導光線光柵，具有大量生產的可能性。 微奈米導光線作為一個新興的研究領域，世界上有許多團隊在探討其特性與找尋其可能應用，而本實驗室也名列其中。我們提出了微小型抽絲塔製作方法，可以製作出更長的微奈米導光線，另外也發展了包覆的技術。於應用面上，我們探討其使用於光互連技術的可行性，也將其製作成了鋸齒狀微光纖光柵，並且具有相當高的靈敏度。這些結果顯示出微奈米導光線可使用於各種不同元件並且具有良好的特性，是未來光電領域的明日之星。

Micro/nano optical wires (MNOWs) were fabricated from an optical fiber preform drawn down to several micrometers (0.5~10 μm) in diameter by the modified fiber drawing tower. MNOWs are flexible, low propagation loss and capable of being mass producted, which can be used to substitute the waveguiding media in various optical devices to miniaturize their size or change their performance. The fiber preform was passed through the furnace with the temperature of 1400~1600℃ and then attached to the feeding and drawing mechanisms. When it was melted by the furnace, different diameters of MNOWs can be drawn by changing the speed ratio of feeding and drawing sides with the calculation of principle of mass conservation. This method can draw the MNOW with 165 cm in length, much longer than other methods (10 cm). Besides, theoretically the MNOW can have the minimal diameter variation after the drawing process becomes stable, providing the possibility of achieving the lowest propagation loss. The effective indices, intensity distribution, optical characteristics and degradation of microfiber were simulated and measured, respectively. The propagation loss of MNOWs we made nowadays is about 0.002 dB/mm, which does not reach the value of conventional optical fiber but can be improved. Furthermore, such an MNOW can be applied to microfiber-based devices with various performances. The MNOW has the strong guiding mechanism and can be easily influenced by the surrounding contaminations. Therefore we tried to use polydimethylsiloxane (PDMS) as the coating material to embed it into a cylindrical layer. It was found that there was no observable degradation after the fabrication of 18 hours. These experimental results imply that the MNOW has the potential to be fabricated to fiber-like devices with mass production capability and long-term reliability. After the fabrication of various microfiber-based devices, we need a technique to connect one another to realize complete functions and performances. Therefore the splicing technique of MNOWs is the best choise. Two MNOWs were attached to each other by van der Waals force and then the coupling region was heated by electric arc. Under the condition of suitable distance, electric current and time, two MNOWs could be melted and form an MNOW. The splicing loss could be down to 0.16 dB, enough for the demand of connecting two devices. Considering microfiber-based devices, we tried to apply MNOWs to optical interconnects. MNOW has the propagation loss lower than that of the silicon waveguide (0.1 dB/mm) and could be used as optical waveguide in photonic integrated circuit. The positions of MNOWs were patterned by optical lithography and therefore the performance could be controlled exactly and repeatability could be obtained. Moreover, due to the way of placement, it had the potential of achieving three dimensional optical interconnect. The patterned grooves were fabricated on PDMS and the MNOWs were placed into it to form the micro-ring resonator with the Q value of 104. This result was similar to that made from silicon as guiding medium, implying the MNOW is feasible of replacing silicon waveguide Another application of MNOW is corrugated long period microfiber grating (C-LPMFG). Fiber sensors have the characteristic of electromagnetic immunity, i.e., many sensors can be placed together without interfering each other. However, the size of fiber sensor limits its application to be placed into small region. Hence we need a microfiber-based grating. The optical fiber was fabricated with surface corrugated periodical structures by using semiconductor process. Due to strong guiding characteristic the MNOW can become C-LPMFG, which can be used as refractive index and temperature sensors. In particular, because of the modes in MNOW is different from that of optical fiber, it has higher sensitivity of approximately 2100 nm/RIU when sensing surrounding refractive index. Besides, the fabrication method of C-LPMFG enables it to be simultaneously made for a large amount, representing mass production ability. Microfiber, as a burgeoning research topic, is studied for its characteristics and applications by many research groups, and we are one of them. We developed the fabrication method using modified fiber drawing tower to make much longer MNOW and the coating method was also presented. For the applications, we tried the feasibility of using MNOW into optical interconnect and, on the other hand, made it into C-LPMFG with high sensitivity. These results show that MNOW can be used in various optical devices with good properties, which is the future star in the field of optoelectronics.