具寬度變化的多模干涉元件

Abstract

中文摘要 以往多模干涉結構的元件，僅止於廣泛的學術應用；因具有一些缺點，所以主要功能僅僅只是分光而已。然而多模干涉結構的許多優點，卻是不可取代的，因此改進這些缺點對實際的應用上，是極其重要的。 多模干涉元件的第一個缺點為波長敏感，此缺點會限制其在實際上的用途；因元件在網路上的應用時，通常都需要降低波長敏感性，以減低對光源波長準確性的要求；有鑑於此，本論文提出多模干涉分光器的寬頻標準；基本原理是建立在縮短其多模干涉區的長度差，並使之小於建設性干涉的光點大小；透過模擬後發現，操作頻寬可涵蓋光纖通信全波段(1.26-1.61um)，且長度小於Y形分支結構的分光器。 一般多模干涉元件，只有全相等與完全不相等的分光功能而已；當元件被要求需要不同比例的分光時，若使用一般多模干涉元件，只能使用串接來達成，如此將造成製作成本過高；此限制即為多模干涉元件的第二個缺點。 所以本論文提出一個寬度函數為任意次方二元函數的多模干涉耦合器，而其中包含了傳統的、蝴蝶形的、與拋物線形的結構，使得新提出的多模干涉元件，可以用來等功率與不等功率的分光，只需選擇適當的次方，即可實現許多不同形式的等功率與不等功率的分光功能。 最後，將新提出的多模干涉結構與波導陣列光柵元件，兩者結合為一，以顯現應用在週期性波導陣列光柵元件，其效能上之提升。而主要的創意是，新提出的週期性波導陣列光柵元件，其每一個傳輸波導，都是由多模干涉結構與錐形波導所構成；模擬結果證實，新提出的週期性波導陣列光柵元件，1dB的頻寬可達通道間隔的ㄧ半，且不均勻度小於1dB。因此成功得到具有平坦化傳輸頻帶與均勻化頻譜響應的週期性波導陣列光柵元件 簡而言之，本論文將多模干涉的結構改良，以提升其效能，使之同時在學術與實際的用途上，都有極大的優勢。

Abstract Multimode interference (MMI) structures were widely studied previously. The functions of interest were mainly for power splitting due to some disadvantages. However, the MMI structures have many advantages which are not replaceable. Therefore, it is important to improve the disadvantages for practical applications. The first disadvantage of MMI device is wavelength sensitive, which leads to restricted applications. To reduce the wavelength insensitivity in optical network applications, the demand of light source with a specific wavelength is needed, especially for power splitters. For this reason, a wideband criterion for MMI splitter is proposed. The basic principle is based on reducing the interference length difference and making it shorter than the spot size for constructive mode interference. Simulation results show the spectra of proposed MMI splitter is wide enough (1.26-1.61um) to cover the fiber communication band and the size of the splitter is more compact than that of the conventional MMI or Y-branch splitter. MMI devices were usually used for all-equal or all-unequal power splitting. When the device is used for different varieties of power splitting, interconnection of the devices make the fabrication cost to high to be realizable. These limitations are the second disadvantage of MMI device. In this work, MMI couplers with a width of arbitrary-exponent binomial function are proposed. The proposed structure includes the conventional, butterfly, and parabolic structure such that the proposed MMI device can be used for equal and unequal power splitting. With a proper choice of the exponent of a binomial function, several equal and unequal optical powers splitting ratio can be obtained. For comparison, the optical power splitting ratios, total transmission, and coupler length of the proposed MMI couplers are discussed. Finally, proposed MMI structures and arrayed waveguide grating (AWG) devices are combined to demonstrate the application in the improvement of cyclic arrayed waveguide grating devices. The basic idea is that each access waveguide of the proposed cyclic AWG consists of an MMI region and a taper waveguide. Simulation results show that the 1dB-bandwidth of the proposed device can be as wide as one half of the channel spacing and the corresponding nonuniformity is smaller than 1dB. Therefore, flat-top passband and uniform spectral response can be obtained. The improvement of the performance of MMI structure makes it of great interest for both academic studies and practical applications.