對稱式光柵耦合器之寬頻研究

Abstract

光柵最常用的功能除了將入射波的能量耦合至介質波導中之外，還能將介質波導中的表面波轉化成存在於自由空間中的漏波。由於近年來在光學積體電路中耦合元件的需求增加，光柵耦合器扮演著重要的角色。然而，傳統的分析大多著重於在固定頻率條件之下的耦合現象。因此在此文中，我們延續性地論述在寬頻操作的分析之下，對於多種對稱式的光柵耦合器探討其出射光的光束寬度，將在1μm到2μm的波帶上維持的一平坦趨勢。之所以選擇對稱形式的光柵，是由於該對稱形狀，將抑制第二諧振的激發，因此在原本應該出現第二空間諧振(n=-2)的波長處，其能量並沒有如預期般的被激發出來，其光束寬度的寬頻表現也不會因而受到破壞。我們並更深一步地討論，在不同形狀的光柵中，此一波帶在寬頻操作下平坦度的變化。我們發現，正規矩型式(duty cycle = 0.5)的光柵耦合器，如預期地表現出最小平坦度差異，而三角式光柵則顯示出差異性較大之特性。

The function of gratings is to couple the incident plane wave energy into the dielectric waveguide or to change the bound surface waves in the dielectric waveguide into leaky waves in free space, and this structure plays a significant role nowadays since the interfacial elements between optical integrated circuits is required urgently. However, the conventional analysis examines the coupling behavior at a fixed value of wavelength. This work, thereby, presents an extended analytical approach to the outgoing beam width on broadband operation, and reveals these properties are remained nearly constant over a wavelength band from 1μm to 2μm. The reason for choosing the symmetric structure is that symmetrical grating avoids excitation of the second (n=-2) space harmonic and therefore, the behavior of broadband would not be deteriorated. Moreover, we find the canonic rectangular (duty cycle = 0.5) gratings exhibit the smallest difference of flatness, but the symmetric triangular ones, however, present larger variation.