利用液晶相位調制於光纖耦合與調變光衰減之一種電控可調式透鏡耦合器

Abstract

我們利用液晶相位調制開發出一種電控可調式透鏡耦合器，此透鏡耦合器同時具備光纖耦合(fiber coupling)與調變光衰減(variable optical attenuation or VOA)之功能。基本原理主要是利用電控液晶相位調制以控制耦合入光纖之光束大小(beam size) ，液晶相位調制器主要利用透過外加電場驅動液晶指向矢軸向分佈以提供等效透鏡的相位分佈，依據屈光度(optical power)的不同來調整耦合入光纖之光束大小。對多模光纖而言，當入射光束小於光纖核心大小時，液晶相位調制器之功能為光纖耦合，當入射光束大於光纖核心大小時，液晶相位調制器之功能為電控VOA 。對單模光纖而言，液晶相位調制器較難達到耦合入單模光纖之光束大小，因此設計上我們使用漸進式固態透鏡(GRIN lens)輔助使入射光束小到可耦合入單模光纖，並利用液晶相位調制器使入射光光束變大以達到VOA功能。我們從高斯光束光學出發推導出液晶相位調制器屈光度與光纖之耦合效率的關係並與實驗結果作比較與討論。此研究進一步可應用在多波道等化器(wavelength-division-multiplexed equalizer)與增益平坦化放大器(gain-flattened amplifier)。

We developed an electrically tunable lens coupler adopting a liquid crystal (LC) phase modulation for both of fiber coupling and variable optical attenuation (VOA). The operating principle is that the beam size of the incident light coupling to the fiber is modulated by the LC phase modulation because LC directors are reoriented by applied electric fields and then result in the electrically tunable optical power. For multimode fibers, the LC phase modulator is a lens coupler when the incident beam size is smaller than the core size of the fiber and the LC phase modulator is a VOA when the beam size is larger than the core size of the fiber. For single mode fibers, we adopt a LC phase modulator with a commercial GRIN lens in order to further reduce the coupling beam size for fiber coupling. The LC phase modulator enlarges the beam size for VOA. In this thesis, the related experiments are performed and optical analysis is discussed based on beam optics. The potential applications of this study are wavelength-division-multiplexed equalizer and gain-flattened amplifier.