長週期光纖光柵(Long-period fiber grating, LPFG)已被廣泛研究與應用在光纖通訊系統，但是光纖並不適合做小型且低成本的元件，故無法滿足量產以及積體化的需求，為了突破長週期光柵在光纖上的限制，長週期波導光柵(Long-period waveguide grating, LPWG)被提出研究以及實現。由於LPWG在波導材料選擇上比LPFG豐富許多，因此能展現出更加靈活性的設計和製作於光學波導應用上。由於傳統的光柵濾波器所工作的單一波長已經無法滿足需要過濾多個波長的特殊頻譜，進而衍生出如波分多工器(Wevelength-division multiplexing, WDM)此類工作波長為多波長元件，但此類元件多利用兩個或兩個以上的獨立濾波器去達到其工作原理，在積體化上將會變得更加困難，因此本實驗利用串級式的3-dB長週期光柵來達到多個抑制頻帶，則可滿足積體化的需求。
本實驗利用二次質子交換成功地在LiNbO3基板上製作出串級式長週期波導光柵元件，其元件週期為Λ=50μm。第一次質子交換主要是製作平面波導披覆層，其溫度為280oC，時間為4小時，接著熱退火溫度為400oC，時間為2小時，而第二次質子交換主要是製作波導層，其溫度為280oC，時間為2小時。本實驗利用三次質子交換以及質子交換濕式蝕刻法(proton-exchanged wet etching)兩種不同的技術來實現光柵結構。其一經由第三次質子交換製作出相位光柵(phase grating)，其溫度為280oC，時間為30分鐘；其二經由質子交換濕式蝕刻法製作出波紋光柵(corrugated grating)，蝕刻速率為0.011μm/min，蝕刻深度為0.1μm。
本實驗利用串級式長週期光柵來實現窄頻帶多通道的穿透頻譜，即為梳形濾波器(comb filter)之穿透頻譜。可藉由改變兩組光柵之間的相隔距離d來調整頻譜通道間距S(channel spacing)，本實驗對頻譜通道間距S的變化進行量測，同時相隔距離d的改變由12至22mm。量測結果顯示元件之光波長抑制對比度(dip contrast)最大可達到18.25dB，而在相隔距離d=22mm時，可達到最小通道間距S=2.45nm，可觀察到通道間距S隨著相隔距離d的增加而縮小。

We propose a design of the cascade long-period waveguides gratings (LPWGs) on lithium niobate (LiNbO3) substrates. In this work, the channel waveguides are buried in slab cladding by two-step proton exchange, and the gratings are subsequently produced by three-step proton exchange or proton-exchanged wet etching.
A narrow-band multichannel transmission spectrum formed by a cascade long-period waveguides gratings reveals a typical transmission spectrum of a comb filter. The spectral channel spacing S can be adjusted by simply changing the separation d between the first and the second gratings. The variation of the spectral channel spacing S is measured when the grating separation d is changed from 12 to 22mm. The measurement results demonstrate the maximum dip contrast is close to 18.25dB and the narrowest channel spacing S is about 2.45nm in the case of the grating separation d is 22mm. Note that the channel spacing S decreases with increasing grating separation d.

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第五章
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