長週期光柵(Long-period grating, LPG)著重在基本波導模態(fundamental guided mode)和批覆層模態(cladding mode)之間的耦合特性。傳統上利用光纖來做長週期光柵會有幾何形狀和材料選擇上的限制，為了要消除光纖在製作上所面臨的限制和實現元件積體化，長週期波導光柵(Long period waveguide grating, LPWG)也因此被提出研究和實現。
一般傳統長週期波導光柵是以低折射率材質建構完成，在本論文中，高折射率材料矽基材首度被用來探討長週期光波導光柵製作的可行性，主要考量的製程是以非等向性濕式蝕刻矽晶圓之技術，蝕刻並製作LPWG於絕緣層覆矽(SOI)晶圓上。製作長週期波導光柵主要是以矽脊狀波導作為光波導核心，其批覆層的材料則是利用電漿輔助式化學氣相沉積法(PECVD)，藉由控制SiH4的流量，於SOI基板上沉積出一系列的非晶矽(a-Si)薄膜，並成功尋找一折射率接近矽卻又低於矽的非晶矽，以此做為元件批覆層，搭配長週期波導光柵之元件理論，計算出光柵週期。之後再利用濕式蝕刻克服塑膠光罩上線寬的限制，以其蝕刻之特性製做低於20μm之線寬，分別為20、18、15、12、10及8μm，成功降低了波導核心層之模態數目，並將相關參數導入於模擬軟體中，並設計出六組不同光柵週期之元件，分別為Λ20=100 μm、Λ18=107 μm、Λ15=93 μm、Λ12=95 μm、Λ10=109 μm和Λ8=91 μm。在後續的量測過程當中，其元件尚未導入偏振光時共振波長範圍皆在1563nm至1578nm之間，而在所有LPWG元件當中以線寬為8μm之元件對比度(contrast)為最佳，可達20dB，而半高全寬(FWHM)則以線寬為10μm之元件的表現為最佳，約為3.3nm。而加入極化控制器後產生極化偏振光，共振波長範圍亦在1563nm至1580nm之間，於橫向電場極化中，以線寬為8μm之元件對比度達30dB為最佳，而半高全寬則以線寬為12μm之元件以1.76nm為最窄;於橫向磁場極化中，以線寬為10μm之元件對比度14.5dB為最深，而半高全寬則以線寬為10μm之元件以1.32nm表現為最佳。

Long-period gratings (LPGs) are functioned based on the light coupling between the core guiding modes and the cladding modes at specific wavelengths (resonance wavelengths). However, the conventional FBG implemented on the optical fiber inevitably faces the geometry and material constraints associated with the fiber, which impose significant limitations on the device functionalities. To bypass the foregoing constraints, a long-period waveguide grating (LPG) employing a waveguide structure has been developed instead to provide an additional flexibility needed in designing various LPG-based devices.
In general, the traditional long-period waveguide gratings were manufactured using low refractive index materials. In this thesis, high-refractive index silicon was used for the first time to explore its practicality for the long-period waveguide gratings fabrication. Specifically, the device was etched and patterned on SOI wafer via an anisotropic wet etching technique and eventually the long-period waveguide gratings were successfully fabricated with silicon ridge waveguide incorporated as an optical waveguide core layer. In addition, the cladding layer based on amorphous silicon with refractive index slightly lower than the crystalline silicon was deposited using plasma-enhanced chemical vapor deposition (PECVD) by judiciously controlling the flow rate of SiH4. With the amorphous silicon used as the cladding layer, the gratings with pitch as long as tens of micrometer could now be defined and patterned using the conventional photolithography. As mentioned previously, the wet etching was adopted to overcome the line width restriction entailed by the use of a much cheaper plastic mask; making the device feature size less than 20μm easily realizable, specifically, waveguides with a line width of 20, 18, 15, 12, 10 and 8μm had all been successfully fabricated to cut down the number of guided modes present in the core region. Additionally, a commercial software was used to design gratings with six different pitches needed, namely, Λ20=100μm, Λ18=107μm, Λ15=93μm, Λ12=95μm, Λ10=109μm and Λ8=91μm. The subsequent experimental results demonstrate that the LPWG devices appear to resonate within a wavelength range between 1563 and 1578nm, and the waveguide width of 8μm has delivered a dip contrast as high as 20 dB, while the device with the waveguide width of 10μm has its FWHM measured as narrow as 3.3nm. Then the experimental results with polarization controller inserted into the measurement setup show that the devices resonate within a wavelength range between 1563 and 1580nm, and the waveguide width of 8μm has delivered a dip contrast as high as 30 dB, while the device with the waveguide width of 12μm has its FWHM measured as narrow as 1.76nm with input light polarized as transverse electric (TE) wave. With transverse magnetic (TM) polarized wave provided as an input, the waveguide width of 10μm yields a dip contrast as high as 14.5 dB, while the device with the waveguide width of 10μm has its FWHM measured as narrow as 1.32nm.

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