基於線性自由載子吸收與非線性克爾效應之奈米矽摻雜富矽氧化矽/富矽氮化矽/富矽碳化矽全光環形調變器

Nano-scale Si doped Si-rich SiOx/ Si-rich SiNx/Si-rich SiCx ring waveguide based all-optical modulator using linear free-carrier absorption and nonlinear Kerr effect

蘇聖斌(Sheng-Pin Su)

指導教授 ： 林恭如

國立臺灣大學/電機資訊學院/光電工程學研究所/碩士(2014年)

https://doi.org/10.6342/NTU.2014.01120

摘要

我們模擬、製作以及量測分別以摻雜奈米矽的富矽氧化矽/富矽碳化矽/富矽氮化矽為基材的光波導元件作為環形線性自由載子吸收和非線性克爾效應全光調變器。
我們首先研究了奈米矽摻雜之富矽氧化矽直線光波導在相同的光通量下以不同波長作為激發光源的自由載子吸收係數表現。在觀察到將激發光源之波長從532奈米縮短至325奈米時，自由載子吸收係數將從0.11 cm-1增加至3.44 cm-1。接著，在直線光波導加上微米等級之環形共振腔，藉由精準的訊號載波波長控制，使其調變深度可更進一步由52.5%上升至63.5%。此外，我們發現並且比較元件被激發後穿透譜的位移以及共振腔的質量因子劣化，將此兩種效應考慮後模型數值分析所相對應的載子濃度與量測值有很高程度的一致性。由於以自由載子吸收當作開關機制的調變器會受到載子活期的影響，使其調變速度被限制在1奈秒以上。隨後，我們進一步的利用非線性克爾效應做為調變機制，來達到調變速度增加之效果。藉成長奈米矽摻雜之富矽碳化矽製作微米環形共振腔，使材料擁有更高的非線性折射係數以達成有效的克爾效應調變深度。利用電漿輔助化學氣相沈積法成長碳化矽薄膜時重新調控了矽與碳的成分，使矽的比例比傳統碳化矽高出37.2%。藉由拉曼光譜分析其結晶性佐證了過量矽原子以量子點的型式存在。根據量子侷限效應，奈米矽的存在將使材料的非線性折射係數大幅上升。利用光束傳播法(BPM)來模擬於通訊波長可單模操作波導結構參數，經由電子束蝕刻製作之環形共振腔波導在傳輸波段擁有質量因子高達Q=22800。最後，利用注入之外部調變高功率非歸零開關鍵控激發光源誘發非線性克爾效應達成全光載波調變，在3 W之尖峰功率下其調變速度可達12 Gbit/s。因為環形波導共振點的紅位移以及共振腔的激發光放大，此奈米矽摻雜之富矽碳化矽在傳輸波長之有效非線性折射率達到n2=3.14×10-13 cm2/W。
第三個部分，在奈米矽摻雜富矽氮化矽微米環形共振腔之克爾開關當中，我們利用相同之高功率非歸零開關鍵控激發光源也能產生訊號格式正反轉換之全光調變。奈米矽摻雜的富矽氮化矽相較於矽塊材擁有較大的能隙，使其雙光子吸收係數降低至儀器可觀測之下限。奈米矽量子侷限效應同樣使典型氮化矽薄膜材料的非線性折射係數大幅上升兩個數量級以上。材料分析發現此奈米矽摻雜富矽氮化矽擁有比傳統氮化矽超出23.4%的矽含量，過量的矽原子並以更小的矽團簇(precursor)形式存在於氮化矽中。此奈米矽摻雜之富矽氮化矽經過計算後發現在傳輸波長具有非線性克爾係數n2=2.17×10-13 cm2/W，經由高功率非歸零開關鍵控激發光源達到非線性克爾效應閥值強度以上可成功完成位元率高達12 Gbit/s的全光載波調變。

關鍵字

矽量子點 ； 環形共振腔 ； 光波導 ； 自由載子吸收 ； 克爾效應 ； 全光調變器 ； 非歸零開關鍵控

並列摘要

In this thesis, we simulate, fabricate and analyze the Si-QD doped Si-rich SiOx/ Si-rich SiCx/ Si-rich SiNx micro-ring waveguide resonator based all-optical modulator using linear free-carrier absorption and nonlinear Kerr effect.
In the chapter 2, by integrating with a ring-resonator waveguide, the Si quantum dot doped SiOx strip-loaded waveguide based free-carrier absorption modulator with enhanced FCA loss modulation is demonstrated by different pumping wavelengths. The micro-ring waveguide resonator induced a dark-comb like throughput transfer function in wavelength domain, in which function with a Q-factor of 6×103 can be blue-shifted by varying the photo-excited electron-hole plasma density. When transmitting optical data stream at the central wavelength of any notch in the dark-comb throughput function, the output data-stream can be inverted by pumping the micro-ring waveguide resonator to up-shift the notch away from its original wavelength. As a result, this maximize the extinction ratio of output data stream. The largest FCA loss and highest free-carrier density can be enhanced up to 3.44 cm-1 and 9.29×1016 cm-3, respectively, at excitation intensity as high as 6.7 W/cm2. By adding the micro-ring waveguide resonator, the modulation depth can be further enhanced from 52.5% to 63.5% by up-shifting the transfer function of the micro-ring waveguide resonator with the photo-excited e-h plasma. The maximal wavelength of the transmittance notch shift can be up to 0.033 nm under the same pumping intensity at wavelength of 1563.42 nm. The excited free-carriers inside the micro-ring waveguide resonator is obtained to be ~5.25×1015 cm-3. The pumping wavelength dependent transmission notch linewidth is broadened from 0.26 nm to 0.3 nm when the ring waveguide resonator is pumped by 325-nm HeCd laser.
Due to the speed of FCA based modulator is limited by the carrier lifetime which the modulation speed of SiOx:Si-QD is ~ 1 μs. As a result, the ultrafast nonlinear optical Kerr effect is employed in the following work.
In the chapter 3, the ultrafast optical Kerr switch with a Si-rich SiC micro-ring resonator is demonstrated. In addition, the nonlinear refractive index of Si-rich SiC at telecommunication wavelengths is firstly estimated by the resonance red-shift, which is still unknown in the literature. With the 12 Gbit/s NRZ-OOK optical pump data-stream, the Si-rich SiC micro-ring resonator shows a great applicability in the real optical communication system. From the maximal inverted probe signal, the red-shift of resonance dip is 0.07 nm corresponding to refractive index change of 1.2×10-4. By the red-shift of resonance dip, the nonlinear refractive index of Si-rich SiC is estimated to be n2=3.14×10-13 cm2/W, which is several orders magnitude larger than that in SiC. From the analyses of XPS and the RSS, the excessive Si concentration of 37.2% which exist in the form of Si-QDs is observed. The existence of Si-QDs buried in the Si-rich SiC matrix can effectively result in a huge enhancement on the nonlinear refractive index, which can be explained by the quantum confinement effect. From the BPM analysis under single-mode condition, the waveguide width and height of 600 nm and 300 nm is determined. The fabricated micro-ring waveguide resonator is obtain with Q=22800 and the transmittance drop of nearly 60% at a wavelength of 1551.08 nm.
In the chapter 4, the 12 Gbit/s optical Kerr switch has been demonstrated with a Si-rich SiN micro-ring resonator in the first time. The Si-rich SiN with excessive Si of 23.4% is grown by PECVD process with fluence ratio of [SiH4]/[NH3] equals 0.9. The fabricated mirco-ring resonator of Q=11000 is observed, which provides a field enhancement inside the micro ring resonator. By introducing the 12 Gbit/s NRZ-OOK data stream as the optical pump format, the ultra-fast response up to 83 ps of the Si-rich SiN micro-ring resonator shows a great applicability in the real-world optical communication system. The SNR is degraded from 9.66 dB to 5.32 dB after the wavelength conversion. According to the ultrafast response of nonlinear Kerr effect, the nonlinear refractive index of Si3N4:Si-QD at near-infrared wavelengths for optical telecommunications is instantly modified by the input optical data stream to cause the red-shift on the resonance of the micro-ring, thus providing a high-speed optical switch up to 12 Gbit/s via the cross-wavelength amplitude modulation effect. By analyzing the resonance dip red-shift of 0.13 nm corresponding toδn=2.2×10-4, the nonlinear refractive index of the Si-rich SiN is estimated as n2=δn/Ir=2.17×10-13 cm2/W, which is one order and two orders of magnitude larger than that in Si and SiN, respectively.

並列關鍵字

Ring resonator ； Waveguide ； Free-carrier absorption ； Kerr effect ； All-optical modulator ； NRZ-OOK

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