基於非線性雙光子吸收與克爾效應之碳化矽/碳化矽摻鍺全光調變器

SiCx/ SiCGe waveguide all-optical modulator based on nonlinear two photon absorption and Kerr effect

黃博基(Bo-Ji Huang)

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(2015年)

https://doi.org/10.6342/NTU.2015.10690

摘要

近年來為了實現全光高速訊號與數據傳輸，矽光子學被視為相當重要的研究主題。在此研究中，我們利用非線性雙光子吸收引發自由載子吸收以及光學克爾效應兩種調變機制，以碳化矽與摻鍺碳化矽製備超高速全光波導調變器。
首先，我們藉由高強度脈衝光源引發富矽碳化矽中之雙光子吸收效應進而產生自由載子。自由載子將吸收耦合入波導之探測光源使之進行波長轉換以達皮秒等級之全光波導調變器，該效應稱之為跨吸收光學調變機制。然而，我們進一步觀察到微弱的非線性克爾效應所造成之波長紅移導致該調變機制之調變深度受到影響。在此研究中，我們發現富矽碳化矽中之矽量子點可提供一相當大的跨吸收調變機制使之可應用於全光調變器與光學邏輯閘之全光開關。我們首度成功利用鑲嵌矽量子點之富矽碳化矽製備全光脈衝歸零開關訊號之反向格式轉換器，其調變位元率可達 1.2 Gbit/s。
接著，我們使用三種不同組成比例之碳化矽製備全光波導調變器，其中包括富矽碳化矽、碳化矽與富碳碳化矽。我們將調變位元率為1.2 以及12 Gbit/s 之全光脈衝歸零開關訊號序列個別耦合進入此三種波導中以展示全光訊號調變。實際上，我們於富矽碳化矽中觀察到非對稱的反向訊號形狀，其消光比僅有8.7 dB。於接近標準組成比例之碳化矽波導中，由於環形共振腔結構內之自由載子吸收較弱，當探測光集中於環形共振腔內將使得訊號序列進行波長轉換，消光比高達14 dB，但其直線波導中仍存在強烈的雙光子吸收現象。然而我們在富矽碳化矽波導中觀察到對稱性的正向及反向格式轉換。為了瞭解元件最大的頻寬限制，我們將調變位元率為12 Gbit/s 之全光脈衝歸零開關訊號序列耦合入該波導。其中我們可於富矽碳化矽中觀察到非線性克爾效應所產生之對稱性正向以及反向訊號調變，其消光比高達20 dB。值得注意的是，由於我們將組成比例調整至富矽碳化矽使得雙光子吸收誘發自由載子吸收效應被抑制，因此富矽碳化矽全光調變器具有最高的調變頻寬，能夠傳輸位元率為12 Gbit/s之全光脈衝歸零開關訊號。
最後，我們於碳化矽中摻鍺以降低自由載子活期，進而提升全光調變器之調變頻寬，並展示快速雙光子吸收之全光調變開關波長轉換及格式反轉。藉由將鍺摻雜入碳化矽主體內以及進一步降低入射脈衝能量，可有效降低碳化矽摻鍺之自由載子時間至皮秒等級。當泵浦能量由0.5 nJ降低至22 pJ時，載子活期成功地由350 ps降低至10 ps。由實驗結果觀察，即使注入較低的入射光能量，仍可以產生跨吸收調變效應達到超快全光開關。為了確認其調變頻寬，我們將調變位元率由1.2增加至6 Gbit/s 之脈衝歸零開關訊號序列耦合入該波導，並成功以摻鍺碳化矽之全光波導調變器傳輸全光脈衝歸零開關訊號。

關鍵字

雙光子吸收 ； 非線性克爾效應 ； 碳化矽 ； 碳化矽摻鍺 ； 全光調變器

並列摘要

Development of Si-based all optical waveguide modulator for transmitting ultrafast optical signal becomes an important research topic in Si photonics. In this thesis, all-optical modulation with wavelength conversion and data inversion has been demonstrated by using the SiCx and SiCGe waveguides.
At first, the Si quantum-dots (QDs) doped Si-rich SiCx waveguide modulator is demonstrated for the first time to perform the 1.2 Gbit/s all-optical format inversion with a pulsed return-to-zero on-off-keying (PRZ-OOK) data-stream. The sub-bandgap cross-absorption-modulation (XAM) has been preliminarily observed and confirmed as a new kind of wavelength conversion process to enable ultrafast optical switching in Si-QDs. The XAM effect induces a wavelength-converted picosecond all-optical switching between pump and probe signals, which is attributed to the free-carrier absorption induced by the two-photon-absorption effect. The pump-probe analysis also shows a weak nonlinear Kerr effect with an ultrafast switching response from the changing envelope of the XAM probe pulse at red-shifted wavelength. These observations declare that the nano-scale Si-QDs can provide sufficiently large XAM effect to enable the ultrafast all-optical switching capability.
Afterwards, the all-optical waveguide modulator is fabricated by the nonstoichiometric SiCx with different composition ratio, including Si-rich SiCx, nearly stoichiometric SiC, and C-rich SiCx. The modulation mechanism is changed from the TPA effect to the nonlinear Kerr effect by detuning the nonstoichiometric SiCx from Si-rich to C-rich condition. The inversely modulated probe data stream reveals an asymmetric bit shape data with an extinction ratio (ER) of only 8.7 dB in Si-rich SiCx. When coinciding the probe wavelength with the transmission dip, the high-speed wavelength conversion of data stream with an ER of 14 dB can be observed by eliminating the FCA effects in the nearly stoichiometric SiCx micro-ring waveguide. To completely suppress the trailing edge, a symmetrically converted and inverted data with high on/off extinction at 1.2 Gbit/s can be observed in the C-rich SiCx waveguide modulator. To demonstrate the ultrafast all-optical modulation, the wavelength-converted and sign-reversible PRZ-OOK data switching at bit rate up to 12 Gbit/s can be delivered via the C-rich SiCx waveguide modulator due to strong Kerr nonlinearity of C-rich SiCx. Because of the suppressed TPA induced FCA effect, the C-rich SiCx based waveguide exhibits a better performance on responding the continuously incoming on-level bits than that of the nearly stoichiometric SiCx and Si-rich SiCx waveguides.
Eventually, the SiCGe based ultrafast all-optical waveguide modulator enables the wavelength conversion and format inversion with a PRZ-OOK data-stream has been demonstrated for the first time. Under the operation of intensive optical pulse illumination, the TPA effect can be observed when the total energy of two incident photons exceeds beyond the bandgap energy of SiCGe, in which the induced free carriers in the SiCGe will absorb the probe beam to reduce its throughput intensity. As a result, the probe beam can be inversely modulated to cause XAM by intensive pump pulse induced TPA/FCA effect. The picosecond effective carrier lifetime of the SiCGe can be obtained by adding the Ge content into the SiC matrix with a reduced pumping energy. By reducing the pumping energy from 0.5 nJ to 22 pJ, the carrier lifetime shortens from 350 to 10 ps, which can provide sufficient large XAM effect to enable the ultrafast all-optical switching.

並列關鍵字

two photon absorption ； nonlinear Kerr effect ； SiC ； SiCGe ； all-optical modulator

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