| 摘要: | 矽晶微共振腔技術近十年來已被廣泛的運用來實現晶載光學梳。並同時應用於光通訊、光感測、頻率分析、時域任意波、光學測距、飛秒極短脈衝的產生。利用高效率微共振腔中的非線性產生寬頻、高同調光譜，近來此技術也被實現在雙光學梳光譜。相較於過去精密光譜分析需要利用色散光譜儀或者邁克森干涉儀，均需要長時間的量測，同時有著移動光柵或平台所造成的誤差。晶載雙光學梳提供了很好的解決方式。利用二光學梳略為不同共振頻率 (frep1 和frep2)所產生的差頻(Δf)，可以將光頻降轉至射頻範圍，將此訊號以高頻光偵測器及示波器記錄其干涉訊號並用傅立葉轉換至射頻，藉此得到光譜吸收訊號。此方式提供了精準且快速的光譜量測。然而過去晶載光學梳之雙光學梳光譜必須利用在波導中產生光孤立子(soliton)，此方式需要複雜的光源及異常色散共振腔調變操作。光學梳之產生會先進入低同調的混沌模態，再利用波長微調或微加熱器調變的方式進入高同調、孤立子模態。此計畫預計利用同調雙光學梳操作於正常色散範圍，透過不同波導模態耦合的方式產生所需之調變增益以激發光學梳。此作法和孤立子的方式不同在於此同調光學梳為非鎖模狀態-時域上非短脈衝。然而此操作依然提供了雙光學梳光譜所需之同調性。 ;Microresonator-based frequency comb generation has been widely applied in optical communication, optical sensing, frequency metrology, arbitrary waveform generation, optical ranging, and astronomy. Through nonlinear interaction in high-quality factor (high-Q) microresonators, a broad comb spectrum can be generated with low intensity noise and high coherence. Recently, the technique has demonstrated the possibility to operate on-chip dual-comb spectroscopy with two microresonators either in serial or in parallel. For traditional spectral measurement, it requires slow dispersive spectrometer with calibration or bulky Michelson interferometer with moving components for Fourier-transform infrared spectrometer (FTIR). Dual-comb spectroscopy alleviated these requirements. By introducing slightly different repetition rate frep1 and frep2 (with different microresonator geometries), these dual combs provide beatnotes and downmixs the optical frequency to radio-frequency (nΔf=n|frep1-frep2|, n is integer) in the interferometry through a fast photodiode and real-time scope. The beatnotes are then reconstructed by fast Fourier transform and therefore the absorption of the chemical / biological sample could be determined. However, those studies required soliton state operation for dual-comb spectroscopy. With delicate pump operation, the comb is transitioned from chaos state to coherent state with multiple soliton, and eventually to a single soliton state. A stable, narrow linewidth, and fine-tuned laser is required with optimized thermal and laser tuning control. Therefore, it strongly limited the tunability for both comb repetition rate difference (Δf=|frep1-frep2|) and spectral resolution (determined by frep1 and frep2). In this project, we propose a simple, tunable on-chip dual-comb operation in waveguide normal dispersion regime. Through mode-coupling, comb could be initiated with high temporal coherence. Even without mode-locking (short pulse operation), this dual-comb offers the potential for future on-chip dual-comb spectroscopy. Two microresonators in serial are fabricated by sharing the same pump lines at a single bus waveguide. A thermal heater on the microresonator or auxiliary ring will be studied to tune the coupling wavelength and match the pump line at selective position. Similar to the soliton operation, these microresonators are designed with slight different repetition rate, providing optical beatnotes and downmixing the optical frequency to radio-frequency (RF). The goal is to achieve noise-free dual-comb spectra with controllable repetition rate from 100 GHz to 200 GHz. The repetition rate difference is designed to be 1GHz to 2GHz (different chips) which results in a spectral bandwidth up to 4THz with a >20 GHz fast photodiode / real-time scope. In addition, with this simple coherent operation, the repetition rate could be possibly tuned by a second thermal heater. We expect the tunability of repetition rate difference is up to 250MHz which provides a 1.3X tunability for spectral bandwidth. |
