| 摘要: | 同時具有單縱模(Single Longitudinal Mode, SLM)及單橫模的雷射具有極窄頻、高強度、高準值性的光源輸出，可被廣泛應用於光譜學、太空遙測和光纖通訊。傳統欲達成單縱模輸出，可使用方法有短共振腔內置一個或數個共振腔(Intra-cavity etalons)作為篩頻元件或是利用Fox-Smith干涉儀的架構[0.1]以共振腔(etalon)選取單頻共振，另外又可由環型共振腔並置入光單通道如法拉第旋轉器(Faraday rotator)，即共振腔內光波以行進波(traveling wave)而非駐波(standing wave)傳遞，藉以抑制同質性拓寬(homogeneous broadening)不可避免因為空間燒洞(spatial-hole burning)所產生的橫模，達到極窄頻單縱模雷射輸出。 上述方法雖可達成極窄頻單縱模輸出，卻不可避免存在如共振腔內元件過多造成較高的端面損耗或環型共振腔架構複雜且架設不易之問題。本論文實驗為以體積全像布拉格光柵(Volume Bragg Grating ,VBG) 取代光參量振盪器(Optical Parametric Oscillator,OPO)的介電質輸出耦合鏡(Dielectric output coupler / Mirror, DM)，可窄化光參量振盪器訊號光之頻寬。在使用較低尖端功率雷射作泵浦光源下，光參量振盪器的增益曲線近似同質性拓寬，若使用電光模態轉換器(Electro-Optic Polarization Mode Converter,EO PMC)調制窄頻寬訊號光的偏振態使往返訊號光偏振態正交，取代傳統複雜元件和架構，以單一元件和線性共振腔達到極窄頻單縱模光參量振盪器。 本論文實驗比較光參量振盪器於駐波操作和電光偏振模態轉換器調制成行進波操作下訊號光之頻譜、模態以及脈寬上之變化。發現操作在行進波條件下，較低增益與相對較高增益頻譜半高寬都有一倍的窄化效果:相對低增益頻譜半高寬從駐波操作下的0.02奈米在行進波操作下窄化至0.01奈米(光譜儀解析極限)，相對高增益頻譜半高寬從0.1奈米窄化至0.05奈米。另外，光參量振盪器的M2值透過電光偏振模態轉換器亦有濾波優化的效果，從駐波操作下的2.47至行進波操作下的1.25。Single Longitudinal Mode (SLM) lasers have narrow linewidth, high degree of brightness and directionality properties, and they can be applied to many areas such as spectroscopy, remote sensing and fiber communic- ating. SLM can be achieve according to many traditional methods such as insert intra-cavity etalons in the short cavity, and constructing Fox-Smith interferometer oscillating single frequency with etalon. After that, we can use the ring cavity and Faraday rotator to let the electromagnetic waves become traveling waves instead of standing waves in the laser resonator to eliminate spatial-hole burning effect in homogeneous broadening mechanism to get rid of unwanted transverse modes and achieve SLM with narrow linewidth. Although those methods mentioned above can achieve SLM, many problems such as high insertion loss and complicated alignment in the ring cavity still remain. This thesis demonstrates another way to attain the SLM: Volume Bragg Grating (VBG) is a linewidth narrowing element, and it can be used as the output coupler of Optical Parametric Oscillator’s (OPO’s) resonator to reduce the output linewidth of OPO signal. If the nonlinear gain medium is pumped by the relatively lower peak power laser in this case, the gain profile of OPO is similar to homogeneous broadening. Next, we employee two Electro-Optic Polarization Mode Convertors (EO PMCs) to modulate the polarization states of narrow linewidth forward and backward signal waves become orthogonal. Finally, the signal waves reach SLM in a compact manner with linear cavity and few elements compares with traditional setups. We investigate and compare the OPO signals in their spectra, mode profiles and pulse durations with OPO signals operated at traveling wave mode and standing wave mode, which are controlled by EO PMCs. Under the traveling wave operation, the spectra linewidth (FWHM) of OPO signals were narrower both in the relatively lower gain regime (0.02nm to 0.01nm) and the relatively higher gain regime (0.1nm to 0.05nm) than under the standing wave operation. Meanwhile, the M2 of OPO was improved by two EO PMCs from 2.47 to 1.25. |
