| 摘要: | 模態分為兩種，縱模 (Longitudinal mode)及橫模 (Transverse mode)。縱模，其特性為時間軸上的變化，影響其時間同調性；橫模，其特性為空間軸上的變化，影響其空間同調性。為了使雷射有好的空間同調性以及方便光波的傳導與聚焦，絕大多數的雷射架構都會將橫模設定為TEM00的基模態，因此對於模態的研究大多是針對縱模來進行相關的實驗研究，本論文實驗即是以產生單縱模為主要研究方向。
單模雷射具有高強度、頻譜寬度極窄、高準直性等特性，多被廣泛應用在光譜學、太空遙測、光纖通訊系統等。本論文實驗利用體積式布拉格光柵 (Volume Bragg Grating ,VBG)當作第二面輸出耦合鏡，取代介電質輸出耦合鏡，藉此窄化光參量震盪器之訊號光頻寬，同時利用晶體兩端設計之電光偏振模態轉換器調制 (Electro-Optic Polarization Mode Converter, EO PMC)其偏振，當光參量產生器產生訊號光，經過一段等效為四分之一波板之鈮酸鋰晶體，再透過電光偏振模態轉換器調制偏振角度，如此在共振腔來回共振之下，使光波在共振腔傳播時不產生駐波 (Standing wave)，而是以行進波(Traveling wave)的形式在共振腔中傳遞，藉此抑制同質性拓寬所造成的空間燒洞 (Special-hole burning)現象所產生的橫模。
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頻譜及模態之變化，發現操作在行進波條件相較於駐波條件下，頻譜半高寬有明顯的窄化效果，頻譜半高寬從駐波操作下的0.1奈米在行進波操作下窄化至0.01奈米以下(光譜儀解析極限)。
;The modes are divided into two types: Longitudinal mode and Transverse mode. Longitudinal mode: its characteristics for the time axis changes, affecting its temporal coherence; transverse mode: its characteristics for the changes in the spatial axis, affecting its spatial coherence. In order to make the laser has a good spatial coherence, facilitates the transmission and focus of light, the vast majority of the laser will be set to the transverse mode as TEM00 fundamental mode, To produce a single longitudinal mode is the main research direction of the experimental study.
Single-mode lasers with high intensity, very narrow spectral width, high collimation and other characteristics, are widely used in spectroscopy, space telemetry, optical fiber communication systems. In this study, the Volume Bragg Grating (VBG) is used as the second output coupling mirror to replace the dielectric output coupling mirror, which narrows the signal optical bandwidth of the optical parametric oscillator, then we use Electro-Optic Polarization Mode Converter ( EO PMC) to modulate polarization as two polarization rotators on both ends of optical parametric gain medium (OPGM), so that the light waves in the cavity will not produce standing wave but in the form of traveling wave transmission in the cavity , which inhibits the homogeneity broadening that caused by the special-hole burning phenomenon produced transverse mode.
In this study, the comparison of the optical spectrum and the mode of the optical parametric oscillator in the standing wave operation and the electro-optical polarization modulator is carried out under the condition of the traveling wave. Compared with the standing wave, the half-height width has a narrowing effect, and the spectrum half-width is from 0.1 nanometer under standing wave operation narrowed to 0.01 nanometers below (our optical spectrometer resolution limit) under traveling wave operation. |
