Repetition-Rate Stability Improvement of a Frequency Comb Laser with a Cesium-Stabilized 822-nm Diode Laser

在精密量測中光的頻率標準是很重要的，無論在從事基礎科學或工業應用都會用的上，例如基礎物理常數、光譜量測、原子分子及天文上的量測等。早期的頻率標準大多利用原子分子的吸收譜線來定義，無法涵蓋大範圍波段；不過透過飛秒雷射以及光子晶體光纖非線性效應的發展，已經可以涵蓋可見光到紅外光波段進行精密的測量。 本實驗使用鈦藍寶石雷射(Ti:Sapphire Laser)為鎖模雷射(Mode-Locked Laser)產生飛秒等級的脈衝，建立一個飛秒光頻梳雷射(Femtosecond Frequency Comb Laser)：也就是讓頻率上縱模與縱模之間的頻率差是固定的。鎖模雷射的第n個縱模頻率可以表示為: 的形式，其中frep為脈衝重複率(Repetition Rate)，fo為頻差(Offset Frequency)，當我們將鎖模雷射打入光子晶體光纖(Photonic Crystal Fiber)將鈦藍寶石雷射的頻寬拉長一個八度音(octave)，接著用自我參考(self-reference)的方法取出fo，當我們同時把frep以及fo都控制住時頻率上分布就上梳子一樣，可以當成一個頻率標準;然而於n的值很大有10^6的等級因此考慮nfrep時我們的頻率標準仍有一定的不準度，我們將藉由一個銫原子半導體穩頻雷射來與光梳雷射取拍頻當作誤差訊號回授控制雷射改善光頻梳雷射的不準度。Frequency standard plays a crucial role in metrology. It is applied in fundamental science and industry (such as measurement of fundamental physics constants, spectrum, and astronomy). In the past, definition of frequency standard was based on the spectral line of atoms, but this method got involved in broad bandwidth region. Thanks to the development of femtosecond mode-locked laser and photonic crystal fiber, frequency standard nowadays covers from visible to infrared region. In this article we use a Ti:Sapphire laser as a femtosecond mode-locked laser to build up a femtosecond frequency comb laser. Its feature is the same intervals between frequency modes, and that’s where the word, “comb”, comes from. The optical frequency of the nth mode of the mode-locked laser can be expressed as:fn=nfrep+fo, where frep is repetition rate and fo is offset frequency. We apply method of self- reference to build up the comb laser. However, there’s a certain uncertainty of fn, since n has a order of magnitude about 10^6 so as to the error-bond of n. For this concern, beat note between a frequency stabilized diode laser at 822nm and our comb laser is served as an error signal to feedback and reduce the uncertainty of the comb laser