在精密量測與現代科技應用中，例如物理常數的定義，原子與分子結構上的研究，光通訊系統，衛星通訊系統等等。絕對頻率的量測是非常重要的。過去，在可見光的頻率標準都是利用原子或分子的吸收譜線來定義，只限於某些特定頻率，無法涵蓋整個可見光範圍。藉由飛秒雷射和光子晶體光纖的發展，科學家已經可以建立涵蓋可見光到近紅外的光頻梳，可精密測量穩頻雷射的絕對頻率。

本實驗是利用鎖模雷射(mode-locked) 產生的脈衝光在時域上約皮秒或飛秒在頻率域上的特性是縱模之頻率間隔是固定的。因其頻率間隔就像梳子般固定所以稱之為光頻梳(frequency comb)。鎖模雷射的縱模頻率為nfrep+□□□，欲建立一光頻梳，必須同時控制脈衝重複率 frep (repetition rate)和頻差□□□(offset frequency)，我們利用光子晶體光纖(photonic crystal fiber)，將飛秒摻鈦藍寶石雷射的頻譜拉寬至涵蓋一個八度音(octave)。再利用self-reference干涉法來得到頻差□，我們藉由控制雷射腔長和泵浦光強度分別穩定脈衝重複率 frep和頻差□。穩頻後的脈衝重複率的頻率擾動約3.63 mHz，而頻差穩定度則小於10 mHz。我們利用飛秒光頻梳量測碘分子在R(56) 32-0的譜線超精細躍遷a10頻率。目前我們系統的最後結果其量測頻率的精準度約為10-11，而頻率標準源銣原子鐘的準確度為5x10-11。

Abstract

Absolute frequency measurement is of great interest not only for metrology applications but also for high-resolution spectroscopy, optical communications, and physical constant definitions. Frequency standards at optical region based on atomic or molecular absorptions provide high accuracy, but it can’t cover all optical spectrum are limited to some special frequencies. With the progresses of the femtosecond lasers and photonic crystal fibers, the frequency comb from visible to near infrared region is well established. It can accurately measure the absolute frequency of stabilized laser.

Mode-locked lasers generate ultrashort optical pulses by establishing a fixed relationship across a broad spectrum of frequency. Mode-locked lasers have resulted in the generation of optical pulse with picosecond or femtosecond duration at time domain. The pulses train generated by a mode-locked laser has a frequency spectrum that consists of a discrete, regular spaced series of sharps lines, known as a frequency comb at frequency domain. The frequency of the nth comb line is nfrep+□□□Establishing an optical frequency comb by simultaneously stabilizes the repetition rate and offset frequency. We used photonic crystal fiber to generate a spectrum that spans an octave. And obtain the offset frequency by self-reference interferometer. We stabilized the repetition rate and offset frequency by controlling the cavity length and the pump power. The frequency fluctuation of the stabilized repetition frequency is 3.63 mHz. The stabilized offset frequency fluctuation is small than 10 mHz. We used the optical frequency comb to measure of a10 component of Iodine R(56) 32-0. The accuracy of our femtosecond comb system is 10-11. The accuracy of rubidium frequency standard is 5x10-11.

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