飛秒光頻梳的改進

The improvement of femtosecond optical frequency comb

李明翰(Ming-Han Lee)

指導教授 ： 施宙聰

國立清華大學/電機資訊學院/光電工程研究所/碩士(2008年)

https://doi.org/10.6843/NTHU.2008.00261

摘要

在精密量測與現代科技應用中，例如物理常數的定義、基本物理定律的檢測、原子與分子結構上的研究、光通訊系統等等。光頻標準的建立及絕對光頻的量測是非常重要的。過去，光頻標準是建立在少數的原子分子的躍遷頻率上，再藉由非常複雜的頻率鏈﹝frequency chain﹞擴張其涵蓋範圍。然而，隨著飛秒光頻梳的發展，提供了一個非常簡易可靠的方法，頻率涵蓋範圍從可見光延伸到近紅外光，甚至還可藉由一些非線性的光學方法，將頻率範圍延伸到深紫外其中紅外的頻段。
本論文是以實驗室原有的光頻梳﹝optical frequency comb﹞為基礎，而加以改進使其更加穩定。我們先利用鎖模雷射﹝mode-locked﹞產生約 44 飛秒的脈衝，再利用長約 15 公分的光子晶體光纖﹝photonic crystal fiber﹞，將頻譜拉寬超過一個八度音﹝octave﹞，波長範圍從 500 nm 到 1550 nm。最後測頻的部分我們使用光柵取代原本的濾波片，藉由光柵的繞射效應把待測光取出並量測。我們利用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 2.3 x 10-12 @ 1 s。

關鍵字

光頻梳 ； 飛秒雷射

並列摘要

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. The pulse duration is about 44 fs 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 n×frep+δ. Establishing an optical frequency comb by simultaneously stabilizes the repetition rate and offset frequency. We used the 15-cm-long photonic crystal fiber to generate a spectrum that spans from 500 nm to 1550 nm. 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. 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 2.3 x 10-12 @ 1 s.

並列關鍵字

optical frequency comb ； femtosecond laser

參考文獻

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