Abstract
The optical frequency comb (OFC) as “frequency rulers” is well established in optical frequency metrology for ten years. It greatly improve the precision in optical frequency measurement. OFC can be considered as a composing of the evenly spaced “comb line” in the frequency domain. Due to the frequency of comb line is equal to a multiple of repetition rate plus the offset frequency, the good stabilization of the the repetition rate and the offset frequency can let each comb line viewed as a frequency stabilized laser. On the other hand, the extremely broad bandwidth of OFC provides the great tool for molecular spectroscopy. The dual-comb spectroscopy using the beating signal of two OFC with different repetition rate can let the optical spectrum down-converted into a microwave frequency region, where becomes accessible to digital signal processing. Then we can obtain the absorption spectrum and the phase delay information by analyzing the microwave spectrum. Compared to conventional CW spectroscopy, this spectroscopy method has some advantages as below: 1. Measuring a broadband spectrum at once. 2. Acquiring ont only the absorption spectrum but also the phase delay information. 3. The fast measuring time, which has the ability to measure the time-dependent system. For these reasons, the dual-comb spectroscopy had been developed recently. In this thesis, we use this technology to apply to molecular spectroscopy. In this work, we study the dual-comb multi-heterodyne spectroscopy by using two Industrial Technology Research Institute’s erbium-doped fiber OFC. The two OFC’s repetition rates and offset frequencies were stabilized through the phase locked loop on the microwave local frequency calibrated by GPS. The two OFC’s repetition rate controled at 400.2 MHz and 400 MHz and the difference of repetition rates was 200 kHz. At present, we can resolve individual RF comb lines and obtain the Acetylene P branch absorption spectrum. The absorption line centers in our results differ by greater than the overall ±300 MHz systematic uncertainty. The worse precision was possibly contributed from the unknown oscillation baseline on our spectrum and the roughly 400 MHz spaced spectrum sampling. In the future, we will overcome these shortcomings to achieve the sufficient precision in molecular spectroscopy.