| Summary: | 碩士 === 國立清華大學 === 光電工程研究所 === 99 === In this thesis, we observe the saturation spectrum of N2O 0001←0000 fundamental band R(10) transition by a CW mid-infrared MgO:PPLN-based difference frequency generation (DFG) source pumped by a Ti:Sapphire laser and a Nd:YAG laser. The absolute frequency of R(10) transition is also determined by the femtosecond optical frequency comb (OFC).
We present two methods to stabilize the frequency of DFG source. Method 1: Ti:S scan in which we stabilize the frequency of Nd:YAG laser on the a10 component, R(56) 32-0 transition iodine molecule 127I2, and the frequency of DFG source is scanned and locked by tuning the frequency of Ti:Sapphire laser. Method 2: YAG scan in which we stabilize the frequency of Ti: Sapphire laser using a thermally isolated Fabry-Perot cavity, and the frequency of the DFG source is scanned and locked by tuning the frequency of the Nd:YAG laser.
The absolute frequency of N2O transition is equal to the difference of two laser frequencies. In Ti:S scan method, we assume that the frequency of Nd:YAG laser is a known value, so we only measure the frequency of Ti:Sapphire laser by the OFC. And in YAG scan method, we have to measure the frequencies of Ti:Sapphire laser and Nd:YAG laser simultaneously. However, in Ti:S scan, the frequency of Nd:YAG laser isn’t a fixed value actually and it is different for different frequency locking condition.
After measuring the Nd:YAG laser, the frequency difference of the absolute frequency measurements using Ti:S scan and YAG scan is approximate 1 kHz. Due to the monolithic non-planar ring oscillator structure of Nd:YAG laser, its frequency stability is better than Ti:Sapphire laser. Our system has better resolution by adopting the YAG scan method. In addition, we have to improve the feedback system to increase the accuracy in measuring the absolute frequency.
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