Frequency Stabilization of CO Laser using Optogalvanic Lamb-dip

• Main Authors: Yu-Hung Lien, 連育宏
• Other Authors: Jow-Tsong Shy
• Format: Others
• Language: en_US
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/67068159817199776625

博士 === 國立清華大學 === 物理學系 === 91 === The optogalvanic spectroscopy of carbon monoxide moleucle (CO) and its application on frequency stabilization of CO laser are presented in the dissertation. Both DC and RF optogalvanic spectroscopy were studied and the saturation dip was pursued for the better frequency accuracy and stability. The DC optogalvanic spectroscopy was first studied in a low pressure discharge tube which the gas mixture of CO, N2 and various noble gases flowed through. The operating pressure was kept between 0.8 and 1.3 Torr to reduce the pressure broadening; the positive column region of the discharge was cooled to -60 C. The optimal S/N ratio was about 600 with the gas mixture CO : N2 : Ar = 1 : 84 : 11 and the total pressure was 1.344 Torr. No saturation dip was observed yet. It was believed that the pressure broadening should be responsible for the missing saturation dip. The RF optogalvanic spectroscopy was studied in order to lower operating pressure. There were three different RF circuits were constructed. Two of them were based on the design of Colpitts oscillator and one was based on a push-pull oscillator. Low or no nitrogen concentration was preferred for RF optogalvanic signal. All three circuits could operate down to 600 mTorr with little noise degradation. Cooling the discharge tube by spraying liquid nitrogen was once tested on Colpitts-I circuit and severely lowered the signal. The maximum S/N ratio was about 2190 for Colpitts-II circuit at amplitude modulation frequency 255 Hz. The saturation dip was observed by both Colpitts optogalvanic circuits and the depth was typically less than 5%. By wavelength modulation technique, the frequency of CO laser could be stabilized at the zero points, which was corresponding to the absorption line center of CO, of derivative spectroscopic signal. The limit of frequency stability was about 149 kHz and the practical stability was better than 300 kHz when the laser was stabilized by first order derivative spectroscopic signal. The third order derivative spectroscopic signal was also obtained but not used to stabilize laser frequency due to low S/N ratio. The extraordinary optogalvanic signals around some specific CO laser wavelengths were only found by Colpitts-II circuit. The origins of these extraordinary signals were discussed but yet identified. At last, the outlook of the experimental works is presented.