1. Institute of Laser Spectroscopy, State Key Laboratory of Quantum Optics and Quantum Optics Devices, Shanxi University, Taiyuan 030006, China
2. Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan 030006, China
3. Middle School Affiliated to Shanxi University, Taiyuan 030006, China
Abstract：Noise-immune cavity-enhanced optical heterodyne molecular spectroscopy (NICE-OHMS) is the most sensitive laser absorption spectroscopy technology. It has obtained dozens of impressive detection sensitivities in a low pressure environment. However, when the measurement is taken at atmospheric pressure, its detection sensitivity becomes much worse. One of the dominant reasons is that the conditions for obtaining the maximum amplitude of NICE-OHMS signal at atmosphereare different from those under low pressure. In this paper, the theory of NICE-OHMS at atmospheric pressure is analyzed. The parametersthat affect the amplitude of the signal, including cavity length (L), modulation factor (β), and detection phase (θ), have been analyzed in order to figure out the best experimental conditions. Among them, due to the use of DeVoe-Brewer technology in NICE-OHMS, the modulation frequency (νｍ) is locked to the free spectral region (FSR) of the Fabry-Parot (FP) cavity. As a result, the cavity lengthnot only affects the NICE-OHMS signal amplitude, but also determines the value of νｍ. The results show that when the cavity length increases, the spectral components of the carrier and the sidebands overlap with each other due to the decrease of νｍ, which results in the decease of the amplitude of lineshape functions. And the amplitude of the NICE-OHMS signal at absorption phase increases gradually with the increase of the cavity length. While the amplitude at dispersion phase increases at the beginning, and reaches the maximum value when the cavity length is equal to 8 cm, then decreases with the cavity length. Modulation coefficient β affects the magnitude of laser carrier and sidebands, as consequence, affects the signal lineshape. As the cavity length increases, the β value for the maximum signal amplitude also increases. At the same cavity length, the β value for the maximum amplitude of the dispersion signal is smaller than that for absorption signal, which is easier to achieve by using an electro-optic modulator. Finally, the feasibility of the parameters has been analyzed. The half-width at half-maximumof spectrum at atmospheric is determined by the pressure broadening, which isaround 3 GHz, and the spectral coverage is larger than 10 GHz. The frequency tuning range of the distributed feedback semiconductor laser (DFB) and external cavity diode laser (ECDL) can reach to 30 GHz, whiletheirlarge laser line width deteriorates the PDH locking performance. The line width of whispering wall mode laser (WGM) and erbium-doped fiber laser (EDFL), the conventional light source in high-sensitive NICE-OHMS system, is in the order of 100 Hz. However, as so far, the frequency tuning range of NICE-OHMS system based on WGM is only 5 GHz, while that based on EDFL is only 3 GHz. When the cavity length is longer than 8 cm, a WGM laser can be used. When the cavity length is greater than 25 cm, an EDFL laser can be used. For a PZT with a flexible length of 25 μm, which is commonly used in the design of optical cavities, the frequency range of the corresponding cavity mode gradually decreases as the cavity length increases. At a typical cavity length of 40 cm, the frequency sweep range is greater than 12 GHz.
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