Wang Yin, Nikodem Michal, Wysocki Gerard
Electrical Engineering Department, Princeton University, Princeton, New Jersey 08544, USA.
Opt Express. 2013 Jan 14;21(1):740-55. doi: 10.1364/OE.21.000740.
A new detection method for Faraday rotation spectra of paramagnetic molecular species is presented. Near shot-noise limited performance in the mid-infrared is demonstrated using a heterodyne enhanced Faraday rotation spectroscopy (H-FRS) system without any cryogenic cooling. Theoretical analysis is performed to estimate the ultimate sensitivity to polarization rotation for both heterodyne and conventional FRS. Sensing of nitric oxide (NO) has been performed with an H-FRS system based on thermoelectrically cooled 5.24 μm quantum cascade laser (QCL) and a mercury-cadmium-telluride photodetector. The QCL relative intensity noise that dominates at low frequencies is largely avoided by performing the heterodyne detection in radio frequency range. H-FRS exhibits a total noise level of only 3.7 times the fundamental shot noise. The achieved sensitivity to polarization rotation of 1.8 × 10(-8) rad/Hz(1/2) is only 5.6 times higher than the ultimate theoretical sensitivity limit estimated for this system. The path- and bandwidth-normalized NO detection limit of 3.1 ppbv-m/Hz(1/2) was achieved using the R(17/2) transition of NO at 1906.73 cm(-1).
本文提出了一种用于顺磁分子物种法拉第旋转光谱的新检测方法。使用外差增强法拉第旋转光谱（H-FRS）系统，在无需任何低温冷却的情况下，展示了在中红外波段接近散粒噪声极限的性能。进行了理论分析，以估计外差和传统FRS对极化旋转的最终灵敏度。使用基于热电冷却的5.24μm量子级联激光器（QCL）和碲镉汞光电探测器的H-FRS系统对一氧化氮（NO）进行了传感。通过在射频范围内进行外差检测，很大程度上避免了在低频占主导的QCL相对强度噪声。H-FRS的总噪声水平仅为基本散粒噪声的3.7倍。实现的对极化旋转的灵敏度为1.8×10^(-8) rad/Hz^(1/2)，仅比该系统估计的最终理论灵敏度极限高5.6倍。利用1906.73 cm^(-1)处NO的R(17/2)跃迁，实现了3.1 ppbv-m/Hz^(1/2)的路径和带宽归一化NO检测极限。
