Shot-noise-limited doppler-broadened noise-immune cavity-enhanced optical heterodyne molecular spectrometry

Shot-noise-limited Doppler-broadened (Db) noise-immune cavity-enhanced optical heterodyne molecular spectrometry (NICE-OHMS) has been realized by implementation of balanced detection. A characterization of the system based on Allan–Werle plots of the absorption coefficient, retrieved by fitting a model function to data, shows that the system has a white noise equivalent absorption per unit length per square root of bandwidth of 2.3 × 10−¹³ cm⁻¹ Hz⁻¹∕², solely 44% above the shot noise limit, and a detection sensitivity of 2.2 × 10⁻¹⁴ cm⁻¹ over 200 s, both being unprecedented for Db NICE-OHMS. The white noise response follows the expected inverse square root dependence on power that is representative of a shot-noise-limited response, which confirms that the system is shot-noise-limited.