Asymmetric degradation of amplitude and phase detection under laser noise in FM spectroscopy

In frequency modulation (FM) spectroscopy employing IQ demodulation, phase detection under ideal conditions typically affords a superior signal-to-noise ratio (SNR) relative to amplitude detection. Here, however, we report a marked asymmetry in the presence of laser phase noise: the phase channel exhibits substantially more rapid SNR degradation than does the amplitude channel. Although phase detection resolves hyperfine transitions in rubidium vapor with high sensitivity under stable operation, the onset of a noisy mode reverses this hierarchy, causing severe deterioration of the phase spectrum while the amplitude spectrum retains a higher apparent SNR but may exhibit additional features that originate from intrinsic laser characteristics rather than atomic transitions. An analysis of the integration inherent in IQ demodulation reveals the underlying mechanism: amplitude detection performs a vector average robust to zero-mean phase fluctuations, merely scaling the signal by a coherence factor; phase detection, by contrast, computes a ratio of averaged quadratures whose variance diverges as phase fluctuations increase. Collectively, this work establishes the superiority of phase detection under stable conditions, identifies its vulnerability to phase noise, and furnishes a theoretical framework for the observed asymmetric degradation.