Quantum Limits and Robustness of Nonlinear Intracavity Absorption   Spectroscopy
  We investigate the limits of intracavity absorption spectroscopy with nonlinear media. Using a common theoretical framework, we compare the detection of a trace gas within an undriven cavity with gain near and above threshold, a driven cavity with gain kept just below threshold, and a cavity driven close to the saturation point of a saturable absorber. These phase-transition-based metrology methods are typically quantum-limited by spontaneous emission, and we compare them to the empty cavity shotnoise-limited case. Although the fundamental limits achievable with nonlinear media do not surpass the empty cavity limits, we show that nonlinear methods are more robust against certain technical noise models. This recognition may have applications in spectrometer design for devices operating in non-ideal field environments. 
John K. Stockton and Ari K. Tuchman