Theoretical limits of the sensitivity thresholds of intracavity laser absorption spectroscopy

A comparison is made of the ultimate capabilities of two intracavity laser spectroscopy methods in which the width of the laser emission spectrum is much greater than or much smaller than the width of the absorption lines. Several laser models are used to demonstrate that quantum noise of laser radiation ensures that the minimum absorption coefficients detectable by these two methods are the same if modulation is employed. A detailed analysis is made of the recording of the laser radiation and the total noise of the photodetector current is found more accurately allowing for the shot noise of the photodetector and also for the quantum noise of the laser output radiation. Conditions are given for optimizing intracavity laser spectroscopy from the point of view of the sensitivity threshold.