Quasi-critical coupling between spatially resolved laser eigenstates: A novel approach to the measurement of intracavity absorption

A novel method of intracavity absorption measurement is demonstrated that is linear in absorber concentration. Strong competition between spatially resolved laser eigenstates is theoretically and experimentally shown to provide conveniently controlled and highly efficient optical leverage for intracavity sensing. In a first experimental realization, a He-Ne laser at 3.39 mm demonstrates a sensitivity of 3 3 109 molecules of methane in vacuum in the volume of the beam in the absorption cell, which corresponds to 0.6 part per billion times meters of CH4 in 1 atm of air. Good agreement is obtained between the experimental results and the theoretical predictions from a model of the laser dynamics based on Landau potentials associated with Lamb’s equations.