Measuring small absorptions by exploiting photothermal self-phase modulation

We present a method for the measurement of small optical absorption coefficients. The method exploits the deformation of cavity Airy peaks that occur if the cavity contains an absorbing material with a nonzero thermorefractive coefficient dn/dT or a nonzero expansion coefficient [a.sub.th]. Light absorption leads to a local temperature change and to an intensity-dependent phase shift, i.e., to a photothermal self-phase modulation. The absorption coefficient is derived from a comparison of time-resolved measurements with a numerical time-domain simulation applying a Markov-chain Monte Carlo algorithm. We apply our method to the absorption coefficient of lithium niobate doped with 7 mol. % magnesium oxide and derive a value of [[alpha].sub.LN] = (5.9 [+ or -] 0.9) x [10.sup.-4]/cm. Our method should also apply to materials with much lower absorption coefficients. Based on our modeling, we estimate that, with cavity finesse values of the order of [10.sup.4], absorption coefficients of as low as [10.sup.-8]/cm can be measured. OCIS codes: 120.0120, 120.5060, 120.6810.