Photothermal self-phase-modulation technique for absorption measurements on high-reflective coatings

We propose and demonstrate a new measurement technique for the optical absorption of high-reflection coatings. Our technique is based on photothermal self-phase modulation and exploits the deformation of cavity Airy peaks that occurs due to coating absorption of intracavity light. The mirror whose coating is under investigation needs to be the input mirror of a high-finesse cavity. Our example measurements were performed on a high-reflection SiO2-Ta2O5 coating in a three-mirror ring-cavity setup at a wavelength of 1064nm.Theoptical absorption of the coatingwasdetermined to be alpha = (23.9 ± 2.0) · 10-6 per coating. Our result is in excellent agreement with an independently performed laser calorimetry measurement that gave a value of alpha = (24.4 ± 3.2) · 10 -6 per coating. Since the self-phase modulation in our coatingabsorptionmeasurement affectsmainlythe propagationthroughthe cavityinput mirror, ourmeasurement result is practically uninfluenced by the optical absorption of the other cavity mirrors