Dynamics of light-induced thermomechanical mirror deformations in high-finesse Fabry–Perot microresonators

Light recirculating inside an optical resonator can spontaneously trigger persistent cyclical thermomechanical mirror deformations. We have observed and characterized the dynamics of such deformations in high-finesse Fabry–Perot microcavities built following three distinct designs. The designs differed in form factor and confinement geometry, incorporating either (A) two nominally planar bulk mirrors, (B) one planar and one microconcave bulk mirror, or (C) one microconcave bulk mirror and one microconcave mirror at the tip of an optical fiber. For all cases, the cavity transmission exhibited bistability and high-frequency (∼MHz), high-amplitude pulsations for input powers greater than ≈10  mW at a cavity finesse of ≈30000. A theoretical analysis reveals that these pulsations are the result of the competition between photothermal expansion and photothermal refraction in the mirror coatings that induces “slow-fast” dynamics. We model these dynamics for the three different cavities and show that their varying duty cycles and periods are consistent with light-induced heating and heat dissipation conforming to the cavity mode spot size and the mechanical mirror support structure.