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Measuring the shift in resonant frequency of a coated tapered optical fiber can be used to estimate the dramatic increase in the temperature at the end of the optical fiber when the light is confined at the fiber apex. The resonance frequency of the tapered fiber was measured as a function of the fiber length and the coating profile. The good agreement between the experimental values and the mechanical model based on a finite element method allows one to propose a simple empirical approximation of the frequency shift. Varying the injected power in the fiber was found to shift the fiber frequency due to the thermal expansion and temperature dependence of Young’s modulus of both the metal coating and the fiber core. A thermal model for the temperature distribution along the fiber is proposed and was found to agree with the experimental values when the temperature does not exceed 200 °C. Above 200 °C, the nonlinearity between the frequency and the injected power is correlated to the coating damage observed by electron microscopy. This behavior has been applied to investigate the phase transition in a thermoplastic polymer as a function of the temperature of the fiber apex. © 2004 American Institute of Physics.