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Optoelectronic devices are particularly sensitive to temperature changes induced by the absorption of light and the passage of current. In order to study the thermal issues arising in a InGaAsP-based Mach-Zehnder (MZ) optical modulator, a nonlinear finite-element thermal model of the device was constructed. The model considers the variation with temperature of both the thermal conductivity of the semiconductors composing the device and the optical absorption. To that effect, the optical absorption was measured inside the waveguide as a function of temperature, An experimental method using liquid crystals to measure the surface temperature was also developed. Both were used to evaluate the temperature inside a variable optical attenuator present on the modulator. Good agreement with the model and the experiment is found over a wide range of operating conditions. These tools are expected to play a key role in understanding thermal issues in future photonic devices, in view of the desire to integrate multiple devices on a common substrate and the continuous increase of the optical powers in fiber systems.