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We have found that the extinction ratio (ER) of an electroabsorption modulator integrated with a distributed feedback laser strongly depends on the incident power to the modulator. Our theoretical consideration based on full device simulation reveals that the temperature change in the modulator dominates the phenomenon. To obtain the intrinsic extinction characteristics of the modulator, we use microscopic theory, which takes into account quantum mechanical many-body interactions in semiconductors and makes it possible to exclude unknown experimental fitting parameters. A heat-flux analysis is performed for the calculation of the temperature distribution in the entire device. As the incident power to the modulator increases, the photocurrent also increases, leading to an increase in the temperature of the modulator by some tens of degrees Celsius by Joule heating. The increase red-shifts the absorption spectrum of the modulator, and the detuning, which is the difference between the lasing wavelength and the band-edge wavelength of the modulator, becomes small, leading to a larger ER. The calculated results are in good agreement with the experiment, showing the validity of our discussion, and indicate that the ER of electroabsorption modulators is strongly affected by the internal heating of the device.