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We investigate thermal effects in widely tunable laser transmitters based on an integrated single chip design. The chip contains a sampled-grating distributed Bragg reflector (SG-DBR) laser monolithically integrated with a semiconductor optical amplifier (SOA) and an electroabsorption modulator (EAM). The thermal impedance of the ridge structure is evaluated through simulation and experiment, and thermal crosstalk between sections is examined. Heating of the mirrors by neighboring sections is found to result in unintentional offsets in wavelength tuning. Thermal effects in the EAM are examined in depth. A positive feedback mechanism causes local temperature rise at the modulator input, with the potential to trigger catastrophic thermal runaway. A self-consistent finite-element model is developed to simulate the EAM temperature profile and device performance. This model is used to optimize the device, resulting in integrated EAMs that achieve a dissipated power limit in excess of 300mW.