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The presence of relatively short time constants (<20 nS) in the impulse response of the thermal chirp of directly modulated DFB lasers is shown to significantly reduce the deleterious effects of interferometric noise in fiber-optic systems with multipaths. Compared with the chirp-free case of external modulation, thermal chirp can improve crosstalk tolerance by as much as 5 dB for eight interferers at 622 Mb/s and 13 dB at 80 Mb/s. This improvement was studied both theoretically and experimentally for several lasers. Simulations results match the experimental data, while two simplified semi-analytical models provide qualitative description.