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We report on the use of butterfly-type fiber-Bragg-grating external cavity semiconductor laser (FGL) modules in dense wavelength-division multiplexing (DWDM) applications. In a dense wavelength-multiplexing experiment, we demonstrated that successive four-channel multiplexing with 25-GHz spacing and with almost the same peak power was realized by using the FGLs. The lasing wavelength of each channel was tuned to the corresponding wavelength grids with an accuracy of ±1 pm. In the DWDM transmission with 25-GHz channel spacing and 2.5-Gb/s direct modulation using a standard single mode fiber (SMF), a good bit-error ratio (BER) performance without floor phenomenon was achieved up to 300 km. There was no degradation of transmission characteristics caused by the optical crosstalk from adjacent channel. Even in the DWDM transmission where channel spacing was 12.5 GHz, good transmission characteristics were maintained up to 300 km and little degradation of BER performance occurred at 300 km compared with the single-channel transmission. In addition, we investigated the influence of external temperature change on the FGL reliability using temperature-cycling test. As a result, we clarified that the characteristic degradation of FGL after this test was sufficiently small and that this device had a high tolerance to the severe external temperature variation.