Fig. 1. Transfer functions of two modulators. (a) PI curve of the directly modulated laser; (b) transfer function of MZM

Fig. 2. Theoretical diagram and experimental device for two modulation modes. (a) Schematic of direct modulation; (b) schematic of external modulation; (c) experimental setup

Fig. 3. Comparison of the link losses of the BTB links under the two modes. The inset is an expansion near the power saturation

Fig. 4. Comparison of SFDR of the BTB links under two modes

Fig. 5. Frequency responses under direct modulation with different fiber lengths. (a) BTB link; (b) 1 km fiber; (c) 6 km fiber; (d) 10 km fiber

Fig. 6. Frequency responses under external modulation with different fiber lengths. (a) BTB link; (b) 1 km fiber; (c) 6 km fiber; (d) 10 km fiber