Fig. 1. (a) Schematic of the proposed structure of one laser unit and (b) calculated normalized optical intensity of the lasing mode along the cavity of one laser unit. (SS, side section; GS, gain section; SCH-MQW, separate confinement hetero-structure-multi-quantum well; BG, Bragg grating; BCB, benzocyclobutene; WG, waveguide).

Fig. 2. (a) Schematic of the grating designed by the REC technique and (b) sampling periods of the four in-series lasers (π-EPS: equivalent π phase shift).

Fig. 3. Picture of the DMTL chip under the microscope (SOA, semiconductor optical amplifier; LD, laser diode).

Fig. 4. (a) Microscopic top view of the DMTL COS and (b) superimposed optical spectra of all four lasers (DMTL, directly-modulated tunable in-series laser array; GCPW, ground coplanar waveguide; COS, chip on submount).

Fig. 5. (a) Optical spectra of LD1 as a function of I_GS when I_SOA and I_SS are both fixed at 30 mA and (b) optical spectra of LD1 as a function of I_SS when I_SOA and I_GS are fixed at 30 and 70 mA, respectively (I_SOA, current injected into the semiconductor optical amplifier; I_GS, current injected into the gain section; I_SS, current injected into the side section).

Fig. 6. (a) Power-current-voltage curve of LD1 with I_SOA of 10 mA and (b) laser output power as a function of the I_SOA for the four lasers with the I_GS and I_SS set at 80 and 30 mA (I_SOA, current injected into the semiconductor optical amplifier; I_GS, current injected into the gain section; I_SS, current injected into the side section).

Fig. 7. (a) Lasing spectra of the tuned 12 channels and (b) tuned temperature of the TEC (left) and SMSRs of all the 12 channels (right) (SMSR, side mode suppression ratio).

Fig. 8. (a) EO response of LD1 when I_GS is varied from 40 to 100 mA and (b) EO response of all the four lasers when I_GS is 100 mA (GS, gain section; I_GS, current injected into the GS).

Fig. 9. Measured back-to-back 25 Gb/s NRZ optical eye diagrams and 25 Gb/s NRZ eye diagrams after 10 km transmission for all 12 channels (Ch., channel; BtB, back-to-back).

Fig. 10. (a) Electrical eye diagram of the 50 Gb/s PAM4 signal after the electrical amplifier and (b) measured 50 Gb/s PAM4 optical eye diagram for channel 10.