Fig. 1. Diagram of the few-mode transmitter. DML, directly modulated laser. MMI, multimode interference coupler. W is the width of the MMI section. Lπ is the beat length of the two lowest-order modes. W₁ and W₂ are the widths of Port4 and Port3, respectively.

Fig. 2. Optical fields of the MMI-based mode converter/multiplexer. (a), (b) in the X-Z plane and (c), (d) in the X-Y plane.

Fig. 3. Epitaxial structure of the few-mode transmitter. SCH, separate confinement heterostructure; QW, quantum well; QB, quantum barrier.

Fig. 4. PL spectrum of the material. The PL wavelength for the passive and active sections were around 1.2 and 1.55 μm, respectively.

Fig. 5. SEM pictures of (a) the butt-joint interface and (b) MQWs, and (c) the microscope picture of the transmitter.

Fig. 6. Near-field pattern of (a) TE0 mode and (b) TE1 mode measured at the common output Port3 of the transmitter, and the excited (c) LP01 mode and (d) LP11 mode measured from a 200 m two-mode fiber, respectively.

Fig. 7. PIV curve, spectrum, and frequency response characteristic of DML1 (TE1 channel). (a) PIV curve, (b) optical spectrum, (c) −3  dB bandwidth.

Fig. 8. PIV curve, spectrum and frequency response characteristic of DML2 (TE0 channel). (a) PIV curve, (b) optical spectrum, (c) −3  dB bandwidth.

Fig. 9. 10 Gbit/s eye diagram of the (a) TE0 mode channel and (b) TE1 mode channel.

Fig. 10. BER curves at 10 Gbit/s for the TE0 mode channel and TE1 mode channel.