We suggest a compact and efficient duplexer using the multimode interference (MMI) effect and the extraneous self-imaging phenomenon for gigabit-capable passive optical network and gigabit Ethernet passive optical network applications. To experimentally evaluate the suggested duplexer, silica-based MMI couplers were tested using light sources with wavelengths of 1310 and $1490\mathrm{nm}$ . From the ex perimental results, this device showed relative output powers of $-0.3$ and $-0.7\mathrm{dBm}$ for the light sources with 1490 and $1310\mathrm{nm}$ wavelengths, respectively. The return power was measured to be less than $-40\mathrm{dBm}$ .