The fiber-to-the-home (FTTH) systems are growing fast these days, where two different wavelengths are used for
upstream and downstream traffic, typically 1310nm and 1490nm. The duplexers are the key elements to separate these
wavelengths into different path in central offices (CO) and optical network unit (ONU) in passive optical network (PON).
Multimode interference (MMI) has some benefits to be a duplexer including large fabrication tolerance, low-temperature
dependence, and low-polarization dependence, but its size is too large to integrate in conventional case. Based on the
silicon photonics platform, ultra-short silicon MMI duplexer was demonstrated to separate the 1310nm and 1490nm
lights. By studying the theory of self-image phenomena in MMI, the first order images are adopted in order to keep the
device short. A cascaded MMI structure was investigated to implement the wavelength splitting, where both the light of
1310nm and 1490nm was input from the same port, and the 1490nm light was coupling cross the first MMI and output at
the cross-port in the device while the 1310nm light was coupling through the first and second MMI and output at the
bar-port in the device. The experiment was carried on with the SOI wafer of 340nm top silicon. The cascaded MMI was
investigated to fold the length of the duplexer as short as 117μm with the extinct ratio over 10dB.