  On-chip high extinction ratio Mach-Zehnder interferometers (MZI) have always
attracted interest from researchers as it can be used in many applications in
astrophotonics, optical switching, programmable photonic circuits, and quantum
information. However, in previous research studies, ultra-high extinction ratio
on-chip MZIs have only been achieved by using a multi-stage MZI approach. In
this paper, we investigate a high extinction ratio single-stage MZI based on
two cascaded multimode interferometers (MMI). We determine that TM noise is an
important factor that can prevent us from achieving a high extinction ratio
MZI. By introducing a bend-based TM filter without additional loss, we
experimentally demonstrate that such a TM filter can improve the maximum
extinction ratio of the MMI-MZI by more than 10 dB. With the TM filter, we
report a record high 61.2 dB extinction ratio in a single stage, thermally
tunable MMI-MZI with only 1.5 dB insertion loss and more than 60nm bandwidth.
These results pave the way for many interesting applications.
