The extinction ratio (or contrast) is the ratio between the optical power transmitted at the output ofthe interferometer in the ON state (Pmax) and the interferometer in the OFF state (Pmin) 

ER (dB) = 10 log Pmax/Pmin

The extinction rate can be degraded if the two arms of the Mach-Zehnder interferometer are not strictly identical (loss imbalance) or if there is a coupling of evanescent waves between the optical modes going through the two arms of the interferometer. The multimodality of the waveguides is also an important factor in the degradation of the extinction rate. In general, sensitivity to the fabrication process is often at the root of these problems.

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