Integrated In-Band OSNR Monitor Based on Asymmetrical Parallel-MZIs for WDM signals

We propose a novel in-band optical signal-to-noise ratio (OSNR) monitor based on asymmetrical parallel Mach-Zehnder interferometers (A-PMZI). By exploiting two different lengths of delay arms in the A-PMZI, the monitor requires no prior knowledge of the noise-free coherence properties of the signal. The A-PMZI is fabricated on a SOI platform and demonstrated for OSNR monitoring of 4 × 28 GBd NRZ-QPSK WDM signals. Experimental results show that OSNR measurements from 7.7 to 24.8 dB within error of ±0.5 dB are achieved for all four channels. Due to the advantages of high fabrication tolerance and broad bandwidth of multimode interferometer couplers in the A-PMZI, the monitor works well in the whole C-band. Moreover, the monitor is insensitive to input optical power and chromatic dispersion.