Investigation of OSNR monitoring technique based on the beat noise detection

We propose an optical signal-to-noise ratio (OSNR) monitoring technique based on the beat noise detection. The signal under test (SUT) is filtered by a band pass filter (BPF) at the signal center frequency. The incident power of the monitor module is adjusted to keep constant. Through measuring the noise power at low radio frequencies (RF), OSNR can be monitored. The monitoring performance for 80 Gb/s non-return-to-zero differential quadrature phase shift keying (NRZ-DQPSK) signal is experimentally demonstrated. OSNR of 8~33.5 dB can be monitored with a 25.4 dB dynamic range (DR) at 100 MHz RF. The impacts of the BPF bandwidth, received power, RF position, chromatic dispersion (CD), and first order polarization mode dispersion (PMD) on the monitoring performance are experimentally investigated. The bandwidths of the receivers can be reduced to tens of MHz.