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Asynchronous linear optical sampling for monitoring impairments in multilevel signal modulation format generation

We propose a general method to monitor the impairments in high speed multilevel signal modulation format generation induced by improper setting and time-drifting of modulator bias offset based on signal constellation diagram. Using asynchronous linear optical sampling technique, we can obtain both the amplitude and phase information of the signal under test with a high time resolution and data speed transparency. We successfully recovered the constellation diagrams of 10Gbaud quadrature phase shift keying (QPSK) after a series proposed signal processing algorithms. Several common types of constellation distortion were observed with the manual tuning of the modulator bias. It proved the effectiveness of the proposed method.