Effects of the Nonlinearity Caused by 'MZM-WDM' Structure in Time-Wavelength Interleaved Photonic Analog-to-Digital Converters.

Abstract

The effects of the nonlinearity caused by the structure of ‘Mach-Zehnder modulator (MZM) - wavelength division demultiplexer (WDM)’ in Time-Wavelength Interleaved Photonic Analog-to-Digital Converters is studied for the first time. Even if the MZM is biased at quadrature, the narrow passband width of the WDM after modulation may result in the presentation of even harmonics in the spectrum of the sampling results. As the frequency of the signal being sampled increases and the passband width of the WDM decreases, the magnitudes of the even harmonics increase. As a result, the spur-free dynamic range (SFDR) would be determined by the magnitudes of the signal and the 2nd harmonic instead of the 3rd harmonic. When the passband width of the WDM is less than 100 GHz, the variation of the SFDR can be as much as 6 dB over 0~25 GHz-frequency range of the signal being sampled. Theoretical analysis is verified by the simulation and experimental results. When the passband width of the WDM is greater than 150 GHz, the SFDR over 0~25 GHz-RF range is always dominated by the 3rd harmonic. When the passband width is 50 GHz, the SFDR is dominated by the 2nd harmonic for RF frequencies above 10 GHz, and can be as much as 20 dB higher than the 3rd harmonic term at 25 GHz-RF frequency. The requirement for the passband width of the WDM can be specified according to the frequency range of the signal being sampled to avoid the presentation of even harmonics and optimize the design of TWIPADCs.