The cavity-less electro-optic combs (EOCs), recognized for exceptional tunability, stability and high power, are a crucial enabler for the fields such as optical communications, precision measurement and metrology, and microwave photonics. This work systematically investigates the fundamental physical factors that govern the spectral flatness via ultrafast measurements and modeling simulations. The ultrafast analysis results demonstrate that, the finite effective modulation extinction ratio of the electro-optic intensity modulators will result in generation of coherent spectral components with identical frequencies but varying phases and amplitudes in ultrashort temporal scale, finally lead to remarkable spectral interference and further intensity fluctuations across the combs spectrum. Furthermore, the established mathematical relationship between the spectral flatness and the modulation extinction ratio of the intensity modulators exhibits a nonlinear dependence up to the third order. Cascading intensity modulators has been exploited to mitigate the spectral interference and improve the modulation extinction ratio, which has been verified by using home-made high sensitive autocorrelator and frequency-resolved optical gating (FROG), and finely spectral flatness of 0.54 dB among 11 lines has been achieved, which recognized for the first time that modulation extinction ratio related spectral interference phenomenon play a subtle role in EOCs generation. Furthermore, photonic analog-to-digital converters (PADCs) have been investigated and an obvious enhancement in signal-to-noise-and-distortion (SINAD) is achieved, These findings will provide crucial theoretical and experimental support for optimizing EOCs performance, and advance the development and application.

Optical time-division demultiplexing photonic analog-to-digital converters (PADCs) represent a compelling paradigm for breaking the bandwidth–resolution trade-off of conventional electronic ADCs, yet inter-channel crosstalk induced by the finite extinction ratio of demultiplexing modulators imposes critical limitations on channel response flatness. Prior investigations have generally ascribed periodic response ripples exclusively to backend bandwidths narrower than half the total sampling rate, while the distortion behavior under wider bandwidths remains inadequately clarified. In this numerical work, we establish a full-link numerical model of a two-channel time-division demultiplexing PADC operating at a total sampling rate of 10 GS/s and systematically characterize crosstalk-induced frequency responses under four representative backend bandwidths (2.5, 5, 7.5, and 10 GHz) with an extinction ratio of 10 dB. Results demonstrate that prominent 10 GHz periodic ripples persist in the channel response even when the backend bandwidth exceeds half the sampling rate. The underlying spectral mechanism is elucidated: demultiplexed comb lines follow a 10 GHz periodic strength distribution, and sampling-generated sidebands inherit the amplitude of their parent comb teeth; the superposition of strong and weak sidebands within the passband governs the ripple magnitude. These findings furnish fundamental insights into crosstalk physics and deliver actionable guidance for device selection and system-level optimization in high-performance PADC implementations.