Versatile Photonic Spectrograms for Ultrafast Real-Time Broadband Microwave Signal Analysis

Joint time-frequency representations (JTFRs), such as the spectrogram, provide key information about the spectral evolution of a system. This is essential for instance to analyze telecommunication channels or sensing targets. Modern electronics cannot obtain JTFRs in real time without gaps for microwave signals with tens of gigahertz of instantaneous bandwidth and nanosecond temporal features. Previous photonics approaches are rigid in their permissible design specifications and cannot simultaneously address all performance requirements. We demonstrate the connection between two real-time, gapless, analog photonics spectrograms with versatile performance exceeding any previous technique, electronic, or otherwise. The time-lens spectrogram (TLS) and the Talbot array illuminator spectrogram (TAIS) are implemented via linear phase transformations using identical hardware to time-map the dynamic spectral information of microwave signals. The TLS enables analysis over broad bandwidths (>100 GHz) with sharp time resolutions (tens of picoseconds), but a limited number of frequency analysis points. Conversely, the TAIS is limited in bandwidth and time resolution but allows for hundreds of frequency analysis points, enabling sub-gigahertz resolution. With these spectrograms, we measure high-speed transients with sub-nanosecond time resolution, multichannel nanosecond frequency hopping communication signals, and signals with bandwidths much higher than that of the detector and analog-to-digital converter employed.