Low-Latency Short-Time Fourier Transform of Microwave Photonics Processing

Short-time Fourier transform (STFT) is a vital tool to observe spectral dynamics of non-stationary signals. Conventional STFT based on digital signal processing (DSP) usually suffers from a limited acquisition frame rate within 10 ⁶ Fourier transforms per second (FTs/s). Thus, it is unable to fully intercept transient information in time and frequency shorter than a few microseconds. Further extending the acquisition frame rate can be achieved by employing the dispersion-based all-optical spectral analysis approach, while the optical dispersion usually introduces a large latency. Here, we experimentally report a low-latency STFT of microwave photonic processing, where a passive fiber loop is used to provide high value equivalent dispersion. Consequently, the latency of the STFT processing can be greatly reduced. The spectrograms of microwave signals, including instantaneous information in both time and frequency, are acquired in a real-time manner, with an acquisition frame rate of about $5 \times {10}^8$ FTs/s and a processing latency of less than 0.2 ${{\bf \mu s}}$ . High temporal resolution within tens of nanosecond range can be achieved, allowing us to capture 30-ns duration frequency transients in real time with a instantaneous bandwidth of about 530 MHz. Our proposed STFT processing scheme is promising for real-time and high-speed spectral dynamic analysis for non-stational signals.