Ultra-fast optical time-domain transformation techniques

Owing to their inherent high-speed acquisition, ultra-short pulses are the major provider of big data in communications, metrology, spectroscopy, imaging and sensing. This has paved the way for various artificial intelligence and machine learning applications. The development of ultra-fast optical time-domain transformation (UO-TDT) techniques - specifically, time-stretched dispersive Fourier transform, temporal imaging and time-stretch imaging - has revolutionized photonic information acquisition. Dispersion-based frequency-to-time transformation of ultra-short pulses enables optical signal spectra to be converted into the time domain for ultra-fast spectral characterization. High-speed acquisition of information encoded on optical spectra means that new measurement techniques can effectively converge photonic and digital technologies. This Primer introduces the fundamental concepts and experimental set-up of various UO-TDT techniques with the associated transformation mechanisms. Recent advances in UO-TDT for various applications, from spectroscopy to velocimetry, and novel breakthroughs in ultra-fast imaging or quantum science are focused on. A range of experimental results are discussed, alongside an examination of reproducibility and limitations of the methods. Finally, a perspective on promising emerging techniques is provided.