Ultrafast optical spectral characterization by temporal imaging systems

Abstract

Real-time characterization of spectral information in ultrafast optics is of great merit for numerous applications in both fundamental scientific research and real-world engineering. In the context of optical science, real-time spectral analysis is highly desired to trace the non-repetitive and transient optical dynamics for facilitating the understanding of the related physical mechanism. In this thesis, we start from space-time duality, the theoretical framework, to illustrate ultrafast optical spectral characterization techniques based on temporal imaging systems. Instead of relying on digital algorithm of fast Fourier transform in data post-processing to extract optical spectra, optical Fourier transform can be physically conducted in real-time based on either temporal far-field diffraction or time-lens focusing effect. On the one hand, as a direct application of temporal far-field diffraction, dispersive Fourier transform (DFT) technique performs optical Fourier transform by mapping the spectral profile of the signal-under-test onto its temporally stretched waveform envelope through chromatic dispersion. On the other hand, it is well established that a time lens can perform optical Fourier transform by focusing different frequency components of the incident light onto the different temporal locations at the temporal focal plane. In both scenarios, with the spectral information readily encoded to the temporal waveform envelope, the real-time optical Fourier analysis is thus enabled by consecutive time-domain waveform acquisition with a high-speed real-time oscilloscope. The research contribution in this dissertation is comprised of two parts. Firstly, we applied standardized DFT technique to experimentally unveil the soliton spectral dynamics triggered by the periodical soliton collision inside a mode-locked fiber laser. And the numerical simulations validate the experimental observation and provide additional insights from temporal domain into the related physical mechanism of the observed optical phenomenon. This study of soliton collision not only demonstrates the role of dispersive wave in sophisticated soliton interaction inside the laser cavity, but also strengthens the understanding on the soliton’s inherent stability. Secondly, based on time-lens focusing, two advanced temporal imaging systems are accordingly developed for the broadband real- time optical spectral analysis. One of the temporal systems, named as parametric spectro- temporal analyzer (PASTA), utilized optical parametric process to create time lens with only a single stage of four-wave-mixing, and achieved a 30-nm spectral measurement range with 48-pm resolution at tens-of-MHz frame rate. While the other system realized broadband real-time spectral analysis of 70-nm spectral measurement range with 42.5-pm resolution under the repetition rate of 10-MHz, by simply using electro-optic modulators to construct a programmable gating-assisted time lens. To conclude, this dissertation both presents the engineering design of advanced temporal imaging systems, and demonstrates the real-time spectral analysis in practical applications with ultrafast processing speed.