Real-time ultrafast spectral characterization of supercontinuum

Abstract

Supercontinuum (SC) source, which exhibits an ultra-wide optical bandwidth, has been advancing innovations in a wide range of photonic applications. Since its first discovery in 1970, experimental demonstration of shot-to-shot broadband spectral characterization has been extremely difficult due to the lack of high-speed spectral measurement techniques. Stochastic numerical simulation has thus been frequently used as an alternative approach to investigate the noise-induced spectral intensity fluctuations and phase term of single shot SC. Traditional experimental approaches are only able to measure the averaged SC spectra at the speed of about 1000 fps for array-based spectrometers. Therefore they are only capable of capturing the average of SC spectra. This partially explains why the properties of shot-to-shot SC spectral stability have long been missing in most commercial laser source specifications.

Until nowadays, the physics of SC generation processes is almost clearly understood from the established theoretical models and experiments in a wide range of optical media. The well-known intertwined dispersion and nonlinearity render the laser output broadened and distorted respectively to form white light, generally known as SC.

However, its broadband spectra will not trivially benefit the optical technologies that focuses on ultrafast non-repetitive phenomena, unless the properties of the intrinsic shot-to-shot spectral fluctuations and spectral coherence of SC are fully characterized. This is particularly true for the SC source used in ultrafast imaging and spectroscopy to study the non-repetitive dynamics.

Driven by the intense interest to stabilize the shot-to-shot fluctuations and improve the coherence of SC, several seeding methods have been proposed to counteract with the noise-driven effects at the onset of SC generation, ranging from pulse seeding to continuous wave (CW) seeding. Such investigations of the dynamical spectral broadening enhance our understandings of the noise role in SC generation and more importantly help us to search for the clues of realizing stable SC source.

Given the research and knowledge gaps in the experimental study of the shot-to-shot broadband spectral characterization of CW-seeded SC, this thesis work aims to employ various statistical spectral characterization methods for the study of real-time shot-to-shot CW-seeded SC. The thesis is organized in the following chapters. Chapter 1 elaborates the motivation of stable SC and ultrafast optical spectral measurement techniques. The chapter also reviews the key applications of OTS techniques and highlights the relevant SC seeding mechanisms. Chapter 2 introduces the statistical spectral characterization methods which will be used as the SC performance metrics. Chapter 3 illustrates the real-time ultrafast spectral characterization experiments and discusses the different SC seeding effects in terms of the performance metrics discussed previously. Chapter 4 describes a new method which could be a potential candidate for measuring SC coherence in real-time at ultrafast acquisition speed. Lastly, Chapter 5 concludes the simulations and experiments, summarizes the new experimental discoveries and their significance brought by the ultrafast spectral analysis enable by OTS technique.