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Fiber-optics reconfigurable temporal intensity optical pulse shaping using chromatic dispersion: methods and applications.


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Huh, Jeong Hyun (2018). Fiber-optics reconfigurable temporal intensity optical pulse shaping using chromatic dispersion: methods and applications. Thèse. Québec, Université du Québec, Institut national de la recherche scientifique, Doctorat en télécommunications, 166 p.

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Optical pulse shaping methods have been developed over the past decades as a key enabling technology for ultrafast science and technology. These techniques enable precise synthesis and control of the temporal shape of optical pulses with durations down to the femtosecond regime, allowing high-speed processing of optical signals without the need of an optical to electrical converter (OEC), as well as generation of ultrafast arbitrary optical waveforms according to user specifications. The latter is of great interest for a broad range of applications, including ultrafast optical telecommunication and information processing, spectroscopy, nonlinear fiber optics, and high-field physics, among many others. The focus of this thesis is on picosecond pulse shaping, and the methods at the core of the work are mainly based on linear photonics signal processing since this approach provides not only simple configurations but also higher energy efficiency than nonlinear photonics signal processing and superior noise performance. In a linear optical pulse shaper, arbitrary waveforms are typically synthesized based on a direct Fourier-domain technique, where the spectrum of a short optical pulse is re-shaped according to the Fourier transform of the target temporal waveform. A customized optical filter is used for this purpose, with a linear spectral response that is simply obtained from the relationship between the input optical spectrum and the target. However, the direct Fourier-domain synthesis method has two critical limitations: First, the maximum duration of the generated pulses is limited by the finest frequency resolution of the pulse shaper or filter, typically below ~10 GHz and shorter than 100 ps for the maximum duration. Moreover, the spectral coverage of the output pulse is dictated by the target pulse duration and as such, for synthesis of long temporal waveforms, a large portion of the input pulse spectrum, i.e., its energy, may need to be filtered out. Thus, the energy efficiency of the process is severely reduced for the synthesis of longer pulse durations, an issue that can be of critical importance for many applications, particularly those that exploit nonlinear effects. The photonic implementation of the spectral filtering stage is another main topic of research. Previously reported photonic signal processors are usually designed to perform a specific function with none or very limited programmability. For general-purpose signal generation and processing, however, a photonic signal processor should be able to perform multiple functions with high reconfigurability. The design concept of the most widely used reconfigurable optical pulse shaper is based on free-space optical processing. In this approach, the spectral components of an incident optical pulse are first decomposed angularly (e.g., by bulk-optics diffraction gratings), focused, and then linearly modulated by spatial amplitude and/or phase masks. By using programmable spatial light modulators (SLMs) or acoustic-optic modulators (AOMs), reconfigurable optical pulse shapers have been successfully demonstrated with resolutions well into the femtosecond range. Central constraints of this approach include the need for an efficient coupling between fiber and free-space optics, and the requirement for multiple spatially-separated modulation points, which translate into a relatively lossy and bulky configuration. In addition, the approach is constrained by the limited update rate of SLMs, typically below the kHz range for liquid-crystal SLMs and a few tens of kHz for acoustic-optic modulators (AOMs). The main objective of this Thesis is to develop linear optics methods for high-speed reconfigurable temporal intensity optical pulse shaping systems in order to overcome the limitations of conventional approaches and explore innovative applications. Generally, the proposed techniques in this Thesis are based on fiber-optics dispersion-induced frequency-to-time mapping (FTM) combined with spectral amplitude and/or phase modulation of either coherent or incoherent light waves. Dispersion induced FTM is the time-domain analogue of spatial-domain Fraunhofer diffraction in the far-field, where the input signal‟s frequency components are linearly distributed along the time-domain by the mere process of linear chromatic dispersion. In Chapter 2, a novel linear-optics method for reconfigurable optical pulse shaping based on dispersive FTM is proposed for overcoming the above-mentioned critical limitations of the direct Fourier-domain technique. This method can provide a greatly increased flexibility to achieve a desired pulse duration, independently of the frequency resolution of the linear spectral shaping stage, while enabling an optimization of the energy efficiency of the shaping process for any target pulse duration. High-quality parabolic optical pulses with a temporal duration ranging from ~25 ps to ~400 ps are generated using the proposed optical pulse shaping method, and these are utilized for realization of an optically programmable time-lens (an important basic building block for temporal optical signal processing) through nonlinear cross-phase modulation (XPM). In Chapter 3, an all-fiber simple configuration is demonstrated for fully arbitrary (including asymmetric) programmable picosecond optical pulse shaping in the picosecond regime. The method is based on multi-level phase-only filtering, which is programmed directly in the time domain using a single high-speed electro-optic phase modulator. This time-domain optical pulse shaper provides the unique, additional capability of achieving high-speed pulse-shape update rates into the sub-GHz range, and solid potential for integration. Chapter 4 deals with an incoherent light temporal shaping system where the temporal shape of the output waveform is determined by the cross-correlation between the spectrum of a broadband incoherent light and a temporal intensity modulation pattern, exploiting the concept of dispersion-induced time-spectrum convolution. The process is specifically designed and developed for demonstration of a practically relevant application, namely reconfigurable real-time identification of optical spectrum patterns, e.g., for dynamic spectroscopy or related uses. This operation is implemented by simply programming the temporal modulation signal to match the target spectral pattern and, in the proof-of-concept experiments, near-infrared spectral pattern recognition is achieved directly in the optical domain without the need for any further numerical post-processing at an update rate of 650 kHz, over a bandwidth of 1.5 THz with a spectral resolution of ~12 GHz. The conducted research can provide useful guidelines for practical realization of dispersion-based optical pulse shaping technologies as well as contribute to the development of novel fiber-optics photonics signal processors and their numerous applications.

Type de document: Thèse Thèse
Directeur de mémoire/thèse: Azaña, José
Mots-clés libres: optique linéaire; intensité temporelle d'impulsions optiques; OPS; optical pulse shaping
Centre: Centre Énergie Matériaux Télécommunications
Date de dépôt: 04 mars 2019 16:01
Dernière modification: 04 mars 2019 16:01
URI: https://espace.inrs.ca/id/eprint/7870

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