Applications and Optimization of Optical Time Lenses based on Four-Wave Mixing in Highly Nonlinear Fibre

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Optical Fourier transformations enabled by the versatile time lens (quadratic phase modulator), have been demonstrated for numerous optical signal processing applications. Applications include ultrafast optical oscilloscopes, high resolution spectralanalysers, and the processing of ultrahigh-speed communication signals, to enable e.g. such varied applications as phase regeneration for wavelength-division multiplexing (WDM) signals, conversion between spectrally efficient formats and receivers with reduced complexity for advanced optical multiplexing formats. Four-wave mixing (FWM) is showing promise as a method to implement time lenses, for its ability to impart a parabolic phase profile with high fidelity, and to transparently process and preserve arbitrary amplitude and phase information at ultrahigh data rates. Many time lens demonstrations have been based on FWM with linearly chirped pump pulses in highly nonlinear fibre (HNLF), due to the low fibre losses and high attainable efficiency. However, HNLF tend to exhibit random dispersion fluctuations which can reduce the FWM efficiency severely, and no two HNLFs are exactly alike. So far, no simple and generic method for systematic performance characterization of HNLF for FWM with chirped pump pulses have been proposed. Hence, time lenses have been optimized using complex systems on an ad-hoc basis. Such optimizations are non-generic and time consuming, and have to be repeated for every new experiment. The work presented in this thesis concerns the optimization, characterization anddemonstration of FWM-based time lenses in HNLF for broadband processing of highspeed communication signals. The first part of the work involves detailed FWM characterizations of different HNLF variations for continuous-wave (CW) and pulsedpumps, which leads to recommendations of HNLF for different applications, and to a novel generic method based on only two tunable CW lasers, which allows for accurate prediction of the FWM performance in HNLF with chirped pump pulses.Then, a composite dispersion-flattened HNLF (DF-HNLF) is proposed and assembled to mitigate the effects of dispersion fluctuations on the FWM performance, with the aim of enabling broadband and efficient time lens operation. The fibre is demonstrated for time lens processing of all 32 channels in a 1.6 THz input WDM superchannel with uniform efficiency. The last part of the thesis begins with a detailed numerical investigation and experimental demonstration of a novel all-optical orthogonal frequency-division multiplexing (OFDM) receiver based on spectral magnification, which allows for direct bandpass filtering of the OFDM subcarriers, in a manner similar to standard WDM receivers. Finally, a matchedpair of composite DF-HNLF are used for the first demonstration of time lenses for advanced optical modulation formats. In this demonstration we perform 4×spectral magnification for quadrature amplitude modulation (QAM) signals up to16-QAM, with >18 nm operational bandwidth observed. The bit-error rate measurements indicate that cascaded time lenses based on FWM may be suitable for higher order modulation formats.
Original languageEnglish
Number of pages112
Publication statusPublished - 2017

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