Browsing by Author "Viljoen, Ruan"

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    Single-beam coherent anti-Stokes Raman scattering combining polarization shaping with spectral focusing
    (Stellenbosch : Stellenbosch University, 2021-03) Viljoen, Ruan; Rohwer, Erich G.; Neethling, Pieter H.; Stellenbosch University. Faculty of Science. Dept. of Physics.
    ENGLISH ABSTRACT: Single-beam coherent anti-Stokes Raman scattering (SB-CARS is a compact approach to CARS which employs a single broadband laser source to probe molecular vibrations. A variety of light sources can be used, one of which is a photonic crystal fibre (PCF pumped by a femtosecond laser. The SB-CARS setup presented herein utilises a polarization main-taining all-normal dispersion PCF (PM-ANDi-PCF pumped by a femtosecond oscillator. The oscillator produces 80 fs pulses with a low average power of 1W at a repetition rate of 80 MHz. When these pulses are used to pump the PM-ANDi-PCF, supercontinuum pulses are produced with a much broader bandwidth (150 nm at the same repetition rate. The supercontinuum pulses are pulse-to-pulse phase stable with high peak intensi-ties, perfect for fast acquisition of spectra in applications of non-linear spectroscopy and microscopy. As a result of using a broadband laser source, the produced CARS spectrum is also broad and dominated by a non-resonant background which is generated by a four-wave mixing non-linear process. The SB-CARS setup utilises an active shaping approach which allows for a multitude of approaches, that can reduce the non-resonant background, to be implemented within the same setup. A spatial light modulator in a 4-f shaper affords the ability to shape the supercontinuum pulses at will and also enables the integration of pulse characterisation techniques into the same SB-CARS setup. Implementation of a phase-only temporal ptychography technique, i2PIE, in a SB-CARS setup is reported on for the first time. Pulse characterisation is a vitally important component of this setup, as produced supercontinuum pulses are initially temporally broad, because of dispersion in the optical fibre, and need to be compressed for applications in spectroscopy. Comparison of i2PIE to another phase-only technique, multiphoton intrapulse interference phase scan (MIIPS, shows remarkable improvements in compression. Second harmonic spectra generated in a beta-barium borate crystal when the supercontinuum pulses are compressed using phase reconstructed with i2PIE and MIIPS, showed an increase of integrated spectral intensity by a factor of 4. When applied to SB-CARS measurements, the integrated intensity of SB-CARS spectra increased by a factor 6.5 when i2PIE was used. The improvement was not limited to spectral intensity yield, but also showed an improvement of signal-to-background in extracted CARS spectra by a factor of 4 when i2PIE was used. Different SB-CARS strategies were implemented in the same setup and evaluated through simulation and experimentation to illustrate the versatility of the SB-CARS apparatus. A new spectral focusing SB-CARS approach that utilises the polarization shaping and phase shaping capabilities of an SLM, is introduced and compared to other SF techniques. Non-resonant background is reduced in this technique by using a narrow probe decoupled from the pump and Stokes fields. Spectral focusing is achieved with phase shaping of the pump which focuses the bandwidth to mostly target a single Raman resonance. Results from spectral measurements demonstrate that the new technique (PP-QPSF) yields spectra with an overall increased S:B. This new technique is evaluated spectroscopically and shows promise for an application in SB-CARS microscopy.
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    Supercontinuum pulse characterisation and compression
    (Stellenbosch : Stellenbosch University, 2017-03) Viljoen, Ruan; Rohwer, Erich G.; Neethling, Pieter H.; Stellenbosch University. Faculty of Science. Dept. of Physics
    AFRIKAANSE OPSOMMING : In hierdie projek word die infrastruktuur van 'n hoë-resolusie nie-lineêre mikroskoop ontwikkel. Spesifiek word daar gekyk na generering van breëband lig deur middel van 'n fotoniese-kristalvesel (PCF) en 'n puls kompressor. Polarisasie en drywing karaktereienskappe van die PCF word ondersoek om optimale werks omstandighede vas te stel. Vir hierdie optimale werks omstandighede benodig ons breëband lig met die hoogste drywing deursette. Afhangend van die pomp drywing, kan die bandwydte van die lig van 12 nm tot sowat 200 nm verbreed. Dit stel ons instaat om die pomp pulslengte van 80 fs na onder 10 fs saam te pers. Vir die doeleindes van die projek is die 'multiphoton intrapulse interference phase scan' of MIIPS die ideale metode om ons breëband pulse te karakteriseer en saam te pers. Ons wys dat ons die toepaslikheid van hierdie metode numeries en eksperimenteel getoets het. Ons het vasgestel vanaf ons simulasies dat ons in staat is om die fase verhoudings van spektrale komponente vas te stel. Eksperimente wat uitgevoer is met lig met 'n breëband van rondom 80 nm wys dat die MIIPS algoritme in staat is om die superkontinue pulse se fase vas te stel en in die proses die pulse saam te pers. `n Meting van die GVD van 'n blokkie SF6 glas met 'n bekende dispersie is gebruik as bykomende inligting om die effektiwiteit van MIIPS te illustreer. 'n SLM IFROG meting is uitgevoer op die kort pulse as 'n toets hoe kort die pulse is na 'n MIIPS meting. Hierdie metode het gewys dat die pulse lengte van die pikosekonde regime na ∼ 23 fs saam gepers is. Hierdie pulse is naby aan die Fourier limiet van ∼ 12 fs vir hierdie bandwydte.