CW Stimulated Raman Scattering Generation and Phase-Locking of Raman Comb using Hypocycloid-shaped Kagome HC-PCF

  • Meshaal Alharbi

Student thesis: Doctoral ThesisPhD


This thesis presents several milestones towards the development of an all-fibre photonic waveform synthesiser. The synthesiser design relies on the generation of multiple-octave wide coherent Raman comb in hydrogen confined in hollow-core photonic crystal fibres (HC-PCF), with the ultimate aim to generate the comb of continuous wave (CW) spectral components, thus paving the way to optical wave synthesis with a comparable control as in electronics. The milestones achieved in this thesis entitle the development of new state-of-the-art HC-PCF, the first demonstration of intra-pulse waveform synthesis using transient stimulated Raman scattering (SRS), strong Stokes generation in the CW regime and finally a report on novel dynamics of the hydrogen molecules that enabled the generation of high power Stokes with ultra-narrow linewidth. Within the continuous endeavour in the group of GPPMM, the development of hollow core photonic crystal fibres (HC-PCF) is an essential key element towards the realisation and the development of an optical waveform synthesiser based on the generation and the synthesis of optical frequency combs using stimulated Raman scattering (SRS) process in gas filled HC-PCF. Two types of HC-PCF are developed and fabricated for this objective. The first type is the photonic bandgap (PBG) HC-PCF that is used as a host for selective generation of rotational first-order Stokes in the first stage of the waveform synthesis process due to its low optical loss and narrow transmission bandwidth. The second type is the Kagome lattice HC-PCF, which is used in the second stage of waveform synthesis due to its ultra-broadband transmission. The guidance mechanism of this type of fibre, known as Inhibited Coupling (IC), is examined, through the study of the effect of the newly introduced hypocycloid core shape on confinement loss theoretically and experimentally. A reduction in optical loss figures from a typical value of ~100 dB/km down to ~17 dB/km is achieved by enhancing the negative curvature of the core-contour. In addition, a systematic theoretical and experimental study on the effect of the number of cladding rings upon the confinement and bending loss in Kagome HC-PCF is performed to gain a thorough understanding of the IC guidance mechanism. The two developed fibres have enabled the development of an all-fibre based system, where the generation of intra-pulse periodic train pulse waveform with 17.6 THz repetition rate and ~ 26fs pulse duration are demonstrated. This was achieved by generating a Raman comb using a compact HC-PCF based system and a micro-chip pulsed laser. The experimental parameters were engineered so the Raman process is in a highly transient regime so to amplify from the quantum noise only a single spatio-temporal mode (STM) to the macroscopic level. We experimentally demonstrated the role of this STM amplification in enhancing the phase-locking of the comb spectral components, and subsequently the intra-pulse optical waveform generation. Furthermore, the results show a long lived persistence of the Raman coherence, thus hinting to a possible pulse-to-pulse mode locking. Such findings make this novel Raman excitation an ideal possible alternative to high harmonic generation (HHG) in gases for the field of attoscience. Towards the aim of generating ultra-broad comb in the CW regime, we generated first rotational Stokes in the CW regime with output power higher than 50 W. The Stokes laser power stability and the thermal distribution along the coupled fibre are reported. In addition, the linewidth measurements of the forward and backward Stokes are presented. A novel mechanism of stimulated Raman scattering is reported. The model is based on optically induced nanostructured Raman gain, whereby the population difference is saturated but in 1D periodic sub-wavelength sections over a long interaction length. The results show a multi-watt forward and backward Stokes emission with structured spectrum, and linewidth as narrow as ~ 10 kHz (>5 orders of magnitude shorter than the Raman linewidth as expected from conventional SRS). Observation of rich dynamics that include Rabi splitting, molecular motional sideband and inter-sideband four wave mixing, and finally AC stark induced molecule acceleration is reported.
Date of Award19 Nov 2014
Original languageEnglish
Awarding Institution
  • University of Bath
SupervisorAbdelfatah Benabid (Supervisor) & William Wadsworth (Supervisor)

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