A periodically tapered waveguides technique is an emerging potential route to establish quasi-phase-matching schemes in third-order nonlinear materials for efficient on-demand parametric interactions. In this paper, I investigate this method in enhancing spontaneous photon-pair emission in microstructured fibres and planar waveguides with sinusoidally varying cross sections. To study this process for continuous and pulsed-pump excitations, I have developed a general robust quantum model that takes into account self- and cross-phase modulations. The model shows a great enhancement in photon-pair generation in waveguides with a small number of tapering periods that are feasible via the current fabrication technologies. I envisage that this work will open a new area of research to investigate how the tapering patterns can be fully optimised to tailor the spectral properties of the output photons in nonlinear guided structures.
A periodically tapered waveguides technique is an emerging potential route to establish quasi-phase-matching schemes in third-order nonlinear materials for efficient on-demand parametric interactions. In this paper, I investigate this method in enhancing spontaneous photon-pair emission in microstructured fibres and planar waveguides with sinusoidally varying cross sections. To study this process for continuous and pulsed-pump excitations, I have developed a general robust quantum model that takes into account self- and cross-phase modulations. The model shows a great enhancement in photon-pair generation in waveguides with a small number of tapering periods that are feasible via the current fabrication technologies. I envisage that this work will open a new area of research to investigate how the tapering patterns can be fully optimised to tailor the spectral properties of the output photons in nonlinear guided structures.
Quasi-phase-matching periodically-tapered waveguides (PTWs) can enable efficient on-demand third-order parametric interactions using right combinations of the tapering period and modulation amplitude [1]. Similar to periodically-poled ferroelectric crystals, this new technique eliminates the stringent constraints imposed by conventional methods on the frequencies, mode profiles, and polarisations of the interacting photons. An example of PTWs is the sinusoidally-tapered fibres that have been exploited in manuiplating supercontinuum generation and modulation instability [2]. In this work, I have developed a rigorous quantum model to investigate spontaneous four-wave mixing (SFWM) inside these tapered waveguides [3]. The right combinations between the modulation amplitude Δd and tapering period ΛTthat enhances the expected number of photons (N̂s) at the targeted wavelengths are portrayed in Fig. 1(a), for fibres with same number of periods M. The values of (N̂s) are normalised to the case when Δd = 0, to quantify the enhancement in photon-pairs generation using the PTW-technique in comparison to uniform fibres. For only M = 50, (N̂s) is remarkably enhanced by 35 dB. The output spectrum of the photon-pairs is featured as a narrow sinc-function with very weak sidelobes that are significantly diminished for large number of periods, as depicted in Fig. 1(b). The 2D representation of the spectrum as a function of the photon-pairs wavelengths (λs, λi) is shown in panel (c). In this plot, the pump is assumed to be a monochromatic at a frequency satisfies the energy conservation. The corresponding (N̂s) for a Gaussian-pulse pump source with an input energy 1 nJ and a full-width-half-maximum 4 ps is portrayed in Fig. 1(d). Using the Schmidt decomposition analysis, the spectral-purity is 0.74. This shows the ability of the PTW-technique in producing highly-efficient relatively-pure single photons at any on-demand frequencies without applying any bandpass filters. This work will also open a new direction of research to investigate how the tapering patterns can be fully optimised to tailor the spectral properties of the output photons in third-order nonlinear guided structures.
In this paper, we present a thorough investigation for a spontaneous parametric four-wave mixing process in third-order nonlinear waveguides with various continuous tapering patterns. It has been previously shown that these devices can quasi-phase-match the four-wave-mixing process and enhance its conversion efficiency by orders of magnitude. By altering the tapering profile curve we found that these devices can enable single-photon sources with either narrow or broadband spectral widths at on-demand frequencies. Using our model, we were also able to identify the waveguide length at which the single-photon spectral purity is maximised.
In this paper, we present a thorough investigation for a spontaneous parametric four-wave mixing process in third-order nonlinear waveguides with various continuous tapering patterns. It has been previously shown that these devices can quasi-phase-match the four-wave mixing process and enhance its conversion efficiency by orders of magnitude. By altering the tapering profile curve we found that these devices can enable single-photon sources with either narrow or broadband spectral widths at on-demand frequencies. Using our model, we were also able to identify the waveguide length at which the single-photon spectral purity is maximized.
Integrated quantum photonics benefits from photon-pair sources that generate photons directly in waveguides, where they can be efficiently collected, routed, and manipulated. Here, we investigate spontaneous four-wave mixing (SFWM) in a fused tapered-fibre microcoupler formed from two single-mode fibres, and report four contributions. First, we observe SFWM photon pairs in this multimode device: its elliptical central region supports three spatial mode profiles, each with two polarisations, giving six guided modes, and pumping it near 800 nm yields two photon pairs at 648/1048 nm and 660/1021 nm. Second, because several allowed SFWM processes produce similar wavelengths and wavelength alone does not identify their origin, we distinguish the processes by combining phase-matching calculations and selection rules with two independent polarisation measurements: the dependence of the coincidence rate on pump polarisation and polarisation tomography of the generated photons. Third, within the present geometrical model, we find that the 660/1021 nm pair is generated by co-polarised pump photons in a process occurring entirely within a single higher-order mode, whereas the 648/1048 nm pair is generated by orthogonally polarised pump photons in an intermodal process coupling a fundamental and a higher-order mode; the signal photons from the two processes have similar output polarisations, while the idler photons are nearly orthogonal. Fourth, extending the analysis beyond the measured operating point, we theoretically identify pairs of simultaneous SFWM processes driven by a common pump that could generate either polarisation entanglement or composite spatial-polarisation entanglement, depending on the pump wavelength. More generally, this work provides a practical strategy for identifying intermodal SFWM processes in multimode waveguides when spectral information alone is insufficient.
The spontaneous decay of a pump photon into a tripartite entangled photon state (photon triplets) has been extensively studied in theory. Observation of these photon triplets at optical frequencies is of interest for fundamental science and future applications of quantum optics, such as quantum computing and quantum cryptography. This decay is known as spontaneous third-order parametric down-conversion (TOPDC). Experimentally, TOPDC has not been demonstrated in the optical frequency domain. TOPDC is enabled by the third-order nonlinearity of the material in which the interaction takes place. In optical fibres, the effective nonlinearity of the glass is increased manifold. The reason for this is the long light-matter interaction length and the tight confinement of light in the waveguide. Optical fibres are therefore the ideal platform for the observation of TOPDC, provided that phase matching can be achieved. In this thesis, the use of submicrometre fibre tapers is studied in the context of TOPDC, both in experiment and theory. Comparing different fibre-based TOPDC sources, fibre tapers rank among the highest in terms of expected triplet generation rate. At the same time, the background noise generated in these fibres is very low, which leads to a high expected three-photon coincidence-to-accidental ratio (CAR) on the order of 7000. In the experiments related to TOPDC, the precision of manufacturing fibre tapers is studied. A novel gas-based mechanism for dispersion tuning is demonstrated. Without this mechanism the manufacturing precision of the fibre tapers would not suffice to observe TOPDC in an experiment. The tuning mechanism is demonstrated using third-harmonic generation, which is the inverse process of TOPDC. The resonant third-harmonic radiation generated around 460nm can be tuned at a rate of −0.118nm/bar. This is achieved by introducing pressurised argon in the taper environment. The background noise from fibre tapers is characterised. The effect of the tuning mechanism on the three-photon-state and the signal-to-noise ratio is studied in theory. In another part of the thesis, gas-filled suspended-core fibres (SCFs) are used as a spectrally tunable source of entangled photon pairs (biphotons). Generation of biphotons via spontaneous four-wave mixing (FWM) in fibres offers great advantages over the generation in bulk crystals, which is the commonly employed technique. The most important advantages are the excellent control of the spatial mode structure, the extra degrees of freedom for dispersion design and the possibility to integrate the fibres into existing networks. Yet, fibre biphoton sources lack the convenient spectral tunability of crystal-based sources. Introducing pressurised gas into the hollow channels of the SCF is demonstrated to be a convenient and broadband spectral tuning mechanism. The FWM-sidebands can be continuously tuned at a rate of −0.3 THz/bar, in the quantum and in the seeded scenario. A high two-photon-CAR exceeding 10000 is measured for the biphoton state, and the frequency entangled joint-spectral intensity is measured. Finally, seeded FWM is used to characterise the core size of high-NA submicrometre fibres to a nanometre precision. If ultrafast pulses are used instead of nanosecond pulses, this will present a non-destructive and high precision characterisation technique for such fibres. The results of this work present an important step towards realising TOPDC at optical frequencies in the near future. The experimental realisation of a broadly tunable biphoton source combines the convenient tuning mechanism found in crystal-based sources with the advantages of fibre-based biphoton generation. Such tunable fibre biphoton sources can therefore be competitive with crystal-based systems for quantum optics applications.