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Optics Letters

Dispersion tuning in sub-micron tapers for third-harmonic and photon triplet generation

Jonas Hammer, Andrea Cavanna, Riccardo Pennetta, Maria V. Chekhova, Philip St.J. Russell, Nicolas Y. Joly

Published May 7, 2018• 24 citations
Publisher

Abstract

Precise control of the dispersion landscape is of crucial importance if optical fibers are to be successfully used for the generation of three-photon states of light-the inverse of third-harmonic generation (THG). Here we report gas-tuning of intermodal phase-matched THG in sub-micron-diameter tapered optical fiber. By adjusting the pressure of the surrounding argon gas up to 50 bars, intermodally phase-matched third-harmonic light can be generated for pump wavelengths within a 15 nm range around 1.38 μm. We also measure the infrared fluorescence generated in the fiber when pumped in the visible and estimate that the accidental coincidence rate in this signal is lower than the predicted detection rate of photon triplets.

DOI
10.1364/ol.43.002320

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Precise control of the dispersion landscape is of crucial importance if optical fibers are to be successfully used for the generation of three-photon states of light-the inverse of third-harmonic generation (THG). Here we report gas-tuning of intermodal phase-matched THG in sub-micron-diameter tapered optical fiber. By adjusting the pressure of the surrounding argon gas up to 50 bars, intermodally phase-matched third-harmonic light can be generated for pump wavelengths within a 15 nm range around 1.38 μm. We also measure the infrared fluorescence generated in the fiber when pumped in the visible and estimate that the accidental coincidence rate in this signal is lower than the predicted detection rate of photon triplets.
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.
All-fiber systems for third harmonic generation are of great interest because they can be used for the inverse process, namely, the generation of entangled photon triplets. Usually, chromatic dispersion prevents phase matching between the incident and generated radiation when they are both guided in an LP01-like mode. Here, we present a hybrid photonic crystal fiber that has been designed for phase matched third harmonic generation from 1596 to 532 nm in single-lobed modes. The third harmonic radiation is guided by an all-solid bandgap microstructure, while the pump frequency is confined by conventional total internal reflection. The fiber is also suitable for the generation of photon triplet states.
Near-infrared (NIR) single-photon sources have become the backbone of quantum technology. There is further potential to demonstrate the next-generation quantum technology applications by extending the wavelengths to the mid-infrared (mid-IR) region. This interest is driven by the extraordinary characteristics of the mid-IR wavelength spectrum. In this paper, we propose germania-doped and ZBLAN fibers that exploit spontaneous four-wave mixing (SpFWM) to generate a single photon-pair in the ∼2 µm to ∼3.5 µm wavelength range. The phase-matching conditions at different wavelengths can be achieved by employing both the same-mode and mixed-mode schemes (intra and intermodal) for the idler, pump, and signal wavelengths. For the intramodal scheme, we exploited the fiber’s high NA to produce closely spaced non-degenerate modes (such as LP 11a /LP 11b and LP 21a /LP 21b ) and then used combinations of these modes (from LP 11a to LP 11b or LP 21a to LP 21b ) for all three signal, pump, and idler wavelengths to achieve the phase-matching. For the intermodal scheme, we borrowed different modes for the signal, pump (including different modes for both pump photons), and idler wavelengths. The mixed-mode schemes allow for a larger core and provide greater flexibility in tuning the idler and signal wavelengths to mitigate the effects of Raman scattering and residual pump photons (∼20–25 THz from the pump frequency), albeit with increased complexity and reduced efficiency. We developed an understanding of the mixed-mode phase-matching scheme and built a model to optimize mode selection for phase-matching. Our investigations reveal, for the first time, the untapped potential of fiber-based mid-IR entangled single photon-pair generation.