Modal noise appears due to the non-uniform and unstable distribution of light intensity among the finite number of modes in multimode fibres. It is an important limiting factor in measuring radial velocity precisely by fibre-fed high-resolution spectrographs. The problem can become particularly severe as the fibre's core become smaller and the number of modes that can propagate reduces. Thus, mitigating modal noise in relatively small core fibres still remains a challenge. We present here a novel technique to suppress modal noise. Two movable mirrors in the form of a galvanometer re-image the mode-pattern of an input fibre to an output fibre. The mixing of modes coupled to the output fibre can be controlled by the movement of mirrors applying two sinusoidal signals through a voltage generator. We test the technique for four multimode circular fibres: 10 and 50 $\mu$m step-index, 50 $\mu$m graded-index, and a combination of 50 $\mu$m graded-index and 5:1 tapered fibres (GI50t). We present the results of mode suppression both in terms of the direct image of the output fibre and spectrum of white light obtained with the high-resolution spectrograph. We found that the galvanometer mitigated modal noise in all the tested fibres, but was most useful for smaller core fibres. However, there is a trade-off between the modal noise reduction and light-loss. The GI50t provides the best result with about 60 per cent mitigation of modal noise at a cost of about 5 per cent output light-loss. Our solution is easy to use and can be implemented in fibre-fed spectrographs.

Optical fibres are a powerful tool in astronomical applications, routinely used in spectroscopy and interferometry [1], [2]. Fibres can also be post-processed to create transitions in size, which adds functionality. Transitions (also known as ‘tapers’) describe regions of fibre where the scale changes along a length. If the change is gradual, the mode field diameter of the guided light can change adiabatically. This is necessary for devices such as photonic lanterns, fused fibre couplers and fibre sensors. We present an all-fibre wavefront sensor, to sense the local tip-tilt of a wavefront, as an example of a fibre transition for astronomical applications.

Multimode fibers (MMFs) can facilitate acquisition of higher resolution images than multicore fibers (MCFs) [1]. MMF imaging is hindered by the transmission matrix (TM) of the MMF changing if its path is altered, and access to the distal-end is required for recalibration, although work is ongoing to address this [2], [3]. In [4] we showed that an MCF terminated with a photonic lantern (PL) [5] may provide some of the sought-after advantages of MMF imaging. We also suggested they might enable interrogation of the TM without access to the multimode (MM) distal end for imaging modalities requiring coherent beam shaping. Here, we demonstrate that following calibration, polarization maintaining (PM) MCF PLs facilitate controlled coherent beam shaping at the distal end after the path of the PL is altered, using only information gained from the proximal end.

We investigated the impact of taper length on light transmission through tapered graded-index fibres. We tested commercial fibres from Thorlabs and a custom graded-index fibre using both coherent and incoherent light sources. Our experimental results show optimum performance for taper transition lengths of 25 mm, although our simulations suggest further improvement may be possible for even shorter transition lengths. We also measured the modal noise power fluctuations caused by bending the fibre. Here, we observe that the custom fibre tapers have the highest transmission but suffer from the most modal noise. Accordingly, we find that the commercial graded-index fibre tapers promise practical usage as a beam mode-field converter, as they have lower power fluctuations but retain relatively high transmission if compared to commercial small core step-index fibre.

We achieve ultrabroad quantum frequency conversion in a photonic crystal fiber, converting a 1,551 nm heralded single photon to one with wavelengths spanning the range 1,226 - 1,483 nm. We confirm non-classical timing correlations with the herald photon, and for conversion to 1,300 nm we measure a g (2) (0) t,t = 0.25(6).

We report on a tapered three-core optical fibre that can be used as a tip-tilt wavefront sensor. In this device, a coupled region of a few millimetres at the sensing tip of the fibre converts fragile phase information from an incoming wavefront into robust intensity information within each of the cores. The intensity information can be easily converted to linear wavefront error over small ranges, making it ideal for closed loop systems. The sensor uses minimal information to infer tip-tilt and is compatible with remote detector arrays. We explore its application within adaptive optics and present a validation case to show its applicability to astronomy.

Low-noise frequency conversion of single photons is a critical tool in establishing fibre-based quantum networks. We show that a single photonic crystal fibre can achieve frequency conversion by Bragg-scattering four-wave mixing of source photons from an ultra-broad wavelength range by engineering a symmetric group velocity profile. Furthermore, we discuss how pump tuning can mitigate realistic discrepancies in device fabrication. This enables a single highly adaptable frequency conversion interface to link disparate nodes in a quantum network via the telecoms band.

We measured the transmission of tapered and untapered optical fibres as a function of input beam numerical aperture at 635 nm. The tapered fibres were fabricated with an adiabatic tapering process from graded and step index fibres with 50 mu m core diameters to form a 100 mm long taper with 5:1 taper ratio. We tested tapered graded-index and step-index fibres fabricated from commercial Thorlabs products and a custom graded-index taper. The 5:1 tapered graded-index fibre can give a transmission greater than 0.4 for Thorlabs and 0.6 for the custom taper. We simulated the transmission of the tapered fibres and found reasonable agreement with the measured graded-index tapered fibre results across the numerical aperture range of interest. Experimentally, step-index tapered fibres performed relative poorly and considerably below modelling expectations. Based on our examinations this arises because the properties of step-index fibre were not robust to the tapering process. Suitably tapered graded-index fibres may offer a new route for efficient focal ratio reduction of fibre optic signals, e.g., in fibre-fed spectrographs, though we stress that our measurements have been limited to monochromatic light in this work.

We report a resource-efficient scheme in which a single pump laser was used to achieve frequency conversion by Bragg-scattering four-wave mixing in a photonic crystal fiber. We demonstrate bidirectional conversion of coherent light between S r + 2 P 1 / 2 → 2 D 3 / 2 emission wavelength at 1092 nm and the telecommunication C band with conversion efficiencies of 4.2% and 37% for up- and down-conversion, respectively. We discuss how the scheme may be viably scaled to meet the temporal, spectral, and polarization stability requirements of a hybrid light–matter quantum network.

We show that through engineering symmetric group velocity, frequency conversion by Bragg-scattering four-wave mixing can be phase matched over ultra-broad band- widths, enabling highly adaptable frequency conversion interfaces for quantum networks.

We report the experimental observation of nonlinear multimode beam cleaning at the multimode output of a photonic lantern by coupling fs-laser pulses into each core of a photonic lantern made from a 7-core multicore fiber.

We report a single-stage bidirectional interface capable of linking Sr+ trapped-ion qubits in a long-distance quantum network. Our interface converts photons between the Sr+ emission wavelength at 422 nm and the telecom C band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single-photon level with efficiencies of 9.4% and 1.1%, respectively. Furthermore, we demonstrate noise levels that are low enough to allow for genuine quantum operation in the future.

We report a single-stage bi-directional interface capable of linking Sr$^+$ trapped ion qubits in a long-distance quantum network. Our interface converts photons between the Sr$^+$ emission wavelength at 422 nm and the telecoms C-band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single photon level with efficiencies of 9.4 $%$ and 1.1 $%$ respectively. Furthermore, we demonstrate that the noise introduced during the conversion process is sufficiently low to implement high-fidelity interconnects suitable for quantum networking.

We report a single-stage bi-directional interface capable of linking Sr+ trapped ion qubits in a long-distance quantum network. Our interface converts photons between the Sr+ emission wavelength at 422 nm and the telecoms C-band to enable low-loss transmission over optical fiber. We have achieved both up- and down-conversion at the single photon level with efficiencies of 9.4 % and 1.1 % respectively. Furthermore, we demonstrate that the noise introduced during the conversion process is sufficiently low to implement high-fidelity interconnects suitable for quantum networking.

We present a single-stage bi-directional interface capable of linking Sr+ trapped ion qubits emitting single photons at 422 nm with the telecoms C-band. We achieve external up(down) conversion efficiencies of 9.4%(1.1%).

Thomas A. Wright, Robert J.A. Francis-Jones, Corin B.E. Gawith, Jonas N. Becker, Patrick M. Ledingham, Peter G.R. Smith, Joshua Nunn, Peter J. Mosley, Benjamin Brecht, and Ian A. Walmsley Centre for Photonics and Photonic Materials, Department of Physics, University of Bath, Bath, BA2 7AY, UK Optoelectronics Research Centre, University of Southampton, SO17 1BJ, UK and Clarendon Laboratory, University of Oxford, Parks Road, Oxford, OX1 3PU, UK (Dated: February 23, 2018)

Third-order nonlinear processes require phase matching between the interacting fields to achieve high efficiencies. Typically in guided-wave $\chi^{(3)}$ platforms this is achieved by engineering the dispersion of the modes through the transverse profile of the device. However, this limits the flexibility of the phase matching that can be achieved. Instead, we analyze four-wave mixing in a pair of asymmetric waveguides and show that phasematching may be achieved in any $\chi^{(3)}$ waveguide by coupling of a nondegenerate pump from an adjacent waveguide. We demonstrate the additional flexibility that this approach yields in the case of photon-pair generation by spontaneous FWM, where the supermode dispersion may be modified to produce pure heralded single photons -- a critical capability required for example by silicon platforms for chip-scale quantum photonics.