Control over the transverse structure and long-distance fiber propagation of light at the single-photon level.

Quantum entanglement is arguably the cornerstone which differentiates the quantum realm from its classical counterpart. While entanglement can reside in any photonic degree of freedom, polarization permits perhaps the most straightforward manipulation due to the widespread availability of standard optical elements such as waveplates and polarizers. As a step towards a fuller exploitation of entanglement in other degrees of freedom, in this work we demonstrate control over the transverse spatial structure of light at the single-photon level. In particular we integrate in our setup all the technologies required for: (i) fibre-based photon pair generation, (ii) deterministic and broadband single-photon spatial conversion relying on a passive optical device, and (iii) single-photon transmission, while retaining transverse structure, over 400 m of few-mode fibre. In our experiment, we employ a mode selective photonic lantern multiplexer with the help of which we can convert the transverse profile of a single photon from the fundamental mode into any of the supported higher-order modes. We also achieve conversion to an incoherent or coherent addition of two user-selected higher order modes by addressing different combinations of inputs in the photonic lantern multiplexer. The coherent nature of the addition, and extraction of usable orbital angular momentum at the single-photon level, is further demonstrated by far-field diffraction through a triangular aperture. Our work could enable studies of photonic entanglement in the transverse modes of a fibre and could constitute a key resource quantum for key distribution with an alphabet of scalable dimension.

Wavelength-selective switch with direct few mode fiber integration.

The first realization of a wavelength-selective switch (WSS) with direct integration of few mode fibers (FMF) is fully described. The free-space optics FMF-WSS dynamically steers spectral information-bearing beams containing three spatial modes from an input port to one of nine output ports using a phase spatial light modulator. Sources of mode dependent losses (MDL) are identified, analytically analyzed and experimentally confirmed on account of different modal sensitivities to fiber coupling in imperfect imaging and at spectral channel edges due to mode clipping. These performance impacting effects can be reduced by adhering to provided design guidelines, which scale in support of higher spatial mode counts. The effect on data transmission of cascaded passband filtering and MDL build-up is experimentally investigated in detail.

A complex multi-notch astronomical filter to suppress the bright infrared sky.

A long-standing and profound problem in astronomy is the difficulty in obtaining deep near-infrared observations due to the extreme brightness and variability of the night sky at these wavelengths. A solution to this problem is crucial if we are to obtain the deepest possible observations of the early Universe, as redshifted starlight from distant galaxies appears at these wavelengths. The atmospheric emission between 1,000 and 1,800 nm arises almost entirely from a forest of extremely bright, very narrow hydroxyl emission lines that varies on timescales of minutes. The astronomical community has long envisaged the prospect of selectively removing these lines, while retaining high throughput between them. Here we demonstrate such a filter for the first time, presenting results from the first on-sky tests. Its use on current 8 m telescopes and future 30 m telescopes will open up many new research avenues in the years to come.

Ultrafast laser inscription of an integrated photonic lantern.

We used ultrafast laser inscription to fabricate three-dimensional integrated optical transitions that efficiently couple light from a multimode waveguide to a two-dimensional array of single mode waveguides and back. Although the entire device has an average insertion loss of 5.7 dB at 1539 nm, only ≈0.7 dB is due to mode coupling losses. Based on an analysis which is presented in the paper, we expect that our device should convert a multimode input into an array of single modes with a loss of ≈2.0 dB, assuming the input coupling losses are zero. Such devices have applications in astrophotonics and remote sensing.

Silica aerogel core waveguide.

We have selectively filled the core of hollow photonic crystal fibre with silica aerogel. Light is guided in the aerogel core, with a measured attenuation of 0.2 dB/cm at 1540 nm comparable to that of bulk aerogel. The structure guides light by different mechanisms depending on the wavelength. At long wavelengths the effective index of the microstructured cladding is below the aerogel index of 1.045 and guidance is by total internal reflection. At short wavelengths, where the effective cladding index exceeds 1.045, a photonic bandgap can guide the light instead. There is a small region of crossover, where both index- and bandgap-guided modes were simultaneously observed.

Control of surface modes in low loss hollow-core photonic bandgap fibers.

We report on the fabrication and characterization of hollow-core photonic bandgap fibers that do not suffer from surface mode coupling within the photonic bandgap of the cladding. This enables low attenuation over the full spectral width of the bandgap--we measured a minimum loss of 15 dB/km and less than 50 dB/km over 300 nm for a fiber operating at 1550 nm. As a result of the increased bandwidth, the fiber has reduced dispersion and dispersion slope--by a factor of almost 2 compared to previous fibers. These features are important for several applications in high-power ultrashort pulse compression and delivery. Realizing these advances has been possible due to development of a modified fabrication process which makes the production of low-loss hollow-core fibers both simpler and quicker than previously.

All-fiber LP11 mode convertors.

We describe two all-fiber devices for converting light from the fundamental mode to the second-order set of modes in optical fibers. The first is made by controlled hole collapse in a photonic crystal fiber, and the second is a twisted fused coupler made from few-moded conventional fiber. As well as having applications within fiber optics, the devices can be used to generate azimuthally polarized free-space beams.

Wavelength-independent all-fiber mode converters.

We have used two different photonic crystal fiber (PCF) techniques to make all-fiber mode converters. An LP(01) to LP(11) mode converter was made by the ferrule technique on a drawing tower, and an LP(01) to LP(02) mode converter was made by controlled hole inflation of an existing PCF on a tapering rig. Both devices rely on adiabatic propagation rather than resonant coupling; so high extinction was achieved across a wide wavelength range.

All-fiber anamorphic core-shape transitions.

We demonstrate low-loss anamorphic transitions between circular and rectangular fiber cores with aspect ratios up to 5:1, and hence improved coupling from a diode laser by using only a spherical lens. Differential hole pressurization and localized heating of a stock photonic crystal fiber inflates the holes at different rates. Some holes are plugged in the fiber end face where pressure is applied, so they remain at ambient pressure. Alternatively, holes of different sizes expand at different rates because the effective pressure due to surface tension differs.

Supercontinuum generation system for optical coherence tomography based on tapered photonic crystal fibre.

We report smooth and broad continuum generation using a compact femtosecond Ti:Sapphire laser as a pump source and a tapered photonic crystal fibre as a nonlinear element. Spectral output is optimized for use in optical coherence tomography, providing a maximum longitudinal resolution of 1.5 microm in free space at 809 nm centre wavelength without use of additional spectral filtering.

Enhanced visible continuum generation from a microchip 1064nm laser.

We demonstrate a cascaded nonlinear process using pump conversion to 742 nm by four-wave mixing in the normal dispersion regime then continuum generation by modulation instability to generate bright single-mode visible continuum with an average power up to -20 dBm/nm, from a compact 1064 nm infrared source in a monolithic single-mode photonic crystal fibre with a tapered section in one end.

Multimode fiber devices with single-mode performance.

A taper transition can couple light between a multimode fiber and several single-mode fibers. If the number of single-mode fibers matches the number of spatial modes in the multimode fiber, the transition can have low loss in both directions. This enables the high performance of single-mode fiber devices to be attained in multimode fibers. We report an experimental proof of concept by using photonic crystal fiber techniques to make the transitions, demonstrating a multimode fiber filter with the transmission spectrum of a single-mode fiber grating.

Splice-free interfacing of photonic crystal fibers.

We report a new method for making low-loss interfaces between conventional single-mode fibers and photonic crystal fibers (PCFs). Adapted from the fabrication of PCF preforms from stacked tubes and rods, this method avoids the need for splicing and is versatile enough to interface to virtually any type of index-guiding silica PCF. We illustrate the method by forming interfaces to two problematic types of PCF, highly nonlinear and multicore. In particular, we believe this to be the first method capable of individually coupling light into and out of all the cores of a fiber with multiple closely spaced cores, without input or output cross talk.

Photonic bandgap with an index step of one percent.

Early work suggested that very large refractive index contrasts would be needed to create photonic bandgaps in two or three dimensionally periodic photonic crystals. It was then shown that in two-dimensionally periodic structures (such as photonic crystal fibres) a non-zero wavevector component in the axial direction permits photonic bandgaps for much smaller index contrasts. Here we experimentally demonstrate a photonic bandgap fibre made from two glasses with a relative index step of only 1%.

Guidance properties of low-contrast photonic bandgap fibres.

We investigate the guidance properties of low-contrast photonic band gap fibres. As predicted by the antiresonant reflecting optical waveguide (ARROW) picture, band gaps were observed between wavelengths where modes of the high-index rods in the cladding are cutoff. At these wavelengths, leakage from the core by coupling to higher-order modes of the rods was observed directly. The low index contrast allowed for bend loss to be investigated; unlike in index-guiding fibres, anomalous "centripetal" light leakage through the inside of the bend can occur.

Hole inflation and tapering of stock photonic crystal fibres.

We report controlled hole expansion in photonic crystal fibres (PCFs) by heating the fibre while the holes were pressurised. This was done by post-processing an existing fibre, not during fibre fabrication. Small holes in an endlessly single-mode (ESM) PCF were inflated to become large holes. The large-hole PCF was then tapered to produce a "cobweb" PCF with a small highly-nonlinear core, interfaced to the ESM PCF at both ends by gradual transitions. The loss was less than 0.4 dB in the complete structure, which was used to demonstrate supercontinuum generation when pumped with a fs Ti:sapphire laser.

Low-loss deposition of solgel-derived silica films on tapered fibers.

Films of porous silica are deposited on the uniform waists of tapered fibers in minutes by a modified solgel dip coating method, inducing less than 0.2 dB of loss. The coated tapers are an ideal platform for realizing all-fiber devices that exploit evanescent-field interactions with the deposited porous film. As an example we demonstrate structural long-period gratings in which a periodic index variation in the film arises from the porosity variation produced by spatially varying exposure of the waist to a scanned CO2 laser beam. The long period grating is insensitive to temperature up to 800 degrees C.

Supercontinuum generation in submicron fibre waveguides.

Submicron-diameter tapered fibres and photonic crystal fibre cores, both of which are silica-air waveguides with low dispersion at 532 nm, were made using a conventional tapering process. In just cm of either waveguide, ns pulses from a low-power 532-nm microchip laser generated a single-mode supercontinuum broad enough to fill the visible spectrum without spreading far beyond it.