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Raman spectroscopy is an attractive technique for simultaneous analysis of multiple gas species. Exploiting hollow-core microstructured optical fibres (HC-MOFs) for enhanced gas-light interaction length allows for extremely low detection limits of, e.g., 0.2 ppm methane [1] . Recently, we reported that extremely small changes in the refractive indices of the gases inside the core and cladding regions (as induced by differential gas pressure) significantly impact the optical properties of these fibres [2] . Here we exploit this effect to almost double the Raman signal.

We report an overview of our recent progress in the design and fabrication of soft-glass based antiresonant hollow core fibers (AR-HCFs) for laser delivery in the mid-IR ( $5-11\ \mu\mathrm{m}$ ). The fibers are fabricated by extrusion and subsequent drawing, using two different glasses: tellurite (70TeO 2 -13ZnO-10BaO-7Na 2 O), and chalcogenide (IG3, Ge 30 As 13 Se 32 Te 25 ). The fabricated tellurite AR-HCFs can operate from 5 to $7\ \mu \mathrm{m}$ wavelength, and the IG3 AR-HCFs can operate further up to $11\ \mu\mathrm{m}$ wavelength. The numerical and experimental losses of the fundamental mode for the fabricated fibers are on the few dB/m level. Numerical modeling study indicates that, by improving the uniformity and shape of the capillaries, losses down to 0.1 dB/m in the mid-IR (5 to $11\ \mu\mathrm{m}$ ) should be possible, making this solution attractive for the mid-IR laser delivery, such CO, CO 2 and quantum-cascade lasers.

We review our recent work on the modelling, fabrication and characterization of hollow-core photonic bandgap fibers. We discuss the modal content of these fibers, as well as the opportunities and challenges presented by modal interactions in space division multiplexed transmission applications.

We demonstrate a longitudinally resolved Optical Side Scattering Radiometry (OSSR) technique to measure discrete and distributed scattering loss with >60dB dynamic range and 5cm spatial resolution. We apply it to a record-length 11km hollow core photonic band-gap fibre.

We present for the first time the group velocity dispersion of multiple distinguishable modes propagating in 3, 7 and 19 cell hollow core photonic bandgap fibres. Measurements are made by direct phase extraction from spectral domain low coherence interferometry.

4×10Gbps WDM transmission over low loss HC-PBGF at 2μm wavelengths is demonstrated using a 2mm long QCSE based InP MZM with Vπ of 6V and ER>20dB. An OSNR of 25dB is required to achieve error free transmission.

An optical transmission loop experiment over hollow-core fiber is demonstrated with 1.25Gb/s OOK, 40G DP-QPSK, 100G DP-QPSK modulation schemes over distances of 31.0 km, 74.8 km, 55.8 km respectively, using two fiber spans with 2.7 km and 3.5 km length.

We present an all-optical flexible disaggregated flat DCN architecture utilizing Hollow-Core Bandgap Fibre, reconfigurable 4×16/8×12 Spectrum Selective Switches and FPGA-based switch and interface intra/inter-blade cards enabling chip-level access while utilizing cut-through low-latency tuneable WDM/WDM-TDM.

We report a novel OFDM-transmitter operating in the emerging 2µm waveband. Sub-FEC limit transmission of a 32QAM signal over 500m of both solid and hollow-core fiber was achieved and the generation of 30Gbits 64QAM demonstrated.

This paper presents WDM transmission at 2µm over 1.15km of HC-PBGF with wavelength channels selected to span a 36.3nm waveband. A total capacity of 81Gbit/s was achieved using 4 x 12.5Gbit/s NRZ-OOK external modulation and 4 x 7.7Gbit/s 4-ASK Fast-OFDM direct modulation signals.

X-ray computational tomography is demonstrated as a powerful non-destructive tool to image the internal structure of a hollow core photonic band-gap fibre and its preforms. The technique is applied to measure the deformation within a splice with unprecedented detail.

We present a new S2-based technique with potential for near real-time (1s) characterization of the modal-content of multimode fibers. We also demonstrate the identification and removal of measurement artifacts originating from reflections from optical components.

Hollow core photonic bandgap fibres (HC-PBGFs) are emerging as contenders in the race to identify the best solution for next-generation high capacity transmission systems, owing to their low nonlinearity, ultimate-low signal latency, an ideal match to thulium-doped fibre amplifiers and to the promise to deliver ultralow loss, potentially comparable or even below the level of state-of-the-art single mode fibres. Recently, substantial progress in the development of HC-PBGFs has been reported; however, for these fibres to stand a realistic chance of competing with solid transmission fibres, the key challenges of loss reduction over a sufficiently wide transmission bandwidth, as well as the ability to produce fibres in multi-hundred kilometre lengths must be addressed. A predicted route to reduce loss is by increasing the core size, e.g. from 19-cell to 37-cell. In this work we review recent progress in the development of HC-PBGFs based on a 37-cell core geometry, including the realization of fibres with low loss, wide bandwidth, low modal crosstalk and low bending loss, which have paved the way to data transmission experiments using mode division multiplexing. We also investigate the issue of longitudinal uniformity in HC-PBGFs and report a systematic study of defects observed in these fibres - a first step towards their removal and thus towards the demonstration of multi-kilometre lengths of defect free fibres.

We describe low loss (50dB/km at 3.3µm) and low bend sensitivity mid-IR hollow core-photonic bandgap fiber. Gas sensing applications are highlighted by a methane spectrum recorded in our fiber.

A CW fiber laser is stabilized to near-infrared transitions of 12C2H2 inside kagome fibers and characterized with a frequency comb. ±10 kHz accuracy is achieved, and performance in sealed photonics microcells is explored.

We demonstrate what we believe is the first hollow fiber gas laser based on population inversion. A single-cell-defect Kagome fiber with a core diameter of about 40 microns was filled with a few torr of acetylene (12C 2 H 2 ). Acetylene has absorption transitions in the attractive telecommunication C band region and recently lasing was observed at about 3 microns from a gas cell when optically pumped. Pumping the gas filled fiber (1.65 m) with micro Joule pulses from a 5-ns OPO resulted in the emission of two laser lines at 3123.2 nm and 3162.4 nm. The pump (-1521 nm) was in the guiding regions of the fiber (loss ∼ 0.75 dB/m) while the laser output occurred in weakly guided modes (∼ 20 dB/m). This laser combines attractive features of both fiber and gas lasers, such as the confinement of the pump and laser over long interaction lengths in a compact configuration, the many possible emission wavelengths of various gases, high damage threshold, and the potential for coherent beam combining of mutually incoherent pump sources. While this first laser demonstration used a pulsed pump, the gas filled fiber laser is particularly attractive for pumping with CW laser sources.