Hollow core large mode area fibre employing a subwavelength grating reflector

Hollow core large mode area fibres are ideal candidates to guide light at high powers while avoiding non-linear effects and, as such, they are generating much scientific interest. A variety of fibres have been investigated, including tube lattice photonic bandgap fibres and Kagomé-latticed photonic crystal fibres. One of the major challenges in obtaining low loss hollow core fibres is related to the unavoidable perturbations induced by the coupling between the core and cladding modes which is responsible for the increase of leakage loss. Recent approach based on the insertion of additional antiresonant elements demonstrates the significance of fibre geometrical parameters and shows leakage loss of an order of ~10-4 dB/m. In this paper, we present preliminary results of a novel approach to fibres that guide light in a large hollow core, starting from the high index contrast grating reflector platform. Subwavelength gratings have been used to achieve broadband mirrors with reflectivity greater than 99%. Importantly, the physical dimensions of the grating must be smaller than the wavelength of incident light, which implies that the diffraction order of interest is 0th. Under a surface normal incidence on diffraction grating, evanescent orders in the direction parallel to the grating period overlap with the leaky mode of the grating leading to the effect of guided mode resonance and a destructive interference effect between the two grating modes, which results in high reflection

Bragg grating design method for the implementation of an optimised symmetric add/drop multiplexer

An efficient and simple method is proposed for the design of a novel Bragg grating based optical add/drop multiplexer. Central to this method is the existence of generic engineering curves that relate for any BG of a specific apodization profile, its penetration slope α = Lp/Lg to its reflectivity. The implemented by this method, interferometric device based on a full-cycle waveguide coupler exhibits symmetric and simultaneously optimised add/drop responses, of zero loss

Short-wavelength transmission-loss suppression in fibre Bragg gratings

Fibre Bragg Gratings (FBGs) are known to suffer from short-wavelength, transmission losses due to resonant coupling into backward-propagating cladding modes. Figure 1 shows a typical transmission spectrum of a 10cm standard FBG. The cladding mode losses increase with grating reflectivity and could eventually impose severe limitations in the use of FBGs. The problem can be quite acute in the case that FBG wavelength-multiplexing is required. So far, several attempts have been made to eliminate the short-wavelength, transmission losses and improve grating performance. In all cases, the resonant coupling of the forward-propagating core mode to the backward-propagating cladding modes is minimised by reducing the coupling strength. In this paper, we report on a novel method for reducing cladding-mode transmission losses in standard FBGs. We show that short-wavelength, transmission losses can be practically eliminated by damping the resonant excitation of the cladding modes. The damping is achieved by properly introducing a substantial propagation loss into the cladding modes. For maximum effect, the core mode should experience no extra propagation losses. By applying a thin lossy layer on the fibre cladding surface, a reduction of cladding-mode-losses of about 12dB was achieved

Effects of high temperature and pressure on silica optical fibre sensors

We report on the effects of liquids at high temperature and pressure on silica optical fibres, sensors and gratings. We propose that the diffusion of molecules into the silica and the resultant expansion of the network are responsible for observed fibre expansions of up to 0.2% and Bragg wavelength increases of 2nm at 1525nm. Amorphous carbon hermetic coating has shown a reduction of these effects by an order of magnitude at 300°C. These results have strong implications for the deployment of fibre sensors in oil wells

Group velocity equalisation in multimode waveguides using inverse scattering designs

In this paper, using an inverse scattering approach, we describe how the selection of mode effective indices and thus phase velocities can be used to control group velocity in a waveguide. As such it is shown that differential group delay can be equalised or minimised over a wavelength of choice. A particular feature of the new designs is the development of rings and a peaked core which may split depending upon the number of guided modes. These designs show characteristics comparable with commercially available fibres but with refractive index profiles that differ from typical graded-index designs

Second-generation erbium-doped fibre amplifiers for advanced optical communications

The erbium doped fibre amplifier (EDFA) [1] has seen rapid development and is now established as a key component in future optical networks. To date, EDFA development has been driven by specific requirements, primarily the single-channel, point-to-point fibre links. There are, however, many more applications which may require, for example, either broadband or narrowband amplifiers and signal-processing functions. In addition, non-telecommunication applications can benefit enormously by the use of EDFAs

Terahertz bandwidth photonic Hilbert transformers and implementations in ultra wideband single-sideband filters

Planar Bragg grating based photonic Hilbert transformers (PHTs) with THz bandwidths are proposed and practically demonstrated. An X-coupler, PHT, and a flat-top reflector are incorporated, demonstrating 2THz all-optical single-sideband filters. Devices are fabricated via a direct UV grating writing technique on a silica-on-silicon platform

Self-starting passive mode-locked fibre ring laser exploiting non-linear polarisation switching

Nonlinear birefringence effects in an all-fibre ring laser cavity have been exploited to produce self-starting passive mode-locking with pulse durations lying in the nanosecond region

Twisted Hi-Bi fiber DFB lasers with controllable output polarization

We demonstrate that single polarization, hi-bi fiber DFB lasers are in general characterized by elliptical state of polarization, due to the in-built fiber birefringence axis rotation. Externally applied birefringence-axis twist is shown to provide accurate control of the output SOP. Continuous tuning from circular to linear polarization, with PER of ~40dB has been demonstrated

Side-pumped WGM milled microstub resonator laser

Whispering Gallery Mode (WGM) resonators are promising candidates for realization of ultra-small micro lasers. Conventional 3D resonators such as micro toroids [1], micro spheres [2, 3] and microbottles [4] are well studied as both passive and active devices. The excitation and signal collection are mostly done using evanescently coupled micro-sized fibers [5], integrated waveguides [3] and\or collecting the scattered light [6]. As the spectrum and coupling efficiency highly depend on the excitation position, the coupling system requires precise alignment, and packaging is complex. Here, we demonstrate a completely new micro laser based on WGM generated in an Yb3+-doped stub resonator side-pumped at 976nm wavelength