Semiconductor filled microstructured optical fibres with single mode guidance

Microstructured optical fibre (MOF) technology has generated new opportunities for the implementation of optical fibres with novel properties and functions [1]. It has been shown that silica MOFs make excellent 3D templates for semiconductor material deposition inside the capillary voids [2]. Recently a silicon MOF was designed and fabricated that had a high refractive index micron sized core, but yet only supported two guided modes [3]. This structure was realised via the complete filling of a hollow core photonic bandgap fibre (PBGF) with silicon so that the original air guiding PBGF was converted to a total internal reflection guiding fibre. Here, we extend the investigation by using a finite element method to model the optical properties of semiconductor filled MOFs of similar structures, with the aim to achieve broadband single mode guidance. Strategies to achieve single mode guidance both through the MOF template design and the selective filling of the voids of the original PBGF with semiconductor materials of different indices (silicon, silicon nitride, germanium) are proposed and investigated numerically. In particular, by selectively filling MOF templates with cladding rods that have a slightly raised index over that of the core, index guiding single mode operation can be observed in high index micron sized cores. Small index differences are achievable by controlling the nitrogen content in SiNx and an example of a single mode semiconductor MOF is shown in Figure 1, where the confinement loss of the fundamental mode is ~106 lower than the lowest order cladding mode

Ultrafast optical control using the Kerr nonlinearity in hydrogenated amorphous silicon microcylindrical resonators

Microresonators are ideal systems for probing nonlinear phenomena at low thresholds due to their small mode volumes and high quality (Q) factors. As such, they have found use both for fundamental studies of light-matter interactions as well as for applications in areas ranging from telecommunications to medicine. In particular, semiconductor-based resonators with large Kerr nonlinearities have great potential for high speed, low power all-optical processing. Here we present experiments to characterize the size of the Kerr induced resonance wavelength shifting in a hydrogenated amorphous silicon resonator and demonstrate its potential for ultrafast all-optical modulation and switching. Large wavelength shifts are observed for low pump powers due to the high nonlinearity of the amorphous silicon material and the strong mode confinement in the microcylindrical resonator. The threshold energy for switching is less than a picojoule, representing a significant step towards advantageous low power silicon-based photonic technologies

Mid-infrared transmission properties of step index and large mode area ZnSe microstructured optical fibers

ZnSe microstructured fibers have been designed and fabricated using silica capillaries and an air-silica photonic band-gap optical fiber as high-pressure microfluidic templates for semiconductor growth via chemical fluid deposition. We examine their transmission properties over a wide spectral range

Effect of core size on nonlinear transmission in silicon optical fibers

The nonlinear transmission properties of two hydrogenated amorphous silicon fibers with core diameters of 5.7µm and 1.7µm are characterized. The measured Kerr nonlinearity, two-photon absorption and free-carrier parameters will be discussed in relation to device performance

Self-similar propagation of parabolic pulses in normal-dispersion fiber amplifiers

Pulse propagation in high-gain optical fiber amplifiers with normal group-velocity dispersion has been studied by self-similarity analysis of the nonlinear Schrödinger equation with gain. For an amplifier with a constant distributed gain, an exact asymptotic solution has been found that corresponds to a linearly chirped parabolic pulse that propagates self-similarly in the amplifier, subject to simple scaling rules. The evolution of an arbitrary input pulse to an asymptotic solution is associated with the development of low-amplitude wings on the parabolic pulse whose functional form has also been found by means of self-similarity analysis. These theoretical results have been confirmed with numerical simulations. A series of guidelines for the practical design of fiber amplifiers to operate in the asymptotic parabolic pulse regime has also been developed

Tunable coaxial resonators based on silicon optical fibers

Thermal tuning of a coaxial fiber resonator with a silica cladding surrounding an inner silicon core is investigated. By pumping the silicon with below bandgap light, it is possible to redshift the WGM resonances

Hypoxaemia in patients with pulmonary arterial hypertension during simulated air travel

SummaryBackgroundRecent air travel recommendations suggest patients with precapillary pulmonary hypertension (PCPH) in New York Heart Association (NYHA) functional class 3 and 4 should have in-flight oxygen without the need for pre-flight testing. However it remains unclear as to how best to determine patients fitness to fly.MethodsThis study (i) investigates the effect of hypoxic challenge testing (HCT) on the arterial oxygen levels in a cohort of 36 patients with PCPH and (ii) compares the relative frequency with which FC and HCT predict the requirement for in-flight oxygen.ResultsThe degree of arterial hypoxaemia induced by HCT (fall in partial pressure of oxygen in arterial blood (PaO2) 2.36 kPa, 95% CI 2.06–2.66 kPa) was similar to the drop observed in other published studies of chronic respiratory diseases.Following current air travel recommendations based on FC, 25 patients of the cohort would require in-flight oxygen whilst 10 subjects failed the HCT. Fourteen subjects had flown post-diagnosis. Of these, nine subjects should have had in-flight oxygen based on FC but were asymptomatic without. Also one who passed the HCT had developed symptoms during the flight whilst three who failed the HCT were asymptomatic flying without in-flight oxygen.ConclusionsHypoxaemia induced by simulated air travel in patients with PCPH is similar to that seen in published studies of patients with other chronic respiratory diseases. HCT failed to predict correctly who had developed symptoms during an aircraft flight in a significant minority of the study subjects. Similarly guidelines based on functional class result in a major increase in the proportion of patients being advised to use oxygen, many of whom had been asymptomatic on previous flights without it. More work is required to improve prediction of need for in-flight oxygen in patients with PCPH

Towards in-fiber silicon photonics

The state of the art of silicon optical fibers fabricated via the high pressure chemical deposition technique will be reviewed. The optical transmission properties of step index silicon optical fibers will be presented, including investigations of the nonlinearities that can be used for all-optical signal processing. In addition, alternative complex fiber geometries that permit sophisticated control of the propagating light will be introduced

Optical detection and modulation at 2µm-2.5µm in silicon

Recently the 2µm wavelength region has emerged as an exciting prospect for the next generation of telecommunications. In this paper we experimentally characterise silicon based plasma dispersion effect optical modulation and defect based photodetection in the 2-2.5µm wavelength range. It is shown that the effectiveness of the plasma dispersion effect is dramatically increased in this wavelength window as compared to the traditional telecommunications wavelengths of 1.3µm and 1.55µm. Experimental results from the defect based photodetectors show that detection is achieved in the 2-2.5µm wavelength range, however the responsivity is reduced as the wavelength is increased away from 1.55µm

Ultra-smooth lithium niobate micro-resonators by surface tension reshaping

Thermal treatment of micro-structured lithium niobate substrates at temperatures close to, but below the melting point, allows surface tension to reshape a preferentially melted surface zone [1] of the crystal to form ultra-smooth single crystal toroidal or spherical structures. Such structures, an example of which is shown in figure 1, are suitable for the fabrication of photonic micro-resonators with low scattering loss. The thermally treated material maintains its single crystal nature after the thermal treatment because the bulk remains solid throughout the process acting as seed during the recrystallization process which takes place during the cooling stage. The single crystal nature of the reshaped material has been verified by piezoresponse force microscopy, Raman spectroscopy and chemical etching. The inherent properties of lithium niobate crystals (optically nonlinear, piezoelectric and electro-optic) makes the resultant micro-resonator extremely suitable for sensing applications, for the production of micro-lasers (if doped with Er or Nd), for nonlinear frequency generation and finally for switching/modulation and tunable spectral filtering in optical telecommunications. The transformation of the initial surface micro-structures to the resulting resonator structure is a temperature dependent process as the surface tension acts on the surface melted layer of the crystal, Experimental investigation and modelling of the thermal treatment as well as investigation of the performance of these microresonators is underway to establish full control of the fabrication process for practical applications