Bend and index insensitive long period grating in progressive three layered optical fibre

A long period fibre grating written in a progressive three-layered optical fibre is shown to exhibit an attenuation band that has a very low bending sensitivity (5.1×10 –2 nm m) compared to normal step-index fibre, and is also insensitive to changes in the refractive index of the surrounding medium. Applications to sensing and telecommunications are discussed.

Controlling soliton refraction in optical lattices

We show in the framework of the 1D nonlinear Schrödinger equation that the value of the refraction angle of a fundamental soliton beam passing through an optical lattice can be controlled by adjusting either the shape of an individual waveguide or the relative positions of the waveguides. In the case of the shallow refractive index modulation, we develop a general approach for the calculation of the refraction angle change. The shape of a single waveguide crucially affects the refraction direction due to the appearance of a structural form factor in the expression for the density of emitted waves. For a lattice of scatterers, wave-soliton interference inside the lattice leads to the appearance of an additional geometric form factor. As a result, the soliton refraction is more pronounced for the disordered lattices than for the periodic ones.

Long period grating directional bend sensor based on asymmetric index modification of cladding

A long period grating (LPG) written in a standard optical fibre was modified by using a femtosecond laser to induce an asymmetric change in the cladding's refractive index. This device produced blue and red wavelength shifts depending on the orientation of applied curvature, with maximum sensitivities of -1.6 nm m and +3.8 nm m, suggesting that this type of LPG may be useful as a shape sensor.

A long period grating directional bend sensor incorporating index modification of the cladding

Long period gratings written into a standard optical fibre were modified by a femtosecond laser, which produced an asymmetric change to the cladding's refractive index resulting in a directional bend sensor.

Long period grating directional bend sensor based on asymmetric index modification of cladding

A long period grating (LPG) written in a standard optical fibre was modified by using a femtosecond laser to induce an asymmetric change in the cladding's refractive index. This device produced blue and red wavelength shifts depending on the orientation of applied curvature, with maximum sensitivities of -1.6 nm m and +3.8 nm m, suggesting that this type of LPG may be useful as a shape sensor.

Direct design of high channel-count fiber Bragg grating filters with low index modulation

A novel method for designing high channel-count fiber Bragg gratings (FBGs) is proposed. For the first time, tailored group delay is introduced into the target reflection spectra to obtain a more even distribution of the refractive index modulation. This approach results in the reduction of the maximum refractive index modulation to physically realizable levels. The maximum index modulation reduction factors are all greater than 5.5. This is a significant improvement compared with previously reported results. Numerical results show that the thus designed high channel-count FBG filters exhibit superior characteristics including 30 dB channel isolation, a flat-top and near 100% reflectivity in each channel. © 2012 Optical Society of America.

A long period grating directional bend sensor incorporating index modification of the cladding

Long period gratings written into a standard optical fibre were modified by a femtosecond laser, which produced an asymmetric change to the cladding's refractive index resulting in a directional bend sensor.

Localized surface plasmon fiber device coated with carbon nanotubes for the specific detection of CO2

We explored the potential of a carbon nanotube (CNT) coating working in conjunction with a recently developed localized surface plasmon (LSP) device (based upon a nanostructured thin film consisting of of nano-wires of platinum) with ultra-high sensitivity to changes in the surrounding index. The uncoated LSP sensor’s transmission resonances exhibited a refractive index sensitivity of Δλ/Δn ~ -6200nm/RIU and ΔΙ/Δn ~5900dB/RIU, which is the highest reported spectral sensitivity of a fiber optic sensor to bulk index changes within the gas regime. The complete device provides the first demonstration of the chemically specific gas sensing capabilities of CNTs utilizing their optical characteristics. This is proven by investigating the spectral response of the sensor before and after the adhesion of CNTs to alkane gases along with carbon dioxide. The device shows a distinctive spectral response in the presence of gaseous CO2 over and above what is expected from general changes in the bulk refractive index. This fiber device yielded a limit of detection of 150ppm for CO2 at a pressure of one atmosphere. Additionally the adhered CNTs actually reduce sensitivity of the device to changes in bulk refractive index of the surrounding medium. The polarization properties of the LSP sensor resonances are also investigated and it is shown that there is a reduction in the overall azimuthal polarization after the CNTs are applied. These optical devices offer a way of exploiting optically the chemical selectivity of carbon nanotubes, thus providing the potential for real-world applications in gas sensing in many inflammable and explosive environments. © (2015) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

Subwavelength Index Engineering for SOI Waveguides

Photonic structures with a sub-wavelength pitch, small enough to suppress diffraction, can behave as equivalent homogenous materials that can be engineered to exhibit a specific refractive index and dispersion. Here we discuss the design of a variety of integrated photonic devices, ranging from grating couplers to multimode interference couplers, for which the use of sub-wavelength structures enables unique characteristics. We will place special emphasis on the design and experimental demonstration of multi-mode interference couplers with an unprecedented bandwidth beyond 200nm at telecom wavelengths.

The optical properties of metamaterials

This thesis studies the parametric investigation, polarisation dependence and characterization of fishnet structure at near infrared wavelengths. Detailed simulations are performed to understand the behaviour of the structure at near infrared and optical wavelengths. Simulations are performed to obtain negative refractive index of the fishnet structure formed from nanoimprint lithography (NIL) by taking into account the effect of substrate and polymethyl methacrylate (PMMA) beneath it. Two different structures have been designed and fabricated of varying dimensions using NIL and their resonant wavelength measured in the near infrared at 1.45 µm and 1.88 µm. Simulations suggest that a negative refractive index real part with the magnitude -0.24 is found at 1.53 µm and this decrease to a maximum magnitude of -0.57 at 1.9 µm. The PMMA and suppressed pillars are here responsible for the increasing material losses and limiting the value of negative refractive index. An analytical approach has been suggested to characterise fishnet structures at oblique incidence. The expressions for an absorbing medium are rewritten for an alternative definition of refractive index. The expressions are initially validated for a dielectric slab and a metal film. These results provide the possibility that this proposal may yield a general algorithm for obtaining the complex reflection and transmission coefficients for artificial structures. FDTD simulations have been extensively used in this thesis to understand the optical metamaterials and their characterization.

Quantum Dots As Biophotonics Tools.

This chapter provides a short review of quantum dots (QDs) physics, applications, and perspectives. The main advantage of QDs over bulk semiconductors is the fact that the size became a control parameter to tailor the optical properties of new materials. Size changes the confinement energy which alters the optical properties of the material, such as absorption, refractive index, and emission bands. Therefore, by using QDs one can make several kinds of optical devices. One of these devices transforms electrons into photons to apply them as active optical components in illumination and displays. Other devices enable the transformation of photons into electrons to produce QDs solar cells or photodetectors. At the biomedical interface, the application of QDs, which is the most important aspect in this book, is based on fluorescence, which essentially transforms photons into photons of different wavelengths. This chapter introduces important parameters for QDs' biophotonic applications such as photostability, excitation and emission profiles, and quantum efficiency. We also present the perspectives for the use of QDs in fluorescence lifetime imaging (FLIM) and Förster resonance energy transfer (FRET), so useful in modern microscopy, and how to take advantage of the usually unwanted blinking effect to perform super-resolution microscopy.

Near-infrared third-order nonlinearity of PbO-GeO(2) films containing Cu and Cu(2)O nanoparticles

We report measurements of the nonlinear (NL) refractive index n(2) of lead-germanium films (LGFs) containing Cu and Cu(2)O nanoparticles (NPs). The thermally managed eclipse Z-scan technique with 150 fs pulses from a laser operating at 800 nm was used. The NL refractive index measured, n(2)=6.3x10(-12) cm(2)/W has electronic origin and the NL absorption coefficient alpha(2) is smaller than 660 cm/GW. The figure of merit n(2)/lambda alpha(2) is enhanced by more than two orders of magnitude in comparison with the result for the LGFs without the copper based NPs. (C) 2008 American Institute of Physics.

Photonic Band Gaps in One-Dimensionally Ordered Cold Atomic Vapors

We experimentally investigate the Bragg reflection of light at one-dimensionally ordered atomic structures by using cold atoms trapped in a laser standing wave. By a fine-tuning of the periodicity, we reach the regime of multiple reflection due to the refractive index contrast between layers, yielding an unprecedented high reflectance efficiency of 80%. This result is explained by the occurrence of a photonic band gap in such systems, in accordance with previous predictions.

Femtosecond third-harmonic generation in a glass ceramic containing sodium niobate nanocrystals

The third-harmonic optical susceptibility, chi((3))(3 omega; omega, omega, omega) of a silicate glass ceramic containing sodium niobate nanocrystals was measured for incident broadband light with central frequency omega corresponding to 1900nm. Absolute values of |chi((3))| and the dispersion of the refractive index from 600 to 1900nm were measured using the spectrally resolved femtosecond Maker fringes technique. The experiments show that |chi((3))| is 1 order of magnitude larger than silica, and it grows by similar to 50% when the volume fraction occupied by the nanocrystals increases up to 40%. (C) 2011 Optical Society of America

Observation of cooperative Mie scattering from an ultracold atomic cloud

Scattering of light at a distribution of scatterers is an intrinsically cooperative process, which means that the scattering rate and the angular distribution of the scattered light are essentially governed by bulk properties of the distribution, such as its size, shape, and density, although local disorder and density fluctuations may have an important impact on the cooperativity. Via measurements of the radiation pressure force exerted by a far-detuned laser beam on a very small and dense cloud of ultracold atoms, we are able to identify the respective roles of superradiant acceleration of the scattering rate and of Mie scattering in the cooperative process. They lead, respectively, to a suppression or an enhancement of the radiation pressure force. We observe a maximum in the radiation pressure force as a function of the phase shift induced in the incident laser beam by the cloud's refractive index. The maximum marks the borderline of the validity of the Rayleigh-Debye-Gans approximation from a regime, where Mie scattering is more complex. Our observations thus help to clarify the intricate relationship between Rayleigh scattering of light at a coarse-grained ensemble of individual scatterers and Mie scattering at the bulk density distribution.