Design of Photonic Bandgap Fibre with Novel Air-Hole Structure

We introduce PBGFs with the cladding made of our newly designed quasi-hexagonal air holes and demonstrate how it actually operates. This cladding structure is introduced for the first time to the best of our knowledge, and is realized by making use of the hydrofluoric acid＇s corrosive properties. The fibre corrosion can be accurately controlled, thus opening us the gate for the design and fabrication of new PB. GFs with more complex and more efficient cladding structures. Numerical results and actual simulations indicate that PBGFs built with this cladding structure have improved b~dgap properties and guiding bands as wide as 500 nm have been theoretically reached. Using the same method, we have also been able to design two other types of PBGFs with improved cladding structure.     

Characteristics of Photonic Bandgap Fibres with Hollow Core＇s Inner Surface Coated by a Layer Material

Hollow core＇s inner surface coating in a photonic bandgap fibre （PBCF） is investigated by means of finite element method. The coat material and thickness-dependence dispersion curve and group velocity dispersion are numerically studied. The coating with materials of low index or small thickness will rise up the dispersion curve but will not induce surface modes. However, coating with materials of high index or big coat thickness will induce surface modes and avoided-crossings. By varying coat material＇s refractive index and thickness, the appearances of surface modes and avoided-crossings can be changed. It is found that the avoided-crossing can enormously enlarge the negative dispersion which can find applications in dispersion compensation. We numerically achieve a negative dispersion as large as -21416.15ps/nm/km. The results give a physical insight into the propagation properties of PBGFs with the hollow core coated by a layer of material and are of crucial significance in the applications of PBGF coating.     

Dispersion properties of hollow-core photonic bandgap fibers based on a square lattice cladding

In this paper the dispersion properties and confinement loss of hollow-core photonic bandgap fibers (PBGFs) based on a square lattice (SL) with rounded square air-holes was investigated for the first time, by using a full-vector finite element method (FEM). The waveguide group velocity dispersion (GVD) curves with different core diameter D, air hole size d, rounded diameter d_c and hole pitch Λ are presented. The influence of the number of cladding rings on dispersion and confinement loss were also calculated. It was found that as Λ or d increases, the width of PBG becomes wider, and that D and the number of cladding rings have a smaller influence on waveguide GVD. The ratio between bandgap width and central wavelength in our simulation is about 38.1%, which is larger than that of hollow-core PBGFs with triangular lattice (TL) (∼25%). By simulation, the desired zero dispersion wavelength or desired dispersion slope could be obtained by properly choosing the value of d_c or Λ. Compared to TL PBGF, at least nine cladding rings is needed to achieve the confinement loss less than 0.1 dB/m for future application.     

Reducing Insertion Loss of Photonic Crystal Couplers by Suppressing the Remained Power

We find that the increasing coupling strength can lead to a decreasing density of power inside the waveguide of a photonic crystal waveguide （PC-WG） directional coupler, which is called the mode power remaining phenomenon. This phenomenon is detrimental to achieving low insertion loss of the coupler. An improved structure of the PC- WG directional coupler is proposed by simply increasing the radii of air holes in the post coupling region. The simulation results demonstrate that the insertion loss can be reduced dramatically by suppressing the remained power, and therefore both the short coupling length （8μm） and low insertion loss （lower than 0.5dB） can be obtained.     

Influence of Non-uniform Temperature Field on Spectra of Fibre Bragg Grating

We simulate the spectrum characteristics of fibre Bragg grating （FBG） with non-uniform temperature using the transmission matrix method, and the results are analysed. It is found that firstly the modulated coefficient of average refractive index is a very important parameter that influences the spectrum characteristic of the fibre Bragg grating, and secondly the spectrum curves are different in different temperature fields at the same parameter. Hence, we can determine the metrical temperature by analysing the spectrum of fibre Bragg grating.     

An Integrative Biosensor Based on Contra-Directional Coupling Two-dimensional Photonic Crystal Waveguides

We propose an integrative biochemical sensor utilizing the dip in the transmission spectrum of a normal singleline defect photonic crystal （PC） waveguide, which has a eontra-directional coupling with another PC waveguide. When the air holes in the PC slab are filled with a liquid analyte with different refractive indices, the dip has a wavelength shift. By detecting the output power variation at a certain fixed wavelength, a sensitivity of 1.2 × 10^-4 is feasible. This structure is easy for integration due to its plane waveguide structure and omissible pump source. In addition, high signal to noise ratio can be expected because signal transmits via a normal single-line defect PC waveguide instead of the PC hole area or analyte.     

Beaming effect from increased-index photonic crystal waveguides

We study the beaming effect of light for the case of increased-index photonic crystal (PhC) waveguides, formed through the omission of low-dielectric media in the waveguide region. We employ the finite-difference time-domain numerical method for characterizing the beaming effect and determining the mechanisms of loss and the overall efficiency of the directional emission. We find that, while this type of PhC waveguide is capable of producing a highly collimated emission as was demonstrated experimentally, the inherent characteristics of the structure result in a restrictively low efficiency in the coupling of light into the collimated beam of light.     

Fibre Optical Parametric Amplification in Defect Bragg Fibres with Zero Dispersion Slow Light Effect

Nonlinearity enhancement by slow light effect and strong light confinement in defect Bragg fibres is demonstrated and anMysed in applications of fibre optical parametric amplifiers. Broadband low group velocity and zero dispersion as well as the strong light confinement by band gap enhances the nonlinear coefficient up to more than one order than the conventional high nonlinear fibres. Moreover, the zero dispersion wavelength of coupled core mode can be designed arbitrarily, under which the phase-matching bandwidth of the nonlinear process can be extended.     

Spatially Inhomogeneous Gain Modification in Photonic Crystals

The electric and magnetic energy distributions in photonic crystals （PC） are calculated by using the plane wave expansion （PWE） method. Even though the total electric and magnetic energy in each unit cell of photonic crystals are equal to each other, the ratio of electric and magnetic energy densities varies depending on the local position. Based on Fermi＇s golden rule, the optical gain is analysed in the full quantum framework that takes the nonuniform energy density ratio into account. This nonuniform energy density ratio in photonic crystals, defined in an equal form as gain modification factor, leads to spatially inhomogeneous modification of optical gain. Results reported in the paper provide a new perspective for analysing gain characteristics, as well as the lasing properties, in photonic crystals.     

Zero-Dispersion Slow Light with Wide Bandwidth in Photonic Crystal Coupled Waveguides

By introducing an adjustment waveguide besides the incident waveguide, zero-dispersion slow light with wide bandwidth can be realized due to anticrossing of the incident waveguide mode and the adjustment waveguide mode. The width of the adjustment waveguide （W2） and the hole radii of the coupling region （r＇） will change the dispersion of incident waveguide mode. Theoretical investigation reveals that zero dispersion at various low group velocity vg in incident waveguide can be achieved. In particular, proper W2 and r＇ can lead to the lowest vg of 0.0085c at 1550 nm with wide bandwidth of 202 GHz for zero dispersion.     

Antiresonant guiding microstructured optical fibers for sensing applications

A novel refractometric sensor utilizing unique spectral properties of antiresonant-guiding microstructured optical fibers is proposed. The sensor operation is based on the wavelength shift of the transmission spectrum in response to the refractive index change of a sample loaded in the air-holes of the microstructured optical fiber. Refractive index changes on the order of 0.1% can be detected using less than a nanoliter of a sample.     

Highly Birefringent Honeycomb Photonic Bandgap Fibre

A new structure of highly birefringent honeycomb photonic bandgap fibres （PBGFs）, including an elliptical air hole in its solid core, is proposed and analysed by using full vectorial plane wave expansion method. From the numerical results it is confirmed that the proposed PBGF. has birefringence of the order of 10^-3. Moreover, there are two single-polarization single-mode ranges at varying normalized wavelength, in one of which only the slowaxis mode exists, and in the other only the fast-axis mode exists, which has not been achieved in index-guiding photonic crystal fibres so far.     

Engineering absolute band gap in anisotropic hexagonal photonic crystals

We analyze the band gap spectra of two-dimensional anisotropic photonic crystals created by a hexagonal lattice of rods covered by an interfacial layer (e.g. tellurium tubes). Using the plane-wave numerical expansion method, we study the modification of the band gap spectrum when the rods are infiltrated with other material, and discuss the optimization strategies leading to the maximum value of the absolute band gap.     

Band-gap properties of two-dimensional low-index photonic crystals

We study the band-gap properties of two-dimensional photonic crystals created by a lattice of rods or holes conformed in a symmetric or asymmetric triangular structure. Using numerical plane-wave method, we calculate a minimum value of the refractive-index contrast for opening both partial and full two-dimensional spectral gaps for both TM- and TE-polarized waves. We also analyze the effect of ellipticity of rods and holes and their orientation on the threshold value and the relative size of the band gaps.     

Absolute band gap properties in two-dimensional photonic crystals composed of air rings in anisotropic tellurium background

We analyze the band gap spectra of two-dimensional photonic crystals created by square, triangular and honeycomb lattices of air rings with different geometrical shapes and orientations in anisotropic tellurium background. Specifically, five different shapes of rings (circular, hexagonal, elliptical, rectangular and square) are considered. Using the numerical plane wave method, we discuss the maximization of the absolute photonic band gap width as a function of air ring parameters with different shapes and orientations in three types of lattices.     

Non-Degenerate Four-Wave Mixing in Microstructure Fibres

Non-degenerate four wave mixing based on third-order susceptibility X^3 in high nonlinearity microstructure fibres is experimentally demonstrated. The Stokes and anti-Stokes peaks are observed simultaneously by launching 10-fs pulses from an 80Ohm Ti：sapphire laser into the fibre.     

Fabrication of Pure Silica Core Multimode Ultraviolet Optical Fibre Probes by Tube Etching

As a light wave-guide component for transmitting ultraviolet （UV） laser pulses, pure silica core UV fibre probes have attracted a great deal of attention in the near-field optical data storage and bio-medical studies. We fabricate UV fibre probes with tips in dimension of about 2-5μm and taper angle 16° by the tube etching method, using 40% HF acid as etching solution and xylene as overlayer. Probes produced have curvy configuration with smooth surface. The yield of fine probes is rather high and etching operation greatly simplified. With higher damage threshold, pure silica core multimode UV fibre probes can be coupled into more laser power. In addition, using UV light reduces the cutoff wavelength of the fibre probes, which is in favour of increasing the transmission efficiency of the probe. Furthermore, the larger tip dimension helps to enhance the light throughput either. The advances of fabrication technique of UV optical fibre probe may further support the studies of UV light data storage, pulsed laser biosurgery and UV photolithography.     

Inhibition of two-photon absorption due to dipole-dipole interaction in nanoparticles

We have investigated the inhibition of two-photon absorption in photonic crystals doped with an ensemble of four-level nanoparticles. The particles are interacting with one another by the dipole-dipole interaction. Dipoles in nanoparticles are induced by a selected transition. Numerical simulations have been performed for an isotropic photonic crystal. Interesting phenomena have been predicted such as the inhibition of the two-photon absorption due to the dipole-dipole interaction. It has also been found that the inhibition effect can be switched on and off by tuning a decay resonance energy within the energy band of the crystal. A theory of dressed states has been used to explain the results.     

Large forbidden angles for a two-dimensional photonic crystal of connected-honeycomb lattice

The off-plane band structures of a two-dimensional photonic crystals of connected-honeycomb lattice are calculated by the plane-wave expansion method. To investigate how the band gaps vary with the off-plane angle, an effective refractive-index model is employed to work out the corresponding angular gap map. We find that the connected-honeycomb lattice can provide forbidden angles taking up as much as 58.6% of the solid angle. The result will be helpful in the design of photonics devices incorporating two-dimensional photonic crystal structures.     

Tunable full band gap in two-dimensional anisotropic photonic crystals infiltrated with liquid crystals

We analyze the tunability of full band gap in two-dimensional photonic crystals created by square and triangular lattices of anisotropic tellurium rods in air background, considering that the rods are infiltrated with liquid crystal. Using the plane-wave expansion method, we study the variation of full band gap by changing the optical axis orientation of liquid crystal.