All-solid silica-based photonic crystal fibers

An index-guiding all-solid photonic crystal fiber (PCF) composed entirely of silica material is proposed in this paper. The core of this optical fiber is composed of pure silica, and the cladding consists of doped silica rod in the background of pure silica. The dependence of confinement loss on the diameter of the doped rods, the number of doped-rod rings, and the doping level is investigated numerically. In addition, the proposed fiber possesses a shorter cutoff wavelength as compared with the air/silica PCF, which is directly confirmed by the V parameter, and explained based on a scalar approximation method. Furthermore, the bending loss for the fiber is predicted. A low-loss single-mode all-solid silica-based PCF with a large-mode-area is possible by the appropriate selection of configuration parameters.     

Design of defect-core in highly birefringent photonic crystal fibers with anisotropic claddings

The influence of defect-core on the birefringence and confinement losses of rectangular-lattice photonic crystal fibers are investigated numerically by applying the multipole method. Numerical results illustrate that the birefringence in such fibers is determined not only by the arrangement of air holes in the cladding but also the shape of the core. It is found that asymmetry of the core represented by its rectangular shape implies a higher effective index of the mode that is parallel with the longer side of the rectangle, whereas the anisotropic rectangular-lattice cladding gives rise to just the opposite effect and thus the resulting birefringence can be controlled by a proper combinations of both mechanisms. In particular, effect of the asymmetry of the core on the birefringence is dominant for shorter wavelength. Increased birefringence and reduced confinement loss can be achieved, if we form the core by the omission of several air holes in a row to reduce its negative effect on the birefringence. On the other hand, when asymmetry is increased in the other direction, a negative birefringence at shorter wavelength can be achieved. This occurs due to the fact that asymmetry of the core at higher frequencies overcomes the effect of the asymmetric cladding. As a result, its possible to achieve zero birefringence in anisotropic cladding photonic crystal fiber with an asymmetric core.     

Design of Microstructured Optical Fibres with Elliptical Air Holes for Properties： Single-Polarization Single-Mode and Nearly Zero Ultra-Flattened Dispersion

By using the complex finite element method （FEM） under perfectly matched layer （PML） boundary conditions, dispersion properties of microstructured optical fibres （MOFs） with elliptical air holes are analysed by changing the pitch and sizes of air holes belonging to the inner three rings. Meanwhile, the confinement loss of the fundamental mode is engineered to achieve the single-polarization single-mode transmission. Based on this analysis, a novel design of MOFs for properties of the single-polarization single-mode and the nearly zero ultraflattened dispersion between lpskm^-1 nm^-1 in the wavelength range of 1.2-1.6μm is presented for the first time.     

The spectral sensitivity of long period gratings fabricated in elliptical core D-shaped optical fibre

Long period gratings (LPGs) were written into a D-shaped optical fibre that has an elliptical core with a W-shaped refractive index profile and the first detailed investigation of such LPGs is presented. The LPGs’ attenuation bands were found to be sensitive to the polarisation of the interrogating light with a spectral separation of about 15 nm between the two orthogonal polarisation states. A finite element method was successfully used to model many of the behavioural features of the LPGs. In addition, two spectrally overlapping attenuation bands corresponding to orthogonal polarisation states were observed; modelling successfully reproduced this spectral feature. The spectral sensitivity of both orthogonal states was experimentally measured with respect to temperature and bending. These LPG devices produced blue and red wavelength shifts depending upon the orientation of the bend with measured maximum sensitivities of −3.56 and +6.51 nm m, suggesting that this type of fibre LPG may be useful as a shape/bend orientation sensor with reduced errors associated with polarisation dependence. The use of neighbouring bands to discriminate between temperature and bending was also demonstrated, leading to an overall curvature error of ±0.14 m⁻¹ and an overall temperature error of ±0.3 °C with a maximum polarisation dependence error of ±8 × 10⁻² m⁻¹ for curvature and ±5 × 10⁻² °C for temperature.     

Characteristics of side-polished thermally expanded core fiber and its application as a band-edge filter with a high cut-off property

Characteristics of the side-polished thermally expanded core (TEC) fiber have been investigated theoretically and experimentally. The effect of core expanding on the transmission of the side-polished TEC fiber is predicted theoretically and demonstrated experimentally. The side-polished TEC fiber covered with an external medium whose chromatic dispersion is much different from the fiber materials, is applied to a band-edge filter with a high cut-off. The relationship between the expanded core diameter and the performance of the band-edge filter was measured and discussed.     

Optimization of Mechanically Induced Long-Period Fibre Grating Transmission Spectrum

A new double-layer grating template is designed to reduce the out-of-band loss as much as 1.8dB when the loss of LPo3 reaches 10.2 dB. Meanwhile, we propose a method to remove the sidelobes in the transmission spectra by the adjustment of the thickness of pressure plates. The plate-thickness-induced shift of resonant wavelength and the attenuation of loss peak intensity when removing sidelobes can be modified by the fibre distance and contact point on the pressure plates.     

Polarization State Evolution in Fibre Polarization Transformer Influenced by Phase Difference

In accordance with the intrinsic structure of controllably-spun birefringent-fibre-based fibre polarization transformer （FPT）, the Jones vector is calculated from point to point along the polarization transforming fibre by the cascade differential phase retarder model. It is the first time using this concise method to examine the phasedifference effect on the evolution of state of polarization （SOP） inside this special fibre component. Both the extinction ratio and orientation angle of SOP are calculated to give out a whole evolution history from linear polarization light at the slow spun end into circular polarization light at the fast spun end, and vice versa. The influence of phase-difference is discussed on the polarization transforming performance and further referential conclusion is provided for design and test of the FPT component.     

Cascaded Optical Buffer Based on Nonlinear Polarization Rotation in Semiconductor Optical Amplifiers

A cascaded buffer based on nonlinear polarization rotation in semiconductor optical amplifiers is proposed, which is suitable for fast reconfiguration of buffering time at picoseconds. With the proposed buffer, sixty different buffer times are demonstrated at 2.5Gb/s.     

Temperature and pressure sensitivities of the highly birefringent photonic crystal fiber with core asymmetry

We investigated a particular design of a highly birefringent PCF with attractive features for pressure sensing applications. A plane-wave method together with the finite element method were used to numerically calculate phase and group modal birefringence, pressure and temperature sensitivities of our fiber. The simulation results together with the experiments demonstrate a considerable difference between a very high phase birefringence (B ∼ 10⁻³) and a very low negative group birefringence (G −10⁻³). Our fiber exhibits a low and positive temperature sensitivity (K_T < 0.1 rad/(K⋅m)), and relatively high and negative mechanical (pressure) sensitivity (K_p ≤ −10 rad/(MPa⋅m)), which supports its possible use as a mechanical sensor that does not require any temperature compensation.     

Supercontinuum Generation with High Birefringence SF6 Soft Glass Photonic Crystal Fibers

A kind of high birefringence SF6 soft glass photonic crystal fiber （HBSF6-PCF） is proposed. The properties of birefringence, dispersion, nonlinear coefficient and the transmission characteristics are studied by the multipole method and the adaptive split-step Fourier method. The numerical results show that the birefringence and the nonlinear coefficient reach the orders of 10^-2 and 10^-1, respectively. In addition, the HBSF6-PCFs can generate very smooth supercontinuum spectra when illuminated with femtosecond pulsed light of 1064 nm. It is found that up to 800nm spectral width （evaluated at -5dB from the peak） is achieved. Therefore, the advantage of the HBSF6-PCFs is such that a high birefringence, a high nonlinearity and a smooth supercontinuum are perfectly combined in them.     

High-Flexible Single-Polarization Single-Mode Photonic-Crystal Fibre

We propose a new type of high flexible single-polarization single-mode (SPSM) photonic crystal fibre (PCF). Numerical analysis indicates that the SPSM PCF proposed guides only one polarization mode over a wavelength band broader than the conventional SPSM fibres. The position and the bandwidth of the single polarization mode region depend flexibly on the structure parameters such as the hole diameters d₁ and d₂ and the hole pitch Λ of the SPSM PCFs. A 200-nm SPSM region can be obtained for the SPSM PCF with Λ=2.55μm, d₁/Λ=0.3 and d₂/Λ=0.833 and the modal birefringence at the wavelength 1.31μm can reach 1.8×10^-3.     

Design Guideline of Hollow-Core Fibres with Cobweb Cladding Structure

By using a plane wave expansion method, some important cobweb cladding structure are analysed. Taking a dielectric parameters of designing the hollow-core fibre with material PMMA, for example, the tolerance of the parameters is discussed. The results show that the parameters of the structure possess oneself of a regularity and limit, and have a larger tolerance for the structural parameters in fabrication.     

Transverse Multimode Evolution in Non-Adiabatic Optical Micro/Nanofiber Tapers

The multimode evolution, optical losses and wavelength response of non-adiabatic micro/nano-fiber （MNF） tapers are numerically simulated using a three-dimensional finite-difference beam propagation method. For a non-adiabatic MNF taper, it is illustrated that optical losses vary with the transition region length and the optical wavelength. We explain how the complicated multimode evolutions result in the complicated optical loss and wavelength response properties, especially when the waist diameters are large enough to allow much higher-order modes. These results may offer valuable references for trapping and guiding cold atoms in atom optics and practical application of micro/nano-devices.     

Highly nonlinear photonic-crystal fibers for the spectral transformation of Cr: forsterite laser pulses

We demonstrate novel photonic-crystal fibers (PCFs) fabricated of a highly nonlinear glass. Dispersion profiles and nonlinearity of these fibers are tailored with an array of submicron holes in the fiber core. With the PCF structure designed to provide a nonlinearity on the order of 10³ W⁻¹ km⁻¹ at the radiation wavelength of 1 μm and a fundamental-mode dispersion profile with zero group-velocity dispersion around 1.19 μm, unamplified femtosecond Cr: forsterite laser pulses are efficiently frequency-converted into the 540-1000-nm wavelength range through solitonic spectral-transformation mechanisms and four-wave mixing.     

Limiting efficiencies of second-harmonic generation and cascaded χ processes in quadratically nonlinear photonic nanowires

Quadratically nonlinear waveguides with subwavelength core dimensions are shown to provide limiting efficiencies of second-harmonic generation and three-wave mixing, as well as cascaded χ⁽²⁾ nonlinear-optical interactions. Small-core waveguides made of high-χ⁽²⁾ materials, such as gallium arsenide and indium phosphide, are shown to allow anomalous dispersion to be achieved within the range of wavelengths from 1.3 to 2.0 μm, with the wavelength of zero group-velocity dispersion controlled by the size of the waveguide core.     

Flat Supercontinuum Generation at 1550 nm in a Dispersion-Flattened Microstructure Fibre Using Picosecond Pulse

The generation of a flat supercontinuum of over 80nm in the 1550nm region by injecting 1.6ps 10 GHz repetition rate optical pulses into an 80-m-long dispersion-flattened microstructure fibre is demonstrated. The fibre has small normal dispersion with a variation smaller than 1.5 （ps·nm^-1·km^-1） between 1500 and 1650nm. The generated supercontinuum ranging from 1513 to 1591 nm has the flatness of ±1.5 dB and it is not so flat in the range of several nanometres around the pump wavelength 1552nm. Numerical simulation is also used to study the effect of optical loss, fibre parameters and pumping conditions on supercontinuum generation in the dispersion-flattened microstructure fibre, and can be used for further optimization to generate flat broad spectra.     

Analysis of propagation characteristics for an octagonal photonic crystal fiber (O-PCF)

An octagonal photonic crystal fiber (O-PCF) structure with eight air-holes on the first ring is proposed based on a unit isosceles triangle. The propagation characteristics and cut-off behaviors of the O-PCF and the standard hexagonal PCF (H-PCF) are numerically investigated by combining the vector boundary method and the effective area method. The phase boundaries for cut-off, single-mode, and multi-mode operations between the O-PCF and H-PCF are calculated and compared. It is found that under the same pitch Λ and air filling fraction (AFF) of the air-holes the O-PCF has significantly wider wavelength range operating in single-mode region, more circular-like field distribution, and less confinement loss than the H-PCF.     

Multi-Edge-Written Long-Period Fibre Gratings with Low PDL by Using High-Frequency CO2 Laser Pulses

A novel long-period fibre grating （LPFG） with low polarization-dependent loss （PDL） is fabricated by using a multi-edge exposure method with high frequency CO2 laser pulses. The experimental results show that the PDL of a triple-edge-written LPFG with a peak amplitude of-16.5 dB can be as low as 0.22 dB. These hovel LPFGs can find important applications in optical communication and sensing.     

Tapered photonic crystal fibres: properties, characterisation and applications

Microstructured optical fibres (MOFs) have attracted much interest in recent times, due to their unique waveguiding properties that are vastly different from those of conventional step-index fibres. Tapering of these MOFs promises to significantly extend and enhance their capabilities. In this paper, we review the fabrication and characterisation techniques of these fibre tapers, and explore their fundamental waveguiding properties and potential applications. We fabricate photonic crystal fibre tapers without collapsing the air-holes, and confirm this with a non-invasive probing technique that enables the characterisation of the internal microstructure along the taper. We then describe the fundamental property of such tapers associated with the leakage of the core mode that leads to long-wavelength loss, influencing the operational bandwidth of these tapers. We also revisit the waveguiding properties in another form of tapered MOF photonic wires, which transition through waveguiding regimes associated with how strongly the mode is isolated from the external environment. We explore these regimes as a potential basis for evanescent field sensing applications, in which we can take advantage of air-hole collapse as an extra dimension to these photonic wires.     

A broadband ultra flattened chromatic dispersion microstructured fiber for optical communications

A double-cladding microstructured fiber (MF) is proposed in this paper. The inner cladding of this optical fiber is composed of elliptical air holes and silica. The dependence of dispersion on the diameter of the air holes, the pitch, and the axes of the elliptical holes is investigated numerically. The proposed fiber possesses an ultra flattened dispersion curve over a wide wavelength range, and its dispersion value is small. The effective mode area is approaching to 60 μm², and the confinement loss is as low as <0.025 dB/km at 1550 nm. While choosing suitable structure parameters, an ultra dispersion-flattened MF within a broadband from1000 nm to 1900 nm can be achieved. The dispersion fluctuation is 0.6-1.0 ps/(nm·km) in all S, C and L band.