Nearly zero ultraflattened dispersion in photonic crystal fibers

We present a procedure for achieving photonic crystal fibers with nearly zero ultraflattened group-velocity dispersion. Systematic knowledge of the special guiding properties of these fibers permits the achievement of qualitatively novel dispersion curves. Unlike the behavior of conventional fibers, this new type of dispersion behavior permits remarkably improved suppression of third-order dispersion, particularly in the low-dispersion domain.     

Enhanced nonlinear effects in photonic crystal fibers

Photonic crystal fibers are a new class of single-material optical fibers with wavelength-scale air holes running down the entire fiber length. Photonic crystal fibers were first developed in 1996 and have subsequently been the focus of increasing scientific and technological interest in the field of fiber optics. The manufacturing, principles, basic properties, and some applications of photonic crystal fibers are briefly described in this paper. A review of our recent work on the nonlinear effects in photonic crystal fibers is presented, and special emphasis is placed on such effects as supercontinuum generation, frequency conversion, and solitons observed when femtosecond light pulses propagate in these fibers.     

Birefringent photonic crystal fibers with zero polarimetric sensitivity to temperature

We designed, fabricated, and characterized birefringent holey fibers with zero polarimetric sensitivity to temperature. The sensitivity measurements were carried out in a wide spectral range of 0.68–1.55 μm in fibers with different hole and pitch values and with birefringence induced by a pair of large holes adjacent to the core. Our results show that zero sensitivity to temperature can be obtained at certain wavelengths for the bare fibers with properly adjusted geometrical parameters. Moreover, the spectral measurements of the sensitivity to temperature are in good agreement with the modeling results for all the investigated fibers.     

Experimental investigation of cutoff phenomena in nonlinear photonic crystal fibers

Modal cutoff is investigated experimentally in a series of high-quality nonlinear photonic crystal fibers. We demonstrate a suitable measurement technique with which to determine the cutoff wavelength and verify the technique by inspecting the near field of the modes that may be excited below and above the cutoff. We observe a double-peak structure in the cutoff spectra, which is attributed to splitting of the higher-order modes. The cutoff is measured for seven different fiber geometries with different pitches and relative hole sizes, and very good agreement with recent theoretical work is found.     

Linear and nonlinear properties of multicomponent glass photonic crystal fibers

Processes resulting in supercontinuum generation in multicomponent glass photonic crystal fibers are reviewed in this paper. Multicomponent glass photonic crystal fibers are shown to have a broad transmission range, extending up to 4.5 μm in selected cases. Pumping with a 1240-nm femtosecond pulse at very low sub-nJ energies resulted in soliton formation and dispersive wave generation in a multicomponent PCF sample having a double-core square-lattice structure. These processes were described using a phase-matching model derived from the simulated dispersive properties of the fiber. Third-harmonic generation was observed in the radiation modes of a different cobweb sample with the simultaneous formation of a soliton in the NIR.     

Hydrophobic photonic crystal fibers

We propose and demonstrate hydrophobic photonic crystal fibers (PCFs). A chemical surface treatment for making PCFs hydrophobic is introduced. This repels water from the holes of PCFs, so that their optical properties remain unchanged even when they are immersed in water. The combination of a hollow core and a water-repellent inner surface of the hydrophobic PCF provides an ultracompact dissolved-gas sensor element, which is demonstrated for the sensing of dissolved ammonia gas.     

Elliptical-hole photonic crystal fibers

We study the dispersive properties of photonic crystal fibers (PCF's) with elliptical air holes. The unusual guidance of PCF leads to novel behavior of the birefringence, group-velocity walk-off, and dispersion parameters, including the possibility of zero walk-off with high birefringence in the single-mode regime. A number of these effects are closely tied to the underlying radiation states of the air-hole lattice.     

Extremely large birefringence and shifting of zero dispersion wavelength of photonic crystal fibers

Three different types of photonic crystal fibers have been investigated which promise very large birefringence. The first type fiber is band gap guiding, the second index guiding, while the third type is index guiding with high refractive index circular and elliptical regions in the innermost ring. The birefringence, group velocity dispersion, modal effective index and mode field area of these fibers have been numerically estimated by employing finite difference time domain method. When elliptical regions are introduced in the first and second rings with the combination of small circular regions, each of these proposed fibers exhibits large birefringence with shifted zero dispersion point. Among these three different types of fibers, the band gap guiding photonic crystal fiber promises the largest birefringence (～5.45×10^?2) reported so far. The value of the birefringence and group velocity dispersion of these fibers can be controlled by controlling the hole pitch. Largest birefringence is achieved with a specific value of hole pitch.     

Supercontinuum generation in photonic crystal fibers made from highly nonlinear glasses

The nonlinear propagation of femtosecond pulses and supercontinuum generation in photonic crystal fibers made from glasses with high nonlinear refractive indices are theoretically investigated without the use of the slowly varying envelope approximation. For specifically designed photonic crystal fibers with two zero-dispersion wavelengths, we predict supercontinuum generation due to nonsolitonic radiation emitted by the solitons on the shorter- and on the longer-wavelength sides. The physical mechanism and the peculiarities of such radiation on both spectral sides are studied and explained by the specific phase-matching relations between the solitons and their associated radiation.     

Birefringence-induced splitting of the zero-dispersion wavelength in nonlinear photonic crystal fibers

We investigate how the strongly wavelength-dependent birefringence in nonlinear photonic crystal fibers leads to a splitting in the zero-dispersion wavelength for the two polarizations. We translate the requirements for the maximum splitting of the zero-dispersion wavelength to requirements for transverse structural uniformity by adopting a simple effective-index approach in which the birefringence is calculated in a step-index fiber with an elliptical core. We find that to reduce the splitting to less than 1 nm the birefringence should be less than 2 x 10(-5), resulting in a transverse uniformity requirement of 1-3%, depending on the index step from the core to the cladding.     

Highly nonlinear dispersion-flattened square photonic crystal fibers with low confinement losses

A new simple structure of an index-guiding highly nonlinear dispersion-flattened square photonic crystal fiber (HNDFSPCF) with low confinement losses is proposed. The results reveal that it is possible to design five-rings HNDF-SPCFs with a flattened dispersion of 0.43 ps/(nm·km), low dispersion slope of -0:02 ps/(nm²·km), low confinement loss of approximately 10³ dB/m, and a large nonlinear coefficient of approximately 35W^-1 km^-1 at 1.55 μm. It is also observed that the confinement loss is less than 10^-1 dB/m in the wavelength range of 1.2 –1.7 μm.     

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.     

Highly birefringent photonic crystal fibers

We report a strongly anisotropic photonic crystal fiber. Twofold rotational symmetry was introduced into a single-mode fiber structure by creation of a regular array of airholes of two sizes disposed about a pure-silica core. Based on spectral measurements of the polarization mode beating, we estimate that the fiber has a beat length of approximately 0.4 mm at a wavelength of 1540 nm, in good agreement with the results of modeling.     

Polarization maintaining large nonlinear coefficient photonic crystal fibers using rotational hybrid cladding

In this paper, we present and explore a new hybrid cladding design for improved birefringence and highly nonlinear photonic crystal fibers (PCFs) in a broad range of wavelength bands. The birefringence of the fundamental mode in such a PCF is numerically analyzed using the finite element method (FEM). It is demonstrated that it is possible to design a simple highly nonlinear hybrid PCF (HyPCF) with a nonlinear coefficient of the about 46 W⁻¹ km⁻¹ at a 1.55 μm wavelength. According to simulation, the highest modal birefringence and lowest confinement loss of our proposed structure at the excitation wavelength of λ = 1.55 μm can be achieved at a magnitude of 1.77 × 10⁻² and of the order less than 10² dB/km with only five rings of air-holes in the fiber cladding.     

Influence of group—velocity mismatch on soliton switching in a nonlinear fibre coupler

In this work, the influence of group-velocity mismatch on soliton self-routing pulse switching in a nonlinear fibre coupler is discussed in detail by the use of both variational approach and numerical simulation. The results obtained show that the group-velocity mismatch leads to the relative displacement between the two orthogonal polarization modes, increase of the critical power, and reduction of the elimination-light ratio. For sub-ps pulse, the influence cannot be neglected.     

Optical sensing with photonic crystal fibers

A review of optical fiber sensing demonstrations based on photonic crystal fibers is presented. The text is organized in five main sections: the first three deal with sensing approaches relying on fiber Bragg gratings, long‐period gratings and interferometric structures; the fourth one reports applications of these fibers for gas and liquid sensing; finally, the last section focuses on the exploitation of nonlinear effects in photonic crystal fibers for sensing.     

Group-velocity dispersion in photonic crystal fibers

The dispersion properties of photonic crystal fibers are calculated by expression of the modal field as a sum of localized orthogonal functions. Even simple designs of these fibers can yield zero dispersion at wavelengths shorter than 1.27 mum when the fibers are single mode, or a large normal dispersion that is suitable for dispersion compensation at 1.55 mum.     

Elongation sensitivity of photonic crystal fibers

We determined theoretically a phase elongation sensitivity of the Photonic Crystal Fibers by means of the Multipole Method. The sensitivity was confirmed experimentally with good agreement for LMA-8 PCF.