Second mode transition in microstructured optical fibers: determination of the critical geometrical parameter and study of the matrix refractive index and effects of cladding size

We carried out a numerical study of the second mode transition in finite-sized, microstructured optical fibers (MOFs) for several values of the matrix refractive index. We determined a unique critical geometrical parameter for the second mode cutoff that is valid for all the matrix refractive indices studied. Finite size effects and extrapolated results for infinite structures are described. Using scaling laws, we provide a generalized phase diagram for solid-core MOFs that is valid for all refractive indices, including those of the promising chalcogenide MOFs.     

Ultrasensitive UV-tunable grating in all-solid photonic bandgap fibers

We study the shift of a long period grating’s resonance wavelength with UV induced refractive index changes in an all-solid photonic bandgap fiber. A long period grating is mechanically imprinted in an all-solid photonic bandgap fiber with Germanium doped silica high-index rods in a lower-index silica background. The index of the high-index rods is modified through UV exposure, and we observe that the long period grating’s resonance shifts with the bandgaps. With a sensitivity of 21,000 nanometers per refractive index unit and a 8.8 nm resonance width changes of refractive index of 3 × 10⁻⁶ are in principle detectable     

Optimization of photonic bandgap fiber long period grating refractive-index sensors

Long period gratings in low-index contrast solid-core photonic bandgap fibers are a promising platform for fiber-based fluid refractive index sensing with very low detection limits. We provide a comprehensive investigation of the possibilities for refractive index sensing using that principle in a commercial photonic crystal fiber filled with a fluid: using an acoustic grating, we map out the cladding bands, and use this data to optimize a long period grating’s sensitivity. We then implement the optimized long period grating, again using an acoustic grating, and directly measure its sensitivity to refractive index. We demonstrate a sensitivity of 17,900 nm/RIU (6.94 nm/°C) which corresponds to a smallest detectable index change of the fluid of 8.4 × 10⁻⁶.     

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.     

Tunable spectral enhancement of fiber supercontinuum

We demonstrate tunable spectral enhancement of the supercontinuum generated in a microstructured fiber with a fiber long-period grating. The long-period grating leads to phase distortion and loss that, with subsequent high-intensity propagation in uniform fiber, evolves into an enhancement around the grating's resonant wavelengths. Wavelength tunability is achieved by varying the temperature or the ambient refractive index, and the spectral peak can be extinguished by immersing the grating in index-matching oil.     

Mode field distributions in solid core photonic bandgap fibers

Solid core photonic bandgap fibers (PBGFs) incorporate a microstructure lattice of high index rods in a low index matrix surrounding a defect core formed by one or several missing rods. Liquids, which can have a wide variety of absorption, gain, nonlinear, and thermal properties, have been used as the high index medium in such fibers. The modal interaction with the liquid is thus an important consideration in the design of solid core PBGFs. We numerically investigate the modal overlap with the high index rods and show that it strongly depends on the core size, and that it has only a weak direct dependence on other lattice properties such as fill fraction, number of rings, or index contrast. We apply our results to calculating the effect of material absorption in the fluid on the transmission properties. We present experimental data which quantitatively confirm our numerical predictions.     

Leakage of the fundamental mode in photonic crystal fiber tapers

We report detailed measurements of the optical properties of tapered photonic crystal fibers (PCFs). We observe a striking long-wavelength loss as the fiber diameter is reduced, despite the minimal airhole collapse along the taper. We associate this loss with a transition of the fundamental core mode as the fiber dimensions contract: At wavelengths shorter than this transition wavelength, the core mode is strongly confined in the fiber microstructure, whereas at longer wavelengths the mode expands beyond the microstructure and couples out to higher-order modes. These experimental results are discussed in the context of the so-called fundamental mode cutoff described by Kuhlmey et al. [Opt. Express 10, 1285 (2002)], which apply to PCFs with a finite microstructure.     

Modal cutoff in microstructured optical fibers

We analyze the nature of modal cutoff in microstructured optical fibers of finite cross section. In doing so, we reconcile the striking endlessly single-mode behavior with the fact that in such fibers all propagation constants are complex. We show that the second mode undergoes a strong change of behavior that is reflected in the losses, effective area, and multipolar structure. We establish the parameter subspace in which the fibers are single mode and an accurate value for the limit of the endlessly single-mode regime.     

Dispersion management with microstructured optical fibers: ultraflattened chromatic dispersion with low losses

We numerically demonstrate ultraflattened chromatic dispersion with low losses in microstructured optical fibers (MOFs). We propose using two different MOF structures to get this result. Both structures are based on a subset of a triangular array of cylindrical air holes; the cross sections of these inclusions are circular, and a missing hole in the fiber's middle forms the core. In this MOF structure the diameters of the inclusions increase with distance from the fiber axis until the diameters reach a maximum. With this new design and with three different hole diameters, it requires only seven rings to reach the 0.2-dB/km level at lambda = 1.55 microm with a variation amplitude of dispersion below 3.0 x 10(-2) ps nm(-1) km(-1) of lambda = 1.5-1.6 microm. With the usual MOF (made from holes of identical diameter), we show that at least 18 hole rings are required for losses to decrease to < 1 dB/km at lambda = 1.55 microm.