Abstract:We demonstrate the fabrication of a high performance multimode (MM) to single-mode (SM) splitter or "photonic lantern", first described by Leon-Saval et al. (2005). Our photonic lantern is a solid allglass version, and we show experimentally that this device can be used to achieve efficient and reversible coupling between a MM fiber and a number of SM fibers, when perfectly matched launch conditions into the MM fiber are ensured. The fabricated photonic lantern has a coupling loss for a MM to SM tapered transition of only 0.32 dB which proves the feasibility of the technology.

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We address the long-standing unresolved problem concerning the V -parameter in a photonic crystal fiber (PCF). Formulate the parameter appropriate for a core-defect in a periodic structure we argue that the multi-mode cut-off occurs at a wavelength λ * which satisfies VPCF(λ * ) = π. Comparing to numerics and recent cut-off calculations we confirm this result.In photonic crystal fibers (PCFs) an arrangement of air-holes running along the full length of the fiber provides the confinement and guidance of light. The airholes of diameter d are typically arranged in a triangular lattice[1] with a pitch Λ (see insert in Fig. 2), but e.g. honey-comb[2] and kagome [3,4] arrangements are other options. By making a defect in the lattice, light can be confined and guided along the fiber axis. The guidance mechanism depends on the nature of the defect and the air-hole arrangement. For the triangular lattice with a silica-core light is confined by total-internal reflection [1] whereas for an air-core a photonic-bandgap confines light to the defect.[5] For recent reviews we refer to Ref. 6 and references therein.Both type of PCFs have revealed surprising and novel optical properties. In this work we consider the silicacore PCF (see insert in Fig. 2) which was the one first reported.[1] This structure provides the basis of a variety of phenomena including the endlessly single-mode behaviour, [7] large-mode area PCFs,[8] as well as highly non-linear PCF with unique dispersion properties. [9,10,11] Properties of standard fibers are often parametrized by the so-called V -parameter and the entire concept is very close to the heart of the majority of the optical fiber community (see e.g. Refs. 12, 13). The cut-off properties and the endlessly single-mode phenomena of PCFs can also be qualitatively understood within this framework. [1,7,14,15,16] However, the proper choice of the correct length scale for the V -parameter has, until now, remained unsolved as well as the value of V * that marks the second-order cut-off. In this Letter we clarify this problem and also put recent work on multi-mode cut-off [17,18] into the context of the V -parameter.The tradition of parametrizing the optical properties in terms of the V -parameter stems from analysis of the step-index fiber (SIF). The SIF is characterized by the core radius ρ, the core index n c , and the cladding index n cl which all enter into the parameter V SIF given byBecause of its inverse dependence on the wavelength λ, this quantity is often referred to as the normalized frequency. However, in a more general context, this is somewhat misleading (especially if n c and/or n cl has a strong wavelength dependence) and in this Letter we would like to emphasize a more physical interpretation. To do this, we first introduce the numerical aperture NA (or the angle of divergence θ) given bywhich follows from use of Snell's law for critical incidence at the interface between the n c and n cl regions (see e.g. Refs. 12, 13). Next, we introduce the free-space wavenumber k = 2π/λ and its tr...

We present an electrically controlled photonic bandgap fiber device obtained by infiltrating the air holes of a photonic crystal fiber (PCF) with a dual-frequency liquid crystal (LC) with pre-tilted molecules. Compared to previously demonstrated devices of this kind, the main new feature of this one is its continuous tunability due to the fact that the used LC does not exhibit reverse tilt domain defects and threshold effects. Furthermore, the dual-frequency features of the LC enables electrical control of the spectral position of the bandgaps towards both shorter and longer wavelengths in the same device. We investigate the dynamics of this device and demonstrate a birefringence controller based on this principle.

We numerically study the possibilities for improved large-mode area endlessly single mode photonic crystal fibers for use in high-power delivery applications. By carefully choosing the optimal hole diameter we find that a triangular core formed by three missing neighboring air holes considerably improves the mode area and loss properties compared to the case with a core formed by one missing air hole. In a realized fiber we demonstrate an enhancement of the mode area by ∼ 30 % without a corresponding increase in the attenuation.Applications requiring high-power delivery call for single-mode large-mode area (LMA) optical fibers. While standard-fiber technology has difficulties in meeting these requirements the new class[1] of all-silica photonic crystal fibers (PCF) has a big potential due to their endlessly single-mode properties [2] combined with (in principle) unlimited large effective areas.[3] For recent reviews we refer to Refs. 4, 5.The cladding structure of these PCFs consists of a triangular array of air holes of diameter d and pitch Λ corresponding to an air-filling fraction f = π/(2 √ 3)(d/Λ) 2 . The presence of the air holes results in a strongly wavelength dependent effective index n eff of the cladding and in the short and long wavelength limits we have lim λ≪Λ n eff = n si , lim λ≫Λ n eff = f × n air + (1 − f ) × n si ≡n.