Interstitial Air Holes Research Articles

Enhancement of evanescent field exposure in a photonic crystal fibre with interstitial holes

In this work, a modified photonic crystal fibre (PCF) that we refer to as Sunny PCF with enhanced evanescent field exposure structure is proposed. The Sunny PCF with triangular interstitial air holes surrounding the core region increases the interaction of the guided mode with the air. Full-vectorial finite element method with perfectly matched layer boundary condition is used to design and simulate the sensitivity and confinement loss characteristics of the proposed Sunny PCF. By adding sunny structure to a conventional PCF with air-filling ratio of 0.9, the highest achievable sensitivity with negligible confinement loss can be boosted up to 21.23% from 15.83% at the operating wavelength of 1550 nm. Sunny PCF can achieve the same sensitivity as suspended-core holey fibre with lower confinement loss. A preliminary Sunny PCF has been fabricated to prove the feasibility of the proposed structure.

Dual-bandgap hollow-core photonic crystal fibers for third harmonic generation.

We present two novel hybrid photonic structures made of silica that possess two well-separated frequency bandgaps. The addition of interstitial air holes in a precise location and size allows these bandgaps to open with a ratio of ∼3 between their central frequencies at the air line ck(z)/w=1, thus fulfilling the basic guidance condition for third harmonic generation in hollow-core fibers. In addition, these designs may serve for high-power laser delivery of two well-separated wavelengths, such as visible and near infrared.

Birefringent solid-core photonic bandgap fibers assisted by interstitial air holes

We report on the fabrication and the characterization of a design of highly birefringent solid-core photonic bandgap fibers. A form birefringence is obtained by keeping open all interstitial air holes but two diametrically opposite and close to the fiber core. Group birefringence as high as 0.57×10−3 and 0.47×10−3 at 1.1 and 1.55 μm, respectively, are demonstrated in the first bandgap.

Effect of interstitial air holes on Bragg gratings in photonic crystal fibre with a Ge-doped core

The effect of interstitial air holes on Bragg gratings in photonic crystal fibre (PCF) with a Ge-doped core is numerically investigated by using the beam propagation method (BPM). It is shown that the interstitial air holes (IAHs) can make Bragg resonance wavelength λB shift a little towards short wavelengths and increase λB – λ1 (the wavelength spacing between the main peak with Bragg resonance wavelength λB and the first side peak with wavelength λ1) and the coupling coefficient κ of Bragg resonance. Moreover, when the ratio of air hole diameter (d) to pitch (Λ), d/Λ, is small, IAHs can suppress the cladding mode resonance. When d/Λ is large, IAHs increase the number of mode that could strongly interact with the fundamental mode. By comparing the transmission spectral characteristics of PCF-based fibre Bragg grating (FBG) with IAHs with those without IAHs at the same air-filling fraction, it is clarified that the change of transmission spectral characteristics of PCF-based FBG with IAHs is not due to a simple change in air-filling fraction. It is also closely related to the distribution of interstitial air holes.

Overview on Solid Core Photonic BandGap Fibers

General properties of solid core photonic bandgap fibers are presented as well as general guidelines, found thanks to numerical investigations, helpful for designing this kind of fibers. We give two examples of applications and a new fiber design with interstitial air holes that lead to a significant reduction of both confinement and bend losses. This design allows the light to be guided in the fiber core thanks to a modified total internal reflection mechanism at long wavelengths and by photonic bandgap at short wavelengths. Furthermore, this new design satisfies the phase index matching condition for either second- or third-harmonic generation between two fundamental modes.

Improvements of solid-core photonic bandgap fibers by means of interstitial air holes

We report the fabrication, characterization and modeling of a solid-core photonic bandgap fiber with interstitial air holes between the cladding rods. The presence of these interstitial air holes leads to a great improvement of optical properties for this kind of fiber. Particularly, we demonstrate that confinement losses and bend sensitivity are substantially reduced. Our experimental results for this new solution are well supported by numerical results.

Influence of interstitial air holes in index-guiding photonic crystal fibers on their basic properties

The modal properties of index-guiding photonic crystal fibers (IGPCFs) with and without interstitial air holes (IAHs) are numerically investigated by the multipole method. It is shown that the IAHs can favorably decrease the confinement loss and the effective mode area of IGPCFs, and simultaneously increase the nonlinear coefficient in the IGPCFs. In addition, at the same air hole diameter d and pitch Λ, IGPCFs with IAHs can shift the position of the zero-dispersion wavelengths to the shorter wavelengths and make the group-velocity dispersion more flattened in the region of anomalous dispersion. By comparing the optical properties of the PCFs in the presence/absence of IAHs and at the same air-filling fractions, it is shown that the change in the optical properties induced by the presence of IAHs is different from those achieved by a simple increase of the air-filling fraction, a result attributed to stronger scattering by IAHs.

Impact of interstitial air holes on a wide-bandwidth rejection filter made from a photonic crystal fiber

We have investigated the spectral properties of a band rejection filter made with a long-period fiber grating written in photonic crystal fiber that has interstitial air holes. Experiments showed that only one mode was coupled strongly to the fundamental core mode over a 600 nm spectral range. The central wavelength of the filter could be tuned over that range without being appreciably affected by any other mode. By using the multipole method, we found that the interstitial air holes of the photonic crystal fiber played a critical role in limiting the number of modes that could strongly interact with the fundamental mode and in obtaining well-separated resonance peaks. Excellent agreement between theory and experiment was obtained.

Honeycomb photonic bandgap fibers with and without interstitial air holes

The photonic bandgaps (PBGs) of honeycomb photonic bandgap fibers (HPBFs) with and without interstitial air holes (IAHs) are numerically investigated. It is shown that the IAHs can increase the width of PBGs in HPBFs, and also that at the same moderate total air filling fraction, HPBFs with IAHs produce more uniform PBGs than those without IAHs. The bandgap behavior is qualitatively explained using the node-and-vein concept.