Air-filling Fraction Research Articles

Dispersion Effects of Fiber Bending Performed on Microstructured Fiber

This paper deals with dispersion effects resulting from fiber bending performed on microstructured fiber. It is theoretically possible to achieve significant negative chromatic dispersion by fiber bending. Bending of photonic crystal fiber, performed at specific conditions, can result in coupling modes and non-standard dispersion properties. The presented knowledge should be taken into account during the design of photonic crystal fibers immune to bending. For example, the potential tune of the operating wavelength by the reason of bending should be avoided. Minimum values of negative chromatic dispersion and of a minimumdispersion wavelength can be controlled by the value of bending radius. Among others, dispersion diagrams obtained for different bending and for different air-filling fraction are presented.

Dependence of Frequency Shift of Depolarized Guided Acoustic Wave Brillouin Scattering in Photonic Crystal Fibers

We study through experiment, calculation and simulation the frequency dependence of depolarized GAWBS observed in four different photonic crystal fibers and extend the results to others through simulation. In two of the fibers studied, high frequency modes (>; 1 GHz) are observed in addition to the usual lower frequency torsional radial (TR) modes. The acoustic modes responsible for GAWBS are characterized through a combination of simulations and experiments. The calculations are done using the silica rod approximation, which is valid for PCFs with a low to intermediate air-fill fraction or with a very high air-filling fraction. Further simulations extend the range of photonic crystal fibers in which acoustic modes can be characterized. These are used to study the dependence of the acoustic mode frequency on key fiber parameters such as the ratio of air to silica across the whole cross section (κ) and the lattice pitch (Λ) .

Study of the properties of 2D photonic crystal structures as a function of the air-filling fraction and refractive index contrast

We present a study of the primary band gaps of a variety of two-dimensional photonic crystal lattices over the air-filling fraction (AFF) parameter space for different refractive index contrasts. Our results exhibit a variety of unexpected trends and may be useful for better designs of photonic crystal devices.

High birefringent rhombic-hole photonic crystal fibers

High birefringence induced by rhombic air-hole photonic crystal fibers (PCFs) is numerically analyzed by using the finite-element method. The birefringence of a few kinds of PCFs was investigated with different parameters related to rhombic holes, including the rhombic-hole shape, size, and spacing. It was found that the birefringence of the proposed rhombic-hole PCF in this study is relatively larger than that of an elliptical-hole PCF with the same air-filling fraction (f=0.0375) when the ratio of the rhombic-hole diagonal length is equal to the elliptical-hole ellipticity.

Design of Bimodal PCFs for Interferometric Gas Sensors With High Sensitivity

We theoretically investigate bimodal photonic crystal fibers (PCFs) for the development of interferometric gas sensors (with one mode acting as the reference arm and another as the sensing arm), working either in the intensity measurement mode or in the spectral regime. For each operation mode we define a merit function that characterizes the sensitivity of the fiber taking into account both the interferometric and the evanescent-field nature of these sensing devices. The evaluation of the above merit functions, which involve the calculation of the propagation constant, the group index, and the field profile of the first two guided modes, allows us to identify highly sensitive microstructured fibers for each sensing mode. In this paper, we propose a single birefringent PCF design that exhibits in both regimes extreme sensitivity in a certain wavelength operation range that can be shifted by changing the air-filling fraction.

White light supercontinuum generation in a Y-shaped microstructured tapered fiber pumped at 1064 nm

We report the generation of supercontinuum in a Ge-doped Y-shape tapered fiber pumped at 1064 nm in the ns pump regime. The taper was designed to have long taper transitions and a taper waist with a core diameter of 0.9 mum. The large air-filling fraction and diameter of the air-hole microstructure reduces the confinement loss at long wavelengths so, enabling the extension of the spectrum to longer wavelengths. Along the taper transition the zero-dispersion wavelength decreases as the diameter of the taper becomes smaller. The spectral components generated along the taper transition pump the taper waist, enhancing the generation of short wavelengths. A flat spectrum spanning from 420 nm to 1850 nm is reported.

Design of two kinds of dual-core high birefringence and high coupling degree photonic crystal fibers

We propose two kinds of dual-core high birefringence and high coupling degree photonic crystal fibers (DHBHCD-PCFs) in this paper. The characteristics of birefringence and coupling are studied by multipole method. Numerical results show that the birefringence and the coupling length reach an order of 10 − 2 and 10 − 5 m at 1.55 μm, respectively. It is found that the birefringence and the coupling intensity increase with the increase of air-filling fraction, which is different from other dual-core fibers. The DHBHCD-PCFs with high degree of polarization-maintaining and high coupling degree are helpful for manufacturing minitype photonic apparatus.

Highly birefringent photonic crystal fiber based on a double-hole unit

We propose a novel highly birefringent photonic crystal fiber (PCF) based on a double-hole unit. Because of the effect of the double-hole unit in which double airholes can be effectively viewed as elliptical airholes, the proposed PCF can achieve birefringence similar to that of an elliptical-hole PCF with high birefringence even up to the order of 0.01 and still avoid the inherent challenge to fabricate an elliptical-hole PCF. The proposed PCF also has a lower confinement loss than an elliptical-hole PCF with the same air-filling fraction.

The Second Order Guided Modes Based on Photonic Bandgap Effects in Air/Glass Photonic Crystal Fibers

We introduce a defect site in the periodic structure of a photonic bandgap fiber, to confine and guide the second order mode by photonic bandgap effects. Based on a high air-filling fraction photonic crystal cladding structure, a simplified model with an equivalent air cladding was proposed to explore and analyze the properties of this second order guided mode.

Ultraviolet-visible non-supercontinuum ultrafast source enabled by switching single silicon strand-like photonic crystal fibers

Cherenkov radiation from short photonic crystal fiber with a high air-fill fraction can selectively convert the 1020 nm fs pump pulses from a laser oscillator to the fundamental-mode signal pulses at a significantly shorter wavelength. Across the ultraviolet-visible spectral region, the typical fiber output is characterized by a single isolated Cherenkov band having a multimilliwatt-level average power, a Gaussian-shaped spectrum, and a 3-dB bandwidth of 15 nm. By selecting photonic crystal fibers with smaller cores, the central wavelength of the Cherenkov band can be easily extended to 347 nm in the ultraviolet, in sharp contrast to various supercontinuum or non-supercontinuum fiber sources that have difficulty extending their emission spectra below 400 nm. The supercontinuum generation often associated with fs pulse-pumped fibers is efficiently suppressed by detuning the zero-dispersion wavelength of the photonic crystal fiber far shorter than the pump wavelength, a condition termed as the short nonlinear-interaction condition.

Influence of air-filling fraction on forward Raman-like scattering by transversely trapped acoustic resonances in photonic crystal fibers

Raman-like forward scattering by acoustic phonons transversely trapped in birefringent silica-air photonic crystal fibers is studied. As the air-filling fraction increases, core-confined acoustic resonances become increasingly apparent at higher frequencies (>1.1 GHz), while the number of cladding-confined acoustic modes involved in scattering falls. Two main types of scattering are observed: intramodal (scattering to new frequencies within the same optical mode) and intermodal (frequency-shifted scattering to a different optical mode). It is shown that the twofold symmetric microstructure in a birefringent fiber causes strongly polarization-dependent intramodal scattering. Good agreement is obtained between the experimental measurements and numerical solutions of both the acoustic and electromagnetic wave equations by using a full-vectorial finite-element approach. Phononic bandgaps are found to play a significant role at higher air-filling fractions, leading to the appearance of additional bands in the scattering spectrum.

More than threefold expansion of highly nonlinear photonic crystal fiber cores for low-loss fusion splicing

We have formed low-loss fusion splices from highly nonlinear (HNL) photonic crystal fibers (PCFs) with small cores and high air-filling fractions to fibers with much larger mode field diameters (MFDs). The PCF core was locally enlarged by the controlled collapse of holes around the core while keeping other holes open. The fiber was then cleaved at the enlarged core and spliced to the large MFD fiber with a conventional electric arc fusion splicer. Splice losses as low as 0.36 dB were achieved between a PCF and a standard single-mode fiber (SMF) with MFDs of 1.8 microm and 5.9 microm, respectively.

Generation of supercontinuum and its theoretical study in three-ring silica microstructured optical fibers

We report supercontinuum generation in nonlinear microstructured optical fibers (MOFs) especially fabricated in a two-step stack and draw process having three rings of airholes. High air-filling fraction (>0.9) is obtained in a simple and straightforward way during the drawing process which is essential to enhance nonlinearity. Two of the fabricated samples are characterized and zero dispersion wavelength is tailored to achieve efficient pumping in the anomalous group velocity dispersion regime. The characteristics of the supercontinuum band as observed experimentally show good agreement with the predicted numerically simulated results, where soliton mediated dispersive waves are distinctly observed.

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.

Fusion Splicing Holey Fibers and Single-Mode Fibers: A Simple Method to Reduce Loss and Increase Strength

We demonstrate a novel method for low-loss splicing Ge-doped holey fibers (HF) with subwavelength core size and high numerical aperture fibers by using a conventional fusion splicer. We found that a large overlap distance of the fibers during fusion would decrease the splice loss and increase the splice strength. The lowest splice loss achieved for a fiber with a core diameter of 1.27 mu m and air-filling fraction of >0.95 was ~ 1 dB at 1550 nm, with a bend failure radius of 0.8 cm. With the same method, we also observed improvement in terms of loss and strength with larger core size HF.

Control of the fabrication parameters during the fabrication of photonic crystal fibers

The interplay between the fabrication parameters during the fabrication of photonic crystal fibers（PCF） are numerically analyzed. Relation between these parameters and the final fiber’s geometry is identified. With the cooperative control of drawing temperature and feed speed, the final hole structure can be effectively controlled by tuning the applied pressure. A single-mode PCF with its d/Λ approaching 0.45, and a PCF with high air-filling fraction, whose d/Λ equals 0.8, are successfully fabricated. For fabrication of PCFs with uneven-sized holes, parameter control alone is sometimes inadequate in achieving the desired geometry. Taking a single-polarization single-mode（SPSM） PCF structure as example, the preform’s geometry is redesigned for optimization by inverse calculation. And the required SPSM PCF can thus be fabricated.

Controlling intermodal four-wave mixing from the design of microstructured optical fibers

Intermodal four-wave mixing (FWM) in microstructured optical fibers (MOF) is studied theoretically and experimentally. The dependance of FWM frequency detuning on the geometrical parameters of the fiber, namely the pitch, the core width and the air-filling fraction is derived. We propose to use the results of this investigation to control the position of the Stokes and anti-Stokes waves directly from the fiber transverse structure drawing without the need for time-consuming simulations as in usual design procedures. Stokes sideband can then be freely tuned within the S-, L-, and C- bands with great potential for infrared applications.

All-optical regeneration based on highly nonlinear photonic crystal fiber

An all-optical regeneration based on self-phase modulation in a highly nonlinear photonic crystal fiber is proposed. The dispersion and nonlinearity properties of a series of photonic crystal fibers are analyzed, and the results show that the nonlinearity coefficient is closely related to the structure of the fiber. In this paper, the nonlinearity coefficient is increased by reducing the effective mode area, and a highly nonlinear photonic crystal fiber with a large air-filling fraction is used as nonlinearity medium in optical regeneration. The numerical results show that good optical regeneration results can be obtained by using a relatively short fiber length due to the high nonlinearity of the fiber. The input peak power launched into the photonic crystal fiber and the parameters of the filter have much influence on optical regeneration. To achieve good optical regeneration, those parameters need to meet certain requirements. Furthermore, the transfer characteristic of the regenerator is also discussed. By adjusting the input peak power and filter parameters, the regenerator can deal with input pulses of different pulse widths.

A new method of modeling cladding band structure of air-guiding photonic crystal fibers

Cladding band structure of air-guiding photonic crystal fibers with high air-filling fraction is calculated in terms of fiber shape variation. The fundamental photonic band gap dependence on structure parameters, air-filling fraction and spacing, is also investigated. The numerical results show that the band gap edges shift toward longer wavelength as the air-filling fraction is increased, whereas the relative band gap width increases linearly. For a fixed air-filling fraction, the band gap edges with respect to spacing keep constant. With this method, the simulation results agree well with the reported data.

Analysis of confinement loss in microstructured optical fibers with elliptical air holes

Confinement loss for the fundamental mode in the triangular lattice microstructured optical fibers with elliptical air holes is analyzed using the complex finite element method under the perfectly matched layer boundary condition. It is shown by the numerical results that the confinement losses decrease when the pitch of the air holes, the air-filling fraction and the rings of the air holes increase. In addition, the confinement losses are dependent on the fundamental mode polarization, and the confinement loss ratio of the y-polarized mode compared to the x-polarized mode increases with increasing pitch of the air holes, the air-filling fraction and the hole ellipticity. In the end, two kinds of highly nonlinear microstructured optical fibers that can achieve single-polarization, single mode transmission are designed.