Mode Photonic Crystal Fiber Research Articles

Circularly polarized guided modes in dielectrically chiral photonic crystal fiber

The effect of dielectric chirality on the polarization states and mode indices of guided modes in photonic crystal fiber (PCF) is investigated by a modified plane-wave expansion (PWE) method. Using a solid-core chiral PCF as a numerical example, we show that circular polarization is the eigenstate of the fundamental mode. Mode index divergence between right-handed circularly polarized (RCP) and left-handed circularly polarized (LCP) states is demonstrated. Chirality's effect on mode index and circular birefringence (CB) in such a PCF is found to be similar to that in bulk chiral media.

Experimental excitation and characterization of cladding modes in photonic crystal fiber

We extend the previous theoretical study on the effect of outer silica cladding to the analysis on real field profiles in a practical PCF. Clear field profiles for the first higher-order cladding modes are presented and discussed. The observed mode fields are reproduced by numerical calculations, and it turns out that they correspond to LP(16) mode groups. Optical properties of the observed modes such as lobe direction and polarization are also investigated. The results of this study will be useful in the design of the PCF-based optical devices utilizing cladding mode coupling.

Photonic crystal fibre based high temperature sensor with three-beam path interference

Photonic crystal fibre (PCF) modal interferometers are widely used for sensing applications. Owing to the intrinsic Mach-Zehnder-like dual-mode interference there is a trade-off between sharp spectral response and wide sensing range. A three-beam path interferometer induced by mode-conversion in a single dual mode PCF end face is experimentally demonstrated. Doubled free spectral range is obtained owing to the three-beam path interference, which is suitable in sensing large index change, wide temperature variation and strong applied pressures. As an example, a temperature sensor with the sensitivity of 8.17 pm/ C is demonstrated ranging from room temperature to 520 C. The mechanism and further improvements of the multi-beam path interferometers are also discussed.

Advanced design and optimization of single mode photonic crystal fibers

This paper proposes a combination of differential evolution (DE) and estimation of distribution algorithm (EDA) to design photonic crystal fiber structures with desired properties over the C communication band. In order to determine the properties of PCFs such as dispersion, dispersion slope and loss, an artificial intelligence method, the Nero-Fuzzy system, is applied. In addition, a special cost function which simultaneously includes the confinement loss, dispersion and its slope is used in the proposed design approach. The results revealed that the proposed method is a powerful tool for solving this optimization problem. The optimized PCF exhibits an ultra low confinement loss and low dispersion at 1.55 µm wavelength with a nearly zero dispersion slope over the C communication band.

Design of ultra-low and ultra-flattened dispersion single mode photonic crystal fiber by DE/EDA algorithm

This paper proposes a combination of differential evolution (DE) and estimation of distribution algorithm (EDA) to design photonic crystal fiber structures with desired properties over the C communication band. In order to determine the effective index of propagation of the mode and then, the other properties of structure, a finite difference frequency domain (FDFD) solver is applied. The results revealed that the proposed method is a powerful tool for solving this optimization problem. The optimized PCF exhibits a dispersion of 0.22 ps nm−1 km−1 at 1.55 µm wavelength with a variance of ±0.4 ps nm−1 km−1 over the C communication band and a nearly zero dispersion slope.

Metal-Coated Defect-Core Photonic Crystal Fiber for THz Propagation

Modal solutions for metal-coated defect-core photonic crystal fiber (PCF) with a central air-hole have been obtained by using a full-vectorial finite element method to model the guidance of THz waves. It has been shown that the surface plasmon modes can couple with the defect-core PCF mode to form supermodes, with potential for sensing applications.

Dynamic control of higher-order modes in hollow-core photonic crystal fibers

We present a versatile method for selective mode coupling into higher-order modes of photonic crystal fibers, using holograms electronically generated by a spatial light modulator. The method enables non-mechanical and completely repeatable changes in the coupling conditions. We have excited higher order modes up to LP(31) in hollow-core photonic crystal fibers. The reproducibility of the coupling allows direct comparison of the losses of different guided modes in both hollow-core bandgap and kagome-lattice photonic crystal fibers. Our results are also relevant to applications in which the intensity distribution of the light inside the fiber is important, such as particle- or atom-guidance.

Highly Sensitive Photonic Crystal Fiber-Based Refractive Index Sensing Using Mechanical Long-Period Grating

We experimentally demonstrate a novel refractive index (RI) sensor with the measurand infiltrated into the air channels of a single mode photonic crystal fiber (PCF). A tunable long-period grating is formed in a short length of PCF by use of the periodic mechanical pressure. Resonance shift with the change of the infiltrated liquid is observed with a sensitivity of 4.1times 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-6</sup> RI unit in a refractive index range from 1.32 to 1.39. The thermal stability of the pure-silica-based PCF device shows a three times better performance compared with conventional single mode fiber.

Sensitivity of photonic crystal fiber modes to temperature, strain and external refractive index

Several strong narrowband resonances are observed in the transmission spectra of fiber Bragg gratings photo-written in photonic crystal fiber that has a refractive index-neutral germanium/fluorine co-doped core. Experimental results for the strain, temperature and refractive index sensitivities of these mode resonances are reported and compared to those of conventional single mode fiber. In particular, we identify three kinds of resonances whose relative sensitivities to strain, temperature and refractive index are markedly different and present numerical simulations to explain these properties. Potential multiparameter optical sensor applications of these mode resonances are briefly discussed.

Highly Birefringent Index-Guiding Photonic Crystal Fiber with Squeezed Differently Sized Air-Holes in Cladding

We propose a novel high-birefringence index-guiding photonic crystal fiber (PCF). This PCF is composed of a solid silica core and a cladding with two differently sized squeezed elliptical air-holes. The mode birefringence of a fundamental mode in such PCFs is analyzed numerically by the finite-element method. Numerical results reveal that an extraordinarily high modal birefringence at the excitation wavelength of λ=1550 nm, 2.6×10-2, is acquired. The contributions of the cladding with two different sizes of air-hole ellipticity, the center-to-center distance between the air-holes, and the the number of cladding rings as well as the confinement loss to the birefringence are systematically evaluated. The evolution of birefringence with the structural variations shows that our highly birefringent fiber can be designed in a controlled manner.

Determining Properties of Fabricated Index-Guiding Photonic Crystal Fibers Using SEM Micrograph and Mode Convergence Algorithm

We develop a method for modeling properties of fabricated (realistic) air-silica photonic crystal fibers (PCFs). Our approach involves extracting the transverse refractive index (RI) profile of the drawn PCF from its scanning electron micrograph on which is operated a precise and fast mode-analysis recipe based on a finite difference (FD) field convergence scheme. From the digitized scaled RI distribution, we evaluate propagation characteristics of guided modes of PCFs, examining modal shapes, birefringence, dispersion, and other relevant properties. Naturally, our true-structure study of PCFs using FD algorithm exhibits results that are more close to measured data, establishing its practicality as compared with idealized-structure modeling. To demonstrate the efficacy of our method, we investigate some application-specific experimentally drawn PCFs, well known for their study in the literature. The key results that fairly predict experimental measurements are presented. Besides modeling fabricated fibers, this analysis will be very useful to realize PCFs with targeted specifications using feedback of estimation and characterization of trial fabrications.

Numerical analysis of refractive index sensitivity of long-period gratings in photonic crystal fiber

Abstract The modal coupling properties and resonance spectral response of long-period gratings (LPGs) in index-guiding photonic crystal fiber (PCF) are numerically investigated with respect to its sensitivity to changes in the index of refraction of the measurand in the PCF air channels using a full-vectorial mode solver combined with frequency-domain method. We show that the shift of wavelength resonance of a PCF-LPG structure can be extremely sensitive by appropriately designing the cladding air-channel dimension and geometry as well as the periodicity of the LPG inscribed in an endlessly single mode PCF (ESM-PCF-LPG). Specifically, a sensitivity of 4.3 × 10−8 refractive index unit (RIU) can be obtained in a refractive index range of 1.33 and 1.4 using an ESM-PCF-LPG with hexagonally arrayed 4-ring air channels at the grating periodicity of 1300 μm. A sensitivity of 4.1 × 10−7 RIU can be achieved in the range of 1.0–1.25 employing an ESM-PCF-LPG with similarly arrayed but 5-ring air channels at the grating periodicity of 350 μm. The predicted index sensitivity, coupled with other schemes for selectivity and specificity, opens the ESM-PCF-LPG structure for a variety of sensing and detection applications.

모세관 적층 방법에 의한 광자결정 광섬유의 제작

실리카 코어 주변에 주기적인 공기층을 가지는 광자결정 광섬유는 넓은 파장 영역을 통한 단일모드의 구현 또는 1옥타브 이상의 광대역 연속광 발생과 같은 기존의 광섬유로는 불가능한 독특한 특성을 갖도록 유연하게 설계할 수 있다. 광자결정 광섬유의 설계에 사용되는 변수로는 공기층의 직경과 간격이 있으며 이러한 변수의 조절을 위한 공정을 도입하여 넓은 파장영역을 통한 단일모드 구현과 높은 비선형 특성을 가지는 광자결정 광섬유를 각각 제작 하였고 수치적 계산과 실험을 통해 그 특성을 살펴보았다. Photonic crystal fibers(PCFs) with silica cores within an away of air holes have unique properties. Broad band single-mode and the octave-spanning supercontinuum generation, impossible to achieve in classical fibers, can be realized. The design of PCFs is very flexible. There are two parameters to manipulate: air hole diameter, and lattice pitch. We introduced a fabrication process for control of the parameters to obtain endlessly single mode PCF, which is single mode in a large wavelength range, and highly nonlinear PCF. The numerical analysis and experiments are included.

Effect of macro-bending on resonant wavelength and intensity of long-period gratings in photonic crystal fiber

We report the spectral characteristics of CO(2)-laser inscribed long-period gratings (LPGs) in endlessly single mode photonic crystal fiber (PCF) subject to macro-bending. The coupling modes as a result of bending were studied by examining the shifts of resonant wavelengths, the splits of attenuation bands, and the variation in coupling strength of the transmission spectra. A bending coefficient of ~ 27.9 nm.m was determined in the PCF at 180 degrees rotational orientation relative to the point of laser inscription in the curvature range from 2.6 m(-1) to 3.5 m(-1). Compared with conventional fiber LPGs fabricated using the same method, the PCF-based LPGs possess higher sensitivity both to bending and orientation, making them promising for sensor applications.

Highly birefringent elliptical-hole photonic crystal fiber with squeezed hexagonal lattice

We propose a novel polarization-maintaining index-guiding photonic crystal fiber (PCF). It is composed of a solid silica core and a cladding with squeezed-hexagonal-lattice elliptical air holes. Using a full-vector finite-element method, we study the modal birefringence of the fundamental modes in such PCFs. Numerical result shows that very high modal birefringence with a magnitude of the order of 10(-2) around 1550 nm has been obtained. Furthermore, large normal dispersion appears over a wide range of wavelengths in both orthogonal polarizations.

Application of effective index method to higher order modes of photonic crystal fibers

AbstractThe application of effective index method (EIM) to calculate the modal indices of higher order modes (HOMs) of photonic crystal fibers (PCFs) is investigated. The effective index of the fiber cladding is obtained by a plane wave expansion method because analytical solution is found to induce large errors for PCFs with large air holes. The modal indices of the TE01 and TM01 modes of a PCF are solved using EIM as a demonstration of the applicability of this method. It is shown that there exists a different optimized core for these two modes when compared with the case for the fundamental HE11 mode. © 2007 Wiley Periodicals, Inc. Microwave Opt Technol Lett 49: 567–570, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.22196

Multifrequency third-harmonic generation by red-shifting solitons in a multimode photonic-crystal fiber

While the standard scenario of third-harmonic generation (THG) by a dispersive-wave pump involves the emission of light with a frequency 3omega, thrice the frequency omega of the input pump field, solitons undergoing a continuous shift of their central frequency omega due to the Raman effect in a multimode optical fiber can generate the third harmonic in a different fashion. In the experiments reported here, we provide the first direct experimental evidence of THG by a continuously red-shifting soliton pump by studying the third-harmonic buildup in relation to the spectral evolution of the soliton pump field in a silica photonic-crystal fiber (PCF). We show that solitons excited in a PCF by unamplified femtosecond pulses of a Cr:forsterite laser sweep through the spectral range from 1.25 to 1.63 microm , scanning through a manifold of THG phase-matching resonances with 3omega dispersive waves in PCF modes. As a result, intense third-harmonic peaks build up in the range of wavelengths from 370 to 550 nm at the output of the fiber, making PCF a convenient fiber-format multifrequency source of short-wavelength radiation. Time-resolved fluorescence measurements with photoexcitation provided by the third-harmonic PCF output are presented, demonstrating the high potential of PCF sources for an ultrafast photoexcitation of fluorescent molecular systems in physics, chemistry, and biology.

Analyses of cladding modes in photonic crystal fiber

Characteristics of cladding modes in a photonic crystal fiber (PCF) are numerically analyzed using the plane wave expansion method. The presence of the outer silica ring in the PCF tends to push the optical fields of the cladding modes toward the rim of the PCF, which creates 'ring modes' whose fields are tightly confined in the outer ring. The dispersion of the cladding modes are determined mainly by the dispersive property of the holey cladding structure. The optical field patterns of the cladding modes and the beatlengths between the fundamental mode and the cladding modes are also investigated.

Raman-resonance-enhanced composite nonlinearity of air-guided modes in hollow photonic-crystal fibers

Coherent anti-Stokes Raman scattering (CARS) is used to measure relations between the resonant (Raman) and nonresonant (Kerr-type) optical nonlinearities of air-guided modes in a hollow-core photonic-crystal fiber (PCF). We demonstrate that, due to its interference nature, CARS provides a convenient tool for measuring the contribution of the fiber cladding to the total nonlinearity sensed by air-guided modes in hollow PCFs. On a Raman resonance with molecular vibrations in the gas that fills the fiber core, a two-color laser field is shown to induce optical nonlinearities that are several orders of magnitude higher than the nonresonant Kerr-type nonlinearities typical of air-guided PCF modes.

Solution of the fundamental space-filling mode of photonic crystal fibers: numerical method versus analytical approaches

The accuracy of the solution of the fundamental space-filling mode of photonic crystal fibers by scalar and vectorial analytical approaches and its effect on the effective index models are investigated. Using a plane wave method as a benchmark, we show that the optimal choice of the radius of the equivalent circular unit cell used in the approximations is different for the two approaches and this value has a great effect on the accuracy of the solution of the fundamental space-filling mode. We also show that the vectorial approach with a properly defined value of the radius is highly accurate over a wide parameter range, whereas the scalar approach causes the main error in the scalar effective index model. We also confirm that a fully vectorial effective index model is accurate and efficient in the case of photonic crystal fibers with large air filling fractions.