Conventional Photonic Crystal Fiber Research Articles

Predication of negative dispersion for photonic crystal fiber using extreme learning machine

Abstract The photonic crystal fiber (PCF) is a resourceful optical device that can be used in various applications. The dispersion is a major impediment for such optical waveguides. We propose a modified PCF that evaluates the negative dispersion coefficient−3126 ps/(nm–km) at 1.55 μm wavelength. The precise value calculation of the design parameters is helpful to improve the desired output. The machine learning approaches are now more in fashion to predicate such parameters. The dispersion parameters are obtained for three different PCF models as conventional PCF with fixed radius air holes and type 1 and type 2 models with dual radius air holes. Further, the negative dispersion of a type-I PCF is modeled using an extreme learning machine (ELM) as a regression task and its performance is tested against benchmark models such as support vector machine with linear and radial basis function kernel function, Gaussian process regression, and artificial neural network. Results indicate superior performance of ELM as a regressor both, in terms of prediction and computation time.

Highly efficient modified dual D-shaped PCF polarization filter

In this work, a highly efficient transverse-electric (TE) pass polarizer based on dual D-shaped photonic crystal fiber (PCF) is designed and analyzed. The PCF birefringence between the two polarized modes is efficiently controlled by adjusting the core region. Additionally, the dual D-shaped layers are coated by gold films to enhance the filtering characteristics. Therefore, the surface plasmon (SP) modes at the metal/dielectric interface are strongly coupled to the quasi-transverse-magnetic (TM) core mode with high confinement loss in comparable to the conventional D-shaped PCF. However, no coupling occurs with the quasi-TE core mode. Thus, a TE-pass polarizer is realized with an insertion loss (IL) of −0.39 dB/cm and −0.57 dB/cm at the two-telecommunication wavelengths of 1300 nm and 1550 nm, respectively. Moreover, the reported device has a wide operation bandwidth that extends from λ = 1.2 µm to λ = 1.6 µm where the crosstalk (CT) is less than −21 dB.

Performance improvement of a resonant fiber optic gyroscope with a hybrid photonic crystal fiber ring resonator.

The photonic crystal fiber (PCF) with better thermal stability and polarization-maintaining performance is applied to the resonant fiber optic gyroscope (RFOG). A hybrid fiber ring resonator is fabricated using the conventional polarization-maintaining fiber (PMF) and PCF with close mode field diameters (MFDs) to reduce MFD-mismatch-induced splicing loss. Furthermore, the temperature-dependent birefringence coefficients of the PMF and the PCF are measured to differ almost four times in absolute value and be opposite in sign. The polarization stability of the hybrid PCF-PMF ring resonator is improved by a factor of 7 compared with the PMF ring resonator with the same length of PMF. The RFOG equipped with this hybrid PCF-PMF ring resonator achieves a bias stability of 1.67°/h.

Development of a method to overcome the power threshold during supercontinuum generation based on an Yb-doped photonic crystal fiber

Optical coherence tomography benefits from the high brightness and bandwidth, as well as the spatial coherence of supercontinuum (SC) sources. The increase of spectral power density (SPD) over conventional light sources leads to shorter measuring times and higher resolutions. For some applications, only a portion of the broad spectral range can be used. Therefore, an increase of the SPD in specific limited spectral regions would provide a clear advantage over spectral filtering. This study describes a method to increase the SPD of SC sources by amplifying the excitation wavelength inside of a nonlinear photonic crystal fiber (PCF). An ytterbium-doped PCF was manufactured by a nanopowder process and used in a fiber amplifier setup as the nonlinear fiber medium. The performance of the fiber was compared with a conventional PCF that possesses comparable parameters. Finally, the system as a whole was characterized in reference to common solid-state laser-based photonic SC light sources. An order-of-magnitude improvement of the power density was observed between the wavelengths from 1100 to 1350 nm.

Light propagation characteristics in photonic crystal fibers with α-power profiles of air hole diameter distributions and their application to fiber collimator

Light propagation characteristics in photonic crystal fibers (PCFs) with α-power profiles of air hole diameter distributions were theoretically investigated. It was clarified that the intensity peak of the beam propagating in the PCF with Gaussian beam excitation varied periodically with little power attenuation. It was found that the envelope of the periodic intensity variation depended on α. We theoretically demonstrated that the PCF with the α-power profile of the air hole diameter distribution could be applied to a collimator for a conventional PCF with uniform air holes in Gaussian beam excitation to reduce coupling loss, where a PCF of appropriate length with the α-power air hole diameter distribution was spliced to a conventional PCF. It was also found that the coupling efficiency was higher for a larger α.

Solid Optical Fiber With Tunable Bandgaps Based on Curable Polymer Infiltrated Photonic Crystal Fiber

We demonstrated the realization and characterization of a solid photonic bandgap fiber (SPBF) with a compact size of about 10 mm and a high wavelength sensitivity of up to −4.034 nm/°C by means of fully infiltrating an ultraviolet curable polymer with a high refractive index of 1.515 into air holes of a photonic crystal fiber (PCF). To the best of our knowledge, it was the first time that the SPBF with tunable bandgaps was fabricated in the conventional index-guiding PCF. Compared with conventional fluid filled PBFs, the proposed SPBF can be stable to temperature and other environmental effects and maintain a large extinction ratio of more than 30 dB within a broad wavelength. The splicing between the SPBF and single mode fibers has been solved. Moreover, it is observed that the bandwidth of bandgap (G2) gradually broadens with the increase in temperature.

A simple structure of all circular-air-holes photonic crystal fiber for achieving high birefringence and low confinement loss

We propose a simple structure of photonic crystal fibers (PCFs) with high birefringence and low confinement loss based on one rectangular centric ring of smaller circular air holes (CAHs) in the fiber core, and three rings of larger CAHs in the fiber cladding. This simple geometry (using all CAHs with two different air hole sizes) is capable of achieving a flexible control of the birefringence, B = 5.501 × 10−3, and ultra-low confinement loss, 7.30 × 10−5 dB/km, at an excitation wavelength of λ = 1550 nm. The birefringence value is ∼5.0 times greater than that obtained for conventional CAH PCF. This simple structure has the added advantage from the view point of easy fabrication, robustness, and cost. A full-vector finite element method combined with anisotropic perfectly matched layers was used to analyze the various fiber structures. We have analyzed four cases of CAH PCFs, focusing on the core asymmetry design as opposed to the conventional approach of CAHs or elliptical air holes on the cladding and core. The robustness against manufacturing inaccuracies of the proposed structure has also been further investigated in this work.

Guiding Properties of a Photonic Quasi-Crystal Fiber Based on the Thue–Morse Sequence

We present a novel microstructured optical fiber having a quasi-periodic distribution of air holes based on the Thue–Morse sequence. The transverse section of these fibers is basically a two-dimensional photonic quasi-crystal that can also provide complete photonic bandgaps without being a perfect periodic structure. Like in the conventional photonic crystal fibers, if the quasi-periodicity is broken by decreasing the size of some air holes or by introducing an extra air hole, the modified holes become defects that localize and guide light along the fiber. The guidance is attributed to the inhibition of transverse radiation produced by the photonic quasi-crystal cladding. Dispersion curves of guided modes for different structural parameters are calculated, along with the transverse intensity distribution of the fundamental mode. In particular, several specially designed Thue–Morse quasi-crystal fibers with nearly zero ultraflattened group-velocity dispersion are presented.

Suspended Ring-Core Photonic Crystal Fiber Gas Sensor With High Sensitivity and Fast Response

We demonstrate the design and fabrication of an in-line evanescent field gas sensor based on a novel suspended ring-core photonic crystal fiber (PCF) to further overcome limitations in sensing capability. The suspended ring-core PCF is made by the stack-and-draw technique with a large central capillary of ~60µm and adapted small capillaries in a larger cladding tube. Sensing capabilities of two variations of the proposed suspended ring-core PCFs were investigated experimentally by detecting absorption lines of acetylene gas. The sensitivity of the two PCF structures was also numerically analyzed using a full-vectorial finite-element method. The dynamic response of the sensors was quantified in detail. The results demonstrated significant improvement in both sensitivity and response time of the suspended ring-core PCF as this special PCF can provide larger gas diffusion and higher overlap between the sensing species and the guided light than either a Ge-doped ring defect PCF or a conventional index-guided PCF.

Fiber-optic Michelson interferometer with high sensitivity based on a liquid-filled photonic crystal fiber

This study proposes an ultracompact and highly sensitive liquid-filled photonic crystal fiber Michelson interferometer (LF-PCFMI) based on material dispersion engineering. Numerical simulations and experimental measurements are performed in the work, and both of their results show that the temperature sensitivity can be more greatly improved than those of conventional photonic crystal fiber interferometers (PCFIs). The experimental results indicate that a very high sensitivity with interference wavelength shifts of almost 27nm within temperature variation of 5°C has been achieved by the configuration. Numerical analysis for the proposed LF-PCFMI also exhibits a good agreement with the results of the experimental measurements.

Tunable Selective Liquid Infiltration: Applications to Low Loss Birefringent Photonic Crystal Fibers (PCF) and Its Single Mode Realization

In this article, we’ve theoretically investigated the application of selective liquid infiltration towards realizing birefringent Photonic Crystal Fiber (PCF) and its operation in the single mode region. Birefringence has been created in a symmetrical structure PCF by infiltrating liquid of certain refractive indices in diagonally opposite air-holes. Different air-hole fraction along with different infiltrating refractive indices has been considered towards studying the effect of the parameters towards birefringence and loss. Cut-off properties for different infiltrating liquid has been performed and it has been found that PCF infiltrated with higher Refractive Indices (RI) liquid is suitable for broader range of single mode operation. The present structure provides very low loss and that can also be minimized with higher number of air-hole rings without affecting the birefringent property. Tunability property of the infiltrating liquid for various temperatures has been studied. By varying the refractive indices of the infiltrating liquid the birefringence can be well tuned. Our research will be useful in designing a birefringent PCF from conventional symmetrical PCF by infiltrating liquids in two diagonally opposite air-holes and proper choice of PCF parameter for single mode operation at a desired wavelength of interest.

Log-periodic-type photonic crystal fiber structures and their propagation characteristics

We propose some modifications in the conventional index-guided photonic crystal fiber (PCF) structure having circular holes of constant radii. In one of the proposed structures, the hole dimensions of the conventional PCF are modified such that the ratio of the radii of the holes in a particular layer with the nearest layer is constant while maintaining the same refractive index in all the holes. In the other structure, we propose to use different dielectrics in different layers of holes in the conventional PCF structure such that the ratio of the refractive index of the dielectric material in the holes in a particular layer with the nearest layer is constant. The simulations of the proposed structures are carried out using OptiFDTD simulator with full-vector mode solver using finite difference time domain method, and the results are compared with the conventional PCF structure having four layers of air holes. It is observed that the proposed structures exhibit lower waveguide dispersion and confinement loss than the conventional PCF structure over a wide range of wavelengths, making them suitable candidates for applications such as long-distance optical communications or high-data rate data transfer applications. One of the proposed structures exhibits large negative dispersion, and it can be used as a dispersion compensating fiber.

Microstructure core photonic crystal fiber for blue extension of supercontinuum generation

We investigate photonic crystal fibers (PCFs) with nanosize airholes (NAHs) in the core for the blue extension of supercontinuum generation. The principle behind the design is to enhance the evanescent wave in the IR part of the supercontinuum. When the zero-dispersion wavelength is fixed around 1μm, the infrared wavelength of 2500nm of the proposed fiber is able to group-velocity match to the short-wavelength of 403nm, which is about 60nm shorter than that of a conventional high-Δ PCF. Simultaneously, the nonlinearity is enhanced about three times. The dependence of PCF characteristics on NAHs is also discussed. Simulated results confirm the possibility of increasing the blueshift of the generated supercontinua in designed PCFs.

Segmented cladding index guiding photonic crystal fiber

In this paper a segmented photonic crystal fiber with segments of varying effective refractive index in the cladding has been proposed. The designs have been analyzed in view of the effect of change in their cladding parameters, such as refractive index contrast, air filling fraction (denoted by normalized air hole diameter), duty cycle of the segments, and core–cladding interface. The periodically varying high and low index regions have been created by considering variations in the air hole size. High index core region has been created by omitting a single air hole from the centre of the lattice. Phosphate glass (n=1.56) has been used as base material. The various designs obtained by varying the controlling parameters, have been investigated for properties such as the effective core and cladding indices, confinement of fundamental mode and higher order mode, and the V parameter values. These parameters have been investigated over near infrared (0.75–1.4μm) to short wavelength infrared (1.4–2.3μm) spectral range using finite difference time domain method. The expression for V parameter for conventional photonic crystal fiber has been extended to the proposed design and single mode cutoff value for the design has been calculated. The proposed design offers robust single mode confinement over specified infrared (0.75–2.3μm) wavelength range and a broad confinement peak has been observed containing the third optical communication window at 1.55μm.

Octagonal dual-concentric-core photonic crystal fiber for C-band dispersion compensation with low conf inement loss

A novel kind of octagonal dual-concentric-core photonic crystal fiber (PCF) is calculated with the finite element method (FEM) and proposed for broadband compensation. In order to realize highly negative dispersion, the hole diameter of the second ring is reduced relative to conventional PCFs. Numerical simulation results show that when the diameters of large holes and small holes are 0.77 \mu m and 0.5 \mu m, respectively, and the pitch of the adjacent rings is 1.1 μm, the negative dispersion can achieve about –840 ps.nm-1.km-1 at 1 550 nm and the dispersion slope can match with single mode fiber (SMF-28) perfectly. This PCF can compensate the positive dispersion and also its slope of about 50 times its length of SMF-28 fiber, which is suitable for C-band optical fiber telecommunication as a dispersion compensation fiber.

Two-core photonic crystal fiber with zero intermodal dispersion

Intermodal dispersion (IMD) in a two-core fiber can distort or even breakup pulses propagating in the fiber, and thus limit the usefulness of the fiber for applications that involve the transmission of ultrashort pulses. In this paper, we analyze a two-core photonic crystal fiber (PCF) for the elimination of IMD. The refractive index of the two cores required is slightly lower than the surrounding index and can serve as an effective parameter for the control of the wavelength at which the IMD disappears. The fiber has a conventional PCF structure and is compatible with the existing PCF fabrication technology.

Guiding and thermal properties of a hybrid polymer-infused photonic crystal fiber

In this work, we investigate the guiding properties of a hybrid polymer (poly-dimethylsiloxane)/silica photonic crystal fiber (PCF). In particular, we demonstrate how the basic guiding properties of a conventional PCF are changed due to the infusion of poly-dimethylsiloxane (PDMS) in its air-holes. We show that PDMS infiltration allows tuning of single mode operation, confinement loss, effective modal area (EMA) and numerical aperture (NA) with wavelength and/or temperature. This is primarily due to the enhancement of evanescent field interaction, lending some important characteristics for designing tunable fiber devices. Numerical calculations were performed for different relative hole sizes, d/Λ (0.35-0.75), of PCF for a 500-1700nm wavelength and 0-100°C temperature range, whereas direct comparison with a conventional air-filled PCF is also shown.

Ge-Doped Defect-Core Microstructured Fiber Design by Genetic Algorithm for Residual Dispersion Compensation

A special microstructured optical fiber, which may be used in a wavelength-division-multiplexing (WDM) optical fiber transmission system for residual chromatic dispersion compensation, is designed and analyzed. The proposed structure is obtained by introducing a small Ge-doped core at the center of a conventional photonic crystal fiber. The behavior of the resulting geometry has been optimized by a genetic algorithm in conjunction with an efficient vectorial finite element formulation. Numerical results show that the designed photonic crystal fiber exhibits flattened negative dispersion over <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex Notation="TeX">$E + S + C + L + U$</tex></formula> wavelength bands with an average dispersion of <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$-$</tex> </formula> 212 <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\hbox{ps} \cdot \hbox{km}^{-1} \cdot \hbox{nm}^{-1}$</tex></formula> .

Design of a large effective mode area photonic crystal fiber with modified rings

A photonic crystal fiber (PCF) is proposed that, through novel design, achieves an enhanced effective mode area. The PCF is composed of two concentric elliptical cores. The central core is un-doped silicon whilst the second, outer core region, is doped. The outer doped region is also bordered by lightly doped half ellipse segments. The effective mode area of the structure was calculated and compared with the mode area of conventional PCF's. Our results show that the mode area and chromatic dispersion are very sensitive to the geometry, dimensions and placement of the lightly doped segments. The chromatic dispersion, bend losses and the nonlinear coefficient are also numerically simulated and presented in this paper.

Sensitivity analysis of refractive index measurement based on intermodal interference in photonic crystal fiber

The sensitivity of an interferometer based on intermodal interference in photonic crystal fiber (PCF) has been investigated theoretically and numerically. Using the intermodal interference theory and the normalized frequency of PCF, we have analyzed the principle of the method for measuring refractive index by PCF interferomter,and obtained a variation, δns, to show the sensitivity of the PCF interferometer. From the analysis of the underlying physics of δns, we find that air holes of the first layer play an important role on the sensitivity.Compared with the conventional PCF structure,the sensitivity can be improved remarkably via employing large air holes with the shape of a combination of half circle and half oval.