Solid-core Bragg Fiber Research Articles

Hole-Assisted Solid Core Bragg Fibers With a High-Index-Contrast Cladding for Broadband Single-Polarization Operation

We propose a hole-assisted solid core Bragg fiber (SCBF) with a high-index-contrast cladding for near-infrared broadband single-polarization transmission in this article. The circular symmetry of a common SCBF is broken by the introduced air holes in the solid core for increasing the modal birefringence and polarization-dependent loss (PDL). In the proposed single-polarization SCBF, the confinement of guided modes predominantly depends on the photonic bandgap effect from the high-index-contrast cladding, and simultaneously rests with the total internal reflection from the interface between the solid core and air holes. The numerical results show that both the enhanced modal birefringence and PDL are strongly related to the refractive index, size and locations of air holes. We demonstrate that the modal birefringence for the fundamental mode can be up to the order of 10−3, and the difference in confinement losses between both polarized fundamental modes is no less than an order of magnitude, which is sufficiently large to support single-polarization operation. Moreover, the single-polarization bandwidth with the transmission loss lower than 0.1 dB/m is as high as 277 nm and the critical bend radius is as low as 1 cm, indicating the excellent performances of high bandwidth and bending resistance in the hole-assisted single-polarization SCBF.

Enhancement of modal birefringence in solid core Bragg fibers with high-index-contrast claddings

Abstract We present two types of highly birefringent solid core Bragg fibers (SCBFs) with high-index-contrast claddings by asymmetrically introducing low index parts (LIPs) into the solid core. The characteristics of their modal birefringence and confinement loss are numerically investigated by using a full-vector finite element method. The simulation results reveal that the modal birefringences in such highly birefringent SCBFs essentially depend not only on the difference in electric field boundary condition between the orthogonal-polarized modes, but also on the photonic bandgap effect. Both types of highly birefringent SCBFs have higher modal birefringence than those with low-index-contrast claddings, owing to the large refractive index (RI) modulation ranges of the LIPs. The modal birefringence of the fundamental mode at the targeted 1550 nm is strongly dependent on the size, RI and location of LIPs. Notably, the linear relationship between the modal birefringence and the RI of LIPs in the y-direction in a highly birefringent SCBF with four LIPs included can serve as a new sensing mechanism for the RI measurement of liquid analytes. We believe that the results would be beneficial to the design and application of highly birefringent SCBFs with high-index-contrast claddings.

Leakage and bend losses in solid-core Bragg fibers

An analytical approach describing the modes of solid-core Bragg fibers is presented. Our method does not assume the periodic structure of a fiber cladding; therefore, it is applicable to a wide class of solid-core Bragg fibers, including bent ones. To check derived formulas, six Bragg fibers with a core diameter from 8.4 to 42 μm and a number of cladding rings from 5 to 15 were fabricated. The measured loss spectra of the straight and bent Bragg fibers agree well with theoretical ones. Based on these results, we show that bending affects leakage losses by two mechanisms: decreasing the reflection of light from interfaces and destroying the constructive interference of reflected waves in the cladding. The role of these mechanisms at low and large (near critical) fiber curvature is discussed.

Design of a Solid-Core Large-Mode-Area Bragg Fiber**Supported by the National Natural Science Foundation of China under Grant No 61405240.

We report the design and simulation results of a solid-core Bragg fiber with 3-bilayer periodic cladding. The simulation results present single mode bandgap guidance, a large effective area of ∼400 μm2 around 1.08 μm, a very low bend loss of 0.038 dB/m at 1.08 μm even under tight bend radius (R = 4 cm), and excellent beam quality. The results indicate that the proposed fiber could be a competitive solution for high-power fiber laser applications.

All photonic bandgap fiber spectroscopic system for detection of refractive index changes in aqueous analytes

We propose and experimentally demonstrate a sensor of refractive index (RI) of aqueous analytes based on a hollow-core low-refractive-index-contrast Bragg fiber. Variations in refractive index of liquid analytes filling the hollow core of Bragg fiber could modify the resonant condition of the modal confinement, thus leading to both spectral shifts and intensity changes in the fiber transmission. As a proof-of-principle demonstration, we characterize sensor performance using a set of NaCl solutions with different concentrations. The experimental sensitivity of the Bragg fiber sensor is found to be ∼1400 nm/RIU (refractive index unit), which is comparable to those of surface plasmon resonance sensors. Moreover, we demonstrate the integration of a Bragg fiber bundle spectrometer in the liquid-core Bragg fiber sensing system to replace the grating-based monochromator, which could potentially lead to significant cost-saving and increased sensing speed. In summary, we demonstrate an all-photonic-bandgap-fiber RI sensing system that uses a hollow core Bragg fiber to hold and probe the analyte, and a solid-core Bragg fiber bundle for spectral interpretation of the transmitted light.

Parabolic pulse generation in a dispersion-decreasing solid-core photonic bandgap Bragg fiber

We analyze the interplay of nonlinearity and dispersion in a dispersion-decreasing photonic bandgap Bragg fiber as a new platform for generating parabolic pulses. A suitably designed linearly tapered, low-index-contrast, solid-core Bragg fiber – amenable to fabrication by conventional modified chemical vapor deposition technology – is shown to yield stable parabolic pulses. The fiber design was optimized through a simple and accurate transfer-matrix formalism and pulse evolution was studied by the well-known split-step Fourier method. Our study revealed feasibility of generating parabolic pulses in such a dispersion-decreasing Bragg fiber of length as short as 1 m. We have also studied the effect of third order dispersion on generated parabolic pulse, which is an important deteriorating factor in such applications. The effective single-mode operation of the proposed device is achieved through appropriate tailoring of the outer cladding layers.

Analysis of Coupling Loss between Laser Diodes and Solid-Core Bragg Fibers

In this paper, using the Galerkin’s numerical method and performing an overlapping integral technique, we calculate the coupling loss coefficients of Laser Diodes (LDs) to solid-core Bragg fibers in terms of lateral, axial and angular displacement. For comparison, results concerning coupling to SMF-28 are also presented. Our calculations show that the coupling of solid-core Bragg fibers to LDs has effectively larger tolerance to lateral and axial displacements compared to conventional SMF-28. It is also found that Bragg fibers are promising to attain low-loss free-space coupling with a long working distance. However, the angular tolerance for LD to Bragg-fiber coupling is lower than that of SMF-28.

Photonic bandgap fibers with resonant structures for tailoring the dispersion

Numerical simulations on different kinds of realistic photonic bandgap fibers exhibiting reversed dispersion slope for the propagating fundamental mode are reported. We show that reversed or flat dispersion functions in a wide wavelength range using hollow-core, air-silica photonic bandgap fibers and solid core Bragg fibers with step-index profile can be obtained by introducing resonant structures in the fiber cladding. We evaluate the dispersion and confinement loss profiles of these fibers from the Helmholtz eigenvalue equation and the calculated fiber properties are used to investigate the propagation of chirped femtosecond pulses through serially connected hollow core fiber compressors.

Guiding in the visible with "colorful" solid-core Bragg fibers

We report on the fabrication and characterization of solid-core all-polymer Bragg fibers consisting of a large-diameter polymethyl methylacrylate (PMMA) core surrounded by 50 alternating PMMA/Polystyrene (PS) polymer layers. By modifying the reflector layer thickness we illustrate that bandgap position can be adjusted at will in the visible. Moreover, such fibers are intensely colored in both the transmission and the outside reflection modes. Potential applications of such fibers are discussed.

Nonlinear Spectral Broadening in Solid-Core Bragg Fibers

We present results of a detailed theoretical study of nonlinear pulse propagation through solid-core photonic bandgap Bragg fibers. Pulse propagation is modeled using the extended nonlinear Schrodinger equation, which is numerically solved by using the split-step Fourier method. The influence of the fiber structure and various parameters of the input pulse on the spectrum of the output pulse are studied, with an aim to achieve Bragg fiber designs that have the potential for nonlinear applications such as continuum pulse generation. It is found that third-order dispersion is dominant in limiting the spectral width of the output pulse. Generation of dispersive waves is also observed at a wavelength close to the center frequency of the pulse, although it does not contribute to the broadening of the frequency spectrum. Simulations show the existence of an optimal length of the Bragg fiber beyond which spectral broadening reaches saturation.