Hollow-core Bragg Fiber Research Articles

Fabrication of mid-infrared hollow-core Bragg fiber and it application in gas sensing

In its application of gas sensing, the transmission band of the hollow-core Bragg fiber should match the main absorption peak of the target gas. In this paper, we introduce the design method of the hollow-core Bragg fiber transmission band and develop a fabrication process supporting its transmission band control. Fiber samples with fundamental transmission bands at 10.6 m and 3.3 m are fabricated, whose transmission losses are 5.9 dB/m and 8.8 dB/m, respectively, measured by the cut-back method. Utilizing the fiber sample with a transmission band of 3.3 m, the injection and the expulsion of CH4/N2 gas are realized and observed by the change of fiber transmission spectrum. The detection limit of the experimental system is measured to be 26 ppm for CH4 by exponential dilution method, demonstrating the feasibility of hollow-core Bragg fiber in its application of gas sensing.

Bragg waveguides with low-index liquid cores

The spectral properties of light confined to low-index media by binary layered structures is discussed. A novel phase-based model with a simple analytical form is derived for the approximation of the center of arbitrary bandgaps of binary layered structures operating at arbitrary effective indices. An analytical approximation to the sensitivity of the bandgap center to changes in the core refractive index is thus derived. Experimentally, significant shifting of the fundamental bandgap of a hollow-core Bragg fiber with a large cladding layer refractive index contrast is demonstrated by filling the core with liquids of various refractive indices. Confirmation of these results against theory is shown, including the new analytical model, highlighting the importance of considering material dispersion. The work demonstrates the broad and sensitive tunability of Bragg structures and includes discussions on refractive index sensing.

High Selectivity Boolean Olfaction Using Hollow-Core Wavelength-Scalable Bragg Fibers

A new odorant detection scheme, based on infrared absorption of volatile organics inside an optofluidic channel array, is discussed in terms of its selectivity. The sensor unit of the array is a hollow core Bragg fiber that selectively (spectrally) guides an incident continuum radiation. The presence of infrared absorbing molecules in the channel results in the quenching of the otherwise transmitted signal. Each fiber unit in the array is designed and fabricated so that it is sensitive to specific chemical bonds and the bond environment, but at the same time, each fiber is also broadly sensitive to a large number of chemicals due to their infrared absorbance spectra. The cumulative array response data, using an appropriate threshold, enable selective binary sampling of the infrared fingerprint of hundreds of molecules. The selectivity of the system is quantitatively investigated with computer simulations and found to be exponentially increasing with the number of fibers in the array. Relatively simple data analysis using binary logic combined with the high selectivity of the novel scheme paves the way for ubiquitous application of electronic noses in toxic gas detection, food quality control, environmental monitoring, and breath analysis for disease diagnostics.

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.

Analytical expression of core and cladding material losses effect in Bragg fibers using the perturbation theory

We present an analytical expression of fiber loss caused by core and cladding material losses for the TE mode of hollow-core and nonhollow-core Bragg fibers using the perturbation theory. The fiber loss agrees well with previous numerical results. Under the quarter-wave stack (QWS) condition, the expression gives the explicit dependence of the fiber loss on the fiber structural parameters, the wavelength, and the mode. The combined effect of the material with the confinement losses is also studied. It is quantitatively revealed that the core material loss affects the fiber loss much more than the cladding material losses, depending on the optical power confinement factor. The position of the minimum fiber loss corresponds well to the condition satisfying a generalized QWS condition even in the nonhollow-core Bragg fibers.

Liquid-core low-refractive-index-contrast Bragg fiber sensor

We propose and experimentally demonstrate a low-refractive-index-contrast hollow-core Bragg fiber sensor for liquid analyte refractive index detection. The sensor operates using a resonant sensing principle—when the refractive index of a liquid analyte in the fiber core changes, the resonant confinement of the fiber guided mode will also change, leading to both the spectral shifts and intensity changes in fiber transmission. As a demonstration, we characterize the Bragg fiber sensor using a set of NaCl solutions with different concentrations. Strong spectral shifts are obtained with the sensor experimental sensitivity found to be ∼1400 nm/refractive index unit, which is comparable to that of a surface plasmon resonance sensor. Besides using theoretical modeling we show that low-refractive-index-contrast Bragg fibers are more suitable for liquid-analyte sensing applications than their high-refractive-index-contrast counterparts.

Analytical Approach in Finding the Semi-Optimum Hollow-Core Bragg Fiber With Minimum Loss

Since it is quite difficult to analytically optimize hollow-core Bragg fibers to have minimum confinement loss without using the Floquet theorem, which does not hold for cylindrical structures, the cylindrical Bragg mirror is approximated by its planar counterpart. For this reason, first, the optimum planar waveguide formed by sandwiching a hollow layer between 1-D Bragg mirrors is found in this paper. The optimization for the dominant transverse-electric mode is analytically performed by using the pointwise wavenumber closely related to the wave admittance. A closed-form expression is then found to relate the propagation constant of this waveguide to the thickness of the hollow layer. This expression is then modified for the cylindrical structure, and the propagation constant is approximately related to the hollow-core diameter of the Bragg fiber to be optimized. The optimum cladding region is then found as if it were planar and a semi-optimum hollow-core Bragg fiber is proposed. Its superiority over conventional hollow-core Bragg fibers is numerically demonstrated by following a rigorous approach.

High-refractive-index composite materials for terahertz waveguides: trade-off between index contrast and absorption loss

We fabricate polymer compounds from titania-doped polyethylene and characterize their linear optical properties by terahertz time domain spectroscopy. We show that the high concentration of dopants not only enhances the refractive index of the composite material, but it also can dramatically raise its absorption coefficient. We demonstrate that the optimal design of photonic bandgap fibers based on lossy composites depends on finding a compromise between the high-refractive-index (hRI) contrast and corresponding material losses. Finally, fabrication and transmission measurements for the hRI contrast hollow-core Bragg fiber are reported and compared with simulations.

Transmission measurements of hollow-core THz Bragg fibers

We report on the terahertz (THz) spectral characteristics of hollow-core THz Bragg fibers. Two types of high-index contrast Bragg fibers were fabricated: one based on the index contrast between a polymer and air, and the second based on the contrast between a pure polymer and a polymer composite doped with high-index inclusions. The THz transmission of these waveguides is compared to theoretical simulations of ideal and nonideal structures. Waveguide dispersion is low, and total loss measurements allow us to estimate an upper bound of 0.05 cm−1 for the power absorption coefficient of these waveguides in certain frequency bands. We discuss multimode regimes, coupling losses, fabrication difficulties, and how bending losses will ultimately be the discriminant between different THz waveguiding strategies.

Confinement loss, including cladding material loss effects, in Bragg fibers

Confinement loss (CL), including cladding material losses, is comprehensively evaluated for TE, TM, and hybrid modes of hollow-core Bragg fibers. We show that the combined fiber loss tends to approach a loss depending only on CL Lconf for a small number N of cladding pairs, while it approaches a material loss limit value Lmat for large N. Transition point Ntr is defined by a crossing point of the asymptotes of Lconf and Lmat. Lmat and Ntr are investigated as a function of cladding material losses, cladding index contrast, and core radius rc, mainly under the quarter-wave stack condition. Lmat decreases in inverse proportion to the third and first powers of rc for TE and the other three modes for sufficiently large rc. An increase in rc more effectively reduces Lmat than an increase in the cladding index contrast. For rc=2.0, 5.0, and 10.0 μm in the TE01 mode transmission, we attain fiber losses less than about 10−2, 10−3, and 10−4 times cladding material losses, respectively.

Hollow-Core Bragg Fiber for Bio-Sensing Applications

Theoretical analysis of propagation properties in a hollow-core Bragg fiber for bio-sensing applications has been demonstrated. Based on the Bragg fiber we propose a resonant sensor that operates on changes in refractive index of aqueous solution placed inside the hollow core. By using the transfer matrix method we analyzed the confinement loss of the TE01 mode in the hollow-core Bragg fiber. We have shown the influence of the fiber geometry on the changes in the confinement loss. Spectral sensitivity and resolution of the sensor are also presented.

Broadband transmission in hollow-core Bragg fibers with geometrically distributed multilayered cladding

For the first time, the quasiperiodic Bragg fibers with geometrically distributed multilayered cladding are proposed and analyzed. We demonstrate that hollow-core Bragg fibers with quasiperiodic dielectric multilayer cladding can achieve low loss transmission over a broadband wavelength range of more than an octave (from 0.81 μm to 1.7 μm). The periods of the Bragg blocks follows a geometrical progression with a common ratio r<rc, where rc is defined as the critical ratio of the periods of two adjacent Bragg blocks. The arrangement of the quasiperiodic cladding can significantly modify the characteristics of the fiber, leading to a broadening of the guiding range compared to a hollow Bragg fiber with uniform periodic multilayer cladding structure. In general, a larger r value results in a broader guiding range. More Bragg blocks in the cladding and more unit cells in each Bragg block lead to a lower fiber modal loss.

Single-Mode Terahertz Bragg Fiber Design Using a Modal Filtering Approach

This paper proposes a single-mode Bragg fiber design method based on a modal filtering technique and applies it to the design of a hollow-core single-mode Bragg fiber, which can achieve low loss by avoiding material loss. The proposed method exploits the Brewster phenomenon to filter out the TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> mode so that only the fundamental HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> mode exists. Consequently, this proposed method maintains a balance between single-mode propagation and low-loss propagation so that it is specially suitable for terahertz single-mode Bragg fiber design. An explicit relationship between the first bandgap width and the layer thickness is derived. As a result, for a certain design bandwidth requirement, the Bragg fiber parameters can be determined. The proposed design strategy is applied to a hollow-core single HE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> -mode Bragg fiber operating in the 0.65-1.35-THz range with a calculated loss ranging from 0.2 to 1 dB/m.

外径波动下空心布拉格光纤的模式传输特性

外径波动下空心布拉格光纤的模式传输特性

Mid-infrared hollow-core Bragg fiber for trace gas detection

Bragg fiber is one kind of microstructure fiber with one-dimensional photonic crystal layer in its fiber cladding. In this paper, the hollow-core Bragg fiber for trace gas detection, with a transmission band in the mid-infrared band, is designed and fabricated based on semiconductor glass and polymer materials. The measurements of the fiber sample demonstrate its bandgap-guided properties, showing two obvious bandgap guided transmission bands. The central wavelength of the lowest transmission band is 4.4 μm.

Magnetically tunable liquid crystal terahertz switch based on Bragg fiber

A novel magnetically tunable liquid crystal terahertz switch is designed to reduce the coupling loss. The main structure of the switch is the hollow-core Bragg fiber with its cladding constructed by periodically setting of high density polyethylene and nematic liquid crystal E7 The refractive index of the E7 can be changed by the application of outside magnetic field and thus alter the loss property of the Bragg fiber dramatically to achieve its function as a switch. The main advantage of this switch is that it can eliminate the loss of coupling with fibers,and what is more,this switch can select the basic mode and is easy to be controlled. This article uses the finite element method to obtain the parameter of the switch and the simulated value of the extinction ratio reaches 2634 dB.

A new generation of plastic optical fibers and its functional exploiting

A major problem of plastic optical fibers (POFs) is large transmission loss in comparison with silica fibers. After adopting a new optical fiber structure, hollow-core Bragg fiber with cobweb-structured cladding, which can suppress the absorption losses of constituent materials by a factor of about 104–106, the problem of POFs with large losses is solved ultimately. With the advantage of flexibility and easy bending, the POFs with this structure can guide light with low transmission loss for information and energy in the wavelength range of visible light to terahertz (THz) wave (0.4–1000 μm). This new generation of POFs will find many applications.

Guiding Properties of Silica/Air Hollow-Core Bragg Fibers

The guiding properties of realistic silica/air hollow-core Bragg fibers have been investigated by calculating the dispersion curves, the confinement loss spectrum, and the field distribution of the guided modes through a full-vector modal solver based on the finite-element method. In particular, the silica bridge influence on the fundamental mode has been analyzed by comparing the properties of an ideal structure, without the silica nanosupports, and of two realistic fibers, with squared off and rounded air-holes. Simulation results have demonstrated the presence of anticrossing points in the dispersion curves, associated to the transition of the fundamental mode into a surface mode. It has been shown that surface modes are responsible for the sharp loss peaks, also experimentally measured, which pollute the loss spectrum of the fundamental mode and of the higher order modes. Then, the influence on the guiding properties of each geometric characteristic in the hollow-core Bragg fiber cross-section has been deeply investigated, thus showing which parameter it is better to change in order to properly modify the loss values or its spectral behavior. Moreover, in order to improve the loss properties of hollow-core Bragg fibers, the number of silica and air layers in the fiber cladding has been increased, and the layer thickness has been modified. Results have shown that the first change is more effective for the loss reduction, while the second is useful for a spectral shift. Finally, among the different possible applications, the feasibility of a DNA biosensor based on a hollow-core Bragg fiber has been demonstrated.

All-Silica Hollow-Core Microstructured Bragg Fibers for Biosensor Application

The possibility to exploit all-silica hollow-core microstructured Bragg fibers to realize a biosensor useful to detect the DNA hybridization process has been investigated. A Bragg fiber recently fabricated has been considered for the analysis performed by means of a full-vector modal solver based on the finite-element method. Since the DNA molecules necessary for the biosensor realization are in aqueous solution, it has been taken into account a microstructured fiber with water-filled holes. The dispersion curve and the confinement loss spectrum have been calculated in order to understand how a biofilm layer on the inner surface of the fiber holes can modify the fundamental mode properties. The numerical analysis results have successfully demonstrated the DNA bio-sensor feasibility in hollow-core Bragg fibers.

Photonic Crystal Fiber and Waveguide-Based Surface Plasmon Resonance Sensors for Application in the Visible and Near-IR

In the proposed photonic crystal waveguide-based surface plasmon resonance (SPR) sensor, a plasmon wave on the surface of a thin metal film is excited by a Gaussian-like leaky mode of an effectively single-mode photonic crystal waveguide. By judicious design of a photonic crystal waveguide, the effective refractive index of a core mode can be made considerably smaller than that of the core material, thus enabling efficient phase-matching with a plasmon, high sensitivity, and high coupling efficiency from an external Gaussian source, at any wavelength of choice from the visible to near infrared (IR), which is, to our knowledge, not achievable by any other design. Moreover, unlike the case of total internal reflection (TIR) waveguide-based sensors, a wide variety of material combinations can be used to design photonic crystal waveguide-based sensors as there is no limitation on the value of the waveguide core refractive index. This sensor design concept was implemented using planar multilayer photonic crystal waveguides, solid and hollow core Bragg fibers, as well as microstructured photonic crystal fibers. Amplitude and spectral-based methodologies for the detection of changes in the analyte refractive index were devised. Sensor resolution as low as 8.3·10−6 refractive-index unit (RIU) was found for aqueous analyte.