Optical Fiber Structure Research Articles

Development of special optical fibers for evanescent-wave chemical sensing

The paper summarizes results achieved in the Institute of Radio Engineering and Electronics in the field of investigation and fabrication of novel fiber-optic structures for evanescent-wave chemical sensing. Several approaches for increasing the evanescent-wave sensitivity of multimode silica optical fibers to gaseous and liquid chemicals are shown. These approaches make use of the decrease of the core diameter in sectorial and capillary sectorial fibers, modification of trajectories of optical rays in the cores of inverted-graded-index fibers, tailoring of the refractive index of porous layers applied on silica cores, an output mode filter preventing the detection of rays propagating at low axial angles or fibers bent in single or multiple turns.

Inverted-graded index fiber structures for evanescent-wave chemical sensing

The paper deals with a novel multimode fiber-optic structure, the inverted-graded index profile (IGI) fiber, and its feasibility for evanescent-wave chemical sensing. Results of the theoretical analysis of the sensitivity of the IGI fiber to changes of the refractive index and light absorption coefficient of its cladding are shown. Fabrication of IGI fibers doped in the core with GeO 2 or B 2O 3 is described. The evanescent-wave sensitivity of the prepared IGI fibers to changes of the cladding refractive index and light absorption coefficient is given showing its increase in comparison with reference fibers with cores of pure silica and with the same polymer claddings as the IGI fibers.

Optical fiber structures studied by a tapping-mode scanning near-field optical microscope

A tapping-mode scanning near-field optical microscope system with a bent optical fiber probe has been successfully developed to study the near-field optics at fiber end faces. Topographic and near-field intensity images of fiber end faces were obtained simultaneously. A brief and gentle etching process on fiber end faces in a saturated solution of ammonium bifluoride was able to reveal the structures of fibers. The topographic features generated by different local chemistry is closely related to the local doping concentration and the distribution of refractive index. The correlation between fiber structures and their near-field intensity profiles of propagating modes can be acquired directly by this novel method. Fibers of step-index, graded-index, single-mode, multimode, and circular, or noncircular geometry were imaged with a near-field resolution.

Study of optical fibre structures by atomic force microscopy

A novel method using atomic force microscopy (AFM) to study optical fibre structures at the fibre end-face has been successfully developed. The doping concentration profiles of fibres revealed by differential etching speeds in a saturated solution of ammonium bifluoride at room temperature (25°C) were obtained from AFM topographic images. The superior spatial resolution of AFM made it possible to resolve concentric structures a hundred times smaller than the feature, due to the difference in the known refractive index (Δn) of 1×10-3. Fibres with small core diameters and anisotropic structures, such as polarization-maintaining fibres, were studied with ease.

Sputter deposited piezoelectric fiber coatings for acousto-optic modulators

All-fiber optical phase modulators were fabricated by coating standard telecommunication optical fibers with thin films of piezoelectric ZnO. Piezoelectric optical fiber coatings of PbZrxTi1−xO3(PZT) were also produced, and the actuation capability necessary for making optical fiber modulators is demonstrated. Both the ZnO and the PZT fiber coatings were deposited by reactive direct current magnetron sputtering of metal targets. The geometry of the sputtering system, fiber rotation, and reactive sputtering processes that result in sufficiently high deposition rates for deposition of axially symmetric coatings between 0.5 and 7 μm thick were studied. Two types of piezoelectric fiber optic modulator structures, Cr/Au/ZnO/Cr/Au and Ti/Pt/PZT/Au, are currently being investigated. Results on crystalline phase formation, microstructure, and piezoelectric and optical behavior of the coated optical fiber devices are presented. Impedance and phase angle measurements are used to demonstrate how device geometry affects the resonance response of ZnO coated devices. Interferometric measurements have shown that the ZnO-based modulators can produce optical phase shifts as high as 0.70 rad when the modulator is driven at the 196.5 MHz fiber radial resonance.

Synthesis of fiber-optic delay line filters

This paper deals with the synthesis of fiber-optic delay line filters and is organized as follows. We show how filters with negative coefficients can be implemented using positive structures and differential detection at the optoelectronic conversion. This possibility implies that not only lowpass, but bandpass and highpass filters with arbitrary transfer functions may be implemented. Synthesis methods for positive filters based on modified versions of well-known time and frequency domain techniques are presented which take into account the restriction on the positive nature of the filter coefficients. We make use of these results to describe the synthesis procedure for arbitrary filters using fiber-optic structures. Some designs of lowpass, bandpass and highpass filters are presented as examples, and a possible implementation of these structures with a view of future optoelectronic integration is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Cooperative luminescence in an ytterbium-doped silica fibre

We report the first spectral and power-law modeling of cooperative luminescence in an ytterbium-doped optical silica fibre structure enabling the creation of new saturation effects. We believe that the low-concentration transfer originates in the ytterbium-oxide molecule itself and is consequently due to the inherently oxidizing MCVD process.

Characterization of the lightguiding structure of optical fibers by atomic force microscopy

For the first time, atomic force microscopy (AFM) is applied to study the lightguiding structure of optical fibers: the local surface topography of an etched fiber endface is profiled with AFM. The etching rate has been studied as a fingerprint to determine the effect of dopant chemistry and preform fabrication conditions on the fiber structure. Structural and geometrical distortions of the lightguiding structure can be directly measured with high spatial resolution. Furthermore, by quantifying the etched depth in relation to the refractive-index change, a spatial mapping of the refractive-index change can be inferred from the AFM profile. These examples demonstrate the effective use of AFM to elucidate, on a nanometric scale, features of the lightguiding structure that contribute to the performance of light transmission. >

Optical fibre sensor-based smart materials and structures R O Claus (Editor)

Optical fibre sensor-based smart materials and structures R O Claus (Editor)

Gaussian and equivalent-step-index approximations for nonlinear waveguides

The field of the fundamental waveguide mode, when it propagates at extremely high intensities or when the core or cladding material has a large nonlinear coefficient, may be quite significantly distorted from that of the corresponding linear mode. We derive a variational formulation of the scalar wave equation for waveguides with arbitrary nonlinearity in the core and show that this formulation can be used to find simple, but accurate, analytical approximations for these nonlinear fields. In particular, we find Gaussian and equivalent-step-index approximations for a variety of planar waveguide and optical-fiber structures and show how they can be used to calculate quantities such as the effective area and group-velocity dispersion.

Mechanical Behavior of Materials in Microelectronic and Fiber-Optic Structures: Application of Analytical Modeling - Review

The overwhelming majority of studies in microelectronics and fiber-optics are experimental. Not too many apply numerical, mainly finite-element, methods to analyze microelectronic and fiberoptic structures. There is a very small number of papers using analytical modeling. At the same time application of powerful and well-developed analytical methods of Engineering Mechanics often enables one to obtain valuable prior information of the mechanical behavior of materials and structures, interpret empirical data, and to extrapolate the accumulated experience on new designs [1,2]. As a rule, application of analytical modeling results in better understanding of the behavior and performance of a material or structure, and in substantial savings of time and expense. This review, based primarily on the author's research, addresses several basic and practically important problems related to the mechanical behavior of materials and-rational structural design of microelectronic and fiber-optic systems and lending themselves to sufficiently simple analytical solutions.

Efficient fiber-optic structure with applications to sensor arrays

A fiber-optic structure which performs the functions of sensing and telemetry with a minimum of components and with efficient utilization of optical power is described. This structure, referred to as a recursive lattice array, requires N+1 couplers and N fiber sensing loops to realise N sensors. It is shown that for pulsed operation, the duty cycle approaches 100% and the maximum sampling rate is 1/(N+1)T, with T denoting the transit time of a single sensing loop. In the ideal (lossless) case, the power returned to the receiver from any sensor is -10 log 2N referred to the input, compared with previously reported, nonrecursive structures for which this figure-of-merit is -20 log N. Expressions for the optimum coupler tap ratios for two different cases of interest are derived: first, for the case in which all the coupler tap ratios are equal, and second, for the case where they may assume different values. The magnitudes of decaying recirculating terms which add noise to the desired primary returns from each sensor are estimated. Methods for reducing the magnitudes of the undesired terms are outlined.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Design consideration on single-mode fibers suitable for subscriber cables

This paper studies suitable optical single-mode fiber structures for subscriber cables that connect subscribers with a local exchange. Allowable ranges of mode-field radius of the LP 01 mode and effective cutoff wavelength of the LP 11 mode, and tolerable values of fiber outer diameter and core nonconcentricity are presented from the viewpoint of reducing losses due to Rayleigh scattering, structural imperfections, bending, and splices. A transmission speed of 100 Mbit/s in the wavelength regions of 1.3 and 1.5 μm is assumed. The allowable area specified by the parameters of mode-field radius and effective cut-off wavelength is enlarged 2.2 times compared with that for the fibers designed for trunk transmission lines.

Evanescent-wave analysis of general clad graded-index fibers

The evanescent wave theory (ewt) is extended to treat modal propagation along a general graded-index optical-fiber structure surrounded by a uniform cladding. A step or a valley may be present at the core–cladding interface. The fiber core may be specified in a more general form than was the case for untruncated profiles. The reformulation of the original ewt method uses the uniform corrective method, developed by Arnold for WKB calculations, iteratively to specify nonintegral radial mode numbers for the clad index profile. The integral radial mode numbers of the unclad profile are used as first approximations. Our method gives corrected values for propagation constants and group delays of guided modes and complex propagation constants of leaky modes. Numerical results show good agreement with a numerical solution for a parabolic-core fiber. For other profiles they specify accurate asymptotic upper and lower limits for the cladding influence. For leaky modes of a truncated near-parabolic profile we show agreement with results of a simple WKB formulation. A valley, present at the core–cladding interface, causes a reduction in the maximum intermodal group-delay difference from 3.1 to 1.9 nsec/km because of a strong attenuation of high-order (leaky) modes.

Material Structure of Germanium-Doped Optical Fibers and Preforms

The structural characteristics of preforms and optical fibers fabricated by modified chemical vapor deposition were studied by optical, interference, and scanning electron microscopy. It was observed that the structural features resulting from the deposition process are preserved through subsequent processing and appear in the fiber with the exception of a region at the center of the fiber. Here, selective evaporation of dopant material from the inner surface of the deposit results in a refractive index depression on the axis of the optical waveguide.