Monomode Fiber Research Articles

Linear azimuthons in circular fiber arrays and optical angular momentum of discrete optical vortices

We study the field generated in the outer space by the superposition of modes of a regular circular monomode fiber array. It is shown that a supermode of the fiber array generates a discrete optical vortex; the formula for the topological charge of the vortex is obtained depending on the order of the supermode and the number of fibers in the array. The orbital angular momentum carried by an arbitrary superposition of supermodes is shown to equal the weighted sum of partial angular momenta of supermodes. It is shown that for certain combinations of supermodes the angular momentum comprises along with its intrinsic part also the extrinsic constituent. For such combinations precession of the angular momentum about the propagation axis is demonstrated. It is demonstrated that by combining supermodes one can generate in the array stable regularly rotating linear azimuthons. By creating a phased excitation of certain groups of fibers in the array one can control the global soliton-like motion of the excited domain.

Laser cladding of Co-based superalloy coatings: Comparative study between Nd:YAG laser and fibre laser

This paper presents a systematic comparison between the performance of a Yb:YAG monomode fibre laser and a Nd:YAG laser as energy sources in the side laser cladding technique. Experiments were performed with exactly the same working head and experimental configuration. Results obtained allow affirming that a high brightness laser is only useful when very narrow coatings are required. A conventional Nd:YAG laser is a suitable source for large area coatings.

Effect of the spin–orbit interaction on polarization conversion in coupled waveguides

A generalization of perturbation theory for weakly coupled optical waveguides beyond the scalar approximation is suggested that explicitly allows for the spin–orbit interaction embedded in the gradient term of the vector wave equation. The obtained perturbation theory is applied to study the problem of cross-talk in coupled ideal monomode fibres. We demonstrate the absence of cross-talk between orthogonal linear polarizations and the inversion of the circular polarization's sign caused by the spin–orbit interaction. The spin conversion length is determined.

High efficient and narrow linewidth fiber laser based on fiber grating Fabry-Perot cavity

A high Er3+-doped narrow linewidth fiber laser based on fiber Bragg grating Fabry-Perot cavity was demonstrated. The spatial hole burning effect was restrained by a fiber Faraday rotator. Two short fiber Bragg grating Fabry-Perot cavities as narrow bandwidth filters discriminated and selected laser longitudinal modes efficiently. A stable single-frequency 1534.83 nm laser was acquired. Pumped by two 976 nm laser diodes and two-ended output, the fiber laser exhibited a 12 mW threshold. Total 39.5 mW output power and one end 22 mW output power were obtained at the maximum 145 mW pump power. Optical-optical efficiency was 27% and slope efficiency was 29.7%. The output power seemed to be saturated when pump power increased. The 3 dB linewidth of the laser was less than 7.5 kHz, measured by the delayed self-heterodyne method with 15 km monomode fiber. The high power narrow linewidth fiber laser can be used in high resolution fiber sensor systems.

Coupling of a laser diode to monomode elliptic core fiber via upside down tapered microlens on the fiber tip: Estimation of coupling efficiency with consideration for possible misalignments by ABCD matrix formalism

We investigate theoretically the coupling optics involving a laser diode and an elliptic core monomode step index fiber via an upside down tapered lens on the tip of the fiber in absence and presence of possible transverse and angular misalignments. By employing Gaussian field distributions for both the source and the fiber and also the derived ABCD matrix for upside down tapered lens, we formulate analytical expressions for the concerned coupling efficiencies with and without transverse and angular mismatches. Our formalism is very simple in comparison to the other existing methods involving cumbersome numerical integrations. Elliptic core single-mode fiber has already emerged as a potential candidate in polarization conserving fiber optic sensor, coherent fiber optical communication, etc. The investigations have been performed for two different emitting wavelengths of the laser diodes for the said elliptic core monomode fibers. Therefore, such analysis which as per our knowledge is the first theoretical investigation in the context, can also be treated as simple and novel prescription for choosing suitable parameters for the design of upside down tapered lens on the tip of elliptic core single-mode fiber for optimum launch optics. In addition, this analysis will be useful in case of inherent non-circularity that may arise due to fabrication problem in circular core fiber and thereby generates interests among the experimentalists for verification and future investigations on upside down tapered lens.

Helical core optical fibers maintaining propagation of a solitary optical vortex

We study theoretically the mode structure of few-mode helical core optical fibers with small (up to a wavelength order) values of the helix pitch. We demonstrate that in such fibers, at certain values of pitch the fundamental $H{E}_{11}$ mode strongly hybridizes with $l=1$ optical vortices and the forbidden zone appears in the spectra of such hybrid modes, which results in their attenuation. We have shown that within the spectrum gap, the only forward-propagating guiding modes are represented by two circularly polarized optical vortices with opposite polarization and the same topological charge so that upon excitation of such a fiber with a circularly polarized beam, only a solitary optical vortex could be excited in it. Such ``monovortex'' fibers are found to be analogous to conventional monomode fibers that maintain the propagation of the $H{E}_{11}$ mode.

Monomode fibres for long haul transmission systems

Today, monomode fibres seem set to replace graded-index fibre in new high-capacity trunk and undersea system installations. This paper reviews the background to that development and looks forward to the remaining untapped potential that is only now being uncovered.

Time‐domain and spectral‐domain optical coherence tomography in the analysis of brain tumor tissue

Detection of residual tumor during resection of glial brain tumors remains a challenge because of a low inherent contrast of adjacent edematous brain, the surrounding infiltration zone, and the solid tumor. Therefore, new technologies that may facilitate an intraoperative analysis of the tissue at the resection edge are of great interest to neurosurgeons. For ex vivo imaging of gliomas in a mouse model and human biopsy specimens of brain tumors and nervous system tissue we have used a time-domain Sirius 713 Tomograph with a central wavelength of 1,310 nm and a coherence length of 15 microm equipped with a mono mode fiber and a modified optical coherence tomography (OCT) adapter containing a lens system for imaging at a working distance of 2.5 cm. A spectral-domain tomograph using 840 nm and 930 nm superluminescence diodes (SLD) with a central wavelength of 900 nm was used as a second imaging modelity. Both time-domain and spectral-domain coherence tomography delineated normal brain, the infiltration zone and solid tumor in murine intracerebral gliomas. Histological evaluation of H&E sections parallel to the optical plain demonstrated that tumor areas of less than a millimeter could be detected and that not only solid tumor, but also brain invaded by a low-density single tumor cells produced an OCT signal different from normal brain. Spectral-domain OCT (SD-OCT) demonstrated a significantly more detailed microstructure of tumor and normal brain up to a tissue depth of 1.5-2.0 mm, whereas the interpretation of time-domain OCT (TD-OCT) was difficult at a tissue depth >1.0 mm. Because of rapid scanning times SD-OCT data could be acquired as 3D data maps, which allowed a multi-planar analysis of the tumor to brain interface. Similar to our findings in experimental gliomas, images of human nervous system tissue acquired using SD-OCT showed a characteristic signal of normal brain tissue and a detailed microstructure of tumor parenchyma. Spectral-domain OCT of experimental gliomas and human brain tumor specimens differentiates solid tumor, diffusely invaded brain tissue, and adjacent normal brain based on microstructure and B-scan signal characteristics. In conjunction with the rapid image acquisition rates of SD-OCT, this technology carries the potential of a novel intraoperative imaging tool for the detection of residual tumor and guidance of neurosurgical tumor resections.

Characterization of a high birefringence fibre Bragg grating sensor subjected to non-homogeneous transverse strain fields

Several theoretical and experimental studies have been carried out in recent years to characterize the spectral response of an optical fibre Bragg grating (FBG) subjected to non-homogeneous axial strain fields. However, few works have considered the evolution of the spectrum when a FBG fabricated in a high birefringence fibre is subjected to non-homogeneous transverse loading. In this paper, a physical model is described and a numerical simulation based on the piecewise-uniform approach is also used to calculate the response of a FBG fabricated in a high birefringence fibre under some typical non-homogeneous transverse strain fields. Finally, simulated results are analysed and compared with a FBG fabricated in a conventional monomode fibre.

New preparation of gold–silver complexes and optical fibre environmental sensors based on vapochromic [Au2Ag2(C6F5)4(phen)2]n

AbstractComplexes [Au2Ag2(C6F5)4L2]n, where L = 1,10‐phenanthroline (phen), 2,2′‐bipyridine (bipy) or tetrahydrothiophene (tht), have been synthesized by reaction of NBu4[Au(C6F5)2] with Ag(SO3CF3), and the addition of phen or bipy thereafter, or with [Ag(SO3CF3)(tht)]. The organometallic vapochromic material [Au2Ag2(C6F5)4(phen)2]nis isolated as a powder that is able to detect volatile organic compounds such as acetone even in an aqueous solution. The colour of this vapochromic material changes from bright yellow to white in the presence of different donor solvents such as acetone, methanol or ethanol. For the practical construction of an optical fibre sensor, a sol‐gel doped with the vapochromic complex was deposited onto one end of a monomode fibre connected to a coupler. The behaviour of the material was studied at different wavelengths and concentrations of acetone vapours and acetone–water solutions. Changes were detected up to 4 dB in the reflected optical power. Copyright © 2005 John Wiley & Sons, Ltd.

Realistic monomode air-core honeycomb photonic bandgap fiber with pockets

We present a photonic bandgap calculation of monomode air-core fibers with a realistic treatment of "pocket" interstitials that are formed in a honeycomb cladding. The analytic Fourier transformations of different honeycomb-lattice unit cells are used for plane-wave calculations. The consideration of the true pocket geometry within the cladding shifts the fundamental bandgap to higher frequencies and leads to a hollow-core fiber. The core radius necessary for a monomode fiber is approximated and then proved via the full-vectorial solution. The field distribution and polarization state of the fundamental mode for a real honeycomb photonic-crystal fiber (PCF) are given.

Tendências de Padronização de Arquiteturas e Protocolos de Redes Ópticas

The main objective of this work is to analyze the evolution trends for the architecture of optical networks, discuss the state of the art for the protocols required for its development and evaluate the various efforts being carried out by different institutions as the ITU (International Telecommunications Union), lETF (Internet Engineering Task Force) and OIF (Optical Internetworking Forum) in order to achieve standardization. Optical networks arose in the early eighties with the use of the monomode fiber optic cables and were further developed with the introduction of a new technological base composed by new types of optical fibers, optical amplifiers, DWDM (dense wavelength division multiplexing) systems and optical components such as add/drop multiplexers and optical cross-connects. The introduction of these components will make possible the evolution from the simple point-to-point WDM connections to the all optical networks. In this work we analyze how the applications will interact with this physical layer. Such interaction has been demonstrated to be a challenge within the framework of the current architecture of data networks, aiming at offering solutions for the transport of a great volume of traffic in an efficient manner. With this goal in mind, the use of technologies such as MPLS (multiprotocol label switching) and its evolution, the generalized MPLS (GMPLS) is discussed. Finally, we present the in progress lTV version for the architecture optical networks, addressing its merging with the GMPLS.

Optimization of wavelength division multiplexing in N × 160 Gbit/s terrestrial transmission systems

We analyze, from an engineering viewpoint, the prospects of an exploitable upgrade of terrestrial fiber systems based on standard monomode fiber and dispersion compensating units, for future N×160Gbit/s transmission systems. We show that dispersion swing, average dispersion and input pulse power are the key parameters that govern the system performances. We show that whenever the dispersion swing is arranged in a symmetrical setup and the compensation ratio is optimized accordingly, one may obtain a significant improvement of the transmission performances.

Spatial phase stepping wavelength meter

A new way of evaluating the ratio between a reference wavelength radiation and an unknown wavelength radiation in a two-beam interferometer is proposed here. The advantage of two-beam interferometry is the sinusoidal fringe signal for which precise phase detection algorithms exist. Modern algorithms can cope with different sources of errors, and correct them. We recall the principle of the Michelson-type lambdameter using temporal interference and we introduce the Young-type lambdameter using spatial interference. The Young-type lambdameter is based on the acquisition of the interference pattern from two point sources (e.g. two ends of monomode fibres projected onto a CCD camera). The measurement of an unknown wavelength can be achieved by comparing with a reference wavelength. Accurate interference phase maps can be calculated using spatial phase shifting. In this way, each small group of contiguous pixels acts as a single interferometer, and the whole set of pixels corresponds to many hundreds or thousands of interferometric measurement system units. The analysis of uncertainties shows that resolutions better than 10−7 can be achieved. An advantage of the fibre wavelength metre described here is the measurement velocity that takes only a few seconds.

The orbital angular momentum of down-converted photons

We calculate the relative amplitude of the orbital angular momentum (OAM) of entangled photon pairs from the spontaneous parametric down-conversion of a thin quadratic nonlinear crystal. The results show that the amplitude depends on both of the two Laguerre indices l, p. We also discuss the influences of the commonly used holograms and mono-mode fibres for mode analysis. We conclude that only a few dimensions can be explored from the infinite OAM modes of down-converted photon pairs.

Experiments and modelling on electrochemically generated bubbly flows

Despite a large demand in the area of electrochemical processes, e.g., the production of sodium chlorate and electrowinning of zinc, the available literature on the flows of micro-bubbles is limited. An experimental investigation of micro-bubbles two-phase flows has been therefore undertaken. The experiments were performed within a vertical pipe (inner diameter 50 mm). An original electrochemical bubble generator has been specially designed for this study. The liquid phase is a salt solution, which inhibits coalescence, so that the electrochemically generated H 2-bubbles exhibit typical diameters about 100 μm, corresponding to a particle Reynolds number about unity. Both the void fraction and the velocity of the micro-bubbles were measured through image analysis. Velocity measurements were also performed using a new type of sensor, based on a mono-modal fiber optics. The void fraction distribution was found to significantly differ from existing results obtained at higher bubble to pipe diameters ratios. The experimental measurements are also compared with numerical simulations based on a 2-D drift flux model. Finally, the experimental results of the paper demonstrate that the use of the Faraday law is questionable as regards its ability to predict the input gas flow rate.

Measurement of residual internal stresses in optical fiber preforms

A non-destructive experimental procedure is presented which enables the determination of residual thermal stresses in optical fiber preforms. The procedure is based on integrated photoelasticity. We carry out the measurement of the optical retardation using the traditional Senarmont compensation method combined with a fringe shifting technique. The radial distribution of the axial stress is reconstructed using Abel transform. We have investigated two different refractive-index profiles, adopted in the realization of monomode and multimode optical fibers. It was shown that in both cases the results obtained experimentally and those analytically predicted by the Timoshenko elastic model were in good agreement. To obtain accurate experimental results, it was necessary to apply a correction procedure to take into account the fact that the trafectories of the light rays inside the preforms are not straight lines.

Laser microwelding for fiber-chip coupling modules with tapered standard monomode fiber ends for optical communication systems

We design a welding machine for fabricating butterfly (BFY) modules with an optical coupling setup that is well suited for rapid prototyping and small-scale production of InP and GaAs-based waveguide-fed photonic components such as diode lasers, semiconductor amplifiers, optical modulators, and optoelectrical ICs (OEICs). The light of these devices is transferred very efficiently into single-mode fibers using integrated tapered lensed ends. In our setup the mechanical adjustment of the fiber-chip coupling is done semiautomatically by a newly designed coupling machine while welding performs the fixing of the fiber-chip connection. We found a yield of 47% within a shift of 1.0 μm at the initial welding spot. With additional weld adjustment, it was possible to extend the yield of 67%. We introduce, to our knowledge for the first time, an adjustable welding technique, which we call strain-reducing welding, which has several benefits in comparison to the well-known laser hammering technique.

γ-radiation-induced attenuation in photonic crystal fibre

Results are presented of what, to the knowledge of the authors, is the first study of the sensitivity of a photonic crystal fibre (PCF) under steady-state gamma irradiation. Radiation-induced attenuation of a PCF has been measured at 1.55 µm. This new type of fibre showed a higher attenuation level than a standard monomode fibre but had a fast kinetic of recovery after irradiation.

The modified nonlinear Schrödinger equation: facts and artefacts

It is argued that the integrable modified nonlinear Schroedinger equation with the nonlinearity dispersion term is the true starting point to analytically describe subpicosecond pulse dynamics in monomode fibers. Contrary to the known assertions, solitons of this equation are free of self-steepining and the breather formation is possible.