Fabrication process of efficient Yb/sup 3+/,Er/sup 3+/ codoped phosphosilicate fibers by modified chemical vapor deposition (MCVD) combined with the solution doping technique is studied in detail. We show that the process can be adapted to incorporate low viscosity phosphate glass and some important issues in the fabrication process are discussed. These include the sensitive presintering pass. We also report on the fabrication of a low loss all-glass double clad Yb/sup 3+/:Er/sup 3+/ codoped fiber. We explain how we evaluate the fibers and discuss the effect of the ytterbium to erbium concentration ratio on the laser characteristics. Finally, we present results of some investigations into the mechanisms which can affect the efficiency of the lasers, and show that the detrimental up-conversion from the metastable level of the erbium ions is dramatically reduced by the presence of the ytterbium ions.

Fabrication and characterization of Yb/sup 3+/:Er/sup 3+/ phosphosilicate fibers for lasers

This paper investigates possibilities for the practical design of high-performance multimode fibers (MMFs) that can provide bandwidths in excess of 10 GHz/spl middot/km in an overfilled regime of operation. Analysis of standard MMF in an overfilled launch demonstrates that the theoretical bandwidth limitations arise from the influence of cladding on the propagation of the highest order modes. Practical MMF profile designs that overcome this problem are investigated. The standard 50- and 62.5-/spl mu/m fiber profiles are redesigned first to allow for the performance in an overfilled launch with the differential mode delays (DMDs) below 0.055 and 0.250 ns/km, respectively. It is shown that such fibers can exhibit the same or better theoretical bandwidth in an overfilled launch when compared to standard fiber under restricted launch. Elimination of the need for the restricted mode launch in high-performance multimode transmission systems can improve reliability issues and can relax the range of tolerance requirements imposed on terminal equipment, optical components, and link installation. Furthermore, MMFs that can be operated in an overfilled launched are compatible with emerging vertical cavity surface emitting laser (VCSEL) wavelength division multiplexing (WDM) array technologies. A successfully controlled higher order mode DMD also allows for the reduction of MMF core size and /spl Delta/ that can be beneficial for low-cost high-performance single-channel links. It is demonstrated that properly designed reduced core fibers can achieve theoretical DMDs in the range of 0.005-0.02 ns/km. The bend loss properties of redesigned fibers are investigated in detail, showing that the proposed modifications do not lead to significant degradation of bend loss performance. Moreover, they can be manufactured at considerably lower cost while utilizing commercially readily available low-cost VCSELs. Even where the theoretical limit is not achieved by practical fiber making, the reduced core size and /spl Delta/ MMF can provide higher production yield, lower cost, and higher average bandwidth.

Opportunities to enhance multimode fiber links by application of overfilled launch

Seven F-doped- and undoped-silica-core optical fibers were produced by the MCVD-method, the ratio of the O2 and SiCl4 molar flow rates in the vapor-gas mixture during the fabrication of the perform core being varied among the performs. The fibers were γ-irradiated to 8.1 kGy (0.75 Gy/s), radiation-induced absorption (RIA) being measured during and after the irradiation in the spectral range 1.1-1.7 μm. The fiber optical loss spectra were also measured after γ-irradiation to 1.31 MGy. Three RIA mechanisms affecting the near-IR region have been revealed, namely, short-wavelength RIA tails due to the Cl0-center and to the self-trapped holes of the second type (STH2) and a long-wavelength RIA tail due to the self-trapped holes of the first type (STH1). STH2 are argued to result from the strain frozen-in in the fiber glass in the fiber drawing process. The RIA due to STH 1 is shown to anticorrelate with the RIA due to STH2. All the RIA mechanisms revealed are shown to be strongly suppressed by providing a high O2 excess in the vapor-gas mixture during the perform core synthesis. The O2-excess technique is therefore proposed as a very promising one for the development of radiation-resistant fibers.

Enhanced Radiation Resistance of Silica Optical Fibers Fabricated in High O $_{\bf 2}$ Excess Conditions

A process is described for reproducible deposition of porous layers uniform along the preform axis, and the effect of the nature of the solvent on the infiltration of salt solutions into the porous layer is analyzed in relation to the fabrication of fiber preforms with controlled doping level. Data are presented on the variation of the retention volume in the porous layer with sintering temperature.

Doping of Optical Fiber Preforms via Porous Silica Layer Infiltration with Salt Solutions

The dependence of fluorine incorporation into silica glass on conditions of the optical waveguide blank manufacture using the method of modified chemical vapor deposition (MCVD) is studied. The degree of fluorine incorporation at constant consumption of a fluoridizer (SiF4) is shown to increase with an increase in the volume of deposited material. Samples of multimode optical waveguides with a core from pure silica glass and a reflective coating made of fluorine-doped silica glass with a numerical aperture of ∼0.18 are obtained. Optical losses in such optical waveguides reach 0.5 dB/km in the long-wavelength region of the spectrum.

J. Krichhof

Papers

A new MCVD technique for increased efficiency of dopant incorporation in optical fibre fabrication