Two modes have been launched into a conventional 10-m long multimode graded-index optical fiber using spatial filtering techniques. The measured cross talk between the two modes in the baseband is -20 dB at the output end of the fiber. Modal multiplexing thus appears to be possible over short fiber lengths.

Mode division multiplexing in optical fibers

This paper describes an advanced multimode-fiber-link model that was used to aid the development of Telecommunication Industry Association standard specifications for a next-generation 50-/spl mu/m-core laser-optimized multimode fiber. The multimode-link model takes into account the interactions of the laser, the transmitter optical subassembly, and the fiber, as well as effects of connections and the receiver preamplifier. We present models for each of these components. Based on these models, we also develop an efficient and simple formalism for the calculation of the fiber transfer function and the signal at the link output in any link configuration. We demonstrate how the model may be used to develop specifications on transmitters and fibers that guarantee any desired level of performance.

Modeling and simulation of next-generation multimode fiber links

Stable self-trapping of laser beams: Observation in a nonlinear planar waveguide

The role of the mode field diameter in the characterization of single-mode fibers is examined. The most relevant definitions of this parameter are reviewed, and a comparative analysis of methods for its measurement is performed. All the discussed measurement methods have reached a repeatability and reproducibility which are quite satisfying. Emphasis is given to the requirements posed by the new fiber designs, such as the polarization-maintaining structures. Most of the discussed techniques have been industrialized, and a number of instruments based on them are commercially available; however, it is predicted that the evolution of fiber design will impose new requirements on some of these instruments. >

Mode field diameter measurements in single-mode optical fibers

We report on the generation of optical masks that permit the spatial shaping of a laser beam so that one can selectively excite single modes of a multimode parabolic graded-index optical fiber. The amplitude masks are determined by the analytical radial function of the fiber mode in the weakly guiding approximation. Phase reversal is encoded on the transparency through a shifted-grating technique. Effective injection and propagation of a single mode in a largely multimode fiber are shown as examples.

Selective mode excitation in graded-index multimode fiber by a computer-generated optical mask.

The clean propagation of tubular modes over more than 50 m of a multimode optical fibre has been observed. This observation opens the way to multichannel transmission through multimode optical fibres and provides a new fibre testing method.

Observation of tubular modes in multimode graded-index optical fibres

Mode selection in lasers is used in order to provide high-purity easy-to-implement single-mode launching in optical fibres. Mode-to-mode switching automatically maintains centring, scaling and alignment. Application of the principle to selective launching into a weakly multimode fibre (V=4) is demonstrated.

Tunable single-mode excitation in multimode fibres

High-purity selective-mode launching being feasible in graded-index profile multimode fibres, we evaluate in a closed form the effects of unavoidable launching imperfections such as lateral offset, angular misalignment and scale discordance of the launched beam.

Coupling efficiency in selective excitation of tubular modes into graded-index multimode fibres

The effects of periodic axis deformations on propagation in multimode optical fibers with single-mode excitation are investigated numerically and experimentally. The numerical study, based on ray theory, deals with helical rays in the presence of sinusoidal axis deformations for various shapes of index profile. The corresponding experimental observations and results, carried out on tubular modes, confirm the existence of resonance effects between the helical ray period and the fiber axis deformation. This technique permits the observation of mode-to-mode power transfer and provides a sensitive tool to investigate the mode-coupling mechanism in optical fibers.

Microbending effects on monomode light propagation in multimode fibers

The spectral analysis of the optical power transmitted through a bow tie slit rotating in the far-field pattern of a single-mode fiber provides an easy and accurate mean of determining the effective cutoff wavelength of the fiber. The principle of this technique is described. Theoretical considerations show that the ratio between the optical powers carried by the LP/sub 11/ mode and the fundamental mode, respectively, is directly related to the amplitude of two particular lines in the spectrum. The degree of coherence of the analyzed field is shown to be related to the amplitude of a third line. Owing to the sensitivity of the method, the detectable amount of the total power which is contained in the LP/sub 11/ mode is as small as 0.5%. The effects of length and curvature on the effective cutoff wavelength are measured over wide ranges for different kinds of single-mode fibers. The behavior of a quadruple-clad flattened dispersion fiber is shown to be somewhat different from that of a typical depressed cladding fiber. >

Azimuthal far-field analysis for the measurement of the effective cutoff wavelength in single-mode fibers/spl minus/effects of curvature, length, and index profile

P. Facq

Papers

Observation of tubular modes in multimode graded-index optical fibres

Tunable single-mode excitation in multimode fibres

Coupling efficiency in selective excitation of tubular modes into graded-index multimode fibres

Microbending effects on monomode light propagation in multimode fibers

Azimuthal far-field analysis for the measurement of the effective cutoff wavelength in single-mode fibers/spl minus/effects of curvature, length, and index profile