Pulse broadening in near-square-law graded-index fibres

A closed-form expression for pulse broadening in graded-index fibres that have small and circularly symmetric, but otherwise arbitrary, deviations from a square-law, and arbitrary dn/d?, is applied to germania-doped fibres. The range of validity of the theoretical expression is defined by comparison with the results of numerical integration.     

Effect of polarisation on pulse broadening in multimode graded-index optical fibres

A formulation of ray optics that takes the polarisation of electromagnetic waves into account shows that the maximum difference in time delay between corresponding HE and EH modes is, for almost any multimode graded-index fibre, of the order of 10 000 (?n/n)2/V ns/km, where V is the V-number of the fibre and ?n is the variation of the refractive index between axis and cladding. The effect of polarisation on pulse broadening is found to be negligible if the V-number of the fibre is much larger than about 20.     

Pulse broadening in multimode fibres excited by GaAs lasers

Pulse dispersion in multimode fibres depends strongly on mode excitation at the input and mode coupling in the fibre. Measurements on fibres of up to 160 m in length are given: pulse dispersion seems to follow a square-root law with the lengths of the fibres.     

Observation of tubular modes in multimode graded-index optical fibres

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.     

Pulse propagation in multimode optical fibres

Coupled mode theory is unnecessary for the analysis of pulse propagation in multimode optic waveguides. Instead, a very simple ray or continuum equation is derived from elementary plane-wave concepts to account for the effects of absorption loss and scattering. The formalism reduces to a diffusion equation for highly forward directed scatterers.     

Pulse broadening in multimode optical fibres with large ?n/n: numerical results

The r.m.s. impulse response width of germania doped fibres having power-law profiles at the carrier wavelength is evaluated with the help of a numerical method and measured values of dn/d?. For ?n/n?0.02, our result exceeds by more than one order of magnitude that obtained from an analytical formula based on the assumption that n dn/d? varies linearly with n2. The optimum profiles are found to differ very significantly from power-law profiles. Our numerical technique is based on scalar-ray optics. It is applicable to any fibre having a large V-number and a smooth profile.     

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

Profile inhomogeneity in multimode graded-index fibers

Profile parameters (g) of several multimode graded-index fibers have been measured. It was found that g may vary azimuthally by ±0.15 or more in fibers for which the average value is between 1.8 and 2.2     

Pulse dispersion for single-mode operation of multimode cladded optical fibres

Pulse dispersions as low as 0.4 ns/km have been measured in multimode cladded fibres at a normalised frequency V=125 and for a constant bend radius of 5.5 cm. Particularly when the number of launched modes is small, the pulse dispersion, as well as the polarisation and angular width of the output beam, are strong functions of the degree of mode conversion.     

Pulse broadening in long-span dispersion-free single-mode fibres at 1.5 ?m

Dispersion-free single-mode fibres at 1.5 ?m were fabricated by controlling their waveguide dispersion and dopant-dependent material dispersion. Pulse broadening in the 20 km long fibre was measured by using a laser diode operated at 1.5 ?m. Measured pulse broadening was 1.3 ps/km nm, which is in good agreement with theoretical results. The present experiment confirms that it is beneficial to use such fibres for large-capacity and long-distance transmission media.     

Chromatic dispersion measurement in graded-index multimode opticalfibers

The chromatic dispersion observed in a graded-index multimode optical fiber is a function of the radial position of the launched light. When a graded-index multimode optical fiber is overfilled spatially and angularly, the chromatic dispersion observed is an average (composite) chromatic dispersion over all of the possible launches. Under certain assumptions, the relationship between the chromatic dispersion observed for the overfilled fiber and the chromatic dispersion of any restrictive launch can be determined. This relationship can be used to predict the composite graded-index multimode optical fiber chromatic dispersion from the measurement of the chromatic dispersion of a single simplified launch, such as a single-mode fiber launch. Preliminary experimental results confirm this approach     

Mode conversion at splices in multimode graded-index fibers

Mode conversions occurring at a splice in multimode graded-index fibers are investigated theoretically, and their effects on impulse responses are verified experimentally. The relation describing the mode behavior at a splice in existence of a geometrical offset and fiber parameter mismatches is derived by taking a skew ray as well as meridional ray into consideration. Then the mode transfer matrix is obtained to determine the variations in mode power distribution and impulse response due to the mode conversions occuring at a splice. The measured mean delay time difference between lower and higher mode pulse responses and baseband frequency response for spliced graded-index fibers are compared with the theory obtained from the transfer functions of individual fibers and the mode transfer matrix which describes the mode conversions at a splice due to a transverse displacement and fiber parameter mismatches.     

Evolution of power distribution in graded-index fibres

Mode-coupling mechanism in square-law multimode fibres are experimentally studied. Measurements are done using the near- and far-field techniques. Special attention is paid to comparing the coupling strength of the microbending type (?v = ± 1) with that of the elliptic deformation type (?v = ± 2). Typically, the latter is much stronger. The significance of this property in transmission aspects is briefly reviewed.     

Hybrid modes of graded-index optical fibres

The profile dependence of the hybrid wave structure of graded-index optical-fibre modes is discussed. It is shown that the generally accepted definitions for HE and EH modes should be altered for certain classes of refractive-index profile.     

Experimental characterisation of graded-index single-mode fibres

A simple and accurate method is presented for experimental characterisation of graded-index single-mode fibres using their far-field radiation patterns and the `equivalent-step? method.     

Zero total dispersion in graded-index single-mode fibres

The combined effects of composite material dispersion, profile and waveguide dispersion can be made zero at a wavelength which depends on core diameter, refractive-index profile and composition. Wide bandwidths over large repeater spacings are therefore possible although close fabrication tolerances may be required.     

Hydroxyl concentration profile in low-loss graded-index fibres

Low OH-contamination in graded-index fibres was achieved by thin barrier layers of fluorine-doped silica and by chlorine drying in the preform collapse phase. A maximum of OH-concentration was found in the fibre core centre. Attenuation values of about 0.30 dB/km were measured between 1550 nm and 1600 nm.     

Wavelength-dependent differential modal delay in graded-index fibres

We present a technique for measurement of wavelength-dependent differential modal delay in a graded-index fibre. The data are used to derive the excitation-dependent zero-dispersion wavelength. The method offers an accurate calibration procedure to determine the zero-material-dispersion wavelength for given dopant concentration.     

Polarisation preservation in multimode fibres

Birefringent multimode fibres have been found to preserve polarisation except for one mode near cutoff. Experimental observations and theoretical considerations show that polarisation ratios better than 20 dB are possible, offering the potential for building polarisation-sensitive multimode fibre components.     

Bandwidth-limited operation of nonideal graded-index fibres

Using previously established analytical formulas, we consider the increase in bandwidth and loss due to mode coupling for graded-index fibres, with an index gradient that slightly differs from its optimal shape. Results show that the maximum possible length of a typical link can be increased by 100%. if well controlled mode coupling is induced.