Pulse broadening in multimode graded-index fibres

A closed-form expression is obtained for pulse broadening in graded-index fibres with k2(r)=1?r2+?2r4+?+?nr2n. Pulse broadening for k2(r)=1?r2+0.615r4+70r6 and r<0.1 is 12 times smaller than for square-law fibres if material dispersion is neglected.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Uncertainties of the leaky mode correction for near-square-law optical fibres

It is shown that the leaky-mode correction as used, for example, for the near-field scanning technique, becomes uncertain for near-square-law fibres if they deviate from perfect circular symmetry. Results for the transition of the leaky-mode behaviour are presented.     

Prediction of mode dispersion in graded-index single-mode fibres

A Cheby?shev power series method is presented for the calculation of the propagation constant of graded-index fibres. Accurate predictions of the dispersion parameters can be obtained as shown by examples of step and parabolic profiles. Measured index profiles can also be used with local index values found by linear interpolation from nearest reading points. Examples are given to demonstrate the effectiveness of this approach.     

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.     

Compensation of profile dispersion in graded-index optical fibres

As the profile dispersion parameter P of graded-index fibres is wavelength-dependent. Marcatili's condition for achieving high bandwidth is usually satisfied only in a narrow wavelength range, if at all. This behaviour can be compensated by a wavelength-dependent refractive-index profile. This letter presents a profile function that compensates for a broad class of profile dispersions and may become a design criterion for fibres in the future, although its practical applicability is presently restricted by the limited variety of available glasses.     

Group delay in graded-index fibres with non-power-law refractive profiles

The group delay of modes in graded-index fibres is often calculated by the method of Gloge and Marcatili. This method can, however, yield misleading results if the refractive profile of a fibre does not strictly follow a power law of the radius, e.g. if it is close to the optimum profile, but has statistical distortions. The letter presents a new method for the calculation of group delay which has a simplicity similar to that of the Gloge?Marcatili method and can be used in conjunction therewith as an easy way of finding reliable results for non-power-law refractive profiles.     

Mode attenuation at 1.39 ?m in graded-index fibres

The differential mode attenuation ?m(?) in the spectral region of OH absorption (1.2?1.6 ?m) has been studied in SiO2/GeO2 core graded-index fibres. It was found that the mode attenuation due to OH absorption Km(?) at 1.39 ?m had its maximum in the fibre core centre. The comparison between the calculated and measured Km(?) at this wavelength could be used for the determination of OH-ion distribution profile in optical fibres.     

Pulse response of a graded-index optical fibre

An analysis has been made of pulse dispersion in an optical fibre having a continuous radial variation of refractive index. Solutions are presented for Selfoc fibre showing the effects of mode and material dispersion and of group delay. The predicted dispersions range from very low values up to about 1 ns/km, depending on the launching conditions.     

Wavelength dependence of modal dispersion in graded-index optical fibres

We describe a method based on the selective mode excitation at different wavelengths for studying the profile dispersion influence on the modal dispersion. By using this technique, we are able to determine either the optimum wavelength at which the fibre should be operated or the correction needed for optimising the index profile at a given wavelength.     

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.