Far-field profiling of multimode optical fibres

Using a recently published relation between far-field intensity and refractive index profile a monotonically decreasing equivalent profile is calculated from measured far-field data. The resulting smooth function may be fitted to composite or multiple ?-profiles very well.     

Bending losses of multimode optical fibres

The bending loss of rays within circular and slab multimode optical waveguides is determined using only Snell's laws, the generalised Fresnel transmission coefficient and the Goos-Haenchen shift. The generalised transmission coefficient accounts for radiation losses, due to curvature, not included in Fresnel's classical laws.     

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.     

New primary coating method for optical fibres

A new primary coating method for optical fibres has been developed. A locally heated and softened thermoplastic tube, in which the fibre is passing through, is drawn to form a coating layer on the fibre. This method has large advantages in forming a precisely concentric coating layer and fabricating a strong fibre.     

Ray analysis of multimode,optical fibres

A ray analysis of a multimode, step or graded index fibre can be conveniently divided into two complementary sections. Those phenomena associated with the fibre core, such as pulse propagation, source excitation, power absorption, scattering and inhomogeneities are viewed as building blocks on a basis of ray trajectories and ray classes as predicted by Snell’s laws. Cladding phenomena, such as absorption and ray tunnelling are superposable building blocks based on Fresnel’s laws and their generalizations.     

Cascaded multipole switches for single-mode and multimode optical fibres

Multistage moving-fibre array switches which use preferentially etched silicon chips for the precise alignment of the optical fibres have been developed. In particular, a 1×4 single mode, and a 1×8 multimode fibre switch are described, with which average insertion losses of 0.8 and 0.2 dB were achieved, respectively.     

Nylon buffering effects in multimode optical fibres

In this letter the effects of nylon buffering in multimode fibres are examined using a method previously reported, and the design criteria for avoiding the additional loss due to the nylon buffering are shown.     

Polarisation-independent optical circulator coupled with multimode fibres

An experiment on a polarisation-independent optical circulator coupled with graded-index fibres was performed. Insertion losses of 2.5?2.9 dB and isolation ratios of 15?34 dB among nonreciprocal ports were measured, at 1.27 ?m wavelength.     

Current sensing with metal-coated multimode optical fibres

A multimode fibre has been used to detect electrical currents. The optical fibre which serves as the sensor is coated with a thick Al metal jacket. Electromotive forces on a wire in a magnetic field are used to induce differential phase shifts between the fibre modes. Good linearity has been observed in the 5 to 2000 mA current range.     

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.     

Optimum operating wavelength for chromatic equalisation in multimode optical fibres

Complete chromatic equalisation is not possible in a graded-index fibre owing to the variation of material properties across the core. We show that it is still possible to choose an operating wavelength which effectively eliminates first-order material dispersion.     

Efficient power coupling using taper-ended multimode optical fibres

Highly efficient power coupling of up to 97% between a d.h. laser and a multimode step-index fibre is demonstrated by employing a tapered fibre launcher. Also, it is shown theoretically that a tapered end increases the effective numerical aperture of a fibre. For a Lambertian emitter, the use of a taper launcher permits the maximum coupling efficiency allowed by the Liouville theorem.     

Length dependence of joint losses in multimode optical fibres

The dependence of actual joint losses on fibre length is studied in some common cases. Interesting relations with scattering loss are found and a strong dependence on launching conditions is evidenced.     

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.     

Intermode phase scanning for recovering phase-modulated signals on multimode optical fibres

Phase-modulated data transmission in multimode optical fibres is complicated by signal fades due to variations in intermodal phase. The letter describes a very simple technique to recover the data without the use of complex diversity receivers. The method involves varying the relative modal phases at a rate sufficient to ensure at least one signal maximum during each bit period. Thus the input signal is sampled and may be recovered by lowpass filtering. Experimental results demonstrate that the error rate for a single-channel digital receiver can be reduced by several orders of magnitude using this technique.     

Fabrication of low-loss 3 dB couplers with multimode optical fibres

A 3 dB coupler was fabricated using two partially polished fibres which were in contact with each polished portion. The excess insertion loss and the directivity of this coupler are measured to be 0.36 dB and 47.0 dB, respectively.     

Effects of mode conversion on backscattering measurements of multimode optical fibres

The effects of mode conversion on measurements by the backscattering technique is investigated. Analytical relations are obtained in practical cases. Although differences on the power decay deduced between the backscattering and a direct technique are evidenced, some procedures to obtain a reliable loss coefficient are suggested.     

Broadband Raman gain characterisation in various optical fibres

The Raman gain coefficient of several different types of optical fibres has been quantified using a sensitive technique based upon the measurement of backward amplified spontaneous emission. Spectral measurement of the effect of the variation of germanium doping in the fibre is also reported     

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.