Measurement of intermodal delay in a dual-mode optical fibre

A novel measurement technique for intermodal dispersion between the LP01 and LP11 modes of a dual-mode optical fibre is described. A frequency-modulated optical source produces a beat frequency, proportional to intermodal delay, by interference of the two modes at the fibre output. Experimental results agree with theoretical predictions.     

Streak camera study of ultrashort pulse propagation in a weakly multimode optical fibre

A mode-locked dye laser in conjunction with a synchronously operating streak camera has been used to study ultrashort light pulse propagation in a weakly multimode optical fibre. Measurements of the intermodal temporal dispersion allowed the effective core radius a, the refractive index difference ? and the cutoff wavelengths of the LP21 and LP11 mode groups to be determined. The LP21 cutoff wavelength was experimentally verified and the temporal dispersion of the LP01 and LP11 mode groups investigated as a function of the fibre length.     

Power loss caused by random mode coupling in a multimode optical fibre

A method is presented for calculating the effects of random mode coupling in a multimode optical fibre. The calculated results are in good agreement with the experiment, and the relative error in the steady-state power loss is about 1%.     

Multilayer multimode optical fibre

Multilayer multimode fibres are considered by means of the ray theory, and the optimal distribution of refractive index with respect to pulse broadening is found. It is concluded that even with a low number of layers we can reduce pulse broadening significantly. Low sensitivity to the variations of refractive index distribution is predicted.     

Multimode fibers for compensating intermodal dispersion of graded-index multimode fibers

This paper proposes a novel type of multimode dispersion compensation fiber (MM-DCF) by which the intermodal dispersion of multimode fibers (MMFs) can be effectively compensated. A theoretical model that precisely calculates the group velocities of a target MMF and its MM-DCF and the power coupling between the two fibers is applied to confirm the proposal presented here. As a demonstration of the operation of the MM-DCF, an MMF optimized for transmission at a wavelength of 850 nm is compensated by the MM-DCF, and its overfilled launch bandwidth at a wavelength of 1300 nm is enhanced from 0.62 to 2.56GHz/spl middot/km.     

Mode behaviour in a tapered multimode fibre

A derivation based on ray tracing and the local plane-wave approximation is used to show the behaviour of waveguide modes in a linearly tapered multimode fibre having a step-index profile. The azimuthal mode order l is invariant through the taper, and, for suitably designed tapers, the changes in the radial mode order m and the lateral component u are negligible. A formula for designing such tapers is presented.     

Wavelength-switching fibre laser based on multimode fibre Bragg gratings

A multimode fibre Bragg grating (MMFBG) based wavelength-switching erbium-doped fibre laser is reported. With a simple singlemode fibre to MMFBG structure, a novel wavelength-selection technique is developed to achieve the wavelength-switching by changing the mode coupling condition in the multimode fibre. 31 discrete lasing wavelengths are obtained over a wide wavelength range.     

Radiation-hardened pure silica-core fibre optics

Transmission losses induced by ionising radiation in fibre opties having cores drawn from pure synthetic silica may be reduced by pre-exposure to large hardening doses. Characteristics of the hardened fibres are described and the necessary features of the hardening irradiations identified.     

Measurement of baseband frequency response of multimode fibre by using a new type of mode scrambler

For measuring the frequency response of an optical fibre, a mode scrambler utilising a sinusoidal bend and a new set-up based on the attenuation-comparison method have been proposed. Measurements for multimode step-index fibres have revealed that the long-fibre response can be closely predicted from the short-fibre response, using an appropriate choice for the scrambler parameter.     

Calculation of impulse response of multimode optical fibre

A simple and fast method of calculating the impulse response of a multimode optical fibre of arbitrary index profile is presented. In many cases, this method leads to simple analytical expressions which provide fuller understanding on the formation of impulse response.     

Measurement of variation of multimode fibre bandwidth withwavelength in 850 nm region using mode-locked Styryl-9 dye laser

A tuneable mode-locked dye laser has been used to measure the frequency response of a multimode fibre as a function of wavelength in the 850 nm region. The narrow spectral width of the dye laser gives an improved accuracy for large length-bandwidth product fibres. The results are compared with those obtained using both pulsed and CW semiconductor lasers     

Does multimode fibre have a future in data-communications?

The worst case Shannon capacities of OM1, OM2 and OM3 multimode fibre types are calculated. The worst case capacity length products are estimated to be 15 Gigabit per second kilometres (Gbit/s.km) for a single wavelength channel at 1300 nm wavelength and 60 Gbit/s.km for OM3 for operation at 850 nm wavelength     

All-fiber wavelength-tunable acoustooptic switches based on intermodal coupling in fibers

In this paper, we demonstrate a novel all-fiber wavelength-tunable acoustooptic switch utilizing intermodal coupling in a two-mode fiber (TMF). Its all-fiber configuration consisting of a fiber acoustooptic tunable filter and a mode-selective coupler results in the low loss (<2 dB) operation. The operating bandwidth >50 nm, the switching time of 40 /spl mu/sec, and the crosstalk of 20 dB were achieved. By controlling the design parameters of the two-mode fiber, the 3-dB bandwidth of the switched signal was varied from 2.5 nm to >35 nm. A novel all-fiber dynamic optical add-drop multiplexer is also demonstrated using two acoustooptic switches in series.     

Mode conversion due to tilts in multimode optical fibre

Coupled-mode theory is used to study the mode-conversion phenomenon due to a tilt in a multimode fibre. It is shown that there exists a critical angle beyond which power tends to be uniformly distributed among a large number of modes. This critical angle decreases with the increase in the number of modes in the fibre.     

Modal distribution of backscattered light in a step-index multimode fibre

The modal distribution of backscattered light has been measured for a low mode-mixing, step-index fibre. The modal distribution of the Rayleigh backscatter is shown to depend on that of the exciting pulse. The possibility of substantial errors arising in reflectometry measurements of splice and connector losses is demonstrated where the receiver is mode-selective.     

多模掺锗石英光纤中非线性的研究

本文报导了用一台调Q和锁模Nd:YAG激光器泵浦一根长500m,芯径为100μm的锗石英光纤,产生了四级stokes受激拉曼散射和一个由四光子参量混频产生的可见到近红外连续光谱,用色散理论阐明了连续谱的主要贡献者是受激拉曼散射的第一级stokes。讨论了由于非线性过程而引起功率耦合的非常数损耗。     

Power attenuation in bent multimode step-index slab and fibre waveguides

The power attenuation induced by a circular bend in the waveguide axis is calculated using ray tracing and a generailsed power-transmission coefficient for the case of diffuse illumination.     

Dispersion in a low-loss multimode fibre measured at three wavelengths

Short pulses from mode-locked lasers (neodynium/yttrium-iron-garnet and krypton) were used to measure mode and material dispersion in 20 m of a multimode fibre at 1060, 647 and 568 nm. Uniform illumination of the cone of acceptance at the input caused a 200 ps pulse to broaden to 400 ps.