Optical waveguides – With optical coupler – Input/output coupler
Patent
1997-11-06
2000-05-16
Healy, Brian
Optical waveguides
With optical coupler
Input/output coupler
383 27, 383 28, 383 31, 359109, 359116, G02B 626, G02F 100
Patent
active
060647868
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The present invention relates to methods and apparatus for improving the performance of multimode optical fibre communications systems, and in particular to methods and apparatus for launching optical radiation into multimode fibre.
In the late 1970s, and early 1980s, much work was carried out to improve performance of multimode optical fibre communications systems. However, when multimode fibre was replaced by a singlemode fibre, as the medium of choice for use in high bit rate, long distance communications systems, much of this work ceased. Multimode fibre has continued to be used in optical communications for systems operating at lower bit rates, and over shorter distances, for example in building or campus LANs. There is thus a large installed base of multimode fibre, which represents a significant investment.
In recent years the demand for high data rate LANs has increased dramatically, for example to 1 GBit/s and beyond. Thus, even though multimode optical fibre is only utilised over short distances, for example 500 meters, the required data rates cannot be achieved utilising conventional techniques.
A key aspect in determining the bandwidth of a multimode optical fibre communications link, which has been recognised for many years, is the number and distribution of modes within the multimode fibre which are excited, and therefore carry optical energy. See for example Chapter 7 of "Optical Fibres for Transmission" by John E. Midwinter, published by John Wiley & Sons in 1979. If a pure low order singlemode is launched into a multimode fibre, and there is no mode mixing, the bandwidth and other characteristics of the optical communication link will be that of a single-mode fibre, i.e. the link will have high bandwidth. If mode mixing occurs, for example due to fibre profile irregularities, or mechanical perturbations of the fibre, energy will be coupled from the single lowest order mode into higher order modes having lower group velocities, and additional pulse dispersion will inevitably result, leading to a lower overall bandwidth for the communications system. Alternatively, if light is launched into the same multimode fibre in a manner so as to uniformly excite all modes of the multimode fibre (a so called "overfilled launch"), and if no mode mixing occurs, a maximum pulse spread will be seen, and the bandwidth of the communications system will be at a minimum. If mode mixing is introduced to this situation, because individual photons will then spend some time in many different modes, and will have travelled many short distances at different group velocities, less pulse spreading will be experienced. In the ideal case rather than experiencing an increase of pulse spreading which is proportional to the length of the optical communications link, pulse spreading builds up only in proportion of the square root of the length of the optical communications link. Thus, in the early 1980s, although various alternative schemes were investigated (see eg U.S. Pat. Nos. 4,050,782 and 4,067,642), it was generally accepted that it was desirable to launch many modes into a multimode optical fibre, and to ensure that adequate mode mixing occurred in order to achieve a reasonable, and predictable, bandwidth for an optical communications link.
Despite this practical approach, it was however theoretically predicted that if the number and distribution of modes excited within a multimode fibre could be precisely controlled, the bandwidth of the communications link could be improved. For example, see Section 7.6, page 126 of Midwinter's book where it is suggested that controlled mode coupling can be utilised to prevent coupling to the highest order modes of the fibre so as to increase the fibre bandwidth without incurring a loss penalty. Nevertheless, it is stated here that "It must be said, however that experimentally it looks extremely difficult to achieve such a precisely controlled fibre environment, and at the time of writing no reports of experimental testing are known."
In recent years l
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Cunningham David George
Nowell Mark Charles
Raddatz Lutz
White Ian
Healy Brian
Hewlett--Packard Company
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