Optics: measuring and testing – For optical fiber or waveguide inspection
Patent
1998-10-27
1999-08-24
Kim, Robert H.
Optics: measuring and testing
For optical fiber or waveguide inspection
385 37, G01N 2141
Patent
active
059431245
DESCRIPTION:
BRIEF SUMMARY
This invention relates to optical line systems, and in particular, optical line systems that are monitored. Such systems include, for example, optical networks for telecommunications purposes which are monitored for damage or deterioration, and optical sensor lines and networks which are monitored to obtain information about some parameter such as temperature or strain.
In such systems, it has been proposed to incorporate reflectors so that the system can be monitored by a reflectometry method such as optical time domain reflectometry (OTDR), optical frequency domain reflectometry (OFDR) and the like. Examples of such systems are described in WO-A-90/06498 and GB-A-2,280,326. WO-A-90/06498 describes an OTDR method for detecting losses in a passive optical network (PON) that includes a number of partial reflectors such as gratings at different positions along its length. GB-A-2,280,326 describes a method of providing the integrity of optical paths in an optical system, comprising sending an amplitude modulated diagnostic signal into the system. A number of gratings in the system produce a pattern of time spaced echoes unique to the particular part of the system. However, one problem that exists with such systems is the fact that the signal to noise ratio is not the same for all the reflectors, especially if the line system contains a number of branches, and the system may not have sufficient dynamic response to obtain clear signals from the reflectors, especially from those reflectors remote from the reflectometer.
According to the present invention, there is provided an optical line system which comprises: different positions along the optical fibre; and optical fibre for monitoring the system by an optical reflectometry method in which light is reflected by the reflectors: that differ from one another, the reflector that is more distant from the monitoring arrangement having a higher reflectivity than the reflector that is nearer to the monitoring arrangement, so as to give a generally constant signal-to-noise ratio of the reflected light received by the monitoring arrangement.
Normally the optical fibre will contain more than two reflectors, for example three or more in relatively simple systems but in other systems it may contain significantly higher numbers e.g. at least ten reflectors or even twenty or more reflectors. In such systems the reflectivities of the reflectors preferably increase in order of the distance from the monitoring arrangement, and most preferably not only do the reflectivities increase but also the difference between the reflectivities of adjacent reflectors increases in order of distance from the monitoring arrangement.
The line system according to the invention has the advantage that, by increasing the reflectivities of the reflectors at the remote end of the optical fibre the signal-to-noise ratio of the signals received at the monitoring arrangement is significantly improved for those reflectors, and so the difference between the signal-to-noise ratio of the reflectors nearest the monitoring arrangement and the reflectors farthest from the monitoring arrangement is reduced. In view of the reduction in variation of intensity of the reflected signals, it is possible to set upper and/or lower threshold values for the reflected signals thereby eliminating or reducing crosstalk arising from secondary reflections occurring between reflectors.
The particular form in which the reflectivities of the reflectors are distributed along the optical fibre will depend, among other things, on the configuration of the line system, and especially on whether and to what extent it is branched. It is shown below that if one neglects Rayleigh backscattering from the optical fibre, a substantially constant signal-to-noise ratio is obtained from the reflectors in the optical line system if the reflectivities of the reflectors are based on the relationship: ##EQU1## Where: R.sub.n is the reflectivity of the n th reflector from the monitoring arrangement; arrangement, the (n-1)th and the n th reflector
REFERENCES:
patent: 5684297 (1997-11-01), Tardy
BICC Public Limited Company
Kim Robert H.
Nath Gary M.
Nguyen Tu T.
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