Optical: systems and elements – Deflection using a moving element – Using a periodically moving element
Patent
1994-09-19
1996-02-13
Boudreau, Leo H.
Optical: systems and elements
Deflection using a moving element
Using a periodically moving element
359156, 356 731, H04B 1008
Patent
active
054915732
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to optical signal transmission networks, as may be used, for example, in optical fibre telecommunications, and relates, in particular, to a fault location and diagnostic systems for use with such networks.
Over the past decade, optical time domain reflectometry, hereinafter referred to as OTDR, has become an established tool for the location and evaluation of the features on an optical fibre route. For example, OTDR techniques are now commonly used to determine both the position and loss of splices and connectors and the position of line breaks as well as providing a means for making single-ended, real-time system loss measurements.
In use of an optical time domain reflectometer a short pulse of light is launched into an optical fibre under test and the backscattered signal is monitored as a function of the time of flight (or equivalently distance) along the fibre. The magnitude of the backscattered signal depends on the Rayleigh scattering, attenuation, inhomogeneities, splices, components and the optical power level in the fibre. Features in the trace of backscatter versus time may then be correlated with the presence of discontinuities and disruptions in the fibre route.
Whilst OTDR techniques are extremely valuable as a diagnostic tool for the location of faults in simple point to point inks there is now an increasing trend towards more complex passive optical networks.
In such networks a number of branch circuits may emanate from a single optical fibre connected, for example to a telephone exchange, and the branch circuits may be further divided into further branch circuits. Such an arrangement can provide a significant cost reduction by utilising the available bandwidth to share said single exchange fibre amongst a number of customers, thereby reducing the level of exchange equipment and fibre installation costs borne by each customer.
It will, however, be apparent that when a signal from OTDR equipment is transmitted along said single fibre, and thence to the branch circuits, the signal which is returned at any instant, following the transmission of an output signal from the OTDR equipment, will be made up of light backscattered from many points in the network at the same distance from the equipment. This makes the location of a fault in the network difficult and time-consuming.
SUMMARY OF THE INVENTION
An object of the invention is to provide a means of simplifying such fault location.
According, therefore, to the invention, in an optical signal transmission network comprising a main optical fibre having a plurality of branch circuits, and associated with OTDR equipment connected so as to transmit an output signal through said main fibre and said branch circuits, each said branch circuit incorporates, means for causing the signal returned to the OTDR equipment from said branch to be modified in a manner unique to that branch, and the OTDR equipment has means for distinguishing the signals returned from the individual branch circuits.
In one arrangement in accordance with the invention, the OTDR is capable of being tuned to provide signals in a selected wavelength band, and each branch circuit incorporates a filter capable of passing traffic wavelengths, and one or more respective pass wavelengths in the OTDR band, without significant loss, but of attenuating all other wavelengths in the band, the filters being so related that a signal returned to the OTDR equipment from any branch of the network has a wavelength unique to that branch. Accordingly any branch circuit having a fault can be unambiguously identified, and measurement of fault conditions can be carried out as in a point to point, rather than a point to multipoint system.
In an alternative arrangement each branch circuit incorporates means for modulating the output signal from the OTDR equipment at a unique frequency and the OTDR incorporates detector means capable of being tuned to respond to a selected one of individual frequencies of the various branch circuits.
Instead of straightf
REFERENCES:
patent: 5177354 (1993-01-01), Tomita et al.
patent: 5187362 (1993-02-01), Keeble
patent: 5383015 (1995-01-01), Grimes
Boudreau Leo H.
GPT Limited
Negash Kinfe-Michael
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