Radiant energy – Photocells; circuits and apparatus – Optical or pre-photocell system
Patent
1991-06-25
1993-02-16
Nelms, David C.
Radiant energy
Photocells; circuits and apparatus
Optical or pre-photocell system
356 731, H01J 516, G01N 2100
Patent
active
051873621
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to detection and measurement of losses in a branched optical fibre network.
BACKGROUND OF THE INVENTION
It is well known that faults in optical fibres can be located by an OTDR (optical time domain reflectometer). An OTDR launches a pulse of light into a fibre, and backscattered light is monitored for abrupt changes indicative of a fault, the time between pulse launch and the detection of the light at the launch end being indicative of the distance along the fibre that the fault occurs. In addition to back scattered light, there may be a Fresnel reflection from the far end of the fibre. Suggestions have been made to use reflection of an outbound pulse, (see Electronics Letters Vol. 20 No. 8, Gold, Hartog and Payne), regarding it as equivalent to launching a pulse at the far end of the fibre, and monitoring the backscattered light resulting from the reflection to give, in combination with the monitoring of the backscattering from the outbound pulse, in effect a simulated, double-ended OTDR. However, no suggestion has been made to monitor the reflection peak itself to provide information relating to losses.
In branched networks, it would be extremely costly to monitor each line separately from the most diversified end. It is, therefore, desirable to be able to monitor the branch lines from a centralised location, such as an exchange. If an OTDR is used, then the backscattered light from each branch line is combined on its return to the junction of the branches, and it is not possible to determine from which branch line it originated, although a distance from the pulse source is known. Also, in a branched network, the power of the outbound pulse is divided into the branch lines. Thus, the information relating to any branch line has only a strength related to the backscattering from the portion of the pulse in that branch line superimposed on the information from all the other branch lines, which will decrease the resolution thereby reducing the dynamic range of the instrument and sensitivity of attenuation measurement in a particular branch line. In general, an OTDR at present has a backscatter range limitation of approximately 20 dB for a 100 ns pulse width. Thus, if the branch lines are of substantial length and/or diversity, it may not be possible to monitor the entire network by normal OTDR methods. In any event, specific branch line(s) at fault would not be identified.
The aim of the present invention is to enable individual monitoring of branch lines in a branched network upstream from the branching point.
SUMMARY OF THE INVENTION
The present invention provides a method of detecting loss in a branched optical fibre network comprising a first optical fibre and a plurality of second optical fibres each of which is coupled to the first optical fibre, the first optical fibre constituting a main line and the second optical fibres constituting branch lines, a respective reflector being associated with each of the branch lines, the method comprising the steps of launching a pulse into the main line and monitoring the main line for attenuation of reflected signals returning from the reflectors, a monitored attenuation of a given reflected signal being indicative of loss in the branch line associated with the reflector giving rise to that reflected signal.
Advantageously, the reflected signals are monitored with respect to a reference signal. Preferably, the reflected signals are monitored with respect to a reference reflection signal from a location positioned in the main line.
Preferably, pulses of different frequency are launched into the main line, and reflection from reflectors having different wavelength responses is monitored.
The invention also provides apparatus for detecting losses in a branched optical fibre network comprising a first optical fibre and a plurality of second optical fibres each of which is coupled to the first optical fibre, the first optical fibre constituting a main line and the second optical fibres constituting branch lines, the
REFERENCES:
patent: 4713538 (1987-12-01), Theocharous
patent: 4816669 (1989-03-01), Anderson
AEG-Telefunken Kabelwerke AG-Development Center-"on site location of optical fiber defects and evaluation of transmission loss", Nov. 1977 B. Hillerick.
British Telecommunications public limited company
Nelms David C.
Pham Hoa Q.
LandOfFree
Loss detection in a branched optical fiber does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Loss detection in a branched optical fiber, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Loss detection in a branched optical fiber will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2149539