GB2182222A

GB2182222A - Optical fibre testing

Info

Publication number: GB2182222A
Application number: GB08525704A
Authority: GB
Inventors: Kevin Christopher Byron
Original assignee: STC PLC
Current assignee: STC PLC
Priority date: 1985-10-18
Filing date: 1985-10-18
Publication date: 1987-05-07
Anticipated expiration: 2005-10-18
Also published as: GB2182222B; GB8525704D0

Abstract

An optical time domain reflectometry (OTDR) system which uses optical amplification in a fibre under test to provide a means of selectively increasing the backscatter at a particular point along the fibre length. A pump pulse ???p is launched in the fibre (9-Figure 1) and after a set delay a signal pulse ???s is launched into the fibre. (This assumes that the group velocity of the signal pulse is greater than that of the pump pulse. If it is less, the signal pulse is launched first). The difference in wavelength between the two pulses is one Stokes shift. In view of dispersion in the fibre the signal pulse will catch up with the pump pulse and be amplified. The backscatter trace will thus exhibit a corresponding peak (12) where the pulses overlap, thus the backscatter is amplified. By varying the delay between the pump and signal pulses, corresponding sections of the fibre may be examined. <IMAGE>

Description

SPECIFICATION Optical fibre testing This invention relatesto optical fibre testing and in particular to an OTDR (Optical Time Domain Reflectometry) system for obtaining a backscattertrace of an optical fibre.

According to one aspect of the present invention there is provided an optical time domain reflectometry (OTDR) system comprising means to provide an optical signal pulse and to launch it into one end of an optical fibre undertest in use ofthe system, means to cause amplification ofthe optical signal pulseata predetermined pointalongthe length of the fibre in use ofthe system, and means to detect the output from the one end ofthefibre.

According to another aspect of the present inven- tion there is provided an optical time domain reflectometry (OTDR) method of examining an optical fibre including the steps of launching an optical signal pulse into one end ofthefibre, causing amplification ofthe optical signal pulse at a predetermined point along the length of the fibre, and detecting the outputfromthe one end ofthefibre.

Embodiments ofthe invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows schematically an OTDR apparatus ofthe present invention, and Figure 2 shows a backscattertrace obtained with the apparatus of Figure 1.

In a conventional OTDR system pulses from a laser are launched into one end of an optical fibre under test and photons output from the one fibre end are detected. The detector output can be integrated and used to provide a Rayleigh backscattertrace. The backscattertrace is a plot of backscatter signal as a function of optical fibre length, or as a function of time delay which can be directly related to fibre length. The backscattertrace will show the expotential decay of backscatter and a sudden drop in signal level at the (other) end of the fibre end under test which is index matched, for example disposed in an index matching liquid. Similar drops in signal level are obtained from faults and splices in the fibre.

Thus generating a backscattertrace of a fibre enables thefibreto betested forfaults, splices etc. and their location along the length ofthefibre.

The OTDR. system ofthe present invention is based on the above conventional system and is such thatthe backscattercan be selectively increased ata predetermined point along the fibre length in order to facilitate examining of that point for defects etc.

The apparatus illustrated in Figure 1 comprises two laser sources 1 and 2 which are driven buy a pulse generator3, a diffraction grating 4, a lens system 5, a fibre coupler 6, a monochromatorornarrowbandfil- ter7 and a photon counting detection system oran ordinary detector 8. Afibre under test 9 is illustrated as connected to the fibre coupler 6. The pulse gener ator 3 produces first pulses to drive the first laser source 1 and second pulses, which are delayed relat ive to the first pulses, to drive the second laser source 2. The outputs of the laser sources 1 and 2 are focussed onto the diffraction grating 4 by respective lensl0andll.

The apparatus of Figure 1 is operated as follows. A pump pulse at a wavelength X, from laser source 1 is launched into the fibre 9 via the diffraction grating 4, the lens system 5 and the fibre coupler 6. After a set delay a signal pulse at a wavelength X, from laser source 2 is similarly launched intofibre 9. This assumes that the relative spacing ofthe pump and signal wavelengths from the zero dispersion wavelength provides the signal pulse with the greater group velocity in the fibre. If the converse is true then it is the signal pulse. The difference in wavelength between the pump and signal pulses is arranged to be one Stokes shift.For a silica fibre 9 operating at 1 .0im the wavelength difference is approximately 60nm. Owing to dispersion in the fibreS the signal pulse catches up with the pump pulse and therefore is amplified. The backscattertrace will exhibit a peak at the point 12 where the pulses overlap (see Figure 2) and the backscatter signal from point 12 onwards untilthetwo pulses no longeroverlapwill be ata higher level, that is the backscatter signal will be amplified.

By varying the delay between the pump and signal pulses at launch, that is controlling the pulse generator 3 accordingly, selected sections of the fibre 9 may be probed (examined),forexampletolookfor spikes associated with splices or defects. Varying the delay between the pump and signal pulses results in the pulses overlapping at corresponding different distances into the fibre. The signal output from fibre 9 is passed through the monochromator 7, which is set at the signal wavelength Xs, to a photon counting system 8 for maximum sensitivity, although alternatively a narrow band filter and ordinary detector may be employed.

Thus the OTDR system of the present invention is non-linear and employs optical amplification in the fibre undertestto provide a means of selectively increasing the backscatter at a particular point along the fibre length. Spatial resolution can thus be achieved, that is a particular point may be examined whilst the backscatter at neighbouring points is negligible. An advantage of this system is that because there is a localised increase in backscatter where the pulses overlap, the problem of detector saturation at the launch end where the backscatter is high in conventional systems, is overcome.

1. An optical time domain reflectometry (OTDR) system comprising means to provide an optical signal pulse and to launch it into one end of an optical fibre under test in use of the system, means to cause amplification of the optical signal pulse ata predetermined point along the length of the fibre in use of the system, and means to detect the output from the one end of the fibre.

2. An OTDR system as claimed in claim 1, and including means to display the detected output in the form of a backscatter trace, the backscatter signal associated with said predetermined point being amplified.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Optical fibre testing This invention relatesto optical fibre testing and in particular to an OTDR (Optical Time Domain Reflectometry) system for obtaining a backscattertrace of an optical fibre. According to one aspect of the present invention there is provided an optical time domain reflectometry (OTDR) system comprising means to provide an optical signal pulse and to launch it into one end of an optical fibre undertest in use ofthe system, means to cause amplification ofthe optical signal pulseata predetermined pointalongthe length of the fibre in use ofthe system, and means to detect the output from the one end ofthefibre. According to another aspect of the present inven- tion there is provided an optical time domain reflectometry (OTDR) method of examining an optical fibre including the steps of launching an optical signal pulse into one end ofthefibre, causing amplification ofthe optical signal pulse at a predetermined point along the length of the fibre, and detecting the outputfromthe one end ofthefibre. Embodiments ofthe invention will now be described with reference to the accompanying drawings, in which: Figure 1 shows schematically an OTDR apparatus ofthe present invention, and Figure 2 shows a backscattertrace obtained with the apparatus of Figure 1. In a conventional OTDR system pulses from a laser are launched into one end of an optical fibre under test and photons output from the one fibre end are detected. The detector output can be integrated and used to provide a Rayleigh backscattertrace. The backscattertrace is a plot of backscatter signal as a function of optical fibre length, or as a function of time delay which can be directly related to fibre length. The backscattertrace will show the expotential decay of backscatter and a sudden drop in signal level at the (other) end of the fibre end under test which is index matched, for example disposed in an index matching liquid. Similar drops in signal level are obtained from faults and splices in the fibre. Thus generating a backscattertrace of a fibre enables thefibreto betested forfaults, splices etc. and their location along the length ofthefibre. The OTDR. system ofthe present invention is based on the above conventional system and is such thatthe backscattercan be selectively increased ata predetermined point along the fibre length in order to facilitate examining of that point for defects etc. The apparatus illustrated in Figure 1 comprises two laser sources 1 and 2 which are driven buy a pulse generator3, a diffraction grating 4, a lens system 5, a fibre coupler 6, a monochromatorornarrowbandfil- ter7 and a photon counting detection system oran ordinary detector 8. Afibre under test 9 is illustrated as connected to the fibre coupler 6. The pulse gener ator 3 produces first pulses to drive the first laser source 1 and second pulses, which are delayed relat ive to the first pulses, to drive the second laser source 2. The outputs of the laser sources 1 and 2 are focussed onto the diffraction grating 4 by respective lensl0andll. The apparatus of Figure 1 is operated as follows. A pump pulse at a wavelength X, from laser source 1 is launched into the fibre 9 via the diffraction grating 4, the lens system 5 and the fibre coupler 6. After a set delay a signal pulse at a wavelength X, from laser source 2 is similarly launched intofibre 9. This assumes that the relative spacing ofthe pump and signal wavelengths from the zero dispersion wavelength provides the signal pulse with the greater group velocity in the fibre. If the converse is true then it is the signal pulse. The difference in wavelength between the pump and signal pulses is arranged to be one Stokes shift.For a silica fibre 9 operating at 1 .0im the wavelength difference is approximately 60nm. Owing to dispersion in the fibreS the signal pulse catches up with the pump pulse and therefore is amplified. The backscattertrace will exhibit a peak at the point 12 where the pulses overlap (see Figure 2) and the backscatter signal from point 12 onwards untilthetwo pulses no longeroverlapwill be ata higher level, that is the backscatter signal will be amplified. By varying the delay between the pump and signal pulses at launch, that is controlling the pulse generator 3 accordingly, selected sections of the fibre 9 may be probed (examined),forexampletolookfor spikes associated with splices or defects. Varying the delay between the pump and signal pulses results in the pulses overlapping at corresponding different distances into the fibre. The signal output from fibre 9 is passed through the monochromator 7, which is set at the signal wavelength Xs, to a photon counting system 8 for maximum sensitivity, although alternatively a narrow band filter and ordinary detector may be employed. Thus the OTDR system of the present invention is non-linear and employs optical amplification in the fibre undertestto provide a means of selectively increasing the backscatter at a particular point along the fibre length. Spatial resolution can thus be achieved, that is a particular point may be examined whilst the backscatter at neighbouring points is negligible. An advantage of this system is that because there is a localised increase in backscatter where the pulses overlap, the problem of detector saturation at the launch end where the backscatter is high in conventional systems, is overcome. CLAIMS

3. An OTDR system as claimed in claim 1 wherein the means to cause amplification of the optical signal pulse includes an optical pump pulse lasersource and the means providing the optical signal pulse comprises a respective laser source, there being difference in wavelength between the pump pulse and the signal pulse of one Stokes shift.

4. An OTDR system as claimed in claim 3, and including means for launching the pump pulse and the signal pulse consecutively into the fibre undertest with a predetermined time delaytherebetween, which time delay determines the point along the length ofthe fibre atwhich the signal pulse is amplified, the order of launching ofthe pump and signal pulses being determined bythe operating wavelength ofthefibre.

5. An OTDR system as claimed in any one ofthe preceding claims and wherein the output detecting means includes a monochromator set to the optical signal wavelength and a photon counting system.

6. An optical time domain reflectometrysystem substantially as herein described with reference to and as illustrated in Figure 1 ofthe accompanying drawings.

7. An optical time domain reflectometry (OTDR) method of examining an optical fibre including the steps of launching an optical signal pulse into one end of the fibre, causing amplification of the optical signal pulse at a predetermined point along the length of the fibre, and detecting the output from the one end ofthefibre.

8. An OTDR method as claimed in claim 7, which method includes the step of displaying the detected output in the form of a backscatter trace, the back scaftersignal associated with said predetermined point being amplified.

9. An OTDR method as claimed in claim 7 or8 wherein to cause amplification of the optical signal pulse an optical pump pulse is also launched into the fibre undertest, there being difference in wavelength between the pump pulse and the signal pulse of one Stokes shift.

10. An OTDR method as claimed in claim 9 wherein the pump pulse and the signal pulse are launched consecutively into thefibre undertestwith a predetermined time delay therebetween, which time delay determines the point along the length of the fibre at which the signal pulse is amplified, the order of launching of the pump and signal pulses being determined by their reiative group velocities in the fibre.

11. An OTDR method as claimed in any one of claims 7 to 10 wherein the detecting step includes passing the output ofthe fibre through a monochromator setto the signal wavelength and applying the monochromatoroutputto a photon counting system.

12. An optical time domain reflectometry method of examining an optical fibre substantially as herein described with reference to the accompanying drawings.