GB2252154A

GB2252154A - Testing erbium doped optical fibres.

Info

Publication number: GB2252154A
Application number: GB9101745A
Authority: GB
Inventors: Kevin Christopher Byron
Original assignee: STC PLC; Northern Telecom Europe Ltd
Current assignee: STC PLC; Nortel Networks Optical Components Ltd
Priority date: 1991-01-26
Filing date: 1991-01-26
Publication date: 1992-07-29
Also published as: GB9101745D0

Abstract

In optical fibre systems incorporating a number of fibre amplifiers the latter should, ideally, have the same characteristics, e.g. the same erbium concentration along their lengths. To test the uniformity of erbium concentration of a long length of erbium doped fibre before it is cut up into individual fibre amplifier lengths an OTDR technique is employed. In order that such a long length of erbium doped fibre can be tested the OTDR source wavelength is chosen to be off resonance of an erbium absorption band, in particular on the short wavelength side of the absorption peak.

Description

OPTICAL FIBRE AMPLIFIERS This invention relates to optical fibre amplifiers, in particular erbium fibre amplifiers and especially to techniques and systems for measuring the uniformity of erbium concentration in erbium doped fibre.

According to one aspect of the present invention there is provided a method of testing erbium doped fibre for uniformity of erbium doping along its length comprising the steps of employing OTDR techniques to produce a backscatter trace, the OTDR source wavelength being off resonance of an erbium absorption band, non-uniformity of the erbium doping being indicated by a deviation, of the backscatter trace from an exponential path, caused other than by breaks or other physical defects in the fibre.

According to another aspect of the present invention there is provided an OTDR system, for testing erbium doped fibre for uniformity of erbium doping along its length, comprising means to provide an optical signal pulse at a wavelength off resonance of the erbium absorption band and to launch it into one end of an erbium doped fibre under test and means to detect the output from the one end of the fibre.

Embo ments o- the invention will now be described with reference to the accompanying drawings in which Fig. 1 illustrates the fluorescence and absorption spectra of erbium, and Fig. 2 illustrates a backscatter trace for erbium doped fibre.

Erbium doped (typically 50ppm) fibre can be produced in substantial lengths by conventional processing and will normally be stored on a reel before being cut up into the relatively short lengths, say 3 to 10 metres, required for erbium fibre amplifiers. In the case of optical fibre submarine systems, for example, of the order of several hundreds or thousands of kilometres long, a number of erbium fibre amplifiers will be required at spaced apart positions along the length of the system. Ideally each erbium fibre amplifier should have the same characteristics, that is the same erbium concentration along its length.

It is thus proposed that whilst the erbium fibre is still on its reel, that is before a long length is cut up into individual amplifier lengths, the uniformity of the erbium concentration down the fibre length is checked. It is conventional practice to test optical fibres using OTDR (Optical Time Domain Reflectometry) systems. In conventional OTDR systems 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 backscatter trace. The backscatter trace 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 backscatter trace will show the exponential decay of backscatter and a sudden drop in signal level at the (other) end of the fibre, which'other end 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 backscatter trace of a fibre enables the fibre to be tested for faults, splices etc. and their location along the length of the fibre to be determined.

Variations in erbium concentration along the length of an erbium doped fibre will produce corresponding changes in the backscatter trace and thus OTDR techniques can be used to check the uniformity of the erbium doping along the length of fibre.

Conventionally OTDR techniques employed to test transmission optical fibre operate at the wavelength of subsequent operation of the fibre, that is 1.3 or 1.5 microns, where the loss is lowest, so that it can be seen what effect there will be on a transmitted optical signal. However, if 1.5 micron OTDR test wavelengths are used with erbium doped fibre the input signal will be totally absorbed after passing only some ten metres or so down the fibre and so no backscatter information from the whole length of the fibre could be obtained.

Fig. 1 of the drawings illustrates the overlapping fluorescence (gain) 1 and absorption 2 spectra of erbium doped fibre (erbium fibre amplifiers).

It is proposed that OTDR is performed with a laser wavelength at the low wavelength end 3 of the absorption spectrum where there will only be a very small amount of absorption due to erbium, there will of course be a small amount due to the silica of the fibre. Typically the laser wavelength will be of the order of 145 jim and this can be achieved using a diode laser. The wavelength chosen is on the short wavelength side of the absorption peak in order to be away from the fluorescence peak and avoid noise arising from the fluorescence peak.

Fig. 2 of the drawings illustrates a typical OTDR trace obtained using such a laser pulse wavelength with erbium doped fibre. The deviation at 4 from the expected exponential trace suggests that if there is not a break or a bubble in the fibre at that position then there is a change in erbium concentration at that position. In this case only the length 5 of the fibre should be used to make fibre amplifier if they are required to be consistent, i.e. have the same characteristics, for a single system.

Thus while a basic established OTDR technique is employed to check the uniformity of doping of erbium fibre the OTDR laser source wavelength, i.e. the wavelength of operation of the OTDR system, is substantially different to the wavelength normally employed, i.e. that of minimum loss, and is in fact coincident with some part of the absorption band of erbium which will allow OTDR to be performed on erbium fibre. In particular the source wavelength should be of resonance of an erbium absorption band.

Claims

CLAIMS:

1. A method of testing erbium doped fibre for uniformity of erbium doping along its length comprising the steps of employing OTDR techniques to produce a backscatter trace, the OTDR source wavelength being off resonance of an erbium absorption band, non-uniformity of the erbium doping being indicated by a deviation, of the backscatter trace from an exponential path, caused other than by breaks or other physical defects in the fibre.

2. A method as claimed in claim 1, wherein the source wavelength is on the low wavelength side of the absorption peak.

3. A method as claimed in claim 1 or claim 2 wherein the source wavelength is of the order of 1.45 jim

4. An OTDR system, for testing erbium doped fibre for uniformity of erbium doping along its length, comprising means to provide an optical signal pulse at a wavelength off resonance of the erbium absorption band and to launch it into one end of an erbium doped fibre under test and means to detect the output from the one end of the fibre.

5. An OTDR system as claimed in claim 4 wherein said wavelength is on the short wavelength side of the absorption peak.

6. An OTDR system as claimed in claim 5 or claim 6 and including means to display the detected output in the form of a backscatter trace.

7. A method of testing erbium doped fibre for uniformity of erbium doping along its length substantially as herein described with reference to the accompanying drawings.

8. An OTDR system for testing erbium doped fibre for uniformity of erbium doping along its length substantially as herein described with reference to the accompanying drawings.

Amendments to the claims have been filed as follows

9. A.method of testing optical fibre, suitable for use as an optical amplifier, for uniformity of doping along its length, including the step of employing OTDR techniques to produce a backscatter trace, the OTDR source wavelength being off resonance of the dopant absorption band, non-uniformity of the doping being indicated by a deviation of the backscatter trace from an experimental path caused other than by breaks or other physical defects in the fibre.

10. An OTDR system for testing optical fibre, suitable for use as an optical amplifier, for uniformity of doping along its length, comprising means to provide an optical signal pulse at a wavelength of resonance of the dopant absorption band and to launch it into one end of the optical fibre under test and means to detect the output from the one end of the fibre.