OTDR using distributed optical amplification in optical waveguid

Details OTDR using distributed optical amplification in optical waveguid OTDR using distributed optical amplification in optical waveguid

1992-02-24

1994-03-29

McGraw, Vincent P.

Optics: measuring and testing

For optical fiber or waveguide inspection

G01N 2188

Patent

active

052989658

DESCRIPTION:

BRIEF SUMMARY
This invention is related to copending U.S. application Ser. No. 07/541,973 filed Jun. 22, 1990.
This invention relates to optical Time Domain Reflectometry (OTDR), and especially to enhancing the measurement range of OTDR equipment..
In OTDR an optical pulse is launched into an optical fibre (or more generally into a waveguide; in this specification the expression `fibre` is used to include the more general case of a waveguide as well as optical fibres) and backscattered signal returning to the launch end is monitored. In the event that there are discontinuities such as faults in the fibre, the amount of backscattering increases and this is detected in the monitored return signal. Backscattering and reflection also occur from elements such as couplers and so the monitored signal is usually compared with a reference, new peaks and other changes in the monitored signal level being indicative of changes in the fibre path, normally indicating a fault. The time between pulse launch and receipt of the backscattered pulse is proportional to the distance along the fibre to the source of the backscattering, and so OTDR is a useful technique for fault location. However there are presently distance limitations on how far it is possible to measure along a fibre. One of the limitations is the dynamic range of the receiver which has to be sufficiently sensitive to detect the low level of backscattered light returning from the more distant part of the fibre, but also capable of receiving, without damage, much greater intensity reflection and backscattering from fibre locations close to the launch end. In practice OTDR receivers have a 60 dB optical dynamic range, which requires the electronics to have a twofold dynamic range of 120 dB resulting from the optical to electrical conversion. Optical test signals from the OTDR travelling along the fibre are attenuated in both the forward direction (before backscattering) and in the return direction after having been backscattered. This means that the optical one-way measurement range is limited to 30 dB by an electronic dynamic range of 120 dB.
It is desirable to be able to detect faults along fibre of a length exceeding that equivalent to 30 dB, there already being lengths equivalent to 40 dB in existence and further increases being probable. At present it is necessary to utilise OTDR measurements from both ends of a 40 dB span in order to test the full span, and this arrangement is not always convenient, and in any event requires more time and movement of, or additional, equipment.
Increasing the launch power of the pulses would be one way to extend the range of OTDR by increasing the received backscatter from the remote part of the fibre, but this also increases the backscattered and reflected intensity close to the launch end and imposes too great a demand on the optical and electronic, or processing dynamic range of the OTDR.
It is known in the art (GB 2182222A) to provide localised amplification to an OTDR signal pulse at a predetermined point along the fibre length, remote from the launch end, in order to increase the backscatter at this particular point. This method does not provide a solution to the problem of increasing the optical measurement range of an OTDR because the amplification provided to the signal is localised to a point and is insufficient. The amplification provided is fundamentally limited because the length of fibre in which it takes place is short and because only the forward travelling signal is amplified.
An object of the present invention is to enable increased optical measurement range, with particular usefulness over long spans, without requiring a substantially enhanced processing dynamic range.
Accordingly a first aspect of the invention provides an optical time domain reflectometer comprising a source of optical pulses, means for launching the optical pulses into a waveguide and means for optically pumping the waveguide to produce gain at the optical pulse wavelength, characterised in that the gain is distributed over a substantial length of

REFERENCES:
patent: 5013907 (1991-05-01), Bateman
Spirit et al, "Raman-Assisted Long, Distance Optical Time Doman Reflectometry", Elec. Ltrs. vol. 25, No. 25, Dec. 7, 1989, pp. 1687-1688.
Blank et al, "OTDR Performance Enhancement Through Erbium Fibre Amplification", Elec. Ltrs. vol. 25, No. 25, Dec. 7, 1989, pp. 1693-1694.
Blank et al, "Optical Time Domain Reflectometry on Optical Amplifier Systems", J. Lightwave Technology, vol. 7, No. 10, Oct. 1989, pp. 1549-1555.
ECOC'87, Technical Digest, vol. 1, 1987, Y. TGamura et al: "Fiber raman amplifier module with semiconductor laser pump source", pp. 62-65.

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