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Method and apparatus for performing optical time domain reflectometry
| Details |
Inventors: Beller, Josef;
Assignee: Hewlett-Packard Company (Palo Alto, CA)
Primary Examiner: Nelms; David C.
Assistant Examiner: Allen; Stephone B.
Attorney, Agent or Firm: Woodcock Washburn Kurtz Mackiewicz & Norris
In a method and a corresponding apparatus for performing optical time domain reflectometry predetermined time sequences of light signals (A', B') are injected into an optical fiber and the signals backscattered from the fiber are correlated with the injected sequences. The injected time sequences are selected according to a specific code such that after each element in the code represented by "1" or "-1", a predetermined number of elements represented by "0" is provided. Depending on the number of inserted "0"s after each "1" or "-1", sidelobes in the correlation product appear at certain predictable positions. By choosing the numbers of inserted "0"s differently for different injections, the resulting sidelobes appear at different positions in the corresponding correlation products. From the correlation products of such different injections, a composite fiber response can be derived wherein sidelobes are substantially eliminated. The codes used can be derived, for example, from Golay complementary codes. |
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DETAILED DESCRIPTION It is an object of this invention to provide a method and corresponding apparatus for performing optical time domain reflectometry on a light transmitting medium which avoids the disadvantages caused by the above-mentioned non-linearities.
According to the invention, at least two different time sequences of light signals are injected into the light transmitting medium and afterwards correlated with the reflected signals from the medium.
The different injected time sequences can be described as being derived from a common basic sequence by insertion of a predetermined number of light signals of zero power level after each light signal of the basic sequence.
In one embodiment of the invention, if the basic sequence is represented by a sequence of "+1" and "-1", the injected time sequences can be described as being derived from the basic sequence by insertion of a predetermined number of "0"s after each "+1" or "-1".
The different injected time sequences differ from each other by the number of light signals inserted after each light signal of the basic sequence, i.
e.
, by the number of "0"s inserted after each "+1" or "-1".
Thus, a first injected sequence according to the invention is obtained by inserting a first number m of "0"s after each "+1" and "-1" of the basic sequence, and a second injected sequence is obtained by inserting a second number n of "0"s after each "+1" and "-1" of the basic sequence.
The insertion of a predetermined number of light signals of zero power level has the important effect that sidelobes in the correlation result of the backscattered signals with the injected sequences appear at well-defined, predictable positions.
By selecting different numbers of inserted light signals for different sequences of injected light signals, the sidelobe positions in the backscatter spectrum can be varied.
Thus, if a data value obtained during a first injection is obscured by a sidelobe, the true sample value, unaffected by sidelobes, can be obtained by injecting a second sequence with a different number of inserted light signals such that the resulting sidelobe is not at the position of the sample value
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Communications 
