Method and apparatus for optical fiber fault location
First Claim
1. A method of measuring the distance from an input end of a fiber to a discontinuity within the fiber, the method comprising launching a swept frequency optical input signal into the input end of the fiber, the signal having components at wavelengths λ
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1 and λ
2 separated by several nanometers, the λ
1 component having a group velocity different from the group velocity of the λ
2 component, receiving at the fiber input end a corresponding interference signal propagating back from the discontinuity, analyzing the frequency spectrum of the interference signal, and, from a frequency related parameter of said frequency spectrum, deriving the distance from the input end to the discontinuity.
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Abstract
A discontinuity within an optical fiber is located using a reflectometry technique. A swept frequency sinusoidal signal is launched into the fiber, is reflected from the remote discontinuity, and is returned to the fiber input end. The signal consists of the combined outputs of two optical sources identically frequency modulated and having output wavelengths λ1, λ2 separated by several nanometers. The light at λ1 has a group velocity different from the light at λ2 and consequently, reflected light components at λ1 and λ2 interfere with each other at the fiber input end. The interference signal is frequency dependent and from the frequency dependence the distance from the fiber input end to the discontinuity can be calculated. The received signal is dependent also on the difference in group index at λ1 and λ2, this difference being relatively insensitive to temperature, stress and fiber composition. In contrast, the group index for a particular wavelength is sensitive to these parameters and so affects the results of known single wavelength and pulse techniques. In this respect the method proposed is more accurate than known optical time-domain and frequency-domain reflectometry techniques.
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Citations
7 Claims
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1. A method of measuring the distance from an input end of a fiber to a discontinuity within the fiber, the method comprising launching a swept frequency optical input signal into the input end of the fiber, the signal having components at wavelengths λ
-
1 and λ
2 separated by several nanometers, the λ
1 component having a group velocity different from the group velocity of the λ
2 component, receiving at the fiber input end a corresponding interference signal propagating back from the discontinuity, analyzing the frequency spectrum of the interference signal, and, from a frequency related parameter of said frequency spectrum, deriving the distance from the input end to the discontinuity. - View Dependent Claims (2)
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1 and λ
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3. Apparatus for monitoring the position of a discontiuity within an optical fiber, the apparatus comprising means for launching a swept frequency optical signal into one end of the fiber, the optical signal having identically modulated components at wavelengths λ
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1 and λ
2 separated by a few nanometers, said components having different group velocities within the fiber, means for receiving from said one end of the fiber an interference signal propagating back from the discontinuity, said interference signal having amplitude variation caused by interference between said two components, and spectrum analyzing means for analyzing the frequency spectrum of the interference signal whereby, from a frequency related parameter thereof to derive the distance from the fiber input end to the discontinuity. - View Dependent Claims (4, 5, 6, 7)
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1 and λ
Specification