| 摘要 |
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 lambda 1, lambda 2 separated by several nanometers. The light at lambda 1 has a group velocity different from the light at lambda 2 and consequently, reflected light components at lambda 1 and lambda 2 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 lambda 1 and lambda 2, 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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