Localization of faults in an optical fiber

Details Localization of faults in an optical fiber Localization of faults in an optical fiber

1998-12-16

2002-05-14

Font, Frank G. (Department: 2877)

Optics: measuring and testing

For optical fiber or waveguide inspection

Reexamination Certificate

active

06388741

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to the localization of faults in fiber optic systems.
Optical time domain reflectometers (OTDRs) are state of the art instruments to localize failures (e.g. breaks, bends, mechanical stress etc.) in fiber optic systems or networks comprising one or more fiber links e.g. for connecting one or more optical components. The reflectometric principle has evolved to a widely accepted technique for capturing backscatter signals because of its suitability for one-port measurements with distance ranges up to 200 km and even more.
An OTDR injects a pulsed signal into a fiber optic network comprising one or more optical components, e.g. fibers, filters, couplers, or the like. The pulsed signal typically is a laser pulse at a certain laser wavelength. A small amount of the pulsed signal is continuously scattered back in the opposite direction towards the OTDR, representing attenuation, loss, and reflectance in the optical network under test. By measuring the amount of backscattered and/or reflected signal versus time, the loss versus distance of the optical network is measured. The results of one or more individual measurements are usually combined and that combination represents an OTDR trace of the optical network. In a typical OTDR trace received from an OTDR measurement, the x-axis corresponds to the distance between the OTDR and a location in the optical network. The y-axis shows the power level of the reflected signal level revealing details about the optical link loss in the optical network. A detailed description of the current knowledge about OTDRs and analyses of OTDR traces is given in detail by the inventor in the book by Dennis Derickson: “Fiber optic test and measurement”, ISBN 0-13-534330-5, 1998 on pages 434 ff.
The OTDR allows the localization of faults in fiber optic systems by evaluating OTDR traces interpreting faults as specific discontinuities. This evaluation, however, finds a ‘natural’ limitation in particular for spatially extended fiber optic systems (e.g. long fibers) where the backscattered signal eventually becomes lower than the OTDR receiver's noise level.
A further limitation of the applicability of OTDR measurements for localization faults in fiber optic systems occurs when fiber optic links equipped with erbium doped amplifiers (EDFAs) need to be tested. In this environment, the standard OTDR faces some obstacles. Firstly, an EDFA produces considerable optical ASE noise (amplified spontaneous emission) which dazzles a sensitive instrument like an OTDR if no precautions were taken. This problem can be tackled e.g. with a narrow-band optical filter in the receiver path. Secondly, EDFAs commonly contain optical isolators that prevent scattered and reflected light from returning to the input port. Thirdly, the very high signal levels in an EDFA transmission link can show nonlinear interference with an OTDR's probe signal.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an improved possibility to localize faults in fiber optic systems. The object is solved by the independent claims. Preferred embodiments are shown by the dependent claims.
According to the invention, a spatially elongated optical element, such as an optical fiber, is provided with a plurality of reflecting events that are spatially allocated along the optical element.
Possible faults in the optical element are localized by emitting a signal into the optical element, measuring signals reflected from the reflecting events, and comparing the measured reflected signals with expected signals representing the optical element without faults. In case that there are no faults in the optical element, the measured reflected signals will substantially correspond to the expected signals. In case that there are one or more faults in the optical element, the measured reflected signals in a distance behind each one of the one or more faults will be attenuated to a certain degree, or, in other words, show at least a different (e.g. decreased) amplitude, or even disappear, with respect to the expected signals.
The invention can be executed by a system comprising means for emitting an optical signal into the optical element and means for measuring the reflected signals. This can be embodied e.g., by an OTDR as known in the art. The system further comprises means for comparing the measured reflected signals with expected signals representing the optical element without faults. This can be embodied and executed e.g. by a data processing unit which might be part of an OTDR or provides a separate device.
The expected signals can be received or determined, e.g. from a previous measurement, such as an acceptance measurement, or can be calculated or otherwise received for example from theoretical analysis (e.g. simulation or modeling) and/or from information about the fiber (such as physical properties).
This allows that a measurement of signals reflected at one or more of the plurality of the reflecting events provides information about possible faults in the optical element.
The reflecting events can be represented by any location where a change of the refractive index occurs and are preferably gratings such as fiber Bragg gratings.
The reflecting events can be embodied as individual devices, e.g. between successive parts of the optical element such as individual fiber parts, but are preferably incorporated within the optical element, e.g. as fiber Bragg gratings.
In a preferred embodiment, the reflectance value of each reflecting event is determined to depend on the distance to a measuring point at which the reflected signals are measured. Preferably, the reflectance values increase with increasing distance to the measuring point to compensate the attenuation of the element. This allows receiving reflected signals with substantially constant amplitudes independent of the distance of the reflecting event. This also allows reducing an influence of possible broadband noise sources, such as EDFAs, on the measurements.
The reflecting events are preferably selected to provide a defined reflectance characteristics versus wavelength. In a preferred embodiment, reflecting events, such as Bragg gratings, are used which provide a specific center wavelength at which a signal is reflected or partially reflected, while substantially all other wavelengths are not reflected.
In a preferred embodiment, the reflecting events are selected to provide different reflectance characteristics versus wavelength for different wavelengths. This allows employing measurements with different measuring wavelengths to localize individual reflecting events corresponding to the respective wavelength(s). This way the reflectance values of the reflecting events at a specific wavelength can be chosen independently and need not depend on the reflectivity and the associated insertion loss of the reflecting events at the other wavelengths. In that case, measurements for determining the time dependencies of the reflected signals are not required in order to clearly assign a specific reflected signal to a specific reflecting event.
The fault localizing according to the invention can be applied to single optical elements, e.g. for monitoring a specific fiber, and/or to an optical network comprising one or more individual elongated optical elements (e.g. fiber segments) that might be coupled between (optically non-elongated) optical components, such as optical filters, switches, or the like. In case that reflected signals from different reflecting events within the optical system to be measured/monitored exhibit the same characteristics, additional information has to be gathered in order to distinguish the different reflecting events. Such information can be time information e.g. the time between emitting a measuring signal and receiving the reflected signal as automatically determined by an OTDR.
Instead of one reflective event at a given location a pair or more than two reflective events can be grouped together to generate a specific pattern of reflections. Such a pattern

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