CA1137787A - Method and apparatus for optical fiber fault location - Google Patents
Method and apparatus for optical fiber fault locationInfo
- Publication number
- CA1137787A CA1137787A CA000374977A CA374977A CA1137787A CA 1137787 A CA1137787 A CA 1137787A CA 000374977 A CA000374977 A CA 000374977A CA 374977 A CA374977 A CA 374977A CA 1137787 A CA1137787 A CA 1137787A
- Authority
- CA
- Canada
- Prior art keywords
- fiber
- frequency
- discontinuity
- signal
- interference signal
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
- H04B10/07—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems
- H04B10/071—Arrangements for monitoring or testing transmission systems; Arrangements for fault measurement of transmission systems using a reflected signal, e.g. using optical time domain reflectometers [OTDR]
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- Physics & Mathematics (AREA)
- Electromagnetism (AREA)
- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Testing Of Optical Devices Or Fibers (AREA)
Abstract
METHOD AND APPARATUS FOR OPTICAL FIBER FAULT LOCATION
Abstract of the Disclosure A discontinuity within a fiber is located using a reflectometry technique. A swept frequency sinusoidal signal is launched into the fiber from one end and propagates along the fiber. If the discontinuity is reflecting the signal is reflected, while if non-reflecting, backscatter is markedly reduced. Both conditions can be monitored when a signal returned to the fiber input end interferes constructively or destructively with the input signal depending on the phase difference between them. Both the amplitude and phase of the resulting interference signal vary periodically with frequency. From the periodicity, the distance from the input end of the fiber to the discontinuity can be derived.
Abstract of the Disclosure A discontinuity within a fiber is located using a reflectometry technique. A swept frequency sinusoidal signal is launched into the fiber from one end and propagates along the fiber. If the discontinuity is reflecting the signal is reflected, while if non-reflecting, backscatter is markedly reduced. Both conditions can be monitored when a signal returned to the fiber input end interferes constructively or destructively with the input signal depending on the phase difference between them. Both the amplitude and phase of the resulting interference signal vary periodically with frequency. From the periodicity, the distance from the input end of the fiber to the discontinuity can be derived.
Description
~ '7~
This inventlon relates to a method and apparatus for locating faults in optical fibers used in fiber optic communlcations systems. With currently available fiber offering losses of less th~n 1 dBtkm, a repeater spacing or uninterrupted optical cable length of about 25 km can be contemplated. A problem arises if a fiber in the cable should develop a fault such as a break. A method is required for accurately predicting the position of the fiber fault so that repair or replacement can be effected rapidly and with minimum disruption to the buried cable. In a known method of fault location termed optical time-domain reflectometry (OTDR), a discrete pulse is launched into a fiber under test and the time taken for the pulse to propagate to and return from a reflecting fault is measured. Knowing the velocity of light in the material of the fiber, the distance of the fault from the fiber input end can be derived. Although in response to launching the pulse, there will be continuous backscatter from alongthe fiber, the reflectivity of a fiber break can be anything up to 3.5%
depending on the nature of the break, so ensuring a distinctive indication within the backscatter response. With zero reflectivity, the cessation of backscatter is detected.
The signal-to-noise ratio of a reflected pulse depends, among other things, on the distance between the fiber input end and the reflecting discontinuity. Even using averaging techniques, the range of OTDR measurement is limited to about 10 km. In contrast, a desired repeater spacing in the fiber communication line is about 25 km and at these lengths OTDR is much less effective. A technique termed optical frequency-domain reflectometry (OFDR) is now proposed in which instead of a discrete pulse, a swept frequency sinusoidally modulated optical signal :',' ~
., .
is directed into one end of khe fiber under test and, instead of timing the return of a pulse, analysis is performed on an interference signal derived from the reflected signal and the input signal. Both amplitude and phase of the interference signal vary periodically with frequency and from such periodicity, the length of fiber to the discontinuity can be derived. Where in the specification reference is made to a signal reflected from a fiber discontinunity, it will be understood that the discontinuity may be reflecting or non-reflecting. In the latter case the discontinuity causes a detectable reduction in backscatter in the signal returned to the fiber input end.
Apparatus used in performing the method of the invention comprises:- means for launching the swept frequency optical signal into one end of the fiber, means for extracting from the one end of the fiber corresponding light reflected from the discontinuity and for co~bining said extracted light with the swept frequency optical signal to produce an interference signal, and a spectrum analyzer or phasemeter for analyzing the frequency spectrum of said interference signal whereby to determine the length of fiber from said input end to said discontinuity.
The means for launching the swept frequency optical signal can include an injection laser, a laser driver circuit and a tracking generator for controlling the modulation ~requency applied to the laser light output. The tracking generator preferably also has an output to said spectrum analyzer whereby to synchronize operation of the spectrum analyzer and the laser driver circuit.
~ mbodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 is a schematic representation of apparatusaccording to the present invention;
Figure 2 is a schematic representation showing how at various modulation frequencies input and reflected waveforms can interfere destructively (Figure 2a) or constructively (Figure 2b);
Figure 3 indicates the maxima and minima obtained for a fiber having a single, distinctive reflective discontinuity when the input signal is swept over a range of frequencies;
Figure 4 indicates the frequency spectrum obtained for a multiplicity of backscattering elements within the fiber length under test; and Figure 5 shows a phase comparison of the input and reflected signals for a range of frequencies.
Referring in detail to Figure 1, a double heterostructure laser diode 10 is biased above a threshold position to produce continuous wave emission. The laser 10 is driven by a laser driver circuit 12 which is itself under the control of a tracking generator 1~ by means of which the laser 10 is modulated to give a swept frequency output.
The laser optical output is launched into one port 16 of a coupler 18. Although many coupler designs can be used, a suitable coupler is made by twisting a pair of dielectric optical waveguide lengths together, heating the twist region until molten, and then drawing the twist regions so that the fiber lengths coalesce.
One far end port 20 of the coupler is fusion spliced to the fiber 22 under test, the other far end port 2~ being rendered non-reflective by immersion in an oil 26 of matching refractive index.
The near end port 16 is fixed closely enough to the laser diode to - ~ , . . .
guarantee lmW input to the test fiber 22 and the other near end por~ 28 is fixed to direct light from the coupler to an avalanche photodiode (APD) 30. The output from the photodiode is taken to an amplifier 32 and then to a spectrum analyzer 34. Operation o~ the spectrum analyzer 34 and the laser driver circuit 12 is synchronized by the tracking generator 14.
In operation, a swept frequency sinusoidally modulated optical signal is launched into the near end port 16, propagates through the coupler 19 and the fiber 22 under test, is reflected from a distant break 36and is continuously backscattered along the fiber length 22~ The reflected light on returning to the coupler 18 interferes constructively or destructively with the partial reflection produced Prom input light scattered in the coupler.
This interference signal is received at the APD and the amplified signal is displayed at the spectrum analyzer 34.
As shown in Figures 2a and 2b, the output waveforms 40 from laser 10 and the reflected waveform 42 from partial mirror 44 representing break 36, are in a constructive interference condition at detector 30 when 2L = n x v f 0 where L is the length of the fiber n is an integer, v is the velocity of light within the fiber, and f is the frequency.
As shown in Figure 3, consecutive amplitude maxima correspond to consecutive integer values. In khis example consecutive maxima and minima in the frequency response are approximately 25~ kHz apart from which it can be computed that reflection occurs at 4 km from the input end of the 7~
fiber. It will be appreciated from the theory outlined above that the further away a speciFic discontinuity from the input end of the Piber, the lower the separation of freguency maxima or minima in the frequency response of the interFerence signal. Consequently for longer fiber lengths, a narrow band receiver can be used with the attendent advantage of high sensitivity for maximum ranging.
The interference signal from the coupler 18 may be complicated somewhat by continuous backscatter from incremental elements along the length of the fiber 22. The time delay and attenuated signal level of the backscatter varies with the position along the fiber and for a high modulation frequency the resulting interference signal for backscatter alone would be an integrated signal with no clear set of amplitude maxima and minima in the frequency spectrum. Typically the frequency spectrum for backscatter with a non-reflecting fiber end is as shown in Figure 4, the trace being oscillatory, but oscillation being indistinct especially at high frequency. At frequencies with signal periods of the order of 1 nanosecond per km of tested fiber or less, phase scramble is so complete as to prevent acquisition of backscatter data. Thus if the invention is used to search for a single, distinctive fiber discontinuity, a wide range offrequencies can be used. If merely deriving backscatter characteristics of a fiber with no marked fiber discontinuity, the method is advantageously applicable at low frequencies but spectrum analysis will be necessary.
As an alternative to inspecting the spacing of amplitude extrema, the length of a fiber can be derived by inspecting the phase difference between the input end reflected signals over a range of ~7 frequencies. The phase difference can be shown to be ~ fL, v the length L of the fiber being given by the slope of phase difference plotted as a function of frequency. In structural terms, the spectrum analyzer 34 in Figure 1 is replaced by a phasemeter to produce a trace such as that shown in Figure 5. Clearly to derive this trace the frequency progression has been monitored. In fact the phasemeter can only detect the phase change as a fraction of 2~ ~ However the length of the fiber can be obtained by measuring the frequency interval corresponding to a single of excursion of phase difference from -~ to ~r and back again.
The technique can also be applied to a non-reflecting termination, although the phase oscillations are much smaller.
As explained in the introduction to ~he specification, this technique offers a signal-to-noise advantage over the known optical time domain reflectometry method of locating fiber faults. The frequency sweep may take anything from a few seconds to more than a minute in sweeping from O to many hundreds of kHz. Incoming data received in this period can be subjected to averaging techniques to remove noise. In the case of ODTR, a single pulse or series of discrete pulses comprises the complete signal on which noise reduction techniques can be effected. The laser pulse width can be increased but, in the case of semiconductor lasers of, for example, the GaAlAs double heterostructure type, the output power must be reduced to avoid device deterioration. Moreover if the pulse is made longer, effects of pulse broadening during the pulse round trip time in the fiber are more marked. Finally, the pulse width is related to resolution. For example with a pulse width of 100 ns, resolution will inevitably be worse than 10m.
:~3~7 It has been determined that using OFDR the limiting distances at which reflective discontinuities can be detected using conventional ~ibers is significantly greater than for ODTR. Because OFDR
has a signal-to-noise advantage of the orcler of 20 dB over OTDR (i.e. 10 dB in one direction), then for O.9~m lasers with about 3dB/lkm loss, a 3km advantage is gained. For 1.3~m lasers with a 1 - 1~dB/km loss, then a 6 to 10 km improvement can be achieved.
Although in the embodiment described previously a composite optical signal having launched and reflected components is used to derive an electrical analog which is subsequently analyzed, it will be appreciated that distinct electrical analogs corresponding to launched and reflected light could be initially derived and these electrical signals being then combined and analyzed.
;, 7
This inventlon relates to a method and apparatus for locating faults in optical fibers used in fiber optic communlcations systems. With currently available fiber offering losses of less th~n 1 dBtkm, a repeater spacing or uninterrupted optical cable length of about 25 km can be contemplated. A problem arises if a fiber in the cable should develop a fault such as a break. A method is required for accurately predicting the position of the fiber fault so that repair or replacement can be effected rapidly and with minimum disruption to the buried cable. In a known method of fault location termed optical time-domain reflectometry (OTDR), a discrete pulse is launched into a fiber under test and the time taken for the pulse to propagate to and return from a reflecting fault is measured. Knowing the velocity of light in the material of the fiber, the distance of the fault from the fiber input end can be derived. Although in response to launching the pulse, there will be continuous backscatter from alongthe fiber, the reflectivity of a fiber break can be anything up to 3.5%
depending on the nature of the break, so ensuring a distinctive indication within the backscatter response. With zero reflectivity, the cessation of backscatter is detected.
The signal-to-noise ratio of a reflected pulse depends, among other things, on the distance between the fiber input end and the reflecting discontinuity. Even using averaging techniques, the range of OTDR measurement is limited to about 10 km. In contrast, a desired repeater spacing in the fiber communication line is about 25 km and at these lengths OTDR is much less effective. A technique termed optical frequency-domain reflectometry (OFDR) is now proposed in which instead of a discrete pulse, a swept frequency sinusoidally modulated optical signal :',' ~
., .
is directed into one end of khe fiber under test and, instead of timing the return of a pulse, analysis is performed on an interference signal derived from the reflected signal and the input signal. Both amplitude and phase of the interference signal vary periodically with frequency and from such periodicity, the length of fiber to the discontinuity can be derived. Where in the specification reference is made to a signal reflected from a fiber discontinunity, it will be understood that the discontinuity may be reflecting or non-reflecting. In the latter case the discontinuity causes a detectable reduction in backscatter in the signal returned to the fiber input end.
Apparatus used in performing the method of the invention comprises:- means for launching the swept frequency optical signal into one end of the fiber, means for extracting from the one end of the fiber corresponding light reflected from the discontinuity and for co~bining said extracted light with the swept frequency optical signal to produce an interference signal, and a spectrum analyzer or phasemeter for analyzing the frequency spectrum of said interference signal whereby to determine the length of fiber from said input end to said discontinuity.
The means for launching the swept frequency optical signal can include an injection laser, a laser driver circuit and a tracking generator for controlling the modulation ~requency applied to the laser light output. The tracking generator preferably also has an output to said spectrum analyzer whereby to synchronize operation of the spectrum analyzer and the laser driver circuit.
~ mbodiments of the invention will now be described by way of example with reference to the accompanying drawings in which:-Figure 1 is a schematic representation of apparatusaccording to the present invention;
Figure 2 is a schematic representation showing how at various modulation frequencies input and reflected waveforms can interfere destructively (Figure 2a) or constructively (Figure 2b);
Figure 3 indicates the maxima and minima obtained for a fiber having a single, distinctive reflective discontinuity when the input signal is swept over a range of frequencies;
Figure 4 indicates the frequency spectrum obtained for a multiplicity of backscattering elements within the fiber length under test; and Figure 5 shows a phase comparison of the input and reflected signals for a range of frequencies.
Referring in detail to Figure 1, a double heterostructure laser diode 10 is biased above a threshold position to produce continuous wave emission. The laser 10 is driven by a laser driver circuit 12 which is itself under the control of a tracking generator 1~ by means of which the laser 10 is modulated to give a swept frequency output.
The laser optical output is launched into one port 16 of a coupler 18. Although many coupler designs can be used, a suitable coupler is made by twisting a pair of dielectric optical waveguide lengths together, heating the twist region until molten, and then drawing the twist regions so that the fiber lengths coalesce.
One far end port 20 of the coupler is fusion spliced to the fiber 22 under test, the other far end port 2~ being rendered non-reflective by immersion in an oil 26 of matching refractive index.
The near end port 16 is fixed closely enough to the laser diode to - ~ , . . .
guarantee lmW input to the test fiber 22 and the other near end por~ 28 is fixed to direct light from the coupler to an avalanche photodiode (APD) 30. The output from the photodiode is taken to an amplifier 32 and then to a spectrum analyzer 34. Operation o~ the spectrum analyzer 34 and the laser driver circuit 12 is synchronized by the tracking generator 14.
In operation, a swept frequency sinusoidally modulated optical signal is launched into the near end port 16, propagates through the coupler 19 and the fiber 22 under test, is reflected from a distant break 36and is continuously backscattered along the fiber length 22~ The reflected light on returning to the coupler 18 interferes constructively or destructively with the partial reflection produced Prom input light scattered in the coupler.
This interference signal is received at the APD and the amplified signal is displayed at the spectrum analyzer 34.
As shown in Figures 2a and 2b, the output waveforms 40 from laser 10 and the reflected waveform 42 from partial mirror 44 representing break 36, are in a constructive interference condition at detector 30 when 2L = n x v f 0 where L is the length of the fiber n is an integer, v is the velocity of light within the fiber, and f is the frequency.
As shown in Figure 3, consecutive amplitude maxima correspond to consecutive integer values. In khis example consecutive maxima and minima in the frequency response are approximately 25~ kHz apart from which it can be computed that reflection occurs at 4 km from the input end of the 7~
fiber. It will be appreciated from the theory outlined above that the further away a speciFic discontinuity from the input end of the Piber, the lower the separation of freguency maxima or minima in the frequency response of the interFerence signal. Consequently for longer fiber lengths, a narrow band receiver can be used with the attendent advantage of high sensitivity for maximum ranging.
The interference signal from the coupler 18 may be complicated somewhat by continuous backscatter from incremental elements along the length of the fiber 22. The time delay and attenuated signal level of the backscatter varies with the position along the fiber and for a high modulation frequency the resulting interference signal for backscatter alone would be an integrated signal with no clear set of amplitude maxima and minima in the frequency spectrum. Typically the frequency spectrum for backscatter with a non-reflecting fiber end is as shown in Figure 4, the trace being oscillatory, but oscillation being indistinct especially at high frequency. At frequencies with signal periods of the order of 1 nanosecond per km of tested fiber or less, phase scramble is so complete as to prevent acquisition of backscatter data. Thus if the invention is used to search for a single, distinctive fiber discontinuity, a wide range offrequencies can be used. If merely deriving backscatter characteristics of a fiber with no marked fiber discontinuity, the method is advantageously applicable at low frequencies but spectrum analysis will be necessary.
As an alternative to inspecting the spacing of amplitude extrema, the length of a fiber can be derived by inspecting the phase difference between the input end reflected signals over a range of ~7 frequencies. The phase difference can be shown to be ~ fL, v the length L of the fiber being given by the slope of phase difference plotted as a function of frequency. In structural terms, the spectrum analyzer 34 in Figure 1 is replaced by a phasemeter to produce a trace such as that shown in Figure 5. Clearly to derive this trace the frequency progression has been monitored. In fact the phasemeter can only detect the phase change as a fraction of 2~ ~ However the length of the fiber can be obtained by measuring the frequency interval corresponding to a single of excursion of phase difference from -~ to ~r and back again.
The technique can also be applied to a non-reflecting termination, although the phase oscillations are much smaller.
As explained in the introduction to ~he specification, this technique offers a signal-to-noise advantage over the known optical time domain reflectometry method of locating fiber faults. The frequency sweep may take anything from a few seconds to more than a minute in sweeping from O to many hundreds of kHz. Incoming data received in this period can be subjected to averaging techniques to remove noise. In the case of ODTR, a single pulse or series of discrete pulses comprises the complete signal on which noise reduction techniques can be effected. The laser pulse width can be increased but, in the case of semiconductor lasers of, for example, the GaAlAs double heterostructure type, the output power must be reduced to avoid device deterioration. Moreover if the pulse is made longer, effects of pulse broadening during the pulse round trip time in the fiber are more marked. Finally, the pulse width is related to resolution. For example with a pulse width of 100 ns, resolution will inevitably be worse than 10m.
:~3~7 It has been determined that using OFDR the limiting distances at which reflective discontinuities can be detected using conventional ~ibers is significantly greater than for ODTR. Because OFDR
has a signal-to-noise advantage of the orcler of 20 dB over OTDR (i.e. 10 dB in one direction), then for O.9~m lasers with about 3dB/lkm loss, a 3km advantage is gained. For 1.3~m lasers with a 1 - 1~dB/km loss, then a 6 to 10 km improvement can be achieved.
Although in the embodiment described previously a composite optical signal having launched and reflected components is used to derive an electrical analog which is subsequently analyzed, it will be appreciated that distinct electrical analogs corresponding to launched and reflected light could be initially derived and these electrical signals being then combined and analyzed.
;, 7
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for monitoring the position of a reflective discontinuity within an optical fiber, the apparatus comprising:-means for launching a swept frequency optical signal into one end of the fiber;
means for receiving from said one end of the fiber corresponding light reflected from said reflective discontinuity; and measuring means for measuring a frequency related parameter of an interference signal produced by combining the swept frequency optical signal with the received light whereby to determine the length of fiber from said one end to said discontinuity.
means for receiving from said one end of the fiber corresponding light reflected from said reflective discontinuity; and measuring means for measuring a frequency related parameter of an interference signal produced by combining the swept frequency optical signal with the received light whereby to determine the length of fiber from said one end to said discontinuity.
2. Apparatus as claimed in claim 2 in which said measuring means is a spectrum analyzer for identifying extreme in the variation of the interference signal amplitude as a function of frequency.
3. Apparatus as claimed in claim 1 in which said measuring means is a phasemeter for indicating variation of the phase difference between the input signal and the received light as a function of frequency.
4. Apparatus according to claim 1 in which said means for launching the swept frequency optical signal includes an injection laser, a laser driver circuit, and tracking generator having an output controlling the laser driver circuit.
5. Apparatus according to claim 4 in which the tracking generator has an output to said measuring means whereby to synchronize operation of the measuring means and the laser driver circuit.
6. 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 signal into the input end of the fiber, receiving from the input end corresponding light propagating back from the discontinuity, analyzing the frequency spectrum of an interference signal obtained by combining the swept frequency optical signal with the received light and, from a frequency related parameter of said interference signal, deriving the length of the optical fiber from the input end to the discontinuity.
7. A method as claimed in claim 6 in which an electrical analog of said swept frequency is generated, an electrical analog of said received light is generated, and said electrical analogs are combined to produce said interference signal.
8. A method as claimed in claim 6 in which said swept frequency optical signal and said received light combine in an optical coupler and said interference signal comprises an electrical analog of an optical output from said coupler.
9. A method as claimed in claim 6, in which said frequency related parameter in said interference signal is a frequency interval between adjacent like extreme.
10. A method as claimed in claim 6, in which the frequency related parameter in the frequency spectrum is the variation with frequency of the phase difference between the swept frequency optical signal and the received signal propagating back from the discontinuity.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000374977A CA1137787A (en) | 1981-04-08 | 1981-04-08 | Method and apparatus for optical fiber fault location |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000374977A CA1137787A (en) | 1981-04-08 | 1981-04-08 | Method and apparatus for optical fiber fault location |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1137787A true CA1137787A (en) | 1982-12-21 |
Family
ID=4119684
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000374977A Expired CA1137787A (en) | 1981-04-08 | 1981-04-08 | Method and apparatus for optical fiber fault location |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1137787A (en) |
Cited By (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0448293A1 (en) * | 1990-03-22 | 1991-09-25 | STC Submarine Systems Limited | Method of locating a fault |
-
1981
- 1981-04-08 CA CA000374977A patent/CA1137787A/en not_active Expired
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| EP0448293A1 (en) * | 1990-03-22 | 1991-09-25 | STC Submarine Systems Limited | Method of locating a fault |
| US5184081A (en) * | 1990-03-22 | 1993-02-02 | Northern Telecom Europe | Fault location |
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