Note: Descriptions are shown in the official language in which they were submitted.
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This invention 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 than
1 dB/Km, a repeater spacin~ 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
cable. In a known method of fault location termed optical time-domain
ln 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 along the 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. If the discontinu1ty has zero reflectivity, then cessation
of backscatter can be detected.
In a newly developed method of fault location termed
optical ~requency-domain reflectometry (OFDR), a swept frequency
sinusoidal signal is launched into the fiber from one end and
propagates along it. A signal propagating back from the discontinuity
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
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the input end of the fiber to the ~iscontinuity can be derived. ~FD~
offers a signal-to-noise ratio advan~age over OTDR since, in the
latter, discrete pulses are used whereas in OFDR an extensive frequency
sweep is used and noise reduction techniques can be effected.
Both OTDR and OFDR are, however, dependent on the
refractive index of the fiber which can vary considerably with
temperature, stress and fiber composition. A modification of OFDR
termed dual wavelength optical frequency-domain reflectometry (D~JOFDR)
is now proposed in which any refractive index fluctuation in the fiber
has little effect on the accuracy of fault location. Where in the
specification reference is nlade to a fiber discontinuity, it will be
understood that the discontinuity is reflecting.
According to one aspect of the invention there is
provided apparatus for monitoring the position of a discontinuity in 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 ~1 and ~2
separated by a few nanometers, said components having diFferent group
velocities within the fiber9 means for receiv1ng from said one end o~
2n the fiber an interference si~nal between the two components propagating
back from the discontinuity, said inter~erence signal having amplitude
variation caused hy interference between said two components, and 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.
The apparatus includes a spectrum analyzing means for
identifying extrema in the variation of the interference signal
amplitude as a function of frequency.
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Said means for launching the swept frequency optical
signal preferably comprises a pair of injection lasers having spectral
line outputs, respective driver circuits for the lasers, and a tracking
~enerator having an output controlling the laser driver circuits. The
tracking generator can have an output to said spectrum analyzing means
whereby to synchronize operation of the analyzing means and the laser
driver circuits. Means should be provided to maintain the pair o~
injection lasers at the same temperature to stabilize their wavelengths
According to another aspect of the invention there is
provided a method of measuring the distance from an input end of a
fiber to a discontinuity within the fiber, the method comprising
launchincl a swept frequency optical input signal into the input end of
the fiber, the signal having components at wavelengths A1 and ~2
separated by several nanometers, the ~1 component having a group
velocity different from the group velocity of the ~2 component within
the fiber, the method further comprising receiving at the fiber input
end a corresponding interference signal between the two components
propagating back from the discontinuity, analyzing the frequency
spectrum of the interference signal, and, from a frequency relatecl
parameter of said interference signal deriving the distance from the
input end to the discontinuity.
An embodiment of the invention will now be described by
way of example w1th reference to the accompanying drawings in which:-
Figure 1 is a schematic representation of apparatusaccording to the present invention; and
Figure 2 illustrates the frequency spectrum of an
interference signal used to locate a fiber fault.
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Re~erring in detail to Figure 1~ two double hetero-
structure laser diodes 10 are biased above lasing threshold to produce
continuous wave emission. The lasers are mounted on a common heatsink
(not shown) so are at the same temperature as one another. The lasers
10 have spectral line outputs with wavelengths ~1 and ~2 separated by
20 nanometers and centered around 1.1 ~m. The lasers 10 are driven by
laser driver circuits 12 which are themselves under the control of a
trackin~ generator 14 by means of which the lasers are modulated to
give an identically modulated swept frequency output.
The laser outputs are launched into ports 16 of a coupler
18. Although many coupler designs can be used, a suitable coupler is
made by twisting dielectric optical waveguide lengths together, heating
a central twist region until molten and then drawing the twist region
so that the fiber length coalesce.
One Far end port 26 of the coupler is fusion spliced to
the fiber 22 under test, the other far end ports 24 being rendered
non-reflective by immersion in oil 27 of matching refractive index.
The near end ports 16 are fixed closely enough to the laser diodes 10
to guarantee 1 mW input to the test flber 22, and another near end port
20 2~ is fixed to direct li~ht from the coupler to an avalanche photodiode
(APD) 30. The output from the photodiode 30 is taken to an amplifier
34. The output of the amplifier 34 is taken through a tunable
narrowband filter 38 under the control of the tracking generator 14 and
then to an amplitude detector 40, the output of which is then displayed
on a CRT 46 as a function of the sweep frequency.
Alternately, the frequency analysis components of the
circuit can be substituted by a standard spectrum analy~er if desired.
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In operation, a swept frequency, sinusoidally modulated
optical signal having components at ~1 and ~2 is launched into the near
end ports 16, propagates through the coupler 18 and the fiber 22 under
test, (being continuously backscattered along the way), and is finally
reflected at a distant break 36. The components ~1 and ~2 have
different group velocities within the fiber. Consequently, the
components interfere constructively or destructively, such interference
bein~ evident in the reflected signal received at near end port 28 of
the coupler. The interference signal is received at the APD 30 and
amplified. The amplitude of the resulting interference signal
generated during the frequency sweep is then displayed on the CRT. The
resulting signal can be shown to have amplitude:-
S(f) ~Ae-j2~fL(N1-N2)/C+Be+i2~fL(N1-N2)/c~
where: A and B are the powers of the reflected signals at
wavelengths ~1 and ~2 respectively;
f is the frequency of the input signal;
N1 and N2 are group indices of the fiber at
wavelengths ~1 and ~2 respectively;
L is the distance from the fiber input end to the
discontinuity; and
c is the speed of light in vacuum.
With reference to Figure 2, and assuming the powers of
the reflected cowponents are erual, (A-B), the amplitude of the
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interference signal exhibits a sharp minimum at
f d L~ ( i i )
From the equation (ii), L, the distance from the fiber
input end to the break, can be computed. As shown in the example of
Figure 2, the first minimum appears at approximately 187.5 MHz from
which it can be calculated that reflection occurs at a distance 1 km
from the fiber input end.
For comparison, the frequency of first constructive
interference in an OFDR method is:
fo ~No (iii)
where No is the group index at the single wavelength.
Comparing the two equations (ii) and (iii) above, it will
be seen that the frequency fO is inversely proportional to the group
index of the fiber whereas frequency fd is inversely proportional to
the difference between the group indices Nl and N2. As indicated
previously, the individual indlces may fluctuate markedly, mainly
because of temperature but also because of stress and composltiona1
variation along the len~th of the fiber. While this may affect fO,
there is negl1gible effect on fd since any change in group index at ~1
Is accompanied by an equivalent change in group index at ~2 so the
dlfference in group indices, and therefore the difference in group
velocity on which fiber length measurement is based, remains
substantially constant. Also the frequency fd of the DWOFDR method
corresponds to a sharp minlmum which can be detected with greater
; accuracy than the frequency fO of the OFDR method because the extrema
of the latter are flat~
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For the 20 nm separation between ~1 and ~ 2~ the
differ~nce N2-N1 is of the order of 0.~004. From equation (ii) above,
a fault 1 km distant from ~he fiber inpu~ end requires a 187l5 MHz
modulation signal. The method can thus be used on current high grade
telecommunication fiber whose bandwidth is greater than 187.5 MHz - km.
If a lower wavelength separation is used then the necessary modulation
frequency is higher. On the other hand, if too high a wavelength
separation is used, then the group velocities of the two components
will be sub~ject to disparate variations owing to fluctuations of
temperature stress and fiber composition, and thus the primary
advantage of the invention will be lost. Because high modulation
frequencies are necessary in DWOFDR, resulting electronic constraints
limit the minimum detection distance making DWOF~R suitable for medium
to long range operations.
In the embodiment described, the components at
wavelen~ths ~1 and ~2 combine optically and an electrical analog of the
resulting interference signal is produced by the APD 30.
Alternatively, the components at ~1 and ~2 are separated using a
wavelength division demultiplexer, the individul optical signals are
used to generate separate electrical analog signals and these are then
combined together to generate an electrical lnterference signal on
which spectrum analysls is performed.
Although the primary advantage of the method of this
invention is in aerial cable to minimize the influence of temperature
fluctuation on measurement accuracy, the me~hod should find application
in compensating for stress or inhomogeneity of buried or submarine
fibers.
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To a first approximation thermal sensitivity of this
method can be reduced to zero at a "medium" wavelength 1 +2 2
of 1.1 ~m. Optimum wavelength, which can be determined from the fiber
refractive index, should be advantageously close to the minimum
attenuation region of most fibers used for telecommunications purposes.
The maximum range of DWOFDR depends on the wavelength used, on the
coupled power and on receiver sensitivity, The range obtainable
increases with wavelength.
It should be mentioned that the interference signal
corresponding to a reflective discontinui~y, may be complicated
somewhat by continuous backscatter from incremental elements along the
length of the fiber. However, incoming data during the frequency sweep
can be subjected to averaging techniques to remove this noise.
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