Note: Descriptions are shown in the official language in which they were submitted.
2043946
OPTICAL TIME DOMAIN REFLECTOME
This invention relates to an optical time domain reflectometer, or OTDR, and
to a
method of using an OTDR for determining characteristics of an optical fiber
path in an
optical communications system.
Background of the Invention
It is well known to use an OTDR to determine or monitor loss characteristics
of an
optical fiber path in an optical communications system. With increasing use of
optical
communications, and with extension of optical fiber communications to
subscribers'
premises, there is an increasing need for such monitoring in a convenient and
effective
manner. In particular, it is desirable to facilitate central monitoring of the
characteristics of
optical fibers leading to many subscribers' premises from a central location,
without
gaining access to the remote ends of the fibers, as is done with automatic
loop testing on
conventional copper subscriber lines. In So et al. United States Patent No.
4,911,515,
issued March 27, 1990, entitled "Optical Fiber Communications System With
Optical
Fiber Monitoring", and assigned to Northern Telecom Limited, there is
described an
OTDR arrangement which facilitates such central monitoring.
Optical fibers which are currently used in optical communications systems are
predominantly "single mode" fibers, in that one mode (LPpI) is propagated over
large
distances with relatively little attenuation or loss, whereas other, higher
order, modes are
heavily attenuated over such distances, so that only the one mode is
effectively propagated
over a long length of an optical fiber communications path. At any
discontinuity in an
optical fiber path, such as occurs at an optical fiber splice or optical
connector, a portion
of the light travelling in the fiber core is lost, the majority of the lost
light being transferred
from the LPpI mode to the LPl l mode. As is known, the LP11 mode travels
faster than
the LPpI mode, but is relatively quickly attenuated.
A problem arises, however, if two discontinuities occur only a relatively
short
distance apart. In such a situation, at the second discontinuity the LP11 mode
can have
sufficient power that some of it can be converted back into the LPpl mode,
this recoupled
light then being propagated along the fiber and interfering with the desired
LPpI mode
signal, with which it is no longer synchronized due to the different
velocities of the LPpI
and LPl1 modes between the two discontinuities. The net effect of this is
modal
interference, or modal noise, which appears as a wavelength dependent loss of
the optical
fiber path.
Such relatively closely spaced discontinuities may occur in a variety of
situations,
for example with repeated splicing of an optical fiber, with the use of
optical fiber patch
cords, or with the use of certain types of optical connectors which
incorporate a short
length of fiber to facilitate field assembly of the connectors. Generally, in
any situation
CA 02043946 2002-02-06
2
where there is less than a few meters of fiber between two successive
discontinuities, there is a potential for modal interference as described
above.
OTDRs conventionally used for optical communications systems
have a resolution of, at best, about 0.1 m, and accordingly are unable to
resolve
between discontinuities closer apart than this, merely indicating the combined
loss of the two discontinuities as though there is only a single
discontinuity.
Accordingly, conventional OTDRs fail to assist in determining the existence
and location of such discontinuities. It should be appreciated that the
combined
loss of two closely spaced discontinuities may not be particularly great, but
the
modal interference may be sufficient to cause significant degradation of the
optical communications path, leading to excessive transmission error rates.
Furthermore, it should be appreciated that the wavelength dependent nature of
the loss due to this modal interference may mean that the errors are
intermittent
or vary with time, due to small changes in the wavelength of the light being
transmitted via the fiber.
An object of this invention, therefore, is to provide an improved
OTDR which facilitates the detection of such closely spaced discontinuities.
According to one aspect of this invention, there is provided an
optical time domain reflectometer (OTDR) comprising an optical source for
producing an optical signal, and coupling means for coupling the optical
signal
to an optical fiber path, the optical source being tunable over a range of
wavelengths in the region of a prescribed wavelength for said optical fiber
path;
means for detecting light from the optical fiber path; means responsive to the
detected light for determining loss-distance characteristics of the optical
fiber
path; control means for controlling the optical source to vary the wavelength
of
the optical signal over said range; and display means responsive to the
control
means for displaying information representative of the determined loss-
distance
characteristics for a plurality of different wavelengths of the optical signal
within said range.
Thus the invention provides an OTDR which can operate at a
plurality of different optical signal wavelengths, to provide loss-distance
characteristics for these wavelengths which indicate any wavelength dependent
CA 02043946 2000-OS-O1
3
loss of the optical fiber path, which wavelength dependent loss may be
produced by closely spaced discontinuities as discussed above.
The means for producing an optical signal conveniently
comprises a tunable optical source controlled by the control means.
Advantageously the tunable optical source comprises a
semiconductor laser and the control means comprises means, such as thermo-
electric cooling means, for controlling an operating temperature of the
semiconductor laser.
The OTDR rnay also include means for monitoring the
1(1 wavelength of the optical signal.
According to another aspect, the invention provides a method
of using an OTDR for determining characteristics of an optical fiber path,
comprising the steps of producing an optical signal using an optical source,
the optical source being tunable over a range of wavelengths in the region of
1 ~~ a prescribed wavelength for said optical fiber path; coupling the optical
signal
to the optical fiber path; detecting light from the optical fiber path; in
response to the detected light, determining loss-distance characteristics of
the
optical fiber path; controlling the optical source using a control means to
vary
a wavelength of the optical signal over said range; and in response to the
2(1 control means, displaying information representative of the determined
loss-
distance characteristics for a plurality of different wavelengths of the
optical _
signal within said range.
The determined characteristics may comprise modal interference
and/or chromatic dispersion of the optical fiber path.
25 Brief Description of the Drawings
The invention will be further understood from the following
description with reference to the accompanying drawings, in which:
Fig. l is a graph illustrating the wavelength dependent
attenuation of an optical fiber path due to two closely spaced
discontinuities;
3(1 Fig. 2 is a block diagram illustrating an OTDR in accordance
with an embodiment of this invention;
Fig. 3 is a graph illustrating results of operation of the OTDR
of Fig. 2;
CA 02043946 2000-OS-O1
3a
Fig. 4 schematically illustrates parts of an optical source which
may be used in the OTDR of Fig. 2;
Fig. 5 is a graph illustrating operation of the OTDR of Fig. 2
for determining chromatic dispersion; and
Fig. 6 schematically illustrates parts of a wavelength division
multiplexed optical communications system with which the OTDR of Fig. 2
may be advantageously used.
Description of the Preferred Embodiment
Fig. :l is a graph which illustrates a typical variation in
attenuation of a single mode optical fiber path, as a function of wavelength
in
the region of 1300 nn at which the optical fiber path is intended to operate,
due to the presence of two discontinuities spaced relatively closely apart; in
this case the discontinuities are two optical fiber splices spaced 15 mm
apart.
As is iillustrated in Fig. 1, the attenuation or loss of an optical
signal propagated via this optical fiber path is of the order of a few dBs,
but
varies by a significant amount with small changes in wavelength, up to about
0.8 dB as illustrated for a wavelength change of 20 nn. The particular
characteristics and wavelength dependence of the loss depend upon the nature
and spacing of the discontinuities, but in any event there may be a wavelength
dependence of the loss which is not determinable using conventional OTDRs.
Furthermore, the close spacing of the discontinuities means that these are not
resolvable using conventional OTDRs.
In an optical communications system in which such an optical
2-'i path is present, the loss due to the two closely spaced discontinuities
may not
initially be a problerr~. For example, if the optical signal has a wavelength
of
1280 nn, then as illustrated in Fig. 1, the loss is at a minimum and possibly
acceptable level, considering the total loss for the
4
system as a whole. However, with changes in operating temperature, and with
aging and
possible substitution of equipment, the optical signal wavelength may change
to a point at
which the additional loss, due to the wavelength dependence as shown in Fig.
l, is
sufficient to cause degradation of the optical signal propagation, resulting
in increased bit
error rates. Such degradation may be intermittent, and consequently very
difficult to
diagnose unless there is an appreciation that it is caused by modal
interference due to the
closely spaced discontinuities.
Referring to Fig. 2, there is illustrated in a block diagram an OTDR, in
accordance
with an embodiment of this invention, which can be used to determine faults in
an optical
fiber path such as the two closely spaced discontinuities as discussed above.
The OTDR
comprises a tunable optical source 10, an optical coupler 12, an optical
detector 14, a
signal processing control unit 16, a display 18, and an optical fiber
connector 20 to which
an optical path to be tested, represented in Fig. 2 by a fiber 22, is coupled.
The OTDR
may also optionally include a wave meter 24 as shown in broken lines in Fig.
2. For
monitoring of many optical fiber paths in an optical fiber communications
system, the
OTDR may be arranged as described in United States Patent No. 4,911,515
already
referred to.
As is known in the art, in a conventional OTDR an optical signal at a single
wavelength is coupled to an optical fiber path, and light which is back-
scattered and
reflected due to faults is detected to provide a loss-distance characteristic
for the path. The
OTDR of Fig. 2 operates in a similar manner, with the additional feature that
the
wavelength of the optical signal is controllably varied to provide a plurality
of loss-
distance characteristics for different wavelengths, so that any wavelength
dependent
nature of the path, which is typically due to closely spaced discontinuities
as discussed
above, is clearly determined. As in conventional OTDRs, the loss-distance
characteristics
can be displayed by the display 18 in any convenient form, for example in
graphical
and/or tabular form as desired. For clarity and simplicity, in the drawings
these
characteristics are illustrated in graphical form, and the following
description is worded
accordingly.
The tunable optical source 10 is controlled by the signal processing control
unit 16
to produce an optical signal at a desired and controlled wavelength. This
optical signal is
coupled via the coupler 12 to the connector 20 and thence to the optical fiber
path 22 to be
tested. The optical signal is also coupled to the wave meter 24 when this is
provided, the
wave meter 24 measuring the wavelength of the optical signal and providing a
corresponding electrical signal to the control unit 16. In the absence of the
wave meter
24, the OTDR is calibrated so that the wavelength of the optical signal
generated by the
source 10 is precisely determined by the control unit 16.
s
Light which is back-scattered and reflected in the optical fiber path 22 is
coupled
via the connector 20 and optical coupler 12 to the detector 14, where it is
detected to
provide a corresponding electrical signal to the signal processing control
unit 16. The unit
16 provides to the display the resulting loss-distance characteristic for the
particular
wavelength of the optical signal. This process is performed for two or more
different
wavelengths, which result in different characteristics if there is a
wavelength dependent
loss of the optical fiber path 22. The location and nature of such a
wavelength dependent
loss is then readily apparent from the information displayed by the display
18, as is
further described below.
Fig. 3 illustrates a graphical display resulting from operation of the OTDR of
Fig. 2 at two different wavelengths, ~ 1 and ~2, shown by continuous and
broken lines
respectively, with an optical fiber path 22 having two closely spaced optical
splices at a
distance of about 2 km from the OTDR. In a manner well known in the OTDR art,
the
graph illustrates relative intensity as a function of distance along the
optical fiber path,
is showing an initial loss 30 due to the connector 20, a slope 32 due to
attenuation of the
optical signal in the fiber, a loss 34 due to the splices, and a final spike
36 due to an end
connector of the optical fiber path. As already discussed, the OTDR display
does not
resolve the two splices due to their close separation, and accordingly for
each wavelength
7~1 or 7~2 these appear in the display as a single fault.
As Fig. 3 illustrates, however, there is a significant difference between the
loss 34
determined by the OTDR at the two different wavelengths ~1 and A2, the
wavelength
dependent attenuation of the two closely spaced splices being appreciably
greater at the
wavelength A2 than at the wavelength A1, as explained with reference to Fig.
1.
Accordingly, the display produced by the OTDR clearly indicates that at this
point there is
a fault having a wavelength dependent attenuation, such as is caused by modal
interference due to two closely spaced discontinuities.
Although not further described here, it should be appreciated that the OTDR
may
provide, from the wavelength dependent loss characteristics which it
determines for any
fault in the optical fiber path, not only the location and attenuation of the
fault, but also an
indication of the nature of the fault. For example, the control unit 16 may be
provided
with stored information typical of the wavelength dependent attenuation of
particular
situations, and may correlate the wavelength dependent information relating to
each fault,
such as that represented by the loss 34, with this stored information to
provide an
indication of what situation is to be expected at the location of the fault.
Such information
3s is of considerable value to service personnel in identifying and rectifying
faults.
The OTDR of Fig. 2 differs from conventional OTDRs in particular in that its
optical source 10 is tunable, the wavelength tuning being controlled by the
control unit 16.
Various forms of tunable optical source, such as semiconductor lasers, are
known and
6
may be used as the source 10. It is convenient, however, for the source 10 to
be
constituted by a conventional semiconductor laser whose temperature is
controlled to
control the wavelength of the optical signal.
Fig. 4 illustrates parts of a known semiconductor laser device package, which
comprises a semiconductor laser 40 and a temperature sensor 42 mounted on a
semiconductor therno-electric cooler 44 which is in turn mounted on a thermal
interface
46. An optical signal produced by the semiconductor laser is coupled via an
optical
system represented by a lens 48 to an optical fiber 50, and light emitted from
a back face
of the semiconductor laser is monitored by a light-sensitive diode 52.
Conventionally, the
thermo-electric cooler 44 is controlled to maintain a constant temperature, as
monitored by
the temperature sensor 42, of the semiconductor laser and hence a constant
wavelength of
the optical signal.
Using such a semiconductor laser device package to constitute the tunable
optical
source 10 of the OTDR of Fig. 2, the temperature of the semiconductor laser 40
is
controlled by the control unit 16, via the thermo-electric cooler 44, to
produce the optical
signal with variable, controlled, wavelengths. The semiconductor laser 40 has
an
operating temperature range of about -20 °C to about 80 °C and
produces the optical
signal with a wavelength which changes at a rate of about 0.3 nmi/°C,
so that the
wavelength of the optical signal can be readily varied through a range of 30
nm. As can
be seen from Fig. 1, this range is sufficient for any wavelength dependent
losses in an
optical fiber path being tested to be clearly identified.
In the event that a greater tuning range is desired, for example for testing a
wavelength division multiplexed optical communications system as discussed
below, the
temperature control described above can be replaced or supplemented by using a
tunable
semiconductor laser of known fore or by using a plurality of semiconductor
lasers of
different wavelengths which are selectively used to provide the optical
signal. In any
event, the optical signal wavelength may be controllably varied either in a
continuous
manner or in discrete steps over a desired range, so that the OTDR provides
wavelength
dependent loss information for the optical fiber path being tested.
The OTDR of Fig. 2 can also be used for determining chromatic dispersion of
single mode optical fiber paths. Although chromatic dispersion test equipment
already
exists, such equipment is expensive and requires access to both ends of the
optical fiber
path. In contrast, the OTDR of Fig. 2 only requires access to one end of the
optical fiber
path, and can serve the dual purpose of fault location and chromatic
dispersion
measurement.
For measurement of the chromatic dispersion of a relatively long optical fiber
path
(chromatic dispersion is only significant for relatively long paths), the OTDR
of Fig. 2 is
operated as described above with two different wavelengths of the optical
signal, for
~~,~ ~c~,~ r_
1/
example separated by 30 nm as discussed above. A resulting display for the two
wavelengths ~1 and ~2, assuming no wavelength dependent loss, is illustrated
in Fig. 5
using solid and broken lines respectively in a similar manner to the
illustration in Fig. 3.
The end reflections for the two wavelengths are separated from one another,
due to
chromatic dispersion, by a distance d which is determined by the OTDR. The
distance d
provides a measurement of the chromatic dispersion. For a typical chromatic
dispersion
of approximately 3 ps/nn~/km, a wavelength difference of 30 nm as discussed
above, and
an optical fiber path of 20 km with an optical signal speed of approximately
0.2 m/ns, the
distance d determined by the OTDR is 0.72 m, which is well within the
resolution of the
OTDR.
Fig. 6 illustrates parts of a wavelength division multiplexed optical
communications system of known form, in which a wavelength division
demultiplexer
(WDM) 60 couples wavelength multiplexed optical signals having respective
wavelengths
~1 to 7.n from an optical fiber path 62 to respective optical network
interfaces (ONI) 64.
With such an arrangement, the optical signal from a conventional OTDR coupled
to the
optical fiber path 62 at its end remote from the ONIs 64 can be routed to only
one of the
ONIs 64 according to its wavelength, but the OTDR will be unable to determine
characteristics of all of the other optical fiber paths beyond the WDM 60. In
contrast,
using an OTDR with a tunable optical source 10 as described above with
reference to
Fig. 2, the wavelength of the optical signal can be tuned selectively to be
routed to each of
the ONIs 64 in turn, so that the characteristics of all of the optical paths
leading to the
ONIs 64 can be determined by the remote OTDR. As the range of wavelengths Al
to an
will generally exceed the thermal tuning range of a single semiconductor laser
as described
above with reference to Fig. 4, in this case the tunable optical source
preferably comprises
a conventional tunable laser or a plurality of semiconductor lasers whose
optical signals
have the different wavelengths A1 to an and are used selectively by the OTDR.
Numerous modifications, variations, and adaptations may be made to the
described embodiments of the invention without departing from the scope of the
invention
as defined in the claims.
