Remotely controlled optical time domain reflectometer serving a plurality of fiber optic cables

This application discloses a system in which a single optical time domain reflector is used to monitor a plurality of cables. In one embodiment two lasers having different wavelengths are separately fired into different cables, and the returning signals are fed into the appropriate port of a wavelength dependent coupler. In another embodiment, each laser simultaneouly checks two cables having different lengths, the length difference permitting identification of the cable in which a fault has occurred.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus used in the maintenance of fiber optic 
communication networks. 
Such networks include a multiplicity of stations, called repeater huts, 
into each of which a plurality of fiber optic cables extend. Each such 
cable, which extends underground, contains a multiplicity of fiber optic 
strands, which carry the communication signals. Stations, or repeater 
huts, are underground chambers located perhaps 40 to 50 kilometers apart. 
The signals coming into a station on one cable are amplified, switched, 
and otherwise regenerated inside each hut, and then sent out on another 
cable. 
Optical Time Domain Reflectometers (OTDRs) are, in effect, "laser radar" 
instruments which test the condition of fiber optic strands by injecting 
laser beam pulses into the strand at one point, and then receiving their 
returned reflections. The time elapsed from the sending to the returning 
of the pulse indicates the distance from the OTDR to the point of 
reflection. Variations in the energy of the returning pulses are used to 
diagnose the condition of the fiber optic strand. These diagnoses are used 
for several purposes: accurate splice analysis, pinpointing fault 
locations, cable acceptance testing, cable installation, and end-to-end 
tests. Most of these purposes relate to installation and testing of the 
initial system. 
However, the pinpointing of fault locations is of extreme importance when 
damage accidentally occurs to an existing cable installation, usually as 
the result of mechanical accident. 
OTDRs may be carried to the sites of work, for use in diagnosing fiber 
optics cable problems. However, the assignee of this application has 
pioneered a remote control OTDR system, in which a central computer is 
separately linked to a plurality of OTDRs, each of which is located inside 
a station, or repeater hut, and each of which has its own computer linked 
to the central computer. Such a system provides many advantages, such as 
continuous remote monitoring of the cable links, triggering of an alarm if 
a problem occurs, and fast fault location from a remote central office 
terminal. 
One problem with the remote control OTDR system is its expensiveness, due 
to the need for a separate OTDR at each station in the network. 
SUMMARY OF THE INVENTION 
The present invention permits each OTDR to serve a plurality of stations, 
thus substantially reducing total network cost. 
Each locally installed OTDR has a wavelength multiplexing unit combined 
with a plurality of laser sources which send different wavelength signals. 
Thus, for example, the OTDR in one station can be instructed from the 
central control to send a signal having a higher wavelength; and that 
signal will automatically check the fiber optics cable extending in one 
direction from the station containing the OTDR. Or, the same OTDR can be 
instructed from the central control to send a signal having a lower 
wavelength; and that signal will automatically check the fiber optics 
cable extending in a different direction from the same station. 
In other words, the cable being checked is identified by its assigned laser 
wavelength. This arrangement is very successful with two cables, and might 
be used with a larger number of cables which are all linked to the same 
OTDR-containing station. 
Another version of the invention increases from two to four the number of 
cables serviced by a single OTDR. This is accomplished by sending the 
signal from each of two differing wavelength laser sources simultaneously 
into two cables, one of which is longer than the other. The sum of the two 
returning signals can be used to determine: (a) whether a fault, or break, 
has occurred in either of the two cables, (b) which of the two cables is 
damaged, and (c) where the damage has occurred.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS 
FIG. 1 shows a remote station 20, which is in a network of such stations 
linked to a central control computer 22. Each station is entered by at 
least two fiber optic cables (not shown) containing a large number of 
separate fiber optic strands, which have been suitably spliced to maintain 
maximum transmission efficiency. 
Each cable contains one or more spare fiber optic strands, which are 
available for use in OTDR monitoring and damage location. The remote OTDR 
(23 in FIG. 1) permits continuous monitoring of the integrity of a fiber 
optic cable link between unmanned installations. Upon detection of signal 
loss between two repeater locations, an alarm sounds at the central 
control, and the central computer automatically comands the remote OTDR to 
scan the affected fiber. This enables the central office operator to view 
the backscatter signature of the damaged link on a terminal. Analysis and 
fault location can be carried out immediately, to greatly minimize any 
delay in getting a repair crew to the site of the fault. 
Locating in a minimum amount of time the position of a damaged or broken 
fiber connection realizes tremendous savings, by minimizing the time 
needed to repair communications breakdowns. However, the communications 
operators want to monitor as many lines as possible with as few OTDRs as 
possible. They could use remotely controlled fiber optic switches, but 
these are nearly as costly as the OTDRs. And fiber optic switches have the 
additional disadvantage that they cause relatively high signal loss. 
The present invention permits a fifty percent, and in one version a 
seventy-five percent, reduction in the number of OTDRs required for 
complete network monitoring. 
FIG. 2 shows the basic components of a station-located OTDR. A CPU 24 is in 
control of the OTDR and in communication with central station 22 (FIG. 1). 
The CPU controls a laser driver 26, which causes laser beam 28 to be 
delivered via a directional coupler 30 to the fiber strand or strands 32 
which serve the OTDR. 
Reflected laser signals returning from the fiber or fibers 32 are directed 
by the directional coupler 30 to a detector 34, from which the returning 
signal goes to a receiver 36, and then to a digital averager 38. From the 
digital averager, the returning signal is directed to the CPU 24, and 
thence to a display 40 which may transmit to the central station. 
The information supplied by the OTDR is used to locate unnatural changes in 
the laser energy moving back and forth through the fiber. There is a 
natural attenuation in the laser energy throughout its travel. In fact, 
the energy level of the returning pulses at detector 34 may be in the 
neighborhood of 0.00001 of the energy level leaving laser 28. The purpose 
of the digital averager 38 is to separate the weak returning laser signal 
from the much greater noise level in which it is buried. The returning 
pulsed signals are digitized and added together. The noise, on the other 
hand, is uncorrelated (random) in its timing; and the net effect of the 
noise will tend to reach a zero average value. By continuing long enough 
in adding up the digitized, coherent signals, a sufficient value is 
obtained to provide useful information. 
The outgoing pulses generate reflections from every part of the fiber, but 
substantial scattering occurs. Normally, on a display graph, the energy of 
the recurring signal shows a gradually declining straight curve. A greater 
decline generally occurs at a splice. If an abrupt signal decline occurs 
at any point, a problem is indicated. The cable may have been damaged; 
damage to an OTDR fiber generally indicates damage to all the fibers in 
the cable. Also, a sudden increase in signal may indicate damage, due to 
increased Fresnel reflection. 
An ideal fiber would provide, in the logarithm domain, a straight line 
dropping to the right, with small splice drops periodically. The slope of 
the line indicates the attenuation of the fiber in decibles per kilometer. 
Distance and location are always indicated by elapsed time. 
Clearly, a monitoring system having a remote OTDR in every repeater station 
would provide an ideal network for locating and promptly correcting faults 
in fiber optic cable systems. But the costs of the OTDRs in such a system 
are a significant deterrent. So a problem to be addressed is trying to 
gain the full benefits with fewer OTDRs. One possible solution, as already 
stated, is to use fiber optic switching systems interconnecting the OTDRs. 
But such systems would be very costly, and have high signal loss. 
The present invention provides a simple, but elegant, arrangement for 
cutting the costs by almost 50%, with very slight sacrifice of efficiency. 
The basic concept is to use a plurality of laser wavelengths, transmitted 
through a wavelength division multiplexer whose two entry ports are 
selectively responsive to different wavelengths. Such couplers are 
available, for example, from Gould Electronics. They are referred to as 
"Singlemode Fused Wavelength Division" couplers. They are intended for use 
"whenever it is necessary to combine or separate two optical data sources 
of differing wavelengths with virtually no optical loss". 
Before explaining the value of such wavelength division in solving the 
problem of the present application, it will be helpful to describe the 
functioning of a standard coupler, such as the coupler 30 in FIG. 2. 
FIG. 3 diagrams such a directional coupler, which provides a 50/50 
beam-splitting effect, analagous to the beamsplitters used in 
interferometers. Referring to FIG. 3, light traveling into Port 1 will 
divide equally between Ports 3 and 4. The same is true for light traveling 
into Port 2. Light traveling in the reverse direction into either Port 3 
or Port 4 will also split up equally and exit from Ports 1 and 2. 
The coupling system used in the present invention incorporates a wavelength 
division multiplexer (or wavelength dependent coupler), which is the 
special type of directional coupler referred to above. A wavelength 
division multiplexer changes the ratio of the light exiting its output 
ports as a function of the wavelength of the light. If 1300 nanometer 
wavelength light is traveling into Port 1, then approximately 99% of it 
will exit from Port 3 and only 1% will exit from Port 4. If the wavelength 
is changed to 1550 nanometers, then approximately 99% will be emitted from 
Port 4 and only 1% from Port 3. Again, these properties are reciprocal 
when the light travels in the opposite direction. 
FIG. 4 illustrates an embodiment of the present invention. Three parts, 
relatively inexpensive, are added to the OTDR of FIG. 2. The added parts 
are a second laser, a second directional coupler, and a wavelength 
division multiplexer (WDM). With this system, two test fibers are used, 
thus permitting two fiber optic cables to be monitored by a single OTDR. 
A 1300 nanometer wavelength laser is shown at 50. Its output is fed through 
a standard directional coupler 52, which outputs 50% of the laser energy 
at each of two ports 54 and 56. The energy through Port 54 is connected by 
a fiber optic connector 58 to a first fiber optic test strand 60 (test 
fiber #1). Note that the energy leaving the second port 56 of coupler 52 
is essentially wasted; but this splitting effect is common to all OTDRs. 
The OTDR signal in test fiber #1 is part of one fiber optic cable. The 
returning laser signal from that cable is directed through directional 
coupler 52, and 50% of its energy exits along path 62 to the inlet port 90 
of a wavelength division multiplexer 64. From the multiplexer 64, almost 
the entire returning signal is sent along path 66 to a detector 68. 
If the operator desires to monitor the second fiber optic cable, test fiber 
#2 will be used. In this case a 1550 nanometer wavelength laser 70 will be 
fired. Its beam is fed through a second directional coupler 72, which 
directs 50% of the energy along path 74 through a fiber optic connector 76 
into a second fiber optic test strand 78. Note that a second port 80 of 
coupler 72 receives 50% of the energy, which is essentially wasted. 
The OTDR signal in test fiber #2 is in a second fiber optic cable leading 
in a different direction from the cable containing test fiber #1. The 
returning signal from the second cable is directed through directional 
coupler 72, and 50% of the energy is carried along path 73 to the inlet 
port 92 of the wavelength division multiplexer 64. From the multiplexer 
64, almost the entire returning signal is sent along path 66 to detector 
68. 
The standard directional couplers 52 and 72 are referred to as wavelength 
independent couplers. The multiplexer 64 is referred to as a wavelength 
dependent coupler. Its internal construction is such that the 1300 
nanometer wavelength signal entering its port 90 will be so split that 
approximately 99% of its energy exits from detector port 66, and only 1% 
exits from the other port (not shown). Also, the internal construction of 
multiplexer 64 is such that the 1550 nanometer wavelength signal entering 
its port 92 will be so split that approximately 99% of its energy exits 
from detector port 66, and only 1% from the other port (not shown). 
FIGS. 5 and 6 show more details of the wavelength independent (WIC) and 
wavelength dependent (WDC) couplers. In FIG. 5, WIC 52a has two ports 100 
and 102 at one end, and two ports 104 and 106 at the other end. WIC 72a 
has two ports 108 and 110 at one end, and two ports 112 and 114 at the 
other end. 
The WDM 64a has two ports at one end: 116 is connected to detector 68a, and 
118 is unconnected. Its two ports at the other end are 102 and 110, which 
are connected, respectively, to WIC 52a and to WIC 72a. Seven splices are 
indicated in FIG. 5 by Xs. 
FIG. 6 shows the couplers and splices of FIG. 5 held on a coupler mount 120 
by Sealastic adhesive. The WDC and the two WICs are indicated by the same 
numerals as in FIG. 5; and the seven splices are indicated by the numeral 
122. 
The optical structure just described depicts the arrangement that allows 
two test fibers to be addressed by simply energizing the appropriate 
laser. If any laser is fired, half of its power is transmitted to a fiber 
under test. Half of the power returned from the fiber under test is 
directed to the Wavelength Division Multiplexer, and its wavelength is 
passed with 99% efficiency to the detector of the instrument. 
The benefit of this arrangement is that it introduces almost no excess 
optical loss compared with a conventional OTDR, and two fibers can be 
examined without the need for an expensive optical switch. The extra 
directional coupler, laser, and WDM, that are required, are relatively 
inexpensive by comparison. A user can thus monitor the same number of 
fibers with half of the remote OTDRs that would otherwise be required. 
FIG. 7 shows a network having a central control station 150, eight repeater 
huts, and four remotely controlled OTDR units located in four repeater 
huts 152. Each OTDR monitors two fiber optic cables 154. 
A more ambitious arrangement is illustrated in FIGS. 8-10. In the 
description of FIG. 4, it was pointed out that the 50% output signals at 
port 56 of WIC 52 and at port 80 of WIC 72 are wasted. 
It is suggested that these ports can also be utilized, permitting one OTDR 
to monitor four separate fiber optic cables. FIG. 8 shows one-half of such 
a four cable OTDR system, in which each laser monitors two test fibers. 
Two of the four test fibers are indicated at 162 and 166. In order for the 
operator to distinguish between the two returning test signals from fibers 
162 and 166, and between the two test signals from the other laser (not 
shown), the technique used is a differentiation in the total fiber length 
of the two signals from the same WIC. With appropriate software, this 
length differential can be used to identify which test fiber has a fault, 
or break, and where the problem is located. 
In FIG. 8, the diagram shows a laser source 50b, similar to laser 50 in 
FIG. 4, which sends radiation through a WIC coupler 52b. Fifty percent of 
the laser energy exits port 54b of coupler 52b and travels into a fiber 
optic connector 160 and test fiber 162. At the same time, the other fifty 
percent of the laser energy exits port 56b of coupler 2b. and travels into 
a fiber optic connector 164 and test fiber 166. 
Returning reflected signals from both test fibers 162 and 166, which are in 
different fiber optic cables, are directed back through WIC coupler 52b, 
and thence along path 68 to a detector 170. 
FIGS. 9 and 10 are display graphs showing how the subsystem of FIG. 8 
permits fault location in two cables. As stated above, the two test fibers 
162 and 166 have different lengths (as do the cables in which they are 
respectively embedded). FIG. 9 is a simplified display graph of the 
reflected signals from test fibers 162 and 166. Assume that test fiber 162 
is the shorter fiber, and that test fiber 166 is the longer fiber. The 
signal from each fiber starts at a relatively high level at the left of 
the figure. In general, each test fiber signal has a gradual, straight 
line decline towards the right of the figure, except where splice losses, 
or the like, are indicated at points 180. The end of fiber 162 is 
indicated by the total loss of signal at point 182. The end of fiber 166 
is indicated by the total loss of signal at point 184. The sudden rises in 
signal at the end of each test fiber are a common result of the Fresnel 
effect. 
The normal signals of the two test fibers, as shown in FIG. 9, are summed 
and stored in computer memory. FIG. 10 is a display graph of the sum of 
the signals of the two test fibers 162 and 166; and it also illustrates 
the result of a fault in the longer test fiber 166. 
The summed signal in FIG. 10 shows the same normal drop locations 180 as in 
FIG. 9. At the end of the shorter test fiber 162, FIG. 10 shows about a 
fifty percent drop in signal at point 186, following a Fresnel effect 
rise. Normally, if no fault has developed, the signal would continue its 
gradual decline from point 186 to the end of the longer test fiber 166. 
However, FIG. 10 also illustrates one possible break location at point 188. 
Since the signal disappears at a point to the right of point 186, it is 
known that the fault is in the longer test fiber 166; and its distance 
from the OTDR is indicated by the total length of the signal graph. 
If the fault occurs to the left of point 186, the test fiber which is 
damaged is indicated by the total length of the signal graph. The fault 
will cause a drop at a new point in the display graph of the summed 
signals. The distance of the fault from the OTDR is indicated by the 
length of the signal graph between the OTDR and the new point of signal 
reduction. If there is some signal extending to, and ending at, the end of 
the shorter test fiber 162, it is assumed that the fault has occurred in 
the longer test fiber 166. On the other hand, if there is some signal 
extending to the end of the longer test fiber 166, it is assumed that the 
fault has occurred in the shorter test fiber 162. 
The technique just described does not permit fine measurements of fiber 
conditions. But it will respond to, and locate, a break in either test 
fiber. By adding two more test fibers, both tested by the second laser in 
FIG. 4, it is possible to locate faults or breaks in four separate fiber 
optics cables, using a single OTDR. 
The two test fibers shown in FIG. 8 are simultaneously checked by firing a 
single laser. If four test fibers are to be checked by the same OTDR, each 
laser in FIG. 4 will check two fibers. As previously stated, the two 
lasers in FIG. 4 are fired at different, non-overlapping, times, in order 
to avoid mixing of the detector signals. In a four test fiber system, the 
wavelength dependent coupler 64 of FIG. 4 receives the time-separated 
returning signals from each of the two wavelength independent couplers 52 
and 72, and sends these signals through to the same detector. 
From the foregoing description, it will be apparent that the apparatus and 
method disclosed in this application will provide the significant 
functional benefits summarized in the introductory portion of the 
specification. 
The following claims are intended not only to cover the specific 
embodiments disclosed, but also to cover the inventive concepts explained 
herein with the maximum breadth and comprehensiveness permitted by the 
prior art.