Path integrity proving in optical communications systems

A method of proving the integrity of optical paths in an optical communication system wherein the same diagnostic optical signal is transmitted along each path from one end, and each path is provided at its other end with a grating pattern, for example, for producing a pattern of time spaced echoes of the diagnostic signal unique to that path. In one particular embodiment the diagnostic signal comprises an amplitude modulated optical signal and the echoes are produced at a time spacing such that the modulation components of the echoes are in-phase at a different frequency for each path.

BACKGROUND OF THE INVENTION 
This invention relates to optical communication systems. 
More particularly the invention relates to methods and apparatus for 
proving the integrity of optical paths in an optical communication system. 
A known such method is to build into terminal equipment at the end of each 
path the ability to respond in predetermined manner to a diagnostic 
optical signal transmitted thereto from the other end of the path, 
detection of the response at the other end being taken as proof of the 
path integrity. One difficulty which arises with such a method is that it 
may not be possible to distinguish between a failure in the terminal 
equipment and an optical path failure. 
One solution to this difficulty is to provide a passive device in the form 
of an optical reflector means at the end of each path, each reflector 
means being tuned to a different optical frequency, detection of the 
reflection of a diagnostic optical signal of that frequency being taken as 
proof of the integrity of that path. The problem with this solution is 
that it uses too much bandwidth, especially because of temperature 
stability limitations. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a method and apparatus 
whereby this problem is overcome. 
According to the present invention there is provided a method for proving 
the integrity of optical paths in an optical communication system 
comprising: transmitting a diagnostic optical signal along each path from 
one end; producing at the other end of each said path a plurality of 
echoes of said diagnostic signal; and detecting at said one end said 
echoes of said diagnostic signal; the diagnostic signal in each path 
having the same optical frequency but the time spacing of the echoes in 
each path being different. 
According to a second aspect of the invention there is provided an 
apparatus for proving the integrity of optical paths in an optical 
communication system comprising: transmitting means for transmitting a 
diagnostic signal of a predetermined optical frequency along each said 
path from one end thereof; echo producing means for producing time-spaced 
echoes of the diagnostic signal at the other end of each path, the time 
spacing of the echoes produced in each path being different, and detection 
means for detecting said time-spaced echoes at said one end of each path. 
In one particular embodiment of the invention said diagnostic optical 
signal is an amplitude modulated optical signal and the time spacing of 
the echoes in each path is such that the modulation components of the 
echoes are in-phase at a different modulating frequency for each path. 
In one such particular embodiment the echoes are produced by a pair of 
strong reflector means spaced by a distance equal to half the wavelength 
of the modulating frequency, the reflector means nearer the source of the 
diagnostic signal being partly transmissive so that the pair of reflectors 
operate in the manner of a Fabry-Perot resonator tuned to the modulating 
frequency. In such an arrangement the reflector means are preferably tuned 
to the optical frequency of the diagnostic signal. 
In another such particular embodiment of the invention the echoes are 
produced by a closed optical loop of length equal to the wavelength of the 
modulating frequency or a multiple thereof. 
In another particular embodiment of the invention said diagnostic signal is 
in the form of a pulse at said optical frequency and said echoes 
constitute a train of echo pulses having a unique time spacing for each 
path. In such an arrangement the echoes are preferably formed by a series 
of weak reflector means positioned along the path at spacings 
corresponding to the time spacings of the pulses in the echo pulse train 
required to be produced in that path by the diagnostic signal.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, the system comprises a fibre optic network 1 which 
connects a main terminal 3 via the network to each of a number of 
satellite terminals 5. 
In order to prove the integrity of the paths between the main terminal 3 
and each of the terminals 5 the terminal 3 transmits an optical diagnostic 
signal. 
In the first method to be described the diagnostic signal comprises a 
signal of a predetermined optical frequency, e.g. of the order of 
2.times.10.sup.14 Hz, which is amplitude modulated at a frequency which is 
swept linearly through a band of frequencies, typically in the range from 
100 MHz to 1 GHz. 
Referring to FIG. 2, at the terminal end of each path there is provided a 
device for producing time spaced echoes in response to the diagnostic 
signal comprising a pair of spaced strong reflector means 7 and 9. Each of 
the reflector means 7, 9 suitably comprises an optical grating structure, 
as illustrated in FIG. 2, the spacing of the lines in each grating being 
half the wavelength of the predetermined optical frequency of the 
diagnostic signal so that the grating operates as a reflector tuned to 
that frequency. 
The two reflector means 7, 9 in each path are spaced from one another by a 
different distance for each path such that the double pass delay between 
the reflector means in each path has a different value equal to the period 
of a different frequency in the modulation frequency band. 
The sum of the reflections of the diagnostic signal produced by the 
reflector means 7, 9 in any particular path can be calculated using a 
Fabry-Perot type calculation, assuming the diagnostic signal source to be 
incoherent and 100% modulation depth, as follows: 
Taking the modulation envelope of the transmitted diagnostic signal at the 
reflector means 7 nearer the main terminal 3 to be given by Exp 
i(wt-kz)+1, where t is time, w is the angular frequency of the modulating 
signal and kz is a phase factor, the sum of the reflections is given by: 
##EQU1## 
where L is the double pass delay, R is the reflectivity of the reflector 
means 7, T(=1-R) is the transmissivity of the reflector means 7, and B is 
the reflectivity of the reflector means 9. 
The sum (1) can be split into a DC component given by 
EQU R+T.sup.2 B+RT.sup.2 B.sup.2 +R.sup.2 T.sup.2 B.sup.3 + (2) 
and an alternating component given by 
EQU Expi(wt-kz)!{R+T.sup.2 B Expi(-kL)!+RT.sup.2 B.sup.2 Expi(-2kL)!+R.sup.2 
T.sup.2 B.sup.3 Exp i(-3kL)!+ . . . } (3) 
which becomes 
EQU R-T.sup.2 /R+T.sup.2 /R{1/(1-{RB Expi(-kL)!})} (4) 
and which is equal to the DC component (2) when L=0 
FIGS. 3A and 38 illustrate the variation of the depth of modulation and the 
phase of the sum of the reflections with variation of modulating frequency 
for different values of the reflectivity R and a value of B of one. In the 
FIGS. 3A, 3B the depth of modulation curves are vertically spaced, for the 
sake of clarity but, in fact, all the curves peak at the same value (i.e. 
at a value at which the depth of modulation of the sum of the reflections 
is equal to the depth of modulation of the diagnostic signal) as shown for 
the curves for reflectivities of 0.8 and 0.65. 
FIG. 4 shows similar curves for reflectivities R of reflector means 7 of 
values 0.95, 0.90 and 0.80 and a reflectivity B of reflection means 9 of 
value 0.95, the peak value of these curves, of course, being less than in 
FIG. 3A. 
As can be seen from FIGS. 3 and 4, the depth of modulation of the sum of 
the reflections periodically peaks in each curve with increasing 
frequency, the peaks occurring at values of the modulating frequency for 
which the period of the modulating frequency is an integral multiple of L, 
i.e. at values of the modulating frequency for which the spacing of the 
reflector means 7 and 9 is equal to an integral multiple of half the 
wavelength of the modulating frequency. 
The sharpness of the modulation depth peaks decreases with decreasing 
reflectivity R whilst the amplitude of the peaks increases with decreasing 
reflectivity R and decreases with decreasing reflectivity B. With 
appropriate choice of R and B, up to about fifty different peaks may be 
distinguished, i.e. up to fifty different modulating frequencies may be 
identified. 
The integrity of any particular path in the system can thus be proved by 
determining whether the depth of modulation of the sum of the reflected 
signals produced at the main terminal 3 in response to transmission from 
terminal 3 of a diagnostic signal having a modulation frequency 
corresponding to the spacing of the reflector means 7 and 9 at the 
terminal end of that path exceeds a predetermined value. 
In practice, proving path integrity will involve detecting and analysing, 
at the main terminal 3, the reflection signals produced by sweeping the 
modulating frequency of the diagnostic signal through the above-mentioned 
modulating frequency band, and thereby determining those modulating 
frequencies present in the reflection signals for which the depth of 
modulation of the reflected signals exceeds the predetermined value. Each 
such modulating frequency corresponds to a different path of the system, 
and a determination that the depth of modulation of the reflected signals 
for a particular modulating frequency exceeds the predetermined value 
proves the integrity of the corresponding path. 
As shown in FIGS. 3 and 4, in addition to the depth of modulation of the 
sum of the reflections exhibiting peaks at particular modulating 
frequencies dependent on the spacing of reflection means 7 and 9, the rate 
of change of the phase of the sum of the reflections also exhibits peaks 
at particular modulating frequencies. Hence, in order to prove path 
integrities, instead of determining the modulating frequencies at which 
the depth of modulation of the reflection signals exceeds a predetermined 
value, the modulating frequencies at which the rate of change of phase of 
the reflection signals with change of modulating frequency exceeds a 
predetermined value may be determined. 
It will be appreciated that if the optical frequency chosen for the 
diagnostic signal is outside the range of optical frequencies to be 
received and/or transmitted by the terminal means 5, the reflector means 
7, 9 associated with a terminal 5 may be positioned directly in the optic 
fibre path connecting the terminal 5 into the system, as shown in FIG. 2. 
However, to avoid this restriction the reflector means 7, 9 may be coupled 
into the terminal path via a 3 db coupler 11 as illustrated in FIG. 5. 
Where it is desired that each terminal 5 is connected by separate receive 
and transmit fibres, the reflector means 7 and 9 may be connected in 
various configurations. One such configuration is shown in FIG. 6 where 
13, 15 and 17 are 3 db couplers. A second such configuration is shown in 
FIG. 7 where 19, 21 and 23 are 3 db couplers. 
In the above described embodiments, instead of using spaced reflector means 
7, 9 to produce the required time-spaced echoes of the diagnostic signal, 
the echoes may be produced by closed fibre loops of a length such as to 
produce the required delay L. FIG. 8 illustrates one such arrangement, 
functionally corresponding to the arrangement of FIG. 6, using a fibre 
loop 25 and four 3 db couplers 27, 29, 31 and 33, and FIG. 9 illustrates a 
second such arrangement, functionally corresponding to the arrangement of 
FIG. 7, using a fibre loop 35 and three 3 dbs couplers 37, 39 and 41. 
It will be appreciated that arrangements using closed optical fibre loops, 
as illustrated in FIGS. 8 and 9 are broadband devices in the sense that 
they operate over a wide band of optical frequency without modification. 
Referring now to FIG. 10, in the second embodiment to be described the 
diagnostic signal comprises a narrow pulse of predetermined optical 
frequency and the time spaced echoes are produced by a series of low 
reflectivity, e.g. 2%, reflector means 45, 47, 49, 51 and 53 disposed at 
spaced positions at the terminal end of the fibre path 55 whose integrity 
is to be proved, the reflector means spacings being different for each 
path to be proved. Each reflector means suitably comprises a grating. 
In response to each diagnostic signal pulse the reflector means 45 to 53 
produce a train of reflected pulses at time spacings corresponding to the 
physical spacings of the reflector means. It will be appreciated that this 
train is constituted by the first reflections at each reflector means, 
subsequent reflections being of relatively negligible amplitude. 
The integrity of any particular path 55 is proved by the detection at the 
main terminal 3 end of that path 55 of a train of pulses corresponding to 
the spacings of the reflector means 45 to 53 in that path 55. 
The spacings of the reflector means are suitably chosen so that the 
reflections of a diagnostic signal pulse from the reflector means 47 to 53 
are at time delays with respect to the reflection of that pulse from the 
reflector means 45 which are different integral multiples of the duration 
of the diagnostic signal pulse. The reflected signal for each path then 
exhibits a series of time slots of a duration equal to the duration of a 
diagnostic signal pulse, in each of which slots a reflected pulse is 
present or absent. Hence, the train of reflected pulses in each path can 
then be considered as a representation of a binary number where the 
presence of a reflected pulse represents 1 and the absence of a reflected 
pulse represents 0, or vice versa. 
In order to facilitate the separate detection of the different trains which 
will be produced in overlapping relationship in a multi-terminal system, 
the spacings of the reflector means are chosen in accordance with known 
techniques such as the code division multiple access (CDMA) technique.