Suppression of stimulated scattering in optical time domain reflectometry

Optical time domain reflectometry methods and apparatus are proposed in which the back-scattered optical radiation used to produce output signals is restricted to that resulting from Rayleigh scattering of light launched into a fiber 2 at a first wavelength and that in an anti-Stokes spectral band resulting from Raman or Brillouin scattering of optical radiation at the first wavelength. A first set of output signals produced in dependence upon the anti-Stokes back-scatter may be normalized to the geometric mean of a second set of output signals, produced in dependence upon the Rayleigh back-scatter at the first wavelength, and a third set of output signals, produced in dependence upon Rayleigh back-scatter resulting from light launched into the fiber at the anti-Stokes wavelength. Growth in the intensity of optical radiation in a first Stokes (Raman or Brillouin) spectral band may be inhibited, so as to suppress stimulated scattering, for example by making the fiber 2 high loss at the Stokes wavelength and/or launching into the fiber 2, simultaneously with the pulse at the first wavelength, an additional pulse at a third wavelength equal to that of a Stokes spectral line resulting from inelastic scattering of optical radiation at the first Stokes wavelength. The first wavelength may be chosen so as to minimize the transmission loss, in view of the average losses at the first and detected wavelengths or the increase with wavelength of the launch power which can be used before stimulated scattering has a specified effect.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to optical time domain reflectometry (OTDR) 
methods and apparatus for carrying out such methods. 
2. Description of the Prior Art 
In OTDR light at a first wavelength (.lambda..sub.0) is launched into one 
end of an optical fibre and optical radiation back-scattered along the 
fibre is measured. In distributed sensing using OTDR the back-scattered 
radiation is used to measure respective values of a physical parameter at 
different locations along the fibre, which is deployed in a region of 
interest. In optical time domain reflectometers, which are used for 
characterising fibres in their production environments or in installed 
cables, the back-scattered optical radiation is used, for example, to 
locate faults in the fibre or to measure the attenuation characteristics 
of the fibre. 
The back-scattered signals may result from either elastic or inelastic 
scattering processes. Rayleigh scattering produces elastically scattered 
signals with a wavelength distribution substantially the same as the 
injected signal (.lambda..sub.0). Brillouin and Raman scattering on the 
other hand are inelastic scattering processes which each produce pairs of 
spectral bands. Each pair of first order bands comprises one (the Stokes 
band) centred on a longer wavelength (.lambda..sub.+1) than that of the 
injected signal (.lambda..sub.0) and the other (the anti-Stokes band) 
centred on a shorter wavelength (.lambda..sub.-1) than that of the 
injected signal, such that the pair is centred on the injected signal 
wavelength. The spectrum would normally contain several successive orders 
(at wavelengths .lambda..sub..+-.n, where n=1, 2, 3 . . . ) resulting from 
a particular scattering process, the intensity of the bands decreasing as 
the order increases. In some silica-based materials the Raman spectrum 
contains more than one band of significant intensity, for example, in a 
binary P.sub.2 O.sub.5 .multidot.SiO.sub.2 glass, P.sub.2 O.sub.5 has a 
band around 1390 cm.sup..sup.-1 in addition to the main silica band at 
around 440 cm.sup.-1. 
For an injected signal of 904 nm in silica, Brillouin anti-Strokes and 
Brillouin Strokes back-scattered signals are shifted by about 0.05 nm from 
the injected signal, and the first order Raman Stokes and Raman 
anti-Stokes back-scattered signals are shifted by about 34 nm. The 
wavelength shifts for the Brillouin and Raman scattered signals are 
respectively about 0.058 nm and 50 nm for a 1.06 .mu.m signal, and about 
0.084 nm and 100 nm for a 1.53 .mu.m injected signal in silica. 
The Brillouin and Raman back-scattered signals have intensities dependent 
on physical parameters, such as temperature. Typically for silica fibres 
at room temperature, the Raman Stokes and anti-Stokes signals are less 
intense than those of the Brillouin back-scattered signals, the Raman 
signals having first order intensities which are lower than the Rayleigh 
backscatter signal at 1.064 .mu.m by about 18 dB and 28 dB respectively, 
compared to the Brillouin signals which are about 13 to 16 dB lower than 
the Rayleigh backscatter signal. With 7 ns, 50W pulses at 1.06 .mu.m in an 
industry-standard multi .mu.mode fibre (50 .mu.m diameter core, 125 .mu.m 
cladding, graded-index core and a numerical aperture of 0.20), the power 
of Raman anti-Stokes wavelength light resulting from back-scattering near 
the receiving/injecting end of the fibre is about 50 nW. 
In a known OTDR method of distributed sensing, such as that described in 
U.S. Pat. No. 4,823,166, a 1.5W modulated optical signal of wavelength 854 
nm at 4 kHz and having a pulse width of 40 ns is injected into one end of 
an optical fibre of more than 1 km in length. A back-scattered signal is 
returned to the first end and comprises the aforementioned elastically and 
inelastically back-scattered signals which are then filtered to remove 
substantially all but the Raman anti-Stokes signal which passes to 
detecting means for measurement of its intensity, referenced to the total 
back-scatter signal. From the change in intensity with the elapsed time 
from the injected signal, the distribution of a particular physical 
parameter such as the temperature along the fibre may be deduced. 
In an earlier known OTDR method, such as that described in GB-2140554, 
pulsed light is launched into one end of an optical fibre and 
back-scattered Raman Strokes and anti-Stokes signals are separated and 
measured. Ratios of the measurements are then obtained from which a 
temperature distribution for the fibre is derived. 
In a further known OTDR method such as that described in U.S. Pat. No. 
5,217,306, optical signals of wavelength 1.32 .mu.m from a source 
comprising a diode-pumped solid state laser are sent through a length of 
optical fibre with enhanced Raman scattering properties, an attenuator and 
an optical filter to emit therefrom a test signal of wavelength 1.40 .mu.m 
for injection into a sensing optical fibre for measuring temperature 
therealong. The conversion of the wavelength between 1.32 .mu.m and 1.40 
.mu.m is achieved by Stimulated Raman Scattering (SRS) of the first 
wavelength to produce the second, longer wavelength. Raman anti-Stokes and 
Raman Stokes signals of respective wavelengths 1.32 .mu.m and 1.50 .mu.m 
which are subsequently back-scattered from positions along the sensing 
optical fibre are then detected and processed in the same way as the first 
mentioned OTDR method. 
The range of test signals in optical fibres is limited by dispersion and 
attenuation. For a given fibre, therefore, the test signal is desirably 
selected to be at a wavelength corresponding to a minimum in the 
attenuation/dispersion characteristics of the fibre. For a fibre material 
such as GeO.sub.2 doped silica, a dispersion minimum for the material 
itself occurs at a wavelength of 1.3 .mu.m, and an attenuation coefficient 
minimum of about 0.2 dB/km at a wavelength .lambda.=1.55 .mu.m. 
An OTDR distributed sensing system suitable for long range sensing, which 
uses injected wavelengths in the range 1.51 .mu.m to 1.59 .mu.m, is 
described in the applicant's British patent application no. 9307660.2 
filed on Apr. 14, 1993, the disclosure of which is hereby incorporated by 
reference. 
Long-range sensing requires high power sources. However, at high power, 
non-linear optical effects appear. A particular problem is stimulated 
Raman scattering which converts the wavelength launched into the fibre to 
the first order Stokes wavelength, mainly in the forward direction. As 
shown in FIGS. 1(A) and 1(B) of the accompanying drawings, which are 
graphs illustrating the variation in the intensity of optical radiation in 
an industry-standard single mode fibre (index difference 0.35%, cut-off 
wavelength 1200 nm) at a test wavelength .lambda..sub.0 (dashed line 200) 
of 1530 nm and the first Stokes Raman wavelength .lambda..sub.+1 (solid 
line 201) with distance along the fibre for launch powers of 1W and 3W 
respectively, the stimulated emission grows along the fibre, which for 
long-range sensing could be many kilometers in length, until eventually 
substantially all the light launched into the fibre is converted to the 
Stokes wavelength. The values given in FIGS. 1(A) and 1(B) are for a 
typical fibre and are dependent on the design of fibre and test wavelength 
used. The rate of growth is proportional to the intensity (power/area) of 
the light launched into the fibre and to parameters of the glass, and is 
therefore inversely proportional to the area over which the power is 
confined. 
Owing to stimulated Raman scattering, in distributed sensors operating at 
sufficiently high power levels (e.g. for long-range applications), the 
signal at the Stokes wavelength becomes significantly greater than it 
would be without non-linear effects and this distorts the measurements 
where the Stokes signal is used as a reference. In addition, because power 
is lost from the light launched into the fibre through conversion to the 
Stokes wavelength, the signal at the anti-Stokes wavelength is 
correspondingly weakened. Furthermore, because of the significant build-up 
of the Stokes power in the forward direction, the connectors in the fibre 
reflect a large amount of this power, which the filters in the receiver 
transmit. The strong signal reflected from the connectors can therefore 
distort the preampllfier output over subsequent fibre sections. FIG. 2 of 
the accompanying drawings, which shows the back-scattered signals 
(normalised to unity at Okm) at the anti-Stokes (chain line 202) and 
Stokes (dashed line 203) wavelengths over a distance of 50,000 meters and 
the ratio of those two signals (solid lane 204) for an injected power of 
3W at 1530 nm in a typical single-mode fibre, illustrates how the Stokes 
signal increases with distance along the fibre, eventually overcoming the 
normal effects of fibre attenuation, and thus distorts the 
anti-Stokes/Stokes ratio. FIG. 3 of the accompanying drawings compares the 
total forward travelling power at the Stokes wavelength (dashed line 205) 
when Raman gain is taken into account with that which has a purely linear 
(spontaneous) origin (solid line 206) in a typical single-mode fibre for 
an injected pulse of 1.0W at 1530 nm. 
By way of example, for a non-dispersion-shifted single mode fibre having a 
core refractive index of 1.45 and a numerical aperture of 0.1, using an 
injected signal of 1.53 .mu.m at which it has an attenuation of 0.197 
dB/km (0.292 dB/km at the anti-Stroke Raman wavelength of 1.43 .mu.m and 
0.311 dB/km at the Stokes wavelength of 1.64 .mu.m), a 2% departure from 
the value for linear operation, for example, in the ratio of the 
anti-Stokes to Stokes signal gives an error of roughly 3.degree. C. in 
distributed temperature sensing applications. For such a fibre 50,000 
meters in length, using an injected signal of wavelength 1.53 .mu.m, the 
non-linearity error begins to exceed 2% at a launch power between 0.9W and 
1.0W, the 2% error being exceeded at a distance of 39,650 m at a power of 
1W and at 12,050 m at a power of 2W. Although these values are very 
sensitive to a number of factors, including very small changes in fibre 
loss, they serve to illustrate the effect of stimulated Raman scattering 
on measurements taken using distributed sensing. 
The early onset of stimulated Raman scattering can also be a problem in 
optical time domain reflectometers in which it is especially important to 
ensure that the optical fibre is operated in the linear regime, since the 
referencing used in distributed sensing is not available. 
Thus, to avoid the onset of stimulated Raman scattering, heretofore the 
power of the light launched into the fibre has been restricted, with the 
result that the range over which measurements can be taken is limited. 
Similar problems arise owing to stimulated Brillouin scattering. 
BRIEF SUMMARY OF THE INVENTION 
According to a first aspect of the present invention, there is provided an 
optical time domain reflectometry method of sensing respective values of a 
physical parameter at different locations along an optical fibre, deployed 
through a region of interest, in which method optical radiation at a first 
wavelength is launched into the fibre and back-scattered optical radiation 
in at least one preselected spectral band used to produce output signals 
dependent upon the values being sensed, wherein none of the spectral bands 
used to produce said output signals lies about a second wavelength which 
is equal to the wavelength on which a first order Stokes spectral band, 
which results from inelastic scattering in the fibre of optical radiation 
at said first wavelength, is centred, and relative growth in the intensity 
in the fibre of optical radiation at said second wavelength is inhibited, 
thereby to suppress conversion in the fibre through stimulated scattering 
of optical radiation at said first wavelength to optical radiation at said 
second wavelength. 
Since measurements are not made on the basis of optical radiation at the 
Stokes (second) wavelength, the system can be designed so as to be high 
loss at that wavelength, thereby reducing the intensity of light at this 
wavelength in the fibre and delaying the onset of stimulated scattering. 
In addition, the wavelength range over which the fibre must be low loss is 
narrower. Furthermore, use of the Rayleigh scattered signal (at the first 
wavelength) as a reference, rather than the Stokes signal, advantageous in 
that it is a much stronger signal, allowing reductions in the measurement 
time required and improvements in measurement accuracy. 
Such a method can be carried out using optical time domain reflectometry 
apparatus for sensing respective values of a physical parameter at 
different locations through a region of interest which embodies a second 
aspect of the present invention, which apparatus comprises an optical 
fibre adapted to be deployed through said region of interest, source means 
for launching optical radiation at a first wavelength into the fibre, and 
detection means for receiving optical radiation back-scattered along the 
fibre from which respective values of the physical parameter can be 
derived, wherein filtering means are provided for restricting the 
back-scattered radiation used to derive said values to one or more 
preselected spectral bands, none of which lies about a second wavelength 
which is equal to the wavelength on which a first order stokes spectral 
band, which results from inelastic scattering in the fibre of optical 
radiation at said first wavelength, is centred, and the said source means 
and/or the said optical fibre are selected and arranged so as to inhibit 
growth in the intensity in the fibre of optical radiation at said second 
wavelength, thereby to suppress conversion in the fibre through stimulated 
scattering of optical radiation at said first wavelength to optical 
radiation at said second wavelength. 
Preferably, the one of the preselected spectral bands is an anti-Stokes 
band resulting from Raman or Brillouin scattering in said fibre and/or a 
spectral band which lies about said first wavelength. 
The physical parameter to be measured may be, but is not necessarily, 
temperature. 
Inhibition of the growth in intensity of optical radiation at said second 
wavelength may be achieved by selecting said first wavelength in 
accordance with a predetermined attenuation characteristic of the fibre 
such that said first wavelength and/or the one or more preselected 
spectral bands lie adjacent to a local attenuation coefficient minimum but 
said second wavelength is displaced from said local attenuation 
coefficient minimum, optical radiation at said second wavelength thereby 
being attenuated to a significantly greater extent than optical radiation 
at said first wavelength or in the or each spectral band. 
This may be carried out by arranging that the said second wavelength 
coincides with an infra-red absorption maximum in the attenuation 
characteristic of the fibre or lies substantially on the short-wavelength 
edge of such an absorption maximum, or with a local absorption maximum in 
that characteristic corresponding to naturally-occurring impurities in the 
fibre, such as hydroxide ions, or doped impurities in the fibre, such as 
rare earth metal ions. In the case of hydroxide ions, the first wavelength 
could conveniently be 1320 nm from a Nd:YAG source. 
Alternatively, or additionally, the optical fibre may be provided with 
built-in gratings for reflecting optical radiation at said second 
wavelength, and/or at least a section of the fibre may be bent so as to 
induce bending loss in the fibre at said second wavelength, and/or the 
waveguide structure of the fibre may be designed such that loss is high at 
the second wavelength. 
A preferred approach, which may be used instead of or in addition to those 
described above, is to launch optical radiation at a third wavelength, 
equal to the wavelength on which a first order Stokes spectral band, which 
results from inelastic scattering in the fibre of optical radiation at the 
said second wavelength, is centred, into the fibre simultaneously with 
said optical radiation at said first wavelength, thereby to cause optical 
radiation at said second wavelength to be converted to optical radiation 
at said third wavelength through stimulated scattering, so as to achieve 
the desired inhibition in growth of optical radiation at said second 
wavelength. 
According to a third aspect of the present invention, there is provided an 
optical time domain reflectometry method of characterising an optical 
fibre, in which optical radiation at a preset first wavelength is launched 
into the fibre and optical radiation back-scattered along the fibre is 
used to measure a selected characteristic of the fibre, wherein, so as to 
inhibit growth in the intensity in the fibre of optical radiation at a 
second wavelength, equal to the wavelength on which a first order Stokes 
spectral band, which results from inelastic scattering in the fibre of 
optical radiation at said preset first wavelength, is centred, optical 
radiation at a third wavelength is launched into the fibre simultaneously 
with optical radiation at said preset first wavelength, said third 
wavelength being selected so as to be equal to the wavelength on which a 
first order Stokes spectral band, which results from inelastic scattering 
in the fibre of optical radiation at said second wavelength, is centred, 
thereby causing optical radiation at said second wavelength to be 
converted to optical radiation at said third wavelength and so suppressing 
conversion in the fibre of optical radiation at said preset first 
wavelength to optical radiation at said second wavelength through 
stimulated scattering. 
The use of optical radiation at such a third wavelength is particularly 
useful where other techniques cannot be used, for example in 
characterising fibres using an optical time domain reflectometer where it 
is not possible to employ any technique which requires the use of anything 
other than a standard fibre, specified test wavelength or preset fibre 
deployment state. 
A method embodying the third aspect of the present invention may be carried 
out using optical time domain reflectometry apparatus for characterising 
an optical fibre, which embodies a fourth aspect of the present invention 
and comprises source means operable to launch optical radiation at a 
preset first wavelength into the fibre, and detection means operable to 
detect optical radiation back-scattered along the fibre which is used to 
measure a selected characteristic of the fibre, wherein, so as to inhibit 
growth in the intensity in the fibre of optical radiation at a second 
wavelength, equal to the wavelength on which a first order Stokes spectral 
band, which results from inelastic scattering in the fibre of optical 
radiation at said preset first wavelength, is centred, the source means 
are also operable to launch optical radiation at a third wavelength into 
the fibre simultaneously with optical radiation at said preset first 
wavelength, said third wavelength being equal to the wavelength on which a 
first order Stokes spectral band which results from inelastic scattering 
in the fibre of optical radiation at said second wavelength is centred, 
thereby causing optical radiation at said second wavelength to be 
converted to optical radiation at said third wavelength and so suppressing 
conversion in the fibre of optical radiation at said preset first 
wavelength to optical radiation at said second wavelength through 
stimulated scattering. 
In a method embodying either the first or the third aspect of the present 
invention in which light at a third wavelength is launched into the fibre 
with light at the first wavelength, to inhibit growth in intensity in the 
fibre of optical radiation at a fourth wavelength, equal to the wavelength 
on which a first order Stokes spectral band, which results from the 
inelastic scattering in the fibre of optical radiation at the said third 
wavelength, is centred, optical radiation at a fifth wavelength, equal to 
the wavelength on which a first order Stokes spectral band, which results 
from inelastic scattering in the fibre of optical radiation at the said 
fourth wavelength, is centred, may be launched into the fibre 
simultaneously with the optical radiation at said third wavelength, 
thereby to cause optical radiation at said fourth wavelength to be 
converted to optical radiation at said fifth wavelength through stimulated 
scattering. This technique increases the maximum power at which radiation 
may be launched into the fibre before the onset of problems resulting from 
stimulated scattering. 
According to a fifth aspect of the present invention there is provided an 
optical time domain reflectometry method of sensing respective values of a 
physical parameter at different locations along an optical fibre, deployed 
through a region of interest, in which method optical radiation at a 
preselected wavelength is launched into the fibre and back-scattered 
optical radiation in first and second preselected spectral bands, centred 
on respective first and second wavelengths, is used to produce output 
signals dependent upon the values being sensed, wherein said preselected 
wavelength is selected in accordance with a predetermined attenuation 
characteristic of said fibre such that an average attenuation value, 
determined by taking the average of the respective attenuation losses at 
said first and second wavelengths, is substantially minimised. In this way 
an increase in sensing range can be obtained. 
A method embodying the fifth aspect of the present invention may be carried 
out using optical time domain reflectometry apparatus for sensing 
respective values of a physical parameter at different locations through a 
region of interest which embodies a sixth aspect of the present invention, 
which apparatus comprises an optical fibre adapted to be deployed through 
said region of interest, source means for launching optical radiation at a 
preselected wavelength into the fibre, detection means for receiving 
optical radiation back-scattered along the fibre from which respective 
values of the physical parameter can be derived, and filtering means for 
restricting the back-scattered radiation used to derive said values to 
first and second preselected spectral bands, centred on respective first 
and second wavelengths, wherein said preselected wavelength is such that 
an average attenuation value, determined by taking the average of the 
respective attenuation losses at said first and second wavelengths, is 
substantially minimised. 
An increase in sensing range can also be obtained using a method embodying 
a seventh aspect of the present invention, according to which there is 
provided an optical time domain reflectometry method for sensing 
respective values of a physical parameter at different locations along an 
optical fibre, deployed through a region of interest, in which method 
optical radiation at a first preselected wavelength is launched into the 
fibre and back-scattered optical radiation in at least one preselected 
spectral band is used to produce output signals dependent upon the values 
being sensed, wherein none of the spectral bands used to produce said 
output signals lies about a second wavelength which is equal to the 
wavelength on which a first order Stokes spectral band, which results from 
inelastic scattering in the fibre of optical radiation at said first 
preselected wavelength, is centred and said first preselected wavelength 
is selected so as to substantially minimise, for a preset length of said 
fibre, the value of a power loss variable for the fibre concerned, which 
variable is determined by subtracting from a first wavelength-dependent 
function, giving the total power loss along the preset length of fibre of 
forward- and backward-travelling optical radiation, a second 
wavelength-dependent function giving the maximum forward-travelling power 
which can be launched into said fibre such that power loss due to 
stimulated scattering in the fibre does not exceed a predetermined value, 
both rune%ions being expressed in logarithmic units. 
Such a method may be carried out using optical time domain reflectometry 
apparatus for sensing respective values of a physical parameter at 
different locations through a region of interest which embodies an eighth 
aspect of the present invention, which apparatus comprises an optical 
fibre adapted to be deployed through said region of interest, source means 
for launching optical radiation at a preselected wavelength into the 
fibre, detection means for receiving optical radiation back-scattered 
along the fibre from which respective values of the physical parameter can 
be derived, and filtering means for restricting the back-scattered 
radiation used to derive said values to one or more preselected spectral 
bands, wherein none of the spectral bands used to produce output signals 
lies about a second wavelength which is equal to the wavelength on which a 
first order Stokes spectral band, which results from inelastic scattering 
in the fibre of optical radiation at said first preselected wavelength, is 
centred and said preselected wavelength is such that, for a preset length 
of said fibre, the value of a power loss variable for the fibre concerned 
is substantially minimised, which variable is determined by subtracting 
from a first wavelength-dependent function, giving the total power loss 
along the said preset length of fibre of forward- and backward-travelling 
optical radiation, a second wavelength-dependent function giving the 
maximum forward-travelling power which can be launched into said fibre 
such that power loss due to stimulated scattering in the fibre does not 
exceed a predetermined value, both functions being expressed in 
logarithmic units. 
According to a ninth aspect of the present invention there is provided an 
optical time domain reflectometry method of sensing respective values of a 
physical parameter at different locations along an optical fibre, deployed 
through a region of interest, in which method optical radiation at a first 
wavelength is launched into the fibre and back-scattered optical radiation 
in first and second spectral bands, centred respectively on said first 
wavelength and a second wavelength equal to the wavelength of an 
anti-Stokes spectral band which results from inelastic scattering in the 
fibre of optical radiation at said first wavelength, is used to produce 
respective first and second sets of output signals, wherein, 
non-simultaneously with optical radiation at said first wavelength, 
optical radiation substantially at said second wavelength is launched into 
the fibre and back-scattered optical radiation in a third spectral band, 
centred on said second wavelength, is used to produce a third set of 
output signals, and a final set of output signals, dependent upon the 
values being sensed, is produced by normalising the first set of output 
signals to the geometric mean of the second and third sets of output 
signals. 
This technique increases the accuracy with which losses may be calibrated 
and also substantially removes the effects of changes in the fibre 
properties along its length. 
Such a method may be carried out using optical time domain reflectometry 
apparatus for sensing respective values of a physical parameter at 
different locations through a region of interest which embodies a tenth 
aspect of the present invention, which apparatus comprises an optical 
fibre adapted to be deployed through said region of interest, source means 
operable selectively to launch optical radiation at a first wavelength 
into the fibre, detection means for receiving optical radiation 
back-scattered along the fibre, and signal processing means for using such 
back-scattered optical radiation in first and second spectral bands, 
centred respectively on said first wavelength and a second wavelength 
equal to the wavelength of an anti-Stokes spectral band which results from 
an inelastic scattering in the fibre of optical radiation at said first 
wavelength, to produce respective first and second sets of output signals, 
wherein said source means are also selectively operable to launch optical 
radiation substantially at said second wavelength into the fibre, 
non-simultaneously with optical radiation at said first wavelength, and 
said signal processing means are operable to use resulting back-scattered 
optical radiation in a third spectral band, centred on said second 
wavelength, to produce a third set of output signals, and are further 
operable to produce a final set of output signals, dependent upon the 
values being sensed, by normalising the first set of output signals to the 
geometric mean of the second and third sets of output signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The apparatus of FIG. 4A is suitable for carrying out the method embodying 
the first aspect of the present invention and comprises an optical source 
1 arranged for launching pulses of optical radiation at a first wavelength 
.lambda..sub.0 into one end A, towards the other end B, of any one of a 
number of selected sensing fibres 2 deployed in a region of interest, via 
a first directional coupler 4a and appropriate launching optics 3. 
A proportion of the injected radiation is back-scattered along the fibre 
and is guided back towards the launching end A. Typically, for 80 ns, 1W 
pulses at a wavelength of 1550 nm injected into a single mode fibre having 
a numerical aperture 0.11, the back-scattered anti-Stokes wavelength 
signal has a power of about 600 pW. The back-scattered light is directed, 
via a second directional coupler 4b, to optical filtering and detecting 
means 6. In this embodiment optical radiation in two preselected spectral 
bands, one of which is that at .lambda..sub.0 due to Rayleigh scattering 
and the other, at .lambda..sub.-1, is the first order anti-Stokes Raman 
band, are measured. Accordingly, the optical filtering and detecting means 
6 conveniently comprise two filters 5a, 5b, and two detectors 7a, 7b, 
dedicated to respective wavelengths .lambda..sub.-1, .lambda..sub.0. 
Instead of two separate filters 5a, 5b, a moving filter arrangement may be 
provided, but the desired filtering may also be achieved using one or more 
of any device which selectively reflects, absorbs, scatters, deflects, 
polarises or otherwise passes light in one or more preselected spectral 
bands, including a small proportion (1-20%) of the Rayleigh back-scatter, 
but not the remainder of the spectrum. Furthermore, the filtering 
device(s) may be combined with the couplers 4a, 4b. 
Although use of the Rayleigh and anti-Stokes Raman spectral bands is 
preferred, any suitable spectral bands may be used excluding those at a 
second wavelength .lambda..sub.+1 equal to the longer of two wavelengths 
of optical radiation which result from inelastic scattering (Raman and/or 
Brillouin) in the fibre of light at wavelength .lambda..sub.0. 
Accordingly, since measurements are not made on the basis of optical 
radiation at .lambda..sub.+1, the system can be designed so as to be high 
loss at .lambda..sub.+1, thereby reducing the intensity of light at this 
wavelength in the fibre and delaying the onset of stimulated scattering. 
In addition, the wavelength range over which the fibre must be low loss is 
narrower. In this respect, for a .lambda..sub.0 of between 1550 to 1650 
nm, the separation between each of the wavelengths .lambda..sub.0, 
.lambda..sub.-1, .lambda..sub.+1 is of the order of 100 nm. The fibre loss 
is not uniform over this wavelength range, or over the intensity range of 
the signals, and the overall system losses are thus higher than if one 
were operating closer to the centre of the transmission window. Over 
several tens of kilometers even small differences (for example, 0.1 dB/km) 
can have a profound effect on system performance. 
Use of the Rayleigh scattered signal as a reference, rather than the Stokes 
signal, is also advantageous in that it is a much stronger signal. Thus, 
the measurement time required to acquire the reference can be reduced, and 
the sensitivity of the ratio of anti-Stokes to Rayleigh signals is 
enhanced. Furthermore, reflections from connectors, also strong in 
Rayleigh back-scatter, can be dealt with in the preamplifier design, by 
sacrificing sensitivity (noise performance) for greater dynamic range. 
The directional couplers 4a, 4b could be all optical fibre dichroic beam 
splitters (also known as wavelength division multiplexers) which separate 
the forward and backward travelling waves and the wavelengths of 
predetermined values, or devices serving a similar purpose formed of bulk 
optics, integrated optics, or a mixture of bulk optics and fibre optics. 
For use with a single mode fibre 2, as shown in FIG. 4(A), respective fused 
taper couplers 4a, 4b may conveniently be used. The transmission 
characteristic of the first directional coupler 4a is shown in FIG. 4(B). 
The coupling coefficient of this device 4a may be designed to vary 
sinusoidally with wavelength in the wavelength range of interest, so as to 
transmit a high fraction of the light entering port 4 to port 2, provided 
this light is at the test wavelength .lambda..sub.0 (or higher, even order 
wavelengths .lambda..sub..+-.2, . . . )., a small but finite proportion (1 
to 20%) of the return signal at .lambda..sub.0 between ports 2 and 1, and 
a high fraction of the light at wavelength .lambda..sub.-1 entering port 2 
to port 1. 
Thus, in practice, .lambda..sub.0 should pass from ports 4 to 2 with 
minimum loss and .lambda..sub.-1 should pass from ports 2 to 1 with 
minimum loss. This is possible because the wavelengths are different. 
Since .lambda..sub.0 should also pass from ports 2 to 1, some limitations 
should be imposed on the transmission at the same wavelength from ports 4 
to 2, but in practice, because the Rayleigh scatter is so much more 
intense than the Raman scatter, the dilemma can be resolved by sacrificing 
the transmission efficiency for the Rayleigh scattering (i.e. from 2 to 1 
at .lambda..sub.0). 
The transmission characteristic of the second directional coupler 4b is 
such that a high fraction of the light at .lambda..sub.-1 entering port 3 
is transmitted to port 4 and a high fraction of the light at 
.lambda..sub.0 entering port 3 is transmitted to port 1, but that little 
light at .lambda..sub.-1 is transmitted from port 3 to port 1 and little 
light at .lambda..sub.-0 is transmitted from port 3 to port 4. 
The detectors 7a, 7b are followed by respective low noise preamplifiers 8 
(8a and 8b) (and possibly further stages of amplification and electrical 
filtering, not shown). The electrical signals thus produced are converted 
into digital signals and processed by a processor 9 which produces 
therefrom a set of readings, in this embodiment based on a ratio of the 
anti-Stokes and Rayleigh signals, representative of a distribution along 
the fibre 2 from one end A to its other end B. FIGS. 5(A) and 5(B) show 
such a distribution of temperature, when the measurements have been 
calibrated and adjusted for non-temperature dependent variations in 
attenuation. This process is preferably repeated and averaged over many 
returned pulses to calculate to a sufficient accuracy the temperature 
distribution along the fibre. The processor 9 may also control, for 
example, the source 1, the selected fibre 2 or the filter 5. Ideally, for 
each laser pulse, samples from every resolvable point along the fibre are 
acquired; that is, the whole fibre is measured quasi-simultaneously. This 
is less time-consuming than the alternative of sampling at a single point 
in the fibre until an adequate signal-to-noise ratio is achieved and then 
moving the sampling point along the fibre. The results may be further 
improved by performing the measurement from each end of the fibre to 
separate variations in the signal caused by temperature fluctuations from 
those from fibre loss, and for this reason some of the fibres 2 shown in 
FIG. 4(A) are looped back on themselves (but obviously this need not be 
so). By calculating the geometric mean of the back-scattered signals 
measured from both ends of the fibre returning from a particular location, 
the effects of any propagation losses can be eliminated, leaving only the 
effects of changes in the back-scatter of the injected signal, i.e. 
changes of numerical aperture or the scattering coefficient in the 
spectral band of interest. 
A preferred method of determining and therefore reducing the effects of 
propagation losses in the fibre on the results, whilst also collecting 
data relevant to the measured sensed, is to make two sets of measurements, 
one at the test wavelength .lambda..sub.0 in which the spectral bands at 
.lambda..sub.0 and .lambda..sub.-1 are measured, and the other at 
wavelength .lambda..sub.-1 in which the spectral band at .lambda..sub.-1 
is measured. 
In particular, the sensing (for example, of temperature) is carried out by 
launching light at wavelength .lambda..sub.0 and detecting the 
corresponding anti-Stokes radiation at wavelength .lambda..sub.-1. In 
order to characterise the fibre at the time of the measurement two further 
measurements are carried out at the relevant wavelengths, namely 
.lambda..sub.0 and .lambda..sub.-1 '. The wavelength .lambda..sub.-1 ' 
coincides with the anti-Stokes wavelength of .lambda..sub.-1, but is 
denoted .lambda..sub.- ' to allow for processing corrections to be made if 
errors occur in the wavelength selection process. In most practical 
circumstances, however, these errors are negligible. The first of these 
measurements can be obtained by also measuring the Rayleigh scattering 
when light at .lambda..sub.0 is launched into the fibre for the sensing 
measurement (either simultaneously or subsequently) and in the second case 
by launching at .lambda..sub.-1 ' and measuring the Rayleigh scattering. 
These two measurements, at .lambda..sub.0 and .lambda..sub.-1 ', not only 
calibrate the losses in situ as accurately as possible, but also remove, 
to a substantial degree, the effect of changes of fibre properties (e.g. 
numerical aperture or core diameter) along the fibre. The sensitivity to 
these effects is reduced by a factor of around 20, primarily because they 
affect all three measurements in a similar way. For example an increase in 
the fibre numerical aperture along its length (e.g., resulting from 
process tolerances) would lead to an increase in the capture efficiency of 
the scattered light, i.e. to an increase in the back-scatter signal which 
would be interpreted as an increase in temperature if only the 
.lambda..sub.0 to .lambda..sub.-1 scattering process were considered. To 
compute the sensed parameter, a waveform obtained by normalising the 
.lambda..sub.0 to .lambda..sub.-1 scattering measurement to the geometric 
mean of the two reference measurements, namely the .lambda..sub.0 to 
.lambda..sub.0 and .lambda..sub.-1 ' to .lambda..sub.-1 ' back-scatter 
measurements, may be used. These suffer almost identical distortions from 
changes in fibre properties, and thus largely eliminate this type of 
error. For example, if at a point along the fibre the numerical aperture 
were to vary by 10%, this would without normalization result in a change 
in back-scatter level of 22%, which in turn would result (in temperature 
sensing) in an error of about 28.degree. C.; however, using the proposed 
signal processing method, such an error would be reduced to less than 
1.degree. C. 
Although two separate sources may be used to provide light at the 
wavelengths .lambda..sub.0, .lambda..sub.-1 ' respectively, it is 
convenient to use a single laser source at .lambda..sub.-1 ' and to 
generate .lambda..sub.0 by stimulated conversion in a fibre associated 
with the source, as shown in FIG. 6. 
In the arrangement of FIG. 6, light at wavelength .lambda..sub.-1 ' 
generated by a source 1 is launched into port 1 of a first coupler D 
operable to transmit a small proportion (approximately 10%) of the light 
at .lambda..sub.-1 ' to its port 2, to which there is connected one end of 
a delay line 21, and the remainder of the light (about 90%) to its port 3, 
which is connected to one end of a length of a fibre 22 having a high 
Raman gain. The Raman shifting fibre 22 converts most of the light at 
.lambda..sub.-1 ' to the first Raman Stokes wavelength, i.e. to light at 
.lambda..sub.0, through stimulated Raman scattering, but no significant 
proportion to the second Raman Stokes wavelength. The other end of the 
Raman shifting fibre 22 is connected to a band pass filter 52, operable to 
output light at .lambda..sub.0. 
It would be possible to launch the entire laser output at .lambda..sub.-1 ' 
into the Raman shifting fibre 22 and to separate out the .lambda..sub.-1 ' 
and .lambda..sub.0 outputs (both of which are required) afterwards; this 
is slightly less desirable, however, because the power remaining at 
.lambda..sub.-1 ' is likely to be far less stable than the original power, 
which leads to practical difficulties in setting up and operating the 
instrument. 
The length of Raman shifting fibre 22 required can be modest, especially if 
polarisation-maintaining fibre with a very large refractive index 
difference is used with a commercially available polarisation-maintaining 
(high-birefingence) single mode fibre, (e.g. HB1550 from Fibercore Ltd, 
Chandler's Ford, UK) having a numerical aperture of 0.15, a launched power 
of about 100W at .lambda..sub.-1 '=1530 nm is sufficient to obtain 
efficient conversion to the first Stokes wavelength, .lambda..sub.0 =1640 
nm, in a fibre length of around 130 m. Using a standard single mode fibre, 
which is usually far cheaper, typically 50W pulses would be required to 
obtain a high conversion efficiency in 800 m of fibre, or 100W in 400 m of 
fibre. 
For reasons which will be described later, for a range of around 30 km the 
optimum value of the wavelength .lambda..sub.0 is around 1640-1650 nm. 
This coincides with the first Raman Stokes wavelength of an Erbium-doped 
fibre laser (Erbium-doped fibres have peak gains around 1530-1540 nm), so 
a Q-switched Erbium-doped fibre laser operating at 1537 nm, giving a 
.lambda..sub.0 of 1648 nm, would be a particularly convenient choice of 
source for this range. 
A preferred way of generating .lambda..sub.0 from .lambda..sub.-1 ' is 
shown in FIG. 7. As in FIG. 6, light at .lambda..sub.-1 ' is split by the 
first coupler D into two components, small proportion (about 10%) of the 
light passing from port 1 to port 3, which is connected to an optional 
delay line 21 and an optional bandpass filter 51 operable to output light 
at .lambda..sub.-1 ', and the remainder (about 90%) being transmitted from 
port 1 to port 2. Port 2 is connected to one end of a length of wavelength 
shifting fibre 22, the other end of which is reflective so that light at 
.lambda..sub.-1 ' is effectively scattered by twice that length of fibre 
22, being converted to .lambda..sub.0 as it does so. This allows the 
length of wavelength shifting fibre 22 required to be halved. Furthermore, 
this arrangement is suitable for the case where the anti-Stokes Brillouin 
line is to be measured, Brillouin scattering being fundamentally a 
backward process, in which case the said other end of the fibre 22 need 
not be reflective. The coupler D is operable to transmit light at 
.lambda..sub.0 from port 2 to port 4, which is connected to a bandpass 
filter 52 operable to transmit light at .lambda..sub.0. 
It will be appreciated that the issues of launching and collecting the 
various wavelengths may be difficult to resolve, if, in effect, a 
singly-triggered source is used, i.e, one in which the various wavelengths 
are generated by the same source and cannot be switched on and off 
independently. Such difficulties can only be resolved by some form of 
switching. 
The switching might be accomplished at the output of the source 1, by means 
of a controllable filter (e.g., mechanically, using a filter wheel, 
Fabry-Perot etalon or monochromator arrangement, or using a variety of 
means of tuning the transmission electrically, e.g. acousto-optics, 
electro-optics, liquid crystal cell and many others). Alternatively, the 
switching could be accomplished by separating the optical signals into 
separate paths and switching these on and off using electro-optic, 
acousto-optic or even mechanical switches. 
Although the switch could comprise integrated optics, It is conveniently a 
mechanical fibre-to-fibre switch, since this is low loss, relatively cheap 
and available. One such arrangement, which is relatively simple to 
implement, is a loop and input select switch 10 shown in FIG. 6. In this 
case, it is necessary to rely on the cross-talk of the switch being very 
low (so that only one fibre is addressed at any one time) and also on the 
very low probability that an aligned position for one input fibre will 
result in the second input fibre also launching light into another fibre 
of the output bundle. 
The arrangement shown in FIG. 6 employs a network of fibre fused taper 
couplers A, B, C and D. Coupler A has its port 1 connected to the bandpass 
filter 52 passing .lambda..sub.0, its port 2 connected to the switch 10 
and its port 4 connected to port 2 of coupler B. Coupler B has its port 1 
connected to a combined filter 5b and detector 7b for receiving light at 
.lambda..sub.0, its port 4 connected to another combined filter 5a and 
detector 7a for receiving light at .lambda..sub.-1, and its port 3 
connected to port 1 of coupler C. Coupler C has its port 2 connected to 
the switch 10 and its port 4 connected to the delay line 21. The 
transmission characteristics of couplers A, B, C and D are desirably, but 
not essentially, as shown in the following table:- 
______________________________________ 
COUPLER PORTS .lambda..sub.0 
.lambda..sub.-1 
______________________________________ 
A 1-2 & 3-4 10% 0% 
1-3 & 2-4 90% 100% 
B 1-2 & 3-4 100% 10% 
1-3 & 2-4 0% 90% 
C 1-2 & 3-4 any 50% 
1-3 & 2-4 any 50% 
D 1-2 & 3-4 any 10% 
1-3 & 2-4 any 90% 
______________________________________ 
It is important to ensure that the path from the sensing fibre 2 through 
couplers A to B to the .lambda..sub.-1 detector 7a has the lowest possible 
loss. Some compromise has been made to allow a nominal 10% transmission 
from port 3 to 4 of coupler B to let the Rayleigh back-scatter, when light 
at .lambda..sub.-1 has been launched into the fibre 2, to reach the 
.lambda..sub.-1 detector 7a. The values given for the coupler 
transmissions are indicative only (100% means "as high as possible", 0% 
"as low as possible", 90% "not quite full transmission" and 10% "a small 
but finite transmission"). In FIG. 6 any unused coupler inputs are 
terminated for low reflection. They may be used for power monitoring. 
Although the arrangements described with reference to FIGS. 6 and 7 are 
entirely based on fused-taper couplers, which are preferred owing to the 
low losses available with such couplers, clearly the same functions can be 
implemented in bulk optics (or even in integrated optics). 
Alternatively, separate sources may be used to generate pulses at 
.lambda..sub.0 and .lambda..sub.-1, in which case items 21, 22 and 52, and 
coupler D would be removed and the source emitting at .lambda..sub.-1 
would be connected directly to port 4 of coupler C, and the source 
emitting at .lambda..sub.0 would be connected to port 1 of coupler A. 
Although it should be noted that the actual values obtained will obviously 
vary in dependence upon the sensing fibre used and the conditions in which 
measurements are made, by way of example FIGS. 8, 9 and 10 illustrate 
results obtained using the signal processing method described above in an 
arrangement comprising a single mode, step-index, non-dispersion-shifted 
fibre having a core refractive index of 1.45, a numerical aperture of 
0.123 at distances below 22.5 km and 0.135 above that distance, a cutoff 
wavelength of 1.33 .mu.m, and a nominal core radius of 4.15 .mu.m (but 
raised by 10% above that value between 15 km and 18 km and beyond 36 km), 
when .lambda..sub.0 is 1.648 .mu.m, the Raman shift is 440 cm.sup.-1 and 
the error between .lambda..sub.-1 ' and .lambda..sub.-1 is assumed to be 
negligible. FIG. 8 shows the variation in the anti-Stokes back-scatter 
signal (normalised to 1 at 0 km) with distance in this fibre, without any 
allowance made for propagation losses; the departures from a straight 
line, on this logarithmic plot, are attributable to the above-mentioned 
discontinuities in the characteristics of the fibre and to the temperature 
being increased in a single section of fibre, between 45 km and 47 km. 
FIG. 9 shows the signal with the effect of the propagation losses removed; 
however, variation in the back-scatter capture fraction along the fibre 
still causes distortion. When the anti-Stokes back-scatter signal is 
divided, point-by-point, by the geometric mean of the two Rayleigh scatter 
measurements at .lambda..sub.0 and .lambda..sub.-1 ', the graph shown in 
FIG. 10 is obtained. 
The curve in FIG. 10, which is the function containing the temperature 
information retrieved using the proposed form of signal processing, shows 
the anticipated error caused by changes in the fibre core diameter (10% 
change at 15-17 km), fibre numerical aperture (10% change at 22.5 km) and 
the combined effect of raised numerical aperture and core diameter beyond 
36 km. To get a feel for scale, the feature between 45 km and 47 km shows 
the effect of a localised 10.degree. C. increase in temperature. As can be 
seen, the changes in core diameter and numerical aperture result in a 
measurement error of just over 1.degree. C. 
The techniques used in the prior art for calibration of the fibre with 
respect to variation in attenuation due to factors independent of the 
physical parameter being measured, for example the scattering coefficient 
or capture fraction of the fibre, and for improving the spatial resolution 
along the fibre, may be employed in apparatus embodying the present 
invention. 
The fibres 2 used in apparatus embodying the present invention may be any 
one of the multi-mode, single mode or single polarisation types, depending 
upon the particular application. Unless otherwise indicated, the 
embodiments described in the present application employ a fibre which is 
low loss, dispersion compensated, and has a linear attenuation coefficient 
minimum at 1550 nm. The attenuation coefficient for such a fibre type is 
shown in FIG. 11. 
For maximum dispersion compensation, the fibre is desirably a single mode 
fibre. Whilst not always giving the best possible performance, 
measurements on single mode fibres also have the advantage of using 
comparatively cheap fibre and, more importantly, often fibre which is 
already installed along the structure to be monitored, for example, power 
cables. 
A source 1 suitable for use with such a fibre 2 produces pulses of optical 
radiation having a typical half power duration of 40 to 80 ns at a 
selected wavelength, and has a peak power which can be varied between 
around 1W to many tens of Watts. The source is conveniently but not 
necessarily an amplitude modulated semiconductor laser, the output of 
which is amplified by an Er (Erbium)-doped fibre amplifier before being 
launched into the sensing fibre. This source has the benefit of being 
available from many suppliers of telecommunications equipment and, 
moreover, it allows the average power launched into the fibre to be 
increased by employing pulse compression coding schemes. Such schemes are 
relatively simple to implement owing to the ease of modulation of the 
semiconductor laser by the variation of its bias current. 
Alternatively, the source 1 may be a diode-pumped solid state loser, in 
particular a Q-switched Er-doped fibre laser, the output spectral width of 
which is preferably below 20 nm with a pulse half power width which can be 
controlled by varying the length of the fibre in its cavity. Such a 
Q-switched laser, which is cheaper than the source mentioned above, may be 
pumped at 980, 1480 or 810 nm using a semiconductor laser or at 514 nm 
using other lasers such as Ti:sapphire lasers, certain dye lasers and the 
like. Semiconductor lasers are likely to be the most convenient pumping 
source, being potentially cheap, energy efficient and compact. 
Another alternative is for the source 1 to be a bulk-optic Er:glass laser. 
Control of the pulse width of the source used is required to maximise, for 
particular fibre types, the injected energy by increasing its peak power 
and/or duration since this determines the temperature resolution of the 
device whilst maintaining a minimum spatial resolution which is degraded 
with increased pulse duration. 
To achieve high loss at .lambda..sub.+1, whilst retaining low loss 
properties of the fibre at the wavelengths of interest, one or more of the 
following techniques may be employed (although it should be noted that 
other suitable techniques, not described below, could be used). The 
techniques are described for the case where the measurements are to be 
based on the Rayleigh scattered signal at .lambda..sub.0 and the 
anti-Stokes Raman signal at .lambda..sub.-1 ' but can readily be adapted 
(where necessary) for situations where other spectral bands are to be 
used. 
Greater attenuation at the Stokes wavelength .lambda..sub.+1 can be 
achieved by selecting .lambda..sub.0 such that .lambda..sub.0 and 
.lambda..sub.-1 lie roughly symmetrically about a transmission window, 
i.e. on either side of a local attenuation minimum. However, 
.lambda..sub.0 can be selected such that .lambda..sub.+1 coincides with a 
local absorption maximum (or lies substantially on the short wavelength 
edge of such a maximum), owing for example to the IR absorption band or 
absorption bands caused by naturally-occurring or doped impurities in the 
fibre. Conveniently the long wavelength edge of the transmission band at 
1550 nm is the IR absorption band (see FIG. 11), and .lambda..sub.0 can be 
selected such that the loss at .lambda..sub.+1 is at least 3 times higher 
than if .lambda..sub.0 and .lambda..sub.-1 were placed symmetrically about 
the centre of the transmission window, resulting in an appreciable 
increase in the usable power level. Local absorption maxima occur in 
fibres owing to naturally-occurring impurities such as hydroxide ions and 
hydrogen gas. The local absorption maximum for hydroxide ions is at 1390 
nm, so a .lambda..sub.0 of about 1310 nm could conveniently be used (or 
1320 nm using a Nd:YAG source). Where it is possible to employ a special 
fibre it may be doped, with rare earth ions for example, so as to provide 
a local attenuation maximum at .lambda..sub.+1. Similarly advantage could 
be taken of the local absorption maxima provided by dopants already in the 
fibre. 
Another possibility is to provide a special fibre having built-in gratings, 
for example formed by colour-centre generation under UV illumination. 
These built-in gratings may be formed by modulating the refractive index 
profile of the core of the fibre, as shown in FIG. 12. The depth d.sub.m 
of this modulation may be typically 3.times.10.sup.-4 or less. Such 
gratings are reflective in a highly wavelength-specific manner, and may be 
incorporated either continuously or at intervals throughout the fibre so 
as to prevent the build-up of power at .lambda..sub.+1. 
Alternatively, it may be possible to arrange for .lambda..sub.+1 to have 
substantially higher losses due to bending than .lambda..sub.0 or 
.lambda..sub.-1. This may be done without perturbing the fibre, by 
choosing .lambda..sub.0 to be as long as possible relative to the cut-off 
wavelength of the fibre. In practice a given fibre will suffer increasing 
bend loss as the wavelength increases from the centre of the transmission 
window. If possible, bends could deliberately be induced in the fibre, 
either along its entire length by some form of structure in the cable, for 
example a helical arrangement, a corrugation inside the cable or random 
roughness, or at intervals along the fibre, for example by applying a 
periodic mechanical perturbation, as shown in FIGS. 13(A) and 13(B). In 
any case the bends are chosen in their amplitude, radius of curvature and 
periodicity to increase the loss at .lambda..sub.+1 whilst having a 
minimal effect on the other wavelengths of interest. 
One method of applying a periodic mechanical perturbation to a fibre is 
shown in FIG. 13(A). Using this method a typical installed single mode 
fibre, in which a short length, for example a few tens of meters, has been 
bent into a coil of carefully chosen diameter d.sub.1 and housed in a 
protective enclosure, can have a loss of around 10 dB at .lambda..sub.+1, 
with an increased loss of only 0.1 dB at .lambda..sub.0. 
Another method of applying such a perturbation to a fibre is shown in FIG. 
13(B). The amplitude d.sub.2 and the period s are chosen so as to maximise 
attenuation at .lambda..sub.+1, while minimising attenuation at 
.lambda..sub.0 and .lambda..sub.-1, while minimising attenuation at 
.lambda..sub.0 and .lambda..sub.-1, for example by using a fibre with a 
refractive index profile as shown in FIG. 14. Typically, the relationship 
between the core radius (a) and the inner cladding radius (a') would be: 
EQU 1.2a&lt;a'&lt;3a. 
The refractive indices of the core, inner cladding and outer cladding are 
represented by n.sub.1, n.sub.1 ', and n.sub.2 and would typically have 
the following relationship: 
EQU 5(n.sub.1 -n.sub.2)&gt;n.sub.2 -n.sub.1 '&gt;0.2(n.sub.1 -n.sub.2). 
A further technique for inducing high loss at .lambda..sub.-1 is to launch 
a "guard" signal at a wavelength .lambda..sub.+2, where 
.lambda..sub..degree.2 =.lambda..sub.+1 +.lambda..sub.s (.lambda..sub.s 
equals the wavelength shift due to an inelastic scattering process), 
simultaneously into the fibre with a signal at .lambda..sub.0. In this 
way, as soon as light at .lambda..sub.+1 is generated by spontaneous 
emission it will be converted by stimulated scattering to light at 
.lambda..sub.+2,thereby preventing the build-up of light at 
.lambda..sub.+1 and hence delaying the onset of problems due to stimulated 
scattering of light at .lambda..sub.0. 
This technique will be explained further with reference to FIG. 15(A), 
which shows guard signal generating means 12 which generate a guard signal 
at .lambda..sub.+2, the second order Raman Stokes wavelength. The guard 
signal generating means 12 are desirably placed between the source 1 and 
first directional coupler 4a in the apparatus of FIG. 4(A) such that 
optical radiation is launched into the coupler 4a simultaneously at the 
two wavelengths .lambda..sub.0 and .lambda..sub.+2. 
The source signal .lambda..sub.0 entering the guard signal generating means 
12 in FIG. 15(A) is split into two components 121, 122, either by a signal 
splitter 123, for example one made from bulk optics or an all-fibre 
directional coupler such as a fused taper coupler or polished coupler. The 
components 121, 122 then pass through respective lengths of coiled fibre 
21, 22, one comprising a fibre 22 with enhanced Raman scattering 
properties and the other 21 acting as a delay line with low Raman gain 
characteristics. The Raman scattering properties of fibre 22 may be 
enhanced in a number of ways. For example, the fibre 22 may be doped with 
GeO.sub.2, or may chosen so as to have a large refractive index difference 
(for example, owing to a large concentration of GeO.sub.2 additive), which 
results in a smaller mode field and hence an increase in the optical 
intensity, or may be chosen so as to be a polarisation maintaining fibre, 
thereby doubling the stimulated scattering gain coefficient compared to a 
non-polarisation maintaining fibre (the length of which would have to be 
double that of the polarisation maintaining fibre to achieve the same 
Raman gain, given the same conditions). Conversely, the delay line fibre 
21 can be made to have reduced Raman gain by keeping the index difference 
and GeO.sub.2 concentration in the core to moderate levels and choosing a 
non-polarisation maintaining fibre. The signals passing through the high 
Raman gain fibre 22 are converted through stimulated Raman scattering from 
.lambda..sub.0 to .lambda..sub.+2, the signals passing through the delay 
line fibre 21 emerging therefrom having an approximately similar time 
delay as the scattered signals in the other fibre. The two separate 
signals at .lambda..sub.0, .lambda..sub.+2 then pass, via filters 151, 
152, to a common output 124, such as a dichroic coupler or other suitable 
means, at which the separate signals are recombined. If no filtering 151, 
152 is included, many orders of Stokes and anti-Stokes signals may be 
emitted which can reduce the effectiveness of the guard signal. 
If it could be possible to propagate signals without the increased 
attenuation that exists at the second order (.lambda..sub.+2) in the 
fibre, the range limitation resulting from stimulated Raman scattering 
could be lifted and the power limitation on the source, at the first 
wavelength .lambda..sub.0, would be then limited by other effects, In 
theory, several orders of magnitude in the range and received power from 
back-scattered signals could be gained. In practice, however, the power at 
the second order .lambda..sub.+2 is itself converted to a third order 
.lambda..sub.+3, owing to stimulated scattering as it propagates along the 
sensing fibre, and its action as a guard pulse gradually diminishes. 
Nevertheless, very useful gains in transmitting power can be achieved, 
and, furthermore, the onset of Raman scattering at .lambda..sub.+2 may be 
suppressed or delayed as described above with reference to the first 
embodiment, or indeed a third pulse at a fourth order .lambda..sub.+4 
(=.lambda..sub.+3 +scattering shift) could also be launched simultaneously 
with the second order .lambda..sub.+2 to restrict the losses from the 
signal at the second order .lambda..sub.+2. 
This third pulse (.lambda..sub.+4) may be derived from the .lambda..sub.+2 
pulse in the same way as the .lambda..sub.+2 pulse is derived from the 
.lambda..sub.0 pulse, or may be supplied from an independent source. A 
coupler 113, shown in detail in FIG. 15(B), can be connected to the port 2 
of coupler 124 of FIG. 15(A), so that the .lambda..sub.+4 pulse may be 
launched into the fibre. Alternatively, coupler 113 could be inserted 
between the coupler 124 and either of the filters 151 and 152. 
The preferred power at each order depends, of course, on the attenuation of 
the relevant wavelength in the fibre concerned, necessitating that it be 
possible to set the power of each signal at each wavelength independently 
of the others in order to maximise the propagation of the test signal at 
the first wavelength. The wavelength of the guard pulse must also lie 
within the bandwidth of the Raman gain coefficient for the fibre, which 
for silica fibres is quite broad (200 cm.sup.-1 or so), in order that 
sufficient Raman scattering occurs at each wavelength. The width or 
duration of the guard pulse should be chosen to be greater than that of 
the source pulse in order to allow for chromatic dispersion of the two 
signals over the lengths of interest. 
The pulse width of the guard signal may be increased by a fibre network 
arrangement 112 such as shown in FIG. 15(C), which is inserted between the 
Raman gain fibre 22 and the filter 152. The fibre network arrangement 112 
comprises a beam splitter for splitting the light emerging from the Raman 
gain fibre 22 into two components and two fibres 1221, 1222, for receiving 
respective components of the signals, one of the fibres 1222 being longer 
than the other such that the component passing through it is delayed by 
approximately half the pulse width. The two components are then recombined 
and passed to the filter 152. 
Although the embodiments discussed above have been described largely with 
reference to prevention of stimulated Raman scattering, it should be noted 
that similar techniques could be employed so as to inhibit growth in the 
fibre of light at the first order Brillouin Stokes wavelength, thereby 
delaying the onset of problems due to stimulated Brillouin Scattering. 
As mentioned earlier, stimulated scattering also decreases the linear range 
of measurements taken when characterising fibres using optical time domain 
reflectometers. Since in this case measurements must be carried out at a 
pre-specified wavelength on a standard fibre in a well specified 
deployment state, there is no opportunity to employ the techniques 
discussed with reference to the first embodiment, for example, selecting 
an appropriate source wavelength, doping the fibre, or inducing bending 
losses. However, launching a second pulse into the fibre as a guard signal 
would in most circumstances be unobjectionable, and would increase the 
linear measurement range. 
Using methods and apparatus embodying the present invention, increased 
power may therefore be launched into the fibre to increase the sensing 
range before non-linear effects start to dominate. 
Generally, the test wavelength (for use in apparatus which may or may not 
be such as that shown in preceding Figures) is also chosen so as to 
minimise the transmission loss for the particular fibre and conditions 
used, as will be explained below. 
When choosing a test wavelength, account is desirably taken of the average 
value of the losses at the test and detected wavelengths. When detecting a 
Brillouin line the frequency shift is so small that there is no 
appreciable change in loss between the wavelengths, unless it is 
introduced artificially (e.g. narrow band grating), but for Raman lines, 
however, the frequency shift is such as to result in significant changes 
in attenuation. FIG. 16 is a graph illustrating the loss of a single mode 
fibre (step-index, non-dispersion-shifted, core refractive index 1.45) as 
a function of wavelength. The solid curve 207 represents the loss at the 
test wavelength .lambda..sub.0, and the dotted curve 208 shows the loss at 
the wavelength .lambda..sub.-1 of the Raman anti-Stokes signal resulting 
from a test pulse at wavelength .lambda..sub.0. As expected, the solid 
curve 207 has a minimum around 1550 nm and the anti-Stokes loss is at a 
minimum if .lambda..sub.0 is around 1650 nm (such as to generate an 
anti-Stokes signal at the loss minimum of the fibre). As shown by the 
dashed curve 209, which represents the average loss 
[.alpha.(.lambda..sub.0)+.alpha.(.lambda..sub.-1)]/2 at the two 
wavelengths .lambda..sub.0, .lambda..sub.-1, for a given .lambda..sub.0, 
the minimum overall loss is for a test wavelength somewhere in between, at 
around 1590 nm. 
However, the choice of test wavelength is not always that simple, so the 
attenuation of the fibre increases rapidly with increasing wavelength 
beyond 1550 nm owing to the infra-red absorption, so that the power that 
can be launched into the fibre before non-linear effects take place is 
also a function of wavelength. FIG. 17 is a graph illustrating, as a 
function of test wavelength, the power level at which the fibre loss 
increases on average (over 80 km) by 0.01 dB/km owing to stimulated Raman 
scattering in an Industry-standard single mode fibre (NA=0.124, 
cutoff=1334 nm). As can be seen, a substantial increase in launched power 
must be used as the test wavelength increases. This must be balanced 
against considerations of fibre attenuation; such a comparison can only be 
made for a particular length of sensing fibre. By way of example, FIG. 
18(A) shows the fibre losses (solid curve 210) for a 30 km single mode 
fibre (dB total loss after two-way propagation), together with a benefit 
factor (dotted curve 211; also shown in dB, normalised to 0 dB at 1550 nm) 
attributable to the variation of allowable launch power with test 
wavelength, FIG. 18(B) showing the same data on an expanded scale. The 
dashed curve 212 (given by the difference between the solid and dotted 
curves) shows how including the improved launch power at the longer 
wavelength shifts the optimum wavelength from 1590 nm to 1640 nm, for 30 
km of fibre. It should be noted that the use of artificial methods to 
suppress the stimulated scattering could alter this value. 
Thus, if one is not able to supply enough power to be limited by non-linear 
effects, one can choose a preferred test wavelength corresponding to the 
minimum overall fibre loss, i.e. where the sum or average of the losses at 
the test and detected wavelengths (or the sum or average of the losses at 
the test and the most critical of the detected wavelengths, where there is 
more than one detected wavelength) is minimised. Where all signals lie 
close to each other (e.g. Brillouin scattering) this preferred wavelength 
is around 1550 nm. For Raman scattering, the optimum test wavelength is 
around 1590 nm, at which wavelength the sum or average of the losses at 
the probe and anti-Stokes wavelengths is minimised. 
When ample source power is available, the preferred test wavelength may be 
altered slightly, as discussed above, because a changed test wavelength 
might result in an increase in the allowable launch power to a greater 
extent than the (linear) fibre loss is increased by the change in the test 
wavelength. In other words, the wavelength is preferably chosen to give 
the best throughput at a defined distance in the fibre, when the effect of 
fibre loss on the maximum launch power is taken into account. Owing to the 
availability of convenient sources, an especially convenient wavelength is 
around 1620 nm to 1680 nm which is generated from sources suitable for the 
low-loss window of silica fibres around 1550 nm. 
The criterion used in preparing FIG. 17, that is limiting the power level 
to one which causes a certain average increase in optical loss at the test 
wavelength over a certain fibre length, is a somewhat crude means of 
determining the best power to launch into the fibre. Assuming that 
sufficient source power is available, then a more precise way of 
optimising the system performance is to adjust the power launched into the 
fibre to maximise the test power conveyed to the most remote point to be 
monitored. There is a clear optimum, since, at lower powers than a certain 
value, more power could be launched in and therefore more returned; at 
higher powers, the benefit of more initial power is more than offset by 
higher losses due to non-linear effects. FIGS. 19 and 20 show the 
(calculated) variation of Rayleigh back-scatter power returned from a 
given distance along a standard single mode fibre (non-dispersion-shifted, 
designed for operation at 1550 nm) as a function of the power launched 
(for a pulse width of 80 ns). Owing to the non-linear effects, this is not 
a monotonic function of launched power; for each distance, there is an 
optimum power which maximises the back-scatter (Rayleigh or anti-Stokes 
Raman) returned from that point. 
In the case where back-scattered signals at the anti-Stokes and Stokes 
wavelengths are both employed and source power is low enough to ignore 
stimulated scattering, it can be more difficult to optimise the probe 
wavelength, since (a) three wavelengths are now involved, (b) the strength 
of the anti-Stokes and Stokes wavelengths, when generated, differ, and (c) 
the losses differ. In general, one would arrange the incident wavelength 
to be roughly in the center of the transmission loss window (e.g. 1550 
nm). In this case, the performance would be worse than in the approach 
proposed above, since the Raman wavelengths would both suffer higher 
transmission losses than the incident wavelength. This discussion is of 
course primarily relevant to Raman scattering, since the wavelength 
separation for Brillouin is far narrower. 
It should be noted that there are circumstances where one might wish to 
suppress stimulated scattering, even if not working at a wavelength which 
minimises the transmission loss. One example might be where the desired 
range (distance coverage) of the sensor is relatively short, but the 
performance must be optimised in other respects (for short measurement 
time or high spatial resolution); in this case one would wish to transmit 
as much power as possible. There may be other reasons for not working at a 
wavelength which minimises the loss. For example, in short distances, the 
loss of the fibre itself becomes less significant, or there may be other 
considerations, such as the intensity of the back-scatter signal which 
increases as the wavelength is reduced, or detector efficiency, which 
improves markedly below about 1100 nm where silicon detectors may be used. 
Similarly, there may be occasions where, owing to the use of a test 
wavelength which minimises the transmission loss and hence increases the 
range over which sensing can be carried out before stimulated scattering 
effects become significant, it is not necessary or desirable to design the 
system so as to suppress stimulated scattering in the manner discussed 
earlier.