Optical fiber sensing system

An optical fibre sensing system for monitoring and/or measuring temperatures or pressures distributed around an optical fibre sensing loop. Light source and coupler are provided for applying continuous wave light to the loop so that the light propagates simultaneously in opposite directions around the loop with the resultant light output from the loop being detected by a detector. Light pulse generating source and coupler cause light pulses to propagate in one direction only around the loop and by so doing produce transient variations in the propagation constant along the looped fibre in dependence upon the distributed temperature or pressure around the loop. The detector detects the phase changes occurring in the continuous wave light received from the sensing loop due to variations in temperature or pressure around the loop.

BACKGROUND OF THE INVENTION 
This invention relates to optical fibre sensing systems for monitoring 
and/or measuring temperatures or pressures distributed over a 
pre-determined path. 
In our co-pending British Patent Application No. 8609733 (published under 
Ser. No. 2,207,236 on Jan. 25, 1989), to which attention is hereby 
directed, an optical fibre sensing system comprises an optical 
interferometer having measurement and reference optical paths in parallel 
relationship, the measurement path comprising an optical fibre extending 
over the path along which distributed temperature or pressure is to be 
measured, continuous wave light generating means producing coherent 
continuous wave light which propagates in one direction along the 
measurement and reference paths of the interferometer, pulse light 
generating means for producing light pulses which propagate along the 
measurement path only in the direction opposite to that in which the 
continuous wave light propagates to produce transient variations in the 
propagation constant (or its directly related phase change coefficient) of 
the optical fibre at points therealong according to the temperature or 
pressure at said points, and detector means for detecting the resultant 
output from the interferometer which will vary with time in dependence 
upon the temperature or pressure at distributed points along the 
measurement optical fibre path. 
SUMMARY OF THE INVENTION 
The present invention is directed to an optical sensing system of generally 
similar form to that forming the subject of the above-mentioned co-pending 
application but which exhibits improved stability and is inherently 
balanced with potentially better phase-noise characteristics. 
According to the present invention there is provided an optical fibre 
sensing system for monitoring and/or measuring temperatures or pressures 
distributed over a pre-determined path, in which the pre-determined path 
comprises an optical fibre defining a sensing loop, in which means are 
provided for applying continuous wave light to the sensing loop so that 
the light propagates simultaneously in opposite directions around said 
loop with the resultant light output from the loop being detected by 
detector means and in which pulse generating means is provided for 
generating light pulses which propagate in one direction only around the 
sensing loop and by so doing produce transient variations in the 
propagation constant (or phase change coefficient) along the looped 
optical fibre in dependence upon the distributed temperature or pressure 
around the loop and in which the detector means detects the phase changes 
occurring in the continuous wave light received from the sensing loop due 
to variations in temperature or pressure around the loop.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT 
Referring to FIG. 1 of the drawings, the optical sensing system illustrated 
comprises a Sagnac interferometer including an optical fibre sensing loop 
1 to which continuous wave light from a suitable light source 2 (e.g. 
laser, light-emitting diode or incandescent lamp) is applied through a 
directional optical fibre coupler 3. The directional coupler 3 allows like 
components of the continuous wave light to propagate in opposite 
directions simultaneously around the sensing loop 1 with a proportion of 
the light returning to the directional coupler 3 being fed to a detector 4 
where coherent interference between the light components is detected in 
the form of a detected signal amplitude which in turn is dependent on the 
relative phase of each of the continuous wave components received by the 
detector 4. 
In the absence of any physical disturbance around the sensing loop 1, due 
for example to changes in temperature or pressure at points therealong, 
the path lengths for the like components travelling around the loop in 
opposite directions will be effectively the same and therefore there will 
be no changes in the relative phase of the components, that is to say the 
components will be in phase with one another. The sensing system of the 
present invention, however, includes a pulse laser source 5 which is 
arranged to produce one or more short intense light pulses which are fed 
into the sensing loop 1 through an optical fibre directional coupler 6. 
This intense pulse or pulses propagate around the sensing loop 1 in the 
counter clockwise direction as viewed in FIG. 1 and will disturb the 
balance of the sensing loop and thereby cause changes in the propagation 
constant of the sensing loop fibre at points therearound in dependence 
upon the temperature or pressure at such points. A time varying imbalance 
will therefore occur in the sensing loop and this imbalance will depend 
both on the intensity of the pulse or pulses and the temperature of, or 
pressure exerted on, the optical fibre in that region of the loop through 
which the clockwise propagating component of the continuous wave light has 
passed when it is encountered by the anti-clockwise light pulse. After 
disturbing the balance of the sensing loop to produce changes in the 
propagation constant thereof the intense pulse or pulses, as the case may 
be, may be removed by means of a filter 7 before reaching the detector 4. 
However, the wavelength of the light pulse could alternatively be outside 
the response range of the detector 4 in which case the filter 7 may be 
dispensed with. For example, a 1.3 um laser for producing pulses could be 
used in conjunction with a silicon detector. 
If distributed temperature is to be measured, the optical Kerr effect may 
be utilised where the propagating light pulse changes the refractice index 
of the loop centre fibre according to temperature. 
By referring to FIG. 2 of the drawings, typical phase changes that occur in 
the interfering light components at the detector under quiescent 
conditions of the loop and when variations in temperature occur along the 
length of the loop can be seen, the latter being shown in dotted form. As 
will be noticed, the variations in phase change which can be measured in 
the detector to determine the distribution of temperature along the 
sensing loop are indicated at M1 and M2. 
In order to prevent any polarisation effects resulting from changes in the 
relative states of polarisation of the continuous wave light components 
propagating around the sensing loop optical fibres and directional 
couplers used may be of the polarisation maintaining (or polarising) type. 
With a view to obtaining the maximum response of the system to the intense 
pulse or pulses injection into the sensing loop 1, the Sagnac loop 
interferometer may be biased by the introduction into the loop 1 of 
biasing means 8. This biasing which may be effected in accordance with any 
of the methods currently used in processing arrangements for gyroscope 
systems disturbs the normal in-phase relationship between the 
counter-propagating components of the continuous wave light received at 
the detector 4 and causes a relative frequency of phase off-set between 
the components. 
The sensing system depicted in FIG. 1 of the drawings may be improved on to 
provide better path reciprocity, as by the use of so-called minimum fibre 
gyroscope configuration for a reciprocal path gyroscope.