Abstract:
An optical fiber temperature sensor comprising: an optical pulse generator; an optical fiber into which said optical pulses are fed; an optical receiver to receive said optical pulses reflected by said optical fiber and to convert them into an electrical signal; a processor which receives said electrical signal and determines the temperature along said optical fiber; said optical receiver comprising a first filter and a second filter to filter said optical pulse reflected by said optical fiber, characterized in that said first and said second filters filter two adjacent portions of anti-Stokes optical signals or of Stokes optical signals.

Description:
BACKGROUND TO THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical fibre temperature sensor and to a method for determining temperature along an optical fibre. 
     2. Technical Background of the Invention 
     It relates in particular to an optical fibre temperature sensor of Raman type for the distributed monitoring of temperature along medium/high voltage electric cables. The sensor is however adaptable to other applications such as fire prevention monitoring within boreholes/tunnels, monitoring of gas/oil pipelines and for all those industrial applications requiring constant continuous temperature control along medium/long distances of interest. 
     Numerous patents are present on the subject of optical fibre sensors for distributed temperature measurement, these discussing the filtration/separation of the different spectral components used in temperature measurement. 
     To provide a reliable measurement of temperature distribution along the fibre sensor an appropriate normalization of the temperature curve must be carried out, i.e. the measurement must be normalized with reference to possible mechanical losses in the fibre sensor. 
     The compensation techniques typically used comprise analysis of the Stokes-anti Stokes spectral components; or analysis of the Rayleigh-anti Stokes spectral components 
     For example, the spectral component of Rayleigh scattering is analyzed and used as reference for temperature distribution compensation. In this respect, analysis of the Rayleigh signal component enables attenuation distribution along the sensor fibre to be determined. Knowing this pattern it is possible, by simultaneously analyzing temperature distribution and attenuation distribution, to discriminate the effects of temperature and those related to possible mechanical stresses or micro-bending phenomena induced by the sensor fibre. In this manner, temperature evaluation along the sensor fibre is more accurate and free of measurement errors. 
     If measurement accuracy is to be improved, several sources with different probe wavelengths could be used. In this manner, by using a suitable filtration system, the attenuation distribution along the fibre can be obtained for each source used. 
     This enables a more accurate reconstruction of attenuation distribution based on the probe wavelength and hence more effective compensation for losses due to mechanical stresses or micro-bending phenomena. 
     The Rayleigh spectral component can also be filtered if a compensation technique is to be used which evaluates the ratio, in terms of power, between the Raman anti-Stokes and Raman Stokes spectral components. This technique typically enables a clear and precise evaluation of temperature distribution but does not normally enable those effects related to temperature to be effectively discriminated from those due to losses of a mechanical nature or to reflection phenomena (splicing or connectors). 
     In both cases those components not used for normalization purposes are filtered out by suitable devices and not used for other purposes. 
     An object of the present invention is to provide an optical fibre temperature sensor and a method for determining temperature along an optical fibre able to obviate the drawbacks of the known art. 
     Another object is to a system which is of low cost while still providing high measurement performance. 
     These and further objects are attained, according to the present invention, by an optical fibre temperature sensor comprising: an optical pulse generator; an optical fibre into which said optical pulses are fed; an optical receiver to receive said optical pulses reflected by said optical fibre and to convert them into an electrical signal; a processor which receives said electrical signal and determines the temperature along said optical fibre; said optical receiver comprising a first filter and a second filter to filter said optical pulse reflected by said optical fibre, characterised in that said first and said second filters filter two adjacent portions of anti-Stokes optical signals or Stokes optical signals. 
     SUMMARY OF THE INVENTION 
     These objects are also attained by a method for determining temperature along an optical fibre, comprising the steps of: generating an optical pulse; feeding said optical pulse to an optical fibre; receiving said optical pulse reflected by said optical fibre and converting it into an electrical signal which determines the temperature along said optical fibre; filtering said optical pulse reflected by said optical fibre; characterised in that the filter step comprises the step of filtering two adjacent portions of anti-Stokes optical signals or Stokes optical signals 
     Further characteristics of the invention are described in the dependent claims. 
     The Applicant has surprisingly noted that the ratio between two adjacent spectral windows pertaining to the Raman spectrum remains constant even when faced with a mechanical disturbance induced in the sensor fibre; preferably, the ratio between the components of two adjacent spectral windows of the Raman anti-Stokes signal. 
     The sensor uses a single mode standard fibre and preferably standard optical components for telecommunications, and enables temperature measurements to be obtained along long distances with excellent performance, excellent reliability and low cost. 
     Preferably, standard commercial filters complying with ITU (International Telecommunication Union) specifications are used, in particular recommendations G.671 and G.694.1. In this manner there is greater reliability of components required for implementing the sensor, and hence reliability of the sensor itself. In this respect, such standard devices are subjected to severe tests (Telcordia GR-1221). 
     The proposed scheme preferably enables not only the temperature distribution to be evaluated but also the attenuation distribution along the sensor fibre. The two measurements are made simultaneously but totally independently. The proposed optical scheme also enables application of a compensation technique, noted surprisingly by the Applicant, which enables the temperature measurement to be normalized with reference to possible mechanical losses in the sensor fibre. In this manner all the effects of a mechanical nature introduced by the fibre can be compensated, so increasing the precision and accuracy of the measurements effected. 
     The characteristics and the advantages of the present invention will be apparent from the ensuing detailed description of one embodiment thereof, illustrated by way of non-limiting example in the accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows schematically a block scheme of an embodiment of a sensor according to the present invention; 
         FIG. 2  shows schematically the complete optical spectrum reflected by a sensor fibre, according to the present invention; 
         FIG. 3  shows schematically the optical spectrum reflected by a sensor fibre relative to just the Rayleigh signal, according to the present invention; 
         FIG. 4  shows schematically the optical spectrum reflected by a sensor fibre relative to the Raman anti-Stokes and Stokes signals, according to the present invention; 
         FIG. 5  shows schematically a block scheme of an embodiment of a filter according to the present invention; 
         FIG. 6  shows schematically the attenuation curves of the optical filtration circuit, according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference to  FIG. 1 , an optical fibre temperature sensor according to the present invention comprises an optical pulse generator  10  consisting of a laser diode, for example DFB (distributed feedback) operating in the third window. The optical pulse may be amplified to a suitable power level using an optical amplifier  11 , for example EDFA (Erbium doped fibre amplifier). The amplified optical pulse is coupled to one end of the sensor fibre  13  via a 3-port optical circulator  12 . Other pulse-fibre coupling devices can be used. 
     The sensor fibre  13  is a standard optical fibre for telecommunications, for example a single mode fibre (SMF). 
     The retrodiffused signal from the sensor fibre  13  is addressed by the circulator  12  to an OADM (optical add drop multiplexer)  14 , typically used in DWDM (dense wavelength division multiplexer) telecommunications systems. 
     This component  14 , preferably formed by thin film or integrated optics technology, presents two output channels  15  and  16 , with passband and stopband functions respectively. 
     The retrodiffused signal is of the type shown in  FIG. 2 , in which the Rayleigh scattering component peak, the Raman Stokes component on the right side and the Raman anti-Stokes component on the left side can be seen. 
     The attenuation curves for the two channels are complementary and enable the Rayleigh scattering component to be separated from the remaining retrodiffused spectral components, in particular of the Raman Stokes and Raman anti-Stokes component. 
     In particular, the filter which provides the signal to the channel  15  has a passband function, enabling just the Rayleigh component to be separated from the total optical spectrum of the retrodiffused signal, as in  FIG. 3 . This filter is characterised by a narrow passband, preferably a few hundreds of GHz. The chosen passband width must ensure that always and only the Rayleigh component is filtered. It must therefore be evaluated such that possible variations in the emission wavelength λ 0  of the generator  10  (caused by component thermal drift, component ageing, sensitivity to feed current) do not render this filtration ineffective. Experimentally it has been evaluated as using probe pulses of length typically tens of nanoseconds, thus taking into consideration all the causes of the aforesaid wavelength variations, to achieve adequate filtration a filter must be used with a drop channel characterised preferably by a passband of 200 GHz. 
     The Rayleigh signal is then detected by using a suitable photodetection system  17 , preferably a photo diode with a TIA (trans-impedance amplifier) preamplification stage. 
     The electrical signal provided by the photodetection system  17  is applied to a processor  18 . 
     From the received signal, the processor  18  is able, in known manner, to evaluate the attenuation distribution along the sensor fibre  13 . 
     The filter which provides the signal to the channel  16  is used to attenuate just the Rayleigh spectral component, while leaving the remaining spectral components virtually unaltered, these being fed to an optical filtration circuit  20  to be suitably selected, to determine the temperature distribution along the sensor fibre. 
     The filter  20  comprises two passband filters centered on two adjacent portions (spectral windows) pertaining to the Raman spectrum of anti-Stokes optical signals, and produces two optical signals  21  and  22 , visible in  FIG. 4 . 
     The two adjacent portions are bands of about 50 nm width each, and are spaced apart by about 15 nm. Specifically, the band of the signal  21  is about 60 nm and the band of the signal  22  is about 40 nm. 
     Alternatively two adjacent portions of Stokes optical signals can be used, although characterised by lesser temperature sensitivity. 
     The optical signals  21  and  22  are fed to corresponding photodetection systems  23  and  24 , preferably an APD (avalanche photo diodes). 
     The electrical signals provided by the photodetection systems  23  and  24  are applied to the processor  18 . 
     The processor  18  determines the temperature of the sensor fibre  13  using a compensation technique, i.e. a method enabling the temperature measurement to be normalized against possible mechanical losses in the sensor fibre (mechanical stresses such as micro/macro bending phenomena, pressure, splicing). 
     The processor  18  determines the relationship between the electrical signals provided by the photodetection systems  23  and  24 . 
     When reproduced on a suitable graph, the two signals represent the so-called OTDR curve; by means of the following so-called OTDR equation, this curve relates the power of the retrodiffused signal to the distance at which the signal was generated:
 
 P   Ram     —     AS ( I )= P exp(−α(λ 0 ))Γ( T ,λ)τexp(−α(λ AS ))
 
     where: 
     P is the power of the pulse launched into the fibre, 
     α(λ 0 ) is the fibre attenuation coefficient at the excitation signal wavelength, 
     α(λ AS ) is the fibre attenuation coefficient at the retrodiffused Raman anti-Stokes signal wavelength, 
     τ is the excitation pulse duration and Γ(T,λ) is the spontaneous Raman scattering (SRS) efficiency, a function of the wavelength and of the temperature T. 
     The first operation computed on the acquired curves is the conversion from linear scale to logarithmic scale using the following equations:
 
 AS   — 1(dB)=20 log(as — 1)
 
 AS   — 2(dB)=20 log(as — 2)
 
where AS_ 1  and AS_ 2  are the linear scale curves.
 
     At this point a simple ratio operation is carried out on the two curves. Operating in logarithmic scale, the operation carried out is one of subtraction. The curve obtained from the ratio of AS_ 1  to AS_ 2  can be now converted into a temperature curve. This operation is applicable using a conversion coefficient known as sensitivity, calculated experimentally, which relates the signal power to temperature. The last operation carried out is filtration; using a mobile media filter the measurement resolution can be improved without altering the signal band. 
     The proposed compensation technique is able to completely separate the mechanical effects induced by the fibre from the effects due to temperature alone. 
     The validity of the proposed technique is confirmed by different experimental measurements which have demonstrated to the Applicant that the ratio between two adjacent spectral windows pertaining to the Raman spectrum remains constant even when faced with a mechanical disturbance induced along the sensor fibre. In particular, it has been noted that this value is substantially constant even against different mechanical variations. 
     In contrast, analyzing the ratio between two windows pertaining one to the Raman spectrum and the other to the Rayleigh spectrum, it is noted that this value changes to introduce an error contribution into the temperature determination. 
     According to the present invention, in addition to the facility for effecting a simultaneous temperature and attenuation measurement, a more precise temperature measurement can be obtained, to the advantage of overall system performance. 
     If an attenuation measurement is not required, but only a temperature measurement, the scheme of  FIG. 1  can be used, by eliminating the multiplexer  14  and the photodetection system  17  and connecting the channel  16  directly to the circulator  12 . 
     In a preferred embodiment, the optical filtration circuits  14  and  20  comprise standard telecommunications components, for example formed by thin film technology, which enable temperature measurements to be obtained along long distances with excellent performance, optimal reliability and low cost. 
     The filters are normally used for signal-pump coupling in optical amplifiers, with stopband WDM filters typically used for attenuation/rejection of signals characterised by a narrow optical spectrum. 
     With reference to  FIG. 5 , the signal originating from the circulator  12  is directed to a stopband filter  30  with its band between 1579 nm and 1580 nm, to eliminate Rayleigh component residues. 
     The attenuation curves for the two channels of the filter  30  are complementary, in the unused filter branch the Rayleigh signal being present such as that present in the channel  15 . The filter  30  used was, for example, a filter marketed by Messrs. JDSU with the symbol DWS-2F-514-3P-2-0. 
     The filter  30  is followed by a WDM filter  31 . 
     The filter  31  presents two output channels, respectively of passband function supplying the signal  21 , corresponding to one of the two spectral windows pertaining to the Raman anti-Stokes signal, to the photodetection system  23 , and of stopband function, supplying the signal to the next filter  32 . 
     The attenuation curves of the two channels of the filter  31  are complementary and enable the band C (1525 nm-1565 nm) to be separated from the entire optical spectrum. In particular, the passband function enables the entire optical spectrum to be attenuated with the exception of just the band C; in contrast, the stopband function enables just the band C to be attenuated, leaving the remaining spectral components virtually unaltered. The channel of the filter  31  of passband function is used to select a spectral region with a wavelength between 1525 nm and 1565 nm. The filter  31  used was, for example, a filter marketed by Messrs. JDSU with the symbol FWS-F00-10. 
     The filter  32  presents two output channels, of passband function and, respectively, of stopband function, which is not used. 
     The purpose of the filter  32  is to select the other of the two spectral regions pertaining to the Raman anti-Stokes band, i.e. the signal  22 , characterised by a wavelength less than 1510 nm, a limit fixed by the characteristics of the WDM C/L filter, and to simultaneously attenuate the spectral components not yet attenuated by the filter  31 . The filter  31  used was, for example, a filter marketed by Messrs. JDSU with the symbol FWS-A00-2-1-0. 
     The optical signal  21  supplied to the photodetection system  23  is a signal within the band between about 1525 nm and 1565 nm. 
     The optical signal  22  supplied to the photodetection system  24  is a signal within the band between about 1450 nm and 1510 nm. 
     The filter must ensure virtually total attenuation of the retrodiffused Rayleigh signal, typically 30 dB higher than the Raman anti-Stokes component, and the simultaneous attenuation of the Raman Stokes component, typically 3 dB higher than the Raman anti-Stokes component. 
     Any Rayleigh signal residues introduce undesirable off-sets into the temperature measurement which alter the measurement accuracy and precision. The Rayleigh signal must therefore be attenuated to at least 20 dB below the typical powers of the Raman anti-Stokes component, the implemented filter having therefore to ensure total Rayleigh component attenuation of at least 50 dB. 
     Any Raman Stokes signal residues reduce the measurement temperature resolution ΔT. To ensure a temperature resolution ΔT of the order of 1° C. the Raman Stokes signal residue must have a power level which is 0.15% less than the total power of the Raman anti-Stokes component. The implemented filter must therefore ensure a total Raman Stokes component attenuation of at least 30 dB. (0.15%=28 dB−&gt;28 dB+3 dB=31 dB). 
     On the basis of these assumptions and from an analysis of the attenuation curves of the implemented optical filtration circuit, visible in  FIG. 6 , an optimum wavelength range has been able to be defined for Raman temperature sensor operation. 
     In particular it can be noted that a first limitation on the wavelength λ 0  is fixed by the cut-off wavelength of the passband functioning channel of the filter  31 . This wavelength, in accordance with ITU specifications, is set at 1565 nm (upper end of band C equal to 1525-1560 nm). Below this wavelength, it would not be possible to obtain the necessary attenuation for the Rayleigh signal component. Moreover because of the previous assumptions, an attenuation of at least 30 dB on the Raman Stokes component and at least 50 dB on the Rayleigh component must be achieved. Although the first condition is easily obtainable on both channels, the second condition is more limiting, especially on the channel  21 , and hence sets a second constraint on the operative wavelength λ 0 . It can be noted that the minimum attenuation value (50 dB) is obtained at 1570 nm. 
     If it is decided to shift the operative wavelength λ 0  towards the far infrared region, the advantage in terms of attenuation of those spectral components not concerned in the measurement is counteracted by the loss in terms of useful signal attenuation. In this respect, because of infrared absorption phenomena, the average attenuation of the sensor fibre increases considerably, with an obvious penalization of the Raman anti-Stokes components analyzed for temperature determination. 
     Moreover, by shifting the operative wavelength λ 0  towards the far infrared region, because of the particular architecture of the implemented optical filtration circuit increasingly smaller portions of the Raman anti-Stokes signal are analyzed, to the detriment of overall sensor temperature resolution ΔT. The power of the excitation signal also decreases. Hence those spectral components not required for the measurement must be attenuated while simultaneously maximizing the analyzed Raman anti-Stokes signal portions, to ensure a temperature resolution ΔT of the order of 1° C. It has been verified experimentally that above an operative wavelength λ 0  corresponding to 1580 nm, it is not possible to obtain these temperature resolution ΔT values. 
     For the aforesaid reasons, the present invention proposes an operative wavelength range for the operation of a Raman temperature sensor, in particular stating that the probe signal wavelength λ 0  must be between 1570 nm and 1580 nm. 
     The choice of an optimum wavelength λ 0  range for the operation of a Raman optical fibre temperature sensor enables a sensor to be formed using standard optical components satisfying the specifications set by the ITU (International Telecommunication Union) in terms of certified quality and reliability. 
     The system conceived in this manner is susceptible to numerous modifications and variants, all falling within the scope of the inventive concept; moreover all details can be replaced by technically equivalent elements.