Temperature and/or strain sensors comprise one or more sensing optical fibers using one or more of those optical effects, mentioned above. Some prior art sensors are only able to measure one parameter, either temperature or strain. Other prior art sensors are capable of measuring both parameters, but with a poor precision. Other prior art sensors are very sophisticated and expensive to use.
In the sensing optical fiber that is part of a sensor, Brillouin shift(s) are generated by acoustic phonons within the sensing optical fiber. These Brillouin shifts present variations caused by temperature and strain variations on the sensing optical fiber.
Long-range sensors, capable of simultaneous measuring strain and temperature, especially if they are distributed sensors, have many applications for measurements. These sensors are in particular useful in the power and oil industries and also for structural health monitoring, for example, in electricity supply cables. Both Raman and Brillouin effects in the sensing optical fibers can be exploited for the purpose of sensing temperature and/or strain variations.
The Raman intensity in an optical fiber is only sensitive to temperature variation. On the contrary, the intensity and frequency shift of the spontaneous Brillouin scattering signal are sensitive to temperature and strain experienced by the optical fiber. Frequency shift is measured by Brillouin Optical Time Domain Reflectometer (BOTDR).
The main difficulty for the development of Brillouin scattering based sensing systems is to differentiate and quantify the impact of both strain and temperature on the Brillouin frequency shift. Indeed, it is difficult to know whether the observed frequency shift is caused by the change of strain or by the change of temperature. In a laboratory environment, the temperature is essentially constant and its effects can generally be neglected when measuring strain. In many field conditions, this is, however, not the case.
According to a first prior art, described in an article by Bao&Chen, Sensors, 2011, 11, 4152-4187, two different optical fibers are used. In this first prior art, it is proposed to use two optical fibers in the same sensing system. The first optical fiber measures the effect of both strain and temperature, while the other optical fiber is isolated from any strain effects, so that it can be used to monitor the temperature only. The strain is then deduced by comparison of the results for both optical fibers. One drawback of this first prior art is the need for two different optical fibers. Another drawback of this first prior art is the complicated management of results obtained from different optical fibers in order to be able to deduce both temperature and strain variations.
According to a second prior art, described in patent application EP 2110651, two optical fibers having different core and clad compositions are used. In this second prior art, a method and system for simultaneously measuring strain and temperature characteristics of an object are disclosed. This involves the attachment to the object of a pair of optical fibers having different refractive indices. The optical fibers are connected together at at least one end thereof, and laser light is directed into at least one end of the optical fibers. The Brillouin frequency of each of the optical fibers is measured. Then, the strain and temperature characteristics are calculated, based on the coefficients of strain and temperature and the measured Brillouin frequencies of the optical fibers. Here again, this second prior art presents drawbacks which are similar to the drawbacks of the first prior art.
According to a third prior art, described in patent application US 2008/0084914, a very specific optical fiber with two cores, or several optical fibers are used as sensing optical fibers. This type of sensor has a structure which enables accurate temperature measurement in a wide temperature range including a low-temperature region. It seems suitable for independently and accurately determining temperature variations and strains appearing in an object to be measured. In particular, the sensor section has such a structure that the variation of the Brillouin spectrum in response to a disturbance differs between the waveguides. Thus, by simultaneously monitoring the Brillouin spectra that vary in a different manner in a plurality of waveguides, it seems possible not only to measure accurately the temperature in a wide temperature range including a low-temperature region, but also to make distinction between the strain and temperature. Use of a very specific optical fiber or of several optical fibers makes this third prior art present comparable drawback to first prior art.
According to a fourth prior art, described in patent application WO 2010/011211, a sensing system is known using two optical sources generating respectively two optical signals at two different wavelengths in order to get two different Brillouin shifts. This sensing system presents the drawback of being relatively complex.
According to a fifth prior art, described in an article by T. R. Parker and al., IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 34, NO. 4, APRIL 1998, 645-659, a sensing system is known using, simultaneously, both Brillouin gain and Brillouin shift: The analysis of both Brillouin gain and Brillouin shift provides simultaneous temperature and strain measurements using the standard communication optical. A drawback of this fifth prior art is its lack of precision due to insufficient spatial, temperature and strain resolution. Besides, using Brillouin gain, makes the sensing system of this fifth prior art very sensitive to optical fiber aging.
According to a sixth prior art, described in an article by Yu, Q and al., Opt. Lett. 2004, 29, 17-19, to use of three parameters is known, viz. the Brillouin frequency, the Stokes power, and the Brillouin spectral width. These three parameters can be simultaneously used for temperature and strain measurements, but a very specific and expensive optical fiber is needed to obtain a good sensing precision. Because controlling the stability of this very specific optical fiber over a long length is very difficult, the corresponding sensing system—using this very specific sensing optical fiber—cannot be used for distributed sensors over a long distance. Besides, this sensing system appears to be rather sensitive to optical fiber aging.
According to a seventh prior art, described in an article by V. Lanticq, PhD thesis, 2009, http://tel.archives-ouvertes.fr/pastel-00006220/, the use of a special single mode optical fiber is known. Said special single mode optical fiber presents a double Brillouin shift due to two germanium doped zones respectively in clad and core. These two zones are respectively doped with different concentrations of doping element. Since the two doped zones are doped with the same doping element, and since the difference of concentrations is not very high, the two Brillouin shifts present close temperature and strain evolutions. Therefore, discriminating between temperature and strain contributions is very difficult to perform. Obtained precision is consequently poor.
According to a eighth prior art, described in patent application US 2003/0103549, a sensing system using successively several sections of different optical fibers is contemplated. Here, each optical fiber presents a doped core with one doping element. The optical fiber line, obtained from different optical fibers concatenated together, presents several Brillouin shifts which can be used to discriminate between temperature and strain contributions. One drawback of this eighth prior art is the complexity of a sensing system based on the concatenation of several different optical fibers together. Another drawback of this eighth prior art is that its sensing system only gives a global temperature and strain measurement and not distributed measures all along the sensing optical fibers: this sensing system is a local sensor and not a distributed sensor.