Abstract:
Apparatus for a spatially resolved temperature measurement, with at least one optical fiber ( 6 ) for the spatially resolved temperature measurement, and at least one laser light source ( 2 ) producing light ( 3, 23 ) which can be coupled into the optical fiber ( 6 ), wherein the portions of the light ( 3, 23 ) backscattered in the optical fiber ( 6 ) can be coupled out of the optical fiber ( 6 ) and evaluated. The apparatus further includes means for reducing polarization-induced effects, wherein the means may be, for example, a polarization modifier ( 4 ) capable of at least partially depolarizing the light ( 3 ).

Description:
This is a continuation application of PCT/EP2009/001879, filed on Mar. 14, 2009, claiming priority to DE 10 2008 023 777.9 filed on May 15, 2008. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to an apparatus for spatially resolved temperature measurement according to the preamble of claim  1 . 
     (2) Description of Related Art 
     An apparatus of the aforedescribed type is disclosed, for example, in EP 0 692 705 A1. Fiber-optic temperature measurement systems (Distributed Temperature Sensing—DTS) can employ optical effects in optical fibers for spatially resolved temperature measurements. For example, the effect caused by Raman scattering can be used. The radiation from a narrowband source of electromagnetic radiation (e.g., from a laser) is inelastically scattered in the fiber material. The ratio of the intensities of the scattered radiation with a wavelength shorter than the excitation wavelength (anti-Stokes scattered radiation) and of the scattered radiation with a wavelength longer than the excitation wavelength (Stokes scattered radiation) is temperature-dependent and can be used to determine the temperature. By using frequency-domain techniques (Optical Frequency Domain Reflectometry—OFDR), which are described in EP 0 692 705 A1 and EP 0 898 151 A2, or pulse techniques (Optical Time-Domain Reflectometry—OTDR), the temperature can be determined along the fiber with spatial resolution. Such a temperature measurement systems can be used, for example, for monitoring fires in tunnels and ducts, for monitoring power cables and pipelines, and in the oil and gas exploration. 
     A DTS device generally includes, in addition to the corresponding coupling optics, the following essential optical components: 
     a laser light source, 
     a spectral splitter for coupling the light from a laser light source into the optical fiber used for the measurement and for separating the Raman scattered light portions of the laser light backscattered from the optical fiber, 
     an optical fiber used for the measurement, 
     a spectral splitter for separating Stokes and anti-Stokes scattered light, 
     filters for the Stokes and the anti-Stokes scattered light, 
     detectors for the Stokes and the anti-Stokes scattered light. 
     Instead of two filters, changeable or interchangeable filters can also be used for the Stokes and the anti-Stokes scattered light. When using interchangeable filters, both channels are measured sequentially. This is disadvantageous for the measurement time, but may have cost advantages as well as advantages for the accuracy, because identical channels are used for both signals. 
     A DTS device can principally be constructed mostly as free space optics. However, fiber-optic setups are frequently employed for a number of practical reasons (efficiency, stability). 
     One problem with spatially resolved temperature measurements using optical fibers is the change of the polarization along the fiber. This occurs mostly, but not exclusively, with single mode fibers. The exciting radiation is typically polarized. Because the Raman scattering can also be polarized, the Raman scattered light portions returned from the fiber can also be polarized. The Raman scattered light is detected with spectral splitters, filters and possibly other polarization-dependent components. The result of the measurement can therefore depend on the polarization. 
     In multimode fibers, the different modes propagate with slightly different velocities, and the effects of the fiber on the polarization are also mode-dependent. A more or less homogeneous mixture of different polarization states is formed over longer distances. The problem associated with polarization effects in DTS measurements hence exists predominantly in single mode fibers and in measurements with multimode fibers having few modes or short lengths. 
     In the fiber, the polarization plane can be rotated or the polarization can be changed in other ways by effects, such as stress-induced birefringence. The measured signals then depend not only in the desired manner on the local temperature, but also on the local polarization at the measurement location or the change of the polarization on the path through the fiber. Even if the polarization effects affect the measured quantities only slightly, they can still have a significant effect on the determination of the temperature, possibly reaching, for example, several ° C. Such effects can therefore limit the temperature resolution of DTS devices. In particular in devices operating with single mode fibers, modulations on the temperature curves with an amplitude of several ° C. and a wavelength of several meters to several 10 m are observed. These modulations are caused by rotations of the polarization plane due to stress-induced birefringence in the fiber material. 
     The problem forming the basis for the present invention is to provide an apparatus of the aforedescribed type which is capable of attaining higher temperature resolution and/or spatial resolution. 
     BRIEF SUMMARY OF THE INVENTION 
     This is attained according to the invention with an apparatus of the aforedescribed type having the characterizing features of claim  1 . The dependent claims recite preferred embodiments of the invention. 
     According to claim  1 , the apparatus includes means for reducing polarization-induced effects. By employing these means, the impact of the aforedescribed polarization effects can be reduced such that the temperature resolution and/or the spatial resolution of the apparatus can be improved. 
     For example, the means for reducing polarization-induced effects may include a polarization modifier, which may at least partially depolarize the light from the at least one laser light source before the light is coupled into the optical fiber, or which may temporally and/or spatially change the polarization state of the light before coupling. 
     The polarization modifier is intended to affect the polarization of the laser light or potentially also of the scattered light, so that the polarization dependence of components, such as spectral splitters and filters is no longer a factor. 
     An ideal polarization modifier would cancel the polarization of the light and therefore operate as a depolarizer. However, this is not possible in all situations or is associated with significant complexity. 
     In particular, coherent radiation is always polarized, so that only its polarization state (linear, circular, elliptic, polarization axes) can be affected. 
     On the other hand, actual depolarization is not required for attaining the desired polarization independence of the detection. Instead, polarization modifiers may be used which temporally and/or spatially change the polarization state and hence produce averaging of different polarization components in the detection. 
     A polarization modifier which is particularly suited for an apparatus according to the invention operates as follows:
         splitting the light into two portions of similar intensity,   rotating the polarization plane of one portion by 90°,   delaying one portion by a path length which is greater than the coherence length of the at least one laser light source, but smaller than the desired spatial resolution of the apparatus,   coupling both portions into the optical fiber used for the temperature measurement.       

     This arrangement has no moving parts, does not require a supply of energy and can be implemented cost-effectively. The delay path length must be greater than the coherence length in order to prevent polarized light to be present after the depolarizer. Because large delay path lengths affect the spatial resolution, their use in high-resolution apparatuses may possibly be limited. However, other solutions can be used for high-resolution apparatuses, for example optically active rotating discs (half-wavelength plates), electroactive cells for changing the polarization, or mechanically stressed optical fibers, which can alter the polarization due to birefringence induced by the mechanical stress. 
     A polarization modifier can generally be constructed mostly from free space optics. However, the polarization modifier may also be constructed with fiber optics. 
     Alternatively or in addition, the means for reducing polarization-induced effects may include at least one filter having properties, in particular its transmission, which differ for two polarization directions, or for each pair of mutually orthogonal polarization directions, by less than 10%, in particular less than 5%, preferably less than 1%. 
     Alternatively or in addition, the means for reducing polarization-induced effects may include at least one spectral splitter with properties, in particular its transmission and/or its reflection, which differ for two polarization directions, or for each pair of mutually orthogonal polarization directions, by less than 10%, in particular less than 5%, preferably less than 1%. 
     These two measures reduce polarization-induced effects on the temperature measurement. 
     The operation of spectral splitters and other wavelength-selective filters can significantly depend on the polarization. This polarization dependence is caused, for example, by polarization-dependent reflection and diffraction at oblique light incidence. 
     One approach for reducing the polarization dependence of thin-film filters is the use of special layer designs which have very similar properties at the respective wavelength for both polarization components. 
     Another approach is the use of small angles of incidence. The filters are polarization-independent for normal incidence. At small angles of incidence, for example less than 10°, polarization effects can be sufficiently small so as to allow precise temperature measurements with an apparatus according to the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present invention will become evident from the following description of preferred exemplary embodiments with reference to the appended drawings, wherein: 
         FIG. 1  is a schematic diagram of a first embodiment of an apparatus according to the invention; 
         FIG. 2  shows an exemplary embodiment of a polarization modifier; 
         FIG. 3  shows a detail of a schematic view of a second embodiment of an apparatus according to the invention; and 
         FIG. 4  is a schematic diagram of a third embodiment of an apparatus according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Identical elements or elements performing the same function are indicated in the figures with identical references symbols. 
       FIG. 1  illustrates an embodiment of an apparatus according to the invention with a laser light source  2  controlled by control means  1 . The light  3  from the laser light source  2  passes through a polarization modifier  4  which can depolarize the light  3  or temporally and/or spatially change the polarization state of the light  3 . After passing through the polarization modifier  4 , the light  3  is coupled by coupling means, which include a spectral splitter  5  and for example a lens  6 , into an optical fiber  7  used for the temperature measurement. 
     The lens  6  and the spectral splitter  5  also operate as decoupling means and can transmit the backscattered portions of the light  3  generated by the laser light source  2  to schematically indicated evaluation means  8 . The evaluation means  8  include, for example, a spectral splitter  9  for the laser wavelength and the Raman scattered radiation as well as two detectors  10 ,  11  for the Stokes and the anti-Stokes scattered radiation, with unillustrated filters being arranged before the detectors  10 ,  11 . The evaluation means  8  further include measurement electronics  12 . Optionally, a detector for the Rayleigh wavelength can also be provided. 
     The filters can be constructed so that they have similar transmission characteristics for mutually orthogonal linear polarizations. For example, the transmission at a Raman wavelength to be detected for two or for each pair of mutually orthogonal polarization directions may differ by less than 1%. The polarization-dependent effect of the filters on the temperature measurement is hereby minimized. 
     The spatially resolved temperature measurement in the optical fiber  7  can here be performed with a method corresponding to the OFDR method described in EP 0 692 705 A1. In particular, the light  3  from the laser light source  2  can be frequency-modulated, and a Fourier transformation can be performed in the evaluation means  8 . 
       FIG. 1  also shows a connection  34  between the control means  1  of the laser light source  2  and the measurement electronics  12 . This connection is used for synchronizing the laser light source  2  and the measurement electronics  12 . 
       FIG. 2  shows an exemplary embodiment of a polarization modifier  4 . The illustrated polarization modifier  4  includes a polarization beam splitter  13  and two Faraday mirrors  14 ,  15 , with each of the Faraday mirrors including a mirror and a polarization rotator. The polarization rotator can be implemented as a 45° Faraday rotator or a quarter-waveplate. 
     The light  3  from the laser light source  2  is incident on the polarization beam splitter  13  from the left side in  FIG. 2 . The light  2  should have a linear polarization  16  which is oriented at an angle of 45° with respect to the parallel and orthogonal polarization or a vertical direction in  FIG. 2 , respectively. A first portion  17  of the light  3  is reflected upward by the polarization beam splitter  13 . This first portion  17  has a polarization  18  corresponding to a parallel polarization. A second portion  19  of the light  3  passes unimpededly through the polarization beam splitter  13 . The second portion  19  has a polarization  20  which corresponds to an orthogonal polarization. 
     The first portion  17  is reflected by the first Faraday mirror  14  downward in  FIG. 2 , whereby its polarization is rotated by 90°, thus producing a orthogonal polarization  21 . The second portion  19  is reflected by the second Faraday mirror  15  to the left in  FIG. 2 , whereby its polarization is also rotated by 90°, thus producing a parallel polarization  22 . When the two portions  17 ,  19  are once more incident on the polarization beam splitter  13 , they are combined by the polarization beam splitter  13  and exit therefrom downward in  FIG. 2 . 
     The optical path of the first portion  17  from the polarization beam splitter  13  through the first Faraday mirror  14  back to the polarization beam splitter  13  is hereby shorter than the optical path of the second portion  19  from the polarization beam splitter  13  through the second Faraday mirror  15  back to the polarization beam splitter  13 . This is attained, in particular, with a greater distance between the polarization beam splitter  13  and the second Faraday mirror  15  compared to the distance between the polarization beam splitter  13  and the first Faraday mirror  14 . The resulting optical path difference of the portions  17 ,  19  should be greater than the coherence length of the light  3 . 
     In this situation, the light  23  exiting from the polarization beam splitter  13  downward in  FIG. 2  (see also  FIG. 1 ) has a component with orthogonal polarization  24  as well as a component with a parallel polarization  25 , which are not mutually coherent. This depolarizes the light  23  in an ideal situation. 
     According to a preferred embodiment of the present invention, both the polarization beam splitter  13  as well as the entire polarization modifier  4  may be constructed of fiber optic components. It would be possible to construct the polarization modifier  4  as a single-piece fiber optic module which can be connected, in particular, by way of optical fibers with the other components of the apparatus according to the invention for a spatially resolved temperature measurement. 
     The polarization modifier  4  depicted in  FIG. 2  is only one of many possible examples. This and other examples of suitable polarization modifiers are disclosed in US 2007/0297054. 
       FIG. 3  shows a spectral splitter  26  which can be used instead of the spectral splitter  5  of  FIG. 1 . The spectral splitter  26  is tilted with respect to the vertical  27  in  FIG. 3  by an angle α less than 10°. The angle of incidence under which the light  3  is incident on the spectral splitter  26  is then also less than 10°. The portions  28  of the light  3  backscattered from the optical fiber  6  are reflected by the spectral splitter  26  at an angle  2 α and coupled into an optical fiber  29  which is constructed to supply the detected components  28  to the evaluation means  8 . 
     Due to the almost orthogonal incidence on the spectral splitter  26 , the spectral splitter  26  operates substantially polarization-independent. 
     The embodiment of an apparatus according to the invention illustrated in  FIG. 4  includes, in addition to a first laser light source  2 , a second laser light source  30  which is also controlled by the control means  1 . The two laser light sources  2 ,  30  have a different polarization, in particular a mutually orthogonal linear polarization, and are not mutually coherent. The light  3 ,  31  from the laser light sources  2 ,  30  is combined by a polarization coupler  32  and coupled into the optical fiber  7  by way of the spectral splitter  5  and the lens  6 . The portions of the light  3 ,  31  produced by the laser light sources  2 ,  30  and backscattered in the optical fiber  7  are supplied via the lens  6  and the spectral splitter  5  to schematically indicated evaluation means  8 . The evaluation means  8  include, for example, a filter  33  for the Raman scattered radiation and a detector  10  for the Stokes scattered radiation. The filter  33  is here constructed as an interchangeable filter, so that the two channels (Stokes and anti-Stokes scattered radiation) can be measured consecutively. Additionally, the evaluation means  8  include measurement electronics  12 . 
     Because the light  3 ,  31  backscattered in the optical fiber  6  has portions with mutually orthogonal linear polarization which are not mutually coherent, polarization-dependent effects known from conventional devices are by and large eliminated. 
       FIG. 4  also indicates a connection  34  between the control means  1  of the laser light sources  2 ,  30  and the measurement electronics  12 . This connection is used for synchronizing the laser light sources  2 ,  30  with the measurement electronics  12 .