WO2009092436A1

WO2009092436A1 - Distributed temperature sensing using two wavelengths differing by a raman shift of a waveguide

Info

Publication number: WO2009092436A1
Application number: PCT/EP2008/050635
Authority: WO
Inventors: Bernd Nebendahl
Original assignee: AP Sensing GmbH
Current assignee: AP Sensing GmbH
Priority date: 2008-01-21
Filing date: 2008-01-21
Publication date: 2009-07-30
Anticipated expiration: 2010-07-21

Abstract

A distributed temperature sensing apparatus (100) for measuring a temperature distribution along a waveguide (102), the distributed temperature sensing apparatus (100) comprising an electromagnetic radiation source (104) adapted for generating electromagnetic radiation of two wavelengths (λ 1, λ 2) to be coupled into the waveguide (102), wherein the two wavelengths (λ1, λ 2) differ by the Raman shift of the material of the waveguide (102), and an evaluation unit(106)adapted for deriving information indicative of a temperature distribution along the waveguide (102) based on an evaluation of the electromagnetic radiation after propagation through the waveguide (102).

Description

DESCRIPTION

DISTRIBUTED TEMPERATURE SENSING USING TWO WAVELENGTHS DIFFERING BY A RAMAN SHIFT OF A WAVEGUIDE

BACKGROUND ART

[0001] The present invention relates to distributed temperature sensing.

[0002] Distributed temperature sensing (DTS) systems are optoelectronic devices which measure temperatures by means of optical fibers functioning as linear sensors. Temperatures are recorded along the optical sensor cable, thus not at points, but as a continuous profile. A high accuracy of temperature determination is achieved over long distances. Measurement distances of several kilometers can be achieved. The temperature dependence of the Raman effect can be used for a DTS measurement.

[0003] GB 2,400,906 discloses a method of obtaining a distributed measurement which comprises deploying an optical fibre in a measurement region of interest, and launching into it a first optical signal at a first wavelength and at a high power level, a second optical signal at a second wavelength, and a third optical signal at the first wavelength and at a low power level. These optical signals generate backscattered light at the second wavelength arising from Raman scattering of the first optical signal which is indicative of a parameter to be measured, at the first wavelength arising from Rayleigh scattering of the first optical signal, at the second wavelength arising from Rayleigh scattering of the second optical signal, and at the first wavelength arising from Rayleigh scattering of the third optical signal. The backscattered light is detected to generate four output signals, and a final output signal is derived by normalizing the Raman scattering signal to a function derived from the three Rayleigh scattering signals, which removes the effects of wavelength-dependent and nonlinear loss.

[0004] EP 0,300,529 discloses a method of measuring temperature which comprises launching input pulses of light into a temperature sensing element and deriving the temperature at a position in the element from the intensity of light scattered at said position, a part of the element being maintained at a known temperature in order to provide a reference for deriving temperature measurements at other positions in the element, thereby to avoid difficulties with calibration of the apparatus used to carry out the method.

[0005] However, conventional DTS systems may lack sufficient accuracy, under undesired circumstances.

DISCLOSURE

[0006] It is an object of the invention to provide a sufficiently accurate distributed temperature sensing system. The object is solved by the independent claims. Further embodiments are shown by the dependent claims.

[0007] According to an exemplary embodiment, a distributed temperature sensing (DTS) apparatus for measuring a temperature of a waveguide is provided, the distributed temperature sensing apparatus comprising a waveguide (such as an optical fiber), an electromagnetic radiation source (such as one or more lasers) adapted for generating electromagnetic radiation of two wavelengths to be coupled (particularly subsequently, i.e. first only electromagnetic radiation of the first wavelength, after that only electromagnetic radiation of the second wavelength) into the waveguide, wherein the two wavelengths differ essentially (for instance with a deviation of less than ten percent, particularly of less than five percent, more particularly of less than one percent) by the Raman shift of the material of the waveguide, and an evaluation unit (such as a microprocessor) adapted for deriving information indicative of a temperature (particularly indicative of a temperature distribution) of the waveguide based on an evaluation of the electromagnetic radiation after propagation through the waveguide (particularly after inelastic scattering of portions of the electromagnetic radiation at various sections of the waveguide, resulting in a backscattered power distribution, wherein the electromagnetic radiation is influenced in a temperature dependent manner during scattering by an interaction with the material of the waveguide, thereby generating or annihilating phonons which modulates the backscattered energy pattern in a characteristic way).

[0008] According to another exemplary embodiment, a distributed temperature sensing method for measuring a temperature of a waveguide is provided, the method comprising coupling electromagnetic radiation of two wavelengths into the waveguide, wherein the two wavelengths differ essentially by the Raman shift of the material of the waveguide, and deriving information indicative of a temperature of the waveguide based on an evaluation of the electromagnetic radiation after propagation through the waveguide.

[0009] According to still another exemplary embodiment, a software program or product is provided, preferably stored on a data carrier, for controlling or executing the method having the above mentioned features, when run on a data processing system, such as a computer.

[0010] Embodiments of the invention can be partly or entirely embodied or supported by one or more suitable software programs, which can be stored on or otherwise provided by any kind of data carrier, and which might be executed in or by any suitable data processing unit. Software programs or routines can be preferably applied in the context of DTS systems. The DTS system control scheme according to an embodiment of the invention can be performed or assisted by a computer program, i.e. by software, or by using one or more special electronic optimization circuits, i.e. in hardware, or in hybrid form, i.e. by means of software components and hardware components.

[0011] In the context of this application, the term "distributed temperature sensing" may particularly denote a mechanism of determining a temperature distribution along an extension of a component or device under test such as a waveguide, particularly an optical fiber. For that purpose, pulses of electromagnetic radiation such as light or infrared radiation may be injected into the waveguide for interaction with the waveguide. In dependence of the local temperature at the specific portions of a waveguide, the radiation-matter interaction properties and the phonon generation/annihilation properties of this portion of the waveguide will be modified characteristically, and a pattern of scattered electromagnetic radiation may be detected at a detector in a manner which is characteristic for the temperature distribution. Thus, by analyzing the scattered electromagnetic radiation, it is possible to derive information indicative of the temperature distribution along the waveguide.

[0012] The term "waveguide" may particularly denote any member capable of guiding electromagnetic radiation to propagate along a defined path. Depending on the wavelength of the electromagnetic radiation to be transported through the waveguide, the waveguide may be an optical fiber, made for instance of fused silica glass, for transporting visible and infrared radiation.

[0013] The "Raman shift of a material of a waveguide" may particularly denote a material specific frequency shift between injected primary electromagnetic radiation from an electromagnetic radiation source and secondary electromagnetic radiation after propagation through the waveguide, wherein the frequency shift may result from the generation of a phonon (which is some kind of lattice vibration) or from an annihilation of a phonon. Thus, the Raman shift may originate from the Raman effect.

[0014] According to an exemplary embodiment, a distributed temperature sensing (DTS) analysis may be performed by injecting for instance pulsed electromagnetic radiation beams of two different wavelengths (preferably one after the other, i.e. only one wavelength at a time) into a waveguide and by measuring properties of reflected radiation. For that purpose, the Stokes line (or Stokes distribution) of one of the two wavelengths (particularly of the beam having the shorter wavelength) may be measured, and the anti-Stokes lines (or anti-Stokes distribution) of the other one of the electromagnetic radiation beams (particularly of the beam having the longer wavelength) is measured. By selecting the primary wavelengths of the two injected beams to differ by the Raman shift of the waveguide material, the effects of wavelength dependent damping and refractive index along the waveguide may be significantly suppressed or eliminated. Therefore, artifacts resulting from a wavelength dependent damping and refractive index along the waveguide may be reduced by a proper selection of the frequency difference between the two primary electromagnetic radiation beams. For example, a higher frequency first beam may be introduced in the waveguide, and the Stokes line resulting after wave-matter interaction may be measured, resulting in a higher frequency input wave and a lower frequency output wave. Correspondingly, a lower frequency second beam may be introduced in the waveguide, and the anti-Stokes line resulting after wave-matter interaction may be measured, resulting in a lower frequency input wave and a higher frequency output wave. Thus, the wave matter based frequency shifts of opposite signs may compensate different damping and refractive index of the input and scattered waves.

This allows to obtain better comparable sets of measurement data, since different damping characteristics and refractive index at different wavelengths along a waveguide such as an optical fiber may be at least partially compensated or equilibrated.

[0015] In contrast to conventional single input wavelength approaches in which beams related to a Stokes line and an anti-Stokes line experience different attenuation and propagation velocity of the corresponding electromagnetic radiation beam (in time and/or in space) when traveling to a detector, exemplary embodiments apply sequential stimulus signals having frequencies f1 and f2, whereas f2-f1 =v, v being the

Raman shift of the material. A result is that the detector "sees" essentially the same attenuation and propagation velocity for both measurements, and fiber attenuation and dispersion effects can be cancelled out at least partially.

[0016] Next, further exemplary embodiments of the distributed temperature sensing apparatus will be explained. However, these embodiments also apply to the distributed temperature sensing method and to the software program or product.

[0017] The waveguide which may be part of the distributed temperature sensing apparatus and may comprise an optical fiber, particularly an optical fiber made of silica glass (SiO₂). The distributed temperature sensing apparatus may be adjusted for functioning with a specific waveguide, so that a delta between the wavelengths required for cancelling out damping and refractive index artifacts may be considered when designing the electromagnetic radiation source. Since the value of the Raman shift depends on the material of the waveguide, this shift should be adapted to the waveguide material, either at the factory or by the user.

[0018] For example, the waveguide may have a length of up to 12 kilometers and more. In such a configuration, a waveguide may be arranged along an extension of a structure the temperature of which shall be measured or monitored. For example, the waveguide may be aligned along a tunnel so as to detect a possible fire in a tunnel at a very early stage resulting in a local temperature increase of a corresponding portion of the waveguide. Alternatively, it is possible to align the waveguide along a borehole for oil or the like, so as to allow to monitor a temperature in a buried portion or even at the remote end of the borehole. Further alternatively, the waveguide may be buried and aligned parallel to a current supply line (or a data communication line) below the earth, so that local damage of a current line resulting in a local increase of the temperature may be detected easily, or the transferred power along the power supply line may be increased until a maximum acceptable temperature value is sensed by the waveguide. For such applications, it may be advantageous to take measures or provide mechanisms to ensure a proper thermal coupling between the waveguide and the structure to be monitored.

[0019] A wavelength of electromagnetic radiation generated by the electromagnetic radiation detector may be tunable to thereby allow for generating electromagnetic radiation of one of the two wavelengths at a time with a single radiation source. Therefore, a single unit for generating both wavelengths may be provided which can be brought in an operation mode to selectively generate an electromagnetic radiation (pulse) of the first wavelength at a first time, and of a second wavelength at a second time. Particularly, the electromagnetic radiation generator may be a wavelength tunable laser. By such a configuration with only a single unit, a simple operation and a compact design may be achieved.

[0020] Alternatively, the electromagnetic radiation source may comprise two separate electromagnetic radiation generation units each adapted for generating electromagnetic radiation of a particular one of the two wavelengths. In such an embodiment, two different apparatuses may be used to generate electromagnetic radiation (pulses) of the two wavelengths which allows for the use of very simple electromagnetic radiation sources which may be very small in size and cheap. For example, the two separate electromagnetic radiation generation units may be two separate lasers, particularly may be two separate laser diodes.

[0021] The electromagnetic radiation source may be adapted for generating electromagnetic radiation of only one of the two wavelengths at a time. For example, in a first sequence of a DTS analysis, electromagnetic radiation of the first wavelength may be generated and may be injected in a pulsed manner into the waveguide. In a subsequent second sequence, only the electromagnetic radiation source of the other wavelength may be used to inject radiation into the waveguide for a separate second measurement phase. By separating the two measurement sequences, undesired interaction or crosstalk between the two traveling signals of different wavelengths may be avoided. Further, the unambiguous assignment of a reflection signal to one of the two wavelengths may be easier. With such a sequence, it may be possible to measure an anti-Stokes line and a Stokes line, one after the other.

[0022] The electromagnetic radiation source may be adapted for generating essentially monochromatic electromagnetic radiation at a time. Electromagnetic radiation emitted, for instance, by a laser may be considered as essentially monochromatic electromagnetic radiation. Such an essentially monochromatic or narrow band electromagnetic radiation beam may allow to perform the DTS measurement in a very simple and accurate manner, since broadened exciting distributions of electromagnetic radiation may also smear out the Stokes and anti- Stokes lines to be analyzed, which can therefore be prevented according to an exemplary embodiment.

[0023] The electromagnetic radiation source may be adapted for generating electromagnetic radiation of two wavelengths which differ by a difference between a center (for instance a geometrical center of gravity) of the Rayleigh peak and a center (for instance a geometrical center of gravity) of the Stokes peak of the material of the waveguide. Although in theory, the Stokes peak and the anti-Stoke peaks as well as the Rayleigh peak of electromagnetic radiation propagating through a waveguide may be very narrow lines (the Rayleigh peak is assigned to the primary electromagnetic radiation beam, and originates from elastic scattering of the lattice; the Stokes peak and the anti-Stokes peak relate to the generation and annihilation, respectively, of a phonon depending on the solid-state properties of the material of the waveguide), broadening of such peaks may occur in practice. To properly cover the entire area of the Rayleigh peak, the Stokes peak and also of the anti-Stokes peak, an integration over such a distribution being the realistic fingerprint of the theoretically assumed narrow peak may be performed. Thus, a center of gravity of the Rayleigh distribution and of the (anti-)Stokes distribution may be calculated which can be obtained by a summation or integration of the intensity values of the respective distributions over the extension of the distribution, divided by the peak distribution width. By considering such broadening effects, the accuracy may be further increased.

[0024] The electromagnetic radiation source may be adapted for generating electromagnetic radiation of two wavelengths (or frequencies) differing by about 10.5 THz. By such a frequency shift between the two primary electromagnetic radiation beam pulses, proper damping and refractive index compensation in accordance with a waveguide made of fused silica glass can be obtained. Fused silica glass may be a preferred material for optical fibers. A frequency distance of 10.5 THz may be obtained, for instance, by generating pairs of wavelengths by the electromagnetic radiation source of approximately 1064 nm and 1026 nm, or of approximately 1310 nm and 1373 nm, or of approximately 1480 nm and 1560 nm. These wavelength pairs have turned out to be particularly appropriate, since they can be generated with reasonable effort due to the availability of commercial laser diodes.

[0025] The electromagnetic radiation source may be adapted for generating optical light or infrared light as the electromagnetic radiation. Although these two wavelength ranges may be preferred due to the ease of operation of electromagnetic radiation in the corresponding frequency ranges, it is also possible to use other frequency ranges such as UV, X-rays, microwaves, etc.

[0026] The electromagnetic radiation source may be adapted for generating electromagnetic radiation pulses of the two wavelengths. For example, subsequent pulses may be spaced in time by 40 μs, and can be repeated to result in a total measurement time of one or several seconds. An accurate time resolution can be achieved, since each individual electromagnetic radiation pulse may be evaluated separately. This allows a measurement of the temperature distribution essentially in real time.

[0027] The evaluation unit may be adapted for deriving information indicative of a temperature distribution along the waveguide based on an evaluation of an integral over a plurality of electromagnetic radiation pulses of the two wavelengths generated by the electromagnetic radiation source after propagation through the waveguide. By not relying only upon a single pulse but averaging over multiple such pulses, artifacts and noise during an individual pulse can be averaged out with high precision, allowing to derive more meaningful results.

[0028] The evaluation unit may be adapted for deriving spatially dependent information indicative of a spatial temperature distribution along the waveguide. For that purpose, an algorithm may be stored in the evaluation unit or may be accessible by the evaluation unit which allows to calculate back, from the distribution of Stokes and anti-Stokes lines, particularly from an area relation there between, the temperature distribution along the waveguide. Such algorithms are known as such but can be applied with significantly increased accuracy according to exemplary embodiments, since waveguide dependent damping and refractive index artifacts are efficiently suppressed.

[0029] The evaluation unit may be adapted for deriving information indicative of a temperature distribution along the waveguide based on an evaluation of a Stokes peak after propagation of electromagnetic radiation of the smaller one of the two wavelengths through the waveguide and of the anti-Stokes peak after propagation of electromagnetic radiation of the larger one of the two wavelengths through the waveguide. By this pair wise correlation of Stokes peak and smaller wavelength primary beam, and anti-Stokes peak and larger wavelength primary beam, the corresponding artifacts may be corrected or compensated, thereby allowing for a high precision measurement.

[0030] The evaluation unit may be adapted for deriving information indicative of a temperature distribution along the waveguide based on an evaluation of a Stokes peak after propagation of electromagnetic radiation of only the smaller one of the two wavelengths through the waveguide and of the anti-Stokes peak after propagation of electromagnetic radiation of only the larger one of the two wavelengths through the waveguide. By, at a time, injecting only one of the two wavelengths into the waveguide and performing the corresponding measurement, any crosstalk may be prevented and a reliable assignment of a measurement signal to a primary electromagnetic radiation beam is possible, thereby increasing the accuracy.

[0031 ] The evaluation unit may be adapted for deriving information indicative of a temperature distribution along the waveguide based on an evaluation of a ratio between an area of the Stokes peak and an area of the anti-Stokes peak. Such an area ratio is a numerically easily calculable parameter and is a source of meaningful information needed for deriving the temperature information.

[0032] The distributed temperature sensing apparatus may comprise a detection unit adapted for detecting the electromagnetic radiation after propagation through the waveguide and for supplying corresponding detection signals to the evaluation unit. Such a detector may be a photodiode, or the like. It is also possible to use a CCD camera or a CMOS camera.

[0033] A single detection unit may be provided for detecting electromagnetic radiation of the two wavelengths. By synergetically combining the detection tasks for detecting both wavelengths in one common detection unit, the DTS apparatus may be manufactured with small dimensions and in a compact manner. Furthermore, conventionally occurring artifacts resulting from different detectors which may have different properties or which may be calibrated in a differing manner may be eliminated.

[0034] The single detection unit may be adapted for detecting electromagnetic radiation of the two wavelengths in a multiplexed manner. Therefore, in a first sequence, the detection unit may detect secondary electromagnetic radiation originating from the primary beam with the higher frequency, and in a second sequence the detection unit may detect a secondary electromagnetic radiation beam originating from a primary electromagnetic radiation beam of the smaller frequency, or vice versa.

[0035] The DTS sensing apparatus may comprise a switch adapted for selectively switching the detection unit into a selectable electromagnetic propagation path corresponding to electromagnetic radiation of one of the two wavelengths. Such a switch may make the redundant provision of corresponding optical elements dispensable, since the switch may simply modify the optical path in accordance with the specific measurement sequence.

[0036] In one embodiment, the switch may comprise a movable, particularly a longitudinally shiftable, optical switch mechanically connected to the waveguide and movable between two positions for selectively coupling electromagnetic radiation of one of the two wavelengths into the waveguide. With such a scenario, it is possible to couple two different electromagnetic radiation sources to one and the same waveguide by mechanically actuating the switch. [0037] Particularly, the switch may comprise a prisma and an at least partially reflective mirror, the prisma being movable without a mechanical connection to the waveguide between two positions for selectively coupling electromagnetic radiation of one of the two wavelengths into the waveguide. Such a prisma may be rotatable (see Fig. 8, Fig. 9) or longitudinally shiftable (see Fig. 6, Fig. 7) relative to the waveguide for selectively coupling electromagnetic radiation of one of the two wavelengths into the waveguide, and/or for selectively coupling reflected electromagnetic radiation from the waveguide to the detector. As examples for such a prisma, a retroflective prisma or a parallel offset prisma (POP) may be implemented.

[0038] The distributed temperature sensing apparatus may comprise a wavelength selective filter arranged between the waveguide and the detection unit. Such a wavelength selective filter may be a low pass filter (allowing only frequencies below a threshold value to pass the filter), a high pass filter (allowing only frequencies above a threshold value to pass the filter), or a band-pass filter (allowing only frequencies below an upper threshold value and above a lower threshold value to pass the filter), or may be switchable between different filter configurations.

[0039] The wavelength selective filter may be adapted to be operable in a low pass operation mode and a high pass operation mode. The low (frequency) pass operation mode may be adjusted during propagation of primary electromagnetic radiation of the smaller one of the two wavelengths through the waveguide thereby allowing for measurement of the Stokes line. The high (frequency) pass operation mode of the wavelength selective filter may be actuated during propagation of primary electromagnetic radiation of the larger one of the two wavelengths through the waveguide, thereby allowing for a measurement of the anti-Stokes lines.

[0040] According to an exemplary embodiment, the wavelength selective filter may be a wavelength division multiplexing (WDM) filter.

[0041 ] The distributed temperature sensing apparatus may comprise a temperature sensor and a reference coil coupled to the waveguide and having a temperature measurable by the temperature sensor. The temperature sensor may be adapted for supplying the evaluation unit with temperature information indicative of the temperature distribution along the reference coil portion as a basis for a correction or calibration of the evaluation procedure. By taking this measure, the accuracy of the evaluation unit may be further increased, since the temperature sensed at the reference coil may allow to correct for thermal disturbation effects in the entire arrangement.

[0042] The distributed temperature sensing apparatus may be adapted as a single- ended distributed temperature sensing apparatus or a double-ended distributed temperature sensing apparatus.

[0043] The Raman effect may be denoted as a solid-state body process in which a frequency shift occurs in a material dependent manner due to an inelastic scattering of photons at the lattice, thereby generating or annihilating lattice vibrations. In an ideal single crystal, this results in discrete shifted lines, whereas a distribution occurs in a real solid-state body.

[0044] In a DTS system according to an exemplary embodiment, light is injected into a waveguide and the reflected light is detected. Considering inelastic scattering effects, the ratio between the Stokes line and the anti-Stokes line may be taken to derive information regarding the temperature to be evaluated (which, via a Boltzmann distribution, has an influence on the areas of the Stokes and the anti-Stokes line). Thus, it is possible to measure the temperature along the waveguide in a spatially dependent manner.

[0045] In contrast to conventional approaches, in which the damping and refractive index of the radiation is wavelength-dependent which has a disturbing influence of the ratio between Stokes and anti-Stokes lines, such a difference may be compensated at least partially according to exemplary embodiments by performing two measurements with wavelengths shifted with respect to one another.

[0046] According to an exemplary embodiment, it is possible to measure backscattering power values in the nW range, and to measure the temperature in a 4 km measurement distance with a spatial accuracy of around 1 m. The measurement may be performed to obtain a time resolution of only several seconds by sending pulses each 40 μs and receiving the corresponding response of the waveguide. The measured power may be integrated over several seconds, to suppress artifacts. [0047] In the context of a single-ended measurement arrangement, the insertion and detection of light occurs at the same end of the waveguide. In a double-ended principle, the insertion and detection occurs at the same end of the waveguide, but measurements are taken from both sides, resulting in more information at the cost of a slightly increased effort. In other words, the measurement may be performed first on the left-hand end of a waveguide, and then on a right-hand end of a waveguide.

[0048] According to an exemplary embodiment, the wavelengths of the exciting electromagnetic radiation are selected so that a geometric center of gravity (which depends on the material of the waveguide) of the Stokes line at a first wavelength essentially corresponds to the anti-Stokes lines at the other wavelength. Therefore, the damping and refractive index in the forward direction and in the backward direction along the waveguide may be compensated efficiently.

[0049] Therefore, according to an exemplary embodiment, a light source can be provided having two wavelengths which are shifted with respect to one another by the Raman shift of the light fiber so that an evaluation unit may be provided with all necessary information for deriving the temperature characteristics of the waveguide. In a corresponding method, it is possible to irradiate a waveguide with the two different wavelengths one after the other, first measuring Stokes and then anti-Stokes, or vice versa. Thus, a calibration-free measurement may be obtained.

[0050] According to an exemplary embodiment, a filter with a separation characteristics between the two wavelengths may be implemented, thereby allowing to filter out radiation which is not needed for an actual evaluation procedure.

[0051] According to an exemplary embodiment, it is possible to have only a single detection unit such as a photodiode which simplifies the design.

[0052] Thus, a DTS system may be provided allowing for a compensation of a wavelength dependent damping and refractive index by the use of two properly shifted wavelength values.

BRIEF DESCRIPTION OF DRAWINGS

[0053] Other objects and many of the attendant advantages of embodiments of the present invention will be readily appreciated and become better understood by reference to the following more detailed description of embodiments in connection with the accompanied drawings. Features that are substantially or functionally equal or similar will be referred to by the same reference signs.

[0054] Fig. 1 to Fig. 15 show distributed temperature sensing apparatuses according to exemplary embodiments.

[0055] Fig. 16 gives an overview over the Raman effect, OTDR technology, and a coding scheme.

[0056] Fig. 17 is a diagram showing that the intensity ratio of Stokes and anti- Stokes lines only depends on fundamental and material constants and on temperature.

[0057] Fig. 18 illustrates an instrument.

[0058] Fig. 19 illustrates a dual-ended DTS apparatus.

[0059] The illustration in the drawing is schematically.

[0060] In the following, referring to Fig. 1 , a distributed temperature sensing (DTS) apparatus 100 for measuring a temperature distribution along a waveguide 102 according to an exemplary embodiment will be explained.

[0061] The distributed temperature sensing apparatus 100 comprises an electromagnetic radiation source 104 for generating electromagnetic radiation of two wavelengths A₁, A₂ to be coupled into the waveguide 102, wherein the two wavelengths A₁, A₂ exactly differ by the Raman shift of the material of the waveguide 102 (silica glass). Beyond this, an evaluation unit 106 (such as a microprocessor or a CPU, central processing unit) is provided which is adapted for deriving information indicative of a temperature distribution along the waveguide 102 based on an evaluation of electromagnetic radiation emitted by the electromagnetic radiation source 104, reflected at a certain portion of the waveguide 102 after propagation through the waveguide 102 and having traveled back to a detector 108. In the present embodiment, the waveguide 102 is an optical fiber made of silica glass.

[0062] In the embodiment of Fig. 1 , the electromagnetic radiation generator 104 is a tunable laser being tunable to allow to generate either electromagnetic radiation of a first wavelength A₁ or of a second wavelength A₂. The tunable laser 104 is configured such that, at a time, it generates either the wavelength A₁ or the wavelength A₂ and directs or couples electromagnetic radiation having exactly one of these two wavelengths A₁, A₂ at a time into the optical fiber 102. The two wavelengths A₁, A₂ differ by a distance between a geometric center of gravity of a Rayleigh peak and a geometrical center of gravity of a Stokes peak of the material of the waveguide 102. This can be derived mathematically by fitting a spectrum such as the one shown in Fig. 16, on the left-hand side.

[0063] The frequencies of the electromagnetic radiation of the wavelengths A₁, A₂ differ by 10.5 THz. In the present embodiment, A₁ = 1064 nm, and A₂ = 1026 nm.

[0064] The electromagnetic radiation source 104 emits pulses of the infrared radiation and couples corresponding beam via an optical coupler 110 into the waveguide 102. The electromagnetic radiation propagates along the waveguide 102 and is, for instance due to inconsistencies of the material of the waveguide 102, partially reflected along the waveguide 102. The wave-matter (radiation A₁, A₂, material of the waveguide 102) interaction properties, such as inelastic scattering of the photons at the lattice of the waveguide 102, is temperature dependent and results in a temperature-dependency of the intensities of Stokes line and anti-Stokes line, respectively, which are shifted to lower/higher frequencies with respect to a Rayleigh line (resulting from elastic scattering) due to phonon excitation/annihilation.

[0065] The reflected electromagnetic radiation travels back opposite to its initial propagation direction, again passes the optical coupler 110 and is detected by a detection unit 108 such as a photodiode.

[0066] The evaluation unit 106 is coupled for a unidirectional or bidirectional data communication with the detection unit 108, is coupled for a unidirectional or a bidirectional data communication with the electromagnetic radiation source 104, and is coupled for a unidirectional or a bidirectional data communication with an input/output unit 120. Via the input/output unit 120, which may be a graphical user interface (GUI), a user may communicate with the apparatus 100. For instance, the input/output unit

120 may comprise input elements such as keypads, buttons, a joystick, etc., and may comprise an output unit such as a display device, for instance an LCD device or a plasma display device.

[0067] The electromagnetic radiation source 104 emits a sequence of pulses of the wavelength A₁ or of pulses of the wavelength A₂. The detection unit 108 accumulates or integrates the detection signals of a respective wavelength A₁ or A₂ over a time of, for instance, some seconds, thereby integrating over several pulses having a distance and time of for instance 40 μs. Based on the ratio of the areas of the Stokes line and the anti-Stokes line each of which can be detected on the basis of one of the wavelengths A₁, A₂, the evaluation unit 106 may derive information indicative of a temperature distribution along the waveguide 102. For this purpose, conventional evaluation or analysis methods can be used as disclosed in, for instance, GB 2,400,906, EP 0,300,529, etc. However, the pair wise assignment of high frequency primary beam and Stokes line detection, and of low frequency primary beam and anti-Stokes line detection, allows to compensate for frequency dependent damping and refractive index effects, since an average frequency of selected primary beam and detected secondary beam is equilibrated.

[0068] According to an exemplary embodiment, the difference between A₁ and A₂ essentially equals to a Raman shift of the waveguide 102 material, so that wavelength- dependent refraction indices and the wavelength-dependent damping or attenuation along the waveguide 102 may be suppressed by evaluating the Stokes peak after propagation of electromagnetic radiation of the smaller one of the two wavelengths A₁, A₂ and evaluating of the anti-Stokes peak of the larger one of the two wavelengths A₁, A₂. In other words, for the smaller input wavelength, the shift to a higher wavelength and therefore the Stokes line will be detected. For the larger input wavelength, a shift to a smaller wavelength and therefore to the anti-Stokes line is detected. Thus, the propagation and back propagation direction results in a compensation of damping and refractive index artifacts, thereby rendering the DTS result more reliable.

[0069] Fig.2 shows a distributed temperature sensing apparatus 200 according to another exemplary embodiment.

[0070] Fig. 2 shows the device 200 in a first operation mode, and a second operation mode 300 is shown in Fig. 3. In the first operation mode, an electromagnetic radiation unit 202 emits a primary beam of electromagnetic radiation which is coupled via a 2x4 switch 206 and a WDM 208 towards a reference coil 210. The reference coil 210 is a piece of fiber of a known temperature and comprises an electrical temperature sensor (not shown) which may measure the local temperature and may supply the evaluation unit 106 (not shown in Fig. 2 and Fig. 3) with corresponding control information. Fig. 2 further shows a device under test 212, which can be the waveguide 102 itself or another optoelectronic element.

[0071] When propagating along the optical fiber 102, parts of the monochromatic electromagnetic radiation will be reflected in a backward direction. Due to wave-matter interaction, in contrast to the primary electromagnetic radiation beam, the secondary (back reflected) electromagnetic radiation beam has a wavelength distribution as indicated in Fig. 2 and particularly comprises a Stokes line, an anti-Stokes line and a Rayleigh line.

[0072] The WDM 208 in Fig . 2 is as a low pass filter between the upper left port and the port on the right and a high pass filter between the lower left port and the right port and therefore only provides signals representing the Stokes line to the photodetector 108, via the 2x4 switch 206 and a 1x2 switch 214, when the source 202 having the larger one of the two primary frequencies is activated. Therefore, in the configuration of Fig. 2, only the Stokes line is measured after activating the system 200 with the electromagnetic radiation having the higher frequency.

[0073] In the configuration of Fig. 3, electromagnetic radiation having a lower frequency is emitted by the second electromagnetic radiation unit 204, passed via the 2x4 switch 206 and the WDM 208 as well as through the reference coil 210 into the optical fiber 102, and after interaction with the DUT 212 is reflected back via the waveguide 102, the reference coil 210, the WDM 208 passes the 2x4 switch 206 and the 1x2 switch 214 and is detected by the photodiode 108. Thus, in the configuration of Fig. 3, only the anti-Stokes line is measured.

[0074] A ratio is then calculated between the areas of the Stokes- and anti-Stokes lines detected with the configuration of Fig. 2 and Fig. 3. Thus, the embodiment of Fig.

2 and Fig. 3 provides for an auto-calibrating DTS opto-design. Two laser sources 202, 204 are used emitting frequencies which are separated by the Raman shift of the optical fiber 202.

[0075] The configuration of Fig. 2 and Fig. 3 has the advantage that a differential attenuation and chromatic dispersion of the DUT fiber cancels out without any assumptions. Only one wavelength selective coupler 208 may be sufficient. Only one photodiode 108 is sufficient. The switching task may be realized with different optical configurations such as a POP (parallel offset prisma) resulting in aligned beams for a photodiode, and the offset will result in an angle error (which is not an issue for an APD, avalanche photodiode).

[0076] The two laser diodes 202, 204 may be provided with a similar design/pulse performance and may emit frequencies spaced by 10.5 THz (which can be obtained for instance by the wavelength combination of 1064 nm + 1026 nm, 1310 nm + 1373 nm, 1480 nm + 1560 nm). The spacing of 10.5 Hz is adapted for fused silica glass which is a preferred material for telecommunication fibers.

[0077] In the following, referring to Fig. 4 and Fig. 5, a distributed temperature sensing apparatus 400 according to an exemplary embodiment will be explained, which is shown in a first operation mode in Fig. 4 and in a second operation mode 500 in Fig. 5.

[0078] In the embodiment of Fig. 4, a 3x2 fiber switch is obtained by ports 402, a movable part 404 attached to flexible optical waveguides connected to ports 406 and a

WDM 408. The movable optical switch can be brought to a state as shown in Fig. 4 in which the laser diode 202 is active and generates an electromagnetic radiation beam, and the photodiode 108 detects the Stokes line. In the configuration shown in Fig. 5, the laser diode 204 is active and generates an electromagnetic radiation beam which is transmitted via the optical switch 404 through the entire system and is detected concentrated on an anti-Stokes line in the photodetector 108. Fig. 4 and Fig. 5 show an auto-calibrating DTS custom fiber switch design (butt-to-butt).

[0079] In the following, referring to Fig. 6, a distributed temperature sensing apparatus 600 is shown in a first operation mode, wherein Fig. 7 shows a second operation mode 700. [0080] In the embodiment of Fig. 6 and Fig. 7, a mechanically movable offset prisma 602, 604 is provided in free beam optic, as indicated by reference numeral 602, 604. Furthermore, mirrors 606, 608 are provided between the mechanically movable offset prisms 602, 604 on the one hand and the optical fiber 102 on the other hand. Fig. 6 and Fig. 7 show an auto-calibrating DTS free space opto-design.

[0081] In the configuration shown in Fig. 6, the laser diode 202 is active and generates electromagnetic radiation which is transmitted via the prisma 604 and the mirrors 608, 606 into the optical fiber 102, and is subsequently directed via the mirror 606 and the prisma 602 to the photodiode 108 for detection of the Stokes line.

[0082] Fig. 7 shows another operation mode of the system 600, as indicated by reference numeral 700. In this operation mode, the mechanically movable prisms 602, 604 are moved outside of the propagation path to be inactive so that the laser diode 204 generates light which is transmitted via the mirror 606 into the fiber 102, and after reflection via the mirror 606 and the mirror 608 to the detector 108. It should be ensured that the two laser beams from laser diodes 204 and 202 are aligned parallel for switching to work properly in this embodiment.

[0083] In the following, referring to Fig. 8 and Fig. 9, a distributed temperature sensing apparatus 800 according to an exemplary embodiment will be explained, which is shown in a first operation mode in Fig. 8 and which is shown in a second operation mode 900 in Fig. 9.

[0084] The embodiment of Fig. 8 and Fig. 9 is an auto-calibrating DTS free space opto-design. In the embodiment of Fig. 8, a POP (parallel offset prisma) 802 is shown which can be rotated to adjust different beam configurations.

[0085] In the operation mode shown in Fig. 8, the laser diode 202 generates an electromagnetic radiation beam which is reflected by the mirror 608 and the mirror 606 to be introduced into the optical fiber 102. After reflection, this light beam is transmitted via the mirror 606 and is reflected at the POP 802 to thereby allow the photodiode 108 to detect the Stokes line.

[0086] In contrast to this, in the embodiment of Fig. 9, the POP 802 is rotated by 180° so as to allow to introduce light from the laser diode 204 via the mirror 606 into the optical fiber 102, and the reflected radiation passes the mirrors 606, 608, the POP 802 and is detected by the photodiode 108 as an anti-Stokes line.

[0087] The mirrors 606, 608 may be dielectric filters (which may have an appropriate surface layer structure), thereby forming a multi-cavity dielectric filter having the desired optical properties. It should be ensured that the two laser beams from laser diodes 204 and 202 are aligned parallel for switching to work properly in this embodiment.

[0088] In the following, referring to Fig. 10 and Fig. 11 , a distributed temperature sensing apparatus 1000 according to another exemplary embodiment will be explained, which is shown in a first operation mode in Fig. 10 and which is shown in a second operation mode indicated by reference numeral 1100 in Fig. 1 1.

[0089] The configuration of Fig. 10 and Fig. 11 allows for an auto-calibrating DTS opto-design having an improved selectivity using the same filter more than once. The steepness of the raising and falling edge may be improved with the configuration of

Fig. 10 and Fig. 11 , and the selectivity may be improved due to the use of more filters.

[0090] Particularly, the two configurations of Fig. 10 and Fig. 9, which essentially correspond to the configurations shown in Fig. 8 and in Fig. 9, respectively, comprise an additional filter 1002 (which may also be denoted as a mirror) which is brought into the optical path between mirrors 608, 1002 on the one hand and the optical fiber 102 on the other hand. It should be ensured that the two laser beams from laser diodes 204 and 202 are aligned parallel for switching to work properly in this embodiment.

[0091] Fig. 12 and Fig. 13 show a distributed temperature sensing apparatus 1200 according to another exemplary embodiment, which is shown in a first operation mode in Fig. 12 and in a second operation mode 1300 in Fig. 13.

[0092] In this embodiment, a retroflective prisma 1202 is provided which can be rotated to allow the measurement of the Stokes line with the photodiode 108 when the laser diode 202 emits electromagnetic radiation (see Fig. 12), and allows to measure an anti-Stokes line when the laser diode 204 emits electromagnetic radiation (see Fig. 13).

[0093] The embodiment of Fig. 12 and Fig. 13 differs from the embodiment of Fig. 10 and Fig. 11 by the use of a retroflective prisma 1202 in contrast to a POP. It should be ensured that the two laser beams from laser diodes 204 and 202 are aligned parallel for switching to work properly in this embodiment.

[0094] In the following, referring to Fig. 14 and Fig. 15, a distributed temperature sensing apparatus 1400 according to another exemplary embodiment will be explained, which is shown in a first operation mode in Fig. 14 and which is shown in a second operation mode 1500 in Fig. 15.

[0095] In the configuration of Fig. 14 and Fig. 15, a POP 1402 is provided as well as an inclined mirror 1404 and a further inclined mirror 1406. The embodiment of Fig. 14 and Fig. 15 allows for an auto-calibrating DTS opto-design for a realistic setup. One might choose an almost perpendicular angle of incidence due to polarization effects, also the photodiode 108 need not be in the same height as the laser diodes 202, 204, instead it can be above allowing smaller angles of rotation. It should be ensured that the two laser beams from laser diodes 204 and 202 are aligned parallel for switching to work properly in this embodiment.

[0096] The left-hand side of Fig. 16 shows a diagram 1600 having an abscissa 1602 along which a return signal wavelength is plotted, and having an ordinate 1604 along which a return signal is plotted.

[0097] A peak 1606 relates to incident light. Brillouin scattering is indicated by a peak 1610 and by a peak 1608. A region 1614 indicates a Raman anti-Stokes signal which is temperature-dependent. A Raman Stokes signal 1612 is temperature- dependent as well. A ratio of the intensities of the signals 1614, 1612 allows to derive information to calculate the temperature T. For this purpose, the intensities of the Stokes and the anti-Stokes lines may be measured, which requires an integration over the broad smeared out Stokes and anti-Stokes lines 1614, 1612.

[0098] A scheme 1650 illustrates OTDR technology (Optical Time Domain Reflectometer). The scheme 1650 of Fig. 16 shows a laser 1652, a pulse generator 1654, a detector 1656, an amplifier 1658 and an analyzing circuitry and display intelligence unit 1660.

[0099] Additionally, a coding circuit 1680 is shown in Fig. 16 as well as a source 1682, a code generator 1684, a detector 1686 and a signal processing unit 1688.

[00100] With the OTDR technology 1650, it is possible to measure the back-intensity versus time, to thereby calculate scattering effects along the fiber. With the coding circuit 1680, it is possible to correlate a back traveling signal with a modulation sequence (Golay). Thus, a DTS system may be provided using a low power laser.

[00101] Fig. 17 shows a diagram 1700 indicating interaction of light and matter, namely a Raman spectrum of a fiber. The intensity ratio of Stokes and anti-Stokes only depends on fundamental and material constants and on temperature. Diagram 1700 has an abscissa 1702 along which a frequency is plotted in THz. Along an ordinate 1704 a backscatter power in μW is plotted.

[00102] An instrument concept 1800 is shown in Fig. 18. A laser source 1802, an anti-Stokes detector 1804 and a Stokes detector 1806 are shown, as well as optical elements 1810, 1808, 1812. An optical fiber 102 is shown having a reference coil region 210, and Raman scattering is indicated by various arrows pointing in very different directions. Pulses having a width of 10 ns and a distance of 40 μs are shown as well.

[00103] The pulse length may be less than 10 ns for a 1 m spatial resolution. The bandwidth of receivers can be larger than 50 MHz. The total time for one shot may be around 40 μs, for instance in the case of a 4 km long fiber 102.

[00104] Fig. 19 shows a dual-ended DTS apparatus 1900.

[00105] The apparatus 1900 comprises a DTS control engine 1902, a DTS measurement engine 1904, a 1x4 switch 1906, a first loop 1908 and a second loop 1910. A dual-ended configuration allows for an automatic correction of fiber inhomogeneities like connectors, bending losses, hydrogen intrusion, etc. Differential loss may vary over distance and time. High measurement reliability may be obtained (if a fiber break occurs, the measurement is carried on, based on the two single measurements). Furthermore, the allowed channel configuration for dual-end measurement is loop 1 with channel 1 and 2, and loop 2 with channels 3 and 4. A dual- end measurement has to be set up as a sequence.

[00106] It should be noted that the term "comprising" does not exclude other elements or features and the "a" or "an" does not exclude a plurality. Also elements described in association with different embodiments may be combined. It should also be noted that reference signs in the claims shall not be construed as limiting the scope of the claims.

Claims

1. A distributed temperature sensing apparatus (100) for measuring a temperature distribution along a waveguide (102), the distributed temperature sensing apparatus (100) comprising

an electromagnetic radiation source (104) adapted for generating electromagnetic radiation of two wavelengths (A₁, A₂) to be coupled into the waveguide (102), wherein the two wavelengths (A₁, A₂) differ by the Raman shift of the material of the waveguide (102);

an evaluation unit (106) adapted for deriving information indicative of a temperature distribution along the waveguide (102) based on an evaluation of the electromagnetic radiation after propagation through the waveguide (102).

2. The distributed temperature sensing apparatus (100) of claim 1 ,

comprising the waveguide (102).

3. The distributed temperature sensing apparatus (100) of claim 2,

wherein the waveguide (102) comprises an optical fiber, particularly an optical fiber made of silica glass.

4. The distributed temperature sensing apparatus (100) of claim 2 or any one of the preceding claims,

wherein the waveguide (102) has a length of at least 100 meters, particularly has a length of at least 1 kilometer.

5. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein a wavelength (A₁, A₂) of electromagnetic radiation generated by the electromagnetic radiation generator (104) is tunable to thereby allow for generating electromagnetic radiation of one of the two wavelengths (A₁, A₂) at a time.

6. The distributed temperature sensing apparatus (100) of claim 5,

wherein the electromagnetic radiation generator (104) is a wavelength tunable laser.

7. The distributed temperature sensing apparatus (200) of claim 1 or any one of the preceding claims,

wherein the electromagnetic radiation source comprises two separate electromagnetic radiation generation units (202, 204) each adapted for generating electromagnetic radiation of a particular one of the two wavelengths (A₁, A₂).

8. The distributed temperature sensing apparatus (200) of claim 7,

wherein the two separate electromagnetic radiation generation units (202, 204) are two separate lasers, particularly are two separate laser diodes.

9. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the electromagnetic radiation source (104) is adapted for generating electromagnetic radiation of only one of the two wavelengths (A₁, A₂) at a time.

10. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the electromagnetic radiation source (104) is adapted for generating essentially monochromatic electromagnetic radiation at a time.

11. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the electromagnetic radiation source (104) is adapted for generating electromagnetic radiation of two wavelengths (A₁, A₂) which differ by a difference between a center of the Rayleigh peak and a center of the Stokes peak of the material of the waveguide (102).

12. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the electromagnetic radiation source (104) is adapted for generating electromagnetic radiation of two wavelengths (A₁, A₂) which differ by a difference between a geometrical center of gravity of the Rayleigh peak and a geometrical center of gravity of the Stokes peak of the material of the waveguide (102).

13. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the electromagnetic radiation source (104) is adapted for generating electromagnetic radiation of two wavelengths (A₁, A₂) differing by 10.5 THz.

14. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the electromagnetic radiation source (104) is adapted for generating electromagnetic radiation of two wavelengths (A₁, A₂) from the group consisting of the wavelength pairs of 1064 nm and 1026 nm, 1310 nm and 1373 nm, and 1480 nm and 1560 nm.

15. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the electromagnetic radiation source (104) is adapted for generating one of the group consisting of optical light and infrared radiation as the electromagnetic radiation.

16. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the electromagnetic radiation source (104) is adapted for generating electromagnetic radiation pulses of two wavelengths (A₁, A₂).

17. The distributed temperature sensing apparatus (100) of claim 16,

wherein the evaluation unit (106) is adapted for deriving information indicative of a temperature distribution along the waveguide (102) based on an evaluation of an integral over a plurality of electromagnetic radiation pulses of the two wavelengths (A₁, A₂) generated by the electromagnetic radiation source (104) after propagation through the waveguide (102).

18. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the evaluation unit (106) is adapted for deriving spatially dependent information indicative of a temperature distribution along the waveguide (102).

19. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the evaluation unit (106) is adapted for deriving information indicative of a temperature distribution along the waveguide (102) based on an evaluation of a Stokes peak after propagation of electromagnetic radiation of the smaller one of the two wavelengths (A₁, A₂) through the waveguide (102) and of an anti-Stokes peak after propagation of electromagnetic radiation of the larger one of the two wavelengths (A₁, A₂) through the waveguide (102).

20. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

wherein the evaluation unit (106) is adapted for deriving information indicative of a temperature distribution along the waveguide (102) based on an evaluation of a

Stokes peak after propagation of electromagnetic radiation of only the smaller one of the two wavelengths (A₁, A₂) through the waveguide (102) and of an anti- Stokes peak after propagation of electromagnetic radiation of only the larger one of the two wavelengths (A₁, A₂) through the waveguide (102).

21. The distributed temperature sensing apparatus (100) of claim 19 or any one of the preceding claims,

wherein the evaluation unit (106) is adapted for deriving information indicative of a temperature distribution along the waveguide (102) based on an evaluation of a ratio between an area of the Stokes peak and an area of the anti-Stokes peak.

22. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

comprising a detection unit (108) adapted for detecting the electromagnetic radiation after propagation through the waveguide (102) and for supplying corresponding detection signals to the evaluation unit (106).

23. The distributed temperature sensing apparatus (100) of claim 22,

wherein a single detection unit (108) is provided for detecting electromagnetic radiation of the two wavelengths (A₁, A₂) after propagation through the waveguide (102).

24. The distributed temperature sensing apparatus (100) of claim 23,

wherein the single detection unit (108) is adapted for detecting electromagnetic radiation of the two wavelengths, after propagation through the waveguide (102), in a multiplexed manner.

25. The distributed temperature sensing apparatus (100) of claim 22 or any one of the preceding claims,

wherein the detection unit (108) comprises a photodiode.

26. The distributed temperature sensing apparatus (200) of claim 22 or any one of the preceding claims,

comprising a switch (206, 214) adapted for selectively switching the detection unit (108) into a selectable electromagnetic propagation path corresponding to electromagnetic radiation of one of the two wavelengths (A₁, A₂).

27. The distributed temperature sensing apparatus (400) of claim 26,

wherein the switch comprises a movable, particularly a longitudinally shiftable, optical switch (404) mechanically connected to the waveguide (102) and movable between two positions for selectively coupling electromagnetic radiation of one of the two wavelengths (A₁, A₂) into the waveguide (102).

28. The distributed temperature sensing apparatus (600) of claim 26,

wherein the switch comprises a prisma (602, 604) and an at least partially reflective mirror (606, 608), the prisma (602, 604) being movable without a mechanical connection to the waveguide (102) between two positions for selectively coupling electromagnetic radiation of one of the two wavelengths into the waveguide (102).

29. The distributed temperature sensing apparatus (800) of claim 28,

wherein the prisma (802) is rotatable or longitudinally shiftable relative to the waveguide (102) for selectively coupling electromagnetic radiation of one of the two wavelengths (A₁, A₂) into the waveguide (102).

30. The distributed temperature sensing apparatus (800, 1200) of claim 28 or any one of the preceding claims,

wherein the prisma comprises one of the group consisting of a retroflective prisma (1202) and a parallel offset prisma (802).

31. The distributed temperature sensing apparatus (200) of claim 22 or any one of the preceding claims,

comprising a wavelength selective filter (208) arranged between the waveguide (102) and the detection unit (108).

32. The distributed temperature sensing apparatus (200) of claim 31 ,

wherein the wavelength selective filter (208) is adapted to be operable in a low pass operation mode selectively enabling electromagnetic radiation having a frequency below a first threshold to pass the wavelength selective filter (208) in the low pass operation mode, and is adapted to be operable in a high pass operation mode selectively enabling electromagnetic radiation having a frequency above a second threshold to pass the wavelength selective filter (208) in the high pass operation mode.

33. The distributed temperature sensing apparatus (200) of claim 31 or any one of the preceding claims,

wherein the wavelength selective filter (208) is adapted to be operable in a low pass operation mode during propagation of electromagnetic radiation of the smaller one of the two wavelengths (A₁, A₂) through the waveguide (102) and is adapted to be operable in a high pass operation mode during propagation of electromagnetic radiation of the larger one of the two wavelengths (A₁, A₂) through the waveguide (102).

34. The distributed temperature sensing apparatus (200) of claim 31 or any one of the preceding claims,

wherein the wavelength selective filter (208) is a wavelength division multiplexing filter.

35. The distributed temperature sensing apparatus (200) of claim 1 or any one of the preceding claims,

comprising a temperature sensor and a reference coil (210) coupled to the waveguide (102) and having a temperature measurable by the temperature sensor;

wherein the temperature sensor is adapted for supplying the evaluation unit (106) with temperature information indicative of the temperature at the reference coil (210) portion as a basis for a correction of the information derived by the evaluation unit (106).

36. The distributed temperature sensing apparatus (100) of claim 1 or any one of the preceding claims,

adapted as one of the group consisting of a single-ended distributed temperature sensing apparatus and a double-ended distributed temperature sensing apparatus.

37. A distributed temperature sensing method for measuring a temperature distribution along a waveguide (102), the method comprising coupling electromagnetic radiation of two wavelengths (A₁, A₂) into the waveguide (102), wherein the two wavelengths (A₁, A₂) differ by the Raman shift of the material of the waveguide (102);

deriving information indicative of a temperature distribution along the waveguide (102) based on an evaluation of the electromagnetic radiation after propagation through the waveguide (102).

38. The method of claim 37,

comprising installing the waveguide (102) in one of the group consisting of a tunnel, a borehole, a subterrestrial data communication line and a subterrestrial power supply line.

39. A software program or product, preferably stored on a data carrier, for controlling or executing the method of any one of the preceding claims, when run on a data processing system (106) such as a computer.