Apparatus and method for measuring temperatures by using optical fiber

Abstract: An optical fiber (13) is installed to pass through a plurality of measurement places. Predetermined positions in the optical fiber are set at a predetermined temperature. When an optical signal is radiated on the optical fiber, Raman scattering occurs at various portions of the optical fiber. The intensities of the Raman scattered light components depend on temperatures. Backscattered light components of the Raman scattered light are sampled and stored in a memory. A temperature distribution on the optical fiber (13) is obtained on the basis of the stored data. The temperatures of the measurement places can be specified on the basis of the predetermined temperature on the obtained temperature distribution as a reference. The temperatures of the measurement places can be corrected on the basis of the predetermined temperature.

Description: The present invention relates to a measuring apparatus and method for measuring the temperature, humidity, temperature distribution, or the like in a measurement region by radiating an optical signal onto the incidence end of an optical fiber arranged in the measurement region and analyzing backscattered light. Generally, when an optical signal having a frequency ω is radiated on a substance, the optical signal is scattered by the substance and a part of the scattered optical signal returns toward the radiation point. If this returning light is observed by optical time domain reflectometry (OTDR), light components having frequencies different from frequency ω, e.g., frequencies ω + ωr, and ω ± mωr (m = an integer) are measured in addition to a light component having the frequency ω, i.e., Raman scattered light is measured. The intensity of Raman scattered light depends on temperatures. Since Raman scattered light is influenced by trace amounts of impurities or various molecules in an optical fiber, the application of Raman scattered light to temperature measurement was originally considered as difficult. However, with the progress of the manufacture and application techniques of optical fibers, use of optical fibers as thermometers has been studied. A technique of using an optical fiber as a thermometer is disclosed, for example, in "New fiber optic distributed temperature sensor", SPIE Vol. 798 Fiber Optic Sensors II (1987) pp. 131 to 136, or in UK Patent Application GB 2140554A. The disclosures of these references are incorporated in the specification. In such a thermometer, backscattered light from each point of an optical fiber is weak, and noise is mixed in this weak optical signal. In addition, optical fibers slightly differ in characteristics from each other. For these reasons, it is very difficult to discriminate temperature measurement positions of an optical fiber and to accurately measure temperatures at the respective positions. Under the circumstances, a technique of removing noise by averaging data obtained by radiating optical signals thousands to tens of thousands times is proposed. Even with this technique, however, high-fidelity measurement is very difficult to perform. Even if measurement is performed by Optical Frequency Domain Reflectometory (OFDR) instead of OTDR, accurate detection of a measurement position and accurate measurement of a temperature at the measurement position are difficult to perform for the same reasons as in measurement by OTDR. In addition, in a conventional system, the intensities of backward Raman scattered light components are stored in an internal memory once, and data processing is subsequently performed. For this reason, unnecessary signals are stored in the internal memory. This undesirably increases the required memory capacity. Prior art document US-A-4 859 065 discloses a method of measuring temperature which comprises the steps of consecutively launching input pulses of light at two different wavelengths into a temperature sensing element in the form of an elongate optical fiber, passing Roman scattered light from said element and deriving from said input pulses to intensity detectors, obtaining from the detectors output signals indicative of the intensity of the Raman scattered light; and processing said output signals to provide a temperature measurement. A known position along the optical fiber is maintained at a known temperature to provide a reference for deriving temperature measurements at other positions along the optical fiber. Further, prior art document "Proceedings of the SPIE", vol. 566, 1985, pages 249 to 256 describes an optical time domain reflectometry technique which uses the Raman Spectrum of backscattered light to give the temperature distribution along a conventional optical fiber. Temperatures from 77K to 800K can be measured. The spatial resolution is better than 3 m, with sensor lengths of up to 1 km. Finally, prior art document "Proceedings of the SPIE, vol. 1172, 1989, pages 194 to 205 discloses application of fiber-optic distributed temperature sensing to process monitoring and control environment. The measurement technique utilizes a modified optical time-domain reflectometer arrangement, where an excitation pulse is launched into the fiber, and a temperature-sensitive property of the backscatter is analyzed as a function of time. From this information the temperature profile along the length of the fiber is deduced. The fiber itself is used as the sensor, and the temperature-sensitive property measured is the spontaneous anti-Stokes Raman scattering of the fiber core material. In order to improve measurement accuracy the fiber is arranged in form of a coil at the respective measurement points, and an averaging of the temperature is performed over the length of each coil. The present invention has been made in consideration of the above situation, and has as its object to provide an improved fiber optic distributed temperature sensor, i.e. it is an object of the present invention to provide a measuring apparatus and method which can accurately measure a temperature and a humidity at an arbitrary measurement place in a measurement region and the temperature distribution of the region, and can increase measurement sensitivity and resolution. In order to solve this object the present invention provides an apparatus and a method as specified in claims 1 and 10, respectively. With the above-described arrangement, according to the apparatus and method of the present invention, the optical fiber means is installed to form a meshlike structure in order to obtain, for each measurement point defined by a crossing point of the meshlike structure, two temperature measurements which are then averaged. Therefore, the temperatures and humidities at a large number of measurement portions in a wide measurement region can be accurately measured. Thus, the resolution, sensitivity, and the like of the apparatus and method of the present invention can be improved. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1 and 2 are views for explaining a principle of measurement of temperatures by means of an optical fiber; Fig. 3 is a block diagram showing an arrangement of an OTDR commonly used in embodiments of the present invention; Figs. 4 to 7B are views for explaining the first embodiment of the present invention; and Fig. 8 is a view for explaining a second embodiment of the present invention. Embodiments of the present invention will be described below. A common principle of measurement of temperatures on which the following embodiments are based will be described first. When light having a frequency ω is radiated on a substance having a frequency ωr at which molecular vibration or lattice vibration occurs, scattered light components having a frequency ω + ωr as well as the frequency ω are radiated from the substance. Especially, when a high-intensity laser beam is radiated on the substance, stimulated light having the frequency ω + ωr or ω ± mωr (m = an integer) is generated due to stimulated emission. A part of this stimulated light propagates in the incident direction of the laser beam. That is, backscattered light is generated. The intensities of light components included in the backscattered light and having the frequency ω + ωr, ω ± mωr, and the like depend on temperatures at positions where the light components are stimulated. Therefore, positions where the backscattered light components are generated and data such as temperatures at the positions can be obtained by measuring the intensities of the back-scattered light components having the frequencies ω + ωr and ω ± mωr, and performing proper processing. A basic arrangement and operation of a thermometer based on the above-described principle will be described below with reference to Fig. 1. Referring to rig. 1, a light source 11 generates a light pulse S. The light pulse S is radiated on the incidence end of an optical fiber 13 through a beam splitter 12. Raman scattered light components having frequencies ω + ωr and ω ± mωr are sequentially generated at various positions of the optical fiber 13, e.g., positions t1, ... tn-1, and tn. As a result, backscattered light components return from the respective positions t1, ... tn-1, and tn to the incidence end of the optical fiber 13. The light components returning to the incidence end are reflected by the beam splitter 12 and are guided to a signal processor 14. Assume that temperatures at the positions t1, ... tn-1, and tn are respectively represented by T1, ... Tn-1, and Tn, and T1 = T2 = ... = Tn-1 = Tn. In this case, since the loss of scattered light is increased with an increase in distance from the incidence end, the signal level is gradually decreased. Fig. 2 shows the intensities of the Raman scattered light components received by the signal processor 14. The signal processor 14 measures the time interval between the instant at which a laser beam is radiated and the instant at which a scattered light component returns to the incidence end. In addition, the signal processor 14 obtains the position at which a backscattered light component is generated on the basis of a measured time interval and a light velocity (obtained from a refractive index) in the optical fiber 13. The backscattered light received by the signal processor 14 includes light components having frequencies ω, ω + ωr, ω ± mωr, and the like. The signal processor 14 extracts the light components having the frequencies ω + ωr, ω + mωr, or ω + ωr and ω - mωr by using a characteristic filter, a multiplexer, or the like, and obtains (calculate) temperatures on the basis of the intensities of the extracted light components and known calculation method. Note that if an element serving as both a light source and a photodetector is used, the beam splitter 12 can be omitted. An arrangement of the light source 11, the beam splitter 12, the optical fiber 13, and the signal shown in Figs. 4 to 8, will be described below with reference to Fig. 3. The light source 11 is constituted by, e.g., a laser oscillator, a light-emitting diode, or the like. The light source 11 outputs a pulse of light having the frequency ω in response to a command from the signal processor 14. This light pulse is supplied to the optical fiber 13 through the beam splitter 12. The beam splitter 12 is constituted by, e.g., a combination of two prisms. The signal processor 14 includes a CPU 141 for outputting various commands on the basis of a sequence program. Backscattered light components of Raman scattered light generated in the optical fiber 13, which return to the incidence end of the optical fiber 13, are incident on an extracting circuit 142 through the beam splitter 12. The extracting circuit 142 extracts light components having the frequencies ω, ω + ωr, and to ω ± mωr from the incident light. The light component having the frequency ω is supplied to an optical-to-electrical converter 143. The light components having the frequencies ω + ωr and to ω ± mωr are supplied to an optical-to-electrical converter 144. The optical-to-electrical converters 143 and 144 respectively output electrical signals having signal levels corresponding to the intensities of the input light components. The electrical signal obtained by the optical-to-electrical converter 144 upon conversion, i.e., the signal based on Raman scattering is supplied to a high speed analytical processor 146 directly or through a switch circuit 145. The high speed analytical processor 146 measures time intervals (positions) and signal intensities equivalent to temperatures corresponding to the time intervals on the basis of a timing signal which is input in synchronism with an operation command from the CPU 141 to the light source 11 and clock signals from a timer 149, and stores them in a built-in memory. A file 148 stores data, e.g., data obtained by mapping the positions of heat sources in a measurement region (to be described later). A data processor 147 performs desired processing by using the data stored in the high speed analytical processor 146 and the data in the file 148, as needed. The first embodiment of the present invention will be described below with reference to Figs. 4 to 7. Fig. 4 shows a method of installing an optical fiber 13 when temperature measurement positions are distributed on a flat surface. The optical fiber 13 is installed to form a meander so as path in a measurement region 20 in one direction and to zigzag to substantially perpendicularly cross the zigzag path, thus finally forming a mesh-like structure. Portions of the optical fiber 13 which cross each other constitute measurement portions 31a, 31b, .... Fig. 5 shows a case wherein a plurality of mesh-like optical fibers, each of which is formed in the same manner as described with reference to Fig. 4, are arranged such that the optical fiber 13 is formed into a three-dimensional structure. Fig. 6 schematically shows the arrangement in Fig. 5. In temperature measuring apparatuses having the arrangements shown in Figs. 4 and 5, the temperatures of the measurement portions 31a, 31b, ... are measured in the following procedure with the heaters or cooling devices. Reference signals representing only positions or positions and temperatures are acquired from the respective measurement portions 31a, 31b, .... A method of acquiring reference signals will be described below. The measurement portions 31a, 31b, ... are separately or simultaneously set at a predetermined temperature, which is sufficiently different from the temperature of the measurement region 20, by using the heaters, the cooling devices, or the like. Subsequently, the light source 11 is driven to radiate the light pulse S on the optical fiber 13. The intensity data of backscattered light components which return to the incidence end upon Raman scattering in the optical fiber 13 are supplied to the high speed analytical processor 146 through the characteristic filter or extracting circuit 142 (Fig. 3) and the optical-to-electrical converter 145 (Fig. 3), and are time-serially processed. In this case, back-scattered light components having intensities different from those of other light components are supplied from the respective measurement portions 31a, 31b, ... at time t1, time t2, ... (reference positions), ..., as shown in Fig. 7A. The data processor 147 measures (calculates) temperatures (reference temperatures) at the positions corresponding to time t1, time t2, ... on the basis of signal levels at time t1, time t2, ..., and sequentially stores the time and the temperature in the file 148 (Fig. 3). When the reference positions and temperatures are acquired, the heaters, the cooling devices, or the like are detached, and the measurement portions 31a, 31b, ... are at the same temperature as that of the measurement region 20. In actual measurement of temperatures, the respective measurement portions 31a, 31b, ... are not set at a specific temperature. A light pulse is radiated on the incidence end of the optical fiber 13. The signal processor 14 receives and stores backward Raman scattered light components. In addition, the signal processor 14 measures a time from the instant at which the light pulse is incident on the incidence end, and detects the intensities of backward Raman scattered light components at the reference times t1, t2, t3, ..., thereby measuring the temperatures of the measurement portions 31a, 31b, ... on the basis of the detected intensities. Since two data are obtained from one measurement portion, a more accurate value can be obtained by averaging the two data. The actually measured temperatures may be corrected (calibrated) by using the above-described reference temperatures. In Figs. 4 and 5, all the crossing portions of the mesh-like structures are used as the measurement portions, but only some of the crossing portions may be used as measurement portions. The second embodiment of the present invention will be described below with reference to Fig . 8. When the temperature or the like of a measurement region 20 is to be measured from a remote place, temperatures at positions between the incidence end of an optical fiber 13 and the measurement region 20 need not be measured. In the arrangement shown in Fig. 8, a time corresponding to a length Lng × 2/C ( C is light velocity) is stored in the file . The high speed analytical processor 146 measures a time from the instant at which the light source 11 emits a light pulse. The high speed analytical processor 146 starts to sample backscattered light components and to store the sampled data in the internal memory at a time point at which the time stored in the file 148 has elapsed. By employing such an arrangement, the memory capacity of the high speed analytical processor 146 can be reduced, and the efficiency of data processing can be improved. Note that if the intervals between measurement portions are long, these intervals may be defined as non-measurement portions Lng so that sampling of data in the intervals is stopped In the above-described embodiments, if a reference temperature is set at the measurement portions 31a, 31b, ..., a connection portion including a weld portion may be formed before or after, or before and after the portion at which the reference temperature is set in order to obtain a distinctive reference position signal from backscattered light. The arrangement is especially effective when the reference temperature is not much different from the temperature of the measurement region 20. In addition, crystal members which easily cause Raman scattering or crystal members sensitive to temperatures may be arranged at proper portions of the optical fiber 13 in the measurement region 20. In the above embodiments, measurement of temperatures is mainly described. However, measurement of humidities can be performed in the same manner. In the above embodiments, an OTDR is used as the signal processor 14. However, the present invention is not limited to the above embodiments. For example, a well known domain reflectometry such as an optical frequency domain reflectometry (OFDR) may be used. An OFDR is a device for obtaining positions where Raman scattering occurs on the basis of the phase differences between, e.g., an incident wave and backscattered light components by using a modulation wave instead of the light pulse S. The time intervals between the instant of radiation of a light pulse and the instant of reception of backward Raman scattered light components are difficult to measure unless they are of certain values. For this reason, an OTDR is effective when the optical fiber 13 is long. Such a problem is not posed in an OFDR, and hence it can be used regardless of the length of the optical fiber 13. As has been described above, according to the embodiments of the present invention, the following various effects can be obtained. In the embodiments shown in Figs. 4 and 5, an optical fiber is two-dimensionally or three-dimensionally arranged, and predetermined positions of the optical fiber are set at a predetermined temperature so as to acquire data in advance. With this arrangement, the temperatures and humidities at a large number of measurement portions in a wide measurement region can be accurately measured in actual measurement. In the embodiments shown in Fig . 8 , since no data processing is performed for backscattered light components from non-measurement portions of an optical fiber, the memory capacity of a signal processor can be reduced, and the data processing speed can be increased. In each embodiment, the propagation time of light in an optical fiber can be accurately measured.

Claims: An apparatus for measuring a temperature of optical fiber means (13) by radiating light on said optical fiber means (13) and measuring Raman scattered light generated in said optical fiber means (13), which is installed to pass through at least one place where a temperature is to be measured and transmits light, said apparatus further comprising: light source means (11, 12) for radiating light on said optical fiber means (13); receiving means (12; 142 to 145) for receiving Raman scattered light, which is generated in said optical fiber means (13) in response to the light radiated on said optical fiber means (13) as a signal; calculating means (146 to 149) for analyzing the signal received by said receiving means (12; 142 to 145) and calculating a position at which the Raman scattered light is generated and a temperature at said position; and isothermal chambers for maintaining parts of the optical fiber means (13) at a predetermined reference temperature, wherein: said optical fiber means (13) is installed to pass one measurement place a plurality of times, and said calculating means (146 to 149) includes means for receiving Raman scattered light from portions, in one measurement place, of said optical fiber means (13) at different timings, obtaining temperatures on the basis of the received Raman scattered light, so that a plurality of measured temperatures are obtained in said one measurement place, characterized in that said optical fiber means (13) is installed to form a meshlike structure in order to obtain, for each measurement point defined by a crossing point of said meshlike structure, two temperature measurements, the calculating means being arranged to average said two measured temperatures for each measurement point. An apparatus according to claim 1, characterized in that said isothermal chambers set one or both of two portions located on both sides of the portion passing through the place where a temperature is to be measured at the predetermined reference temperature. An apparatus according to claim 1, characterized in that said isothermal chambers set a temperature which is considerably different from an estimated temperature at the place where a temperature is to be measured. An apparatus according to claim 1, characterized in that said calculating means (146 to 149) specifies the position, at which the Raman scattered light is generated, on the basis of the signal representing the predetermined temperature. An apparatus according to claim 1, characterized by means for setting an amount of Raman scattered light at a predetermined position of said optical fiber means (13) to be different from that of Raman scattered light at a peripheral portion. An apparatus according to claim 1, characterized by storage means for controlling said receiving means (12; 142 to 145) and said calculating means (146 to 149) to obtain and store data representing the predetermined positions from the Raman scattered light, wherein: said calculating means (146 to 149) comprises: means for specifying the Raman scattered light from a measurement position on the basis of the data stored in said storage means upon actual temperature measurement, and means for calculating a temperature on the basis of the specified Raman scattered light. An apparatus according to claim 1, characterized in that said receiving means comprises: means (146) for storing sampling the received Raman scattered light; means (146) for storing a sampled signal; and means for inhibiting Raman scattered light from a non-measurement portion of said optical fiber means from being stored in said storage means (146). An apparatus according to claim 1, characterized in that said calculating means (146 to 149) obtains a propagation time of an optical signal in said optical fiber means (13) on the basis of the signal representing the position. An apparatus according to claim 1, characterized in that said calculating means (146 to 149) includes means for measuring a humidity from a temperature difference between two adjacent portions. A method comprising the steps of: installing an optical fiber (13) to pass through at least one temperature measurement place; radiating an optical signal on said optical fiber (13) and continuously generating Raman scattered light; receiving the Raman scattered light, and measuring a temperature of said optical fiber (13) on the basis of the received Raman scattered light; and adding a signal representing a predetermined position or temperature of said optical fiber (13) to the Raman scattered light, the signal being constituted by the Raman scattered light, wherein: the step of installing said optical fiber (13) includes the step of installing said optical fiber (13) so as to pass through one measurement place a plurality of times, and the measuring step includes the step of obtaining temperatures from Raman scattered light from a plurality of portions on one measurement place, characterized in that in said installing step, said optical fiber (13) is installed to form a meshlike structure in order to obtain, for each measurement point defined by a crossing point of said meshlike structure, two temperature measurements which are then averaged in said measuring step. A method according to claim 10, characterized in that the measuring step includes the step of specifying a position at which the Raman scattered light is generated on the basis of the signal. A method according to claim 10, characterized in that the adding step includes the step of setting a predetermined position of said optical fiber (13) at a predetermined temperature. A method according to claim 10, characterized in that the adding step includes the step of arranging portion , at which a rate of occurrence of Raman scattering is different from that at a peripheral portion thereof, at a predetermined position of said optical fiber (13). A method according to claim 11, characterized in that the measuring step includes the steps of: storing an intensity of received Raman scattered light; and calculating a temperature from the stored content, an intensity of Raman scattered light from a non-measurement portion of said optical fiber (13) being not stored in the storing step.

Inventors: WADA ICHIRO, WADA, ICHIRO, WADA, ICHIRO, C/O INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: G01K11
IPC codes: G01K11, G01K1