JP2008232929A - Method of expanding measurement range in distribution temperature sensor using light wavelength detection method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve a distribution type temperature sensing system having performance in which dynamic range is 5,000 or more, light amount variation is 0.5 nW or more by a light reception element with variation of 0.1 pm of wavelength from the sensor, and the number of sensors connectable to the system is 100 or more at 50 nm of bandwidth of C band, the performance extremely surpassing those of conventional distribution type temperature sensing systems. <P>SOLUTION: The sensor is constituted by connecting a Bragg grating to a drop port of a double ring resonator. Incident/emitting ports of the sensor are used as resonator incident/emitting ports. Light from a wide band light source is incident on a sensor group in which the plurality of sensors are connected. The reflected light is detected by a wavelength detector using an interferometer such as a Fabry-Perot interferometer in which a resonant wavelength is variable by an electric signal. Accordingly, it extremely surpasses the performance of the conventional distribution type sensing systems in both aspects of the dynamic range and the number of multiplicity (number of sensors connectable to the system). <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

The present invention relates to a technical field of a distributed optical fiber sensor using an optical fiber Bragg grating (hereinafter referred to as FBG).

  Background arts of the present invention include first, second, third, and fourth background arts. First, the first background art will be described. A distributed temperature sensor using the prior art will be described with reference to FIG. Light from the broadband light source 3 is input to a single mode fiber (hereinafter referred to as SMF) through an optical directional coupler 4 and one or more FBGs are drawn on the SMF. Since the temperature to be detected is linked to the reflection center wavelength of the FBG of the sensor, the temperature of each sensor can be measured by measuring these reflection center wavelengths. The system is designed so that the reflection center wavelengths of each FBG do not overlap each other over the entire measurement range, including their bandwidth. The reflected light from the FBG traces back the SMF and returns to the light source side, and is input to the wavelength detector 1 such as a Fabry-Perot interferometer type wavelength detector by the optical directional coupler 4 installed immediately before the light source. Is done. The reflection center wavelength of each FBG is measured by the Fabry-Perot interferometer. The Fabry-Perot interferometer is a narrow band comb-type bandpass filter. This pass band can be changed, for example, by using a piezoelectric element, and the interval between the half mirrors of the interferometer can be changed by the voltage applied to the piezoelectric element repeatedly. For example, if the applied voltage is changed in a sawtooth shape, the Fabry-Perot The narrow band comb bandpass filter spectrum of the interferometer also changes periodically. The Fabry-Perot interferometer is designed so that the free spectral range (hereinafter referred to as FSR) is wider than the entire wavelength band occupied by the multiple FBGs used. Further, one of the plurality of passing center wavelengths of the Fabry-Perot interferometer is swept by FSR by the change of the voltage applied to the piezoelectric element. As a result, the reflected center wavelength of the reflected light of each FBG is measured when the applied voltage is maximized by observing the amount of light emitted from the Fabry-Perot interferometer linked to the applied voltage to the piezoelectric element. can do. On the other hand, the relationship between the applied voltage and the center wavelength of the transmission spectrum of a single Fabry-Perot interferometer existing in the entire wavelength band occupied by the plurality of FBGs has been measured in advance. The reflection center wavelengths of the plurality of FBGs can be measured by measuring the applied voltage that maximizes the amount of light (see Non-Patent Document 1). If the relationship between the reflection center wavelength of each sensor and the temperature is measured in advance and stored as data in a programmable read only memory (hereinafter referred to as PROM) as shown in FIG. 6, for example, the wavelength temperature conversion unit 2 in FIG. This is the memory to be configured. The wavelength temperature converter 2 is connected to the wavelength detector 1 and outputs the temperature of each sensor corresponding to the wavelength of each sensor input.

Next, the second background art will be described. This technology is related to a ring resonator that has been studied for wavelength division multiplexing in the field of optical communications. The ring resonator is a narrow-band add / drop optical filter for wavelength division multiplexing communication, and the two optical directional couplers are formed so that an optical waveguide loop can be formed by using two 2-input 2-output optical directional couplers. Is connected. Of the four input / output terminals of the optical directional coupler remaining unconnected, one is used as a light incident port, one is a through port, one is a drop port, and the other is used as an add port. The transmittance from the light incident port to the through port shows a repeated comb-shaped band reject filter characteristic. Further, the transmittance from the light incident port to the drop port shows repeated comb-shaped bandpass filter characteristics. Further, the transmission characteristics from the add port to the through port also show comb-shaped bandpass filter characteristics (see Non-Patent Document 3, Non-Patent Document 4, and Non-Patent Document 5).

Next, the third background art will be described. The third background art is a technique for drawing a Bragg grating on an optical waveguide. A case where SiO ₂ doped with germanium in the core is used has been reported (see Non-Patent Document 6). Although it is not a Bragg grating, it has been reported that the refractive index of a core made of Ta ₂ O ₅ —SiO ₂ can be trimmed with ultraviolet rays (see Non-Patent Document 3).

P. Eigenraam, B. S. Douma, A. P. Koopman, Applications of Fiber Optic Sensors & Instrumentation in the Oil and Gas Industry, in Proc. Of OFS-13, pp602-607, 1999 A. D. Kersey, T. A. Berkoff, and W. W. Morey, Multiplexed fiber Bragg grating strain-sensor system with a fiber Fabry- Perot wavelength filter, Optics Lett.Vol. 18, No.16, pp.1370-1372, 1993 "Microring resonator type optical routing element" Yasuo Kokubun, Applied Physics, Vol.72, No.11, pp1364-1373, 2003 S. Suzuki, K. Oda, and Y. Hibino, Integrated-Optic Double-Ring Resonators with a Wide Spectral Range of 100GHz, J. Lightwave Technolo., Vol.13, no.8, pp.1766-1771, 1995 BE Little, ST Chu, W. Pan, D. Ripin, T. Kaneko, Y. Kokubun and E. Ippen, Vertically Coupled Glass Microring Resonator Channel Dropping Filter, IEEE Photon. Technol. Lett., Vol. 11, no.2 , pp215-217, 1999 Y. Sano, T. Hirayama, JK Kurihara, T. Goto, K. Taniguchi, J. Nishii, K. Kintaka, and T. Yoshino, Planar Waveguide Bragg Grating Pressure Sensor on a Micro-Machined Silicon Diaphragm in Proc. Of OFS -16 pp694-697 2003 Japanese Patent Application 2006-288631

The operating principle of the distributed temperature sensor system according to the present invention is the same as that of Patent Document 1 except for the following points. That is, two ring resonators are connected in series to expand the FSR. It is known that the double ring resonator can increase the FSR as described in Non-Patent Document 4. On the other hand, according to Patent Document 1, a WBG is connected to a drop port of a ring resonator, an input is an incident port of the ring resonator, and an output is an output port of the ring resonator. By making light from a broadband light source enter the connected sensor group and combining the wavelength of the reflected light from the sensor group with a piezoelectric element such as a Fabry-Perot interferometer with a wavelength detector consisting of an interferometer with a variable resonance wavelength. There is a known method that can detect a wavelength even when the wavelength changes. By combining these two technologies, the present invention can far surpass the performance of a conventional distributed sensing system in terms of both dynamic range and multiplex number (number of sensors connectable to the system).

By using the above-mentioned means, the present invention can achieve a dynamic range of 5000 or more, which far exceeds the performance of the conventional optical fiber type distributed sensor system, as described in paragraph 0009, and the light receiving element with a change in wavelength from the sensor of 0.1 pm. A sensor system with a change in the amount of light of 0.5 nW or more and a sensor system with a performance of 100 or more with a C-band bandwidth of 50 nm can be connected to the system.

FIG. 1 shows an overall configuration of a distributed temperature measurement system according to an embodiment of the invention. FIG. 2 shows the configuration of a sensor using a double ring resonator which is the key hardware of the system shown in FIG. In FIG. 1, light emitted from the broadband light source 3 enters the SMF via the SMF and the optical directional coupler 4. One or more sensors are connected to this SMF in series. The broadband light incident on the sensor is reflected as a line spectrum and enters the wavelength detector 1 including a Fabry-Perot interferometer via the optical directional coupler 4. The wavelength of the incident light is measured by this detector. The reflection wavelength of each sensor changes due to the temperature change of each sensor for reasons described later. Accordingly, the temperature at each sensor can be measured by detecting the reflected wavelength of each sensor with the wavelength detector 1. As shown in FIG. 1, the double ring resonator includes two ring waveguides RW1, RW2, and three optical directional couplers DC1, DC2, and DC3. Next, a method for manufacturing an actual double ring resonator will be described with reference to FIG.

Manufacturing method may be a method of a core of a SiO ₂ waveguide doped with germanium is known by Non-patent document 4, Ta is known by Non-patent document 5 ₂ O ₅ - manufactured core by SiO ₂ The method to do is also good. Here, a method known from Non-Patent Document 5 will be described. A SiO ₂ film is formed on the silicon substrate 6 by CVD (Chemical Vapor Deposition) as an under clad layer, and Ta ₂ O ₅ -SiO ₂ is formed by RF sputtering as a layer corresponding to the core of the linear waveguide 8 thereon. Film. Then, a core is formed by dry etching using a Cr mask and CF ₄ . The core refractive index can be controlled to various values by the% molar ratio of Ta ₂ O ₅ and SiO ₂ . For example, when Ta ₂ O ₅ is 30% molar ratio SiO ₂ is 70% molar ratio, the refractive index of Ta ₂ O ₅ -SiO ₂ is 1.7825. After the core is formed, an SiO ₂ film is further formed to form an intermediate cladding 9, and the linear waveguide 8 is completed by this process. It is formed by RF sputtering SiO ₂ - Further Ta ₂ O ₅ to form a ring waveguide RW thereon. And form the core of the ring waveguide RW1, RW2 by dry etching using the same as a Cr mask and CF ₄ as in the straight waveguide 8. A SiO ₂ film is further formed as an overcladding layer 10 on the core of the ring waveguide to complete a double ring resonator. Although not indicated by symbols in FIG. 2, the optical directional couplers DC2 and DC3 correspond to portions where the linear waveguide 8 and the ring waveguides RW1 and RW2 are spatially close to each other. DC1 corresponds to a portion where two ring waveguides RW1 and RW2 are spatially close to each other.

Next, we will describe how to make WBG. This method utilizes the fact that the refractive index changes when the core is irradiated with the second harmonic of an argon laser having a wavelength of 244 nm (see Non-Patent Document 3). WBG can be configured by irradiating the core with this ultraviolet light through a phase mask. Of course, even if the core of the straight waveguide 8 is made of germanium-doped SiO ₂ , the WBG 14 can be formed by the same method (see Non-Patent Document 6).

FIG. 9 is a diagram showing the correlation of the spectrum of the distributed temperature measurement system of FIG. 1 using the technique of the present invention. The system is designed so that the reflection center wavelength from each sensor does not overlap with each other over the entire measurement range including the bandwidth of the WBG constituting the sensor. The Fabry-Perot interferometer is designed so that the FSR is wider than the entire wavelength band occupied by the multiple WBGs used. Further, one of the plurality of passing center wavelengths of the Fabry-Perot interferometer is swept by FSR by a change in the voltage applied to the piezoelectric element. As such a resonant wavelength variable Fabry-Perot interferometer and its controller, for example, FFP-TF2 and FFP-C manufactured by Micron Optics can be used. As a result, the reflected center wavelength of the reflected light of each sensor is measured when the applied voltage is maximized by observing the amount of light emitted from the Fabry-Perot interferometer linked to the applied voltage to the piezoelectric element. can do. On the other hand, the relationship between the applied voltage and the center wavelength of the transmission spectrum of a single Fabry-Perot interferometer existing in the entire wavelength band occupied by the plurality of WBGs has been measured in advance. By measuring the applied voltage that maximizes the amount of light, the reflection center wavelengths of the plurality of WBGs can be measured. On the other hand, the relationship between the reflection center wavelength of each sensor and the temperature is measured in advance in the same manner as the method described in the fourth background art, and stored as data in, for example, PROM. The wavelength temperature converter 2 is composed of the PROM as shown in FIG. 6 and a microcomputer (CPU) for controlling the PROM. With the above configuration, the wavelength temperature converter 2 is connected to the wavelength detector 1 and outputs the temperature of each sensor corresponding to the wavelength of each sensor input. Obviously, if the variable wavelength range of the Fabry-Perot interferometer is narrow and the reflected wavelength region of all the sensors cannot be covered, a plurality of these may be used.

In addition to the field of health monitoring of so-called building structures (buildings, bridges, iron bridges, etc.) that attempt to maintain and maintain the building structures before they are fatally damaged, the invention described above is aerospace. For example, it can be applied to the field of failure prediction of a casing such as a wing.

It is a figure showing the whole system of a first embodiment of the present invention. The figure which shows the 1st example of a structure of the sensor of this invention. The figure which shows an example of the change with the wavelength of the reflectance of the conventional sensor which consists of a single ring resonator and WBG (waveguide Bragg grating), and the enlarged figure (b) Figure (a) showing an example of the transmission spectrum to the drop port of a single ring resonator, (b) shows an example of the transmission spectrum to the drop port of another single ring resonator, Figure (c) shows the transmission spectrum to the drop port of the double ring resonator when these two rings are combined and used as a double ring resonator. Transmission spectrum characteristics to the drop port of a double ring resonator with the characteristics of Fig. 4 (enlarged view) The figure which shows an example of the wavelength temperature conversion part for making the wavelength data from each sensor into the temperature data of each sensor Diagram showing the configuration of a conventional distributed optical fiber temperature sensor system using FBG Diagram showing the configuration of a conventional temperature sensor using a single ring resonator and WBG The figure which shows the spectrum of each place for demonstrating operation | movement of the distributed optical fiber temperature sensor system of this invention

Explanation of symbols

1 Wavelength detector
2. Wavelength temperature change part
3. Broadband light source
4 ... Optical directional coupler
SMF ・ ・ Single mode optical fiber
WBG ・ ・ Bragg grating drawn on optical waveguide
5 ・ ・ ・ Double ring resonator
IP: Incident port
DP ... Drop port
OP: Output port
DC1, DC2, DC3 ・ ・ Optical directional coupler
RW, RW1, RW2 ... Ring waveguide
6 ... Silicon substrate
7 ... Under clad layer
70 ・ ・ ・ Clad
8, 11 ... Linear waveguide
9 ... Intermediate cladding layer
10 ... Over clad layer

Claims (4)

Light from a broadband light source is incident on an optical directional coupler or circulator, and light emitted from the optical directional coupler or circulator is guided to one sensor via an optical signal transmission line composed of an optical fiber or an optical waveguide, or a plurality The sensor is guided to a series circuit connected in series using an optical signal transmission line, and the reflected light from one or more sensors follows the reverse path and is guided to a wavelength detector via an optical directional coupler or circulator. Sensors are input by inputting the reflection spectra from these sensors measured by the detector into a storage device comprising a memory storing the relationship between the reflection wavelength and temperature of the sensor measured in advance and a control unit for controlling the memory. Is a temperature measurement system for measuring a temperature change or temperature to be detected, wherein each sensor is An element in which a Bragg grating is drawn in a waveguide (hereinafter referred to as WBG: Waveguide Bragg Grating) is connected to a drop port of the double ring resonator, and the optical signal transmission line is connected to an incident port and an output port of the double ring resonator. Point-type or distributed-type temperature measurement system characterized by being connected The point type or distributed temperature measurement system according to claim 1, wherein the full width at half maximum of the WBG is narrower than the free spectrum range of the double ring resonator. 3. The point-type temperature measurement system according to claim 2, wherein a range of fluctuation of a specific spectrum in a comb-shaped transmission spectrum between an incident port and a drop port of a double ring resonator corresponds to a temperature measurement range to be sensed. And a temperature measuring system that is narrower than the reflected wavelength band of the WBG connected to the double ring resonator of each sensor. 4. The distributed temperature measurement system according to claim 3, wherein the range of fluctuation of one specific spectrum of the comb-shaped transmission spectrum between the incident port and the drop port of the double ring resonator constituting each sensor is the respective sensor. Is compatible with the temperature measurement range of the target and is narrower than the reflection wavelength band of the WBG connected to the double ring resonator of each sensor, and at the same time, the reflection wavelength bands of these multiple WBGs do not overlap each other Distributed temperature measurement system

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