JP4586167B2 - Ultra-narrow band sensor in distributed measurement system using optical wavelength detection method and method for increasing the number of sensors that can be connected to the system - Google Patents

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).

  As the background art of the present invention, the first background art and the second background art will be described. First, the first background art will be described. A distributed temperature sensor using the prior art will be described with reference to FIG. The light from the broadband light source 1 is input to the single mode optical fiber SMF through the optical directional coupler 2, and one or more FBGs are drawn on the SMF. Since the temperature to be detected has a correlation with 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. Reflected light from the FBG traces back the SMF and returns to the broadband light source 1 side, and is composed of an optical interferometer such as a Fabry-Perot interferometer by the optical directional coupler 2 installed immediately before the light source. It is input to the wavelength detector 3 (see Non-Patent Document 1). The reflection center wavelength of each FBG is detected, that is, measured by the wavelength detector 3. The Fabry-Perot interferometer is a narrow band comb-type bandpass filter. On the other hand, when a piezoelectric element is used and a sawtooth voltage is applied to the piezoelectric element, the piezoelectric element is displaced by the voltage. Therefore, if the half mirror of the interferometer is connected to the piezoelectric element, two elements can be obtained. The interval of the semi-transparent mirror can be changed. Therefore, if the voltage applied to the piezoelectric element is periodically changed, the narrow band comb-shaped bandpass filter spectrum of the Fabry-Perot interferometer can be periodically changed. FIG. 5 is a diagram showing the mutual relationship of the spectrum of the distributed temperature measurement system using this conventional technique. A Fabry-Perot interferometer constituting the wavelength detector 3 is designed so that a free spectrum range (hereinafter referred to as FSR) wider than the entire wavelength band occupied by a plurality of FBGs to be used is designed. Further, the system design is performed so that 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 reflection center wavelength of the reflected light of each FBG is maximized 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. It becomes possible to measure by measuring separately. This is because 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. This is because the reflection center wavelengths of the plurality of FBGs can be measured by measuring the applied voltage that maximizes the amount of emitted light. 4 includes a memory such as a programmable read only memory (hereinafter, PROM). The relationship between the reflection center wavelength of each sensor and the temperature is measured in advance, and this is stored in the memory as data. Thereby, the wavelength temperature converter 4 is connected to the wavelength detector 3 and outputs the temperature of each sensor corresponding to the wavelength of each sensor input.

Next, the second background art will be described. This is a technique known from Non-Patent Document 2. FIG. 6 is a system configuration diagram of a distributed temperature sensor using the second background art. The difference from the first background art is the wavelength detector 3. Everything else is the same. The feature is that the wavelength detector 3 of the first background art mechanically sweeps the interference center wavelength of a Fabry-Perot interferometer using a piezoelectric element or the like, but this second background art is a Fabry-Perot type. The use of an arrayed waveguide grating AWG instead of an interferometer eliminates mechanical moving parts, improves the reliability of the wavelength detector 3 over the first background art, and at the same time enables high-speed wavelength detection by parallel signal processing. It is a big feature. Next, the operation of the wavelength detector composed of the AWG will be described with reference to FIG. The reflection center wavelength of the FBG is λ _bi , and the transmission center wavelengths of the adjacent channel m and channel m + 1 of the AWG are λ _am and λ _{a, m + 1} , respectively. When λ _bi is equal to the center wavelength of λ _am and λ _{a, m + 1} , the amount of light emitted from channel m and channel m + 1 of the AWG is the same, so the current value after photoelectrically converting them with the light receiving element The ratio is 1. If λ _bi is shifted to the short wavelength side, the ratio is smaller than 1, and if it is shifted to the long wavelength side, the ratio is larger than 1. lambda _bi is a system designed to vary in the range λ _am <λ _bi <for λ _{a, m + 1.} The FBG reflection center wavelength λ _bi has a one-to-one correspondence with the FBG temperature. Therefore, the relationship between the temperature of the FBG and the ratio is also a one-to-one relationship. This relationship is obtained in advance at the system production stage and stored in a storage device such as a PROM. The wavelength temperature converter 4 in FIG. 6 is this storage device. Using the system configured as described above, as shown in the figure, the microcomputer 7 performs a ratio calculation using the current value after photoelectric conversion by the light receiving element 5 and the preamplifier 6, and the calculation result is stored in the storage device. The output of the storage device becomes the temperature of the FBG at that time. In general, an AWG has two or more channels, so that the reflection center wavelengths of multiple FBGs can be measured simultaneously in parallel. This is the second background art.
AD Kersey, TA Berkoff, and WW Morey, Multipled fiber Bragg grating strain-sensor system with a fiber Fabry-Perot wavelength filter, Optics Lett., Vol.18, No.16, PP.1370-1372, 1993 Y. Sano and T. Yoshino, Fast Optical Wavelength Interrogator Employing Arrayed Waveguide Grating for Distributed Fiber Bragg Grating Sensors, J. Lightwave Technol., Vol.21, pp.132-139, 2003 S. Suzuki, K. Oda, and Y. Hibino, Integrated-optic Double-Ring Resonators with a Wide Spectral Range of 100 GHz, J. Lightwave Tecnolo., Vol.13, no.8, pp.1766-1771, 1995 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, pp.694-696, 2003 Japanese Patent Application No. 2006-288631, Invention name: Optical wavelength detection type physical quantity sensor using ring resonator and Bragg grating, Inventor: Yasuichi Sano, Patent applicant: National College of Technology, Representative: Ichiro Kono

Light from a broadband light source is incident on an optical directional coupler or optical circulator, and light emitted from the optical directional coupler or optical circulator is guided to one sensor via an optical signal transmission line made of SMF, or a plurality of sensors A sensor is led to a series circuit connected in series using SMF, and reflected light from one or a plurality of sensors follows a reverse path and is guided to the wavelength detector via the optical directional coupler or the optical circulator. A measuring system for measuring a temperature to be detected by a sensor by measuring a wavelength of reflected light from these sensors in a detector, wherein each sensor includes a first ring waveguide and a second Bragg grating. The ring waveguide is coupled with the first optical directional coupler, and at the same time, the first ring waveguide is formed from the SMF via the second optical directional coupler. A first ring resonator is formed by coupling to the optical signal transmission line so that a transmission band filter characteristic from an incident port to a drop port of the ring resonator becomes a comb filter characteristic having a narrow band characteristic. By determining the ring length of the first ring waveguide and making the reflection bandwidth of the Bragg grating in the second ring waveguide narrower than the FSR of the first ring resonator, the Bragg grating in the waveguide In the reflection band, the spectral characteristic of the sensor is a single narrow-band reflection spectral characteristic. By making the optical path lengths of the cores the same, a flat spectrum is obtained, in other words, adjacently connected sensors are constructed. The incident spectrum to the ring resonator to be formed is a flat spectrum, and at the same time, the wavelength detector has a narrow band filter characteristic equivalent to that of the sensor, and the reflection band of the Bragg grating in the waveguide is different for each sensor. Furthermore, it is a solution to the problem of the present invention that the measurement range of each sensor is the bandwidth of the Bragg grating in the waveguide of each sensor.

The overall configuration and operation of the distributed temperature measurement system will be described as a first embodiment with reference to FIGS. In FIG. 1, light emitted from the broadband light source 1 enters the SMF via the SMF and the optical directional coupler 2 (the optical directional coupler may be a circulator). N sensors (N is an integer of 1 or more) are connected to the SMF in series via the SMF. The reflected light from one or a plurality of sensors follows the reverse path and is guided to the wavelength detector 3 via the optical directional coupler 2, and the sensor detects the reflected spectrum from these sensors in the detector. Measure the power temperature. In each sensor, the first ring waveguide RW1 and the second ring waveguide RW2 incorporating the Bragg grating WBG are coupled by the first optical directional coupler DC1, and the first ring waveguide RW1 is coupled to the second ring waveguide RW1. A first ring resonator is formed by coupling to the optical signal transmission line made of the SMF via an optical directional coupler DC2, and a transmission band filter characteristic from the incident port IP to the drop port DP of the ring resonator By setting the ring length of the first ring waveguide RW1 to, for example, about millimeters, a narrow band transmission comb filter characteristic as shown in the spectrum S2 of FIG. By making the bandwidth of the Bragg grating WBG in the second ring waveguide RW2 narrower than the FSR of the first ring resonator as shown in the spectra S3 and S6 in FIG. The reflected reflection spectral characteristic is assumed to be a single narrowband reflection spectral characteristic of FWHM as shown in spectrums S4 and S7 in FIG. 2 in the order of a picometer. This single narrow-band reflection spectrum is one of the transmission spectra from the incident port IP to the drop port DP of the ring resonator, but at the same time this is the light incident from the drop port DP toward the incident port IP. Since it should be equal to the transmission spectrum, the single narrow-band reflection spectrum reflected by the WBG propagates from the drop port DP to the optical directional coupler 2 via the incident port IP . That is, the reflection spectrum as a sensor is a single narrow-band reflection spectrum as shown by the spectra S4 and S7 in FIG. Since this reflection spectrum has a single narrow band characteristic, the sensor of the present invention has a FWHM with a much wider FWHM than the change in the energy density of the reflection spectrum with respect to the wavelength near the center wavelength of the reflection spectrum. It is much larger than the conventional distribution measurement system using FBG. The wavelength detector 3 also has a narrow-band filter characteristic in order to more accurately grasp the change in energy density with respect to the wavelength of the reflection spectrum. As reported in Non-Patent Document 1 described in the background art, this characteristic is provided by using, for example, a Fabry-Perot interferometer that can sweep the comb-type transmission filter characteristic by an electric signal. This wavelength detection method is the same as the method described in Non-Patent Document 1 above. The WBG reflection band of each sensor varies with temperature changes, but the system is designed so that this reflection band varies from sensor to sensor in any operating temperature range of the system. Furthermore, the measurement range of each sensor is the WBG bandwidth of each sensor. The above is the configuration of the embodiment.

Next, the case where the light incident from the SMF is in the WBG transmission band will be described. FIG. 11 is an equivalent circuit of the sensor in this case. In the figure, the optical path length of the ring resonator composed of the optical directional coupler DC1, the optical directional coupler DC2, and the ring waveguide RW1 and the optical path length of the ring waveguide RW2 are designed to be equal. The light spectrum emitted from the drop port DP of the ring resonator is a comb spectrum as shown in the spectrum (b) of FIG. The spectrum incident on DC1 from the add port AP of the resonator is the same as the spectrum emitted from the drop port DP because the optical path lengths of both rings are the same, as shown in FIG. 12 (d). That is, the same optical spectrum as the optical spectrum that has passed through the ring resonator through the drop port DP enters from the add port AP. Without the ring waveguide RW2, the spectrum emitted from the output port OP of the ring resonator shows band rejection characteristics for each wavelength determined by the free spectrum range of the ring resonator, as shown in the spectrum (c) of Fig. 12. Since the emitted optical spectrum enters from the add port AP of the ring resonator, as a result, the output spectrum of the output port OP of the ring resonator becomes flat as shown by the spectrum (e) in FIG. As described above, the system design is made so that the reflection bands of the WBGs do not overlap in order not to overlap the measurement ranges of the sensors.

It is obvious that the sensor itself can be manufactured by a planar optical waveguide manufacturing technique using so-called silicon or quartz as described in Patent Document 1 filed by the inventors of the present application. Also, it is described in the same document that the WBG can be manufactured by irradiating a germanium-doped quartz core with spatially periodic ultraviolet rays. It is known from Non-Patent Document 3 that the temperature dependence of the spectrum of a ring resonator using a quartz core doped with germanium is 12 pm / ° C. Furthermore, the temperature dependence of the WBG using a germanium-doped quartz core is also 11.5 pm / ° C, and the temperature dependence of the spectrum of the ring resonator is almost 11.5 pm / ° C. be equivalent to. Therefore, in the case of the system shown in FIG. 1, when the sensor temperature changes, the line spectrum of the ring resonator shifts by approximately the same amount as the reflection center wavelength of WBG. Therefore, the central wavelength interval between adjacent WBGs must be determined in consideration of the shift amount of this wavelength. If the operating temperature range is 100 ° C., this shift amount becomes 1.2 nm at 100 ° C. × 12 pm / ° C. If the light source bandwidth is 50 nm and the WBG bandwidth is 0.1 nm, the number of sensors that can be multiplexed with this system is 50 nm / (100 + 1200) pm = 38. Since the wavelength resolution of each sensor is 0.1 pm as described in paragraph 0006, the dynamic range is 1.2 nm / 0.1 pm = 12000.

Next, a distributed temperature measurement system according to the second embodiment will be described. The system configuration is the same as in FIG. 1 except for the sensor unit. The sensor part of this embodiment is shown in FIG. Instead of WBG, FBG with low temperature characteristics with low temperature characteristics (eg, Fiber Lab, FBG1550-ATMPKG type) is prepared, and this is connected to the optical waveguide WG that constitutes DC1 with SMF, for example, with an optical adhesive. The operation as a sensor is basically the same as in the first embodiment. In this case as well, the optical path lengths of the ring resonator and the ring waveguide RW2 are designed to be the same in the transmission band of the FBG. From the viewpoint of optical loss, assuming that the insertion loss between the incident port IP and the outgoing port OP of the sensor shown in FIG. 3 is 0.5 dB and the allowable loss until the light emitted from the optical directional coupler 2 returns is 50 dB. 50 dB / (0.5 dB × 2) = 50 sensors can be connected. If this loss is very small and can be ignored, the temperature characteristics of the FBG can be ignored, so the number of sensors that can be connected is 50 nm / (0.1 +0) nm = 500, which greatly increases the number of sensors that can be connected. . In this case, however, the dynamic range is 0.1 nm / 0.1pm = 1000. If an optical amplifier with an amplification factor of 50 dB is inserted immediately before the optical wavelength detector 3, the allowable loss is 100 dB, so the number of sensors that can be connected is 100.

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.

First embodiment of the present invention Operation principle of the system in the first embodiment of the present invention Second embodiment of the present invention Main configuration of the first prior art system Spectrum of each part to show the operation of the system using the first prior art Main configuration of the second prior art system Spectrum of each part to show the operation of the second prior art system Example of reflection spectrum of WBG and transmission spectrum between incident port and drop port of ring resonator Example of reflected light spectrum from incident port to optical directional coupler 2 side Example of reflected light spectrum from incident port to optical directional coupler 2 side (enlarged view) Sensor transmission circuit when light coming from SMF is in the WBG transmission band Optical spectrum at each point when light incident from SMF is in the WBG transmission band

Explanation of symbols

1 Broadband light source
2 ... Optical directional coupler
3 Wavelength detector
4 ... Wavelength temperature converter
5. Light receiving element
6 ... Preamplifier
7 ... Microcomputer
SMF ・ ・ Single mode optical fiber
IP: Ring resonator incident port
DP ・ ・ ・ Ring resonator drop port
AP ・ ・ ・ Ring resonator add port
OP: Ring resonator output port
FBG ・ ・ Optical fiber Bragg grating
WBG ・ ・ Optical waveguide Bragg grating
RW1 Ring ring 1
RW2 ・ ・ Ring waveguide 2
DC1 ・ ・ Optical directional coupler
DC2 ・ ・ Optical directional coupler
WG ・ ・ Optical waveguide
CN ・ ・ Optical waveguide and SMF coupling part
AWG array waveguide grating

Claims (3)

Light from a broadband light source is incident on an optical directional coupler or optical circulator, and light emitted from the optical directional coupler or optical circulator is guided to one sensor via an optical signal transmission line composed of a single mode optical fiber, Alternatively, a plurality of sensors are led to a series circuit connected in series using a single-mode optical fiber, and the reflected light from the one or more sensors follows a reverse path to the wavelength via the optical directional coupler or the optical circulator. A measurement system that is guided to a detector and measures the temperature to be detected by the sensor by measuring a reflection spectrum from the sensor, and each sensor includes a first ring waveguide and a Bragg grating. The second ring waveguide is coupled by the first optical directional coupler and at the same time the first ring guide A first ring resonator is formed by coupling a path to the optical signal transmission line composed of the single mode optical fiber via a second optical directional coupler, and from the incident port to the drop port of the ring resonator. The ring length of the first ring waveguide is determined such that the transmission band filter characteristic of the first ring waveguide is a comb filter characteristic having a narrow band characteristic, and the reflection bandwidth of the Bragg grating in the second ring waveguide is In the reflection band of the Bragg grating in the waveguide, the spectral characteristic of the sensor becomes a single narrow-band reflection spectral characteristic, and at the same time, the Bragg grating in the waveguide The transmission spectrum from the entrance port to the exit port of the first ring resonator in the transmission band By making the optical path length the same, a flat spectrum is obtained. In other words, the incident spectrum to the ring resonator constituting the adjacent sensor is made a flat spectrum, and at the same time, the wavelength detector has a narrow band equivalent to the sensor. The system has a filter characteristic, the reflection band of the Bragg grating in the waveguide is different for each sensor, and the measurement range of each sensor is the bandwidth of the Bragg grating in the waveguide of each sensor. Distributed temperature measurement system that can increase the number of sensors that can be connected to the sensor. 2. The distributed temperature measuring system according to claim 1, wherein the first ring resonator has a narrow band characteristic of a picometer order by setting the length of the first ring waveguide to about millimeters. 2. The distributed temperature measurement system according to claim 1, wherein an interferometer such as a Fabry-Perot interferometer is used as the wavelength detector.

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