WO2015008332A1 - 光ファイバセンシング光学系及び光ファイバセンシングシステム - Google Patents
光ファイバセンシング光学系及び光ファイバセンシングシステム Download PDFInfo
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- WO2015008332A1 WO2015008332A1 PCT/JP2013/069310 JP2013069310W WO2015008332A1 WO 2015008332 A1 WO2015008332 A1 WO 2015008332A1 JP 2013069310 W JP2013069310 W JP 2013069310W WO 2015008332 A1 WO2015008332 A1 WO 2015008332A1
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
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- G—PHYSICS
- G01—MEASURING; TESTING
- G01D—MEASURING NOT SPECIALLY ADAPTED FOR A SPECIFIC VARIABLE; ARRANGEMENTS FOR MEASURING TWO OR MORE VARIABLES NOT COVERED IN A SINGLE OTHER SUBCLASS; TARIFF METERING APPARATUS; MEASURING OR TESTING NOT OTHERWISE PROVIDED FOR
- G01D5/00—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable
- G01D5/26—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light
- G01D5/32—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light
- G01D5/34—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells
- G01D5/353—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre
- G01D5/35306—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement
- G01D5/35309—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer
- G01D5/35316—Mechanical means for transferring the output of a sensing member; Means for converting the output of a sensing member to another variable where the form or nature of the sensing member does not constrain the means for converting; Transducers not specially adapted for a specific variable characterised by optical transfer means, i.e. using infrared, visible, or ultraviolet light with attenuation or whole or partial obturation of beams of light the beams of light being detected by photocells influencing the transmission properties of an optical fibre using an interferometer arrangement using multiple waves interferometer using a Bragg gratings
Definitions
- the present invention relates to an optical fiber sensing optical system and an optical fiber sensing system used for measuring a physical quantity to be measured using an optical fiber.
- a diffraction grating FBG formed in an optical fiber is proposed.
- An FBG method using (Fiber Bragg Grating) can be mentioned.
- This FBG is formed so as to have a predetermined period (grating period) in the longitudinal direction of the fiber by refractive index modulation produced by irradiating a specific core portion of the optical fiber with an interference pattern of ultraviolet laser light.
- the FBG reflects only light of a specific wavelength that matches the grating period, and passes light of other wavelengths.
- a change in the specific wavelength reflected by a change in the grating period accompanying a change in the external environment is detected to detect a change in the external environment.
- the refractive index changes and the wavelength of the reflected light changes.
- the change in the wavelength of the return light it is possible to detect ground fluctuation, liquid temperature, displacement and strain of the structure, and the like.
- Patent Document 1 discloses a technique for accurately identifying the presence, position, and size of an impact on a composite material using FBG.
- Patent Document 2 discloses a technique for measuring distortion of a measurement object using FBG.
- Variation factors include light source emission power, fiber insertion loss, receiver sensitivity and amplifier fluctuations, optical energy loss due to bending of optical fiber (bending loss), and optical energy loss due to connectors connecting optical fibers.
- bending loss optical energy loss due to bending of optical fiber
- connectors connecting optical fibers.
- the reflected light from the filter is reflected as compensation light in order to compensate for the intensity fluctuation due to the fluctuation of the optical measurement system among the intensity of the transmitted light of the wavelength filter for detecting the fluctuation of the reflected wavelength from the FBG due to the environment.
- the intensity of transmitted light is divided by the intensity of light.
- the reason why the reflected light is used as compensation light is to improve the detection sensitivity because the transmitted light and the reflected light are complementary characteristics.
- a circulator that separates the traveling wave and the reflected wave in the optical fiber has been used to prevent the reflected light from the filter from returning to the light source (FIG. 1 of Patent Document 3).
- the circulator and the filter are strongly dependent on polarization, and the measurement value fluctuates due to the polarization fluctuation in the optical fiber, and the S / N of the measurement value deteriorates conversely by using the reflected light as the compensation light. I understood that.
- the present invention has been made in view of the above problems, and provides an inexpensive optical fiber sensing system capable of performing highly accurate measurement by preventing the influence of disturbances other than the above-described physical quantities to be measured, particularly the influence of polarization fluctuations.
- the task is to do.
- An optical fiber sensing optical system includes a light source that outputs search light having a continuous wavelength component, a reflection unit that generates a detection light by selecting a part of the continuous wavelength component, and the detection light. Includes a wavelength discriminating unit that generates transmitted light whose intensity changes in accordance with a change in the wavelength, and irreversibly transmits the search light to the reflecting unit, and includes the wavelength discriminating unit for the detection light.
- the search light from the light source is irreversibly transmitted to the reflecting means, it is possible to avoid the light source being affected by the reflected light.
- the specific wavelength in the continuous wavelength is selected and reflected by the reflecting means reflecting the state of the object to be measured
- the state of the object to be measured in the detection light can be determined by specifying the wavelength reflected by the wavelength discriminating means. You can get information about.
- the wavelength discriminating means converts the wavelength difference into transmitted light intensity by the difference in transmittance. However, the intensity of transmitted light is also affected by fluctuations in characteristics of the light source and the optical fiber or optical device through which the light passes.
- the detection light from the FBG is taken out as it is instead of the reflected light having a high polarization dependency as in the prior art.
- the optical fiber sensing optical system according to claim 2 is the optical fiber sensing optical system according to claim 1, wherein the optical axis of the detection light incident on the wavelength discriminating unit is on the incident side of the wavelength discriminating unit. It is preferably substantially perpendicular to the interface.
- the reflectance and transmittance of light differ depending on the polarization component perpendicular to the incident surface (S-polarized light) and the component parallel to the incident surface (P-polarized light), but the incident angle to the interface is 0, that is, relative to the incident-side interface. If it is perpendicular, the reflectance and transmittance of S-polarized light and P-polarized light are equal, and thus the polarization dependence disappears.
- the optical fiber sensing optical system according to claim 3 is the lamp optical fiber sensing optical system according to claim 1 or 2, wherein the light source, the reflection means, the first light distribution means, and the second light distribution means. And the optical power transmission between the wavelength discriminating means is performed by a single mode optical fiber, the delay means is constituted by a single mode optical fiber, and the first light distributing means is a first input to which the search light is input.
- the second light distribution means has a 21st port to which the detection light output from the 13th port is input, a 22nd port to output the detection light to the wavelength discrimination means, and Preferably comprises a, having a first 23 port for outputting to the outside the detection light as the compensation light.
- the optical fiber sensing optical system includes an optical device and a single mode fiber that connects the optical device.
- the first light distribution means transmits the search light input to the eleventh port to the twelfth port, and the light is transmitted to the FBG with a single mode fiber.
- Detection light from the FBG is transmitted from the 12th port to the 13th port.
- the detection light from the 13th port is input to the 21st port of the second light distribution means and transmitted to the 22nd port connected to the wavelength discrimination means and the 23rd port outputting the detection light to the outside.
- Single mode fibers are connected to the 22nd port and the 23rd port, and the detection light is guided directly or indirectly to the wavelength discriminating means and the outside, respectively.
- An optical fiber sensing optical system is the optical fiber sensing optical system according to the third aspect, wherein the first light distribution means is connected to the twelfth port from the eleventh port to the twelfth port.
- a circulator that transmits optical power from the first port to the thirteenth port, and the second optical distribution means is a coupler that divides the optical power input to the 21 port into the 22 port and the 23 port. Is preferred.
- the optical fiber sensing optical system according to the present invention can prevent the detection light from the FBG from returning to the light source and causing laser oscillation by using a circulator as the first light distribution means. Moreover, cost reduction can be achieved by using a coupler for the second light distribution means.
- the optical fiber sensing optical system according to claim 5 is the optical fiber sensing optical system according to claim 3, wherein the first light distribution means is an optical isolator that forwards from the light source to the FBG, and A coupler for branching light from the twelfth port to the thirteenth port and the eleventh port, and the second optical distribution means sends the optical power input to the twenty-first port to the twenty-second port and the twenty-third port. It is preferable that the coupler be divided.
- the optical fiber sensing optical system according to the present invention can reduce costs while preventing laser oscillation of the light source by using an optical isolator and a coupler instead of a circulator as a branching unit.
- An optical fiber sensing optical system is the optical fiber sensing optical system according to any one of the third to fifth aspects, wherein a plurality of the reflecting means are provided, and an optical fiber having a predetermined length is mutually connected. It is preferable to be connected.
- the optical fiber sensing optical system according to the present invention can measure a plurality of physical quantities by connecting a plurality of reflecting means.
- an optical fiber sensing system according to any one of the first to sixth aspects in which there is a difference in the extraction time between the compensation light and the detection light from the optical fiber sensing optical system according to any one of the first to sixth aspects. It is preferable to include a calculation unit that receives light with two light receivers and calculates a ratio of the intensity of the transmitted light to the compensation light based on an output from the light receiver.
- the optical fiber sensing system receives light with a single light receiver in a state where there is a difference in transmission time between transmitted light and detection light, it is difficult for the light receiving system to vary.
- the optical fiber sensing optical system according to claim 8 is the optical fiber sensing optical system according to claim 7, wherein the light source is driven based on a pseudo random number, and the output is signal-processed by a correlator. Is preferred.
- the light source is driven with a pseudo-random number and the output from the light receiver is processed by the correlator, a sharp pulse is generated at the time when the correlation is obtained, and the size of the light receiver is Is proportional to the light input to.
- the level variation of the net transmitted light that compensates for the variation of the optical fiber sensing optical system including the polarization variation is known, and the wavelength corresponding to the physical state of the object to be measured is determined from there. You can know the fluctuation.
- the output from the optical fiber sensing optical system is not easily affected by the fluctuation of the polarization during the light propagation, and the optical fiber sensing is aimed at reducing the cost of the optical fiber sensing optical system capable of measuring with high accuracy.
- An optical system and an optical fiber sensing system using the optical system can be provided.
- FIG. 1 is a diagram illustrating an overall configuration of the optical fiber sensing system according to the first embodiment.
- the optical fiber sensing system includes an electrical measurement system 10 and an optical fiber sensing optical system 30 connected thereto.
- An optical fiber sensing optical system includes: a light source; an FBG that is a reflection unit having a reflection wavelength that varies according to a change in a measurement environment; a wavelength gradient filter that is a wavelength discrimination unit that discriminates a selected wavelength; and a wavelength discrimination unit.
- Delay means for providing a time difference between transmitted light from the light and compensation light for compensating the transmitted light, and an optical component that organically connects them. The actual coupling is realized by optical power transmission using a single mode optical fiber.
- FBG 1 via an optical coupler connected to the optical fiber 62, FBG 2 via an optical coupler connected to the optical fiber 63, and FBG 3 via an optical coupler connected to the optical fiber 64 are reflection means. By placing an appropriate distance between the installation locations of the FBGs, the reflected light from each can be separated in time.
- the electrical measurement system 10 includes a PN code generator, a light source driver, a PD (photodiode) 6, a preamplifier, an A / D converter, a correlation circuit, a signal processor, and the like.
- an optical fiber sensing optical system including optical fibers 61, 65, 66, 67, 68, a circulator 1, an optical coupler (optical coupler, optical splitter) 2, a wavelength tilt filter 3, and a dummy fiber 4, An optical fiber sensing system is configured.
- the circulator 1 is a first light distribution means and has an eleventh port, a twelfth port and a thirteenth port.
- the optical coupler 2 is a second light distribution means and has a 21st port, a 22nd port and a 23rd port.
- the wavelength tilt filter is a filter having a tilt characteristic in which a change in wavelength corresponds to a change in transmittance, and serves as a wavelength discrimination means.
- the circulator 1 is used not to return the reflected light from the light measurement system to the light source, and the direction from the light source to the FBG is the forward direction. Therefore, as shown in FIG. 2, an isolator 11 and an optical coupler 12 can be combined as an inexpensive configuration that does not return light to the light source.
- the isolator 11 has a forward direction from the light source to the FBG, and this configuration greatly reduces the light returning to the light source.
- the PN code generator generates an M-sequence pseudo-random code (hereinafter referred to as a pseudo-random code) having a code length N at a predetermined chip speed, and sends a pseudo-random signal to the signal processing circuit of the light source driver and the correlator. .
- an M-sequence pseudo-random code is used.
- the present invention is not limited to this.
- another pseudo-random code such as a Gold sequence may be used.
- a pseudo-random code generated by sampling white noise may be used.
- the light source driver is driven by the pseudo random signal from the PN code generator, and performs PN modulation on the light emitted from the broadband light source 5 based on the pseudo random signal from the PN code generator (hereinafter, after modulation)
- the search light is introduced into the optical fiber 61.
- the broadband light source 5 having a continuous wavelength component is preferably a high-power superluminescent diode (SLD) that can be used easily compared to an ASE light source using a fiber.
- SLD superluminescent diode
- a light emitting diode and a multimode fiber having a large core diameter may be combined.
- the circulator 1 has three input / output ports and has irreversible transmission characteristics. That is, the light incident on the first port (P11) is output from the second port (P12), and the light incident on the second port (P12) is output from the third port (P13). However, the second port (P12) to the first port (P11), the third port (P13) to the second port (P12) and the first port (P11), and the first port (P11) To the third port (P13) is blocked and not transmitted.
- the circulator 1 outputs the light incident from the optical fiber 61 to the through main line 62, while outputting the reflected light entering the circulator 1 from the through main line 62 to the optical fiber 65. Further, the optical coupler 2 outputs the light incident from the optical fiber 65 to the optical fiber 68 via the optical fiber 66 and the dummy fiber 4.
- the search light introduced into the optical fiber 61 passes through the through main line 62, is branched by the optical coupler, and enters the FBGs 1, 2, and 3, respectively.
- the FBGs 1, 2, and 3 light having a wavelength corresponding to the physical quantity to be measured is reflected.
- the wavelength gradient filter 3 changes the transmitted light intensity according to the wavelength of the return light from the FBG1, FBG2, and FBG3.
- the wavelength tilt filter 3 it is preferable to use a band pass filter in which a dielectric multilayer filter is deposited on the end face of the optical fiber, an edge filter, or the like.
- the search light that has entered the P11 that is the entrance of the isolator 11 enters the optical fiber 62 and is transmitted to the FBG.
- the light reflected by the FBG and incident on the P12 is branched to the P11 direction and the P13.
- the light branched in the direction of P11 is blocked by the isolator 11, and the light to the light source 5 is greatly attenuated.
- the configuration shown in FIG. 2 is advantageous in terms of cost because the sensitivity of reflected light from the FBG is divided using an isolator and an optical coupler instead of a circulator, so that the detection sensitivity is lowered.
- FBGs 1, 2 and 3 are fiber Bragg gratings, and are respectively installed with a distance sufficiently longer than the distance resolution of the electrical measurement system 10. In this embodiment, it is set so that light of equal energy is distributed to each of the FBGs 1, 2 and 3.
- the FBGs 1, 2, and 3 reflect light having a wavelength ⁇ o as detection light when there is no change in the physical quantity to be measured. When the physical quantity of the measurement target changes, the wavelength of the detection light is shifted according to the change of the physical quantity of the measurement target.
- the installation interval between the FBGs 1, 2 and 3 may be four times or more the distance resolution determined by the chip speed of the PN code generator 20.
- the time difference of the reflected light between the FBGs is a time difference that reciprocates the distance between the FBGs.
- the distance between the FBGs is preferably sufficiently longer than the length of a dummy fiber described later so that the detection light and the compensation light are not mixed by the photodetector.
- FIG. 4 is a graph showing changes in the wavelength of the return light with respect to temperature, where the horizontal axis represents wavelength (nm) and the vertical axis represents reflection gain (mV) with respect to incident light.
- the wavelength of the reflected light changes as shown in FIG. Since the light source is a portion where the emission spectrum of the SLD shown in FIG. 3 is almost flat, the level of the reflected light is substantially uniform in the region where the reflection wavelength of the FBG changes.
- the change in the wavelength of the reflected light from the FBG is changed in the intensity of the transmitted light. Can be detected as
- the residual noise is increased in an optical system in which transmitted light and reflected light are spatially separated by using an obliquely incident wavelength gradient filter in the optical coupler 2 portion, in addition to cost reduction without using an expensive circulator. I found out.
- FIG. 5 is a diagram showing a transmittance (T) indicated by a dotted line and a reflectance (R) indicated by a solid line with respect to the wavelength of light incident on the wavelength tilt filter 3.
- T transmittance
- R reflectance
- the sensitivity of wavelength discrimination can be doubled by using reflected light from the wavelength tilt filter as compensation light to compensate for the variation factors of the measurement system.
- the use of the reflected light from the wavelength tilt filter as the compensation light has a big problem due to fluctuations in the polarization of the measurement system. Therefore, this time, the reflected light (detection light) from the FBG is used as the compensation light, not the reflected light from the wavelength tilt filter.
- the dummy fiber 4 as a delay line gives an optical path difference of four times or more of the distance resolution of the electrical measurement system 10 between the detection light and the transmitted light separated by the optical coupler 2.
- the length of the dummy fiber 4 may be at least four times the distance resolution determined by the chip speed of the PN code generator.
- the dummy fiber 4 is provided in the optical fiber 68, but the dummy fiber 4 may be provided in the optical fiber 66 or the optical fiber 67.
- the optical coupler 2 divides the detection light from the optical fiber 65 into compensation light and light directed to the wavelength tilt filter.
- the PD 6 comprises a photo diode or the like, and receives transmitted light and compensation light at different times with a time difference corresponding to an optical path difference given between them.
- the preamplifier amplifies the transmitted light and compensation light received by the PD 6.
- the A / D converter converts an analog voltage applied to the transmitted light and the compensation light amplified by the preamplifier into a digital signal.
- the correlation circuit obtains a correlation between the electric signal from the A / D converter and the pseudo random code from the PN code generator, thereby increasing the gain for the physical measurement light according to the intensity of the transmitted light and the compensation light, respectively. calculate. This is a calculation means for calculating the ratio between the two.
- the intensity ratio of (transmitted light / compensated light) is obtained from the gain at the peak of the transmitted light and the gain at the peak of the compensated light calculated by the correlation circuit, and the wavelength shift is calculated from this ratio, Detect physical quantity information to be measured.
- the search light introduced into the optical fiber 61 passes through the through main line 62, is branched by the optical coupler, and enters the FBGs 1, 2, and 3, respectively.
- the FBGs 1, 2, and 3 light having a wavelength corresponding to the physical quantity to be measured is reflected.
- the light from the FBGs 1, 2 and 3 is incident on the electrical measurement system 10 with a time difference corresponding to the installation interval.
- the return light from the FBG 1 installed at a position closest to the electrical measurement system 10 enters the circulator 1 and is output to the optical fiber 65.
- the detection light output to the optical fiber 65 enters the optical coupler 2 and is output to the optical fiber 66.
- the light output to the optical fiber 66 enters the wavelength tilt filter 3 and is separated into transmitted light and reflected light whose intensity changes complementarily according to the wavelength.
- the transmitted light travels through the optical fiber 67 and is received by the PD 6. Further, the detection light from the FBG enters the coupler 2 via the optical fiber 65, and a part of the light is output as compensation light to the optical fiber 68, and the optical path that is equal to or higher than the distance resolution of the electrical measurement system 10 by the dummy fiber 4. After the difference is given, the light is received by the photodetector PD6.
- the transmitted light and the compensation light are amplified by a preamplifier, converted to an electrical signal by an A / D converter, and sent to a correlation circuit.
- the electrical signal from the A / D converter is correlated with the pseudo-random code sent from the PN code generator, and the respective gains of the compensation light and the transmitted light are calculated. Signal processing is performed.
- the level difference between the peak gain of transmitted light and the peak gain of compensation light is obtained.
- the wavelength shift is calculated based on this level difference, and the physical quantity to be measured is calculated from the shift amount.
- the detection lights from the FBGs 2 and 3 are processed in the same manner as described above, and the physical quantities of the respective measurement objects are detected.
- the horizontal axis represents wavelength (nm), and the vertical axis represents transmittance (dB) and reflectance (dB).
- the wavelength shift from the center wavelength ⁇ o is represented by ⁇ .
- the dotted line indicates the transmittance of light incident on the wavelength-gradient filter 3 with respect to the wavelength.
- the dotted line is represented by the following equation.
- the solid line indicates the reflectance of the light incident on the wavelength-gradient filter 3 with respect to the wavelength, and is normalized by the following expression when normalized.
- the transmittance and the reflectance change complementarily, and when one increases, the other decreases, and the sum is always 1 as represented by the following equation.
- the ratio of transmittance and reflectance It becomes.
- the wavelength shift amount ⁇ is It becomes.
- the return light from the FBG is separated into reflected light and transmitted light at a reflectance and transmittance according to the wavelength by the wavelength tilt filter 3, and the wavelength shift is performed from the level ratio of these two lights. It can be calculated.
- both the reflected light and transmitted light of the wavelength tilt filter are used to efficiently extract the influence of the change in wavelength on the change in received light power.
- residual noise which is not only an intensity change accompanying a wavelength change, hinders improvement in measurement sensitivity.
- a countermeasure is to use detection light from the FBG as compensation light without passing through the filter instead of the reflected light from the wavelength tilt filter.
- Variations in transmittance and reflectance due to polarization variations can be understood as variations from the characteristics of transmittance and reflectance at normal incidence. Equivalently, it can be seen that the filter characteristics change like the transmittance (T ′) indicated by the thin solid line and the reflectance (R ′) indicated by the thin dotted line in FIG. On the other hand, considering the characteristics of the thick broken line and the solid line with normal incidence, it can be seen that the wavelength of the detection light is fluctuating.
- the reflected light is complementary to the transmitted light, so that the sensitivity is also increased with respect to the polarization variation, and the noise component due to the polarization variation is easily affected. That is, in order to increase the detection sensitivity, the conventional method using reflected light from the wavelength discriminator as compensation light has a weak point that it is highly sensitive to noise. Therefore, as a countermeasure, by using detection light instead of reflected light as compensation light, the denominator of Equation 6 becomes constant, and the sensitivity to polarization fluctuations is reduced and S / N is improved.
- the suspected cause of residual noise is polarization fluctuation when the search light and the detection light pass through an optical device including an optical fiber.
- the characteristics of the wavelength tilt filter are susceptible to polarization fluctuations. Therefore, the characteristics of the wavelength tilt filter that seems to cause noise due to the fluctuation of the polarization were investigated.
- Fig. 6 shows the measurement system.
- the optical output from the SLD of the light source was connected to the FBG through a circulator and a 100 m optical fiber through a polarization controller that controls the polarization of the light output from the SLD of the light source. Reflected light from the FBG is input to the optical filter through the circulator.
- Measured object is a filter that seems to cause polarization fluctuations that cause noise. Since the sum of the transmitted light power and the reflected light power of the filter can be assumed to be constant, the polarization dependence of the transmitted light becomes the polarization dependence in the reverse direction of the reflected light.
- the three-port filter to be measured has a collimating optical system that converts the light from the fiber into a parallel beam in the main body and the optical axis of the incident light at a predetermined angle with respect to the interface on the incident side of the filter. It consists of a tilted interference filter. Light incident at a predetermined angle with respect to the incident interface of the interference filter is reflected at a predetermined angle on the opposite side and transmitted to the back surface.
- the light from the outside is substantially perpendicular to the interface on the incident side of the filter.
- Both the 2-port filter and the 3-port filter have a pigtail for connecting the internal optical system to an external optical fiber.
- the polarization plane during transmission through a single-mode optical fiber is not preserved, so the polarization plane fluctuates due to disturbance. If it fluctuates in units of time larger than the time constant of the measurement system, the measured value will change each time it is measured.
- Fig. 7 compares the fluctuations in the transmitted light intensity of the 2-port filter and 3-port filter when they are close to the actual measurement system.
- the amplitude of power fluctuation is 0.05 dBm
- the fluctuation range of transmitted light intensity is 0.95 dBm. Yes, there is a difference of about 1 dBm.
- the reflected light from the FBG is vertically incident on the wavelength tilt filter, and the reflected light from the FBG is used as the compensation light instead of the reflected light from the wavelength tilt filter, thereby greatly improving the sensitivity of the measurement system. It was.
- an optical fiber sensing optical system and an optical fiber sensing system that enable measurement with higher sensitivity by reducing noise accompanying polarization fluctuation as compared with the conventional method.
- Circulator 2 Optical coupler (Optical coupler, Optical splitter) 3 Wavelength inclined filter: Wavelength discrimination means 4 Dummy fiber: Delay means 5 SLD: Light source 6 PD: Light receiver 10 Electrical measurement system 30 Optical measurement system (optical fiber sensing optical system) 61-68 optical fiber 71 FBG1 72 FBG2 73 FBG3
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- Physics & Mathematics (AREA)
- General Physics & Mathematics (AREA)
- Optical Transform (AREA)
- Length Measuring Devices By Optical Means (AREA)
Abstract
Description
本実施形態では、M系列の擬似ランダム符号を用いるものとしたが、これに限定されず、例えばGold系列等の他の擬似ランダム符号でも良い。またホワイト雑音をサンプリングして生成した擬似ランダム符号でも良い。
て、透過光強度が変化する。波長傾斜フィルタ3としては、光ファイバの端面に誘電体多層膜フィルタを蒸着させたバンドパスフィルタや、エッジフィルタ等を使用することが好ましい。
距離分解△L=100/fc (m)
(但し、fcはPN符号生成器20のチップ速度で単位はMHzである)
例えば、チップ速度が10MHzである場合、上記の△LFは10mとなるので、BG1,2及び3の設置間隔は、それぞれ40m以上であれば、各FBGからの反射光の間には往復で80m分以上の時間差が生じる。FBG間の距離は検出光と補償光が光検出器で混ざらないように、後述するダミーファイバの長さよりも十分長い距離が好適である。
△L=100/fc (m)
(但し、fcはPN符号生成器20のチップ速度)
例えば、チップ速度が10MHzである場合、△Lは10mになるのでダミーファイバ4は40m以上であれば良い。
次に、上記レベル差から波長のシフト量を算出する方法を、図5を参照しながら説明する。図5は、横軸に波長(nm)を、縦軸に透過率(dB)及び反射率(dB)をそれぞれとったものである。図5では中心波長λoからの波長のずれをδで表わしている。
ここで、測定対象の物理量が変化してFBGが反射させる光の波長が△λだけシフトし、λ=λ0+δ=λ0+△λに変化した場合、透過率と反射率との比は、上式より
以上のようにして、FBGからの戻り光を、波長傾斜フィルタ3によって波長に応じた反射率と透過率で反射光と透過光とに分離し、これら2つの光のレベル比から波長のシフトを算出する事ができる。
波長変化に伴う強度変化だけでない残留の雑音が逆に測定感度の向上を妨げていることが分かった。対策は、上記の波長傾斜フィルタからの反射光の代わりにFBGからの検出光をフィルタを介さずに補償光として使用することである。
2 光カプラ(光結合器、光分岐器)
3 波長傾斜フィルタ:波長弁別手段
4 ダミーファイバ:遅延手段
5 SLD:光源
6 PD:受光器
10 電気測定系
30 光学測定系(光ファイバセンシング光学系)
61~68 光ファイバ
71 FBG1
72 FBG2
73 FBG3
Claims (8)
- 連続波長成分を有する探索光を出力する光源と、
前記連続波長成分の一部の波長成分を選択して検出光を生成する反射手段と、
前記検出光が入射され、その波長の変化に応じて強度の変化する透過光を生成する波長弁別手段と、
前記探索光を前記反射手段へ非可逆的に伝送すると共に前記検出光を前記波長弁別手段を含む光学系へ伝送する第1光分配手段と、
該第1光分配手段から前記波長弁別手段を含む光学系に伝送された検出光をさらに前記波長弁別手段への光及び補償光に分割する第2光分配手段と、
前記補償光及び前記透過光の外部への取り出し時刻の間に時間差を生成する遅延手段と、を備えることを特徴とする光ファイバセンシング光学系。 - 前記波長弁別手段へ入射する前記検出光の光軸が前記波長弁別手段の入射側の界面に対して略垂直であることを特徴とする請求項1に記載の光ファイバセンシング光学系。
- 前記光源、前記反射手段、前記第1光分配手段、前記第2光分配手段、及び前記波長弁別手段の間の光パワー伝送がシングルモード光ファイバで行われ、前記遅延手段がシングルモード光ファイバで構成され、
前記、第1光分配手段が、前記探索光が入力される第11ポート、前記第11ポートへ入力された探索光を出力するとともに前記検出光が入力される第12ポート、及び前記第12ポートへ入力された前記検出光を出力する第13ポートを有し、
前記第2光分配手段が、前記第13ポートから出力された前記検出光が入力される第21ポート、前記検出光を前記波長弁別手段へ出力する第22ポート、及び前記検出光を前記補償光として外部へ出力する第23ポートを有することを特徴とする請求項1又は2に記載の光ファイバセンシング光学系。 - 前記第1光分配手段が、前記第11ポートから前記第12ポートへ、前記第12ポートから前記第13ポートへ光パワーを伝送するサーキュレータであり、
前記第2光分配手段が、前記21ポートへ入力された光パワーを、前記22ポート及び前記23ポートへ分割する結合器であることを特徴とする請求項3に記載の光ファイバセンシング光学系。 - 前記第1光分配手段が、前記光源から前記FBGへ順方向である光アイソレータ及び前記第12ポートから前記第13ポートと前記第11ポートへ光を分岐する結合器から成り、
前記第2光分配手段が、前記21ポートへ入力された光パワーを、前記22ポート及び前記23ポートへ分割する結合器であることを特徴とする請求項3に記載の光ファイバセンシング光学系。 - 前記反射手段が複数設けられ、所定の長さの光ファイバで相互に接続されることを特徴とする請求項3乃至5の何れかに記載の光ファイバセンシング光学系。
- 請求項1乃至6何れかに記載の前記光ファイバセンシング光学系から出力される前記補償光と前記検出光との取り出し時刻に差が有る状態で一つの受光器で受光し、前記受光器からの出力により前記補償光に対する前記透過光の強度の比を算出する算出手段を備えることを特徴とする光ファイバセンシングシステム。
- 前記光源が、擬似乱数に基づいて駆動され、前記出力が相関器により信号処理されることを特徴とする請求項7に記載の光ファイバセンシングシステム。
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EP13889464.7A EP3023747A4 (en) | 2013-07-16 | 2013-07-16 | OPTICAL GLASS FIBER MEASURING SYSTEM AND GLASS FIBER MEASURING SYSTEM |
PCT/JP2013/069310 WO2015008332A1 (ja) | 2013-07-16 | 2013-07-16 | 光ファイバセンシング光学系及び光ファイバセンシングシステム |
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