JP4808546B2 - Optical fiber sensor - Google Patents

Description

  The present invention relates to an optical fiber sensor using an optical fiber.

Sensors using optical fibers are easy to measure at remote locations and are resistant to electromagnetic noise, and are thus being applied in various fields. Above all, a sensor that uses a change in light intensity has advantages such as a simple configuration and a low price.
For example, Patent Document 1 discloses a temperature detection device having a structure in which a temperature-sensitive color change material whose light absorption or scattering characteristics change according to temperature is applied around a small-diameter portion formed in the middle of an optical fiber to provide a temperature-sensitive layer. Is described. In Patent Document 2, a light reflector is provided at a position facing the front end surface of the optical fiber, and the light absorption or scattering characteristics change depending on the temperature between the front end surface of the optical fiber and the light reflector. A temperature sensor having a structure in which a temperature-sensitive part is provided by being filled with a temperature-changing material is described.
However, the temperature detection device of Patent Document 1 is difficult to manufacture with high accuracy because the characteristics greatly depend on the shape of the small diameter portion. In addition, it is necessary to use a special material for the temperature-sensitive layer, and characteristic deterioration due to repeated use is also observed. Therefore, it has been difficult to maintain high measurement accuracy over a long period. Also in Patent Document 2, there is a problem in terms of long-term reliability because a special material is used for the temperature sensitive part.
Also, a structure such as a hetero-core type optical fiber (Non-patent Document 1 or the like) has been proposed, but it has low sensitivity to temperature and cannot be used as a thermometer. Also, when measuring strain, it is necessary to bend the fiber, and there is a problem that the apparatus configuration becomes complicated as compared with strain detection in the pulling direction in the fiber axis direction.
Patent Documents 3 and 4 describe a structure in which a graded index optical fiber is inserted between single-mode optical fibers. However, the purpose is to avoid the fiber fuse phenomenon by lowering the optical energy density in the graded index optical fiber.
In Patent Document 5, the output light of a light emitting diode is split into two by a coupler, and one is made incident on a signal optical fiber clamped by two plate members, and the other is made incident on a reference optical fiber. An apparatus is described that measures the amount of strain by comparing the transmitted light intensity of fibers.
JP-A-9-159548 JP-A-9-43066 IEICE, Society Conference, September 2005, B-13-26 JP 2004-86127 A Japanese Patent Laid-Open No. 2005-77549 US Pat. No. 5,020,379

  The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical fiber sensor that can be measured by a change in light intensity, is easy to manufacture, and has high reliability.

To solve the above problems, the present invention includes a sensing portion consists graded index optical fiber, Ri transmission path on the transmission side and the reception side to the sensing unit Do from the single-mode optical fiber, the center wavelength of the used light source The length L of the sensing unit made of the graded index optical fiber satisfies the following formula with respect to the pitch Λ of light propagating while repeating diffusion and collection in the graded index optical fiber: An optical fiber sensor is provided.
(1 + 2N) Λ / 8−Λ / 16 <L <(1 + 2N) Λ / 8 + Λ / 16
However, N is a positive integer (N = 1, 2, 3,...).
In addition, the present invention provides the optical fiber sensor, wherein the optical fiber sensor is used by measuring a change in light intensity transmitted through the sensing unit.
In addition, the present invention provides the optical fiber sensor, wherein a plurality of the sensing units are arranged in series, and a single mode optical fiber is inserted therebetween.
In the present invention, a first single-mode optical fiber that serves as both transmission and reception transmission paths is connected to one side of the sensing unit, and a tip is provided to the other side of the sensing unit. The optical fiber sensor is provided, wherein a second single mode optical fiber treated with a reflection end is connected.
In addition, the present invention compares and calculates the reference light intensity obtained by monitoring the light intensity of the light source and the sensing light intensity obtained by monitoring the light intensity incident from the light source and transmitted through the optical fiber sensor, so that the temperature of the sensing unit is calculated. There is provided a temperature measurement method characterized by measuring a change.
The present invention also provides the above-described optical fiber sensor, a light source that makes light incident on the optical fiber sensor, a first photodetector that monitors the light intensity of the light source as a reference light intensity, and the light source that is made incident from the light source. A second photodetector for monitoring the intensity of light transmitted through the optical fiber sensor as the intensity of sensing light, and a comparison operation circuit for comparing and calculating the intensity of the sensing light with the intensity of the reference light and outputting a temperature change of the sensing unit. A temperature measuring system is provided.
The present invention also compares the reference light intensity monitored for the light intensity of the light source with the sensing light intensity monitored for the light intensity incident from the light source and transmitted through the optical fiber sensor, and applied to the sensing unit. A tension measuring method is provided that measures the measured tension.
The present invention also provides the above-described optical fiber sensor, a light source that makes light incident on the optical fiber sensor, a first photodetector that monitors the light intensity of the light source as a reference light intensity, and the light source that is made incident from the light source. A second photodetector that monitors the intensity of light transmitted through the optical fiber sensor as the intensity of sensing light, and a comparison operation circuit that compares the sensing light intensity with the reference light intensity and outputs the tension applied to the sensing unit; A tension measuring system is provided.

  According to the present invention, a highly accurate sensor capable of measuring the temperature of an optical fiber and the tension applied to the optical fiber can be realized with a very simple configuration.

Hereinafter, the present invention will be described in detail.
As schematically shown in FIG. 1, the optical fiber sensor of the present invention uses a graded index optical fiber 1 manufactured so that the refractive index of the core 1a continuously changes in the radial direction of the optical fiber as a sensing unit. The single-mode optical fiber 2 in which only the LP ₀₁ mode propagates in the portion of the core 2a in the used wavelength band is connected to both ends of the sensing unit. In the following description, the graded index optical fiber may be abbreviated as GI fiber, and the single mode optical fiber may be abbreviated as SM fiber.

  FIGS. 1A and 1B show a case where the GI fiber 1 has the same diameter as the SM fiber 2, and FIG. 1C shows a case where the GI fiber 1 has a larger diameter than the SM fiber 2. Since the GI fiber and the SM fiber are connected by fusion splicing, axial misalignment between the connected optical fibers can be suppressed, so that a highly reliable optical fiber sensor can be obtained. At this time, if the diameters of the GI fiber and the SM fiber are different from each other, it causes a shaft misalignment at the time of fusion.

  In FIG. 1, the curve shown in the portion of the GI fiber 1 schematically shows the propagation of light incident on the GI fiber from the SM fiber. That is, light incident on the GI fiber 1 from the SM fiber 2 on the transmission side (left side in FIG. 1) propagates while repeating diffusion and collection in the GI fiber 1. This is because the refractive index of the GI fiber has a refractive index distribution that varies in the radial direction, and thus has an effect similar to a lens. FIG. 1A shows a case where the length L of the GI fiber 1 is set so that light incident on the SM fiber 2 on the receiving side (right side in FIG. 1) from the GI fiber 1 is in a condensing condition. FIG. 1B shows a case where the length L of the GI fiber 1 is set to a length that deviates from the condensing condition. When the length L of the GI fiber 1 is set so that the light incident on the SM fiber 2 on the receiving side is in the condensing condition, most of the light propagated through the GI fiber 1 is coupled to the SM fiber 2 again. However, when it deviates from the condensing condition, a part of the light propagated through the GI fiber 1 is coupled with the cladding mode of the SM fiber 2 and leaks to the outside of the optical fiber. For this reason, when the lengths of the GI fibers are different, the intensity of light propagating from the SM fiber on the transmission side to the optical fiber on the reception side through the GI fiber changes.

  2 and 3 show the results of estimating the intensity change when the length of the GI fiber part (GI part length) is changed. FIG. 2 shows a case where the core diameter of the GI fiber is 50 μm, and FIG. 3 shows a case where the core diameter of the GI fiber is 320 μm. As can be seen from FIGS. 2 and 3, the transmittance (relative ratio) of the optical fiber sensor periodically changes with respect to the length of the GI fiber portion. Further, when the core diameter of the GI fiber is increased, the amplitude of diffusion / condensation of light propagating through the GI fiber is increased and the period is also increased. Therefore, as can be seen from the comparison between FIG. 2 and FIG. 3, when the core diameter of the GI fiber is increased, the period of periodic transmittance change due to the change in the length of the GI fiber portion is increased. Also, the change in transmittance is different. Thus, by changing the core diameter of the GI fiber, it is possible to adjust the characteristics of the optical fiber sensor.

  The optical path length (also referred to as optical distance) of the GI fiber portion is the refractive index n of the GI fiber (approximately, the refractive index at the core center can be used as a representative value) and the length of the GI fiber portion. Expressed by the product of L. That is, when these physical quantities change due to temperature change or tension change of the GI fiber, the optical path length of the GI fiber part changes, and the transmitted light intensity of the optical fiber sensor changes. For example, when the temperature T of the GI fiber changes, the optical path length Lo = n × L shows the temperature dependence obtained by the following equation (1) by taking the differentiation of the temperature T.

dLo / dT = L × dn / dT + n × dL / dT (1)

Since dn / dT, n, and dL / dT on the right side of Equation (1) are physical constants that are uniquely determined when the optical fiber to be used and the wavelength to be used are determined, in order to increase the characteristic variation due to temperature change, the GI fiber It can be seen that the length (actual length) L of the portion may be increased.
Since the optical path length Lo of the GI fiber portion also changes depending on the tension applied to the GI fiber, it is also possible to measure the change in tension as the change in light intensity, similar to the change in temperature. At the time of measurement, an environment is created in which the influence of other factors that cause disturbance is not applied to the sensing unit as much as possible so that the light intensity changes based on the change of the parameter to be measured. That is, when measuring temperature, do not apply tension to the GI fiber. When measuring tension, measure the environment so that the temperature of the GI fiber does not change, or measure the ambient temperature with another sensor. Correct the effect of temperature.
Further, since the refractive index n of the GI fiber has a wavelength dependence, the transmitted light intensity of the optical fiber sensor also changes depending on the wavelength. Thus, the optical fiber sensor of the present invention can be used as a temperature sensor or a tension sensor by optimizing the core diameter and length L of the GI fiber and the wavelength (use wavelength) of light used for measurement. it can.

  As the mode field diameter (MFD) of the SM fiber 2 is smaller, the wavelength dependency at the point of recombination from the GI fiber 1 to the SM fiber 2 on the receiving side increases, and therefore the rate of change in transmitted light intensity tends to increase. is there. Therefore, in order to increase the rate of change of transmitted light intensity due to temperature change or tension change to increase the sensitivity of the optical fiber sensor, it is desirable that the MFD of the SM fiber is small, but the insertion loss increases as the MFD decreases. Considering the sensitivity of the optical fiber sensor, the MFD of the SM fiber is desirably 12 μm or less. In consideration of insertion loss, the MFD of the SM fiber is preferably 5 μm or more.

  As described above, the length L of the portion of the GI fiber 1 is so long that the rate of change in transmitted light intensity due to temperature change or tension change increases, and the sensitivity of the optical fiber sensor can be increased. As a result, the distortion of the diffusion / condensation characteristics inside the GI fiber increases, and the condensing characteristics are lost. Therefore, when used as an optical fiber sensor, the length L of the GI fiber 1 is desirably 5 mm or more and 40 mm or less. The preferred range of the length L of the GI fiber 1 varies somewhat depending on the core diameter of the GI fiber 1, but is approximately the same when the core diameter is 100 μm or less.

  As the core diameter (core diameter) of the GI fiber 1 is smaller, the insertion loss of the optical fiber sensor can be reduced, but the sensitivity as the optical fiber sensor is also reduced. For this reason, considering the sensitivity, the core diameter of the GI fiber 1 is preferably 30 μm or more. Moreover, since the outer diameter (cladding diameter) of a normal optical fiber is 125 μm, the core diameter of the GI fiber 1 is desirably 120 μm or less from the viewpoint of insertion loss. In particular, a GI fiber having a core diameter of about 50 μm to 70 μm is widely used for optical communication, and is particularly desirable from the viewpoint of availability, price, and characteristics.

  In the optical fiber sensor of the present invention, light from a light source is incident on a configuration composed of SM fiber-GI fiber-SM fiber, and the change in transmitted light intensity is measured. At this time, in order to increase the sensor sensitivity, it is necessary to optimize the characteristics of the light source (center wavelength / bandwidth) and the length of the GI fiber portion. In order to obtain high sensitivity as an optical fiber sensor, the length L of the GI fiber serving as the sensing unit is such that the light having the center wavelength of the light source used is coupled with the SM fiber on the receiving side as shown in FIG. It is desirable to set the position so that it is halfway between the antinodes and nodes of light propagating in the GI fiber. When the pitch Λ of light propagating through the GI fiber at the center wavelength of the light source used is defined as twice the node-to-node distance as shown in FIG. 1, the length L of the GI fiber 1 is given by the following formula ( Conditions satisfying 2) are preferred. However, in Formula (2), N is a positive integer (N = 1, 2, 3,...).

(1 + 2N) Λ / 8−Λ / 16 <L <(1 + 2N) Λ / 8 + Λ / 16 (2)

If the light source spectrum bandwidth is too wide, the change in light intensity due to the shift of the wavelength spectrum of the optical fiber sensor becomes small. For example, in the spectrum of the embodiment shown in FIG. 14 (GI fiber length L = 20.092 mm), the transmittance is minimum at a wavelength of 1400 nm and the transmittance is maximum at a wavelength of 1600 nm. In this example, the wavelength at which the transmittance is maximum (this may be referred to as “maximum transmittance wavelength” in this specification) and the wavelength at which the transmittance is minimum (this is referred to as “minimum transmittance” in this specification). The difference from the “wavelength” is about 200 nm. When the spectral bandwidth of the light source is 200 nm or more, which is the difference, the sensitivity of the optical fiber sensor is deteriorated.
That is, the spectral bandwidth of the light source used with the optical fiber sensor of the present invention needs to be equal to or less than the “difference between the maximum transmittance wavelength and the minimum transmittance wavelength”, preferably “the maximum transmittance wavelength and the minimum transmittance”. It is less than half of the “difference from wavelength”. Therefore, the spectral bandwidth of the light source is desirably 100 nm or less. As a light source showing such a spectral bandwidth, a laser diode (LD), a light emitting diode (LED), various lamps, spontaneous emission light of a laser medium, or the like can be used. When a light source having a wider spectral bandwidth than desired is used, it can be used by making light of a desired spectral bandwidth incident on the optical fiber sensor of this configuration in combination with a band pass filter.

  The optical fiber sensor of the present invention can be measured even when the fused portion of the SM fiber and the GI fiber and the GI fiber portion are recoated and bent. For confirmation, an optical fiber sensor having an SM-GI-SM configuration was prepared using an SM fiber with an MFD of 8 μm and a GI fiber with a core diameter of 50 μm. The change was not confirmed, and it could be used as an optical fiber sensor. When the optical fiber sensor is used in a bent state, a smaller thermometer can be configured as compared with a case where the optical fiber sensor is stored on a straight line.

  The optical fiber sensor of the present invention can be improved in sensitivity by being fixed to a base material having a large linear expansion such as aluminum. For example, in the SM-GI-SM configuration, when the SM fiber portions at both ends are fixed to aluminum with an adhesive or the like, and the GI fiber portion expands and contracts as the aluminum expands and contracts due to temperature changes, the characteristic changes due to temperature changes In addition to this, characteristic fluctuation due to tension change is also added, so that the sensitivity as a thermometer can be increased. In this case, it is desirable to fix the optical fiber sensor after adjusting the tension when fixing the optical fiber sensor to the substrate so that the GI fiber does not loosen in the operating temperature range. In addition, when the GI fiber portion is fixed with an adhesive, distortion occurs and affects the diffusion / condensing characteristics of light propagating in the GI fiber. Therefore, when fixing the optical fiber sensor to the base material, the SM fiber portion It is desirable to fix with.

  As described above, when the sensor part is made small, the sensing part of the optical fiber sensor can be bent and installed. On the other hand, when the sensor sensitivity is increased, the sensor part is fixed on a straight line on a substrate having a large linear expansion. Therefore, it is possible to flexibly select a packaging structure according to a desired application.

  As described above, the longer the length of the GI fiber portion, the higher the sensitivity can be. However, if the length is too long, the distortion of the diffusion / condensing characteristics inside the GI fiber increases, and the condensing characteristics are reduced. As a result, a good sensor cannot be obtained. In order to solve this problem, the optical fiber sensor of the present invention may be configured such that a plurality of sensing units made of GI fibers are inserted and SM fibers are inserted between them, as in SM-GI-SM-GI-SM. . By adopting such a configuration, the distortion of the mode field is eliminated, so that beautiful diffusion / condensing characteristics can be obtained again with the second-stage GI fiber. In addition, since the length of the GI fiber portion can be increased as a whole, the sensitivity as an optical fiber sensor can be improved. That is, it is possible to provide a highly sensitive sensor while suppressing the influence of distortion of the diffusion / condensing characteristics.

An example of a method for manufacturing the optical fiber sensor of the present invention will be described with reference to FIG. First, the GI fiber 1 having a length longer than the desired length L is prepared, and the SM fiber 2 is fusion-spliced to one end of the GI fiber 1 as shown in FIG. Then, as shown in FIG. 4 (b), after cutting the length of the GI fiber 1 to a desired length, the other SM fiber 2 is fused and connected to the cut surface 6, thereby obtaining the light of the present invention. A fiber sensor 3 is obtained. As a method for fusion-splicing the fibers, for example, the end faces of the fibers are butted together and heated by an arc discharge 5 between the electrodes 4 and 4.
When cutting the length of the GI fiber 1 to a desired length, the SM fiber 2 connected to one end of the GI fiber 1 is placed on a precision stage, and the fusion splice 7 is used by using a microscope. It is possible to cut with high accuracy by confirming the length of.

  FIG. 5 shows a configuration example of a measurement system that measures temperature and tension using the optical fiber sensor of the present invention. In the measurement system 10 shown in FIG. 5, the output light from the light source 11 is branched into two by the coupler 12 at a light intensity of 50%. One of the lights branched by the coupler 12 is incident on the first photodetector 13 that monitors the light intensity of the light source 11. The other of the lights branched by the coupler 12 enters the optical fiber sensor 3 and then enters the second photodetector 14 that monitors the light intensity of the sensing light transmitted through the sensing unit. In the case of this measurement system 10, the second photodetector 14 is connected to the tip of an SM fiber serving as a transmission path on the receiving side.

In FIG. 5, the “x” mark displayed between the GI fiber 1 and the SM fiber 2 represents a fusion splicing portion between the fibers 1 and 2. When laying the SM fiber 2 as a transmission path, the optical fiber C is used to connect the SM fiber 2 drawn from one end of the cable C to the sensing unit 1 and to connect the SM fiber 2 drawn from the other end of the cable C. The coupler 12 is connected. Further, the transmission path on the transmission side and the reception side may be configured by fusion-connecting a plurality of SM fibers 2. For example, when an optical fiber sensor 3 prepared by fusion-bonding SM fibers 2 and 2 having appropriate lengths to both ends of a GI fiber 1 having a predetermined length is prepared, A method of fusion-connecting the SM fiber 2 of the fiber sensor 3 and the SM fiber 2 of the optical cable C may be used.
In the measurement system of the present invention, it is preferable to use a single mode optical fiber as an optical fiber other than the GI fiber 1 of the sensing unit, for example, an optical fiber connecting the light source 11 and the coupler 12. Further, when a coupler obtained by melting and stretching an optical fiber is used as the coupler 12, it is preferable to use a coupler manufactured from a single mode optical fiber.

  The first light detector 13 monitors the intensity of the light incident on the light detector 13 as the light intensity of the light source 11, converts it into an electric signal, and outputs it to the comparison operation circuit 15. The second photodetector 14 monitors the intensity of the light incident on the photodetector 14 as the sensing light intensity, converts it to an electrical signal, and outputs it to the comparison operation circuit 15. As these photodetectors 13 and 14, a photodiode (PD), an optical power meter, or the like can be used. The electric signals indicating the light intensity monitored by the photodetectors 13 and 14 can be amplified to an appropriate level as an input to the comparison operation circuit 15 by passing through the photo sensor amplifiers 16 and 17, respectively.

  The comparison calculation circuit 15 compares the reference light intensity monitored by the first photodetector 13 with the sensing light intensity monitored by the second photodetector 14 and outputs the calculation result as a measured value. Output to the device 18. For example, the comparison operation circuit 15 calculates the transmittance at the sensor portion in which the change in the light source intensity is corrected by taking the ratio of the electrical signal p1 representing the reference light intensity and the electrical signal p2 representing the sensing light intensity. Then, the calculation result can be output based on the calculation processing based on the “relationship between measured value and transmittance” obtained in advance. When the measured value is the temperature of the sensing unit, data indicating the relationship between the temperature and the transmittance is prepared in advance as “the relationship between the measured value and the transmittance”. Further, when the measured value is the tension applied to the sensing unit, data indicating the relationship between the tension of the optical fiber and the transmittance is prepared in advance as “the relationship between the measured value and the transmittance”. Thereby, the system which measures and outputs the change of the temperature of the sensing part 1, and tension | tensile_strength can be constructed | assembled.

Further, in the optical fiber sensor having the SM-GI-SM configuration, when the tip end portion of the SM fiber is subjected to the reflection end treatment, the light reciprocates at the reflection end, and SM-GI-SM-reflection end-SM-GI. Since it passes through the same GI fiber part twice like -SM, the sensor sensitivity can be increased. FIG. 6 shows an example of the measurement system 20 having a configuration in which the reflection end R is provided in the optical fiber sensor 3. In FIG. 6, the same elements as those shown in FIG.
In the case of the measurement system 20 shown in FIG. 6, the output light from the light source 11 is branched into two by the coupler 12 at a light intensity of 50%. One of the lights branched by the coupler 12 is incident on the first photodetector 13 that monitors the light intensity of the light source 11. The other of the lights branched by the coupler 12 is incident on the optical fiber sensor 3, passes through the sensing unit, and then reflects the reflected light from the reflection end R as a second photodetector that monitors the light intensity of the sensing light. 14 is incident. In the case of this measurement system 20, since the SM fiber 2 serves as both transmission and reception transmission paths, the reflected light is incident on the second photodetector 14 through the coupler 12.

  As a method for treating the reflection end of the optical fiber, there is a method in which a reflection film made of a metal or a dielectric multilayer film is formed on the tip of the optical fiber by vapor deposition or sputtering. It is also possible to configure the transmission side and reception side transmission paths to be the same SM fiber by treating one end of the GI fiber as a reflection end, such as SM-GI-reflection end-GI-SM. However, in the case of the optical fiber sensor of the present invention, since it is desired to adjust the length of the GI fiber as the sensing unit accurately, it is necessary to perform the processing accurately when the reflection end is provided in the GI fiber. Therefore, it is difficult to manufacture the reflection end with high accuracy. For this reason, when providing a reflective end, it is preferable to provide at the front-end | tip of SM fiber (specifically, the edge part on the opposite side to the side connected to GI fiber).

As described above, according to the optical fiber sensor of the present invention, a highly accurate sensor capable of measuring temperature and optical fiber tension can be realized with a very simple configuration. A sensor that is easy to manufacture and highly reliable can be provided at low cost.
The optical fiber sensor of the present invention can be combined with general devices such as a light source and a detector to constitute a measurement system based on a change in light intensity, and can easily carry out measurement from a remote location.

《SMFD MFD》
In order to confirm the characteristic change due to the difference in mode field diameter (MFD) of the SM fiber connected to both ends of the GI fiber, the SM fiber is fused to each end of the GI fiber having a core diameter of 50 μm by using an optical fiber fusion machine. Then, an optical fiber sensor having an SM-GI-SM configuration was produced. The length L of the GI fiber portion of the optical fiber sensor is about 20 mm. 7 and 8 show the spectrum (transmission wavelength dependence) when the temperature of the GI fiber is changed in the range of 0 ° C to 100 ° C. FIG. 7 shows the case of MFD = 10 μm (L = 20.02 mm), and FIG. 8 shows the case of MFD = 8 μm (L = 20.092 mm). In either case, a shift in spectrum can be confirmed due to temperature change, but the intensity difference in transmittance due to wavelength change increases as the MFD of the SM fiber decreases. This is because the smaller the MFD, the greater the wavelength dependency where recombination from the GI fiber to the SM fiber occurs.

<Length of GI fiber>
In order to confirm the change in characteristics due to the difference in the length L of the GI fiber portion, SM fibers (MFD 8 μm) are fused to both ends of the GI fiber (core diameter 50 μm) using an optical fiber fusion machine, respectively. An optical fiber sensor having a SM configuration was prepared, and a spectrum (wavelength dependence of transmittance) when the temperature of the GI fiber was changed in the range of 0 ° C. to 100 ° C. was measured. 9 shows a case where L is about 5 mm, FIG. 10 shows a case where L is about 20 mm, FIG. 11 shows a case where L is about 30 mm, and FIG. 12 shows a case where L is about 40 mm. When the length L of the GI fiber portion is 5 mm, since the optical path length Lo of the GI fiber portion is short, the characteristic variation due to wavelength dependency and temperature change is small as shown in FIG. 9, and the sensitivity as an optical fiber sensor is not good. I can confirm that. When the length L is 20 mm or 30 mm, as shown in FIG. 10 and FIG. 11, the transmittance varies greatly due to temperature change, and the sensitivity as an optical fiber sensor can be increased. However, when the length L is increased to 40 mm, it can be confirmed that the spectrum of the wavelength characteristic is disturbed as shown in FIG. For this reason, it is preferable that the length of a GI fiber part is 5 mm or more and 40 mm or less. More preferably, it is 20 mm or more and 30 mm or less.

<< Light source characteristics and GI fiber length >>
13 to 15 show an SM-GI-SM optical fiber sensor in which an SM fiber with an MFD of 8 μm and a GI fiber with a core diameter of 50 μm are used, and the SM fibers are fused to both ends of the GI fiber. And the spectrum was measured when the temperature of the GI fiber portion was changed in the range of 0 ° C to 100 ° C. At this time, spontaneous emission light obtained when the erbium-doped optical fiber was excited with an LD light source having a wavelength of 1.48 μm was used as the light source. 13 shows a case where the length L of the GI fiber portion is 20.002 mm, FIG. 14 shows a case where the length L of the GI fiber portion is 20.092 mm, and FIG. 15 shows a case where the length L of the GI fiber portion is The case of 20.18 mm is indicated respectively.
As shown in FIGS. 13 to 15, as the length L of the GI fiber portion becomes as long as 20.002 mm, 20.092 mm, 20.18 mm, the transmittance spectrum shifts to the short wavelength side. I can confirm. At this time, paying attention to the light transmittance at 1550 μm, which is the center wavelength of the light source, it can be seen that the transmittance is increased by increasing the length L of the GI fiber. This is because, as shown in FIG. 1, the light propagating while repeatedly diffusing and condensing in the GI fiber is coupled with the SM fiber on the receiving side, and when L = 20.002 mm, the antinode position, L = 20.18 mm corresponds to a node position.
FIG. 16 shows the results of measuring the temperature dependence of transmitted light intensity using these optical fiber sensors of L = 20.002 mm, 20.092 mm, and 20.18 mm. In the case of L = 20.002 mm and L = 20.18 mm, the light intensity change with respect to the temperature change is small and is not monotonous change, so it is difficult to use as an optical fiber sensor. On the other hand, when the length of the GI fiber is L = 20.092 mm, the light intensity of the transmitted light monotonously increases as the temperature rises, and the intensity change rate is about ± 10%. Can be used. Therefore, in order to obtain high sensitivity as an optical fiber sensor, the length L of the GI fiber serving as the sensing unit propagates through the GI fiber at the position where the light at the center wavelength of the light source used is coupled to the SM fiber on the receiving side. It is desirable to set it to be halfway between the light belly and the node.
Further, FIG. 17 shows the results of measuring a plurality of samples with L = 20.092 mm and measuring the temperature dependence of the transmitted light intensity for each sample. Almost the same curve is obtained for all samples, and the reproducibility of sensor fabrication can be confirmed.

<Use as tension sensor>
In FIG. 18, an SM fiber having an MFD of 8 μm and a GI fiber having a core diameter of 50 μm are used, and SM fibers are fused to both ends of the GI fiber to fabricate an optical fiber sensor having an SM-GI-SM configuration. The spectrum was measured when the tension (unit: Newton (N)) of the GI fiber portion was changed in the range of 0N to 2.25N. At this time, spontaneous emission light obtained when the erbium-doped optical fiber was excited with an LD light source having a wavelength of 1.48 μm was used as the light source. The length L of the GI fiber portion was 20.092 mm. As a method of applying tension to the GI portion, a method of pulling SM fibers at both ends was used.
As shown in FIG. 18, the light intensity of the transmitted light monotonously increases as the tension increases, and the intensity change rate is about ± 10%. Therefore, by measuring the transmitted light intensity of the optical fiber sensor, It can be used as a tension sensor. In order to obtain high sensitivity as a tension sensor, the length L of the GI fiber serving as the sensing unit is such that the position where the light at the center wavelength of the light source used is coupled to the SM fiber on the receiving side is the position of the light propagating in the GI fiber. It is desirable to set it so that it is halfway between the belly and the node.
In addition, FIG. 19 shows the results of measuring a plurality of samples with L = 20.09 mm and measuring the dependency of the transmitted light intensity on the tension for each sample. Almost the same curve is obtained for all the samples, the characteristic fluctuation is small, and the reproducibility of sensor fabrication can be confirmed.

  The present invention can be used to measure temperature and tension from a remote location.

(A), (b), (c) is a schematic diagram which shows the optical fiber sensor of this invention. It is a graph which shows an example of the relationship between the length of a GI fiber part, and the transmittance | permeability (relative ratio) when the core diameter of GI fiber is 50 micrometers. It is a graph which shows an example of the relationship between the length of a GI fiber part, and the transmittance | permeability (relative ratio) when the core diameter of GI fiber is 320 micrometers. (A), (b), (c) is drawing explaining an example of the method of manufacturing the optical fiber sensor of this invention. It is a schematic block diagram which shows an example of the measurement system using the optical fiber sensor of this invention. It is a schematic block diagram which shows the 2nd example of the measurement system using the optical fiber sensor of this invention. It is a graph which shows an example of the spectrum at the time of changing the temperature of GI fiber part in the range of 0 degreeC-100 degreeC, when MFD of SM fiber is 10 micrometers. It is a graph which shows an example of the spectrum at the time of changing the temperature of GI fiber part in the range of 0 degreeC-100 degreeC, when MFD of SM fiber is 8 micrometers. It is a graph which shows an example of the spectrum at the time of changing the temperature of GI fiber part in the range of 0 to 100 degreeC, when the length of GI fiber part is about 5 mm. It is a graph which shows an example of the spectrum at the time of changing the temperature of a GI fiber part in the range of 0 degreeC-100 degreeC, when the length of a GI fiber part is about 20 mm. It is a graph which shows an example of the spectrum at the time of changing the temperature of GI fiber part in the range of 0 to 100 degreeC, when the length of GI fiber part is about 30 mm. It is a graph which shows an example of the spectrum at the time of changing the temperature of GI fiber part in the range of 0 to 100 degreeC, when the length of GI fiber part is about 40 mm. It is the graph which showed the spectrum of the transmittance | permeability and light source intensity in the case where the length of the GI fiber portion is 20.000 mm. It is the graph which showed the spectrum of the transmittance | permeability and light source intensity in the case where the length of the GI fiber portion is 20.092 mm. When the length of a GI fiber part is 20.18 mm, it is the graph which showed the spectrum of the transmittance | permeability and light source intensity in piles. It is a graph which shows an example of the result of having measured the temperature dependence of light intensity change. It is a graph which shows the reproducibility of the temperature dependence of a light intensity change. It is the graph which showed the change of the transmittance | permeability spectrum at the time of changing the tension | tensile_strength of a GI fiber part, and the spectrum of light source intensity. It is a graph which shows the reproducibility of the dependence of the light intensity change with respect to tension | tensile_strength.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Graded index optical fiber (GI fiber), 2 ... Single mode optical fiber (SM fiber), 3 ... Optical fiber sensor, 10 ... Measurement system, 11 ... Light source, 13 ... 1st photodetector, 14 ... 1st 2 photodetectors, 15... Comparison operation circuit, 20.

Claims (8)

Sensing unit consists graded index optical fiber, Ri transmission path on the transmission side and the reception side to the sensing unit Do from the single-mode optical fiber,
The length L of the sensing unit made of the graded index optical fiber with respect to the pitch Λ of light propagating while repeating diffusion and condensing in the graded index optical fiber at the center wavelength of the light source used is: An optical fiber sensor characterized by satisfying
(1 + 2N) Λ / 8−Λ / 16 <L <(1 + 2N) Λ / 8 + Λ / 16
However, N is a positive integer (N = 1, 2, 3,...). The optical fiber sensor according to claim 1, wherein the optical fiber sensor is used by measuring a change in light intensity transmitted through the sensing unit. The optical fiber sensor according to claim 1, wherein a plurality of the sensing units are arranged in series, and a single mode optical fiber is inserted therebetween. A first single-mode optical fiber that serves as both transmission and reception transmission paths is connected to one side of the sensing unit, and a second end having a tip end reflected at the other side of the sensing unit. The optical fiber sensor according to claim 1, wherein a single mode optical fiber is connected. The reference light intensity obtained by monitoring the light intensity of the light source is compared with the sensing light intensity obtained by monitoring the light intensity incident from the light source and transmitted through the optical fiber sensor according to any one of claims 1 to 4. A temperature measuring method characterized by measuring a temperature change of the sensing unit. An optical fiber sensor according to any one of claims 1-4, a light source for incident light to the optical fiber sensor, a first optical detector for monitoring the light intensity of the light source as reference light intensity, the light source A second photodetector that monitors the intensity of light that is more incident and transmitted through the optical fiber sensor as sensing light intensity, and compares the sensing light intensity with the reference light intensity and outputs a temperature change of the sensing unit A temperature measurement system comprising a comparison operation circuit. The reference light intensity obtained by monitoring the light intensity of the light source is compared with the sensing light intensity obtained by monitoring the light intensity incident from the light source and transmitted through the optical fiber sensor according to any one of claims 1 to 4. A tension measuring method characterized by measuring the tension applied to the sensing unit. An optical fiber sensor according to any one of claims 1-4, a light source for incident light to the optical fiber sensor, a first optical detector for monitoring the light intensity of the light source as reference light intensity, the light source A second photodetector for monitoring the intensity of light that is incident and transmitted through the optical fiber sensor as the intensity of sensing light, and the tension applied to the sensing unit by comparing the sensing light intensity with the reference light intensity. A tension measurement system comprising a comparison operation circuit for outputting.

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