Optical fiber temperature sensor and temperature measuring method

An optical fiber temperature sensor employs a holey fiber. The holey fiber has an optical fiber concentrated part that a predetermined length of the holey fiber is concentrated in a spot region that is included in a space to be measured in temperature distribution. The optical fiber concentrated part has a bend formed by looping or winding the holey fiber.

The present application is based on Japanese patent application No. 2004-371442, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to temperature measurement using an optical fiber and, particularly, to an optical fiber temperature sensor and a temperature measuring method.

2. Description of the Related Art

An optical fiber temperature sensor is known in which a temperature distribution along the longitudinal direction of an optical fiber is measured based on a change in transmitted light when light is inputted to an optical fiber. By placing such an optical fiber in a space such as a surface or inside of a temperature measuring object, a temperature distribution of the space can be measured. An optical fiber thermometer is also known which converts a change in transmitted light into a temperature distribution along the longitudinal direction. When the optical fiber (or optical fiber temperature sensor) is connected to the optical fiber thermometer, the temperature distribution can be measured in known method. Herein, the explanation of a principle for the temperature measurement is omitted.

However, there is a restriction as to distance resolution in the measurement results by the optical fiber thermometer. The measurement result of a point on an optical fiber is obtained as an average value of temperatures in a range of a predetermined length lying before and after the point. Namely, the length range corresponds to the distance resolution. It is impossible to measure in accurate resolution the difference of temperatures on multiple points in a region narrower than the distance resolution. Thus, the accuracy of optical fiber thermometer depends on a data sampling interval (i.e., interval in distance) in addition to the distance resolution.

FIG. 1is a plain view showing a conventional optical fiber temperature sensor.

An optical fiber121is linearly placed in a space of 5 m in length. When the data sampling interval of the optical fiber thermometer is 1 m, a temperature at five points can be measured at intervals of 1 m.

JP-A-9-210809 discloses an example of an optical fiber temperature sensor.

JP-A-2002-249335 discloses an example of a holey fiber.

In order to measure a temperature (called spot temperature) in a spot region narrower than a distance resolution in a space where an optical fiber is placed by using the optical fiber with the distance resolution and an optical fiber thermometer, a method may be employed in which a predetermined length portion of an optical fiber which corresponds to the data sampling interval is concentrated in the spot region. In this case, since the predetermined length portion of the optical fiber falls within the spot region, the spot temperature can be accurately measured.

Thus, if an optical fiber concentrated part is formed to allow the predetermined length portion of the optical fiber to be concentrated in the narrow spot region, the optical fiber must have a bend. The narrower the spot region becomes, the greater the bend is (i.e., the bend radius becomes smaller). For example, when it is desired that a temperature distribution in the space as shown inFIG. 1is measured at points as many as possible, the bend radius becomes smaller since the spot region becomes narrower as the number of the points increases.

However, in general, an optical fiber is subjected to optical transmission loss caused by the bend. As the bend radius decreases, the optical transmission loss generated at the bend portion increases. Further, when there are multiple bend portions on the entire length of an optical fiber, optical transmission losses at the respective bend portions are superposed to increase the entire length of the optical fiber. Thus, if the number of bend portions increases, the optical transmission loss increases in total.

As such, if the optical transmission loss is increased by reducing the bend radius or increasing the number of the bend portion, the temperature measurement of the optical fiber thermometer is disturbed.

For example, when a loop portion with a bend radius less than 100 mm is made, a large optical transmission loss is generated and therefore it is impossible to increase the number of the loop portion. Further, since the large optical transmission loss is generated at the loop portion, the entire length of the optical fiber needs to be reduced to suppress the total optical transmission loss. Further, due to the large optical transmission loss generated at the loop portion, temperature measurement will be disturbed and the accuracy of measurement will lower.

As such, even though it is desired to wholly and accurately measure the temperature distribution of a space, a small number of measurement points must be located in the space.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an optical fiber temperature sensor that can be applied to a narrowed spot region to increase the number of measurement points.

It is a further object of the invention to provide a temperature measuring method that can accurately measure the temperature distribution of a measurement space by using the optical fiber temperature sensor.

According to one aspect of the invention, an optical fiber temperature sensor comprises:

a holey fiber,

wherein the holey fiber comprises an optical fiber concentrated part that a predetermined length of the holey fiber is concentrated in a spot region that is included in a space to be measured in temperature distribution.

It is preferred that the optical fiber concentrated part comprises a bend formed by looping or winding the holey fiber.

It is preferred that the optical fiber concentrated part comprises the bend formed with a bend radius d to satisfy:
d≧ΔL×x/(2π(L−L0))
where x is an entire length of the holey fiber, ΔL is an optical transmission loss per one loop or winding, L0is an optical transmission loss in case of no bend in the holey fiber, and L is a maximum acceptable optical transmission loss.

The optical fiber temperature sensor may further comprise a sheet on which the optical fiber concentrated part is formed.

According to another aspect of the invention, a temperature measuring method comprises the steps of:

placing an optical fiber near to a spot region of a temperature measuring object; and

measuring a temperature at the spot region based on a change in transmitted light when light is inputted into the optical fiber,

wherein the optical fiber comprises a holey fiber, and

the holey fiber comprises an optical fiber concentrated part that a predetermined length of the holey fiber is concentrated in the spot region.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2is a constructional diagram showing an optical fiber temperature sensor in a preferred embodiment of the invention.

As shown inFIG. 2, the optical fiber temperature sensor of the invention employs an optical fiber1that allows the measurement of temperature distribution along the longitudinal direction of the optical fiber1based on a change in transmitted light to be inputted to the optical fiber1. A predetermined length portion of the optical fiber1is concentrated at an optical fiber concentrated part2which is located in a relatively narrow spot region A of a space (i.e., all regions inFIG. 2) where the optical fiber1is placed. A holey fiber1ais used as the optical fiber1. Thus, the optical fiber temperature sensor is composed to measure the spot temperature in the spot region A.

The internal structure, mechanical property and optical property of the holey fiber1aare known. For example, they are disclosed in JP-A-2002-249335, entire contents of which are incorporated herein by reference.

As shown inFIG. 2, one end of the holey fiber1ais connected to an optical fiber thermometer3. The opposite end of the holey fiber1ais opened or terminated. The holey fiber1awith a whole length x is placed in a space length F from the one end to the opposite end of the holey fiber1a.

The optical fiber concentrated part2is formed by looping or winding the predetermined length portion of the holey fiber1a.InFIG. 2, the four optical fiber concentrated parts2are formed separately at intervals of a pitch S. Another example to continuously form the optical fiber concentrated part2is shown inFIG. 5, which is explained later.

The optical fiber concentrated part2is formed of a loop by winding the holey fiber1a.Although the number of winds may be any number, one loop is made at each of the optical fiber concentrated parts2in this embodiment.

Although the loop needs not to be a perfect circle, in this embodiment, the loop of the optical fiber concentrated part2is made to be a circle with a radius of d such that it has the minimum bend radius of d and the holey fiber1ais placed concentrated in a region as narrow as possible.

In this embodiment, the pitch S is set to be S=2d such that the adjacent optical fiber concentrated parts2are in contact with each other. However, the adjacent optical fiber concentrated parts2may be separated from each other.

In the above configuration, the entire length x of the optical fiber1is represented as x=F+4×2πd since it is obtained by adding the respective loop lengths, 4×2πd, to the linear distance F of the space. If the loops are wound while being shifted as shown inFIG. 5without connecting the adjacent loops with a line as shown inFIG. 2, the entire length x of the optical fiber1can be approximated as x=F+2πd×T, where T is the number of winds.

The bend radius d of the optical fiber concentrated part2is set to be a value to satisfy:
d≧ΔL×(x−F)/(2π(L−L0))  (2)
where x is the entire length of the optical fiber1, F is the linear distance of the space, ΔL is an optical transmission loss per one loop, L0is an optical transmission loss in case of no bend, L is a maximum acceptable optical transmission loss to satisfy the accuracy in temperature measurement, and π is the circular constant.

In explaining the derivation of the above expression (2) taking into account an example as shown inFIG. 5, since the optical fiber1is continuously looped over the entire length F of the space,
x=F2πd×T(3)
gives the entire length x of the optical fiber1, where T is the number of winds.

Then, provided that the maximum acceptable optical transmission loss L required is L, an optical transmission loss per one loop is ΔL, and the number of winds is T,
L≧L0+ΔL×T(4)
is to be satisfied.

Substituting T into the expression (4) by modifying the expression (3),
L−L0≧ΔL×(x−F)/2πd(5)

Since L−L0≧0is given from the definition,
d≧ΔL×(x−F)/(2π(L−L0))  (2)
is obtained.

In order to allow the optical transmission loss of the entire length of the optical fiber1to be within the maximum acceptable optical transmission loss L, the bend radius d can be obtained by using the entire length x of the optical fiber1, the maximum acceptable optical transmission loss L, the optical transmission loss ΔL per one loop, and the optical transmission loss L0in case of no bend.

FIG. 11is a characteristic diagram showing optical transmission loss in a holey fiber and a single-mode fiber, where the relationship between a bend radius and a bend loss per one loop is shown.

InFIG. 11, the full lines show the relationship between the bend radius d and the optical transmission loss ΔL in a typical holey fiber. As shown, the optical transmission loss ΔL decreases as the bend radius d increases, and the optical transmission loss ΔL increases as the bend radius d decreases.

The dotted lines show the relationship between the bend radius and the optical transmission loss ΔL in a typical single-mode fiber. As shown, it will be appreciated that even with a relatively large bend radius, the optical transmission loss ΔL is large, and the optical transmission loss ΔL becomes significantly large as the bend radius d decreases.

A comparative example and an example of the invention are compared to know how long the entire length x of the optical fiber can be secured. In the comparative example, provided that the maximum acceptable optical transmission loss L required by the optical fiber thermometer3is 10 dB, when a single-mode fiber is wounded with a bend radius of 10 cm, the number T of loops cannot be more than11since the optical transmission loss ΔL per one loop is 0.9 dB/loop. As a result, the entire length x of the optical fiber has no more than 1 m. Therefore, it is not practical.

In the example of the invention, the number T of loops can be as many as 100,000 since the optical transmission loss ΔL per one loop is 0.0001 dB/loop. As a result, the entire length x of the optical fiber can have 9420 m.

As described above, since the invention employs the holey fiber1aas the optical fiber1, optical signal that has intensity sufficient to allow the accurate temperature measurement at the optical fiber thermometer3can be obtained even when the bend radius d decreases. Further, the spot region can be narrower since the bend radius d can be reduced. Further, since the increase of optical transmission loss caused by the bend is relatively small, the number of measurement points can be increased and the entire length of the optical fiber1can be also increased. Further, in the invention, even when the size of a temperature measuring object is smaller than the conventional spot region, the spot region can be applied to such a size of the temperature measuring object.

FIG. 3is a cross sectional view showing an optical fiber cable to be used as a modification of the optical fiber in the invention.

The optical fiber1as shown inFIG. 2needs not to be a simple body of the holey fiber1a.For example, as shown inFIG. 3, it may be the optical fiber cable26that a spacer22with a groove23on the periphery is formed around a tension member21, the holey fiber1ais placed in the groove23, and the spacer22is covered by winding a tape24and a sheath25is formed thereon. Thus, by using the optical fiber cable26as the optical fiber1, the holey fiber1acan be protected from being broken or deformed even if an object in the space hits the optical fiber cable26.

FIG. 4is side view showing an optical fiber concentrated part in the preferred embodiment of the invention.

As shown inFIG. 4, the optical fiber concentrated part2may be formed by winding the holey fiber1aaround a columnar or cylindrical core material31such as a cable and a pipe. The way of winding the holey fiber1aaround the columnar or cylindrical core material31is easy in manufacture as compared to a case that it is wound with no core material. Also, it is easy to separately or continuously form the optical fiber concentrated part2in the longitudinal direction of the core material31. Herein, the composition of the holey fiber1aand the core material31wound around there is called a cable-shaped sensor32.

FIGS. 5 and 6are plain views showing temperature sensor sheets with an optical fiber concentrated part in the preferred embodiment of the invention.

As shown inFIGS. 5 and 6, the temperature sensor sheet42may be constructed by forming the optical fiber concentrated part2on a sheet41with a predetermined length. The holey fiber1acan be secured on the sheet41by using adhesives such as jelly, silicone, or by a method of laminating or wiring.

Alternatively, the holey fiber1amay be placed in a sheet-like case called a sheet case. By thus configuring the temperature sensor sheet42, the entire thickness can be reduced. Therefore, the temperature sensor sheet42becomes easy to install at the surface or inside of a temperature measuring object. Especially, when it is installed at the surface of the temperature measuring object, heat propagation from the temperature measuring object can be enhanced.

Alternatively, when the sheet41is made of a material with a high flexibility, it can be in close contact with the temperature measuring object even when the temperature measuring object is curved in outer shape. Also, it can easily form the optical fiber concentrated part2separately or continuously in the longitudinal direction of the sheet41. Further, since the temperature sensor sheet42has a width equal to the spot region A in the width direction of the sheet41, for the temperature measuring object with a flat plane, the flat plane can be closely covered with the temperature sensor sheet42. Thereby, temperature distribution at the entire flat plane can be measured.

The difference between the two embodiments inFIGS. 5 and 6is that the optical fiber concentrated part2inFIG. 6is separately formed while the optical fiber concentrated part2inFIG. 5is continuously formed. InFIG. 6, each of the multiple optical fiber concentrated parts2includes the holey fiber1awith multiple loops (though it may be one loop) wound substantially in concentric form on the plane. Thus, since the multiple optical fiber concentrated parts2are formed at intervals, spot regions A1, A2, A3and A4can be provided at intervals.

InFIG. 5, the holey fiber1ais formed winding a first loop on the plane, the next loop being wound while shifting the loop center by a small pitch, the next loop being wound while shifting the loop center in like manner, such that the loops are formed shifted in one direction. Although the holey fiber1aof the embodiment inFIG. 4is formed spiral, it also allows the continuous formation of the optical fiber concentrated parts2.

By thus forming the continuous optical fiber concentrated parts2, a number of spot regions A1-1, A1-2, A1-3, . . . , A2-1, A2-2, A2-3, . . . can be obtained infinitely as shown inFIG. 5. Although resolution is not so excellent since adjacent spot regions A1-1and A1-2are overlapped, spot regions A1-1and A2-2are perfectly separated without being overlapped. Thus, separate spot regions can be secured as many as possible in a length F′ (i.e., the length of the temperature sensor sheet42) of the continuous optical fiber concentrated parts2. Further, after disposing the temperature sensor sheet42in the measurement space, the distance (i.e., the distance from the end of the optical fiber to the center of the optical fiber loop being located within the spot region) of the measurement point can be desirably controlled by the optical fiber thermometer3.

FIG. 7is a plain view showing a spot sensor with an optical fiber concentrated part in the preferred embodiment of the invention.

As shown, the optical fiber concentrated part2is placed in a case61, which is provided with a fixing hole62to penetrate through the case61. By using the fixing hole62, the case61can be attached onto the surface of a temperature measuring object or onto a wall or a support in the measurement space. The embodiment thus constructed is herein called a spot sensor63. By suitably disposing the spot sensor63midway on the optical fiber1, temperature at that point can be measured accurately.

FIG. 8is an illustrative diagram showing an optical fiber temperature sensor of the invention to be installed on a real measurement site. InFIG. 8, two different temperature measuring objects81,82(both of which have a structural body) can be subjected to temperature measurement by one optical fiber1(i.e., the holey fiber1a) extending from the optical fiber thermometer3. The temperature measuring object81is formed columnar at its base portion and dome-shaped at its top portion. In order to measure the surface temperature distribution of the temperature measuring object81at intervals of a small distance, the temperature sensor sheet42as shown inFIG. 5or6is wound around the temperature measuring object81to cover the entire surface. The temperature measuring object82is provided with the temperature sensor sheet42or cable-shaped sensor32as shown inFIG. 4and the spot sensor63as shown inFIG. 7, which are inserted into or embedded in the temperature measuring object82.

As seen from the embodiment inFIG. 8, the temperature sensor sheet42with a small thickness and a good flexibility can be advantageously used to measure the surface temperature of a structural body with a curved surface like the temperature measuring object81. In the invention, since temperature distribution can be measured at a sampling interval shorter than the data sampling interval of the optical fiber thermometer3, different temperatures can be accurately measured at the surface of the temperature measuring object81.

Further, the spot sensor63. can be advantageously used to accurately measure a temperature at a local point.

The comparison of the invention and the conventional technique will be explained below.

FIG. 9is a plain view showing a continuous optical fiber concentrated part of the invention to be placed in a space.

As shown inFIG. 9, the optical fiber1as the continuous optical fiber concentrated part2is provided in a 5 m long space. For example, 5 m of the long optical fiber1is wound per 1 m of the space. If the data sampling interval of the optical fiber thermometer3is 1 m, temperature can be measured at five points per 1 m of the space, i.e., at 25 points for the entire space. Thus, a distance resolution of 20 cm in space length can be obtained.

FIG. 10is a graph showing a temperature distribution for the comparison between the invention and prior art.

In comparing the invention with the conventional optical fiber temperature sensor (prior art example) as shown inFIG. 1, the measurement data of temperature distribution obtained from both examples are shown together inFIG. 10. However, in order to facilitate the visual comparison, the measurement data of the invention example are plotted somewhat lower than the real data.

As shown inFIG. 10, in prior art, temperature at 5 points only can be measured at intervals of 1 m since the data sampling interval of the optical fiber thermometer3is 1 m. In contrast, in the invention, temperature at 25 points can be measured since the distance resolution of 20 cm in space length is obtained due to the winding of the 25 m long optical fiber1as shown inFIG. 9even when the data sampling interval of the optical fiber thermometer3is 1 m. Thus, the data (depicted in full lines) of the invention can reflect a slight fluctuation in temperature, while the data (depicted in dotted lines) of prior art, which are obtained by linear interpolation between measurement points, cannot reflected such a fluctuation in temperature.

The invention can be altered variously other than the above embodiments.

For example, the holey fiber1amay be made of glass or plastics. Instead of the holey fiber1a, a low-loss optical fiber with a property similar to the holey fiber1amay be used. The low-loss optical fiber is defined in that it has an optical transmission loss of 0.01 dB/loop or less at a bend radius of d=10 mm, and an optical transmission loss ΔL per one loop at a band radius d to satisfy:
ΔL<4774×d−5.68(6)
where d≦50 mm is given.

FIG. 12is a constructional diagram showing an optical fiber temperature sensor in the preferred embodiment of the invention.

As shown inFIG. 12, a single-mode fiber111is connected to the optical fiber thermometer3, and the holey fiber1ais connected through an optical fiber connecting box112to the single-mode fiber111. Since the holey fiber1ais used for the optical fiber concentrated part2where the bend radius needs to be reduced, the same effects as the above embodiments can be obtained. In addition, the single-mode fiber111connected to the optical fiber thermometer3is used only for optical signal transmission and needs not be bent. Therefore, it does not cause an increase in optical transmission loss.

The optical fiber thermometer3is frequently provided with a reference optical fiber inside or outside thereof so as to determine a relative temperature in addition to the optical fiber1installed in the space, temperature measuring object. The optical fiber temperature sensor of the invention can be also used as the reference optical fiber.

<Advantages of the Invention>

The invention can be used for the measurement of temperature distribution in planar body or space other than the structural body and stereoscopic space. Also, it can be used for the monitoring of temporal change in temperature distribution by measuring the temperature distribution continuously or at intervals of a given time.

The optical fiber temperature sensor of the invention can be applied to a temperature measuring object with a small diameter and a large length such as a power cable with a small diameter or an electrical wire with a regular diameter so as to monitor its temperature.

The optical fiber temperature sensor of the invention can be applied to the surface or inside of a pipeline in the axis or radial direction thereof.

The optical fiber temperature sensor of the invention can be applied to a spherical temperature measuring object.

The invention can be applied to the field of strain measurement or vibration measurement with an enhanced distance resolution while converting a change in transmitted light of the optical fiber into a strain or vibration distribution along the longitudinal direction.