JP2008298581A - Optical fiber sensor - Google Patents

Abstract

Provided is a simple and inexpensive high-precision optical fiber sensor that suppresses polarization dependence.
A sensing unit in which a relative distance between a light source and a reflection surface having an optical fiber end surface changes according to a physical quantity such as pressure and temperature, and a first optical fiber that transmits light from the light source to the sensing unit A second optical fiber and a third optical fiber that respectively transmit light reflected by the reflecting surface to a plurality of light receiving units, and an arithmetic processing unit that calculates the physical quantity from a ratio of electrical signals from the light receiving units, Each end face of the fiber is fixed at an angle θ between the longitudinal direction of the optical fiber and the normal to the reflecting surface, the second optical fiber and the third optical fiber are parallel, and the first optical fiber and the first optical fiber The fixed angles of the second and third optical fibers are symmetrically arranged with respect to the normal to the reflecting surface, the first to third optical fibers are single mode at the wavelength used, and the light source, the first optical fiber end surface, During the depot Optical fiber sensor, characterized in that the riser has been inserted.
[Selection] Figure 3

Description

  The present invention relates to an optical fiber sensor using detection of a change in light intensity, and in particular, achieves high measurement accuracy by suppressing polarization dependence with an inexpensive and simple device configuration.

  Conventionally, electrical sensors have been widely used as sensors for measuring physical quantities such as displacement, temperature, and pressure. However, the electric sensor needs to be supplied with power, and is easily affected by electromagnetic noise. In contrast, optical sensors that use optical fibers convert measurement signals to optical signals and transmit them to remote locations via optical fibers, enabling signal transmission without being affected by electromagnetic noise. An error is also small and very high-precision measurement is possible (for example, refer to patent documents 1 to 9).

  FIG. 1 is a diagram illustrating an example of a conventional optical pressure sensor. This optical pressure sensor senses light from the light source 1, the sensing unit 3 in which the relative distance between the reflection surface 4 a of the reflector 4 and the end face of the optical fiber changes according to physical quantities such as pressure and temperature, and the light from the light source 1. The first optical fiber 2 for transmission transmitted to the unit 3 and the light reflected by the reflecting surface 4a of the sensing unit 3 are transmitted to the two light receiving units (first and second light receiving units 7, 9), respectively. From the second optical fiber 5 and the third optical fiber 6, the amplifiers 8 and 10 for amplifying the signals from the first and second light receiving units 7 and 9, and the amplified first and second light receiving units 7 and 9 It is comprised from the arithmetic processing part 11 which calculates a physical quantity from the ratio of an electric signal. The end surfaces of the first to third optical fibers 2, 5, 6 facing the reflecting surface 4 a are fixed so as to have an angle θ between the longitudinal direction of the optical fiber and the normal to the reflecting surface 4 a, and the second optical fiber 5 and the third optical fiber 6 are parallel to each other, and the fixed angles of the first optical fiber 2 and the second and third optical fibers 5 and 6 are symmetric with respect to the normal to the reflecting surface 4a.

FIG. 2 shows an enlarged view of the sensing unit 3. The light receiving second optical fiber 5 and the third optical fiber 6 are parallel, and the first optical fiber 2 and the second and third optical fibers 5 and 6 are fixed at an angle θ with respect to the normal to the reflecting surface 4a. It is fixed symmetrically. The outgoing light from the first optical fiber 2 is reflected by the reflecting surface 4a, and the reflected light coupled to the second optical fiber 5 and the third optical fiber 6 is transmitted to the first and second light receiving parts 7 and 9, respectively. Thus, the respective light intensities P1 and P2 are measured, and the arithmetic processing unit 11 calculates the light intensity ratio F (P1, P2) = (P1−P2) / (P1 + P2). Here, since the light intensity changes depending on the relative distance between the reflecting surface 4a and the end face of the optical fiber, the light intensity ratio F (P1, P2) changes, and the sensing unit 3 changes according to physical quantities such as pressure and temperature. By adopting a structure that performs this, it is possible to detect these physical quantities.
With this optical pressure sensor, it is possible to configure the measurement device with an inexpensive one, and the measured signal processing can be easily performed. When measuring the light intensity, there is a problem that the light intensity change due to factors other than the position fluctuation of the reflector, such as the light intensity fluctuation of the light source and the transmission loss, will result in a measurement error. In addition to making it smaller, it is possible to improve the measurement accuracy by compensating for the ratio of the light intensity received by a plurality of optical fibers.
Japanese Patent Publication No. 6-8724 JP-A-57-108605 JP-A-2-57909 JP-A-3-243822 JP-A-2-49115 JP-A 63-169521 US Pat. No. 5,068,527 US Pat. No. 4,996,418 US Pat. No. 4,249,076 Japanese Patent Laid-Open No. 11-352158 JP 2004-301769 A

  In the conventional configuration, the reflected light is received by two fibers arranged in parallel and the ratio of the light intensities is taken. Therefore, it has been considered that there is no need to consider the influence of the change in the polarization state. For example, in Patent Documents 1 to 9, a light-emitting diode (hereinafter referred to as LED) light source is used, but there is no description regarding the degree of polarization of the light source, and it is considered that the measurement accuracy is hardly affected, and the influence thereof. Did not recognize itself.

  However, the present inventors have examined the degree of polarization and measurement accuracy of the light source in detail, and it has been confirmed for the first time that the measurement accuracy is deteriorated even with the generally considered degree of polarization of the LED light source. It was found that the greater the fiber fixing angle, the greater the effect.

  In the sensors disclosed in Patent Documents 1 to 9, the output optical fiber and the light receiving optical fiber have separate structures, and when the polarization state of the output light changes, the intensity of the received light changes. There is a problem that accuracy deteriorates. In addition, a structure in which a specific angle is given to the outgoing optical fiber and the receiving optical fiber has been proposed, but when reflected at an angle, it becomes more susceptible to changes in the polarization state, Deterioration of measurement accuracy will increase.

  Further, in Patent Documents 3, 4, 5, 7, 8, and 9, the device configuration uses a plurality of LEDs and utilizes the difference in wavelength. However, since the number of parts increases, the device configuration is complicated and expensive. There is a problem.

As described above, a simple and inexpensive high-precision optical fiber sensor that suppresses the polarization dependence of a light source, particularly a light source having an inherently low degree of polarization, such as an LED, has not been realized.
The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a simple and inexpensive high-precision optical fiber sensor that suppresses polarization dependence.

In order to achieve the above object, the present invention provides a sensing unit in which a relative distance between a light source and a reflection surface and an optical fiber end surface changes according to a physical quantity such as pressure and temperature, and light from the light source is input to the sensing unit. The physical quantity is calculated from the ratio of the first optical fiber to be transmitted, the second optical fiber and the third optical fiber that respectively transmit the light reflected by the reflecting surface of the sensing unit to the plurality of light receiving units, and the electrical signal from the light receiving unit. An arithmetic processing unit,
Each end surface of the first to third optical fibers opposed to the reflecting surface is fixed so as to have an angle θ between the longitudinal direction of the optical fiber and the normal to the reflecting surface, and the second optical fiber and the third optical fiber Are parallel and the fixed angles of the first optical fiber and the second and third optical fibers are symmetrically arranged with respect to the normal to the reflecting surface,
The first to third optical fibers are single mode at the wavelength used,
An optical fiber sensor is provided in which a depolarizer is inserted between a light source and a first optical fiber end face.

  In the optical fiber sensor of the present invention, it is preferable to use an LED as a light source.

  In the optical fiber sensor of the present invention, the depolarizer is preferably a fiber depolarizer using a polarization maintaining fiber.

  In the optical fiber sensor of the present invention, the fiber type depolarizer is formed by fusion-bonding two polarization maintaining fibers L1 and L2 in a state where the birefringence main axis by the stress applying portion is shifted by 45 °. When the fiber length L1: L2 of the polarization maintaining fiber is 1: 2, it is preferable that 0.1 m ≦ L1 ≦ 10 m.

  In the optical fiber sensor of the present invention, the degree of polarization of the emitted light is preferably 20% or less.

According to the present invention, a simple, inexpensive, and highly accurate optical fiber sensor can be realized by a light intensity measurement method using a three-core array that is easy to measure.
In addition, since the LED light source and the depolarizer are combined, even when a fiber fixing angle is applied, measurement with high accuracy is possible without being affected by fluctuations in the polarization state.
Further, by using a fiber type depolarizer as the depolarizer, the coupling with the transmission path fiber becomes easy.
In addition, when the fiber length L1: L2 of the polarization maintaining fiber used for the fiber depolarizer is 1: 2, it is possible to reduce the polarization by setting 0.1 m ≦ L1 ≦ 10 m.

Embodiments of an optical fiber sensor of the present invention will be described below with reference to the drawings.
FIG. 3 is a block diagram showing a first embodiment of the optical fiber sensor of the present invention. The optical fiber sensor according to the present embodiment includes a light source 1, a sensing unit 3 having a reflection surface 4 a of a reflection plate 4, and a relative distance between the light fiber 1 and the end surface of the optical fiber changing according to physical quantities such as pressure and temperature. First light fiber 2 (outgoing port) that transmits the light of the first to the sensing unit 3, and second light that transmits the light reflected by the reflecting surface 4a of the sensing unit 3 to the first and second light receiving units 7 and 9, respectively. An arithmetic processing unit 11 that calculates the physical quantity from the ratio of the electrical signals from the first and second light receiving units 7 and 9 amplified by the amplifiers 8 and 10, the light source 1, It is comprised from the depolarizer 12 inserted between 1 optical fiber 2 end surfaces.

  In the sensing unit 3, each end face of the first to third optical fibers 2, 5, 6 opposed to the reflecting surface 4a is an angle formed between the longitudinal direction of the optical fiber and the normal to the reflecting surface, as shown in FIG. The second optical fiber 5 and the third optical fiber 6 are parallel to each other, and the fixed angle between the first optical fiber 2 and the second and third optical fibers 5 and 6 is reflected. They are arranged symmetrically with respect to the normal to the surface 4a. The first to third optical fibers 2, 5, and 6 are single mode at the wavelength to be used.

  With the above-described configuration of the present embodiment, a simple, inexpensive, and highly accurate optical fiber sensor can be realized with a light intensity measurement method using a three-core array that is easy to measure. In the sensor using the light intensity measurement method, if the angle of the fiber is not attached to the reflecting surface 4a, the influence of the change in the polarization state is small, and an LED having a small degree of polarization is generally used as the light source 1. Was able to achieve the target accuracy. However, when the fiber fixing angle is set, the present inventors generally deteriorate the measurement accuracy even if the polarization state of the LED having a small degree of polarization is changed, and the influence is caused by the size of the fixing angle. It was confirmed to be proportional.

  Moreover, in sensors other than the light intensity measurement method, as disclosed in Patent Document 10 and Patent Document 11, a depolarizer is inserted to suppress the influence thereof. However, when a laser diode (hereinafter referred to as LD) is used as the light source, the spectral line width is also as thin as about 1 nm, and it is difficult to effectively depolarize it, which is difficult in practical use. In the case of a super luminescent diode (hereinafter referred to as SLD), the spectral line width is wider than that of LD, and the half-width is usually 10 nm or more. It can be used by managing it. Thus, the wider the full width at half maximum of the light source spectrum, the greater the depolarization effect, and it is desirable that it be about 1 nm or more. However, since these light sources are more expensive than LEDs, it is desirable to use LED light sources in order to provide an inexpensive sensor device.

  Accordingly, the present inventors have invented an optical fiber sensor having a three-core array configuration in which an inexpensive LED having a small degree of polarization and a depolarizer are combined. As a result, even in a configuration with a fixed fiber angle, deterioration in measurement accuracy due to a change in polarization state is suppressed, and a highly accurate optical fiber sensor independent of the size of the fixed fiber angle is realized.

  The depolarizer 12 is an element that can be depolarized by changing the polarization of the light emitted from the light source 1 to an infinite number of random polarizations. In this embodiment, the depolarizer 12 is a polarization maintaining fiber depolarizer that uses a polarization maintaining fiber. Using. This polarization maintaining fiber type depolarizer is obtained by fusion splicing a fiber length L1 to L2 ratio of 1: 2 with the birefringent main axis shifted by about 45 ° by the stress applying portion. By inserting, it is possible to forcibly depolarize the polarization state of the light source and suppress the influence of the change in the polarization state. This polarization maintaining fiber type depolarizer is usually used with L1 connected to the light source side. The depolarizer 12 is not limited to the one using the polarization maintaining fiber, and may be one using a crystal plate having optical anisotropy.

  In general, the degree of polarization of light emitted from an LED light source is considered to be smaller than that of an LD or SLD, but actually varies in a range of about 5 to 50% due to manufacturing variations. Therefore, when an LED light source is used for the optical fiber sensor, the polarization state with a degree of polarization of about 50% may change at the maximum, thereby degrading the measurement accuracy. Here, instead of reducing the polarization degree of the light source, a method of suppressing the change in the polarization state is also conceivable. However, in order to maintain the polarization state of the entire transmission path, an expensive special fiber called a polarization maintaining fiber is used. This is not desirable because it would be necessary. Further, the optimum lengths of the fiber lengths L1 and L2 when the depolarizer 12 is manufactured vary depending on the light source to be used, but when a 1300 nm band LED is used and the ratio of the fiber lengths L1 and L2 is 1: 2, L1 is It is desirable that it is 0.1 m or more. Further, since the polarization maintaining fiber is very expensive, it is desirable that L1 is 10 m or less.

  As an influence on the measurement accuracy due to the change in the polarization state, for example, when an LED light source having a polarization degree of about 40% is used and the polarization state changes at a fiber fixed angle θ = 6 °, the achievable measurement accuracy is ± 0.4% F. S. However, when the depolarizer 12 is inserted, the degree of polarization of the emitted light becomes 5% or less, and the measurement accuracy is ± 0.05% F.S. S. To an extent. In addition, the influence on the measurement accuracy when the degree of polarization of the light emitted from the light source changes by about 0 to 50% has a proportional relationship, and when θ = 0 ° with no fiber fixing angle, the measurement accuracy is polarized. When the degree is 50%, a maximum of about ± 0.2% F.F. S. It becomes. However, when the fiber fixing angle is set, when θ = 6 °, the measurement accuracy is about ± 0.6% F. max. S. When θ = 10 °, the maximum ± 1% F.S. S. It gets worse. Thus, as the fiber fixing angle θ between the outgoing fiber and the light receiving fiber is larger, the influence of the change in the polarization state on the measurement accuracy is larger. Therefore, when an LED light source is used and a fiber fixing angle is applied, the measurement accuracy is ± 0.2% F.S. S. The following is difficult to achieve and requires the use of a depolarizer 12. Measurement accuracy ± 0.25% F.V. S. The degree of polarization of the emitted light necessary to achieve the above is 18% or less when the fixed angle θ = 6 °, and 12% or less when the fixed angle θ = 10 °.

  A variation in the degree of polarization of commercially available LED light sources was confirmed. As a measurement method, ten 1310 nm band LED light sources were used, and measurement was performed using an optical polarization analyzer (manufactured by Agilent, HP8509) in a state where a rated voltage and current were applied. Table 1 shows the measurement results.

  From the results in Table 1, it was confirmed that the degree of polarization of the LED light source varies from 5 to 50%.

  The suppression effect of the polarization degree on the fiber length of the depolarizer 12 was confirmed. A depolarizer 12 was produced by using a polarization maintaining fiber and preparing a plurality of sets having a ratio of fiber lengths L1 and L2 of 1: 2 and fusion-bonding each so that the stress applying portion has an angle of about 45 °. . FIG. 4 shows the measurement system. The LED 13 having a degree of polarization of about 50% was used, the fiber length L1 side of the depolarizer 12 was connected to the LED 13 emission side, and the degree of polarization of the emitted light from the fiber length L2 side was measured by the optical polarization analyzer 14.

  FIG. 5 shows the measurement results. In FIG. 5, the horizontal axis represents the fiber length L1, and the vertical axis represents the degree of polarization measured by an optical polarization analyzer. When the fiber length is adjusted from L1 = 0.1 m, L2 = 0.2 m to L1 = 10 m, L2 = 20 m, the degree of polarization of the outgoing light is suppressed to 3% or less, and the fiber length L1 is 0.1 m or more. If it exists, it confirmed that the effect of the depolarizer 12 was fully acquired.

  In the three-core array structure of the optical fiber sensor of the present invention, the measurement accuracy was compared when a light source having a polarization degree of about 40% was used and no depolarizer was inserted. FIG. 6 shows the measurement configuration. A polarization controller 15 was inserted on the emission side of the LED light source 1 to change the polarization state in the first optical fiber 2. The relative distance between the reflecting surface 4a and the end face of the optical fiber is constant, and the fiber length of the depolarizer 12 is L1 = 0.7 m and L2 = 1.4 m. The light emitted from the first optical fiber 2 is received by the second optical fiber 5 and the third optical fiber 6, the respective light intensities P1 and P2 are measured, and the light intensity ratio F (P1, P2) = (P1- P2) / (P1 + P2) was calculated, and the measurement accuracy was calculated. Here, the measurement accuracy refers to the difference from the average value of the light intensity ratio F (P1, P2) over the entire measurement time, and the maximum value (full span: F) at which the light intensity ratio F (P1, P2) changes within the measurable range. .. S.).

  FIG. 7 shows the measurement results. In FIG. 7, the horizontal axis represents measurement time, and the vertical axis represents measurement accuracy. As shown in FIG. 7A, when the depolarizer 12 is not inserted, the measurement accuracy is ± 0.4% F.S. S. It became about. As shown in FIG. 7B, when the depolarizer 12 is inserted, the measurement accuracy is ± 0.05% F.S. S. The following improvements were confirmed.

  The relationship between the degree of polarization and the measurement accuracy was confirmed by calculating the measurement accuracy using LED light sources having various degrees of polarization. The measurement system was the same as in FIG. 6, and the degree of polarization from the LED light source was varied from 1 to 50%, and the measurement accuracy under each condition was measured.

  FIG. 8 shows the measurement results. In FIG. 8, the horizontal axis represents the degree of polarization of the emitted light, and the vertical axis represents the measurement accuracy. When the fixed angle θ = 6 ° of the three-core array is solid, the solid line is shown, and when the fixed angle θ = 10 ° is shown is there. Thus, there is a proportional relationship between the degree of polarization and the measurement accuracy, and the greater the fixed angle of the optical fiber, the greater the influence of the degree of polarization on the measurement accuracy. The target measurement accuracy is ± 0.25% F. S. In order to realize the above, it was confirmed that when the fixed angle θ = 6 °, the degree of polarization is 18% or less, and when the fixed angle θ = 10 °, the degree of polarization is 12% or less.

It is a block diagram which shows an example of the conventional optical fiber sensor. It is an enlarged view of the sensing part of the optical fiber sensor shown in FIG. It is a block diagram which shows one Embodiment of the optical fiber sensor of this invention. 6 is a configuration diagram of a depolarizer testing apparatus performed in Example 2. FIG. It is a graph which shows the test result of Example 2, and shows the relationship between the fiber length L1 of a depolarizer, and a polarization degree. 6 is a configuration diagram of a test apparatus used in a comparative test performed in Example 3. FIG. The result of Example 3 is shown, (a) is a graph showing the relationship between measurement accuracy and measurement time when no depolarizer is used, and (b) shows the relationship between measurement accuracy and measurement time when using a depolarizer. It is a graph. It is a graph which shows the result of Example 4 and shows the relationship between polarization degree and measurement accuracy.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Light source, 2 ... 1st optical fiber, 3 sensing part, 4 ... Reflecting plate, 4a ... Reflecting surface, 5 ... 2nd optical fiber, 6 ... 3rd optical fiber, 7 ... 1st light-receiving part, 8, 10 ... Amplifier, 9 ... second light receiving unit, 11 ... arithmetic processing circuit, 12 ... depolarizer.

Claims (5)

A sensing unit in which a relative distance between a light source and a reflection surface and a fiber end surface changes according to a physical quantity such as pressure and temperature, a first optical fiber that transmits light from the light source to the sensing unit, and sensing unit reflection A second optical fiber and a third optical fiber that respectively transmit the light reflected by the surface to the plurality of light receiving units, and an arithmetic processing unit that calculates the physical quantity from a ratio of electric signals from the light receiving unit,
Each end surface of the first to third optical fibers opposed to the reflecting surface is fixed so as to have an angle θ between the longitudinal direction of the optical fiber and the normal to the reflecting surface, and the second optical fiber and the third optical fiber Are parallel and the fixed angles of the first optical fiber and the second and third optical fibers are symmetrically arranged with respect to the normal to the reflecting surface,
The first to third optical fibers are single mode at the wavelength used,
An optical fiber sensor, wherein a depolarizer is inserted between the light source and the end face of the first optical fiber.   2. The optical fiber sensor according to claim 1, wherein a light emitting diode is used as a light source.   3. The optical fiber sensor according to claim 1, wherein the depolarizer is a fiber type depolarizer using a polarization maintaining fiber.   The fiber depolarizer is formed by fusion splicing two polarization maintaining fibers L1 and L2 with the birefringence main axis of the stress applying portion shifted by 45 °, and the fiber length L1 of these two polarization maintaining fibers. The optical fiber sensor according to claim 3, wherein when L2 is 1: 2, 0.1 m ≦ L1 ≦ 10 m.   5. The optical fiber sensor according to claim 1, wherein the degree of polarization of the emitted light is 20% or less.

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