KR102548444B1 - Double-clad fiber Mach-Zehnder interferometer based on long-period fiber grating and apparatus and method for simultaneous measuring of bending and temperature insensitive to strain using thereof - Google Patents

Abstract

The present invention relates to an apparatus and method for simultaneously measuring bending and temperature using an optical fiber. According to the present invention, the sensing performance of a sensor is limited due to cross-sensitivity between temperature and bending, and in particular, bending of a sensor unit In order to solve the problems of conventional fiber Bragg grating (FBG) and long period fiber grating (LPFG) based measurement devices and methods, which have limitations in that measurement results are inaccurate due to strain being applied simultaneously when strain is applied, By irradiating a double-cladding optical fiber (DCF) with carbon dioxide (CO ₂ ) laser pulses at regular intervals and periodically modulating the core refractive index of the DCF, two LPFGs manufactured by connecting them in series at regular intervals are configured, so that a single sensor unit is used. A long-period fiber optic grating (LPFG)-based double-clad optical fiber (DCF) Mach-Zehnder interferometer ( MZI) and a device and method for simultaneous measurement of bending and temperature independent of strain using the same are provided.

Description

Double-clad fiber Mach-Zehnder interferometer based on long-period fiber grating and apparatus and method for simultaneous measuring of bending and temperature independent of strain using the Mach-Zehnder interferometer based on long-period fiber grating and apparatus and method for simultaneous measuring of bending and temperature insensitive to strain using its}

The present invention relates to an apparatus and method for simultaneously measuring bending and temperature using an optical fiber, and more particularly, to a fiber Bragg grating (FBG) and a long period optical fiber. Existing fiber optic grating-based sensors that measure specific physical quantities using long-period fiber grating (LPFG), etc., have limited sensing performance due to cross sensitivity between temperature and bending. In order to solve the problems of prior art optical fiber grating-based measurement devices and methods that have limitations in that measurement results are inaccurate due to simultaneous application of strain when applied, bending and temperature applied to the sensor unit can be simultaneously measured. At the same time, even if a strain is applied to the sensor unit during measurement, a long-period fiber optic grating (LPFG)-based double-clad fiber (DCF) Mach-Zehnder interferometer configured to perform simultaneous measurement regardless of the strain. It relates to the Zehnder interferometer (MZI) and a device and method for simultaneous measurement of bending and temperature independent of strain using the same.

In addition, the present invention, as described above, even if a strain is applied to the sensor unit during measurement, in order to simultaneously measure bending and temperature regardless of the strain, a carbon dioxide (CO ₂ ) laser at a regular period in a double cladding optical fiber (DCF) It consists of connecting two LPFGs manufactured by periodically modulating the core refractive index of DCF by illuminating pulses in series at regular intervals, so that a single sensor unit is used without the need to use multiple sensor units. It enables simultaneous measurement of two physical quantities, improves the convenience of the manufacturing process and reduces manufacturing costs compared to conventional FBG, and can be manufactured without a mask by using a carbon dioxide laser, making it possible to manufacture low-cost LPFG. Since there is no change in the spectrum of the LPFG even when a tensile/compressive strain is applied when this is applied, a long period fiber optic grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) configured to improve the accuracy of simultaneous bending and temperature measurement ) and a device and method for simultaneous measurement of bending and temperature independent of strain using the same.

Recently, an optical fiber sensor for measuring a specific physical quantity using an optical fiber has been commercialized and is widely used in various fields.

More specifically, the principle of measuring a physical quantity using an optical fiber sensor is that light propagating through an optical fiber changes its intensity, wavelength, phase, polarization, etc. by a change in an external physical quantity. When it is experienced, a specific physical quantity is measured by converting it into an electrical signal through an optoelectronic transducer or the like and analyzing the external physical quantity applied to the optical fiber.

In addition, as a representative example of such an optical fiber sensor, there is an optical fiber grating-based sensor. When a grating is formed by periodically changing the refractive index in the core region where light actually propagates in the optical fiber, the refractive index of the grating part or the grating period according to the change in the external physical quantity It uses the principle that the characteristics of the light passing through the fiber optic grating are also changed by changing the .

Here, as described above, an example of the prior art related to a measuring device and method for measuring a specific physical quantity using an optical fiber is, for example, a "Mach Zender" as presented in Korean Patent Publication No. 10-2010-0095252. There is an interferometric optical fiber, a manufacturing method thereof, and a sensor including the same.

More specifically, Korean Patent Laid-Open Publication No. 10-2010-0095252 discloses an optical fiber including a core portion and a cladding portion surrounding the core portion, and splits a light beam incident on the optical fiber into two or more optical sights having a phase difference. By being configured to include a cavity capable of generating a furnace therein, it is possible to provide an optical fiber that exhibits Mach-Zender interference characteristics that are sensitive to changes in physical quantities such as temperature, strain, and acceleration, even though it is based on a single strand, thereby efficiently detecting changes in various physical quantities At the same time as possible, it relates to a Mach-Zehnder interferometric optical fiber that can be manufactured at a simple process and low cost, a manufacturing method thereof, and a sensor including the same.

In addition, as described above, another example of the prior art related to a measuring device and method for measuring a specific physical quantity using an optical fiber is, for example, as presented in Korean Patent Registration No. 10-0367297 "Optical Fiber Fabry Pero ( Fabry-Perot) interferometric temperature measurement device".

More specifically, the above Korean Patent Registration No. 10-0367297 discloses a sensing means having a main sensor and an auxiliary sensor that differentially cause a light phase change according to a temperature change of an object to be measured, and a main sensor and an auxiliary sensor. The modulated light is incident and the optical means that converts the reflected light from the main sensor and the auxiliary sensor into electrical signals and converts each electrical signal from the optical means into a phase, and calculates the temperature of the object to be measured based on the phase value. When detecting a change in light phase corresponding to a change in temperature of an object to be measured, including a signal processing means, information on the position of the cycle to which the phase change of the main sensor belongs is transferred to an auxiliary sensor whose cycle is tens to hundreds of times slower. Fiber optic Fabry-Perot interferometer type configured to expand the application area while maintaining the precision of the high-precision fiber optic interferometer type temperature sensor for the main sensor by being configured to stably maintain the precision of the main sensor by accepting it from the main sensor It is about a temperature measuring device.

As described above, various measuring devices and methods using an optical fiber sensor have been proposed in the prior art to measure a specific physical quantity, but the optical fiber-based measuring devices and methods of the prior art as described above have the following problems.

More specifically, as a representative example of an optical fiber grating constituting an optical fiber sensor, a fiber Bragg grating (FBG) manufactured by changing the refractive index in a core of a general single mode fiber (SMF) at a period of 1 μm or less ), and these FBGs have a characteristic of reflecting light corresponding to a specific resonant wavelength without passing through, and this resonant wavelength is determined by the effective refractive index of the optical fiber core and the period of the grating.

Therefore, when an external physical quantity is applied to the FBG and the effective refractive index of the fiber core or the period of the grating is changed, a shift occurs in the resonance wavelength of the FBG. By measuring this through various demodulation methods, the change in the physical quantity applied to the FBG can be calculated. there is.

In addition, in addition to the above-mentioned FBG, Long-Period Fiber Grating (LPFG), which has a much larger grating period than FBG, has been steadily researched along with FBG, receiving a lot of attention as an important optical passive device in the field of optical communication and optical sensor. there is.

Such an LPFG can be manufactured by periodically modulating the refractive index of an optical fiber core with a period ranging from 100 to 1000 μm, and by changing grating parameters such as grating period, grating intensity, and phase shift, desired sensitivity and transmission. ) performance, and can be fabricated through methods such as UV laser exposure, electric arc discharge, and carbon dioxide laser irradiation.

Furthermore, because LPFG has advantages such as electromagnetic interference immunity, insulation, waterproofness, light weight, multiplexing function, and fast response, LPFGs manufactured under various grid conditions are resistant to temperature, bending, strain (longitudinal tension), and torsion. It has been used as a sensor unit to measure mechanical and physical variables such as (torsion/twist) and refractive index.

In particular, among these measurement variables, bending is one of the essential and key measurement variables, and the fiber optic bending sensor will be useful for structural safety diagnosis in various application fields such as autonomous driving, aerospace, architecture and civil engineering, wind power and hydroelectric power generation. can

However, in both FBG and LPFG-based sensors, there is a problem that cross-sensitivity between temperature and bending can limit the sensing performance of the sensor. Therefore, the temperature cross-dependence of the sensor must be resolved in order to accurately measure bending, and this temperature cross-over of LPFG In order to solve the dependency problem, studies have been conducted to simultaneously measure variables such as strain and temperature, torsion and temperature, and refractive index and temperature. In particular, there are many situations where strain is simultaneously applied when bending is applied to the sensor unit. Therefore, in the case of a bending sensor, it is required to minimize the influence of the strain accompanying bending application.

That is, as described above, conventional optical fiber grating-based measuring devices and methods for measuring a specific physical quantity using a fiber Bragg grating (FBG) or a long period fiber grating (LPFG) have a sensor sensitivity due to the cross sensitivity between temperature and bending. There is a problem that the sensing performance of is limited, and in general, only one physical quantity can be measured with one sensor, so there is a problem that the number of sensors must be increased by that much to measure a plurality of physical quantities.

In addition, the prior art optical fiber grating-based measurement devices and methods, such as the FBG or LPFG, have a problem in that measurement results are inaccurate due to strain being simultaneously applied when bending is applied to the sensor unit.

Accordingly, in order to solve the problems of the prior art fiber optic grating-based measuring devices and methods as described above, a plurality of physical quantities applied to the sensor unit, such as bending and temperature, must be simultaneously measured with a single sensor. At the same time, it is desirable to propose a new optical fiber lattice-based simultaneous measurement device and method for simultaneous measurement of bending and temperature, which is configured to perform simultaneous measurement regardless of strain even if strain is applied to the sensor unit during measurement. A device or method that satisfies all the requirements has not yet been presented.

Korean Patent Publication No. 10-2010-0095252 (2010.08.30.) Korean Registered Patent Publication No. 10-0367297 (2002.12.23.)

The present invention is intended to solve the problems of the prior art as described above, and therefore, an object of the present invention is that sensing performance is limited due to cross sensitivity between temperature and bending, and in particular, strain when bending is applied to the sensor unit. In order to solve the problems of the prior art optical fiber grating-based measuring devices and methods, which have limitations in that measurement results are inaccurate due to simultaneous application of strain, it is possible to simultaneously measure the bending and temperature applied to the sensor unit, A long-period optical fiber grating (LPFG)-based double-cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) configured to perform simultaneous measurement regardless of strain even if strain is applied to the sensor unit during measurement, and strain-independent bending and It is intended to provide a device and method for simultaneous temperature measurement.

In addition, another object of the present invention, as described above, even if strain is applied to the sensor unit during measurement, in order to simultaneously measure bending and temperature regardless of strain, carbon dioxide (CO ₂ ) By irradiating laser pulses and periodically modulating the refractive index of the core of DCF, it is composed of connecting two LPFGs in series at regular intervals. Simultaneous measurement of physical quantities is possible, it is possible to improve the convenience of the manufacturing process and reduce the manufacturing cost compared to the existing FBG, and it is possible to manufacture low-cost LPFG by using a carbon dioxide laser, which can be manufactured without a mask. long-period optical fiber grating (LPFG)-based double-cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) configured to improve the accuracy of simultaneous bending and temperature measurement because there is no change in the spectrum of the LPFG even when tensile / compressive strain is applied, and It is intended to provide a device and method for simultaneous measurement of bending and temperature independent of the strain used.

In order to achieve the above object, according to the present invention, a long-period fiber grating (LPFG) configured to simultaneously measure bending and temperature regardless of strain applied to the sensor unit. In a Double-Clad Fiber (DCF) Mach-Zehnder interferometer (MZI) based double-clad fiber (DCF), two cores of the double-clad fiber (DCF) are connected in series at a predetermined interval. A long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) is provided, characterized in that it is configured to include a long period fiber grating (LPFG).

Here, the LPFG is formed by periodically modulating the core refractive index of the DCF by illuminating the DCF with a carbon dioxide (CO ₂ ) laser pulse at a predetermined period. do.

In addition, the long-period optical fiber grating (LPFG)-based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) measures two dips having different cladding-mode orders in the transmission spectrum of the DCF MZI, respectively. It is characterized in that it is configured to measure bending and temperature simultaneously by selecting a sensor indicator (indicator dip) for measuring bending and temperature, and measuring the amount of wavelength shift by bending and temperature of the sensor marker.

In addition, the long-period optical fiber grating (LPFG)-based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) is formed so that the diameter of the inner cladding of the DCF is larger than a predetermined standard, so that the strain during measurement Even if this is applied, it is characterized in that it is configured to enable simultaneous measurement of bending and temperature regardless of strain.

Here, the long-period optical fiber grating (LPFG)-based double-cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) is characterized in that the inner cladding diameter of the DCF is formed to be 80% or more of the outer cladding diameter.

Alternatively, the long-period optical fiber grating (LPFG)-based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) is characterized in that the inner cladding diameter of the DCF is formed to be 100 μm or more.

Moreover, the long-period fiber optic grating (LPFG)-based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) is configured to include three or more long-period fiber optic gratings (LPFG) in the core of the DCF.

In addition, according to the present invention, in the simultaneous bending and temperature measuring device configured to simultaneously measure bending and temperature regardless of strain applied to the sensor unit, the sensor unit includes the above-described long-period fiber optic grating (LPFG). )-based double cladding optical fiber (DCF) is provided with a simultaneous bending and temperature measuring device characterized in that it is configured to include a Mach-Zehnder interferometer (MZI).

In addition, according to the present invention, in the simultaneous bending and temperature measurement method configured to simultaneously measure bending and temperature regardless of the strain applied to the sensor unit, the long-period fiber optic grating (LPFG)-based double cladding described above Implementing the sensor unit using a optical fiber (DCF) Mach-Zehnder interferometer (MZI); and simultaneously measuring bending and temperature through the sensor unit.

As described above, according to the present invention, two LPFGs manufactured by periodically modulating the refractive index of the core of the DCF by irradiating the double-cladding optical fiber (DCF) with carbon dioxide (CO ₂ ) laser pulses at regular intervals. long-period optical fiber grating (LPFG)-based double-cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) configured to enable simultaneous measurement of bending and temperature regardless of strain by connecting in series and strain-independent bending using the same And by providing a temperature simultaneous measurement device and method, the sensing performance is limited due to the cross sensitivity between temperature and bending, and in particular, when bending is applied to the sensor unit, strain is simultaneously applied, resulting in inaccurate measurement results. It is possible to solve the problems of conventional fiber Bragg grating (FBG) and long period fiber grating (LPFG)-based measuring devices and methods, which have limitations.

In addition, according to the present invention, as described above, even if a strain is applied to the sensor unit during measurement, a long period fiber optic grating (LPFG) based double cladding optical fiber (DCF) Mach-Zender configured to simultaneously measure bending and temperature regardless of the strain By providing an interferometer (MZI) and a device and method for simultaneous measurement of bending and temperature independent of strain using the same, it is possible to simultaneously measure two physical quantities using a single sensor unit without using a plurality of sensor units, compared to the existing FBG. It is possible to improve the convenience of the manufacturing process and reduce the manufacturing cost, and by using a carbon dioxide laser, it is possible to manufacture LPFG at low cost because it can be manufactured without a mask. In addition, even if tensile/compressive strain is applied when bending is applied, the spectrum of LPFG changes Since there is no measurement, the accuracy of simultaneous measurement of bending and temperature can be improved.

1 is a diagram schematically showing the overall configuration of a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to an embodiment of the present invention.
2 is a diagram showing the results of measuring the transmission spectrum of the DCF MZI shown in FIG. 1;
FIG. 3 is a diagram schematically showing the overall configuration of an experimental device for measuring the bending response, temperature response, and strain response, respectively, for the DCF MZI shown in FIG. 1.
4 is a diagram showing the measurement results for the bending response with respect to selected sensor markers.
5 is a diagram showing measurement results for temperature response with respect to selected sensor markers.
6 is a diagram showing measurement results for strain response with respect to selected sensor markers.
7 is a diagram showing the transmission spectrum of the fabricated DCF MZI measured at room temperature without external perturbation.
8 is a diagram showing spectral changes due to bending of DA, DB, and DC measured for a curvature range of 1.484 to 4.451 m ⁻¹ at room temperature.
FIG. 9 is a diagram showing spectral changes according to temperature of DA, DB, and DC measured for an ambient temperature range of 30 to 100° C. without applying bending.
FIG. 10 is a diagram showing the bending and temperature-dependent wavelength shifts of DA, DB, and DC measured in a curvature range of 1.484 to 4.451 m ⁻¹ and an ambient temperature range of 30 to 100° C.
11 is a diagram showing spectral changes due to strain of DA, DB, and DC.
12 is a diagram showing ΔC and ΔT values measured by an optical fiber sensor according to an embodiment of the present invention with respect to ΔC and ΔT values applied using two marker valleys (DA and DB).
13 is a table showing the comparison results of sensor characteristics, including sensitivities to bending, temperature, and deformation, and simultaneous measurement functions of an optical fiber sensor according to an embodiment of the present invention and an optical fiber sensor proposed in the prior art.

Hereinafter, with reference to the accompanying drawings, detailed implementation of a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) and a strain-independent bending and temperature simultaneous measurement device and method using the same according to the present invention An example is described.

Here, it should be noted that the contents described below are only one embodiment for carrying out the present invention, and the present invention is not limited to the contents of the embodiments described below.

In addition, in the following description of the embodiments of the present invention, with respect to parts that are identical or similar to the contents of the prior art or are determined to be easily understood and implemented at the level of those skilled in the art, the detailed descriptions are provided in order to simplify the description. It should be noted that .

That is, in the present invention, as will be described later, sensing performance is limited due to cross sensitivity between temperature and bending. In order to solve the problems of the prior art optical fiber grating-based measuring devices and methods, which have limitations, it is possible to simultaneously measure the bending and temperature applied to the sensor unit, and at the same time, even if strain is applied to the sensor unit during measurement, regardless of the strain It relates to a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) configured to perform simultaneous measurement, and an apparatus and method for simultaneous measurement of bending and temperature independent of strain using the same.

In addition, as will be described later, the present invention, as will be described later, irradiates carbon dioxide (CO ₂ ) laser pulses on a double-cladding optical fiber (DCF) at regular intervals to periodically modulate the refractive index of the core of the DCF to create two LPFGs at regular intervals. By being configured by connecting in series with a plurality of sensor units, it is possible to simultaneously measure two physical quantities using a single sensor unit without using a plurality of sensor units, and it is possible to improve the convenience of the manufacturing process and reduce manufacturing costs compared to the existing FBG, carbon dioxide In addition to being able to manufacture low-cost LPFG because it can be manufactured without a mask by using a laser, even when tensile/compressive strain is applied when bending is applied, there is no change in the spectrum of LPFG, so that the accuracy of simultaneous bending and temperature measurement can be improved. It relates to a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) and an apparatus and method for simultaneous measurement of bending and temperature independent of strain using the same.

Continuing, with reference to the drawings, the details of the long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) and the strain-independent bending and temperature simultaneous measurement device and method using the same according to the present invention. explain about.

First, referring to FIG. 1, FIG. 1 is a diagram schematically showing the overall configuration of a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to an embodiment of the present invention.

That is, as shown in FIG. Using DCF MZI, which is formed in a form in which two LPFGs are connected in series at a regular interval (L) to the core of DCF, which consists of cladding and inner cladding, as a sensor head, the sensor Bending and temperature applied to the unit can be simultaneously measured, and in addition, even if strain, that is, tension in the longitudinal direction is applied to the sensor unit during measurement, simultaneous measurement can be performed regardless of it.

Here, the LPFG, which is a key element of the sensor unit, is manufactured by periodically modulating the refractive index of the core of the DCF by illuminating the DCF with carbon dioxide (CO ₂ ) laser pulses at regular intervals. DCF MZI can be implemented by manufacturing two at intervals and connecting two LPFGs in series.

More specifically, LPFG generally has several discontinuous loss dips over a wavelength band of hundreds of nanometers by co-directional mode coupling, and DCF MZI is two LPFG Since are connected in series, the modes coupled from the core to the cladding in the first LPFG can be recombined into the core mode when they meet the second LPFG after being guided a certain distance to the cladding in the area without the LPFG.

At this time, the recombined cladding modes and the core modes that proceed as they are without mode coupling in the first LPFG and reach the second LPFG meet each other in the core region of the second LPFG, causing interference.

In the DCF MZI interference spectrum, two valleys with different cladding-mode orders can be selected as indicator dips for simultaneous measurement of bending and temperature. It is possible to measure the bending and temperature response independently.

That is, referring to FIG. 2, FIG. 2 is a diagram showing the result of measuring the transmission spectrum of the DCF MZI according to the embodiment of the present invention shown in FIG.

As shown in FIG. 2, the transmission spectrum of the DCF MZI according to an embodiment of the present invention manufactured using a carbon dioxide laser as described above shows transmission minima at 1522.8 nm and 1590.4 nm, and in this embodiment, these losses The valleys were designated as DA and DB, respectively, and used as sensor markers.

Next, referring to FIGS. 3 to 6, first, FIG. 3 is the overall configuration of the experimental apparatus for measuring the bending response, temperature response, and strain response, respectively, for the DCF MZI according to the embodiment of the present invention shown in FIG. It is a diagram schematically showing.

In addition, FIGS. 4 to 6 are graphs showing the results of measuring the bending response, temperature response, and strain response for the sensor markers selected as described above through the experimental device shown in FIG. 3 .

Here, in FIGS. 4 to 6, FIG. 4 is a measurement result for a bending response, FIG. 5 is a measurement result for a temperature response, and FIG. 6 shows a measurement result for a strain response, respectively.

That is, the present inventors fabricated the DCF MZI according to the embodiment of the present invention as described above with reference to FIGS. The bending, temperature, and strain responses were respectively measured for a curvature range of 1.48 to 4.45 m ^-1 and a temperature range of 30 to 110 °C.

As a result of the measurement, first, as shown in FIG. 4, as the bending applied to the DCF MZI at room temperature increased, DA and DB showed a blue transition and a red transition, respectively, and the bending sensitivity of DA and DB from the measured bending response They are given as -911.3 pm/m ^-1 and 233.3 pm/m ^-1 respectively.

In addition, as shown in FIG. 5, unlike the bending response, DA and DB showed different amounts of red transition as the ambient temperature increased. At this time, the sensor unit was kept flat so that no bending was applied, and the measured From the temperature response, the temperature sensitivities of DA and DB are given as 56.4 pm/℃ and 74.6 pm/℃, respectively.

As can be seen from the measurement results of FIGS. 4 and 5, the two sensor markers exhibit linear and independent responses to bending and temperature, and therefore, using the bending and temperature sensitivities of the two sensor markers determined from FIGS. 4 and 5 It is possible to simultaneously measure the amount of bending and temperature change applied to the DCF MZI by measuring the amount of wavelength shift due to bending and temperature of the two sensor markers.

In particular, as shown in FIG. 6, as a result of measuring the wavelength change according to the strain application of the sensor markers in the strain range of 0 to 1136 με, the strain sensitivities of DA and DB were -0.048 pm/με and 0.045 pm/με, respectively. measured, and the amount of wavelength shift according to the strain application is so small that it hardly affects the results of simultaneous measurement of bending and temperature.

Therefore, the DCF MZI according to the embodiment of the present invention configured as described above is expected to be used as a sensor unit that can accurately measure bending and temperature at the same time without being affected by external strain.

As described above, the long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) and simultaneous bending and temperature measuring device using the same according to an embodiment of the present invention are the core of the double cladding optical fiber (DCF) It is configured to measure the bending and temperature applied to the sensor unit at the same time by using the DCF MZI implemented in the form of a long-period fiber optic grating (LPFG) connected in series as a sensor head. At this time, during simultaneous measurement Even if strain, that is, tension in the longitudinal direction is applied to the sensor unit, it has the advantage of being able to perform simultaneous measurement regardless of it.

To this end, the LPFG, which is a key element of the sensor unit, is manufactured by periodically modulating the refractive index of the core of the DCF by illuminating the DCF with a carbon dioxide laser pulse at regular intervals. Two are fabricated and connected in series to implement DCF MZI.

In addition, LPFG generally has several discontinuous loss dips over a wavelength band of hundreds of nanometers due to co-directional mode coupling, and the DCF according to an embodiment of the present invention In the MZI, since two LPFGs are connected in series, the mode coupled from the core of the first LPFG to the cladding can be recombined into the core mode when it encounters the second LPFG after guiding a certain distance to the cladding in the region where there is no LPFG.

At this time, the recombined cladding modes and the core modes that proceed as they are without mode coupling in the first LPFG and reach the second LPFG meet each other in the core region of the second LPFG, causing interference.

In the transmission spectrum of DCF MZI, two dips with different cladding-mode order can be selected as indicator dips for simultaneous measurement of bending and temperature. Letting the bones be Bone I and Bone II, it is possible to measure the flexural and temperature responses independently for the two marker bones.

Here, if the measured bending response is linear, the bending sensitivities (S _B,I and S _B,II ) of valley I and valley II can be obtained, and similarly, if the measured temperature response is linear, the temperature of valley I and valley II The sensitivities (S _T,I and S _T,II ) can be obtained.

That is, if the bending response and temperature response of the above two bones (bones I and II) are independent, that is, if S _B,I /S _T,I is not equal to S _B,II /S _T,II , the sensor Bending applied to the part and temperature change can be obtained simultaneously through a system of binary linear equations.

In particular, the LPFG made of DCF has a characteristic that is insensitive to externally applied strain as the diameter of the inner cladding of the DCF increases. When using a DCF of 80% or more of the diameter of the outer cladding, it is possible to fabricate an LPFG whose strain sensitivity is reduced by more than 20 times compared to LPFG fabricated on a general single-mode fiber (SMF).

Since the DCF MZI based on the LPFG fabricated on the strain-insensitive DCF is very small in the amount of wavelength shift by the additionally applied strain, the strain does not greatly affect the measurement results of bending and temperature.

Therefore, a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to an embodiment of the present invention manufactured using a DCF having a large internal cladding diameter and a device and method for simultaneous bending and temperature measurement using the same Simultaneous measurement of bending and temperature can be performed regardless of strain.

That is, by configuring the sensor unit using the long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to an embodiment of the present invention configured as described above, bending and temperature regardless of strain It is possible to easily implement a simultaneous bending and temperature measuring device capable of performing simultaneous measurement of, and in addition, by implementing such a simultaneous bending and temperature measuring device and performing simultaneous measurement of bending and temperature, according to an embodiment of the present invention Simultaneous measurement of bending and temperature can be easily implemented.

Here, in the above-described embodiment with reference to FIGS. 1 to 4, the long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) of the present invention uses two long period fiber gratings (LPFG) connected in series. Although the present invention has been described as an example of a case configured to include, the present invention is not necessarily limited to the contents of the above-described embodiment, that is, the present invention is, for example, three or more LPFGs in the core of the DCF. It should be noted that it may be configured by modifying and changing in various forms as needed by those skilled in the art within the scope not departing from the spirit and essence of the present invention, such as may be configured including.

Continuing, the contents and results of experiments performed by actually implementing a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to an embodiment of the present invention configured as described above will be described. do.

That is, the present inventors actually implemented a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to an embodiment of the present invention configured as shown in FIG. 1 and proved its performance through experiments. did

More specifically, in implementing the configuration of the DCF MZI shown in FIG. 1, both ends of the DCF are fusion-spliced with a single-mode optical fiber (SMF), and the three layers of the DCF (ie, the core, inner cladding, and outer cladding) are The diameters were formed to be 9, 105 and 125 μm, respectively.

The sensor unit composed of two identical LPFGs was fabricated by irradiating the DCF with a 10.6 μm CO ₂ laser (SYNRAD FSVI30SAC) by the line-by-line technique, and the spot of a focused laser beam with an average power of ~ 2.78 W. (Spot) size was adjusted to ~ 70 μm using an F-θ lens installed on a 2D optical scanner (MINI SCAN).

In addition, for the simultaneous measurement of bending and temperature in the manufacture of LPFG, to obtain different perturbation responses in bending and temperature responses, resonant wavelengths widely spread over the wavelength domain with different cladding modes It was aimed to obtain as many attenuation dips as possible with a Band Rejection Ratio (BRR) of about 5 dB at the resonance wavelength, which is the first for the fabrication of a DCF MZI composed of two identically formed LPFGs. is the second prerequisite.

In the manufacture of each LPFG, grating pitch, mark speed and While maintaining other grid fabrication conditions, such as the number of scanning cycles, in a fixed state and changing the number of grids, LPFG was manufactured to determine the optimal number of grids. After determining the number of grids, the first requirement described above The grating pitch that best satisfies the was adjusted and selected.

Through the optimization process of these grating fabrication conditions, the grating pitch, the number of gratings (lattice length), the engraving speed, and the number of laser scan cycles of the first LPFG were determined to be ~ 551.02 ㎛, 40 (~ 21.49 mm), 40.0 mm/s, and 1, respectively. It became.

During one laser scan cycle, the CO ₂ laser beam moves from top to bottom in both directions (ie, without lateral movement) perpendicular to the longitudinal direction of the fiber to create each index-modulated layer. and bottom to top).

The spectral characteristics of the two LPFGs constituting the sensor head should be as similar as possible for a clear and high-contrast interference spectrum of the in-fiber MZ interference of the sensor head. Therefore, the second LPFG Under the same fabrication conditions, a ~86.20 mm long grid-free region (i.e., the separation between the two LPFGs) was formed in between.

During the entire fabrication process of DCF MZI, after fixing one end of the DCF to an optical fiber holder mounted on a motorized linear stage, a slight lengthwise tension is applied to the optical fiber and an accurate grating period is applied. A 10 g weight was hung from the other end to secure it.

In addition, in-situ monitoring of the transmission spectrum of the DCF MZI was performed using a broadband light source (Fiberlabs FL7701) and an optical spectrum analyzer (OSA, Yokogawa AQ6370C).

The guided core mode derived from the LPFG is coupled into multiple radiative cladding modes by the interaction between the fundamental core mode and the forward propagating cladding mode. (coupled).

Since this mode coupling differs depending on the wavelength, different losses can be obtained depending on the wavelength, and the phase-matching condition of the conventional LPFG can be expressed as Equation 1 below.

[Equation 1]

where λ _res is the resonance wavelength, Λ is the grating period, and n _co,eff and ni _cl,eff are the guided core mode and the ith cladding mode of the LPFG, respectively. Each represents an effective refractive index.

The resonance dip of a conventional LPFG exhibits a highly linear bending or temperature response within a specific curvature or temperature range, and these multiple dampings with different cladding-mode orders The multiple attenuation dips do not have the same bending or temperature sensitivity.

In two LPFGs connected in series, when each LPFG has a BRR of 3dB at λ _res , the core mode at λ _res is 50% coupled with the cladding mode while passing through the first LPFG, and the coupled cladding mode The uncoupled core modes (i.e., the remaining 50% of the initial core modes) continue to propagate along the core while p is guided through the cladding of the grating-free-region.

When the coupled cladding mode encounters the second LPFG, it is coupled back to the core mode and interferes with the uncoupled core mode induced along the core in the first LPFG, and the core and cladding in the grating-free region is the MZ interferometer in the fiber. forms two optical paths of

The phase difference ΔΨ between the two forward propagation modes induced along the two optical paths can be expressed as in [Equation 2] below.

[Equation 2]

where λ, L _g and L represent the free-space wavelength, grating length, and center-to-center separation, respectively, and Δn _g,eff = n _g,eff,co - n _g,eff,cl , where n _g,eff,co and n _g,eff,cl represent the effective refractive indices of the core and cladding modes within the grating region, respectively.

Also, Δn _eff = n _eff,co - n _eff,cl , where n _eff,co and n _eff,cl represent the effective refractive indices of the core and cladding modes in the lattice-free region (outside the lattice region), respectively.

In addition, ΔΨ determines the interference fringe generated by the LPFG series pair, and in the fabricated DCF MZI, the grating strength of each LPFG is adjusted to an average of ~3.60 dB in the wavelength range of 1460 ~ 1600 nm did

Since the grating strength is not high, the differential index variation (eg, |Δn _g,eff - Δn _eff |) is expected to be less than 5%, and the fringe spacing is 3] can be approximated.

[Equation 3]

Here, Δm represents the effective group refractive index.

A change in Δm due to a perturbation may change the interference pattern wavelength spacing and thus shift the wavelength of the interference spectrum.

That is, referring to FIG. 7, FIG. 7 shows the transmission spectrum of the fabricated DCF MZI measured at room temperature (eg ΔT = 0) without external perturbation (eg ΔC = 0 and Δε = 0) (black indicated by solid lines).

Here, Δε represents the change in strain applied to the DCF MZI, and for comparison, the transmission spectrum of the first LPFG fabricated on the DCF is also shown as a purple solid line.

The measured wavelength spacing of the interference spectrum was ~5.1 nm, and this wavelength spacing can be adjusted by changing L, and the measured insertion loss of the fabricated DCF MZI was ~1.88 dB, which is mainly due to the difference between SMF and DCF. It occurred due to fusion splicing loss.

In the transmission spectrum of the first LPFG, the three resonant valleys near 1470, 1520 and 1590 nm are all associated with the LP ₀₃ and LP ₀₄ cladding modes, and among these three valleys, the leftmost and rightmost valleys in the spectrum are LP ₀₃ and LP, respectively. ₀₄ It is mainly affected by the cladding mode.

Therefore, likewise for the DCF MZI, the MZ interference in the fiber between the LP ₀₃ and LP ₀₄ cladding modes and the uncoupled core mode mainly determines the interference spectrum, while the LP ₀₃ cladding mode mainly affects the interference spectrum around 1470 nm. , the interference spectrum around 1590 nm is mainly generated from the LP ₀₄ cladding mode. In other words, the interference spectrum is more influenced by the LP ₀₃ cladding mode and less influenced by the LP ₀₄ cladding mode in the shorter wavelength region.

Therefore, as shown in FIG. 7, three damping valleys having resonance wavelengths of λ _DA = ~ 1462.3 nm, λ _DB = ~ 1522.8 nm, and λ _DC = ~ 1589.4 nm were selected as sensor markers, respectively, and DA, DB, and DC , and the measured BRRs of the three marker valleys (DA, DB and _DC ) at λ _DA , λ _DB and λ DC were ~7.40, ~15.53 and ~10.28 dB, respectively.

In addition, the present inventors chose these three markers to keep as far apart on the spectrum as possible from each other in order to distinguish the cladding modes affecting each marker.

Here, the LP ₀₃ and LP ₀₄ cladding modes are expected to have a strong influence on DA and DC, respectively, and thus the effect of the two cladding modes on each sensor marker is different, so the bending or temperature sensitivity is expected to be different depending on the sensor marker. expected

For two valleys (D1 and D2) selected among the three marker valleys (DA, DB, and DC), assuming that the bending and temperature responses of D1 are independent of those of D2 and that the bending and temperature responses are linear, the following Bending and temperature changes (ie, ΔC and ΔT) can be simultaneously measured using a sensitivity coefficient matrix as shown in [Equation 4] of [Equation 4].

[Equation 4]

Here, Δλ _D1 and Δλ _D2 represent wavelength shifts of D1 and D2, respectively.

S _C,D1 = Δλ _D1 /ΔC| _ΔT=0 and S _C,D2 = Δλ _D2 /ΔC| _ΔT=0 is the bending sensitivity coefficient of D1 and D2 and S _T,D1 = Δλ _D1 /ΔT| _ΔC=0 and S _T,D2 = Δλ _D2 /ΔT| _ΔC=0 represents temperature sensitivity coefficients of D1 and D2, respectively.

Therefore, these four sensitivity coefficients can be determined from the independent bending and temperature responses measured for each selected marker valley, and the inverse matrix of the sensitivity coefficient matrix as shown in [Equation 5] below and the measured wavelength shift (Δλ _D1 and Simultaneous measurements of ΔC and ΔT can be made using Δλ _D2 ).

[Equation 5]

Here, Det = S _C,D1 S _T,D2 - S _T,D1 S _C,D2 is the determinant of the sensitivity coefficient matrix.

Subsequently, the contents and results of the experiment using the DCF MZI produced as described above will be described.

That is, the present inventors conducted an experiment using the experimental apparatus as shown in FIG. 3 to measure the bending, temperature and strain responses of the DCF MZI manufactured as described above, and to measure the transmission spectrum of the DCF MZI Spectral changes due to perturbations in the transmission spectrum were again monitored using a broadband light source and OSA.

At this time, the resolution bandwidth and sampling point of the OSA were 0.02 nm and 7501, respectively. First, in order to investigate the bending response, one end of the sensor unit was attached to a fixed stage and a translation stage A 10 g weight was suspended at the other end fixed to the sensor to keep the sensing fiber straight during measurement.

In addition, in order to apply bending, the moving stage was slightly moved in the direction of the right arrow in FIG. 3, and the distance D between the holders of the two stages was ~ 17.6 cm when no bending was applied.

In addition, the curvature C due to bending is given by C = R ^-1 = (24ΔD/D ³ ) ^1/2 , where R and ΔD are the curvature radius of the sensor unit and the displacement of the moving stage, respectively ( displacement).

The length D, which is determined by the sum of the lengths of the lattice-free region and the SMF segments spliced at both ends of the two LPFG and DCF MZI, was selected considering the measurement accuracy and resolution of the sensor.

Next, referring to FIG. 8 , FIG. 8 is a diagram showing spectral changes due to bending of DA, DB, and DC measured for a curvature range of 1.484 to 4.451 m ^-1 at room temperature.

As shown in FIG. 8, DA and DB move to a shorter wavelength region as the curvature increases, showing a blueshift from 1462.30 to 1461.08 nm for λ _DA and from 1522.84 to 1520.06 nm for λ _DB . Conversely, DC moved to a longer wavelength region as the curvature increased, and λ _DC showed a redshift from 1589.44 to 1590.12 nm.

The bending-induced change in λ _DA , λ _DB or λ _DC is due to the bending-induced change in the effective refractive index difference (Δn _eff ) and the length of the grating-free region (L - L _g ), DCF The bending sensitivity of the marker bone λ _dip of MZI can be derived from [Equation 2], and Δn _g,eff and its temperature dependence (dΔn _g,eff /dT) are similar to Δn _eff and dΔn _eff /dT It can be approximated as in [Equation 6] below.

[Equation 6]

Next, to investigate the temperature response of DA, DB, and DC, the sensor unit was placed in a thermal chamber, and the spectral changes of the three marker valleys were measured in the ambient temperature range of 30 to 100 °C (in increments of 5 °C). range), and while observing the temperature response, a slight tension was applied to the DCF MZI so that the sensor part was kept upright and not affected by bending due to temperature.

That is, referring to FIG. 9 , FIG. 9 is a diagram illustrating spectral changes according to temperature of DA, DB, and DC measured for the above ambient temperature range without applying bending.

As shown in Fig. 9, λ _DA , λ _DB and λ _DC increased from 1462.00 to 1466.24 nm, 1523.76 to 1527.80 nm, and 1587.58 to 1592.84 nm, respectively, and unlike the bending response, all marker valleys became longer as the temperature increased. It shifted to the wavelength region and showed a red shift, and these temperature-induced changes in λ _DA , λ _DB or λ _DC occur due to temperature-induced changes in Δn _eff and (L - L _g ).

In addition, as in the case of bending sensitivity, the temperature sensitivity of the marker valley λ _dip can be approximated as shown in [Equation 7] below.

[Equation 7]

Next, referring to FIG. 10, FIG. 10 shows the bending of the three markers measured in the curvature range of 1.484 to 4.451 m ^-1 and the ambient temperature range of 30 to 100 ° C. and the wavelength shift by temperature, respectively. It is a drawing that represents

10, red, blue, and green rectangular or circular symbols represent the measured values of λ _DA , λ _DB , and λ _DC, respectively, and the red, blue, and green solid lines represent the linearity of the measured data for DA, DB, and DC, respectively. Regression analysis results are shown, and error bars of measured points indicate the measured wavelength change of each sensor marker.

As shown in FIG. 10, DA, DB, and DC exhibit sufficiently linear bending responses in the above curvature range with adjusted R ² values of ~ 0.9901, ~ 0.9912, and ~ 0.9915, respectively, and the evaluated DA, The bending sensitivities of DB and DC were S _C,DA = -427.63 pm/m ^-1 , S _C,DB = -911.29 pm/m ^-1 , and S _C,DC = 233.33 pm/m ^-1 , respectively. It can be confirmed that the bending sensitivity is different, and the mean variation of DA, DB, and DC was ~ 0.102, ~ 0.12, and ~ 0.082 nm, respectively.

Similarly, as shown in Fig. 10, the three marker valleys exhibited highly linear temperature responses in the above temperature range with adjusted R ² values of ~0.9991, ~0.9983, and ~0.9994, respectively, and the estimation of DA, DB, and DC The obtained temperature sensitivities are S _T,DA = 60.49 pm/℃, S _T,DB = 56.44 pm/℃, and S _T,DC = 74.61 pm/℃, respectively, indicating that the temperature sensitivities of the three marker bones are also different from each other, and DA , the average fluctuations of DB and DC were ~0.095, ~0.061 and ~0.047 nm, respectively.

In addition, considering the ratio of temperature sensitivity and bending sensitivity, S _C,DA /S _T,DA , S _C,DB /S _T,DB and S _C,DC /S _T,DC are approximately -7.07, −16.15 and 3.13 °C/m ⁻¹ _respectively , and the apparent difference between _{these ratios} is the bending and The temperature response reaffirms that the bones are uncorrelated with each other.

As described above, the linearity and independence of the sensor response to bending and temperature, which were experimentally confirmed, were determined by DCF using the perturbation-induced wavelength shift of two sensor markers selected from DA, DB, and DC. It allows simultaneous measurement of the bending applied to the MZI and the temperature change (ΔC and ΔT).

In addition, since there are three marker valleys, it is possible to select two of them to obtain a sensitivity coefficient matrix for three cases. In particular, three possible combinations of sensor markers are "DA and DB", "DB and DC" and If “DA and DC” is selected, the sensitivity coefficients obtained as described above are S _C,DA , S _C,DB , S _C,DC , S _T,DA , S _T,DB and S _T, The above [Equation 5] can be changed to the following [Equation 8] to [Equation 10] by substituting each with _DC .

[Equation 8]

[Equation 9]

[Equation 10]

Moreover, in order to confirm the stability of the fiber optic sensor according to an embodiment of the present invention against unwanted perturbation, the condition number κ = │M│ │M ^-1 │ was evaluated for each sensitivity coefficient matrix .

Here, ΘMΠ is the norm of the matrix M, and the calculated condition numbers of the sensitivity coefficient matrix in [Equation 8] to [Equation 10] were ~ 32.89, ~ 10.92, and ~ 5.16, respectively, and these Since the condition number is much lower than the case presented in the prior art, that is, a large condition number means a matrix with a bad condition (low stability), the sensitivity coefficient matrix of the optical fiber sensor according to an embodiment of the present invention has a good condition. (high stability) can be seen.

In addition, we also investigated the strain response using the same experimental setup used for the close examination of the bending and temperature response as shown in FIG. 3 .

More specifically, the fiber length to which the strain is applied is equal to D (= ~ 17.6 cm) as used for the bending response, and the moving stage is used to apply the axial strain to the DCF MZI as shown in FIG. Moving in the direction of the right arrow, strain was applied from 0 to 1136 με (in increments of 114 με) at room temperature.

That is, referring to FIG. 11, FIG. 11 is a diagram illustrating spectrum changes due to strains of DA, DB, and DC.

As shown in FIG. 11, if the longitudinal displacement of the moving stage is ΔD, the longitudinal strain applied to the DCF MZI is ΔD/D0, and if the applied strain is gradually increased, A very small change can be observed in the interference spectrum of the DCF MZI containing the three marker valleys.

In addition, for an applied strain of 1136 με, the three markers showed a maximum wavelength shift of 0.06 nm, and the strain sensitivities of DA, DB, and DC evaluated based on linear regression analysis for the measured wavelength shift were respectively With -0.019, -0.048 and 0.045 pm/με, they exhibited excellent strain insensitivity (<0.05 pm/με) compared to the strain sensitivity (~ 1.2 pm/με) of prior art LPFG.

That is, the wavelength shift due to this strain is so small that it has little effect on the results of bending and temperature measurements. From bending and temperature measurements using DB, (0.045 pm/με)×(100 με)/(233.33 pm/m ⁻¹ ) = ~ 0.019 m ⁻¹ and (-0.048 pm/με)×(100 με)/(56.44 pm/°C) = ~ 0.085°C, respectively (worst-case assumption).

As such, it is believed that the lower strain sensitivity than the LPFG of the prior art is related to the outer cladding dimension of the DCF. It is estimated.

This assumption is supported by previous studies in which LPFG was formed on another DCF having an inner cladding diameter about 2 times smaller than the DCF used in this example, which showed a strain sensitivity of ~ 0.8 pm/με. In the DCF MZI, the large internal cladding diameter of the DCF and the small size occupied by the external cladding are considered to be the cause of the low strain sensitivity.

Finally, the present inventors, 1.484 ~ 3.925 m ^-1 Applied curvature (ΔC) range and applied temperature (ΔT) range of 15 ~ 75 ℃ Random 16 (ΔC, ΔT) for simultaneous measurement for the point did

To this end, 10 simultaneous measurements were performed for 4 different values of ΔC (= 1.484, 2.570, 3.317, 3.925 m ^-1 ) and 4 values of ΔT (= 15, 35, 55, 75 °C). And the results are shown in Figure 12.

That is, referring to FIG. 12, FIG. 12 shows the values of ΔC and ΔT measured by the optical fiber sensor according to an embodiment of the present invention with respect to the values of ΔC and ΔT applied using the two marker valleys DA and DB. It is a drawing that represents

Here, in FIG. 12, the error bar of the measured point represents the mean deviation of the applied parameter values.

In addition, the standard deviation of the (ΔC, ΔT) values measured within the above measurement range is ~ 0.059 m ^-1 and ~ 0.85 ° C for DA and DB, respectively, and ~ 0.075 for DB and DC, respectively. m ^-1 and ~ 1.31 ° C, and ~ 0.086 m ^-1 and ~ 0.64. ° C for DA and DC, respectively.

Through these measurement results, it can be seen that the optical fiber sensor according to the embodiment of the present invention can perform separate measurement of bending and temperature with sufficient measurement accuracy.

Moreover, by considering the standard deviation of simultaneous measurement, bending sensitivity, strain insensitivity, etc., DA and DB among the markers in the selected three cases can be determined as optimal sensor markers.

Regarding the detectable minimum level (C or T), the present inventors use |Δλ _DA | for detection of ΔC or ΔT. ≥ 20 pm, |Δλ _DB | ≥ 20 pm and |Δλ _DC | It was confirmed that at least one of the three inequalities of ≥ 20 pm must be satisfied, because the minimum detectable wavelength shift that can be detected by the resolution bandwidth of OSA is 20 pm.

In addition, in the sensor system of this embodiment, the minimum detectable bending and temperature are functions of ΔC and ΔT that do not have fixed values, where assuming that ΔC or ΔT is 0, the minimum detectable bending or temperature is optimal Using the markers (ie DA and DB), it is estimated to be ~ 0.022 m ^-1 or ~ 0.331 °C, respectively.

In addition, the present inventors, based on the above [Equation 8] to [Equation 10], integer multiples of the detectable wavelength shift (20 pm) (eg, δλ _DA , δλ _DB and δλ _DC = 20 N , N is an integer) for several discrete wavelength shifts of the three marker troughs, the bending and temperature resolutions of the optical fiber sensor according to an embodiment of the present invention were derived.

From the calculation results, the estimated bending and temperature resolutions of the optical fiber sensor according to the embodiment of the present invention were about -0.0026 m ^-1 and 0.312 ° C when using the optimal sensor marker, respectively, and in particular, the wavelength accuracy of 1 pm. When using an optical sensing interrogator (Micron Optics HYPERION si155) with wavelength accuracy, these resolutions can be improved to about -0.0001 m ^-1 and ~ 0.016 °C, respectively.

That is, referring to FIG. 13, FIG. 13 shows sensor characteristics including sensitivity to bending, temperature, deformation, and simultaneous measurement of the optical fiber sensor according to an embodiment of the present invention and the optical fiber sensors proposed in the prior art. It is a diagram showing the results of the comparison in a table.

Here, in the comparison results shown in FIG. 13, PCF is a photonic crystal fiber, HST is a hump-shaped taper, and PSS is a peanut-shaped structure. The optical fiber sensor according to the embodiment is shown in the Cascaded LPFG's on DCF section at the bottom, and the rest are optical fiber sensors presented in the prior art. The highest value was applied.

As shown in the table of Fig. 13, the fiber optic sensor according to the embodiment of the present invention is not superior to the prior art fiber optic sensors in terms of bending sensitivity, but this does not affect the simultaneous measurement accuracy, which is not affected by the perturbation of the sensor markers. Strain insensitivity, which is a unique feature of the optical fiber sensor according to the embodiment of the present invention, is sufficiently guaranteed by the independence of perturbation-induced response and low condition number, and the measurement accuracy and stability can be improved.

As described above, in the present invention, an optical fiber sensor capable of simultaneously measuring strain-insensitive bending and temperature using an LPFG manufactured by a CO ₂ laser in DCF was proposed. To this end, As sensor markers for simultaneously measuring bending and temperature, three (i.e., DA, DB, and DC) were selected from multiple transmission valleys of the interference spectrum of the DCF MZI fabricated according to an embodiment of the present invention.

In addition, the measured bending sensitivities of DA, DB, and DC were -427.53, -911.29, and 233.33 pm/m ^-1 in the curvature range of 1.484 to 4.451 m ^-1 , and the measured temperature sensitivity was in the temperature range of 30 to 100 °C. were 60.49, 56.44 and 74.61 pm/°C, respectively, and thus, due to linear and independent responses to bending and temperature, two of the three valleys (a total of three cases) by transforming the measured wavelength shift into predetermined bending and temperature sensitivities. It is possible to simultaneously estimate the bending applied to the sensor unit and the temperature change.

In particular, in the present invention, strain-induced spectral changes of the three marker valleys were also investigated in the strain range of 0 to 1136 με, and the strain sensitivities evaluated were -0.019, -0.048 and 0.045 pm/με, and these sensitivities is more than 25 times lower than that of conventional LPFG, which greatly reduces the influence of strain on bending and temperature measurement results.

Finally, as a result of simultaneous measurement of bending and temperature at 16 random points within the applied curvature range of 1.484 to 3.925 m ^-1 and the applied temperature range of 15 to 75 °C, two optimal values selected from the three marker valleys were obtained. In the case of markers (DA and DB), the standard deviations of the measured bending and temperature values were ~0.059 m ^-1 and ~0.85 °C, respectively.

From these experimental results, the DCF MZI according to the embodiment of the present invention is a substantially strain-insensitive and cost-effective sensor unit for structure health monitoring in various industrial fields. ) is expected to contribute beneficially to the discrimination of bending and temperature for

Therefore, as described above, a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to an embodiment of the present invention and a strain-independent bending and temperature simultaneous measurement device and method using the same can be implemented. Thereby, according to the present invention, carbon dioxide (CO ₂ ) laser pulses are irradiated at regular intervals to the double cladding optical fiber (DCF) to periodically modulate the refractive index of the core of the DCF to form two LPFGs at regular intervals. long-period optical fiber grating (LPFG)-based double-cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) configured to enable simultaneous measurement of bending and temperature regardless of strain by connecting in series and strain-independent bending using the same And by providing a temperature simultaneous measurement device and method, the sensing performance is limited due to the cross sensitivity between temperature and bending, and in particular, when bending is applied to the sensor unit, strain is simultaneously applied, resulting in inaccurate measurement results. It is possible to solve the problems of conventional fiber Bragg grating (FBG) and long period fiber grating (LPFG)-based measuring devices and methods, which have limitations.

In addition, according to the present invention, as described above, even if a strain is applied to the sensor unit during measurement, a long period fiber optic grating (LPFG) based double cladding optical fiber (DCF) Mach-Zender configured to simultaneously measure bending and temperature regardless of the strain By providing an interferometer (MZI) and a device and method for simultaneous measurement of bending and temperature independent of strain using the same, it is possible to simultaneously measure two physical quantities using a single sensor unit without using a plurality of sensor units, compared to the existing FBG. It is possible to improve the convenience of the manufacturing process and reduce the manufacturing cost, and by using a carbon dioxide laser, it is possible to manufacture LPFG at low cost because it can be manufactured without a mask. In addition, even if tensile/compressive strain is applied when bending is applied, the spectrum of LPFG changes Since there is no measurement, the accuracy of simultaneous measurement of bending and temperature can be improved.

In the above, through the embodiments of the present invention as described above, a long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to the present invention and a strain-independent bending and temperature simultaneous measurement device using the same, and Although the details of the method have been described, the present invention is not limited only to the contents described in the above embodiments, and therefore, the present invention, the design needs and It goes without saying that various modifications, changes, combinations, and substitutions are possible depending on various other factors.

Claims (9)

Long-Period Fiber Grating (LPFG)-based double-clad fiber (DCF) Mach-Zehnder interferometer configured to simultaneously measure bending and temperature regardless of the strain applied to the sensor unit (Mach-Zehnder interferometer; MZI),
It is configured to include two long-period optical fiber gratings (LPFG) connected in series at a predetermined interval to the core of the double-cladding optical fiber (DCF),
The LPFG,
By irradiating the DCF with a carbon dioxide (CO ₂ ) laser pulse at a predetermined period and periodically modulating the refractive index of the core of the DCF to be configured to be respectively formed,
In addition to being able to simultaneously measure bending and temperature using a single sensor unit, even if a longitudinal strain is applied to the sensor unit during measurement, simultaneous measurement can be performed regardless of the strain. Characterized in that Long period fiber grating (LPFG) based double cladding fiber (DCF) Mach-Zehnder interferometer (MZI).
delete According to claim 1,
The long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI),
In the transmission spectrum of the DCF MZI, two dips having different cladding-mode orders are selected as sensor dips for measuring bending and temperature, respectively,
Long period fiber optic grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) .
delete According to claim 3,
The long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI),
A long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI), characterized in that the inner cladding diameter of the DCF is formed to be 80% or more of the outer cladding diameter.
delete According to claim 5,
The long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI),
A long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI), characterized in that the long period fiber grating (LPFG) is configured to include three or more of the long period fiber grating (LPFG) in the core of the DCF.
In the simultaneous bending and temperature measuring device configured to simultaneously measure bending and temperature regardless of the strain applied to the sensor unit,
The sensor unit,
Claim 1, claim 3, claim 5, claim 7 characterized in that it is configured to include a long-period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to any one of claims 1, 3, 5, and 7 and a temperature simultaneous measuring device.
In the simultaneous bending and temperature measurement method configured to simultaneously measure bending and temperature regardless of the strain applied to the sensor unit,
Implementing the sensor unit using the long period fiber grating (LPFG) based double cladding optical fiber (DCF) Mach-Zehnder interferometer (MZI) according to any one of claims 1, 3, 5, and 7; and
Bending and temperature simultaneous measurement method characterized in that it is configured to include the step of performing simultaneous measurement of bending and temperature through the sensor unit.

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