KR20090034509A - Optical waveguide strain sensors with a bragg reflector on a flexible substrate - Google Patents

Abstract

An optical waveguide strain sensor is provided to bend a plastic substrate by directly forming a polymer optical waveguide and a Bragg grating on a flexible plastic substrate. A polymer optical waveguide is formed on a top part of a flexible substrate(110). The optical waveguide has a clade(120) and a core(130). One or more polymer Bragg grating(140) is formed on the optical waveguide. Two or more Bragg gratings are connected to a single optical waveguide. A polymer of the polymer waveguide has a negative thermo-optic coefficient. The flexible substrate has a positive coefficient of thermal expansion.

Description

Optical waveguide strain sensors with a Bragg reflector on a flexible substrate

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a strain sensor attached to a predetermined object and measuring strain values according to deformation of the object. More particularly, the present invention relates to a polymer optical waveguide strain sensor having a Bragg grating integrated on a plastic substrate.

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a flexible substrate Bragg grating optical waveguide strain sensor, and more particularly, to a flexible substrate Bragg which is freely bent and has excellent tensile properties by directly integrating a polymer optical waveguide and a Bragg grating on a relatively flexible plastic substrate. A grating optical waveguide strain sensor.

Optical sensors known to date have generally used optical fibers, but in the present invention, polymer optical waveguides are used, and flexible plastic materials are used as substrates, not rigid substrates such as silicon or glass.

Flexible display technology includes a flexible display technology that has recently attracted high interest. [C. D. Sheraw, L. Zhou, J. R. Huang, D. J. Gundlach, and T. N. Jackson, Appl. Phys. Lett., 80, 1088 (2002); V. Vorflusev and S. Kumar, Science, 283, 1903 (2002)]

This technology is a technology that can be folded and stored in a large area display, and also used as a display when it is needed, which is a new concept that can be applied to various products with a new concept.

Optical device technology using a flexible substrate is a method applied in the process of manufacturing an electro-optic polymer optical modulator. The SU-8 photoresist uses a very weak adhesion to the gold surface, so that the device can be easily separated from the substrate at a desired stage.

This makes it possible to fabricate an optical device on a flexible substrate without affecting the process conditions of the device by completely fabricating a device having a difficult manufacturing process on a silicon substrate and then separating it in the final step. [W. H. Steier, H.-C. Song, M.-C. Oh, and S.-W. Ahn, "Low-voltage flexible optic polymer modulators," US patent, US 2005/0249445 A1; H.-C. Song, M.-C. Oh, S.-W. Ahn, and W.-H. Steier, "Flexible low-voltage electro-optic polymer modulators," Applied Physics Letters, Vol. 82, pp. 4432-4434, June 2003]

However, the above patent technique proposes a method for fabricating an electro-optic modulator on a flexible substrate and, as in the present embodiment described below, is substantially unrelated to the Bragg grating optical waveguide strain sensor.

Strain sensors using Bragg grating-type optical devices have been mostly manufactured using optical fiber made of silica.

This is because, due to the convenience of the optical fiber manufacturing process, the cost of the optical fiber is very low and the process of forming the Bragg grating is relatively simple by irradiating an interference pattern using a UV pulse laser.

A number of patents have been registered for a method of measuring the strain applied to an optical fiber sensor by irradiating the light emitted from the broadband light source to the Bragg grating sensor and detecting the wavelength of the light reflected therefrom. [H. Tsuda, "Optical fiber strain sensor device and strain detection system," US patent 7,002,672 B2, Feb. 2006; B. Moslehi, R. J. Black, H. J. Shaw, and K. Toyama, "Multiplexable fiber-optic strain sensor system with temperature compensation capability," US patent 6,895,132 B2, May 2005; L. K. Wang, Y.-J. Chiang, and C.-C. Yang, "Dual fiber Bragg grating strain sensor system," US patent 6,829,397, Dec. 2004]

However, the conventional silica optical fiber has a limit in the tensile strength of the material, so if a large strain is generated, the tension of the optical fiber sensor does not follow the tension of the actual object, causing a problem of slipping of the adhesive site.

On the other hand, the plastic optical sensor can solve the above problems because the tension range is very wide.

The Bragg grating sensor can be manufactured in the form of measuring the strains of several points at the same time using one light source by connecting gratings having different periods to each other. Each Bragg grating sensor reflects only light of a specific wavelength, so measuring the change of a specific wavelength at the output can independently drain strain from a specific point. [I. Bennion, J. Williams, C. Groves-Kirkby, and L. Zhang, "Strain sensing," US patent 6,816,638 B1]

The polymer optical waveguide device has a problem in that it has a temperature dependency in which the characteristics of the sensor change with temperature. In order to overcome this temperature dependency, a form that supplements the characteristic change according to the temperature change by using a separate temperature measuring sensor is widely used.

However, even in this case, if the temperature sensitivity of the sensor element is too large, it is difficult. In order to reduce this temperature dependency, studies have been published that cancel out the thermal expansion and optical refractive index of polymer materials. [N. Keil et al., "Athermal all-polymer arrayed-waveguide grating multiplexer," Electronics Letters, Vol. 37, pp. 579-580, April 2001]

The results show reduced temperature dependence of arrayed waveguide devices made of polymeric materials, but the results for Bragg grating devices are unknown.

As described above, in the case of the strain sensor, which is widely used or researched in the past, the use of the strain sensor is weak because its tensile strength is hard to bend freely due to the characteristics of the material, and in the case of the conventional strain sensor, the strain sensor is larger than 10,000 με. Not only the measurement of the tensile value is practically impossible, but it is also a little cumbersome or difficult to measure the strain value in the multi-axis direction.

Moreover, in the case of the conventional strain sensor, there is a problem that the temperature dependence of the device is rather high, which also limits its use.

SUMMARY OF THE INVENTION An object of the present invention is to provide a flexible substrate Bragg grating optical waveguide strain sensor that is freely bent and excellent in tensile strength by fabricating the polymer optical waveguide and Bragg grating directly on a relatively flexible plastic substrate, and a method of manufacturing the same.

Another object of the present invention is to provide a device on a plastic substrate to freely increase the volumetric expansion with temperature to cancel the thermo-optic effect, thereby reducing the temperature dependency of the polymer Bragg grating device, the flexible substrate Bragg grating optical waveguide strain sensor And to provide a method for producing the same.

It is still another object of the present invention to provide a flexible substrate Bragg grating optical waveguide strain sensor capable of measuring a large tensile value of 10,000 με or more, which is difficult to measure by a conventional silica fiber Bragg grating, and a method of measuring the same.

It is still another object of the present invention to provide a flexible substrate Bragg grating optical waveguide strain sensor and a method for measuring the strain value in the multi-axis direction more accurately and easily.

The flexible substrate optical waveguide strain sensor of the present invention is a strain sensor attached to a predetermined object to measure the strain value generated by deformation of the object, a flexible substrate made of plastic; A polymer optical waveguide formed on the flexible substrate; And at least one polymer Bragg grating formed on the optical waveguide.

In the flexible substrate optical waveguide strain sensor of the present invention, it is preferable that two or more Bragg gratings having different angles and different grating periods are connected to a single optical waveguide, each having a different grating period and having a relative angle of 90 degrees and More preferably, it consists of three Bragg gratings of 45 degrees. In addition, the thickness of the flexible substrate is preferably 80 to 200㎛.

The polymer of the polymer optical waveguide has a negative thermo-optic coefficient, and the flexible substrate has a positive thermal expansion coefficient so that the flexible substrate optical waveguide strain sensor has a low temperature dependency. There is this.

In the strain measuring method of attaching the flexible substrate optical waveguide strain sensor of the present invention to a predetermined object and measuring the strain value generated by the deformation of the object, the strain measuring method using the optical waveguide of the present invention (a) broadband optical Incident to a core of the flexible substrate optical waveguide strain sensor; (b) detecting the output light output from the core of the flexible substrate optical waveguide strain sensor to obtain a transmission intensity for each wavelength of the output light; And (c) measuring a shifted value of the center wavelength of light reflected by the Bragg grating of the flexible substrate optical waveguide strain sensor based on the transmission intensity for each wavelength to obtain strain values and direction information generated by deformation of the object. There is a feature that is performed, including;

Tensile of 10,000με or more of the object may be measured by the strain measuring method, and the strain measuring method may be measured in the three-axis direction in the step (c) by a single wavelength-specific transmission intensity of the step (b). There is a characteristic that can obtain strain information of.

The manufacturing method for manufacturing the flexible substrate optical waveguide strain sensor of the present invention comprises the steps of: (i) forming a film containing Au on the Si substrate; (ii) forming a SU-8 polymer layer over the Au-containing film and over the Si substrate; (iii) forming a plastic substrate layer on top of the SU-8 polymer layer; (iv) forming a polymer optical waveguide and a Bragg grating on the plastic substrate layer; And (v) cutting up to the SU-8 polymer layer formed on the Au-containing film of step (ii) to obtain a flexible substrate optical waveguide strain sensor.

After the step (iii), it is preferable to further include forming a SU-8 polymer layer on the plastic substrate layer, the thickness of the plastic substrate layer of the step (iii) is preferably 80 to 200 ㎛. .

According to the present invention, by directly integrating the polymer optical waveguide and Bragg grating on a relatively flexible plastic substrate, there is an effect of free bending and excellent tensile properties.

In addition, it is possible not only to measure a large tensile value of 10,000 με or more, which is difficult to measure by conventional silica fiber Bragg gratings, but also to measure strain values in the multi-axis direction more accurately and easily.

In addition, by providing a polymer optical waveguide and a polymer Bragg grating on the plastic substrate, there is an advantage of having a low temperature dependency by the effect of offsetting the thermal expansion coefficient of the plastic substrate and the thermo-optic coefficient of the optical waveguide.

Hereinafter, a flexible substrate optical waveguide strain sensor, a method of manufacturing the strain sensor, and a strain measuring method using the strain sensor will be described in detail with reference to the accompanying drawings. The drawings introduced below are provided by way of example so that the spirit of the invention to those skilled in the art can fully convey. Accordingly, the present invention is not limited to the drawings presented below and may be embodied in other forms. Also, like reference numerals denote like elements throughout the specification.

At this time, if there is no other definition in the technical terms and scientific terms used, it has a meaning commonly understood by those of ordinary skill in the art to which the present invention belongs, the gist of the present invention in the following description and the accompanying drawings Descriptions of well-known functions and configurations that may be unnecessarily blurred are omitted.

1 is a schematic perspective view of a flexible substrate optical waveguide strain sensor 100 fabricated on a flexible substrate according to an embodiment of the present invention, Figure 2 is a flexible substrate optical waveguide strain sensor 100 shown in FIG. And a device having an optical fiber 210 for input and output of an optical signal attached to the surface 220 of an object to measure strain.

As can be seen in Figure 1 of the flexible substrate optical waveguide strain sensor of the present invention is formed on the optical waveguide consisting of the clad 120 and the core 130 on the flexible substrate 110, a plastic substrate, on the optical waveguide The Bragg grating 140 is formed. At this time, the Bragg grating 140 having the periodic grating structure reflects a specific wavelength satisfying the Bragg reflection condition. When the flexible substrate optical waveguide strain sensor of the present invention is attached to a predetermined object as shown in FIG. The deformation of an object causes the lattice constant of the Bragg grating to change, thereby changing a specific wavelength to be reflected. Therefore, by measuring the change in the wavelength reflected by the Bragg grating, it is possible to measure the deformation of the object. Therefore, the portion where the Bragg grating 4 is formed is a sensing surface 150 for measuring the actual strain.

In addition, contacting the sensing surface 150 with another material from the outside causes a change in the effective refractive index of the optical waveguide on which the Bragg grating 140 is formed, causing a change in the Bragg reflection wavelength. Due to this property, it can be used as a sensor for measuring the presence or content of a specific substance.

In order to increase the tension and flexibility of the sensor, and to increase physical durability and ease of handling, the thickness of the flexible substrate 110 may be 80 to 200 μm.

As described above, a thin substrate, a poly-waveguide, and a polymer Bragg grating made of a flexible plastic as described above may be easily bent or stretched when an external force is applied, thereby causing breakage and adhesion of an adhesive surface attached to an object. The tensile strength of the flexible substrate 110 may be transferred to the optical waveguide and the tensile property of the Bragg grating 140 as it is, thereby changing the period of the Bragg grating 140 to a large value. Will be. As a result, a sensor capable of measuring a large tensile value of 10,000 με or more, which is difficult to measure by a conventional technology, can be realized.

On the other hand, the strain measuring optical sensor is used for the safety diagnosis of the structure, the tank crack condition monitoring of LNG ships, the strain measurement on the fuselage of the automobile plane, etc. The degree was very small. However, in the present invention, since the flexible substrate 110 is applied, there is an advantage that can be easily applied under any environment. That is, since the tensile property is relatively excellent, there is an advantage that it can be applied to various use environments, that is, the surface of various objects.

As can be seen in Figure 2, the flexible substrate optical waveguide strain sensor 100 of the present invention can be used by attaching to the surface 220 of the specific object to measure the strain. In this case, the strain information measured by the flexible substrate optical waveguide strain sensor 100 is a strain value where the sensing surface 150 described in FIG. 1 is located among the strains of the object, and the direction in which the Bragg grating is formed (arrow direction in FIG. 2). ) Is the strain value.

As such, after the strain sensor 100 is attached to the surface 220 of the object, the optical fiber 210 for input / output of the broadband light source may be connected to the strain sensor 100 to measure that deformation of the object surface occurs.

In addition, since the strain sensor 100 is manufactured on the flexible substrate 110, that is, the plastic substrate, the strain sensor 100 may be attached to the metal or plastic surface more easily than the conventional silica optical fiber material. In addition, even if the tension or shrinkage of the attached object is more than 10,000 με it has the advantage that can accurately measure the degree of change without slipping with the surface.

3 is an output spectrum of the strain sensor 100 attached to the surface of an object, and is a graph showing the change in intensity (spectrum of output light) according to the wavelength of light output as the object contracts or expands.

As shown in FIG. 3, the flexible substrate optical waveguide strain sensor 100 according to the present embodiment attached to a surface of a material capable of contracting and expanding may include a light source having a broad wavelength band from the outside. When input to 130, the optical signal around the Bragg reflection wavelength is attenuated and all remaining wavelengths are transmitted to show the spectral characteristics of the output light as shown in FIG.

Referring to FIG. 3, when a transmission spectrum (reference transmission spectrum) exhibits the same characteristics as “reference” of FIG. 3 before the deformation of an object, when an external force acts to shrink the Bragg grating 100 interval. (When the shrinkage stress is applied to the strain sensor attached to the object surface by the deformation of the object), the center wavelength of the reflected light is shortened to show a transmission spectrum such as "shrink".

On the other hand, when tensile force is applied externally (when tensile stress is applied to the strain sensor attached to the object surface by deformation of the object), the central wavelength of the reflected light becomes long and the transmission spectrum is shown as "elongation". When the change in the transmission spectrum is detected by converting it into an electrical signal using an optical filter or an AWG from the outside, the degree of shrinkage or tension to be measured can be measured.

On the other hand, as an advantage of the flexible substrate optical waveguide strain sensor 100 according to the present embodiment, it is possible to implement a compact low-cost sensor by integrating various types of sensing surface 150 on one plastic substrate.

In other words, in the flexible substrate optical waveguide strain sensor according to the present embodiment, when a plurality of sensing surfaces having different grating periods are integrated to have various relative angles and connected to one optical waveguide, they are manufactured on a single plastic flexible substrate at various points. The generated strain can be measured simultaneously with one light source. Especially when measuring strain values, it is very useful to measure strain applied at different angles at once.

4 shows a plurality of sensing surfaces 151, 152, 153, and 154 composed of a plurality of Bragg gratings having different periods (Λ1, Λ2, Λ3, Λ4), as shown, with different angles on one flexible substrate. Another embodiment of the present invention implemented by sorting. The configuration as shown in Figure 4 of the present invention has the advantage that the three-axis strain can be measured and measured at various positions of the object.

As shown in FIG. 4, the flexible substrate optical waveguide strain sensor 400 has four sensing surfaces 151, 152, 153, and 154, respectively, based on a single sensing surface 151. It is arranged to have a 90-degree angle, and each structure has a different period so that it is possible to measure the strain values applied at different angles at the same time.

As described above with reference to FIG. 1, the sensing surface refers to a portion where a Bragg grating is formed on an optical waveguide, and the angle of the sensing surface itself is 0 degrees in a direction where the Bragg grating is formed. 4, the grating constituting the Bragg grating is generally formed perpendicular to the traveling direction of the light on the optical waveguide, so that the direction in which the Bragg grating is formed and the direction in which the optical waveguide core is formed are the same.

Since the four sensing surfaces 151, 152, 153, and 154 are connected to a single optical waveguide and integrated on a single flexible substrate, when the broadband light is input to the core of the strain sensor 400, the three-axis strain information It can be obtained through a single measurement, strain information can be obtained at various positions on the object surface, and if the measurement is repeated, the strain information that changes in real time can be obtained in-situ.

The key feature of the flexible substrate optical waveguide strain sensor 400 of the present invention described above with reference to FIG. 4 is that a plurality of sensing surfaces are integrated into a single sensor, and an object surface to be measured by adjusting a position at which a sensing surface is formed. It is to measure the strain values occurring at various positions at the same time, and to obtain the direction information (vector) of the measured strain values by adjusting the angle of the sensing surface, and the Bragg lattice constituting the sensing surface has different lattice constants. In this way, a single measurement can simultaneously obtain reliable measurement results of individual sensing surfaces.

In addition, as shown in FIG. 4, when the sensing surfaces having Bragg gratings having different periods are arranged to have an angle of 0 degrees, 45 degrees, and 90 degrees to each other, strain information in the 3-axis direction, which is very useful in strain sensing, is simultaneously displayed. In addition, since multiple sensing surfaces are integrated in a single sensor, the relative angle between the sensing surfaces can be set precisely from the beginning.

When using a light sensor to sense certain physical quantities, such as strain or pressure, changing the characteristics of the sensor with ambient temperature complicates the use of the sensor. In general, in order to suppress the change in the characteristics of the optical sensor due to temperature, the external sensor is used to accurately fix the operating temperature of the optical sensor, or the temperature is measured by another sensor to correct the characteristic according to the temperature. .

However, if the temperature dependency of the sensor is too large, there is a difficulty in the temperature correction as described above.

The flexible substrate optical waveguide strain sensor of the present invention has an advantage of reducing such temperature dependency. Polymer optical waveguide materials have the property of decreasing optical refractive index with increasing temperature. This is in contrast to silica materials, where the refractive index increases with temperature. In addition, the plastic substrate has a property of expanding in volume or length with increasing temperature. As a result, the optical device fabricated on the plastic substrate has a characteristic of canceling the decrease in the refractive index that occurs when the temperature increases with the increase in the length. In other words, as the temperature increases, the change in the optical path length, expressed as the product of the optical refractive index and the actual length, is almost negligible.

This property has been reported to be implemented in an arrayed waveguide grating (AWG) device using a plastic substrate. [N. Keil et al., "Athermal all-polymer arrayed-waveguide grating multiplexer," Electronics Letters, Vol. 37, pp. 579-580, April 2001].

However, the application of the Bragg grating to the purpose of reducing the temperature dependency of the sensor by using such a characteristic can be said to be the first proposed in the present invention. The flexible substrate optical waveguide strain sensor of the present invention comprises a polymer optical waveguide and a polymer Bragg grating on a plastic flexible substrate, so that the negative thermo-optic coefficient of the polymer of the polymer optical waveguide and the amount of the flexible substrate The thermal expansion coefficient of makes the sensor low temperature dependent. Figure 5 shows the temperature dependence of the Bragg grating optical waveguide device, it can be seen that the temperature dependence is significantly different depending on the type of material used as the substrate. The Bragg grating device fabricated on a silicon substrate has a large temperature dependence of about -0.241 nm / ° C due to the thermo-optic effect of the polymer itself, whereas the temperature dependence of the Bragg grating device using the polymer substrate of the present invention (NOA / SU-8) It was confirmed experimentally that can be reduced to -0.011 nm / ℃. Therefore, the polymer of the polymer optical waveguide has a negative thermo-optic coefficient, the flexible substrate has a positive thermal expansion coefficient of the present invention, the flexible substrate optical waveguide strain sensor is a conventional silicon substrate Compared with the device fabricated on the substrate, temperature dependence of 10 branch 1 or less is obtained.

FIG. 6 is a flowchart illustrating a strain measuring method using a flexible substrate optical waveguide strain sensor of the present invention.

Since the flexible substrate optical waveguide strain sensor of the present invention measures strain by the movement of reflected light by the Bragg grating, it is preferable to measure the strain using a broadband light source. As shown in the flowchart of FIG. 6, the strain measuring method of the present invention includes the steps of (a) injecting broadband light into the core of the flexible substrate optical waveguide strain sensor (s10); (b) detecting output light output from the core of the flexible substrate optical waveguide strain sensor (s20) to obtain transmission intensity for each wavelength of the output light (s30); And (c) measuring a shifted value of the center wavelength of light reflected by the Bragg grating of the flexible substrate optical waveguide strain sensor based on the transmission intensity for each wavelength to obtain strain values and direction information generated by deformation of the object. Obtaining step (s50); there is a feature that is performed.

In addition, the steps (s10 to s40) may be repeatedly performed (s50) to measure the strain value changing in real time in-situ, even in the case of using a single sensing surface as shown in FIG. The direction information of the strain can be obtained according to the formation direction of the grating, and when a plurality of sensing surfaces are used as shown in FIG. .

7 is a flowchart illustrating a manufacturing process of the flexible substrate optical waveguide strain sensor of the present invention. As shown in FIG. 7, the manufacturing method for manufacturing the flexible substrate optical waveguide strain sensor of the present invention includes (i) forming a film containing Au on the Si substrate; (ii) forming a SU-8 polymer layer over the Au-containing film and over the Si substrate; (iii) forming a plastic substrate layer on top of the SU-8 polymer layer; (iv) forming a polymer optical waveguide and a Bragg grating on the plastic substrate layer; And (v) cutting the SU-8 polymer layer formed on the Au-containing film of step (ii) to obtain a flexible substrate optical waveguide strain sensor. ) Features. In the above manufacturing method, a feature of the manufacturing method of the present invention is to fabricate a strain sensor by integrating an optical waveguide and Bragg grating on a plastic substrate by using a non-adhesion between SU-8 polymer and Au.

In detail, the flexible substrate optical waveguide strain sensor of the present invention can be fabricated by directly integrating devices on a thin plastic substrate, but for the fabrication of a fine structure such as Bragg grating optical waveguide, the post lift-off proposed in this embodiment is proposed. The process is effective. As shown in FIG. 5, the post lift-off process is a method of fabricating a device using a non-adhesion property of the Au surface, which is a characteristic of the SU-8 polymer. During the device fabrication process, the SU-8 polymer is bonded to the surface of the Si substrate. After the device fabrication is complete, the Si substrate is bonded to the SU-8 polymer, and the polymer thin film remaining on the Au surface is lifted off. The flexible substrate optical element is completed.

In the following description, details of processes such as deposition of metal films, etching, application, curing and removal of polymers and photosensitive materials, patterning for cores, Bragg grating patterning, dicing, etc. may unnecessarily obscure the subject matter of the present invention. Although the detailed description is omitted, it may be performed using a technique substantially similar to a conventional semiconductor manufacturing method.

The manufacturing method of the flexible substrate optical waveguide strain sensor according to the present embodiment is largely, the manufacturing steps of the flexible substrate (Figs. 7 (a) and 7 (b)), the step of manufacturing the optical waveguide formed with Bragg grating (Fig. 7 (c) to 7 (e)) and a dicing step (FIG. 7 (f)) for separating the completed strain sensor from the Si substrate.

First, considering the size of the device to be finally completed on the Si substrate 510 on which Au / Cr 520 is deposited, a pattern is formed by photo-lithography using AZ 5214 photoresist (PR), and Au / Cr Cr is removed to allow the SU-8 polymer to adhere to the exposed Si surface.

After coating SU-8 2015 with cyclopentanone at a ratio of 2: 1 on a patterned Si substrate, the solution was coated to a thickness of 6 μm and cured to form a SU-8 layer (530), and a NOA 61 to be used as a flexible substrate (540). Is cured after coating to 100 μm thickness. Finally, SU-8 is once again coated 550 to 6 μm thick to increase the surface hardness of polymer flexible substrate 540.

Next, an optical waveguide having a Bragg grating 570 and a lip shape is manufactured on the polymer multilayer thin film structure manufactured through the steps (FIGS. 7A and 7B). First, ZPU13-430 is coated with a thickness of 10 μm to manufacture a lower cladding 560, and then a Bragg grating 570 is manufactured on the fabricated lower cladding.

Bragg gratings can be fabricated using laser beam interferometers and nanoimprinting processes. These two processes are widely used in the semiconductor processing field, and will be briefly described below.

In the method using a laser beam interferometer, a lattice structure is formed by dividing TSMR PR using a holographic interference method using an Ar laser (Coherent Innova70, 488nm) light source, and then inscribe a lattice pattern on the lower cladding using O ₂ plasma.

On the other hand, in the nanoimprinting method, a lattice pattern fabricated on a Si substrate is copied to a poly (dimethylsiloxane) (PDMS) mold, and then the lattice structure is fabricated by covering it on the uncured lower cladding polymer and irradiating UV.

After fabricating the Bragg grating, LPS-154 was coated with a thickness of 3 μm to prepare a core layer 580, and an optical waveguide pattern was fabricated thereon with AZ5214 PR, and then etched to a depth of 0.8 μm to 0.9 μm using an O ₂ plasma. . When the ZPU13-430 is coated on the finished core layer 580 to a thickness of 10 μm to make the upper cladding 590, the optical waveguide structure is completed.

The completed optical element (strain sensor, 100 or 400) is separated from the Si substrate through a lift-off process as shown in FIG. 7 (f) and forms a cross section for optical input / output. In this case, when the SU-8 polymer and the Si substrate are bonded to each other, the NOA61 540 and the SU-8 layer 530 are separated from the Au surface 520 to complete the flexible substrate optical waveguide strain sensor.

For reference, after the post lift-off process, the flexible substrate may be weakly bent due to stress release, but it does not cause loss in the optical waveguide. However, the phenomenon that the position of the wavelength reflected by the Bragg grating slightly shifted from the design position occurred. After the post lift-off process, the flexible substrate device itself is less stable and requires packaging to maintain stability. To this end, the stability of the device can be improved by attaching a piece of glass to the end of the optical waveguide, polishing the cross section of the waveguide, and then connecting the optical fibers.

As described above, the strain sensor, the strain measuring method, or the strain sensor manufacturing method of the present invention has been described by specific embodiments such as specific components, processes, and the like, and the embodiments and drawings, which are provided to help a more general understanding of the present invention. However, the present invention is not limited to the above-described embodiments, and various modifications and variations are possible to those skilled in the art to which the present invention pertains.

Therefore, the spirit of the present invention should not be limited to the described embodiments, and all the things that are equivalent to or equivalent to the claims as well as the following claims will belong to the scope of the present invention. .

1 is a perspective view showing one structure of a flexible substrate optical waveguide strain sensor of the present invention,

2 is a view showing an example of the seal of the flexible substrate optical waveguide strain sensor of the present invention,

3 is an example showing the transmission intensity according to the wavelength of the output light of the flexible substrate optical waveguide strain sensor of the present invention,

Figure 4 is an example showing another structure of the flexible substrate optical waveguide strain sensor of the present invention,

5 is a result of measuring the temperature dependency of the Bragg grating optical waveguide device according to the substrate type,

FIG. 6 is a flowchart illustrating a strain measuring method using a flexible substrate optical waveguide strain sensor of the present invention.

7 is a flowchart illustrating a method of manufacturing the flexible substrate optical waveguide strain sensor of the present invention.

* Description of the symbols for the main parts of the drawings *

100, 400: flexible substrate optical waveguide strain sensor

110: substrate 120: clad

130: core 140: Bragg grating

150, 151, 152, 153, 154: sensing surface

210: optical fiber

Claims (5)

In the strain sensor for attaching to a predetermined object and measuring the strain value generated by the deformation of the object, Flexible substrates made of plastic; A polymer optical waveguide formed on the flexible substrate; And At least one polymer Bragg grating formed on the optical waveguide; A flexible substrate optical waveguide strain sensor comprising a. The method of claim 1, The flexible substrate optical waveguide strain sensor is characterized in that two or more Bragg gratings having different angles and different grating periods are connected to a single optical waveguide. The method of claim 2, The flexible substrate optical waveguide strain sensor includes three Bragg gratings having different lattice periods and having relative angles of 90 degrees and 45 degrees. The method of claim 1, The polymer of the polymer optical waveguide has a negative thermo-optic coefficient, and the flexible substrate has a positive thermal expansion coefficient so that the flexible substrate optical waveguide strain sensor is fabricated on an existing silicon substrate. A flexible substrate optical waveguide strain sensor, characterized in that it has a low temperature dependency compared to a conventional device. In the manufacturing method for manufacturing the flexible substrate optical waveguide strain sensor of any one of claims 1 to 4, (i) forming a film containing Au on the Si substrate; (ii) forming a SU-8 polymer layer over the Au-containing film and over the Si substrate; (iii) forming a plastic substrate layer on top of the SU-8 polymer layer; (iv) forming a polymer optical waveguide and a Bragg grating on the plastic substrate layer; And (v) obtaining a flexible substrate optical waveguide strain sensor by cutting to the SU-8 polymer layer formed on the Au-containing film of step (ii); Method of manufacturing an optical waveguide strain sensor, characterized in that it is produced, including.

Images