Optical fiber temperature sensor

Abstract

The utility model discloses an optical fiber temperature sensor, belongs to the technical field of optical fiber sensing, and aims to solve the problem of low sensitivity of the existing optical fiber temperature sensor. The optical fiber temperature sensor based on cascade connection of the two different interferometers comprises a wide-spectrum light source, a spectrograph and a series connection structure, the series structure comprises a first interferometer, an optical fiber circulator, an optical fiber coupler and a second interferometer; light emitted by the broadband light source enters the first interferometer through the optical fiber circulator, and reflected light of the first interferometer enters the second interferometer through the optical fiber circulator and the optical fiber coupler; reflected light of the second interferometer is received by the spectrograph after passing through the optical fiber coupler; wherein the first interferometer is used as a sensing interferometer and the second interferometer is used as a reference interferometer. The technology can improve the sensitivity of the optical fiber sensor, and can be applied to the field of optical fiber sensing.

Description

[0001] This application is a divisional application. The application date of the original application is July 4, 2023. The application number is 202321736150.X. The name of the invention is: A fiber optic temperature sensor based on the cascade of two different interferometers. Technical Field

[0002] The utility model relates to the technical field of optical fiber sensing, in particular to an optical fiber temperature sensor. Background Art

[0003] Optical fiber sensors have the advantages of simple manufacturing, small size, high stability, low power loss, strong anti-corrosion and anti-interference performance, and high sensitivity. They have been widely used in various sensing fields, such as energy industry, aerospace, environmental monitoring, medicine, etc. Among them, optical fiber Fabry-Perot sensors and Michelson sensors stand out among many sensors. They have the advantages of simple manufacturing, good stability, compact structure and high sensitivity. They can realize real-time, efficient and large-scale continuous monitoring of environmental parameters. They have attracted widespread attention from researchers and can be applied to the field of marine environmental monitoring.

[0004] However, the sensitivity of the above-mentioned existing optical fiber temperature sensors is relatively low, and it is necessary to propose an optical fiber sensing technology that can improve the measurement sensitivity. Utility Model Content

[0005] The utility model aims to provide an optical fiber temperature sensor to solve the problem of low sensitivity of the existing optical fiber temperature sensor.

[0006] To achieve the above-mentioned purpose, the utility model provides the following technical solutions: an optical fiber temperature sensor, comprising a broadband light source, a spectrometer and a series structure; the series structure comprises a first interferometer, an optical fiber circulator, an optical fiber coupler and a second interferometer; the light emitted by the broadband light source is used to pass through the optical fiber circulator and enter the first interferometer, so that the reflected light of the first interferometer enters the second interferometer after passing through the optical fiber circulator and the optical fiber coupler; the reflected light of the second interferometer is used to be received by the spectrometer after passing through the optical fiber coupler, so that the spectrometer obtains an interference spectrum formed by the first interferometer and the second interferometer connected in series; wherein the first interferometer is used as a sensing interferometer, and the second interferometer is used as a reference interferometer; the sensing head of the second interferometer comprises a second single-mode optical fiber segment, a dual-core optical fiber segment and a single-hole side-core optical fiber segment; the second single-mode optical fiber segment is used to transmit the reflected light of the first interferometer to the optical fiber circulator and the optical fiber coupler, and then enters the second interferometer; the reflected light of the second interferometer is used to be received by the spectrometer after passing through the optical fiber coupler, so that the spectrometer obtains an interference spectrum formed by the first interferometer and the second interferometer connected in series; One end of the fiber segment serves as the input / output end of the second interferometer 304, and the other end is fused to one end of the dual-core fiber segment; wherein, the portion between the second predetermined position of the second single-mode fiber segment 51 and the third predetermined position of the dual-core fiber segment is a tapered area, the tapered area includes the fusion point between the second single-mode fiber segment and the dual-core fiber segment, and the minimum outer diameter of the tapered area corresponds to the fusion point, so that the light entering the second single-mode fiber segment is divided into two beams at the fusion point and enters the double cores of the dual-core fiber segment respectively; the other end of the dual-core fiber segment is fused to one end of the single-hole side-core fiber segment, a second air cavity is provided in the single-hole side-core fiber segment, the core-air interface between the dual-core fiber segment and the second air cavity serves as the third interface, and the interface between the core of the single-hole side-core fiber segment and the external air serves as the fourth interface.

[0007] Furthermore, the first interferometer is a Fabry-Perot interferometer, and the second interferometer is a Michelson interferometer.

[0008] Furthermore, the light entering the second single-mode optical fiber segment through the input / output end of the second interferometer is divided into two beams by the tapered area: one beam enters the first core of the dual-core optical fiber segment and is reflected by the core-air interface of the third interface as the third reflected light; the other beam enters the second core of the dual-core optical fiber segment and then enters the core of the single-hole side-core optical fiber segment, and is reflected by the fourth interface as the fourth reflected light, so that the fourth reflected light interferes with the third reflected light.

[0009] The present application proposes a temperature sensor, which is composed of two interferometers of different structures connected in series. The first interferometer is formed by fusing a single-mode optical fiber to a hollow-core optical fiber, wherein one end of the hollow-core optical fiber is filled with PDMS, wherein the PDMS is sensitive to temperature and serves as a sensing cavity. The second interferometer is a Michelson interferometer made by fusing a single-mode optical fiber through tapering deformation and a double-core optical fiber, which is insensitive to temperature and serves as a reference cavity. The first interferometer and the second interferometer are in a series relationship, and the free spectral ranges of the first interferometer and the second interferometer are close and unequal, thereby improving the sensitivity of the sensor to temperature measurement.

[0010] The utility model has the beneficial effects of improving the sensitivity of the optical fiber sensor and improving the measurement sensitivity by adopting the serial connection mode of the Fabry-Perot interferometer and the Michelson interferometer. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 It is a structural schematic diagram of the optical fiber temperature sensor based on the cascade connection of two different interferometers of the utility model;

[0012] Figure 2 It is a schematic diagram of the structure of the sensor head of the first interferometer of the utility model;

[0013] Figure 3 It is a schematic diagram of the structure of the sensor head of the second interferometer of the utility model;

[0014] Figure 4 The serial structure interference spectrum obtained by the optical fiber temperature sensor of the utility model;

[0015] Figure 5 is the interference spectrum of the first interferometer;

[0016] Figure 6 is the interference spectrum of the second interferometer;

[0017] Figure 7 is the interference spectrum of the first interferometer at different temperatures;

[0018] Figure 8 The interference spectra at 40°C and 41°C are shown. DETAILED DESCRIPTION

[0019] The following will be combined with the drawings in the embodiments of the utility model to clearly and completely describe the technical solutions in the embodiments of the utility model. Obviously, the described embodiments are only part of the embodiments of the utility model, not all of the embodiments. Based on the embodiments in the utility model, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the utility model.

[0020] An embodiment of the utility model provides an optical fiber temperature sensor, comprising a broadband light source, a spectrometer and a series structure; the series structure comprises a first interferometer, an optical fiber circulator, an optical fiber coupler and a second interferometer; the light emitted by the broadband light source is used to pass through the optical fiber circulator and enter the first interferometer, so that the reflected light of the first interferometer enters the second interferometer after passing through the optical fiber circulator and the optical fiber coupler; the reflected light of the second interferometer is used to be received by the spectrometer after passing through the optical fiber coupler, so that the spectrometer receives the interference spectrum formed by the first interferometer and the second interferometer connected in series; wherein the first interferometer is used as a sensing interferometer, and the second interferometer is used as a reference interferometer.

[0021] Combine the following Figure 1 An embodiment of the above-mentioned optical fiber temperature sensor based on the cascade connection of two different interferometers is described.

[0022] like Figure 1 As shown, the optical fiber temperature sensor based on the cascade connection of two different interferometers of the present invention includes a broadband light source 10 , a spectrometer 20 and a series structure 30 .

[0023] The series structure 30 includes a first interferometer 301 , a fiber circulator 302 , a fiber coupler 303 and a second interferometer 304 .

[0024] As an example, the wavelength range of the light emitted by the broadband light source 10 is, for example, 1400 nm-1600 nm (including boundary values).

[0025] See also Figure 1 The light emitted by the wide-spectrum light source 10 is used to pass through the fiber circulator 302 (entering from port 1 of the fiber circulator and exiting from port 2), and enter the first interferometer 301 (the interferometer is sensitive to temperature and serves as a sensing interferometer), so that the reflected light of the first interferometer 301 passes through the fiber circulator 302 (entering from port 2 of the fiber circulator 302 and exiting from port 3) and the fiber coupler 303, and then enters the second interferometer 304 (the sensor is not sensitive to temperature and serves as a reference interferometer); the reflected light of the second interferometer 304 is used to be received by the spectrometer 20 after passing through the fiber coupler 303, so that the spectrometer 20 receives the interference spectrum formed by the first interferometer 301 and the second interferometer 304 connected in series.

[0026] The first interferometer 301 is used as a sensing interferometer, and the second interferometer 304 is used as a reference interferometer.

[0027] During the sensing process, the temperature-sensitive interferometer (i.e., the sensing interferometer) is placed in a temperature-controlled box. When the temperature changes, the refractive index changes, causing the band to change accordingly. However, the refractive index of the reference interferometer remains unchanged or changes so little as to be negligible, whether at room temperature or when the temperature changes in the temperature-controlled box.

[0028] In an embodiment of the present invention, the first interferometer is filled with PDMS, which has good thermal expansion and thermo-optical properties and is a temperature-sensitive material that is relatively sensitive to temperature. The second interferometer may use quartz optical fiber, which is relatively insensitive or not sensitive enough to temperature compared to the first interferometer.

[0029] In this way, the free spectral ranges of the first interferometer 301 and the second interferometer 304 are close but not equal, and the spectrum received by the spectrometer is the interference spectrum of the two interferometers in series, and an envelope phenomenon occurs. The sensing sensitivity of the envelope is much greater than the sensing sensitivity of a single sensor.

[0030] As an example, the first interferometer 301 may be a FPI (Fabry-Perot Interferometer) interferometer, and the second interferometer 304 may be a Mechelson interferometer.

[0031] like Figure 2 As shown, the sensor head of the first interferometer 301 may include, for example, two sections: a first single-mode optical fiber section 41 and a hollow-core optical fiber section 42 .

[0032] One end of the first single-mode optical fiber segment 41 serves as the input / output end (ie, the input end or the output end) of the first interferometer 301 , and the other end is fused with one end of the hollow-core optical fiber segment 42 .

[0033] The other end face of the hollow-core optical fiber segment 42 is an oblique section to ensure that the reflected light at the interface here has less influence than the first two reflected lights (i.e., the reflection direction of the reflected light after incident on the oblique section does not coincide with the incident direction) and can be ignored. The angle between the oblique surface of the oblique section and the axis of the hollow-core optical fiber segment 42 is, for example, between 30 and 60 degrees, for example, 45 degrees.

[0034] The fusion surface between the first single-mode fiber segment 41 and the hollow-core fiber segment 42 is used as the first interface M1 between the two; a first air cavity is provided in the hollow-core fiber segment 42 from the first interface M1 to the first predetermined position of the hollow-core fiber segment 42, and PDMS is solidified from the first predetermined position to the other end face of the hollow-core fiber segment 42; the interface between the first air cavity and PDMS corresponding to the first predetermined position is used as the second interface M2. The distance between the first interface M1 and the second interface M2 is L1, and L1 represents the cavity length of the first interferometer.

[0035] PDMS is a temperature-sensitive material made of an elastic polymer (Sylgard184-A) and a hardener (Sylgard184-B). It has very good thermal expansion and thermo-optical properties, as well as good adhesion and chemical inertness. The PDMS injected at first is liquid, which gradually becomes solid under the action of temperature. Therefore, it is easy to combine with optical fiber to prepare a highly sensitive temperature sensor.

[0036] As an example, a portion of the light entering the first single-mode optical fiber segment 41 through the input / output end of the first interferometer 301 is reflected by the first interface and serves as the first reflected light, and another portion of the light passes through the first interface and enters the first air cavity and is reflected by the second interface and serves as the second reflected light, so that interference occurs between the first reflected light and the second reflected light.

[0037] In addition, if Figure 3 As shown, the sensor head of the second interferometer 304 includes a second single-mode optical fiber segment 51 , a dual-core optical fiber segment 52 , and a single-hole side-core optical fiber segment 53 .

[0038] The single-hole side-core optical fiber is a special optical fiber, which is made of special materials and has special functions. The special optical fiber can be made according to needs, such as photonic crystal fiber (cross section similar to honeycomb), panda-type polarization-maintaining fiber (cross section similar to panda), eccentric hole-assisted dual-core optical fiber (cross section with two cores and one air hole), etc. In the embodiment of the utility model, the core (i.e., the side core) of this single-hole side-core optical fiber is not at the center of the interface, and has a hole.

[0039] One end of the second single-mode optical fiber segment 51 serves as the input / output end of the second interferometer 304, and the other end is fused to one end of the dual-core optical fiber segment 52; wherein, the portion between the second predetermined position of the second single-mode optical fiber segment 51 and the third predetermined position of the dual-core optical fiber segment 52 is a tapered area, the tapered area includes the fusion point between the second single-mode optical fiber segment 51 and the dual-core optical fiber segment 52, and the minimum outer diameter of the tapered area corresponds to the fusion point, so that the light entering the second single-mode optical fiber segment 51 is divided into two beams at the fusion point and then enters the two cores of the dual-core optical fiber segment 52 respectively.

[0040] The other end of the double-core fiber segment 52 is fused with one end of the single-hole side-core fiber segment 53. A second air cavity is provided in the single-hole side-core fiber segment 53. The core-air interface between the double-core fiber segment 52 and the second air cavity serves as a third interface M3 between the two. The core (i.e., the side core) of the single-hole side-core fiber segment 53 is connected to the outside air (i.e., Figure 3The interface between the air on the right side of the single-hole side core optical fiber segment 53 shown in the figure is used as the fourth interface M4; wherein one end of the second air cavity is located at the third interface, and the other end is located on the other end surface of the single-hole side core optical fiber segment 53 or other positions. The second predetermined position and the third predetermined position can also be determined by experience or experiment, which will not be repeated here.

[0041] Among them, the distance between the two surfaces M3 and M4 is L2, and L2 represents the cavity length of the second interferometer.

[0042] Thus, the light entering the second single-mode fiber segment 51 through the input / output end of the second interferometer 304 is divided into two beams by the taper region: one beam enters the first fiber core (eg, Figure 3 The fiber core shown in the figure is located at the top), and is reflected by the fiber core-air interface of the third interface as the third reflected light; the other beam enters the second fiber core of the dual-core optical fiber segment 52 (as shown in the figure). Figure 3 The fourth reflected light enters the core of the single-hole side-core optical fiber segment 53 and is reflected by the fourth interface as the fourth reflected light, so that the fourth reflected light interferes with the third reflected light.

[0043] Next, it is introduced how to prepare the sensor heads of the above two interferometers.

[0044] See also Figure 2 The first interferometer (i.e., sensing cavity) sensing head can be obtained, for example, by fusing a single-mode optical fiber with a hollow-core optical fiber, injecting PDMS into the hollow-core optical fiber, heating to solidify the PDMS, and cutting the rear end of the injected PDMS into an oblique cross section to ensure that the reflected light at the interface here has a smaller effect than the first two reflected light beams (i.e., the reflected light at the first interface M1 and the reflected light at the second interface M2) and can be ignored.

[0045] See also Figure 3 The sensor head of the second interferometer (i.e., reference cavity) is a Michelson interferometer made by fusing a single-mode optical fiber with a dual-core optical fiber after it has been deformed by a taper machine. For example, it can be obtained by the following method: aligning and fusing the cut single-mode optical fiber and the dual-core optical fiber, and then cutting the dual-core optical fiber to the required length, that is, the signal light incident on the single-mode optical fiber is exactly parallel to the dual-core optical fiber; aligning and fusing the free end of the dual-core optical fiber with the single-hole side-core optical fiber, and aligning the dual cores of the dual-core optical fiber with the hole and the side core of the single-hole side-core optical fiber, respectively, and cutting the single-hole side-core optical fiber to 100-500 microns.

[0046] The following describes the light beam transmission process and sensing principle of the optical fiber temperature sensor based on the cascade of two different interferometers of the present invention.

[0047] like Figure 1As shown, the incident light emitted by the broadband light source 10 enters the first interferometer 301 through the fiber circulator 302 , is reflected by the first interferometer 301 , and is transmitted to the second interferometer 304 through the fiber circulator 302 and the fiber coupler 303 . The reflected light is received by the spectrometer 20 after passing through the coupler 303 .

[0048] For simplicity, the signal light reflected by the first interface M1 (the interface between the single-mode optical fiber and the air) is called light beam 1 (i.e., the first reflected light described above), the signal light reflected by the second interface M2 (the interface between the air and PDMS) is called light beam 2 (i.e., the second reflected light described above), the signal light reflected by the interface M3 (the interface between the core of the dual-core optical fiber and the air cavity of the single-hole side-core optical fiber) is called light beam 3 (i.e., the third reflected light described above), and the signal light reflected by the interface M4 (the interface between the free end of the single-hole side-core optical fiber and the air) is called light beam 4 (i.e., the fourth reflected light described above).

[0049] Light beam 1 and light beam 2 form a FPI interferometer (ie, the first interferometer), and light beam 3 and light beam 4 form a Michelson interferometer (ie, the second interferometer).

[0050] The interference spectra of the first and second interferometers can be expressed as

[0051]

[0052] Where λ is the wavelength of the incident light, I1 and I2 are the intensities of the two reflected beams (i.e., beam 1 and beam 2) of the first interferometer, I3 and I4 are the intensities of the two reflected beams (i.e., beam 3 and beam 4) of the second interferometer, L1 and L2 are the cavity lengths of the first interferometer and the second interferometer, n1 and n2 are the refractive indices of the medium in the cavity of the first interferometer and the second interferometer. The FPI interferometer and the Michelson interferometer form a series structure. Since the free spectral ranges of the two interferometers are close but not the same, a vernier effect will be generated after the series connection, that is, an envelope will appear in the interference spectrum of the series structure, and the translation of the envelope with temperature is much larger than that of a single interferometer. I 12 (λ) represents the interference spectrum of the first interferometer, I 34 (λ) represents the interference spectrum of the second interferometer.

[0053] The free spectral ranges of the first interferometer and the second interferometer can be expressed as formula (3-1) and formula (3-2):

[0054] FSR1=λ 2 / 2n1L1 (3-1)

[0055] FSR2=λ 2 / 2n2L2 (3-2)

[0056] Wherein, FSR1 represents the free spectral range of the first interferometer, and FSR2 represents the free spectral range of the second interferometer.

[0057] The resulting interference spectrum envelope I en-12 (λ) can be expressed as:

[0058]

[0059] Among them, E 12 is the interference spectrum envelope amplitude, M 12 is the amplification factor of the conventional vernier effect.

[0060] The interference spectrum envelope is Figure 4 As shown, the vertical axis I V It represents the interference spectrum intensity in dBm, and the unit of the horizontal axis λ is nm.

[0061] In addition, through simulation, the reflection spectra of the first interferometer and the second interferometer are obtained as follows: Figure 5 and Figure 6 As shown, I PDMS represents the reflection spectrum intensity of the first interferometer (in dBm), I Mechelson Represents the reflection spectrum intensity of the second interferometer (in dBm). The main simulation parameters are; n1=1.41, n2=1, λ=1.550um, I1=0.0005, I2=0.0001, I3=0.0005, I4=0.001, L1=150um, L2=190um.

[0062] Figure 7 Figure 3 is the interference spectrum of the FPI interferometer (i.e., the first interferometer) when the temperature is 40°C and 41°C respectively. The results show that when the temperature increases from 40°C to 41°C, the interference spectrum of the FPI interferometer moves toward the short-wave direction, i.e., blue shift, and the blue shift is 1.1nm.

[0063] Figure 8 The interference spectrum of the first interferometer and the second interferometer in series shows an obvious envelope phenomenon. When the temperature is increased from 40℃ ( Figure 8 (a)) rises to 41℃( Figure 8 When (b) is used, the interference spectrum envelope moves toward the long-wave direction, that is, red-shifted, with an amount of 19.0 nm, which is 17 times that of a single FPI interferometer.

[0064] In summary, the present application proposes a temperature sensor, which is composed of two interferometers of different structures connected in series, such as Figure 1As shown, the first interferometer 301 is formed by fusing a single-mode optical fiber with a hollow-core optical fiber, wherein one end of the hollow-core optical fiber is filled with PDMS, wherein the PDMS is sensitive to temperature and serves as a sensing cavity. The second interferometer 304 is a Michelson interferometer made by fusing a single-mode optical fiber with a double-core optical fiber after tapering, which is insensitive to temperature and serves as a reference cavity. The first interferometer 301 and the second interferometer 304 are in a series relationship, and the free spectral ranges of the first interferometer 301 and the second interferometer 304 are close and unequal, so a vernier effect will be generated, thereby improving the sensitivity of the sensor to temperature measurement.

[0065] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. An optical fiber temperature sensor, characterized in that: The optical fiber temperature sensor comprises a broadband light source (10), a spectrometer (20) and a series structure (30); the series structure (30) comprises a first interferometer (301), an optical fiber circulator (302), an optical fiber coupler (303) and a second interferometer (304); The light emitted by the broadband light source (10) is used to pass through the optical fiber circulator (302) and enter the first interferometer (301), so that the reflected light of the first interferometer (301) enters the second interferometer (304) after passing through the optical fiber circulator (302) and the optical fiber coupler (303); the reflected light of the second interferometer (304) is used to be received by the spectrometer (20) after passing through the optical fiber coupler (303), so that the spectrometer (20) obtains an interference spectrum formed by the first interferometer (301) and the second interferometer (304) connected in series; wherein the first interferometer (301) is used as a sensing interferometer, and the second interferometer (304) is used as a reference interferometer; The sensing head of the second interferometer (304) comprises a second single-mode optical fiber segment (51), a dual-core optical fiber segment (52) and a single-hole side-core optical fiber segment (53); One end of the second single-mode optical fiber segment (51) serves as the input / output end of the second interferometer (304), and the other end is fused to one end of the dual-core optical fiber segment (52); wherein the portion between the second predetermined position of the second single-mode optical fiber segment (51) and the third predetermined position of the dual-core optical fiber segment (52) is a tapered region, the tapered region includes a fusion point between the second single-mode optical fiber segment (51) and the dual-core optical fiber segment (52), and the minimum outer diameter of the tapered region corresponds to the fusion point, so that light entering the second single-mode optical fiber segment (51) is divided into two beams at the fusion point and then enters the dual cores of the dual-core optical fiber segment (52) respectively; The other end of the dual-core optical fiber segment (52) is fused with one end of the single-hole side-core optical fiber segment (53); a second air cavity is provided in the single-hole side-core optical fiber segment (53); a core-air interface between the dual-core optical fiber segment (52) and the second air cavity serves as a third interface; and an interface between the core of the single-hole side-core optical fiber segment (53) and the outside air serves as a fourth interface.

2. The optical fiber temperature sensor according to claim 1, characterized in that: The first interferometer (301) is a Fabry-Perot interferometer, and the second interferometer (304) is a Michelson interferometer.

3. An optical fiber temperature sensor according to claim 1 or 2, characterized in that: Light entering the second single-mode optical fiber segment (51) through the input / output end of the second interferometer (304) is divided into two beams by the tapered area: one beam enters the first core of the dual-core optical fiber segment (52) and is reflected by the core-air interface of the third interface to serve as the third reflected light; the other beam enters the second core of the dual-core optical fiber segment (52) and then enters the core of the single-hole side-core optical fiber segment (53) to be reflected by the fourth interface to serve as the fourth reflected light, so that interference occurs between the fourth reflected light and the third reflected light.

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