Temperature sensing system

Abstract

The invention relates to the technical field of optical fiber sensing, and discloses a temperature sensing system. The temperature sensing system comprises an optical fiber parameter amplifier used for carrying out four-wave mixing on injected pump light and injected seed light so that the injected seed light is amplified into signal light and idler frequency light is generated; wherein the signal light and the idler frequency light have an intensity correlation characteristic, the signal light is injected into the Sagnac ring, and the idler frequency light is injected into the balance detector; the balance detector is used for differentiating the light intensity of the signal light injected into the Sagnac ring and the light intensity of the idler frequency light, and the differentiated signal is used for temperature measurement; wherein the length and the curvature of the polarization maintaining optical fiber in the Sagnac ring are determined according to the dependency relationship between the preset Sagnac ring transmission strength and the length, the curvature and the Sagnac ring temperature measurement result. According to the temperature sensing system, the problem that the signal-to-noise ratio and the sensitivity of an existing temperature sensor are affected due to limitation of classical noise is solved.

Description

Technical Field

[0001] The invention relates to the technical field of optical fiber sensing, and in particular to a temperature sensing system. Background Art

[0002] Fiber optic sensing technology is widely used in the detection of physical quantities such as temperature, strain, and vibration due to its high sensitivity, anti-electromagnetic interference, and miniaturization potential. Among them, Sagnac interferometry, as a detection method based on optical phase difference, is widely used in fiber optic gyroscopes and distributed fiber optic sensors.

[0003] However, the Sagnac interferometry technique uses a single classical beam input, and the output signal will produce noise, causing the fiber optic sensor to be limited by classical noise, affecting its signal-to-noise ratio and sensitivity. Summary of the invention

[0004] The object of the present invention is to provide a temperature sensing system which can solve the technical problem that the existing temperature sensors are limited by classical noise, thus affecting the signal-to-noise ratio and sensitivity of the sensors.

[0005] To solve the above technical problems, an embodiment of the present invention provides a temperature sensing system, comprising: an optical fiber parametric amplifier, a Sagnac ring and a balanced detector connected in sequence, and the balanced detector is also connected to the optical fiber parametric amplifier; The fiber parametric amplifier is used to perform four-wave mixing on the injected pump light and the injected seed light, so that the injected seed light is amplified into signal light and generates idler light; wherein the signal light and the idler light have intensity correlation characteristics, and the signal light is injected into the Sagnac ring, and the idler light is injected into the balanced detector; The balanced detector is used to differentiate the light intensities of the signal light and the idler light after being injected into the Sagnac ring, and the differentiated signal is used for temperature measurement; wherein the length and curvature of the polarization-maintaining optical fiber in the Sagnac ring are determined by the dependency relationship between the preset Sagnac ring transmission intensity and the length, curvature and temperature measurement results of the Sagnac ring.

[0006] Optionally, the fiber parametric amplifier is composed of two coarse wavelength division multiplexers and a dispersion-shifted fiber located between the two coarse wavelength division multiplexers; wherein the injected pump light and the injected seed light enter the dispersion-shifted fiber through one of the coarse wavelength division multiplexers for four-wave mixing, and are separated into signal light and idler light through another coarse wavelength division multiplexer.

[0007] Optionally, the injected pump light is located in an anomalous dispersion region of the dispersion-shifted optical fiber, so that the injected pump light and the injected seed light undergo four-wave mixing.

[0008] Optionally, the wavelength of the injected pump light is 1550 nm, and the length of the dispersion-shifted optical fiber is 300 m.

[0009] Optionally, the temperature sensing system further includes a spectrum analyzer for generating noise intensity changes of the differential signal, so as to perform temperature measurement through the noise intensity changes.

[0010] Optionally, the Sagnac ring is composed of two fiber holders, a fiber coupler, a single-mode fiber and a polarization-maintaining fiber, and the distance between the two fiber holders is used to determine the curvature of the polarization-maintaining fiber.

[0011] Optionally, the balanced detector is used to use a balanced measurement method or an unbalanced measurement method to differentiate the light intensities of the signal light and the idler light after being injected into the Sagnac ring.

[0012] Optionally, the balanced detector is composed of two photodetectors and a differential amplifier; After being injected into the Sagnac ring, the signal light and the idler light are respectively injected into two photodetectors and differentiated by a differential amplifier.

[0013] The temperature sensing system provided by the present invention has at least the following beneficial effects: Signal light and idler light with intensity correlation characteristics (a quantum entanglement characteristic) are generated through the fiber parametric amplifier, and the Sagnac ring is a traditional temperature sensor that uses Sagnac interference technology. The temperature is measured by using the differential signal between the signal light and the idler light intensities after being injected into the Sagnac ring. This signal processing method based on light intensity detection uses intensity difference measurement technology to replace traditional wavelength modulation. By performing differential operations on the intensities of two correlated optical signals, the quantum characteristics of the intensity-correlated light beam are used to eliminate common noise. The low-noise characteristics of the intensity-correlated light beam can surpass the standard quantum limit and improve the signal-to-noise ratio of the measurement signal.

[0014] At the same time, considering the influence of the curvature and length of the polarization-maintaining fiber of the Sagnac ring on temperature measurement, the length and curvature of the polarization-maintaining fiber in the Sagnac ring are determined by the preset dependence between the Sagnac ring transmission intensity and the length, curvature and temperature measurement results of the Sagnac ring, thereby optimizing the curvature and length of the polarization-maintaining fiber of the traditional Sagnac ring, which can effectively improve the sensitivity of temperature measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] One or more embodiments are exemplarily described by the pictures in the corresponding drawings, and these exemplary descriptions do not constitute limitations on the embodiments.

[0016] Figure 1 A schematic diagram of a temperature sensing system provided according to an embodiment of the present invention Figure 1 ; Figure 2 A schematic diagram of a temperature sensing system provided according to an embodiment of the present invention Figure 2 ; Figure 3 is a schematic diagram of principle analysis of a temperature sensing system provided according to an embodiment of the present invention; Figure 4 is a schematic diagram of the effect of polarization-maintaining optical fiber length on temperature sensitivity provided according to an embodiment of the present invention; Figure 5 is a schematic diagram of the effect of the curvature of a polarization-maintaining optical fiber on temperature sensitivity provided according to an embodiment of the present invention; Figure 6 It is a schematic diagram of the dependence of compression on temperature under equilibrium conditions with and without curvature provided by one embodiment of the present invention; Figure 7 It is a schematic diagram of the dependence of light intensity difference noise power on temperature under the premise of having and not having curvature in a non-equilibrium state provided by one embodiment of the present invention; Figure 8 It is a schematic diagram of the dependence of compression on temperature in a non-equilibrium state with and without curvature according to an embodiment of the present invention. DETAILED DESCRIPTION

[0017] In order to make the purpose, technical scheme and advantages of the embodiments of the present invention clearer, the embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings. However, it will be appreciated by those skilled in the art that in the embodiments of the present invention, many technical details are proposed in order to enable the reader to better understand the present invention. However, even without these technical details and various changes and modifications based on the following embodiments, the technical scheme claimed in the present invention can be implemented. The division of the following embodiments is for the convenience of description and should not constitute any limitation on the specific implementation of the present invention. The various embodiments can be combined and referenced with each other without contradiction.

[0018] An embodiment of the present invention relates to a temperature sensing system. The implementation details of the temperature sensing system of this embodiment are described in detail below. The following content is only provided for the convenience of understanding the implementation details and is not necessary for implementing this solution.

[0019] The temperature sensing system of this embodiment can be Figure 1As shown, it includes: a fiber optic parametric amplifier (FOPA), a Sagnac ring and a balanced detector connected in sequence, and the balanced detector is also connected to the fiber optic parametric amplifier.

[0020] Specifically, the fiber parametric amplifier FOPA is used to perform four-wave mixing on the injected pump light and the injected seed light, so that the injected seed light is amplified into a signal light and an idler light is generated. The fiber parametric amplifier is specifically composed of two coarse wavelength division multiplexers and a dispersion-shifted fiber located between the two coarse wavelength division multiplexers. The injected pump light and the injected seed light enter the dispersion-shifted fiber through one of the coarse wavelength division multiplexers, generating a four-wave mixing process, the injected seed light is amplified into a signal light, and a new idler light is generated at the same time. The signal light and the idler light have an intensity correlation characteristic, forming an intensity-correlated double beam; the second coarse wavelength division multiplexer is used to spatially separate the signal light and the idler light, and the signal light is injected into the temperature sensing element (i.e., the Sagnac ring), with the idler light as a reference. In addition, the wavelength of the injected pump light is 1550nm, which is located in the anomalous dispersion region of the dispersion-shifted fiber, and meets the phase matching condition of four-wave mixing, so that the injected pump light and the injected seed light can be four-wave mixed.

[0021] It can be understood that the signal light is injected into the Sagnac ring, the idler light is injected into the balanced detector as a reference, and the length and curvature of the polarization-maintaining fiber in the Sagnac ring are determined by the preset dependence between the Sagnac ring transmission intensity and the length, curvature and the Sagnac ring temperature measurement results, so as to optimize the curvature and length of the polarization-maintaining fiber of the traditional Sagnac ring. Among them, the Sagnac ring is composed of two fiber clamps, a fiber coupler, a single-mode fiber and a polarization-maintaining fiber, and its detection sensitivity is higher than that of the traditional fiber temperature sensor, and the distance between the two fiber clamps in the Sagnac ring is used to determine the curvature of the polarization-maintaining fiber.

[0022] The balanced detector is used to differentiate the light intensity of the signal light and the idler light after being injected into the Sagnac ring, and the differential signal is used for temperature measurement. The balanced detector is used to differentiate the light intensity of the signal light and the idler light after being injected into the Sagnac ring using a balanced measurement method or an unbalanced measurement method.

[0023] In a specific implementation, the balanced detector is specifically composed of two photodetectors and a differential amplifier, which is used to detect the difference in the signal and reduce the background noise interference. The signal light and the idler light injected into the Sagnac ring are respectively injected into the two photodetectors and differentiated by the differential amplifier.

[0024] The differential signal is input into the spectrometer, which is used to generate the noise intensity change of the differential signal, so as to measure the temperature through the noise intensity change. Therefore, this embodiment detects temperature changes through intensity difference measurement technology, and combines quantum light sources (i.e., injected pump light and injected seed light), which can reduce classical noise and break through the standard quantum limit, thereby improving the signal-to-noise ratio and sensitivity of the temperature sensor.

[0025] In this embodiment, a signal light and an idler light with intensity correlation characteristics (a quantum entanglement characteristic) are generated by a fiber parametric amplifier, and the Sagnac ring is a traditional temperature sensor that uses the Sagnac interference technology. The temperature is measured by using the differential signal between the signal light and the idler light intensities after being injected into the Sagnac ring. This signal processing method based on light intensity detection uses intensity difference measurement technology to replace traditional wavelength modulation. By performing differential operations on the intensities of two correlated optical signals, the quantum characteristics of the intensity-correlated light beam are used to eliminate common noise. Through the low-noise characteristics of the intensity-correlated light beam, the standard quantum limit can be surpassed and the signal-to-noise ratio of the measurement signal can be improved.

[0026] At the same time, considering the influence of the curvature and length of the polarization-maintaining fiber of the Sagnac ring on temperature measurement, the length and curvature of the polarization-maintaining fiber in the Sagnac ring are determined by the preset dependence between the Sagnac ring transmission intensity and the length, curvature and temperature measurement results of the Sagnac ring, thereby optimizing the curvature and length of the polarization-maintaining fiber of the traditional Sagnac ring, which can effectively improve the sensitivity of temperature measurement.

[0027] In a specific embodiment, the temperature sensing system of the present invention can be seen in Figure 2 The structure shown, Figure 2 It includes an injected pump light 1; an injected seed light 2; a coarse wavelength division multiplexer 3 and a coarse wavelength division multiplexer 5; a dispersion-shifted optical fiber 4; a temperature sensing element 6; photodetectors 7 and 8; a differential amplifier 9; and a spectrometer 10. Among them, the coarse wavelength division multiplexer 3, the dispersion-shifted optical fiber 4, and the coarse wavelength division multiplexer 5 constitute a parametric amplifier FOPA; the temperature sensing element is composed of two fiber holders, a 2×2 fiber coupler, a single-mode optical fiber, and a polarization-maintaining optical fiber, and the change in the curvature of the polarization-maintaining optical fiber is achieved by changing the distance between the two fiber holders. The connection method is: the injected pump light 1 and the injected seed light 2 are injected into the FOPA, and the FOPA output signal light passes through the temperature sensing element 6 and the idler light passes through the photodetectors 7 and 8 through the differential amplifier 9, and then the spectrum analyzer 10 is connected to analyze the noise intensity.

[0028] The following specifically describes the effect of the length and curvature of the polarization-maintaining fiber on the temperature measurement of the temperature sensor: Classic temperature sensing, the corresponding schematic diagram is shown in Figure 3 (a), the light beam emitted from the broadband light source 12 is split into two parts by the 2×2 fiber coupler 13 and transmitted in two directions through the Sagnac loop, which is made by splicing a section of polarization-maintaining fiber 14 between two sections of single-mode fiber. Due to the high birefringence effect of the polarization-maintaining fiber, the two beams will accumulate a net phase difference, and the resulting interference spectrum will be measured by the spectrum analyzer 11. The curvature of the polarization-maintaining fiber is calculated by Figure 3 (b), the original length of the polarization-maintaining fiber is L, the distance change between the two fiber holders is ΔL, R is the radius of the circle where the polarization-maintaining fiber is bent to form an arc, and the curvature of the polarization-maintaining fiber is:

[0029] ; The applied external parameters will cause the shift of the interference spectrum, which will change the intensity of a specific wavelength. This intensity demodulation technology can be used to infer the information of the sensing parameters. Ignoring the inherent loss in the Sagnac ring, the dependence of the output transmission intensity I on the net phase difference is:

[0030] ; In the above dependence of the output transmission intensity I on the net phase difference, φ=2πBL / λ is the relative phase difference, B is the refractive index difference between the slow axis and the fast axis of the polarization-maintaining fiber, L is the length of the polarization-maintaining fiber, and λ is the wavelength of the light beam in free space. If the temperature of the polarization-maintaining fiber is changed experimentally and temperature sensing is realized, the length and refractive index of the polarization-maintaining fiber will also change accordingly. By performing a full differential operation on the dependence of the output transmission intensity I on the net phase difference, the dependence of the intensity change on the temperature of the polarization-maintaining fiber can be obtained as follows:

[0031] ; dL / dT and dB / dT are the thermal expansion coefficient and the thermo-optic coefficient, respectively. Since the thermal expansion effect is about two orders of magnitude smaller than the thermo-optic effect, the first term in the above-mentioned dependence of intensity change on the temperature of the polarization-maintaining fiber can be omitted. At the same time, in order to study the influence of curvature on the temperature sensing result, the curvature is added. At this time, the dependence of intensity change on curvature is:

[0032] ; ; In the above relationship between intensity change and curvature, C S It is a combination of fiber and material parameters. r and R are the radius of the polarization-maintaining fiber and the radius of the circle where the polarization-maintaining fiber is bent to form an arc. Combining the above-mentioned dependence of intensity change on curvature, the relationship between intensity change and temperature and curvature can be expressed as:

[0033] ; From the above relationship between intensity change, temperature and curvature, it can be seen that for a specific temperature change ΔT, the effect on intensity change when curvature does not exist is significantly different from that when curvature exists. In addition, the length of the polarization-maintaining fiber, as a pre-factor in the above relationship between intensity change, temperature and curvature, will also affect temperature sensitivity.

[0034] Therefore, the a priori length and curvature of the polarization-maintaining fiber can be used as modulation parameters to improve the temperature sensing performance, see Figure 4 and Figure 5 , respectively show the effect of polarization-maintaining fiber length and curvature on temperature sensitivity. Among them, Figure 4 (a) shows the relationship between intensity transmission and temperature at different polarization-maintaining fiber lengths. Figure 4 (b) shows the temperature sensitivity of three polarization-maintaining fiber lengths; Figure 5 (a) shows the relationship between intensity transmission and temperature under different polarization-maintaining fiber curvatures. Figure 5 (b) shows the temperature sensitivity of five polarization-maintaining fiber curvatures.

[0035] Based on this, the present invention takes into account the influence of the curvature and length of the polarization-maintaining fiber of the Sagnac ring on temperature measurement, optimizes the curvature and length of the polarization-maintaining fiber of the traditional Sagnac ring, and makes the polarization-maintaining fiber adopt a preset length and a preset curvature, which can effectively improve the sensitivity of temperature measurement.

[0036] The following specifically describes the effect of signal light and idler light with intensity correlation characteristics (i.e., intensity-correlated dual beams) on temperature measurement by a temperature sensor: Intensity difference measurement technology is a signal processing method based on light intensity detection. It is widely used in the field of optical sensing, especially in reducing noise and enhancing signal detection sensitivity. Its core idea is to perform differential operations on the light intensity of two correlated optical signals (usually signal light and idler light) to eliminate common noise and improve the signal-to-noise ratio of the measurement signal. It is divided into balanced measurement and unbalanced measurement, and the difference lies in whether an optical attenuator is added to the idler light path.

[0037] Figure 6 To balance the dependence of compression on temperature under the premise of measuring two curvatures, the yellow one is the polarization-maintaining fiber at a curvature of 11.646 m. -1The blue shows the temperature dependence of compression when the curvature is 0 m⁻¹; the blue shows the temperature dependence of compression when the curvature is 0 m⁻¹. The compression value here can be obtained by subtracting the standard quantum limit intensity difference noise power from the intensity difference noise power of the intensity-correlated dual beams. The standard quantum limit is achieved by a dual-channel light source with the same power as the signal light and the idler light. If the compression value is less than 0, it means that the intensity difference noise power of the intensity-correlated dual beams is less than the standard quantum limit, and quantum enhanced temperature sensing can be achieved. 11.646 m -1 The signal-to-noise ratio under curvature conditions is improved by 1.43 dB-1.05 dB=0.38 dB.

[0038] Unlike balanced measurements, in unbalanced measurements, the optical attenuator in the idler path is removed, thus keeping its noise power unchanged. Figure 7 (a) and Figure 7 (b) shows the relationship between the intensity difference noise power of the dual-channel light source and the intensity-correlated dual beam and the temperature. Figure 7 (a) It can be seen that the power of the strong difference noise increases with the increase of temperature. In the temperature range of 36℃~50℃, under the curvature of 11.646 m -1 With the support of , the sensing depth of 15.62 dB is slightly greater than 15.4 dB, indicating an enhancement factor of 0.22 dB. Figure 7 The intensity difference noise power in (b) also increases with the increase of temperature, and its increase rate is faster than Figure 7 (a). Since the intensity-correlated dual beams in the dual-mode compression state are generated by the amplification effect in the four-wave mixing process, they must have more thermal noise than the beam directly from the femtosecond fiber laser. The unattenuated idler light here cannot completely offset the quantum noise of the signal light, resulting in the generation of excess intensity difference noise power. Under this condition, the sensing depth is enhanced from 29.15 dB to 31.49 dB, with an enhancement factor of 2.34 dB. Due to the above amplification effect, Figure 7 The sensing depth in (b) is significantly greater than Figure 7 (a). Therefore, in the absence of curvature preconditions, the sensing depth of the dual-channel light source is 15.4 dB and the temperature sensitivity is 1.1 dB / °C, respectively, enhanced to 29.15 dB and 2.082 dB / °C of the intensity-correlated dual beam, and the enhancement factors of the sensing depth and temperature sensitivity are about 13.75 dB and 0.982 dB / °C, respectively. Under the curvature condition, the corresponding enhancement factors are about 15.87 dB and 1.134 dB / °C, respectively.

[0039] In the case of unbalanced measurement, the compression degree cannot be directly calculated according to the compression degree calculation method of balanced measurement, because Figure 7The data in (a) do not represent the actual standard quantum limit, which can be determined by differential measurement of the equivalent total optical power (the sum of the sensing signal and the reference idler power) and then from Figure 7 Subtract the actual standard quantum limit from (b) to obtain the experimental result Figure 8 Among them, the yellow one is the polarization-maintaining fiber with a curvature of 11.646 m -1 The dependence of compression on temperature; blue is the polarization-maintaining fiber at a curvature of 0 m -1 The dependence of compression on temperature. The curvature condition is 11.646 m -1 When , the compression degree 1.7 dB is greater than the 0.54 dB when there is no curvature. It can be concluded that changing the curvature can improve the signal-to-noise performance of the Sagnac loop.

[0040] Therefore, the temperature measurement by intensity-correlated dual light beams provided by the present invention has a higher signal-to-noise ratio than the traditional optical fiber temperature sensing system.

[0041] In summary, the rapid development of quantum optics has provided a new idea for improving the performance of optical fiber sensing. The present invention significantly improves the temperature sensing sensitivity and signal-to-noise ratio of the Sagnac optical fiber loop by selecting the optimal length and curvature of the polarization-maintaining optical fiber and combining the intensity-correlated dual beams, thereby reducing classical noise, breaking through the standard quantum limit, and significantly improving the sensing performance.

[0042] Among them, by optimizing the curvature and length, the temperature sensitivity can reach 2.249dB / °C, which is about twice as high as that of traditional technologies. By using two beams of light (signal light and idler light) and combining them organically, the signal-to-noise ratio can be improved by 1.43dB in a balanced state and by 1.7dB in an unbalanced state.

[0043] Those skilled in the art can understand that the above embodiments are specific embodiments of the present invention, and in practical applications, various changes can be made to them in form and detail without departing from the spirit and scope of the embodiments of the present invention. Any person skilled in the art can make various changes and modifications without departing from the spirit and scope of the embodiments of the present invention, so the protection scope of the embodiments of the present invention shall be based on the scope defined in the claims.

Claims

1. A temperature sensing system, characterized in that: include: A fiber parametric amplifier, a Sagnac ring and a balanced detector are connected in sequence, and the balanced detector is also connected to the fiber parametric amplifier; The fiber parametric amplifier is used to perform four-wave mixing on the injected pump light and the injected seed light, so that the injected seed light is amplified into signal light and generates idler light; wherein the signal light and the idler light have intensity correlation characteristics, and the signal light is injected into the Sagnac ring, and the idler light is injected into the balanced detector; The balanced detector is used to differentiate the light intensities of the signal light and the idler light after being injected into the Sagnac ring, and the differentiated signal is used for temperature measurement; wherein the length and curvature of the polarization-maintaining optical fiber in the Sagnac ring are determined by the dependency relationship between the preset Sagnac ring transmission intensity and the length, curvature and temperature measurement results of the Sagnac ring.

2. The temperature sensing system according to claim 1, characterized in that: The fiber parametric amplifier is composed of two coarse wavelength division multiplexers and a dispersion-shifted fiber located between the two coarse wavelength division multiplexers; wherein the injected pump light and the injected seed light enter the dispersion-shifted fiber through one of the coarse wavelength division multiplexers for four-wave mixing, and are separated into signal light and idler light through the other coarse wavelength division multiplexer.

3. The temperature sensing system according to claim 2, characterized in that: The injected pump light is located in the anomalous dispersion region of the dispersion-shifted optical fiber, so that the injected pump light and the injected seed light undergo four-wave mixing.

4. The temperature sensing system according to claim 3, characterized in that: The wavelength of the injected pump light is 1550 nm, and the length of the dispersion-shifted optical fiber is 300 m.

5. The temperature sensing system according to claim 1, characterized in that: The temperature sensing system further includes a spectrum analyzer for generating noise intensity variation of the differential signal, so as to perform temperature measurement through the noise intensity variation.

6. The temperature sensing system according to claim 1, characterized in that: The Sagnac ring is composed of two fiber holders, a fiber coupler, a single-mode fiber and a polarization-maintaining fiber, and the distance between the two fiber holders is used to determine the curvature of the polarization-maintaining fiber.

7. The temperature sensing system according to claim 1, characterized in that: The balanced detector is used to use a balanced measurement method or an unbalanced measurement method to differentiate the light intensities of the signal light and the idler light after being injected into the Sagnac ring.

8. The temperature sensing system according to claim 7, characterized in that: The balanced detector is composed of two photodetectors and a differential amplifier; After being injected into the Sagnac ring, the signal light and the idler light are respectively injected into two photodetectors and differentiated by a differential amplifier.

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