Singular point optical temperature sensor based on beat frequency detection

Abstract

The invention discloses a singular point optical temperature sensor based on beat frequency detection. Comprising a wide-spectrum light source, a coupling lens group, a spot size converter, a temperature sensing unit, a photoelectric detector and a lithium niobate film layer which are located in a packaging shell. The broadband light source, the coupling lens group, the spot size converter, the temperature sensing unit and the photoelectric detector are all located on the upper surface of the lithium niobate thin film layer and are sequentially arranged in the light path direction. Light emitted by the wide-spectrum light source sequentially passes through the coupling lens group, the spot size converter and the temperature sensing unit and then is detected and received by the photoelectric detector. The electro-optical effect of lithium niobate is utilized, the resonant frequency of the microcavity is adjusted by applying voltage, the temperature sensing unit is kept at a singular point in an initial state, and the influence of interference of external factors on the sensor is restrained; the anti-space-time inversion symmetric structure is constructed, the sensitivity of the temperature sensor is improved by using the sensitivity of singular points to external perturbation, and the temperature sensor has the advantages of high sensitivity, small size and integratability.

Description

Technical Field

[0001] The invention relates to the field of integrated optics and temperature sensing, and in particular to a singular point optical temperature sensor based on beat frequency detection. Background Art

[0002] As one of the basic state parameters of an object, temperature measurement is closely related to people's lives. With the development of science and technology, various smart devices and micro sensors have been widely used. These products are usually affected by the operating temperature. The change of temperature will affect their accuracy and stability. Especially in the integrated optical sensor with waveguide as the medium, the thermal light and thermal expansion effect of the material will introduce additional phases in the optical sensing, affecting the measurement of the physical quantity that the sensor is sensitive to. Therefore, it is particularly important to accurately measure and control the change of sensor temperature.

[0003] At present, commonly used temperature sensors include thermocouples, thermistors, RTDs (resistance temperature detectors) and IC temperature sensors (temperature integrated circuits). They have their own advantages in terms of sensitivity, stability, power consumption, etc. Among them, thermal resistors and thermocouples have the largest market share. However, in terms of use with other sensors, they can usually only be assembled on the same substrate to achieve temperature monitoring, and the integration compatibility is very poor. In addition, since the temperature transfer takes time, the obtained temperature information cannot accurately reflect the working temperature of the sensor, affecting the measurement of the physical quantity to be measured.

[0004] In recent years, with the advancement of micro-nano processing technology, optical sensors have gradually developed towards integration and miniaturization. Chip-based optical sensors are widely used in the detection of many physical quantities such as angular velocity, acceleration, displacement, temperature, etc., and have the advantages of high sensitivity and resistance to electromagnetic interference. In terms of optical temperature measurement, the current mainstream technologies include optical thermometers based on brightness balance method, laser spectrum temperature measurement technology, holographic interferometry temperature measurement technology, and infrared radiation temperature measurement technology. Although these temperature measurement technologies have the advantages of real-time and non-contact compared to traditional temperature sensors, they are still difficult to be compatible with other sensors based on optical waveguides in principle. Summary of the invention

[0005] Although the thermal light and thermal expansion effects of materials will affect the optical sensor's detection of other physical quantities, by cleverly utilizing this phenomenon, it can also be used as a means to detect temperature changes and achieve simultaneous sensing of temperature and other physical quantities.

[0006] In order to solve the problems in the background technology, the present invention aims at the integration problem of temperature detection in optical sensors and proposes a singular point optical temperature sensor that uses an optical waveguide resonant cavity to construct a non-Hermitian system. The temperature sensor has the advantages of small size, high sensitivity, and easy packaging and integration with optical sensors that detect other physical quantities.

[0007] The temperature sensor of the present invention uses a transmission waveguide and an "8"-shaped waveguide microcavity distributed on both sides of the transmission waveguide to construct a non-Hermitian optical system. The electro-optical effect of lithium niobate is used to change the refractive index of the two "8"-shaped waveguide microcavities by adjusting the voltage applied to the control electrode, so that the temperature sensitive unit is at the singular point of the non-Hermitian system in the initial state. When the ambient temperature changes, the thermo-optical effect and thermal expansion effect of the material cause the equivalent optical path of the microcavity to change, causing the change of the resonant frequency of the microcavity, and the temperature sensitive unit deviates from the singular point, thereby causing the beat frequency of the light output from the transmission waveguide to change. By detecting the change of the beat frequency, the current temperature change can be obtained. The present invention uses an integrated optical waveguide to construct a non-Hermitian optical system, and uses the characteristic that the singular point of the non-Hermitian system is sensitive to environmental disturbances to improve the sensitivity of the optical sensor. In addition, the structure of the waveguide microcavity is the same as that of other optical waveguide sensors in terms of process, so it can be integrated with other optical waveguide sensors into a single chip for coordinated work, thereby improving the compatibility of the temperature sensor.

[0008] The technical solution adopted by the present invention is as follows:

[0009] 1. A singular point optical temperature sensor based on beat frequency detection

[0010] It includes a wide spectrum light source, a coupling lens group, a pattern spot converter, a temperature sensitive unit, a photodetector and a lithium niobate thin film layer located in a packaging shell;

[0011] The broadband light source, coupling lens group, pattern spot converter, temperature sensitive unit and photoelectric detector are all located on the upper surface of the lithium niobate film layer and arranged in sequence along the optical path direction; the light emitted by the broadband light source passes through the coupling lens group, pattern spot converter and temperature sensitive unit in sequence and is detected and received by the photoelectric detector.

[0012] The temperature sensitive unit comprises two microcavity electrode assemblies and a transmission waveguide, wherein the two microcavity electrode assemblies are respectively arranged on both sides of the transmission waveguide, each microcavity electrode assembly comprises an "8"-shaped waveguide microcavity and a pair of electrode pairs, and the two ends of the transmission waveguide are respectively connected to the output end of the mode spot converter and the input end of the photodetector;

[0013] The "8"-shaped waveguide microcavity is a waveguide in the shape of "8", and the vertical center line of the "8"-shaped waveguide microcavity is arranged perpendicular to the transmission waveguide;

[0014] The electrode pair is composed of two control electrodes, and the two control electrodes are arranged on a side of the "8"-shaped waveguide microcavity away from the transmission waveguide.

[0015] Each of the "8"-shaped waveguide microcavity is mainly composed of two straight waveguides and two open racetrack waveguides arranged in a cross shape, one end of the two straight waveguides is respectively connected to the two ends of an open racetrack waveguide, and the other ends of the two straight waveguides are respectively connected to the two ends of another open racetrack waveguide, and the two control electrodes are respectively arranged on both sides of the middle of an open racetrack waveguide away from the transmission waveguide.

[0016] The middle portion of the open racetrack waveguide is arranged as a straight waveguide structure parallel to the transmission waveguide, and the middle portion of the open racetrack waveguide close to the transmission waveguide is close to the transmission waveguide and coupled with the transmission waveguide through evanescent waves.

[0017] The resonant frequency difference between the two "8"-shaped waveguide microcavities is designed as follows:

[0018] |ω1-ω2|=μ1μ2

[0019] Where ω1 and ω2 represent the resonant frequencies of the two “8”-shaped waveguide microcavities, and μ1 and μ2 represent the coupling coefficients between the two “8”-shaped waveguide microcavities and the transmission waveguide.

[0020] According to the relationship between the resonant frequency and the microcavity length, and the calculation formula of the coupling coefficient, the relationship between the cavity lengths of the two "8"-shaped waveguide microcavities is obtained as follows:

[0021]

[0022] Where c is the speed of light in vacuum, n is the refractive index of the waveguide material, L1 and L2 represent the cavity lengths of the two "8"-shaped waveguide microcavities, m is the resonance order determined according to the incident wavelength and L1, and κ1 and κ2 are the energy coupling ratios between the two "8"-shaped waveguide microcavities and the transmission waveguide.

[0023] The broadband light source, coupling lens group and spot converter are coaxially arranged, the output end of the spot converter is connected to the input end of the transmission waveguide of the temperature sensitive unit, and the output end of the transmission waveguide is connected to the input end of the photodetector.

[0024] The pattern spot converter is a waveguide with different widths at the input and output ends. The width of the input end is greater than the width of the output end. The width of the output end is consistent with that of the transmission waveguide and is connected to the input end of the transmission waveguide.

[0025] The singular point optical temperature sensor further comprises a silicon substrate and a silicon dioxide buried layer, the lithium niobate thin film layer is arranged on the silicon dioxide buried layer, and the silicon dioxide buried layer is arranged on the silicon substrate.

[0026] 2. A temperature detection method of a singular point optical temperature sensor

[0027] In the initial state, a feedback voltage is applied to the control electrode, and the refractive index of the two "8"-shaped waveguide microcavities is changed by utilizing the electro-optical effect of the lithium niobate material, thereby adjusting the resonant frequency difference between the two "8"-shaped waveguide microcavities, so that the temperature sensitive unit is at the singular point of the non-Hermitian system in the initial state;

[0028] In the working state, the light emitted by the wide-spectrum light source is detected and received by the photoelectric detector after passing through the coupling lens group, the mode spot converter, and the temperature sensitive unit in sequence;

[0029] When the temperature sensitive unit senses a change in ambient temperature, the thermo-optical effect of the temperature sensitive unit material causes the refractive index to change, and the expansion of the temperature sensitive unit material causes the cavity length of the "8"-shaped waveguide microcavity to change, so that the equivalent optical path of the "8"-shaped waveguide microcavity changes, causing the deviation of the resonant frequency of the "8"-shaped waveguide microcavity. Since the cavity lengths of the two "8"-shaped waveguide microcavities are slightly different, the deviation amounts of the resonant frequencies of the two "8"-shaped waveguide microcavities are also different, resulting in a change in the difference in the resonant frequencies of the two "8"-shaped waveguide microcavities.

[0030] This change in the resonant frequency difference causes the temperature-sensitive unit to deviate from the singularity of the non-Hermitian system, which manifests itself as a splitting of the output spectrum in the transmission waveguide, resulting in a beat frequency signal that changes with temperature. The photodetector detects this beat frequency signal and substitutes the corresponding beat frequency-temperature according to the pre-fitted curve to obtain the temperature change, which is then combined with the initial temperature to obtain the current temperature.

[0031] Beneficial effects of the present invention:

[0032] The present invention concentrates all devices on an optical chip, which reduces the volume of the temperature sensor and has a high degree of integration compared to traditional optical temperature measurement methods. The temperature sensitive unit based on the optical waveguide can be easily integrated with other optical sensors and has strong compatibility.

[0033] The present invention constructs two "8"-shaped waveguide microcavities with the same loss. The two "8"-shaped waveguide microcavities are indirectly coupled through the transmission waveguide in the middle. It is a typical non-Hermitian system that satisfies the anti-parity-time reversal symmetry conditions. The characteristic that the singular points of the non-Hermitian system are sensitive to external disturbances can be effectively utilized to improve the sensitivity of the temperature sensor.

[0034] The two "8"-shaped waveguide microcavities constructed by the present invention allow light to have both clockwise and counterclockwise parts when transmitted therein, effectively eliminating the interference of the Sagnac effect caused by the rotation of the earth and other possible rotations on the temperature sensitive unit, and improving the signal-to-noise ratio and anti-interference ability of the sensor.

[0035] The "8"-shaped waveguide microcavity of the present invention uses two cross-shaped straight waveguides to connect two open racetrack waveguides, avoiding energy crosstalk that may be generated when light is transmitted in the two straight waveguides, and ensuring the unidirectionality of light transmission in the microcavity.

[0036] The control electrode of the present invention utilizes the electro-optical effect of lithium niobate material and adjusts the refractive index of the microcavity electrode coverage area by applying different voltages, thereby changing the optical path of the microcavity. Therefore, the resonant frequency of the microcavity can be accurately adjusted to keep the temperature sensitive unit at the singularity point in the initial state, thereby suppressing the influence of the temperature sensitive unit deviating from the singularity point due to interference from preparation errors or other factors such as the environment on the sensor performance, and reducing the preparation process requirements of the waveguide. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 It is a schematic diagram of the overall structure of the singular point optical temperature sensor based on beat frequency detection of the present invention;

[0038] Figure 2 It is a schematic diagram of the structure of the temperature sensitive unit in the present invention;

[0039] Figure 3 It is a graph showing the change of the resonant frequency difference of the resonant cavity and the change of the output beat frequency caused by temperature in the present invention;

[0040] In the figure: 1. broadband light source, 2. coupling lens group, 3. pattern spot converter, 4. temperature sensitive unit, 5. photodetector, 6. packaging shell, 7. lithium niobate thin film layer, 41. open racetrack waveguide, 42. straight waveguide, 43. control electrode, 44. transmission waveguide, 45. silicon dioxide buried layer, 46. silicon substrate. DETAILED DESCRIPTION

[0041] The present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0042] like Figure 1 , Figure 2 As shown, the singular point optical temperature sensor includes a wide spectrum light source 1, a coupling lens group 2, a pattern spot converter 3, a temperature sensitive unit 4, a photodetector 5 and a lithium niobate thin film layer 7 located in a packaging shell 6;

[0043] The broadband light source 1, the coupling lens group 2, the pattern spot converter 3, the temperature sensitive unit 4 and the photodetector 5 are all located on the upper surface of the lithium niobate thin film layer 7 and are arranged in sequence along the optical path; the light emitted by the broadband light source 1 passes through the coupling lens group 2, the pattern spot converter 3, the temperature sensitive unit 4 in sequence and is detected and received by the photodetector 5.

[0044] All waveguide structures of the pattern spot converter 3 and the temperature sensitive unit 4 are formed by etching the lithium niobate thin film layer 7 , and the broadband light source 1 , the coupling lens group 2 and the photodetector 5 are located in the fixed groove formed by etching the lithium niobate thin film layer 7 .

[0045] The temperature sensitive unit 4 includes two microcavity electrode assemblies and a transmission waveguide 44. The transmission waveguide 44 is in a strip shape. The two microcavity electrode assemblies are approximately symmetrically arranged on both sides of the transmission waveguide 44. Each microcavity electrode assembly includes an "8"-shaped waveguide microcavity and a pair of electrodes. The two ends of the transmission waveguide 44 are respectively connected to the output end of the spot converter 3 and the input end of the photodetector 5.

[0046] The “8”-shaped waveguide microcavity is a waveguide whose main body is in the shape of “8”, and the vertical center line of the “8”-shaped waveguide microcavity passing through the two inner rings is arranged perpendicular to the transmission waveguide 44;

[0047] One electrode pair is composed of two control electrodes 43 , and the two control electrodes 43 are arranged on one side of the “8”-shaped waveguide microcavity away from the transmission waveguide 44 .

[0048] Each "8"-shaped waveguide microcavity is mainly composed of two straight waveguides 42 and two open racetrack waveguides 41 arranged in a cross shape, one end of the two straight waveguides 42 is respectively connected to the two ends of an open racetrack waveguide 41, and the other ends of the two straight waveguides 42 are respectively connected to the two ends of another open racetrack waveguide 41, and the two control electrodes 43 are respectively arranged on both sides of the middle of an open racetrack waveguide 41 away from the transmission waveguide 44.

[0049] The middle portion of the open racetrack waveguide 41 is configured as a linear waveguide structure parallel to the transmission waveguide 44 , and the middle portion of the open racetrack waveguide 41 close to the transmission waveguide 44 approaches the transmission waveguide 44 and couples with the transmission waveguide 44 through evanescent waves.

[0050] The spacing between the “8”-shaped waveguide microcavity and the transmission waveguide 44 determines the coupling coefficient between the two. The temperature sensitive unit 4 is at a singular point and requires that the coupling coefficient and the resonant frequency of the waveguide microcavity satisfy the relationship:

[0051] |ω1-ω2|=μ1μ2

[0052] Wherein ω1 and ω2 represent the resonant frequencies of the two “8”-shaped waveguide microcavities, respectively, and μ1 and μ2 represent the coupling coefficients between the two “8”-shaped waveguide microcavities and the transmission waveguide 44, respectively.

[0053] In practical applications, the temperature sensitive unit 4 is at a singular point, which places very high demands on the process. Due to the existence of micro-nano processing errors, the above relationship is usually difficult to meet. In addition, interference from external factors such as the environment often causes it to deviate from the singular point, affecting the detection of temperature. The control electrode 43 applies a corresponding feedback voltage and uses the electro-optical effect of the lithium niobate material to change the refractive index of the two "8"-shaped waveguide microcavities. Therefore, the resonant frequency of the waveguide microcavity can be adjusted so that the temperature sensitive unit 4 is at a non-Hermitian system singular point in the initial state. The design of the control electrode 43 reduces the requirements for the preparation process of the waveguide and also suppresses the influence of external factors such as the environment on the temperature sensor.

[0054] During the operation of the singularity point optical temperature sensor:

[0055] In the initial state, due to the interference of external factors such as preparation errors and environment, the temperature sensitive unit 4 may deviate from the singular point of the non-Hermitian system.

[0056] During operation, a corresponding feedback voltage is applied to the control electrode 43, and the electro-optical effect of the lithium niobate material is used to change the refractive index of the two "8"-shaped waveguide microcavities, thereby adjusting the resonant frequency difference between the two "8"-shaped waveguide microcavities, so that the temperature sensitive unit 4 is at the singular point of the non-Hermitian system in the initial state, and the sensor is in a working state.

[0057] In the working state, the light emitted by the wide-spectrum light source 1 is detected and received by the photodetector 5 after passing through the coupling lens group 2, the pattern spot converter 3, and the temperature sensitive unit 4 in sequence;

[0058] When the temperature sensitive unit 4 senses a change in ambient temperature, the thermo-optical effect of the material of the temperature sensitive unit 4 causes the refractive index to change, and the expansion of the material of the temperature sensitive unit 4 causes the cavity length of the "8"-shaped waveguide microcavity to change, causing the equivalent optical path of the "8"-shaped waveguide microcavity to change, thereby causing the deviation of the resonant frequency of the "8"-shaped waveguide microcavity. Since the cavity lengths of the two "8"-shaped waveguide microcavities are slightly different, the deviation amounts of the resonant frequencies of the two "8"-shaped waveguide microcavities are also different, causing a change in the resonant frequency difference of the two "8"-shaped waveguide microcavities.

[0059] According to the parameter requirements of the temperature sensitive unit 4 at the singular point in the previous text, the change of this resonant frequency difference causes the temperature sensitive unit 4 to deviate from the singular point of the non-Hermitian system, which manifests as the splitting of the output spectrum in the transmission waveguide 44, bringing a beat frequency signal that changes with temperature. The photodetector 5 detects this beat frequency signal, and the temperature change can be obtained by substituting the corresponding beat frequency-temperature according to the pre-fitted curve, and then combined with the initial temperature to obtain the current temperature.

[0060] No voltage is applied to the control electrode 43, and the singular point optical temperature sensor is in a non-working state.

[0061] The resonant frequency difference between the two "8"-shaped waveguide microcavities is designed as:

[0062] |ω1-ω2|=μ1μ2

[0063] Wherein ω1 and ω2 represent the resonant frequencies of the two “8”-shaped waveguide microcavities, and μ1 and μ2 represent the coupling coefficients between the two “8”-shaped waveguide microcavities and the transmission waveguide 44, respectively;

[0064] According to the relationship between the resonant frequency and the microcavity length, and the calculation formula of the coupling coefficient, the relationship between the cavity lengths of the two "8"-shaped waveguide microcavities is obtained as follows:

[0065]

[0066] Wherein c is the speed of light in vacuum, n is the refractive index of the waveguide material, L1 and L2 represent the cavity lengths of the two "8"-shaped waveguide microcavities, m is the resonance order determined according to the incident wavelength and L1, κ1 and κ2 are the energy coupling ratios between the two "8"-shaped waveguide microcavities and the transmission waveguide 44, which can be flexibly determined within the range of 0-1.

[0067] In actual design, the energy coupling ratio can be set as κ1=κ2, and then the length L1 of the first "8"-shaped waveguide microcavity can be set according to the sensor size requirements. The resonance order m is obtained according to the wavelength of the light source used. Substituting it into the above formula can obtain the length L2 of the second "8"-shaped waveguide microcavity designed.

[0068] There is no coupling when light is transmitted in the straight waveguide 42. The light transmitted in this cross-shaped "8"-shaped waveguide microcavity has clockwise and counterclockwise parts, thereby eliminating the change in the cavity resonance frequency caused by the Sagnac effect brought about by the rotation of the earth and other rotations.

[0069] The two "8"-shaped waveguide microcavities in the temperature sensitive unit 4 have the same loss and are only energy coupled with the middle transmission waveguide 44. The energy coupling is completed through the evanescent wave. There is no direct energy coupling between the two "8"-shaped waveguide microcavities, but indirect coupling is generated through the middle transmission waveguide 44, which is used to construct a non-Hermitian system that satisfies anti-parity-time reversal symmetry.

[0070] The broadband light source 1 , the coupling lens group 2 and the spot converter 3 are coaxially arranged. The output end of the spot converter 3 is connected to the input end of the transmission waveguide 44 of the temperature sensitive unit 4 , and the output end of the transmission waveguide 44 is connected to the input end of the photodetector 5 .

[0071] The positions and heights of the broadband light source 1 and the coupling lens group 2 in the transmission direction are based on the maximum efficiency of light source coupling.

[0072] The broadband light generated by the broadband light source 1 is coupled into the spot converter 3 through the coupling lens group 2 and then enters the input end of the transmission waveguide 44 from the output end of the spot converter 3. A part of the light passing through the transmission waveguide 44 is coupled to the middle of the open racetrack waveguide 41 of the "8"-shaped waveguide microcavity on both sides, and then transmitted along the "8"-shaped cavity of the "8"-shaped waveguide microcavity, and coupled back to the transmission waveguide 44 again. This process is repeated, and another part of the light passing through the transmission waveguide 44 is output to the photodetector 5, and the photodetector 5 detects the corresponding beat frequency signal.

[0073] The mode spot converter 3 is a waveguide with different widths at the input and output ends, and its length is determined by simulation to minimize the light transmission loss. It can be etched together with the transmission waveguide 44. The width of the input end is greater than the width of the output end. It is located after the coupling lens group 2 and adapts to the mode field after being shaped by the coupling lens group 2. The width of the output end is consistent with the transmission waveguide 44 and is connected to the input end of the transmission waveguide 44 to adapt to the general waveguide mode field to improve the energy coupling efficiency.

[0074] The coupling lens group 2 is composed of two or more lenses, and is used to shape the output mode field of the broadband light source 1 into a mode field that is more easily adapted to the end face of the mode spot converter 3, thereby improving the energy coupling efficiency and facilitating the integrated packaging of the sensor chip.

[0075] Broad spectrum light sources include SLD and ASE light sources.

[0076] The singular point optical temperature sensor also includes a silicon substrate 46 and a silicon dioxide buried layer 45. The temperature sensitive unit 4 of the singular point optical temperature sensor is formed by a lithium niobate thin film layer 7. The lithium niobate thin film layer 7 is arranged on the silicon dioxide buried layer 45. The silicon dioxide buried layer 45 is arranged on the silicon substrate 46. The preparation process of the temperature sensor is the same as that of a general optical waveguide sensor. It can be processed together with other optical waveguide sensors and has strong compatibility.

[0077] A temperature detection method of a singular point optical temperature sensor:

[0078] In the initial state, a feedback voltage is applied to the control electrode 43, and the refractive index of the two "8"-shaped waveguide microcavities is changed by utilizing the electro-optical effect of the lithium niobate material, thereby adjusting the resonant frequency difference between the two "8"-shaped waveguide microcavities, so that in the initial state, the temperature sensitive unit 4 is at a non-Hermitian system singular point;

[0079] In the working state, the light emitted by the wide-spectrum light source 1 is detected and received by the photodetector 5 after passing through the coupling lens group 2, the pattern spot converter 3, and the temperature sensitive unit 4 in sequence;

[0080] When the temperature sensitive unit 4 senses a change in ambient temperature, the thermo-optical effect of the material of the temperature sensitive unit 4 causes a change in the refractive index, and the expansion of the material of the temperature sensitive unit 4 causes a change in the cavity length of the "8"-shaped waveguide microcavity, which changes the equivalent optical path of the "8"-shaped waveguide microcavity, causing a deviation in the resonant frequency of the "8"-shaped waveguide microcavity. Since the cavity lengths of the two "8"-shaped waveguide microcavities are slightly different, the deviations of the resonant frequencies of the two "8"-shaped waveguide microcavities are also different, resulting in a change in the difference in the resonant frequencies of the two "8"-shaped waveguide microcavities.

[0081] This change in the resonant frequency difference causes the temperature sensitive unit 4 to deviate from the singularity point of the non-Hermitian system, which manifests itself as a splitting of the output spectrum in the transmission waveguide 44, resulting in a beat frequency signal that changes with temperature. The photodetector 5 detects this beat frequency signal and substitutes the corresponding beat frequency-temperature according to the pre-fitted curve to obtain the temperature change, which is then combined with the initial temperature to obtain the current temperature.

[0082] In a specific embodiment, the temperature sensitive unit 4 of the singular point optical temperature sensor based on beat frequency detection is designed as follows:

[0083] In this example, the wide spectrum light source 1 adopts an SLD light source with a central wavelength of 1550 nanometers and a spectrum width greater than 30 nm. The optical power coupling ratio between the two "8"-shaped waveguide microcavities and the transmission waveguide 44 is 90:10, that is, 90% of the light is coupled from the transmission waveguide 44 into the "8"-shaped waveguide microcavity. According to the conditions of the singular point of the non-Hermitian system, it can be known that the total cavity length of the two "8"-shaped waveguide microcavities needs to satisfy the relationship:

[0084]

[0085] In the formula, c represents the speed of light in vacuum, n represents the refractive index of the waveguide material, L1 and L2 represent the cavity lengths of the two "8"-shaped waveguide microcavities, respectively, and m is the resonance order determined according to the incident wavelength and L1. Specifically, after determining the wavelength λ and the energy coupling ratio between the "8"-shaped waveguide microcavity and the transmission waveguide 44, the length L1 of the first "8"-shaped waveguide microcavity can be given according to the size requirements of the actual sensor, and the resonance order m can be calculated using the resonance condition. Substituting each parameter into the above formula, the design length L2 of the second "8"-shaped waveguide microcavity can be obtained.

[0086] When the ambient temperature changes, due to the thermo-optical effect and thermal expansion effect of the material, the change in the microcavity optical path caused by the temperature change ΔT can be written as:

[0087]

[0088] In the formula, δ(nL 1，2) represents the change in the equivalent optical path of the two “8”-shaped waveguide microcavities caused by temperature change, n represents the refractive index of the waveguide material, L 1，2 They represent the cavity lengths of the two “8”-shaped waveguide microcavities, dn / dT represents the thermo-optic coefficient of the material, and dL 1，2 / dT represents the rate of change of the length of the two "8"-shaped waveguide microcavities with temperature, ΔT represents the change in temperature, and α represents the thermal expansion coefficient of the material.

[0089] Using the relationship between the cavity length and the resonant frequency, the change in resonant frequency caused by temperature can be written as:

[0090]

[0091] In the formula, δω 1，2 Respectively represent the changes in the resonant frequencies of the two “8”-shaped waveguide microcavities, ω 1，2 They represent the resonant frequencies of the two "8"-shaped waveguide microcavities, c represents the speed of light in vacuum, and m is the resonance order.

[0092] It can be seen from the above formula that since the initial resonant frequencies of the two “8”-shaped waveguide microcavities are different, the changes in the resonant frequencies caused by temperature are also different. The temperature change will cause the temperature sensitive unit 4 to deviate from the singular point, causing a change in the output.

[0093] Assuming that ω2>ω1 in this example, the change in the resonant frequency difference between the two “8”-shaped waveguide microcavities caused by temperature is:

[0094]

[0095] In the formula, δ ω represents the change of the resonant frequency difference, δω1 represents the change of the resonant frequency of one of the “8”-shaped waveguide microcavities, δω2 represents the change of the resonant frequency of the other “8”-shaped waveguide microcavity, ω1 represents the resonant frequency of one of the “8”-shaped waveguide microcavities, and ω2 represents the resonant frequency of the other “8”-shaped waveguide microcavity.

[0096] like Figure 3 (a) Plotting the change in the resonant frequency difference δ ω As ΔT changes, it can be seen that a temperature change of only 0.02 degrees Celsius will cause a change in the resonant frequency difference of the two "8"-shaped waveguide microcavities by δ ω In 10 7 rad / s, which will cause a significant change in the output beat frequency.

[0097] Furthermore, the beat frequency change caused by the temperature change can be calculated. At the singular point, the eigenfrequency of the temperature sensitive unit 4 is:

[0098]

[0099] In the formula, ω ± represents the eigenfrequency of the temperature sensitive unit 4, j represents the imaginary unit, and g is the loss coefficient of the two "8"-shaped waveguide microcavities. It can be seen that the two eigenfrequencies of the temperature sensitive unit 4 are the same, and there is no beat frequency. When the temperature changes by ΔT, the eigenfrequency of the temperature sensitive unit 4 changes to:

[0100]

[0101] So the beat frequency signal is:

[0102]

[0103] Where Δω represents the beat frequency signal, represents the coupling coefficient between one of the “8”-shaped waveguide microcavities and the transmission waveguide 44, represents the coupling coefficient between another “8”-shaped waveguide microcavity and the transmission waveguide 44. It can be seen that the change in the beat frequency Δω and the resonant frequency difference δ ω is proportional to the square root of ω It is proportional to the temperature change ΔT, so there is a linear relationship between the square of the beat frequency Δω and the temperature change ΔT.

[0104] Figure 3 (b) The variation of the beat frequency signal with ΔT is plotted.

[0105] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

[0106] The present invention proposes a singular point optical temperature sensor based on beat frequency detection, which is helpful to improve the integration of temperature sensors. The present invention utilizes the characteristics of non-Hermitian optical systems with anti-parity-time reversal symmetric structures to propose a temperature sensing scheme that is easy to detect, and utilizes the characteristics that the singular points of non-Hermitian systems are sensitive to external disturbances, which greatly improves the sensitivity of the temperature sensor. In the design of the temperature sensitive unit 4, two "8"-shaped waveguide microcavities are used to make the light in the "8"-shaped waveguide microcavity have both clockwise and counterclockwise states, offsetting the signal interference that may be caused by the Sagnac effect caused by the rotation of the earth or other rotations, and improving the anti-interference ability of the temperature sensitive unit 4. The present invention has a simple structure, high integration, and strong compatibility. It can be easily integrated with other optical sensor chips and is expected to become a new solution for optical temperature measurement.

Claims

1. A singular point optical temperature sensor based on beat frequency detection, characterized in that: It comprises a wide-spectrum light source (1), a coupling lens group (2), a pattern spot converter (3), a temperature sensitive unit (4), a photodetector (5) and a lithium niobate thin film layer (7) located in a packaging shell (6); The broadband light source (1), coupling lens group (2), pattern spot converter (3), temperature sensitive unit (4) and photoelectric detector (5) are all located on the upper surface of the lithium niobate thin film layer (7) and are arranged in sequence along the light path direction; light emitted by the broadband light source (1) passes through the coupling lens group (2), pattern spot converter (3) and temperature sensitive unit (4) in sequence and is then detected and received by the photoelectric detector (5).

2. The singular point optical temperature sensor according to claim 1, characterized in that: The temperature sensitive unit (4) comprises two microcavity electrode assemblies and a transmission waveguide (44), the two microcavity electrode assemblies are respectively arranged on both sides of the transmission waveguide (44), each microcavity electrode assembly comprises an "8"-shaped waveguide microcavity and a pair of electrode pairs, and the two ends of the transmission waveguide (44) are respectively connected to the output end of the pattern spot converter (3) and the input end of the photodetector (5); The "8"-shaped waveguide microcavity is a waveguide in the shape of "8", and the vertical center line of the "8"-shaped waveguide microcavity is arranged perpendicular to the transmission waveguide (44); The pair of electrodes is composed of two control electrodes (43), and the two control electrodes (43) are arranged on a side of the "8"-shaped waveguide microcavity away from the transmission waveguide (44).

3. The singular point optical temperature sensor according to claim 2, characterized in that: Each of the "8"-shaped waveguide microcavities is mainly composed of two straight waveguides (42) and two open racetrack waveguides (41) arranged in a cross shape, one end of the two straight waveguides (42) is respectively connected to the two ends of an open racetrack waveguide (41), the other ends of the two straight waveguides (42) are respectively connected to the two ends of another open racetrack waveguide (41), and the two control electrodes (43) are respectively arranged on both sides of the middle of an open racetrack waveguide (41) away from the transmission waveguide (44).

4. The singular point optical temperature sensor according to claim 3, characterized in that: The middle portion of the open racetrack waveguide (41) is arranged as a linear waveguide structure parallel to the transmission waveguide (44), and the middle portion of the open racetrack waveguide (41) close to the transmission waveguide (44) is close to the transmission waveguide (44) and coupled with the transmission waveguide (44) through evanescent waves.

5. The singular point optical temperature sensor according to claim 2, characterized in that: The resonant frequency difference between the two "8"-shaped waveguide microcavities is designed as follows: |ω1-ω2|=μ1μ2 Wherein ω1 and ω2 represent the resonant frequencies of the two "8"-shaped waveguide microcavities, and μ1 and μ2 represent the coupling coefficients between the two "8"-shaped waveguide microcavities and the transmission waveguide (44). According to the relationship between the resonant frequency and the microcavity length, and the calculation formula of the coupling coefficient, the relationship between the cavity lengths of the two "8"-shaped waveguide microcavities is obtained as follows: Wherein c is the speed of light in vacuum, n is the refractive index of the waveguide material, L1 and L2 represent the cavity lengths of the two "8"-shaped waveguide microcavities, m is the resonance order determined according to the incident wavelength and L1, and κ1 and κ2 are the energy coupling ratios between the two "8"-shaped waveguide microcavities and the transmission waveguide (44).

6. The singular point optical temperature sensor according to claim 2, characterized in that: The broadband light source (1), the coupling lens group (2) and the pattern spot converter (3) are coaxially arranged; the output end of the pattern spot converter (3) is connected to the input end of the transmission waveguide (44) of the temperature sensitive unit (4); and the output end of the transmission waveguide (44) is connected to the input end of the photodetector (5).

7. The singular point optical temperature sensor according to claim 2, characterized in that: The pattern spot converter (3) is a waveguide with different widths at the input and output ends, the width of the input end is greater than the width of the output end, the width of the output end is consistent with that of the transmission waveguide (44), and is connected to the input end of the transmission waveguide (44).

8. The singular point optical temperature sensor according to claim 1, characterized in that: The singular point optical temperature sensor further comprises a silicon substrate (46) and a silicon dioxide buried layer (45), the lithium niobate thin film layer (7) is arranged on the silicon dioxide buried layer (45), and the silicon dioxide buried layer (45) is arranged on the silicon substrate (46).

9. A temperature detection method applied to the singular point optical temperature sensor according to any one of claims 1 to 8, characterized in that: In the initial state, a feedback voltage is applied to the control electrode (43), and the refractive index of the two "8"-shaped waveguide microcavities is changed by utilizing the electro-optical effect of the lithium niobate material, thereby adjusting the resonant frequency difference between the two "8"-shaped waveguide microcavities, so that in the initial state, the temperature sensitive unit (4) is at a non-Hermitian system singular point; In the working state, light emitted by the wide-spectrum light source (1) passes through the coupling lens group (2), the pattern spot converter (3), and the temperature sensitive unit (4) in sequence, and is then detected and received by the photoelectric detector (5); When the temperature sensitive unit (4) senses a change in ambient temperature, the thermo-optical effect of the material of the temperature sensitive unit (4) causes a change in the refractive index, and the expansion of the material of the temperature sensitive unit (4) causes a change in the cavity length of the "8"-shaped waveguide microcavity, causing a change in the equivalent optical path of the "8"-shaped waveguide microcavity, resulting in a deviation in the resonance frequency of the "8"-shaped waveguide microcavity. Since the cavity lengths of the two "8"-shaped waveguide microcavities are slightly different, the deviation amounts of the resonance frequencies of the two "8"-shaped waveguide microcavities are also different, resulting in a change in the resonance frequency difference of the two "8"-shaped waveguide microcavities. The change of the resonant frequency difference causes the temperature sensitive unit (4) to deviate from the singular point of the non-Hermitian system, which manifests itself as a splitting of the output spectrum in the transmission waveguide (44), resulting in a beat frequency signal that changes with temperature. The photodetector (5) detects the beat frequency signal, and the temperature change can be obtained by substituting the corresponding beat frequency-temperature according to the pre-fitted curve, and then combined with the initial temperature to obtain the current temperature.

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