An optical temperature sensor based on beat frequency detection

Abstract

This invention discloses a singularity optical temperature sensor based on beat frequency detection. It includes a broadband light source, a coupling lens group, a modulus converter, a temperature-sensitive unit, a photodetector, and a lithium niobate thin film layer, all located within a packaged housing. The broadband light source, coupling lens group, modulus converter, temperature-sensitive unit, and photodetector are all located on the upper surface of the lithium niobate thin film layer and arranged sequentially along the optical path. Light emitted from the broadband light source passes sequentially through the coupling lens group, modulus converter, and temperature-sensitive unit before being detected and received by the photodetector. This invention utilizes the electro-optic effect of lithium niobate, adjusting the resonant frequency of the microcavity by applying a voltage to maintain the temperature-sensitive unit at a singularity point in its initial state, thus suppressing the influence of external factors on the sensor. The constructed inverse parity-time reversal symmetric structure utilizes the sensitivity of the singularity point to external perturbations, improving the sensitivity of the temperature sensor, and offering advantages such as high sensitivity, small size, and integrability.

Description

Technical Field

[0001] This invention relates to the field of integrated optics and temperature sensing, and more specifically to a singularity optical temperature sensor based on beat frequency detection. Background Technology

[0002] As one of the fundamental state parameters of an object, temperature measurement is closely related to people's lives. With the development of technology, various intelligent devices and miniature sensors have been widely used. These products are usually affected by operating temperature; temperature changes will affect their accuracy and stability. This is especially true in integrated optical sensors using waveguides as a medium, where the thermal and optical effects of materials and thermal expansion will introduce additional phase into the optical sensing, affecting the measurement of the physical quantities that the sensor is sensitive to. Therefore, accurately measuring and controlling changes in sensor temperature is particularly important.

[0003] Currently, commonly used temperature sensors include thermocouples, thermistors, RTDs (resistance temperature detectors), and ICs (temperature integrated circuits). Each type has its advantages in terms of sensitivity, stability, and power consumption, with RTDs and thermocouples holding the largest market share. However, when used in conjunction with other sensors, they typically can only be mounted on the same substrate for temperature monitoring, resulting in poor integration compatibility. Furthermore, because temperature transfer takes time, the obtained temperature information cannot accurately reflect the sensor's operating temperature, affecting the measurement of the physical quantity being measured.

[0004] In recent years, with the advancement of micro-nano fabrication 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, and temperature, offering advantages such as high sensitivity and resistance to electromagnetic interference. In optical temperature measurement, current mainstream technologies include optical thermometers based on brightness equalization methods, laser spectroscopy temperature measurement, holographic interferometry temperature measurement, and infrared radiation temperature measurement. While these temperature measurement technologies offer advantages such as real-time performance and non-contact operation compared to traditional temperature sensors, they are still fundamentally incompatible with other sensors based on optical waveguides. Summary of the Invention

[0005] Although the thermo-optical and thermal expansion effects of materials can affect the detection of other physical quantities by optical sensors, this phenomenon can be cleverly utilized as a means of detecting temperature changes, enabling the synchronous sensing of temperature and other physical quantities.

[0006] To address the problems in the background art, this invention proposes a singularity optical temperature sensor that utilizes an optical waveguide resonant cavity to construct a non-Hermitian system. This temperature sensor has the advantages of small size, high sensitivity, and ease of integration with optical sensor packaging for detecting other physical quantities.

[0007] This invention's temperature sensor utilizes a transmission waveguide and figure-eight-shaped waveguide microcavities distributed on both sides of the transmission waveguide to construct a non-Hermitian optical system. Utilizing the electro-optic effect of lithium niobate, the refractive index of the two figure-eight-shaped waveguide microcavities is altered by adjusting the voltage applied to the control electrodes, so that the temperature-sensitive unit is initially located at a singularity point of the non-Hermitian system. When the ambient temperature changes, the thermo-optic effect and thermal expansion effect of the material alter the equivalent optical path of the microcavity, causing a change in the microcavity's resonant frequency. The temperature-sensitive unit deviates from the singularity point, resulting in a change in the beat frequency of the light output from the transmission waveguide. By detecting this change in beat frequency, the current temperature change can be obtained. This invention utilizes an integrated optical waveguide to construct a non-Hermitian optical system. By leveraging the sensitivity of the non-Hermitian system's singularity point to environmental disturbances, the sensitivity of the optical sensor is improved. Furthermore, the structure of the waveguide microcavities is fabricated similarly to other optical waveguide sensors, thus allowing integration with other optical waveguide sensors onto a single chip, improving the temperature sensor's compatibility.

[0008] The technical solution adopted in this invention is as follows:

[0009] I. A singularity optical temperature sensor based on beat frequency detection

[0010] It includes a broadband light source, a coupling lens group, a mode converter, a temperature-sensitive unit, a photodetector, and a lithium niobate thin film layer, all located within the package housing.

[0011] The broadband light source, coupling lens group, mode converter, temperature sensitive unit and photodetector are all located on the upper surface of the lithium niobate thin film layer and arranged sequentially along the optical path; the light emitted from the broadband light source passes through the coupling lens group, mode converter and temperature sensitive unit in sequence and is then detected and received by the photodetector.

[0012] The temperature-sensitive unit includes two microcavity electrode assemblies and a transmission waveguide. The two microcavity electrode assemblies are respectively arranged on both sides of the transmission waveguide. Each microcavity electrode assembly includes an "8"-shaped waveguide microcavity and a pair of electrodes. The two ends of the transmission waveguide are respectively connected to the output end of the mode converter and the input end of the photodetector.

[0013] The “8”-shaped waveguide microcavity is a waveguide in the shape of an “8”, with the vertical centerline of the “8”-shaped waveguide microcavity arranged perpendicular to the transmission waveguide;

[0014] The electrode pair consists of two control electrodes arranged on the side of the figure-eight waveguide microcavity away from the transmission waveguide.

[0015] Each of the “8”-shaped waveguide microcavities mainly consists of two straight waveguides arranged in a cross shape and two open racetrack waveguides. One end of each of the two straight waveguides is connected to both ends of one open racetrack waveguide, and the other end of each of the two straight waveguides is connected to both ends of the other open racetrack waveguide. Two control electrodes are respectively arranged on both sides of the middle of one open racetrack waveguide, which is far away from the transmission waveguide.

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

[0017] The design of the resonant frequency difference between the two figure-eight waveguide microcavities is as follows:

[0018] |ω1-ω2|=μ1μ2

[0019] Where ω1 and ω2 represent the resonant frequencies of the two figure-eight waveguide microcavities, and μ1 and μ2 represent the coupling coefficients between the two figure-eight waveguide microcavities and the transmission waveguide, respectively.

[0020] Based on the relationship between the resonant frequency and the microcavity length, and the formula for calculating the coupling coefficient, the relationship between the cavity lengths of the two figure-eight 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 figure-eight waveguide microcavities, m is the resonance order determined by the incident wavelength and L1, and κ1 and κ2 are the energy coupling ratios between the two figure-eight waveguide microcavities and the transmission waveguide, respectively.

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

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

[0025] The singularity optical temperature sensor further includes a silicon substrate and a silicon dioxide buried layer, wherein the lithium niobate thin film layer is disposed on the silicon dioxide buried layer, and the silicon dioxide buried layer is disposed on the silicon substrate.

[0026] II. A Temperature Detection Method for a Singularity 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 figure-eight waveguide microcavities is changed by utilizing the electro-optic effect of lithium niobate material, thereby adjusting the resonant frequency difference between the two figure-eight waveguide microcavities, so that the temperature-sensitive unit is at the non-Hermitian singularity point in the initial state.

[0028] In operation, the light emitted from the broadband light source passes sequentially through the coupling lens group, the mode converter, and the temperature-sensitive unit before being detected and received by the photodetector.

[0029] When the temperature-sensitive unit senses a change in ambient temperature, the thermo-optical effect of the temperature-sensitive unit material causes a change in refractive index. The expansion of the temperature-sensitive unit material causes a change in the cavity length of the figure-eight waveguide microcavity, which in turn changes the equivalent optical path of the figure-eight waveguide microcavity, causing a deviation in the resonant frequency of the figure-eight waveguide microcavity. Since the cavity lengths of the two figure-eight waveguide microcavities are slightly different, the deviations in the resonant frequencies of the two figure-eight waveguide microcavities are also different, resulting in a change in the resonant frequency difference between the two figure-eight waveguide microcavities.

[0030] This change in resonant frequency difference causes the temperature-sensitive element to deviate from the singularity of the non-Hermitian system, which manifests as a split in the output spectrum in the transmission waveguide, resulting in a beat frequency signal that varies with temperature. The photodetector detects this beat frequency signal, and by substituting the corresponding beat frequency-temperature into the pre-fitted curve, the amount of temperature change can be obtained. Then, combined with the initial temperature, the current temperature can be obtained.

[0031] The beneficial effects of this invention are:

[0032] This invention integrates all components onto an optical chip, which reduces the size of the temperature sensor compared to traditional optical temperature measurement methods. It has a high degree of integration, and the temperature sensing unit based on optical waveguides can be easily integrated with other optical sensors, resulting in strong compatibility.

[0033] This invention constructs two figure-eight waveguide microcavities with the same loss. The two figure-eight waveguide microcavities are indirectly coupled through a transmission waveguide in the middle. It is a typical non-Hermitian system that satisfies the inverse parity-time reversal symmetry condition. It can effectively utilize the characteristic that the singularity of non-Hermitian system is sensitive to external disturbances to improve the sensitivity of temperature sensor.

[0034] The two figure-eight waveguide microcavities constructed in this invention allow light to travel in both clockwise and counterclockwise directions simultaneously, effectively eliminating the interference of the Sagnac effect caused by the Earth's rotation and other possible rotations on the temperature-sensitive unit, thereby improving the signal-to-noise ratio and anti-interference capability of the sensor.

[0035] The figure-eight waveguide microcavity of the present invention uses two cross-shaped straight waveguides to connect two open racetrack-shaped waveguides, which avoids energy crosstalk that may occur when light is transmitted in the two straight waveguides and ensures the unidirectionality of light transmission in the microcavity.

[0036] The control electrode of this invention utilizes the electro-optic effect of lithium niobate material. By applying different voltages to adjust the refractive index of the microcavity electrode coverage area, the optical path of the microcavity is changed. Therefore, the resonant frequency of the microcavity can be precisely adjusted, keeping the temperature-sensitive unit at the singular point in the initial state. This suppresses the influence of fabrication errors or other factors such as the environment on the sensor performance caused by the temperature-sensitive unit deviating from the singular point, and reduces the requirements for waveguide fabrication process. Attached Figure Description

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

[0038] Figure 2 This is a schematic diagram of the temperature-sensitive unit structure in this invention;

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

[0040] In the figure: 1. Broadband light source, 2. Coupled lens group, 3. Mode converter, 4. Temperature sensitive unit, 5. Photodetector, 6. Package shell, 7. Lithium niobate thin film layer, 41. Open racetrack waveguide, 42. Straight waveguide, 43. Control electrode, 44. Transmission waveguide, 45. Silica buried layer, 46. Silicon substrate. Detailed Implementation

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

[0042] like Figure 1 , Figure 2 As shown, the singularity optical temperature sensor includes a broadband light source 1, a coupling lens group 2, a speckle converter 3, a temperature sensing unit 4, a photodetector 5, and a lithium niobate thin film layer 7, all located within the package housing 6.

[0043] The broadband light source 1, the coupling lens group 2, the mode 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 arranged sequentially along the optical path direction; the light emitted from the broadband light source 1 passes through the coupling lens group 2, the mode converter 3, and the temperature sensitive unit 4 in sequence before being detected and received by the photodetector 5.

[0044] All waveguide structures of the mode converter 3 and temperature-sensitive unit 4 are formed by etching of lithium niobate thin film layer 7. Broadband light source 1, coupling lens group 2 and photodetector 5 are located in the fixed groove formed by etching of 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 strip-shaped. The two microcavity electrode assemblies are arranged approximately symmetrically 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 mode converter 3 and the input end of the photodetector 5.

[0046] The waveguide is a figure-eight shaped microcavity. The vertical center line of the two internal rings of the figure-eight shaped waveguide microcavity is arranged perpendicular to the transmission waveguide 44.

[0047] A pair of electrodes consists of two control electrodes 43, which are arranged on the side of the figure-eight waveguide microcavity away from the transmission waveguide 44.

[0048] Each figure-eight waveguide microcavity mainly consists of two straight waveguides 42 arranged in a cross shape and two open racetrack waveguides 41. One end of each of the two straight waveguides 42 is connected to both ends of one open racetrack waveguide 41, and the other end of each of the two straight waveguides 42 is connected to both ends of the other open racetrack waveguide 41. Two control electrodes 43 are respectively arranged on both sides of the middle of one open racetrack waveguide 41, which is far away from the transmission waveguide 44.

[0049] The middle part of the open racetrack waveguide 41 is configured as a straight waveguide structure parallel to the transmission waveguide 44. The middle part of the open racetrack waveguide 41 near the transmission waveguide 44 is close to the transmission waveguide 44 and is coupled through an evanescent wave.

[0050] The spacing between the figure-eight waveguide microcavity and the transmission waveguide 44 determines their coupling coefficient. The temperature-sensitive element 4 is located at a singularity, requiring the coupling coefficient to satisfy a relationship with the resonant frequency of the waveguide microcavity:

[0051] |ω1-ω2|=μ1μ2

[0052] Where ω1 and ω2 represent the resonant frequencies of the two figure-eight waveguide microcavities, and μ1 and μ2 represent the coupling coefficients between the two figure-eight waveguide microcavities and the transmission waveguide 44.

[0053] In practical applications, ensuring that the temperature-sensitive unit 4 is located at a singular point places high demands on the manufacturing process. Due to the existence of micro-nano fabrication errors, the aforementioned relationship is often difficult to satisfy. Furthermore, interference from external factors such as the environment often causes it to deviate from the singular point, affecting temperature detection. The control electrode 43, by applying a corresponding feedback voltage, utilizes the electro-optic effect of lithium niobate material to change the refractive index of the two figure-eight waveguide microcavities. Therefore, the resonant frequency of the waveguide microcavities can be adjusted so that the temperature-sensitive unit 4 is initially located at a non-Hermitian singular point. The design of the control electrode 43 reduces the requirements for waveguide fabrication 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 preparation errors and interference from external factors such as the environment, the temperature-sensitive unit 4 may deviate from the singularity of the non-Hermitian system.

[0056] During operation, a corresponding feedback voltage is applied to the control electrode 43, and the refractive index of the two figure-eight waveguide microcavities is changed by utilizing the electro-optic effect of lithium niobate material, thereby adjusting the resonant frequency difference between the two figure-eight waveguide microcavities, so that the temperature sensing unit 4 is in the non-Hermitian singularity point in the initial state, and the sensor is in the working state.

[0057] In operation, the light emitted from the broadband light source 1 passes sequentially through the coupling lens group 2, the mode converter 3, and the temperature sensitive unit 4 before being detected and received by the photodetector 5.

[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 a change in refractive index. The expansion of the material of the temperature-sensitive unit 4 causes a change in the cavity length of the figure-eight waveguide microcavity, which in turn changes the equivalent optical path of the figure-eight waveguide microcavity. This causes a deviation in the resonant frequency of the figure-eight waveguide microcavity. Since the cavity lengths of the two figure-eight waveguide microcavities are slightly different, the deviation in the resonant frequency of the two figure-eight waveguide microcavities is also different, causing a change in the resonant frequency difference between the two figure-eight waveguide microcavities.

[0059] Based on the parameter requirements of the temperature-sensitive unit 4 located at the singular point mentioned above, this change in resonant frequency difference causes the temperature-sensitive unit 4 to deviate from the singular point of the non-Hermitian system. In the transmission waveguide 44, this manifests as a splitting of the output spectrum, resulting in a beat frequency signal that changes with temperature. The photodetector 5 detects this beat frequency signal, and by substituting the corresponding beat frequency-temperature into the pre-fitted curve, the amount of temperature change can be obtained. Then, combined with the initial temperature, the current temperature can be obtained.

[0060] When no voltage is applied to the control electrode 43, the singularity optical temperature sensor is in a non-operating state.

[0061] The design of the resonant frequency difference between the two figure-eight waveguide microcavities is as follows:

[0062] |ω1-ω2|=μ1μ2

[0063] Where ω1 and ω2 represent the resonant frequencies of the two figure-eight waveguide microcavities, and μ1 and μ2 represent the coupling coefficients between the two figure-eight waveguide microcavities and the transmission waveguide 44, respectively.

[0064] Based on the relationship between the resonant frequency and the microcavity length, and the formula for calculating the coupling coefficient, the relationship between the cavity lengths of the two figure-eight waveguide microcavities is obtained as follows:

[0065]

[0066] 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 figure-eight waveguide microcavities, m is the resonance order determined based on the incident wavelength and L1, and κ1 and κ2 are the energy coupling ratios between the two figure-eight waveguide microcavities and the transmission waveguide 44, which can be flexibly determined within the range of 0-1.

[0067] In practical design, the energy coupling ratio can be set first with κ1 = κ2. Then, the length L1 of the first figure-eight waveguide microcavity can be set according to the sensor size requirements. The resonant order m can be obtained according to the wavelength of the light source used. Substituting it into the above formula, the length L2 of the second figure-eight waveguide microcavity can be obtained.

[0068] Light does not couple when it propagates in the straight waveguide 42. The light propagating in this cross-shaped “8” waveguide microcavity has clockwise and counterclockwise components, thus eliminating the change in cavity resonant frequency caused by the Sagnac effect due to the Earth’s rotation and other rotations.

[0069] The two figure-eight 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 accomplished through evanescent waves. There is no direct energy coupling between the two figure-eight waveguide microcavities, but indirect coupling is generated through the middle transmission waveguide 44. This 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 mode converter 3 are arranged coaxially. The output end of the mode 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 position and height of the broadband light source 1 and the coupling lens group 2 in the transmission direction are determined by maximizing the coupling efficiency of the light source.

[0072] The broadband light generated by the broadband light source 1 is coupled into the mode 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 mode converter 3. A portion of the light passing through the transmission waveguide 44 is coupled to the middle of the open racetrack-shaped waveguide 41 of the figure-eight waveguide microcavities on both sides, and then propagates along the figure-eight cavity of the figure-eight waveguide microcavities, and is coupled back to the transmission waveguide 44 again. This process is repeated. The other portion 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 converter 3 is a waveguide with different widths at the input and output ends. Its length is determined by simulation to minimize optical transmission loss. It can be etched together with the transmission waveguide 44. The width at the input end is greater than the width at 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 at the output end is consistent with that of the transmission waveguide 44 and is connected to the input end of the transmission waveguide 44 to adapt to general waveguide mode fields and improve energy coupling efficiency.

[0074] The coupling lens group 2 consists 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 converter 3, thereby improving energy coupling efficiency and facilitating the integration and packaging of the sensor chip.

[0075] Broadband light sources include SLD and ASE light sources.

[0076] The singularity optical temperature sensor also includes a silicon substrate 46 and a silicon dioxide buried layer 45. The temperature-sensitive unit 4 of the singularity optical temperature sensor is formed by a lithium niobate thin film layer 7, which is disposed on the silicon dioxide buried layer 45, which is disposed on the silicon substrate 46. The fabrication process of this temperature sensor is the same as that of a general optical waveguide sensor, and it can be processed together with other optical waveguide sensors, thus having strong compatibility.

[0077] A temperature detection method for a singularity 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 figure-eight waveguide microcavities is changed by utilizing the electro-optic effect of lithium niobate material, thereby adjusting the resonant frequency difference between the two figure-eight waveguide microcavities, so that the temperature sensitive unit 4 is at the non-Hermitian singularity point in the initial state.

[0079] In operation, the light emitted from the broadband light source 1 passes sequentially through the coupling lens group 2, the mode converter 3, and the temperature sensitive unit 4 before being detected and received by the photodetector 5.

[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 refractive index. The expansion of the material of the temperature-sensitive unit 4 causes a change in the cavity length of the figure-eight waveguide microcavity, which in turn changes the equivalent optical path of the figure-eight waveguide microcavity, causing a deviation in the resonant frequency of the figure-eight waveguide microcavity. Since the cavity lengths of the two figure-eight waveguide microcavities are slightly different, the deviation in the resonant frequency of the two figure-eight waveguide microcavities is also different, resulting in a change in the resonant frequency difference between the two figure-eight waveguide microcavities.

[0081] This change in resonant frequency difference causes the temperature-sensitive unit 4 to deviate from the singularity of the non-Hermitian system, which manifests as a split in the output spectrum in the transmission waveguide 44, resulting in a beat frequency signal that varies with temperature. The photodetector 5 detects this beat frequency signal, and by substituting the corresponding beat frequency-temperature into the pre-fitted curve, the amount of temperature change can be obtained. Then, combined with the initial temperature, the current temperature can be obtained.

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

[0083] In this example, broadband light source 1 uses an SLD light source with a center wavelength of 1550 nm and a spectral width greater than 30 nm. The optical power coupling ratio between the two figure-eight waveguide microcavities and the transmission waveguide 44 is 90:10, meaning that 90% of the light is coupled from the transmission waveguide 44 into the figure-eight waveguide microcavities. According to the conditions for singularities in non-Hermitian systems, the total cavity length of the two figure-eight waveguide microcavities must satisfy the following 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 figure-eight waveguide microcavities, and m is the resonance order determined based on the incident wavelength and L1. Specifically, after determining the wavelength λ and the energy coupling ratio between the figure-eight waveguide microcavity and the transmission waveguide 44, the length L1 of the first figure-eight waveguide microcavity can be given according to the actual sensor size requirements. The resonance order m can be calculated using the resonance condition. Substituting the parameters into the above formula, the design length L2 of the second figure-eight 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 optical path of the microcavity caused by the temperature change ΔT can be written as:

[0087]

[0088] In the formula, δ(nL) 1，2The ) represents the change in the equivalent optical path of the two figure-eight waveguide microcavities caused by temperature changes, n represents the refractive index of the waveguide material, and L represents the change in the equivalent optical path of the two figure-eight waveguide microcavities. 1，2 dn / dT represents the cavity lengths of the two figure-eight waveguide microcavities, dn / dT represents the thermo-optic coefficient of the material, and dL represents the cavity lengths of the two figure-eight waveguide microcavities. 1，2 / dT represents the rate of change of the cavity length of the two figure-eight waveguide microcavities with temperature, ΔT represents the amount of temperature change, and α represents the coefficient of thermal expansion 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 ω represents the change in the resonant frequency of the two figure-eight waveguide microcavities, respectively. 1，2 These represent the resonant frequencies of the two figure-eight waveguide microcavities, respectively, where c represents the speed of light in a vacuum, and m is the resonant order.

[0092] As can be seen from the above equation, since the initial resonant frequencies of the two figure-eight waveguide microcavities are different, the amount of change in the resonant frequency caused by temperature is also different. Temperature change will cause the temperature-sensitive unit 4 to deviate from the singular point, resulting in a change in the output.

[0093] Assuming ω2 > ω1 in this example, the change in the resonant frequency difference between the two figure-eight waveguide microcavities caused by temperature is:

[0094]

[0095] In the formula, δ ω The values ​​represent the change in resonant frequency difference. δω1 represents the change in the resonant frequency of one of the figure-eight waveguide microcavities, and δω2 represents the change in the resonant frequency of the other figure-eight waveguide microcavity. ω1 represents the resonant frequency of one of the figure-eight waveguide microcavities, and ω2 represents the resonant frequency of the other figure-eight waveguide microcavity.

[0096] like Figure 3 (a) The change in resonant frequency difference δ was plotted. 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 δ between the two figure-eight waveguide microcavities. ω In 10 7 The magnitude is on the order of rad / s, which will cause a significant change in the output beat frequency.

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

[0098]

[0099] In the formula, ω ± Let represent the eigenfrequency of temperature-sensitive element 4, j represent the imaginary unit, and g be the loss coefficient of the two figure-eight waveguide microcavities. It can be seen that the two eigenfrequencys of temperature-sensitive element 4 are the same, and there is no beat frequency. When the temperature changes by ΔT, the eigenfrequency of temperature-sensitive element 4 changes as follows:

[0100]

[0101] Therefore, the beat frequency signal is obtained:

[0102]

[0103] Where Δω represents the beat frequency signal. This represents the coupling coefficient between one of the figure-eight waveguide microcavities and the transmission waveguide 44. This represents the coupling coefficient between another figure-eight waveguide microcavity and transmission waveguide 44. It can be seen that the change in beat frequency Δω is related to the change in resonant frequency difference δ. ω It is proportional to the square root, while δ ω It is proportional to the temperature change ΔT, therefore 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 was plotted.

[0105] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

[0106] This invention proposes a singularity-based optical temperature sensor, which helps improve the integration of temperature sensors. Utilizing the characteristics of a non-Hermitian optical system with an inverse parity-time reversal symmetry structure, this invention proposes a convenient temperature sensing scheme. Furthermore, by leveraging the sensitivity of the singularity of a non-Hermitian system to external disturbances, the sensitivity of the temperature sensor is greatly enhanced. In the design of the temperature-sensitive unit 4, two figure-eight waveguide microcavities are used, allowing light to simultaneously exhibit both clockwise and counterclockwise states within the microcavities. This cancels out signal interference that may be caused by the Sagnac effect due to the Earth's rotation or other rotations, improving the anti-interference capability of the temperature-sensitive unit 4. This invention has a simple structure, high integration, and strong compatibility, allowing for easy integration with other optical sensing chips, and is expected to become a novel solution for optical temperature measurement.

Claims

1. A singularity optical temperature sensor based on beat frequency detection, characterized in that: It includes a broadband light source (1), a coupling lens group (2), a mode converter (3), a temperature-sensitive unit (4), a photodetector (5), and a lithium niobate thin film layer (7) located in the package housing (6). The broadband light source (1), coupling lens group (2), mode converter (3), temperature sensitive unit (4) and photodetector (5) are all located on the upper surface of the lithium niobate thin film layer (7) and arranged sequentially along the optical path direction; the light emitted from the broadband light source (1) passes through the coupling lens group (2), mode converter (3) and temperature sensitive unit (4) in sequence and is then detected and received by the photodetector (5); The temperature-sensitive unit (4) includes 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 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 mode converter (3) and the input end of the photodetector (5). The “8”-shaped waveguide microcavity is a waveguide in the shape of an “8”, and the vertical center line of the “8”-shaped waveguide microcavity is arranged perpendicular to the transmission waveguide (44); The electrode pair consists of two control electrodes (43), which are arranged on the side of the figure-eight waveguide microcavity away from the transmission waveguide (44).

2. The singularity optical temperature sensor as described in claim 1, characterized in that: Each of the “8”-shaped waveguide microcavities is mainly composed of two straight waveguides (42) arranged in a cross shape and two open racetrack waveguides (41). One end of each of the two straight waveguides (42) is connected to both ends of one open racetrack waveguide (41), and the other end of each of the two straight waveguides (42) is connected to both ends of the other open racetrack waveguide (41). Two control electrodes (43) are respectively arranged on both sides of the middle of one open racetrack waveguide (41) away from the transmission waveguide (44).

3. The singularity optical temperature sensor as described in claim 2, characterized in that: The middle part of the open racetrack waveguide (41) is set as a straight waveguide structure parallel to the transmission waveguide (44). The middle part of the open racetrack waveguide (41) close to the transmission waveguide (44) is close to the transmission waveguide (44) and is coupled through evanescent waves.

4. The singularity point optical temperature sensor as described in claim 1, characterized in that: The design of the resonant frequency difference between the two figure-eight waveguide microcavities is as follows: in These represent the resonant frequencies of the two figure-eight waveguide microcavities. These represent the coupling coefficients between the two figure-eight waveguide microcavities and the transmission waveguide (44), respectively; Based on the relationship between the resonant frequency and the microcavity length, and the formula for calculating the coupling coefficient, the relationship between the cavity lengths of the two figure-eight waveguide microcavities is obtained as follows: in The speed of light in a vacuum The refractive index of the waveguide material, , These represent the cavity lengths of the two figure-eight waveguide microcavities. It is based on the incident wavelength and A given resonance order, , These represent the energy coupling ratios between the two figure-eight waveguide microcavities and the transmission waveguide (44).

5. The singularity point optical temperature sensor as described in claim 1, characterized in that: The broadband light source (1), the coupling lens group (2) and the speckle converter (3) are arranged coaxially. The output end of the speckle 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).

6. The singularity point optical temperature sensor as described in claim 1, characterized in that: The modal converter (3) is a waveguide with different widths at the input and output ends. The width at the input end is greater than the width at the output end, and the width at the output end is consistent with that of the transmission waveguide (44) and is connected to the input end of the transmission waveguide (44).

7. The singularity optical temperature sensor as described in claim 1, characterized in that: The singularity optical temperature sensor further includes a silicon substrate (46) and a silicon dioxide buried layer (45), wherein the lithium niobate thin film layer (7) is disposed on the silicon dioxide buried layer (45) and the silicon dioxide buried layer (45) is disposed on the silicon substrate (46).

8. A temperature detection method applied to the singularity optical temperature sensor according to any one of claims 1-7, characterized in that: In the initial state, a feedback voltage is applied to the control electrode (43), and the refractive index of the two figure-eight waveguide microcavities is changed by the electro-optic effect of lithium niobate material, thereby adjusting the resonant frequency difference between the two figure-eight waveguide microcavities, so that the temperature sensitive unit (4) is at the non-Hermitian singularity point in the initial state. In operation, the light emitted from the broadband light source (1) passes through the coupling lens group (2), the mode converter (3), and the temperature sensitive unit (4) in sequence before being detected and received by the photodetector (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 refractive index. The expansion of the material of the temperature-sensitive unit (4) causes a change in the cavity length of the figure-eight waveguide microcavity, which in turn changes the equivalent optical path of the figure-eight waveguide microcavity, causing a deviation in the resonant frequency of the figure-eight waveguide microcavity. Since the cavity lengths of the two figure-eight waveguide microcavities are slightly different, the deviation of the resonant frequency of the two figure-eight waveguide microcavities is also different, resulting in a change in the resonant frequency difference between the two figure-eight waveguide microcavities. This change in resonant frequency difference causes the temperature-sensitive unit (4) to deviate from the singularity of the non-Hermitian system, which manifests as a split in 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 the temperature change can be obtained by substituting the corresponding beat frequency-temperature into the pre-fitted curve. The current temperature can then be obtained by combining the initial temperature.

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