An arbitrary band frequency stabilization device and method based on a dual interferometer

Abstract

This invention discloses an arbitrary-band frequency stabilization device based on a dual interferometer, comprising: a dual-fiber interferometer module for generating interference signals related to temperature and frequency changes; a temperature control module for temperature control of the dual interferometer module; and a control module for receiving the interference signals and outputting control commands accordingly. The dual interferometer module includes an equal-arm interferometer and an unequal-arm interferometer. The equal-arm interferometer responds to temperature changes and outputs a first interference signal; the unequal-arm interferometer responds to frequency changes and outputs a second interference signal. The control module receives the first interference signal and outputs a first control command to the temperature control module to improve temperature control accuracy. The control module receives the second interference signal and outputs a second control command to a laser to adjust the laser frequency. This device can effectively reduce costs while maintaining a stable and continuously tunable laser frequency.

Description

Technical Field

[0001] This invention relates to the field of quantum precision measurement, specifically to an arbitrary band frequency stabilization device and method based on a dual interferometer. Background Technology

[0002] The stability of laser frequency is crucial in many applications, such as precision spectroscopy, quantum information processing, and high-resolution optical sensing. Currently, Pound-Drever-Hall (PDH) technology combined with precise optical cavities (such as Fabry-Perot cavities) and saturable absorption spectroscopy is commonly used to control frequency fluctuations, thereby reducing laser frequency ripple and achieving narrower linewidths. While PDH combined with optical cavities can achieve extremely high frequency locking precision, the high cost of the precision optical cavity used as the frequency reference and its extreme sensitivity to external disturbances such as vibration and temperature changes necessitate complex vibration isolation and temperature control systems to ensure performance, placing extremely stringent requirements on the experimental environment. Saturable absorption spectroscopy uses atomic and molecular spectral lines as absolute frequency references, giving it excellent long-term stability; however, it is also limited by this limitation—the number of molecular and atomic spectral lines available for reference in nature is extremely limited. This means it can only lock onto discrete, specific frequencies, lacking the flexibility to stably lock frequencies across any wavelength band. When lasers need to maintain long-term stability in frequency ranges far from atomic and molecular spectral lines, although their linewidth stability does not need to reach the level of PDH ultrafine frequency locking, it is still necessary to ensure that the laser frequency can be stably controlled across all bands. Therefore, a low-cost frequency stabilization device applicable to all frequency bands is needed to achieve stable frequency control of the laser over long periods of time. Summary of the Invention

[0003] The purpose of this invention is to overcome the shortcomings of the prior art and to propose an arbitrary band frequency stabilization device and method based on a dual interferometer, which can effectively reduce costs while maintaining the laser frequency stable and continuously tunable.

[0004] To achieve the above objectives, the technical solution specifically adopted by the present invention is as follows:

[0005] An arbitrary band frequency stabilization device based on a dual interferometer includes:

[0006] The dual-fiber interferometer module is used to generate interference signals related to temperature and frequency changes;

[0007] A temperature control module is used to control the temperature of the dual interferometer module;

[0008] The control module is used to receive interference signals and output control commands accordingly.

[0009] The dual interferometer module includes an equal-arm interferometer and an unequal-arm interferometer;

[0010] The equal-arm interferometer is used to respond to temperature changes and output a first interference signal;

[0011] The unequal-arm interferometer is used to respond to frequency changes and output a second interference signal;

[0012] The control module receives the first interference signal and outputs a first control command to the temperature control module to improve the temperature control accuracy.

[0013] The control module receives the second interference signal and outputs a second control command to the laser to adjust the laser frequency.

[0014] Preferably, both the equal-arm interferometer and the unequal-arm interferometer are Mach-Zehnder fiber interferometers.

[0015] Preferably, the unequal-arm interferometer includes a reference arm and an experimental arm, the optical path difference of which is provided by an additional optical fiber of fixed length; the reference arm and experimental arm of the equal-arm interferometer are of equal length.

[0016] Preferably, the optical path difference of the unequal-arm interferometer is 3 meters; the arm length of the equal-arm interferometer is 10 meters.

[0017] Preferably, the experimental arms of the unequal-arm interferometer and the equal-arm interferometer are wound together on the same heat-conducting surface to sense the same temperature field.

[0018] Preferably, the heat conductor is a copper cylinder with heating elements attached to its upper and lower surfaces; the temperature control module controls the heating elements to heat the copper cylinder.

[0019] Preferably, the copper cylinder and the experimental arm wound around it are placed in an insulated box made of extruded polystyrene board with low thermal conductivity.

[0020] Preferably, the control module includes a first PID module and a second PID module;

[0021] The first PID module is used to calculate and output the first control command to the setpoint of the temperature control module based on the first interference signal;

[0022] The second PID module is used to calculate and output the second control command to the laser based on the second interference signal.

[0023] Preferably, the device further includes a differential detection unit for converting the optical interference signals output by the equal-arm interferometer and the unequal-arm interferometer into electrical signals.

[0024] Preferably, an optical isolator, a half-wave plate, a polarization beam splitter, an optical fiber coupler, and several beam splitters are sequentially arranged between the laser and the dual-fiber interferometer module.

[0025] This invention also provides an arbitrary band frequency stabilization method based on a dual interferometer, comprising the following steps:

[0026] Step 1: Construct a dual-fiber interferometer consisting of an equal-arm interferometer and an unequal-arm interferometer, wherein the experimental arms of the equal-arm interferometer and the unequal-arm interferometer are located in the same controlled temperature region;

[0027] Step 2: Control the temperature of the controlled temperature area to a preset value;

[0028] Step 3: The polarized light output from the laser is split by an optical fiber beam splitter and input into an equal-arm interferometer and an unequal-arm interferometer respectively, thereby obtaining the first interference signal output by the equal-arm interferometer and the second interference signal output by the unequal-arm interferometer.

[0029] Step 4: Based on the first interference signal, a temperature control compensation signal is generated by the first PID module. The zero point of the first interferometer signal is used as the set value, and the actual output signal of the first interferometer is compared with it to generate an error signal. Then, the temperature of the controlled temperature area is controlled by the PID algorithm in a closed loop, so as to stabilize the temperature of the controlled temperature area within a fluctuation range better than that when only temperature control is used.

[0030] Step 5: Based on the second interference signal, a frequency control signal is generated by the second PID module. The zero point of the second interferometer signal is used as the set value, and the actual output signal of the second interferometer is compared with it to generate an error signal. Then, the frequency of the laser is controlled by the PID algorithm in a closed loop to achieve frequency stabilization.

[0031] Preferably, the relationship between the interference phase φ of the unequal-arm interferometer and the laser frequency f satisfies:

[0032]

[0033] Where n is the refractive index of the optical fiber, L is the fixed optical path difference between the two arms of the interferometer, c is the speed of light, and f is the laser frequency.

[0034] Preferably, in step 3, the laser emitted by the laser is sequentially passed through an optical isolator, a half-wave plate, and a polarization beam splitter, splitting it into two polarized beams. One of these polarized beams is selected as the input beam and coupled into a polarization-maintaining fiber via an optical fiber coupler. This beam is then split by a first optical fiber beam splitter, and then enters different Mach-Zehnder fiber interference paths through second and third optical fiber beam splitters: interference path one has unequal arm lengths of 3 meters, and interference path two has equal arm lengths of 10 meters. Finally, the two optical signals are combined by fourth and fifth optical fiber combiners, respectively, and output as a first interference signal and a second interference signal.

[0035] This invention has the following characteristics and beneficial effects:

[0036] First, using a differential detector to collect phase information and converting the interference phase change into a voltage amplitude change is beneficial for data acquisition and analysis.

[0037] 2. Temperature changes can be monitored using an equal-arm interferometer, whose equal-arm characteristic can effectively suppress common-mode gain such as frequency changes.

[0038] Third, the output of the equal-arm interferometer is used as the temperature bias, and the output of the temperature sensor is used as the temperature reference. The two are fused according to a certain weight to improve the temperature control accuracy and make the output light intensity of the equal-arm interferometer fluctuate within the linear region.

[0039] Fourth, use an unequal-arm interferometer to monitor frequency changes and enhance the frequency response by increasing the optical path difference.

[0040] Fifth, the output of the unequal-arm interferometer is combined with the PID algorithm to perform closed-loop feedback of the laser frequency, so as to achieve long-term stable frequency control.

[0041] 6. The experimental arms of the unequal-arm interferometer and the equal-arm interferometer are wound together on the surface of a copper cylinder to sense the same, uniform, and stable temperature field.

[0042] 7. Attach two identical heating elements to the upper and lower surfaces of a copper cylinder, respectively. Through the design of a symmetrical structure, the cylinder and its symmetrical points will perceive the same temperature change.

[0043] 8. The arm length of the equal-arm interferometer is 3.33 times that of the unequal-arm interferometer, so that the equal-arm interferometer has a stronger temperature response.

[0044] 9. Using an interferometer with mixed arm lengths to control the laser frequency enables separate monitoring of temperature and frequency changes, thereby improving frequency stabilization accuracy. Attached Figure Description

[0045] Figure 1 This is a system block diagram according to an embodiment of the present invention;

[0046] Figure 2 This is a device diagram according to an embodiment of the present invention;

[0047] Figure 3 This is a temperature response diagram of an equal-arm interferometer according to an embodiment of the present invention;

[0048] Figure 4 This is a frequency response diagram of an equal-arm interferometer according to an embodiment of the present invention;

[0049] Figure 5 This is a temperature response diagram of an unequal-arm interferometer according to an embodiment of the present invention;

[0050] Figure 6 This is a frequency response diagram of the unequal-arm interferometer according to an embodiment of the present invention;

[0051] Figure 7 This is a diagram showing the PID effect of the unequal-arm interferometer in an embodiment of the present invention.

[0052] Figure 8 This is a theoretical prediction diagram for an embodiment of the present invention. Detailed Implementation

[0053] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0054] Example 1

[0055] This embodiment provides an arbitrary band frequency stabilization device based on a dual interferometer, such as... Figure 1 As shown, the device includes a dual-fiber interferometer module, a temperature control module, and a control module.

[0056] Specifically, the dual-fiber interferometer module includes an equal-arm interferometer and an unequal-arm interferometer, both of which are Mach-Zehnder fiber interferometers.

[0057] Furthermore, the unequal-arm interferometer consists of a reference arm and an experimental arm, with the experimental arm being 3 meters longer than the reference arm, forming a fixed optical path difference to enhance the response to changes in laser frequency.

[0058] Equal-arm interferometer: Its reference arm and experimental arm are of equal length, both 10 meters, with a symmetrical structure. It is used to suppress common-mode noise such as frequency and mainly responds to temperature changes.

[0059] In a further embodiment, the experimental arms of the equal-arm interferometer and the unequal-arm interferometer are located in the same controlled temperature region.

[0060] Specifically, the experimental arms of the two interferometers are uniformly wound around the surface of the same copper cylinder. The cylinder has a diameter of 10 cm and a thickness of 2 cm, and heating plates are attached to its upper and lower surfaces to achieve a uniform and stable temperature field.

[0061] It should be noted that in this embodiment, the experimental arms are uniformly wound around the surface of a copper cylinder to obtain a consistent, uniform, and stable temperature field. To further ensure the stability of the temperature field, the copper block is placed in a foam box made of extruded polystyrene board with low thermal conductivity to minimize the impact of external heat interaction on temperature stability. The entire apparatus is also placed in an insulated box made of extruded polystyrene board with low thermal conductivity to prevent the reference arms from being affected by changes in ambient temperature.

[0062] Furthermore, the temperature control module includes a heating device, a temperature control device, and a temperature sensing device.

[0063] In this embodiment, the heating device uses a thermoelectric cooler (TEC), which achieves heating or cooling by changing the direction of the current;

[0064] The temperature sensing device is a 10k negative temperature coefficient thermistor, which has high sensitivity;

[0065] The temperature control unit uses an HTC1500 controller, which adjusts the TEC power based on the voltage signal fed back by the thermistor to achieve closed-loop temperature control with a temperature control accuracy of up to 0.0015℃.

[0066] Furthermore, the control module includes a first PID module and a second PID module:

[0067] The first PID module is used to calculate and output the first control command to the setpoint of the temperature control module based on the first interference signal.

[0068] The second PID module is used to calculate and output the second control command to the laser based on the second interference signal.

[0069] As you can understand, the optical path in this embodiment is as follows: Figure 2 As shown, the laser beam emitted from the laser passes sequentially through an optical isolator, a half-wave plate, and a polarization beam splitter, splitting into two polarized beams. One of these polarized beams is selected as the input beam and coupled into a polarization-maintaining fiber via an optical fiber coupler. This beam is then split by the first optical fiber beam splitter and subsequently enters different Mach-Zehnder fiber interference paths via the second and third optical fiber beam splitters: interference path one has unequal arm lengths of 3 meters, and interference path two has equal arm lengths of 10 meters. Finally, the two optical signals are combined by the fourth and fifth optical fiber combiners and output. The output interference signal is converted into an electrical signal by a differential detector.

[0070] Example 2

[0071] This embodiment provides an arbitrary band frequency stabilization method based on a dual interferometer, the specific method is as follows:

[0072] Step 1: Construct a system consisting of a 10-meter equal-arm and a 3-meter unequal-arm Mach-Zehnder fiber interferometer. The experimental arms of the 10-meter equal-arm and the 3-meter unequal-arm are uniformly wound around the surface of a copper cylinder with a diameter of 10 cm and a thickness of 2 cm. Heating plates are attached to the upper and lower surfaces of the cylinder to ensure uniform heating of the fiber optic cable wound around the cylinder surface.

[0073] In this embodiment, equal-arm interferometers and unequal-arm interferometers are used to monitor temperature changes and frequency changes, respectively.

[0074] It should be noted that the monitoring principle is as follows: for an equal-arm interferometer, its interference phase... It is mainly affected by the changes in optical path length and refractive index between the two arms:

[0075]

[0076] in , The optical path lengths of the two arms can be considered equal and almost constant in this embodiment. , These represent the refractive indices of the two arms, respectively. Since the entire apparatus is housed in a foam box made of extruded polystyrene board with low thermal conductivity, it is thermally isolated from the external environment, so the refractive index of its reference arm can be considered almost constant. However, when the experimental arm of the equal-arm interferometer is temperature-controlled, due to the thermo-optic effect, the refractive index of the experimental arm changes with small temperature fluctuations, causing a change in the interference phase and thus a change in the output interference light intensity that varies with temperature. Furthermore, due to its symmetrical structure, when the laser frequency fluctuates or both arms are subjected to common environmental disturbances, these disturbances act on the two beams in a common-mode manner and are effectively canceled out during interference. Based on the thermo-optic effect, the temperature responsiveness of the interference phase can be derived. The value is: 784.51 Theoretical predictions show that the change in light intensity with temperature is as follows: Figure 3 As shown. Assume the temperature difference between the experimental arm and the reference arm is 10. The change in optical path length of a 10-meter optical fiber is meter, the change in refractive index of optical fiber The frequency response of the interference phase can be obtained. for: Theoretical predictions show that the change in light intensity with frequency is as follows: Figure 4 As shown. Because the frequency fluctuation of the laser itself is generally in the hundreds... The magnitude range is such that the frequency response is a very small quantity relative to the temperature response, and therefore can be ignored.

[0077] Therefore, for an equal-arm interferometer, the main factor causing phase change is temperature, and the relationship between light intensity and phase can be expressed as:

[0078]

[0079] in, It is the amplitude of light intensity. The change in temperature. The initial phase, the specific relationship is as follows: Figure 3 As shown.

[0080] For unequal-arm interferometers, their interference phase Mainly affected by optical path difference Refractive index change and frequency changes Influence:

[0081]

[0082] in, In this embodiment, the fixed optical path difference between the two arms of the unequal-arm interferometer is given by the length of the additional optical fiber. In this case, the change in the interference phase, i.e., the output light intensity of the unequal-arm interferometer, can be approximated as being related only to the frequency change of the laser and the refractive index change due to temperature. Related to, and related to, optical path difference Proportional. In this embodiment, the optical path difference... The value is 3m, from which the interferometer's temperature response can be obtained. The value is: 235.37 Theoretical predictions show that the change in light intensity with temperature is as follows: Figure 5 As shown. Frequency responsivity for: Theoretical predictions show that the change in light intensity with frequency is as follows: Figure 6 As shown.

[0083] For an unequal-arm interferometer, both temperature and frequency cause phase changes, and the relationship between light intensity and phase can be expressed as follows:

[0084]

[0085]

[0086] in, It is the amplitude of light intensity. The change in temperature. For this variable, frequency The initial phase, the specific relationship is as follows: Figure 5 and Figure 6 As shown.

[0087] Step 2: Activate the temperature control system to stabilize the temperature of the copper column near the set point. In this embodiment, the temperature is set to 25°C.

[0088] Understandably, in order to make the output of the unequal-arm interferometer as unaffected as possible by frequency changes, temperature control is necessary to reduce the interference of temperature fluctuations on the interferometer's output signal.

[0089] Step 3: The polarized light output from the laser is split by an optical fiber beam splitter and input into an equal-arm interferometer and an unequal-arm interferometer respectively, thereby obtaining the first interference signal output by the equal-arm interferometer and the second interference signal output by the unequal-arm interferometer.

[0090] Understandably, in order to control temperature and frequency separately, it is necessary to acquire temperature and frequency information separately. Both temperature and light intensity information are contained in the interferometer signal, so it is necessary to acquire the signals of the first and second interferometers.

[0091] Step 4: After the temperature control process stabilizes, input the output signal of the 10-meter equal-arm Mach-Zehnder fiber interferometer to the PID module, and then fine-tune the set point of the temperature control system through the PID module, so as to stabilize the temperature of the copper column within a fluctuation range better than when using temperature control alone.

[0092] Specifically, the frequency stabilization principle of the unequal-arm interferometer is as follows: when the refractive index and the optical path difference between the two arms remain constant, the interference phase is approximately proportional only to the frequency; and the output light intensity signal has a cosine mapping with the interference phase, meaning the output light intensity signal can reflect frequency changes. To obtain the best frequency stabilization effect, the zero point of the interference curve (i.e., the place with the largest slope) is used as the set value, and the actual light intensity is compared with it to generate an error signal. Then, a PID algorithm is used to perform closed-loop feedback control of the laser frequency. It is important to note that the PID cannot distinguish whether the phase change is caused by temperature or frequency. If the temperature fluctuation is too large, the frequency change will be submerged in the temperature fluctuation, which will cause the PID algorithm to misjudge, resulting in frequency loss of control.

[0093] Understandably, to avoid impacting frequency control, the phase change introduced by temperature must be as small as possible. However, conventional temperature control is limited by temperature sensors, with a control accuracy of 0.05 degrees Celsius being the limit. This is mainly due to insufficient sensor sensitivity, which cannot detect even minute temperature fluctuations. A 0.05-degree Celsius temperature change also alters the refractive index of the optical fiber, resulting in optical path drift, producing a complete interference cycle even if the frequency remains constant. In this case, the temperature-introduced disturbance is perceived by the PID controller as a drastic frequency change, causing the laser's output frequency to oscillate with temperature fluctuations instead of being stably controlled at a specific point, failing to achieve the desired frequency control effect. Therefore, further improvements in temperature control accuracy are needed. In contrast, a 10m equal-arm interferometer has an interference phase almost unaffected by laser frequency changes and, compared to a 3m unequal-arm interferometer, possesses a stronger temperature response, capable of detecting even minute temperature changes. Therefore, the output of the equal-arm interferometer can be fed back to the temperature control module via a PID module. A small voltage signal related to temperature changes, output by the PID, can be superimposed on the reference voltage of the temperature sensor. This improves the temperature sensitivity of the temperature control device, thereby further enhancing the temperature control accuracy and stabilizing the temperature at a level that prevents the equal-arm interferometer from exhibiting abnormal behavior. The range of phase shift fluctuations.

[0094] Understandably, when the PID module uses the output light intensity of the equal-arm interferometer for further feedback control of the temperature, the interference phase of the equal-arm interferometer (mainly affected by temperature) will fluctuate within a very small range. However, since the experimental arms of both the equal-arm and unequal-arm interferometers are uniformly wound around the surface of the same copper cylinder, they can simultaneously sense the same temperature change. Furthermore, because the unequal-arm interferometer has a lower response to temperature changes than the equal-arm interferometer, it can significantly reduce the interference phase change caused by temperature, thus ensuring that the interference phase is mainly affected by frequency fluctuations, thereby guaranteeing the measurement accuracy and stability of the system.

[0095] Step 5: Input the output signal of the 3-meter unequal-arm Mach-Zehnder fiber interferometer to another feedback control PID module, and apply the output of the PID module to the laser to adjust its frequency, so as to achieve a high stability of the laser frequency within a small fluctuation range over a long period of time.

[0096] Understandably, with the combined effect of temperature control and the PID module, temperature fluctuations are confined to a very small range, and the output of the unequal-arm interferometer is considered to be dominated by frequency changes. At this point, under the action of the PID module, the phase change is limited to near the setpoint most sensitive to frequency changes, thereby achieving long-term stability of the laser frequency and effective reduction of the linewidth, as expected. Figure 7 As shown.

[0097] To demonstrate the effectiveness of the above technical solution, this embodiment uses software to perform theoretical verification. Figure 8 In the diagram, (a) represents the output light intensity of the unequal-arm interferometer considering only a 15MHz frequency fluctuation (the short-term frequency drift of the laser under normal conditions). Figure 8 (b) represents the output light intensity when the temperature is stable within a fluctuation range of 0.05 degrees Celsius under the influence of temperature control (i.e., the temperature change that the thermistor can detect), and only the unequal arm interferometer is present. Figure 8 In (c), when the equal-arm interferometer is present, under the action of the PID feedback control module, the theoretically predicted temperature control accuracy will stabilize within a temperature fluctuation range of 0.0009 degrees Celsius, which can effectively improve the accuracy and stability of temperature control. The temperature and frequency curves corresponding to the unequal-arm fiber optic interferometer at this time are as follows: Figure 8 As shown in (d) in the figure; at this time, the monotonically linear mapping relationship between the output light intensity and frequency of the interferometer is maintained.

[0098] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A frequency stabilization method for arbitrary bands based on a dual interferometer, characterized in that, Includes the following steps: Step 1: Construct a dual-fiber interferometer consisting of an equal-arm interferometer and an unequal-arm interferometer, wherein the experimental arms of the equal-arm interferometer and the unequal-arm interferometer are located in the same controlled temperature region; The relationship between the interference phase φ of the unequal-arm interferometer and the laser frequency f satisfies: ; Where n is the refractive index of the optical fiber, L is the fixed optical path difference between the two arms of the interferometer, c is the speed of light, and f is the laser frequency; The relationships between the unequal-arm interferometer and changes in temperature and frequency, and between light intensity and phase, are expressed as follows: ; ; in, It is the amplitude of light intensity. The change in temperature. The change in frequency. This is the initial phase; Step 2: Control the temperature of the dual-fiber interferometer using the temperature control module to bring the temperature of the controlled temperature area to a preset value; Step 3: Split the laser beam output from the laser and input it into the equal-arm interferometer and the unequal-arm interferometer respectively, so as to obtain the first interference signal output by the equal-arm interferometer and the second interference signal output by the unequal-arm interferometer; Step 4: Based on the first interference signal, a temperature control compensation signal is generated by the first PID module. The zero point of the first interferometer signal is used as the set value, and the actual output signal of the first interferometer is compared with it to generate an error signal. Then, the temperature of the controlled temperature area is controlled by the PID algorithm in a closed loop, so as to stabilize the temperature of the controlled temperature area within a fluctuation range better than that when only temperature control is used. Step 5: Based on the second interference signal, a frequency control signal is generated by the second PID module. The zero point of the second interferometer signal is used as the set value, and the actual output signal of the second interferometer is compared with it to generate an error signal. Then, the frequency of the laser is controlled by the PID algorithm in a closed loop to achieve frequency stabilization. The first PID module is used to calculate and output a first control command to the setpoint of the temperature control module based on the first interference signal; The second PID module is used to calculate and output a second control command to the laser based on the second interference signal.

2. The method according to claim 1, characterized in that, Both the equal-arm interferometer and the unequal-arm interferometer are Mach-Zehnder fiber interferometers.

3. The method according to claim 2, characterized in that, The unequal-arm interferometer includes a reference arm and an experimental arm, the optical path difference of which is provided by an additional optical fiber of fixed length; the reference arm and experimental arm of the equal-arm interferometer are of equal length.

4. The method according to claim 3, characterized in that, The optical path difference of the unequal-arm interferometer is 3 meters; the arm length of the equal-arm interferometer is 10 meters.

5. The method according to claim 3 or 4, characterized in that, The experimental arms of the unequal-arm interferometer and the equal-arm interferometer are wound together on the same heat-conducting surface to sense the same temperature field.

6. The method according to claim 5, characterized in that, The heat conductor is a copper cylinder with heating elements attached to its upper and lower surfaces; the temperature control module controls the heating elements to heat the copper cylinder.

7. The method according to claim 6, characterized in that, The copper cylinder and the experimental arm wrapped around it were placed in an insulated box made of extruded polystyrene board with low thermal conductivity.

8. The method according to claim 1, characterized in that, It also includes a differential detection unit for converting the optical interference signals output by the equal-arm interferometer and the unequal-arm interferometer into electrical signals.

9. The method according to claim 1, characterized in that, Between the laser and the dual-fiber interferometer module, there are sequentially arranged an optical isolator, a half-wave plate, a polarization beam splitter, an optical fiber coupler, and several beam splitters.

10. The method according to claim 1, characterized in that, In step 3, the laser emitted from the laser passes sequentially through an optical isolator, a half-wave plate, and a polarization beam splitter, splitting it into two polarized beams. One of these polarized beams is selected as the input beam and coupled into a polarization-maintaining fiber via an optical fiber coupler. The beam is split by the first optical fiber beam splitter and then enters different Mach-Zehnder fiber interference paths through the second and third optical fiber beam splitters: interference path one has unequal arm lengths of 3 meters, and interference path two has equal arm lengths of 10 meters. Finally, the two optical signals are combined by the fourth and fifth optical fiber combiners, respectively, and output the first interference signal and the second interference signal.

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