Dispersion-induced ultrasensitive gas sensor and detection method based on micro-nano optical fiber

Abstract

The present invention discloses a dispersion-induced ultra-sensitive gas sensor and detection method based on micro-nano optical fibers. This sensor, which belongs to the technical field of optical interferometric measurement devices, comprises a first laser, a Mach-Zehnder interferometer, a second laser, and a signal processing system. The first laser is used to generate probe light, and the second laser is used to generate pump light. The Mach-Zehnder interferometer includes a fiber coupler, a micro-nano optical fiber, and a fiber collimator. The micro-nano optical fiber is located in the gas sensing arm of the Mach-Zehnder interferometer. The present invention utilizes a single-mode micro-nano optical fiber to transmit the probe light. When the pump light propagates through the micro-nano optical fiber, it produces a large evanescent field, a strong photothermal effect, and excellent transient sensing response. By adjusting the distance between the end faces of the fiber collimator, the Mach-Zehnder interferometer is enhanced using dispersion-induced methods. This achieves ultra-high refractive index measurement sensitivity using a relatively short micro-nano optical fiber, successfully resolving the challenges faced by existing technologies.

Description

Technical Field

[0001] The present invention relates to a dispersion-induced ultra-sensitive gas sensor based on micro-nano optical fiber and a detection method, belonging to the technical field of optical interference measurement devices. Background Art

[0002] Human breath contains hundreds of volatile organic compounds, some of which have been identified as biomarkers for specific diseases and metabolic disorders. Therefore, breath gas analysis can be used for noninvasive disease diagnosis and metabolic status monitoring. However, the disease markers in breath gas are not only present in extremely low concentrations but also have a highly complex composition. Therefore, developing gas sensors with high specificity and precision is extremely important.

[0003] Gas detection based on absorption spectroscopy has excellent specificity. Many gas sensors have been designed by detecting changes in light intensity. However, these gas sensors are susceptible to environmental and laser intensity fluctuations. Even for gases with relatively strong absorption, the detection limit is only one part per million.

[0004] Building a mode interferometer based on antiresonant fiber to detect phase changes caused by the photothermal effect can reduce the gas detection limit to the trillionth level. However, to enhance the interaction between light and gas and increase the sensitivity of the refractive index sensor, this solution requires long specialty optical fibers, resulting in high sensor costs. In addition, the gas to be detected must diffuse into the specialty optical fiber before it can be detected, resulting in long detection times, typically on the order of hundreds of seconds.

[0005] Designing gas sensors based on cavity-enhanced photothermal spectroscopy can significantly improve the gas detection limit. However, these free-space Fabry-Perot cavities are not only large in size, but also require very complex optical path adjustments. If detection is carried out based on photothermal spectroscopy of hollow-core fibers, the phase detection sensitivity can be greatly improved when the detection wavelength is locked to the resonant wavelength of the Fabry-Perot cavity. However, the refractive index sensor used in this solution has low sensitivity, and it mainly relies on improving the resolution to reduce the gas detection limit. However, the high cavity finesse greatly compresses the available detection light wavelength range, places high demands on the wavelength of the detection laser and the stability of the sensor, and makes detection difficult. In addition, since the gas to be measured needs to diffuse into the hollow-core fiber, this method also has the problem of long detection time.

[0006] Micro-nano optical fibers are drawn from single-mode quartz optical fibers. When there are only two modes in the micro-nano optical fiber, a mode interferometer can be fabricated. If the interferometer operates near the dispersion inflection point, it has a high sensitivity for refractive index measurement. The gas to be measured absorbs the evanescent waves around the micro-nano optical fiber, generating a photothermal effect that modulates the refractive index of the micro-nano optical fiber. This is detected by the mode interferometer, enabling the design of a gas sensor with fast detection speed and high sensitivity. However, to ensure the presence of two coherent modes in the micro-nano optical fiber, the diameter of the micro-nano optical fiber is large when detecting gas. This results in a weak evanescent wave around the micro-nano optical fiber, reducing the photothermal effect's ability to modulate the refractive index of the micro-nano optical fiber, which is not conducive to reducing the gas detection limit.

[0007] In summary, gas sensing technology based on absorption spectroscopy has made significant progress. However, solutions employing high-sensitivity refractive index detection result in extremely high system costs and slow detection speeds. Solutions employing high-resolution refractive index detection require high system stability, are difficult to detect, and also suffer from the problem of slow detection speeds. Gas sensors based on micro-nano fiber mode interferometers offer good transient response, but their weak photothermal effect hinders lowering the gas detection limit. Addressing these shortcomings in existing technologies has become a pressing technical challenge for those skilled in the art. Summary of the Invention

[0008] In response to the shortcomings of the existing technology, the purpose of the present invention is to provide a dispersion-induced ultra-sensitive gas sensor and detection method based on micro-nano optical fibers. The dispersion-induced method is used to enhance the sensitivity of the Mach-Zehnder interferometer. By using shorter micro-nano optical fibers, not only can ultra-high refractive index measurement sensitivity be obtained and the gas detection limit be reduced, but the system cost can also be greatly reduced, successfully solving the problems of the existing technology.

[0009] The dispersion-induced ultra-sensitive gas sensor based on micro-nano optical fiber of the present invention comprises a first laser, a Mach-Zehnder interferometer, a second laser and a signal processing system. The Mach-Zehnder interferometer comprises an optical fiber coupler, a micro-nano optical fiber and an optical fiber collimator. The optical fiber coupler comprises a first optical fiber coupler and a second optical fiber coupler. The micro-nano optical fiber is located in the sensing arm of the Mach-Zehnder interferometer. The optical fiber collimator connects the first optical fiber coupler and the second optical fiber coupler. The first laser is connected to the first optical fiber coupler, and the second laser is connected to the micro-nano optical fiber. The signal processing system is connected to a second fiber coupler. An air chamber is provided in the Mach-Zehnder interferometer. The micro-nano optical fiber is arranged in the air chamber. The air chamber contains a gas to be measured. The first laser is used to generate detection light. The second laser is used to generate pump light. The outside of the micro-nano optical fiber is connected to a light wave directional transmission device. The gas to be measured absorbs the pump light to generate a photothermal effect, causing the refractive index of the micro-nano optical fiber to change, thereby causing the fringes of the Mach-Zehnder interferometer to drift. The signal processing system is used to analyze the fringes drift to detect the gas to be measured.

[0010] Furthermore, the optical wave directional transmission device is a wavelength division multiplexer, which includes a first wavelength division multiplexer and a second wavelength division multiplexer. The first wavelength division multiplexer is connected to the first optical fiber coupler and the micro-nano optical fiber, and the second wavelength division multiplexer is connected to the micro-nano optical fiber, the second laser and the second optical fiber coupler. The first wavelength division multiplexer is used to ensure that the pump light of the second laser is not incident on the first laser, and the second wavelength division multiplexer is used to ensure that the detection light of the first laser is not incident on the second laser.

[0011] Furthermore, the lightwave directional transmission device is an optical circulator.

[0012] Furthermore, an optical fiber delay line is provided in the Mach-Zehnder interferometer, and the optical fiber delay line is a single-mode optical fiber for the wavelength of the first laser.

[0013] Furthermore, the fiber collimator includes a first fiber collimator and a second fiber collimator. The first fiber collimator is used to collimate the light of the first laser output wavelength, and the second fiber collimator is used to couple the light output from the first fiber collimator into the fiber connection loop.

[0014] Furthermore, the first optical fiber collimator and the second optical fiber collimator are located on the connection loop where the micro-nano optical fiber is located or on the connection loop where the optical fiber delay line is located.

[0015] Furthermore, the end faces of the first optical fiber collimator and the second optical fiber collimator are coated with anti-reflection coatings.

[0016] Furthermore, the distance between the first fiber collimator and the second fiber collimator is adjustable. By adjusting the distance between the first fiber collimator and the second fiber collimator, the interference fringes of the Mach-Zehnder interferometer are split, resulting in a phenomenon in which the two interference fringes have the same interference order.

[0017] Furthermore, the micro-nano optical fiber is a single-mode optical fiber for the first laser wavelength.

[0018] Furthermore, the first laser, the Mach-Zehnder interferometer, the second laser, and the signal processing system are connected via a single-mode optical fiber.

[0019] Furthermore, the first optical fiber coupler and the second optical fiber coupler have a 1×2 or 2×2 structure.

[0020] The detection method of the dispersion-induced ultrasensitive gas sensor based on micro-nano optical fiber described in the present invention comprises the following steps:

[0021] S1: By adjusting the distance between the two end faces of the first fiber collimator and the second fiber collimator, the interference fringes of the Mach-Zehnder interferometer are split, and the interference fringes close to the critical wavelength of the fringe splitting have an ultra-high response sensitivity to the refractive index change of the micro-nano optical fiber;

[0022] S2: The micro-nano optical fiber is surrounded by the gas to be measured. The gas to be measured absorbs the pump light generated by the second laser, generating a photothermal effect, which in turn causes the refractive index of the micro-nano optical fiber to change;

[0023] S3: The refractive index change of the micro-nano optical fiber causes the fringe drift of the Mach-Zehnder interferometer;

[0024] S4: Analyze the fringe drift of the Mach-Zehnder interferometer using the signal processing system to detect the gas to be measured.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] The dispersion-induced ultra-sensitive gas sensor and detection method based on micro-nano optical fibers described in the present invention utilizes single-mode micro-nano optical fibers to transmit detection light. The micro-nano optical fibers have a small diameter, and when the pump light propagates through them, the evanescent field is large, the photothermal effect is strong, and the sensor has excellent transient response. The distance between the end faces of the optical fiber collimator can be adjusted, and the Mach-Zehnder interferometer is enhanced based on the dispersion-induced method. Using shorter micro-nano optical fibers, not only can ultra-high refractive index measurement sensitivity be achieved, lowering the gas detection limit, but also significantly reducing system costs, successfully resolving existing technical challenges. This has the following beneficial effects:

[0027] 1. Strong photothermal effect: The micro-nano optical fiber used is single-mode for the detection light wavelength. The diameter of the micro-nano optical fiber is small, the evanescent wave around the optical fiber is strong, the photothermal effect generated by the absorption of pump light by the gas to be measured is strong, and the refractive index modulation ability of the micro-nano optical fiber is also strong;

[0028] 2. Fast detection speed: The micro-nano optical fiber is surrounded by the gas to be measured, which makes it easy for the gas to absorb the pump light to produce photothermal effect, with excellent transient response and fast detection speed;

[0029] 3. Gas detection limit and low cost: Based on the dispersion-induced method to enhance the sensitivity of the Mach-Zehnder interferometer and the use of shorter micro-nano optical fibers, it can not only achieve ultra-high refractive index measurement sensitivity and reduce the gas detection limit, but also greatly reduce system costs;

[0030] 4. Dynamically adjustable: By adjusting the distance between the fiber collimators, the wavelength of the critical point of fringe splitting of the Mach-Zehnder interferometer can be adjusted to compensate for the impact of environmental changes on the sensitivity of the gas sensor. It can also be used to improve the dynamic range of the gas sensor.

[0031] The dispersion-induced ultra-sensitive gas sensor based on micro-nano optical fibers described in the present invention can achieve high-precision, high-speed, large dynamic range, and low-cost gas detection, solving the problems existing in the prior art. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a connection diagram of the dispersion-induced ultra-sensitive gas sensor and detection method based on micro-nano optical fiber of the present invention;

[0033] Figure 2 This is the mode field distribution diagram of a single-mode micro-nano optical fiber with a diameter of 2 microns;

[0034] Figure 3 This is the mode field distribution diagram of a single-mode micro-nano optical fiber with a diameter of 1 micron;

[0035] Figure 4 This is a spectral drift diagram when measuring refractive index changes in the dispersion-induced ultra-sensitive gas sensor and detection method based on micro-nano optical fibers of the present invention;

[0036] Figure 5 This is a detection principle diagram of the dispersion-induced ultra-sensitive gas sensor and detection method based on micro-nano optical fiber of the present invention;

[0037] In the figure: 1. First laser; 2. First fiber coupler; 3. First wavelength division multiplexer; 4. Micro-nano optical fiber; 5. Second wavelength division multiplexer; 6. Second laser; 7. Signal processing system; 8. Second fiber coupler; 9. Second fiber collimator; 10. Gas chamber; 11. First fiber collimator; 12. Fiber delay line. DETAILED DESCRIPTION

[0038] The present invention will be further described below with reference to the accompanying drawings and embodiments:

[0039] Example 1:

[0040] like Figure 1 As shown, the dispersion-induced ultra-sensitive gas sensor based on micro-nano optical fiber of the present invention includes a first laser 1, a Mach-Zehnder interferometer, a second laser 6 and a signal processing system 7. The Mach-Zehnder interferometer includes a fiber coupler, a micro-nano optical fiber 4 and a fiber collimator. The fiber coupler includes a first fiber coupler 2 and a second fiber coupler 8. The micro-nano optical fiber 4 is connected to the first fiber coupler 2 and the second fiber coupler 8 through a wavelength division multiplexer. The fiber collimator is connected to the first fiber coupler 2 and the second fiber coupler 8. The first laser 1 is connected to the first fiber coupler 2, the second laser 6 is connected to the wavelength division multiplexer, and the signal processing system 7 is connected to the second fiber coupler 8. An air chamber 10 is provided in the Mach-Zehnder interferometer. The micro-nano optical fiber 4 is arranged in the air chamber 10. The air chamber 10 contains a gas to be measured. The gas to be measured absorbs the light emitted by the second laser 6 to generate a photothermal effect, causing the refractive index of the micro-nano optical fiber 4 to change, thereby causing the fringes of the Mach-Zehnder interferometer to drift. The signal processing system 7 is used to analyze the fringe drift to achieve detection of the gas to be measured.

[0041] The micro-nano optical fiber 4 is a single-mode optical fiber for the wavelength of the first laser 1 . Figure 2 This is the mode field distribution diagram of a single-mode micro-nano optical fiber with a diameter of 2 microns. Figure 3 This is the mode field distribution diagram for a single-mode micro-nano fiber with a diameter of 1 micron. It can be seen that the thinner the micro-nano fiber 4, the stronger the evanescent wave. When the pump light propagates through the micro-nano fiber 4, the thinner the micro-nano fiber 4, the stronger the interaction between the light and the gas being measured, which helps enhance the photothermal effect and improve detection sensitivity.

[0042] The micro-nano optical fiber 4 is also externally connected to a wavelength division multiplexer, which includes a first wavelength division multiplexer 3 and a second wavelength division multiplexer 5. The first wavelength division multiplexer 3 is connected to the first fiber coupler 2 and the micro-nano optical fiber 4, and the second wavelength division multiplexer 5 is connected to the micro-nano optical fiber 4, the second laser 6, and the second fiber coupler 8. The first wavelength division multiplexer 3 ensures that the light from the second laser 6 cannot be incident on the first laser 1, and the second wavelength division multiplexer 5 ensures that the light from the first laser 1 cannot be incident on the second laser 6.

[0043] Figure 1 The first wavelength division multiplexer 3 and the second wavelength division multiplexer 5 can be replaced by an optical circulator, which can also realize the functions of the first wavelength division multiplexer 3 and the second wavelength division multiplexer 5 of the present invention.

[0044] An optical fiber delay line 12 is further provided in the Mach-Zehnder interferometer. The optical fiber delay line 12 is a single-mode optical fiber for the wavelength of the first laser 1 .

[0045] The optical fiber collimator includes a first optical fiber collimator 11 and a second optical fiber collimator 9. The first optical fiber collimator 11 can collimate the light of the wavelength emitted by the first laser 1, and the second optical fiber collimator 9 can couple the light emitted by the first optical fiber collimator 11 into the optical fiber.

[0046] The end faces of the first optical fiber collimator 11 and the second optical fiber collimator 9 are coated with anti-reflection coatings.

[0047] By adjusting the distance between the first fiber collimator 11 and the second fiber collimator 9 , the interference fringes of the Mach-Zehnder interferometer can be split, resulting in a phenomenon in which the two interference fringes have the same interference order.

[0048] The first laser 1 , the Mach-Zehnder interferometer, the second laser 6 , and the signal processing system 7 are connected via a single-mode optical fiber.

[0049] The first optical fiber coupler 2 and the second optical fiber coupler 8 have a 1×2 or 2×2 structure.

[0050] The first laser 1 is a detection light, the second laser 6 is a pump source, and the gas to be measured absorbs the pump laser, causing the refractive index of the micro-nano optical fiber 4 to change.

[0051] Ignoring the effects of the first wavelength division multiplexer 3, the second wavelength division multiplexer 5, the first fiber collimator 11, and the second fiber collimator 9, for the mth order dark fringe, the phase difference between the measurement arm and the reference arm of the interferometer can be expressed as:

[0052]

[0053] Where l1, l2, and l3 are the lengths of the air gaps between the micro-nano optical fiber 4, the optical fiber delay line 12, and the first optical fiber collimator 11 and the second optical fiber collimator 9, respectively. n1(λ) and n2(λ) are the effective refractive indices of the micro-nano optical fiber 4 and the optical fiber delay line 12, respectively. If the photothermal effect causes the effective refractive index of the micro-nano optical fiber to change by d(δn1(λ)), the wavelength of the interferometer will drift by dλ, and dispersion will cause the effective refractive indices of the micro-nano optical fiber 4 and the optical fiber delay line 12 to change by (dn1 / dλ)dλ and (dn2 / dλ)dλ, respectively, where dn1 / dλ and dn2 / dλ are the dispersion coefficients of the micro-nano optical fiber and the optical fiber delay line, respectively. The refractive index measurement sensitivity S of the Mach-Zehnder interferometer can be expressed as:

[0054]

[0055] When the length l1 of the micro-nano optical fiber and the length l2 of the optical fiber delay line are constant, the size of m can be adjusted by adjusting the distance l3 between the first optical fiber collimator and the second optical fiber collimator, so that the denominator of formula (2) approaches 0 under the action of dispersion, and the refractive index measurement sensitivity with a limit approaching ±∞ can be obtained.

[0056] As a preferred example, when the length of the micro-nano optical fiber is 100 mm, the spectral drift of the ultra-sensitive Mach-Zehnder interferometer designed by the present invention when measuring the refractive index change is as follows: Figure 4 As shown. Figure 4 The interferometer fringes split, and the critical point of the fringes splitting is between 1360-1380nm. The interference fringes on both sides of the critical point are equal. The refractive index of the micro-nano fiber increases by 3.06×10 -6 At RIU, the resonance wavelength of the first interference fringe on the left side of the critical point shifts to the long wavelength by about 3.7 nm. The calculated refractive index measurement sensitivity of the Mach-Zehnder interferometer is as high as 1.2×10 6 nm / RIU; the drift direction of the first interference fringe on the right side of the critical point is exactly opposite, and we can obtain a value of approximately -1.2×10 6 The refractive index measurement sensitivity is in nm / RIU. When the interference fringes drift to the critical point, the sensitivity can reach positive and negative infinity.

[0057] Figure 5 This is the detection principle of the dispersion-induced ultra-sensitive gas sensor based on micro-nano optical fiber designed by the present invention. The optical power of the second laser is periodically modulated, and the gas to be measured absorbs the pump light to produce a photothermal effect, causing the refractive index of the micro-nano optical fiber 4 to change periodically, thereby causing the fringes of the interferometer to drift periodically, such as Figure 5 If the wavelength and output power of the first laser are constant, the drift of the interference fringes will cause the optical power output of the interferometer to fluctuate periodically, as shown in Figure 5 By analyzing the changes in the interferometer's output optical power, the gas to be tested can be detected.

[0058] Example 2:

[0059] The detection method of the dispersion-induced ultrasensitive gas sensor based on micro-nano optical fiber described in the present invention comprises the following steps:

[0060] S1: By adjusting the distance between the two end faces of the first fiber collimator 11 and the second fiber collimator 9, the interference fringes of the Mach-Zehnder interferometer are split, and the interference fringes close to the critical wavelength of the fringe splitting have an ultra-high response sensitivity to the refractive index change of the micro-nano optical fiber 4;

[0061] S2: The micro-nano optical fiber 4 is surrounded by the gas to be measured. The gas to be measured absorbs the pump light generated by the second laser 6, generating a photothermal effect, which in turn causes the refractive index of the micro-nano optical fiber 4 to change;

[0062] S3: The refractive index change of the micro-nano optical fiber 4 causes the fringe drift of the Mach-Zehnder interferometer;

[0063] S4: Analyze the fringe drift of the Mach-Zehnder interferometer using the signal processing system 7 to detect the gas to be measured.

[0064] By adjusting the distance between the fiber collimators, the wavelength of the critical point at which the fringe splitting of the Mach-Zehnder interferometer is split can be adjusted, compensating for the effects of environmental changes on the sensitivity of the gas sensor and also improving the dynamic range of the gas sensor. By using a dispersion-induced method to enhance the sensitivity of the Mach-Zehnder interferometer and utilizing shorter micro-nano fibers, not only can ultra-high refractive index measurement sensitivity be achieved, lowering the gas detection limit, but also significantly reducing system costs, enabling high-precision, high-speed, large dynamic range, and low-cost gas detection.

[0065] In the description of the present invention, it should be noted that the terms "up", "down", "left", "right", "inside", "outside", etc., indicating directions or positional relationships, are based on the directions or positional relationships shown in the accompanying drawings. They are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction. Therefore, they cannot be understood as limiting the present invention.

[0066] In the description of the present invention, it should also be noted that, unless otherwise expressly specified or limited, the terms "disposed," "installed," "connected," and "connected" should be understood in a broad sense. For example, they may refer to fixed connections, detachable connections, or integral connections; they may refer to mechanical connections or electrical connections; they may refer to direct connections or indirect connections through an intermediate medium; and they may refer to internal communication between two components. Those skilled in the art will understand the specific meanings of the above terms in the present invention based on specific circumstances.

[0067] The dispersion-induced ultra-sensitive gas sensor and detection method based on micro-nano optical fibers, as described above in conjunction with the accompanying drawings, utilizes single-mode micro-nano optical fibers to transmit pump light, resulting in a strong photothermal effect and excellent transient response. The dispersion-induced method is used to enhance the sensitivity of the Mach-Zehnder interferometer, utilizing shorter micro-nano optical fibers. This not only achieves ultra-high refractive index measurement sensitivity and lowers the gas detection limit, but also significantly reduces system costs, successfully resolving existing technical challenges. However, the present invention is not limited to the described embodiments, and changes, modifications, substitutions, and variations to these embodiments without departing from the principles and spirit of the present invention remain within the scope of protection of the present invention.

Claims

1. A dispersion-induced ultrasensitive gas sensor based on micro-nano optical fiber, characterized by: The invention comprises a first laser (1), a Mach-Zehnder interferometer, a second laser (6) and a signal processing system (7), wherein the Mach-Zehnder interferometer comprises a fiber coupler, a micro-nano fiber (4) and a fiber collimator, wherein the fiber coupler comprises a first fiber coupler (2) and a second fiber coupler (8), wherein the micro-nano fiber (4) is located in a sensing arm of the Mach-Zehnder interferometer, wherein the fiber collimator is connected to the first fiber coupler (2) and the second fiber coupler (8), wherein the first laser (1) is connected to the first fiber coupler (2), wherein the second laser (6) is connected to the micro-nano fiber (4), wherein the second fiber coupler (8) is connected to the signal processing system (7), wherein an air chamber (10) is provided in the Mach-Zehnder interferometer, wherein the micro-nano fiber (4) is provided in the air chamber (10), wherein the gas to be measured is contained in the air chamber (10), wherein the first laser (1) is used for generating detection light, wherein the second laser (6) is used for generating pump light, wherein the outside of the micro-nano fiber (4) is connected to a light wave directional transmission device, wherein the gas to be measured is Absorbing the pump light generates a photothermal effect, causing the refractive index of the micro-nano optical fiber (4) to change, thereby causing the fringes of the Mach-Zehnder interferometer to drift, and using the signal processing system (7) to analyze the fringes drift to achieve detection of the gas to be measured; the Mach-Zehnder interferometer is also provided with an optical fiber delay line (12), and the optical fiber delay line (12) is a single-mode optical fiber for the wavelength of the first laser (1); the optical fiber collimator includes a first optical fiber collimator (11) and a second optical fiber collimator (9), the first optical fiber collimator (11) can collimate the light of the wavelength emitted by the first laser (1), and the second optical fiber collimator (9) can couple the light emitted by the first optical fiber collimator (11) into the optical fiber connection loop; the distance between the first optical fiber collimator (11) and the second optical fiber collimator (9) is adjustable, and by adjusting the distance between the first optical fiber collimator (11) and the second optical fiber collimator (9), the interference fringes of the Mach-Zehnder interferometer are split, resulting in a phenomenon in which two interference fringes have the same interference level.

2. The dispersion-induced ultrasensitive gas sensor based on micro-nano optical fiber according to claim 1, characterized in that: The optical wave directional transmission device is a wavelength division multiplexer, which includes a first wavelength division multiplexer (3) and a second wavelength division multiplexer (5). The first wavelength division multiplexer (3) is connected to the first optical fiber coupler (2) and the micro-nano optical fiber (4), and the second wavelength division multiplexer (5) is connected to the micro-nano optical fiber (4), the second laser (6) and the second optical fiber coupler (8). The first wavelength division multiplexer (3) is used to prevent the pump light of the second laser (6) from entering the first laser (1), and the second wavelength division multiplexer (5) is used to prevent the detection light of the first laser (1) from entering the second laser (6).

3. The dispersion-induced ultrasensitive gas sensor based on micro-nano optical fiber according to claim 1, characterized in that: The light wave directional transmission device is an optical circulator.

4. The dispersion-induced ultrasensitive gas sensor based on micro-nano optical fiber according to claim 1, characterized in that: The end faces of the first optical fiber collimator (11) and the second optical fiber collimator (9) are coated with anti-reflection films.

5. The dispersion-induced ultrasensitive gas sensor based on micro-nano optical fiber according to claim 1, characterized in that: The micro-nano optical fiber (4) is a single-mode optical fiber for the wavelength of the first laser (1).

6. The dispersion-induced ultrasensitive gas sensor based on micro-nano optical fiber according to claim 1, characterized in that: The first laser (1), the Mach-Zehnder interferometer, the second laser (6), and the signal processing system (7) are connected via a single-mode optical fiber.

7. A detection method for a dispersion-induced ultrasensitive gas sensor based on a micro-nano optical fiber according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: S1: By adjusting the distance between the two end faces of the first optical fiber collimator (11) and the second optical fiber collimator (9), the interference fringes of the Mach-Zehnder interferometer are split, and the interference fringes close to the critical wavelength of the fringes splitting have an ultra-high response sensitivity to the refractive index change of the micro-nano optical fiber (4); S2: The micro-nano optical fiber (4) is surrounded by a gas to be measured, and the gas to be measured absorbs the pump light of the second laser (6), generating a photothermal effect, thereby causing the refractive index of the micro-nano optical fiber (4) to change; S3: The refractive index change of the micro-nano optical fiber (4) causes the fringe drift of the Mach-Zehnder interferometer; S4: Analyze the fringe drift of the Mach-Zehnder interferometer using the signal processing system (7) to detect the gas to be measured.

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