Optical fiber sensor for monitoring micro-overcharge strain and temperature of lithium battery and preparation method of optical fiber sensor

Abstract

The invention discloses an optical fiber sensor for monitoring micro-overcharge strain and temperature of a lithium battery and a preparation method of the optical fiber sensor. The sensor is formed by cascading a bending optical fiber sensor (BFOS) and two warped bending optical fiber sensors (WBFOS), and the WBFOS with low temperature-strain crosstalk is prepared by designing non-bending parts at two ends of a tail fiber of the BFOS to be warped by a certain height. The sensor has the advantages of being simple in structure, easy and convenient to prepare and low in cost, multi-sensor cascade multiplexing is supported, crosstalk is avoided, and the complexity and invasiveness of battery monitoring can be effectively reduced; by monitoring the wavelength offset of the interference peak of the sensor, the accurate monitoring of the transverse and longitudinal strain and temperature of the surface of the micro overcharge battery can be synchronously realized, so that early warning is provided for the thermal runaway of the battery.

Description

Technical Field

[0001] This invention relates to the field of battery state monitoring technology, specifically to an optical fiber sensing technology for synchronous monitoring of multiple physical quantities on the battery surface, which is particularly suitable for real-time monitoring of transverse and longitudinal strain and temperature on the surface of lithium batteries during micro-overcharging. Background Technology

[0002] Lithium-ion batteries are widely used due to their high power density, long cycle life, and environmental friendliness; however, thermal runaway accidents are frequent, making early monitoring and warning crucial. Micro-overcharging is a key precursor to battery thermal runaway, easily leading to internal side reactions and heat accumulation. Therefore, accurate monitoring of this process is of great significance. Temperature and strain are two key physical quantities reflecting the micro-overcharge state of lithium batteries. Temperature directly reflects the battery's thermal state, while casing strain characterizes early internal state changes during charging and discharging. Therefore, developing advanced sensing technologies capable of real-time, in-situ monitoring of temperature and strain is essential for achieving early warning of battery thermal runaway.

[0003] Currently, there are four main methods for monitoring strain and temperature during lithium battery cycling: First, implantable thin-film sensing technology, which monitors parameters by utilizing the resistance changes of flexible thin films. It has the advantages of being implantable and small in size, but is susceptible to battery interference, has poor corrosion resistance, is costly, and can damage the battery structure upon implantation. Second, ultrasonic testing technology, which correlates temperature changes and strain information by analyzing ultrasonic signals. It is inexpensive and does not damage the battery structure. However, it cannot directly measure target parameters and requires temperature compensation models to correct the monitoring data. Third, electrochemical impedance spectroscopy, which achieves non-invasive monitoring based on impedance characteristics. It avoids the lag of traditional temperature measurement methods, but is time-consuming and susceptible to interference from the battery's own operating state. Fourth, the combined use of thermocouples and strain gauges. Both types of devices are mature and inexpensive, but both are point-based measurement methods. Thermocouples have weak anti-interference capabilities, and strain gauges exhibit temperature drift. Neither is suitable for long-term online monitoring of batteries.

[0004] Compared to the technologies mentioned above, fiber optic sensors, with their superior advantages such as strong resistance to electromagnetic interference, fast response speed, resistance to electrolyte corrosion, simultaneous and accurate monitoring of temperature and strain, and minimal damage to battery structure, are gradually being applied to the field of temperature and strain monitoring during lithium battery cycling. For example, invention patent CN117030051B discloses a lithium battery storage temperature monitoring system based on distributed fiber optic temperature measurement. This system uses temperature-sensing optical fibers and fiber Bragg grating sensors to work together to achieve temperature monitoring and anomaly handling. However, this system only focuses on the single dimension of temperature monitoring and does not simultaneously collect strain parameters of the battery surface, and its cost is relatively high. The invention patent with publication number CN115574731A proposes a distributed strain measurement system based on optical frequency domain reflection (OFDR). This system achieves two-dimensional distributed measurement of micro-strain by attaching a single optical fiber to the battery surface in a serpentine arrangement. However, this scheme relies on a complex optical interferometry system and a high-speed data demodulation algorithm, which not only results in high cost of sensing and demodulation equipment and low system integration, but also only measures strain and does not simultaneously acquire temperature information of the battery surface, thus limiting its practical application in battery thermo-mechanical coupling monitoring. In addition, the invention patent with publication number CN114421037A proposes a battery internal temperature and strain sensor based on fiber Bragg gratings (FBG). It adopts a combination structure of FBG1 fixed at both ends and FBG2 free at one end to achieve decoupled detection of temperature and strain. However, this sensor needs to be placed inside the battery, and the fabrication process involves complex implantation and sealing processes, which is highly invasive to the battery structure. Furthermore, it does not allow for multi-point and multi-directional deployment on the same battery.

[0005] Existing lithium battery state monitoring technologies primarily utilize fiber optic sensors for normal battery cycling scenarios, rarely monitoring for micro-overcharge states. Furthermore, existing technologies largely employ FBG fiber optic sensors, which suffer from complex fabrication processes and typically monitor only temperature and strain in a single direction, lacking multi-directional monitoring. In contrast, this invention, through a cascaded design and unique arrangement of bent fiber optic sensors (BFOS) and warped-bend fiber optic sensors (WBFOS), not only achieves monitoring of both normal cycling scenarios and micro-overcharge states but also fabricates a fiber optic sensor with a simpler manufacturing process than traditional FBG sensors. Simultaneously, it successfully achieves simultaneous monitoring of lateral and longitudinal strain and temperature under both normal and micro-overcharge states, filling a gap in multi-dimensional monitoring technology and providing a simple, low-cost, and long-term monitoring solution for lithium battery safety status early warning. Summary of the Invention

[0006] This invention discloses a fiber optic sensor for monitoring the strain and temperature of lithium batteries under micro-overcharge and its fabrication method. The sensor consists of one cascaded BFOS and two WBFOS. By designing the non-bent portions of the BFOS pigtails to be raised to a certain height, a WBFOS with low temperature-strain crosstalk is fabricated. This sensor has the advantages of simple structure, simple fabrication process, low cost, support for multi-sensor cascading and multiplexing without crosstalk, and can effectively reduce the complexity and intrusiveness of battery monitoring. By monitoring the wavelength shift of the sensor's interference peak, simultaneous monitoring of the transverse strain, longitudinal strain, and temperature of the battery surface can be achieved.

[0007] The present invention provides an optical fiber sensor for monitoring the micro-overcharge strain and temperature of a lithium battery and its fabrication method. The fabrication steps are as follows: Take a single-mode silica optical fiber, remove part of its coating layer, and cross the two ends of the fiber to form a near-elliptical structure, which is then fixed to the surface of an iron block with tape. Next, pre-shape the near-elliptical structure into a near-parabolic curved structure by flame heating. Then, fine-tune the curved structure by flame until a BFOS with a specific interference peak is obtained. Repeat the above steps to prepare three BFOS with different specific interference peaks. Subsequently, select two of the BFOS, rotate the two ends of their pigtails respectively, and form a permanent warped curved structure by flame shaping treatment, to obtain a warped optical fiber sensor (WBFOS-T) for measuring the transverse strain of the battery and a warped curved optical fiber sensor (WBFOS-L) for measuring the longitudinal strain of the battery. Finally, cascade the three sensors. The cascading must meet the conditions of matching bending diameter, wide spectral interference peak coverage, and non-overlapping interference peak positions to ensure that each sensor can work independently without crosstalk.

[0008] The present invention also includes:

[0009] 1. The single-mode silica optical fiber is 1m long, with a core diameter of 9μm and a cladding diameter of 125μm; a coating layer of 4-6cm in length is removed from the middle of the optical fiber using fiber strippers.

[0010] 2. Connect the BFOS prepared during the process to the spectrometer. Fine-tune the BFOS using a flame below it, while simultaneously observing the spectral curve displayed on the spectrometer until a specific interference peak appears. This peak should have no other interference peaks within 40-60 nm to its left and right. Repeat the above steps to prepare two more BFOS. The positions of the three BFOS interference peaks should meet the following requirements: the interval between the interference peak position of one BFOS and the interference peak position of its nearest neighbor should be within 20-35 nm. Then, treat the two selected BFOS with a flame to achieve a sensor tilt height of 3-7 mm.

[0011] The working principle of this invention is as follows: Due to differences in bending diameter and the opening size at the top of the optical fiber, WBFOS-T, BFOS, and WBFOS-L each form a unique interference peak, enabling effective differentiation within the spectral range without crosstalk. By cascading the three sensors and arranging them orthogonally on the battery surface, and monitoring the shift in the center wavelength of the transmission spectrum of each sensor, synchronous measurement of battery temperature and lateral and longitudinal strain can be achieved. The specific working principle of each sensor is as follows: For BFOS, light emitted from the broadband light source is transmitted through a single-mode optical fiber. At the bend, due to the lack of total internal reflection, it splits into two parts: some light leaks into the cladding, forming a cladding mode transmission, while the remaining light continues to propagate along the fiber core. After the bend, the cladding mode and the core mode couple, and interference occurs due to the optical path difference, resulting in a resonant interference peak in the transmission spectrum. When the external temperature changes, the thermal expansion effect causes the optical path length of the optical fiber to increase with the temperature. At the same time, the thermo-optical effect leads to an increase in the effective refractive index of both the fiber core and the cladding. Moreover, due to the influence of dopants, the rate of increase in the refractive index of the fiber core is higher than that of the cladding. Under the combined effect of thermal expansion and thermo-optical effect, the increase in temperature will cause a redshift of the BFOS interference peak.

[0012] The light transmission mechanism of WBFOS-T and WBFOS-L is similar to that of BFOS: when broadband light passes through the bent section of the fiber, it splits into two parts due to the failure to meet the total internal reflection condition. One part of the light propagates along the fiber core, while the other part leaks into the cladding, forming cladding modes. After passing through the bent section, the two modes couple, and the optical path difference forms a characteristic interference peak in the transmission spectrum. The difference between them and BFOS is that the two sides of the bend have a permanent slope, which makes the order of the cladding modes leaking from the bent section higher than that of BFOS. When the temperature changes, the thermal expansion effect lengthens the optical path of the fiber, and the thermo-optical effect increases the effective refractive index of both the fiber core and the cladding, with a greater increase in the refractive index of higher-order cladding modes, resulting in a decrease in the effective refractive index difference between the core and cladding. This effect outweighs the effect of the increased optical path, ultimately causing the interference peak to exhibit a blue shift characteristic when the temperature rises. When subjected to external strain and stretching, the bending diameter of the sensor increases and the optical path lengthens, resulting in a decrease in the effective refractive index of both the core and cladding layers, with a more significant decrease in the cladding refractive index. The decrease in the core-cladding refractive index difference dominates, thus causing the interference peak to exhibit a blue shift characteristic.

[0013] The temperature-strain decoupling method for WBFOS-T, BFOS, and WBFOS-L is as follows: BFOS is placed on the battery surface coated with thermally conductive silicone grease and is used solely for temperature measurement; WBFOS-T and WBFOS-L are fixed to the battery in the lateral and longitudinal directions respectively using UV adhesive, allowing simultaneous detection of temperature and strain. Because the three sensors are closely orthogonally arranged in the central region of the battery, their temperature environments are approximately identical. The temperature change during battery cycling is calculated using the interference peak drift of BFOS. This temperature value is then substituted into the interference peak drift influence formulas for WBFOS-T and WBFOS-L to calculate the lateral and longitudinal strain changes during battery cycling.

[0014] Compared with the prior art, the present invention has the following advantages:

[0015] 1. This invention fabricates a low-temperature strain crosstalk WBFOS by designing the non-bent portions at both ends of the BFOS pigtail to be raised to a certain height. The fiber optic sensor described in this invention has a simple structure, a simple fabrication process, and long-term operational stability.

[0016] 2. The sensor fabrication method of the present invention is flexible. By adjusting key structural parameters such as the opening size of the BFOS, the bending diameter, and the lifting height of the WBFOS, sensor probes with different performance specifications can be fabricated.

[0017] 3. The WBFOS described in this invention has both temperature and strain sensing sensitivity. By designing two sensing probes for measuring strain to be arranged vertically in the width and length directions of the battery, and placing the sensing probe for measuring only temperature in the width direction of the battery, the synchronous monitoring of the transverse strain, longitudinal strain and temperature of the battery surface can be realized, effectively filling the gap in the prior art that does not simultaneously acquire multi-dimensional parameters. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of BFOS and WBFOS in this invention;

[0019] Figure 2 This is a schematic diagram of a fiber optic sensor arranged on the surface of a lithium battery and the system.

[0020] Figure 3 The transmission spectra of individual WBFOS-T, BFOS, and WBFOS-L sensors and the cascaded transmission spectra of the three sensors in this invention.

[0021] Figure 4 This is a diagram showing the variation of the resonance peaks of WBFOS-T and WBFOS-L with external strain in this invention.

[0022] Figure 5 This is a graph showing the variation of the resonance peak wavelengths of WBFOS-T and WBFOS-L with external strain in this invention.

[0023] Figure 6 The diagram shows the variation of the resonance peaks of WBFOS-T, BFOS, and WBFOS-L with external temperature in this invention.

[0024] Figure 7 The graph shows the variation of the resonance peak wavelength of WBFOS-T, BFOS, and WBFOS-L with external temperature in this invention.

[0025] Figure 8This image shows the synchronous measurement results of battery current, voltage, surface transverse strain, longitudinal strain, and temperature changes during normal charging and discharging and slight overcharging processes by placing the WBFOS-T, BFOS, and WBFOS-L of this invention together with commercial thermocouples in the central region of a ternary soft-pack lithium battery. Detailed Implementation

[0026] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings.

[0027] See Figure 1 This is a schematic diagram of the structure of BFOS and WBFOS in the fiber optic sensor of the present invention, where D is the diameter of the sensor probe and h is the tilt height of WBFOS. Figure 2 This is a schematic diagram of a fiber optic sensor deployed on the surface of a lithium battery and the system. The sensor probe consists of three cascaded components, and the fabrication process is as follows: 1. Select a single-mode silica fiber approximately 1m long (core diameter 9μm, cladding diameter 125μm), and use fiber strippers to remove approximately 4-6cm of the coating layer from its middle section. Cross the two ends of the stripped fiber to form a long, elliptical-like structure, and fix it to an iron block using tape; 2. Use a disposable lighter to provide a flame below the elliptical-like structure, pre-shaping the fiber into a parabolic shape; 3. Connect the fiber to a spectrometer and monitor it in real time. Fine-tune the flame until an interference peak appears at 1258nm (with 40-60 nm on each side). A BFOS (without other interference peaks within 0 nm) was prepared; the above steps were repeated to prepare two more BFOS with interference peaks of 1229 nm and 1281 nm respectively; 4. Since the temperature and strain sensitivities of BFOS differ by 3 orders of magnitude, which easily leads to cross-sensitivity errors, the two BFOS with non-1258 nm interference peaks were selected, and their ends were rotated to make them bend. The non-bent parts were heated with a flame and shaped to a bend height of 3-7 mm to prepare WBFOS-T and WBFOS-L; the temperature and strain sensitivities of the two differ by only 1 order of magnitude, which can reduce measurement errors. Among them, the bending diameter of WBFOS-T, WBFOS-L and BFOS is within 7-12 mm, and the aperture value (defined as: when the top of a parabola coincides with the bending top of the sensor, the coefficient 'a' of the quadratic term in the function formula of the parabola is the aperture value of the BFOS) is within -0.8 to -2.

[0028] See Figure 3The data consists of transmission interference peak spectra of individual WBFOS-T, BFOS, and WBFOS-L, as well as their cascaded configuration. Due to differences in aperture size and bending diameter, the characteristic interference peaks do not overlap and transmission loss is low. The cascaded configuration allows for independent operation without crosstalk. The working principle of each sensor is as follows: In BFOS, broadband light is transmitted through a single-mode fiber to the bending section. Because total internal reflection is not satisfied, the light splits into two parts: some light leaks into the cladding to form a cladding mode, while the remaining light propagates along the fiber core. These two parts couple after passing through the bending section, and interference occurs due to the optical path difference, resulting in a resonant interference peak in the transmission spectrum. With temperature changes, thermal expansion causes the optical path length of the fiber to increase with temperature. The thermo-optical effect increases the effective refractive index of both the core and cladding, with the core refractive index increase being more affected by dopants than the cladding. Under the combined effect of these two factors, the temperature rise causes a redshift in the BFOS interference peaks.

[0029] The light transmission mechanism of WBFOS-T and WBFOS-L is similar to that of BFOS, the difference being that the upturned ends on both sides have permanent slope bends, resulting in higher-order cladding modes leaking from the bends. With increasing temperature, thermal expansion lengthens the optical path, and the thermo-optical effect increases the effective refractive indices of both the core and cladding, with a more significant increase in the refractive index of higher-order cladding modes, leading to a decrease in the effective refractive index difference between the core and cladding. Under this dominant effect, the interference peak exhibits a blue shift. Under strain and stretching, the sensor's bending diameter increases, the optical path lengthens, and both the effective refractive indices of the core and cladding decrease, with a more significant decrease in the cladding refractive index. The decrease in the core-cladding refractive index difference dominates, and the interference peak also exhibits a blue shift.

[0030] Temperature-strain decoupling method: The BFOS is placed on the battery surface coated with thermally conductive silicone grease and is used only for temperature measurement; WBFOS-T and WBFOS-L are fixed to the battery's horizontal and vertical directions respectively with UV adhesive, allowing for simultaneous temperature and strain detection. Because the three are closely orthogonally arranged at the battery center, their temperature environments are approximately uniform. The temperature change during battery cycling is calculated by the BFOS interference peak drift. Substituting this into the formula for the influence of the interference peak drift of WBFOS-T and WBFOS-L, the horizontal and vertical strain changes can be calculated. The changes in the interference peaks of the cascaded WBFOS-T, BFOS, and WBFOS-L when the ambient temperature and strain change are also observed. , , It can be represented as: , , , in, , These are temperature sensitivity and strain sensitivity, respectively. , These are the temperature change and the strain change, respectively. The strain changes of WBFOS-T and WBFOS-L when the ambient temperature and strain change. and It can be represented as:

[0031] See Figure 4 This shows the changes in the sensor's transmission spectrum under different strains. From Figure 4 As can be seen from the left figure, when external strain is applied to WBFOS-T and increases from 0 με to 4462 με, the interference peak of WBFOS-T shifts to blue, while BFOS and WBFOS-L are not affected by strain when no external strain is applied, and the peak value only fluctuates by ±0.08 nm. Figure 4 As can be seen from the right figure, when external strain is applied to WBFOS-L and increases from 0 με to 4425 με, the interference peak of WBFOS-L shifts to blue, while BFOS and WBFOS-T are not affected by strain when no external strain is applied, and the peak value only fluctuates by ±0.08 nm.

[0032] See Figure 5 The transmission spectra of WBFOS-T and WBFOS-L are shown as functions of external strain from 0 με to 4462 με and from 0 με to 4425 με, respectively. From the linear fitting, the strain sensitivities of WBFOS-T and WBFOS-L are obtained as -5.28 × 10⁻⁶. -4 nm / με, -4.715×10 -4 nm / με.

[0033] See Figure 6 This displays the changes in the sensor's transmission spectrum under different temperature variations. Figure 6 As can be seen from the upper left figure, when the external temperature of BFOS is increased from 26℃ to 53.5℃, the interference peak of BFOS redshifts. Figure 6 As can be seen from the upper right figure, when the external temperature of WBFOS-T is increased from 25.5℃ to 53.5℃, the interference peak of WBFOS-T shifts to blue. Figure 6 As shown in the lower left figure, when the external temperature of WBFOS-L is increased from 26℃ to 52℃, the interference peak of WBFOS-L exhibits a blue shift. The three curved channels are cascaded together and then individually calibrated at temperature. When an external temperature is applied to one of the channels, the other two channels are at room temperature and are unaffected by the temperature, with the peak value fluctuating only ±0.08nm.

[0034] See Figure 7The transmission spectra of WBFOS-T, BFOS, and WBFOS-L are shown as functions of ambient temperatures ranging from 26℃ to 53.5℃, 25.5℃ to 53.5℃, and 26℃ to 52℃, respectively. From the linear fitting, the temperature sensitivities of WBFOS-TBFOS and WBFOS-L are -8.5 pm / ℃, 210 pm / ℃, and -4.76 pm / ℃, respectively.

[0035] See Figure 8 This test involves placing WBFOS-T, BFOS, WBFOS-L, and commercial thermocouples at the center of a pouch battery and measuring the battery current, voltage, lateral / longitudinal strain, and temperature changes under specific operating conditions. Test conditions: The basic procedure is constant current (CC) charging to the set cutoff voltage, followed by constant current (CV) charging to 0.02C, and resting for 1 hour; then CC discharging to 2.8V, and resting for 1 hour, completing one cycle. Each condition is repeated 3 times to ensure data repeatability. Specific procedures include normal charge / discharge (cutoff voltage 4.2V, 0.4C, 0.6C rate) and overcharge testing (cutoff voltage 4.6V, 0.4C, 0.6C rate). Figure 7 Data shows that the lateral strain changes are more significant during the battery micro-overcharge expansion process, and the lateral and longitudinal strain change trends are not synchronized; this measurement method can effectively reflect the battery expansion state and provide early warning for battery thermal runaway.

Claims

1. A fiber optic sensor for monitoring micro-overcharge strain and temperature of lithium batteries and its fabrication method, characterized in that: The fiber optic sensor consists of a cascaded Bending Fiber Optic Sensor (BFOS) and two Wavy Bending Fiber Optic Sensors (WBFOS). By designing the non-bent portions of the BFOS pigtails to bend to a certain height, a WBFOS with low temperature-strain crosstalk is prepared. By designing the bending diameter and aperture value of each sensor probe, the spectral peak coverage of each probe is made wide and the interference peak positions do not overlap, so that the cascade loss and overall size of the fiber optic sensor are within a reasonable range, and the independent operation is free of crosstalk. The unique arrangement of the three sensor probes enables real-time monitoring of the transverse strain, longitudinal strain and temperature of the battery surface.

2. The sensor feature according to claim 1, the method for its preparation includes the following steps: (1) Take a single-mode silica optical fiber, strip off part of its coating layer, and cross the two ends of the fiber to form an elliptical structure. Fix it to the surface of the iron block with tape. (2) Apply a flame to the elliptical structure for pre-forming treatment, so that the optical fiber forms a parabolic curved structure. (3) Continue to use the flame to fine-tune the curved structure until a bent fiber sensor (BFOS) with a specific interference peak is obtained. The BFOS is used for battery surface temperature monitoring. Repeat steps (1)-(3) to prepare three BFOS with different specific interference peaks. (4) Select one of them. Two BFOS are rotated at both ends of their tails and flame-shaped to form a permanent warped structure of a certain height, forming a warped fiber sensor (WBFOS). One of them is a warped fiber sensor (WBFOS-T) for measuring the transverse strain of the battery, and the other is a warped fiber sensor (WBFOS-L) for measuring the longitudinal strain of the battery. (5) The three sensors are cascaded. The cascade must meet the following conditions: the bending diameter of each sensor is appropriate, the aperture value is suitable, the spectral interference peak coverage is wide, and the interference peak positions do not overlap.

3. According to claim 1, the three sensor probes are arranged as follows: one BFOS and one WBFOS are placed in the width direction of the lithium battery surface, and the two are placed opposite each other; one WBFOS is placed in the length direction of the lithium battery surface, perpendicular to the first two sensor probes.

4. The method according to claim 2, wherein the single-mode quartz fiber in step (1) has a length of 1m, a core diameter of 9μm, and a cladding diameter of 125μm; and a coating layer of 4 to 6cm in length is stripped at the middle of the fiber using fiber strippers.

5. According to the method of claim 2, the flame in step (2) is generated by a disposable lighter; the flame is placed in the area below the cross optical fiber, and after the optical fiber is heated and softened by the flame, the softened area forms a parabolic shape with uniform stress at the top of the bend under the combined action of gravity and thermal stress, and the bending diameter is controlled to be 7-12mm.

6. According to the method of claim 2, the BFOS in step (3) is connected to a spectrometer, and fine-tuned by a flame below the BFOS while observing the spectral curve displayed by the spectrometer until a special interference peak appears, with no other interference peaks 40-60 nm to the left and right of the interference peak; repeat steps (1)-(3) to prepare two more BFOS, and the positions of the three BFOS interference peaks obtained must meet the following requirements: the position of the interference peak of the BFOS is within 18-35 nm of the position of the interference peak of the nearest BFOS.

7. According to the method of claim 2, the two BFOS selected in step (4) are subjected to flame treatment to make the sensor tilt height reach within 3-7mm.

8. According to the method of claim 2, in step (5), the bending diameter of the three optical fiber sensors is in the range of 7 to 12 mm, the aperture value is in the range of -0.8 to -2, and the corresponding interference peak wavelength satisfies the content of claim 6.

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