Hollow core optical fiber gas cell

Abstract

The application discloses a hollow-core optical fiber gas chamber, which comprises a hollow-core optical fiber, which has a hollow core, a cladding layer and a coating layer from inside to outside, and the hollow core is used for guiding light beam to interact with gas; an air passage cutout extends from the outside of the coating layer to the hollow core along the cross section direction of the hollow-core optical fiber, forms an open channel through the cladding layer, and the hollow-core optical fiber exchanges gas with the outside through the air passage cutout; and a fixing member is used for fixing the hollow-core optical fiber during fiber processing and subsequent use, and the hollow-core optical fiber is fixed on the fixing member. The air passage cutout is transversely arranged on the hollow-core optical fiber, the processing difficulty of the hollow-core optical fiber gas chamber is reduced, the fiber processing efficiency is improved, the product production cost is reduced, and the gas exchange efficiency of the hollow-core optical fiber gas chamber is improved, so that the application requirements in the fields of gas sensing and the like can be met.

Description

Technical Field

[0001] This invention relates to the fields of fiber optics and gas sensing, and in particular to a hollow fiber optic gas chamber. Background Technology

[0002] Hollow-core fiber is a special type of optical fiber with an air or air-filled cavity core. Light primarily propagates within the air channel rather than the solid silica material of traditional optical fibers. Unlike traditional optical fibers that rely on total internal reflection (TIR), hollow-core fiber achieves optical confinement through two mechanisms:

[0003] Photonic bandgap fiber (PBG): Periodic microstructures (such as honeycomb-shaped air holes) form a photonic bandgap, which confines light of a specific wavelength to propagate within the fiber core.

[0004] Anti-resonant fiber (ARF): Thin-walled glass capillaries around the fiber core form an anti-resonant cavity, preventing light of a specific wavelength from leaking into the cladding due to destructive interference.

[0005] This design of hollow optical fiber brings unique advantages such as low latency, low nonlinearity, and high laser damage threshold, making it particularly suitable for fields such as gas sensing, high-power laser transmission, and mid-infrared optics.

[0006] In traditional gas detection or laser frequency stabilization systems, gas cells typically employ free-space optical designs, such as: single-path gas cells: short optical path (usually <50 cm), low sensitivity; long-path gas cells (such as White cells and Herriott cells): increasing the optical path through multiple reflections using mirrors, but with complex structures, difficult adjustment, and susceptibility to vibration. Furthermore, traditional gas cells are bulky and difficult to integrate into compact optical systems, making them unsuitable for miniaturized monitoring systems.

[0007] A hollow fiber optic gas cell is a gas sensing and optical device based on hollow fiber technology. Its core feature is the utilization of long-distance interaction between light and the gas within the hollow core of the fiber to achieve high-sensitivity detection or specific optical functions. Its emergence has solved many problems associated with traditional gas cells.

[0008] Long-distance interaction between light and gas: Light propagates in the core (air or gas-filled) of hollow optical fiber, with an optical path of several meters or even tens of meters, which greatly improves detection sensitivity;

[0009] Compactness and flexibility: Optical fibers can be bent and wrapped, greatly reducing system size and making them suitable for portable or embedded devices;

[0010] Low loss and high damage threshold: Compared with traditional glass optical fiber, hollow optical fiber has lower loss in the mid-infrared and ultraviolet bands and can withstand high-power lasers.

[0011] For hollow fiber optic cells to enter practical applications, the processing difficulty and cost are key concerns. Conventional hollow fiber optic cells are fabricated by drilling holes in the sidewalls of the hollow fiber using femtosecond lasers. Because laser drilling requires alignment with the fiber cladding, extremely precise positioning is necessary. If the position deviates, either the hole will not penetrate to the hollow fiber core, or the hole diameter will be insufficient, affecting gas exchange efficiency, making the cell fabrication extremely difficult.

[0012] Furthermore, due to the small aperture of the perforations, a large number of micro-holes need to be drilled on the optical fiber to ensure gas exchange efficiency, resulting in very low processing efficiency. Patent application number 201710770017.9 describes an unsealed hollow-core photonic crystal fiber, and patent application number 200880005960.X describes an optical waveguide environmental sensor and its manufacturing method. Both utilize axially open hollow-core optical fibers to achieve rapid exchange between the hollow core and the external environment. However, because the outer cladding of this type of fiber is unsealed, there are through channels, making it difficult to directly produce the hollow-core fiber using normal fiber drawing methods. It is also difficult to process axially through channels on the hollow-core fiber. Precise positioning of the channel opening is required to avoid damaging the microstructure capillaries of the inner cladding; otherwise, damage to the microstructure capillaries used for light confinement will result in significant optical loss, rendering the fiber unusable. Summary of the Invention

[0013] In view of this, the present invention addresses the deficiencies of the existing technology, and its main objective is to provide a hollow fiber gas chamber. By opening a lateral ventilation slit on the hollow fiber, the processing difficulty of the hollow fiber gas chamber is reduced, the fiber processing efficiency is improved, and the product production cost is reduced. At the same time, it can improve the gas exchange efficiency of the hollow fiber gas chamber to meet the application needs of gas sensing and other fields.

[0014] To achieve the above objectives, the present invention adopts the following technical solution:

[0015] A hollow optical fiber air cell, comprising:

[0016] Hollow fiber has a hollow core, a cladding, and a coating layer from the inside out. The hollow core is used to guide the interaction between the light beam and the gas.

[0017] A venting slit extends from the outside of the coating layer along the cross-sectional direction of the hollow fiber towards the hollow fiber core, forming an open channel that penetrates the cladding layer. Gas exchange occurs between the outside of the hollow fiber and the hollow fiber core through the venting slit.

[0018] Fixtures are used to secure hollow optical fibers during optical fiber processing and subsequent use. The hollow optical fibers are fixed to the fixtures.

[0019] As a preferred embodiment, the opening width z of the ventilation slit is related to the modulus radius of the hollow fiber core. ωHCF Satisfying Relationship:

[0020] ;

[0021] The fundamental mode of hollow fiber is approximately Gaussian distributed, α gap λ represents the insertion loss of the ventilation cut, and λ represents the wavelength of the transmitted light.

[0022] As a preferred embodiment, the hollow optical fiber has multiple ventilation slits, which are spaced apart along the axial direction of the hollow optical fiber. The distance L between adjacent slits and the gas diffusion time τ satisfy: τ ≈ L² / 4D; where τ is the time required for gas to diffuse from the two ventilation slits to the center of the gas chamber; L is the length of the hollow optical fiber gas chamber in meters; and D is the gas diffusion coefficient in meters. 2 / s.

[0023] As a preferred embodiment, the ventilation cut is formed by making a transverse or / and oblique cut along the outer side of the coating layer toward the hollow fiber core.

[0024] As a preferred embodiment, the hollow fiber is cut laterally and / or obliquely along the outer side of the coating layer toward the hollow fiber core, and the hollow fiber is directly cut off to form the through ventilation cut; the cut fiber is fixed by a fastener to form a spatially coupled optical path.

[0025] As a preferred embodiment, the opening width z of the ventilation slit is 1 μm to 100 μm.

[0026] As a preferred embodiment, the hollow optical fiber is an anti-resonant hollow optical fiber, a photonic bandgap hollow optical fiber, or a dielectric film hollow optical fiber.

[0027] As a preferred embodiment, the cutting depth of the venting slit should at least exceed the top of the inner cladding wall of the hollow optical fiber.

[0028] As a preferred embodiment, the fixing element is a rigid or semi-rigid carrier made of organic polymer, glass or metal, and the hollow optical fiber is fixed to the fixing element by adhesive, binding or embedding; the spatial configuration of the hollow optical fiber is a straight line, circle, ellipse, racetrack shape, polygon or figure-eight shape, and the configuration is maintained by the fixing element.

[0029] As a preferred embodiment, the fastener is made of cured adhesive, which shapes the hollow optical fiber into a coil before curing. After curing, the fastener and the hollow optical fiber form an integrated structure.

[0030] Compared with the prior art, the present invention has significant advantages and beneficial effects. Specifically, as can be seen from the above technical solution,

[0031] First, loss is controllable: optical loss is controlled by precisely calculating and controlling the width of individual vent cutouts. The vent cutout is equivalent to spatial coupling; the fixing components maintain the collimation of the fiber optic path, and mode mismatch loss, lateral offset loss, and angular deviation loss can all be ignored. According to the axial spacing loss calculation formula:

[0032] ;

[0033] The fundamental mode of hollow fiber can be approximated as a Gaussian distribution, α gap The insertion loss is the venting cut, and λ is the light transmission wavelength. ω HCF Let be the radius of the model field.

[0034] The loss caused by different ventilation notch widths (z) at the corresponding wavelength can be calculated. Taking a 1064nm wavelength laser as an example, with a mode field radius of 10µm, a notch width of 10µm only increases the loss by 0.001dB, and 300 notches only increase the loss by 0.3dB, which can fully meet the requirements of the system optical path power budget.

[0035] Secondly, the processing difficulty is greatly reduced and the efficiency is multiplied: In the conventional method of preparing the air cell of hollow fiber, one approach is to drill micro-holes on the side of the hollow fiber to allow the outside to communicate with the hollow fiber core. Since the hollow fiber core is wrapped in a ring by the cladding and coating, drilling holes on the side of the fiber requires precise alignment with the hollow fiber core on the circumferential sidewall outside the coating layer at each location, which makes laser processing very difficult.

[0036] Another approach, as seen in patents with application numbers 201710770017.9 and 200880005960.X, involves axially opening the sidewalls of hollow optical fibers to facilitate rapid exchange between the hollow core and the external environment. Firstly, because the outer cladding of this type of fiber is not sealed, a through-channel exists, making it difficult to directly produce the hollow fiber using normal fiber drawing methods. Secondly, drawing a fully clad hollow fiber first and then creating cuts through chemical etching or laser cutting requires finding a very precise path along the fiber axis to avoid damaging the inner cladding microstructure; otherwise, damage to the microstructure used for light confinement will result in significant optical loss, rendering the fiber unusable. Furthermore, chemical etching inevitably causes some damage to the internal microstructure after penetrating the cladding, and laser cutting also faces the challenge of precisely aligning the hollow fiber core on the circumferential sidewalls outside the coating layer, both contributing to significant processing difficulties.

[0037] By adopting a horizontal or / and oblique cutting method, the precision requirements for processing are greatly reduced. The positional deviation of the cutting has no impact on the yield of finished products, and the error of the cutting depth is also very large. This results in a significant reduction in processing difficulty and a multiple increase in efficiency.

[0038] Third, gas exchange efficiency is greatly improved: After the cladding is penetrated by perforation, the aperture is generally around 5µm, and the air intake area is the circular area created by perforation. However, cutting creates an open semi-circular notch, and the air intake area can easily reach 30 to 50 times the area of ​​the pore. The thicker the fiber core, the greater the improvement in gas replacement efficiency. In addition, the distance L between two adjacent ventilation notches can be flexibly designed according to the gas detection response time. The shorter the distance, the shorter the gas diffusion time, and the shorter the corresponding detection response time.

[0039] The time required for a gas to diffuse to a given concentration can be calculated using Fick's Second Law. Fick's Second Law formula:

[0040] ;

[0041] Among them, ∂ C / ∂ t is The rate of change of concentration C with time t; ∂ 2 C / ∂x 2 Let C be the second spatial derivative of concentration C along the x-direction; D is the gas diffusion coefficient, in m³. 2 / s, which is related to the type of gas, temperature, and pressure.

[0042] In summary, based on the advantages mentioned above, this invention can design a corresponding hollow fiber gas cell according to the optical power budget and gas detection response time requirements of the gas detection system, and can be easily and quickly processed and prepared.

[0043] To more clearly illustrate the structural features and effects of the present invention, a detailed description is provided below in conjunction with the accompanying drawings and specific embodiments. Attached Figure Description

[0044] Figure 1 This is a schematic diagram of the hollow fiber air cell of the present invention;

[0045] Figure 2 This is a schematic diagram of a hollow optical fiber air chamber according to an embodiment of the present invention.

[0046] Explanation of reference numerals in the attached diagram:

[0047] 101. Hollow fiber; 102. Ventilation notch; 103. Fixing component; 104. Hollow fiber core; 105. Cladding; 106. Coating layer. Detailed Implementation

[0048] The present invention is as follows Figures 1 to 2 As shown, a hollow optical fiber air chamber includes a hollow optical fiber 101, a ventilation notch 102, and a fixing member 103, wherein:

[0049] The hollow fiber 101 has a hollow fiber core 104, a cladding 105 and a coating layer 106 in sequence from the inside to the outside. The hollow fiber core 104 is used to guide the interaction between the light beam and the gas. The light is confined in the hollow fiber core 104 for transmission and can interact with the gas entering the hollow fiber core 104. The hollow fiber 101 becomes a gas chamber for the interaction between light and gas.

[0050] The hollow fiber 101 is one of the following: anti-resonant hollow fiber, photonic bandgap hollow fiber, dielectric film hollow fiber, etc., preferably an anti-resonant hollow fiber.

[0051] The ventilation slit 102 extends from the outside of the coating layer 106 along the cross-sectional direction of the hollow fiber 101 toward the hollow fiber core 104, forming an open channel that penetrates the cladding 105. Gas exchange occurs between the outside of the hollow fiber 101 and the hollow fiber core 104 through the ventilation slit 102.

[0052] The ventilation cut 102 is formed along the outer side of the optical fiber coating layer 106 towards the hollow fiber core 104 by transverse cutting, oblique cutting, or other methods, or by directly cutting the hollow fiber 101 to form a through ventilation cut 102. The cut optical fiber is fixed by a fixing component to form a spatially coupled optical path. The insertion loss can be determined according to the insertion loss α of the ventilation cut. gap The calculation is performed using the formula; the opening width z of the ventilation cut 102 is 1-100um; one or more ventilation cuts 102 are provided on the hollow optical fiber 101 at intervals; the cutting depth of the ventilation cut 102 must at least exceed the top of the inner wall of the cladding 105 of the hollow optical fiber 101.

[0053] The fixing member 103 secures the hollow optical fiber 101 during optical fiber processing and subsequent use, fixing the hollow optical fiber 101 to the fixing member 103. The fixing member 103 is a rigid or semi-rigid carrier made of organic polymer, glass, or metal. The hollow optical fiber 101 is fixed to the fixing member 103 by adhesive, binding, or embedding. The spatial configuration of the hollow optical fiber 101 can be straight, circular, square, elliptical, racetrack-shaped, polygonal, or figure-eight shaped, and the configuration is maintained by the fixing member 103. In this embodiment, the fixing member 103 is composed of cured adhesive. Before curing, the adhesive shapes the hollow optical fiber 101 into a coiled shape. After curing, the fixing member 103 and the hollow optical fiber 101 form an integrated structure. The fixing member 103 and the hollow optical fiber 101 can be fixed by one or more of the following methods: adhesive, tape, cable tie. The fixing member 103 and the hollow optical fiber 101 are fixed together. After the hollow optical fiber 101 is fixed and shaped by adhesive, it is demolded from the fixing member 103. The adhesive itself becomes the new fixing member 103 to fix the hollow optical fiber 101 together.

[0054] The following embodiments further illustrate the technical solution in a specific, clear, and complete manner:

[0055] This implementation example Figure 2 As shown, the hollow fiber 101 is an anti-resonant hollow fiber. The hollow core 104 of the hollow fiber 101 has a diameter of 50 μm, the cladding 105 has a diameter of 125 μm, the coating 106 has a diameter of 245 μm, and the mode field diameter is 16 μm. The hollow fiber 101 is 5 meters long. Adhesive is evenly applied to the fixing member 103, and the hollow fiber 101 is coiled into a ring around the fixing member 103. The inner diameter of the fiber ring is 35 mm, and the outer diameter of the ring is 55 mm. Approximately 40 turns are made. After the adhesive solidifies, four ventilation cuts 102 are evenly made on the hollow fiber ring using laser cutting. The cut width Z is 5 μm, and the cut depth reaches the center of the hollow core 104. The remaining uncut portion can still maintain a certain strength of the hollow fiber 101, and the coupled optical path at the cut can also remain collimated. Based on the number of turns, a total of 160 ventilation cuts 102 are formed on the entire hollow fiber.

[0056] The opening width z of the ventilation slit 102 is related to the modulus radius of the hollow fiber core 104. ω HCF Satisfying the relationship (formula for calculating axial spacing loss):

[0057] ;

[0058] The fundamental mode of hollow fiber can be approximated as a Gaussian distribution, α gap The insertion loss is the venting cut, and λ is the light transmission wavelength. ω HCF Let be the radius of the model field.

[0059] In this embodiment, the light transmission wavelength is 1064nm, the mode field radius is 8µm, and the slit width z is 5µm. According to the above formula, the optical loss increase caused by a single venting slit on the hollow fiber is 0.0008dB, and the total loss increase of 160 slits on the entire hollow fiber is 0.13dB. The loss of the 5-meter hollow fiber itself is <0.2dB, and the overall fiber loss is <0.5dB, which can meet the requirements of the system optical path power budget.

[0060] The gas diffusion time in this example is calculated using Fick's Second Law. The formula for Fick's Second Law is:

[0061] ;

[0062] Among them, ∂ C / ∂ t t is the rate of change of concentration C with time t; ∂ 2 C / ∂x2 t is the second spatial derivative of concentration C along the x-direction; D is the gas diffusion coefficient, in meters. 2 / s, which is related to the type of gas, temperature, and pressure;

[0063] In this example, the diameter d of the hollow fiber core is much smaller than the venting notch spacing L, and the radial diffusion velocity is much higher than the axial velocity. Therefore, the gas diffusion direction can be simplified to one dimension, and a one-dimensional diffusion model can be used to approximate the gas replacement time in the hollow fiber chamber. Assuming that the gas diffuses from both venting notches along the axial direction (the length direction of the hollow fiber), the gas diffusion replacement time τ is determined by the one-dimensional diffusion equation:

[0064] ;

[0065] Where τ is the time required for gas to diffuse from the two venting openings to the center of the air chamber; L is the length of the hollow fiber air chamber in meters; and D is the gas diffusion coefficient in meters. 2 / s;

[0066] Taking methane gas as an example, neglecting the effects of convection and airflow, and considering only molecular diffusion, what is the diffusion coefficient of methane in air under ambient temperature of 25℃ and 1 standard atmosphere? D CH4−air ≈2.0×10 -5 [m 2 / s].

[0067] In this example, the hollow fiber optic cell is coiled in a ring around the fixing member 103 to form a fiber optic ring with an inner diameter of 35mm and an outer diameter of 55mm. After processing, the longest distance between two adjacent ventilation cuts 102 of the hollow fiber optic cell is the outermost segment of the fiber optic ring, and this segment distance is:

[0068] L = 3.14 × 55 ÷ 4 = 43.175 [mm];

[0069] Substituting into the simplified one-dimensional equation of Fick's law, we can obtain the diffusion time τ of methane gas:

[0070] τ≈(0.043175) 2 ÷4÷0.00002≈23 [s];

[0071] To shorten the gas detection response time, simply shorten the interval between ventilation incisions.

[0072] The key design feature of this invention is that

[0073] First, loss is controllable: optical loss can be controlled by precisely calculating and controlling the width of a single venting notch. The venting notch is equivalent to spatial coupling, and the fixing device can maintain the collimation state of the optical fiber path, so mode field mismatch loss, lateral offset loss, and angular deviation loss can all be ignored.

[0074] Secondly, the processing difficulty is significantly reduced, and efficiency is multiplied: Conventional methods for fabricating hollow fiber air cells mostly involve drilling micro-holes in the side of the hollow fiber. Drilling holes in the fiber side requires precise alignment of the fiber cladding at each position, demanding extremely high precision in fiber placement. However, the dicing method greatly reduces the precision requirements. Positional deviations during dicing have no impact on the yield rate, and there is a very large tolerance for errors in the dicing depth. This results in a significant reduction in processing difficulty and a multiplied increase in efficiency.

[0075] Third, gas exchange efficiency is greatly improved: After perforation through the cladding, the pore size is typically around 5µm, and the air intake area is the circular area created by perforation. Cutting, on the other hand, creates an open semi-circular notch, easily achieving an air intake area 30 to 50 times larger than the pore area. The thicker the fiber core, the greater the improvement in gas exchange efficiency. Furthermore, based on the gas detection response time, the distance L between two adjacent ventilation notches can be flexibly designed. The shorter the distance, the shorter the gas diffusion time, and the corresponding detection response time...

[0076] It is also shorter.

[0077] In summary, this invention allows for the design of corresponding hollow fiber gas cells based on the optical power budget and gas detection response time requirements of the gas detection system, and enables convenient and rapid fabrication.

[0078] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A hollow optical fiber air cell, characterized in that, include: Hollow fiber has a hollow core, a cladding, and a coating layer from the inside out. The hollow core is used to guide the interaction between the light beam and the gas. A venting slit extends from the outer side of the coating layer along the cross-sectional direction of the hollow fiber towards the hollow fiber core, forming an open channel penetrating the cladding. Gas exchange occurs between the outside of the hollow fiber and the hollow fiber core through the venting slit. The opening width z of the venting slit is approximately equal to the mode field radius of the hollow fiber core. ω HCF Satisfying Relationship: ； The fundamental mode of hollow fiber is approximately Gaussian distributed, α gap The insertion loss is the venting cut, and λ is the light transmission wavelength; The fixing component is used to fix the hollow fiber during fiber processing and subsequent use. The hollow fiber is fixed to the fixing component; the ventilation cut is formed by transverse and / or oblique cutting along the outer side of the coating layer towards the hollow fiber core.

2. The hollow optical fiber air cell according to claim 1, characterized in that, The hollow optical fiber has multiple ventilation slits spaced at intervals along its axial direction. The distance L between adjacent slits and the gas diffusion time τ satisfy the following condition: τ ≈ L² / 4D; where τ is the time required for gas to diffuse from the two ventilation slits to the center of the air chamber; L is the length of the air chamber in the hollow optical fiber (m); and D is the gas diffusion coefficient (m). 2 / s.

3. The hollow optical fiber air cell according to claim 1, characterized in that, The hollow fiber is cut laterally and / or obliquely along the outer side of the coating layer toward the hollow fiber core, and the hollow fiber is directly cut off to form the through ventilation cut; the cut fiber is fixed by a fastener to form a spatially coupled optical path.

4. The hollow optical fiber air cell according to claim 1, characterized in that, The opening width z of the ventilation slit is from 1 μm to 100 μm.

5. The hollow optical fiber air cell according to claim 1, characterized in that, The hollow fiber is an anti-resonant hollow fiber, a photonic bandgap hollow fiber, or a dielectric film hollow fiber.

6. The hollow optical fiber air cell according to claim 1, characterized in that, The cutting depth of the ventilation slit must extend at least beyond the top of the inner cladding wall of the hollow optical fiber.

7. The hollow optical fiber air cell according to claim 1, characterized in that, The fixing component is a rigid or semi-rigid carrier made of organic polymer, glass or metal. The hollow optical fiber is fixed to the fixing component by adhesive, binding or embedding. The spatial configuration of the hollow optical fiber is a straight line, circle, ellipse, racetrack shape, polygon or figure-eight shape, and the configuration is maintained by the fixing component.

8. The hollow optical fiber air cell according to claim 1, characterized in that, The fastener is made of cured adhesive. Before curing, the adhesive shapes the hollow optical fiber into a coiled shape. After curing, the fastener and the hollow optical fiber form an integrated structure.

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