Composite hollow-core optical fiber and application thereof in detection of hydrocarbon gas

Abstract

The invention discloses a composite hollow-core optical fiber and application thereof in detection of hydrocarbon gas, and belongs to the technical field of optical fibers. The composite hollow-core optical fiber sequentially comprises an outer cladding layer, an inner cladding layer and a fiber core from outside to inside. The inner cladding is an anti-resonance area and comprises four anti-resonance units which are uniformly distributed in the circumferential direction, and each anti-resonance unit comprises three layers of circular micro capillary tubes which are coaxially nested in sequence from inside to outside; the outer cladding is a photonic band gap region, comprises a plurality of air holes which are tangent to one another, and is of a hexagonal structure as a whole; the fiber core is filled with air and is formed by encircling a plurality of anti-resonance units in the inner cladding. The composite hollow-core optical fiber has the characteristics of a hollow-core photonic band gap optical fiber and a hollow-core anti-resonance optical fiber, a three-layer circular micro capillary structure is adopted, light can be effectively limited in a fiber core, and transmission loss is reduced; and the composite hollow-core optical fiber can work under the ultra-wide wavelength condition of 5.5-9.5 [mu] m, so that the requirement for detecting toxic hydrocarbon gas is met.

Description

Technical Field

[0001] The invention belongs to the technical field of optical fibers, and in particular relates to a composite hollow-core optical fiber and an application thereof in detecting hydrocarbon gas. Background Art

[0002] Most toxic hydrocarbon gases, such as ethane, butane, dichlorobenzene, etc., have strong absorption lines near the 8 μm band. Long-term inhalation of hydrocarbons can cause damage to the human digestive system, endocrine system, respiratory system, central nervous system and cardiovascular system. Therefore, the wide-spectrum mid-infrared composite hollow-core optical fiber of the present invention can be used to detect these toxic hydrocarbons, and plays an important role in the fields of environmental detection, laser infrared directional interference, differential absorption radar, etc.

[0003] The core and cladding of traditional quartz optical fiber are both based on silica. The principle of light transmission is total internal reflection, which guides the propagation of light waves through the refractive index difference between the core and the cladding. It is currently the most widely used optical fiber in optical applications such as optical communications and optical sensing. However, due to the high loss and small mode field area of ​​solid core optical fiber, researchers have developed hollow core optical fiber, including hollow core photonic bandgap fiber and hollow core antiresonant fiber.

[0004] Hollow-core photonic bandgap fiber is a type of optical fiber that uses the photonic bandgap effect to guide light propagation in the core. Its core is a hollow core structure, replacing the traditional solid core glass material. The unique structure of this optical fiber can reduce the loss of light during transmission and reduce nonlinear effects. It has the characteristics of high-energy laser transmission and precise dispersion control, and is widely used in precision sensing and ultra-short pulse laser output. However, its operating wavelength range is narrow, mainly limited by the photonic bandgap effect, and is not suitable for working in a wide-bandwidth spectrum.

[0005] The working principle of hollow core antiresonant fiber is based on the antiresonance effect. When light propagates between the core and the cladding, certain wavelengths of light are reflected back to the core due to the optical antiresonance effect instead of penetrating into the cladding. This mechanism mainly relies on the thin wall structure surrounding the hollow core in the cladding. These thin walls exhibit antiresonance reflection effect at certain wavelengths, thereby limiting the propagation path of light and concentrating it in the hollow core. The structure of antiresonant fiber usually includes a hollow core surrounded by a cladding composed of multiple thin-walled holes or ring structures. The thickness and arrangement of the thin-wall structure determine the efficiency of guiding photons in the hollow core. It can transmit a wider range of wavelengths and is very suitable for optical communications and ultrafast optics. However, due to the characteristics of the hollow core antiresonant fiber structure, it may be more sensitive to the external environment (such as temperature, mechanical stress, etc.). In some harsh environments, the transmission performance may be affected. Summary of the invention

[0006] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a composite hollow-core optical fiber and its application in detecting hydrocarbon gas, thereby solving the problems in the prior art.

[0007] The purpose of the present invention can be achieved through the following technical solutions: A composite hollow core optical fiber comprises, from outside to inside, an outer cladding, an inner cladding and a core; The inner cladding is an anti-resonance region, including four anti-resonance units uniformly distributed in the circumferential direction, and the anti-resonance units include: three layers of circular microcapillaries coaxially nested in sequence from the inside to the outside; the outer cladding is a photonic band gap region, including a plurality of mutually tangent air holes, and the overall structure is a hexagonal structure; the core is filled with air and is surrounded by a plurality of anti-resonance units in the inner cladding.

[0008] Furthermore, the wall thickness t of the three layers of circular microcapillaries is the same, satisfying the anti-resonance condition: Wherein, λ represents the designed working wavelength, n1 and n0 are the refractive index of the circular microcapillary material and the refractive index of air, respectively, and m is an integer; Furthermore, the circular microcapillary is filled with air, and the base material is As2S3 glass.

[0009] Furthermore, the base material of the outer cladding is As2S3 glass, and the air holes are filled with air.

[0010] Furthermore, the radii of the three layers of circular microcapillaries are 20 μm, 35 μm and 47 μm from inside to outside.

[0011] Furthermore, the diameter of the air hole is 8 μm; the diameter of the outer cladding is 250 μm.

[0012] Furthermore, the radius of the fiber core is 33 μm.

[0013] Furthermore, a perfect matching layer is arranged outside the outer cladding layer, and the material of the perfect matching layer is As2S3 glass with a thickness of 10 μm.

[0014] The above-mentioned composite hollow core optical fiber is used for detecting hydrocarbons.

[0015] Furthermore, the absorption spectrum of the hydrocarbon is 5.5-9.5 μm.

[0016] Beneficial effects of the present invention: 1. The composite hollow-core optical fiber of the present invention has the characteristics of hollow-core photonic bandgap optical fiber and hollow-core antiresonant optical fiber, and adopts a three-layer circular microcapillary structure, which can not only effectively confine light in the fiber core and reduce transmission loss, but also is easy to process and manufacture.

[0017] 2. The composite hollow-core optical fiber of the present invention can work under the ultra-wide wavelength condition of 5.5-9.5 μm, and can be used to detect toxic hydrocarbon gas, monitor air quality, evaluate pollution degree, and promote environmental governance. The loss at a wavelength of 7.1 μm is 1.4×10 -6 dB / m, and the dispersion is flat and close to 0, and the nonlinear effect is less than 0.002 (W·m) -1 , mode field area greater than 2000 μm 2 , the loss is 8.66×10 in the bending radius range of 2-10 cm -3 ~4.10×10 -6 dB / m. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0019] Figure 1 is a schematic cross-sectional view of a composite hollow core optical fiber of the present invention; Figure 2 is a cross-sectional fundamental mode diagram of a composite hollow core optical fiber of the present invention; Figure 3 is a three-dimensional height representation of a cross section of a composite hollow core optical fiber of the present invention; Figure 4 is a graph showing the relationship between the limiting loss of the fundamental mode of the present invention and the change in wavelength; Figure 5 is a graph showing the relationship between fundamental mode dispersion and wavelength of the present invention; Figure 6 is a graph showing the relationship between the fundamental mode mode field area and the wavelength of the present invention; Figure 7 is a graph showing the relationship between the fundamental mode nonlinear effect of the present invention and the wavelength; Figure 8 It is a relationship diagram of bending loss when bending in the y direction at a wavelength of 7.1 μm and bending radius provided by the present invention; Fig. 9 This is a relationship diagram of bending loss when bending in the x direction at a wavelength of 7.1 μm versus bending radius provided by the present invention.

[0020] In the figure, 1-perfect matching layer, 2-outer cladding, 3-air hole, 4-inner cladding, 5-first layer of circular microcapillaries, 6-second layer of circular microcapillaries, 7-third layer of circular microcapillaries, 8-fiber core. DETAILED DESCRIPTION

[0021] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] Example 1 like Figure 1 As shown in (a), a composite hollow-core optical fiber includes, from the outside to the inside, an outer cladding 2, an inner cladding 4 and a core 8.

[0023] The inner cladding 4 is an anti-resonance region, including four anti-resonance units uniformly distributed in the circumferential direction, the anti-resonance units including: a first layer of circular microcapillary 5, a second layer of circular microcapillary 6 and a third layer of circular microcapillary 7 coaxially nested in sequence from the inside to the outside; Figure 1 As shown in (b), the wall thickness t of the first layer of circular microcapillary 5, the second layer of circular microcapillary 6 and the third layer of circular microcapillary 7 is the same, satisfying the anti-resonance condition: Wherein, t represents the wall thickness of the capillary, λ represents the designed working wavelength, n1 and n0 represent the refractive index of the circular microcapillary material and the refractive index of air, respectively, and m is an integer; in this embodiment, t is 1.1 μm; In this embodiment, the diameter d2 of the inner cladding 4 is 100 μm; the radius r1 of the first circular microcapillary 5 is 20 μm, tangent to the inner cladding, and the angle ɵ with the core 8 is 45°; the radius r2 of the second circular microcapillary 6 is 35 μm, concentric with the first circular microcapillary 5 (the part beyond the inner cladding is ignored); the radius r3 of the third circular microcapillary 7 is 47 μm, concentric with the first circular microcapillary 5 (the part beyond the inner cladding is ignored). The first circular microcapillary 5, the second circular microcapillary 6 and the third circular microcapillary 7 are all filled with air, with a refractive index of 1, and the base material is sulfide (As2S3) glass, with a refractive index of 2.38~2.41; The outer cladding 2 is a photonic bandgap region, including a plurality of mutually tangent air holes 3, and presents a hexagonal structure as a whole; in this embodiment, the diameter d1 of the air hole 3 is 8 μm. The diameter d3 of the outer cladding 2 is 250 μm, wherein the base material of the outer cladding 2 is sulfide (As2S3) glass, with a refractive index of 2.38-2.41, and the air holes 3 are filled with air, with a refractive index of 1.

[0024] The fiber core 8 is filled with air, has a refractive index of 1, and a radius of about 33 μm. It is composed of a plurality of antiresonance units in the inner cladding 4 and is the main light-guiding region of the composite hollow-core optical fiber.

[0025] In this embodiment, a perfect matching layer 1 is disposed outside the outer cladding 2; the base material of the perfect matching layer 1 is sulfide (As2S3) glass with a thickness of 10 μm. The perfect matching layer 1 is used to eliminate the environmental difference between the optical fiber model and the actual optical fiber.

[0026] Example 2 This embodiment uses simulation tests to prove the superiority of the composite hollow-core optical fiber in Embodiment 1; The simulation was performed using the finite element simulation software COMSOL Multiphysics, and the finite element method was used to perform theoretical calculations in combination with the boundary absorption condition of the perfect matching layer 1. The change relationship diagrams of the mode field distribution diagram, limiting loss, dispersion, nonlinear coefficient, and bending loss of the composite hollow core optical fiber in Example 1 were obtained by calculation.

[0027] Figure 2 and Figure 3 The two-dimensional mode field distribution and three-dimensional height expression of the composite hollow core fiber at the 8 μm fundamental mode are shown respectively. Figure 2 and Figure 3 It can be seen that when the composite hollow-core optical fiber transmits optical signals, it can concentrate the energy in the core area, indicating that the light is well confined in the core area and the transmitted light has excellent beam quality.

[0028] Figure 4 The figure shows the relationship between the limiting loss of the composite hollow core fiber base film and the wavelength (5.5-9.5 μm). Figure 4 It can be seen that the loss of the composite hollow core fiber is at a low level when the incident wavelength is 5.5-9.5 μm, and the minimum loss is 1.48×10 -6 dB / m, which is much less than the 0.2 dB / km of traditional single-mode optical fiber. It can be seen that it plays an important role in the detection of toxic hydrocarbon gases (absorption spectrum is about 8μm).

[0029] Figure 5The figure shows the relationship between the dispersion of the composite hollow core fiber base film and the wavelength. Figure 5 The dispersion of the composite hollow-core fiber can be seen in the 5.5~9.5 µm band, and its group velocity dispersion value remains stable at -4.50×10 -10 ~5.19×10 -10 ps / (nm·m). It can be seen that when light waves propagate in this optical fiber, there will be no broadening or compression effect of light pulses. It can be used for the transmission of femtosecond or picosecond ultrafast lasers.

[0030] Figure 6 The figure shows the relationship between the mode field area of ​​the composite hollow core fiber base film and the wavelength. Figure 6 It can be seen that the mode field area of ​​the composite hollow core fiber is larger in the 5.5~9.5 µm band, and in the 2075~3940 μm band. 2 This shows that this optical fiber is very suitable for use in high-energy lasers and amplifiers.

[0031] Figure 7 The figure shows the relationship between the nonlinear effect of the composite hollow core fiber base film and the wavelength. Figure 7 It can be seen that the nonlinearity of the composite hollow-core fiber is low in the 5.5-9.5 µm band, and the value is stable at 0.001-0.002 (W·m). -1 It can be seen that the optical fiber can reduce the influence of nonlinear phenomena such as self-phase modulation (SPM), cross-phase modulation (XPM) and four-wave mixing (FWM), thereby maintaining signal quality and reducing distortion and noise in long-distance transmission. It can be applied to long-distance optical fiber communication fields, such as submarine optical cables and long-distance communication networks.

[0032] Figure 8 and Fig. 9 is the relationship between the bending loss of the composite hollow core optical fiber and the bending radius in the 7.1 μm band; Figure 8 The illustration in the figure shows the electric field distribution when bending in the y direction. Fig. 9 The inset in the figure shows the electric field distribution when bending in the x direction. Figure 8 and Fig. 9 It can be seen that the composite hollow core fiber has low bending loss when bending in the x and y directions under the condition of a transmission wavelength of 7.1 μm. When the bending radius is 2 cm, the bending loss of the fiber when bending in the x and y directions is 0.005 dB / m and 0.008 dB / m respectively; when the bending radius is 4 cm, the bending loss of the fiber when bending in the x and y directions is 10 -4 ~10 -6dB / m level. This shows that the composite hollow-core optical fiber in Example 1 has good anti-bending performance, effectively reduces light leakage caused by bending, and is widely used in scenarios requiring extremely flexible and compact wiring.

[0033] In summary, the composite hollow-core optical fiber described in the present invention has a wide spectral bandwidth of 4 μm (5.5-9.5 μm), can transmit optical signals in the long-wave range, and has the characteristics of low dispersion, low nonlinearity and large mode field area, and also has anti-bending properties.

[0034] In the description of this specification, the description with reference to the terms "one embodiment", "example", "specific example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments or examples in a suitable manner.

[0035] The above shows and describes the basic principles, main features and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments, and the above embodiments and descriptions are only for explaining the principles of the present invention. Without departing from the spirit and scope of the present invention, the present invention may have various changes and improvements, and these changes and improvements all fall within the scope of the present invention to be protected.

Claims

1. A composite hollow core optical fiber, characterized in that: From outside to inside, it includes: outer cladding, inner cladding and fiber core; The inner cladding is an anti-resonance region, including four anti-resonance units uniformly distributed in the circumferential direction, and the anti-resonance units include: three layers of circular microcapillaries coaxially nested in sequence from the inside to the outside; the outer cladding is a photonic band gap region, including a plurality of mutually tangent air holes, and the overall structure is a hexagonal structure; the core is filled with air and is surrounded by a plurality of anti-resonance units in the inner cladding.

2. A composite hollow core optical fiber according to claim 1, characterized in that: The wall thickness t of the three layers of circular microcapillaries is the same, satisfying the anti-resonance condition: Wherein, λ represents the designed working wavelength, n1 and n0 are the refractive index of the circular microcapillary material and the refractive index of air, respectively, and m is an integer.

3. A composite hollow core optical fiber according to claim 1 or 2, characterized in that: The circular microcapillary is filled with air, and the substrate material is As2S3 glass.

4. A composite hollow core optical fiber according to claim 1, characterized in that: The base material of the outer cladding is As2S3 glass, and the air holes are filled with air.

5. The composite hollow core optical fiber according to claim 1, characterized in that: The radii of the three layers of circular microcapillaries are 20 μm, 35 μm and 47 μm from inside to outside.

6. The composite hollow core optical fiber according to claim 1, characterized in that: The diameter of the air hole is 8 μm; the diameter of the outer cladding is 250 μm.

7. The composite hollow core optical fiber according to claim 1, characterized in that: The radius of the fiber core is 33 μm.

8. The composite hollow core optical fiber according to claim 1, characterized in that: A perfect matching layer is arranged outside the outer cladding layer. The material of the perfect matching layer is As2S3 glass and the thickness is 10 μm.

9. Use of a composite hollow core optical fiber according to any one of claims 1 to 8 in detecting hydrocarbons.

10. The use according to claim 9, characterized in that: The absorption spectrum of the hydrocarbon is 5.5-9.5 μm.

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