A low-loss, high-birefringence porous core terahertz fiber and its fabrication method

Abstract

This invention discloses a low-loss, high-birefringence porous core terahertz fiber and its fabrication method, comprising a fiber core and a cladding. The fiber core consists of a substrate material and large circular air holes, small circular air holes, and elliptical air holes distributed within it. One large circular air hole is located at the center of the fiber core, and the remaining large circular air holes are symmetrically arranged in an equilateral triangle along the vertical direction of the cross-section. Two elliptical air holes are horizontally located on the left and right sides of the central large circular air hole to introduce asymmetric defects, increase structural birefringence, and achieve polarization-maintaining function. Two small circular air holes are symmetrically arranged between the two elliptical air holes and the central large circular air hole to further reduce the overlap between the mode field and the substrate material, thereby reducing loss. The cladding includes a protective layer coaxial with the fiber core, which is connected to the fiber core by radially arranged support strips. Both the protective layer and the support strips are made of the substrate material.

Description

Technical Field

[0001] This application relates to the field of terahertz fiber technology, specifically to a low-loss, high-birefringence porous core terahertz fiber and its fabrication method. Background Technology

[0002] Terahertz (THz) spectrum (0.1–10 THz) holds great promise for many applications due to its wide bandwidth, molecular fingerprint spectrum, and non-ionizing properties, including communications, imaging, sensing, and spectroscopy. Particularly in communications, with the advent of 6G's integrated sensing and communication technologies, the increasing demand for data transmission capacity and data rates necessitates an inevitable shift in carrier frequencies towards the terahertz band to meet the bandwidth requirements of next-generation wireless systems.

[0003] Currently, internationally, the International Telecommunication Union (ITU) allocated four frequency bands totaling 137 GHz within the 275–450 GHz band for terrestrial terahertz communication services at the 2019 World Radiocommunication Conference. Domestically, core components such as frequency multipliers, mixers, and modems in the 0.1 THz to 0.3 THz range are approaching world-class levels, and several organizations have already demonstrated terahertz wireless communication systems near 0.3 THz. Therefore, 0.1–0.3 THz is the primary frequency band for next-generation communication technologies.

[0004] Although terahertz communication has been proven in free-space wireless links, the high directivity of terahertz beams necessitates careful positioning of transmitters and receivers. Reliable communication in non-static environments (e.g., between moving objects) requires complex beam control solutions. This situation is further exacerbated in geometrically complex environments (such as inside vehicles and buildings), as these environments require highly complex channel modeling. Moreover, terahertz waves are sensitive to water absorption, and atmospheric weather conditions such as rain, snow, and fog directly affect the performance and reliability of wireless terahertz links.

[0005] Given the aforementioned limitations of wireless terahertz communication, short-range terahertz fiber optic links offer an alternative due to their ability to traverse complex geometric paths. Terahertz fiber optic links hold promising applications in reliable vehicle-mounted and airborne connections and internal communications for both military and civilian transportation. Terahertz fiber optics are also crucial in inaccessible or highly protected environments, such as corrosive biological shells and within shelters and bunkers, where flexible fibers facilitate convenient connections between different parts of the same system. Furthermore, terahertz fiber optics can serve as a backup for short-range wireless communication, proving invaluable in areas with severe weather conditions should atmospheric conditions deteriorate.

[0006] Currently, mainstream terahertz fibers include hollow-core waveguides based on the anti-resonant (ARROWs) principle or the photonic bandgap (PBG) principle. Due to the long wavelength of terahertz waves in the 0.1–0.3 THz frequency band, thicker air holes are required as cladding, resulting in a large overall fiber size. Furthermore, the low-loss dielectric material exhibits high rigidity within this larger size, limiting fiber bending. Moreover, some terahertz systems requiring polarization control impose requirements on the polarization-maintaining performance of the fiber. Currently reported terahertz hollow-core anti-resonant fibers and photonic bandgap fibers cannot simultaneously satisfy both high birefringence and low loss. Therefore, designing and fabricating a low-loss, high birefringence terahertz fiber in the 0.1–0.3 THz frequency range holds significant application potential. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention provides a low-loss, high-birefringence porous core terahertz optical fiber and its fabrication method.

[0008] This invention discloses a low-loss, high-birefringence porous core terahertz fiber, comprising a core and a cladding covering the core;

[0009] The fiber core consists of a substrate material and large circular air holes, small circular air holes, and elliptical air holes distributed within it. One large circular air hole is located at the center of the fiber core, and the remaining large circular air holes are symmetrically arranged in an equilateral triangle along the vertical direction of the cross section. Two elliptical air holes are horizontally located on the left and right sides of the central large circular air hole to introduce asymmetric defects, increase the birefringence of the structure, and achieve polarization preservation. Two small circular air holes are symmetrically arranged between the two elliptical air holes and the central large circular air hole to further reduce the overlap between the mode field and the substrate material and reduce losses.

[0010] The cladding includes an outer protective layer coaxially arranged with the fiber core. The protective layer is connected to the fiber core by a plurality of radially arranged support strips. The support strips divide the cavity between the protective layer and the fiber core into a plurality of fan-shaped air holes. Both the protective layer and the support strips are made of base material.

[0011] As a further improvement of the present invention, the diameter d1 of the small circular air hole is: d1 = d 2 / 4, where d2 is the diameter of the large circular air hole; the minor axis of the elliptical air hole is horizontal and the major axis is vertical, the minor axis a of the elliptical air hole is: a=p / 2, and the major axis b is: Where p is the distance between adjacent large circular air holes.

[0012] As a further improvement of the present invention, the substrate material is a polymer material with low absorption loss in the terahertz band.

[0013] As a further improvement of the present invention, the substrate material includes one of high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyethylene (PE), cyclic olefin copolymer (COC), cyclic olefin polymer (COP), polytetrafluoroethylene (PTFE), polypropylene (PP), polystyrene (PS), polycarbonate (PC), polymethyl methacrylate (PMMA), polylactic acid (PLA), and UV-resin materials.

[0014] This invention also discloses a method for fabricating a low-loss, high-birefringence porous-core terahertz fiber, comprising:

[0015] A cylinder containing substrate material particles is placed into a heating furnace. Once the substrate material particles reach a molten state, they are squeezed to expel air. Then, the molten substrate material is squeezed through a structured mold that is complementary to the cross-section of the optical fiber, and a traction device is used to stretch the terahertz optical fiber to the required size.

[0016] Compared with the prior art, the beneficial effects of the present invention are:

[0017] 1. The terahertz fiber of the present invention guides light based on modified total internal reflection. The refractive index of the cladding structure is closer to that of air, which can increase the fiber core's ability to confine light and reduce confinement loss. The setting of protective layer and support strip can effectively prevent the influence of external environment, such as water vapor and dust, which have a significant impact on terahertz wave transmission.

[0018] 2. This invention reduces the overlap between the mode field and the substrate material by adding air holes to the fiber core cross-section, thereby further reducing absorption loss. Using cyclic olefin copolymer (COC) as the substrate material, the total loss is less than 20 dB / m in the 0.1–0.3 THz range, making it suitable for short-distance terahertz wave transmission. Furthermore, by introducing an asymmetric air hole structure in the vertical direction, birefringence is increased, achieving a birefringence of up to 10 in the 0.1–0.3 THz range. -2 ~10 -3 This allows for the maintenance of the bias level.

[0019] 3. In the low-frequency range of 0.1 to 0.3 THz, the fiber of this invention has a smaller overall size compared with the microstructured fibers such as terahertz hollow anti-resonance and hollow Bragg fibers that have been reported so far, and can be bent due to the thinner cladding thickness.

[0020] 4. The terahertz optical fiber of the present invention is manufactured by extrusion-traction integrated fabrication, which has a short cycle, high yield and can achieve long-distance fabrication. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the terahertz optical fiber disclosed in this invention;

[0022] Figure 2 This is a schematic diagram illustrating the structural dimensions of the terahertz optical fiber disclosed in this invention.

[0023] Figure 3 This invention discloses the variation of birefringence with proportional scaling of the fiber core structure.

[0024] Figure 4 This invention discloses the variation of birefringence with frequency under different duty cycles (d2 / p) when the core diameter is 2.2 mm.

[0025] Figure 5 This invention discloses the variation of total loss of X and Y polarization modes with frequency under different duty cycles (d2 / p) when the fiber core diameter is 2.2 mm.

[0026] Figure 6 For different length support bars disclosed in this invention, (a) loss in X-polarization mode; (b) loss in Y-polarization mode; (c) birefringence;

[0027] Figure 7 The final optical fiber structure loss and birefringence disclosed in this invention;

[0028] Figure 8 This is a mode distribution diagram of the final optical fiber structure disclosed in this invention at different frequencies.

[0029] In the picture:

[0030] 1. Protective layer; 2. Fan-shaped air holes; 3. Porous fiber core; 4. Support strip; 5. Elliptical air holes; 6. Small circular air holes; 7. Large circular air holes; 8. Base material.

[0031] t, thickness of protective layer and support strip; L, length of support strip; od, diameter of porous fiber core; d1, diameter of small circular air hole; d2, diameter of large circular air hole; a, minor axis of elliptical air hole; b, major axis of elliptical air hole; p, distance between adjacent large circular air holes; OD, overall diameter of optical fiber. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] The present invention will now be described in further detail with reference to the accompanying drawings:

[0034] like Figures 1-2 As shown, this invention provides a low-loss, high-birefringence porous core terahertz fiber, comprising a porous core 3 and a cladding covering the porous core 3; wherein,

[0035] The porous fiber core 3 of this invention is composed of a substrate material 8 and large circular air holes 7, small circular air holes 6, and elliptical air holes distributed inside it. On the longitudinal section of the optical fiber, one large circular air hole 7 is located at the center of the fiber core 3, and the remaining large circular air holes 7 are arranged symmetrically along the vertical direction of the section in an equilateral triangle configuration. Figure 1 The arrangement shown includes two elliptical air holes 5 horizontally positioned to the left and right of the central large circular air hole, with the minor axis of the elliptical air holes 5 being horizontal and the major axis being vertical. These air holes are used to introduce asymmetric defects, increase structural birefringence, and achieve polarization preservation. Two small circular air holes 6 are symmetrically arranged between the two elliptical air holes 5 and the central large circular air hole, further reducing the overlap between the mode field and the substrate material, thus reducing losses. Furthermore, the diameter d1 of the small circular air holes 6 in this invention is: d1 = d 2 / 4, where d2 is the diameter of the large circular air hole 7; the minor axis a of the elliptical air hole 5 is: a=p / 2, and the major axis b is: Where p is the distance between adjacent large circular air holes.

[0036] The cladding of the present invention includes a protective layer 1 of an outer ring coaxially arranged with the porous fiber core 3. The protective layer 1 is connected to the porous fiber core 3 by a plurality of radially arranged support strips 4. The support strips 4 divide the cavity between the protective layer 1 and the porous fiber core 3 into a plurality of fan-shaped air holes 2. Both the protective layer 1 and the support strips 4 are base materials 8.

[0037] The substrate material 8 used in the preparation of the porous fiber core 3, protective layer 1, and support strip 4 of the present invention is a polymer material with low absorption loss in the terahertz band. Preferably, the substrate material 8 includes one of high-density polyethylene (HDPE), low-density polyethylene (LDPE), polyethylene (PE), cyclic olefin copolymer (COC), cyclic olefin polymer (COP), polytetrafluoroethylene (PTFE), polypropylene (PP), polystyrene (PS), polycarbonate (PC), polymethyl methacrylate (PMMA), polylactic acid (PLA), and UV-resin materials.

[0038] This invention provides a method for fabricating a low-loss, high-birefringence porous-core terahertz fiber, comprising:

[0039] A cylinder containing substrate material particles is placed into a heating furnace. Once the substrate material particles reach a molten state, they are squeezed to expel air. Then, the molten substrate material is squeezed through a structured mold that is complementary to the cross-section of the optical fiber, and a traction device is used to stretch the terahertz optical fiber to the required size.

[0040] Example:

[0041] The cross-section of the terahertz fiber of the present invention is as follows: Figure 1 As shown, the substrate material 8 is a cyclic olefin copolymer (COC), and the absorption loss of the COC block material can be expressed by the following formula:

[0042] α mat = -5.205 × f 3 +7.757×f 2 -3.797×f+0.695

[0043] Where f is the frequency, in THz; α mat The unit is dB / cm.

[0044] The total loss of a terahertz fiber consists of the sum of absorption loss and confinement loss.

[0045] The absorption loss of terahertz fiber can be expressed by the following formula:

[0046] EML = α mat ·η·10 2 (dB / m)

[0047]

[0048] Where S1 is the total cross-sectional area of ​​the optical fiber, S2 is the area of ​​the material portion, and P z This is the Poynting vector perpendicular to the fiber cross-section.

[0049] The confinement loss of terahertz fiber can be expressed by the following formula:

[0050]

[0051] Where Im(n) eff ) is the imaginary part of the effective refractive index of this mode, and the birefringence is represented by B = |n x -n y | indicates.

[0052] The diameter of the large circular air hole in the fiber core is d2 = 3.28 mm, the distance between adjacent large circular air holes is p = 4.1 mm, and the diameter of the small circular air hole 6 is d1: d1 = d 2 / 4; The minor axis a of the elliptical air hole 5 is: a = p / 2, and the major axis b is: The distance from the center of the elliptical air hole 5 to the center of the fiber core is 1.65p. The center of the small circular air hole 6 is located between the large circular air hole 7 and the elliptical air hole 5 at the center of the fiber core.

[0053] For porous terahertz fibers, the core size, the size of the air holes in the core, and the cladding thickness all affect the fiber loss. First, a full-vector finite element method was used for analysis and calculation. With the length of support strip 4 L = 2 mm and the thicknesses of protective layer 1 and support strip 4 t = 0.1 mm, the birefringence variation was observed by scaling the core diameter od and the number of air holes in the core proportionally. Figure 3 As shown, it can be observed that when the core diameter is 1.3–2.2 mm, the birefringence is around 10. -2 In terms of magnitude, considering that a larger core diameter can improve the ability to confine light, the core diameter od is set to 2.2 mm.

[0054] Secondly, with a core diameter od = 2.2 mm, the birefringence and total loss of the optical fiber were compared under different duty cycles (d² / p), such as... Figure 4 and Figure 5 As shown, it can be observed that when the duty cycle (d2 / p) is 0.8, the optical fiber exhibits high birefringence and low loss over a wider terahertz frequency range.

[0055] Finally, the influence of the support strip length L on fiber loss is analyzed, such as... Figure 6 As shown, it can be observed that the losses in both X-polarization and Y-polarization modes decrease with increasing support strip length. This is mainly because the increased support strip length leads to increased cladding thickness, reducing the fiber confinement loss. However, changes in cladding thickness have almost no effect on birefringence. To ensure a smaller fiber size, the support strip thickness is set at 3.5 mm, resulting in an overall fiber diameter of less than 10 mm.

[0056] In summary, the final fiber structure exhibits a total loss of less than 20 dB / m in the 0.1–0.3 THz frequency range and a birefringence of 10. -2 ~10 -3 Magnitude, such as Figure 7 As shown. The low-loss, high-birefringence porous-core terahertz fiber of this invention can achieve polarization-maintaining transmission of terahertz waves over short distances. Simultaneously, the overall diameter of the fiber is smaller than that of hollow-core fiber, allowing for good bending performance. Figure 8 This shows the pattern distribution at different frequencies.

[0057] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A low-loss, high-birefringence porous-core terahertz optical fiber, characterized in that, Includes a fiber core and a cladding layer covering the fiber core; The fiber core is composed of a base material and large circular air holes, small circular air holes and elliptical air holes distributed inside it. One large circular air hole is located at the center of the fiber core, and the remaining large circular air holes are arranged symmetrically in the vertical direction of the cross section in an equilateral triangle manner. Two elliptical air holes are located horizontally on the left and right sides of the large circular air hole in the center, and two small circular air holes are symmetrically arranged between the two elliptical air holes and the large circular air hole in the center. The cladding includes an outer protective layer coaxially arranged with the fiber core. The protective layer is connected to the fiber core by a plurality of radially arranged support strips. The support strips divide the cavity between the protective layer and the fiber core into a plurality of fan-shaped air holes. Both the protective layer and the support strips are made of base material.

2. The low-loss, high-birefringence porous core terahertz fiber as described in claim 1, characterized in that, The diameter d1 of the small circular air hole is: d1 = d 2 / 4, where d2 is the diameter of the large circular air hole; the minor axis of the elliptical air hole is horizontal and the major axis is vertical, the minor axis a of the elliptical air hole is: a=p / 2, and the major axis b is: Where p is the distance between adjacent large circular air holes.

3. The low-loss, high-birefringence porous core terahertz fiber as described in claim 1, characterized in that, The substrate material is a polymer material with low absorption loss in the terahertz band.

4. The low-loss, high-birefringence porous core terahertz fiber as described in claim 3, characterized in that, The substrate material includes one of high-density polyethylene, low-density polyethylene, polyethylene, cyclic olefin copolymers, cyclic olefin polymers, polytetrafluoroethylene, polypropylene, polystyrene, polycarbonate, polymethyl methacrylate, polylactic acid, and UV-resin materials.

5. A method for fabricating a low-loss, high-birefringence porous-core terahertz optical fiber as described in any one of claims 1 to 4, characterized in that, include: The cylinder containing the base material particles is placed into the heating furnace. After the base material particles reach a molten state, they are squeezed to expel the air. Then, the molten substrate material is extruded through a structured mold that is complementary to the fiber cross-section, and the terahertz fiber of the required size is stretched by a traction device.

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