Low-loss and high-birefringence porous core terahertz optical fiber and preparation method thereof

Abstract

The invention discloses a low-loss and high-birefringence porous core terahertz optical fiber and a preparation method thereof. The low-loss and high-birefringence porous core terahertz optical fiber comprises a fiber core and a cladding, the fiber core is composed of a substrate material and large circular air holes, small circular air holes and elliptical air holes which are distributed in the substrate material, one large circular air hole is located in the center of the fiber core, and the rest large circular air holes are symmetrically arranged in the vertical direction of the cross section in a regular triangle mode; the two elliptical air holes are horizontally positioned on the left side and the right side of the large circular air hole in the center and are used for introducing asymmetric defects, increasing structure birefringence and realizing a polarization maintaining function; the two small circular air holes are symmetrically arranged between the two elliptical air holes and the central large circular air hole and are used for further reducing the overlapping degree of the mode field and the substrate material and reducing the loss; the cladding comprises a protective layer coaxial with the fiber core, the protective layer is connected with the fiber core through radial supporting strips, and the protective layer and the supporting strips are both made of substrate materials.

Description

Technical Field

[0001] The present application relates to the technical field of terahertz optical fibers, and in particular to a low-loss, high-birefringence porous core terahertz optical fiber and a preparation method thereof. Background Art

[0002] The terahertz (THz) spectrum (0.1-10 THz) has great prospects in many applications, including communications, imaging, sensing and spectroscopy, due to its wide bandwidth, molecular fingerprint spectrum and non-ionization characteristics. Especially in the field of communications, with the proposal of 6G intersensory integration, the data transmission capacity and the user's demand for information transmission rate are gradually increasing. In order to meet the bandwidth requirements set by the next generation of wireless systems, the shift of carrier frequency to the terahertz band is inevitable.

[0003] At present, in the international field, the International Telecommunication Union (ITU) allocated 4 frequency bands totaling 137 GHz in the 275-450 GHz frequency band for terrestrial terahertz communication services at the 2019 World Radio Conference. In China, core components such as frequency multipliers, mixers, and modems in the range of 0.1THz to 0.3THz are close to the world's first-class level, and many units have demonstrated terahertz wireless communication systems near 0.3THz, so 0.1-0.3THz is the main frequency band for the next generation of communication technology.

[0004] Although terahertz communications have been demonstrated in free-space wireless links, the transmitter and receiver need to be carefully positioned due to the high directivity of the terahertz beam; reliable communications in non-static environments (for example, between moving objects) require complex beam control solutions; this situation is further exacerbated in geometrically complex environments (such as vehicles and inside buildings) because these environments require highly complex channel modeling. In addition, terahertz waves are sensitive to water absorption, and atmospheric weather conditions such as rain, snow, and fog will directly affect the performance and reliability of wireless terahertz links.

[0005] Given the above limitations of wireless terahertz communications, short-distance terahertz fiber links can be used as an alternative because they can cross complex geometric paths. Terahertz fiber links have attractive application prospects in reliable vehicle-mounted, airborne connections and internal communications in the military and civilian transportation fields. Terahertz fiber is also extremely critical in environments that are difficult to reach or require high protection, such as corrosive biological shells and inside shelters and bunkers. Flexible optical fibers can easily connect different parts of the same system. Terahertz fiber can also serve as a backup means of short-distance wireless communication. If the atmospheric conditions suddenly deteriorate, it can play a role, which is of great significance for areas with severe weather.

[0006] The mainstream terahertz optical fibers at present include hollow-core waveguides based on the antiresonance (ARROWs) principle or the photonic bandgap (PBG) principle. Since the terahertz wave wavelength in the 0.1-0.3THz frequency band is relatively long, thicker air holes are required as cladding, which leads to a larger overall optical fiber size. At the same time, low-loss dielectric materials have greater rigidity at larger sizes, so the bending of the optical fiber is limited. In addition, some terahertz systems that require polarization control have requirements for the polarization-maintaining performance of optical fibers. The terahertz hollow-core antiresonance optical fibers and photonic bandgap optical fibers reported so far cannot simultaneously meet the requirements of high birefringence and low loss. Therefore, designing and preparing a low-loss, high-birefringence terahertz optical fiber in the 0.1-0.3THz frequency range has important application prospects. Summary of the invention

[0007] In view of the shortcomings of the existing problems, the present invention provides a low-loss, high-birefringence porous core terahertz optical fiber and a preparation method thereof.

[0008] The present invention discloses a low-loss, high-birefringence holey core terahertz optical fiber, comprising a core and a cladding covering the core;

[0009] The fiber core is composed of a base material and large circular air holes, small circular air holes and elliptical air holes distributed inside the base material. A 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 form of regular triangles along the vertical direction of the cross section. Two elliptical air holes are horizontally located on the left and right sides of the large circular air hole in the center, so as to introduce asymmetric defects, increase structural birefringence, and realize polarization-maintaining function. The two small circular air holes are symmetrically arranged between the two elliptical air holes and the large circular air hole in the center, so as to further reduce the overlap between the mode field and the base material and reduce the loss.

[0010] The cladding includes an outer ring protective layer coaxially arranged with the fiber core, the protective layer is connected to the fiber core through a plurality of radial support strips, the support strips separate the cavity between the protective layer and the fiber core into a plurality of fan-shaped air holes, and the protective layer and the support strips are both base materials.

[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 short axis of the elliptical air hole is horizontal and the long axis is vertical, the short axis a of the elliptical air hole is: a=p / 2, and the long 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 base 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] The present invention also discloses a method for preparing a low-loss, high-birefringence porous core terahertz optical fiber, comprising:

[0015] A barrel filled with base material particles is placed in a heating furnace, and after the base material particles reach a molten state, the air is expelled by squeezing; then, the base material melt is continued to be squeezed through a structured mold complementary to the optical fiber cross-section, and the terahertz optical fiber of the required size is stretched through a traction device.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

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

[0018] 2. The present invention reduces the overlap between the mode field and the base material by adding air holes on the core cross section, further reducing the absorption loss. With cycloolefin copolymer (COC) as the base material, the total loss is less than 20 dB / m in the range of 0.1 to 0.3 THz, and can be used for short-distance terahertz wave transmission. By introducing an asymmetric air hole structure in the vertical direction, the birefringence is increased, and the birefringence can reach 10 in the range of 0.1 to 0.3 THz. -2 ~10 -3 order of magnitude, thereby achieving the polarization maintaining function.

[0019] 3. In the low frequency range of 0.1 to 0.3 THz, the overall size of the optical fiber of the present invention is smaller than that of the currently reported terahertz hollow-core anti-resonance, hollow-core Bragg and other microstructured optical fibers, and bending can be achieved due to the thin cladding thickness.

[0020] 4. The terahertz optical fiber of the present invention is prepared by an integrated extrusion-pulling process, which has a short cycle, a high yield and can achieve long-distance preparation. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0022] Figure 2 It is a schematic diagram of the structural dimensions of the terahertz optical fiber disclosed in the present invention;

[0023] Figure 3 The birefringence disclosed in the present invention changes with the proportional scaling of the core structure;

[0024] Figure 4 The figure shows the variation of birefringence with frequency at different duty ratios (d2 / p) when the core diameter of the fiber disclosed in the present invention is 2.2 mm;

[0025] Figure 5 When the core diameter of the fiber disclosed in the present invention is 2.2 mm, the total loss of the X and Y polarization modes varies with frequency at different duty ratios (d2 / p);

[0026] Figure 6 (a) X polarization mode loss; (b) Y polarization mode loss; (c) birefringence under different lengths of support strips disclosed in the present invention;

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

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

[0029] In the figure:

[0030] 1. Protective layer, 2. Fan-shaped air holes, 3. Porous fiber core, 4. Support bar, 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 bar, L, length of support bar, od, diameter of porous core, d1, diameter of small circular air hole, d2, diameter of large circular air hole, a, short axis of elliptical air hole, b, long axis of elliptical air hole, p, distance between adjacent large circular air holes, OD, overall diameter of optical fiber. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are 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.

[0033] The present invention is further described in detail below in conjunction with the accompanying drawings:

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

[0035] The porous fiber core 3 of the present invention is composed of a base material 8 and large circular air holes 7, small circular air holes 6 and elliptical air holes 5 distributed therein. In the longitudinal section of the optical fiber, a 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 in the vertical direction of the section in the form of an equilateral triangle, such as Figure 1 The arrangement shown in the figure is as follows: two elliptical air holes 5 are horizontally located on the left and right sides of the large circular air hole in the center, and the short axis of the elliptical air hole 5 is in the horizontal direction and the long axis is in the vertical direction, which is used to introduce asymmetric defects, increase the structural birefringence, and realize the polarization-maintaining function; two small circular air holes 6 are symmetrically arranged between the two elliptical air holes 5 and the large circular air hole in the center, which are used to further reduce the overlap between the mode field and the substrate material and reduce the loss. Further, the diameter d1 of the small circular air hole 6 of the present 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 an outer ring protective layer 1 coaxially arranged with the porous fiber core 3. The protective layer 1 is connected to the porous fiber core 3 through a plurality of radial support strips 4. The support strips 4 separate the cavity between the protective layer 1 and the porous fiber core 3 into a plurality of fan-shaped air holes 2. The protective layer 1 and the support strips 4 are both base materials 8.

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

[0038] The present invention provides a method for preparing a low-loss, high-birefringence porous core terahertz optical fiber, comprising:

[0039] A barrel filled with base material particles is placed in a heating furnace, and after the base material particles reach a molten state, the air is expelled by squeezing; then, the base material melt is continued to be squeezed through a structured mold complementary to the optical fiber cross-section, and the terahertz optical fiber of the required size is stretched through a traction device.

[0040] Example:

[0041] The cross section of the terahertz optical fiber of the present invention is as follows Figure 1 As shown, the substrate material 8 is cycloolefin copolymer (COC), and the absorption loss of the COC bulk 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, unit is THz; α mat The unit is dB / cm.

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

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

[0046] EML=α mat ·η·10 2 (dB / m)

[0047]

[0048] Among them, S1 is the cross-sectional area of ​​the entire optical fiber, S2 is the area of ​​the material part, and P z is the Poynting vector perpendicular to the fiber cross section.

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

[0050]

[0051] Among them, Im(n eff ) is the imaginary part of the effective refractive index of the mode, and the birefringence is expressed 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 d1 of the small circular air hole 6 is: 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 1.65·p, and 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 terahertz optical fibers with porous cores, the core size, core air hole size, and cladding thickness all have an impact on the optical fiber loss. First, the full vector finite element method is used for analysis and calculation. When the length of the support bar 4 is L = 2 mm, and the thickness of the protective layer 1 and the support bar 4 is t = 0.1 mm, the core diameter od and the core air hole are proportionally scaled. The birefringence change law is as follows: Figure 3 As shown in Figure 2, it can be found that when the core diameter is 1.3 to 2.2 mm, the birefringence is 10 -2 Considering that a larger core diameter can improve the light binding ability, the core diameter od is 2.2mm.

[0054] Secondly, under the condition of core diameter od = 2.2 mm, the birefringence and total loss values ​​of the optical fiber under different duty ratios (d2 / p) are compared, such as Figure 4 and Figure 5 As shown, it can be found that when the duty cycle (d2 / p) is 0.8, the optical fiber has high birefringence and low loss in a wider terahertz frequency range.

[0055] Finally, the influence of the length L of the support bar on the optical fiber loss is analyzed, such as Figure 6 As shown in the figure, it can be found that as the length of the support bar increases, the losses of the X polarization mode and the Y polarization mode both tend to decrease. The main reason is that as the length of the support bar increases, the cladding thickness increases, and the limiting loss of the optical fiber decreases. However, the change in cladding thickness has almost no effect on the birefringence. In order to ensure a smaller optical fiber size, the thickness of the support bar is determined to be 3.5mm, so that the overall diameter of the optical fiber is less than 10mm.

[0056] In summary, the total loss of the final optical fiber structure is less than 20dB / m in the frequency range of 0.1~0.3THz, and the birefringence is within 10 -2 ~10 -3 Magnitude, such as Figure 7 The low-loss, high-birefringence holey core terahertz optical fiber of the present invention can realize polarization-maintaining transmission of terahertz waves in a short distance. At the same time, the overall diameter of the optical fiber is smaller than that of the hollow core optical fiber, and good bending can be achieved. Figure 8 is the distribution of modes at different frequencies.

[0057] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A low-loss, high-birefringence holey core terahertz optical fiber, characterized in that: comprising a core and a cladding covering the 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 the base material, wherein a large circular air hole is located at the center of the fiber core, and the remaining large circular air holes are symmetrically arranged in a regular triangle along the vertical direction of the cross section, and two of the elliptical air holes are horizontally located on the left and right sides of the large circular air hole in the center, and two of the 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 ring protective layer coaxially arranged with the fiber core, the protective layer is connected to the fiber core through a plurality of radial support strips, the support strips separate the cavity between the protective layer and the fiber core into a plurality of fan-shaped air holes, and the protective layer and the support strips are both base materials.

2. The low-loss, high-birefringence holey core terahertz optical fiber according to 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 short axis of the elliptical air hole is horizontal and the long axis is vertical, the short axis a of the elliptical air hole is: a=p / 2, and the long axis b is: Where p is the distance between adjacent large circular air holes.

3. The low-loss, high-birefringence holey core terahertz optical fiber according to 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 holey core terahertz optical fiber according to claim 3, characterized in that: The base material includes one of high-density polyethylene, low-density polyethylene, polyethylene, cycloolefin copolymer, cycloolefin polymer, polytetrafluoroethylene, polypropylene, polystyrene, polycarbonate, polymethyl methacrylate, polylactic acid and UV-resin materials.

5. A method for preparing a low-loss, high-birefringence holey core terahertz optical fiber as claimed in any one of claims 1 to 4, characterized in that: include: Put the barrel containing the base material particles into the heating furnace, and squeeze the base material particles to expel the air after they reach a molten state; Then, the molten base material is extruded through a structured die complementary to the optical fiber cross section, and the terahertz optical fiber of the desired size is stretched by a pulling device.

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