JP2019050537A - Photonic crystal fiber - Google Patents

Abstract

To widen a single mode band width while lowering a propagation loss of a photonic crystal fiber.SOLUTION: A photonic crystal fiber includes a fiber body 101, a core region 102 placed in a central part of the fiber body 101, and a clad region 103 surrounding the core region 102. The photonic crystal fiber includes six first holes 104 placed in a first clad region adjacent to the core region 102, and multiple second holes 105 placed in a second clad region surrounding the first clad region. Assuming an inter-grid distance of a triangular lattice is λ, a diameter of the core region 102 is 1.244λ, a diameter of the first hole 104 is 0.756λ, and a diameter of the second hole 105 is 0.42λ.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a photonic crystal fiber comprising a plurality of air holes extending along a fiber axis.

  At present, metal waveguides are generally used to exchange and propagate signals between electrical components in the terahertz (THz) band. However, the following three problems exist in metal waveguides. First, metal waveguides suffer from high propagation losses. Long distance propagation is a difficult point. Second, metal waveguides are not flexible because they are made of metal, and there is a problem that the flexibility of connection between parts is lost. Third, metal waveguides have a narrow single-mode band, and can not propagate broadband signals.

  In order to solve these problems, photonic crystal fibers (PCF) have been proposed (see Non-Patent Document 1). As shown in FIG. 4, the PCF includes a core portion 202 uniformly filled with a dielectric material at the center of the cross section of the fiber body 201, and a cladding portion 203 around the core portion 202. In the cladding portion 203, a plurality of holes 204 extending along the fiber axis are periodically arranged in a sectional view.

  Since PCF uses a dielectric material, low propagation loss can be realized if a material with no conductor loss and a low dielectric loss tangent (tand δ) is used. In addition, since it is a nonmetallic material, it is possible to have flexibility. Furthermore, since a PCF-specific endless single mode can be expressed, a wide band single mode band can be realized. The dielectric loss tangent indicates the degree of electrical energy loss in the dielectric.

M. GOTO et al., "Teflon Photonic Crystal Fiber as Terahertz Waveguide", Japanese Journal of Applied Physics, vol. 43, no. 2B, pp. L317-L319, 2004.

  However, the conventional PCF has a trade-off relationship between the propagation loss and the single mode band. As shown in the simulation results in FIG. 5, when the diameter of the holes is small [(a) in FIG. 5], the propagation loss is large because the energy confinement in the core is weak, but the effective dielectric constant between core and cladding Because the difference is small, the single mode band is wide [Fig. 5 (b)]. In addition, since the high-order mode stands above 500 GHz, the transmission loss of the high-order mode is not shown in the scale of FIG. 5 (b).

  On the other hand, as shown in FIG. 6, when the diameter of the hole is large [(a) in FIG. 6], the propagation loss is small because the energy confinement in the core is strong, but the effective dielectric constant difference between core and cladding The single mode band is narrow because of large [Figure 6 (b)].

  As described above, in the conventional PCF, there is a problem that it is difficult to obtain a wide single mode band with low propagation loss.

  The present invention has been made to solve the above problems, and it is an object of the present invention to widen a single mode band in a state where the propagation loss of the photonic crystal fiber is lowered.

  A photonic crystal fiber according to the present invention comprises a fiber body, a core region disposed at a central portion of the fiber body, a cladding region surrounding the core region, and a plurality of hollows extending along a fiber axis formed in the cladding region. And a two-dimensional periodic structure of a plurality of periodically arranged holes in a cross section perpendicular to the fiber axis of the cladding region, the plurality of holes being a first cladding region adjacent to the core region And six first holes disposed in the first cladding region and a plurality of second holes disposed in the second cladding region surrounding the first cladding region, the first Each of the holes is disposed at the apex of a regular hexagon centered on the central portion of the core region, and the center of each of the six first holes and the plurality of second holes is a cross section perpendicular to the fiber axis , On the grid points of the triangular grid In the cross section perpendicular to the fiber axis, assuming that the lattice distance of the triangular lattice is λ, the diameter of the core region is 1.244 λ, the diameter of the first air hole is 0.756 λ, and the second air hole is Has a diameter of 0.42λ.

  In the photonic crystal fiber, the plurality of second holes may be 30 or more.

  In the above-mentioned photonic crystal fiber, the fiber main body may be made of fluorine resin.

  As described above, according to the present invention, the diameter of the core region is 1.244 λ and the diameter of the first hole is 0.756 λ with respect to the lattice distance λ of the triangular lattice in which the holes are disposed. Since the diameter of the second hole is 0.42 λ, an excellent effect can be obtained that the single mode band can be broadened in a state where the propagation loss of the photonic crystal fiber is lowered.

FIG. 1 is a cross-sectional view showing the configuration of a photonic crystal fiber according to an embodiment of the present invention. FIG. 2 is an explanatory view showing the result of simulating the frequency characteristic of the photonic crystal fiber of the embodiment. FIG. 3: is explanatory drawing which shows the result of having compared the photonic crystal fiber in embodiment, and a metal waveguide. FIG. 4 is a cross-sectional view showing the configuration of a conventional photonic crystal fiber. FIG. 5 is an explanatory view showing the result of simulating the frequency characteristic of the conventional photonic crystal fiber. FIG. 6 is an explanatory view showing the result of simulating the frequency characteristic of the conventional photonic crystal fiber.

  Hereinafter, the photonic crystal fiber in the embodiment of the present invention will be described with reference to FIG. The photonic crystal fiber comprises a fiber body 101, a core region 102 disposed at the center of the fiber body 101, and a cladding region 103 surrounding the core region 102. As is well known, a photonic crystal fiber comprises a core region 102 and a cladding region 103, and comprises a plurality of holes extending along the fiber axis formed in the cladding region 103, along the fiber axis. It has a uniform refractive index distribution. The plurality of holes have a two-dimensional periodic structure periodically arranged in a cross section perpendicular to the fiber axis of the cladding region 103.

  The photonic crystal fiber according to the present invention comprises six first holes 104 disposed in the first cladding region adjacent to the core region 102 and a plurality of the first cladding regions disposed in the second cladding region surrounding the first cladding region. And 2 holes 105 are provided. All the first holes 104 have the same diameter, and all the second holes 105 have the same diameter. In FIG. 1, the inside of the dashed dotted circle is the first cladding region, and the outside is the second cladding region.

  In the cross section perpendicular to the fiber axis, each of the six first holes 104 is arranged at the apex of a regular hexagon centered on the central portion of the core region 102. In the cross section perpendicular to the fiber axis, the centers of the six first holes 104 and the plurality of second holes 105 are arranged at lattice points of the triangular lattice. In the cross section of the photonic crystal fiber, an equilateral triangle is formed by the center of each of three adjacent holes.

  In the embodiment, a ring-shaped portion is formed by the six first holes 104 at the innermost side, a ring-shaped portion by the twelve second holes 105 is formed adjacent to the outer side, and adjacent to the outer side. An annular portion is formed of eighteen second holes 105. In the embodiment, the number of second holes 105 is set to thirty. A ring-shaped portion formed of 24 second holes 105 may be formed outside the ring-shaped portion formed of 18 second holes 105. Similarly, a ring of 30 second holes 105 may be formed on the outside of the ring of 24 second holes 105.

  In the cross section perpendicular to the fiber axis, assuming that the lattice distance of the triangular lattice (length of one side of an equilateral triangle) is λ, the diameter of the core region 102 is 1.244 λ, and the diameter of the first hole 104 Is set to 0.756λ, and the diameter of the second holes 105 is set to 0.42λ. As a result, low loss can be realized while the single mode band has a relative band of 100% or more with respect to the center frequency.

  For example, the fiber body 101 is made of a fluorocarbon resin, the inter-grating distance is 1.5 mm, the diameter of the core region 102 is 1.866 mm, and the diameter of the first holes 104 is 1.134 mm. The diameter of the second holes 105 may be 0.63 mm. Also, the diameter of the fiber body 101 may be 15 mm.

  According to the photonic crystal fiber in the above-described embodiment, as described below, the single mode band can be broadened in a state in which the propagation loss is reduced.

  About the photonic crystal fiber of the embodiment which set it as each dimension mentioned above and made the 2nd cavity 105 30 pieces, the result of having simulated the frequency characteristic is explained. As shown in (b) of FIG. 2, the minimum loss was −0.031 dB / 2 mm, and the single mode band was 300 GHz. On the other hand, the minimum loss of the conventional photonic crystal fiber described with reference to FIG. 6 is -0.024 dB / 2 mm, and the single mode band is about 100 GHz. In the conventional photonic crystal fiber, the diameter of all the holes is 1.134 mm, and the other conditions are the same.

  As can be seen from the simulation results, according to the photonic crystal fiber in the embodiment, the single mode band can be extended by three times or more while maintaining the propagation loss at the same level as the conventional photonic crystal fiber. From the characteristics of the photonic crystal fiber, it is considered that if the number of the second holes 105 is 30 or more, the above-mentioned effect can be sufficiently obtained.

  Next, the comparison between the photonic crystal fiber and the metal waveguide in the embodiment in which each dimension is set to 30 and the number of second holes 105 is 30, will be described with reference to FIG. Here, each dimension of the metal waveguide used waveguide standard WR3. While the minimum loss of the metal waveguide is -0.24 dB / cm, the photonic crystal fiber (PCF) in the embodiment is as small as -0.17 dB / cm. In addition, the single mode band of the metal waveguide is 150 GHz. On the other hand, the photonic crystal fiber (PCF) in the embodiment is twice as large as 300 GHz as shown in the electromagnetic field distribution of the photonic crystal fiber in (b) and (c) of FIG. As shown in the electromagnetic field distribution of the photonic crystal fiber of (a) of FIG. 3, light confinement is weak in a band near a frequency of 100 GHz, and the single mode is not obtained.

  As described above, it is an object of the present invention to realize low loss while the single mode band of the photonic crystal fiber has a relative band of 100% or more with respect to the center frequency. The design concept of each parameter of the photonic crystal fiber corresponding to such purpose is as follows. First, the cutoff frequency on the low frequency side (for example, the portion of 100 GHz of the solid line in FIG. 3) is determined by the diameter of the core region. As the diameter of the core region is larger, the cutoff frequency on the low frequency side shifts to the low frequency side, that is, it transmits from the lower frequency. The diameter of the core region is about the wavelength.

  On the other hand, in the single mode band, the waveguide parameter called V value needs to be 2.4 or less. This V value is a function of the effective refractive index difference of the core region and the cladding region, and in the case of a photonic crystal fiber, has frequency characteristics (or wavelength deviation). Specifically, the higher the frequency, the larger the V value, and when the V value exceeds 2.4, higher order modes begin to stand up. The frequency at this time is the cutoff frequency on the high frequency side of the single mode. In this state, the propagation characteristics of the fundamental mode do not necessarily deteriorate.

  The factors that determine the frequency characteristics of the effective refractive index difference are the diameter and spacing (interlattice distance) of the holes in the cladding region. Although theoretical analysis on this factor does not yet exist, as a result of intensive investigations by the inventors, the above-described lattice distance λ, the diameter of the core region, the diameter of the first hole, and the second space We found a relationship with the diameter of the hole.

  As described above, according to the present invention, the diameter of the core region is 1.244 λ and the diameter of the first hole is 0.756 λ with respect to the lattice distance λ of the triangular lattice in which the holes are disposed. Since the diameter of the second air holes is 0.42 λ, it is possible to widen the single mode band in a state in which the propagation loss of the photonic crystal fiber is lowered.

  The present invention is not limited to the embodiments described above, and many modifications and combinations can be made by those skilled in the art within the technical concept of the present invention. It is clear.

  101 ... fiber body, 102 ... core region, 103 ... cladding region, 104 ... first hole, 105 ... second hole.

Claims (3)

A fiber body,
A core region located at the center of the fiber body;
A cladding region surrounding the core region;
A plurality of holes extending along a fiber axis formed in the cladding region;
In a cross section perpendicular to the fiber axis of the cladding region, it has a two-dimensional periodic structure by the plurality of holes periodically arranged,
The plurality of vacancies are six first vacancies disposed in a first cladding region adjacent to the core region, and a plurality of second vacancies disposed in a second cladding region surrounding the first cladding region. Equipped with holes and
In a cross section perpendicular to the fiber axis, each of the six first holes is arranged at an apex of a regular hexagon centered on a central portion of the core region;
In a cross section perpendicular to the fiber axis, centers of each of the six first holes and the plurality of second holes are arranged at lattice points of a triangular lattice;
In the cross section perpendicular to the fiber axis, assuming that the lattice distance of the triangular lattice is λ, the diameter of the core region is 1.244 λ, and the diameter of the first air hole is 0.756 λ. The diameter of the air holes is 0.42 λ. A photonic crystal fiber characterized in that In the photonic crystal fiber according to claim 1,
30. A photonic crystal fiber, wherein the plurality of second holes are 30 or more. In the photonic crystal fiber according to claim 1 or 2,
A photonic crystal fiber, wherein the fiber body is made of a fluorocarbon resin.