Zero-dispersion and dispersion-flattened photonic crystal fiber

Abstract

To provide an optical fiber which realizes a slope having a wavelength dispersion value of nearly zero and a zero-dispersion slope over a wide band with a wavelength of around 1.1 μm to 1.9 μm.SOLUTION: A first clad layer 22 has 6 holes with a diameter of d1 and a second clad layer 24 has 12 holes with a diameter of d2. The distance of holes adjacent to each other in the first clad layer and in the second clad layer, and the distance of the holes in the second layer adjacent to the holes in the first clad layer is both ΛB and the following condition is satisfied.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a photonic crystal fiber having a plurality of air holes extending along the fiber axis. The zero-dispersion, dispersion-flattened photonic crystal fiber according to the present invention can achieve characteristics such as a chromatic dispersion value close to zero and a zero dispersion slope over a wide bandwidth. [Background technology]

[0002] It is known that photonic crystal fibers can achieve optical properties that are not possible with conventional optical fibers by changing the spacing and size of the holes arranged in the cladding.

[0003] For example, it is known that a photonic crystal fiber can shift the zero dispersion wavelength to the shorter wavelength side compared to conventional optical fibers due to its large waveguide dispersion caused by air holes arranged in the cladding.

[0004] Furthermore, for example, Non-Patent Document 1 suggests that in a holey fiber (photonic crystal fiber) with a hole diameter d and an adjacent hole distance Λ, a region of flat dispersion exists when Λ is 2.3 μm and d / Λ is around 0.3.

[0005] On the other hand, it is known that even with conventional optical fibers, it is possible to realize optical fibers with zero dispersion and dispersion flatness around the wavelength of 1.55 μm by using a ring core structure. Such optical fibers are known to be effective in generating supercontinuum light using a 1.55 μm light source as a seed light source.

[0006] Meanwhile, Patent Document 1 states that, as a fiber for a supercontinuum light source, "the center-to-center spacing of the cladding features / holes is referred to as the pitch (Λ), and the microstructured fiber is characterized by the core dimensions and the ratio of the cladding feature dimensions to the spacing or pitch (Λ) of the cladding features. By adjusting the cladding feature dimensions and pitch, the zero dispersion wavelength (ZDW) of the fiber can be adjusted." Such photonic crystal fibers have a zero dispersion wavelength that is shorter than that of conventional optical fibers, and also have a small core diameter and high nonlinearity, so they are known to be effective in generating supercontinuum light with a seed light source having a wavelength of 1.06 μm.

[0007] As such, fibers for generating supercontinuum light either have zero dispersion on the short wavelength side, or zero dispersion and flat dispersion near a wavelength of 1.55 μm, but no optical fiber with chromatic dispersion near zero and zero dispersion slope over the broad wavelength band from 1.1 μm to 1.9 μm has been realized to date. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2020-74045 A (Patent No. 6974518) [Non-patent literature]

[0009] [Non-Patent Document 1] Tanya M.Monro et al,JOURNAL OF LIGHTWAVE TECHNOLOGY,VOL.17,NO.6,JUNE 1999 Summary of the Invention [Problem to be solved by the invention]

[0010] It has been recognized that photonic crystal fiber can shift the zero-dispersion wavelength to shorter wavelengths and achieve dispersion flatness over a narrow band. However, no optical fiber actually exists that achieves near-zero chromatic dispersion and zero dispersion slope over a broad band from 1.1 μm to around 1.9 μm. Furthermore, there are no guidelines regarding the structure of photonic crystal fiber to realize such an optical fiber. [Means for solving the problem]

[0011] In view of the above problems, the present invention has found that by shifting the zero dispersion wavelength to 1.1 μm toward the short wavelength side by using the holes in the first layer of the photonic crystal fiber and making the quartz part around the holes in the second layer function as a ring core, dispersion flat can be realized over a wide band around 1.55 μm, and an optical fiber with a chromatic dispersion value near zero and a zero dispersion slope from 1.1 μm to 1.9 μm can be realized. The inventors have also found an appropriate relationship between the hole spacing and the hole diameters in the first and second layers to realize a photonic crystal fiber with zero dispersion at a wavelength of 1.55 μm and dispersion flat from 1.1 μm to 1.9 μm. Furthermore, the inventors have found that in the above relationship, it is important that the hole diameter in the cladding region of the first layer falls within a certain range with respect to manufacturing variations in such a zero-dispersion, dispersion-flat photonic crystal fiber.

[0012] More specifically, the zero-dispersion and dispersion-flattened photonic crystal fiber according to the present invention is In a cross section perpendicular to the axis of the fiber body, A solid portion disposed at the center; The solid portion has three clad layers, namely, a first clad layer, a second clad layer, and a leakage prevention clad layer, which are successively formed from the center of the solid portion toward the outside in the radial direction, each cladding layer having a hole aligned along the axis of the fiber body; The first cladding layer has a diameter d1 (μm) The six holes are At the vertices of a regular hexagon Has The second cladding layer has a diameter d2 (μm)The twelve holes are At the vertices and center of the sides of a regular hexagon Has The distance between adjacent holes in each of the first cladding layer and the second cladding layer, and the distance between holes in the second cladding layer adjacent to the holes in the first cladding layer are both Λ B (μm) and is characterized in that the conditions of formulas (1) to (3) are satisfied.

[0013]

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[0014] The zero-dispersion, dispersion-flattened photonic crystal fiber according to the present invention has at least n cladding layers (n≧3), and the holes in the first cladding layer shift the zero-dispersion wavelength to the shorter wavelength side, while the ring core of the quartz part surrounding the holes in the second cladding layer makes it possible to flatten dispersion over a wide band around a wavelength of 1.55 μm. B Within a certain range, it is possible to almost uniquely determine the hole diameters in the first and second cladding layers of a fiber that has a chromatic dispersion value close to zero and zero dispersion slope characteristics in the wavelength range from 1.1 μm to 1.9 μm. In addition, it is possible to prevent light with longer wavelengths from spreading and leaking into the cladding region outside the holes in the second cladding layer by using holes in the third cladding layer and beyond.

[0015] This type of zero-dispersion, dispersion-flat photonic crystal fiber has a dispersion value close to zero and is dispersion-flat in the wavelength range from 1.1 μm to 1.9 μm, and can use any wavelength, such as 1.06 μm or 1.55 μm, as a seed light source.In addition, since it has flat dispersion over a wide band, it has excellent spread of supercontinuum light during propagation through the optical fiber.

[0016] Furthermore, regarding the manufacturing variability of such a zero-dispersion, dispersion-flattened photonic crystal fiber, it is sufficient that the hole diameter of the first cladding region falls within a certain range, and redundancy during actual fiber manufacturing can be improved, thereby providing an effect of providing a zero-dispersion, dispersion-flattened photonic crystal fiber that is easier to manufacture. [Brief description of the drawings]

[0017] [Figure 1] 1 is a schematic diagram showing a cross section of a zero-dispersion, dispersion-flattened photonic crystal fiber according to the present invention. [Diagram 2] FIG. 2 is an enlarged view of the cladding region. [Diagram 3] FIG. 4 is a diagram illustrating the distance between holes and the hole diameter of each cladding layer. [Figure 4] FIG. 13 is a graph showing the results of simulating the chromatic dispersion characteristics while changing the value of d2 / ΛB when ΛB=2.0 μm and d1 / ΛB=0.2. [Diagram 5] FIG. 13 is a graph showing the results of simulating the chromatic dispersion characteristics while changing the value of d2 / ΛB when ΛB=2.0 μm and d1 / ΛB=0.3. [Figure 6] FIG. 13 is a graph showing the results of simulating the chromatic dispersion characteristics while changing the value of d2 / ΛB when ΛB=2.0 μm and d1 / ΛB=0.4. [Figure 7] FIG. 13 is a graph showing the results of simulating the chromatic dispersion characteristics while changing the value of d2 / ΛB when ΛB=2.0 μm and d1 / ΛB=0.5. [Figure 8] This figure shows the zero dispersion (D=0) and flat dispersion (S=0) lines for each ΛB. [Figure 9] 1 is a graph showing the relationship between ΛB and d2 / ΛB. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The zero-dispersion, dispersion-flattened photonic crystal fiber according to the present invention will be described below with reference to drawings and examples. Note that the following description illustrates one embodiment of the present invention and one example, and the present invention is not limited to the following description. The following description can be modified within the scope of the present invention. In addition, embodiments and examples obtained by appropriately combining the technical means disclosed in different embodiments and examples are also included in the technical scope of the present invention. In addition, all documents described in this specification are incorporated herein by reference. In this specification, when a numerical range is described as "A to B," the description intends "A or more and B or less."

[0019] <Fiber configuration> Fig. 1 shows a cross-sectional view of a zero-dispersion, dispersion-flattened photonic crystal fiber 1 (hereinafter simply referred to as "photonic crystal fiber 1") according to the present invention. The axis of this photonic crystal fiber 1 runs perpendicularly from the front side to the back side of the page. Therefore, Fig. 1 shows a cross-section perpendicular to the axis of photonic crystal fiber 1.

[0020] Photonic crystal fiber 1 has solid portion 20 disposed at the center of glass material 12, and cladding region 10 provided around it. Width 12w of glass material 12 outside cladding region 10 may be any size.

[0021] Fig. 2 shows an enlarged view of the cladding region 10. The photonic crystal fiber 1 (see Fig. 1) has a solid portion 20 at the center of the glass material 12. The solid portion 20 is a fiber that has a circular cross section and extends in the axial direction. A first cladding layer 22, a second cladding layer 24, and a leakage prevention cladding layer 40 are formed around the solid portion 20. Fig. 2 shows that the leakage prevention cladding layer 40 is composed of three layers, namely, a third cladding layer 26, a fourth cladding layer 28, and a fifth cladding layer 30.

[0022] Here, the cladding layers refer to regions that are sequentially provided from the center of solid portion 20 toward the outside in the radial direction so as to envelop the inner cladding layers, and each cladding layer has holes provided along the axis of photonic crystal fiber 1 (fiber body). In other words, it can be said that the cladding layers are sequentially formed from the center of solid portion 20 toward the outside in the radial direction.

[0023] In the following description, the reference numerals of each cladding layer refer to the holes in that cladding layer, but when referring to the holes themselves, an alphabetical subscript is added to the reference numerals. For example, the holes in the first cladding layer 22 are referred to as holes 22a, 22b, etc.

[0024] The numbers of holes provided in the first cladding layer 22 and the second cladding layer 24 are fixed, 6 and 12, respectively. The holes in the first cladding layer 22 are arranged at the vertices of a regular hexagon, and the holes in the second cladding layer 24 are arranged at the vertices and the centers of the sides of the regular hexagon. Therefore, the distance between adjacent holes in the first cladding layer 22 and the distance between adjacent holes in the second cladding layer 24 are equal. Here, the distance between the holes refers to the distance between the centers of the holes. Also, "adjacent" refers to holes that are in a relationship such that the distance between the holes is the shortest.

[0025] In the photonic crystal fiber 1 of the present invention, the distance between adjacent holes in the first cladding layer 22 is equal to the distance between adjacent holes in the second cladding layer 24. This distance between holes is defined as Λ B Let us assume that.

[0026] Furthermore, in the photonic crystal fiber 1 of the present invention, the distance between adjacent holes in the first cladding layer 22 and the second cladding layer 24 is also Λ B That is, in any combination, three adjacent holes in the first cladding layer 22 and the second cladding layer 24 are located at the vertices of an equilateral triangle.

[0027] As described above, the cladding region 10 has at least three cladding layers, and the diameter of the hole belonging to the nth cladding layer is defined as dn (n is a natural number). Therefore, the diameters of the holes in the first cladding layer 22 and the second cladding layer 24 are d1 and d2, respectively. In the photonic crystal fiber 1 of the present invention, the diameters of the holes in the first cladding layer 22 and the second cladding layer 24 are equal to each other. That is, the diameters of all six holes (diameter of the holes) in the first cladding layer 22 are d1, and the diameters of all twelve holes in the second cladding layer 24 are d2.

[0028] There are no particular limitations on the distance between the holes and the diameter of the holes in the leak-proof cladding layer 40. The leak-proof cladding layer 40 is provided to prevent light passing through the fiber from leaking outside the cladding region 10, or to minimize the leakage. Therefore, as long as this purpose is achieved, there are no particular limitations on the distance between the holes and the diameter of the holes.

[0029] Incidentally, forming the leakage prevention cladding layer 40 from three cladding layers as shown in Fig. 2 can be said to be one of the preferable configurations of the leakage prevention cladding layer 40. The configuration of Fig. 2 will be described in detail below.

[0030] In the configuration shown in Fig. 2, 6k air holes are arranged in the kth (1 <= k <= n: k is a natural number) cladding layer. Here, n is the total number of cladding layers, including the cladding layers in the leakage prevention cladding layer 40. In Fig. 2, in the third cladding layer 26 and subsequent cladding layers, the air holes are arranged at the vertices and sides of a hexagon. In the third cladding layer 26 and subsequent cladding layers, the air hole diameters of each layer are d1 to d5 as described above (d3 and d4 are omitted in Fig. 2).

[0031] Moreover, in the cladding layers after the third cladding layer 26, the hole diameters are all the same. That is, in FIG. 2, d3 to d5 are all the same hole diameter. Moreover, d3 to d5 are formed for the purpose of confining light that spreads outward from the second cladding layer 24 and leaks as the wavelength becomes longer, and have almost no effect on the dispersion characteristics of the photonic crystal fiber 1. It is sufficient that the holes have a size sufficient to prevent light leakage. For example, it is sufficient to satisfy the relationship of formula (4).

[0032]

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[0033] In the photonic crystal fiber 1 according to the present invention, the zero-dispersion wavelength is shifted to the short wavelength side by adjusting the hole diameter d1 in the first cladding layer 22, and the ring core made of the surrounding quartz part (glass material 12) by adjusting the hole diameter d2 in the second cladding layer 24 realizes a chromatic dispersion value near zero and a zero dispersion slope over a wide band around a wavelength of 1.55 μm. Furthermore, the cladding layers after the third cladding layer 26 confine light that spreads outward beyond the second cladding layer 24 as the wavelength becomes longer. For example, light leakage can be prevented by satisfying the relationship in formula (4).

[0034] 3 shows an enlarged view of a part of FIG. 2. The holes in the first cladding layer 22 to the fifth cladding layer 30 are shown with the solid portion 20 at the center. Here, the distance Λ between adjacent holes belonging to the same cladding layer is B is expressed as L mm (1 ≦ m ≦ n-1: m is a natural number), where n is the total number of cladding layers. Also, the distance Λ between adjacent holes belonging to the cladding layer one layer outside is B is represented as Lmm+1. In Fig. 3, L11, L22, L33, L44, and L55 are adjacent air hole distances belonging to the same cladding layer, and L12, L23, L34, and L45 are adjacent air hole distances belonging to adjacent cladding layers. In one example of the photonic crystal fiber 1 according to the present invention, all of these adjacent air hole distances are the same Λ B It is.

[0035] <Numerical simulator> Next, a numerical simulation is used to determine the characteristics of the photonic crystal fiber 1 according to the present invention. The numerical simulator is commercially available software, BeamPROP from the former Rsoft company. In addition, the scalar wave equation is solved by the imaginary axis beam propagation method assuming that the horizontal and vertical directions are degenerate.

[0036] First, to verify the validity of the numerical simulator, we calculated the chromatic dispersion characteristics of a normal single-mode fiber. The calculated dispersion value was 17 ps / (nm km) at a wavelength of 1.55 μm, which is consistent with the measured value, and we can say that the calculation results of the numerical simulator are valid.

[0037] <Calculation results> Using the above-mentioned numerical simulator, a numerical simulation was performed under the conditions shown in Figs. 1 to 3.

[0038] Λ B is 2.0μm, d3 / Λ B = 0.8, d1 / Λ B , d2 / Λ B While varying each of these, the dispersion value (ps / (nm·km)) was calculated and plotted against the wavelength (μm) of light at the cross section of the photonic crystal fiber 1. Some of the results are shown in Figs.

[0039] Figure 4 shows the relationship between d1 / Λ B =0.2, Figure 5 shows d1 / Λ B =0.3, Fig. 6 shows d1 / Λ B =0.4, Fig. 7 shows d1 / Λ B = 0.5, respectively. B = 0.2 to 0.8.

[0040] Referring to Figure 6, d2 / Λ B When = 0.3 (shown by symbol A), the dispersion was -1.4 to 4.8 (ps / (nm km)) over the wide wavelength range from 1.1 μm to 1.9 μm, and a chromatic dispersion value of nearly zero was achieved in this wavelength band. In addition, the average dispersion slope at each wavelength in 0.05 μm increments between 1.1 μm and 1.9 μm was 0.0066 (ps / (nm km)), and a zero dispersion slope was achieved.

[0041] In the photonic crystal fiber 1 of the present invention, a chromatic dispersion value near zero means that the dispersion value is -5 (ps / (nm·km)) to 5 (ps / (nm·km)), and a zero dispersion slope means that the average dispersion slope in the target wavelength range is 0.01 (ps / (nm·km)) or less.

[0042] Please refer to Fig. 8. Fig. 8 shows the relationship between the distance Λ between adjacent vacancies and the calculation results shown in Figs. B The calculation results are shown when the thickness is changed to 1.75 μm, 1.5 μm, and 1.25 μm (Λ B = 2.0μm is also shown). The solid line is the line where the dispersion value at a wavelength of 1.55μm is zero (denoted as "D = 0"), and the dashed line is the line where the average dispersion slope at wavelengths of 1.1μm to 1.9μm is zero (denoted as "S = 0"). Note that when calculating the average dispersion slope, the dispersion slope at each wavelength in 0.05μm increments between 1.1μm and 1.9μm was calculated and the average value was used. The intersection of the solid line and the dashed line is the point where zero dispersion and zero dispersion slope can be formed. This is called the zero point. The zero point is the point where the dispersion value at a wavelength of 1.55μm is zero and the average dispersion slope at wavelengths of 1.1μm to 1.9μm is zero.

[0043] Looking at this graph, d1 / Λ B We can see that the zero points are in the range from 0.4 to 0.42. In other words, it can be expressed as equation (2).

[0044]

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[0045] In Fig. 9, this zero point is shown on the horizontal axis Λ B (distance between adjacent holes), vertical axis is d2 / Λ B This point can be summarized by the relationship in equation (3).

[0046]

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[0047] In addition, Λ B By examining the range of, we were able to confirm the zero point within the range of equation (1).

[0048]

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[0049] Also, d1 = 0.4Λ B In the case of Λ B is 1.79 or more, d1>d2. Similarly, d1=0.42Λ B In the case of Λ B is 1.83 or more, and d1>d2. Therefore, at least Λ B If Λ is 1.79 or more, the relationship d1>d2 is satisfied, and zero dispersion and flat dispersion slope characteristics can be obtained. B is Λ B is sufficiently large, resulting in a large core diameter, which provides excellent connectivity with conventional optical fibers.

[0050] From the above, in the photonic crystal fiber 1 having the configuration of Figures 1 to 3, a design having the relationship of equations (1) to (3) can achieve zero dispersion at a wavelength of 1.55 μm and zero dispersion slope characteristics over the wavelength range from 1.1 μm to 1.9 μm.

[0051]

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[0052] An example of the cladding layer within the leakage prevention cladding layer 40 can be realized by satisfying formula (4).

[0053]

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[0054] The photonic crystal fiber according to the present invention can be suitably used as a fiber for a supercontinuum light source. [Explanation of symbols]

[0055] 1. Zero-dispersion and dispersion-flattened photonic crystal fiber 1. Photonic crystal fiber 10 Cladding Region 12 Glass material 12w (glass material) width 20 Solid part 22 First cladding layer 24 Second cladding layer 26 3rd cladding layer 28 4th cladding layer 30 5th cladding layer 40 Leak-proof cladding layer

Claims

[Claim 1] In a cross section perpendicular to the axis of the fiber body, A solid portion disposed at the center; The solid portion has three clad layers, namely, a first clad layer, a second clad layer, and a leakage prevention clad layer, which are successively formed from the center of the solid portion toward the outside in the radial direction, each cladding layer having a hole aligned along the axis of the fiber body; The first cladding layer has a diameter d １ The six holes each having a diameter of 1 μm are located at the vertices of a regular hexagon, The second cladding layer has a diameter d ２ The 12 holes having a diameter of 1 μm are located at the vertices and the center of the sides of a regular hexagon, The distance between adjacent holes in each of the first cladding layer and the second cladding layer, and the distance between holes in the second cladding layer adjacent to the holes in the first cladding layer are both Λ Ｂ (μm) and satisfies the conditions of formulas (1) to (3). [Number 100]

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