JP2005140857A - Dispersion-flat fiber - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a DFF that is simple in structure and that can be manufactured at a low cost. <P>SOLUTION: The DFF 10 has a pair of voids 12a, 12a in a clad 12. With this pair of holes 12a, 12a, the dispersion value of the DFF 10 shows no wavelength dependency. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a dispersion flat fiber (hereinafter referred to as “DFF”).

  As light propagates through the optical fiber, chromatic dispersion occurs. The wavelength dispersion includes material dispersion and structure dispersion. Material dispersion is caused by the fact that the refractive index of the glass constituting the core and cladding of the optical fiber depends on the wavelength of light. On the other hand, structural dispersion is caused by the wavelength dependence of the confinement state of light in the core. When the refractive index changes with the wavelength, the refractive index felt by the wave packet, that is, the group refractive index also changes with the wavelength. Further, when the light pulse propagates through the optical fiber, it proceeds by a value obtained by dividing the speed of light by the group refractive index. Therefore, when the group refractive index shows wavelength dependence, the light pulse travels at different speeds depending on the wavelength, and arrives at the output end in the optical fiber at different times. In a single mode fiber (hereinafter referred to as “SMF”), the change in the confinement state with respect to the wavelength is relatively small, and the group refractive index changes mainly due to material dispersion. In the SMF, at a wavelength longer than the zero-dispersion wavelength of 1.3 μm, the group refractive index shows a larger value as the wavelength becomes longer, and shows a convex curve change downward with respect to the wavelength. On the other hand, by using a confinement structure of the optical fiber so as to eliminate the wavelength change of the group refractive index due to material dispersion, a DFF having no wavelength dependency of the group refractive index can be realized. In DFF, since all wavelengths travel through the optical fiber at the same speed, optical pulses of different wavelengths can be transmitted without delay or distortion. In addition, the interaction between wavelengths due to the nonlinear optical effect in the optical fiber is increased, which can be used for generation of supercontinuum light.

  For example, Non-Patent Document 1 discloses a technique for reducing the wavelength dependence of the dispersion value by making the fiber structure a quadruple structure. The DFF 50 disclosed in Non-Patent Document 1 is made of quartz glass, and as shown in FIG. 13A, a core 51, a first cladding 52 provided so as to cover the core 51, and a first cladding A second clad 53 provided so as to cover 52, a third clad 54 provided so as to cover the second clad 53, and a fourth clad 55 provided so as to cover the third clad 54. ing. Therefore, light propagates under the influence of the first cladding 52. The core 51 and the second clad 53 are made of a material that increases the refractive index of quartz glass such as germanium, and the first clad 52 and the third clad 54 are made of a material that lowers the refractive index of quartz glass such as fluorine. Doped. FIG. 13B shows a refractive index profile of the DFF 20 in the fiber cross section. As shown in FIG. 13B, the refractive index of the first cladding 52 is smaller than the refractive index of the core 51.

  Non-Patent Document 2 discloses a technique for reducing the wavelength dependence of the dispersion value by providing holes in the cladding. As shown in FIG. 14, the DFF 60 disclosed in Non-Patent Document 2 includes a core 61 and a clad 62 provided so as to cover the core 61, and the clad 62 extends in plural along the core 61. The photonic crystal structure is formed in the fiber radial direction by the holes 62a, 62a,. Then, a low refractive index portion 62b corresponding to one hole is formed so as to be adjacent to the core 61 and to divide the fiber cross section into three equal parts, and each low refractive index portion 62b is made of fluorine or the like. A material that lowers the refractive index of quartz glass is doped. Therefore, holes 62a, 62a,... And three low refractive index portions 62b, 62b, 62b are provided so as to be adjacent to the core 61 in the fiber cross section.

Generally, when the wavelength of propagating light (hereinafter referred to as “propagating light”) becomes longer, the mode field diameter becomes larger. Therefore, the propagating light is affected by the clad 52 described in Patent Document 1 and the holes 62a and the low refractive index portions 62b described in Patent Document 2. As a result, the group refractive index by the structure draws a convex graph upward with respect to the wavelength, and becomes smaller as the wavelength becomes longer. That is, in the DFFs 50 and 60, the group refractive index due to the material and the group refractive index due to the structure exhibit completely opposite wavelength dependence. In addition, the amount of change of the group refractive index with respect to the wavelength due to the material and the amount of change of the group refractive index with respect to the wavelength due to the structure exhibit completely opposite behavior. Therefore, the group refractive index in the DFFs 50 and 60 does not depend on the wavelength, and the amount of change of the group refractive index with respect to the wavelength does not depend on the wavelength. As described above, a complicated optical fiber confinement structure such as the DFFs 50 and 60 is adopted so as to eliminate the wavelength change of the group refractive index due to the material.
BJ Ainslie et al, "A Novel Design of a Dispersion Compensating Fiber", J. of Lightwave Technorol., Vol.LT-4, No.8, 1986 p.967-979 KP Hansen et al, "Fully Dispersion Controlled Triangular-Core Nonlinear Photonic Crystal Fiber", OFC, vol.8, No.11, 2003 PD2-1

  However, the DFFs 50 and 60 have a complicated structure in the fiber cross section as shown in FIGS. Therefore, design and manufacture are not easy. In addition, it is expensive to manufacture.

  The present invention has been made in view of this point, and an object of the present invention is to provide a DFF having a simple structure in a fiber cross section and capable of being manufactured at low cost.

  Invention of Claim 1 is a dispersion | distribution flat fiber provided with the core and the clad which covers this core, Comprising: As for the said clad, the void | hole which makes dispersion value substantially constant irrespective of the wavelength of the light to propagate is said It is characterized by being formed along the core.

  According to said structure, the wavelength dependence of the dispersion value of propagation light becomes small by forming a void | hole in a clad. In the conventional DFF, four types of clads are provided so that a region where the refractive index distribution is larger than that of quartz glass and a region where the refractive index distribution is smaller than that of quartz glass appear alternately twice from the center of the fiber, or the fiber Some of the plurality of holes constituting the photonic crystal structure in the radial direction are doped with a material such as fluorine that lowers the refractive index of quartz glass. As described above, the fiber structure in the fiber cross section is complicated and cannot be easily designed and manufactured. However, in the DFF of the present invention, the structure of the fiber in the fiber cross section is simple because it is only necessary to form holes in the cladding so that the dispersion value of propagating light does not exhibit wavelength dependence. In addition, since only holes are formed in the optical fiber, it can be manufactured at low cost.

  The invention according to claim 2 is characterized in that a dispersion value of propagating light is −3 to 3 (ps / nm / km).

  According to said structure, the delay of the optical pulse by the wavelength in optical fiber propagation and the distortion of an optical pulse spectrum can be made small. Further, the interaction of the nonlinear optical effect can be increased in the length direction.

  The invention according to claim 3 is characterized in that a pair of the holes are provided so as to sandwich a core having a relative refractive index difference of 3% or less.

  According to the above configuration, the relative refractive index difference is a small value of 3% or less, and the dispersion value does not depend on the wavelength and shows a value close to 0. In addition, the plane of polarization of the propagating light is retained.

  The invention according to claim 4 is characterized in that the pair of holes are provided at substantially the same interval as a mode field diameter of the propagating light in a direction in which the pair of holes is not provided. To do.

The invention of claim 5 is characterized in that the cut-off wavelength λ _c (μm) and the mode field diameter x (μm) satisfy Formula 1.

(Where n: refractive index, Δ: relative refractive index difference (%) of the core, λ: wavelength of propagating light (μm), Dm: material dispersion value at the wavelength of propagating light (ps / nm / km) )
The above two configurations are specific examples for acting as a DFF with good performance. The mode field diameter means the mode field diameter in the direction perpendicular to the direction connecting the centers of the pair of holes (hereinafter referred to as “mode field diameter in the y-axis direction”).

  The DFF of the present invention has a simple structure in the fiber cross section and can be manufactured at low cost.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Embodiment 1 of the Invention
In the first embodiment, the structure, manufacturing method, and principle of dispersion flat of the DFF 10 will be described.

  First, the structure of the DFF 10 will be described.

  FIG. 1 shows a cross section of the DFF 10.

  The DFF 10 is made of quartz glass, and includes a solid core 11 and a clad 12 provided so as to cover the core 11. The core 11 is doped with a material that increases the refractive index of quartz glass such as germanium. The relative refractive index difference of the core 11 is 3% or less. Here, if the relative refractive index difference of the core 11 is 3% or less, the propagation light is not scattered so violently, so that the transmission loss does not increase so that the practical use becomes difficult. A pair of air holes 12 a and 12 a are formed in the clad 12 so as to sandwich the core 11. The interval between the core 11 and the holes 12a (hereinafter referred to as “core-hole interval”) is 1.0 to 2.5 μm. The holes 12a and the holes 12a are provided with an interval approximately equal to the mode field diameter in the y-axis direction.

  Next, a method for manufacturing the DFF 10 will be briefly described.

  In the first manufacturing method, first, one quartz glass rod doped with germanium, two quartz capillaries, and a plurality of pure quartz glass rods are prepared.

  Next, a base material is manufactured by bundling a quartz glass rod, a quartz capillary, and a glass rod made of pure quartz in a close-packed state. At this time, the quartz glass rod is disposed at the center position of the base material. Then, two quartz capillaries are arranged so as to sandwich the quartz glass rod. A plurality of pure quartz glass rods are disposed so as to surround the quartz glass rod and the quartz capillary.

  Subsequently, the base material is drawn by heating and stretching. At this time, between the quartz glass rod and the quartz capillary, between the quartz glass rod and the pure quartz glass rod, between the quartz capillary and the pure quartz glass rod, and between the pure quartz glass rods are fused together. The DFF 10 is manufactured to have a reduced diameter and formed such that the quartz glass rod corresponds to the core, the quartz capillary corresponds to the hole, and the pure quartz glass rod corresponds to the cladding.

  In the second manufacturing method, first, a solid quartz glass rod doped with germanium at the center of the rod is prepared.

  Next, using a drill, two holes are made in the rod cross section and penetrated in the longitudinal direction of the rod. At this time, two substantially circular holes are formed so as to sandwich the portion doped with germanium.

  Subsequently, the base material is drawn by heating and stretching. At this time, the DFF 10 is manufactured in which the germanium-doped portion corresponds to the core, the hole opened by the drill corresponds to the hole, and the quartz glass portion corresponds to the cladding.

  Finally, the principle of the dispersion flat in the DFF 10 will be described.

The DFF 10 is formed with a pair of air holes 12a and 12a spaced apart from the core 11 by 1.0 to 2.5 μm (hereinafter referred to as “close to the core”). Further, the relative refractive index difference of the core 11 is 3% or less. Therefore, light is propagated under the influence of each hole 12a. Here, the refractive index of each air hole 12 a is substantially the same as the refractive index of air, and is smaller than the refractive index of the core 11. As a result, the contribution due to structural dispersion and the contribution due to material dispersion are offset, and the group refractive index does not exhibit wavelength dependence. Therefore, the speed of the propagation light of the DFF 10 does not show wavelength dependence, and the dispersion value does not show wavelength dependence. In addition, when the core-hole spacing or the relative refractive index difference of the core 11 is changed, the influence of propagating light from each hole 12a differs. Therefore, the dispersion value of the DFF 10 can be changed. Therefore, the structure of the DFF 10 can be determined so that the dispersion value is relatively close to 0, specifically, −3 to 3 (ps / nm / km). FIG. 2 shows the relationship between the mode field diameter in the y-axis direction (indicated as “x” in FIG. 2) and the value of the central term in Equation 1 (indicated as “y” in FIG. 2). FIG. Then, the value on the vertical axis of the graph of FIG. 2 can be obtained by substituting the value of the actually measured cutoff wavelength λ _c into the central term of Equation 1. The solid line is a graph when the dispersion value of the entire DFF 10 is 3 (ps / nm / km), and the broken line is a graph when the dispersion value of the entire DFF 10 is −3 (ps / nm / km). An optical fiber showing the value of x and the value of y existing in the region surrounded by the two curves, that is, the cutoff wavelength λ _c , has a dispersion value that does not depend on the wavelength, and the dispersion value is close to zero. Will be shown. And DFF10 is formed so that the space | interval of the hole 12a and the hole 12a may become substantially the same as the mode field diameter in a y-axis direction. Therefore, if the mode field diameter is determined, the distance between the holes 12a is determined, and the fiber structure of the DFF 10 is determined.

  As described above, by forming the pair of hole diameters 12a and 12a in the vicinity of the core 11, the dispersion value of the propagation light of the DFF 10 does not depend on the wavelength. As shown in FIGS. 13A and 14, the conventional DFF has a structure that is very similar to a quadruple clad structure or a photonic crystal structure, and thus the fiber structure is complicated. As a result, designing and manufacturing are difficult, and manufacturing costs are high. However, in the DFF 10 of the present invention, it is only necessary to form the pair of holes 12a and 12a so as to be close to the core 11, so that the fiber structure is simple and can be manufactured at low cost. Further, the dispersion value can be set to 0 by changing values such as the core-hole interval, the interval between the holes 12a and 12a, and the relative refractive index difference of the core 11.

Further, the value of the relative refractive index difference is relatively small at 3%. Here, a case is considered where light is propagated through an optical fiber made of pure quartz provided with only holes. In this case, the group refractive index due to structural dispersion draws a convex graph upward with respect to the wavelength, and shows a larger value as the wavelength becomes longer. Therefore, the group refractive index due to material dispersion eliminates the wavelength dependence, that is, draws a downward convex graph with respect to the wavelength, and shows a smaller value as the wavelength becomes longer. The value of the relative refractive index difference of the core must be determined such that the amount of change in the group refractive index due to structural dispersion with respect to the wavelength cancels the amount of change in the group refractive index relative to the wavelength due to material dispersion. Therefore, as the number of holes provided in the clad increases, the value of the relative refractive index difference of the core must be increased. However, if the relative refractive index difference of the core is 3% or more, the propagation light is scattered violently, so that the transmission loss increases, and it is difficult to put it to practical use. Therefore, the relative refractive index difference of the core needs to be 3% or less, and in that case, the number of holes is 2 or less. Therefore, another effect that the relative refractive index difference of the core can be suppressed to a small value of 3% or less is brought about.

In addition, as shown in FIG. 1, the pair of holes 12a and 12a are formed so as to sandwich the core 11 in the fiber cross section, so that the refractive index of the fiber cross section shows anisotropy. That is, by forming the pair of air holes 12a, 12a, another effect of maintaining the polarization plane of the propagating light is brought about. If holes are provided so that the difference in refractive index between the two directions orthogonal to each other in the cross section of the fiber is 10 ⁻⁵ or more, the DFF 10 functions reliably as a polarization maintaining optical fiber.

  Here, the number of holes depends on the dispersion value of the optical fiber before the holes are formed. For example, when the number of holes increases, the amount of change with respect to the wavelength of the group refractive index in the holes due to the structure increases. Therefore, when the amount of change in the group refractive index with respect to the wavelength change is larger than that of the DFF 10 before forming the pair of holes 12a, 12a, it is necessary to provide two or more holes. On the other hand, when the number of vacancies is reduced, the amount of change of the group refractive index with respect to the wavelength in the vacancies due to the structure becomes smaller. Therefore, when the magnitude of the change in the group refractive index with respect to the wavelength change is smaller than that of the DFF 10 before forming the pair of holes 12a, 12a, it is only necessary to provide one hole.

<< Embodiment 2 of the Invention >>
In the second embodiment, the structure and manufacturing method of the DFF 20 will be described.

  First, the structure of the DFF 20 will be described.

  FIG. 3 shows a cross section of the DFF 20.

  The DFF 20 is made of quartz glass, and includes a solid core 21 and a clad 12 provided so as to cover the core 21. The core 21 is doped with a material that increases the refractive index of quartz glass such as germanium. The relative refractive index difference of the core 21 is 3% or less. Here, if the relative refractive index difference of the core 11 is 3% or less, the propagation light is not scattered so violently, so that the transmission loss does not increase so that the practical use becomes difficult. A hole 22 a is formed in the cladding 22 so as to be close to the core 21. That is, the DFF 20 differs from the DFF 10 in the first embodiment in the number of holes.

  Next, a method for manufacturing the DFF 20 will be briefly described. Since the structure is slightly different from the DFF 10 of the first embodiment, the differences will be described below.

  In the first manufacturing method, one quartz glass rod, a plurality of pure quartz glass rods, and one quartz capillary are prepared. When the base material is produced by bundling them, a quartz capillary is disposed next to the quartz glass rod. Other points are substantially the same as the manufacturing method of the DFF 10 in the first embodiment.

  In the second manufacturing method, one hole is made in the quartz glass rod cross section using a drill. The hole is formed next to the portion doped with germanium in the rod cross section. Other points are substantially the same as the manufacturing method of the DFF 10 in the first embodiment.

  As described above, the DFF 20 includes the pores 22 a so as to be close to the core 21 as in the DFF 10. For this reason, the DFF 20 provides substantially the same effect as that described in the first embodiment. As described in the first embodiment, the DFF 20 is suitable when the dispersion value of the optical fiber before forming the holes is relatively small.

<< Embodiment 3 of the Invention >>
In the third embodiment, the structure and manufacturing method of the DFF 20 will be described.

  First, the structure of the DFF 30 will be described.

  FIG. 4 shows a cross section of the DFF 30.

  The DFF 30 is made of quartz glass, and includes a solid core 31 and a clad 32 provided so as to cover the core 31. The core 31 is doped with a material that increases the refractive index of quartz glass such as germanium. Three holes 32 a, 32 a, 32 a are formed in the clad 32 so as to be close to the core 31, and each hole 32 a is formed to be a vertex of an equilateral triangle centering on the core 31. Yes. That is, the DFF 30 has more holes than the DFF 10 in the first embodiment.

  Next, a method for manufacturing the DFF 30 will be briefly described. Since the structure is slightly different from the DFF 10 of the first embodiment, the differences will be described below.

  In the first manufacturing method, one quartz glass rod, a plurality of pure quartz glass rods, and three quartz capillaries are prepared. And when bundling these together to produce the base material, in the rod cross section, so that the three quartz capillaries become the apex of the equilateral triangle, and so that the quartz glass rod becomes the center of the equilateral triangle, Three quartz capillaries are arranged. Other points are substantially the same as the manufacturing method of the DFF 10 in the first embodiment.

  In the second manufacturing method, three holes are formed around a portion doped with germanium by using a drill. At this time, in the rod cross section, the three holes are opened so that the three holes become the vertices of the equilateral triangle, and the germanium-doped portion becomes the center of the equilateral triangle. Other points are substantially the same as the manufacturing method of the DFF 10 in the first embodiment.

  As described above, the DFF 30 includes the pores 32 a, 32 a, and 32 a so as to be close to the core 31 like the DFF 10. Therefore, the DFF 30 provides substantially the same effect as that described in the first embodiment. As described in the first embodiment, the DFF 30 is suitable when the dispersion value of the optical fiber before forming the holes is relatively large.

<< Embodiment 4 of the Invention >>
In the fourth embodiment, the structure and manufacturing method of the DFF 40 will be described.

  First, the structure of the DFF 40 will be described.

  FIG. 5 shows a cross section of the DFF 40.

  The DFF 40 is made of quartz glass, and includes a solid core 41 and a clad 42 provided so as to cover the core 41. The core 41 is doped with a material that increases the refractive index of quartz glass such as germanium. Four holes 42 a, 42 a, 42 a, 42 a are formed in the clad 42 so as to be close to the core 41, and each hole 42 a is positioned at a vertex of a square centering on the core 41. That is, the DFF 40 has more holes than the DFF 10 in the first embodiment.

  Next, a method for manufacturing the DFF 40 will be briefly described. Since the structure is slightly different from the DFF 10 of the first embodiment, the differences will be described below.

  In the first manufacturing method, one quartz glass rod, a plurality of pure quartz glass rods, and four quartz capillaries are prepared. When the base material is produced by bundling them, the four quartz capillaries are at the top of the square and the quartz glass rod is at the center of the square in the rod cross section. A quartz capillary is provided. Other points are substantially the same as the manufacturing method of the DFF 10 in the first embodiment.

  In the second manufacturing method, four holes are formed around a portion doped with germanium by using a drill. At this time, in the rod cross section, the four holes are formed so that the four holes become the apexes of the square, and the germanium-doped portion becomes the center of the square. Other points are substantially the same as the manufacturing method of the DFF 10 in the first embodiment.

As described above, the DFF 40 includes the pores 42 a, 42 a, 42 a, 42 a so as to be close to the core 41 like the DFF 10. Therefore, the DFF 40 provides substantially the same effect as that described in the first embodiment. As described in the first embodiment, the DFF 40 is suitable when the dispersion value of the optical fiber before forming the holes is relatively large.
<< Other Embodiments >>
As described in the first to fourth embodiments, the number of holes is not limited to 1 to 4 because it depends on the dispersion value of the optical fiber before forming the holes. At that time, when a hole is provided so that the difference in refractive index between two directions orthogonal to each other in the cross section of the fiber is 10 ⁻⁵ or more, the optical fiber functions as a DFF whose dispersion value is close to zero. At the same time, it also functions as a polarization maintaining optical fiber.

  Specific examples will be described.

<< Embodiment 1 of the Invention >>
Eight DFFs having substantially the same structure as in the first to fourth embodiments are prepared. Table 1 shows the number of holes, Δ, 2a, W, d, and dW of the eight DFFs. As shown in Table 1, these eight DFFs are divided into two groups, which differ only in the value of Δ, group (A) and group (B). The four DFFs in each group are empty. Only the number of holes is different. Δ is the relative refractive index of the core, and 2a, W, d, and dW are the core diameter, the gap interval, the hole diameter, and the core-hole interval, as shown in FIG.

  Light having a wavelength of 1.45 to 1.65 (μm) was propagated to the DFFs of group (b) and group (b), and the wavelength dependence of the dispersion value with respect to the number of holes was examined.

  FIG. 7 shows the wavelength dependence of the dispersion value of the DFF of the group (A), and FIG. 8 shows the wavelength dependence of the dispersion value of the DFF of the group (B).

  From Table 1, the dispersion value of a DFF having two holes (hereinafter referred to as “DFF having two holes”) is 1.3 (ps / nm / km) in the group (A). The group (b) shows 1.4 (ps / nm / km), and it can be seen that the dispersion value of the DFF with two holes is close to 0 in both groups. 7 and 8, it can be seen that the dispersion value of the DFF having two holes does not show wavelength dependency. From these facts, it can be said that the DFF with two holes functions as a DFF having a dispersion value close to 0 and works well. Further, since the wavelength dependence of the dispersion value with respect to the number of holes is substantially the same in FIGS. 7 and 8, it can be said that Δ does not affect the performance of the DFF.

<< Embodiment 2 of the Invention >>
Five DFFs having substantially the same structure as that of the first embodiment and different only in the value of dW were prepared. Table 2 shows the number of holes, Δ, 2a, W, d, and dW of the DFF used in Example 2. The definitions of Δ, 2a, W, d, and dW are the same as those in the first embodiment.

  DW having a dW of 1 (μm), 2 (μm), 3 (μm), 4 (μm), and 6 (μm) propagates light having a wavelength of 1.45 to 1.65 (μm), and dW The wavelength dependence of the dispersion value was investigated.

  FIG. 9 shows the wavelength dependence of the dispersion value in a DFF having dW values of 1 (μm), 2 (μm), 3 (μm), 4 (μm), and 6 (μm).

  From FIG. 9, it is referred to as a DFF having a dW of 2 (μm) (hereinafter referred to as “DFF having a dW = 2 (μm)”. The dW is 1 (μm), 3 (μm), 4 (μm), 6 (μm). In this case, the dispersion value is not dependent on the wavelength and is almost 0 (ps / nm / km). The dispersion value of DFF with dW = 1 (μm) shows a smaller value as the wavelength becomes longer, whereas the dispersion value of DFF with dW> 3 (μm) shows a larger value as the wavelength becomes longer. Show different wavelength dependences.

  The dispersion slope shown in Table 2 indicates the slope of the graph of FIG. 9 at a wavelength of 1.55 (μm). FIG. 10 shows the dW dependence of the dispersion slope.

From Table 2 and FIG. 10, the dispersion slope of DFF with dW = 2 (μm) is very small as 0.008 (ps / nm ² / km). And since the dispersion slope which a normal optical fiber shows is 0.07-0.08 (ps / nm < ² > / km), it can be said that DFF of dW = 2 (micrometer) acts as DFF with sufficient performance. Further, in order to function as a DFF with good performance, the dispersion slope may be −0.02 to 0.02 (ps / nm ² / km). Therefore, from FIG. 10, the dW is 1.0 to 2.5. (Μm) It can be said that it is preferably 1.6 to 2.3 (μm).

<< Embodiment 3 of the Invention >>
Three DFFs (hereinafter referred to as “A”, “B”, and “C”), respectively, having substantially the same structure as that of the first embodiment and different only in the value of Δ were prepared. Table 3 shows the numbers of vacancies of A, B, and C used in Example 3, and values of Δ, 2a, W, and d. The definitions of Δ, 2a, W, and d are the same as those in the first embodiment.

Using A, B, and C shown in Table 3, the mode field diameter MFDy and the cutoff wavelength λ _c in the y-axis direction were measured.

From Tables 1 and 2, it can be seen that A, B, and C are optical fibers that function as a DFF with good performance, and are optical fibers that exhibit dispersion values close to zero. From this and the measurement results shown in Table 3, it can be seen that MFDy and W must be substantially the same in order to function as a DFF with good performance. Further, if the values of MFDy and λ _c are substituted into Equation 1, since they exist in a region sandwiched between the solid line and the dotted line shown in FIG. 2, MFDy and λ satisfying Equation 1 are used in order to function as a DFF with good performance. It can be seen that it must have _c .

<< Embodiment 4 of the Invention >>
The wavelength dependence of mode birefringence was examined using A, B, and C shown in Example 3, and the polarization maintaining performance of A, B, and C was examined. In addition, the wavelength dependence of the power leakage amount in the core was investigated using A, B, and C, and the bending loss of A, B, and C was estimated. The amount of leakage of the power in the core represents the amount of reduction in the ratio of the power of the light propagating in the core to the power of the propagating light in dB units.

  11 and 12 show the results of wavelength dependence of mode birefringence and in-core power leakage.

  From FIG. 11, it can be seen that the polarization maintaining performance is improved as the order of A, B, and C, that is, the value of Δ increases. In particular, C has substantially the same performance as the polarization maintaining performance of the PANDA type fiber. A, B, and C show substantially the same structure as the DFF used in Examples 1 and 2 above. Therefore, it can be seen that A, B and C function as a DFF and also function as a polarization maintaining optical fiber from FIG. Although the polarization maintaining performance of A is inferior to that of the PANDA type fiber, dW = 1.4 (μm) is sufficient to show the fiber structure of A, and the optical fiber is broken during the manufacture of A. It can be said that there is a low risk. The effective core area and mode field diameter are substantially the same in the conventional DFF and A. For this reason, when connecting to a single mode optical fiber, the connection loss may be lower when connected to A than when connected to DFF.

  From FIG. 12, since the power leakage amount in the core is about 1 (dB), it can be said that about 90% of the power of propagating light in A, B, and C is confined in the core. If the power of leaking light is large, there is a risk that loss occurs when the optical fiber is bent, and light does not propagate. However, from FIG. 12, since the power of the leaked light is about 10% of the power of the propagating light, the risk of such a loss occurring is low, and therefore A, B, and C can be effectively used commercially. It can be said.

<Action>
From Examples 1 and 2, it can be seen that in a DFF having two holes and dW = 2 (μm), the dispersion value does not depend on the wavelength of propagating light and is close to zero. Further, it can be seen from Example 3 that the DFF used in Examples 1 and 2 has another effect that the polarization plane of the propagating light is maintained.

  In addition, it can be seen from Example 1 that the dispersion value of the propagation light of the DFF with two holes does not show wavelength dependency and shows a value close to 0, but the number of holes is not limited to this. . For example, when the number of holes increases, the amount of change with respect to the wavelength of the group refractive index in the holes due to the structure increases. Therefore, when the amount of change in the group refractive index with respect to the wavelength change is larger than that of the DFF 10 before forming the pair of holes 12a, 12a, it is necessary to provide two or more holes. On the other hand, when the number of vacancies is reduced, the amount of change of the group refractive index with respect to the wavelength in the vacancies due to the structure becomes smaller. Therefore, when the magnitude of the change in the group refractive index with respect to the wavelength change is smaller than that of the DFF 10 before forming the pair of holes 12a, 12a, it is only necessary to provide one hole.

  The present invention is applicable to a polarization maintaining dispersion flat transmission line, a fiber amplifier, a fiber laser, a mode-locked laser, a supercontinuum light source, and the like.

It is a fiber cross-sectional view of DFF10 in Embodiment 1. It is a graph which shows the relationship between the dispersion value in the core 11 of DFF10 in Embodiment 1, and the mode field diameter in a y-axis direction. It is a fiber cross-sectional view of DFF20 in Embodiment 2. It is a fiber cross-sectional view of DFF30 in Embodiment 3. It is a fiber cross-sectional view of DFF40 in Embodiment 4. It is explanatory drawing of the parameter showing the structure of DFF in Examples 1-3. It is a graph which shows the relationship between the number of holes, and the wavelength dependence of a dispersion value. It is a graph which shows the relationship between the number of holes, and the wavelength dependence of a dispersion value. It is a graph which shows the relationship between the core-hole space | interval and the wavelength dependence of a dispersion value. It is a graph which shows the relationship between a core-hole space | interval and a dispersion | distribution slope. It is a graph which shows the relationship between the relative refractive index of a core, and the wavelength dependence of a mode birefringence. It is a graph which shows the relationship between the relative refractive index of a core, and the wavelength dependence of the amount of power leaks in a core. It is the fiber cross-sectional view of DFF50 in a prior art example, and the refractive index distribution figure in a cross section. It is a fiber cross-sectional view of DFF60 in a prior art example.

Explanation of symbols

10, 20, 30, 40, 50, 60 DFF
11, 21, 31, 41, 51, 61 Core 12, 22, 32, 42, 62 Cladding 12a, 22a, 32a, 42a, 62a Hole 52 First cladding 53 Second cladding 54 Third cladding 55 Fourth cladding 62b Low refractive index

Claims (5)

A dispersion flat fiber comprising a core and a clad covering the core,
The dispersion flat fiber, wherein the clad is formed with a hole along the core that makes a dispersion value substantially constant irrespective of a wavelength of propagating light. The dispersion flat fiber according to claim 1,
A dispersion flat fiber, wherein a dispersion value of propagating light is -3 to 3 (ps / nm / km). The dispersion flat fiber according to claim 2,
A dispersion flat fiber, wherein a pair of the holes are provided so as to sandwich a core having a relative refractive index difference of 3% or less. The dispersion flat fiber according to claim 3, wherein
A dispersion flat fiber, wherein a distance between the core and the hole in a cross section of the fiber is 1.0 to 2.5 μm. The dispersion flat fiber according to claim 4, wherein
A dispersion flat fiber, characterized in that the cutoff wavelength λ _c (μm) and the mode field diameter x (μm) satisfy Equation 1.

(Where n: refractive index, Δ: relative refractive index difference (%) of the core, λ: wavelength of propagating light (μm), Dm: material dispersion value at the wavelength of propagating light (ps / nm / km) )

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