JP4490464B2 - Optical fiber and manufacturing method thereof - Google Patents

Description

  The present invention relates to an optical fiber used for optical signal propagation and a manufacturing method thereof, and more particularly to an optical fiber used as a transmission medium in optical communication and a manufacturing method thereof.

  The transmission characteristics of the optical fiber are controlled by giving different refractive indexes to the core region and the cladding region constituting the optical fiber and changing the refractive index distribution in the radial direction of the optical fiber. Generally, the refractive index distribution is set by adding germanium or fluorine (hereinafter referred to as an additive material) to pure quartz glass and changing the refractive index of the portion to which the additive material is added. However, the amount of change in the refractive index that can be realized by the addition of the above-described additive material is approximately −3% to +3 with respect to the refractive index of pure quartz glass from the viewpoint of manufacturability of the optical fiber preform and suppression of increase in transmission loss. Limited to a range of%.

  On the other hand, for example, in Non-Patent Document 1, the quartz glass constituting the optical fiber extends regularly in the longitudinal direction, in other words, along the light propagation direction around the axis, and regularly in a cross section perpendicular to the longitudinal direction. A technique for realizing an optical waveguide structure by providing a hole region that is disposed in the region to reduce the effective refractive index of the region is disclosed. According to the optical fiber disclosed in Non-Patent Document 1, it is possible to realize a refractive index change of 10% or more by providing the hole region. For this reason, in the optical fiber having the hole area, the design area of transmission characteristics can be greatly expanded.

T.A.Birks, et al. `` Endlessly single-mode photonic crystalfiber '', OPTICS LETTERS, July 1, 1997, p.961-963

  However, in the optical fiber having a hole region disclosed in Non-Patent Document 1, a hollow quartz glass tube extending along the longitudinal direction is formed to form the hole region, or a circle constituting the optical fiber. There has been a problem that complicated steps in manufacturing are required, such as periodically forming holes extending along the longitudinal direction in the columnar quartz glass. In addition, the light propagation direction, such as the periodicity and diameter of the holes formed in the hole region in the radial direction, and variations in the holes in the radial direction affect the transmission characteristics of the optical fiber having the hole region. There was a problem of giving. Furthermore, the use of quartz glass having a hollow structure in which holes are formed limits the capacity of the optical fiber base material that can be produced, which is not suitable for mass production. .

  In view of the background as described above, an object of the present invention is to provide an optical fiber excellent in controllability of transmission characteristics and a simple manufacturing method of the optical fiber.

  An optical fiber according to a first aspect of the present invention for solving the above-mentioned problem is a core region having an arbitrary refractive index distribution, and is lower and more uniform than the core region on the radially outer side of the core region. A first clad region having a refractive index and a second clad region which is spaced radially from the first clad region and spaced apart from the first clad region and has a lower and uniform refractive index than the core region. A clad region, and a buffer region formed by arranging a plurality of gap adjusting elements extending along the light propagation direction in a gap provided between the first clad region and the second clad region. Features. Thereby, the controllability of the transmission characteristic superior to the optical fiber based on the refractive index distribution control by the conventional material addition can be realized.

The optical fiber according to the first invention, before SL gap adjusting element, characterized in that it is a quartz glass having a uniform refractive index is formed in a cylindrical shape. Thereby, a gap having a desired distribution can be easily provided in the buffer region.

The optical fiber according to the first invention, before Symbol relationship between the radius and the radius of the cylindrical quartz glass of the first cladding region, and the number of the columnar quartz glass, depending on the desired gap occupancy It is characterized by being set. Thereby, control of a gap occupation rate can be performed.

An optical fiber according to a second invention is characterized in that, in the first invention, a waveguide structure for main light is formed by the core region and the first cladding region. Thereby, the characteristic change by the local nonuniformity in the optical fiber cross section and longitudinal direction of the gap | interval in a buffer area | region can be suppressed.

An optical fiber manufacturing method according to a third invention is the optical fiber manufacturing method according to the first invention, wherein the core step and the first cladding region are formed in the first step and the first step. A second step of disposing a cylindrical quartz glass having a predetermined radius and number on the outer periphery of the first cladding region to form a buffer region, and a first step for a hollow cylindrical quartz glass constituting the second cladding region And the third step of inserting the core region, the first cladding region and the buffer region formed in the second step, and the first step, the second step and the third step. And a fourth step of melt-stretching the core region, the first cladding region, the buffer region, and the second cladding region. Conventionally, an optical fiber preform is formed in the first to third steps, and the core region, the first cladding region, the buffer region, and the second cladding region are melt-stretched in the fourth step. An optical fiber having controllability of transmission characteristics superior to that of an optical fiber based on the refractive index distribution control by adding the above material can be manufactured by a simple method.

  According to the optical fiber according to the present invention, it is possible to obtain transmission characteristics that cannot be realized by an optical fiber based on conventional material addition by providing a buffer region composed of a plurality of gap adjusting elements and gaps. The effect of becoming.

  Specifically, by controlling the occupation ratio of the gap in the buffer region, it is possible to reduce the bending loss in the conventional optical fiber by two orders of magnitude or more. Further, by controlling the radius of the first cladding region in accordance with the electric field strength, there is an effect that the mode field diameter and the chromatic dispersion characteristic equivalent to those of the conventional optical fiber can be maintained or reduced and varied.

  Further, according to the optical fiber manufacturing method of the present invention, the core region and the first cladding region equivalent to those of the conventional optical fiber can be formed, and at the same time, a plurality of core regions and the first cladding region can be formed radially outward. Since the buffer region is formed by arranging the cylindrical quartz glass, the hole processing and the hole forming process are not required, compared with the conventional optical fiber having a periodic arrangement of a plurality of holes. There is also an effect that it is easy to form a large-sized optical fiber preform for production.

  Furthermore, according to the optical fiber according to the present invention, since the main light waveguide structure is formed by the core region and the first cladding region, the optical fiber that forms the waveguide structure based on the periodic arrangement of holes. As compared with the above, there is an effect that it is difficult to receive characteristic changes due to local non-uniformity in the cross section of the optical fiber and in the longitudinal direction of the gap in the buffer region.

  Hereinafter, embodiments of the present invention will be described with reference to FIGS. FIG. 1 is a sectional structural view showing an example of an optical fiber according to the present embodiment, FIG. 2 is an explanatory view showing a manufacturing process of the optical fiber according to the present embodiment, and FIG. 3 is another example of the optical fiber according to the present embodiment. FIG.

  As shown in FIG. 1, an optical fiber 1 according to this embodiment includes a core region 11 having an arbitrary refractive index distribution based on material addition, and is lower and more uniform than the core region 11 provided on the outer periphery of the core region 11. The first cladding region 12 having an appropriate refractive index, the buffer region 13 provided on the outer periphery of the first cladding region 12, and a lower and uniform refractive index than the core region 11 provided on the outer periphery of the buffer region 13. The second clad region 14 is provided. The optical fiber 1 is assumed to have a main light waveguide structure formed by the core region 11 and the first cladding region 12.

  In the present embodiment, the buffer region 13 is a cylindrical region provided between the first cladding region 12 and the second cladding region 14, and is formed in a columnar shape as a gap adjusting element and has a uniform refractive index. A plurality (60 in FIG. 1) of quartz glass (hereinafter referred to as columnar quartz glass) 131 and the columnar quartz glasses 131 or between the columnar quartz glass 131 and the first cladding region 12 or the second cladding region 14. And a plurality of gaps 132 formed at the same time. In the present embodiment, the columnar quartz glass 131 is laminated regularly in the circumferential direction and the radial direction (in FIG. 1, 20 pieces in the circumferential direction and three layers in the radial direction).

  A procedure for manufacturing the optical fiber 1 according to this embodiment shown in FIG. 1 will be described with reference to FIG. In the method for manufacturing the optical fiber 1 according to the present embodiment, as shown in FIG. 2A, the core region 11 and the first cladding region 12 are first formed as a first step. Here, the batch formation of the core region 11 and the first cladding region 12 can be performed by a vapor phase method or the like used in the production of an optical fiber preform based on the conventional material addition.

  Next, as a second step, as shown in FIG. 2B, a cylindrical quartz glass 131 is laminated on the outer periphery of the first cladding region 12 formed in the first step, and the cylindrical quartz glass. The buffer region 13 composed of 131 and the gap 132 is formed.

  Thereafter, as a third step, as shown in FIG. 2C, the core region 11, the first cladding region 12 and the buffer region 13 formed in the first step and the second step are replaced with the second cladding region 14. It inserts in the cylindrical quartz glass which comprises.

  Finally, as a fourth step, the optical fiber preform composed of the core region 11, the first cladding region 12, the buffer region 13, and the second cladding region 14 formed in the third step is melt-drawn. The optical fiber 1 according to the present embodiment can be manufactured through the above steps.

  The buffer region is not limited to the structure shown in FIG. 1, but two types of cylindrical quartz glass having different radii instead of the buffer region 13 shown in FIG. 1 and described above, such as the optical fiber 2 shown in FIG. 20 231 and 232 are laminated in the circumferential direction one by one in the radial direction, and a region constituted by the cylindrical quartz glasses 231 and 232 and a gap 233 formed by these is constituted as a buffer region 23. Various changes can be made without departing from the spirit of the present invention.

  Further, the number N1 in the circumferential direction and the number N2 in the radial direction can be arbitrarily combined. In FIG. 1, the first columnar quartz glass 131 forming the buffer region 13 is Although the example which mutually adjoins by the direction was shown, the form arrange | positioned in the state which has a mutual gap may be sufficient.

  The gap 132 is generally an air layer, but may be filled with a liquid having a specific refractive index, a resin, or the like. Furthermore, the core region 11 and the first cladding region 12 have an equal and uniform refractive index, and the core region 11, the first cladding region 12, and the buffer region 13 form a main light guiding structure. It does not matter. Moreover, the cross section orthogonal to the longitudinal direction of the cylindrical quartz glass 131 is not limited to a perfect circle and may be an ellipse or the like.

  An embodiment of the present invention will be described in detail with reference to FIGS. FIG. 4 is an explanatory view showing the structural parameters of the optical fiber according to the present embodiment. FIG. 5 is a graph and a diagram showing the relationship between the number N1 in the circumferential direction and the normalized radius r2 / r1 in the optical fiber according to the present embodiment. 6 is a graph showing the relationship between the gap distance Λ and the normalized radius r2 / r1 in the optical fiber according to this embodiment. FIG. 7 shows the relationship between the gap occupancy and the normalized radius r2 / r1 in the optical fiber according to this embodiment. FIG. 8 is a graph showing the relationship between the bending loss and the gap occupancy at the wavelength of 1550 nm in the optical fiber according to the present embodiment. FIG. 9 is the mode field diameter (MFD) at the wavelength of 1550 nm in the optical fiber according to the present embodiment. ) And the normalized radius r1 / a with respect to the core radius a, FIG. 10 is a graph showing the relationship between the chromatic dispersion at the wavelength of 1550 nm and the core radius a in the optical fiber according to the present embodiment. That is a graph showing the relationship between the normalized radius r1 / a.

  The optical fiber according to this embodiment has the same structure as the optical fiber 1 shown in FIG. 1 and described above, and the same members as those shown in FIG. Description is omitted.

  As shown in FIG. 4, the diameter of the core region 11 of the optical fiber 1 is 2a, the radius of the first cladding region 12 is r1, the radius of the cylindrical quartz glass 131 constituting the buffer region 13 is r2, and the first cladding region 12 And the maximum circumferential length of the gap 132 (hereinafter referred to as the gap distance) formed by two cylindrical quartz glasses 131 adjacent to each other along the outer peripheral surface of the first cladding region 12 is defined as Λ. The gap distance Λ is the same length as the circumference of the outer periphery of the first cladding region at the central angle 2θ.

  As shown in FIG. 5 and FIG. 6, lamination is performed along the circumferential direction as the value of the normalized radius r 2 / r 1, that is, the radius r 2 of the cylindrical quartz glass 131 with respect to the radius r 1 of the first cladding region 12 increases. The number of cylindrical quartz glasses 131 to be reduced decreases and the gap distance Λ increases.

  As shown in FIG. 7, as the normalized radius r2 / r1 increases, the gap occupying ratio, which is the ratio occupied by the gap 132 in the buffer region 13, increases. Note that the solid line, broken line, and alternate long and short dash line shown in FIG. 7 are values when the number of laminated layers N2 in the radial direction of the cylindrical quartz glass 131 is one layer, two layers, and three layers, respectively. 5-7, it can be seen that the gap occupancy can be controlled by a combination of the normalized radius r2 / r1, the circumferential lamination number N1 and the radial lamination number N2 of the cylindrical quartz glass 131.

  As an example, the optical fiber 1 shown in FIG. 1 has a step-type refractive index distribution in which the refractive index of the core region 11 has a relative refractive index difference of 0.35% with respect to the refractive index of the first cladding region 12. The case will be described. In addition, the refractive index of the quartz glass which forms the 1st cladding area | region 12, the 2nd cladding area | region 14, and the columnar quartz glass 131 was made into the refractive index of a pure quartz glass. The gap 132 provided in the buffer region 13 is filled with air. Furthermore, the diameter 2a of the core region 11 was 9 μm.

  The vertical axis shown in FIG. 8 is a bending loss (loss increase amount) when bending with a radius of 5 mm is applied 10 turns. The condition that the gap occupancy is zero corresponds to the case where the buffer region does not exist, that is, only the core region and the cladding region based on the conventional material addition exist.

  As shown in FIG. 8, in this embodiment, it can be seen that the bending loss is reduced exponentially as the gap occupancy increases. This is because, in the optical fiber 1 of this embodiment, the effective refractive index in the buffer region 13 is reduced by the presence of the plurality of gaps 132, and the confinement of the electric field distribution in the optical fiber 1 inside the second cladding region 14 is improved. The effect of this is not dependent on the shape or number of the gaps 132. In addition, since the effect is realized by reducing the effective refractive index in the entire buffer region 13, it also depends on the non-uniformity of the local gap 132 in the longitudinal direction of the optical fiber 1 or in a cross section perpendicular to the longitudinal direction. do not do. That is, the gap occupancy can be controlled by setting the normalized radius r2 / r1 and the number of cylindrical quartz glasses 131 disposed in the buffer region 13 to desired values.

  In the present embodiment, for example, by realizing a gap occupancy ratio of 30% or more, it becomes possible to improve the bending loss by two orders of magnitude or more compared to the conventional optical fiber. When the structure of the optical fiber 1 shown in FIG. 1 is used, a gap occupancy of 30% or more can be easily realized by suitably combining the normalized radius r2 / r1 and the radial stacking number N2 from FIG. I understand.

  As shown in FIG. 9, the mode field diameter (MFD) increases exponentially as the value of the normalized radius r1 / a, that is, the radius r1 of the first cladding region 12 with respect to the radius a of the core region 11 increases. It has increased. Here, the normalized radius r1 / a shown as infinite on the horizontal axis shown in FIG. 9 corresponds to the case where the buffer region does not exist, that is, only the core region and the cladding region based on the conventional material addition exist.

  From FIG. 9, it can be seen that the mode field diameter (MFD) rapidly decreases in the region where the normalized radius r1 / a is 2 or less. This is because confinement of the electric field distribution in the core region 11 and the first cladding region 12 is improved with the provision of the buffer region 13.

  Here, in FIG. 11, the normalized electric field strength and the normalized radius r1 / a with respect to the core radius in the optical fiber having only the core region and the cladding region based on the conventional material addition, that is, the optical fiber without the buffer region of the present invention. Show the relationship. The characteristic shown in FIG. 11 is an electric field distribution at a wavelength of 1550 nm. From FIG. 11, it can be seen that, for example, at the point where the normalized radius r1 / a is 1.5, the normalized electric field intensity attenuates to 20% or less. That is, when the radius r1 of the first cladding region is 1.5 times the radius a of the core region, a normalized electric field strength of about 20% exists on the circumference of the first cladding region.

  As shown in FIG. 11, the normalized electric field strength in the conventional optical fiber in the region where the normalized radius r1 / a is 2 is about 8%. Therefore, in the optical fiber 1 of the present embodiment, the radius r1 of the first cladding region 12 is set to a region where the intensity of the electric field distribution formed in the core region 11 and the first cladding region 12 is 8% or less. The field diameter (MFD) characteristic equivalent to that of the conventional optical fiber can be maintained. Conversely, by setting the radius r1 of the first cladding region 12 to a region where the intensity of the electric field distribution formed in the core region 11 and the first cladding region 12 is 8% or more, the vicinity of the core region of the electric field distribution It is also possible to realize confinement in

  As shown in FIG. 10, in this embodiment, the chromatic dispersion decreases exponentially as the normalized radius r1 / a increases. Here, the normalized radius r1 / a shown as infinite on the horizontal axis corresponds to the case where there is no buffer region, that is, only the core region and the cladding region based on the conventional material addition.

  FIG. 10 shows that the chromatic dispersion increases rapidly in the region where the normalized radius r1 / a is 2.5 or less. This is because confinement of the electric field distribution inside the first cladding region 12 occurs with the provision of the buffer region 13, and the waveguide dispersion characteristics are affected.

  Further, from FIG. 11, the normalized electric field strength in the conventional optical fiber in the region where r1 / a is 2.5 is about 3%. Therefore, in the optical fiber 1 of the present embodiment, the radius r1 of the first cladding region 12 is set to a region where the intensity of the electric field distribution formed in the core region 11 and the first cladding region 12 is 3% or less. Thus, it becomes possible to maintain the same wavelength dispersion characteristic as that of the conventional optical fiber. Conversely, by setting the radius r1 of the first cladding region 12 to a region where the intensity of the electric field distribution formed in the core region 11 and the first cladding region 12 is 3% or more, the chromatic dispersion characteristics are varied. It is also possible.

  In particular, the chromatic dispersion characteristic of 30 ps / nm · km or more obtained at the wavelength of 1550 nm obtained by this example is a characteristic that cannot be realized in the conventional optical fiber based on the addition of materials due to the restriction on the variable amount of the refractive index change. This is considered as one of the operational effects associated with the provision of the buffer region 13 in the optical fiber 1 of the present embodiment.

  As described above, according to the optical fiber 1 according to the present embodiment, the relationship between the hole and the gap adjusting element that has been conventionally produced in the holey optical fiber is reversed, and the portion that has been a hole in the past is obtained. The buffer region 13 is configured with the gap adjusting element as a gap, that is, the gap, which is the conventional gap adjusting element, and the buffer region 13 is disposed between the first cladding region 12 and the second cladding region 14. Furthermore, by adopting a configuration in which the occupation ratio of the gap 132 in the buffer region 13 is controlled, it becomes possible to improve the controllability of transmission characteristics and realize an optical fiber that is resistant to bending.

  The present invention relates to an optical fiber used for optical signal propagation and a method for manufacturing the same, and is particularly suitable for application to an optical fiber used as a transmission medium in optical communication and a method for manufacturing the same.

1 is a cross-sectional structure diagram of an optical fiber according to an embodiment of the present invention. It is explanatory drawing which shows the manufacturing process of the optical fiber which concerns on embodiment of this invention. It is a sectional structure figure of other optical fibers concerning an embodiment of the present invention. It is explanatory drawing which shows the structural parameter of the optical fiber which concerns on the Example of this invention. It is a graph which shows the relationship between the lamination | stacking number N1 of the circumferential direction in the optical fiber which concerns on the Example of this invention, and normalized radius r2 / r1. It is a graph which shows the relationship between the gap | interval distance (LAMBDA) in the optical fiber which concerns on the Example of this invention, and the normalization radius r2 / r1. It is a graph which shows the relationship between the gap occupation rate in the optical fiber which concerns on the Example of this invention, and normalized radius r2 / r1. It is a graph which shows the relationship between the bending loss with respect to the light of wavelength 1550nm and the gap occupation rate in the optical fiber which concerns on the Example of this invention. It is a graph which shows the relationship between the mode field diameter (MFD) with respect to the light of wavelength 1550nm in the optical fiber which concerns on the Example of this invention, and the normalized radius r1 / a with respect to a core radius. It is a graph which shows the relationship between the wavelength dispersion | distribution with respect to the light of wavelength 1550nm in the optical fiber which concerns on the Example of this invention, and the normalization radius r1 / a with respect to a core radius. It is a graph which shows the relationship of the normalized radius r1 / a with respect to the normalized electric field strength and core radius in the optical fiber which has only the core area | region and clad area | region based on the conventional material addition.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Optical fiber 11 Core area | region 12 1st clad area | region 13,23 Buffer area | region 131,231,232 Cylindrical quartz glass 132,233 Gap | interval 14 Second clad area | region

Claims (3)

A core region having an arbitrary refractive index profile;
A first cladding region radially outward of the core region and having a low and uniform refractive index compared to the core region;
A second cladding region that is spaced radially from the first cladding region and spaced apart from the first cladding region and has a lower and uniform refractive index than the core region;
A buffer region formed by arranging a plurality of gap adjusting elements extending along the light propagation direction in a gap provided between the first cladding region and the second cladding region ;
The gap adjusting element is a quartz glass formed in a cylindrical shape and having a uniform refractive index,
An optical fiber, wherein the radius and number of the cylindrical quartz glass are set according to a desired gap occupancy . The optical fiber according to claim 1, wherein a waveguide structure for main light is formed by the core region and the first cladding region. In the manufacturing method of the optical fiber according to claim 1,
A first step of forming a core region and a first cladding region;
A second step of forming a buffer region by disposing a columnar quartz glass having a predetermined radius and number on the outer periphery of the first cladding region formed in the first step;
A third step of inserting the core region, the first cladding region and the buffer region formed in the first step and the second step into the hollow cylindrical quartz glass constituting the second cladding region;
A fourth step of melting and stretching the core region, the first cladding region, the buffer region, and the second cladding region formed in the first step, the second step, and the third step. A method for manufacturing an optical fiber.

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