JP4398491B2 - Optical fiber - Google Patents

Description

The present invention relates to an optical fiber, and more particularly, even when given a bending with a small radius of curvature relative to the optical fiber main body, an optical fiber that enables transmission of optical signals in single-mode.

  As a known technique that enables transmission of an optical signal in a single mode even when a small radius of curvature is given to the optical fiber body, Non-Patent Document 1 discloses a “photonic crystal fiber” shown in FIG. Is disclosed. As shown in FIG. 1, quartz glass 13 is formed around the core region 11. An effective cladding region is formed between the core region 11 and the air hole by providing a plurality of air holes 12 having a refractive index of “1” so as to surround the single mode core region 11. Is done.

  FIG. 2 is a refractive index distribution diagram of the photonic crystal fiber of FIG. In FIG. 2, reference numeral 14 denotes a refractive index of the core region 11, reference numeral 15 denotes a refractive index of the air hole 12, and reference numeral 16 denotes a refractive index of the quartz glass 13.

  As can be seen from FIG. 2, the presence of air holes 12 with a very low refractive index outside the cladding region stabilizes the confinement of the electric field strength distribution even when the optical fiber body is bent. Yes.

  Patent Document 1 discloses a “trench optical fiber” shown in FIG. 3 as another structure in which the confinement of the electric field strength distribution is stabilized.

  In FIG. 3, in the trench optical fiber disclosed in Patent Document 1, a material having a low refractive index (low refractive index material) is located 20 μm away from the center of the single-mode core 21 surrounded by the quartz glass 22. ) 23 is arranged.

  FIG. 4 is a refractive index profile of the trench optical fiber of FIG. In FIG. 4, reference numeral 24 represents the refractive index of the core 21, reference numeral 25 represents the refractive index of the low refractive index material 23, and reference numeral 26 represents the refractive index of the quartz glass 22.

  As can be seen from FIG. 4, in the trench type optical fiber disclosed in Patent Document 1, the electric field strength distribution is confined when the optical fiber body is bent by the low refractive index material 23 arranged 20 μm away from the core 21. We are trying to stabilize.

Furthermore, Patent Document 2 discloses an optical fiber having a refractive index distribution similar to that shown in FIG. 2 for the purpose of providing an optical fiber that has a small loss due to bending and is easy to connect with a general transmission optical fiber. The ratio r ₂ / r ₁ of the refractive index volume of the low refractive index material 23 and the radius r ₁ of the core 21 to the distance r ₂ from the center of the core 21 to the low refractive index material 23 is a predetermined value. It is disclosed to be within range.

That is, in Patent Document 2, the refractive index volume is set to 25% for reducing the reduction of the mode field diameter for facilitating connection with a general transmission optical fiber and for suppressing the loss due to bending. [mu] m ² or more to 110% · μm ² or less, that the ratio r ₂ / r ₁ 2.5 to 4.5 is disclosed.

US Pat. No. 7,164,835 Japanese Patent No. 3835833 K. Tajima et al, "Low peak photonic crystal fibers", Proc. ECOC2003., Th4.1.6.

  As described above, in terms of improving resistance to bending, the method using the above-described technique is effective, but even with these methods, problems still need to be improved. In particular, there is a demand for cost reduction of optical fiber fabrication that is resistant to bending, such as shortening the fabrication time of the optical fiber preform and fabrication by a simpler method.

  Specifically, the “photonic crystal fiber” described in Non-Patent Document 1 generally requires a special manufacturing method for providing a plurality of holes in an optical fiber preform. It is difficult to produce only by the conventional optical fiber manufacturing method.

  In addition, in the “trench optical fiber” disclosed in Patent Document 1, it is necessary to provide a low refractive index material different from the cladding in a region relatively far from the center of the single mode core. Further, it takes time to produce a preform as an optical fiber preform made of a low refractive index material.

  Furthermore, in Patent Document 2, as described above, the main focus is on facilitating the connection with a general transmission optical fiber, which is resistant to bending loss. There is no mention of shortening. Therefore, in order to improve the convenience for the user, it is desired to produce the optical fiber preform in a shorter time.

The present invention has been made in view of the above problems, and an object of the present invention is to provide an optical fiber capable of single-mode transmission with low bending loss at a low cost by producing an optical fiber preform in a short time. There is to do.

To achieve the above object, a first aspect of the present invention, the core, a first cladding and a second optical fiber Ru comprising a cladding, disposed at the axial center of the pre-Symbol optical fiber, A first material having a first refractive index; a second material having a second refractive index smaller than the first refractive index disposed on an outer periphery of the first material; and the second material And a third material having a third refractive index smaller than the second refractive index, disposed on the outer periphery of the material, wherein the first material is the core, and the second material is The first clad, the third material is the second clad, and the core has a physical outer diameter (only a fundamental mode is excited when the core is excited in a predetermined wavelength band). 2 * a1) and a relative refractive index difference Δ1 between the first cladding and the first cladding. Outermost position Rudd (a2) is a distance from the center axis of the front Symbol optical fiber a2, the second cladding has a relative refractive index difference Δ2 of the first cladding, The physical outer diameter (2 * a1) of the core is 3.8 μm ≦ (2 * a1) <10 μm, and the outermost peripheral position (a2) of the first cladding is 5 μm ≦ (a2) ≦ 10 μm And the outermost peripheral position (a2) of the first cladding and a2 / a1 which is the ratio of a1 satisfy the relationship of 1.14 ≦ (a2 / a1) <2.5, the relative refractive index difference .DELTA.1 is 0.1% ≦ Δ1 ≦ 0.4%, the relative refractive index difference Delta] 2 is Ri -1.0% ≦ Δ2 ≦ -0.2% der, the core Each of the refractive index and the refractive index of the first cladding has a constant value. When the optical fiber is not bent, the first cladding is closed. Functions as Rad, when bending the optical fiber, the second cladding is characterized that you function as a second cladding.

The invention according to claim 2 is the invention according to claim 1, further comprising a fourth material having a fourth refractive index higher than the third refractive index, disposed on the outer periphery of the third material. And the thickness (d) of the second cladding is a value of three times the predetermined wavelength ≦ d ≦ 40 μm .

According to a third aspect, the invention of claim 2, wherein said fourth material is characterized by a strength retention body before Symbol optical fiber.

  The invention according to claim 4 is the invention according to claim 3, wherein the fourth material is a strength holder made of pure quartz.

  The invention according to claim 5 is the invention according to claim 3, wherein the fourth material is a resin having the fourth refractive index.

  The invention according to claim 6 is the invention according to any one of claims 1 to 5, wherein the first material is quartz to which at least one of Ge, P, Sn, and B elements is added, and the second material. The material is pure quartz, and the third material is F element, B element, or quartz to which two elements are mixedly added.

According to a seventh aspect, in the invention described in any one of claims 1 to 6, the outer diameter of the body of the previous SL optical fiber is equal to or less than 55μm and not more than 125 [mu] m.

  The invention according to claim 8 is the invention according to any one of claims 1 to 7, characterized in that the predetermined wavelength band is a wavelength band of 1.31 μm.

According to the present invention, in the optical fiber in which the first clad and the second clad are sequentially arranged on the outer periphery of the single mode core, a low power separate from the first clad is provided in a region relatively close to the center of the single mode core. Since the second clad, which is a refractive index material, is arranged and the relative refractive index difference between the first clad and the second clad is set large, the optical fiber preform can be manufactured in a short time and the bending loss can be reduced at a low cost. It is possible to provide an optical fiber capable of small single mode transmission.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings described below, components having the same function are denoted by the same reference numerals, and repeated description thereof is omitted.

(First embodiment)
FIG. 5 is a diagram showing the electric field distribution of a single mode in a single mode optical fiber measured at a wavelength of 1.31 μm. Compared with the Gaussian fitting curve, the half-value width of the actually measured single mode electric field distribution is narrower. If the same measurement is performed at the same wavelength as the cutoff wavelength of the single mode optical fiber, a half-value width equivalent to a Gaussian fitting curve can be obtained. However, since the wavelength used in actual optical communication is far from the cutoff wavelength, the above result can be obtained with good reproducibility. Therefore, considering the bending of the optical fiber, if the low refractive region provided outside the cladding is provided relatively close to the center of the core, the single mode is maintained even when the optical fiber is bent. It can be seen that an optical signal can be transmitted. Also, there is no air hole having a refractive index of “1”, which requires a manufacturing cost.

  That is, in one embodiment of the present invention, by setting parameters as described below, excessive loss due to bending is suppressed even when the low refractive index region is arranged relatively close to the center of the core, as shown in FIG. Such a good single mode (fundamental mode) electric field distribution can be obtained. Therefore, it is possible to reduce the volume of the optical fiber preform while reducing the excess loss and enabling good single mode transmission, leading to cost reduction. In particular, it is effective for an optical fiber having a transmission distance of about 10 km.

Figure 6 is a cross-sectional view of an optical fiber rather based on the first embodiment. 7 is a refractive index profile of an optical fiber rather based on the first embodiment.

In FIG. 6, a core 111 as a first material for single mode transmission is provided at the center. Further, sequentially on the outside of the core 111 to the present embodiment is arranged at the axial center of the engagement Ru optical fiber, a first cladding 121 as the second material, the second clad 131 as a third material is provided. In the present embodiment, the core 111 and the first cladding 121 serve as an optical fiber preform. The core 111, the first clad 121, and the second clad 131 may be made of a material that is usually used for an optical fiber, such as quartz glass, an organic material such as polymer or acrylic, and the like.

  In FIG. 7, reference numeral 112 denotes the refractive index of the core 111, reference numeral 122 denotes the refractive index of the first cladding 121, and reference numeral 132 denotes the refractive index of the second cladding 131. As can be seen from FIG. 7, the refractive index 122 of the first cladding 121 is smaller than the refractive index 112 of the core 111, and the refractive index 132 of the second cladding 131 is smaller than the refractive index 122 of the first cladding 121.

The core 111 has a relative refractive index difference Δ1 between the physical outer diameter (2 * a1) excited only in the fundamental mode and the first cladding 121 when the core is excited in a wavelength band of 1.31 μm. . Further, the outermost peripheral position of the first cladding 121 (a2), the distance from the central axis of the upper Symbol optical fiber is a2. Further, the outermost peripheral position of the second cladding 131 (a3) is equal to the outer diameter position of the upper Symbol optical fiber has a relative refractive index difference Δ2 of the first cladding 121.

The physical arrangement relationship is as follows.
The physical outer diameter (2 * a1) of the core 111 is preferably 3.8 μm ≦ (2 * a1) <10 μm, and the outermost peripheral position (a2) of the first cladding 121 is 5 μm ≦ (a2) ≦ 10 μm. preferably, and the first outermost peripheral position of the cladding 1 2 1 (a2), which is the ratio of the a1 a2 / a1 satisfies the 1.14 ≦ (a2 / a1) < 2.5 relationship It is preferable.

  As a result, the second cladding 131 can be provided in a region relatively close to the core 111.

  In addition, in each refractive index relationship, the relative refractive index difference Δ1 is preferably 0.1% or more and 0.4% or less, and the relative refractive index difference Δ2 is −1.0% or more and −0.2%. The following values are preferred. Thus, the single mode condition is adjusted by Δ1, and the optical signal can be guided even when bent by Δ2 having a negative value.

  In the present embodiment, additives (for example, Ge, P, Sn, and B elements) added to the core 111 are selected so as to increase the refractive index of a base material such as quartz. The first cladding 121 is pure quartz.

  In the present embodiment, the F element is added to quartz for the second cladding 131, but the present invention is not limited to this. For example, any additive such as element B that can lower the refractive index of quartz may be used. In the present embodiment, the refractive index can be further reduced by adding the B element to the second cladding 131 in addition to the F element.

In such a configuration, the core 111 functions as a core for single mode transmission, and when the optical fiber is not bent, the first cladding 121 functions as a cladding, and when bent, the second cladding 131 is the second. A dual guide structure is obtained.

  In the present embodiment, the optical signal wavelength for single mode transmission (the wavelength of the optical signal that excites the specified mode of the core 111 and performs single mode transmission) is set to the C-Band band (1530 nm to 1560 nm) or the L-Band band (1570 nm). ˜1610 nm), or alternatively, any of the 1300 nm band can be designed. Further, the mode field diameter can be set to 8.0 to 10.0 μm. That is, the mode field diameter of the above-mentioned specified mode can be set to 8.0 to 10.0 μm.

Further , the outer diameter of the optical fiber can be 55 μm or more and 125 μm or less.

In the present embodiment, the second cladding 131 that functions as a low refractive index region for reducing bending loss is disposed relatively close to the center of the core 111. Therefore, the volume of the optical fiber preform composed of the core 111 and the first cladding 121 can be reduced while realizing a reduction in bending loss. Thus, since the volume of the optical fiber preform can be reduced, the production time of the optical fiber preform can be shortened. Further, since the second clad 131 as the low refractive index material can be made of general glass, it can be easily formed around the optical fiber preform. Therefore, the engaging Ru optical fiber to this embodiment, a strong bending loss condition, it is possible to more quickly and easily manufactured.

Further, as described above, since the optical fiber preform can be made small , the optical fiber can be made thin while obtaining the same bending loss reduction effect as that of the prior art.

What is important in the present embodiment is that the second cladding 131, which is a low refractive index material that contributes to the reduction of bending loss, is arranged as close as possible to the center of the core 111 in order to shorten the manufacturing time of the optical fiber preform. that is to first outermost peripheral position of the cladding 1 2 1 (a2) is as small as possible. That is, it is important to make the outermost peripheral position (a2) of the first cladding 1 2 1 as small as possible while the optical fiber functions well as a dual guide structure.

In the present embodiment, Delta] 2 -1.0% or more, since a large value than -0.2%, it is possible to reduce first the outermost peripheral position of the cladding 1 2 1 (a2), as a result, The volume of the optical fiber preform can be reduced.

  Thus, from the viewpoint of reducing the volume of the optical fiber preform, it is important to set Δ2 to −1.0% or more and −0.2% or less, and from the viewpoint of reducing bending loss. It is important to arrange a low refractive index material as close to the core as possible. In the present embodiment, in order to realize such a configuration, the relationship of 3.8 μm ≦ (2 * a1) <10 μm, 5 μm ≦ (a2) ≦ 10 μm, 1.14 ≦ (a2 / a1) <2.5 It is preferable to satisfy.

  In particular, in one embodiment of the present invention, it is important that the ratio a2 / a1 is less than 2.5. In the present embodiment, as described above, in order to reduce the volume of the optical fiber preform when 1.9 μm ≦ a1 <5 μm, it is necessary to reduce the ratio a2 / a1 as much as possible. Thus, in consideration of reducing the volume of the optical fiber preform, it is preferable to set the ratio a2 / a1 to less than 2.5.

  In this embodiment, in order to give the relative refractive index difference Δ1 between the core and the first cladding, for example, when quartz is used for the first cladding, Ge or the like is added to the core. When the relative refractive index difference Δ1 is reduced, the ratio a2 / a1 is reduced. At this time, in order to reduce the relative refractive index difference Δ1, the addition amount must be controlled to a strictly small value when the Ge or the like is added to the core. That is, in order to reduce the ratio a2 / a1, it is necessary to strictly control the additive to the core. Therefore, in one embodiment of the present invention, it is preferable to set the ratio a2 / a1 to 1.14 or more in consideration of such strict control.

  In an embodiment of the present invention, by satisfying 1.14 ≦ a2 / a1 <2.5, it is possible to reduce the electric field distribution characteristics in a good single mode and the excessive loss due to bending, and the optical fiber mother The volume of the material can be reduced. Thus, cost reduction can be achieved.

  That is, what is important in the present embodiment is to shorten the time for producing the optical fiber preform and simplify the optical fiber producing process by reducing the volume of the optical fiber preform. Therefore, by setting the parameters according to the present embodiment, the low refractive index region can be disposed relatively close to the core while achieving low bending loss and good single mode, and the optical fiber. The volume of the base material can be reduced.

(Second Embodiment)
In the present embodiment, in order to improve the moisture resistance of the optical fiber, a strength holder is disposed on the outer periphery of the second cladding. F element and B element are said to be weak against humidity or relatively low in mechanical strength, but since quartz is a material resistant to humidity, F element and B element are added as the strength holder. It is possible to improve moisture resistance by using no pure quartz. Therefore, the range of use environment can be expanded.

  In this embodiment, a member such as a strength holder is arranged on the outer periphery of the second cladding, and the optical fiber preform is the core, the first cladding, and the second cladding.

  In the first embodiment, the second clad, which is a low refractive index material, is disposed relatively near the core to reduce the bending loss, and the relative refractive index difference Δ2 between the first clad and the second clad is increased. By increasing the size, the volume of the optical fiber preform is reduced. In the present embodiment, in addition to the above configuration, the thickness (width) of the second cladding is optimized in order to reduce the volume of the optical fiber preform composed of the core, the first cladding, and the second cladding.

Figure 8 is a cross-sectional view of an optical fiber rather based on the second embodiment. 9 is a refractive index profile of an optical fiber rather based on the second embodiment.

  In FIG. 8, a core 111 as a first material for single mode transmission is provided at the center, and a first clad 121 as a second material and a second as a third material are sequentially formed outside the core 111. A clad 131 is provided, and finally a strength holding body 141 is provided as a fourth material. In the present embodiment, the core 111, the first cladding 121, and the second cladding 131 are optical fiber preforms. For the core 111, the first clad, the second clad, and the strength holder, materials usually used for optical fibers such as quartz glass, organic substances such as polymer and acrylic can be used.

  In FIG. 9, reference numeral 112 denotes the refractive index of the core 111, reference numeral 122 denotes the refractive index of the first cladding 121, reference numeral 132 denotes the refractive index of the second cladding 131, and reference numeral 142 denotes the refraction of the strength holder 141. Rate. As can be seen from FIG. 9, the refractive index 122 of the first cladding 121 is smaller than the refractive index 112 of the core 111, and the refractive index 132 of the second cladding 131 is smaller than the refractive index 122 of the first cladding 121. The refractive index 142 of the strength holder 141 is equivalent to the refractive index 122 of the first cladding 121.

  In the present embodiment, the refractive indexes of the strength holder 141 and the first cladding are made equivalent, but the present invention is not limited to this. In the present embodiment, as an example, the strength holder 141 is made of pure quartz to improve moisture resistance, and the first cladding 121 is also made of pure quartz. Therefore, the refractive index of the strength holder 141 and the first cladding 121 is high. In other words, when the first cladding 121 is doped quartz, the refractive indexes of the first cladding 121 and the strength holder 141 are not equivalent.

The core 111 has a relative refractive index difference Δ1 between the physical outer diameter (2 * a1) excited only in the fundamental mode and the first cladding 121 when the core is excited in a wavelength band of 1.31 μm. . Further, the outermost peripheral position of the first cladding 121 (a2), the distance from the central axis of the upper Symbol optical fiber is a2. Further, the outermost peripheral position of the second cladding 131 (a3), the distance from the central axis of the upper Symbol optical fiber is a3, and has a relative refractive index difference Δ2 of the first cladding 121.

For physical placement relationship,
The physical outer diameter (2 * a1) of the core 111 is preferably 3.8 μm ≦ (2 * a1) <10 μm, and the outermost peripheral position (a2) of the first cladding 121 is 5 μm ≦ (a2) ≦ 10 μm. preferable. The thickness (d) of the second cladding 131 is preferably 3.93 μm ≦ d ≦ 40 μm, and more preferably {24−0.8 * a2} μm ≦ d ≦ 40 μm. The first outermost peripheral position of the cladding 1 2 1 (a2), the ratio a2 / a1 of the a1 may preferably satisfy the 1.14 ≦ (a2 / a1) < 2.5 relationship. Further, the outermost peripheral position of the strength retention member 141 (a4) is equal to the outer diameter position of the upper Symbol optical fiber. With this structure, the second cladding 131 can be provided in a region relatively close to the core 111.

  In the present embodiment, it is preferable that the thickness d of the second cladding is at least three times the wavelength of the light used. In the present embodiment, since light having a wavelength of 1.31 μm is used, the width d is preferably 3.93 μm or more.

  In the present embodiment, as described above, since light having a wavelength of 1.31 μm is used, the preferable minimum value of the width d is 3.93 μm. It goes without saying that the minimum value is determined according to the above. For example, when designing to transmit light having a wavelength of 1.55 μm, a preferable minimum value of the width d is 4.65 (3 × 1.55 μm) μm.

  In addition, in the optical fiber according to the present embodiment, the width d is preferably 40 μm or less, considering the form without the strength holder 141.

  Further, regarding the respective refractive index relationships, the relative refractive index difference Δ1 is preferably 0.1% to 0.4%, and the relative refractive index difference Δ2 is preferably −1.0% to −0.2%. Thus, the single mode condition is adjusted by Δ1, and the optical signal can be guided even when bent by Δ2 having a negative value.

  FIG. 10 is a diagram illustrating an example in which the distribution of the basic mode based on the present embodiment is calculated. Δ1 = 0.36%, Δ2 = −0.4%, a1 = 4.4 μm, a2 = 10 μm, a2 / a1 = 2.27, and the thickness of the second cladding 131 (d = a3- a2) is 10 μm and 20 μm. As can be seen from FIG. 10, both d = 10 μm and 20 μm show good fundamental mode distribution. Furthermore, since the half width is not different from the basic mode, there is no problem in the connectivity of the normal optical fiber t.

  Note that the numerical value of 20 μm is included in {24−0.8 * a2} μm ≦ d ≦ 40 μm, which is the range of the more preferable width d described above, but does not include 10 μm. As is apparent from FIG. 10, it is clear that the fundamental mode distribution is stronger in the case where the thickness of the second cladding 131 is 20 μm, and the optical fiber is bent in this state. Even in such a case, since the confinement is strong, the excess loss due to bending becomes lower, so the above range is more preferable.

  FIG. 11 is a diagram showing the relationship between excess loss and the distance that light propagates in a bent optical fiber according to the present embodiment. In FIG. 11, R is a radius of curvature of the bent optical fiber, and Δ1 = 0.36%, Δ2 = −0.4%, and a1 = 4.4 μm, a2 = 10 μm, a2 / a1 = 2. 27, and the thickness (d = a3-a2) of the second cladding 131 is 15 μm.

  From FIG. 11, even if the curvature radius R changes between 3 and 8 mm, there is almost no change in excess loss due to bending, and if an optical fiber set to each parameter characteristic of the present invention is used, the optical fiber is It can be seen that even when bent, light can be propagated with almost no loss.

  In this embodiment, by setting the width of the second clad as described above, as can be seen from FIGS. 10 and 11, excess loss due to bending can be reduced while obtaining a good single mode.

  In the present embodiment, additives (for example, Ge, P, Sn, and B elements) added to the core 111 are selected so as to increase the refractive index of a base material such as quartz. The first cladding 121 and the strength holder 141 are each pure quartz.

  In the present embodiment, the F element is added to quartz for the second cladding 131, but the present invention is not limited to this. For example, any additive such as element B that can lower the refractive index of quartz may be used. In the present embodiment, the refractive index can be further reduced by adding the B element to the second cladding 131 in addition to the F element.

In such a configuration, the core 111 acts as a core for single mode transmission, when not bend off Aiba, in the case where the first clad 121 acts as a cladding, bending, the second cladding 131 is a second A dual guide structure is obtained.

  In the present embodiment, the optical signal wavelength for single mode transmission (the wavelength of the optical signal that excites the specified mode of the core 111 and performs single mode transmission) is set to the C-Band band (1530 nm to 1560 nm) or the L-Band band (1570 nm). ˜1610 nm), or alternatively, any of the 1300 nm band can be designed. Further, the mode field diameter can be set to 8.0 to 10.0 μm. That is, the mode field diameter of the above-mentioned specified mode can be set to 8.0 to 10.0 μm.

  In the present embodiment, as in the first embodiment, the second cladding 131 that functions as a low refractive index region for reducing bending loss is disposed relatively close to the center of the core 111. Furthermore, in this embodiment, in order to reduce the volume of the optical fiber preform composed of the core 111, the first cladding 121, and the second cladding 131, Δ2 is set to −1.0% as in the first embodiment. As described above, the thickness d is set to a large value of −0.2% or less, and the thickness d which is the width of the first cladding 131 is made as thin as possible. By controlling Δ2 and thickness d in this way, the volume of the optical fiber preform can be reduced, and therefore the manufacturing time of the optical fiber preform can be shortened.

In addition, since the strength holder 141 can be made of general pure quartz, it can be easily formed around the optical fiber preform. Therefore, the engaging Ru optical fiber to this embodiment, a strong bending loss condition, it is possible to more quickly and easily manufactured.

  What is important in the present embodiment is that the volume of the optical fiber preform (in this embodiment, the core 111, the first cladding 121, and the second cladding) is reduced while reducing the bending loss, as in the first embodiment. Is to make it smaller.

  In order to reduce the volume of the optical fiber preform according to the present embodiment, the width of the second cladding 131 that is a low refractive index material may be reduced. However, if the width of the second cladding 131 is reduced, the volume of the optical fiber preform is reduced. However, if the relative refractive index difference Δ2 is not increased, the second cladding 131 functions as a substantial cladding when the optical fiber is bent. There are times when it stops. Therefore, when the relative refractive index difference Δ2 is small, it is necessary to increase the thickness d, which is the width of the second cladding 131, in order to make the second cladding 131 function well. In Patent Document 2, since the relative refractive index difference corresponding to the second cladding of the present embodiment is set small, it is necessary to increase the thickness (width) of the member corresponding to the second cladding. In the disclosed optical fiber, there is a limit in reducing the volume of the optical fiber preform.

  However, in the present embodiment, in response to the desire to reduce the volume of the optical fiber preform, the thickness d of the second cladding 131 is reduced, and the dual guide structure can be maintained well even when the thickness d is reduced. By setting Δ2 to a large value of −1.0% or more and −0.2% or less, the second clad functions well and the bending loss is reduced while reducing the volume of the optical fiber preform. Can do.

  In the present embodiment, the strength holder 141 is used as a member disposed on the outer periphery of the second clad 131, but is not limited to this. In this embodiment, when a member having a higher refractive index than the second clad 131 that functions as a clad in some cases when the optical fiber is bent is disposed on the outer periphery of the second clad 131, the optical fiber preform In order to make the second clad 131 function as a substantial clad when the optical fiber is bent while reducing the volume of the optical fiber, the width of the second clad 131 is made smaller and the relative refractive index difference Δ2 is made larger. It is. Therefore, the member disposed on the outer periphery of the second cladding 131 may be any material as long as it has a higher refractive index than the second cladding 131, such as doped quartz or resin.

It is sectional drawing of the conventional photonic crystal fiber. It is a refractive index distribution map of the conventional photonic crystal fiber. It is sectional drawing of the conventional trench type optical fiber. It is a refractive index distribution map of the conventional trench type optical fiber. It is a single mode electric field distribution (actual measurement and Gaussian fitting) figure concerning one Embodiment of this invention. To a first embodiment of the present invention is a cross-sectional view of a based rather optical fiber. It is a refractive index profile of an optical fiber rather based on the first embodiment of the present invention. To a second embodiment of the present invention is a cross-sectional view of a based rather optical fiber. It is a refractive index profile of an optical fiber rather based on the second embodiment of the present invention. It is a figure which shows the relationship between the intensity | strength of the fundamental mode based on the 2nd Embodiment of this invention, and the thickness d of the 2nd clad. It is a figure which shows the relationship between the distance which light propagates in the excess loss based on the 2nd Embodiment of this invention, and the bent optical fiber.

Explanation of symbols

111 Core 121 First clad 131 Second clad 141 Strength holder

Claims (8)

Core, a first cladding and a second optical fiber Ru comprising a cladding,
Disposed at the axial center of the pre-Symbol optical fiber, the first material having a first refractive index,
A second material having a second refractive index smaller than the first refractive index, disposed on an outer periphery of the first material;
A third material having a third refractive index smaller than the second refractive index, disposed on the outer periphery of the second material,
The first material is the core;
The second material is the first cladding;
The third material is the second cladding;
The core has a physical outer diameter (2 * a1) excited only in a fundamental mode when the core is excited in a predetermined wavelength band, and a relative refractive index difference Δ1 between the first cladding and the core. And
Outermost peripheral position of the first cladding (a2) is a distance from the center axis of the front Symbol optical fiber a2,
The second cladding has a relative refractive index difference Δ2 with respect to the first cladding,
The physical outer diameter (2 * a1) of the core is 3.8 μm ≦ (2 * a1) <10 μm,
The outermost peripheral position (a2) of the first cladding is 5 μm ≦ (a2) ≦ 10 μm,
And a2 / a1 which is the ratio of the outermost peripheral position (a2) of the first clad and the a1 satisfies the relationship of 1.14 ≦ (a2 / a1) <2.5,
The relative refractive index difference Δ1 is 0.1% ≦ Δ1 ≦ 0.4%,
The relative refractive index difference Delta] 2 is Ri -1.0% ≦ Δ2 ≦ -0.2% der,
The refractive index of the core and the refractive index of the first cladding each have a constant value,
When not bend the optical fiber, the first cladding functions as a cladding, when bending the optical fiber, optical fiber characterized that you function second clad as the second cladding. Disposed on the outer periphery of the front Symbol third material, further comprising a fourth timber fees with the third fourth refractive index higher than the refractive index of,
Said second cladding thickness (d) is an optical fiber according to claim 1 you wherein 3 times the value ≦ d ≦ 40 [mu] m der Rukoto of the predetermined wavelength. It said fourth material, according to claim 2, wherein the optical fiber, which is a strength retention body before Symbol optical fiber. 4. The optical fiber according to claim 3, wherein the fourth material is a strength holder made of pure quartz. The optical fiber according to claim 3, wherein the fourth material is a resin having the fourth refractive index. The first material is quartz to which at least one of Ge, P, Sn, and B elements is added,
The second material is pure quartz;
6. The optical fiber according to claim 1, wherein the third material is F element, B element, or quartz added with a mixture of the two elements. The outer diameter of the body of the pre-Symbol optical fiber, the optical fiber according to any one of claims 1 to 6, characterized in that at most 55μm and not more than 125 [mu] m. The optical fiber according to any one of claims 1 to 7, wherein the predetermined wavelength band is a wavelength band of 1.31 µm.

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