Polarization-maintaining fiber

Abstract

A polarization-maintaining fiber (1) comprises a core (11), an inner layer (12) which surrounds the core (11) without a gap, a pair of stress-applying parts (13) which are disposed at positions such that the core (11) is sandwiched therebetween, and cladding (14) in which the inner layer (12) and the pair of stress-applying parts (13) are embedded, wherein: when light having a wavelength of 1.55 μm propagates through the core (11), the mode field diameter of the light is not more than 9.3 μm; when the length of said fiber is 0.25 m and said fiber is wound once around a mandrel having a radius of 2 mm such that the slow axes of the stress-applying parts (13) are perpendicular to a surface of the mandrel, a cutoff wavelength is not less than 1.32 μm; and the product of the area of a cross section of the inner layer (12) which is perpendicular to the lengthwise direction and the average relative refractive index difference of the entire inner layer (12) with respect to the cladding (14) is -36 to 0%μm2 <sp / >.

Description

Polarization-Maintaining Fiber

[0001] The present invention relates to polarization-maintaining fibers.

[0002] In recent years, there has been an increasing need for polarization-maintaining fibers as optical devices such as coherent optical transceivers and CPO (Co-packaged Optics) switch modules, and as lines for coherent optical transceivers, in optical transmission and reception modules that require the polarization-maintaining characteristics of optical devices such as CPO modules. In particular, in the case of lines for coherent optical transceivers, optical digital coherent communication is used to transmit a large amount of information through a single optical fiber, and optical devices that perform optical digital coherent communication are polarization-dependent. For this reason, polarization-maintaining optical fibers are sometimes used to connect optical devices to lines connecting external light sources.

[0003] An example of a polarization-maintaining fiber is described in Patent Document 1 below.

[0004] Japanese Patent Application Laid-Open No. 2000-221341

[0005] In recent years, the number of optical devices such as optical transceivers has increased in response to the expansion of communication capacity, and efforts have been made to miniaturize optical devices by combining multiple optical devices and electronic components into a single package. To meet these needs, there is a demand for polarization-maintaining optical fibers that can suppress bending loss (macrobend loss) even when bent slightly.

[0006] Bending loss correlates with the cutoff wavelength. The IEC 60793-1-44 standard defines the cutoff wavelength as the wavelength at which LP11 mode light exceeds 19.3 dB for an optical fiber with a fiber length of 2 m and wound once around a mandrel with a diameter of 280 mm. When multiple optical devices or electronic components are packaged together, the optical fiber may be used while being bent to a length shorter than that specified by the standard and with a bending diameter smaller than that specified by the standard. However, when an optical fiber measured according to the standard is used in a state shorter than the above-mentioned length, the cutoff wavelength becomes longer, which may result in small loss of LP11 mode light and make single-mode communication difficult.

[0007] Therefore, an object of the present invention is to provide a polarization-maintaining fiber that can suppress bending loss and prevent the cutoff wavelength from becoming longer when the optical fiber is shortened and bent slightly during use.

[0008] In order to solve the above problems, aspect 1 of the present invention provides a fiber comprising a core, an inner layer surrounding the core with no gaps, a pair of stress-applying portions arranged at positions sandwiching the core, and a cladding containing the inner layer and the pair of stress-applying portions, wherein when light having a wavelength of 1.55 μm propagates through the core, the mode field diameter of the light is 9.3 μm or less, the fiber length is 0.25 m, and when the fiber is wound once around a mandrel having a radius of 2 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, the cutoff wavelength is 1.32 μm or more, and the product of the area of ​​the inner layer in a cross section perpendicular to the longitudinal direction and the average relative refractive index difference of the entire inner layer with respect to the cladding is -36% μm. ２ More than 0%μm ２ The polarization-maintaining fiber is characterized by the following:

[0009] Since the mode field diameter is 9.3 μm or less, the light confinement power of the core is large, and therefore, with this polarization-maintaining fiber, bending loss can be reduced compared to when the mode field diameter is larger than 9.3 μm.

[0010] The longer the cutoff wavelength, the greater the light confinement force of the core tends to be. Therefore, as described above, when the fiber length is 0.25 m and the fiber is wound once around a mandrel with a radius of 2 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, and the cutoff wavelength is 1.32 μm or more, bending loss can be more easily suppressed, even when the polarization-maintaining fiber is bent with a small bending diameter and used, compared to a polarization-maintaining fiber having a cutoff wavelength of less than 1.32 μm when the fiber is wound once around a mandrel with a radius of 2 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel.

[0011] Furthermore, the product of the area of ​​the inner layer in a cross section perpendicular to the longitudinal direction and the average relative refractive index difference of the entire inner layer with respect to the cladding is -36% μm ２In the above cases, the influence of the change in bending radius of the polarization-maintaining fiber 1 on the cutoff wavelength tends to be larger than the influence of the change in fiber length on the cutoff wavelength. ２ In addition, when the above product is 0% μm, the change in cutoff wavelength relative to the change in fiber length can be suppressed. ２ When the product is 0% μm or less, ２ Compared with the case where the diameter is larger, the light confinement force can be increased, and bending loss can be suppressed even if the polarization-maintaining fiber is bent slightly.

[0012] As described above, according to the present invention, a polarization-maintaining fiber can be provided that can suppress bending loss and prevent the cutoff wavelength from becoming longer when the optical fiber is shortened and used with a small bend.

[0013] 1 is a diagram showing a polarization-maintaining fiber according to an embodiment of the present invention, and FIG. 2 is a diagram showing the distribution of the relative refractive index difference of the polarization-maintaining fiber with respect to the cladding.

[0014] Preferred embodiments of the polarization-maintaining fiber according to the present invention will be described in detail below with reference to the drawings. The embodiments exemplified below are intended to facilitate understanding of the present invention and are not intended to limit the present invention. The present invention can be modified and improved within the scope of the claims. Note that, for ease of understanding, the scale of each drawing may differ from the scale described below.

[0015] 1 is a cross-sectional view showing the transverse section of a polarization-maintaining fiber according to this embodiment. The transverse section is a cross section perpendicular to the central axis C of the polarization-maintaining fiber 1.

[0016] As shown in Fig. 1, the polarization-maintaining fiber 1 is an optical fiber capable of maintaining the polarization plane of light propagating through a core 11, and includes the core 11, an inner layer 12 tightly surrounding the core 11, a pair of stress-applying portions 13 positioned on either side of the core 11, and a cladding 14 enclosing the inner layer 12 and the stress-applying portions 13. Note that since the core 11 is surrounded by the inner layer 12, the cladding 14 also encloses the core 11. The polarization-maintaining fiber 1 may also include a coating layer that covers the cladding 14. A polarization-maintaining fiber 1 configured in this manner is sometimes called a PANDA (Polarization-maintaining AND Absorption-reducing) fiber. In other words, it is sometimes called a PANDA-type polarization-maintaining fiber.

[0017] The core 11 has a columnar shape extending in the direction of the central axis C of the polarization-maintaining fiber 1, which passes through approximately the center of the cladding 14. Therefore, the central axis C of the polarization-maintaining fiber and the central axis of the core are approximately the same. Therefore, in the following description, the central axis of the core 11 and the polarization-maintaining fiber 1 will be referred to as the central axis C. The number of cores 11 may be one, or, unlike in FIG. 1, may be two or more.

[0018] 2 is a diagram showing the distribution of the relative refractive index difference of the polarization-maintaining fiber 1 with respect to the cladding 14 along the fast axis direction. In Fig. 2, the refractive index distribution from the central axis C of the polarization-maintaining fiber 1 to the outer periphery of the cladding 14 without passing through the stress-applying parts 13 is shown by a dashed line.

[0019] The core 11 is a region that includes the central axis C and has a refractive index higher than that of the cladding 14. The core 11 is made of, for example, silica glass doped with an up-dopant such as germanium (Ge). The relative refractive index difference Δ of the core 11 with respect to the cladding 14 is １１ is preferably 0.39% or more. １１ is 0.39% or more, the relative refractive index difference Δ １１ In comparison with the case where the relative refractive index difference Δ is less than 0.39%, the light confinement force in the core 11 can be increased, and the leakage of light from the core 11 can be suppressed. １１ is more preferably 0.43% or more. １１is 0.43% or more, the relative refractive index difference Δ １１ In comparison with the case where the relative refractive index difference Δ is less than 0.43%, the light confinement force in the core 11 can be increased, and the leakage of light from the core 11 can be suppressed. １１ is preferably 0.50% or less. １１ is 0.50% or less, the relative refractive index difference Δ １１ Compared to when the relative refractive index difference Δ is greater than 0.50%, it is possible to prevent the mode field diameter of light propagating through the core 11 from becoming too small, and to suppress light loss at the connection point with other optical fibers or optical devices. Therefore, even if the strict connection loss condition required as one of the target values ​​for the optical device described above is met, for example, when the polarization-maintaining fiber 1 is housed in the optical device or connected to another optical fiber, it is possible to realize a polarization-maintaining fiber 1 that can more reliably satisfy the conditions to the extent that it can be applied to the optical device or the other optical fiber. １１ It is more preferable that the relative refractive index difference Δ is 0.44% or less. １１ is 0.44% or less, the relative refractive index difference Δ １１ Compared with the case where the relative refractive index difference Δ is greater than 0.44%, the mode field diameter of light propagating through the core 11 can be prevented from becoming too small, and the loss of light at the connection point with another optical fiber or an optical device can be suppressed. １１ It is preferable that the relative refractive index difference Δ is 0.39% or more and 0.44% or less, since this makes it possible to balance the light confinement force and the light loss at the connection point. １１ It is more preferable that the relative refractive index difference Δ is 0.41% or more and 0.44% or less, in order to achieve a balance between the light confinement force and the light loss at the connection point. １１ is the maximum relative refractive index difference Δ of the core 11 １１The average relative refractive index difference is the average relative refractive index difference in the range that satisfies the relative refractive index difference of 90% or more of the maximum. Examples of the above-mentioned "optical device" include (1) optical devices for telecommunications, such as transceivers for intercity networks and submarine networks, or optical devices for datacom, such as CPO (Co-packaged Optics) modules (CPO switch modules, etc.) and pluggable transceivers (coherent optical transceivers, etc.), (2) optical devices for amplifiers, and (3) optical devices for sensors. The term "optical device" described below is used to include the optical devices in (1) to (3) above.

[0020] In this embodiment, the maximum relative refractive index difference Δ １１ max, and the relative refractive index difference of the core 11 decreases from that position toward the central axis C. In this embodiment, the relative refractive index difference of the core 11 at the central axis C is approximately 90% of the maximum relative refractive index difference of the core 11. Note that the distribution of the relative refractive index difference of the core 11 is not limited to the example in FIG. 2 .

[0021] The cross-sectional shape of the core 11 is generally circular. However, the cross-sectional shape of the core 11 is not limited to circular, and may be, for example, elliptical, crescent, or non-circular. The cross-sectional shape of the core 11 here refers to the shape of a cross section perpendicular to the central axis C of the polarization-maintaining fiber 1. The radius of the core is preferably 4.7 μm or less. The radius of the core 11 is preferably 4.1 μm or more. The radius of the core 11 is more preferably 4.1 μm or more and 4.7 μm or less. The radius of the core 11 may be, for example, half the average value of the diameter of the core 11 in the direction perpendicular to the central axis C, or the relative refractive index difference Δ １１ The radius of the core 11 is not particularly limited as long as it satisfies conditions 1 to 3 described below.

[0022] The inner layer 12 extends along the direction of the central axis C, contacts the outer peripheral surface of the core 11, and surrounds the core 11. Furthermore, the inner layer 12 of this embodiment contacts each of the stress-applying portions 13. Therefore, the outer periphery of the inner layer 12 consists of a part of a circle having an outer periphery with a diameter larger than the shortest distance from the central axis C to the stress-applying portion 13, and a part contacting the stress-applying portion 13. Note that the inner layer 12 may be separated from the stress-applying portion 13 as long as it satisfies conditions 1 to 3 described below. Note that, as shown in FIG. 2, the relative refractive index difference Δ of the inner layer 12 with respect to the cladding 14 is １２ The diameters at the two points where the value of φ is zero can be the inner diameter and the outer diameter of the inner layer 12, respectively.

[0023] In FIG. 2, the relative refractive index difference Δ １２ Although an example is shown in which the relative refractive index difference Δ １２ may be zero. That is, the inner layer 12 of the polarization-maintaining fiber 1 may have the same refractive index as the cladding 14. Therefore, the inner layer 12 and the cladding 14 may be made of the same material. The relative refractive index difference Δ １２ is preferably −0.05% or more and 0% or less, and more preferably −0.03% or more and 0% or less. １２ is -0.05% or more, the relative refractive index difference Δ １２ In comparison with the case where the relative refractive index difference Δ １２ is -0.03% or more, the relative refractive index difference Δ １２ In comparison with the case where the relative refractive index difference Δ is less than −0.03%, the effect of the coefficient A described later can be easily obtained. １２ may be less than 0%. １２ is the average relative refractive index difference of the entire inner layer 12. The relative refractive index difference Δ １２ When the inner layer 12 is a depressed layer, the light confinement power of the core 11 is large, and the relative refractive index difference Δ １２ The bending loss can be suppressed compared to when the relative refractive index difference Δ of the inner layer 12 is equal to or greater than zero. １２is equal to or greater than zero, the refractive index of the inner layer 12 and the refractive index of the cladding 14 are equal. In this case, if the refractive index profile of the core 11 is approximately constant in the radial direction, the refractive index profile of the core 11 of the polarization-maintaining fiber 1 may be called a step type, and if the refractive index profile of the core 11 decreases from the central axis C of the core 11 toward the outer circumferential surface of the core 11, the refractive index profile of the core 11 of the polarization-maintaining fiber 1 may be called a GI type. Note that the refractive index profile of the core 11 of the polarization-maintaining fiber 1 is not limited to the step type or GI type described above, but may also be a ring type, a double step type, or a tapered type.

[0024] In addition, a trench-type optical fiber is known that has a refractive index lower than that of the cladding and is provided with a trench layer that is spaced apart from the core and surrounds the outer surface of the core. When the inner layer 12 is a depressed layer, the structure is simpler because no layer is required between the core and the trench layer.

[0025] The relative refractive index difference Δ of the inner layer 12 １２ is negative, the outer diameter of the inner layer 12 is preferably 15 μm or more. When the refractive index of the inner layer 12 does not change substantially in the radial direction, an outer diameter of the inner layer 12 of 15 μm or more tends to make the polarization-maintaining fiber 1 more resistant to bending than when the outer diameter of the inner layer 12 is less than 15 μm. Furthermore, the outer diameter of the inner layer 12 is preferably 20 μm or less. An outer diameter of the inner layer 12 of 20 μm or less tends to achieve the effect of coefficient A, which will be described later, compared to when the outer diameter of the inner layer 12 exceeds 20 μm. The thickness of the inner layer 12, which is the distance from the inner circumference to the outer circumference, is preferably 11 μm or more and 18 μm or less.

[0026] The pair of stress-applying portions 13 have a columnar shape extending along the direction of the central axis C. In this embodiment, the refractive index of the stress-applying portions 13 is lower than the refractive index of the clad. The stress-applying portions 13 are made of, for example, boron oxide (B ２ O ３The stress-applying portion 13 is made of silica glass doped with a down dopant such as SiO 2 . The cross-sectional shape of the stress-applying portion 13 may be an isosceles trapezoid with its upper base (shorter base) facing the core 11. In this case, one or both of the upper and lower bases (longer bases) of the stress-applying portion 13 may be arc-shaped, bulging in a direction away from the core 11. A polarization-maintaining fiber 1 having stress-applying portions 13 shaped like an isosceles trapezoid is sometimes called a "bow-tie polarization-maintaining fiber." In this way, the polarization-maintaining fiber 1 is not limited to the above-mentioned PANDA polarization-maintaining fiber, but may also be a bow-tie polarization-maintaining fiber.

[0027] The relative refractive index difference Δ of the stress-applying portion 13 with respect to the cladding 14 １３ is preferably −0.70% or less. With such a relative refractive index difference, the stress-applying parts 13 can function as low refractive index layers in the slow axis direction along which the stress-applying parts 13 are arranged, and leakage of light from the core 11 can be further suppressed. In addition, the relative refractive index difference Δ １３ is −0.70% or less, the relative refractive index difference Δ of a typical polarization-maintaining fiber 1 that can maintain the polarization plane of light propagating through the core 11. １３ In addition, the relative refractive index difference Δ １３ is preferably −0.90% or more. １３ When the relative refractive index difference Δ is −0.90% or more, the amount of boron oxide added can be sufficiently small, so that deliquescence of the stress-applying portion 13 can be suppressed. １３ It is more preferable that the relative refractive index difference Δ is not less than −0.70% and not more than −0.90%. １３ The above-mentioned content can be asserted when the diameter of the clad 14 is mainly 80 μm±1 μm to 125 μm±1 μm.

[0028] The cross-sectional shape of the stress-applying portion 13 in this embodiment is generally circular, as shown in FIG. 1 . However, the cross-sectional shape of the stress-applying portion 13 may also be generally elliptical, with the arrangement direction of the pair of stress-applying portions 13 as the minor axis direction. The cross-sectional shape of the stress-applying portion 13 is not limited to circular or elliptical, and may be, for example, crescent or noncircular. The cross-sectional shape of the stress-applying portion 13 here refers to the shape of a cross section perpendicular to the central axis C of the polarization-maintaining fiber 1. Here, when the diameter of the cladding 14 described below is 80 μm±1 μm, the diameter of the stress-applying portion 13 along the slow axis direction is preferably 22.0 μm or more, more preferably 24.8 μm or more, and even more preferably 30.0 μm or more. This allows sufficient stress to be applied to the core 11, thereby enabling the polarization-maintaining function to be suitably achieved. Furthermore, the diameter is preferably 27.0 μm or less. A diameter of the stress-applying portion 13 of 27.0 μm or less can suppress a decrease in the propagation loss of light. This allows the stress-applying portions 13 to be suitably disposed inside the cladding 14, which has a diameter of 80 μm±1 μm. Furthermore, when the diameter of the cladding 14 is 80 μm±1 μm, the diameter of the stress-applying portions 13 is preferably 22.0 μm or more and 27.0 μm or less, and more preferably 24.8 μm or more and 27.0 μm or less. Furthermore, by making the diameter of the stress-applying portions 13 22.0 μm or more, the range of diameters of the stress-applying portions 13 of a general polarization-maintaining fiber 1 that can maintain the polarization plane of light propagating through the core 11 can be covered. In the cases of Examples 1 to 5 described below, the diameter of the stress-applying portions 13 can be 24.0 μm or more and 26.0 μm or less.

[0029] Furthermore, when the diameter of the cladding 14 described below is 125 μm±1 μm, the diameter of the stress-applying portion 13 along the slow axis direction is preferably 30.0 μm or more. This allows sufficient stress to be applied to the core 11, and as a result, the polarization-maintaining function can be suitably achieved. Furthermore, the above diameter is preferably 40.0 μm or less. This allows the stress-applying portion 13 to be suitably disposed inside the cladding 14 having a diameter of 125 μm±1 μm. Furthermore, when the diameter of the cladding 14 is 125 μm±1 μm, the above diameter is preferably 30.0 μm or more and 40.0 μm or less.

[0030] The stress-applying parts 13 are each spaced apart from the core 11. This reduces the possibility that the core 11 will be unexpectedly deformed by stress from the stress-applying parts 13 when manufacturing the polarization-maintaining fiber 1 by melt-drawing. Furthermore, if the core 11 is in contact with the stress-applying parts 13, the boron oxide added to the stress-applying parts 13 may cause a deterioration in transmission loss, but since the core 11 is spaced apart from the stress-applying parts 13, this deterioration in transmission loss can be suppressed.

[0031] When the diameter of the cladding 14 is 80 μm±1 μm, the distance from the central axis C of the core 11 to the center of the stress-applying portion 13 is preferably 15.0 μm or more. Furthermore, this distance is preferably 20.0 μm or less. Furthermore, this distance is preferably 15.0 μm or more and 20.0 μm or less. When this distance is 20.0 μm or less, the stress-applying portion 13 is closer to the core 11 than when this distance is greater than 20.0 μm. Therefore, the stress applied to the core 11 is stronger, and the polarization-maintaining function is enhanced. Therefore, the above-mentioned distance range of a typical polarization-maintaining fiber 1 that can maintain the polarization plane of light propagating through the core 11 can be covered. Furthermore, the stress-applying portion 13, which has a lower refractive index than the cladding 14, is closer to the core 11. Therefore, the function of the stress-applying portion 13 as a low-refractive-index layer in the slow-axis direction can be enhanced, and light leakage from the core 11 can be further suppressed. Furthermore, by setting the distance to 15.0 μm or more, an increase in loss due to boron oxide can be prevented. Furthermore, when the diameter of the cladding 14 is 125 μm±1 μm, the distance from the central axis C of the core 11 to the center of the stress-applying portion 13 is preferably 25.0 μm or more. Furthermore, the distance is preferably 30.0 μm or less. Furthermore, when the stress-applying portion 13 is in contact with the inner layer 12 as in this embodiment, this is preferable from the viewpoint of reducing polarization crosstalk, and when the stress-applying portion 13 is separated from the inner layer 12, this is preferable from the viewpoint of suppressing propagation loss. Furthermore, the ratio obtained by dividing the distance from the central axis C of the core 11 to the end of the stress-applying portion 13 by the radius of the mode field diameter (MFD) is preferably 1.05 or more. Since this ratio is 1.05 or more, the stress-applying portion 13 is positioned outside the mode field, and therefore, when boron oxide is added to the stress-applying portion 13, deterioration in transmission loss due to the boron oxide can be further suppressed. The distance between the end of the core 11 on the stress-applying part 13 side and the end of the stress-applying part 13 on the core 11 side is preferably 0.5 μm or more. １３The distance from the central axis C of the core 11 to the center of the stress-applying portion 13 and the diameter of the stress-applying portion 13 along the slow axis direction are not particularly limited. That is, the effects of the present invention can be obtained as long as the conditions 1 to 3 described below are satisfied within a range in which the polarization plane of light propagating through the core 11 can be maintained (functioning as a polarization-maintaining fiber). In the cases of Examples 1 to 5 described below, the distance can be 17.0 μm or more and 18.0 μm or less. The distance between the end of the core 11 on the stress-applying portion 13 side and the end of the stress-applying portion 13 on the core 11 side refers to the distance from the point included in the core 11 that is closest to the stress-applying portion 13 to the point included in the stress-applying portion 13 that is closest to the core 11.

[0032] The cladding 14 has a columnar shape extending along the direction of the central axis C. As described above, the refractive index of the cladding 14 is lower than that of the core 11, equal to or higher than that of the inner layer 12, and higher than that of the stress-applying portion 13. The cladding 14 is made of, for example, silica glass.

[0033] The cross-sectional shape of the cladding 14 in this embodiment is generally circular. However, the cross-sectional shape of the cladding 14 is not limited to this and may be, for example, elliptical, crescent, or non-circular. Note that the cross-sectional shape of the cladding 14 here refers to the shape of a cross section perpendicular to the central axis C of the polarization-maintaining fiber 1.

[0034] The diameter of the cladding 14 is preferably approximately 80 μm±1 μm, i.e., 79 μm to 81 μm, and more preferably 80 μm. In this case, for example, the installation area can be reduced when the fiber is housed in an optical transceiver or when applied to an amplifier or sensor, thereby enabling high-density packaging. Furthermore, in this case, the rigidity of the polarization-maintaining fiber 1 can be reduced, thereby reducing the decrease in mechanical strength of the polarization-maintaining fiber 1 when twisted. Alternatively, the diameter of the cladding is preferably 125 μm±1 μm, i.e., 124 μm to 126 μm, and more preferably 125 μm. In this case, a polarization-maintaining fiber with approximately the same diameter as optical fibers commonly used in communication infrastructure can be realized. Even if a ±1 μm difference in the diameter of the cladding 14 occurs when the diameter of the cladding 14 is 80 μm or 125 μm, the optical and mechanical characteristics of the optical fiber will be roughly the same as when the cladding diameter is 80 μm or 125 μm, or the variation will be within the margin of error and will not significantly affect the optical and mechanical characteristics of the optical fiber. For example, in an optical fiber with a cladding diameter of 125 μm, the ±1 μm difference in the diameter of the cladding is the same as the cladding diameter tolerance specified in the optical fiber standard (ITU-T). Furthermore, the ±1 μm difference in the diameter of the cladding in an optical fiber with a cladding diameter of 80 μm may also correspond to the above tolerance value. Furthermore, the diameter of the cladding 14 may be 79 μm or greater and 126 μm or less. When the cross section of the cladding 14 is non-circular, the diameter of the cladding 14 is, for example, the average value of the diameters of the cladding 14 in the direction perpendicular to the central axis C.

[0035] Furthermore, in the present embodiment, the center of the circle that forms the outer periphery of the core 11 coincides with the center of the circle that forms the outer periphery of the cladding 14, but this is not limited thereto, and it is sufficient that the center of the circle that forms the outer periphery of the core 11 is included in the center of the cladding 14. Therefore, when the diameter of the cladding 14 satisfies the range of 79 μm to 126 μm, it is sufficient that the center of the circle that forms the outer periphery of the core 11 is included in the center of the cladding 14. Here, the center of the cladding 14 refers to a circle with a radius of 0.6 μm, and the inner region of the circle whose center coincides with the center of the circle that forms the outer periphery of the cladding 14.

[0036] In view of its use in cases where multiple optical devices or electronic components are integrated into a single package, the fiber length of the polarization-maintaining fiber 1 may be less than 2 m, may be 0.5 m or less, or may be 0.5 m or less and 0.25 m or more. Furthermore, the fiber length of the polarization-maintaining fiber 1 is preferably 0.01 m or more. The fiber length of the polarization-maintaining fiber 1 may be 2 m or more.

[0037] When an optical fiber is bent at a small radius, light tends to leak, shortening the cutoff wavelength. On the other hand, when the length of the optical fiber is shortened, the cutoff wavelength lengthens. A depressed layer, which has a refractive index lower than that of the cladding and is in contact with and surrounds the core, may be provided. In this case, the light confinement force of the core is greater than when the depressed layer is not provided. Therefore, the bending loss of light propagating through the core is reduced. Furthermore, when a depressed layer is provided, the extent of the shortening of the cutoff wavelength may be smaller when the optical fiber is bent at a small radius than when the depressed layer is not provided. On the other hand, when a depressed layer is provided, the extent of the lengthening of the cutoff wavelength may be greater when the optical fiber is shortened than when the depressed layer is not provided. Regardless of whether a depressed layer is provided, when an optical fiber measured in accordance with the IEC 60793-1-44 standard is bent and used at a diameter smaller than the 280 mm specified in the standard, the cutoff wavelength becomes shorter, and single-mode light can be propagated. However, when an optical fiber measured in accordance with the above standard is used at a length shorter than the 2 m specified in the standard, the cutoff wavelength becomes longer, and the loss of LP11 mode light becomes small, which may make single-mode communication difficult.

[0038] Therefore, the polarization-maintaining fiber 1 of this embodiment having the above configuration satisfies the following conditions 1 to 3.

[0039] <Condition 1> When light having a wavelength of 1.55 μm propagates through the core 11, the mode field diameter of the light is 9.3 μm or less.

[0040] By satisfying this condition, the light confinement effect can be improved compared to when the mode field diameter at a wavelength of 1.55 μm is larger than 9.3 μm. Therefore, by satisfying condition 1, bending loss can be suppressed even when the polarization-maintaining fiber is bent slightly compared to when the mode field diameter at a wavelength of 1.55 μm is larger than 9.3 μm.

[0041] <Condition 2> The fiber has a length of 0.25 m, and when wound once around a mandrel with a radius of 2 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, the cutoff wavelength is 1.32 μm or more.

[0042] In optical fibers, the longer the cutoff wavelength, the greater the light confinement power of the core tends to be. Therefore, by having a cutoff wavelength of 1.32 μm or more when wound once around a mandrel with a radius of 2 mm as described above, even when the polarization-maintaining fiber is used while being bent with a small bending diameter, bending loss can be reduced compared to a polarization-maintaining fiber having a cutoff wavelength of less than 1.32 μm when wound once around a mandrel with a radius of 2 mm. Here, "one turn" refers to winding the polarization-maintaining fiber around a mandrel with a radius of 2 mm once. Furthermore, "one turn" is synonymous with one turn.

[0043] <Condition 3> The area of ​​the cross section of the inner layer 12 perpendicular to the longitudinal direction and the relative refractive index difference Δ of the entire inner layer 12 with respect to the cladding 14 １２ The product of the average of ２ More than 0%μm ２ The following is the result.

[0044] This product is called the refractive index volume of the inner layer 12. If the refractive index volume of the inner layer 12 is -36% μm ２ In the above cases, the influence of the change in bending radius of the polarization-maintaining fiber 1 on the cutoff wavelength tends to be larger than the influence of the change in fiber length on the cutoff wavelength. Therefore, when the polarization-maintaining fiber 1 is bent slightly, the refractive index volume is reduced to -36% μm ２ In addition, when the refractive index volume of the inner layer 12 is 0% μm, the change in the cutoff wavelength relative to the change in the fiber length can be suppressed compared to when the refractive index volume of the inner layer 12 is 0% μm ２ When the refractive index volume is 0% μm or less, ２ Therefore, when the refractive index volume of the inner layer 12 is −36% μm, the optical confinement force can be increased, and the bending loss can be suppressed even when the polarization-maintaining fiber is bent slightly. ２ More than 0%μm ２By satisfying this condition, when the polarization-maintaining fiber 1 is used after being shortened and bent slightly, bending loss can be suppressed and the cutoff wavelength can be prevented from becoming longer.

[0045] Next, an example of the polarization-maintaining fiber 1 will be described.

[0046] The polarization-maintaining fibers of Comparative Example 1, Comparative Example 2, and Examples 1 to 5 were subjected to the following measurements.

[0047] <Mode field diameter> The mode field diameter was measured when light with a wavelength of 1.55 μm propagated through the core. The measurement was performed using the variable aperture method in accordance with IEC 60793-1-45. Light with a wavelength of 1.55 μm that was guided through the core 11 was used to measure the mode field diameter. For this reason, the "mode field diameter" and "mode field diameter at a wavelength of 1.55 μm" described above and below may refer to the mode field diameter of light with a wavelength of 1.55 μm when the light propagates through the core.

[0048] <Cladding Diameter> The cladding diameter was measured by the transmitted near-field method based on IEC 60793-1-20.

[0049] <Fiber Cutoff Wavelength> The cutoff wavelength when a polarization-maintaining fiber with a fiber length of 2 m is wound around a mandrel with a diameter of 280 mm is referred to as the fiber cutoff wavelength. The fiber cutoff wavelength was determined based on IEC 60793-1-44 by measuring the wavelength at which the higher-order mode is attenuated by 19.3 dB when a polarization-maintaining fiber 1 cut to a fiber length of 2 m is wound around a mandrel with a diameter of 280 mm once. At this time, the polarization-maintaining fiber was wound around the mandrel so that the slow axis of the polarization-maintaining fiber was perpendicular to the surface of the mandrel. The mandrel is a rod-shaped jig with a perfectly circular cross section.

[0050] <Effective Cutoff Wavelength> The cutoff wavelength when a polarization-maintaining fiber with a fiber length of 0.25 m is wound around a mandrel with a radius of 2 mm once is referred to as the effective cutoff wavelength. The effective cutoff wavelength was measured using the optical system and calculation method used to measure the fiber cutoff wavelength. The polarization-maintaining fiber was cut to a fiber length of 0.25 m and wound around a mandrel with a radius of 2 mm once. The polarization-maintaining fiber was wound around the mandrel so that the slow axis of the polarization-maintaining fiber was perpendicular to the surface of the mandrel. The effective cutoff wavelength was determined as the wavelength at which the ratio of the total power of light, including higher-order modes, to the power of light in the fundamental mode was 0.1 dB. The mandrel was a rod-shaped jig with a perfectly circular cross section.

[0051] <Relative refractive index difference Δ １１ > Relative refractive index difference Δ １１ is the relative refractive index difference of the core with respect to the cladding. Figure 2 is a diagram showing the distribution of the relative refractive index difference along the fast axis direction. １１ was calculated as the average of the relative refractive index differences in a region that is 90% or more of the maximum value of the relative refractive index difference of the core with respect to the cladding, using the following method: That is, the refractive index distribution of the core in the fast axis direction of the polarization-maintaining fiber was obtained by measurement using an interferometry, the relative refractive index difference distribution on the fast axis was calculated, the maximum value of the relative refractive index difference of the core with respect to the cladding was determined, and then the average of the relative refractive index differences in a region that is 90% or more of the maximum value of the relative refractive index difference of the core with respect to the cladding was calculated.

[0052] <Relative refractive index difference Δ １２ > Relative refractive index difference Δ １２ is the relative refractive index difference of the inner layer with respect to the cladding. １２ is the relative refractive index difference of the core, Δ １１ Similarly, the refractive index distribution of the inner layer in the fast axis direction of the polarization-maintaining fiber was obtained by measurement using an interferometric method, and the relative refractive index difference distribution on the fast axis was calculated and found as the average relative refractive index difference of the entire inner layer relative to the cladding.

[0053] <Relative refractive index difference Δ13> The relative refractive index difference Δ13 is the relative refractive index difference of the stress-applying portion with respect to the cladding. As with the relative refractive index difference Δ11, the relative refractive index difference Δ13 was obtained by measurement using an interference method as the average of the relative refractive index differences in a region where the refractive index distribution in the slow axis direction of the stress-applying portion is 50% or less of the minimum value. That is, the refractive index distribution of the stress-applying portion in the slow axis direction of the polarization-maintaining fiber was obtained by measurement using an interference method, the relative refractive index difference distribution on the slow axis was calculated, and the minimum value of the relative refractive index difference of the stress-applying portion with respect to the cladding was obtained, and then the average of the relative refractive index differences in a region where the refractive index distribution in the slow axis direction of the stress-applying portion is 50% or less of the minimum value of the refractive index distribution in the slow axis direction of the stress-applying portion was obtained.

[0054] <Distance from the central axis of the core to the stress-applying portion> The distance from the central axis of the core to the stress-applying portion was obtained by measuring the fiber end face with an optical microscope.

[0055] <Diameter of Stress-Applying Portion> The diameter of the stress-applying portion was obtained by measuring the end face of the fiber with an optical microscope.

[0056] <Refractive index volume of inner layer> The refractive index volume of the inner layer is determined by observing the end face of the polarization-maintaining fiber with an optical microscope to determine the cross-sectional area of ​​the inner layer perpendicular to the longitudinal direction of the polarization-maintaining fiber, and multiplying the determined cross-sectional area of ​​the inner layer by the relative refractive index difference Δ １２ The area was calculated by multiplying the area by the stress-applying portion. Here, the values ​​of the inner diameter and outer diameter of the inner layer 12 obtained during the refractive index distribution measurement in Figure 2 were used as the values ​​of the inner diameter and outer diameter of the inner layer 12, respectively. When the stress-applying portion penetrated into the inner layer, the area was calculated for the region of the inner layer 12 excluding the region of the stress-applying portion. In each example and comparative example, the stress-applying portion penetrated into the inner layer, so the above area was calculated as described above. In Example 1 and Example 4, the refractive index of the inner layer and the refractive index of the cladding were equal, so no calculation was performed and the refractive index volume of the inner layer was set to zero.

[0057] <Effective bending loss> In this specification, the bending loss of light with a wavelength of 1.55 μm per turn when wound around a mandrel with a radius of 2 mm or a mandrel with a radius of approximately 2 mm is referred to as effective bending loss. Effective bending loss was measured based on IEC 60793-1-47, when a polarization-maintaining fiber was wound once so that the slow axis was perpendicular to the surface of a mandrel with a radius of 2 mm (when the bending radius was 2 mm), the power of the light in LP01 mode from the light source at that time and the power of the light in LP01 mode from the light source in a state before winding were measured as a reference, and the bending loss per turn was calculated from the difference between these two powers. Note that, for bending losses when the fiber cutoff wavelength exceeds 1.55 μm, the fiber was made longer in length than 2 m and arbitrarily bent, so that the cutoff wavelength in that state was 1.55 μm or less, and this state was used as a reference, and the difference in power was measured when the fiber was wound once around a mandrel with a radius of 2 mm. The mandrel is a rod-shaped jig with a perfectly circular cross section. For example, the "bending radius" is not limited to 2 mm, as long as the effective bending loss can be suppressed to 0.20 dB / turn or less even when the bending radius is greater than 2 mm and less than or equal to 2.5 mm. The mandrel with a radius of approximately 2 mm refers to measurement conditions under which the effective bending loss can be suppressed to 0.20 dB / turn or less, regardless of whether the bending radius is 2 mm or less or equal to 2 mm or less.

[0058] <Fiber Cutoff Wavelength a-Effective Cutoff Wavelength b> The fiber cutoff wavelength measured above was designated as a, and the effective cutoff wavelength was designated as b, and the value of a-b was calculated.

[0059] <Coefficient A> A represents the rate of change [μm] in cutoff wavelength when the logarithm of the fiber length is changed. The fiber length is X [m], the cutoff wavelength of the polarization-maintaining fiber is Y [μm], the natural logarithm of X is LN(X), and B is the estimated value [μm] of the cutoff wavelength of the polarization-maintaining fiber at a fiber length of 1 m. When the fiber length X is 1 m or longer, the cutoff wavelength Y is the cutoff wavelength when the fiber is wound once around a mandrel with a diameter of 280 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel. When the fiber length X is less than 1 m, the cutoff wavelength Y is the cutoff wavelength when the fiber is wound 0.5 times around a mandrel with a diameter of 280 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel. For each of the polarization-maintaining fibers of Comparative Example 1, Comparative Example 2, and Examples 1 to 5, the cutoff wavelength Y was measured at each fiber length X, including at least three points where the fiber length X was 1 m, more than 1 m but not more than 3 m, and less than 1 m, and the coefficients A and B were found by the least squares method based on LN(X) and Y, yielding the following equation: Y = A × LN(X) + B Note that although the number of turns differs between fiber lengths of 1 m or more and fiber lengths of less than 1 m, the bending loss at a diameter of 280 mm is very small, so the measurement results for each case are generally linear, and the effect of changes in the number of turns on the above equation is negligibly small.

[0060] The results of the measurements are shown in Table 1.

[0061] As shown in Table 1, the effective cutoff wavelength of Comparative Example 1 is smaller than the effective cutoff wavelength of 1.32 μm of Example 1, which has the shortest effective cutoff wavelength among the Examples. Therefore, the polarization-maintaining fiber of Comparative Example 1 had a larger effective bending loss than the polarization-maintaining fibers of each Example. In other words, it was found that an effective cutoff wavelength of 1.32 μm or more can suppress the effective bending loss compared to polarization-maintaining fibers with effective cutoff wavelengths smaller than 1.32 μm.

[0062] The refractive index volume of Comparative Example 2 is −36% μm, which is the refractive index volume of Example 3, which has the smallest refractive index volume among the Examples. ２is smaller than . That is, the refractive index volume of Comparative Example 2 has a large absolute value, and the light confinement force of the core is large. Therefore, the extent of change in the cutoff wavelength when the length of the optical fiber is shortened can be larger than the extent of change in the cutoff wavelength when the optical fiber is bent at a small radius. This is because the polarization-maintaining fiber of Comparative Example 2 has a coefficient A of −0.045, which is smaller than −0.035 of Example 3, which is the smallest coefficient A value among the examples. On the other hand, Example 3 has a coefficient A of −0.035, which prevents the effective cutoff wavelength from becoming longer. Therefore, in optical devices such as optical devices for telecommunications or datacoms, such as CPO modules and pluggable transceivers, the polarization-maintaining fiber of Comparative Example 2 may be difficult to use, whereas the polarization-maintaining fiber of Example 3 may be fully usable.

[0063] Next, Comparative Example 1 and Example 1 are compared. Comparative Example 1 and Example 1 have approximately the same mode field diameter. However, the effective cutoff wavelength of Comparative Example 1 is 1.23 μm, while the effective cutoff wavelength of Example 1 is 1.32 μm. Therefore, the effective bending loss of Comparative Example 1 is 0.35 dB / turn, while the effective bending loss of Example 1 is 0.20 dB / turn. Therefore, in optical devices for telecommunications or datacom optical devices such as CPO modules and pluggable transceivers, the polarization-maintaining fiber of Comparative Example 1 may be difficult to use, whereas the polarization-maintaining fiber of Example 1 may be fully usable.

[0064] In addition, the core relative refractive index difference Δ １１ In Example 5, where the relative refractive index difference Δ １１ is preferably 0.39 or more.

[0065] In Example 3, the refractive index volume of the inner layer was −36% μm ２The difference a-b between the fiber cutoff wavelength and the effective cutoff wavelength is 0.16, and the effective bending loss of Example 3 is suppressed to 0.06 dB / turn. The shorter the effective cutoff wavelength is relative to the fiber cutoff wavelength, the longer the fiber cutoff wavelength can be, so the effective bending loss can be suppressed. In addition, the refractive index volume of the inner layer is -36% μm ２ If so, the refractive index volume of the inner layer is -36% μm ２ Compared with the case where the effective cutoff wavelength is smaller, the effective cutoff wavelength can be prevented from becoming longer, and the effective bending loss can be reduced.

[0066] Some of the aspects of the present invention described above are listed below.

[0067] Aspect 1 of the present invention is a fiber optic optical fiber comprising a core 11, an inner layer 12 surrounding the core 11 without any gaps, a pair of stress-applying portions 13 arranged at positions sandwiching the core 11, and a cladding 14 containing the inner layer 12 and the pair of stress-applying portions 13, wherein when light having a wavelength of 1.55 μm propagates through the core 11, the mode field diameter of the light is 9.3 μm or less, the effective cutoff wavelength is 1.32 μm or more, and the refractive index volume of the inner layer 12 is −36% μm ２ More than 0%μm ２ The polarization-maintaining fiber is characterized by the following:

[0068] In the polarization-maintaining fiber 1, the mode field diameter is 9.3 μm or less, and therefore the light confinement power of the core 11 is large. Therefore, the polarization-maintaining fiber 1 can suppress bending loss more effectively than when the mode field diameter is larger than 9.3 μm.

[0069] Furthermore, the longer the cutoff wavelength, the greater the light confinement power of the core 11. Therefore, by having an effective cutoff wavelength of 1.32 μm or more as described above, even when the polarization-maintaining fiber is used while being bent with a small bending diameter, bending loss can be more easily suppressed compared to a polarization-maintaining fiber having an effective cutoff wavelength of less than 1.32 μm.

[0070] In addition, the refractive index volume is -36% μm ２In the above cases, the influence of the change in bending radius of the polarization-maintaining fiber 1 on the cutoff wavelength tends to be larger than the influence of the change in fiber length on the cutoff wavelength. Therefore, when the polarization-maintaining fiber is bent slightly, the refractive index volume becomes -36% μm ２ Compared to when the refractive index volume is smaller, the change in cutoff wavelength relative to the change in fiber length can be suppressed. Therefore, it is possible to realize a polarization-maintaining fiber 1 that can be fully put to practical use in optical devices such as optical devices for telecom and datacom that require particularly small bending (for example, transceivers and CPO modules in compact optical devices aimed at high-density packaging). ２ By being less than or equal to 0%, the refractive index volume is 0% μm ２ Compared with the case where the diameter is larger, the light confinement force can be increased, and bending loss can be suppressed even if the polarization-maintaining fiber is bent slightly.

[0071] Furthermore, according to aspect 1, the effective bending loss can be suppressed to 0.20 dB / turn or less. Therefore, it is possible to realize a polarization-maintaining fiber 1 that can be fully practically used in optical devices such as optical devices for telecom and datacom applications (e.g., transceivers and CPO modules in compact optical devices designed for high-density packaging), which require particularly short fiber lengths. Even in the case where a small bending radius of about 2 mm is required as a guideline for the target values ​​of the optical devices described above, it is possible to realize a polarization-maintaining fiber 1 that can meet the conditions for application to the optical devices, for example, when the polarization-maintaining fiber 1 is housed in the optical devices.

[0072] A second aspect of the present invention is the polarization-maintaining fiber of the first aspect, characterized in that when the fiber is wound around a mandrel having a radius of 2 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, the loss per turn of light having a wavelength of 1.55 μm propagating through the core 11 is 0.20 dB or less.

[0073] In this case, the effective bending loss can be more reliably suppressed to 0.20 dB / turn or less.

[0074] Aspect 3 of the present invention is the polarization-maintaining fiber of Aspect 1 or 2, characterized in that when the fiber length is X [m], and when the fiber length is 1 m or more, the fiber is wound once around a mandrel having a diameter of 280 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, and when the fiber length is less than 1 m, the fiber is wound 0.5 times around a mandrel having a diameter of 280 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is Y [μm], and when the natural logarithm of X is LN(X), and the coefficient A [μm] is calculated using the least squares method based on LN(X) and Y, the coefficient A [μm] given by the following equation is -0.035 or more and 0 or less: Y=A×LN(X)+B (B: estimated value [μm] of the cut-off wavelength at a fiber length of 1 m).

[0075] By setting the coefficient A within the above range, when the polarization-maintaining fiber 1 is shortened, the rate at which the cutoff wavelength increases with the logarithmic change in the fiber length can be more reliably suppressed.

[0076] A fourth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to third aspects, characterized in that the effective cutoff wavelength is 1.55 μm or less.

[0077] Because the effective cutoff wavelength is 1.55 μm or less, even when the fiber is 0.25 μm long and wound once around a mandrel with a radius of 2 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, light with a wavelength of 1.55 μm can be propagated in single mode.

[0078] A fifth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to fourth aspects, characterized in that the difference ab between the fiber cutoff wavelength and the effective cutoff wavelength is 0.16 μm or more.

[0079] When the difference a-b is 0.16 μm or more, even for polarization-maintaining fibers with fiber cutoff wavelengths longer than 1.55 μm, the effective cutoff wavelength is easily set to 1.55 μm or less. Therefore, even for polarization-maintaining fibers with fiber cutoff wavelengths longer than 1.55 μm, when the fiber length is 0.25 μm and the fiber is wound once around a mandrel with a radius of 2 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, light with a wavelength of 1.55 μm is easily propagated in single mode. Furthermore, because the fiber cutoff wavelength can be set to a wavelength longer than 1.55 μm, bending loss can be further suppressed.

[0080] In addition, in the sixth aspect of the present invention, the relative refractive index difference Δ １２ 6. The polarization-maintaining fiber according to any one of Aspects 1 to 5, wherein the .lambda. is −0.05% or more and 0% or less.

[0081] Relative refractive index difference Δ １２ is -0.05% or more, the coefficient A can be easily set to -0.035 or more. Therefore, when the polarization-maintaining fiber 1 is shortened, the rate at which the cutoff wavelength increases with the logarithm of the fiber length can be more reliably suppressed. １２ is −0.05% or more, the difference a−b can be easily set to 0.16 μm or more. Therefore, even if the polarization-maintaining fiber has a fiber cutoff wavelength longer than 1.55 μm, when the fiber is 0.25 μm long and wound once around a mandrel with a radius of 2 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, it is easy to propagate light with a wavelength of 1.55 μm in single mode.

[0082] In addition, in the seventh aspect of the present invention, the relative refractive index difference Δ １１ 7. The polarization-maintaining fiber according to any one of Aspects 1 to 6, wherein the .lambda.

[0083] Relative refractive index difference Δ １１ is 0.39% or more, the relative refractive index difference Δ １１ The optical confinement power of the core 11 tends to be greater than when the value is less than 0.39, and bending loss can be more reliably suppressed.

[0084] An eighth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to seventh aspects, characterized in that the fiber cutoff wavelength is 1.55 μm or more.

[0085] When the fiber cutoff wavelength is 1.55 μm or more, the effective cutoff wavelength can be more easily set to 1.32 μm or more as in aspect 1, compared to when the fiber cutoff wavelength is less than 1.55 μm.

[0086] In addition, in a ninth aspect of the present invention, the refractive index volume of the inner layer 12 is -20% μm ２ More than 0%μm ２ 9. The polarization-maintaining fiber of any one of Aspects 1 to 8, wherein:

[0087] In addition, in the tenth aspect of the present invention, the relative refractive index difference Δ １２ 10. The polarization-maintaining fiber according to any one of Aspects 1 to 9, wherein the .lambda..times ...

[0088] When the refractive index volume of the inner layer 12 is within the range of aspect 9 or aspect 10, the coefficient A of aspect 3 can be set to be greater than or equal to −0.01 and less than −0.1, and when the polarization-maintaining fiber 1 is shortened, the rate at which the cutoff wavelength increases with the logarithmic change in the fiber length can be further reduced.

[0089] An eleventh aspect of the present invention is the polarization-maintaining fiber according to any one of the first to tenth aspects, characterized in that the effective cutoff wavelength is 1.46 μm or less.

[0090] In this case, even if the polarization-maintaining fiber is bent at a radius of 2 mm for use, and the bending is relaxed so that the radius becomes greater than 2 mm, up to a maximum of approximately 3 mm, the cutoff wavelength at that time will only increase to approximately 1.54 μm, and light with a wavelength of 1.55 μm can be propagated in single mode.

[0091] A twelfth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to eleventh aspects, characterized in that the fiber cutoff wavelength is 1.67 μm or less.

[0092] In this case, even if the difference a−b is 0.16 μm or more, when the fiber has a length of 0.25 μm and is wound once around a mandrel with a radius of 2 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, light with a wavelength of 1.55 μm can easily be propagated in single mode.

[0093] A thirteenth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twelfth aspects, characterized in that the effective cutoff wavelength is 1.43 μm or more.

[0094] In this case, the loss of light with a wavelength of 1.55 μm propagating through core 11 per turn when wound around a mandrel with a radius of 2 mm can be 0.09 dB or less. Therefore, even when wound around a mandrel with a radius of 2 mm, the loss of the light can be reduced to a value of two decimal places or less, an order of magnitude smaller.

[0095] A fourteenth aspect of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 13, characterized in that when wound around a mandrel with a radius of 2 mm, the loss of light having a wavelength of 1.55 μm propagating through the core 11 per turn is 0.09 dB or less.

[0096] In this case, the bending loss can be more reliably suppressed to 0.09 dB or less.

[0097] A fifteenth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to fourteenth aspects, characterized in that when light having a wavelength of 1.55 μm propagates through the core 11, the mode field diameter of the light is 8.2 μm or more.

[0098] In this case, optical connection loss can be reduced and the polarization-maintaining fiber 1 can be fully practically used in optical devices such as telecom and datacom optical devices, such as CPO modules and pluggable transceivers. Furthermore, since the mode field diameter is larger than when the mode field diameter is less than 8.2 μm, light propagating from other optical fibers or optical devices can be more easily incident on the polarization-maintaining fiber at the connection point with other optical fibers or optical devices. This reduces connection loss. Therefore, even if the strict connection loss requirement is required as one of the target values ​​for the optical device described above, it is possible to realize a polarization-maintaining fiber 1 that can more reliably meet the requirements for application to the optical device or other optical fiber when the polarization-maintaining fiber 1 is housed in the optical device or spliced ​​with the optical fiber.

[0099] A sixteenth aspect of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 15, characterized in that the following (A) is satisfied: (A) When light having a wavelength of 1.55 μm propagates through the core 11, the mode field diameter of the light is 9.1 μm or less, the fiber has a length of 0.25 m, and when wound once around a mandrel having a radius of 2 mm, the cutoff wavelength is 1.32 μm or more, and the product of the area of ​​the inner layer 12 in a cross section perpendicular to the longitudinal direction and the average relative refractive index difference of the entire inner layer 12 with respect to the cladding 14 is -36% μm. ２ More than 0%μm ２ The following is the result.

[0100] Here, the variable that affects the coefficient A [μm] is the refractive index volume, and in the range of the above-mentioned aspect 1, the value of the refractive index volume is -36% μm ２ Even if the coefficient A is reduced to -0.035, the coefficient A can still be -0.035.

[0101] A seventeenth aspect of the present invention is the polarization-maintaining fiber of any one of the first to fifteenth aspects, characterized in that the following (B) is satisfied: (B) When light having a wavelength of 1.55 μm propagates through the core 11, the mode field diameter of the light is 9.3 μm or less, the fiber has a length of 0.25 m, and when wound once around a mandrel having a radius of 2 mm, the cutoff wavelength is 1.43 μm or more, and the product of the area of ​​the inner layer 12 in a cross section perpendicular to the longitudinal direction and the average relative refractive index difference of the entire inner layer 12 with respect to the cladding 14 is -36% μm. ２ -35% μm or more ２ The following is the result.

[0102] An eighteenth aspect of the present invention is the polarization-maintaining fiber of any one of the first to seventeenth aspects, characterized in that the relative refractive index difference Δ11 of the core 11 with respect to the cladding 14 is 0.44% or less.

[0103] In addition, in aspect 19 of the present invention, the refractive index volume of the inner layer 12 is -36% μm ２ -20%μm or more ２ 19. The polarization-maintaining fiber of any one of Aspects 1 to 18, wherein:

[0104] Aspect 20 of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 19, characterized in that when wound around a mandrel with a radius of 2 mm, the loss of light with a wavelength of 1.55 μm propagating through core 11 per turn is 0.04 dB or more.

[0105] A twenty-first aspect of the present invention is the polarization-maintaining fiber of any one of the first to twentieth aspects, characterized in that the difference a−b between the fiber cutoff wavelength and the effective cutoff wavelength is 0.35 μm or less.

[0106] Aspect 22 of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 21, characterized in that, when the fiber length is X [m], and when the fiber length is 1 m or more, the stress-applying portion is wound once around a mandrel having a diameter of 280 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, and when the fiber length is less than 1 m, the stress-applying portion is wound 0.5 times around a mandrel having a diameter of 280 mm so that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, the cutoff wavelength is Y [μm], and when the natural logarithm of X is LN(X), and the coefficient A [μm] obtained by the least squares method based on LN(X) and Y is given by Y=A×LN(X)+B (B: estimated value [μm] of the cutoff wavelength at a fiber length of 1 m). A coefficient A [μm] is −0.035 or more and −0.01 or less.

[0107] Aspect 23 of the present invention is a polarization-maintaining fiber according to any one of Aspects 1 to 22, characterized in that the center of the circle that forms the outer periphery of core 11 is included in the center of cladding 14, the center of cladding 14 is a circle with a radius of 0.6 μm, and the center of this circle is an inner region of the circle that coincides with the center of the circle that forms the outer periphery of cladding 14.

[0108] As described above, according to the present invention, a polarization-maintaining fiber can be provided that can suppress bending loss and prevent the cutoff wavelength from becoming longer when the optical fiber is shortened and bent slightly during use, and is expected to be used as a polarization-maintaining fiber for communications in fields such as optical communications, a polarization-maintaining fiber for amplifiers in fields such as amplifiers, and a polarization-maintaining fiber for sensors in fields such as measurement.

Claims

1. A core, an inner layer that surrounds the core without a gap, a pair of stress applying portions disposed at positions sandwiching the core, and a cladding that encloses the inner layer and the pair of stress applying portions, wherein when light having a wavelength of 1.55 μm propagates through the core, the mode field diameter of the light is 9.3 μm or less, the fiber length is 0.25 m, and when wound once around a mandrel having a radius of 2 mm such that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is 1.32 μm or more, and the product of the area in a cross-section perpendicular to the longitudinal direction of the inner layer and the average of the relative refractive index difference of the entire inner layer with respect to the cladding is -36%μm ２ or more and 0%μm ２ or less. The polarization maintaining fiber is characterized by this.

2. The polarization-maintaining fiber according to claim 1, wherein when the slow axis of the stress applying portion is wound around a mandrel with a radius of 2 mm so as to be perpendicular to the surface of the mandrel, the optical loss per winding is 0.20 dB or less.

3. Let the fiber length be X [m]. When the fiber length is 1 m or more, it is wound once around a mandrel with a diameter of 280 mm so that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel. When the fiber length is less than 1 m, it is wound 0.5 times around a mandrel with a diameter of 280 mm so that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel. Let the cut-off wavelength in this case be Y [μm], the natural logarithm of X be LN(X), and when obtained by the least squares method based on LN(X) and Y, Y = A × LN(X) + B (B: estimated value of the cut-off wavelength at a fiber length of 1 m [μm]), and the coefficient A [μm] represented by this is -0.035 or more and 0 or less. The polarization-maintaining fiber according to claim 1 is characterized by this.

4. The polarization-maintaining fiber according to claim 1 or 2, wherein when the fiber length is 0.25 m and it is wound once around a mandrel with a radius of 2 mm so that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is 1.55 μm or less.

5. The polarization-maintaining fiber according to claim 1 or 2, wherein the difference between the cut-off wavelength when the fiber length is 2 m and it is wound once around a mandrel with a diameter of 280 mm so that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, and the cut-off wavelength when the fiber length is 0.25 m and it is wound once around a mandrel with a radius of 2 mm so that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, is 0.16 μm or more.

6. The polarization-maintaining fiber according to claim 1 or 2, wherein the relative refractive index difference of the inner layer with respect to the cladding is -0.05% or more and 0% or less.

7. The polarization-maintaining fiber according to claim 1 or 2, wherein the relative refractive index difference of the core with respect to the cladding is 0.39% or more.

8. The polarization-maintaining fiber according to claim 1 or 2, wherein when the fiber length is 2 m and it is wound once around a mandrel with a diameter of 280 mm so that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is 1.55 μm or more.

9. The product of the area in the cross-section perpendicular to the longitudinal direction of the inner layer and the average of the specific refractive index differences of the entire inner layer with respect to the cladding is -20% µm ２ or more and 0% µm ２ or less. The polarization-maintaining fiber according to claim 1 or 2, characterized in that.

10. The polarization maintaining fiber according to claim 1 or 2, wherein the difference in relative refractive index between the inner layer and the clad is -0.03% or more and 0% or less.

11. The polarization maintaining fiber according to claim 1 or 2, wherein when the fiber length is 0.25 m and it is wound once around a mandrel with a radius of 2 mm such that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is 1.46 μm or less.

12. The polarization maintaining fiber according to claim 1 or 2, wherein when the fiber length is 2 m and it is wound once around a mandrel with a diameter of 280 mm such that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is 1.67 μm or less.

13. The polarization maintaining fiber according to claim 1 or 2, wherein when the fiber length is 0.25 m and it is wound once around a mandrel with a radius of 2 mm such that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is 1.43 μm or more.

14. The polarization maintaining fiber according to claim 1 or 2, wherein when it is wound around a mandrel with a radius of 2 mm such that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, the optical loss per turn is 0.09 dB or less.

15. The polarization maintaining fiber according to claim 1 or 2, wherein when the light with a wavelength of 1.55 μm propagates through the core, the mode field diameter of the light is 8.2 μm or more.

16. The polarization-maintaining fiber according to claim 1 or 2, characterized by satisfying the following (A) or (B). (A) When the light with a wavelength of 1.55 μm propagates through the core, the mode field diameter of the light is 9.1 μm or less, the fiber length is 0.25 m, and when wound once around a mandrel with a radius of 2 mm such that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is 1.32 μm or more, and the product of the area of the cross-section perpendicular to the longitudinal direction of the inner layer and the average of the relative refractive index difference of the entire inner layer with respect to the cladding is -36%μm ２ or more and 0%μm ２ or less. (B) When the light with a wavelength of 1.55 μm propagates through the core, the mode field diameter of the light is 9.3 μm or less, the fiber length is 0.25 m, and when wound once around a mandrel with a radius of 2 mm such that the slow axis of the stress-applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is 1.43 μm or more, and the product of the area of the cross-section perpendicular to the longitudinal direction of the inner layer and the average of the relative refractive index difference of the entire inner layer with respect to the cladding is -36%μm ２ or more and -35%μm ２ or less.

17. The polarization maintaining fiber according to claim 3, wherein the difference in relative refractive index between the core and the clad is 0.39% or more.

18. The polarization maintaining fiber according to claim 3 or 17, wherein the difference in relative refractive index between the inner layer and the clad is -0.05% or more and 0% or less.

19. The polarization maintaining fiber according to claim 3 or 17, wherein when the fiber length is 2 m and it is wound once around a mandrel with a diameter of 280 mm such that the slow axis of the stress applying portion is perpendicular to the surface of the mandrel, the cut-off wavelength is 1.55 μm or more.

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