Polarization-maintaining fiber

Abstract

A polarization-maintaining fiber (1) is characterized by: being provided with a core (11), a pair of stress impartment parts (12a), (12b) which are disposed at positions holding the core (11) therebetween, and a cladding (13) which surrounds the core (11) and the pair of stress impartment parts (12a), (12b); and having a torsion section (1A) where the stress impartment parts (12a), (12b) rotationally extends in a helical shape permanently around the core (11). In the torsion section (1A): helical torsion of the stress impartment parts (12a), (12b) per fiber length of 47.1 mm consists of not more than one round of turn; the mode field diameter at a wavelength of 1.55 μm is 10.6 μm or less; and the cutoff wavelength is 1.24 μm or more and less than 1.55 μm when the fiber length is 2 m and the bending radius is 140 mm.

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 lines between optical devices such as coherent optical transceivers and CPO (Co-packaged Optics) switch modules and external light sources, which are used in optical transceiver modules that require the polarization-maintaining characteristics of optical devices such as CPO switch modules and lines for coherent optical transceivers. 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 fibers are often used to connect optical devices to lines between external light sources.

[0003] In this situation, in recent years, studies have been conducted to address the increasing number of optical devices such as optical transceivers in response to increased communication capacity, and to reduce the size of optical devices by combining multiple optical devices and electronic components into a single package. In order to meet these needs, there is a demand for polarization-maintaining fibers that can suppress bending loss (macrobend loss) even when bent slightly.

[0004] An example of a polarization-maintaining fiber is described in Patent Document 1 below.

[0005] Japanese Patent Application Laid-Open No. 2000-221341

[0006] A polarization-maintaining fiber having a structure in which a core is sandwiched between a pair of stress-applying portions is known. Such a polarization-maintaining fiber is sometimes called a PANDA fiber. The stress-applying portions are made of glass doped with boron oxide, for example, and have a lower refractive index than the cladding. Such a polarization-maintaining fiber may suffer from a deterioration in macrobend loss characteristics when a user applies the polarization-maintaining fiber to an optical device. Therefore, there is a demand for a polarization-maintaining fiber that suppresses a deterioration in macrobend loss characteristics even when a user applies the polarization-maintaining fiber to an optical device.

[0007] Therefore, an object of the present invention is to provide a polarization-maintaining fiber that suppresses deterioration in macrobend loss characteristics even when a user applies the polarization-maintaining fiber to an optical device.

[0008] In order to solve the above-mentioned problems, Aspect 1 of the present invention is a polarization-maintaining fiber comprising: a core; a pair of stress-applying portions arranged at positions sandwiching the core; and a cladding that contains the core and the pair of stress-applying portions; the stress-applying portions have a twist section in which they permanently rotate helically around the core; in the twist section, the helical twist of the stress-applying portions per 47.1 mm of fiber length is one turn or less; the mode field diameter at a wavelength of 1.55 μm is 10.6 μm or less; and the cutoff wavelength when the fiber length is 2 m and the bending radius is 140 mm is 1.24 μm or more and less than 1.55 μm.

[0009] By having a mode field diameter of 10.6 μm or less at a wavelength of 1.55 μm like this polarization-maintaining fiber, the light confinement force can be increased due to the smaller mode field diameter compared to when the mode field diameter at that wavelength is greater than 10.6 μm. Furthermore, by having a cutoff wavelength of 1.24 μm or more when the fiber length is 2 m and the bending radius is 140 mm, the light confinement force can be increased compared to when the cutoff wavelength is less than 1.24 μm. This can suppress the loss of light propagating through the core.

[0010] Furthermore, since this polarization-maintaining fiber has the above-described mode field diameter and cutoff wavelength configuration, it can increase the light confinement power as described above. Therefore, compared to a polarization-maintaining fiber not having this configuration, it can suppress light leakage even in the twisted section with the above-described number of twist rotations, and for example, even when a user applies the polarization-maintaining fiber to an optical device, it can suppress deterioration in macrobend loss characteristics.

[0011] As described above, according to the present invention, it is possible to provide a polarization-maintaining fiber that suppresses deterioration in macrobend loss characteristics even when a user applies the polarization-maintaining fiber to an optical device, for example.

[0012] 1A and 1B are diagrams illustrating a polarization-maintaining fiber according to an embodiment of the present invention, a twisted section of the polarization-maintaining fiber, a distribution of the relative refractive index difference along the fast axis direction, and a distribution of the relative refractive index difference along the slow axis direction.

[0013] 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 without departing from the spirit of the invention. Note that, for ease of understanding, the scales of the drawings may differ from those used in the following description.

[0014] Fig. 1 is a diagram showing a polarization-maintaining fiber according to this embodiment. In Fig. 1, (a) is a cross-sectional view showing the transverse cross section of the polarization-maintaining fiber according to this embodiment, and (b) is a diagram showing the refractive index profile along line A-A of the cross section shown in (a) of Fig. 1. The transverse cross section is a cross section perpendicular to the central axis C of the polarization-maintaining fiber 1.

[0015] 1A, the polarization-maintaining fiber 1 includes a core 11, a pair of stress-applying portions 12a and 12b arranged on either side of the core 11, and a cladding 13 that encloses the core 11 and the pair of stress-applying portions 12a and 12b. The polarization-maintaining fiber 1 may also include a coating that covers the cladding 13. 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.

[0016] 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 13. 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 axes of the core and the polarization-maintaining fiber will be referred to as the central axis C. The refractive index n11 of the core is higher than the refractive index n13 of the cladding 13. The core 11 is made of, for example, silica glass doped with an up-dopant such as germanium (Ge). The number of cores 11 may be one, or two or more.

[0017] The relative refractive index difference Δ of the core 11 with respect to the cladding 13 １１ is preferably 0.32% or more. １１ is 0.32% or more, the relative refractive index difference Δ １１ In comparison with the case where the relative refractive index difference Δ is less than 0.32%, 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 0.33% or more. １１ is 0.33% or more, the relative refractive index difference Δ １１ In comparison with the case where the relative refractive index difference Δ is less than 0.33%, the light confinement force in the core 11 can be increased, and leakage of light from the core 11 can be suppressed. １１ is preferably 0.40% or less. This can prevent the mode field diameter of light propagating through the core 11 from becoming too small, and can suppress light loss at the connection point with other optical fibers or optical devices. Therefore, the relative refractive index difference Δ １１is preferably 0.32% or more and 0.40% or less, in order to achieve a balance between the light confinement power and the light loss at the connection point, and more preferably 0.33% or more and 0.40% or less. Examples of the above-mentioned "optical device" include, for example, (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. In the following description, the term "optical device" is used to include the optical devices in (1) to (3) above.

[0018] 1, in the polarization-maintaining fiber 1 of this embodiment, the refractive index profile of the core 11 relative to the cladding 13 is step-like. However, this refractive index profile is not limited to step-like and may be, for example, a GI-type profile. Furthermore, the refractive index profile of the core 11 relative to the cladding 13 may have other shapes.

[0019] The cross-sectional shape of the core 11 in this embodiment 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 4.4 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 4.4 μm or less. The radius of the core 11 is, for example, half the average diameter of the core 11 in the direction perpendicular to the central axis C. The radius of the core 11 is not particularly limited as long as the cutoff wavelength satisfies any one of the preferable conditions for the cutoff wavelength, which will be described later, <Condition 2>, (3), or (4), which will be described later.

[0020] The pair of stress-applying portions 12a, 12b have a columnar shape extending along the direction of the central axis C, but may have a shape other than a columnar shape. As will be described later, the pair of stress-applying portions 12a, 12b may have at least a part of a twisted section along the longitudinal direction that permanently rotates in a spiral around the core 11. Furthermore, the pair of stress-applying portions 12a, 12b may have a twisted section along the longitudinal direction that permanently rotates in a spiral around the core 11 over the entire longitudinal area of ​​the polarization-maintaining fiber 1. In this embodiment, the refractive index n12 of the stress-applying portions 12a, 12b is lower than the refractive index n13 of the cladding. The stress-applying portions 12a, 12b are made of, for example, boron oxide (B ２ O ３ The stress-applying portions 12a, 12b are made of silica glass doped with a down dopant such as SiO 2 . The cross-sectional shape of the stress-applying portions 12a, 12b may be an isosceles trapezoid with the 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 portions 12a, 12b may be arc-shaped, bulging in a direction away from the core 11. A polarization-maintaining fiber 1 having stress-applying portions 12a, 12b with an isosceles trapezoidal shape is sometimes called a "bowtie 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 bowtie polarization-maintaining fiber.

[0021] The relative refractive index difference Δ of the stress-applying portions 12a and 12b with respect to the cladding １２ is preferably −0.78% or less. With such a relative refractive index difference, the stress-applying parts 12 a and 12 b can function as low refractive index layers in the S-axis direction along which the stress-applying parts are arranged, and leakage of light from the core 11 can be further suppressed. In addition, the relative refractive index difference Δ １２ is preferably −1.0% or more. １２ When the relative refractive index difference Δ is −1.0% or more, the amount of boron oxide added can be sufficiently small, and therefore deliquescence of the stress-applying portions 12 a and 12 b can be prevented. １２ is more preferably −1.0% or more and −0.78% or less.

[0022] In this embodiment, the cross-sectional shape of the stress-applying portions 12a and 12b is generally circular as shown by the solid line in FIG. 1 , or generally elliptical as shown by the dashed line, with the arrangement direction of the stress-applying portions 12a and 12b as the minor axis direction. However, the cross-sectional shape of the stress-applying portions 12a and 12b is not limited to circular or elliptical, and may be, for example, crescent or noncircular. Note that the cross-sectional shape of the stress-applying portions 12a and 12b 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 13 described below is 125 μm±1 μm, the diameter t of the stress-applying portions 12a and 12b along the S-axis direction is preferably 24.8 μm or more, 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 preferably realized. Furthermore, the diameter t is preferably 40.0 μm or less, more preferably 38.0 μm or less, and even more preferably 34.4 μm or less. This allows the stress-applying portions 12 a, 12 b to be suitably disposed inside the cladding 13 having a diameter of 125 μm±1 μm. Furthermore, when the diameter of the cladding 13 is 125 μm±1 μm, the diameter t is preferably 24.8 μm or more and 40.0 μm or less, preferably 24.8 μm or more and 38.0 μm or less, preferably 24.8 μm or more and 34.4 μm or less, or preferably 30.0 μm or more and 40.0 μm or less, preferably 30.0 μm or more and 38.0 μm or less, and preferably 30.0 μm or more and 34.4 μm or less.

[0023] Furthermore, when the diameter of the cladding 13 described below is 80 μm±1 μm, the diameter t of the stress-applying portions 12 a, 12 b along the S-axis direction is preferably 22.0 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 diameter t is preferably 27.0 μm or less. This allows the stress-applying portions 12 a, 12 b to be suitably disposed inside the cladding 13, which has a diameter of 80 μm±1 μm. Furthermore, when the diameter of the cladding 13 is 80 μm±1 μm, the diameter t is preferably 22.0 μm or more and 27.0 μm or less.

[0024] The stress-applying parts 12a and 12b 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 12a and 12b when manufacturing the polarization-maintaining fiber 1 by melt-drawing. Furthermore, if the core 11 is in contact with the stress-applying parts 12a and 12b, and boron oxide is added to the stress-applying parts 12a and 12b, the boron oxide may cause a deterioration in transmission loss. However, since the core 11 is spaced apart from the stress-applying parts 12a and 12b, this deterioration in transmission loss can be suppressed.

[0025] The interval between the stress-applying portions 12a and 12b is 2a, and the distance from the central axis C of the core 11 to the stress-applying portions 12a and 12b is a. Here, the distance a refers to the distance from the center of the core 11 to the point included in the stress-applying portion 12a that is closest to the core 11, or the distance from the center of the core 11 to the point included in the stress-applying portion 12b that is closest to the core 11. The distance a is preferably 10 μm or less. When the distance a is 10 μm or less, the stress-applying portions 12a and 12b, which have a lower refractive index than the cladding 13, are closer to the core 11 than when the distance a is greater than 10 μm. This can further enhance the function of the stress-applying portions 12a and 12b as low-refractive-index layers in the S-axis direction, and can further suppress light leakage from the core 11. Note that the ratio of the distance a to the radius of the mode field diameter MFD (a / (MFD / 2)) is preferably 1.05 or more. When the ratio of the distance a to the radius of the mode field diameter MFD (a / (MFD / 2)) is 1.05 or more, the stress-applying portions 12a, 12b are disposed outside the mode field, and therefore, when boron oxide is added to the stress-applying portions 12a, 12b, deterioration in transmission loss due to the boron oxide can be suppressed. Note that the distance between the core 11 and the stress-applying portions 12a, 12b is preferably 1 μm or more. Note that the size of the diameter t is not particularly limited as long as the distance a satisfies any one of the conditions (1), (2), (3), or (4) described below.

[0026] The cladding 13 has a columnar shape extending along the direction of the central axis C. As described above, the refractive index n13 of the cladding 13 is lower than the refractive index n11 of the core 11 and higher than the refractive index n12 of the stress-applying portions 12 a and 12 b. The cladding 13 is made of, for example, silica glass.

[0027] The cross-sectional shape of the cladding 13 in this embodiment is generally circular. However, the cross-sectional shape of the cladding 13 is not limited to this and may be, for example, elliptical, crescent, or non-circular. Note that the cross-sectional shape of the cladding 13 here refers to the shape of a cross section perpendicular to the central axis C of the polarization-maintaining fiber 1.

[0028] The diameter b of the cladding 13 is preferably approximately 80 μm±1 μm, i.e., 79 μm or more and 81 μm or less, 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 a 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 b of the cladding 13 is preferably 125 μm±1 μm, i.e., 124 μm or more and 126 μm or less, 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 13 occurs when the diameter of the cladding 13 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 13 may be 79 μm or greater and 126 μm or less. When the cross section of the cladding 13 is non-circular, the diameter b of the cladding 13 is, for example, the average value of the diameters of the cladding 13 in a direction perpendicular to the central axis C.

[0029] 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 13, but this is not limitative, and the center of the circle that forms the outer periphery of the core 11 may be included in the center of the cladding 13. Therefore, when the diameter of the cladding 13 satisfies the range of 79 μm to 126 μm, the center of the circle that forms the outer periphery of the core 11 may be included in the center of the cladding 13. Here, the center of the cladding 13 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 13.

[0030] The fiber length of the polarization-maintaining fiber 1 is preferably less than 50 m, may be less than 10 m, or may be 300 mm or less. The fiber length of the polarization-maintaining fiber 1 is preferably 47.1 mm or more, and may be, for example, 2 m or more. When the polarization-maintaining fiber 1 is used for a sensor, commercially available polarization-maintaining fibers are often used with lengths of 50 m or more. This is because the longer the polarization-maintaining fiber, the better the measurement resolution of the sensor. If a polarization-maintaining fiber for a sensor is used for several meters, the measurement resolution may decrease, making it difficult to use as a sensor. However, polarization-maintaining fibers for communications and amplifiers do not require the same high measurement resolution as polarization-maintaining fibers for sensors. Furthermore, when connecting various optical devices to external light sources, as described above, the fiber length is preferably less than 50 m to prevent the polarization-maintaining fiber from interfering with the various optical devices. Furthermore, when connecting a polarization-maintaining fiber in a wound state, the fiber length may be not only less than 50 m, but may also be a single-digit value of more than 300 mm but less than 10 m, taking into account accommodation in various optical devices, and when connecting a polarization-maintaining fiber without winding it, the fiber length may be 300 mm or less, so it is preferable to determine the fiber length according to the connection state. Therefore, a fiber length of less than 50 m does not need to be long, 50 m or more, and the polarization-maintaining fiber may be less likely to interfere with various optical devices.From the above, taking into consideration the cases of 23.55 mm and 31.4 mm as fiber lengths described below, the fiber length of the polarization-maintaining fiber 1 is preferably 23.55 mm or more and less than 50 m, preferably 23.55 mm or more and less than 10 m, preferably 23.55 mm or more and less than 300 mm, preferably 31.4 mm or more and less than 50 m, preferably 31.4 mm or more and less than 10 m, preferably 31.4 mm or more and less than 300 mm, preferably 47.1 mm or more and less than 50 m, preferably 47.1 mm or more and less than 10 m, preferably 47.1 mm or more and less than 300 mm, preferably 2 m or more and less than 50 m, preferably 2 m or more and less than 10 m, and preferably more than 300 mm and less than 10 m.

[0031] The polarization-maintaining fiber 1 also has a twisted section along its longitudinal direction in which the stress-applying portions 12a and 12b permanently rotate helically around the core 11. Figure 2 is a diagram showing the twisted section 1A of the polarization-maintaining fiber 1. In the twisted section 1A, the stress-applying portions 12a and 12b are generally parallel and helically oriented around the central axis C of the core 11.

[0032] "Permanently rotating in a spiral" means that the stress-applying portions 12a and 12b rotate in a spiral around the core 11 even when no external force is applied to the polarization-maintaining fiber 1. Such twisted section 1A can be formed by rotating the preform that will become the polarization-maintaining fiber 1 when manufacturing the polarization-maintaining fiber 1 by drawing. That is, when the glass that will become the polarization-maintaining fiber 1 is melted and drawn, the unsolidified glass in the stress-applying portions 12a and 12b is twisted, and the twisted glass solidifies, thereby forming the twisted section 1A. In this way, the twisted section 1A is a section in which a permanent twist is applied to the stress-applying portions 12a and 12b.

[0033] In the twisted section 1A, the helical twist of the stress-applying portions 12a, 12b is within one turn per 47.1 mm of the fiber length of the polarization-maintaining fiber 1. Furthermore, in the twisted section 1A, it is more preferable that the helical twist of the stress-applying portions 12a, 12b is within one turn per 62.8 mm of the fiber length of the polarization-maintaining fiber 1. Furthermore, in the twisted section 1A, it is preferable that the helical twist of the stress-applying portions 12a, 12b is greater than 1 / 50 of a turn per meter of the fiber length of the polarization-maintaining fiber 1. In other words, it is preferable that the helical twist of the stress-applying portions 12a, 12b is greater than one turn per 50 m of the fiber length of the polarization-maintaining fiber 1. Furthermore, in the twisted section 1A, it is preferable that the twist is greater than 1 / 25 of a turn per meter of the fiber length. In FIG. 2, the scale of the polarization-maintaining fiber 1 in the longitudinal direction is different from the actual scale for ease of understanding.

[0034] In the twisted section 1A, when the helical twist of the stress-applying portions 12 a, 12 b per meter of fiber length is greater than 1 / 50 rotation, the stress-applying portions 12 a, 12 b rotate helically, which can prevent light propagating through the core from concentrating in only a specific bending direction, such as the F-axis direction, from leaking out, thereby suppressing an extreme increase in macrobend loss. In this way, it is possible to suppress the macrobend loss from becoming too dependent on the bending direction, which can eliminate the need to pay attention to the bending direction when using the polarization-maintaining fiber 1, and can make handling easier than when the fiber is twisted within 1 / 50 rotation per meter.

[0035] Furthermore, the polarization-maintaining fiber 1 preferably has another twisted section in which the stress-applying portions 12a, 12b rotate helically around the core 11 in the direction opposite to the twist direction of the stress-applying portions 12a, 12b in the twisted section 1A. In the twisted section 1A, a permanent twist is applied as described above, and a slight stress due to this twist remains. Therefore, when no external force is applied to the polarization-maintaining fiber 1, the polarization-maintaining fiber 1 may bend slightly in the twisted section 1A. However, according to the above configuration, since the other twisted sections are twisted in the opposite direction to the twisted section 1A, when no external force is applied to the polarization-maintaining fiber 1, the polarization-maintaining fiber 1 may bend slightly in the other twisted section in the direction opposite to the slight curvature of the twisted section 1A. Therefore, as described above, when the polarization-maintaining fiber 1 has the twisted section 1A and other twisted sections, the polarization-maintaining fiber 1 can approach a straight line as a whole when no external force is applied to the polarization-maintaining fiber 1, compared to when the polarization-maintaining fiber 1 has only the twisted section 1A. Therefore, it is preferable that the lengths of the twisted section and the other twisted sections are approximately the same, and it is even more preferable that they are the same. It is also preferable that the absolute value of the amount of helical twist of the stress-applying portions 12a, 12b per unit length is approximately the same, and it is even more preferable that they are the same. It is also preferable that the twisted section 1A and the other twisted sections are provided alternately. The twist periods of the twisted section 1A and the other twisted sections may be different. Furthermore, even if the polarization-maintaining fiber 1 includes the twisted section 1A and the other twisted section, if the twisted section 1A and the other twisted section are arranged at the same period, they can be considered to have equivalent configurations when their optical characteristics are measured. Therefore, the optical characteristics, such as macrobend loss, of the polarization-maintaining fiber 1 including the twisted section 1A and the polarization-maintaining fiber 1 including the other twisted section can be roughly equivalent to each other.

[0036] The amount of helical twist can be determined, for example, by immersing the polarization-maintaining fiber 1 in matching oil and observing the polarization-maintaining fiber 1 from the side, or by observing both end faces of the polarization-maintaining fiber 1. In the latter case, by observing the positions of the stress-applying portions 12 a and 12 b on both end faces of the polarization-maintaining fiber 1, the amount of helical twist can be determined by estimating the number of twist turns from the amount of relative positional deviation between the two stress-applying portions.

[0037] The polarization-maintaining fiber 1 having the above configuration satisfies the following conditions 1 and 2.

[0038] <Condition 1> The mode field diameter at a wavelength of 1.55 μm is 10.6 μm or less.

[0039] By satisfying this condition, the mode field diameter at a wavelength of 1.55 μm is smaller than when the mode field diameter is greater than 10.6 μm, and the light confinement effect can be improved accordingly.

[0040] <Condition 2> The cutoff wavelength is 1.24 μm or more and less than 1.55 μm when the fiber length is 2 m and the bending radius is 140 mm. Note that the cutoff wavelength here refers to the cutoff wavelength of light in the LP11 mode when the fiber length is 2 m and the bending radius is 140 mm.

[0041] A cutoff wavelength of 1.24 μm or more can improve the light confinement effect compared to a cutoff wavelength of less than 1.24 μm. Furthermore, if the above-mentioned mode field diameter satisfies condition 1 and the cutoff wavelength satisfies condition 2, the refractive index distribution can be determined naturally.

[0042] Furthermore, since the cutoff wavelength is less than 1.55 μm, single-mode optical transmission can be achieved when the wavelength used is 1.55 μm or more. In particular, a polarization-maintaining fiber applicable to coherent optical transceivers using wavelengths of about 1.55 μm and optical fibers and optical devices using wavelengths of 1.55 μm or more can be realized.

[0043] Furthermore, it is preferable that the polarization-maintaining fiber 1 satisfies at least one of the following conditions (1) and (2) by satisfying <Condition 1> and <Condition 2>. Note that macrobend loss is sometimes called bending loss. (1) The bending radius is 7.5 mm, and the macrobend loss at a wavelength of 1.55 μm is 0.67 dB / turn or less. (2) The bending radius is 10 mm, and the macrobend loss at a wavelength of 1.55 μm is 0.16 dB / turn or less.

[0044] Unless otherwise specified, the term "bend" used in the above bending radius and the like refers to a state in which the polarization-maintaining fiber 1 is bent without any twist other than a permanent twist. The same applies hereinafter. A fiber length of 47.1 mm is the fiber length when one turn is made with a bending radius of 7.5 mm, and a fiber length of 62.8 mm is the fiber length when one turn is made with a bending radius of 10 mm. Here, "one turn" refers to winding the polarization-maintaining fiber one turn around a mandrel with a diameter of 7.5 mm, or winding the polarization-maintaining fiber one turn around a mandrel with a diameter of 10 mm. Furthermore, "one turn" is synonymous with one turn.

[0045] By satisfying the above condition (1), the polarization-maintaining fiber 1 can be realized to suppress the macrobend loss value to or below the loss tolerance required for an optical device, such as a coherent optical transceiver, even when a user applies the polarization-maintaining fiber to the optical device under conditions where the required bend radius is approximately 7.5 mm or more, which is one of the target values ​​for the optical device. Here, the case where a user applies the polarization-maintaining fiber to the optical device, which will be described later, includes, for example, when the polarization-maintaining fiber 1 is housed in various optical devices. The above-mentioned loss tolerance value includes, for example, a value that is applicable to various optical devices. Furthermore, as long as the above conditions 1 and 2 are satisfied, the measurement may be performed under the condition of 0.5 turns in the above condition (1) of the above condition 3. In this case, the fiber length of the polarization-maintaining fiber 1 may be less than 47.1 mm (e.g., 23.55 mm). In this case, the macrobend loss value may be a value proportional to 0.5.

[0046] Furthermore, by making the polarization-maintaining fiber 1 satisfy the above condition (2), it is possible to realize a polarization-maintaining fiber that can suppress the macrobend loss value to below the loss tolerance required for the optical device, etc., even when a user applies the polarization-maintaining fiber to an optical device under conditions where a required bend radius is about 10 mm or more, which is one guideline for the target value of an optical device that requires an integrated form with a bend radius greater than about 7.5 mm. Furthermore, as long as the above conditions 1 and 2 are satisfied, measurement may be performed under the condition of 0.5 turns in the above condition (2) of the above condition 3, and in this case, the fiber length of the polarization-maintaining fiber 1 may be less than 62.8 mm (e.g., 31.4 mm), in which case the macrobend loss value may be a value proportional to 0.5.

[0047] Furthermore, as described above, by making the diameter t of the stress-applying portions 12a and 12b along the S-axis direction 24.8 μm or more, the light confinement force can be made greater than when the diameter t is smaller than 24.8 μm, and it can be made easier to satisfy at least one of the conditions (1) and (2) above.

[0048] Next, an example of the polarization-maintaining fiber 1 will be described.

[0049] The polarization-maintaining fibers of Examples 1 to 12 and Comparative Examples 1 to 3 were measured for the following items.

[0050] <Fiber Cutoff Wavelength> The cutoff wavelength when the polarization-maintaining fiber has a fiber length of 2 m and is wound around a diameter of 280 mm once is referred to as the fiber cutoff wavelength. The fiber cutoff wavelength was measured based on IEC 60793-1-44 as 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. The polarization-maintaining fiber was wound around the mandrel so that its slow axis was perpendicular to the surface of the mandrel. The mandrel is a rod-shaped jig with a perfectly circular cross section. Here, the polarization-maintaining fiber 1 was wound around the mandrel so that its slow axis was perpendicular to the surface of the mandrel. In the table below, R indicates the radius of the mandrel.

[0051] <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 based on IEC 60793-1-45. Light with a wavelength of 1.55 μm guided through the core 11 was used to measure the mode field diameter.

[0052] <Cladding Diameter> The cladding diameter was measured by the transmitted near-field method based on IEC 60793-1-20.

[0053] <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 3 is a diagram showing the distribution of the relative refractive index difference along the fast axis direction. １１was 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.

[0054] <Relative refractive index difference Δ １２ > Relative refractive index difference Δ １２ is the relative refractive index difference of the stress-applying portion with respect to the cladding. Figure 4 is a diagram showing the distribution of the relative refractive index difference along the slow axis direction. １２ is the relative refractive index difference of the core, Δ １１ Similarly, in the region where the refractive index distribution in the slow axis direction of the stress-applying portion is 50% or less of the minimum value (0.5Δ in FIG. 3 ), １２ From the area corresponding to min, Δ １２ The refractive index distribution of the stress-applying portions in the slow axis direction of the polarization-maintaining fiber was obtained by measurement using an interferometer, the relative refractive index difference distribution on the slow axis was calculated, the minimum value of the relative refractive index difference of the stress-applying portions with respect to the cladding was found, and then the average relative refractive index difference in the region where the minimum value of the refractive index distribution of the stress-applying portions in the slow axis direction was satisfied was calculated.

[0055] <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.

[0056] <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.

[0057] <Macrobend Loss> In this specification, the bending loss of light with a wavelength of 1.55 μm per turn when a polarization-maintaining fiber is wound around a mandrel with a radius of 7.5 mm or 10 mm is referred to as macrobend loss. Based on IEC 60793-1-47, when the polarization-maintaining fiber 1 is wound once so that the slow axis is perpendicular to the surface of a mandrel with a radius of 7.5 mm or 10 mm (when the bending radius is 7.5 mm or 10 mm), the power of the light in the LP01 mode from the light source at that time and the power of the light in the LP01 mode from the light source in a state before winding as a reference are measured, and the bending loss per turn is calculated from the difference between these two powers. The mandrel is a rod-shaped jig with a perfectly circular cross-sectional shape. In the table below, R indicates the radius of the mandrel.

[0058] In measuring the macrobend loss, the bending loss was measured when the polarization-maintaining fiber was wound around a mandrel with a bending radius of 10 mm without any twisting and with one twist per turn of 62.8 mm of fiber length per turn. Furthermore, the bending loss was measured for these polarization-maintaining fibers when the polarization-maintaining fiber was wound around a mandrel with a bending radius of 7.5 mm without any twisting and with one twist per turn of 47.1 mm of fiber length per turn. In other words, the bending loss of light per turn at a wavelength of 1.55 μm was measured when the bending radius was 7.5 mm and the twist was one turn per 47.1 mm of fiber length, and when the bending radius was 10 mm and the twist was one turn per 62.8 mm of fiber length. The polarization-maintaining fibers of Examples 1 to 12 and Comparative Examples 1 to 3 did not have twisted sections 1A. However, when the optical characteristics of the polarization-maintaining fibers were measured, the configuration in which the twist was applied as described above and the configuration in which the stress-applying portions 12a and 12b permanently rotate helically around the core 11 were considered to be equivalent to each other. Therefore, the optical characteristics of the former polarization-maintaining fiber can be roughly equivalent to those of the latter polarization-maintaining fiber 1. The measurement results are shown in Tables 1 to 3. Here, the refractive index profile of the core relative to the cladding in the polarization-maintaining fibers of Examples 1 to 12 and Comparative Examples 1 to 3 is step-like.

[0059]

[0060]

[0061]

[0062] As shown in Tables 1 to 3, Examples 1 to 12 satisfy Conditions 1 and 2. Specifically, by satisfying Condition 1, these polarization-maintaining fibers can have a smaller mode field diameter at a wavelength of 1.55 μm than when the mode field diameter is greater than 10.6 μm, thereby improving the light confinement effect. Furthermore, by satisfying Condition 2, these polarization-maintaining fibers can have a better light confinement effect than when the cutoff wavelength is less than 1.24 μm. This improved light confinement effect reduces bending loss. Furthermore, when these polarization-maintaining fibers satisfy Condition 3, the macrobend loss value of optical devices such as coherent optical transceivers can more reliably satisfy the allowable range. Here, the allowable range refers to, for example, a range in which the deterioration of macrobend loss characteristics can be suppressed to a degree that allows a user to apply the polarization-maintaining fiber to the optical device, even when the user applies the polarization-maintaining fiber to the optical device.

[0063] The polarization-maintaining fiber of Comparative Example 1 has a large mode field diameter and does not satisfy Condition 1. Furthermore, this polarization-maintaining fiber does not satisfy Condition 3, and compared to the polarization-maintaining fibers of Examples 8 and 12, which have the largest macrobend loss among Examples 1 to 12, the macrobend loss (1) of Condition 3 is about 3.7 times larger and the macrobend loss (2) of Condition 3 is about 3.0 times larger. For this reason, it may be difficult to use the polarization-maintaining fiber of Comparative Example 1 in optical devices that satisfy the above-mentioned tolerance range, such as coherent optical transceivers.

[0064] The polarization-maintaining fiber of Comparative Example 2 has a relative refractive index difference Δ １１ The relative refractive index difference Δ １１ Therefore, the relative refractive index difference Δ １１ is preferably 0.32% or more, as described above, and more preferably 0.33% or more. １１ As described above, it is preferable that the content of Si is 0.40% or less.

[0065] The polarization-maintaining fiber of Comparative Example 3 has a small cutoff wavelength and does not satisfy Condition 2. Furthermore, this polarization-maintaining fiber does not satisfy Condition 3, and compared to the polarization-maintaining fibers of Examples 8 and 12, which have the largest macrobend losses, the macrobend loss (1) of Condition 3 is about 1.9 times larger and the macrobend loss (2) of Condition 3 is about 1.6 times larger. For this reason, it may be difficult to use the polarization-maintaining fiber of Comparative Example 3 in optical devices such as coherent optical transceivers.

[0066] Furthermore, Tables 1 to 3 show that the mode field diameter at a wavelength of 1.55 μm is preferably 10.3 μm or less in polarization-maintaining fiber 1. Furthermore, Tables 1 to 3 show that the mode field diameter at a wavelength of 1.55 μm is more preferably 9.6 μm or less. In this case, as is clear from each example, macrobend loss can be further suppressed.

[0067] Furthermore, Tables 1 to 3 show that the mode field diameter at a wavelength of 1.55 μm in the polarization-maintaining fiber 1 is preferably 8.9 μm or greater. In this case, the larger mode field diameter compared to a mode field diameter less than 8.9 μm can facilitate light propagating from other optical fibers or optical devices being incident on the polarization-maintaining fiber at the splice point with other optical fibers or optical devices. This can reduce splice loss. Therefore, in this case, a balance can be maintained between the effect of satisfying the macrobend loss characteristics under the specified twist conditions in at least one of Condition 3 (1) and (2) above and the effect of suppressing splice loss with other optical fibers or optical devices. Considering the beam size and focusing lens of a laser light source used in an optical transceiver (such as a coherent optical transceiver) operating at a wavelength of 1.55 μm, a mode field diameter of 8.9 μm or greater is preferred to reduce splice loss to a level that allows operation in the optical transceiver. In addition, the mode field diameter of the optical waveguide to which the polarization-maintaining fiber is actually connected is often 8.9 μm or more and 9.6 μm or less. This is because the mode field diameter of the single-mode fiber connected to the optical waveguide is often 8.9 μm or more and 9.6 μm or less. Therefore, from the perspective of reducing the connection loss with the optical waveguide, the mode field diameter of the polarization-maintaining fiber 1 is preferably 8.9 μm or more and 9.6 μm or less. Furthermore, taking into account the beam size and focusing lens of the laser light source to which the polarization-maintaining fiber is actually connected, the mode field diameter is preferably 8.9 μm or more and 9.6 μm or less in order to reduce the connection loss with the laser light source to a level that allows operation when applied to an optical transceiver.

[0068] Furthermore, in the polarization-maintaining fiber 1, it is preferable that the mode field diameter is less than 10 μm. Having a mode field diameter of less than 10 μm, which is a single-digit value, can prevent the optical element (lens, spot size converter, etc.) for converting the mode field diameter from becoming too complex and large when the optical element is disposed between the optical fiber and the modulator. In particular, when an optical fiber with a cladding diameter of 80±1 μm is used, the entire optical device including the optical fiber, the optical element for converting the mode field diameter, and the modulator can be made smaller. Therefore, in order to balance the connection loss with the optical element for converting the mode field diameter and the miniaturization of the optical element for converting the mode field diameter, it is more preferable that the mode field diameter is 8.9 μm or more and less than 10 μm.

[0069] Furthermore, Tables 1 to 3 show that the cutoff wavelength of the polarization-maintaining fiber 1 is preferably 1.24 μm or more and 1.39 μm or less. When the fiber length is short, for example, 1 m or less, the cutoff wavelength may shift toward longer wavelengths. Therefore, to reduce the possibility that a shift in the cutoff wavelength toward longer wavelengths due to the fiber length will cause the cutoff wavelength to exceed the operating wavelength and make single-mode transmission at the operating wavelength difficult, it is preferable that there be a certain margin between the operating wavelength and the upper limit of the cutoff wavelength measured at a fiber length of 2 m. Therefore, when the cutoff wavelength is, for example, 1.24 μm or more and 1.39 μm or less as described above, even if the cutoff wavelength shifts toward longer wavelengths, such as when the fiber length is short, the cutoff wavelength can be prevented from exceeding the operating wavelength. Therefore, the possibility that single-mode transmission at the operating wavelength will become difficult can be reduced.

[0070] The cutoff wavelength of the polarization-maintaining fiber 1 may be greater than 1.39 μm and less than 1.55 μm. The cutoff wavelength of the polarization-maintaining fiber 1 can be set closer to the wavelength band in use, improving the light confinement effect compared to when the cutoff wavelength is 1.39 μm or less. Therefore, even when a user applies the polarization-maintaining fiber to an optical device, the macrobend loss value can be reduced compared to when the cutoff wavelength is 1.39 μm or less.

[0071] Furthermore, it is preferable that the cutoff wavelength of the polarization-maintaining fiber 1 is 1.36 μm or more and less than 1.55 μm. In this case, an appropriate balance can be maintained between a higher light confinement effect compared to when the cutoff wavelength is less than 1.36 μm and an effect that can reduce the possibility of single-mode transmission becoming difficult at the wavelength used compared to when the cutoff wavelength is greater than 1.39 μm and less than 1.55 μm.

[0072] Furthermore, Tables 1 to 3 show that the cutoff wavelength of the polarization-maintaining fiber 1 is preferably 1.36 μm or more and 1.39 μm or less. In this case, a good balance can be maintained between a high light confinement effect and an effect that can reduce the possibility of single-mode transmission becoming difficult at the wavelength used, compared to when the cutoff wavelength is less than 1.36 μm. Furthermore, compared to when the cutoff wavelength is greater than 1.39 μm, the possibility of single-mode transmission becoming difficult at the wavelength used can be reduced.

[0073] Furthermore, from Tables 1 to 3, in the polarization-maintaining fiber 1, the diameter of the cladding 13 is 124 μm or more and 126 μm or less, the mode field diameter at a wavelength of 1.55 μm is 10.2 μm or more and 10.6 μm or less, the cutoff wavelength is 1.33 μm or more and 1.39 μm or less, and the relative refractive index difference of the core 11 with respect to the cladding 13, Δ １１ It is preferable that the content of Si is 0.32% or more and 0.34% or less.

[0074] Furthermore, from Tables 1 to 3, in the polarization-maintaining fiber 1, the diameter of the cladding 13 is 79 μm or more and 81 μm or less, the mode field diameter at a wavelength of 1.55 μm is 8.9 μm or more and 9.6 μm or less, the cutoff wavelength is 1.24 μm or more and 1.39 μm or less, and the relative refractive index difference of the core 11 with respect to the cladding 13, Δ １１ It is preferable that the content of Si is 0.33% or more and 0.40% or less.

[0075] Furthermore, it is preferable that the polarization-maintaining fiber 1 has a mode field diameter of 10.6 μm or less at a wavelength of 1.55 μm, and a cutoff wavelength of 1.26 μm or more and less than 1.41 μm.

[0076] Furthermore, in the polarization-maintaining fiber 1, it is preferable that the mode field diameter at a wavelength of 1.55 μm is greater than 9.2 μm and not greater than 10.6 μm, and the cutoff wavelength is not less than 1.26 μm and less than 1.55 μm.

[0077] Furthermore, it is preferable that the polarization-maintaining fiber 1 satisfy any one of the following conditions (A) to (C): (A) The mode field diameter at a wavelength of 1.55 μm is 9.6 μm or less, and the cutoff wavelength is 1.24 μm or more and less than 1.55 μm. (B) The mode field diameter at a wavelength of 1.55 μm is greater than 9.6 μm and 10.5 μm or less, and the cutoff wavelength is 1.33 μm or more and less than 1.55 μm. (C) The mode field diameter at a wavelength of 1.55 μm is greater than 10.5 μm and 10.6 μm or less, and the cutoff wavelength is 1.36 μm or more and less than 1.55 μm.

[0078] In order to reduce the macrobend loss value when the mode field diameter is large, it is conceivable to increase the cutoff wavelength. Therefore, by satisfying at least one of the above conditions (A) to (C), at least one of (1) and (2) of Condition 3 can be more reliably satisfied. For example, as shown in Tables 1 to 3, when the mode field diameter is 10.6 μm, the cutoff wavelength range satisfying at least one of (1) and (2) of Condition 3 is 1.36 μm or more and less than 1.55 μm. However, when the mode field diameter is less than 10.6 μm, the light confinement effect can be improved compared to when the mode field diameter is 10.6 μm. Therefore, if the cutoff wavelength range satisfying at least one of (1) and (2) of Condition 3 when the mode field diameter is 10.6 μm is satisfied, at least one of (1) and (2) of Condition 3 can be necessarily satisfied within that cutoff wavelength range, even when the mode field diameter is less than 10.6 μm. Furthermore, when the above-mentioned mode field diameter is less than 10.6 μm, if the cutoff wavelength range is 1.24 μm or more, at least one of (1) and (2) of Condition 3 can be satisfied even if the cutoff wavelength is less than 1.36 μm.

[0079] Furthermore, it is preferable that the polarization-maintaining fiber 1 satisfies the following condition (3) instead of (1) or (2): (3) The bending radius is 7.5 mm, and the macrobend loss at a wavelength of 1.55 μm is 0.27 dB / turn or less.

[0080] Furthermore, when condition (3) is satisfied, it is preferable that the mode field diameter is 10.3 μm or less. It is also preferable that the mode field diameter is greater than 9.6 μm. Therefore, it is more preferable that the mode field diameter is greater than 9.6 μm and less than 10.3 μm. Furthermore, when condition (3) is satisfied, it is preferable that the cutoff wavelength is 1.36 μm or more and less than 1.55 μm.

[0081] There is an ITU-T standard G657.A2 for low-bend-loss single-mode optical fibers that are required to support access networks and general transport networks, and that have strict macrobend loss conditions. This standard specifies that the macrobend loss value is 0.5 dB when the fiber is wound around a mandrel once, with a bend radius of 7.5 mm at a wavelength of 1.55 μm. By satisfying the above condition (3) as can be seen from Tables 1 to 3, the polarization-maintaining fiber 1 can achieve a value equal to or less than the standard value, even though it has a twisted section 1A.

[0082] Furthermore, it is preferable that the polarization-maintaining fiber 1 satisfies the following condition (4) instead of (1) or (2): (4) The bending radius is 10 mm, and the macrobend loss at a wavelength of 1.55 μm is 0.07 dB / turn or less.

[0083] Furthermore, when condition (4) is satisfied, it is preferable that the mode field diameter is 9.6 μm or less. Furthermore, when condition (4) is satisfied, it is preferable that the cutoff wavelength is 1.24 μm or more and less than 1.55 μm.

[0084] The ITU-T standard G657.A2 specifies that the macrobend loss is 0.1 dB when the fiber is wound around a mandrel one turn at a wavelength of 1.55 μm and a bending radius of 10 mm. By satisfying the above condition (4) as can be seen from Tables 1 to 3, the polarization-maintaining fiber 1 can achieve a value equal to or less than the value specified by the ITU-T standard, even though it has a twisted section 1A. Furthermore, the ITU-T standard G657.B3, which specifies stricter macrobend loss conditions than the ITU-T standard G657.A2, specifies that the macrobend loss is 0.08 dB when the fiber is wound around a mandrel one turn at a wavelength of 1.55 μm and a bending radius of 10 mm. By satisfying the above condition (4), the polarization-maintaining fiber 1 can achieve a value equal to or less than the value specified by the standard, even though it has a twisted section 1A. Furthermore, if the polarization-maintaining fiber satisfies at least one of (3) and (4) of Condition 3, the macrobend loss can be small enough to support access networks, general transport networks, and the like.

[0085] Some of the aspects of the present invention described above are listed below.

[0086] A first aspect of the present invention is a polarization-maintaining fiber comprising a core 11, a pair of stress-applying portions 12 a, 12 b arranged at positions sandwiching the core 11, and a cladding 13 containing the core 11 and the pair of stress-applying portions 12 a, 12 b, wherein the stress-applying portions have a twist section in which they permanently rotate helically around the core, and in the twist section, the helical twist of the stress-applying portions per 47.1 mm of fiber length is one turn or less, the mode field diameter at a wavelength of 1.55 μm is 10.6 μm or less, and the cutoff wavelength when the fiber is 2 m long and has a bending radius of 140 mm is 1.24 μm or more and less than 1.55 μm.

[0087] By having a mode field diameter of 10.6 μm or less at a wavelength of 1.55 μm, as in the polarization-maintaining fiber 1, the light confinement force can be increased compared to when the mode field diameter at that wavelength is greater than 10.6 μm. Furthermore, by having a cutoff wavelength of 1.24 μm or more when the fiber length is 2 m and the bending radius is 140 mm, the light confinement force can be increased compared to when the cutoff wavelength at that wavelength is less than 1.24 μm. This reduces the loss of light propagating through the core. Furthermore, by having a cutoff wavelength of less than 1.55 μm, single-mode optical transmission can be achieved when the operating wavelength is 1.55 μm or greater. Therefore, a polarization-maintaining fiber can be realized that is particularly applicable to coherent optical transceivers operating at wavelengths of approximately 1.55 μm, and optical fibers and optical devices operating at wavelengths of 1.55 μm or greater. Furthermore, because the polarization-maintaining fiber 1 has the above-described mode field diameter and cutoff wavelength, the light confinement force can be increased as described above. Therefore, even in a twist section with the above-mentioned number of twist rotations, light leakage can be suppressed. For example, even if a user can apply the polarization-maintaining fiber to various optical devices, degradation of macrobend loss characteristics can be suppressed. This is because, when the twist section 1A includes the stress-applying portions 12a and 12b, in which the stress-applying portions 12a and 12b permanently rotate helically around the core 11, and the helical twist of the stress-applying portions 12a and 12b per 47.1 mm of the fiber length in the twist section 1A is within one rotation, the twist of the stress-applying portions 12a and 12b tends to be loose. In this case, leakage of light propagating through the core when the fiber is bent in the S-axis direction by the stress-applying portions 12a and 12b can be prevented. Therefore, bending loss due to bending in the F-axis direction, which would normally be a large bending loss, can be suppressed and can be kept to at most the average bending loss in the S-axis and F-axis directions, which may result in suppressing an extremely large macrobend loss.

[0088] The cutoff wavelength is a wavelength value measured at the fiber cutoff wavelength when the fiber length is 2 m, but by using the method described in the following document, which calculates the loss per unit length of each wavelength in the LP11 mode from the refractive index structure, it is possible to calculate the cutoff wavelength at any length even if the fiber length is less than 2 m. Therefore, since the polarization-maintaining fiber 1 of this embodiment can measure the cutoff wavelength, it can be used even if the fiber length is less than 2 m. 16th Opto-Electronics and Communications Conference "LP11 mode attenuation behavior of optical fibers with trench-cladding"

[0089] Aspect 2 of this embodiment is the polarization-maintaining fiber according to Aspect 1, characterized in that in the twisted section, the helical twist of the stress-applying portion per 62.8 mm of fiber length is one turn or less.

[0090] Aspect 3 of this embodiment is the polarization-maintaining fiber of Aspect 1 or 2, characterized in that the relative refractive index difference of the core with respect to the cladding is 0.32% or more.

[0091] 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 relative refractive index difference of the core with respect to the cladding is 0.33% or more.

[0092] 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 relative refractive index difference of the core with respect to the cladding is 0.40% or less.

[0093] A sixth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to fifth aspects, wherein the distance from the central axis of the core to the stress-applying portion is 10 μm or less.

[0094] A seventh aspect of the present invention is the polarization-maintaining fiber according to any one of the first to sixth aspects, characterized in that the relative refractive index difference of the stress-applying portions with respect to the cladding is −0.78% or less.

[0095] An eighth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to seventh aspects, wherein the mode field diameter is 10.3 μm or less.

[0096] A ninth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to eighth aspects, characterized in that the mode field diameter is 9.6 μm or less.

[0097] A tenth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to ninth aspects, wherein the mode field diameter is 8.9 μm or more.

[0098] An eleventh aspect of the present invention is the polarization-maintaining fiber according to any one of the first to tenth aspects, wherein the cutoff wavelength is 1.24 μm or more and 1.39 μm or less.

[0099] A twelfth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to eleventh aspects, wherein the cutoff wavelength is greater than 1.39 μm and less than 1.55 μm.

[0100] A thirteenth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twelfth aspects, wherein the cutoff wavelength is 1.36 μm or more and less than 1.55 μm.

[0101] A fourteenth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to thirteenth aspects, wherein the cutoff wavelength is 1.36 μm or more and 1.39 μm or less.

[0102] A fifteenth aspect of the present invention is the polarization-maintaining fiber of any one of the first to fourteenth aspects, wherein in the twisted section, the helical twist of the stress-applying portion per meter of fiber length is greater than 1 / 50 rotation.

[0103] Aspect 16 of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 15, wherein the cladding has a diameter of 124 μm or more and 126 μm or less, the mode field diameter is 10.2 μm or more and 10.6 μm or less, the cutoff wavelength is 1.33 μm or more and 1.39 μm or less, and the relative refractive index difference of the core with respect to the cladding is 0.32% or more and 0.34% or less.

[0104] Aspect 17 of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 16, wherein the cladding has a diameter of 79 μm or more and 81 μm or less, the mode field diameter is 8.9 μm or more and 9.6 μm or less, the cutoff wavelength is 1.24 μm or more and 1.39 μm or less, and the relative refractive index difference of the core with respect to the cladding is 0.33% or more and 0.40% or less.

[0105] Aspect 18 of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 17, characterized in that it satisfies any one of the following conditions (A) to (C): (A) the mode field diameter is 9.6 μm or less, and the cutoff wavelength is 1.24 μm or more and less than 1.55 μm; (B) the mode field diameter is greater than 9.6 μm and less than 10.5 μm, and the cutoff wavelength is 1.33 μm or more and less than 1.55 μm; (C) the mode field diameter is greater than 10.5 μm and less than 10.6 μm, and the cutoff wavelength is 1.36 μm or more and less than 1.55 μm.

[0106] A nineteenth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to eighteenth aspects, characterized in that the bending radius is 7.5 mm and the macrobend loss at a wavelength of 1.55 μm is 0.67 dB / turn or less.

[0107] A twentieth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to nineteenth aspects, characterized in that the bending radius is 10 mm and the macrobend loss at a wavelength of 1.55 μm is 0.16 dB / turn or less.

[0108] A twenty-first aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twenty aspects, wherein the mode field diameter is less than 10 μm.

[0109] A twenty-second aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twenty-first aspects, wherein the mode field diameter is greater than 9.6 μm.

[0110] A twenty-third aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twenty-second aspects, wherein the fiber length is 47.1 mm or more.

[0111] A twenty-fourth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twenty-third aspects, wherein the fiber length is less than 50 m.

[0112] A twenty-fifth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twenty-fourth aspects, wherein the fiber length is less than 10 m.

[0113] A twenty-sixth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twenty-fifth aspects, wherein the fiber length is less than 300 mm.

[0114] A twenty-seventh aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twenty-sixth aspects, wherein the refractive index profile of the core relative to the cladding is a step-like or GI-type profile.

[0115] A twenty-eighth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twenty-seventh aspects, wherein the stress-applying portions have a diameter of 24.8 μm or more.

[0116] A twenty-ninth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to twenty-eighth aspects, wherein the stress-applying portion has a diameter of 38.0 μm or less.

[0117] Aspect 30 of the present invention is the polarization-maintaining fiber according to any one of aspects 1 to 29, wherein the stress-applying portion has a diameter of 34.4 μm or less.

[0118] A thirty-first aspect of the present invention is the polarization-maintaining fiber according to any one of the first to thirty aspects, wherein the core has a radius of 4.4 μm or less.

[0119] A thirty-second aspect of the present invention is the polarization-maintaining fiber according to any one of the first to thirty-first aspects, wherein the core has a radius of 4.1 μm or more.

[0120] A thirty-third aspect of the present invention is the polarization-maintaining fiber according to any one of the first to thirty-second aspects, wherein the relative refractive index difference of the stress-applying portions with respect to the cladding is −1.0% or more.

[0121] Aspect 34 of the present invention is the polarization-maintaining fiber according to any one of Aspects 1 to 33, wherein the ratio of the distance from the central axis of the core to the stress-applying portion to the radius of the mode field diameter is 1.05 μm or greater.

[0122] A thirty-fifth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to thirty-fourth aspects, characterized in that the diameter of the cladding is 79 μm or more and 126 μm or less.

[0123] A thirty-sixth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to thirty-fifth aspects, characterized in that the diameter of the cladding is 124 μm or more and 126 μm or less.

[0124] A thirty-seventh aspect of the present invention is the polarization-maintaining fiber according to any one of the first to thirty-sixth aspects, characterized in that the diameter of the cladding is 79 μm or more and 81 μm or less.

[0125] Aspect 38 of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 37, characterized in that it has the twisted section and another twisted section, and the other twisted section has a twist period different from that of the twisted section.

[0126] Aspect 39 of the present invention is the polarization-maintaining fiber according to any one of Aspects 1 to 38, wherein the twisted section and the other twisted section have approximately the same length.

[0127] A fortieth aspect of the present invention is the polarization-maintaining fiber according to any one of the first to thirty-ninth aspects, wherein the twisted sections and the other twisted sections are provided alternately.

[0128] Aspect 41 of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 40, wherein in the twisted section, the helical twist of the stress-applying portion per meter of fiber length is greater than 1 / 25 rotation.

[0129] Aspect 42 of the present invention is the polarization-maintaining fiber of any one of Aspects 1 to 41, characterized in that the center of the circle that constitutes the outer periphery of the core is included in the center of the cladding, the center of the cladding 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 constitutes the outer periphery of the cladding.

[0130] As described above, according to the present invention, for example, a polarization-maintaining fiber can be provided in which the deterioration of macrobend loss characteristics is suppressed even when a user can apply the polarization-maintaining fiber to the optical device, and it is expected that the polarization-maintaining fiber will 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 polarization-maintaining fiber comprising: a core; a pair of stress-applying parts arranged at positions sandwiching said core; and a cladding containing said core and said pair of stress-applying parts; said stress-applying parts have a twisted section in which they permanently rotate in a spiral around said core; in said twisted section, the spiral twist of said stress-applying parts per 47.1 mm of fiber length is within one turn; the mode field diameter at a wavelength of 1.55 μm is 10.6 μm or less; and when the fiber length is 2 m and the bending radius is 140 mm, the cutoff wavelength is 1.24 μm or more and less than 1.55 μm.

2. The polarization-maintaining fiber according to claim 1, characterized in that in the twisted section, the helical twist of the stress-applying portion is within one turn per 62.8 mm of fiber length.

3. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the relative refractive index difference of the core with respect to the cladding is 0.32% or more.

4. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the relative refractive index difference of the core with respect to the cladding is 0.33% or more.

5. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the relative refractive index difference of the core with respect to the cladding is 0.40% or less.

6. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the distance from the central axis of the core to the stress-applying portion is 10 μm or less.

7. A polarization-maintaining fiber according to claim 1 or 2, characterized in that the relative refractive index difference of said stress-applying portion with respect to said cladding is -0.78% or less.

8. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the mode field diameter is 10.3 μm or less.

9. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the mode field diameter is 9.6 μm or less.

10. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the mode field diameter is 8.9 μm or more.

11. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the cutoff wavelength is 1.24 μm or more and 1.39 μm or less.

12. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the cutoff wavelength is greater than 1.39 μm and less than 1.55 μm.

13. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the cutoff wavelength is 1.36 μm or more and less than 1.55 μm.

14. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the cutoff wavelength is 1.36 μm or more and 1.39 μm or less.

15. The polarization-maintaining fiber according to claim 1 or 2, characterized in that in the twisted section, the helical twist of the stress-applying portion per meter of fiber length is greater than 1 / 50 rotation.

16. A polarization-maintaining fiber according to claim 1 or 2, characterized in that the diameter of the cladding is 124 μm or more and 126 μm or less, the mode field diameter is 10.2 μm or more and 10.6 μm or less, the cutoff wavelength is 1.33 μm or more and 1.39 μm or less, and the relative refractive index difference of the core with respect to the cladding is 0.32% or more and 0.34% or less.

17. A polarization-maintaining fiber according to claim 1 or 2, characterized in that the diameter of the cladding is 79 μm or more and 81 μm or less, the mode field diameter is 8.9 μm or more and 9.6 μm or less, the cutoff wavelength is 1.24 μm or more and 1.39 μm or less, and the relative refractive index difference of the core with respect to the cladding is 0.33% or more and 0.40% or less.

18. The polarization-maintaining fiber according to claim 1 or 2, characterized in that any one of the following (A) to (C) is satisfied: (A) the mode field diameter is 9.6 μm or less, and the cutoff wavelength is 1.24 μm or more and less than 1.55 μm; (B) the mode field diameter is greater than 9.6 μm and less than 10.5 μm, and the cutoff wavelength is 1.33 μm or more and less than 1.55 μm; (C) the mode field diameter is greater than 10.5 μm and less than 10.6 μm, and the cutoff wavelength is 1.36 μm or more and less than 1.55 μm.

19. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the bending radius is 7.5 mm and the macrobend loss at a wavelength of 1.55 μm is 0.67 dB / turn or less.

20. The polarization-maintaining fiber according to claim 1 or 2, characterized in that the bending radius is 10 mm and the macrobend loss at a wavelength of 1.55 μm is 0.16 dB / turn or less.

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