Polarization maintaining optical fiber

Abstract

A polarization-maintaining optical fiber (1) is provided with a core (11), stress application sections (12a, 12b), and a cladding (13), and has a mode field diameter of 10.6 [mu] m or less at a wavelength of 1.55 [mu] m, and a cutoff wavelength of 1.24 [mu] m or more and less than 1.55 [mu] m when the optical fiber length is 2 m and the bending radius is 140 mm, and satisfies at least one of (1) and (2). (1) The macrobend loss at a wavelength of 1.55 [mu] m is 0.67 dB / 1 turn or less when the bending radius is 7.5 mm and the fiber length is twisted for one turn per 47.1 mm. (2) The macrobend loss of the polarization maintaining fiber at a wavelength of 1.55 [mu] m is 0.16 dB / 1 turn or less when the bending radius is 10 mm and the fiber length is twisted for one turn per 62.8 mm.

Description

Technical Field

[0001] This invention relates to a polarization-maintaining optical fiber. Background Technology

[0002] In recent years, the demand for polarization-maintaining fibers in optical transceiver modules, such as coherent optical transceivers and CPO (Co-packaged Optics) switching modules, which require ensuring polarization-maintaining characteristics of optical devices, has been increasing. This is especially true in the case of coherent optical transceiver circuitry, where optical digital coherent communication is used to transmit more information over a single fiber. The optical devices used for optical digital coherent communication exhibit polarization dependence. Therefore, polarization-maintaining fibers are frequently used in the connections between optical devices and external light sources.

[0003] In this context, in recent years, research has been conducted on the increasing number of optical devices such as optical transceivers with increasing communication capacity, as well as the miniaturization of optical devices that integrate multiple optical devices and electronic components into a single package. To meet this demand, a polarization-maintaining fiber is needed that can suppress bending loss (macro-bending loss) even with minimal bending.

[0004] The following patent document 1 describes an example of a polarization-maintaining optical fiber.

[0005] Patent Document 1: Japanese Patent Application Publication No. 2000-221341 Summary of the Invention

[0006] (a) Technical problems to be solved As a polarization-maintaining fiber, there is a known structure in which the core is held between a pair of stress-applying parts. Such a polarization-maintaining fiber is sometimes called PANDA fiber. As a stress-applying part, for example, one known stress-applying part uses boron oxide-added glass with a lower refractive index than the cladding. Therefore, in such a polarization-maintaining fiber, light propagating in the core has the property of not easily leaking to the stress-applying part side. Furthermore, when the polarization-maintaining fiber is bent in the S-axis direction, the leakage of light from the F-axis direction, which is perpendicular to the S-axis, does not change much compared to the case where the polarization-maintaining fiber is not bent. However, sometimes a twist is applied to the polarization-maintaining fiber. In this case, the bending direction of the polarization-maintaining fiber is sometimes in a direction other than the S-axis direction. There are no stress-applying parts other than in the S-axis direction, or the distance from the core to the stress-applying parts is large. Therefore, when the polarization-maintaining fiber is bent in a direction other than the S-axis direction, light propagating in the core easily leaks from that bending direction. Therefore, when a twist is applied to the polarization-maintaining fiber, and the polarization-maintaining fiber is bent, light propagating in the core easily leaks from a direction other than the S-axis direction. Therefore, when twisting is applied to a polarization-maintaining fiber, the macro-bending loss characteristic, which is a fiber property, tends to decrease easily. Thus, there is a need for a polarization-maintaining fiber that can suppress the decrease in macro-bending loss characteristics even when twisting is applied.

[0007] Therefore, the object of the present invention is to provide a polarization-maintaining fiber that can suppress the reduction of macrobending loss characteristics even when twisting is applied to the polarization-maintaining fiber.

[0008] (II) Technical Solution To solve the above-mentioned technical problems, the present invention provides a polarization-maintaining optical fiber, characterized in that it comprises: a fiber core; a pair of stress-applying portions disposed at a position for clamping the fiber core; and a cladding that encloses the fiber core and the pair of stress-applying portions. The polarization-maintaining optical fiber has a mode field diameter of 10.6 μm or less at a wavelength of 1.55 μm, and when the fiber length is 2 m and the bending radius is 140 mm, the cutoff wavelength of the polarization-maintaining optical fiber is 1.24 μm or more and less than 1.55 μm, satisfying at least one of the following (1) and (2).

[0009] (1) With a bending radius of 7.5 mm and a twist of 1 turn for every 47.1 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.67 dB / 1 turn.

[0010] (2) With a bending radius of 10 mm and a twist of 1 turn for every 62.8 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.16 dB / 1 turn.

[0011] (III) Beneficial Effects As with this polarization-maintaining fiber, by having a mode field diameter of 10.6 μm or less at a wavelength of 1.55 μm, the light containment force can be increased due to the reduced mode field diameter compared to cases where the mode field diameter at the same wavelength is greater than 10.6 μm. Furthermore, with a fiber length of 2 m and a bending radius of 140 mm, the cutoff wavelength of this polarization-maintaining fiber is 1.24 μm or greater, which, compared to cases where the cutoff wavelength is less than 1.24 μm, increases the light containment force. Therefore, the loss of light propagating in the fiber core can be suppressed. In summary, compared to polarization-maintaining fibers that do not meet the above-described structure, the polarization-maintaining fiber of the present invention can suppress the reduction of macro-bending loss characteristics even when twisting is applied to the polarization-maintaining fiber.

[0012] As described above, according to the present invention, it is possible to provide a polarization-maintaining fiber that can suppress the reduction of macrobending loss characteristics even when twisting is applied to the polarization-maintaining fiber. Attached Figure Description

[0013] Figure 1 This is a diagram illustrating a polarization-maintaining optical fiber according to an embodiment of the present invention.

[0014] Figure 2 It is a diagram showing the distribution of the relative refractive index difference along the fast axis.

[0015] Figure 3 It is a diagram showing the distribution of the relative refractive index difference along the slow axis. Detailed Implementation

[0016] The preferred embodiments of the polarization-maintaining optical fiber of the present invention will now be described in detail with reference to the accompanying drawings. The embodiments illustrated below are intended to facilitate understanding of the invention and are not intended to limit its scope. The invention can be modified and altered within the scope of the claims without departing from its spirit. Furthermore, for ease of understanding, the scale of the figures may differ from that described in the following description.

[0017] Figure 1 This is a diagram illustrating the polarization-maintaining fiber used in this embodiment. Figure 1 In the diagram, (a) is a cross-sectional view showing the polarization-maintaining fiber of this embodiment, and (b) is a cross-sectional view showing the cross-section of the fiber. Figure 1 The diagram shows the refractive index distribution of the AA line in the cross section shown in (a). Furthermore, the cross section refers to a section orthogonal to the central axis C of the polarization-maintaining fiber 1.

[0018] like Figure 1As shown in (a), the polarization-maintaining fiber 1 includes: a core 11; a pair of stress-applying portions 12a and 12b disposed on both sides of the core 11; and a cladding 13 enclosing the core 11 and the pair of stress-applying portions 12a and 12b. Furthermore, the polarization-maintaining fiber 1 may also include a cladding portion covering the cladding 13. This type of polarization-maintaining fiber 1 is sometimes referred to as PANDA (Polarization-maintaining and Absorption-reducing) fiber. In other words, it is sometimes called a PANDA-type polarization-maintaining fiber.

[0019] The fiber core 11 has a cylindrical shape extending along the central axis C of the polarization-maintaining fiber 1, which passes approximately through the center of the cladding 13. Therefore, the central axis C of the polarization-maintaining fiber and the central axis of the fiber core are approximately the same. Therefore, the central axis C of both the fiber core and the polarization-maintaining fiber will be used as the central axis in the following description. The refractive index n11 of the fiber core is higher than the refractive index n13 of the cladding 13. The fiber core 11 is made of quartz glass, for example, with added ascending dopant such as germanium (Ge). There can be one or more fiber cores 11.

[0020] The relative refractive index difference Δ between the core 11 and the cladding 13 11 Preferably, it is 0.32% or higher. This is determined by the relative refractive index difference Δ. 11 It is above 0.32%, and is related to the relative refractive index difference Δ. 11 Compared to cases with a refractive index difference of less than 0.32%, this increases the light-blocking force towards the fiber core 11, thus suppressing light leakage from the fiber core 11. Furthermore, the relative refractive index difference Δ 11 More preferably, it is 0.33% or higher. This is determined by the relative refractive index difference Δ. 11 It is above 0.33%, and is related to the relative refractive index difference Δ. 11 Compared to cases with a refractive index less than 0.33%, this increases the light-blocking force towards the fiber core 11, thus suppressing light leakage from the fiber core 11. Furthermore, the relative refractive index difference Δ 11 Preferably, it is 0.40% or less. This can suppress the mode field diameter of light propagating in the fiber core 11 from becoming too small, and can suppress light loss at the connection points with other optical fibers and optical devices. Therefore, since a balance can be achieved between the light containment force and the light loss at the connection points, the relative refractive index difference Δ 11Preferably, the content is 0.32% or more and 0.40% or less, more preferably 0.33% or more and 0.40% or less. In addition, as examples of the above-mentioned "optical devices", for example, telecommunications optical devices such as transceivers for intercity networks and submarine networks, or data communication optical devices such as CPO (Co-packaged Optics) modules (CPO switch modules, etc.) and plug-in transceivers (coherent optical transceivers, etc.), (2) amplifier optical devices, (3) sensor optical devices, etc. The term "optical device" as used below is used to include the optical devices in (1) to (3) above.

[0021] like Figure 1 As shown, in the polarization-maintaining fiber 1 of this embodiment, the refractive index distribution of the core 11 relative to the cladding 13 is stepped. However, this refractive index distribution is not limited to a stepped shape; for example, it can also be a GI-type distribution. Furthermore, the refractive index distribution of the core 11 relative to the cladding 13 can also be other shapes.

[0022] The cross-sectional shape of the fiber core 11 in this embodiment is approximately circular. However, the cross-sectional shape of the fiber core 11 is not limited to a circle; for example, it may be elliptical, crescent-shaped, or other non-circular shapes. Furthermore, the cross-sectional shape of the fiber core 11 here refers to the shape of a cross-section orthogonal to the central axis C of the polarization-maintaining fiber 1. Furthermore, the radius of the fiber core is preferably 4.4 μm or less. In addition, the radius of the fiber core 11 is preferably 4.1 μm or more. Furthermore, the radius of the fiber core 11 is more preferably 4.1 μm or more and 4.4 μm or less. The radius of the fiber core 11 refers to, for example, half the value of the average diameter of the fiber core 11 in the direction perpendicular to the central axis C. Furthermore, the radius of the fiber core 11 is not particularly limited as long as the cutoff wavelength described later satisfies any of the preferred conditions for the cutoff wavelength in <condition 2> or (3) or (4) described later.

[0023] The pair of stress-applying portions 12a and 12b are columnar in shape, extending along the aforementioned central axis C, but may also be other shapes. In this embodiment, the refractive index n12 of the stress-applying portions 12a and 12b is lower than the refractive index n13 of the cladding 13. The stress-applying portions 12a and 12b are made of quartz glass, for example, with added boron oxide (B2O3) or other dopant. Here, the cross-sectional shape of the stress-applying portions 12a and 12b may also be an isosceles trapezoid with the upper base (shorter base side) facing the fiber core 11. In this case, one or both of the upper base and lower base (longer base side) of the stress-applying portions 12a and 12b may be an arc shape bulging away from the fiber core 11. The polarization-maintaining fiber 1 with the isosceles trapezoidal shape of the stress-applying portions 12a and 12b is sometimes referred to as a "bowtie-type polarization-maintaining fiber". Thus, polarization-maintaining fiber 1 is not limited to the PANDA type polarization-maintaining fiber mentioned above, but can also be a bow-tie type polarization-maintaining fiber.

[0024] The relative refractive index difference Δ between the stress-applying portions 12a and 12b and the cladding 13 12 Preferably, it is -0.78% or less. With such a relative refractive index difference, in the S-axis direction along the direction in which the stress-applying portions 12a and 12b are arranged, the stress-applying portions 12a and 12b can function as low-refractive-index layers, suppressing light leakage from the fiber core 11. Furthermore, the relative refractive index difference Δ... 12 Preferably, it is -1.0% or higher. This is determined by the relative refractive index difference Δ. 12 For values ​​above -1.0%, within a relative refractive index difference Δ 12 With a concentration of -1.0% or higher, the amount of boron oxide added can be sufficiently small to prevent deliquescence of the stress-applied portions 12a and 12b. Furthermore, the relative refractive index difference Δ... 12 More preferably, it is -1.0% or more and -0.78% or less.

[0025] The cross-sectional shapes of the stress application parts 12a and 12b in this embodiment are as follows: Figure 1The solid lines indicate a roughly circular shape, or the dashed lines indicate a roughly elliptical shape with the arrangement direction of the stress-applying portions 12a and 12b as the minor axis. However, the cross-sectional shape of the stress-applying portions 12a and 12b is not limited to a circle or an ellipse; for example, it can be crescent-shaped or other non-circular shapes. Furthermore, the cross-sectional shape of the stress-applying portions 12a and 12b here refers to the shape of a cross-section orthogonal to the central axis C of the polarization-maintaining fiber 1. Here, when the diameter of the cladding 13 (described later) is 125 μm ± 1 μm, the diameter t of the stress-applying portions 12a and 12b along the S-axis is preferably 24.8 μm or more, more preferably 30.0 μm or more. This allows sufficient stress to be applied to the fiber core 11, resulting in appropriate polarization-maintaining functionality. Additionally, 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. Therefore, stress application portions 12a and 12b can be appropriately disposed inside the cladding layer 13 with a diameter of 125 μm ± 1 μm. Furthermore, when the diameter of the cladding layer 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, and preferably 30.0 μm or more and 40.0 μm or less, preferably 30.0 μm or more and 38.0 μm or less, preferably 30.0 μm or more and 34.4 μm or less.

[0026] Furthermore, when the diameter of the cladding 13 (described later) is 80 μm ± 1 μm, the diameter t of the stress application portions 12a and 12b along the S-axis is preferably 22.0 μm or more. This allows sufficient stress to be applied to the fiber core 11, resulting in adequate polarization maintenance. Additionally, the diameter t is preferably 27.0 μm or less. This allows the stress application portions 12a and 12b to be appropriately arranged inside the cladding 13 with 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.

[0027] The stress-applying portions 12a and 12b are separated from the fiber core 11. Therefore, when manufacturing the polarization-maintaining fiber 1 by melt stretching, the possibility of accidental deformation of the fiber core 11 due to stress from the stress-applying portions 12a and 12b can be reduced. Furthermore, when the fiber core 11 is in contact with the stress-applying portions 12a and 12b, the addition of boron oxide to the stress-applying portions 12a and 12b can sometimes lead to deterioration of transmission loss. However, by separating the fiber core 11 from the stress-applying portions 12a and 12b, the aforementioned deterioration of transmission loss can be suppressed.

[0028] The spacing between stress-applying portions 12a and 12b is 2a, and the distance from the central axis C of the fiber core 11 to the stress-applying portions 12a and 12b is a. Here, distance a refers to the distance from the center of the fiber core 11 to the point closest to the fiber core 11 among the points included in stress-applying portion 12a, or the distance from the center of the fiber core 11 to the point closest to the fiber core 11 among the points included in stress-applying portion 12b. This distance a is preferably 10 μm or less. By having a distance a of 10 μm or less, compared to the case where the distance a is greater than 10 μm, the stress-applying portions 12a and 12b, which have a lower refractive index than the cladding 13, are closer to the fiber core 11. Therefore, the stress-applying portions 12a and 12b in the S-axis direction can function more effectively as low-refractive-index layers, and can suppress light leakage from the fiber core 11. Furthermore, it is preferable that the ratio of distance a to half (radius) of the mode field diameter MFD (a / (MFD / 2)) is 1.05 or more. With a ratio (a / (MFD / 2)) of 1.05 or higher, the stress application portions 12a and 12b are positioned outside the mold field. Therefore, the deterioration of transmission loss caused by boron oxide when it is added to the stress application portions 12a and 12b can be further suppressed. Furthermore, the spacing between the fiber core 11 and the stress application portions 12a and 12b is preferably 1 μm or more. In addition, the size of the diameter t is not particularly limited as long as the distance a satisfies any one of conditions (1), (2), (3), or (4) of <Condition 3> described later.

[0029] The cladding 13 has a columnar shape extending along the aforementioned central axis C. As described above, the refractive index n13 of the cladding 13 is lower than the refractive index n11 of the fiber core 11, but higher than the refractive index n12 of the stress-applying portions 12a and 12b. The cladding 13 is made of, for example, quartz glass.

[0030] The cladding 13 in this embodiment has a generally circular cross-sectional shape. However, the cross-sectional shape of the cladding 13 is not limited to this; for example, it can also be elliptical, crescent-shaped, or other non-circular shapes. Furthermore, the cross-sectional shape of the cladding 13 here refers to the shape of the cross-section orthogonal to the central axis C of the polarization-maintaining fiber 1.

[0031] The diameter b of the cladding 13 is preferably approximately 80 μm ± 1 μm, that is, 79 μm or more and 81 μm or less, more preferably 80 μm. In this case, for example, the installation area when housed in an optical transceiver or applied to a sensor can be kept small, thus enabling high-density installation. In addition, in this case, the rigidity of the polarization-maintaining fiber 1 can be kept small, thus reducing the decrease in the mechanical strength of the polarization-maintaining fiber 1 when it is twisted. In addition, in this case, when housed in optical devices, especially when the polarization-maintaining fiber 1 is wound, the polarization-maintaining fiber 1 tends to be easily twisted, raising concerns about an increase in macro-bending loss. However, even in this case, by satisfying the following conditions 1 and 2 with the polarization-maintaining fiber 1, a small-diameter polarization-maintaining fiber can be realized, which can suppress the loss value below that required by various optical devices, even when the polarization-maintaining fiber is twisted when housed in optical devices, etc. Alternatively, the cladding diameter b is preferably 125 μm ± 1 μm, that is, 124 μm or more and 126 μm or less, more preferably 125 μm. In this case, a polarization-maintaining fiber with approximately the same diameter as the optical fiber commonly used in communication infrastructure can be achieved. Regarding the ± 1 μm of the cladding diameter described above, even if there is a difference of ± 1 μm when the cladding diameter is set to 80 μm or 125 μm, the optical and mechanical properties of the optical fiber will be approximately the same as when the cladding diameter is 80 μm or 125 μm, or the variation in the degree of error will not have a significant impact on the optical and mechanical properties of the optical fiber. For example, in an optical fiber with a cladding diameter of 125 μm, the ± 1 μm of the cladding diameter is also the same size as the tolerance value of the cladding diameter specified in the optical fiber standard (ITU-T). In addition, the ± 1 μm of the cladding diameter in an optical fiber with a cladding diameter of 80 μm can also be equivalent to the aforementioned tolerance value. Furthermore, the diameter of the cladding 13 can be 79 μm or more and 126 μm or less. In addition, when the cross-sectional shape of the cladding 13 is not circular, the diameter b of the cladding 13 refers to, for example, the averaged value of the diameter of the cladding 13 in the direction perpendicular to the aforementioned central axis C.

[0032] Furthermore, in this embodiment, the center of the circle constituting the outer periphery of the core 11 coincides with the center of the circle constituting the outer periphery of the cladding 13, but this is not a limitation; the center of the circle constituting the outer periphery of the core 11 only needs to be contained within the central portion of the cladding 13. Therefore, when the diameter of the cladding 13 is 79 μm or more and 126 μm or less, the center of the circle constituting the outer periphery of the core 11 only needs to be contained within the central portion of the cladding 13. Here, the central portion of the cladding 13 refers to the inner region of a circle with a radius of 0.6 μm, the center of which coincides with the center of the circle constituting the outer periphery of the cladding 13.

[0033] Furthermore, the fiber length of polarization-maintaining fiber 1 is preferably less than 50m, and can be less than 10m or less than 300mm. Additionally, the fiber length of polarization-maintaining fiber 1 is preferably 47.1mm or more, for example, it can be 2m or more. When polarization-maintaining fiber 1 is used as a sensor, commercially available polarization-maintaining fibers are typically used with a length of 50m or more. This is because there is a tendency for the longer the polarization-maintaining fiber, the higher the measurement resolution of the sensor. If the polarization-maintaining fiber used for sensors is only a few meters long, the measurement resolution may decrease, making it difficult to use as a sensor. However, in the case of polarization-maintaining fibers used for communication or amplifiers, high measurement resolution is not required as much as in the case of polarization-maintaining fibers used for sensors. Furthermore, when connecting various optical devices and external light sources, as mentioned above, to avoid interference between the polarization-maintaining fiber and various optical devices due to fiber length, it is preferable to have a length of less than 50m, as described above. Furthermore, sometimes the fiber length is set to less than 50m when connected with the polarization-maintaining fiber in a wound state, and also, considering the accommodation of various optical devices, a single-digit value greater than 300mm and less than 10m is set. Additionally, sometimes the fiber length when connected without winding is set to less than 300mm. Preferably, the fiber length is determined based on the connection state. Therefore, a fiber length of more than 50m is unnecessary; a length of less than 50m makes it less likely for the polarization-maintaining fiber to interfere with various optical devices. In summary, considering the fiber lengths of 23.55 mm and 31.4 mm (described later), the fiber length of 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 greater than 300 mm and less than 10 m.

[0034] The polarization-maintaining fiber 1 with the above structure satisfies the following conditions 1 to 3.

[0035] <Condition 1> The mode field diameter at a wavelength of 1.55 μm is less than 10.6 μm.

[0036] By satisfying this condition, the mode field diameter is smaller compared to the case where the mode field diameter is greater than 10.6 μm at a wavelength of 1.55 μm, which correspondingly improves the light containment effect.

[0037] <Condition 2> With an optical fiber length of 2m and a bending radius of 140mm, the cutoff wavelength of this polarization-maintaining fiber is greater than 1.24μm and less than 1.55μm. Furthermore, the cutoff wavelength mentioned here refers to the cutoff wavelength of LP11 mode light when the optical fiber length is 2m and the bending radius is 140mm.

[0038] By using a cutoff wavelength of 1.24 μm or higher, the light containment effect can be improved compared to cases where the cutoff wavelength is less than 1.24 μm. Furthermore, as long as the mode field diameter satisfies condition 1 and the cutoff wavelength satisfies condition 2, the refractive index distribution can naturally be determined.

[0039] Furthermore, by setting the cutoff wavelength to less than 1.55 μm, single-mode optical transmission can be achieved when using wavelengths above 1.55 μm. In particular, it can be applied to coherent optical transceivers using wavelengths around 1.55 μm, optical fibers using wavelengths above 1.55 μm, and polarization-maintaining fibers for optical devices.

[0040] <Condition 3> It must satisfy at least one of the following (1) and (2). In addition, macro bending loss is sometimes referred to as bending loss.

[0041] (1) With a bending radius of 7.5 mm and a twist of 1 turn for every 47.1 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.67 dB / 1 turn.

[0042] (2) With a bending radius of 10 mm and a twist of 1 turn for every 62.8 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.16 dB / 1 turn.

[0043] Furthermore, the fiber length of 47.1 mm refers to the fiber length when wound with a bending radius of 7.5 mm for one turn, and the fiber length of 62.8 mm refers to the fiber length when wound with a bending radius of 10 mm for one turn. Here, "one turn" means: winding the polarization-maintaining fiber once on a mandrel with a diameter of 7.5 mm, or winding the polarization-maintaining fiber once on a mandrel with a diameter of 10 mm. In addition, "one turn" is synonymous with 1 turn.

[0044] By satisfying the conditions described in (1) above with polarization-maintaining fiber 1, a polarization-maintaining fiber can be realized that, under the condition that the bending radius required as a reference for the target value of optical devices such as coherent optical transceivers is about 7.5 mm or more, even when torsion is applied, the macro-bending loss value can be suppressed to below the loss tolerance value required by the optical device. Here, the case where the user applies the polarization-maintaining fiber to the optical device, as described later, can be exemplified by the case where the polarization-maintaining fiber 1 is housed in various optical devices. In addition, the loss tolerance value described above can be exemplified by a value that allows it to be applied to various optical devices. Furthermore, as long as conditions 1 and 2 above are satisfied, in condition (1) above, the measurement can also be performed under the condition that the number of turns is 0.5 turns. In this case, the fiber length of the polarization-maintaining fiber 1 can also be less than 47.1 mm (for example, 23.55 mm). In this case, the macro-bending loss value can be a value proportional to 0.5.

[0045] Furthermore, by satisfying the conditions in (2) above with polarization-maintaining fiber 1, a polarization-maintaining fiber can be realized that, under the condition that the bending radius required as a benchmark for an optical device with a bending radius greater than about 7.5 mm is about 10 mm or more, even when torsion is applied, the macro-bending loss value can be suppressed to below the loss tolerance value required by the optical device. In addition, as long as conditions 1 and 2 above are satisfied, in condition (2) above with condition 3 above, it is also possible to measure under the condition that the number of turns is 0.5 turns. In this case, the fiber length of the polarization-maintaining fiber 1 can also be less than 62.8 mm (for example, 31.4 mm). In this case, the macro-bending loss value can be a value proportional to 0.5.

[0046] Furthermore, as described above, when the diameter t of the stress application portions 12a and 12b along the S-axis is 24.8 μm or more, compared to the case where the diameter t is less than 24.8 μm, the light containment force can be increased, and at least one of the conditions (1) and (2) above can be satisfied more reliably.

[0047] Next, an embodiment of polarization-maintaining fiber 1 will be described.

[0048] The following items were measured for the polarization-maintaining optical fibers of Examples 1 to 12 and Comparative Examples 1 to 3.

[0049] <Fiber cutoff wavelength> The cutoff wavelength of a 2m long polarization-maintaining fiber wound once with a diameter of 280mm is called the fiber cutoff wavelength. Regarding the fiber cutoff wavelength, based on IEC 60793-1-44, the wavelength at which the higher-order mode attenuation of a 2m long polarization-maintaining fiber 1 is 19.3dB when wound once on a mandrel with a diameter of 280mm is measured is the fiber cutoff wavelength. In this case, the polarization-maintaining fiber is wound onto the mandrel with its slow axis perpendicular to the surface of the mandrel. Furthermore, the aforementioned mandrel is a rod-shaped tool with a circular cross-section. Here, the polarization-maintaining fiber 1 is wound onto the aforementioned mandrel with its slow axis perpendicular to the surface of the aforementioned mandrel. R in the following table represents the radius of the mandrel.

[0050] <Mold field diameter> The mode field diameter of light with a wavelength of 1.55 μm propagating in the fiber core was measured. This measurement was based on IEC 60793-1-45 and was performed using the Variable Aperture Method. The mode field diameter was determined using light with a wavelength of 1.55 μm waveguided within fiber core 11.

[0051] <Cladon diameter> The diameter of the cladding was determined based on IEC60793-1-20 by the transmitted near-field method.

[0052] <Relative refractive index difference Δ 11 > Relative refractive index difference Δ 11 It is the difference in refractive index between the core and the cladding. Figure 2 This is a graph showing the distribution of the relative refractive index difference along the fast axis. Relative refractive index difference Δ 11 The average relative refractive index difference in the region representing more than 90% of the maximum relative refractive index difference between the core and cladding is obtained using the following method. Specifically, the refractive index distribution of the fiber core along the fast axis is obtained through interferometry; the relative refractive index difference distribution along the fast axis is calculated; and after determining the maximum relative refractive index difference between the core and cladding, the average relative refractive index difference in the region representing more than 90% of the maximum relative refractive index difference between the core and cladding is calculated.

[0053] <Relative refractive index difference Δ 12 > Relative refractive index difference Δ 12 It is the relative refractive index difference between the stress-applied part and the cladding. Figure 3 This is a graph showing the distribution of the relative refractive index difference along the slow axis. Relative refractive index difference Δ 12 The relative refractive index difference Δ with the fiber core 11Similarly, the region that satisfies the minimum value of the refractive index distribution in the slow axis direction of the stress-applied portion (from...) Figure 3 The equivalent of 0.5Δ 12 min to the region equivalent to Δ 12 The average of the relative refractive index difference in the region (min range) is calculated. That is, by measuring the refractive index distribution of the stress-applied portion in the slow axis direction of the polarization-maintaining fiber based on interferometry, the relative refractive index difference distribution on the slow axis is calculated, and after determining the minimum value of the relative refractive index difference between the stress-applied portion and the cladding, the average of the relative refractive index difference in the region less than 50% of the minimum value of the refractive index distribution in the slow axis direction of the stress-applied portion is calculated.

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

[0055] <Diameter of the stress-applying part> The diameter of the stress-applying section was obtained by measuring the fiber end face using an optical microscope.

[0056] <Macro bending loss> In this specification, the bending loss of 1.55 μm wavelength light per turn when the polarization-maintaining fiber is wound once on a mandrel with a radius of 7.5 mm or 10 mm is referred to as macro-bending loss. Regarding macro-bending loss, based on IEC 60793-1-47, when the polarization-maintaining fiber 1 is wound once with its slow axis perpendicular to the surface of the mandrel with a radius of 7.5 mm or 10 mm (bending radius of 7.5 mm or 10 mm), the power of the LP01 mode light from the light source at this time is measured, and the power of the LP01 mode light from the light source in the unwound state is measured as a reference. The bending loss per turn is calculated by the difference between these two powers. Furthermore, the aforementioned mandrel is a rod-shaped tool with a circular cross-section. R in the following table represents the radius of the mandrel.

[0057] In the macrobending loss measurement, the bending loss was measured when the polarization-maintaining fiber was wound around the mandrel with a bending radius of 10 mm without any twist, and when the same twist was applied to each turn with a fiber length of 62.8 mm per turn. Here, the refractive index distribution of the core relative to the cladding in the polarization-maintaining fibers of Examples 1-12 and Comparative Examples 1-3 was stepped. Furthermore, regarding these polarization-maintaining fibers, the bending loss was measured when the polarization-maintaining fiber was wound around the mandrel with a bending radius of 7.5 mm without any twist, and when the same twist was applied to each turn with a fiber length of 47.1 mm per turn. In summary, in other words, the bending loss per turn of light at a wavelength of 1.55 μm was measured when the bending radius was 7.5 mm and the fiber length twisted once every 47.1 mm. The bending loss per turn of light at a wavelength of 1.55 μm was also measured when the bending radius was 10 mm and the fiber length twisted once every 62.8 mm. Furthermore, regarding the bending loss without twist, the bending loss was measured with the slow axis perpendicular to the core axis. The results are shown in Tables 1-3.

[0058] As shown in Tables 1-3, Examples 1-12 satisfy all conditions 1-3. Specifically, by satisfying condition 1, these polarization-maintaining fibers can reduce the mode field diameter compared to cases where the mode field diameter at a wavelength of 1.55 μm is greater than 10.6 μm, thereby improving the light containment effect. Furthermore, by satisfying condition 2, these polarization-maintaining fibers can improve the light containment effect compared to cases where the cutoff wavelength is less than 1.24 μm. This improved light containment effect suppresses bending loss. Moreover, by satisfying condition 3, these polarization-maintaining fibers can achieve a macro-bending loss value within an acceptable range in optical devices such as coherent optical transceivers. Here, the acceptable range refers to a range where, even when twisting is applied to the polarization-maintaining fiber when it is housed in various optical devices, the reduction in macro-bending loss characteristics can be suppressed to a level where the user can apply the polarization-maintaining fiber to the optical device.

[0059] The polarization-maintaining fiber of Comparative Example 1 has a large mode field diameter, which does not satisfy condition 1. Furthermore, this polarization-maintaining fiber does not satisfy condition 3. Compared to the polarization-maintaining fiber of Example 8 or Example 12, which has the highest macrobending loss among Examples 1-12, its macrobending loss relative to condition 3(1) is approximately 3.7 times, and its macrobending loss relative to condition 3(2) is approximately 3.0 times. Therefore, the polarization-maintaining fiber of Comparative Example 1 is difficult to use in optical devices such as coherent optical transceivers that meet the above-mentioned permissible range.

[0060] The polarization-maintaining fiber of Comparative Example 2 does not meet condition 3. Compared with the polarization-maintaining fiber of Example 8 or Example 12, which has the largest macrobending loss, its macrobending loss relative to condition 3(1) is about 2.8 times, and its macrobending loss relative to condition 3(2) is about 2.1 times. Therefore, the polarization-maintaining fiber of Comparative Example 2 is difficult to use in optical devices such as coherent optical transceivers that meet the above-mentioned allowable range.

[0061] For the polarization-maintaining fiber in Comparative Example 2, the relative refractive index difference Δ between the core and the cladding is... 11 The relative refractive index difference Δ between the polarization-maintaining fibers of Examples 1 to 12 11 Small. Therefore, the relative refractive index difference Δ between the core 11 and the cladding 13 is small. 11 As described above, the value is preferably 0.32% or more, and more preferably 0.33% or more. Furthermore, the relative refractive index difference Δ... 11 As mentioned above, it is preferably below 0.40%.

[0062] The polarization-maintaining fiber of Comparative Example 3 has a small cutoff wavelength, thus failing to meet condition 2. Furthermore, this polarization-maintaining fiber does not meet condition 3; compared to the polarization-maintaining fibers of Example 8 or Example 12, which have the highest macro-bending loss, its macro-bending loss relative to condition 3(1) is approximately 1.9 times, and its macro-bending loss relative to condition 3(2) is approximately 1.6 times. Therefore, the polarization-maintaining fiber of Comparative Example 3 is difficult to use in optical devices such as coherent optical transceivers that meet the aforementioned permissible range.

[0063] Furthermore, according to Tables 1-3, in polarization-maintaining fiber 1, a mode field diameter of 10.3 μm or less at a wavelength of 1.55 μm is preferred. Additionally, according to Tables 1-3, a mode field diameter of 9.6 μm or less at a wavelength of 1.55 μm is preferred. In this case, as shown in the various embodiments, macrobending loss can be further suppressed.

[0064] Furthermore, according to Tables 1 to 3, in polarization-maintaining fiber 1, a mode field diameter of 8.9 μm or more at a wavelength of 1.55 μm is preferred. In this case, compared to the case where the mode field diameter is less than 8.9 μm, the mode field diameter is larger, and therefore, at the connection point with other optical fibers or optical devices, light propagating from other optical fibers or optical devices can easily enter the polarization-maintaining fiber. Therefore, connection loss can be reduced. Therefore, in this case, a balance can be maintained between the following two effects: the effect of macro-bending loss characteristics under the specified torsion conditions that satisfy at least one of conditions (1) and (2) of the above-mentioned condition 3, and the effect of suppressing connection loss with other optical fibers or optical devices. In addition, considering the beam size of the laser source used in an optical transceiver (coherent optical transceiver, etc.) in the 1.55 μm wavelength band, and the focusing lens, a mode field diameter of 8.9 μm or more is preferred in order to reduce the connection loss to a level that allows operation when applied to such an optical transceiver. Furthermore, the mode field diameter of the optical waveguide in the actual polarization-maintaining fiber connection is mostly 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 aforementioned optical waveguide is mostly 8.9 μm or more and 9.6 μm or less. Therefore, from the viewpoint of reducing the connection loss with the aforementioned 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. Additionally, considering the beam size and focusing lens of the laser source in the actual polarization-maintaining fiber connection, in order to reduce the connection loss with the aforementioned laser source to a level that allows operation in an optical transceiver, a mode field diameter of 8.9 μm or more and 9.6 μm or less is preferred.

[0065] Furthermore, in polarization-maintaining fiber 1, a mode field diameter of less than 10 μm is preferred. By using a single-digit value such as a mode field diameter of less than 10 μm, when an optical element (lens, spot size converter, etc.) for mode field diameter conversion is placed between the fiber and the modulator, the complexity and size of this optical element can be suppressed. In particular, when using fiber with a cladding diameter of 80 ± 1 μm, the overall optical device including the fiber, the optical element for mode field diameter conversion, and the modulator can be miniaturized. Therefore, considering the balance between connection loss and miniaturization of the optical element for mode field diameter conversion, a mode field diameter of 8.9 μm or more and less than 10 μm is more preferable.

[0066] Furthermore, according to Tables 1 to 3, in polarization-maintaining fiber 1, a cutoff wavelength of 1.24 μm or more and 1.39 μm or less is preferred. When the fiber length is short, for example, 1 m or less, the cutoff wavelength may sometimes shift towards a longer wavelength. Therefore, in order to reduce the possibility that the cutoff wavelength exceeds the usable wavelength due to the shift towards a longer wavelength caused by fiber length, making single-mode transmission at the usable wavelength difficult, it is preferable that there is a certain margin between the upper limit of the cutoff wavelength measured with a fiber length of 2 m and the usable wavelength. 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 towards a longer wavelength in cases such as when the fiber length is short as described above, it is possible to suppress the cutoff wavelength from exceeding the usable wavelength. Therefore, the possibility of difficulty in performing single-mode transmission at the usable wavelength can be reduced.

[0067] Furthermore, in polarization-maintaining fiber 1, the cutoff wavelength can also be greater than 1.39 μm and less than 1.55 μm. By using such a cutoff wavelength for polarization-maintaining fiber 1, the cutoff wavelength can be made close to the usable wavelength band, and the light containment effect can be improved compared to the case where the cutoff wavelength is below 1.39 μm. Therefore, even when twisting is applied to polarization-maintaining fiber 1, the macrobending loss value can be reduced compared to the case where the cutoff wavelength is below 1.39 μm.

[0068] Furthermore, in polarization-maintaining fiber 1, a cutoff wavelength of 1.36 μm or higher and less than 1.55 μm is preferred. In this case, a balance can be appropriately maintained between two effects: a higher light containment effect compared to the case where the cutoff wavelength is less than 1.36 μm, and an effect that reduces the possibility of single-mode transmission at the usable wavelength being difficult compared to the case where the cutoff wavelength is greater than 1.39 μm and less than 1.55 μm.

[0069] Furthermore, according to Tables 1 to 3, in polarization-maintaining fiber 1, a cutoff wavelength of 1.36 μm or higher and 1.39 μm or lower is preferred. In this case, a balance can be appropriately maintained between two effects: a higher light containment effect compared to cases where the cutoff wavelength is less than 1.36 μm, and an effect that reduces the possibility of single-mode transmission at usable wavelengths. Additionally, compared to cases where the cutoff wavelength is greater than 1.39 μm, the possibility of single-mode transmission at usable wavelengths is reduced.

[0070] Furthermore, according to Tables 1 to 3, in the polarization-maintaining fiber 1, preferably, 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 Δ between the core 11 and the cladding 13 is... 11 It is between 0.32% and 0.34%.

[0071] Furthermore, according to Tables 1 to 3, in the polarization-maintaining fiber 1, preferably, 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 Δ between the core 11 and the cladding 13 is... 11 It is between 0.33% and 0.40%.

[0072] In addition, in polarization-maintaining fiber 1, it is preferred that the mode field diameter at a wavelength of 1.55 μm is less than 10.6 μm, and the cutoff wavelength is greater than 1.26 μm and less than 1.41 μm.

[0073] In addition, in polarization-maintaining fiber 1, it is preferred that the mode field diameter at a wavelength of 1.55 μm is greater than 9.2 μm and less than 10.6 μm, and the cutoff wavelength is greater than 1.26 μm and less than 1.55 μm.

[0074] In addition, the polarization-maintaining fiber 1 preferably satisfies any one of the following (A) to (C).

[0075] (A) The mode field diameter at a wavelength of 1.55 μm is less than 9.6 μm, and the cutoff wavelength is greater than 1.24 μm and less than 1.55 μm.

[0076] (B) The mode field diameter at a wavelength of 1.55 μm is greater than 9.6 μm and less than 10.5 μm, and the cutoff wavelength is greater than 1.33 μm and less than 1.55 μm.

[0077] (C) The mode field diameter at a wavelength of 1.55 μm is greater than 10.5 μm and less than 10.6 μm, and the cutoff wavelength is greater than 1.36 μm and less than 1.55 μm.

[0078] In cases with a large mode field diameter, to reduce the macrobending loss, a longer cutoff wavelength can be considered. Therefore, by satisfying at least one of the conditions (A) to (C) above, at least one of conditions (1) and (2) of condition 3 can be satisfied more reliably. For example, according to Tables 1 to 3, in the case of a mode field diameter of 10.6 μm, the range of cutoff wavelengths that satisfy at least one of conditions (1) and (2) of condition 3 is 1.36 μm or more and less than 1.55 μm. However, in the case of a mode field diameter less than 10.6 μm, the light blocking effect can be improved compared to the case of a mode field diameter of 10.6 μm. Therefore, if the range of cutoff wavelengths that satisfy at least one of conditions (1) and (2) of condition 3 is satisfied in the case of a mode field diameter of 10.6 μm, then even in the case of a mode field diameter less than 10.6 μm, at least one of conditions (1) and (2) of condition 3 can necessarily be satisfied within that cutoff wavelength range. Furthermore, if the cutoff wavelength range is greater than 1.24 μm, then even if the cutoff wavelength is less than 1.36 μm, at least one of conditions (1) and (2) of condition 3 can be satisfied.

[0079] In addition, polarization-maintaining fiber 1 preferably satisfies the following (3) instead of (1) and (2) of <condition 3>.

[0080] (3) With a bending radius of 7.5 mm and a twist of 1 turn for every 47.1 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.27 dB / 1 turn.

[0081] Furthermore, when condition (3) is satisfied, the mode field diameter is preferably 10.3 μm or less. Additionally, the mode field diameter is preferably greater than 9.6 μm. Therefore, a mode field diameter greater than 9.6 μm and less than 10.3 μm is more preferred. Furthermore, when condition (3) is satisfied, the cutoff wavelength is preferably 1.36 μm or more and less than 1.55 μm.

[0082] There exists an ITU-T standard, G657.A2, for low-bending-loss single-mode optical fiber with stringent macro-bending loss requirements, which is necessary to support access networks, general transmission networks, etc. This standard specifies a macro-bending loss of 0.5 dB when the fiber is wound once around a mandrel at a wavelength of 1.55 μm and a bending radius of 7.5 mm. By satisfying the above-mentioned (3) read from Tables 1 to 3, even with the polarization-maintaining fiber twisted, values ​​below those specified in this standard can be obtained.

[0083] In addition, polarization-maintaining fiber 1 preferably satisfies the following (4) instead of (1) and (2) of <condition 3>.

[0084] (4) With a bending radius of 10 mm and a twist of 1 turn for every 62.8 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.07 dB / 1 turn.

[0085] Furthermore, when condition (4) is satisfied, the mode field diameter is preferably 9.6 μm or less. Also, when condition (4) is satisfied, the cutoff wavelength is preferably 1.24 μm or more and less than 1.55 μm.

[0086] The aforementioned ITU-T standard G657.A2 specifies a macrobending loss of 0.1 dB when the wavelength is 1.55 μm, the bending radius is 10 mm, and the fiber is wound once on a mandrel. By satisfying (4) above, as read from Tables 1 to 3, a value below this standard can be obtained even when the polarization-maintaining fiber is twisted. Furthermore, ITU-T standard G657.B3, which specifies more stringent macrobending loss conditions than ITU-T standard G657.A2, specifies a macrobending loss of 0.08 dB when the wavelength is 1.55 μm, the bending radius is 10 mm, and the fiber is wound once on a mandrel. By satisfying (4) above, a value below this standard can be obtained even when the polarization-maintaining fiber is twisted.

[0087] In addition, by satisfying at least one of conditions (3) and (4) of polarization-maintaining fiber, sufficiently small macrobending loss can be achieved to support access networks, general transmission networks, etc.

[0088] The following are some of the embodiments of the present invention described above.

[0089] The first embodiment of the present invention is a polarization-maintaining optical fiber, characterized in that it comprises: a fiber core 11; a pair of stress-applying portions 12a and 12b disposed at a position for clamping the fiber core 11; and a cladding 13 enclosing the fiber core 11 and the pair of stress-applying portions 12a and 12b. The polarization-maintaining optical fiber has a mode field diameter of 10.6 μm or less at a wavelength of 1.55 μm, and when the fiber length is 2 m and the bending radius is 140 mm, the cutoff wavelength of the polarization-maintaining optical fiber is 1.24 μm or more and less than 1.55 μm, satisfying at least one of the following (1) and (2).

[0090] (1) With a bending radius of 7.5 mm and a twist of 1 turn for every 47.1 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.67 dB / 1 turn.

[0091] (2) With a bending radius of 10 mm and a twist of 1 turn for every 62.8 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.16 dB / 1 turn.

[0092] For example, in polarization-maintaining fiber 1, the mode field diameter at a wavelength of 1.55 μm is less than 10.6 μm, which increases the light containment force compared to cases where the mode field diameter at the same wavelength is greater than 10.6 μm. Furthermore, with an fiber length of 2 m and a bending radius of 140 mm, the cutoff wavelength of this polarization-maintaining fiber is 1.24 μm or greater, which increases the light containment force compared to cases where the cutoff wavelength at the same wavelength is less than 1.24 μm. Therefore, light loss propagating in the fiber core 11 can be suppressed. Moreover, with a cutoff wavelength less than 1.55 μm, single-mode optical transmission can be achieved when using wavelengths of 1.55 μm or greater. Therefore, it is particularly suitable for applications such as coherent optical transceivers using wavelengths around 1.55 μm, and optical devices using optical fibers and optical devices with wavelengths of 1.55 μm or greater. Therefore, compared with polarization-maintaining fibers that do not satisfy the above structure, the polarization-maintaining fiber 1 of this embodiment can suppress the reduction of macro-bending loss characteristics to the extent that the user can apply the polarization-maintaining fiber to the optical device, even when it is housed in various optical devices or the like.

[0093] Furthermore, the cutoff wavelength is the wavelength value measured when the fiber length is 2m. However, if the method described in the following literature, which calculates the loss per unit length of each wavelength of the LP11 mode based on the refractive index structure, is used, the cutoff wavelength for any length can be calculated even if the fiber length is less than 2m. Therefore, since the polarization-maintaining fiber 1 of this embodiment can measure the cutoff wavelength, it is valid even if the fiber length is less than 2m.

[0094] 16th Opto-Electronics and Communications Conference: “LP11 modeattenuation behavior of optical fibers with trench-cladding” In addition, the second embodiment of this invention is a polarization-maintaining fiber of the first embodiment, characterized in that the relative refractive index difference between the fiber core and the cladding is 0.32% or more.

[0095] In addition, the third embodiment of the present invention is a polarization-maintaining fiber of embodiment 1 or 2, characterized in that the relative refractive index difference between the fiber core and the cladding is 0.33% or more.

[0096] In addition, embodiment 4 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 3, characterized in that the relative refractive index difference between the core and the cladding is less than 0.40%.

[0097] In addition, embodiment 5 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 4, characterized in that the distance from the central axis of the fiber core to the stress application part is 10 μm or less.

[0098] Furthermore, embodiment 6 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 5, characterized in that the relative refractive index difference between the stress-applying portion and the cladding is -0.78% or less.

[0099] In addition, embodiment 7 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 6, characterized in that the mode field diameter is 10.3 μm or less.

[0100] In addition, embodiment 8 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 7, characterized in that the mode field diameter is 9.6 μm or less.

[0101] In addition, embodiment 9 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 8, characterized in that the mode field diameter is 8.9 μm or more.

[0102] In addition, embodiment 10 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 9, characterized in that the cutoff wavelength is 1.24 μm or more and 1.39 μm or less.

[0103] In addition, embodiment 11 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 10, characterized in that the cutoff wavelength is greater than 1.39 μm and less than 1.55 μm.

[0104] In addition, embodiment 12 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 11, characterized in that the cutoff wavelength is 1.36 μm or more and less than 1.55 μm.

[0105] In addition, embodiment 13 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 12, characterized in that the cutoff wavelength is 1.36 μm or more and 1.39 μm or less.

[0106] Furthermore, embodiment 14 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 13, 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 between the fiber core and the cladding is 0.32% or more and 0.34% or less.

[0107] Furthermore, embodiment 15 of the present invention is a polarization-maintaining fiber according to any one of embodiments 1 to 14, 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 between the core and the cladding is 0.33% or more and 0.40% or less.

[0108] In addition, embodiment 16 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 15, characterized in that the mode field diameter is less than 10.6 μm and the cutoff wavelength is greater than or equal to 1.26 μm and less than 1.41 μm.

[0109] In addition, embodiment 17 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 16, characterized in that the mode field diameter is greater than 9.2 μm and less than 10.6 μm, and the cutoff wavelength is greater than 1.26 μm and less than 1.55 μm.

[0110] In addition, the present invention embodiment 18 is a polarization-maintaining optical fiber of any one of embodiments 1 to 17, characterized in that it satisfies any one of the following (A) to (C).

[0111] (A) The mode field diameter is less than 9.6 μm, and the cutoff wavelength is greater than 1.24 μm and less than 1.55 μm.

[0112] (B) The mode field diameter is greater than 9.6 μm and less than 10.5 μm, and the cutoff wavelength is greater than 1.33 μm and less than 1.55 μm.

[0113] (C) The mode field diameter is greater than 10.5 μm and less than 10.6 μm, and the cutoff wavelength is greater than 1.36 μm and less than 1.55 μm.

[0114] In addition, the present invention embodiment 19 is a polarization-maintaining fiber of any one of embodiments 1 to 18, characterized in that it satisfies the following (3).

[0115] (3) With a bending radius of 7.5 mm and a twist of 1 turn for every 47.1 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.27 dB / turn.

[0116] In addition, the present invention, embodiment 20, is a polarization-maintaining fiber of any one of embodiments 1 to 19, characterized in that it satisfies the following (4).

[0117] (4) With a bending radius of 10 mm and a twist of 1 turn for every 62.8 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.07 dB / turn.

[0118] In addition, embodiment 21 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 20, characterized in that the mode field diameter is less than 10 μm.

[0119] In addition, the present invention embodiment 22 is a polarization-maintaining fiber of any one of embodiments 1 to 21, characterized in that the mode field diameter is greater than 9.6 μm.

[0120] In addition, embodiment 23 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 22, characterized in that the length of the optical fiber is 47.1 mm or more.

[0121] In addition, embodiment 24 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 23, characterized in that the length of the optical fiber is less than 50m.

[0122] In addition, embodiment 25 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 24, characterized in that the length of the optical fiber is less than 10m.

[0123] In addition, embodiment 26 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 25, characterized in that the length of the optical fiber is less than 300 mm.

[0124] In addition, embodiment 27 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 26, characterized in that the refractive index distribution of the fiber core relative to the cladding is stepped or GI-type.

[0125] Furthermore, embodiment 28 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 27, characterized in that the diameter of the stress application portion is 24.8 μm or more.

[0126] Furthermore, embodiment 29 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 28, characterized in that the diameter of the stress-applying portion is 38.0 μm or less.

[0127] Furthermore, embodiment 30 of the present invention is a polarization-maintaining optical fiber according to any one of embodiments 1 to 29, characterized in that the diameter of the stress application portion is 34.4 μm or less.

[0128] In addition, embodiment 31 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 30, characterized in that the radius of the fiber core is 4.4 μm or less.

[0129] In addition, embodiment 32 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 31, characterized in that the radius of the fiber core is 4.1 μm or more.

[0130] Furthermore, embodiment 33 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 32, characterized in that the relative refractive index difference between the stress-applying portion and the cladding is -1.0% or more.

[0131] Furthermore, embodiment 34 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 33, characterized in that the ratio of the distance from the central axis of the fiber core to the stress application portion to half the diameter (radius) of the mode field is 1.05 μm or more.

[0132] In addition, embodiment 35 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 34, characterized in that the diameter of the cladding is 79 μm or more and 126 μm or less.

[0133] In addition, embodiment 36 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 35, characterized in that the diameter of the cladding is 124 μm or more and 126 μm or less.

[0134] In addition, embodiment 37 of the present invention is a polarization-maintaining fiber of any one of embodiments 1 to 36, characterized in that the diameter of the cladding is 79 μm or more and 81 μm or less.

[0135] In addition, embodiment 38 of the present invention is a polarization-maintaining optical fiber of any one of embodiments 1 to 37, characterized in that the center of the circle constituting the outer periphery of the fiber core is contained in the center portion of the cladding, the center portion of the cladding is the inner region of a circle with a radius of 0.6 μm, and the center of the circle coincides with the center of the circle constituting the outer periphery of the cladding.

[0136] As described above, according to the present invention, a polarization-maintaining fiber can be provided that can suppress the reduction of macrobending loss characteristics even when twisting is applied to the polarization-maintaining fiber. It is expected to be used as a polarization-maintaining fiber for communication in the field of optical communication, polarization-maintaining fiber for amplifiers in the field of amplifiers, and polarization-maintaining fiber for sensors in the field of measurement.

Claims

1. A polarization-maintaining optical fiber, characterized in that, have: Fiber core; A pair of stress-applying portions are positioned to clamp the fiber core; The cladding, which encloses the fiber core and the pair of stress-applying portions, This polarization-maintaining fiber has a mode field diameter of less than 10.6 μm at a wavelength of 1.55 μm. With an optical fiber length of 2m and a bending radius of 140mm, the cutoff wavelength of this polarization-maintaining fiber is above 1.24μm and below 1.55μm. If at least one of the following (1) and (2) is satisfied, (1) With a bending radius of 7.5 mm and a twist of 1 turn every 47.1 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.67 dB / 1 turn. (2) With a bending radius of 10 mm and a twist of 1 turn for every 62.8 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.16 dB / 1 turn.

2. The polarization-maintaining optical fiber according to claim 1, characterized in that, The relative refractive index difference between the fiber core and the cladding is greater than 0.32%.

3. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The relative refractive index difference between the fiber core and the cladding is greater than 0.33%.

4. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The relative refractive index difference between the fiber core and the cladding is less than 0.40%.

5. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The distance from the central axis of the fiber core to the stress application point is less than 10 μm.

6. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The relative refractive index difference between the stress-applying portion and the cladding is less than -0.78%.

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

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

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

10. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The cutoff wavelength is above 1.24 μm and below 1.39 μm.

11. The polarization-maintaining optical 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.

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

13. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The cutoff wavelength is above 1.36 μm and below 1.39 μm.

14. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The diameter of the cladding is greater than 124 μm and less than 126 μm. The diameter of the mode field is greater than 10.2 μm and less than 10.6 μm. The cutoff wavelength is above 1.33 μm and below 1.39 μm. The relative refractive index difference between the core and the cladding is greater than 0.32% and less than 0.34%.

15. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The diameter of the cladding is greater than 79 μm and less than 81 μm. The diameter of the mode field is greater than 8.9 μm and less than 9.6 μm. The cutoff wavelength is above 1.24 μm and below 1.39 μm. The relative refractive index difference between the core and the cladding is greater than 0.33% and less than 0.40%.

16. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The diameter of the mode field is less than 10.6 μm. The cutoff wavelength is greater than 1.26 μm and less than 1.41 μm.

17. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, The diameter of the mode field is greater than 9.2 μm and less than 10.6 μm. The cutoff wavelength is greater than 1.26 μm and less than 1.55 μm.

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

19. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, Satisfy the following (3) (3) With a bending radius of 7.5 mm and a twist of 1 turn for every 47.1 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.27 dB / 1 turn.

20. The polarization-maintaining optical fiber according to claim 1 or 2, characterized in that, If the following condition (4) is met, then... (4) With a bending radius of 10 mm and a twist of 1 turn for every 62.8 mm of fiber length, the macrobending loss of the polarization-maintaining fiber at a wavelength of 1.55 μm is less than 0.07 dB / 1 turn.

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