WO2018174206A1 - Polarization maintaining fiber, optical device, preform of polarization maintaining fiber, and manufacturing method - Google Patents

Abstract

The present invention implements a polarization maintaining fiber that has a core with a flattened cross section and can be easily manufactured. A polarization maintaining fiber (1) is provided with a core (11), an inner clad (12), two stress application parts (13a-13b), and an outer clad (14). The inner clad (12) is fitted into each of the two stress application parts (13a-13b). The cross section of the core (11) is flattened such that the arrangement direction of the two stress application parts (13a-13b) is used as the major axis thereof.

Description

Polarization maintaining fiber, optical device, base material of polarization maintaining fiber, and manufacturing method

The present invention relates to a polarization maintaining fiber having a flat core. The present invention also relates to an optical device having such polarization maintaining, a base material for such a polarization maintaining fiber, and a method for manufacturing such a polarization maintaining fiber.

In the field of silicon photonics, a polarization maintaining fiber is widely used as a transmission medium for transmitting light input to the silicon waveguide or light output from the silicon waveguide. The polarization maintaining fiber refers to an optical fiber having improved polarization maintaining performance by suppressing coupling between polarization modes. For example, a PANDA (Polarization-maintaining AND Absorption-reducing) fiber in which a stress applying portion for applying stress to the core is provided in the clad is a typical example of a polarization maintaining fiber.

In a polarization maintaining fiber having two stress applying portions, it is preferable that the core has a flat cross section (not an exact circle or a square, but an ellipse or a rectangle) depending on the application. The reason is that, firstly, the polarization maintaining performance can be enhanced by the flat cross section of the core. Second, because the core has a flat cross section, the mode field of the polarization maintaining fiber can be made elliptical. The mode feel of a silicon waveguide is usually elliptical. Accordingly, the polarization maintaining fiber having the elliptical mode field can suppress the connection loss with the silicon waveguide to be smaller than that of the polarization maintaining fiber having the true circular mode field.

Note that the polarization maintaining performance can be enhanced by flattening the core cross section because the longitudinal direction of the core cross section and the direction in which the two stress applying sections are aligned are parallel, and the birefringence due to the stress applying section. And birefringence due to the flattening of the core mutually intensify. When the longitudinal direction of the cross section of the core and the arrangement direction of the two stress applying portions are perpendicular to each other, the birefringence due to the stress applying portion and the birefringence due to the flattening of the core weaken each other. The effect of increasing cannot be obtained.

As a method for manufacturing an optical fiber having a flat core cross section, for example, a manufacturing method described in Patent Document 1 is known. According to Patent Document 1, an optical fiber having an elliptical cross-sectional shape of the core can be manufactured by performing the following steps. Step 1: A first preform is formed by forming a primary cladding portion on the entire outer periphery of a core portion having a true circular cross-sectional shape. Step 2: A second preform is created by cutting off a part of the side of the primary cladding portion of the first preform along the longitudinal direction of the first preform. Step 3: A third preform is created by forming a soot of the secondary cladding part on the entire outer periphery of the primary cladding part of the second preform. Step 4: The third preform (soot rod) is heated by sintering in a dehydrated atmosphere to form a fourth preform. At this time, volume contraction accompanying the disappearance of the hole portion occurs, and the cross-sectional shape of the core portion changes from a perfect circle to an ellipse. Step 5: A fifth preform is created by cutting the fourth preform so that the cross-sectional shape becomes a perfect circle. Step 6: An optical fiber having a core with an elliptical cross-sectional shape is obtained by drawing the fifth preform.

Japanese Patent Publication “JP 2002-365463” (published on December 18, 2002)

However, the manufacturing method described in Patent Document 1 has the following problems.

That is, in the manufacturing method described in Patent Document 1, two external cutting processes are required before the preform is completed. In particular, in order to make the cross-sectional shape of the core into an elliptical shape having a sufficient non-circularity, the external cutting with respect to the first preform is performed by reducing the radius of the externally cut portion to 1 / of the radius of the non-external portion. It is necessary to carry out until it becomes about 2 (refer FIG. 2 of patent document 1 and paragraph 0024). For this reason, there is a problem that the time required for the external cutting is long and cannot be easily manufactured.

The present invention has been made in view of the above problems, and its main purpose is a polarization maintaining fiber having two stress applying portions and a flat core cross section, which can be easily manufactured. The object is to realize a polarization maintaining fiber. Another object of the present invention is to realize an optical device including such a polarization maintaining fiber, a base material of such a polarization maintaining fiber, or a manufacturing method of such a polarization maintaining fiber.

In order to achieve the above object, a polarization maintaining fiber according to an aspect of the present invention includes a core, an inner cladding that encloses the core, two stress applying portions that sandwich the inner cladding from both sides, and the inner surface. A clad and an outer clad that encloses the two stress applying portions, and the inner clad is indented into each of the two stress applying portions, and the cross section of the core has the two stress applying portions. The arrangement direction is flattened so that the arrangement direction is the longitudinal direction.

In order to achieve the above object, a base material of a polarization maintaining fiber according to an aspect of the present invention includes a core, an inner clad that encloses the core, and two stress applying portions that sandwich the inner clad from both sides. An inner cladding and an outer cladding that encloses the two stress applying portions, and each of the two stress applying portions is indented into the inner cladding.

In order to achieve the above object, a method of manufacturing a polarization maintaining fiber according to an aspect of the present invention includes a core, an inner clad that encloses the core, and two stress applying portions that sandwich the inner clad from both sides. A polarization maintaining fiber manufacturing method including a step of drawing a base material including the inner clad and the outer clad including the two stress applying portions, wherein the two stress applying portions are provided in the base material. Are intruded into the inner cladding, and in the polarization maintaining fiber, the inner cladding is recessed into each of the two stress applying portions, and the cross section of the core is the two stress applying portions. The arrangement direction is flattened so that the arrangement direction is the longitudinal direction.

According to one aspect of the present invention, a polarization maintaining fiber that has a flat core cross section and can be easily manufactured can be realized. In addition, according to one aspect of the present invention, an optical device including such a polarization maintaining fiber, a base material for such a polarization maintaining fiber, or a method for manufacturing such a polarization maintaining fiber is realized. be able to.

It is a figure which shows the structure of the polarization maintaining fiber which concerns on one Embodiment of this invention. (A) is sectional drawing which shows the cross section of the polarization maintaining fiber. (B) is a graph which shows the refractive index distribution in the AA 'straight line of the cross section shown to (a). (C) is a graph which shows the refractive index distribution in the BB 'straight line of the cross section shown to (a). It is a figure which shows the manufacturing method of the polarization maintaining fiber shown in FIG. It is a cross-sectional photograph of the polarization maintaining fiber manufactured by the manufacturing method shown in FIG. (A) is a figure which shows the definition of the origin in the base material which concerns on a comparative example. (B) is a figure which shows the definition of the origin in the polarization maintaining fiber which concerns on a comparative example. (A) is a figure which shows the definition of the origin in the base material which concerns on an Example. (B) is a figure which shows the definition of the origin in the polarization maintaining fiber which concerns on an Example. It is a side view of the optical device which can apply the polarization maintaining fiber shown in FIG. (A) is a front view of the board | substrate type | mold optical waveguide with which the optical device shown in FIG. 6 is provided. (B) is a front view of the 1st optical fiber with which the optical device shown in FIG. 5 is provided. (A) is a graph which shows the mode field pattern of the board | substrate type | mold optical waveguide with which the optical device shown in FIG. 6 is provided. (B) is a graph which shows the mode field pattern of the 1st optical fiber with which the optical device shown in FIG. 6 is provided. It is a side view of the polarization maintaining fiber at the time of drawing.

(Polarization-maintaining fiber structure)
The structure of the polarization maintaining fiber 1 according to one embodiment of the present invention will be described with reference to FIG. FIG. 1A is a sectional view showing a transverse section of the polarization maintaining fiber 1. FIG. 1B is a graph showing the refractive index distribution of the polarization maintaining fiber 1 along the line AA ′ in the cross section shown in FIG. FIG. 1C is a graph showing the refractive index distribution of the polarization maintaining fiber 1 along the BB ′ straight line in the cross section shown in FIG.

As shown in FIG. 1A, the polarization maintaining fiber 1 includes a core 11, an inner cladding 12 that encloses the core 11, two stress applying portions 13a to 13b that sandwich the inner cladding 12 from both sides, And an outer clad 14 including the clad 12 and the two stress applying portions 13a to 13b.

The inner cladding 12 is recessed into each of the two stress applying portions 13a to 13b. For this reason, the cross-sectional shape of the inner cladding 12 is circular, whereas the cross-sectional shape of the stress applying portion 13a located on the left side of the inner cladding 12 is a circular shape with a chip on the right side. The cross-sectional shape of the stress applying portion 13b located on the right side is a circular shape with a chip on the left side. In addition, the cross-sectional shape of the core 11 is a flat shape (in this embodiment, a flat circular shape, that is, an elliptical shape) such that the direction in which the two stress applying portions 13a to 13b are aligned is the long axis direction. . Here, the arrangement direction of the two stress applying portions 13a to 13b refers to a direction parallel to a straight line passing through the center of the first stress applying portion 13a and the center of the second stress applying portion 13b.

The core 11 is made of quartz glass to which germanium (Ge) is added. Germanium added to the core 11 has an action of increasing the refractive index of quartz glass. For this reason, the refractive index n1 of the core 11 is higher than the refractive index n0 (about 1.46) of pure quartz glass. The melting viscosity η1 of the core 11 is substantially the same as the melting viscosity η0 of pure quartz glass, or a value slightly smaller than the melting viscosity η0 of pure quartz glass.

In addition, in this embodiment, although the structure which adds germanium to the core 11 as an updopant is employ | adopted, this invention is not limited to this. That is, a configuration in which one or both of phosphorus and aluminum in addition to germanium is added to the core 11 as an updopant may be employed. Or you may employ | adopt the structure which adds germanium oxide, chlorine, etc. to the core 11 as an updopant. Regardless of which updopant is used, the refractive index n1 of the core 11 with respect to the refractive index n4 (substantially the same as the refractive index of pure quartz glass) of the outer cladding 14 to be described later can be adjusted by appropriately adjusting its concentration. The relative refractive index difference can be 1.0% or more. When adopting a configuration in which germanium is added to the core 11, the concentration of germanium in the core 11 can be set to, for example, 10 to 30 wt%.

The inner cladding 12 is made of quartz glass co-doped with phosphorus (P) and fluorine (F). Phosphorus added to the inner cladding 12 has an action of increasing the refractive index of the quartz glass and an action of lowering the viscosity at the time of melting of the quartz glass. On the other hand, the fluorine added to the inner clad 12 promotes the diffusion of germanium added to the core 11 into the inner clad 12 during heating for fusion, and lowers the refractive index of quartz glass. And have. The concentrations of phosphorus and fluorine added to the inner cladding 12 are adjusted so that the refractive index increasing action of phosphorus and the refractive index decreasing action of fluorine cancel each other. For this reason, the refractive index n2 of the inner cladding 12 is substantially the same as that of pure quartz glass. The melting viscosity η2 of the inner cladding 12 is lower than the melting viscosity η0 of pure quartz glass.

In addition, in this embodiment, although the structure which adds phosphorus to the inner side cladding 12 as an updopant is employ | adopted, this invention is not limited to this. That is, a configuration in which germanium (Ge) is added as an updopant to the inner cladding 12 instead of phosphorus may be adopted, or germanium may be added as an updopant to the inner cladding 12 in addition to phosphorus. Even when the fluorine concentration in the inner cladding 12 is high, by adding both phosphorus and germanium to the inner cladding 12, the refractive index n4 of the outer cladding 14 described later (substantially the refractive index of pure quartz glass). The relative refractive index difference of the refractive index n2 of the inner cladding 12 with respect to the same) can be made 0.1% or less. When adopting a configuration in which phosphorus, germanium, and fluorine are added to the inner cladding 12, the concentrations of phosphorus and germanium in the inner cladding 12 are, for example, 0.5 to 2.0 wt% and 1.5 to 5 respectively. 0.0 wt%. The fluorine concentration in the inner cladding 12 may be set so that the relative refractive index difference of the refractive index n2 of the inner cladding 12 with respect to the refractive index n4 of the outer cladding 14 is 0.1% or less.

The two stress applying portions 13a to 13b are each composed of quartz glass to which boron (B) is added. Boron added to the stress applying portions 13a to 13b has an action of lowering the refractive index of the quartz glass and an action of lowering the viscosity at the time of melting of the quartz glass. Therefore, the refractive index n3 of the stress applying portions 13a to 13b is lower than the refractive index n0 of pure quartz glass. Further, the melting viscosity η3 of the stress applying portions 13a to 13b is lower than the melting viscosity η0 of pure quartz glass. Instead of the configuration in which boron (B) is added to the two stress applying portions 13a to 13b, a configuration in which boron oxide (B ₂ O ₃ ) is added to the two stress applying portions 13a to 13b may be employed. . In this case, the concentration of boron oxide in the two stress applying portions 13a to 13b may be set to 15 to 25 mol%, for example.

The outer cladding 14 is made of quartz glass to which a dopant other than chlorine (Cl) is not intentionally added. In other words, the quartz glass constituting the outer cladding 14 is not added with an updopant other than chlorine having a refractive index increasing action and a downdopant having a refractive index lowering action. Here, the chlorine concentration in the outer cladding 14 may be set so that the relative refractive index difference of the refractive index n2 of the inner cladding 12 relative to the refractive index n4 of the outer cladding 14 is 0.1% or less. For this reason, the refractive index n4 of the outer cladding 14 is substantially the same as the refractive index n0 of pure quartz glass. The melting viscosity η4 of the outer cladding 14 is substantially the same as the melting viscosity η0 of pure quartz glass.

As described above, the relationship of n3 <n2≈n4 <n1 is established among the refractive indexes n1, n2, n3, and n4 of the core 11, the inner cladding 12, the stress applying portions 13a to 13b, and the outer cladding 14. . The polarization maintaining fiber 1 has an optical confinement function due to this relationship (particularly n2 <n1).

In addition, a relationship of η3 << η2 << η1 <η4 is established among the melt viscosity η1, η2, η3, and η4 of the core 11, the inner cladding 12, the stress applying portions 13a to 13b, and the outer cladding 14. Here, the relationship of η3 << η2 is established because boron added to the stress applying portions 13a to 13b has a stronger viscosity reducing action than phosphorus added to the inner cladding 12. This is the reason why the polarization maintaining fiber 1 has the polarization maintaining capability (particularly, η3 << η2 << η1) (for the reason, refer to “Method of manufacturing polarization maintaining fiber”). Here, the respective dopant concentrations in the core 11, the inner cladding 12, the stress applying portions 13a to 13b, and the outer cladding 14 may be set so as to satisfy η3 << η2 << η1 <η4. For example, a configuration in which germanium is added to the core 11, a configuration in which phosphorus, germanium, and fluorine are added to the inner cladding 12 is adopted, and boron oxide (B ₂ O ₃ ) is added to the two stress applying portions 13 a to 13 b. When adopting a structure to be added and adopting quartz glass in which a dopant other than chlorine (Cl) is not intentionally added to the outer cladding 14, the following dopant concentrations satisfy η3 << η2 << η1 <η4. It suffices if it is set in this way. That is, the concentration of germanium in the core 11 is set to, for example, 10 to 30 wt%, and the concentrations of phosphorus and germanium in the inner cladding 12 are set to, for example, 0.5 to 2.0 wt% and 1.5 to 5. The concentration of boron oxide in the inner cladding 12 is set to 0 wt% so that the relative refractive index difference of the refractive index n2 of the inner cladding 12 with respect to the refractive index n4 of the outer cladding 14 is 0.1% or less. The concentration of boron oxide in the two stress applying portions 13a to 13b is set to 15 to 25 mol%, for example, and the concentration of chlorine in the outer cladding 14 is set to the ratio of the refractive index n2 of the inner cladding 12 to the refractive index n4 of the outer cladding 14. The refractive index difference may be set to be 0.1% or less, and may be set to satisfy η3 << η2 << η1 <η4.

(Polarization-maintaining fiber manufacturing method)
Next, a manufacturing method of the polarization maintaining fiber 1 shown in FIG. 1 will be described with reference to FIG. FIG. 2 is a diagram illustrating a method for manufacturing the polarization maintaining fiber 1.

First, the base material 1A is prepared. In the base material 1A, for example, two holes are formed in the base material including the core 11, the inner cladding 12, and the outer cladding 14 using a drill tool or the like, and the base materials of the stress applying portions 13a to 13b are formed in the holes. It can manufacture by inserting the rod which becomes. The base material 1 </ b> A also has the same cross-sectional structure as the polarization maintaining fiber 1. However, in the polarization maintaining fiber 1, the inner clad 12 is indented into the stress applying portions 13a to 13b (the inner clad 12 is convex and the stress applying portions 13a to 13b are concave). In 1A, the stress applying portions 13a to 13b (or holes into which the stress applying portions 13a to 13b are inserted) are recessed into the inner cladding 12 (the inner cladding 12 is concave and the stress applying portions 13a to 13b are inserted). Is convex). In the polarization maintaining fiber 1, the cross-sectional shape of the core 11 is elliptical (flattened circular shape), whereas in the base material 1A, the cross-sectional shape of the core 11 is circular.

Next, the polarization maintaining fiber 1B is obtained by melting and stretching the base material 1A. In the polarization maintaining fiber 1B, the core 11, the inner cladding 12, the stress applying portions 13a to 13b, and the outer cladding 14 are all melted.

Next, the polarization maintaining fiber 1B is cooled to obtain the polarization maintaining fiber 1C. In the polarization maintaining fiber 1C, the outer cladding 14 is in a solidified state, and the core 11, the inner cladding 12, and the stress applying portions 13a to 13b are in a molten state. Thus, the outer cladding 14 solidifies before the core 11, the inner cladding 12, and the stress applying portions 13a to 13b because the viscosity η4 of the outer cladding 14 is equal to the core 11, the inner cladding 12, and the stress applying. This is because the viscosities of the parts 13a to 13b are higher than η1, η2, and η3.

Next, the polarization maintaining fiber 1D is obtained by cooling the polarization maintaining fiber 1C. In the polarization maintaining fiber 1D, the core 11, the inner cladding 12, and the outer cladding 14 are in a solidified state, and the stress applying portions 13a to 13b are in a molten state. As described above, the main reason why the core 11, the inner cladding 12, and the outer cladding 14 solidify before the stress applying portions 13a to 13b is that the viscosity η1, η2, and the viscosity of the core 11, the inner cladding 12, and the outer cladding 14, And η4 is higher than the viscosity η3 of the stress applying portions 13a to 13b.

When the core 11 and the inner cladding 12 are solidified, the stress applying portions 13a to 13b are in a molten state. For this reason, the inner cladding 12 is deformed so as to have a circular cross-sectional shape due to surface tension. At this time, the core 11 is deformed so that the cross-sectional shape becomes elliptical due to the stress received from the inner cladding 12.

Finally, the polarization maintaining fiber 1 is obtained by cooling the polarization maintaining fiber 1D. In the polarization maintaining fiber 1, the core 11, the inner cladding 12, the stress applying portions 13a to 13b, and the outer cladding 14 are all solidified. In the polarization maintaining fiber 1, stress acts on the inner clad 12 and the core 11 that are solidified first from the stress applying portions 13 a to 13 b that are solidified later. Due to this stress, the polarization maintaining fiber 1 exhibits a polarization maintaining function. Here, the polarization maintaining fiber 1B described above can be rephrased as a molten base material 1A, and the polarization maintaining fibers 1C and 1D described above can be rephrased as a base material 1A cooled after melting.

FIG. 3 shows a cross-sectional photograph of the polarization maintaining fiber 1 manufactured according to the above manufacturing method. According to this cross-sectional photograph, it is confirmed that the cross-sectional shape of the core 11 is elliptical.

Here, the manufacturing method has been described in which the cross-sectional shape of the core 11 in the base material 1A is circular, so that the cross-sectional shape of the core 11 in the polarization maintaining fiber 1 is elliptical (flattened circular shape). However, the present invention is not limited to this. For example, by setting the cross-sectional shape of the core 11 in the base material 1A to be a square shape, the cross-sectional shape of the core 11 in the polarization maintaining fiber 1 can be a rectangular shape (flat square shape). More generally speaking, the cross-sectional shape of the core 11 in the polarization maintaining fiber 1 manufactured according to the above manufacturing method is a flattened shape of the cross-sectional shape of the core 11 in the base material 1A.

(Examples and Comparative Examples)
A base material in which the core diameter, inner clad diameter, outer clad diameter, stress applying part interval, barrier thickness, hole diameter, stress applying part diameter, and outer peripheral thickness were set as shown in Table 1 below was prepared. The base material having a negative barrier thickness is a base material in which each stress applying portion is recessed in the inner cladding, and is an example. The base material having a positive barrier thickness is a base material in which each stress applying portion is separated from the inner cladding, and is a comparative example. For the definition of the specifications in the base materials according to Comparative Examples 1 to 3, see (a) of FIG. For the definition of the specifications in the base materials according to Examples 1 to 3, see (a) of FIG. In the base materials according to Comparative Examples 1 to 3, as shown in FIG. 4 (a), L represents a half line extending in parallel with the direction of arrangement of the stress applying portions from the center of the base material, and the outer edge of the inner cladding. The distance from the intersection P between the circle constituting the circle and the half line L to the intersection Q between the circle constituting the outer edge of the hole into which the rod serving as the base material of the stress applying portion is inserted and the half line L is the absolute thickness of the barrier Value. On the other hand, in the base materials according to Examples 1 to 3, as shown in FIG. 5 (a), L is a half line extending in parallel with the arrangement direction of the stress applying portions from the center of the base material, and the outer edge of the inner cladding. The absolute value of the barrier thickness is the distance from the intersection point P of the circle that overlaps with the half line L to the intersection point Q of the circle and the half line L that forms the outer edge of the hole into which the rod serving as the base material of the stress applying portion is inserted It becomes.

A polarization maintaining fiber was manufactured from each base material according to the manufacturing method described above. Measure the core diameter (average value of core major axis diameter and core minor axis diameter), inner cladding diameter, outer cladding diameter, stress applying part interval, stress applying part diameter, and barrier thickness of each polarization maintaining fiber. The measurement results shown in Table 1 below were obtained. As for the barrier thickness of the polarization maintaining fibers according to Examples 1 to 3, estimated values estimated from the barrier thickness of the base material are shown in Table 1 below. The core non-circularity is measured by measuring the core major axis diameter and the core minor axis diameter, and the core non-circularity = {(core major axis diameter−core minor axis diameter) / (core major axis diameter and core minor axis diameter Table 1 below shows the values calculated according to For the definition of the specifications in the polarization maintaining fibers according to Comparative Examples 1 to 3, see FIG. 4B. For the definition of the specifications in the polarization maintaining fibers according to Examples 1 to 3, see (b) of FIG. In the polarization maintaining fibers according to Comparative Examples 1 to 3, as shown in FIG. 4B, a half line extending in parallel with the alignment direction of the stress applying portions from the center of the polarization maintaining fiber is denoted as L ′. The distance from the intersection point P ′ between the circle constituting the outer edge of the inner cladding and the half line L ′ to the intersection point Q ′ between the circle constituting the outer edge of the stress applying part and the half line L ′ is the absolute value of the barrier thickness. Become. On the other hand, in the polarization maintaining fibers according to Examples 1 to 3, as shown in FIG. 5B, a half line extending in parallel with the direction in which the stress applying portions are arranged from the center of the polarization maintaining fiber is denoted as L ′. The distance from the intersection point P ′ between the circle constituting the outer edge of the inner cladding and the half line L ′ to the intersection point Q ′ between the circle overlapping the outer edge of the stress applying portion and the half line L ′ is the absolute value of the barrier thickness. .

In Examples 1 to 3, the non-circularity ratio of the core could be 20% or more. In Examples 1 and 2, the non-circularity of the core could be 50% or more. In Example 2, the non-circularity ratio of the core could be 80% or more. That is, it was confirmed that the non-circularity of the core increases as the absolute value of the barrier thickness in the base material increases, that is, as each stress applying portion is deeply inserted into the inner cladding.

Furthermore, mode field patterns were measured for Examples 1 to 3 and Comparative Examples 1 to 3. Specifically, the mode field diameter measurement by the one-dimensional far field pattern method was repeated while rotating the polarization maintaining fiber by 30 °, thereby obtaining the dependency of the mode field diameter on the rotation direction. As a result, the measurement results of 4.0 ± 0.9 μm in Example 1, 4.0 ± 1.3 μm in Example 2, and 4.0 ± 0.4 μm in Example 3 were obtained. These results indicate that an elliptical electric field distribution is formed in Examples 1 to 3. On the other hand, in Comparative Examples 1 to 3, the variation of the mode field diameter (α of 4.0 ± α μm) was 0.3 μm or less. These results indicate that a substantially circular electric field distribution is formed in Comparative Examples 1 to 3.

In the base materials of Examples 1 to 3 and Comparative Examples 1 to 3, the only dopant added to the core was germanium. The germanium concentration in the core was 22 wt%. In these base materials, the dopants added to the inner cladding were phosphorus, germanium, and fluorine. The phosphorus concentration in the inner cladding was 0.8 wt%, and the germanium concentration in the inner cladding was 2.9 wt%. The fluorine concentration in the inner cladding was adjusted so that the relative refractive index difference between the inner cladding and the outer cladding was 0.0%. Moreover, in these base materials, the dopant added to the stress applying portion was boron oxide (B ₂ O ₃ ). The concentration of boron oxide (B ₂ O ₃ ) in the stress applying part was about 20 mol%. The polarization maintaining fiber 1 shown in FIG. 3 is obtained by drawing the base material of Example 1 to which such a dopant is added.

As shown in FIGS. 4A and 5A, in the base material of the polarization maintaining fiber, the side surface of the rod serving as the base material of the stress applying portion and the hole into which the rod is inserted are provided. There is a gap between the inner wall. This gap is eliminated when the rod melts to become low-viscosity glass during drawing, and the low-viscosity glass expands to fill the holes. For this reason, the position and size of the stress applying portion in the cross section of the polarization-maintaining fiber can be estimated from the position and size of the hole in the cross section of the base material.

(Application example)
The polarization maintaining fiber 1 can be suitably used for an optical device including a substrate type optical waveguide and an optical fiber. Such an optical device 2 will be described with reference to FIGS.

FIG. 6 is a side view of the optical device 2. As shown in FIG. 6, the optical device 2 includes a substrate type optical waveguide 21, a first optical fiber 22, and a second optical fiber 23. The substrate-type optical waveguide 21 is optically connected to the first optical fiber 22 by making its end face face one end face of the first optical fiber 22. The second optical fiber 23 is physically and optically connected to the first optical fiber 22 by fusing its end face to the other end face of the first optical fiber 22. A spatial optical system may be provided between the end face of the substrate type optical waveguide 21 and the end face of the polarization maintaining fiber 1 facing each other.

The substrate type optical waveguide 21 is, for example, a silicon waveguide having a silicon core 211. The core diameter of the substrate type optical waveguide 21 is smaller than the core diameter of the first optical fiber 22 described later. Therefore, the mode field diameter of the substrate type optical waveguide 21 is set to the mode of the first optical fiber 22 in the vicinity of the end surface of the core type 211 of the substrate type optical waveguide 21 facing the core 221 of the first optical fiber 22. A mode field diameter conversion unit 212 is provided to match the field diameter.

FIG. 7A is a front view showing an end face 21 a facing the first optical fiber 22 among the end faces of the substrate-type optical waveguide 21. The cross section (end face) shape of the core 211 of the substrate-type optical waveguide 21 is a rectangle having the longitudinal direction in the x-axis direction, as shown in FIG. For this reason, the mode field pattern of the substrate-type optical waveguide 21 is an ellipse whose longitudinal direction is the x-axis direction.

The first optical fiber 22 is, for example, a glass fiber having a glass core 221 and has a polarization maintaining function by a stress applying unit (not shown). The core diameter of the first optical fiber 22 is smaller than the core diameter of the second optical fiber 23 described later. Therefore, the mode field diameter of the first optical fiber 22 is set to the mode of the second optical fiber 23 in the vicinity of the end surface of the core 221 of the first optical fiber 22 facing the core 231 of the second optical fiber 23. A mode field diameter converter 222 is provided for matching with the field diameter.

7B is a front view showing an end face 22a facing the substrate-type optical waveguide 21 among the end faces of the first optical fiber 22. FIG. The cross section (end face) shape of the core 221 of the first optical fiber 22 is an ellipse whose longitudinal direction is the x-axis direction, as shown in FIG. Therefore, the mode field pattern of the first optical fiber 22 has an elliptical shape with the x axis as the longitudinal direction, similar to the mode field pattern of the substrate type optical waveguide 21. For this reason, the connection loss between the substrate-type optical waveguide 21 and the first optical fiber 22 can be kept small.

The second optical fiber 23 is, for example, a glass fiber having a glass core 231 and has a polarization maintaining function by a stress applying unit (not shown). The cross section (end face) shape of the core 231 of the second optical fiber 23 is circular.

The polarization maintaining fiber 1 described above can be suitably used as the first optical fiber 22 in the optical device 2.

Note that fluorine is added to the inner cladding 12 of the polarization maintaining fiber 1 as described above. This fluorine has a function of promoting diffusion of germanium added to the core 11 by heating. For this reason, when the polarization maintaining fiber 1 is used as the first optical fiber 22, the core diameter of the first optical fiber 22 is fused to the heat when the first optical fiber 22 is fused to the second optical fiber 23. It can be enlarged in the vicinity. For this reason, when the polarization maintaining fiber 1 is used as the first optical fiber 22, the mode field diameter converter 222 can be easily realized simply by fusing the first optical fiber 22 to the second optical fiber 23. it can.

8A is a graph showing a mode field pattern of the substrate type optical waveguide 21. FIG. In FIG. 8A, a chain line represents an electric field distribution on a straight line passing through the central axis of the core 211 and parallel to the x axis, and a dotted line is a straight line passing through the central axis of the core 211 and parallel to the y axis. Represents the electric field distribution. FIG. 8B is a graph showing a mode field pattern of the first optical fiber 22 (polarization-maintaining fiber 1). In FIG. 8B, a chain line represents an electric field distribution on a straight line passing through the central axis of the core 221 and parallel to the x axis, and a dotted line is a straight line passing through the central axis of the core 221 and parallel to the y axis. Represents the electric field distribution. Comparing these graphs, it can be seen that the mode field pattern of the substrate type optical waveguide 21 and the mode field pattern of the first optical fiber 22 are in good agreement.

Here, an application example in which the polarization maintaining fiber 1 is used as the first optical fiber 22 in the optical device 2 including the substrate type optical waveguide 21, the first optical fiber 22, and the second optical fiber 23 will be described. However, it is not limited to this. For example, in the optical device 2 in which the substrate type optical waveguide 21 is omitted, that is, in the optical device 2 including the first optical fiber 22 and the second optical fiber 23, the polarization maintaining fiber 1 is used as the first optical fiber 22. Also good. Alternatively, in the optical device 2 in which the second optical fiber 23 is omitted, that is, in the optical device 2 including the substrate type optical waveguide 21 and the first optical fiber 22, the polarization maintaining fiber 1 is used as the first optical fiber 22. Also good.

(Supplementary information on melting viscosity)
As described above, the relationship of η3 << η2 << η1 <η4 is established among the melt viscosity η1, η2, η3, and η4 of the core 11, the inner cladding 12, the stress applying portions 13a to 13b, and the outer cladding 14. The inequality sign “<<” between η3 and η2 means that the difference between η3 and η2 is larger than the difference between η1 and η4, and the difference between η3 and η2 is larger than a specific value. Does not mean. Similarly, the inequality sign “<<” between η2 and η1 means that the difference between η2 and η1 is larger than the difference between η1 and η4, and the difference between η2 and η1 is larger than a specific value. Does not mean that. Hereinafter, the melt viscosity η1, η2, η3, and η4 of the core 11, the inner cladding 12, the stress applying portions 13a to 13b, and the outer cladding 14 will be supplemented with reference to FIG.

FIG. 9 is a side view showing the base material 1A during drawing of the polarization-maintaining fiber 1, a cross-sectional view taken along the line AA ′ serving as the XY plane (hereinafter referred to as the AA ′ cross-section), and BB FIG. 6 is a cross-sectional view of a cross section taken along a line CC 'serving as an XY plane (hereinafter referred to as a CC' cross section). Here, according to FIG. 9, the cross-sectional view of the A-A ′ cross section and the cross-sectional view of the C-C ′ cross section are drawn in the same size for facilitating understanding of the drawing, but the actual sizes are different from each other. As shown in FIG. 9, the base material 1A is reduced in diameter in a wire drawing furnace. The A-A ′ cross section shown in FIG. 9 is a cross section where the diameter reduction starts, that is, a cross section where melting starts. Accordingly, above the section AA ′, the diameter of the base material 1A matches the diameter of the base material 1A before drawing, and below the section AA ′, the diameter of the base material 1A is the base material 1A before drawing. Smaller than the diameter of. The C-C ′ cross section shown in FIG. 9 is a cross section where the diameter reduction ends, that is, a cross section where the melting ends, or a cross section when the polarization maintaining fiber 1 is completed after the solidification ends. Therefore, the diameter of the base material 1A is larger than the diameter of the base material 1A after drawing above the section CC ′, and the diameter of the base material 1A is below the section after drawing CC ′. It corresponds to a diameter of 1A. A BB ′ line shown in FIG. 9 is an imaginary line showing a region where the temperature of the base material 1A is highest in the shape of a line extending in the X-axis direction of FIG. It is located between the CC ′ cross section. The section from the A-A ′ cross section to the C-C ′ cross section of the base material 1A may be called “neck down”. The A-A ′ cross section can be rephrased as a cross section where neck-down starts. Further, the C-C ′ section can be rephrased as a section where the neck-down ends. In addition, the base material 1A after drawing can be rephrased as the polarization maintaining fiber 1.

By the way, in the base material 1A from the start of melting to the end of melting, that is, in the base material 1A from the AA ′ section to the CC ′ section, each of the melt viscosity η1, η2, η3, η4 The value may be different for each position in the Z-axis direction and for each position in the X-axis direction. This is because the melt viscosity η1, η2, η3, and η4 depend on the temperature of the base material 1A, and the temperature of the base material 1A can be different for each position. Therefore, the melt viscosity η1, η2, η3, and η4 at the position (x, y, z) are respectively expressed as η1 (x, y, z), η2 (x, y, z), η3 (x, y, z). ), Η4 (x, y, z). Here, the z-axis is a coordinate axis parallel to the longitudinal direction of the base material 1A, and the x-axis and the y-axis are coordinate axes orthogonal to the longitudinal direction of the base material 1A.

Η1 (z), η2 (z), η3 (z), and η4 (z) in each cross section from the AA ′ cross section to the CC ′ cross section are the melt viscosity η1 (x, y , Z), η2 (x, y, z), η3 (x, y, z), and η4 (x, y, z) as a spatial average. For example, the melt viscosity η1 (z _A ), η2 (z _A ), η3 (z _A ), and η4 (z _A ) in the AA ′ section are respectively the melt viscosity η1 (x _{, y, z a), η2} (x, y, z a), η3 (x, y, z a), η4 (x, y, is defined as a spatial average of _{z a).} In addition, η1 (z _B ), η2 (z _B ), η3 (z _B ), and η4 (z _B ) of a cross section along the BB ′ line (hereinafter referred to as a BB ′ cross section) are B− melt viscosity at η1 at B 'cross section _{(x, y, z B)} , η2 (x, y, z B), η3 (x, y, z B), η4 (x, y, z B) as a space average Defined. The melt viscosity η1 (z _C ), η2 (z _C ), η3 (z _C ), and η4 (z _C ) in the CC ′ section are respectively the melt viscosity η1 (x _{, y, z C), η2} (x, y, z C), η3 (x, y, z C), η4 (x, y, is defined as a spatial average of _{z C).}

Each value of the melt viscosity η1 (z), η2 (z), η3 (z), and η4 (z) in each cross section from the AA ′ cross section to the CC ′ cross section is as per XY plane. Can be different. However, an essential point in realizing the flattening of the core 11 is that the melt viscosity η1 (z), η2 (z), η3 (z) in each section from the AA ′ section to the CC ′ section. ), Not the value of η4 (z), but the melt viscosity η1 (z), η2 (z), η3 (z), η4 (z) in each section from the AA ′ section to the CC ′ section. It is a big and small relationship.

In addition, in this embodiment, although 1 A of base materials which satisfy | fill the following conditions 1 were demonstrated, it is not limited to this. That is, even with the base material 1A that satisfies the following condition 2 or 3, the core 11 can be flattened, similarly to the base material 1A that satisfies the following condition 1.

Condition 1: η3 (z) between the melt viscosity η1 (z), η2 (z), η3 (z), and η4 (z) in an arbitrary cross section from the AA ′ cross section to the CC ′ cross section << η2 (z) << η1 (z) <η4 (z). This magnitude relationship also holds at least as a magnitude relationship of η3 (z) << η2 (z) << η1 (z) <η4 (z) of the melt viscosity in the cooling process. This magnitude relationship also holds at least as a magnitude relationship of η3 (z) <η2 (z) <η1 (z) <η4 (z) of viscosity at the time of cooling.

Condition 2: At least the melting viscosity η1 (z), η2 (z), η3 (z), and η4 (z) in the cooling process, η3 (z) <η1 (z) <η2 (z) <η4 ( The magnitude relationship z) holds.

Condition 3: η3 (z) <η2 (z) = η1 (z) <η4 (at least between the melt viscosity η1 (z), η2 (z), η3 (z), and η4 (z) in the cooling process The magnitude relationship z) holds.

Here, conditions 1 to 3 described above can be summarized as follows. That is, at least the melt viscosity η1 (z), η2 (z), η3 (z), and η4 (z) in the cooling process is between η3 (z) <η2 (z) <η4 (z) and η3 The magnitude relationship of (z) <η1 (z) <η4 (z) is established. Here, before and after the cooling process, the melt viscosity η1 (z), η2 (z), η3 (z), and η4 (z) are between η3 (z) <η2 (z) <η4 (z), In addition, the magnitude relationship of η3 (z) <η1 (z) <η4 (z) is not necessarily established, but the magnitude relationship may be established.

Here, in each of the above conditions 1 to 3, the dopant concentrations in the core 11, the inner cladding 12, the stress applying portions 13a to 13b, and the outer cladding 14 are η3 (z) <η2 (z). <Η4 (z) and η3 (z) <η1 (z) <η4 (z) may be set so as to satisfy. For example, a configuration in which germanium is added to the core 11, a configuration in which phosphorus, germanium, and fluorine are added to the inner cladding 12 is adopted, and boron oxide (B ₂ O ₃ ) is added to the two stress applying portions 13 a to 13 b. When adopting a structure to be added and adopting quartz glass in which a dopant other than chlorine (Cl) is not intentionally added to the outer cladding 14, the following dopant concentrations are respectively η3 (z) <η2 (z) <Η4 (z) and η3 (z) <η1 (z) <η4 (z) may be set so as to satisfy. That is, the concentration of germanium in the core 11 is set to, for example, 10 to 30 wt%, and the concentrations of phosphorus and germanium in the inner cladding 12 are set to, for example, 0.5 to 2.0 wt% and 1.5 to 5. The concentration of boron oxide in the inner cladding 12 is set to 0 wt% so that the relative refractive index difference of the refractive index n2 of the inner cladding 12 with respect to the refractive index n4 of the outer cladding 14 is 0.1% or less. The concentration of boron oxide in the two stress applying portions 13a to 13b is set to 15 to 25 mol%, for example, and the concentration of chlorine in the outer cladding 14 is set to the ratio of the refractive index n2 of the inner cladding 12 to the refractive index n4 of the outer cladding 14. The refractive index difference is set to be 0.1% or less, and η3 (z) <η2 (z) <η4 (z) and η3 (z) <η1 ( z) <η4 (z) may be set so as to satisfy.

In each of the above conditions 1 to 3, the temperature of the base material 1A only needs to be set so as to satisfy the following (Condition 1) and (Condition 2).

(Condition 1) At least the temperatures of the core 11, inner clad 12, stress applying portions 13a to 13b, and outer clad 14 immediately before the cooling process satisfy the melting temperature or higher. (Condition 2) η3 (z) <η2 (z) <η4 (z) at least between the melt viscosity η1 (z), η2 (z), η3 (z), and η4 (z) in the cooling process, and , Η3 (z) <η1 (z) <η4 (z).

Here, “melting viscosity η1 (z), η2 (z), η3 (z), η4 (z) in the cooling process” refers to the BB ′ cross section where the temperature becomes highest and the diameter reduction is completed. Means melt viscosity η1 (z), η2 (z), η3 (z), η4 (z) in an arbitrary cross-section sandwiched between the −C ′ cross-section. Therefore, the molten base material 1A can be cooled mainly between the B-B ′ section and the C-C ′ section.

Further, here, the definition of η1 (z), η2 (z), η3 (z), and η4 (z) in each cross section from the AA ′ cross section to the CC ′ cross section is as follows. Although the spatial average of η1 (x, y, z), η2 (x, y, z), η3 (x, y, z), and η4 (x, y, z) is employed, it is not limited to this. That is, the definitions of η1 (z), η2 (z), η3 (z), and η4 (z) in each cross section from the AA ′ cross section to the CC ′ cross section are as follows: (a) Viscosity at melting in the cross section The minimum value of η1 (x, y, z), η2 (x, y, z), η3 (x, y, z), η4 (x, y, z) can be adopted, and (b) It is also possible to adopt the maximum values of the melt viscosity η1 (x, y, z), η2 (x, y, z), η3 (x, y, z), η4 (x, y, z) in the cross section. , (C) The median value of the melt viscosity η1 (x, y, z), η2 (x, y, z), η3 (x, y, z), η4 (x, y, z) in the cross section is adopted. You can also Even when these definitions are adopted, the core 11 can be flattened as long as the base material 1A satisfies the above condition 1. If the base material 1A satisfies the above condition 2, the core 11 can be flattened.

In the base material 1A satisfying the above condition 2, when the base material 1A is melted, η3 (z) <η2 (z) <η4 (z) and η3 (z) <η1 in an arbitrary cross section The magnitude relationship of (z) <η4 (z) is established. In addition, the polarization maintaining fiber 1 manufactured from the base material 1A satisfying the above condition 2 has an arbitrary cross section of η3 (z) <η2 (z) <η4 (z) when the polarization maintaining fiber 1 is melted. ) And a magnitude relationship of η3 (z) <η1 (z) <η4 (z) is established. That is, the base material 1A used in the manufacturing method described above has η3 (z) <η2 (z) <η4 (z) and η3 (z) <η1 (z) <η4 in an arbitrary cross section when melted. It can be characterized by the fact that the magnitude relationship (z) holds. Similarly, when the polarization maintaining fiber 1 manufactured by the above manufacturing method is melted, η3 (z) <η2 (z) <η4 (z) and η3 (z) <η1 ( z) <η4 (z).

(Supplement about the effect)
The polarization maintaining fiber 1 in which germanium is added to the core 11 and a dopant such as fluorine or phosphorus is co-added to the inner cladding 12 is a core having a circular mode cross section with a mode field diameter larger than that of the polarization maintaining fiber 1. In the case of fusion splicing with another optical fiber, it is possible to reduce the connection loss. There are at least two reasons for this effect.

The first reason is that germanium added to the core 11 is diffused in the inner cladding 12 by heating at the time of fusion splicing, as is widely known as TEC (Thermally Diffused Expanded Core) technology. The mode field diameter of the holding fiber 1 is increased. The second reason is that the cross section of the core of the polarization-maintaining fiber 1 when germanium added to the core 11 due to heating at the time of fusion splicing diffuses into the inner cladding 12 as shown in the following reference. The flatness of the core decreases (for example, the cross section of the core that was an ellipse approaches a perfect circle).

References: H. YOKOTA, et al., “Design of Polarization-Maintaning Optical Fiber Suitable for Thermally-Diffused Expanded Core Techniques,” IEICE TRANS. COMMUN., VOL. E80-B, NO. 4, pp516-521 1997.

In addition, the polarization maintaining fiber 1 in which germanium is added to the core 11 and the dopant is added to the inner clad 12 can suppress the connection loss when the mode field pattern is connected to the elliptical substrate type optical waveguide. There is an effect that can be. This is because the connection between the polarization-maintaining fiber 1 and the substrate type optical waveguide can be realized without heating the polarization-maintaining fiber 1 while keeping the cross section of the core flat. Accordingly, the polarization maintaining fiber 1 in which germanium is added to the core 11 and the dopant is co-added to the inner cladding 12 is (a) a core having a circular mode cross section with a mode field diameter larger than that of the polarization maintaining fiber 1. In the case of fusion splicing with another optical fiber, the mode field diameter of the polarization-maintaining fiber 1 is likely to approach the mode field diameter of the other optical fiber, so that the connection loss can be kept small. b) It can be said that this is an excellent polarization-maintaining fiber having the advantage that the connection loss can be kept small when the mode field pattern is connected to an elliptical substrate type optical waveguide. The optical device 2 (see FIGS. 6 to 8) provided with the polarization maintaining fiber 1 can also be said to be an excellent optical device having the same advantages.

(Summary)
The polarization maintaining fiber according to the present embodiment includes a core, an inner clad that encloses the core, two stress applying portions that sandwich the inner clad from both sides, and the inner cladding and the two stress applying portions. An outer clad, wherein the inner clad is indented into each of the two stress applying portions, and a cross section of the core is flat so that a direction in which the two stress applying portions are arranged is a longitudinal direction. It is characterized by that.

The polarization maintaining fiber configured as described above includes a core, an inner clad enclosing the core, two stress applying portions sandwiching the inner clad from both sides, the inner cladding and the two stress applying portions. An inner cladding, and each of the two stress-applying portions can be easily manufactured by drawing a base material that is recessed in the inner cladding. Therefore, according to the above configuration, a polarization maintaining fiber that has a flat core cross section and can be easily manufactured can be realized. In addition, such a base material is realized by inserting a rod serving as a base material of the stress applying portion into a hole formed by using a drill tool or the like so as to be inserted into the inner cladding. Can do.

In the polarization maintaining fiber according to the present embodiment, it is preferable that the stress applying portion is made of quartz glass to which boron is added.

According to the above configuration, the melting viscosity of the stress applying part can be made significantly smaller than the melting viscosity of pure quartz glass. Thereby, after drawing, the time when the stress applying portion is cured can be set later than the time when the core and the inner cladding are cured. For this reason, the core can be deformed such that the inner clad is deformed so as to be recessed into the stress applying portion and the cross section is flattened.

In the polarization maintaining fiber according to the present embodiment, the core is made of quartz glass doped with germanium, and the inner cladding is doped with fluorine and an updopant that offsets the refractive index lowering effect of fluorine. It is preferable that the quartz glass is formed.

According to the above configuration, since fluorine is added to the inner clad, germanium added to the core by heating can be diffused into the inner clad. That is, according to said structure, the polarization maintaining fiber which has the property that a core expands by heating is realizable. Since the up-dopant that cancels the refractive index lowering effect of fluorine is added to the inner cladding, there is no possibility that the refractive index difference between the core and the inner cladding is lost. Therefore, it is possible to realize a polarization maintaining fiber having a property that the core expands by heating without impairing the optical confinement function.

In the polarization maintaining fiber according to the present embodiment, it is preferable that the updopant includes one or both of phosphorus and germanium.

According to the above configuration, the refractive index lowering effect of fluorine can be offset by the refractive index increasing action of one or both of phosphorus and germanium.

In the polarization maintaining fiber according to the present embodiment, the melt viscosity η1 (z) of the core in each cross section, the melt viscosity η2 (z) of the inner cladding, the melt viscosity η3 (z) of the stress applying portion, And the melt viscosity η4 (z) of the outer cladding have the following magnitude relationship: η3 (z) <η2 (z) <η4 (z) and η3 (z) <η1 (z) <η4 (z) It is preferable to hold.

According to the above configuration, a polarization maintaining fiber that has a flat core cross section and can be manufactured more easily can be realized.

An optical device according to the present embodiment includes the polarization maintaining fiber according to the present embodiment, and an optical waveguide whose end surface is opposed to the end surface of the polarization maintaining fiber, and whose mode field pattern is an ellipse. It is preferable.

According to the above configuration, the mode field pattern of the polarization-maintaining fiber can be made elliptical like the mode field pattern of the optical waveguide. Therefore, an optical device with a small connection loss can be realized.

An optical device according to the present embodiment includes a polarization maintaining fiber according to the present embodiment, an optical fiber having an end surface fused to the end surface of the polarization maintaining fiber, and a mode field diameter of the polarization maintaining fiber. It is preferable to provide an optical fiber larger than the mode field diameter.

According to the above configuration, the core of the polarization-maintaining fiber is made of quartz glass to which germanium is added, and the inner cladding of the polarization-maintaining fiber offsets the fluorine and the refractive index lowering effect of fluorine. A mode field conversion unit that matches the mode field diameter of the polarization maintaining fiber with the mode field diameter of the optical fiber, and the polarization maintaining fiber is It can be formed easily when fused to an optical fiber.

An optical device according to the present embodiment includes a polarization maintaining fiber according to the present embodiment, an optical waveguide having an end surface facing one end surface of the polarization maintaining fiber, and an optical waveguide having an elliptical mode field pattern. An optical fiber whose end face is fused to the other end face of the polarization maintaining fiber, and the mode field diameter of the optical fiber is larger than the mode field diameter of the polarization maintaining fiber. preferable.

According to the above configuration, the mode field pattern of the polarization-maintaining fiber can be made elliptical like the mode field pattern of the optical waveguide. Therefore, the connection loss between the polarization maintaining fiber and the optical waveguide can be reduced. Further, according to the above configuration, the core of the polarization maintaining fiber is configured by quartz glass to which germanium is added, and the inner cladding of the polarization maintaining fiber is fluorine and a refractive index lowering effect of fluorine. A mode field conversion unit that matches the mode field diameter of the polarization maintaining fiber with the mode field diameter of the optical fiber, wherein the polarization maintaining fiber includes: Can be easily formed when it is fused to the optical fiber. Therefore, the connection loss between the polarization maintaining fiber and the optical fiber can be kept small.

In order to achieve the above object, the base material of the polarization maintaining fiber according to the present embodiment includes a core, an inner cladding that encloses the core, two stress applying portions that sandwich the inner cladding from both sides, and the inner surface. A clad and an outer clad that encloses the two stress applying portions, and each of the two stress applying portions is indented into the inner clad.

According to the above configuration, it is possible to realize a polarization-maintaining fiber base material that can easily obtain a polarization-maintaining fiber having a flat cross section by drawing the base material.

In the base material of the polarization maintaining fiber according to the present embodiment, the melting viscosity η1 (z) of the core, the melting viscosity η2 (z) of the inner clad, and the melting viscosity η3 ( z) and the melt viscosity η4 (z) of the outer cladding, η3 (z) <η2 (z) <η4 (z) and η3 (z) <η1 (z) <η4 (z) It is preferable that a magnitude relationship is established.

According to the above configuration, it is possible to realize a polarization maintaining fiber preform that can more easily obtain a polarization maintaining fiber having a flat cross section by drawing the preform.

In order to achieve the above object, a polarization maintaining fiber manufacturing method according to the present embodiment includes a core, an inner cladding that encloses the core, two stress applying portions that sandwich the inner cladding from both sides, and the inner surface. A method of manufacturing a polarization-maintaining fiber including a step of drawing a base material including a clad and an outer cladding including the two stress applying portions, wherein each of the two stress applying portions is provided in the base material. Is intruded into the inner clad, and in the polarization maintaining fiber, the inner clad is indented into each of the two stress applying portions, and the cross-section of the core is an array of the two stress applying portions. It is flat so that the direction may be the longitudinal direction.

According to the above configuration, it is possible to easily manufacture a polarization maintaining fiber having a flat cross section.

In the polarization maintaining fiber manufacturing method according to the present embodiment, the melt viscosity η1 of the core in the cross section between the cross section where the temperature is highest in the polarization maintaining fiber being drawn and the cross section where the diameter reduction is finished ( z), the melting viscosity η2 (z) of the inner cladding, the melting viscosity η3 (z) of the stress applying portion, and the melting viscosity η4 (z) of the outer cladding η3 (z) <η2 ( z) <η4 (z) and η3 (z) <η1 (z) <η4 (z) are preferably satisfied.

According to the above configuration, a polarization maintaining fiber having a flat cross section can be more easily manufactured.

(Additional notes)
The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and the embodiments can be obtained by appropriately combining technical means disclosed in different embodiments. The form is also included in the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 1 Polarization-maintaining fiber 11 Core 12 Inner clad 13a-13b Stress giving part 14 Outer clad 1A Base material of polarization-maintaining fiber 2 Optical device 21 Substrate type optical waveguide (optical waveguide)
22 First optical fiber (Polarization-maintaining fiber)
23 Second optical fiber

Claims (12)

The core,
An inner clad enclosing the core;
Two stress applying portions sandwiching the inner cladding from both sides;
An outer clad including the inner clad and the two stress applying portions,
The inner clad is indented into each of the two stress applying portions, and the cross section of the core is flattened so that the alignment direction of the two stress applying portions is the longitudinal direction.
A polarization-maintaining fiber characterized by that. The stress applying part is made of quartz glass to which boron is added,
The polarization maintaining fiber according to claim 1. The melting viscosity η1 (z) of the core in each cross section, the melting viscosity η2 (z) of the inner cladding, the melting viscosity η3 (z) of the stress applying portion, and the melting viscosity η4 (z) of the outer cladding. Between η3 (z) <η2 (z) <η4 (z) and η3 (z) <η1 (z) <η4 (z) holds.
The polarization maintaining fiber according to claim 1 or 2, wherein The core is made of quartz glass to which germanium is added,
The inner cladding is made of quartz glass to which fluorine and an updopant that cancels the refractive index lowering effect of fluorine are added.
The polarization-maintaining fiber according to any one of claims 1 to 3, wherein: The updopant includes one or both of phosphorus and germanium.
The polarization maintaining fiber according to claim 4. The polarization maintaining fiber according to any one of claims 1 to 5,
An end face is an optical waveguide facing the end face of the polarization maintaining fiber, and the mode field pattern includes an elliptical optical waveguide.
An optical device characterized by that. The polarization maintaining fiber according to claim 4 or 5,
An optical fiber having an end face fused to the end face of the polarization maintaining fiber, and an optical fiber having a mode field diameter larger than the mode field diameter of the polarization maintaining fiber.
An optical device characterized by that. The polarization maintaining fiber according to claim 4 or 5,
An optical waveguide having an end face opposed to one end face of the polarization-maintaining fiber, and an optical waveguide having an elliptical mode field pattern;
An optical fiber having an end face fused to the other end face of the polarization maintaining fiber, and an optical fiber having a mode field diameter larger than the mode field diameter of the polarization maintaining fiber;
An optical device characterized by that. The core,
An inner clad enclosing the core;
Two stress applying portions sandwiching the inner cladding from both sides;
An outer clad including the inner clad and the two stress applying portions,
Each of the two stress applying portions is recessed in the inner cladding,
A base material for a polarization maintaining fiber. The melting viscosity η1 (z) of the core in each cross section, the melting viscosity η2 (z) of the inner cladding, the melting viscosity η3 (z) of the stress applying portion, and the melting viscosity η4 (z) of the outer cladding. Between η3 (z) <η2 (z) <η4 (z) and η3 (z) <η1 (z) <η4 (z) holds.
The base material of the polarization maintaining fiber according to claim 9. Drawing a base material comprising a core, an inner clad enclosing the core, two stress applying portions sandwiching the inner clad from both sides, and an outer clad enclosing the inner clad and the two stress applying portions A method of manufacturing a polarization-maintaining fiber including the step of:
In the base material, each of the two stress applying portions is invaginated in the inner cladding,
In the polarization maintaining fiber, the inner clad is indented into each of the two stress applying portions, and the cross section of the core is flattened so that the alignment direction of the two stress applying portions is a longitudinal direction. Yes,
A method of manufacturing a polarization-maintaining fiber. In the base material during drawing, the melt viscosity η1 (z) of the core and the melt viscosity η2 (z) of the inner clad in the cross section between the cross section where the temperature becomes highest and the cross section where the diameter reduction is finished, Between the melting viscosity η3 (z) of the stress applying portion and the melting viscosity η4 (z) of the outer cladding, η3 (z) <η2 (z) <η4 (z) and η3 (z) < A magnitude relationship of η1 (z) <η4 (z) holds.
The method of manufacturing a polarization maintaining fiber according to claim 11.

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